Suzuki S, Oshimi Y, Sugi H
Department of Physiology, School of Medicine, Teikyo University, Tokyo, Japan.
J Electron Microsc (Tokyo). 1993 Apr;42(2):107-16.
To give information about the changes of cross-bridge (myosin head) orientation during muscle contraction, mechanically skinned frog muscle fibers were rapidly frozen at various states, and the cross-bridges were observed on freeze-etch replicas. The number of cross-bridges per unit length of thick filament in relaxed state was less than one-third of that in contracting and rigor states. The interval between adjacent cross-bridges was maximum around 35 nm, a value close to the crossover repeat of actin helix. The cross-bridge angle distribution, as measured with a digital image processor, showed a peak around 90 degrees in all the states examined. The proportion of cross-bridges with angles around 90 degrees decreased either after stretch or after release of fibers in rigor state. The axial spacing of actin monomers in the thin filament was found to increase with increasing rigor force, to give the thin filament stiffness (times unit length) of about 1.8 x 10(4) pN. These results are discussed in connection with the mechanical properties of cross-bridges.
为了获取有关肌肉收缩过程中横桥（肌球蛋白头部）取向变化的信息，对机械去膜的青蛙肌肉纤维在不同状态下进行快速冷冻，并在冷冻蚀刻复制品上观察横桥。在松弛状态下，每单位长度粗丝上的横桥数量不到收缩状态和僵直状态下的三分之一。相邻横桥之间的间隔最大约为35纳米，这一数值接近肌动蛋白螺旋的交叉重复距离。用数字图像处理器测量的横桥角度分布在所有检测状态下均在90度左右出现峰值。在僵直状态下，纤维拉伸或释放后，角度约为90度的横桥比例会降低。发现细肌丝中肌动蛋白单体的轴向间距随着僵直力的增加而增大，使细肌丝的刚度（每单位长度）约为1.8×10⁴皮牛。结合横桥的力学特性对这些结果进行了讨论。
