Changes in membrane potential and HCO₃⁻ fluxes across the apical membrane of guinea pig pancreatic ducts. (A) Experimental conditions used for the measurement of apical HCO₃⁻ permeability in microperfused interlobular ducts. In this case, the bath perfusate contained 25 mM HCO₃⁻ and was equilibrated with 5% CO₂. This ensured that pCO₂ was constant throughout the system. All other conditions were the same as in Fig. 1 A. (B) Changes in basolateral membrane potential V_b recorded with a conventional microelectrode when the bath K⁺ concentration was switched between 1, 5, and 70 mM. Representative of five experiments. (C) Changes in transepithelial potential difference V_t with the tip of the electrode advanced into the duct lumen. Representative of four experiments. (D) Membrane potential–evoked changes in pH_i recorded under the same conditions as in B. Representative of five experiments.