Activation of fast skeletal muscle: contributions of studies on skinned fibers

The membrane potential of vertebrate twitch fibers closely controls Ca fluxes between intracellular compartments, which in turn control contraction. Recent work on intracellular Ca movement is reviewed in the general context of current efforts to synthesize physiological, biochemical, and structural observations on the contractile mechanism and its regulation, emphasizing the increasing role of functionally skinned fibers in this synthesis. Skinned fiber preparations, with removed or disrupted sarcolemma, bridge the gap between properties of isolated subsystems and their constrained operation in the intact fiber. Recent studies indicate that the surface action potential propagates along the transverse tubules, but not the sarcoplasmic reticulum (SR), which appears to be a distinct intracellular compartment. Voltage-dependent charge movements in the transverse tubules probably control Ca flux across the SR membranes. Current questions concern the mechanism of the signal that bridges the junctional gap between the two membrane systems, the mechanism and properties of the activated Ca efflux to the myofilament space, and the operation of the Ca pump of the SR during activation. New methods applied to intact fibers, cut fibers, skinned fibers, and subcellular systems are yielding the kind of information needed for a complete description of these central steps in excitation-contraction coupling and of Ca regulation of the myofilaments.