Inhibition of MthK activation by H⁺. (A) Representative channel currents recorded over a range of pH at −100 mV. Each of these currents was from different bilayers, which each contained two active channels. For the top of A, no channel activity was observed for 10 min of recording. (B) Po versus [H⁺] at four different [Ca²⁺]: 0.5 mM (open diamond and dash line), 1 mM (open square and solid line), 2 mM (open circle and dotted line), and 10 mM (open triangle and dash-dotted line). Smooth curves represent fits of data at each [Ca²⁺] with Hill equations. Parameters (±SE) for 0.5 mM Ca²⁺ fit: EC₅₀ = 1.47 × 10⁻⁹ ± 1.8 × 10⁻¹⁰ M (pH = 8.8), n_H = 6.1 ± 1.9, and Pomax = 0.95 ± 0.01; for 1 mM Ca²⁺ fit: EC₅₀ = 7.6 × 10⁻⁹ ± 5.9 × 10⁻¹⁰ M (pH = 8.1), n_H = 5.9 ± 10.0, and Pomax = 0.94 ± 0.04; for 2 mM Ca²⁺ fit: EC₅₀ = 1.84 × 10⁻⁸ ± 2.0 × 10⁻⁹ M (pH = 7.7), n_H = 3.5 ± 2.1, and Pomax = 0.94 ± 0.07; for 10 mM Ca²⁺ fit: EC₅₀ = 1.06 × 10⁻⁷ ± 1.3 × 10⁻⁸ M (pH = 7.0), n_H = 2.9 ± 0.45, and Pomax = 0.94 ± 0.02.