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 22Na+ 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 36Cl- into Cl(-)- or HCO3(-)-loaded voltage-clamped BLM vesicles. Na(+)-HCO3- cotransport was assessed by comparing HCO3(-)-driven 22Na uptake with uptake driven by an identical pH gradient. No significant difference was found between 22Na uptake in the presence and absence of HCO3-; 1 mM amiloride inhibited 22Na 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 22Na uptake much more than an equivalent pH gradient, and DIDS inhibited this HCO3- gradient-driven 22Na 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.