Arginine vasopressin (AVP) related water resorption in the collecting duct acts through aquaporins 2 specific water channels. AQP2 are small integral tetrameric plasma membrane proteins. Each of four subunits is selectively permeable to water. In mammals lack of vasopressin stimulation causes AQP2 storage in intracellular vesicles of the principal cells of the renal distal tubule and collecting duct. AVP acts through specific V receptors. Arginine vasopressin receptor binding increases intracellular production of cAMP and increases protein kinase A (PKA) activity.