Iwamoto H
Department of Physiology, School of Medicine, Teikyo University, Tokyo, Japan.
Biophys J. 1995 Sep;69(3):1022-35. doi: 10.1016/S0006-3495(95)79977-5.
The dynamic characteristics of the low force myosin cross-bridges were determined in fully calcium-activated skinned rabbit psoas muscle fibers shortening under constant loads (0.04-0.7 x full isometric tension Po). The shortening was interrupted at various times by a ramp stretch (duration, 10 ms; amplitude, up to 1.8% fiber length) and the resulting tension response was recorded. Except for the earlier period of velocity transients, the tension response showed nonlinear dependence on stretch amplitude; i.e., the magnitude of the tension response started to rise disproportionately as the stretch exceeded a critical amplitude, as in the presence of inorganic phosphate (Pi). This result, as well as the result of stiffness measurement, suggests that the low force cross-bridges similar to those observed in the presence of Pi (presumably A.M.ADP.Pi) are significantly populated during shortening. The critical amplitude of the shortening fibers was greater than that of isometrically contracting fibers, suggesting that the low force cross-bridges are more negatively strained during shortening. As the load was reduced from 0.3 to 0.04 P0, the shortening velocity increased more than twofold, but the amount of the negative strain stayed remarkably constant (approximately 3 nm). This This insensitiveness of the negative strain to velocity is best explained if the dissociation of the low force cross-bridges is accelerated approximately in proportion to velocity. Along with previous reports, the results suggest that the actomyosin ATPase cycle in muscle fibers has at least two key reaction steps in which rate constants are sensitively regulated by shortening velocity and that one of them is the dissociation of the low force A.M.ADP.Pi cross-bridges. This step may virtually limit the rate of actomyosin ATPase turnover and help increase efficiency in fibers shortening at high velocities.
在完全钙激活的去皮兔腰大肌纤维以恒定负荷（0.04 - 0.7×最大等长张力P0）缩短的过程中，测定了低力肌球蛋白横桥的动力学特性。在不同时间通过斜坡拉伸（持续时间10毫秒；幅度高达纤维长度的1.8%）中断缩短过程，并记录由此产生的张力响应。除了速度瞬变的早期阶段，张力响应显示出对拉伸幅度的非线性依赖性；即，当拉伸超过临界幅度时，张力响应的大小开始不成比例地增加，就像在有无机磷酸盐（Pi）存在的情况下一样。这一结果以及刚度测量的结果表明，在缩短过程中，类似于在Pi存在时观察到的那些低力横桥（可能是A.M.ADP.Pi）大量存在。缩短纤维的临界幅度大于等长收缩纤维的临界幅度，这表明低力横桥在缩短过程中受到更大的负应变。当负荷从0.3P0降低到0.04P0时，缩短速度增加了两倍多，但负应变的量保持显著恒定（约3纳米）。如果低力横桥的解离大约与速度成比例地加速，那么负应变对速度的这种不敏感性就能得到最好的解释。与之前的报道一起，这些结果表明，肌肉纤维中的肌动球蛋白ATP酶循环至少有两个关键反应步骤，其中速率常数受缩短速度的敏感调节，其中之一是低力A.M.ADP.Pi横桥的解离。这一步骤实际上可能限制肌动球蛋白ATP酶周转的速率，并有助于提高纤维在高速缩短时的效率。
