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 epithel ia. In these Xenopus sodium reabsorbing cells, aldosterone rapidly activate s preexisting epithelial Na+ channels (XENaC) via a transcriptionally media ted 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-Ras2AG12V led to oocyte maturation characterized b y a decrease in surface area and endogenous Na+ pump function. In both cond itions, the surface expression of exogenous XENaC's was also decreased; how ever, in comparison with progesterone-treated oocytes, XK-ras2A(G12V)-coinj ected oocytes expressed a fivefold higher XENaC-mediated macroscopic Nat cu rrent that was as high as that of control oocytes. Thus, the Na+ current pe r surface-expressed XENaC was increased by XK-Ras2A(G12V). The chemical dri ving force for Na+ influx was not changed, suggesting that XK-Ras2AG12V inc reased the mean activity of XENaCs at the oocyte surface. These observation s raise the possibility that XK-Ras2A, which is the first regulatory protei n known to be transcriptionally induced by aldosterone, could play a role i n the control of XENaC function in aldosterone target cells.