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. This increased by 8.5-fold with Na depletion and decreased by 40% with Na loading. The biochemical parameter most strongly affected by dietary Na was the abundance of the 65-kDa cleaved form of gamma-ENaC at the surface. 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 increases in ENaC subunits were mimicked by administration of aldosterone for 1 wk, but the increase in NCC was not. The amounts of the luminal Na/H exchanger (NHE3) and the luminal Na-K-2Cl cotransporter (NKCC2) did not change significantly. 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 abundance of Na transport proteins in the luminal membrane of the rat kidney was assessed using in situ biotinylation and immunoblotting.
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