Water channel-containing vesicles

Principal cells in the renal collecting duct have V2 receptors on their basolateral membranes that couple to to stimulate adenylyl cyclase activity (Figure 29—3). The resulting increase in cellular cyclic AMP and PKA activity triggers increased insertion of water channel—containing vesicles (WCVs) into the apical membrane and decreased endocytosis of WCVs from the apical membrane. The distribution of WCVs between the cytosolic compartment and the apical membrane compartment thus is shifted in favor of the apical membrane compartment. Because WCVs contain preformed aquaporin 2 water channels, their net shift into apical membranes in response to V2 receptor stimulation greatly increases water permeability of the apical membrane. 

WCVs = Water Channel Containing Vesicles [ = Aquaporin-2 = Water Channel... 

A key determinant of the final urine concentration is antidiuretic hormone (ADH also called vasopressin). In the absence of ADH, the collecting tubule (and duct) is impermeable to water, and dilute urine is produced. However, membrane water permeability of principal cells can be increased by ADH-induced fusion of vesicles containing preformed water channels with the apical membranes (Figure 15-6). ADH secretion is regulated by serum osmolality and by volume status. 

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