Ranatunga K W, Coupland M E
Muscle Contraction Group, Department of Physiology, School of Medical Sciences, University of Bristol, Bristol BS8 1TD, UK.
Adv Exp Med Biol. 2003;538:441-57; discussion 457.
The steady active muscle force is reduced, but the force generation induced by a standard temperature jump becomes 2-3 fold faster with increased inorganic phosphate level, [Pi]. The increase in the rate of force generation also exhibits saturation at higher [Pi] levels and the relation is hyperbolic. These observations are consistent with a kinetic scheme where rapid Pi release by actomyosin crossbridges in muscle is preceded by the force generation step. Such a scheme accounts for the sigmoidal temperature dependence of steady active force and its sensitivity to [Pi]. The [Pi] dependence of force recovery after stretch (positive strain) is also hyperbolic, suggesting that the "pre Pi-release force generation step" is strain-sensitive--as expected. However, length-release (negative strain) force transients are not [Pi] sensitive indicating an asymmetry, but its significance and also the kinetic step underlying force recovery from negative strain remain unclear.
稳定的主动肌肉力量降低，但随着无机磷酸盐水平[Pi]的增加，由标准温度跃升诱导的力量产生速度加快2至3倍。在较高的[Pi]水平下，力量产生速率的增加也呈现出饱和现象，且两者关系呈双曲线。这些观察结果与一种动力学机制一致，即在肌肉中肌动球蛋白横桥快速释放Pi之前存在力量产生步骤。这样的机制解释了稳定主动力量的S形温度依赖性及其对[Pi]的敏感性。拉伸（正应变）后力量恢复的[Pi]依赖性也是双曲线的，这表明“Pi释放前的力量产生步骤”对应变敏感——正如预期的那样。然而，长度释放（负应变）时的力量瞬变对[Pi]不敏感，表明存在不对称性，但其意义以及负应变后力量恢复的潜在动力学步骤仍不清楚。
