Surface expression of sodium channels and transporters in rat kidney: effects of dietary sodium.

The abundance of Na transport proteins in the luminal membrane of the rat kidney was assessed using in situ biotinylation and immunoblotting. When animals were fed an Na-deficient diet for 1 wk, the amounts of epithelial Na channel (ENaC) beta-subunit (beta-ENaC) and gamma-subunit (gamma-ENaC) and Na-Cl cotransporter (NCC) protein in the surface fraction increased relative to controls by 1.9-, 3.5-, and 1.5-fold, respectively. The amounts of the luminal Na/H exchanger (NHE3) and the luminal Na-K-2Cl cotransporter (NKCC2) did not change significantly. The increases in ENaC subunits were mimicked by administration of aldosterone for 1 wk, but the increase in NCC was not. When the animals were fed a high-Na (5% NaCl) diet for 1 wk, the surface expression of beta-ENaC increased by 50%, whereas that of the other membrane proteins did not change, relative to controls. The biochemical parameter most strongly affected by dietary Na was the abundance of the 65-kDa cleaved form of gamma-ENaC at the surface. This increased by 8.5-fold with Na depletion and decreased by 40% with Na loading. The overall 14-fold change reflected regulation of the total abundance of the subunit as well as the fraction of the subunit protein in the cleaved form. We conclude that cleavage of gamma-ENaC and its expression at the apical surface play a major role in the regulation of renal Na reabsorption.