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

为了了解骨骼肌中肌动蛋白细丝的力学性质与力产生之间的关系，利用同步辐射进行了快速X射线衍射研究。主要结果如下。(1)肌动蛋白丝的伸展性和肌肉中产生的力之间的关系是线性的，这表明肌动蛋白丝具有纯粹的弹性。肌动蛋白丝的这种伸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着力的发展而发生。(2)在最初的激活，薄肌动蛋白丝变得缩短轻微，但显着。考虑到该结果，在最大力产生下肌肉中肌动蛋白丝的净延伸性被确定为约0.35%。(3)当斜坡长度拉伸施加到收缩的肌肉时，肌球蛋白横向反射的强度降低和它们的轴向间距变化与力的变化平行，这意味着肌球蛋白横桥的结构改变与力产生机制耦合。为活动肌肉中细肌动蛋白丝的弹性性质和结构变化提供了有力的实验证据。基于这些新的结果，结合肌动蛋白和肌球蛋白的特殊构象变化讨论了肌肉收缩的分子模型。

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.