Deletion of Slc26a6 alters the stoichiometry of apical Cl^-/HCO₃^- exchange in mouse pancreatic duct

To define the stoichiometry and molecular identity of the Cl^-/HCO₃^- exchanger in the apical membrane of pancreatic duct cells, changes in luminal pH and volume were measured simultaneously in interlobular pancreatic ducts isolated from wild-type and Slc26a6-null mice. Transepithelial fluxes of HCO₃^- and Cl^- were measured in the presence of anion gradients favoring rapid exchange of intracellular HCO₃^- with luminal Cl^- in cAMP-stimulated ducts. The flux ratio of Cl^- absorption/ HCO₃^- secretion was ~0.7 in wild-type ducts and ~1.4 in Slc26a6^-/- ducts where a different Cl^-/HCO₃^- exchanger, most likely SLC26A3, was found to be active. Interactions between Cl^-/HCO₃^- exchange and cystic fibrosis transmembrane conductance regulator (CFTR) in cAMP-stimu-lated ducts were examined by measuring the recovery of intracellular pH after alkali-loading by acetate prepulse. Hyperpolarization induced by luminal application of CFTRinh-172 enhanced HCO₃^- efflux across the apical membrane via SLC26A6 in wild-type ducts but significantly reduced HCO₃^- efflux in Slc26a6^-/- ducts. In microper-fused wild-type ducts, removal of luminal Cl^-, or luminal application of dihydro-4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid to inhibit SLC26A6, caused membrane hyperpolarization, which was abolished in Slc26a6^-/- ducts. In conclusion, we have demonstrated that deletion of Slc26a6 alters the apparent stoichiometry of apical Cl^-/ HCO₃ exchange in native pancreatic duct. Our results are consistent with SLC26A6 mediating 1:2 Cl^-/HCO₃^- exchange, and the exchanger upregulated in its absence, most probably SLC26A3, mediating 2:1 exchange.