Extracellular Protons Regulate Human ENaC by Modulating Na+ Self-inhibition

The epithelial Na+ channel, ENaC, is exposed to a wide range of proton concentrations in the kidney, lung, and sweat duct. We, therefore, tested whether pH alters ENaC activity. In Xenopus oocytes expressing human alpha-, beta-, and gamma ENaC, amiloride-sensitive current was altered by protons in the physiologically relevant range ( pH 8.5-6.0). Compared with pH 7.4, acidic pH increased ENaC current, whereas alkaline pH decreased current (pH(50) = 7.2). Acidic pH also increased ENaC current in H441 epithelia and in human primary airway epithelia. In contrast to human ENaC, pH did not alter rat ENaC current, indicating that there are species differences in ENaC regulation by protons. This resulted predominantly from species differences in gamma ENaC. Maneuvers that lock ENaC in a high open-probability state ("DEG" mutation, proteolytic cleavage) abolished the effect of pHon human ENaC, indicating that protons alter ENaC current by modulating channel gating. Previous work showed that ENaC gating is regulated in part by extracellular Na+ ("Na+ self-inhibition"). Based on several observations, we conclude that protons regulate ENaC by altering Na+ self-inhibition. First, protons reduced Na+ self- inhibition in a dose-dependent manner. Second, ENaC regulation by pH was abolished by removing Na+ from the extracellular bathing solution. Third, mutations that alter Na+ self- inhibition produced corresponding changes in ENaC regulation by pH. Together, the data support a model in which protons modulate ENaC gating by relieving Na+ self- inhibition. We speculate that this may be an important mechanism to facilitate epithelial Na+ transport under conditions of acidosis.