Wang Shi-Qiang, Song Long-Sheng, Xu Le, Meissner Gerhard, Lakatta Edward G, Ríos Eduardo, Stern Michael D, Cheng Heping
Laboratory of Cardiovascular Sciences, National Institute on Aging, National Institutes of Health, Baltimore, Maryland 21224, USA.
Biophys J. 2002 Jul;83(1):242-51. doi: 10.1016/S0006-3495(02)75165-5.
For a single or a group of Markov channels gating reversibly, distributions of open and closed times should be the sum of positively weighted decaying exponentials. Violation of this microscopic reversibility has been demonstrated previously on a number of occasions at the single channel level, and has been attributed to possible channel coupling to external sources of free energy. Here we show that distribution of durations of Ca(2+) release underlying Ca(2+) sparks in intact cardiac myocytes exhibits a prominent mode at approximately 8 ms. Analysis of the cycle time for repetitive sparks at hyperactive sites revealed no intervals briefer than approximately 35 ms and a mode at approximately 90 ms. These results indicate that, regardless of whether Ca(2+) sparks are single-channel or multi-channel in origin, they are generated by thermodynamically irreversible stochastic processes. In contrast, data from planar lipid bilayer experiments were consistent with reversible gating of RyR under asymmetric cis (4 microM) and trans Ca(2+) (10 mM), suggesting that the irreversibility for Ca(2+) spark genesis may reside at a supramolecular level. Modeling suggests that Ca(2+)-induced Ca(2+) release among adjacent RyRs may couple the external energy derived from Ca(2+) gradients across the SR to RyR gating in situ, and drive the irreversible generation of Ca(2+) sparks.
对于单个或一组可逆门控的马尔可夫通道，开放时间和关闭时间的分布应该是正加权衰减指数的总和。先前在单通道水平上已多次证明违反这种微观可逆性的情况，并将其归因于通道可能与外部自由能源耦合。在这里，我们表明完整心肌细胞中钙火花背后的钙释放持续时间分布在约8毫秒处呈现出一个突出的模式。对高活性位点重复火花的循环时间分析显示，没有比约35毫秒更短的间隔，且在约90毫秒处有一个模式。这些结果表明，无论钙火花起源于单通道还是多通道，它们都是由热力学不可逆的随机过程产生的。相比之下，平面脂质双层实验的数据与在不对称顺式（4微摩尔）和反式钙（10毫摩尔）条件下兰尼碱受体（RyR）的可逆门控一致，这表明钙火花产生的不可逆性可能存在于超分子水平。模型表明，相邻兰尼碱受体之间的钙诱导钙释放可能将源自肌浆网（SR）跨膜钙梯度的外部能量与原位兰尼碱受体门控耦合，并驱动钙火花的不可逆产生。
