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.