Ras Pathway Activates Epithelial Na(+) Channel and Decreases Its Surface Expression in Xenopus Oocytes

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 with XK-Ras2A rendered constitutively active (XK-Ras2A(G12V)). 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-Ras2A(G12V) led 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-ras2A(G12V)-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-expressed XENaC was increased by XK-Ras2A(G12V). The chemical driving force for Na(+) influx was not changed, suggesting that XK-Ras2A(G12V) increased the mean activity of XENaCs 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.