Reiermann H J, Herzig J W, Rüegg J C
Basic Res Cardiol. 1977 Mar-Jun;72(2-3):133-9. doi: 10.1007/BF01906351.
In the course of MG-dependent ATP splitting by heart actomyosin, an "energy rich" actomyosin-ADP complex is formed, which promotes the incorporation of phosphate 32P into ATP in myofibrils. The rate of this ATP-phosphate exchange reaction depends on the extent of actin-myosin overlap which can be decreased by stretching glycerinated muscle fibres. In heart muscle, the calcium-ion dependence of this reaction is similar to that of the actomyosin ATPase, the tension, and "immediate fibre stiffness" (which is "hookean" and which is a measure for the number of myosin cross-bridges attached to and interacting with actin). These findings suggest that calcium increases the amount of "contractile" actomyosin-ADP complexes. The proportionality between tension and ATPase activity further suggests that the rate-limiting step of the cross-bridge cycle (which determines the molecular turnover number, the "Wechselzahl" of the ATPase) is only little affected by calcium ions. These ions act by recruiting more bridges rather than by accelerating their reactions. In addition, the depressing effect of inorganic phosphate on the contractile tension and its presumable role in energetic insuffciency will be discussed.
在心脏肌动球蛋白依赖镁离子进行ATP分解的过程中，会形成一种“能量丰富”的肌动球蛋白 - ADP复合物，它能促进32P磷酸根掺入肌原纤维中的ATP。这种ATP - 磷酸交换反应的速率取决于肌动蛋白 - 肌球蛋白重叠的程度，而这种重叠程度可通过拉伸甘油处理的肌纤维来降低。在心肌中，该反应对钙离子的依赖性与肌动球蛋白ATP酶、张力以及“即时纤维硬度”（呈“胡克弹性”，是附着于肌动蛋白并与其相互作用的肌球蛋白横桥数量的一种度量）的依赖性相似。这些发现表明，钙离子会增加“收缩性”肌动球蛋白 - ADP复合物的量。张力与ATP酶活性之间的比例关系进一步表明，横桥循环的限速步骤（它决定了ATP酶的分子转换数，即“转换数”）受钙离子的影响很小。这些离子的作用是募集更多的横桥，而不是加速它们的反应。此外，还将讨论无机磷酸对收缩张力的抑制作用及其在能量不足中可能扮演的角色。
