text: The drug amiloride decreases blood pressure by increasing the amount of this ion released by the distal tubule.
full_question: The drug amiloride decreases blood pressure by increasing the amount of this ion released by the distal tubule. In the kidney, the collecting duct of the nephron is a channel for this ion, whose transfer rate is regulated by the macula densa. Channels of either calcium or of this ion generate action potentials used in cell communication within the nervous system. Three of these ions are expelled from a cell- while two potassium ions are taken into the cell- in a namesake pump. For 10 points, name this primary cation in human extracellular fluids.
first_sentence: The drug amiloride decreases blood pressure by increasing the amount of this ion released by the distal tubule.