Physiology_Levy

Physiology_Levy. The base may be hydroxide ions (OH−), formate (HCO2−), oxalate, HCO3− , or sulfate. The positive transepithelial voltage in the lumen, indicated by the plus sign inside the circle in the tubular lumen, is generated by diffusion of Cl− (lumen to blood) across the tight junction. The high [Cl−] of tubular fluid provides the driving force for diffusion of Cl− . Some glucose is also reabsorbed in the second half of the proximal tubule by a mechanism similar to that described in the first half of the proximal tubule, except that the Na+/glucose symporter (SGLT1 gene) transports 2Na+ with one glucose and has higher affinity and lower capacity than the Na+/glucose symporter in the first part of the proximal tubule, depicted in


Physiology_Levy

Physiology_Levy. Fig. 34.1 . In addition, glucose exits the cell across the basolateral membrane via GLUT1 rather than via GLUT2 as in the first part of the proximal tubule (not shown). KCC, KCl symporter. aAtrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) inhibit vasopressin (AVP)-stimulated water permeability. and one or more Cl−-base antiporters (e.g., CFEX). Because the secreted H+ and base combine in the tubular fluid and reenter the cell, operation of the Na+/H+ and Cl−-base antiporters is equivalent to uptake of NaCl from tubular fluid into the cell. Na+ leaves the cell via Na+,K+-ATPase, and Cl− leaves the cell and enters the blood via a K+/ Cl− symporter (KCC) and a Cl− channel in the basolateral membrane.


Physiology_Levy

Physiology_Levy. Some NaCl is also reabsorbed across the second half of the proximal tubule via a paracellular route. Paracellular NaCl reabsorption occurs because the rise in [Cl−] in tubule fluid in the first half of the proximal tubule creates a [Cl−] gradient (140 mEq/L in the tubule lumen and 105 mEq/L in the interstitium). This concentration gradient favors diffusion of Cl− from the tubular lumen across the tight junctions into the lateral intercellular space. Movement of the negatively charged Cl− results in the tubular fluid becoming positively charged relative to blood. This positive transepithelial voltage causes diffusion of positively charged Na+ out of the tubular fluid across the tight junction into blood. Thus in the second half of the proximal tubule, some Na+ and Cl− are reabsorbed across the tight junctions via passive diffusion.


Physiology_Levy

Physiology_Levy. In summary, reabsorption of Na+ and Cl− in the proximal tubule occurs via both paracellular and transcellular pathways. Approximately 67% of the NaCl filtered each day is reabsorbed in the proximal tubule. Of this amount, two-thirds moves across the transcellular pathway, whereas the remaining one-third moves across the paracellular pathway ( Table 34.4 The proximal tubule reabsorbs 67% of the filtered water ( Table 34.5 ). The driving force for water reabsorption is a transtubular osmotic gradient established by reabsorption of solute (e.g., NaCl, Na+-glucose). Reabsorption of Na+ along with organic solutes, HCO3 − , and Cl− from tubular fluid into the lateral intercellular spaces reduces the osmolality of the tubular fluid and increases the osmolality of the lateral intercellular space. The osmotic gradient across the proximal tubule established by these transport processes is only a few mOsm/L (


Physiology_Levy

Physiology_Levy. Fig. 34.4 ). Because the proximal tubule is highly permeable to water, primarily owing to expression of aquaporin water channels (AQP1) in the apical and basolateral membranes, water is reabsorbed across cells by osmosis. In addition the tight junctions in the proximal tubule are also water permeable, so some water is also reabsorbed across the paracellular pathway between proximal tubular cells. Accumulation of fluid and solutes within the lateral intercellular space increases hydrostatic pressure in this compartment. The increased hydrostatic pressure forces fluid and solutes into the capillaries. 2In addition, protein oncotic pressure in the peritubular capillaries (πpc) is elevated because of the process of glomerular filtration (see ). The elevated πpc facilitates uptake of fluid and solute into the capillary.


Physiology_Levy

Physiology_Levy. •Fig. 34.4 Routes of reabsorption of water and solute across the proximal tubule. Transport of solutes, including Na+ , Cl− , and organic solutes, into the lateral intercellular space increases the osmolality of this compartment, which establishes the driving force for osmotic reabsorption of water across the proximal tubule. This occurs because some Na+,K+-ATPase and some transporters of organic solutes, HCO3− , and Cl− are located on the lateral cell membranes and deposit these solutes between cells. Furthermore, some NaCl also enters the lateral intercellular space via diffusion across the tight junction (i.e., paracellular pathway). An important consequence of osmotic water flow across the transcellular and paracellular pathways in the proximal tubule is that some solutes, especially K+ and Ca++ , are entrained in the reabsorbed fluid and thereby reabsorbed by the process of solvent drag.


Physiology_Levy

Physiology_Levy. reabsorption in the proximal tubule. The reabsorbed fluid is slightly hyperosmotic relative to plasma. However, this difference in osmolality is so small it is commonly said that proximal tubule reabsorption is isosmotic (i.e., ≈67% of both the filtered load of solute and water are reabsorbed). Indeed, there is little difference in the osmolality of tubular fluid at the start and end of the proximal tubule. An important consequence of osmotic water flow across the proximal tubule is that some solutes, especially K+ and Ca++ , are entrained in the reabsorbed fluid and thereby reabsorbed by the process of solvent drag (see


Physiology_Levy

Physiology_Levy. Fig. 34.4 ). Reabsorption of virtually all organic solutes, Cl− and other ions, and water is coupled to Na+ reabsorption. Therefore changes in Na+ reabsorption influence reabsorption of water and other solutes by the proximal tubule. This point will be discussed later, notably in , and is especially relevant during volume depletion when increased Na+ reabsorption by the proximal tubule is accompanied by a parallel increase in HCO3 − reabsorption, which can contribute to metabolic alkalosis (i.e., volume contraction alkalosis).


Physiology_Levy

Physiology_Levy. Proteins filtered by the glomerulus are reabsorbed in the proximal tubule. As mentioned previously, peptide hormones, small proteins, and small amounts of larger proteins such as albumin are filtered by the glomerulus. Overall, only a small percentage of proteins cross the glomerulus and enter Bowman’s space (i.e., the concentration of proteins in the glomerular ultrafiltrate is only ≈ 40 mg/L). However, the total amount of protein filtered per day is significant because the glomerular filtration rate (GFR) is so high: Equation 34.1 Filtered protein = GFR ×[Protein] in the ultrafiltrate = 7200 mg/day, or 7 2. g/day


Physiology_Levy

Physiology_Levy. Filtered proteins are reabsorbed in the proximal tubule by endocytosis either as intact proteins or after being partially degraded by enzymes on the surface of proximal tubule cells. Once the proteins and peptides are inside the cell, enzymes digest them into their constituent amino acids, which then leave the cell across the basolateral membrane by transport proteins and are returned to the blood. Normally this mechanism reabsorbs virtually all the proteins filtered, and hence the urine is essentially protein free. However, because the mechanism is easily saturated, an increase in filtered proteins can result in proteinuria (appearance of protein in urine). Disruption of the glomerular filtration barrier to proteins increases the filtration of proteins and 610 SECTION7Berne & Levy Physiology • BOX 34.2 Some Organic Cations Secreted by Proximal Tubule • BOX 34.1 Some Organic Anions Secreted by Proximal Tubule cAMP, cGMPBile saltsHippuratesOxalateProstaglandins: PGE2, PGF2α


Physiology_Levy

Physiology_Levy. Levy Physiology • BOX 34.2 Some Organic Cations Secreted by Proximal Tubule • BOX 34.1 Some Organic Anions Secreted by Proximal Tubule cAMP, cGMPBile saltsHippuratesOxalateProstaglandins: PGE2, PGF2α UrateVitamins: ascorbate, folate


Physiology_Levy

Physiology_Levy. AcetazolamideAcyclovirAmoxicillinCaptoprilChlorothiazideFurosemideLosartanPenicillinProbenecidSalicylate (aspirin)HydrochlorothiazideSimvastatinBumetanideNonsteroidal antiinflammatory drugs (NSAIDs): indomethacin cAMP, cyclic adenosine monophosphate; cGMP, cyclic guanosine monophosphate. results in proteinuria, which is frequently seen with kidney disease. Secretion of Organic Anions and Organic Cations Cells of the proximal tubule also secrete organic anions and organic cations into the tubule fluid. Secretion of organic anions and cations by the proximal tubule plays a key role in regulating the plasma levels of xenobiotics (e.g., a variety of antibiotics, diuretics, statins, antivirals, antineoplastics, immunosuppressants, neurotransmitters, and nonsteroidal antiinflammatory drugs [NSAIDs]) and toxic compounds derived from endogenous and exogenous sources. Many of the organic anions and cations (


Physiology_Levy

Physiology_Levy. Boxes 34.1 34.2 ) secreted by the proximal tubule are end products of metabolism that circulate in plasma. Many of these organic compounds are bound to plasma proteins and thus are not readily filtered. Therefore only a small fraction of these potentially toxic substances are eliminated from the body by excretion resulting from filtration alone. Thus secretion of organic anions and cations, including many toxins from the peritubular capillary into the tubular fluid, promote elimination of these compounds from plasma entering the kidneys. Hence these substances are removed from plasma by both filtration and secretion. It is important to note that when kidney function is reduced by disease, urinary excretion of organic anions and cations is severely reduced, which can lead to increased plasma levels of xenobiotics and potentially toxic accumulation of organic anions and cations.


Physiology_Levy

Physiology_Levy. Water channels called aquaporins (AQPs) mediate transcellular reabsorption of water across many nephron segments. To date, 13 aquaporins have been identified. The AQP family is divided into two groups based on their permeability characteristics. One group (aquaporins) is permeable to water (AQP0, AQP1, AQP2, AQP4, AQP5, AQP6, AQP8, AQP11, and AQP12). The other group (aquaglyceroporins) is permeable to water and small solutes, especially glycerol (AQP3, AQP7, AQP9, AQP10). Aquaporins form tetramers in the plasma membrane of cells, with each subunit forming a water channel. In the kidneys, AQP1 is expressed in the apical and basolateral membranes of the proximal tubule and in portions of the descending thin limb of Henle’s loop. The importance of AQP1 in renal water reabsorption is underscored by studies in which the AQP1 gene was “knocked out” in mice. These mice exhibit increased urine output (polyuria) and reduced ability to concentrate urine. In addition the osmotic water


Physiology_Levy

Physiology_Levy. underscored by studies in which the AQP1 gene was “knocked out” in mice. These mice exhibit increased urine output (polyuria) and reduced ability to concentrate urine. In addition the osmotic water permeability of the proximal tubule is fivefold less in mice lacking APQ1 than in normal mice. AQP7 and AQP8 are also expressed in the proximal tubule. AQP2 is expressed in the apical plasma membrane of principal cells in the collecting duct, and its abundance in the membrane is regulated by arginine vasopressin (AVP) (see ). AQP3 and AQP4 are expressed in the basolateral membrane of principal cells in the collecting duct, and mice deficient in these AQPs (i.e., AQP3 and AQP4 knockout mice) have defects in the ability to concentrate urine (see ). AQPs are also expressed in many other organs in the body, including the lung, eye, skin, secretory glands, and brain, where they play key physiological roles. For example, AQP4 is expressed in cells that form the blood-brain barrier. Knockout of


Physiology_Levy

Physiology_Levy. the body, including the lung, eye, skin, secretory glands, and brain, where they play key physiological roles. For example, AQP4 is expressed in cells that form the blood-brain barrier. Knockout of AQP4 affects the water permeability of the blood-brain barrier such that brain edema is reduced in AQP4-deficient mice after acute water loading and subsequent development of hyponatremia.


