Analyses of transmural difference of myocardial crossbridge dynamics using SPring-8 synchrotron facility

容积依赖性心脏性能增加的机制（Frank-Starling定律）是心血管生理学中一个有趣的主题。Frank-Starling定律的可能机制是肌丝线晶格间距减小，钙与肌钙蛋白的结合亲和力增加，肌动蛋白与肌球蛋白的相互作用率与强结合的交桥增加。然而，心脏具有复杂的三维结构，因此肌丝取向和张力的区域异质性可能在心脏功能的调节中起重要作用。我们利用SPring-8同步加速器揭示了周期性心脏跳动时跨桥动力学的跨壁差异。在收缩期，心外膜和心中交叉桥动力学与左室压力发展很好地协调。在松弛期，心间桥明显早于心外膜桥。预压水平的变化对这一区域差异没有影响。我们还发现，在高负荷和低负荷水平下，心外膜区域的肌凝蛋白晶格间距明显小于心中央区域。假设在心肌细胞的拉伸和释放过程中，晶格体积是恒定的，肌球蛋白晶格间距的减小意味着肌节的延长，反之亦然。因此，无论负荷水平如何，心外膜区域的肌节长度都比心中央区域长。一般而言，心内膜或心深部动作电位的持续时间比心外膜区域长。目前的结果与这种跨壁电活动的异质性相矛盾。然而，肌节长度越长，抽搐收缩持续时间越长。因此，心中区长动作电位和短肌节长度的收缩持续时间似乎与心外膜区短动作电位和长肌节长度的收缩持续时间相当。（297/300字）少

Mechanism of volume dependent increase in cardiac performance (Frank-Starling law) is interesting theme in the cardiaovascular physiology. Possible mechanisms of the Frank-Starling law are decrease in the myofilamet lattice spacing, and increase in the calcium binding affinity to troponin and actin myosin interaction rate with strongly bound crossbrdge. However the heart has complex three-dimensional structure so the regional heterogeneities of myofilament orientation and strain may have important role in regulation of the cardiac performance.We revealed the transmural difference of crossbridge dynamics using the SPring-8 synchrotron facility during cyclic heart beats. During contraction phase, epicardial and midcardial crossbridge dynamics were harmonized well with the left ventricular pressure development. However during relaxation phase, midcardial crossbridge decayed significantly earlier than epicardial one. The changes of the preload level did not affect on this regional differen … More ce of crossbridge decay.We also found the myosin lattice spacing in the epicardial region was significantly smaller than that in the midcardial region at both high and low preload levels. With an assumption that the lattice volume is constant during stretch and release of the cardiomyocyte, decrease in the myosin lattice spacing means the lengthening of the sarcomere and vice versa. Thus the epicardial region would work with longer sarcomere length than the midcardial region regardless of preload levels.Generally the duration of action potential in the endocardial or deeper region is longer than the epicardial region. The present results contradicts to this transmural heterogeneity of electrical activity. However the longer duration of twitch contraction is observed at the longer sarcomere length. Thus the duration of contraction with the long action potential and short sarcomere length in the midcardial region seemed to be comparable with the duration of contraction with the short action potential and long sarcomere length in the epicardial region. (297/300 words) Less