Figure 5. Mean reversal potential data for Na⁺-HCO₃⁻ cotransport currents recorded at different concentrations of extracellular Na⁺.

•, mean values of I_Na,BicE_rev registered at 30 mm (n = 5), 60 mm (n = 5), 90 mm (n = 4) and 138 mm (n = 4) extracellular Na⁺. The dotted line represents the I_Na,BicE_rev calculated using the equation described in the text and assuming a HCO₃⁻ :Na⁺ ratio of 2:1. [HCO₃⁻]_i was assumed to be 13.9 mm (pH_i 7.1). The means ±s.e.m. of the I_Na,BicE_rev recorded at 60, 90 and 138 mm extracellular Na⁺ were fitted to a regression line with a slope of −43 mV mm⁻¹. This value corresponds to a Na⁺-HCO₃⁻ symport with a stoichiometry n value of 2.3. The results are consistent with the presence of a Na⁺-HCO₃⁻ cotransport current with a HCO₃⁻:Na⁺ stoichiometry ratio of 2:1.