Physiology_Levy

Physiology_Levy. ATP ATP ADP ˜-KG NaDC3 OAT 1,2,3 OAT4 OA– ˜-KG MRP2/4 BCRP K+ •Fig. 34.5 Secretion of organic anion (OA−) across the proximal tubule. OA−s enter the cell across the basolateral membrane by one of three OA−/α-ketoglutarate (α-KG) antiporter mechanisms (organic anion transporters, OAT1, OAT2, OAT3). Uptake of α-KG into the cell against its chemical concentration gradient is driven by movement of Na+ into the cell via the Na+-dicarboxylate transporter (NaDC3). The [Na+] inside the cell is low because of the Na+,K+-ATPase in the basolateral membrane, which transports Na+ out of the cell in exchange for K+ . The α-KG recycles across the basolateral membrane on the OATs in exchange for OA− . OA−s leave the cell across the apical membrane by multidrug drug resistance proteins (MRP2 and 4), and by breast cancer resistance protein (BCRP), which require ATP. OAT4 in the apical membrane reabsorbs urate, an organic anion.


Physiology_Levy

Physiology_Levy. The endocytosis of proteins by the proximal tubule is mediated by apical membrane proteins that specifically bind proteins and peptides in tubule fluid. These receptors, called multiligand endocytic receptors, can bind a wide range of peptides and proteins and thereby mediate their endocytosis. Megalin and cubilin mediate protein and peptide endocytosis in the proximal tubule. Both are glycoproteins, with megalin being a member of the low-density lipoprotein receptor gene family. Fig. 34.5 illustrates the mechanisms of organic anion (OA−) transport across the proximal tubule. These secretory pathways have maximum transport rates, low specificity (i.e., they transport many OA−s), and are responsible for secretion of the OA−s listed in Box 34.1.


Physiology_Levy

Physiology_Levy. Box 34.1. OA−s are taken up into the cell across the basolateral membrane against their chemical gradient in exchange for α-ketoglutarate (α-KG) via several OA−/α-KG antiporters, including OAT1, OAT2, and OAT3. α-KG accumulates inside the cells via metabolism of glutamate and by a Na+/α-KG symporter (i.e., the Na+/dicarboxylate transporter [NaDC3]) also present in the basolateral membrane. Thus uptake of OA− into the cell against an electrochemical gradient is coupled to the


Physiology_Levy

Physiology_Levy. Urinalysis is an important and routine tool for detection of kidney disease. A thorough analysis of urine includes macroscopic, microscopic, and biochemical assessments. This is performed by visual assessment of the urine, microscopic examination of urinary sediment, and biochemical evaluation of urinary composition using dipstick reagent strips. The dipstick test is both inexpensive and fast (i.e., <5 minutes) and tests urine for both pH and the presence of many substances (e.g., bilirubin, blood, glucose, ketones, protein). It is normal to find trace amounts of protein in urine, particularly concentrated urine. Urinary proteins are derived from two principal sources: (1) filtration exceeding the reabsorptive capacity of the proximal tubule and (2) synthesis and secretion of Tamm-Horsfall glycoprotein by the thick ascending limb of Henle’s loop. Because the mechanism for protein reabsorption is “upstream” of the thick ascending limb (i.e., in the proximal tubule), the secreted


Physiology_Levy

Physiology_Levy. glycoprotein by the thick ascending limb of Henle’s loop. Because the mechanism for protein reabsorption is “upstream” of the thick ascending limb (i.e., in the proximal tubule), the secreted Tamm-Horsfall glycoprotein appears in urine. However, proteinuria in greater than trace amounts is often indicative of renal disease.


Physiology_Levy

Physiology_Levy. exit of α-KG out of the cell, down its chemical gradient generated by the Na+/α-KG symporter mechanism. The exit of OA−s across the luminal membrane into the tubular fluid are mediated by multidrug resistance proteins 2 and 4 (MRP2/4) and breast cancer resistance protein 1 (BCRP), which require adenosine triphosphate (ATP) for their operation. Recent studies reveal that OAT4 mediates reabsorption of the organic anion urate, the end product of purine catabolism, by the proximal tubule (see


Physiology_Levy

Physiology_Levy. Fig. 34.5 •Fig. 34.6 Organic cation (OC+) secretion across the proximal tubule. OC+s enter the cell across the basolateral membrane primarily by OCT2. Uptake of OC+s into the cell against their chemical concentration gradient is driven by the cell-negative potential difference. OC+s leave the cell across the apical membrane in exchange with H+by electrically neutral multidrug and toxin transporters (MATE1 and MATE2-K) and by multidrug resistance protein (MDR1), which requires ATP. ATP OCT2MATE MDR1 Na+ OC+ OC+ H+ K+ Blood Tubular fluid OC+ ATP ADP


Physiology_Levy

Physiology_Levy. Fig. 34.6 illustrates the mechanism of organic cation (OC+) transport across the proximal tubule. Organic cations, including xenobiotics such as the antidiabetic agent metformin, the antiviral agent lamivudine, and the anticancer drug oxaliplatin, and many important monoamine neurotransmitters including dopamine, epinephrine, histamine, and norepinephrine are secreted by the proximal tubule. Organic cations are taken up into the cell across the basolateral membrane, primarily by the organic cation transporter 2 (OCT2). Uptake of organic cations is driven by the magnitude of the cell-negative potential difference across the basolateral membrane. Organic cation transport across the luminal membrane into the tubular fluid, which is the rate-limiting step in secretion, is mediated primarily by electroneutral multidrug and toxin extrusion transporters (MATEs) and MDR1 (also known as P-glycoprotein), which requires ATP for its operation. These transport mechanisms are nonspecific, and


Physiology_Levy

Physiology_Levy. by electroneutral multidrug and toxin extrusion transporters (MATEs) and MDR1 (also known as P-glycoprotein), which requires ATP for its operation. These transport mechanisms are nonspecific, and several organic cations usually compete for secretion via a given transport pathway.


Physiology_Levy

Physiology_Levy. Henle’s loop reabsorbs approximately 25% of the filtered NaCl and 15% of the filtered water. Reabsorption of NaCl


Physiology_Levy

Physiology_Levy. Because many organic anions compete for the same secretory pathways, elevated plasma levels of one transported anion often inhibit secretion of the others. For example, infusing p-aminohippuric acid (PAH) can reduce secretion of penicillin by the proximal tubule. Because the kidneys are responsible for eliminating penicillin, infusion of PAH into individuals receiving penicillin reduces penicillin excretion and thereby extends its biological half-life. In World War II, when penicillin was in short supply, hippurates were given with penicillin to extend its therapeutic effect. Similar competition is observed for organic cation secretion by the proximal tubule, and elevated plasma levels of one transported cation species can inhibit secretion of the other competing cations. For example, the histamine H2antagonist cimetidine used to treat gastric ulcers is secreted via organic cation transport mechanisms in the proximal tubule. If cimetidine is given to patients receiving procainamide (a


Physiology_Levy

Physiology_Levy. histamine H2antagonist cimetidine used to treat gastric ulcers is secreted via organic cation transport mechanisms in the proximal tubule. If cimetidine is given to patients receiving procainamide (a drug used to treat cardiac arrhythmias), cimetidine reduces urinary excretion of procainamide (also an organic cation) by direct competition for a common secretory pathway. As a consequence, coadministration of cationic drugs competing for the same pathway can increase the plasma concentration of both drugs to levels much higher than those observed when the drugs are given alone. This effect can lead to drug toxicity.


Physiology_Levy

Physiology_Levy. •Fig. 34.7 Transport mechanisms for NaCl reabsorption in the thick ascending limb of the loop of Henle. The positive voltage in the lumen plays a major role in driving the passive paracellular reabsorption of cations. Because the apical membrane is conductive primarily to K+ , the apical membrane voltage is more negative than the basolateral membrane voltage, which is conductive to K+ and Cl− , thereby resulting in a lumen positive transepithelial potential. Mutations in the apical membrane K+ channel (ROMK), the apical membrane 1Na+/1K+/2Cl− symporter (NKCC2), or the basolateral Cl− channel (ClCNKB) cause Bartter syndrome (see the clinical box on Bartter syndrome). CA, carbonic anhydrase.


Physiology_Levy

Physiology_Levy. in the loop of Henle occurs in both the thin and thick ascending limbs, whereas the descending thin limb does not reabsorb NaCl. In contrast, water reabsorption mediated by AQP1 water channels is exclusively restricted to the descending thin limb, whereas the ascending limb is impermeable to water. In addition, divalent cations (e.g., Ca++ and Mg++) and HCO3 − are also reabsorbed in the loop of Henle (see for more details). The thin ascending limb reabsorbs NaCl by a passive mechanism. Reabsorption of water, but not NaCl, in the descending thin limb increases [NaCl] in the tubule fluid entering the ascending thin limb. As the NaCl-rich fluid moves toward the cortex, NaCl diffuses out of the tubule lumen across the ascending thin limb and into the medullary interstitial fluid, down a concentration gradient directed from the tubule fluid to the interstitium (see for details).


Physiology_Levy

Physiology_Levy. The key element in reabsorption of solute by the thick ascending limb is Na+,K+-ATPase in the basolateral membrane (Fig. 34.7 ). As with reabsorption in the proximal tubule, reabsorption of every solute by the thick ascending limb is linked to Na+,K+-ATPase activity. This transporter maintains a low intracellular [Na+], which provides a favorable chemical gradient for the movement of Na+ from tubular fluid into the cell. This movement of Na+ across the apical membrane into the cell is mediated by the 1Na+/1K+/2Cl− symporter (NKCC2), which couples the movement of 1Na+ with 1K+ and 2Cl− . Using the potential energy released by the downhill movement of Na+ and Cl− , this symporter drives the uphill movement of K+ into the cell. K+ channels (ROMK and Maxi-K) in the apical plasma membrane play an important role in reabsorption of NaCl by the thick ascending limb. These K+ channels allow the K+ transported into the cell via the 1Na+/1K+/2Cl− symporter to recycle back into tubule fluid.


Physiology_Levy

Physiology_Levy. play an important role in reabsorption of NaCl by the thick ascending limb. These K+ channels allow the K+ transported into the cell via the 1Na+/1K+/2Cl− symporter to recycle back into tubule fluid. Because the [K+] in tubule fluid is relatively low, K+ recycling is required for continued operation of the 1Na+/1K+/2Cl− symporter. A Na+/H+ anti-porter (NHE3) in the apical cell membrane also mediates Na+ reabsorption as well as H+ secretion (HCO3 − reabsorption) in the thick ascending limb (see also ). The operation of the Na+/H+ antiporter in the apical membrane results in cellular uptake of Na+ in exchange for H+ . The production of H+ inside cells generates HCO3 −, which exits the cell across the basolateral membrane via a Cl−/HCO3 − antiporter (AE2). Na+ leaves the cell across the basolateral membrane via the Na+,K+-ATPase, whereas K+ and Cl− leave the cell via separate pathways in the basolateral membrane (i.e., K+ and Cl− channels and the K+/Cl− symporter).


