Evidence for Cl^- -independent HCO₃^- transport in basolateral membranes of Necturus oxyntopeptic cells

Luminal H⁺ secretion by gastric mucosa is accompanied by basolateral HCO₃^- release. A basolateral Cl^-/HCO₃^- exchanger is known to mediate HCO₃^- extrusion from oxyntopeptic cells during resting and secretagogue-induced apical HCl secretion. From recent work, we hypothesized that there might be a Cl^--independent pathway for basolateral HCO₃^- exit in Necturus oxyntopeptic cells. In this study, we used a fluorescent pH indicator [2′,7′-bis(2-carboxyethyl)-5(6)-carboxyfluorescein] to evaluate Cl^--independent HCO₃^- transport across the basolateral membranes of intact oxyntopeptic cells. Removal of serosal Cl^- increased intracellular pH (pH_i) (7.05 to 7.25), consistent with Cl^--dependent HCO₃^- extrusion. Removal of serosal Na⁺ in the absence of Cl^- resulted in significant acidification of pH_i (7.10 to 6.89), but studies involving amiloride, 4,4′-diisothiocyanostilbene-2,2′-disulfonic acid (BIDS), and 0 HCO₃^--N-2-hydroxyethylpiperazine-N′-2-ethanesulfonic acid-buffered solutions suggest that Na⁺-dependent changes in pH_i are due to Na⁺/H⁺ exchange. Our studies demonstrate a marked concentration-dependent alkalinization when tissues are exposed to increases in serosal K⁺. A substantial part of this alkalinization in response to increases in serosal K⁺ (pH_i 7.00 to 7.46) appears to be a HCO₃^- exit pathway that is independent of both Na⁺ and Cl^-, unaffected by bumetanide or amiloride, but sensitive to DIDS. We propose the presence of a Cl^-- and Na⁺-independent K⁺-dependent HCO₃^- cotransporter in Necturus oxyntopeptic cell basolateral membranes.