Degenerin Sites Mediate Proton Activation of δβγ-Epithelial Sodium Channel*

The δ-subunit of epithelial Na⁺ channels (ENaC) is predominately expressed in brain, heart, and pancreas. The amiloride sensitivity, Na⁺ conductance, and critical domains for gating are characterized as a cross between proton-activated Na⁺ channels and α-ENaC. The hypothesis that external protons may activate human δ-ENaC was addressed by expressing δβγ-hENaC in Xenopus oocytes and evaluating proton-activated current with the two-electrode voltage clamp technique. Our results showed that protons transiently evoked a Na⁺ current with an EC₅₀ of pH 6 overlapped on the basal current of δβγ-hENaC. Proton-activated current was not observed in uninjected oocytes. Studies on gating kinetics revealed that activation, desensitization, and recovery times of proton-activated Na⁺ current were 3.8 ± 0.5 s, 253 ± 9.5 s, and 10 ± 3.6 s, respectively (n = 4–12). Alkali metal cation selectivity of the proton-activated current was identical to that of the basal current of δβγ-hENaC. The metabolic acids, lactate, pyruvate, and formate, modified the proton-activated current, as did hypo-osmotic stress. EDTA, hypo-osmolarity, and lactate enhanced proton activation synergistically. Our results suggest that δ-hENaC subunit is essential for proton-activated current and γ-subunit may potentially regulate the response of δ-hENaC to protons. We have concluded that δβγ-hENaC is a proton-activated cation channel whose closing gate can be regulated by a proton-induced conformational change. Proton-sensitivity of δβγ-hENaC may be an important mechanism for integrating external ischemic signals in inflamed and hypoxic tissues.