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 \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{HCO}_{3}^{-}\) \end{document} transport in many epithelia. As the \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{HCO}_{3}^{-}\) \end{document} 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 \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{HCO}_{3}^{-}\) \end{document} 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– (\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \((\mathrm{Cl}_{o}^{-})\) \end{document}) with \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{HCO}_{3}^{-}\) \end{document}, Br–, \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NO}_{3}^{-}\) \end{document}, or aspartate– inhibited inward CFTR current (Cl– efflux) by ∼50% in a reversible manner. Lowering \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{Cl}_{o}^{-}\) \end{document} 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 \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{Cl}_{o}^{-}\) \end{document}. Single channel recordings from outside-out patches showed that lowering \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{Cl}_{o}^{-}\) \end{document} markedly reduced channel open probability with little effect on unitary conductance. Together, these results indicate that alterations in \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{Cl}_{o}^{-}\) \end{document} alone and not the \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{Cl}^{-}{/}\mathrm{HCO}_{3}^{-}\) \end{document} ratio regulate the gating of CFTR. Physiologically, our data have implications for current models of epithelial \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{HCO}_{3}^{-}\) \end{document} secretion and for the control of pH at epithelial cell surfaces.