Cl-HCO3 exchange and anion conductance in rat duodenal apical membrane vesicles.

Transport mechanisms for Cl and HCO3 anions in the apical membrane of rat duodenal enterocytes have been characterized using brush-border membrane vesicles. 36Cl uptake was stimulated by outwardly directed gradients of OH, HCO3, and Cl anions. The anion exchanger was sensitive to inhibition by 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS) (Ki, 1.3 mmol/l), 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), and furosemide. The process was electroneutral, since voltage clamping the membrane potential to 0 mV or applying a large inside-positive potential had no effect on the magnitude of uptake. The kinetic properties of the exchanger were measured and an apparent Km of 9.8 mM Cl and a Vmax of 134 nmol.mg protein-1.4 s-1 were found. In addition, an electrogenic conductive component of 36Cl uptake was found. This component was dependent on an inside-positive membrane potential and was inhibited by the Cl channel blocker diphenylamine-2-carboxylate. SITS, DIDS, and furosemide had no effect on the electrogenic component of 36Cl uptake. An apparent anion selectivity of SCN greater than I greater than Br greater than Cl greater than HCO3 greater than SO4 greater than Glu greater than PO4 was found. These results support the presence of both Cl-HCO3 exchange and a conductive anion channel in the apical membrane of rat duodenal enterocytes.