Physiology_Levy

Physiology_Levy. 614 SECTION7Berne & Levy Physiology •Fig. 34.8 Transport mechanism for NaCl reabsorption in the early segment of the distal tubule. This segment is impermeable to water. Mutations in the apical membrane NaCl symporter (NCC) cause Gitelman syndrome. ATP H2O NCC Na+ K+ K+ K+ Na+ Cl– Blood Tubular fluid Cl– The voltage across the thick ascending limb is important for reabsorption of several cations. The tubular fluid is positively charged relative to blood because of the unique location of transport proteins in the apical and basolateral membranes. Two points are important: (1) increased NaCl transport by the thick ascending limb increases the magnitude of the positive voltage in the lumen, and (2) this voltage is an important driving force for reabsorption of several cations, including Na+ , K+ , Mg++ , and Ca++ , across the paracellular pathway (see


Physiology_Levy

Physiology_Levy. Fig. 34.7 ). The importance of the paracellular pathway to solute reabsorption is underscored by the observation that inactivating mutations of the tight junction protein claudin-16 reduce reabsorption of Mg++ and Ca++ by the ascending thick limb, even in the presence of a lumen positive transepithelial voltage. In summary, NaCl reabsorption across the thick ascending limb occurs via transcellular and paracellular pathways. Fifty percent of NaCl reabsorption is transcellular, and 50% is paracellular. Because the thick ascending limb does not reabsorb water, owing to a lack of water channels (i.e., AQPs), reabsorption of NaCl and other solutes reduces the osmolality of tubular fluid to less than 150 mOsm/ kg H2O. Thus because the thick ascending limb of Henle’s loop produces a fluid that is dilute relative to plasma, this segment and the adjacent distal tubule (as discussed next) are often collectively referred to as the “diluting segments.”


Physiology_Levy

Physiology_Levy. The distal tubule and collecting duct reabsorb approximately 8% of the filtered NaCl, secrete variable amounts of K+ and H+ , and reabsorb a variable amount of water (≈8%–17%). The initial segment of the distal tubule (early distal tubule) reabsorbs Na+, Cl−, and Ca++ and is impermeable to water ( Fig. 34.8 ). Entry of NaCl into the cell across the apical membrane is mediated by a Na+/Cl− symporter,


Physiology_Levy

Physiology_Levy. Chapter 2, epithelial cells are joined at their apical surfaces by tight junctions (zonula occludens). A number of proteins have now been identified as components of the tight junction, including proteins that span the membrane of one cell and link to the extracellular portion of the same molecule in the adjacent cell (e.g., occludin and claudins), as well as cytoplasmic linker proteins (e.g., ZO-1, ZO-2, and ZO-3) that link the membrane-spanning proteins to the cytoskeleton of the cell. Of these junctional proteins, claudins appear to be major determinants of the permeability characteristics of tight junctions. For example, claudin-16 and claudin-19 are critical determinants of divalent cation permeability of the tight junctions in the thick ascending limb of Henle’s loop. Mutations in human claudin-16 and claudin19 cause familial hypomagnesemia (i.e., low plasma [Mg++]) with hypercalciuria (i.e., increased Ca++ in the urine) and nephrocalcinosis (i.e., calcification of the kidney).


Physiology_Levy

Physiology_Levy. claudin-16 and claudin19 cause familial hypomagnesemia (i.e., low plasma [Mg++]) with hypercalciuria (i.e., increased Ca++ in the urine) and nephrocalcinosis (i.e., calcification of the kidney). Claudin-2 is permeable to water and may be responsible for paracellular water reabsorption across the proximal tubule. Claudin-4 has been shown in cultured kidney cells to control the permeability of the tight junction to Na+ , whereas claudin-15 determines whether a tight junction is permeable to cations or anions. Thus the permeability characteristics of the tight junctions in different nephron segments are determined at least in part by the specific claudins expressed by the cells in that segment.


Physiology_Levy

Physiology_Levy. AVP, water is reabsorbed. By contrast, in the absence of AVP the late distal tubule and collecting duct reabsorb little Bartter syndrome is a set of autosomal recessive genetic diseases characterized by hypokalemia, metabolic alkalosis, and hyperaldosteronism (see Table 34.3 ). Inactivating mutations in the gene coding for the 1Na+/1K+/2Cl− symporter (NKCC2), the apical K+ channel (ROMK), or the basolateral Cl− channel (ClCNKB) decrease both NaCl reabsorption and K+ reabsorption by the ascending thick limb, which in turn causes hypokalemia (i.e., low plasma [K+]) and a decrease in ECFV. The fall in ECFV stimulates aldosterone secretion, which in turn stimulates NaCl reabsorption and H+ secretion by the distal tubule and collecting duct.


Physiology_Levy

Physiology_Levy. Fig. 34.8 ). Na+ leaves the cell via the action of Na+,K+-ATPase, and Cl− leaves the cell via diffusion through Cl− channels and a K+/Cl− symporter (KCC4). Thus dilution of tubular fluid begins in the thick ascending limb and continues in the early segment of the distal tubule. The last segment of the distal tubule (late distal tubule) and the collecting duct are composed of three cell types: principal cells and two types of intercalated cells. As illustrated in


Physiology_Levy

Physiology_Levy. Fig. 34.9, principal cells reabsorb NaCl and water and secrete K+ . Both Na+ reabsorption and K+ secretion by these cells depend on the activity of Na+,K+-ATPase in the basolateral membrane. By maintaining a low intracellular [Na+], the Na+,K+-ATPase provides a favorable chemical gradient for movement of Na+ from tubular fluid into the cell. Because Na+ enters the cell across the apical membrane via diffusion through epithelial Na+-selective channels (ENaCs), the negative voltage inside the cell facilitates entry of Na+, which then exits the cell and enters the blood via the basolateral membrane Na+,K+-ATPase. Reabsorption of Na+ generates a negative luminal voltage across the late distal tubule and collecting duct, which provides the driving force for paracellular reabsorption of Cl− . Intercalated cells secrete either H+ or HCO3 − and play important roles in acid-base homeostasis (see ). The α-intercalated cell (see


Physiology_Levy

Physiology_Levy. Fig. 34.9 , center) secretes H+ and reabsorbs both HCO3 − and K+ and is thus important in regulating acid-base balance (see ). α-Intercalated cells reabsorb K+ by the operation of an H+,K+-ATPase (HKA) located in the apical plasma membrane. In contrast, β-intercalated cells (see Fig. 34.9 , bottom) secrete HCO3 − and reabsorb both H+ and Cl− . Chloride enters the β-intercalated cell across the apical membrane via a Cl−/HCO3 − antiporter (pendrin) and leaves the cell across the basolateral membrane via a Cl− channel. A variable amount of water is reabsorbed across principal cells in the late distal tubule and collecting duct. Water reabsorption in these segments is mediated by the AVP-regulated AQP2 water channel located in the apical plasma membrane and by AQP3 and AQP4 located in the basolateral membrane of principal cells. In the presence of water (see K+ is secreted from blood into tubular fluid by principal cells in two steps (see Fig. 34.9, top).


Physiology_Levy

Physiology_Levy. K+ is secreted from blood into tubular fluid by principal cells in two steps (see Fig. 34.9, top). First, uptake of K+ across the basolateral membrane is mediated by the action of Na+,K+-ATPase. Second, K+ leaves the cell via passive diffusion. Because [K+] inside principal cells is high (≈150 mEq/L) and [K+] in tubular fluid is low (≈10 mEq/L), K+ diffuses down its concentration gradient through apical cell membrane K+ channels (ROMK and BK) into tubular fluid. Although the negative potential inside these cells favors intracellular K+ retention, the electrochemical gradient across the apical membrane promotes secretion of K+ from the cell into tubular fluid (see ). In contrast, K+ reabsorption by α cells is mediated by an H+,K+-ATPase (HKA) located in the apical cell membrane (see


Physiology_Levy

Physiology_Levy. Fig. 34.9 , center). As a consequence these distal nephron segments possess the ability to both secrete and reabsorb K+ via independently regulated mechanisms, which contrasts with the general tendency to reabsorb Na+ along most nephron segments. Regulation of NaCl and Water Reabsorption Quantitatively, angiotensin II, aldosterone, catecholamines, natriuretic peptides, and uroguanylin are the most important hormones regulating NaCl reabsorption and thereby urinary NaCl excretion ( Table 34.6). However, other hormones (including dopamine and adrenomedullin), Starling forces, and the phenomenon of glomerulotubular balance also influence NaCl reabsorption. AVP is the only major hormone that directly regulates the amount of water excreted by the kidneys.


Physiology_Levy

Physiology_Levy. Angiotensin II has a potent stimulatory effect on the isosmotic reabsorption of NaCl and water in the proximal tubule. It also stimulates reabsorption of Na+ in the thick ascending limb of Henle’s loop, as well as the late distal tubule and collecting duct. A decrease in ECFV activates the renin-angiotensin-aldosterone system (see for more details), thereby increasing the plasma concentration of angiotensin II.


Physiology_Levy

Physiology_Levy. Aldosterone is synthesized by the glomerulosa cells of the adrenal cortex and stimulates reabsorption of NaCl by the thick ascending limb of Henle’s loop, the late distal tubule, and the collecting duct. Most of aldosterone’s effect on NaCl reabsorption reflects its action on the late distal tubule and collecting duct. Aldosterone enhances reabsorption of NaCl across principal cells in these segments by four mechanisms: (1) increasing the amount of Na+,K+-ATPase in the basolateral membrane; (2) increasing expression of the sodium channel (ENaC) in the apical cell membrane; (3) elevating Sgk1 (serum glucocorticoid-stimulated kinase; see the molecular box) levels, which also increases the expression of ENaC in the apical cell membrane; and (4) stimulating •Fig. 34.9 Transport pathways in principal cells, α-intercalated cells, and β-intercalated cells of the late segment of the distal tubule and collecting duct. CA, carbonic anhydrase. Principal cells reabsorb Na+and secrete K+. K+is


Physiology_Levy

Physiology_Levy. principal cells, α-intercalated cells, and β-intercalated cells of the late segment of the distal tubule and collecting duct. CA, carbonic anhydrase. Principal cells reabsorb Na+and secrete K+. K+is secreted by two types of K+channels (ROMK and BK) and by a K+/Cl−symporter (KCC1). α-Intercalated cells secrete H+and reabsorb HCO3−and K+, and β-intercalated cells secrete HCO3−and reabsorb H+and Cl−. CO2 + H2O HCO− 3 CA Cl− Cl− H+ ˙-Intercalated cell Pendrin V-ATPase Blood


Physiology_Levy

Physiology_Levy. CAP1 (channel-activating protease, also called prostatin), a serine protease that directly activates ENaCs by proteolysis. Taken together, these actions increase Na+ uptake across the apical cell membrane and facilitate Na+ exit from the cell interior into blood. The increase in reabsorption of Na+ generates a negative transepithelial luminal voltage across the late distal tubule and the collecting duct. This negative voltage in the lumen provides the electrochemical driving force for reabsorption of Cl− across the tight junctions. Aldosterone also stimulates secretion of K+ by the late distal tubule and collecting duct (collectively referred to as the aldosterone-sensitive distal nephron (ASDN) Angiotensin II↑ReninPT, TAL, DT/CD↑NaCl and H2O reabsorptionAldosterone↑Angiotensin II, ↑[K+]pTAL, DT/CD↑NaCl and H2O reabsorption


Physiology_Levy

Physiology_Levy. Angiotensin II↑ReninPT, TAL, DT/CD↑NaCl and H2O reabsorptionAldosterone↑Angiotensin II, ↑[K+]pTAL, DT/CD↑NaCl and H2O reabsorption ANP, BNP, urodilatin↑ECFVCD↓H2O and NaCl reabsorptionUroguanylin, guanylinOral ingestion of NaClPT, CD↓H2O and NaCl reabsorptionSympathetic nerves↓ECFVPT, TAL, DT/CD↑NaCl and H2O reabsorption Dopamine↑ECFVPT↓H2O and NaCl reabsorptionAVP↑Posm, ↓ECFVDT/CD↑H2O reabsorption aAll these hormones act within minutes, except aldosterone, which exerts its action on reabsorption of NaCl with a delay of 1 hour. Aldosterone achieves its maximal effect after a few days.bThe effect on reabsorption of H2O does not include the thick ascending limb.ANP, atrial natriuretic peptide; BNP, brain natriuretic peptide, BP, blood pressure; CD, collecting duct; DT, distal tubule; ECFV, extracellular fluid volume; [K+]p, plasma K+ concentration; Posm, plasma osmolality; PT, proximal tubule; TAL, thick ascending limb. Chapter 36]).


