Schematic model of kidney collecting duct cell. Vasopressin binds to the V2 receptor located in the basolateral membrane of collecting duct principal cells. This is couplet to a heterotrimeric G protein Gs which in turn activates adenylyl cyclase. This causes increase in intracellular cAMP levels leading to activation of Protein Kinase A (PKA). PKA phosphorylates the AQP2 water channel located in intracellular vesicles and this represents a key event stimulating regulated exocytosis of AQP2. Mutation either in the V2R receptor or in AQP2 water channel have been found associated with congenital X-linked NDI and autosomal NDI respectively