Extracellular Chloride Regulates the Epithelial Sodium Channel

The extracellular domain of the epithelial sodium channel ENaC is exposed to a wide range of Cl- concentrations in the kidney and in other epithelia. We tested whether Cl- alters ENaC activity. In Xenopus oocytes expressing human ENaC, replacement of Cl- with SO42-, H2PO4-, or SCN- produced a large increase in ENaC current, indicating that extracellular Cl- inhibits ENaC. Extracellular Cl- also inhibited ENaC in Na+-transporting epithelia. The anion selectivity sequence was SCN- < SO42- < H2PO4- < F- < I- < Cl- < Br-. Crystallization of ASIC1a revealed a Cl- binding site in the extracellular domain. We found that mutation of corresponding residues in ENaC (alpha(H418A) and beta(R388A)) disrupted the response to Cl-, suggesting that Cl- might regulate ENaC through an analogous binding site. Maneuvers that lock ENaC in an open state (a DEG mutation and trypsin) abolished ENaC regulation by Cl-. The response to Cl- was also modulated by changes in extracellular pH; acidic pH increased and alkaline pH reduced ENaC inhibition by Cl-. Cl- regulated ENaC activity in part through enhanced Na+ self-inhibition, a process by which extracellular Na+ inhibits ENaC. Together, the data indicate that extracellular Cl- regulates ENaC activity, providing a potential mechanism by which changes in extracellular Cl- might modulate epithelial Na+ absorption.