Na⁺-dependent HCO₃⁻/HCO₃⁻ exchange under low Cl⁻ concentrations. (A) Cartoon depicting the hypothetical Ae4-mediated HCO₃⁻ transport under low Cl⁻ concentrations when a HCO₃⁻-free external solution (condition i; solution R [Table 2]) was switched to a HCO₃⁻-containing external solution (condition ii; solution S [Table 2]). Note that there is no Ae4-mediated transport under HCO₃⁻-free conditions. Furthermore, given that the stoichiometric coefficients of Ae4-mediated HCO₃⁻ transport under low Cl⁻ conditions are unknown, we have denoted them as x (extracellular) and y (intracellular), respectively. (B) A decrease in the pH_i was observed after the external HEPES-buffered solution was replaced with a CO₂/HCO₃⁻-buffered solution (change of solution R to S [Table 2]). Little, if any, alkalinization was observed in Ae4-transfected cells using a HCO₃⁻-free pipette solution containing low Cl⁻ (4 mM; black circles; n = 6; solution P [Table 2]). In contrast, the initial acidification was followed by a marked intracellular alkalinization in Ae4-transfected cells dialyzed with a pipette solution containing 15 mM HCO₃⁻ + 4 mM Cl⁻ (green circles; n = 7; solution Q [Table 2]). A summary of the results calculated from experiments shown in this figure is given in Table 4. Results are presented as the mean ± SEM. (C) Summary of the alkalinization rates obtained in response to intracellular acidification from Ae4-transfected cells (voltage clamp whole cell recordings) held at −100 mV (green bar; n = 5) and 0 mV (green dashed bar; n = 4) using a HCO₃⁻-containing pipette solution supplemented with 100 µM BCECF (free acid; solution Q [Table 2]). Data are expressed as the mean ± SEM from single cells from at least three different electroporations.