Physiology_Levy

Physiology_Levy. Chapter 36]). Aldosterone secretion is increased by hyperkalemia and by hypovolemia (i.e., reduced ECFV via increased angiotensin II following activation of the renin-angiotensin system [RAS]; see for more details). Aldosterone secretion is decreased by hypokalemia and natriuretic peptides (discussed in more detail next). Through its stimulation of NaCl reabsorption in the collecting duct, aldosterone also indirectly increases water reabsorption by this nephron segment.


Physiology_Levy

Physiology_Levy. The aldosterone paradox. As noted earlier, aldosterone stimulates both NaCl reabsorption and K+ secretion by the collecting duct. Although both a reduction in the ECFV (i.e., hypovolemia, see ) increase aldosterone levels, the physiological response of the kidneys differs in these two conditions. In the setting of ECFV depletion, NaCl excretion by the kidneys is reduced to restore ECFV, without an accompanying change in K+ excretion. By contrast, during hyperkalemia, K+ excretion by the kidneys is increased to normalize plasma [K+], albeit without an accompanying change in NaCl excretion. This phenomenon—the apparent independent effects of aldosterone on urinary Na+ and K+ excretion—is called the aldosterone paradox. The paradox can be explained by the observation that ECFV depletion increases aldosterone release via activation of the RAS, whereas hyperkalemia directly stimulates adrenal release of aldosterone without a requirement for RAS activation. As such, aldosterone increases in


Physiology_Levy

Physiology_Levy. aldosterone release via activation of the RAS, whereas hyperkalemia directly stimulates adrenal release of aldosterone without a requirement for RAS activation. As such, aldosterone increases in both conditions, whereas angiotensin II levels increase only during ECFV depletion and not during hyperkalemia. It is the differential regulation of transport processes in the distal tubule and collecting duct by aldosterone and angiotensin II that accounts for this paradox. The integrated physiological response to a reduction in ECFV is depicted in


Physiology_Levy

Physiology_Levy. Fig. 34.10A . During hypovolemia, angiotensin II stimulates NaCl reabsorption by the proximal tubule (not shown in Figure 34.10A ), and by the early distal tubule by activating WNK (with no lysine [K] kinase), which enhances NaCl reabsorption by activating the Na+/Cl− symporter (NCC). Aldosterone stimulates ENaC-mediated Na+ reabsorption in principal cells of the collecting duct by activating SGK1, which increases ENaC abundance in the apical plasma membrane. In parallel, angiotensin II activates WNK in principal cells, which inhibits K+ secretion via ROMK, thereby preventing increased K+ excretion despite elevated aldosterone levels, which would be expected to promote K+ secretion. Angiotensin II stimulation of proximal tubule NaCl and water reabsorption also reduces delivery of NaCl and fluid to the collecting duct, which also suppresses K+ secretion in this segment (see for more details). The corresponding integrated physiological response to hyperkalemia is depicted in


Physiology_Levy

Physiology_Levy. Fig. 34.10B . During hyperkalemia, aldosterone stimulates ROMK mediated K+ secretion by principal cells in the collecting duct by activating WNK. Because the early distal tubule is not directly responsive to aldosterone, this hormone does not stimulate NaCl reabsorption in this segment. In fact, because angiotensin II levels are not elevated by hyperkalemia, the basal activity of WNK in the early distal tubule is low, resulting in decreased NaCl reabsorption via NCC (see Fig. 34.10B ). This effect of WNK on early distal tubule NaCl reabsorption offsets the stimulatory effect of aldosterone, via SGK1, on ENaC-mediated Na+ reabsorption in principal cells of the collecting duct.


Physiology_Levy

Physiology_Levy. Atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) inhibit NaCl and water reabsorption. Secretion of ANP by the cardiac atria and BNP by the cardiac ventricles are both stimulated by increased ECFV and increased myocardial wall pressure. ANP and BNP reduce blood pressure by decreasing total peripheral resistance and enhancing urinary excretion of both NaCl and water, primarily by increasing RBF and GFR. These natriuretic peptides vasodilate the afferent arterioles and vasoconstrict the efferent arterioles, which increases GFR and thus filtration of NaCl, thereby increasing NaCl excretion (see later discussion of glomerulotubular balance for the mechanism). In addition the increase in RBF decreases the concentration of NaCl in the medullary interstitium, which in turn reduces passive NaCl reabsorption by the thin ascending •Fig. 34.10 Aldosterone paradox (see text for details). +, stimulate; −, inhibit; AII, angiotensin II; NCC, NaCl symporter; ENaC, epithelial sodium


Physiology_Levy

Physiology_Levy. passive NaCl reabsorption by the thin ascending •Fig. 34.10 Aldosterone paradox (see text for details). +, stimulate; −, inhibit; AII, angiotensin II; NCC, NaCl symporter; ENaC, epithelial sodium channel; ROMK, K+ channel; WNK, with no lysine[K] kinases. There are four WNKs in the kidney; details are not given in the figure for clarity and because some of the details of WNK regulation of NCC, ENaC, and ROMK have not been elucidated.


Physiology_Levy

Physiology_Levy. limb of Henle’s loop (see earlier discussion for details on NaCl reabsorption by this segment). ANP and BNP also inhibit NaCl reabsorption by the medullary portion of the collecting duct and inhibit AVP-stimulated water reabsorption across the collecting duct. Moreover, ANP and BNP also reduce secretion of AVP from the posterior pituitary. These actions of ANP and BNP are mediated by the activation of membrane-bound guanylyl cyclase receptors, which increases intracellular levels of the second messenger cyclic guanine monophosphate (cGMP). ANP is a more profound natriuretic and diuretic agent than BNP.


Physiology_Levy

Physiology_Levy. Urodilatin and ANP are encoded by the same gene and have similar amino acid sequences. Urodilatin is a 32–amino acid hormone that differs from ANP by the addition of four amino acids to the amino terminus. Urodilatin is secreted by the distal tubule and collecting duct and is not present in the systemic circulation; thus urodilatin influences only the function of the kidneys. Secretion of urodilatin is stimulated by a rise in blood pressure and an increase in ECFV. It inhibits NaCl and water reabsorption across the medullary portion of the collecting duct. Urodilatin is a more potent natriuretic and diuretic hormone than ANP because some of the ANP that enters the kidneys in blood is degraded by a neutral endopeptidase that has no corresponding effect on urodilatin.


Physiology_Levy

Physiology_Levy. Uroguanylin and guanylin are produced by neuroendocrine cells in the intestine in response to oral ingestion of NaCl. These hormones enter the circulation and inhibit NaCl and water reabsorption by the kidneys via activation of membrane-bound guanylyl cyclase receptors, which


Physiology_Levy

Physiology_Levy. Sgk1 (serum glucocorticoid-stimulated kinase), a serine/threonine kinase, plays an important role in maintaining NaCl and K+ homeostasis by regulating excretion of both NaCl and K+ by the kidneys. Studies in Sgk1 knockout mice reveal that this kinase is required for animals to survive severe NaCl restriction and K+ loading. NaCl restriction and K+ loading enhance plasma [aldosterone], which rapidly (in minutes) increases Sgk1 protein expression and phosphorylation. Phosphorylated Sgk1 enhances ENaC-mediated Na+ reabsorption in the collecting duct, primarily by increasing the number of ENaCs in the apical plasma membrane of principal cells and also by increasing the number of Na+,K+-ATPase pumps in the basolateral membrane. Phosphorylated Sgk1 inhibits Nedd4-2, a ubiquitin ligase that monoubiquitinylates ENaC subunits, thereby targeting them for endocytic removal from the plasma membrane and subsequent destruction in lysosomes. Inhibition of Nedd4-2 by Sgk1 reduces the


Physiology_Levy

Physiology_Levy. ligase that monoubiquitinylates ENaC subunits, thereby targeting them for endocytic removal from the plasma membrane and subsequent destruction in lysosomes. Inhibition of Nedd4-2 by Sgk1 reduces the monoubiquitinylation of ENaC, thereby reducing endocytosis and increasing the number of channels in the membrane. Sgk1 induces the translocation of K+ channels (ROMK) from an intracellular pool to the plasma membrane, and thereby enhances ROMK-mediated K+ secretion by principal cells. These effects of Sgk1 precede the aldosterone-stimulated increase in ENaC, ROMK, and Na+,K+-ATPase abundance, which leads to a delayed (>4 hours) secondary increase in NaCl and K+ transport by the collecting duct. Activating polymorphisms in Sgk1 cause an increase in blood pressure, presumably by enhancing NaCl reabsorption by the collecting duct, which increases the ECFV and thereby blood pressure.


Physiology_Levy

Physiology_Levy. increase intracellular [cGMP]. The involvement of these gut-derived hormones helps explain why the natriuretic response of the kidneys to an oral NaCl load is more pronounced than when delivered intravenously. Catecholamines stimulate reabsorption of NaCl. Catecholamines released from the sympathetic nerves (norepinephrine) and the adrenal medulla (epinephrine) stimulate reabsorption of NaCl and water by the proximal tubule, thick ascending limb of the loop of Henle, distal tubule, and collecting duct. Although sympathetic nerves are not active when ECFV is normal, when ECFV declines (e.g., after hemorrhage), sympathetic nerve activity rises and dramatically stimulates reabsorption of NaCl and water by these four nephron segments.


