Recently, considerable evidence has been accumulated to support the novel view that water homeostasis in the inner ear is regulated via the vasopressin-aquaporin 2 (VP-AQP2) system in the same fashion as in the kidney. Indeed, multiple subtypes of AQPs including AQP-2 are reported to be expressed in the cochlea. However, the mechanism that underlies VP-AQP-2 mediated water homeostasis remains to be elucidated. In the present study, the localizations of AQP-1, -2, -3, -4, -5, -7, -8, -9, and vasopressin type 2 receptor (V2-R) in the stria vascularis (SV) were molecular biologically and immunohistochemically examined to evaluate the role of the AQP water channel system in water homeostasis of the SV. A RT-PCR study revealed that AQPs and V2-R mRNA are expressed in the cochlea. As for their immunohistochemical localization, the AQP-2 protein is expressed on the basal side of the basal cells of the SV, and proteins of AQP-3 and V2-R are expressed on the apical side of the basal cells. AQP-7 and -9 proteins are expressed on the apical side of marginal cells. AQP-4, -5, and -8 protein expressions could not be detected in the lateral wall of the cochlea. From the present results, water flux in the SV is thought to be regulated at the level of the basal cells by vasopressin. Furthermore, such a distribution of AQP-2, -3, and V2-R suggests that VP-AQP-2 mediated water transport might work actively in the basal cells from perilymph towards endolymph containing AQP-1, -7 and -9.
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