Song L S, Wang S Q, Xiao R P, Spurgeon H, Lakatta E G, Cheng H
Laboratory of Cardiovascular Sciences, National Institute on Aging, National Institutes of Health, Baltimore, Md, USA.
Circ Res. 2001 Apr 27;88(8):794-801. doi: 10.1161/hh0801.090461.
To elucidate microscopic mechanisms underlying the modulation of cardiac excitation-contraction (EC) coupling by beta-adrenergic receptor (beta-AR) stimulation, we examined local Ca(2+) release function, ie, Ca(2+) spikes at individual transverse tubule-sarcoplasmic reticulum (T-SR) junctions, using confocal microscopy and our recently developed technique for release flux measurement. beta-AR stimulation by norepinephrine plus an alpha(1)-adrenergic blocker, prazosin, increased the amplitude of SR Ca(2+) release flux (J(SR)), its running integral (integralJ(SR)), and L-type Ca(2+) channel current (I(Ca)), and it shifted their bell-shaped voltage dependence leftward by approximately 10 mV, with the relative effects ranking I(Ca)> J(SR)>integralJ(SR). Confocal imaging revealed that the bell-shaped voltage dependence of SR Ca(2+) release is attributable to a graded recruitment of T-SR junctions as well as to changes in Ca(2+) spike amplitudes. beta-AR stimulation increased the fractional T-SR junctions that fired Ca(2+) spikes and augmented Ca(2+) spike amplitudes, without altering the SR Ca(2+) load, suggesting that more release units were activated synchronously among and within T-SR junctions. Moreover, beta-AR stimulation decreased the latency and temporal dispersion of Ca(2+) spike occurrence at a given voltage, delivering most of the Ca(2+) at the onset of depolarization rather than spreading it out throughout depolarization. Because the synchrony of Ca(2+) spikes affects Ca(2+) delivery per unit of time to contractile myofilaments, and because the myofilaments display a steep Ca(2+) dependence, our data suggest that synchronization of SR Ca(2+) release represents a heretofore unappreciated mechanism of beta-AR modulation of cardiac inotropy.
为阐明β-肾上腺素能受体（β-AR）刺激调节心脏兴奋-收缩（EC）偶联的微观机制，我们使用共聚焦显微镜和我们最近开发的释放通量测量技术，研究了局部Ca(2+)释放功能，即单个横管-肌浆网（T-SR）连接处的Ca(2+)尖峰。去甲肾上腺素加α(1)-肾上腺素能阻滞剂哌唑嗪对β-AR的刺激增加了肌浆网Ca(2+)释放通量（J(SR)）的幅度、其运行积分（integralJ(SR)）和L型Ca(2+)通道电流（I(Ca)），并使其钟形电压依赖性向左移动约10 mV，相对效应排序为I(Ca)>J(SR)>integralJ(SR)。共聚焦成像显示，肌浆网Ca(2+)释放的钟形电压依赖性归因于T-SR连接处的分级募集以及Ca(2+)尖峰幅度的变化。β-AR刺激增加了产生Ca(2+)尖峰的T-SR连接处的比例，并增大了Ca(2+)尖峰幅度，而不改变肌浆网Ca(2+)负荷，这表明在T-SR连接处之间和内部有更多的释放单位被同步激活。此外，β-AR刺激减少了给定电压下Ca(2+)尖峰出现的延迟和时间离散，在去极化开始时释放大部分Ca(2+)，而不是在整个去极化过程中分散释放。由于Ca(2+)尖峰的同步性影响单位时间内向收缩性肌丝的Ca(2+)递送，并且由于肌丝表现出陡峭的Ca(2+)依赖性，我们的数据表明肌浆网Ca(2+)释放的同步化代表了β-AR调节心脏收缩力的一种迄今未被认识的机制。
