Wakabayashi K, Sugimoto Y, Tanaka H, Ueno Y, Takezawa Y, Amemiya Y
Department of Biophysical Engineering, Faculty of Engineering Science, Osaka University, Japan.
Biophys J. 1994 Dec;67(6):2422-35. doi: 10.1016/S0006-3495(94)80729-5.
To clarify the extensibility of thin actin and thick myosin filaments in muscle, we examined the spacings of actin and myosin filament-based reflections in x-ray diffraction patterns at high resolution during isometric contraction of frog skeletal muscles and steady lengthening of the active muscles using synchrotron radiation as an intense x-ray source and a storage phosphor plate as a high sensitivity, high resolution area detector. Spacing of the actin meridional reflection at approximately 1/2.7 nm-1, which corresponds to the axial rise per actin subunit in the thin filament, increased about 0.25% during isometric contraction of muscles at full overlap length of thick and thin filaments. The changes in muscles stretched to approximately half overlap of the filaments, when they were scaled linearly up to the full isometric tension, gave an increase of approximately 0.3%. Conversely, the spacing decreased by approximately 0.1% upon activation of muscles at nonoverlap length. Slow stretching of a contracting muscle increased tension and increased this spacing over the isometric contraction value. Scaled up to a 100% tension increase, this corresponds to a approximately 0.26% additional change, consistent with that of the initial isometric contraction. Taken together, the extensibility of the actin filament amounts to 3-4 nm of elongation when a muscle switches from relaxation to maximum isometric contraction. Axial spacings of the layer-line reflections at approximately 1/5.1 nm-1 and approximately 1/5.9 nm-1 corresponding to the pitches of the right- and left-handed genetic helices of the actin filament, showed similar changes to that of the meridional reflection during isometric contraction of muscles at full overlap. The spacing changes of these reflections, which also depend on the mechanical load on the muscle, indicate that elongation is accompanied by slight changes of the actin helical structure possibly because of the axial force exerted by the actomyosin cross-bridges. Additional small spacing changes of the myosin meridional reflections during length changes applied to contracting muscles represented an increase of approximately 0.26% (scaled up to a 100% tension increase) in the myosin periodicity, suggesting that such spacing changes correspond to a tension-related extension of the myosin filaments. Elongation of the myosin filament backbone amounts to approximately 2.1 nm per half sarcomere. The results indicate that a large part (approximately 70%) of the sarcomere compliance of an active muscle is caused by the extensibility of the actin and myosin filaments; 42% of the compliance resides in the actin filaments, and 27% of it is in the myosin filaments.
为了阐明肌肉中细肌动蛋白丝和粗肌球蛋白丝的可伸展性，我们使用同步加速器辐射作为强X射线源，以及存储磷光板作为高灵敏度、高分辨率的面积探测器，在青蛙骨骼肌等长收缩和主动肌肉稳定拉长过程中，以高分辨率检查了X射线衍射图谱中基于肌动蛋白丝和肌球蛋白丝的反射间距。在粗细肌丝完全重叠长度的肌肉等长收缩过程中，肌动蛋白子午反射间距（约为1/2.7nm-1，对应于细肌丝中每个肌动蛋白亚基的轴向上升）增加了约0.25%。当肌肉拉伸至约为肌丝半重叠状态，然后按比例线性放大至完全等长张力时，间距增加约0.3%。相反，在非重叠长度下激活肌肉时，间距减少约0.1%。收缩肌肉的缓慢拉伸会增加张力，并使该间距超过等长收缩值。按比例放大至张力增加100%，这相当于额外增加约0.26%的变化，与初始等长收缩时的变化一致。综上所述，当肌肉从松弛状态转变为最大等长收缩时，肌动蛋白丝的可伸展性相当于伸长3 - 4nm。对应于肌动蛋白丝右手和左手遗传螺旋螺距的约1/5.1nm-1和约1/5.9nm-1处的层线反射轴向间距，在粗细肌丝完全重叠的肌肉等长收缩过程中，显示出与子午反射类似的变化。这些反射的间距变化也取决于肌肉上的机械负荷，这表明伸长伴随着肌动蛋白螺旋结构的轻微变化，可能是由于肌动球蛋白横桥施加的轴向力所致。在对收缩肌肉施加长度变化过程中，肌球蛋白子午反射的额外小间距变化表示肌球蛋白周期性增加约0.26%（按比例放大至张力增加100%），这表明这种间距变化对应于肌球蛋白丝与张力相关的伸展。每半个肌节中肌球蛋白丝主干的伸长量约为2.1nm。结果表明，主动肌肉肌节顺应性的很大一部分（约70%）是由肌动蛋白丝和肌球蛋白丝的可伸展性引起的；其中42%的顺应性存在于肌动蛋白丝中，27%存在于肌球蛋白丝中。