Physiology_Levy

Physiology_Levy. Dopamine, a catecholamine, is released from dopaminergic nerves in the kidneys and is also synthesized by cells of the proximal tubule. The action of dopamine is opposite that of norepinephrine and epinephrine. Secretion of dopamine is stimulated by an increase in ECFV, and its secretion directly inhibits reabsorption of NaCl and water in the proximal tubule. Adrenomedullin is a 52–amino acid peptide hormone that is produced by a variety of organs, including the kidneys. Adrenomedullin induces a marked diuresis and natriuresis, and its secretion is stimulated by congestive heart failure and hypertension. The major effect of adrenomedullin on the kidneys is to increase GFR and renal blood flow and


Physiology_Levy

Physiology_Levy. Liddle syndrome is a rare genetic disorder characterized by an increase in blood pressure (i.e., hypertension) secondary to an increase in ECFV. Liddle syndrome is caused by activating mutations in either the β or γ subunit of the epithelial Na+ channel (ENaC). These mutations increase the number of Na+ channels in the apical cell membrane of principal cells and thereby the amount of Na+ reabsorbed. In Liddle syndrome, the rate of renal Na+ reabsorption is inappropriately high, which leads to an increase in ECFV and hypertension. There are two different forms of pseudohypoaldosteronism (PHA) (i.e., the kidneys reabsorb NaCl as they do when aldosterone levels are low; however, in PHA, aldosterone levels are elevated). The autosomal recessive form is caused by inactivating mutations in the α, β, or γ subunit of ENaC. The cause of the autosomal dominant form is an inactivating mutation in the mineralocorticoid receptor. Pseudohypoaldosteronism is characterized by an increase in Na+


Physiology_Levy

Physiology_Levy. the α, β, or γ subunit of ENaC. The cause of the autosomal dominant form is an inactivating mutation in the mineralocorticoid receptor. Pseudohypoaldosteronism is characterized by an increase in Na+ excretion, a reduction in ECFV, hyperkalemia, and hypotension. Some individuals with expanded ECFV and elevated blood pressure are treated with drugs that inhibit angiotensin-converting enzyme (ACE) (e.g., captopril, enalapril, lisinopril) and thereby lower fluid volume and blood pressure. Inhibition of ACE blocks degradation of angiotensin I to angiotensin II and thereby lowers plasma angiotensin II levels. The decline in plasma angiotensin II concentration has three effects. First, NaCl and water reabsorption by the nephron (especially the proximal tubule) falls. Second, aldosterone secretion decreases, thus reducing NaCl reabsorption in the thick ascending limb, distal tubule, and collecting duct. Third, because angiotensin is a potent vasoconstrictor, a reduction in its concentration


Physiology_Levy

Physiology_Levy. decreases, thus reducing NaCl reabsorption in the thick ascending limb, distal tubule, and collecting duct. Third, because angiotensin is a potent vasoconstrictor, a reduction in its concentration permits the systemic arterioles to dilate and thereby lower arterial blood pressure. ACE also degrades the vasodilator hormone bradykinin; thus ACE inhibitors increase the concentration of bradykinin, a vasodilatory hormone. ACE inhibitors decrease ECFV and the arterial blood pressure by promoting renal NaCl and water excretion and by reducing total peripheral resistance.


Physiology_Levy

Physiology_Levy. thereby indirectly stimulate excretion of NaCl and water (see earlier discussion about ANP and BNP). Arginine vasopressin (AVP) regulates water reabsorption. It is the most important hormone that regulates reabsorption of water in the kidneys (see ). This hormone is secreted by the posterior pituitary gland in response to an increase in plasma osmolality (1% or more) or a decrease in ECFV (>5%–10% of normal). AVP increases the permeability of the collecting duct to water. It increases reabsorption of water by the collecting duct because of the osmotic gradient that exists across the wall of the collecting duct (see Chapter 35). AVP has little effect on urinary NaCl excretion.


Physiology_Levy

Physiology_Levy. Starling forces regulate reabsorption of NaCl and water across the proximal tubule. As previously described, Na+ , Cl−, HCO3 −, amino acids, glucose, and water are transported into the intercellular space of the proximal tubule. Starling forces between this space and the peritubular capillaries facilitate movement of the reabsorbed fluid into the capillaries. Starling forces across the wall of peritubular capillaries •Fig. 34.11 Starling forces modify proximal tubule solute and water reabsorption. (1) Solute and water are reabsorbed across the apical membrane. This solute and water then cross the lateral cell membrane. Some solute and water reenters the tubule fluid (3), and the remainder enters the interstitial space and then flows into the capillary (2). The width of the arrows is directly proportional to the amount of solute and water moving by pathways 1 to 3. Starling forces across the capillary wall determine the amount of fluid flowing through pathway 2 versus pathway 3.


Physiology_Levy

Physiology_Levy. directly proportional to the amount of solute and water moving by pathways 1 to 3. Starling forces across the capillary wall determine the amount of fluid flowing through pathway 2 versus pathway 3. Transport mechanisms in the apical cell membranes determine the amount of solute and water entering the cell (pathway 1). Pi, interstitial hydrostatic pressure; Ppc, peritubular capillary hydrostatic pressure; πi, interstitial fluid oncotic pressure; πpc, peritubular capillary oncotic pressure. Thin arrows across the capillary wall indicate the direction of water movement in response to each force.


Physiology_Levy

Physiology_Levy. consist of hydrostatic pressure in the peritubular capillary (Ppc) and lateral intercellular space (Pi) and oncotic pressure in the peritubular capillary (πpc) and lateral intercellular space (πi). Thus reabsorption of water as a result of transport of Na+ from tubular fluid into the lateral intercellular space is modified by the Starling forces. Accordingly:


Physiology_Levy

Physiology_Levy. Equation 34.2 where J is flow (positive numbers indicate flow from the intercellular space into blood). Starling forces that favor movement from the interstitium into the peritubular capillaries are πpc and Pi (Fig. 34.11 ). The opposing Starling forces are πi and Ppc. Normally the sum of the Starling forces favors movement of solute and water from the interstitial space into the capillary. However, some of the solutes and fluid that enter the lateral intercellular space leak back into the proximal tubular fluid. Starling forces do not affect transport by the loop of Henle, distal tubule, and collecting duct because these segments are less permeable to water than the proximal tubule is. A number of factors can alter the Starling forces across the peritubular capillaries surrounding the proximal tubule. For example, dilation of the efferent arteriole increases Ppc, whereas constriction of the efferent arteriole decreases it.


Physiology_Levy

Physiology_Levy. An increase in Ppc inhibits solute and water reabsorption by increasing back-leak of NaCl and water across the tight junction, whereas a decrease stimulates reabsorption by decreasing back-leak across the tight junction.


Physiology_Levy

Physiology_Levy. Peritubular capillary oncotic pressure (πpc) is partially determined by the rate of formation of the glomerular ultrafiltrate. For example, if one assumes a constant plasma flow in the afferent arteriole, the plasma proteins become less concentrated in the plasma that enters the efferent arteriole and peritubular capillary as less ultrafiltrate is formed (i.e., as GFR decreases). Hence, πpc decreases. Thus πpc is directly related to the filtration fraction (FF = GFR/renal plasma flow [RPF]). A fall in the FF resulting from a decrease in GFR, at constant RPF, decreases πpc. This in turn increases the backflow of NaCl and water from the lateral intercellular space into tubular fluid and thereby decreases net reabsorption of solute and water across the proximal tubule. An increase in FF has the opposite effect.


Physiology_Levy

Physiology_Levy. The importance of Starling forces in regulating solute and water reabsorption by the proximal tubule is underscored by the phenomenon of glomerulotubular (G-T) balance. Spontaneous changes in GFR markedly alter the filtered amount of Na+ (filtered Na+= GFR × [Na+] in the filtered fluid). Without rapid adjustments in Na+ reabsorption to counter the changes in filtration of Na+ , urinary excretion of Na+ would fluctuate widely and disturb the Na+ balance of the body and thus alter ECFV and blood pressure (see for more details). However, spontaneous changes in GFR do not alter Na+ excretion in urine or Na+ balance when ECFV is normal because of the phenomenon of G-T balance. When body Na+ balance is normal (i.e., ECFV is normal), G-T balance refers to the fact that reabsorption of Na+ and water increases in proportion to the increase in GFR and filtered amount of Na+ . Thus a constant fraction of the filtered Na+ and water is reabsorbed from the proximal tubule despite variations in GFR.


Physiology_Levy

Physiology_Levy. increases in proportion to the increase in GFR and filtered amount of Na+ . Thus a constant fraction of the filtered Na+ and water is reabsorbed from the proximal tubule despite variations in GFR. The net result of G-T balance is to reduce the impact of changes in GFR on the amount of Na+ and water excreted in urine when ECFV is normal.


Physiology_Levy

Physiology_Levy. Two mechanisms are responsible for G-T balance. One is related to the oncotic and hydrostatic pressure differences between the peritubular capillaries and the lateral intercellular space (i.e., Starling forces). For example, an increase in the GFR (at constant RPF) raises the protein concentration in glomerular capillary plasma above normal. This protein-rich plasma leaves the glomerular capillaries, flows through the efferent arterioles, and enters the peritubular capillaries. The increased πpc augments the movement of 1. The four major segments of the nephron (proximal tubule, Henle’s loop, distal tubule, and collecting duct) determine the composition and volume of urine by the processes of selective reabsorption of solutes and water and secretion of some solutes. 2.


Physiology_Levy

Physiology_Levy. 2. Tubular reabsorption of substances filtered by the glomerulus allows the kidneys to retain substances that are essential and regulate their levels in plasma by altering the degree to which they are reabsorbed. Reabsorption of Na+ , Cl− , other anions, and organic anions and cations together with water constitutes the major function of the nephron. Approximately 25,200 mEq of Na+ and 179 L of water are reabsorbed each day. Proximal tubule cells reabsorb 67% of the glomerular ultrafiltrate, and cells of Henle’s loop reabsorb about 25% of the NaCl that was filtered and about 15% of the water that was filtered. The distal segments of the nephron (distal tubule and collecting duct system) have a more limited reabsorptive capacity. However, although the proximal tubule reabsorbs Brown D, et al. New insights into the dynamic regulation of water and acid-base balance by renal epithelial cells. Am J Physiol Cell Physiol. 2012;302:C1421-C1433.


Physiology_Levy

Physiology_Levy. Brown D, et al. New insights into the dynamic regulation of water and acid-base balance by renal epithelial cells. Am J Physiol Cell Physiol. 2012;302:C1421-C1433. Dantzler WH, et al. Urine-concentrating mechanism in the inner medulla: function of the thin limbs of the loops of Henle. Clin J Am Soc Nephrol. 2014;9:1781-1789. solute and fluid from the lateral intercellular space into the peritubular capillaries. This action increases net solute and water reabsorption by the proximal tubule. The second mechanism responsible for G-T balance is initiated by an increase in the filtered amount of glucose and amino acids. As discussed earlier, reabsorption of Na+ in the first half of the proximal tubule is coupled to that of glucose and amino acids. The rate of Na+ reabsorption therefore partially depends on the filtered amount of glucose and amino acids. As the GFR and filtered amount of glucose and amino acids increase, reabsorption of Na+ and water also rises.


Physiology_Levy

Physiology_Levy. In addition to G-T balance, another mechanism minimizes changes in the filtered amount of Na+ . As discussed in , an increase in GFR (and thus in the amount of Na+ filtered by the glomerulus) activates the tubuloglomerular feedback mechanism. This action returns the GFR and filtration of Na+ to normal values. Thus spontaneous changes in GFR (e.g., caused by changes in posture and blood pressure) increase the amount of Na+ filtered for only a few minutes. The mechanisms that underlie G-T balance maintain urinary Na+ excretion constant and thereby maintain Na+ homeostasis (and ECFV and blood pressure) until the GFR returns to normal. the largest fraction of the filtered solutes and water (i.e., 67%), final adjustments in the composition and volume of urine and most of the regulation by hormones and other factors occur primarily in the distal tubule and collecting duct. 3.


Physiology_Levy

Physiology_Levy. 3. Secretion of substances from the blood into tubular fluid is a means for excreting various byproducts of metabolism, and it also serves to eliminate exogenous organic anions and cations (e.g., drugs) and toxins from the body. Many organic anions and cations are bound to plasma proteins and are therefore unavailable for ultrafiltration. Thus secretion is their major route of excretion in urine. 4. Various hormones (including angiotensin II, aldosterone, AVP, natriuretic peptides [ANP, BNP, and urodilatin], uroguanylin, and guanylin), sympathetic nerves, dopamine, and Starling forces regulate reabsorption of NaCl by the kidneys. AVP is the major hormone that regulates water reabsorption. Divers J, Freedman BI. Genetics in kidney disease in 2013: susceptibility genes for renal and urological disorders. Nat Rev Nephrol. 2014;10:69-70. Hoenig MP, Zeidel ML. Homeostasis, the milieu interieur, and the wisdom of the nephron. Clin J Am Soc Nephrol. 2014;9: 1272-1281.


