Estimation of apical HCO₃⁻ permeability. (A) Calculation of the rate of change of pH_i by linear regression at 1-min intervals after step changes in [K⁺]_B. The fitted lines are overlaid on data from the second half of the experiment shown in Fig. 5 D. Together with the midpoint value of pH_i and the intracellular buffering capacity, these data were used to calculate the HCO₃⁻ flux across the apical membrane. (B) Distribution of calculated HCO₃⁻ permeabilities for HCO₃⁻ influx (n = 18 from 5 ducts) and HCO₃⁻ efflux (n = 18 from 5 ducts) pooled from experiments like the one shown in A. The horizontal dotted lines indicate the mean values. (C) Predicted changes in pH_i resulting from changes in V_a induced by switching [K⁺]_B from 1 to 70 mM, and then back to 1 mM. Theoretical curves based on three alternative apical HCO₃⁻ permeability values (0.05, 0.1, and 0.2 µm sec⁻¹) are superimposed on averaged data from four experiments of the type shown in Fig. 5 D. The dotted lines indicate the mean ± SEM. The top panel shows the predicted changes in V_a, assuming a time constant of 45 s.