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-regul ated Na+ transport. The possibility that increased K-Ras2A alone is suffici ent to mimic aldosterone action on Na+ transport also was tested. Aldostero ne treatment increased K-Ras2A protein expression 2.8 fold within 4 h, Acti ve Ras is membrane associated. After aldosterone treatment, 75% of K-Ras wa s localized to the plasma membrane compared with 25% in the absence of ster oid. Aldosterone also increased the amount of active (phosphorylated) mitog en-activated protein kinase kinase likely through K-Ras2A signaling. Steroi d-induced K-Ras2A protein levels and Na+ transport were decreased with anti sense K-ras2A oligonucleotides, showing that K-Ras2A is necessary for the n atriferic actions of aldosterone, Aldosterone-induced Na+ channel activity, was decreased from 0.40 to 0.09 by pretreatment with antisense ras oligonu cleotide, implicating the luminal Na+ channel as one final effector of Ras signaling. Overexpression of K-Ras2A increased Na+ transport approximately 2.2-foId in the absence of aldosterone. These results suggest that aldoster one signals to the luminal Naf channel via multiple pathways and that K-Ras 2A levels are limiting for a portion of the aldosterone-sensitive Na+ trans port.