Piazzesi G, Lombardi V
Dipartimento di Scienze Fisiologiche, Università degli Studi di Firenze, Italy.
Biophys J. 1995 May;68(5):1966-79. doi: 10.1016/S0006-3495(95)80374-7.
The responses of muscle to steady and stepwise shortening are simulated with a model in which actin-myosin cross-bridges cycle through two pathways distinct for the attachment-detachment kinetics and for the proportion of energy converted into work. Small step releases and steady shortening at low velocity (high load) favor the cycle implying approximately 5 nm sliding per cross-bridge interaction and approximately 100/s detachment-reattachment process; large step releases and steady shortening at high velocity (low load) favor the cycle implying approximately 10 nm sliding per cross-bridge interaction and approximately 20/s detachment-reattachment process. The model satisfactorily predicts specific mechanical properties of frog skeletal muscle, such as the rate of regeneration of the working stroke as measured by double-step release experiments and the transition to steady state during multiple step releases (staircase shortening). The rate of energy liberation under different mechanical conditions is correctly reproduced by the model. During steady shortening, the relation of energy liberation rate versus shortening speed attains a maximum (approximately 6 times the isometric rate) for shortening velocities lower than half the maximum velocity of shortening and declines for higher velocities. In addition, the model provides a clue for explaining how, in different muscle types, the higher the isometric maintenance heat, the higher the power output during steady shortening.
利用一个模型模拟了肌肉对持续和逐步缩短的反应，在该模型中，肌动蛋白-肌球蛋白横桥通过两条途径循环，这两条途径在附着-分离动力学以及转化为功的能量比例方面有所不同。小步释放和低速（高负荷）下的持续缩短有利于这样一种循环，即每次横桥相互作用意味着大约5纳米的滑动以及大约每秒100次的分离-重新附着过程；大步释放和高速（低负荷）下的持续缩短有利于另一种循环，即每次横桥相互作用意味着大约10纳米的滑动以及大约每秒20次的分离-重新附着过程。该模型令人满意地预测了青蛙骨骼肌的特定力学特性，比如通过双步释放实验测量的工作冲程的再生速率以及多步释放（阶梯式缩短）过程中向稳态的转变。该模型正确地再现了不同力学条件下的能量释放速率。在持续缩短过程中，对于低于最大缩短速度一半的缩短速度，能量释放速率与缩短速度的关系达到最大值（约为等长速率的6倍），而对于更高的速度则下降。此外，该模型为解释在不同肌肉类型中，等长维持热越高，持续缩短过程中的功率输出就越高这一现象提供了线索。
