The AQP2(aquaporin 2)is a water channel protein located in the kidney collecting tubule. It belongs to the MIP/aquaporin family, some members of which are clustered together on chromosome 12q13. In collecting duct cells, AQP2 trafficking to vasopressin-sensitive vesicles is phosphorylation-independent and phosphorylation of Ser-256 is necessary for expression of AQP2 in the apical membrane. The changes in urinary excretion of aquaporin-2 indicates circulatory blood volume depletion, and estimates the AVP-dependent recovery of circulatory blood volume during the therapeutic period in patients with diabetic ketoacidosis