Abstract
The epithelial Na channel (ENaC) is a heterotrimer whose trafficking to the apical membrane is stimulated by aldosterone. Trafficking is associated with proteolytic cleavage of the α and γ subunits. We examined the kinetics of this process to ascertain whether the observed changes could contribute to the most rapid anti-natriuretic effects (within 1-3 h) of hormone administration in rats. Infusion of aldosterone increased the abundance of cleaved αENaC and γENaC with time constants of 2.2 and 2.3 h, respectively. Decreases in full-length γENaC and increases in full-length αENaC occurred more slowly, with time constants of 22 and 17 h. Decreases in aldosterone also caused rapid decreases in cleaved and slower changes in full-length forms. Kinetic modeling suggested that the major effect of aldosterone on γENaC kinetics was on the transition from a full-length, intracellular (I) to a cleaved, membrane-associated (M) population. This rate is relatively slow (0.002-0.01 h-1) compared with rates of degradation of M (∼0.4 h-1) and I (∼0.04 h-1). Short lifetimes (∼1 h) of channels at the surface were confirmed in a mouse collecting duct cell line (mCCD). Lifetimes of full-length forms of α and γENaC were also short in whole-cell extracts of mCCD cells but were much longer in the cytoplasm of mouse tubule suspensions (10-20 h). We conclude that one effect of aldosterone in the kidney is to increase forward trafficking of ENaC to the apical membrane, where rapid degradation from the surface permits fast regulation of apical channel abundance.