Ras Pathway Activates Epithelial Na+Channel and Decreases Its Surface Expression inXenopusOocytes

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Abstract
The small G protein K-Ras2A is rapidly induced by aldosterone in A6 epithelia. In these Xenopus sodium reabsorbing cells, aldosterone rapidly activates preexisting epithelial Na+channels (XENaC) via a transcriptionally mediated mechanism. In the Xenopus oocytes expression system, we tested whether the K-Ras2A pathway impacts on XENaC activity by expressing XENaC alone or together withXK-Ras2A rendered constitutively active (XK-Ras2AG12V). As a second control,XENaC-expressing oocytes were treated with progesterone, a sex steroid that induces maturation of the oocytes similarly to activated Ras. Progesterone or XK-Ras2AG12Vled to oocyte maturation characterized by a decrease in surface area and endogenous Na+pump function. In both conditions, the surface expression of exogenous XENaC′s was also decreased; however, in comparison with progesterone-treated oocytes,XK-ras2AG12V-coinjected oocytes expressed a fivefold higher XENaC-mediated macroscopic Na+current that was as high as that of control oocytes. Thus, the Na+current per surface-expressedXENaC was increased byXK-Ras2AG12V. The chemical driving force for Na+influx was not changed, suggesting thatXK-Ras2AG12Vincreased the mean activity ofXENaCs at the oocyte surface. These observations raise the possibility that XK-Ras2A, which is the first regulatory protein known to be transcriptionally induced by aldosterone, could play a role in the control of XENaC function in aldosterone target cells.