HCO₃⁻ fluxes across the apical membrane of interlobular ducts isolated from the guinea pig pancreas. (A) Experimental conditions for the detection of membrane potential–evoked HCO₃⁻ fluxes across the apical membrane of microperfused interlobular ducts. It was assumed that, because of the leakiness of the ductal epithelium, changes in bath K⁺ concentration would bring about comparable changes in membrane potential at both apical and basolateral membranes. Concentrations of Cl⁻ and HCO₃⁻ are indicated in mM. Basolateral HCO₃⁻ efflux was inhibited with 0.5 mM H₂DIDS. HCO₃⁻ fluxes across the apical membrane were detected as changes in pH_i. (B) Isolated interlobular duct from guinea pig pancreas cannulated with concentric holding and perfusion pipettes. This configuration allowed independent perfusion of the lumen and bath. Duct cells were loaded with BCECF, and a small region of the duct epithelium (indicated by the rectangle) was selected for measurement of pH_i. (C and D) Membrane potential–evoked changes in pH_i in ducts exposed to different bath K⁺ concentrations. Bath and lumen were first perfused with the standard HEPES-buffered solution, and the luminal solution was then switched to the high-HCO₃⁻ solution containing 125 mM HCO₃⁻ and 24 mM Cl⁻. 0.5 mM dbcAMP was present in the bath perfusate as indicated. The Na⁺ concentration in the bath and luminal solutions was 60 mM throughout, and [K⁺]_B was raised or lowered (1, 5, and 70 mM) by replacement with NMDG. Each trace is representative of four experiments.