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Physiology_Levy. Hoenig MP, Zeidel ML. Homeostasis, the milieu interieur, and the wisdom of the nephron. Clin J Am Soc Nephrol. 2014;9: 1272-1281. Knepper MA, et al. Molecular physiology of water balance. N Engl J Med. 2015;372:1349-1358. Kortenoeven ML, et al. Vasopressin regulation of sodium transport in the distal nephron and collecting duct. Am J Physiol Renal Physiol. 2015;309:F280-F299. McCormick JA, Ellison DH. Distal convoluted tubule. Compr Physiol. 2015;5:45-98. Mount DB. Thick ascending limb of the loop of Henle. Clin J Am Soc Nephrol. 2014;9:1974-1986. Palmer BF. Regulation of potassium homeostasis. Clin J Am Soc Nephrol. 2015;10:1050-1060. Palmer LG, Schnermann J. Integrated control of Na transport along the nephron. Clin J Am Soc Nephrol. 2015;10:676-687. Pearce D, et al. Collecting duct principal cell transport processes and their regulation. Clin J Am Soc Nephrol. 2015;10:135-146.


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Physiology_Levy. Pearce D, et al. Collecting duct principal cell transport processes and their regulation. Clin J Am Soc Nephrol. 2015;10:135-146. Pelis RM, Wright SH. Renal transport of organic anions and cations. Compr Physiol. 2011;1:1795-1835. Pluznick JL, Caplan MJ. Chemical and physical sensors in the regulation of renal function. Clin J Am Soc Nephrol. 2015;10:1626-1635. Roy A, et al. Collecting duct intercalated cell function and regulation. Clin J Am Soc Nephrol. 2015;10:305-324. Theilig F, Wu Q. ANP-induced signaling cascade and its implications in renal pathophysiology. Am J Physiol Renal Physiol. 2015;308:F1047-F1055. Weiner ID, et al. Urea and ammonia metabolism and the control of renal nitrogen excretion. Clin J Am Soc Nephrol. 2015;10: 1444-1458. Brown D, Nielsen S. The cell biology of vasopressin action. In: Taal MW, et al., eds. Brenner and Rector’s The Kidney. 9th ed. Philadelphia: Saunders; 2012.


Physiology_Levy

Physiology_Levy. Brown D, Nielsen S. The cell biology of vasopressin action. In: Taal MW, et al., eds. Brenner and Rector’s The Kidney. 9th ed. Philadelphia: Saunders; 2012. Burckhardt G, Koepsell H. Organic anion and cation transporters in renal elimination of drugs. In: Alpern RJ, et al., eds. Seldin and Giebisch’s The Kidney: Physiology and Pathophysiology. 5th ed. Philadelphia: Academic Press; 2012. Christensen EI, et al. Renal filtration, transport, and metabolism of albumin and albuminuria. In: Alpern RJ, et al., eds. Seldin and Giebisch’s The Kidney: Physiology and Pathophysiology. 5th ed. Philadelphia: Academic Press; 2012. Gamba G, Schild L. Sodium chloride transport in the loop of Henle, distal convoluted tubule, and collecting duct. In: Alpern RJ, et al., eds. Seldin and Giebisch’s The Kidney: Physiology and Pathophysiology. 5th ed. Philadelphia: Academic Press; 2012.


Physiology_Levy

Physiology_Levy. Giebisch G, Satlin L. Regulation of potassium excretion. In: Alpern RJ, et al., eds. Seldin and Giebisch’s The Kidney: Physiology and Pathophysiology. 5th ed. Philadelphia: Academic Press; 2012. Moe OW, et al. Renal transport of glucose, amino acids, sodium, chloride, and water. In: Taal MW, et al., eds. Brenner and Rector’s The Kidney. 9th ed. Philadelphia: Saunders; 2012. Preisig P, et al. Cellular mechanisms of renal tubular acidification. In: Alpern RJ, et al., eds. Seldin and Giebisch’s The Kidney: Physiology and Pathophysiology. 5th ed. Philadelphia: Academic Press; 2012. Sands JM, Layton H. The urine concentrating mechanism and urea transporters. In: Alpern RJ, et al., eds. Seldin and Giebisch’s The Kidney: Physiology and Pathophysiology. 5th ed. Philadelphia: Academic Press; 2012. Upon completion of this chapter the student should be able to answer the following questions: 1.


Physiology_Levy

Physiology_Levy. Upon completion of this chapter the student should be able to answer the following questions: 1. Why do changes in water balance result in alterations in the [Na+] of the extracellular fluid (ECF)? 2. How is the secretion of arginine vasopressin (AVP) controlled by changes in the osmolality of body fluids and in blood volume and pressure? 3. What are the cellular events associated with the action of AVP on the collecting duct, and how do they lead to an increase in the water permeability of this segment of the nephron? 4. What is the role of Henle’s loop in the production of both dilute and concentrated urine? 5. What is the composition of the medullary interstitial fluid, and how does it participate in the process of producing concentrated urine? 6. What are the roles of the vasa recta in the process of diluting and concentrating urine? 7. How is the diluting and concentrating ability of the kidneys quantitated? 8.


Physiology_Levy

Physiology_Levy. 6. What are the roles of the vasa recta in the process of diluting and concentrating urine? 7. How is the diluting and concentrating ability of the kidneys quantitated? 8. Why do changes in Na+ balance alter the volume of extracellular fluid? 9. What is the effective circulating volume, how is it influenced by changes in Na+ balance, and how does it influence renal Na+ excretion? 10. What are the mechanisms by which the body monitors the effective circulating volume? 11. What are the major signals acting on the kidneys to alter their excretion of Na+? 12. How do changes in extracellular fluid volume alter Na+ transport in the different segments of the nephron, and how do these changes in transport regulate renal Na+ excretion? 13. What are the mechanisms involved in formation of edema, and what role do the kidneys play in this process?


Physiology_Levy

Physiology_Levy. 13. What are the mechanisms involved in formation of edema, and what role do the kidneys play in this process? he kidneys maintain the osmolality and volume of the body fluids within a narrow range by regulating excretion of water and NaCl, respectively. This chapter discusses the regulation of renal water excretion (urine concentration and dilution) and NaCl excretion. The composition and volumes of the various body fluid compartments are reviewed in


Physiology_Levy

Physiology_Levy. Control of Body Fluid Osmolality: Urine Concentration and Dilution , water constitutes approximately 60% of the healthy adult human body. Body water is divided into two major compartments—intracellular fluid (ICF) and extracellular fluid (ECF)—that are in osmotic equilibrium because of the high permeability of most cell membranes to water via aquaporins (e.g., AQP1). Water intake into the body generally occurs orally. This may be water contained in beverages as well as water generated during metabolism of ingested foods (e.g., carbohydrates). In many clinical situations, intravenous infusion is an important route of water entry. The kidneys are responsible for regulating water balance and under most conditions are the major route for elimination of water from the body (


Physiology_Levy

Physiology_Levy. The kidneys are responsible for regulating water balance and under most conditions are the major route for elimination of water from the body ( Table 35.1 ). Other routes of water loss from the body include evaporation from cells of the skin and respiratory passages. Collectively, water loss by these routes is termed insensible water loss because the individual is unaware of its occurrence. Production of sweat accounts for the loss of additional water. Water loss by this mechanism can increase dramatically in a hot environment, with exercise, or in the presence of fever ( 35.2 ). Finally, water can be lost from the gastrointestinal tract. Fecal water loss is normally small (≈100 mL/day) but can increase dramatically with diarrhea (e.g., 20 L/ day with cholera). Vomiting can also cause gastrointestinal water losses.


Physiology_Levy

Physiology_Levy. Although water loss from sweating, defecation, and evaporation from the lungs and skin can vary depending on the environmental conditions or during pathological conditions, loss of water by these routes cannot be regulated. In contrast, renal excretion of water is tightly regulated to maintain whole-body water balance. Maintenance of water balance requires that water intake and loss from the body be precisely matched. If intake exceeds losses, positive water balance exists. Conversely, when intake is less than losses, for review of (with its anions Cl− and HCO3 −), these disorders also result steady-state balance). in alterations in plasma or serum [Na+] ( Fig. 35.2 ). One


Physiology_Levy

Physiology_Levy. Fig. 35.2 ). One When water intake is low or water losses increase, the kidneys conserve water by producing a small volume of urine that is hyperosmotic with respect to plasma. When water intake is high, a large volume of hypoosmotic urine is produced. In a normal individual, urine osmolality (Uosm) can vary from approximately 50 to 1200 mOsm/kg H2O, and the corresponding urine volume can vary from approximately 18 L/day to 0.5 L/day. Importantly the kidneys can regulate excretion of water separately from excretion of total solute ( Fig. 35.1 ). The ability to regulate water excretion separate from excretion of solutes (e.g., Na+ , K+ , urea, etc.) is necessary for survival because it allows water balance to be achieved without upsetting the other homeostatic functions of the kidneys.


Physiology_Levy

Physiology_Levy. It is important to recognize that disorders of water balance are manifested by alterations in body fluid osmolality, which are usually measured by changes in plasma osmolality (POSM). Because the major determinant of plasma osmolality is Na+ aFluid intake varies widely for both social and cultural reasons. of the most common fluid and electrolyte disorders seen in clinical practice is an alteration in serum [Na+]. When an abnormal serum [Na+] is found in an individual, it is tempting to suspect a problem in Na+ balance. However, the problem most often relates to water balance, not Na+ balance. As described later, changes in Na+ balance result in alterations in the volume of ECF, not its osmolality.


Physiology_Levy

Physiology_Levy. The following sections discuss the mechanisms by which the kidneys excrete either hypoosmotic (dilute) or hyperosmotic (concentrated) urine. Control of arginine vasopressin secretion and its important role in regulating excretion of water by the kidneys are also explained (see also • Fig. 35.1 Relationships between plasma AVP levels, and urine osmolality, urine flow rate, and total solute excretion. Max, maximum; Min, minimum. (From Koeppen BM, Stanton BA. Renal Physiology. 5th ed. Philadelphia: Elsevier; 2013.) aIn hot weather and during prolonged heavy exercise, water balance is maintained by increased water ingestion. Decreased excretion of water by the kidneys alone is insufficient to maintain water balance. Kidneys excrete 1 L of water Kidneys excrete hyperosmotic in hypoosmotic urine, urine as the individual drinks returning volume to 14 L and water, returning volume to 14 L restoring [Na˜] and osmolality and restoring [Na˜] and osmolality to normal. to normal.


Physiology_Levy

Physiology_Levy. • Fig. 35.2 Response to changes in water balance. Illustrated are the effects of adding or removing 1 L of water from the ECF of a 70-kg individual. Positive Water Balance: (1) Addition of 1 L of water increases the ECFV and reduces its osmolality. The [Na+] is also decreased (hyponatremia). (2) The normal renal response is to excrete 1 L of water as hypoosmotic urine. (3) As a result of the renal excretion of water, the ECFV, osmolality, and [Na+] are returned to normal. Negative Water Balance: (4) The loss of 1 L of water from the ECF decreases its volume and increases its osmolality. The [Na+] is also increased (hypernatremia). (5) The renal response is to conserve water by excreting a small volume of hyperosmotic urine. (6) With ingestion of water, stimulated by thirst, and conservation of water by the kidneys, the ECFV, osmolality, and [Na+] are returned to normal. Size of the boxes indicates relative ECFV. (From Koeppen BM, Stanton BA. Renal Physiology. 5th ed. Philadelphia:


Physiology_Levy

Physiology_Levy. of water by the kidneys, the ECFV, osmolality, and [Na+] are returned to normal. Size of the boxes indicates relative ECFV. (From Koeppen BM, Stanton BA. Renal Physiology. 5th ed. Philadelphia: Elsevier; 2013.)


