Fast BK-type channel mediates the Ca2+-activated K+ current in crayfish muscle

The role of the Ca2+-activated K+ current (I-K(Ca)) in Crayfish opener musc le fibers is functionally important because it regulates the graded electri cal activity that is characteristic of these fibers. Using the cell-attache d and inside-out configurations of the patch-clamp technique, we found thre e different classes of channels with properties that matched those expected of the three different ionic channels mediating the depolarization-activat ed macroscopic currents previously described (Ca2+, K+, and Ca2+-dependent K+ currents). We investigated the properties of the ionic channels mediatin g the extremely fast activating and persistent I-K(Ca). These voltage- and Ca2+ activated channels had a mean single-channel conductance of similar to 70 pS and showed a very fast activation. Both the single-channel open prob ability and the speed of activation increased with depolarization. Both par ameters also increased in inside-out patches, i.e., in high Ca2+ concentrat ion. Intracellular loading with the Ca2+ chelator bis(2-aminophenoxy) ethan e-N, N,N',N'-tetraacetic acid gradually reduced and eventually prevented ch annel openings. The channels opened at very brief delays after the pulse de polarization onset (<5 ms), and the time-dependent open probability was con stant during sustained depolarization (less than or equal to 560 ms), match ing both the extremely fast activation kinetics and the persistent nature o f the macroscopic I-K(Ca). However, the intrinsic properties of these singl e channels do not account for the partial apparent inactivation of the macr oscopic I-K(Ca), which probably reflects temporal Ca2+ variations in the wh ole muscle fiber. We conclude that the channels mediating I-K(Ca) in crayfi sh muscle are voltage- and Ca2+-gated BK channels with relatively small con ductance. The intrinsic properties of these channels allow them to act as p recise Ca2+ sensors that supply the exact feedback current needed to contro l the graded electrical activity and therefore the contraction of opener mu scle fibers.