HCO3 exit across the basolateral membrane of the kidney proximal tubule cell is mediated via an electrogenic Na+: cotransporter. In these experiments, we have studied the effect of internal pH on the activity of the Na+:HCO-cotransport system in basolateral membrane vesicles isolated from rabbit renal cortex. Equilibrium thermodynamics predicts that in the presence of constant intravesicular concentration of Na+, an increasing concentration of HCO3 will be associated with an increasing driving force for Na+:HCO3 cotransport across the vesicles. Our experimental approach was to preequilibrate the membrane vesicles with 1 mM 22Na' at pH1 6.8-8.0 and known concentrations of HCO3. The vesicles were diluted 1:100 into Na+-free solution at pH 7.4 and the net flux of 22Na+ was assayed over 5 s. The results demonstrate that the net flux of Na+ was significantly higher at pH1 7.2 than pH, 8.0 despite much higher IHCO3I at pHi 8.0. This suggests that an internal pHsensitive site regulates the activity of the Na+:HCO-cotransporter. This modifier site inhibits the cotransporter at alkaline pH despite significant base concentration and is maximally functional around physiologic pH. The combination of modifier sites on the luminal Na+/H+ exchanger and the basolateral Na+:HCO-cotransporter should help maintain intracellular pH in a narrow range with changes in extracellular pH. (J.

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