GATING OF RECOMBINANT SMALL-CONDUCTANCE CA-ACTIVATED K+ CHANNELS BY CALCIUM

Small-conductance Ca-activated Kf channels play an important role in m odulating excitability in many cell types. These channels are activate d by submicromolar concentrations of intracellular Ca2+, but little is known about the gating kinetics upon activation by Ca2+. Tn this stud y, single channel currents were recorded from Xenopus oocytes expressi ng the apamin-sensitive clone rSK2. Channel activity was detectable in 0.2 mu M Ca2+ and was maximal above 2 mu M Ca2+. Analysis of stationa ry currents revealed two open times and three closed times, with only the longest closed time being Ca dependent, decreasing with increasing Ca2+ concentrations. In addition, elevated Ca2+ concentrations result ed in a larger percentage of long openings and short closures. Membran e voltage did not have significant effects on either open or closed ti mes. The open probability was similar to 0.6 in 1 mu M free Ca2+. A lo wer open probability of similar to 0.05 in 1 mu M Ca2+ was also observ ed, and channels switched spontaneously between behaviors. The occurre nce of these switches and the amount of time channels spent displaying high open probability behavior was Ca2+ dependent. The two behaviors shared many features including the open times and the short and interm ediate closed times, but the low open probability behavior was charact erized by a different, long Ca2+-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 clo sed and two open states. This model yielded a close representation of the single channel data and predicted a macroscopic activation time co urse similar to that observed upon fast application of Ca2+ to excised inside-out patches.