Cooperative mechanisms in the activation dependence of the rate of force development in rabbit skinned skeletal muscle fibers

Regulation of contraction in skeletal muscle is a highly cooperative process involving Ca2+ binding to troponin C (TnC) and strong binding of myosin cross-bridges to actin. To further investigate the role(s) of cooperation in activating the kinetics of cross-bridge cycling, we measured the Ca2+ dependence of the rate constant of force redevelopment (k(tr)) in skinned single fibers in which cross-bridge and Ca2+ binding were also perturbed. Ca2+ sensitivity of tension, the steepness of the force-pCa relationship, and Ca2+ dependence of k(tr) were measured in skinned fibers that were (1) treated with NEM-S1, a strong-binding, non-force-generating derivative of myosin subfragment 1, to promote cooperative strong binding of endogenous cross-bridges to actin; (2) subjected to partial extraction of TnC to disrupt the spread of activation along the thin filament; or (3) both, partial extraction of TnC and treatment with NEM-S1. The steepness of the force-pCa relationship was consistently reduced by treatment with NEM-S1, by partial extraction of TnC, or by a combination of TnC extraction and NEM-S1, indicating a decrease in the apparent cooperativity of activation. Partial extraction of TnC or NEM-S1 treatment accelerated the rate of force redevelopment at each submaximal force, but had no effect on kinetics of force development in maximally activated preparations. At low levels of Ca2+, 3 muM NEM-S1 increased k(tr) to maximal values, and higher concentrations of NEM-S1 (6 or 10 muM) increased k(tr) to greater than maximal values. NEM-S1 also accelerated k(tr) at intermediate levels of activation, but to values that were submaximal. However, the combination of partial TnC extraction and 6 muM NEM-S1 increased k(tr) to virtually identical supramaximal values at all levels of activation, thus, completely eliminating the activation dependence of k(tr). These results show that k(tr) is not maximal in control fibers, even at saturating [Ca2+], and suggest that activation dependence of k(tr) is due to the combined activating effects of Ca2+ binding to TnC and cross-bridge binding to actin.