INTRACELLULAR PH-REGULATING ION-TRANSPORT MECHANISMS IN PARIETAL-CELL BASOLATERAL MEMBRANE-VESICLES

Na+-H+ and Cl--base exchangers on the parietal cell have been demonstrated by several authors. Controversy exists concerning a basolateral Na+-HCO3-cotransporter in the parietal cell. To clarify this issue, we prepared highly enriched basolateral membrane (BLM) and apical-tubulovesicular membrane (to serve as negative controls) vesicles from rabbit fundic mucosa. Na+-H+ exchange was demonstrated by measuring pH gradient-driven amiloride-sensitive Na-22+ uptake and Na+ gradient-driven proton uptake into voltage-clamped BLM but not into apical-tubulovesicular vesicles. Anion exchange was demonstrated by measuring 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS)-inhibitable influx of Cl-36- into Cl-- or HCO3--loaded voltage-clamped BLM vesicles. Na+-HCO3- cotransport was assessed by comparing HCO3--driven Na-22 uptake with uptake driven by an identical pH gradient. No significant difference was found between Na-22 uptake in the presence and absence of HCO3-; 1 mM amiloride inhibited Na-22 uptake >90% in both conditions, whereas 2 mM DIDS had no effect. In BLM vesicles prepared from rabbit renal cortex, however, a HCO3- gradient stimulated Na-22 uptake much more than an equivalent pH gradient, and DIDS inhibited this HCO3- gradient-driven Na-22 uptake. This indicates that our experimental setup was suitable to detect a Na+-HCO3- cotransporter if present. Our data suggest that the parietal cell BLM contains Na+-H+ exchangers and Cl--HCO3- exchangers but no Na+-HCO3- cotransporter.