Regulation of Na⁺ reabsorption by the aldosterone-induced small G protein K-Ras2A

Xenopus laevis A6 cells were used as model epithelia to test the hypothesis that K-Ras2A is an aldosterone-induced protein necessary for steroid-regulated Na⁺ transport. The possibility that increased K-Ras2A alone is sufficient to mimic aldosterone action on Na⁺ transport also was tested. Aldosterone treatment increased K-Ras2A protein expression 2.8-fold within 4 h. Active Ras is membrane associated. After aldosterone treatment, 75% of K-Ras was localized to the plasma membrane compared with 25% in the absence of steroid. Aldosterone also increased the amount of active (phosphorylated) mitogen-activated protein kinase kinase likely through K- Ras2A signaling. Steroid-induced K-Ras2A protein levels and Na⁺ transport were decreased with antisense K-ras2A oligonucleotides, showing that K-Ras2A is necessary for the natriferic actions of aldosterone. Aldosterone-induced Na⁺ channel activity, was decreased from 0.40 to 0.09 by pretreatment with antisense ras oligonucleotide, implicating the luminal Na⁺ channel as one final effector of Ras signaling. Overexpression of K-Ras2A increased Na⁺ transport approximately 2.2-fold in the absence of aldosterone. These results suggest that aldosterone signals to the luminal Na⁺ channel via multiple pathways and that K-Ras2A levels are limiting for a portion of the aldosterone-sensitive Na⁺ transport.