Physiology_Levy

Physiology_Levy. In the clinical setting, hypoosmolality (a reduction in plasma osmolality) shifts water into cells, and this process results in cell swelling (see ). Symptoms associated with hypoosmolality are related primarily to swelling of brain cells. For example, a rapid fall in Posm can alter neurological function and thereby cause nausea, malaise, headache, confusion, lethargy, seizures, and coma. When Posm is increased (i.e., hyperosmolality), water is lost from cells. Symptoms of an increase in Posm are also primarily neurological and include lethargy, weakness, seizures, coma, and even death.


Physiology_Levy

Physiology_Levy. Symptoms associated with changes in body fluid osmolality vary depending on how quickly osmolality is changed. Rapid changes in osmolality (i.e., over hours) are less well tolerated than changes that occur more gradually (i.e., over days to weeks). Indeed, individuals who have developed alterations in their body fluid osmolality over an extended period of time may be entirely asymptomatic. This reflects the ability of cells over time to either eliminate intracellular osmoles, as occurs with hypoosmolality, or to generate new intracellular osmoles in response to hyperosmolality and thus minimize changes in cell volume of the neurons (see several nephron segments express a different AVP receptor (V2) that mediates the kidneys’ ability to regulate the volume and osmolality of urine. When plasma AVP levels are low, a large volume of urine is excreted (diuresis), and the urine osmolality is less than that of plasma (i.e., dilute).


Physiology_Levy

Physiology_Levy. When plasma levels of AVP are high, a small volume of urine is excreted (antidiuresis), and the urine osmolality is greater than that of plasma (i.e., concentrated). AVP is a small peptide that is 9 amino acids in length (arginine is found at position 8). It is synthesized in neuroendocrine cells located within the supraoptic and paraventricular nuclei of the hypothalamus. The synthesized hormone is packaged in granules that are transported down the axon of the cell and stored in nerve terminals located in the neurohypophysis (posterior pituitary). The anatomy of the hypothalamus and pituitary gland are shown in Fig. 35.3 Secretion of AVP by the posterior pituitary can be influenced by several factors. The primary physiological regulators of AVP secretion are (1) the osmolality of body fluids (osmotic) and (2) the volume and pressure of the vascular system (hemodynamic, or nonosmotic). Other factors that can alter AVP secretion include nausea (stimulates), atrial


Physiology_Levy

Physiology_Levy. The human form of vasopressin is arginine vasopressin (AVP), which is also known as antidiuretic hormone (ADH). AVP, acting though V1 receptors, causes vascular smooth muscle contraction. As described subsequently, aDiuresis is the term used for excretion of a large volume of urine. This may reflect either excretion of a large volume of water (water diuresis), or excretion of a large amount of solute (solute diuresis). bNeurons within the supraoptic and paraventricular nuclei synthesize either AVP or the related peptide oxytocin. AVP-secreting cells predominate in the supraoptic nucleus, whereas oxytocin-secreting neurons are primarily found in the paraventricular nucleus.


Physiology_Levy

Physiology_Levy. 626 SECTION7Berne & Levy Physiology • Fig. 35.3 Anatomy of the hypothalamus and pituitary gland (midsagittal section) depicting the pathways for AVP section. Also shown are pathways involved in regulating AVP secretion. Afferent fibers from the baroreceptors are carried in the vagus and glossopharyngeal nerves. The inset box illustrates an expanded view of the hypothalamus and pituitary gland. Vagus and glossopharyngeal nerves Vasomotor center (medulla oblongata) Cerebellum Baroreceptor input Pituitary Posterior lobe AVP Paraventricular neuronsOsmoreceptors Supraoptic neurons Optic chiasm Anterior lobe natriuretic peptide (inhibits), and angiotensin II (stimulates). A number of drugs, prescription and nonprescription, also affect AVP secretion. For example, nicotine stimulates secretion, whereas ethanol inhibits secretion. Osmotic Control of AVP Secretion


Physiology_Levy

Physiology_Levy. Osmotic Control of AVP Secretion Changes in the osmolality of body fluids (changes as minor as 1% are sufficient) play the most important role in regulating AVP secretion. The receptors that monitor changes in osmolality of body fluids (termed osmoreceptors) are distinct from the cells that synthesize and secrete AVP, and are located in the organum vasculosum of the lamina terminalis (OVLT) of the hypothalamus. The osmoreceptors sense changes in body fluid osmolality by either shrinking or swelling. Recent studies have provided evidence that transient receptor potential vanilloid (TRVP) cation channels are involved in the response of the cells to changes in body fluid osmolality. The osmoreceptors respond only to solutes in plasma that are effective osmoles (see ). For example, urea is an ineffective osmole when the function of osmoreceptors is considered. Thus elevation of the plasma urea concentration alone has little effect on AVP secretion.


Physiology_Levy

Physiology_Levy. When the effective osmolality of the plasma increases, the osmoreceptors send signals to the AVP synthesizing/ secreting cells located in the supraoptic and paraventricular nuclei of the hypothalamus, and AVP synthesis and secretion are stimulated. Conversely, when the effective cAngiotensin II also stimulates AVP secretion; the cells that mediate this response are located in the subfornical organ (SFO).


Physiology_Levy

Physiology_Levy. The gene for AVP is found on chromosome 20. It contains approximately 2000 base pairs with three exons and two introns. The gene codes for a preprohormone that consists of a signal polypeptide, the AVP molecule, neurophysin, and a glycopeptide (copeptin). As the cell processes the preprohormone the signal peptide is cleaved off in the rough endoplasmic reticulum. Once packaged in neurosecretory granules, the preprohormone is further cleaved into AVP, neurophysin, and copeptin molecules. The neurosecretory granules are then transported down the axon to the posterior pituitary and stored in the nerve endings until released. When the neurons are stimulated to secrete AVP, the action potential opens Ca++ channels in the nerve terminal, which raises the intracellular [Ca++] and causes exocytosis of the neurosecretory granules. All three peptides are secreted in this process. Neurophysin and copeptin do not have an identified physiological function.


Physiology_Levy

Physiology_Levy. osmolality of the plasma is reduced, secretion is inhibited. Because AVP is rapidly degraded in the plasma, circulating levels can be reduced to zero within minutes after secretion is inhibited. As a result the AVP system can respond rapidly to fluctuations in body fluid osmolality. Fig. 35.4A illustrates the effect of changes in plasma osmolality on circulating AVP levels. The slope of the relationship is quite steep and accounts for the sensitivity of this system. The set point of the system is the plasma osmolality value at which AVP secretion begins to increase. Below this set point, virtually no AVP is released. The set point varies


Physiology_Levy

Physiology_Levy. CHAPTER 35 Control of Body Fluid Osmolality and Volume among individuals and is genetically determined. In healthy adults it varies from 275 to 290 mOsm/kg H2O (average ≈ 280–285 mOsm/kg H2O). Several physiological factors can also change the set point in a given individual. As discussed later, alterations in blood volume and pressure can shift it. In addition, pregnancy is associated with a decrease in the set point. The mechanism responsible for the set-point shift with pregnancy is not completely understood but is likely due to hormone levels (e.g., relaxin and chorionic gonadotropin) that are elevated during pregnancy. Hemodynamic (Nonosmotic) Control of AVP Secretion


Physiology_Levy

Physiology_Levy. A decrease in blood volume or pressure also stimulates AVP secretion. The receptors responsible for this response are located in both the low-pressure (left atrium and large pulmonary vessels) and the high-pressure (aortic arch and carotid sinus) sides of the circulatory system. Because the low-pressure receptors are located in the high-compliance side of the circulatory system (i.e., venous), and because the majority of blood is in the venous side of the circulatory system, these low-pressure receptors can be viewed as responding to overall vascular volume. The high-pressure receptors respond to arterial pressure. Both groups of receptors are sensitive to stretch of the wall of the structure in which they are located (e.g., cardiac atrial and aortic arch) and are termed baroreceptors. Signals from these receptors are carried in afferent fibers of the vagus and glossopharyngeal nerves to the brainstem (solitary tract nucleus of the medulla oblongata), which is part of the center that


Physiology_Levy

Physiology_Levy. from these receptors are carried in afferent fibers of the vagus and glossopharyngeal nerves to the brainstem (solitary tract nucleus of the medulla oblongata), which is part of the center that ). Signals are then relayed from the brainstem to the


Physiology_Levy

Physiology_Levy. AVP secretory cells of the supraoptic and paraventricular hypothalamic nuclei. The sensitivity of the baroreceptor system is less than that of the osmoreceptors, and a 5% to 10% decrease in blood volume or pressure is required before AVP secretion is stimulated. This is illustrated in Fig. 35.4B . A number of substances have been shown to alter the secretion of AVP through their effects on blood pressure. These include bradykinin and histamine, which lower pressure and thus stimulate AVP secretion, and norepinephrine, which increases blood pressure and inhibits AVP secretion. Alterations in blood volume and pressure also affect the response to changes in body fluid osmolality (see Fig.


Physiology_Levy

Physiology_Levy. Alterations in blood volume and pressure also affect the response to changes in body fluid osmolality (see Fig. 35.4C ). With a decrease in blood volume or pressure, the set point is shifted to lower osmolality values and the slope of the relationship is steeper. In terms of survival of the individual this means that faced with circulatory collapse, the kidneys will continue to conserve water, even though by doing so they reduce the osmolality of the body fluids. With an increase in blood volume or pressure, the opposite occurs. The set point is shifted to higher osmolality values and the slope is decreased. AVP Actions on the Kidneys


Physiology_Levy

Physiology_Levy. AVP Actions on the Kidneys The primary action of AVP on the kidneys is to enhance absorption of water from the tubular fluid by increasing • Fig. 35.4 Osmotic and hemodynamic (nonosmotic) control of AVP secretion. A, Effect of changes in plasma osmolality (constant blood volume and pressure) on plasma AVP levels. B, Effect of changes in blood volume or pressure (constant plasma osmolality) on plasma AVP levels. C, Interactions between osmolar and blood volume and pressure stimuli on plasma AVP levels.


Physiology_Levy

Physiology_Levy. Inadequate release of AVP from the posterior pituitary results in excretion of a large volume of dilute urine (polyuria). To compensate for this loss of water the individual must ingest a large volume of water (polydipsia) to maintain constant body fluid osmolality. If the individual is deprived of water, body fluids will become hyperosmotic. This condition is called central diabetes insipidus or pituitary diabetes insipidus. Central diabetes insipidus can be inherited, although this is rare. It occurs more commonly after head trauma and with brain neoplasms or infections. Individuals with central diabetes insipidus have a urine-concentrating defect that can be corrected by administration of exogenous AVP. The inherited (autosomal dominant) form of central diabetes insipidus is due to numerous mutations in all regions of the AVP gene (i.e., AVP, copeptin and neurophysin). The human placenta produces a cysteine aminopeptidase that degrades AVP. In some women the levels of this


Physiology_Levy

Physiology_Levy. to numerous mutations in all regions of the AVP gene (i.e., AVP, copeptin and neurophysin). The human placenta produces a cysteine aminopeptidase that degrades AVP. In some women the levels of this vasopressinase result in diabetes insipidus. The associated polyuria can be treated by administration of the synthetic AVP analogue desmopressin (DDAVP).


