Selective expression of aquaporin 1, 4 and 5 in the rat middle ear

The middle ear cavity is an air-filled space that must be maintained for ef fective sound transmission to the inner ear. To examine the mechanisms of w ater homeostasis in the middle ear, we investigated whether aquaporins (AQP s), a family of water-permeable channels, were expressed in the middle car. Reverse transcription-polymerase chain reaction and immunoblot analyses re vealed that mRNAs encoding AQP1, 4 and 5 (but not 2 or 3) subtypes were exp ressed in rat middle ear epithelium, AQP1, 4 and 5 were detected as 28-, 30 - and 30-kDa proteins. respectively. Immunohistochemical analysis showed th at AQP1 was localized at capillary endothelial cells and fibroblasts in lam ina propria mucosae.. AQP4 was present solely at the basolateral membrane o f ciliated cells, whereas AQP5 was on the apical surface of ciliated cells as well as of flat and columnar epithelial cells. The characteristic differ ent localizations of AQP1, 4 and 5 subtypes in the middle ear suggest that middle ear water homeostasis requires the coordinated operation of these AQ Ps. (C) 2001 Elsevier Science B.V. All rights reserved.