Ca 2+-Dependent Gating Mechanisms for dSlo, a Large-Conductance Ca 2+-Activated K + (BK) Channel

Abstract

The Ca 2+-dependent gating mechanism of cloned BK channels from Drosophila ( dSlo) 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 Ca 2+ concentration (Ca i 2+) in the range of 5 to 30 μM increases mean open time, increasing Ca i 2+ in this range for dSlo had little effect on mean open time. However, further increases in Ca i 2+ to 300 or 3000 μM then typically increased dSlo mean open time. Kinetic schemes for the observed Ca 2+-dependent gating kinetics of dSlo 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 Ca 2+-dependent kinetics of native BK channels from rat skeletal muscle did not adequately describe the Ca 2+ dependence of dSlo. An expanded version of these schemes which, in addition to the Ca 2+-activation steps, permitted a Ca 2+-facilitated transition from each open state to a closed state, could approximate the Ca 2+-dependent kinetics of dSlo, suggesting that Ca 2+ may exert dual effects on gating.