Feedback inhibition of ENaC: Acute and chronic mechanisms

Abstract

Intracellular [Na+] ([Na+]i) modulates the activity of the epithelial Na channel (ENaC) to help prevent cell swelling and regulate epithelial Na+ transport, but the underlying mechanisms remain unclear. We show here that short-term (60-80 min) incubation of ENaC-expressing oocytes in high Na+ results in a 75% decrease in channel activity. When the β subunit was truncated, corresponding to a gain-of-function mutation found in Liddle's syndrome, the same maneuver reduced activity by 45% despite a larger increase in [Na+]i. In both cases the inhibition occurred with little to no change in cell-surface expression of γENaC. Long-term incubation (18 hours) in high Na+ reduced activity by 92% and 75% in wild-type channels and Liddle's mutant, respectively, with concomitant 70% and 52% decreases in cell-surface γENaC. In the presence of Brefeldin A to inhibit forward protein trafficking, high-Na+ incubation decreased wt ENaC activity by 52% and 88% after 4 and 8 hour incubations, respectively. Cleaved γENaC at the cell surface had lifetimes at the surface of 6 hrs in low Na+ and 4 hrs in high Na+, suggesting that [Na+]i increased the rate of retrieval of cleaved γ ENaC by 50%. This implies that enhanced retrieval of ENaC channels at the cell surface accounts for part, but not all, of the downregulation of ENaC activity shown with chronic increases in [Na+]i.