Urinary excretion of aquaporin-2 in pathological states of water metabolism

Aquaporin-2 (AQP-2) is an arginine vasopressin (AVP)-regulated water channe l in renal collecting duct cells. Approximately 3% of AQP-2 in collecting d uct cells is excreted into urine, Urinary excretion of AQP-2 varies widely in different physiological conditions, and it has a positive correlation wi th plasma AVP levels. Urinary excretion of AQP-2 was significantly increase d by the single injection of AVP in patients with central diabetes insipidu s. The urinary excretion of AQP-2 was one-eighth over in patients with cent ral diabetes insipidus and three times greater in patients with impaired wa ter excretion than that in normal subjects. In a hypertonic saline rest, th e urinary excretion of AQP-2 promptly increased 6-12-fold in normal subject s, but remained low in patients with central diabetes insipidus. In additio n, exaggerated urinary excretion of AQP-2 persisted after an acute water lo ad in patients with impaired water excretion. These results indicate that u rinary excretion of AQP-2 is a potent marker for the diagnosis of water met abolism disorders dependent on AVP.