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Department of Medicine, University of Manchester School of Medicine, Hope Hospital, Salford.

1. The pH dependence of a chloride conductance in the apical membrane of rat duodenal enterocytes was examined. 2. A stepwise reduction of both internal and external pH from 7.4 to 6.8 resulted in a significant stimulation of 36Cl flux driven by an inside-positive membrane potential. 3. A stepwise reduction in pH had no significant effect upon other parameters such as the initial rate of D-[3H]glucose or voltage-independent 36Cl uptake, suggesting a specific effect upon the chloride conductance. 4. The pH-dependent stimulation of 36Cl uptake exhibited saturation kinetics, with an apparent Vmax (maximum velocity) of 5.5 nmol (mg protein)-1 (4 s)-1 and an apparent Km (Michaelis-Menten constant) of 88 nM H+ ions. 5. To determine the site of action of protons upon the conductance the effect of asymmetrically reducing either the internal or external pH was examined. 6. A step reduction of extracellular pH from 7.8 to 6.8 significantly stimulated the rate of 36Cl uptake. In contrast, a step reduction of internal pH from 7.8 to 6.8 was without effect upon the rate of 36Cl uptake. 7. These results suggest that the chloride conductance on the apical membrane of rat duodenal enterocytes is allosterically regulated by protons at an external site.