Membrane of Necturus Gallbladder

The hypothesis of Cl-/HCO s exchange across the apical membrane of the epithelial cells of Necturus gallbladder was tested by means of measurements of extracellular pH (pH.), intracellular pH (pHi), andClactivity (aCli) with ion-sensitive microelectrodes. Luminal pH changes were measured after stopping mucosal superfusion with a solution of low buffering power. Under control conditions, the luminal solution acidifies when superfusion is stopped. Shortly after addition of the Na'/H' exchange inhibitor amiloride (10-3 M) to the superfusate, alkalinization was observed . During prolonged (10 min) exposure to amiloride, no significant pH. change occurred . Shortly after amiloride removal, luminal acidification increased, returning to control rates in 10 min. The absence of Na' in the superfusate (TMA' substitution) caused changes in the same direction, but they were larger than those observed with amiloride. Removal of CI(cyclamate or sulfate substitution) caused a shortlived increase in the rate of luminal acidification, followed by areturn to control values (10-30 min) . Upon re-exposure to Cl-, there was a transient reduction of luminal acidification . The initial increase in acidification produced by CIremoval was partially inhibited by SITS (0 .5 mM). The pHi increased rapidly and reversibly when the Clconcentration o£ the mucosal bathing solution was reduced to nominally 0 mM . The pH; changes were larger in 10 mM HC03Ringer's than in 1 mM HEPES-Ringer's, which suggests that HCO-3 is transported in exchange for CI-. In both HEPESand HC03-Ringer's, SITS inhibited the pHi changes. Finally, intracellular acidification or alkalinization (partial replacement of NaCl with sodium propionate or ammonium chloride, respectively) caused a reversible decrease or increase of aCli . These results support the hypothesis of apical membrane CI-/HC03 exchange, which can be dissociated from Na''/H+ exchange and operates under control conditions . The coexistence at the apical membrane of Na'/H' and CI-/HCO3 antiports suggests that NaCl entry can occur through these transporters .