Burst analysis of MthK gating over a range of pH and [Ca²⁺]. Bursts of openings were classified by calculation of a critical closed interval duration (see Materials and methods), and the number of openings per burst and durations of bursts was analyzed over a range of experimental conditions. (A) Representative distribution of the number of openings per burst from data at 15 mM Ca²⁺, pH 6.5. Data were binned at a resolution of one opening/burst per bin. The solid line represents a fit of the data with a geometric distribution (N(r)=N(1) b^r-1), with mean number of openings per burst (given by 1/1-b) = 9. (B) Increasing [Ca²⁺] increases the number of openings per burst, whereas increasing [H⁺] (decreasing pH) decreases the number of openings per burst. The number of openings per burst increased 6.6-fold per twofold increase in [Ca²⁺], indicating that the frequency of gaps between bursts decreased by that amount to increase channel Po. In general, increasing H⁺ increased the [Ca²⁺] required to reach a given burst length. (C) Representative distribution of burst durations from data at 15 mM Ca²⁺, pH 6.5. The solid line represents a fit of the data with a single-exponential component, with τ = 130 ms. (D) As with the number of openings/burst, increasing [Ca²⁺] increases burst duration, whereas increasing [H⁺] decreases burst duration. Burst duration increased 7.7-fold per twofold increase in [Ca²⁺], and increasing [H⁺] increased the [Ca²⁺] required to reach a given burst duration. Symbols in B and D represent mean ± SEM.