Gating of Recombinant Small-Conductance Ca-activated K(+) Channels by Calcium

Small-conductance Ca-activated K(+) channels play an important role in modulating excitability in many cell types. These channels are activated by submicromolar concentrations of intracellular Ca(2+), but little is known about the gating kinetics upon activation by Ca(2+). In this study, single channel currents were recorded from Xenopus oocytes expressing the apamin-sensitive clone rSK2. Channel activity was detectable in 0.2 μM Ca(2+) and was maximal above 2 μM Ca(2+). Analysis of stationary currents revealed two open times and three closed times, with only the longest closed time being Ca dependent, decreasing with increasing Ca(2+) concentrations. In addition, elevated Ca(2+) concentrations resulted in a larger percentage of long openings and short closures. Membrane voltage did not have significant effects on either open or closed times. The open probability was ∼0.6 in 1 μM free Ca(2+). A lower open probability of ∼0.05 in 1 μM Ca(2+) was also observed, and channels switched spontaneously between behaviors. The occurrence of these switches and the amount of time channels spent displaying high open probability behavior was Ca(2+) dependent. The two behaviors shared many features including the open times and the short and intermediate closed times, but the low open probability behavior was characterized by a different, long Ca(2+)-dependent closed time in the range of hundreds of milliseconds to seconds. Small-conductance Ca- activated K(+) channel gating was modeled by a gating scheme consisting of four closed and two open states. This model yielded a close representation of the single channel data and predicted a macroscopic activation time course similar to that observed upon fast application of Ca(2+) to excised inside-out patches.