Cytosolic pH regulates GCl through control of phosphorylation states of CFTR

Our objective in this study was to determine the effect of changes in luminal and cytoplasmic pH on cystic fibrosis transmembrane regulator (CFTR) Cl- conductance (G(Cl)). We monitored CFTR G(Cl) in the apical membranes of sweat ducts as reflected by Cl- diffusion potentials (V-Cl) and transepithelial conductance (G(Cl)). We found that luminal pH (5.0-8.5) had little effect on the cAMP/ATP-activated CFTR G(Cl), showing that CFTR G(Cl) is maintained over a broad range of extracellular pH in which it functions physiologically. However, we found that phosphorylation activation of CFTR G(Cl) is sensitive to intracellular pH. That is, in the presence of cAMP and ATP [adenosine 5'-O-(3-thiotriphosphate)], CFTR could be phosphorylated at physiological pH (6.8) but not at low pH (similar to 5.5). On the other hand, basic pH prevented endogenous phosphatase(s) from dephosphorylating CFTR. After phosphorylation of CFTR with cAMP and ATP, CFTR G(Cl) is normally deactivated within 1 min after cAMP is removed, even in the presence of 5 mM ATP. This deactivation was due to an increase in endogenous phosphatase activity relative to kinase activity, since it was reversed by the reapplication of ATP and cAMP. However, increasing cytoplasmic pH significantly delayed the deactivation of CFTR G(Cl) in a dose-dependent manner, indicating inhibition of dephosphorylation. We conclude that CFTR G(Cl) may be regulated via shifts in cytoplasmic pH that mediate reciprocal control of endogenous kinase and phosphatase activities. Luminal pH probably has little direct effect on these mechanisms. This regulation of CFTR may be important in shifting electrolyte transport in the duct from conductive to nonconductive modes.