Cheng H, Lederer M R, Lederer W J, Cannell M B
Department of Physiology, University of Maryland, Baltimore School of Medicine 21201, USA.
Am J Physiol. 1996 Jan;270(1 Pt 1):C148-59. doi: 10.1152/ajpcell.1996.270.1.C148.
Local elevations in intracellular calcium ("Ca2+ sparks") in heart muscle are elementary sarcoplasmic reticulum (SR) Ca(2+)-release events. Ca2+ sparks occur at a low rate in quiescent cells but can also be evoked by electrical stimulation of the cell to produce the cell-wide Ca2+ transient. In this study we investigate how Ca2+ sparks are related to propagating waves of elevated cytosolic Ca2+ induced by "Ca2+ overload." Single ventricular myocytes from rat were loaded with the Ca(2+)-sensitive indicator fluo 3 and imaged with a confocal microscope. After extracellular Ca2+ concentration was increased from 1 to 10 mM to produce Ca2+ overload, the frequency of spontaneous Ca2+ sparks, which occur at the t tubule/SR junction, increased approximately 4-fold, whereas the spark amplitude and spatial size increased 4.1-and 1.7-fold, respectively. In addition, a spectrum of larger subcellular events, including propagating Ca2+ waves, was observed. Ca2+ sparks were seen to occur at the majority (65%) of the sites of wave initiation. For slowly propagating Ca2+ waves, discrete Ca(2+)-release events, similar to Ca2+ sparks, were detected in the wave front. These Ca2+ sparks appeared to recruit other sparks along the wave front so that the wave progressed in a saltatory manner. We conclude that Ca2+ sparks are elementary events that can explain both the initiation and propagation of Ca2+ waves. In addition, we show that Ca2+ waves and electrically evoked Ca2+ transients have the same time course and interact with each other in a manner that is consistent with both phenomena having the same underlying mechanism(s). These results suggest that SR Ca2+ release during Ca2+ waves, like that during normal excitation-contraction coupling, results from the spatial and temporal summation of Ca2+ sparks.
心肌细胞内钙离子浓度的局部升高（“钙离子火花”）是肌浆网（SR）基本的钙离子释放事件。在静息细胞中，钙离子火花发生率较低，但也可通过对细胞进行电刺激来诱发，从而产生全细胞钙离子瞬变。在本研究中，我们探究了钙离子火花与“钙离子超载”诱导的细胞溶质钙离子升高的传播波之间的关系。用钙离子敏感指示剂氟罗 3 加载大鼠单个心室肌细胞，并用共聚焦显微镜成像。将细胞外钙离子浓度从 1 mM 提高到 10 mM 以产生钙离子超载后，发生在横管/肌浆网连接处的自发性钙离子火花频率增加了约 4 倍，而火花幅度和空间大小分别增加了 4.1 倍和 1.7 倍。此外，还观察到一系列更大的亚细胞事件，包括传播的钙离子波。在大多数（65%）波起始位点都能看到钙离子火花。对于缓慢传播的钙离子波，在波前检测到类似于钙离子火花的离散钙离子释放事件。这些钙离子火花似乎沿着波前招募其他火花，从而使波以跳跃方式推进。我们得出结论，钙离子火花是基本事件，能够解释钙离子波的起始和传播。此外，我们表明钙离子波和电诱发的钙离子瞬变具有相同的时间进程，并且它们之间的相互作用方式与这两种现象具有相同潜在机制相一致。这些结果表明，钙离子波期间的肌浆网钙离子释放，与正常兴奋 - 收缩偶联期间一样，是由钙离子火花的时空总和引起的。
