The cross-bridge cycle in muscle. Mechanical, biochemical, and structural studies on single skinned rabbit psoas fibers to characterize cross-bridge kinetics in muscle for correlation with the actomyosin-ATPase in solution.

A characteristic and important feature of myocardium is the modulation of tension when stimulated or possibly even when unstimulated. In addition, resistance to stretch and its variation in unstimulated heart muscle is an important factor in myocardial function. These features may occur in some new light when viewed from some recent advances in understanding of cross-bridge action and regulation of muscle. For this reason we give a short review of such advances. Firstly, we summarize some of our earlier results obtained in experiments designed to see whether and to what extent actomyosin ATPase data obtained in solution might apply in muscle. Secondly, we present a recently developed experimental approach to estimate the rate constants that determine the cycling of cross-bridges between weak-binding, 'non-force-generating' states and strong-binding, 'force-generating' states. The estimated rate constants confirm the prediction of cross-bridge models derived from in vitro studies that the step which is rate-limiting in solution also determines the rate of force-generation in the cross-bridge cycle in muscle. Experiments at various Ca++ concentrations imply that a major mechanism of regulation is the control of the transition from the weak-binding, 'non-force-generating' states to the strong-binding, 'force-generating' states while the number of activated interaction sites appears unchanged and always at its maximum. This implies that changes in the force-pCa relation cannot be interpreted without detailed analysis of cross-bridge kinetics, and that factors other than Ca++ may have the potential to modulate muscle activity, both in stimulated and unstimulated muscle, by affecting cross-bridge kinetics.