Novel regulation of cystic fibrosis transmembrane conductance regulator (CFTR) channel gating by external chloride

The cystic fibrosis transmembrane conductance regulator (CFTR) is vital for Cl^- and HCO₃^- transport in many epithelia. As the HCO₃^- concentration in epithelial secretions varies and can reach as high as 140 mM, the lumen-facing domains of CFTR are exposed to large reciprocal variations in Cl^- and HCO₃^- levels. We have investigated whether changes in the extracellular anionic environment affects the activity of CFTR using the patch clamp technique. In fast whole cell current recordings, the replacement of 100 mM external Cl ^- (Cl₀^-) with HCO₃^-, Br^-, NO₃^-, or aspartate^- inhibited inward CFTR current (Cl^- efflux) by ∼50% in a reversible manner. Lowering Cl₀^- alone by iso-osmotic replacement with mannitol also reduced Cl^- efflux to a similar extent. The maximal inhibition of CFTR current was ∼70%. Raising cytosolic calcium shifted the Cl^- dose-inhibition curve to the left but did not alter the maximal current inhibition observed. In contrast, a reduction in the internal [Cl ^-] neither inhibited CFTR nor altered the block caused by reduced Cl₀^-. Single channel recordings from outside-out patches showed that lowering Cl₀^- markedly reduced channel open probability with little effect on unitary conductance. Together, these results indicate that alterations in Cl₀^- alone and not the Cl^-/HCO₃^- ratio regulate the gating of CFTR. Physiologically, our data have implications for current models of epithelial HCO₃^- secretion and for the control of pH at epithelial cell surfaces.