Endocrine Index Glossary  Adrenal Medullary Hormones  Cells in
the adrenal medulla synthesize and secrete epinephrine and
norepinephrine. The ratio of these two catecholamines differs
considerably among species: in humans, cats and chickens,
roughly 80, 60 and 30% of the catecholamine output is
epinephrine. Following release into blood, these hormones bind
adrenergic receptors on target cells, where they induce
essentially the same effects as direct sympathetic nervous
stimulation.  Synthesis and Secretion of Catecholamines
Synthesis of catecholamines begins with the amino acid tyrosine,
which is taken up by chromaffin cells in the medulla and
converted to norepinephrine and epinephrine through the
following steps:  Norepinephine and epinephrine are stored in
electron-dense granules which also contain ATP and several
neuropeptides. Secretion of these hormones is stimulated by
acetylcholine release from preganglionic sympathetic fibers
innervating the medulla. Many types of "stresses" stimulate such
secretion