Aquaporin 2 (AQP2) is a channel protein found in the apical cell membranes and intracellular vesicles of the collecting ducts cells of the kidneys. In the plasma membrane, this protein forms a vasopressin-regulated water channel, which aids in the reabsorption of water. In the presence of vasopressin, a G-protein linked receptor causes the intracellular accumulation of protein kinase A, which in turn causes phosphorylation and subsequent activation of the aquaporin 2 protein. This activation enables the migration of the water channel to the apical membranes of the collecting duct cells, where they carry out the osmotic reabsorption of solute free water, thereby concentrating the urine for excretion.
About 10% of patients with inherited forms of Nephrogenic Diabetes Insipidus (NDI) have been found to carry mutations in the AQP2 gene. In these patients, deficiency of aquaporin 2 function leads to the excretion of highly dilute urine, resulting in severe dehydration and associated complications.