Ultra-fast X-ray diffraction studies on the relationship between the structun change extensibility of the actin filaments and force generation in muscle

为了了解含肌动蛋白细丝的力学性能与骨骼肌受力之间的关系，利用同步辐射进行了快速x射线衍射研究。主要结果描述如下。(1)肌动蛋白丝的延伸性与肌肉中产生的力呈线性关系，表明肌动蛋白丝具有纯弹性。肌动蛋白丝的这种延展性与丝中肌动蛋白单体的扭转运动密切相关，其变化的时间过程随力的发展而发生。(2)初始激活时，肌动蛋白细丝略有缩短，但明显缩短。考虑到这一结果，肌肉中肌动蛋白丝在最大发力下的净伸长率约为0.35%。(3)当斜面长度拉伸作用于收缩肌时，肌球蛋白子午反射强度的降低及其轴向间距的变化与力的变化平行，表明肌球蛋白过桥的结构改变与力的产生机制耦合。本研究为活动肌中肌动蛋白细丝的弹性特性和结构变化提供了有力的实验证据。基于这些新的结果，我们讨论了肌动蛋白和肌球蛋白的特殊构象变化对肌肉收缩的分子模型的影响。

In order to understand the relationship between the mechanical properties of thin actin-containing filaments and force generation in skeletal muscle, the fast X-ray diffraction studies were performed by using synchrotron radiation. The principal results were described below.(1) The relation between the extensibility of the actin filaments and force generated in muscle was linear, establishing that the actin filament has a purely elastic property. Such an extensibility of the actin filaments was closely associated with a torsional motion of the actin monomers in the filament and its time course of the change occurred with the development of force.(2) Upon the initial activation, the thin actin filaments became shortened slightly but significantly. Considering this result, the net extensibility of the actin filaments in muscle under the maximum force generation was determined to about 0.35%.(3) When the ramp length stretch was applied to a contracting muscle, the intensity decrease of the myosin meridional reflections and their axial spacing change run parallel to the change in force, implying that the structural alteration of the myosin crossbridges is coupling with the force-generation mechanism.In this research, firm experimental evidence was provided for the elastic properties and structural changes of the thin actin filaments in active muscle. Based on these new results, the molecular model for muscle contraction was discussed in a combination of peculiar conformational changes of both actin and myosin.