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Conclusion: Our findings suggest that there exists a Na+ transport system in the cochlea consisting of SGK1, Nedd4 isoforms and ENaC, which may work in concert to transport Na+ and to maintain homeostasis in the inner ear as they do in other tight epithelia. Objectives: It has been well demonstrated that the epithelial sodium channel (ENaC) plays important roles in regulation of Na+ reabsorption in the tight epithelia of kidney, colon and lung. Recent studies have provided multiple evidence that the serum glucocorticoid-inducible kinase1(SGK1) and the ubiquitin-protein ligase neural precursor cell-expressed, developmentally downregulated isoforms Nedd4 are involved in ENaC regulation in response to hormones such as aldosterone, vasopressin and insulin. The authors aimed to investigate the cellular localization of Nedd4 isoforms, Nedd4-1/2 and Nedd4-2, and SGK1 in the rat cochlea. Materials and Methods: The expression patterns of Nedd4-1/2, Nedd4-2 and SGK1 in the rat cochlea were studied by immunohistochemistry with the specific polyclonal rabbit antibodies against the rat Nedd4-1/2, Nedd4-2 and SGK1. Results: All three proteins were extensively expressed in various regions of the rat cochlea. They were found in the stria vascularis, spiral ligament, organ of Corti, spiral limbus, spiral ganglion and Reissner's membrane. [ABSTRACT FROM AUTHOR]

Endolymph in membranous labyrinth is a K+-rich and Na+-poor fluid, and perilymph is conversely Na+-rich and K+-poor. Electrolyte transport between endolymph and perilymph is important for regulation of volume and osmotic pressure of the labyrinth. The epithelial sodium channel (ENaC) is a good candidate protein for Na+ transport in the tight epithelia, which has been well demonstrated in other tissues such as kidney, colon and lung. The purpose of the present study was to investigate the cellular localization of ENaC subunits in the rat inner ear immunohistochemically with the specific polyclonal rabbit antibodies against the rat α-, β- and γ-ENaC. All three subunits of ENaC were extensively labeled in the cochlea including the stria vascularis, spiral ligament, organ of Corti, spiral limbus, Reissner''s membrane and spiral ganglion, and in the vestibule including the sensory epithelia and stroma cells of the macula utriculi, macula sacculi and ampullary crest. In conclusion, our results suggest that functional ENaC in the labyrinth may work in concert with other Na+ and K+ transport molecules to regulate endolymph and to maintain homeostasis in the inner ear. [Copyright &y& Elsevier]