Cytosolic pH regulatesG Cl through control of phosphorylation states of CFTR

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Abstract
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 CFTRG 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 CFTRG 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 (∼5.5). On the other hand, basic pH prevented endogenous phosphatase(s) from dephosphorylating CFTR.After phosphorylation of CFTR with cAMP and ATP, CFTRG 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 CFTRG Cl in a dose-dependent manner, indicating inhibition of dephosphorylation. We conclude that CFTRG 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.