Dependence of Ae4 activity on external Na⁺ and K⁺ concentrations. Ae4 activity was measured as the alkalinization rate in response to a reduction in the external [Cl⁻] using Na⁺- and K⁺-containing solutions (change from solution A in Table 1 to external solutions 5, 25, 50, 100, 125, and 150 mM [see Table 3 for ion composition of the Na⁺- and K⁺-containing solutions]). The calculated activities at different cation concentrations were plotted against the cation concentration and then fitted to a Hill function as described in Materials and methods, Eq. 4. The Hill fits are plotted as black lines. Data are expressed as the mean ± SEM from at least four experiments per condition that included at least three different electroporations.