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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]

It has been demonstrated that the epithelial sodium channel (ENaC) may play critical roles in (re)absorbing Na+ from apical plasma membrane in various tissues and cells. Moreover, the serum glucocorticoid-inducible kinase 1 (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 aim of the study was to investigate the cellular localizations of ENaC subunits, SGK1, and Nedd4 isoforms in the cochlea of guinea pig. Ten adult guinea pigs were sacrificed and their cochleas were collected. The expression patterns of ENaC subunits, SGK1 and Nedd4-2 isoforms in the guinea pig cochlea were studied by immunohistochemistry with the specific polyclonal rabbit antibodies against rat α-, β- and γ-ENaC, SGK1, Nedd4-1/2 and Nedd4-2. The results showed that all these proteins were extensively expressed in various regions of the cochlea. They were found in the spiral ligament, organ of Corti, spiral limbus, spiral ganglion and Reissner's membrane with different staining patterns. The results indicated that a Na+ transport system may exist in the cochlea of guinea pig consisting of ENaC, SGK1 and Nedd4, which may work in concert to transport Na+ and to maintain homeostasis in inner ear as it does in other epithelia.