Kress M, Huxley H E, Faruqi A R, Hendrix J
J Mol Biol. 1986 Apr 5;188(3):325-42. doi: 10.1016/0022-2836(86)90158-0.
The pattern given by contracting frog muscle can be followed with high time resolution using synchrotron radiation as a high-intensity X-ray source. We have studied the behaviour of the second actin layer-line (axial spacing of approximately 179 A) at an off-meridional spacing of approximately 0.023 A-1, a region of the diagram that is sensitive to the position of tropomyosin in the thin filaments. In confirmation of earlier work, we find that there is a substantial increase in the intensity of this part of the pattern during contraction. We find that the reflection reaches half its final intensity about 17 milliseconds after the stimulus at 6 degrees C. The changes in the equatorial reflections, which arise from movement of crossbridges towards the thin filaments, occur with a delay of about 12 to 17 milliseconds relative to this change in the actin pattern. In over-stretched muscle, where thick and thin filaments no longer overlap, the changes in the actin second layer-line still take place upon stimulation with a time course and intensity similar to that observed at full overlap. This indicates that tropomyosin movement, in response to calcium binding to troponin, is the first structural step in muscular contraction, and is the prerequisite for myosin binding. A change in intensity similar to that found in contracting muscle is seen in rigor, where tropomyosin is probably locked in the active position. During relaxation the earlier stages in the decrease in intensity of the second actin layer-line take place significantly sooner after the last stimulus than tension decay. In over-stretched muscles the intensity decay is appreciably faster than in the same muscles at rest length, where attached crossbridges may interfere with the return of tropomyosin to its resting position.
利用同步辐射作为高强度X射线源，可以以高时间分辨率跟踪收缩青蛙肌肉给出的图案。我们研究了第二肌动蛋白层线（轴向间距约为179埃）在离子午线间距约为0.023埃^-1处的行为，该图的这一区域对细肌丝中原肌球蛋白的位置敏感。正如早期工作所证实的那样，我们发现在收缩过程中该图案这一部分的强度有显著增加。我们发现，在6摄氏度下刺激后约17毫秒，该反射达到其最终强度的一半。赤道反射的变化是由横桥向细肌丝移动引起的，相对于肌动蛋白图案的这种变化，其延迟约为12至17毫秒。在过度拉伸的肌肉中，粗肌丝和细肌丝不再重叠，肌动蛋白第二层线的变化在刺激时仍会发生，其时间进程和强度与在完全重叠时观察到的相似。这表明，原肌球蛋白响应钙与肌钙蛋白的结合而发生的移动是肌肉收缩的第一个结构步骤，也是肌球蛋白结合的前提条件。在僵直状态下，观察到与收缩肌肉中类似的强度变化，此时原肌球蛋白可能被锁定在活性位置。在松弛过程中，第二肌动蛋白层线强度下降的早期阶段在最后一次刺激后比张力衰减明显更早发生。在过度拉伸的肌肉中，强度衰减明显比处于静息长度的相同肌肉更快，在静息长度时附着的横桥可能会干扰原肌球蛋白回到其静息位置。
