Water Excretion In contrast to the ingestion of water, its excretion is tightly regulated by physiologic factors. The principal determinant of renal water excretion is arginine vasopressin (AVP; formerly antidiuretic hormone), a polypeptide synthesized in the supraoptic and paraventricular nuclei of the hypothalamus and secreted by the posterior pituitary gland. The binding of AVP to V 2 receptors on the basolateral membrane of principal cells in the collecting duct activates adenylyl cyclase and initiates a sequence of events that leads to the insertion of water channels into the luminal membrane. These water channels that are specifically activated by AVP are. | Chapter 046. Sodium and Water Part 2 Water Excretion In contrast to the ingestion of water its excretion is tightly regulated by physiologic factors. The principal determinant of renal water excretion is arginine vasopressin AVP formerly antidiuretic hormone a polypeptide synthesized in the supraoptic and paraventricular nuclei of the hypothalamus and secreted by the posterior pituitary gland. The binding of AVP to V2 receptors on the basolateral membrane of principal cells in the collecting duct activates adenylyl cyclase and initiates a sequence of events that leads to the insertion of water channels into the luminal membrane. These water channels that are specifically activated by AVP are encoded by the aquaporin-2 gene Chap. 334 . The net effect is passive water reabsorption along an osmotic gradient from the lumen of the collecting duct to the hypertonic medullary interstitium. The major stimulus for AVP secretion is hypertonicity. Since the major ECF solutes are Na salts effective osmolality is primarily determined by the plasma Na concentration. An increase or decrease in tonicity is sensed by hypothalamic osmoreceptors as a decrease or increase in cell volume respectively leading to enhancement or suppression of AVP secretion. The osmotic threshold for AVP release is 280-290 mosmol kg and the system is sufficiently sensitive that plasma osmolality varies by no more than 1-2 . Nonosmotic factors that regulate AVP secretion include effective circulating arterial volume nausea pain stress hypoglycemia pregnancy and numerous drugs. The hemodynamic response is mediated by baroreceptors in the carotid sinus. The sensitivity of these receptors is significantly lower than that of the osmoreceptors. In fact depletion of blood volume sufficient to result in a decreased mean arterial pressure is necessary to stimulate AVP release whereas small changes in effective circulating volume have little effect. To maintain homeostasis and a normal plasma Na concentration the .
