Fig. 1: Simplified scheme of vasopressin-regulated water reabsorption in the distal collecting duct.
Arginine vasopressin (AVP) binds to the vasopressin V2 receptor (V2R) expressed on the basolateral membrane of collecting duct cells and initiates a cascade of events that involves receptor-linked activation of G protein, activation of adenylyl cyclase, production of cyclic adenosine monophosphate (cAMP) and stimulation of protein kinase A (PKA). These processes lead to the final step of the antidiuretic action of AVP — the exocytic insertion of aquaporin water channels into the luminal membrane — thereby increasing the water permeability of that membrane and allowing water to flow from the tubule lumen to the hypertonic medullary interstitium. This water transport occurs via AQP2 in the apical membrane and via AQP3 and AQP4 (constitutive water channels) in the basolateral membrane, and enables the formation of concentrated urine.
