AQP2 and Menkes disease: There is considerable evidence that water homeostasis in the inner ear is partly regulated via the vasopressin-aquaporin 2 (VP-AQP2) system3–13 as follows: (1) plasma levels of arginine VP are higher in patients with MD and may depend on the phase that the patient is in3,5,6, (2) acute and chronic application of arginine VP produces EH in guinea pigs and rats4,7,10, (3) V2 receptor mRNA is expressed in the rat and human inner ear8,11–13, and (4) expression of V2 receptor mRNA in the rat inner ear is down-regulated by VP application9.