Mi.III⁺ RBCs exhibited higher Cl⁻/HCO₃⁻ exchange capacity upon HCO₃⁻ stimulation. (A) Intracellular Cl⁻ was labeled by fluorescent dye SPQ, and the intracellular Cl⁻ concentration ([Cl⁻]_out) was measured by the degree of SPQ quenching inside erythrocytes. When the extracellular bicarbonate ([HCO₃⁻]_out) was 5 mM, [Cl⁻]_in increased little with respect to [Cl⁻]_out. [Cl⁻]_in was similar between the control and Mi.III⁺ erythrocytes. In the milieu of 15 mM HCO₃⁻, Mi.III⁺ erythrocytes contained more Cl⁻ and showed higher Cl⁻ permeability than the control cells. Data are expressed as mean ± SE; *P < .05 at 90 mM [Cl⁻]_out. The numbers of donors tested were indicated next to sample labels. (B) The concentrations of intracellular bicarbonate ([HCO₃⁻]_in) were predicted according to Donnan equilibrium. In the milieu of 15 mM HCO₃⁻, Mi.III⁺ erythrocytes also contained more HCO₃⁻ and showed higher HCO₃⁻ permeability.