Campbell K
Department of Veterinary and Comparative Anatomy, Physiology, and Pharmacology, Washington State University, Pullman 99164, USA.
Biophys J. 1997 Jan;72(1):254-62. doi: 10.1016/S0006-3495(97)78664-8.
The rate of muscle force redevelopment after release-restretch protocols has previously been interpreted using a simple two-state cross-bridge cycling model with rate constants for transitions between non-force-bearing and force-bearing states, f, and between force-bearing and non-force-bearing states, g. Changes in the rate constant of force redevelopment, as with varying levels of Ca2+ activation, have traditionally been attributed to Ca(2+)-dependent f. The current work adds to this original model a state of unactivated, noncycling cross-bridges. The resulting differential equation for activated, force-bearing cross-bridges, Ncf, was Ncf = -[g+f(K/(K + 1))] Ncf+f(K/(K + 1))NT, where K is an equilibrium constant defining the distribution between cycling and noncycling cross-bridges and NT is the total number of cross-bridges. Cooperativity by which force-bearing cross-bridges participate in their own activation was introduced by making K depend on Ncf. Model results demonstrated that such cooperativity, which tends to enhance force generation at low levels of Ca2+ activation, has a counter-intuitive effect of slowing force redevelopment. These dynamic effects of cooperativity are most pronounced at low Ca2+ activation. As Ca2+ activation increases, the cooperative effects become less important to the dynamics of force redevelopment and, at the highest levels of Ca2+ activation, the dynamics of force redevelopment reflect factors other than cooperative mechanisms. These results expand on earlier interpretations of Ca2+ dependence of force redevelopment; rather than Ca(2+)-dependent f, Ca(2+)-dependent force redevelopment arises from changing expressions of cooperativity between force-bearing cross-bridges and activation.
先前，释放 - 拉伸方案后肌肉力量重新发展的速率是使用一个简单的双态横桥循环模型来解释的，该模型具有非承载和承载状态之间转换的速率常数f，以及承载和非承载状态之间转换的速率常数g。与不同水平的Ca2 + 激活一样，力量重新发展速率常数的变化传统上归因于Ca(2 +)依赖性f。当前的工作在这个原始模型中增加了未激活的、非循环横桥的状态。由此产生的关于激活的、承载力量的横桥Ncf的微分方程为Ncf = -[g + f(K / (K + 1))] Ncf + f(K / (K + 1))NT，其中K是定义循环和非循环横桥之间分布的平衡常数，NT是横桥的总数。通过使K依赖于Ncf，引入了承载力量的横桥参与自身激活的协同性。模型结果表明，这种协同性在低水平的Ca2 + 激活时倾向于增强力量产生，但对力量重新发展具有减缓作用，这一作用与直觉相反。协同性的这些动态效应在低Ca2 + 激活时最为明显。随着Ca2 + 激活增加，协同效应对于力量重新发展的动力学变得不那么重要，并且在最高水平的Ca2 + 激活时，力量重新发展的动力学反映了协同机制以外的因素。这些结果扩展了早期对力量重新发展的Ca2 + 依赖性的解释；力量重新发展的Ca(2 +)依赖性不是源于Ca(2 +)依赖性f，而是源于承载力量的横桥与激活之间协同性表达的变化。
