Contribution of ambient HCO3- to mucosal protection and intracellular pH in isolated amphibian gastric mucosa.

The relationship between mucosal protection by ambient HCO3- and its impact on intracellular pH was studied in isolated frog gastric mucosa. Closed sacs of gastric mucosa, containing exogenous HCl and pepsin, readily ulcerated when incubated in Ringer solution devoid of HCO3-, but were effectively protected against ulceration when ambient HCO3- was present. This protection was abolished by blocking anion exchange in cell membrane by 4-acetamido-4'-isothiocyanostillbene-2,2'-disulfonic acid or by inhibiting mucosal carbonic anhydrase by azetazolamide. Intracellular pH, measured by the 14C-labeled 5,5-dimethylxazolidine-2,4-dione method, was significantly higher when open sheets of mucosas were incubated in a medium containing HCO3- than when they were incubated in a medium devoid of HCO3- at the same external pH. Furthermore, a positive pH gradient across the cell membrane (intracellular pH was greater than extracellular pH) was generated only if HCO3- was present in the incubation medium. 4-Acetamido-4'-isothiocyano-stillbene-2,2'-disulfonic acid completely abolished these effects of ambient HCO3- on intracellular pH, suggesting that ambient HCO3- contributes to intracellular pH by acting as an intracellular base. In contrast, acetazolamide had no significant influence on these effects, suggesting that carbonic anhydrase acts in the protective mechanism distally to the entry of HCO3- into the cell. The data indicate that ambient HCO3- significantly contributes to intracellular pH in the gastric mucosa, a feature that may enhance the ability of the mucosa to withstand intraluminal acid.