In the inner ear, water homeostasis is of crucial importance to maintain the function of hearing and equilibrium. The morphological characteristics of Ménière's disease, endolymphatic hydrops, reflect the malregulation of water homeostasis in the inner ear. The water channels composed of aquaporin (AQP) proteins are well known to play a very important role in water homeostasis in the body. At least 8 types of AQPs are also reported to be expressed in the inner ear. It is notiable that the AQP-2 water channel, mediated by vasopressin (VP), is localized in the inner ear. In the present paper, it is reviewed how water homeostasis is mediated by the AQP system in the inner ear, and how this AQP system, especially the VP-AQP2 system, is linked to the formation of endolymphatic hydrops.
