Figure 3.

Mg²⁺ shortens the mean latency to the first opening and lengthens mean burst duration of BK channels. For this and all experiments in the paper, the two high-affinity Ca²⁺ binding sites have been disabled by mutation (see Materials and methods). (A and B) Representative traces from a single BK channel in a patch showing examples of sweeps after voltage steps from −100 mV to +100 mV for 400 ms. The channel in this patch was relatively active compared with a typical channel. The sweeps were selected (except for the second plotted sweep) to have at least one opening. The voltage protocol was 600 ms at the prepulse holding potential of −100 mV and then 400 ms at +100 mV, applied once per second. Latencies to opening are marked by double-ended arrows and first burst durations are marked by thick horizontal bars, which appear as dots for short bursts. 10 mM Mg²⁺ shortened mean latency and lengthened mean burst duration. Capacitive transients at the beginning and end of the sweeps have been removed. (C and E) Distributions of latency to first opening. For sweeps without openings, one count was added to the bin at 400 ms, where intervals were counted as ≥400 ms for the fitting. The distributions of latencies to first opening were described with sums of exponentials. For 0 Mg²⁺: τ₁ = 56 ms (area = 0.27) and τ₂ = 575 ms (area = 0.73). For 10 mM Mg²⁺: τ₁ = 31 ms (area = 0.63) and τ₂ = 228 ms (area = 0.37). The distribution for 0 Mg²⁺ (broken line) is overlaid on the distribution for 10 mM Mg²⁺. 10 mM Mg²⁺ shifted the distribution toward shorter latency. (D and F) Distributions of burst duration are plotted. For 0 Mg²⁺: τ₁ = 5.9 ms (area = 1.0). For 10 mM Mg²⁺: τ₁ = 0.04 ms (area = 0.27) and τ₂ = 53 ms (area = 0.73). 10 mM Mg²⁺ shifted the distribution for burst duration to a longer duration. Distributions are scaled to the same number of events to facilitate visual comparisons.