Ca²⁺-dependent opening of MthK. (A) Representative channel currents recorded over a range of [Ca²⁺] at −100 mV. These currents are from the same bilayer, which contained two active channels. (B) P_O over a range of cytoplasmic [Ca²⁺] and pH. Symbols represent the mean Po at particular pH (down triangle, pH 6.5; diamond, pH 6.9; square, pH 7.3; circle, pH 7.7; up triangle, pH 8.1; left triangle, pH 8.5; star, pH 9.0). These symbols corresponding to each pH are used in subsequent figures, as applicable. Solid lines represent fits with Hill equations for data at a given pH. Increasing pH over the range of 6.5–9.0 resulted in roughly parallel shifts of Po versus [Ca²⁺] relations toward lower [Ca²⁺]. (C) Logarithmic plot of fitted EC₅₀(Ca²⁺) values versus pH shows an approximately linear relation. The solid line represents a linear fit to these data points. (D) Variation in fitted Hill coefficients does not correlate with pH. In B–D, all error bars represent standard errors of the means (n ≥ 4).