Inhibition of alpha beta epithelial sodium channels by external protons indicates that the second hydrophobic domain contains structural elements forclosing the pore

We have examined the effect of extracellular protons on the activity of epi thelial sodium channels (ENaCs). We found that alpha beta channels, but not alpha beta gamma or alpha gamma channels, are inhibited by low extracellul ar pH. External protons induced short and long closed states that markedly decreased the open probability of (YP channels. External protons did not ch ange the single-channel conductance or amiloride binding. Analysis of the p roton-induced changes on the kinetics of single channels indicates that at least two protons sequentially hind to the extracellular domain at sites th at are not in the ion pathway. Conformational changes induced by protonatio n of those sites are transmitted to the second hydrophobic domain (M2) of t he subunits to induce closure of the pore. The results suggest that element s located in the carboxy-terminal half of M2 participate in the gating mech anism of ENaCs.