Na⁺-HCO₃⁻ cotransporter activity associated with mouse Ae4 expression under Cl⁻-free conditions. Representative experiments showing pH_i measurements made in nontransfected (green circles; n = 14) and Ae4-transfected cells (black circles; n = 12) in response to a change in the external solution from a Na⁺-containing HEPES-buffered to a Na⁺-containing CO₂/HCO₃⁻-buffered solution (change from solution G to H [Table 1]). Then the external Na⁺ was isotonically replaced by NMDG (change from solution H to I [Table 1]). The experiments were performed under Cl⁻-free conditions, and all solutions were supplemented with 10 µM EIPA to inhibit endogenous Na⁺/H⁺ exchanger activity. Results are presented as the mean ± SEM.