The Ca2+-dependent gating mechanism of cloned BK channels from Drosophila (dSIo) was studied. Both a natural variant (A1/C2/E1/G3/IO) and a mutant (S942A) were expressed in Xenopus oocytes, and single-channel currents were recorded from excised patches of membrane. Stability plots were used to define stable segments of data. Unlike native BK channels from rat skeletal muscle in which increasing internal Ca2+ concentration (Ca(i)/2+) in the range of 5 to 30 μM increases mean open time, increasing Ca2+ in this range for dSIo had little effect on mean open time. However, further increases in Ca2+ to 300 or 3000 μM then typically increased dSIo mean open time. Kinetic schemes for the observed Ca2+-dependent gating kinetics of dSIo were evaluated by fitting two-dimensional dwell-time distributions using maximum likelihood techniques and by comparing observed dependency plots with those predicted by the models. Previously described kinetic schemes that largely account for the Ca2+-dependent kinetics of native BK channels from rat skeletal muscle did not adequately describe the Ca2+ dependence of dSIo. An expanded version of these schemes which, in addition to the Ca2+-activation steps, permitted a Ca2+-facilitated transition from each open state to a closed state, could approximate the Ca2+- dependent kinetics of dSIo, suggesting that Ca2+ may exert dual effects on gating.
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