Changes in aquaporin expression in the inner ear of the rat after i.p. injection of steroids.

The aquaporins (AQPs) are a family of small transmembrane water transporters. It has recently been revealed that they play a role in regulating homeostasis in the inner ear fluids. Steroid therapy is usually administered to patients with inner ear disorders; however, the mechanism of steroid effects has not been clearly determined. To elucidate the points of action of steroids in the inner ear, we recently examined the distributions of AQP isoform mRNAs in the rat inner ear and identified AQP1-6 mRNAs in the rat cochlea and AQP1, 3, 4, 5 and 6 mRNAs in the rat endolymphatic sac by means of reverse transcriptase polymerase chain reaction (PCR). In this study, we investigated changes in expression of AQP mRNAs in the rat inner ear after i.p. injections of steroids using real-time quantitative PCR and found that AQP3 mRNA in the endolymphatic sac was significantly upregulated in both dose- and time-dependent manners. This result suggests that steroids may effect water homeostasis in the rat inner ear via AQPs.