Physiology_Levy

Physiology_Levy. The syndrome of inappropriate AVP (ADH) secretion (SIADH) is a common clinical problem characterized by plasma AVP levels that are elevated above what would be expected on the basis of body fluid osmolality and blood volume and pressure—hence the term inappropriate AVP (ADH) secretion (this is alternatively named syndrome of inappropriate antidiuresis [SIAD]). In addition the collecting duct overexpresses water channels (see below), thus augmenting the effect of AVP to stimulate water retention by the kidney. Individuals with SIADH retain water, and their body fluids become progressively hypoosmotic. In addition, their urine is more hyperosmotic than expected based on the low body-fluid osmolality. SIADH can be caused by infections and neoplasms of the brain, drugs (e.g., antitumor drugs), pulmonary diseases, and carcinoma of the lung. Many of these conditions stimulate AVP secretion by altering neural input to the AVP secretory cells. By contrast, small cell carcinoma of the lung


Physiology_Levy

Physiology_Levy. pulmonary diseases, and carcinoma of the lung. Many of these conditions stimulate AVP secretion by altering neural input to the AVP secretory cells. By contrast, small cell carcinoma of the lung produces and secretes a number of peptides including AVP. Recently, AVP receptor antagonists (e.g., the nonpeptide antagonists conivaptan [Vaprisol] and tolvaptan [Samsca and Jinarc]) have been developed that can be used to treat SIADH and other conditions in which AVP-dependent water retention by the kidneys occurs (e.g., congestive heart failure and hepatic cirrhosis).


Physiology_Levy

Physiology_Levy. the water permeability of the latter portion of the distal tubule and collecting duct. In addition, and importantly, AVP increases the permeability of the medullary portion of the collecting duct to urea. Finally, AVP stimulates NaCl reabsorption by the thick ascending limb of Henle’s loop, distal tubule, and collecting duct.


Physiology_Levy

Physiology_Levy. In the absence of AVP, the apical membrane of principal cells (see ), located in the latter portion of the distal tubule and along the collecting duct, is relatively impermeable to water. This reflects the fact that in the absence of AVP the apical membrane of these cells contains few water channels (aquaporins). Thus in the absence of AVP, little water is reabsorbed by these nephron segments. Binding of AVP to the V2 receptor located in the basolateral membrane of principal cells results in the insertion of aquaporin (AQP2) water channels into the apical membrane, allowing water to enter the cell from the tubule lumen. This water then exits the cell across the basolateral membrane, which is always freely permeable to water owing to the presence of AQP3 and AQP4 water channels. Thus in the presence of AVP, water is reabsorbed from the tubule lumen.


Physiology_Levy

Physiology_Levy. The gene for the V2 receptor is located on the X chromosome. It codes for a 371–amino acid protein that is in the family of receptors that have seven membrane spanning domains and are coupled to heterotrimeric G proteins. As shown in


Physiology_Levy

Physiology_Levy. Fig. 35.5 , binding of AVP to its receptor on the basolateral membrane activates adenylcyclase. The increase in intracellular cyclic adenosine monophosphate (cAMP) then activates protein kinase A (PKA), which results in phosphorylation of AQP2 water channels and also results in increased transcription of the AQP2 gene via activation of a cAMP-response element (CRE). Vesicles containing phosphorylated AQP2 move toward the apical membrane along microtubules driven by the molecular motor dynein. Once near the apical membrane, proteins called SNAREs interact with vesicles containing AQP2 and facilitate fusion of these vesicles with the membrane. Addition of AQP2 to the membrane allows water to enter the cell driven by the osmotic gradient (lumen osmolality < cell osmolality). The water then exits the cell across the basolateral membrane through AQP3 and AQP4 water channels, which are constitutively present in the basolateral membrane. When the V2 receptor is not occupied by AVP, the AQP2


Physiology_Levy

Physiology_Levy. the cell across the basolateral membrane through AQP3 and AQP4 water channels, which are constitutively present in the basolateral membrane. When the V2 receptor is not occupied by AVP, the AQP2 water channels are removed from the apical membrane by clathrinmediated endocytosis, thus rendering the apical membrane impermeable to water. The endocytosed AQP2 molecules may be either stored in cytoplasmic vesicles, ready for reinsertion into the apical membrane when AVP levels in the plasma increase, or degraded.


Physiology_Levy

Physiology_Levy. AVP also regulates long-term expression of AQP2 (and AQP3). When large volumes of water are ingested over an extended period of time (e.g., psychogenic polydipsia), the abundance of AQP2 and AQP3 in principal cells is reduced. As a consequence, when water ingestion is restricted, these individuals cannot maximally concentrate their urine. Conversely, in states of restricted water ingestion, AQP2 and AQP3 protein expression in principal cells increases, thereby facilitating excretion of maximally concentrated urine. It is also clear that expression of AQP2 (and in some instances also AQP3) varies in pathological conditions associated with disturbances in urine concentration and dilution. AQP2 expression is reduced in a number of conditions associated with impaired urine concentrating ability (e.g., hypercalcemia, hypokalemia). By contrast, in conditions associated with water retention (e.g., congestive heart failure, hepatic cirrhosis, pregnancy) AQP2 expression is increased.


Physiology_Levy

Physiology_Levy. CHAPTER 35 Control of Body Fluid Osmolality and Volume 629 • Fig. 35.5 Action of AVP via the V2 receptor on the principal cell of the late distal tubule and collecting duct. See text for details. AC, adenylcyclase; cAMP, cyclic adenosine monophosphate; CREB-P, phosphorylated cAMP response element binding protein; CRE, cAMP response element; P, phosphorylated proteins; AQP2, aquaporin 2; AP2, aquaporin 2 gene. (Adapted from Brown D, Nielsen S. The cell biology of vasopressin action. In: Brenner BM [ed]. The Kidney. 7th ed. Philadelphia: Saunders; 2004.)


Physiology_Levy

Physiology_Levy. AVP also increases the permeability of the terminal portion of the inner medullary collecting duct to urea. This results in an increase in urea reabsorption and an increase in the osmolality of the medullary interstitial fluid, which as described below is needed for maximal urine concentration. The cells of the collecting duct express two types of urea transporters, UT-A1 and UT-A3. UT-A1 is localized to the apical membrane, and UT-A3 is localized primarily to the basolateral membrane. AVP, acting through the cAMP/PKA cascade, increases expression of both UT-A1 and UT-A3. Increasing the osmolality of the interstitial fluid of the renal medulla also increases the permeability of the inner medullary collecting duct to urea. This effect is mediated by the phospholipase C/protein kinase C (PKC) pathway, which increases UT-A1 and UT-A3 expression.


Physiology_Levy

Physiology_Levy. AVP also stimulates reabsorption of NaCl by the thick ascending limb of Henle’s loop and by the distal tubule and cortical segment of the collecting duct. This increase in Na+ reabsorption is associated with increased abundance of three Na+ transporters: the 1Na+/1K+/2Cl− symporter (thick ascending limb of Henle’s loop), the Na+/Cl− symporter (distal tubule), and the Na+ channel (ENaC, in the latter portion of the distal tubule and collecting duct). Stimulation of thick ascending limb NaCl transport may help maintain the hyperosmotic medullary interstitium that is necessary for absorption of water from the medullary portion of the collecting duct (see below).


Physiology_Levy

Physiology_Levy. In addition to affecting the secretion of AVP, changes in plasma osmolality and blood volume or pressure lead to alterations in the perception of thirst. When body fluid osmolality is increased or the blood volume or pressure is reduced, the individual perceives thirst. Of these stimuli, hypertonicity is the more potent. An increase in plasma osmolality of only 2% to 3% produces a strong desire to drink, whereas decreases in blood volume and pressure in the range of 10% to 15% are required to produce the same response.


Physiology_Levy

Physiology_Levy. As already discussed, there is a genetically determined threshold for AVP secretion (i.e., a body fluid osmolality above which AVP secretion increases). Similarly there is a genetically determined threshold for triggering the sensation of thirst. However, the thirst threshold is higher than the threshold for AVP secretion. On average the threshold for AVP secretion is approximately 285 mOsm/kg H2O, whereas the thirst threshold is approximately 295 mOsm/ kg H2O. Because of this difference, thirst is stimulated at a body fluid osmolality where AVP secretion is already maximal. The neural centers involved in regulating water intake (the thirst center) are located in the same region of the hypothalamus involved with regulating AVP secretion. However, it is not certain whether the same cells serve both functions. Indeed the thirst response, like the regulation of


Physiology_Levy

Physiology_Levy. The collecting ducts of some individuals do not respond normally to AVP. These individuals cannot maximally concentrate their urine and consequently have polyuria and polydipsia. This clinical entity is termed nephrogenic diabetes insipidus to distinguish it from central diabetes insipidus. Nephrogenic diabetes insipidus can result from a number of systemic disorders and more rarely occurs as a result of inherited disorders. Many of the acquired forms of nephrogenic diabetes insipidus are the result of decreased expression of AQP2 in the collecting duct. Decreased expression of AQP2 has been documented in the urine concentrating defects associated with hypokalemia, lithium ingestion (35% of individuals who take lithium for bipolar disorder develop some degree of nephrogenic diabetes insipidus), ureteral obstruction, low-protein diet, and hypercalcemia. The inherited forms of nephrogenic diabetes insipidus reflect mutations in the AVP receptor (V2 receptor) gene or the AQP2 gene.


Physiology_Levy

Physiology_Levy. insipidus), ureteral obstruction, low-protein diet, and hypercalcemia. The inherited forms of nephrogenic diabetes insipidus reflect mutations in the AVP receptor (V2 receptor) gene or the AQP2 gene. Approximately 90% of hereditary forms of nephrogenic diabetes insipidus are the result of mutations in the V2 receptor gene, with the other 10% the result of mutations in the AQP2 gene. Since the gene for the V2 receptor is located on the X chromosome, these inherited forms of nephrogenic diabetes insipidus are X-linked. The gene coding for AQP2 is located on chromosome 12 and is inherited as both an autosomal recessive as well as an autosomal dominant defect. As noted in , aquaporins exist as homotetramers. This homotetramer formation explains the difference between the two forms of AQP2-related nephrogenic diabetes insipidus. In the recessive form, heterozygotes produce both normal AQP2 and defective AQP2 molecules. The defective AQP2 monomer is retained in the endoplasmic reticulum of


Physiology_Levy

Physiology_Levy. nephrogenic diabetes insipidus. In the recessive form, heterozygotes produce both normal AQP2 and defective AQP2 molecules. The defective AQP2 monomer is retained in the endoplasmic reticulum of the cell, and thus the homotetramers that do form only contain normal molecules. Thus mutations in both alleles are required to produce nephrogenic diabetes insipidus. In the autosomal dominant form, the defective monomers can form tetramers with normal monomers as well as defective monomers. However, tetramers containing defective monomers are unable to traffic to the apical membrane.


Physiology_Levy

Physiology_Levy. Recently, individuals have been found that have activating (gain-of-function) mutations in the V2 receptor gene. Thus the receptor is constitutively activated even in the absence of AVP. These individuals have laboratory findings similar to those seen in SIADH, including reduced plasma osmolality, hyponatremia (reduced plasma [Na+]), and urine more concentrated than would be expected from the reduced body fluid osmolality. However, unlike SIADH where circulating levels of AVP are elevated and thus responsible for water retention by the kidneys, these individuals have undetectable levels of AVP in their plasma. This new clinical entity has been termed nephrogenic syndrome of inappropriate antidiuresis.