title: 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.
authors.affiliation: a Department of Physiology and Biophysics; Weill-Cornell Medical College ; New York , NY USA.
authors.forename: Ankit B
authors.initials: AB
authors.lastname: Patel
authors.forename: Lei
authors.initials: L
authors.lastname: Yang
authors.forename: Su
authors.initials: S
authors.lastname: Deng
authors.forename: Lawrence G
authors.initials: LG
authors.lastname: Palmer
mesh_terms: D000818:Animals; D004305:Dose-Response Relationship, Drug; D062686:Epithelial Sodium Channel Blockers / Q000494:pharmacology; D053503:Epithelial Sodium Channels / Q000235:genetics / Q000378:metabolism*; D025461:Feedback, Physiological / Q000187:drug effects*; D009865:Oocytes / Q000187:drug effects / Q000378:metabolism; D012964:Sodium / Q000494:pharmacology*; D013329:Structure-Activity Relationship; D014981:Xenopus
chemical_list: D062686:Epithelial Sodium Channel Blockers; D053503:Epithelial Sodium Channels; D012964:Sodium
keywords: CCD, cortical collecting duct; ENaC, epithelial Na channel; INa, amiloride-sensitive current; Liddle's syndrome; Po, open probability; [Na]i, intracellular Na concentration; brefeldin A; intracellular Na+; surface biotinylation