A CL- CONDUCTANCE SENSITIVE TO EXTERNAL PH IN THE APICAL MEMBRANE OF RAT DUODENAL ENTEROCYTES

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 Cl-36 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-[H-3]glucose or voltage-independent Cl-36 uptake, suggesting a specific effect upon the chloride conductance. 4. The pH-dependent stimulation of Cl-36 uptake exhibited saturation kinetics, with an apparent V(max) (maximum velocity) of 5.5 nmol (mg protein)-1 (4 s)-1 and an apparent K(m) (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 Cl-36 uptake. In contrast, a step reduction of internal pH from 7.8 to 6.8 was without effect upon the rate of Cl-36 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.