Mechanisms underlying calcium sparks in cardiac muscle.
Mechanisms underlying calcium sparks in cardiac muscle.
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DOI：
10.1085/jgp.113.3.373
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发表时间：
1999-03
期刊：
影响因子：
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通讯作者：
Soeller C
中科院分区：
文献类型：
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作者：
Cannell MB;Soeller C
Cheng等在单个离体大鼠心肌细胞中发现了钙火花。（1993），同时对来自钙指示剂fluo-3的荧光进行成像（Minta等人，1989年，用显微镜。心肌钙火花与静息fluo-3荧光（F/F 1.0）的大约加倍相关，并占据细胞直径为2 m的微小区域。火花下钙的可能变化的简单平衡计算表明，局部钙在10 ms内达到300 nM的峰值（Cheng et al.，1993年）。然而，由于fluo-3的动力学和动态范围有限，以及显微镜下的模糊，该数字低估了钙的真实变化。摘自Smith等人最近发表的一篇论文。（1998），我们可以估计火花下的钙的真实变化（通过0.5米宽的区域中的显微镜模糊平均）可能是10 M。在行扫描图像中，钙火花以1.6s-1的频率出现（Cheng等人，1993），其调查70 - 100个肌节。然而，钙火花也可以通过膜去极化产生，因此反映了在z线处发生的t管系统和肌浆网之间的各个连接处的兴奋-收缩（E-C）偶联过程（Shacklock et al.，1995; Cheng等人，1996年）。因此，在静息心肌细胞中，火花位点纵向间隔1.8 m。在横向方向上，钙火花位点具有更可变的间距，并且彼此更接近的位点更可能共激活（帕克等人，1996年）。全细胞钙瞬变可以通过钙火花的时空总和来解释（Cannell等人，1994年，1995年）。因此，全细胞肌浆网（SR）钙释放通量应该是SR释放的平均概率（PSpark）乘以平均局部SR释放通量（JRel），但这些变量在传统的全细胞光度测量中不可分离。然而，记录钙火花克服了这个问题，因为该方法提供了对PSpark（其与每单位时间在共焦成像体积中检测到的火花的数量成比例）以及JRel（其可以从钙火花的时空特性估计）的直接测量。Cheng等人对JRel的初步估计。（1993）提出，钙火花可以用10 ms的4 pA的钙通量来解释。随后对钙去除过程的详细数学建模表明，与火花相关的钙的平均通量为3 pA（Blatter等人，1997年）。将这些通量与所涉及的SR释放通道的数量相关是有问题的，因为在E-C偶联期间SR钙释放通道的开放概率的值以及在生理条件下通过通道的钙通量存在相当大的不确定性。尽管如此，这种小的通量立即表明钙火花是由于单个通道或少量通道协同门控（Cheng等人，1993; Blatter等人，1997年）。最近，已经提出单个SR释放通道可以在接近生理条件下传导0.5pA（Mejia-Alvarez等人，1998年），表明火花几乎肯定是由于一个集群的SR释放通道门控一致。这种单个SR释放通道电流的测量仅对所涉及的通道数量设置下限，因为我们不知道在钙火花上升期间SR钙释放通道的开放概率的时间过程。SR钙释放通道门控的时间过程应反映在…
Calcium sparks were discovered in single isolated rat cardiac myocytes by Cheng et al.(1993) while imaging fluorescence from the calcium indicator fluo-3 (Minta et al., 1989) with a confocal microscope. Cardiac muscle calcium sparks are associated with an approximate doubling of the resting fluo-3 fluorescence (F/F 1.0) and occupy a tiny region of the cell 2 m in diameter. A simple equilibrium calculation of the likely change in calcium underlying the spark suggested that the local calcium peaked at 300 nM in 10 ms (Cheng et al., 1993). However, this figure underestimates the true change in calcium because of the limited kinetics and dynamic range of fluo-3, as well as blurring by the microscope. From a recent paper by Smith et al.(1998), we can estimate that the true change in calcium underlying the spark (averaged by microscope blurring in a region 0.5 m across) may be 10 M. Calcium sparks occur at a frequency of 1.6 s 1 in line scan images (Cheng et al., 1993), which survey 70–100 sarcomeres. However, calcium sparks can also be produced by membrane depolarization and therefore reflect the process of excitation–contraction (E–C) coupling at individual junctions between the t-tubular system and the sarcoplasmic reticulum that occur at the z-line (Shacklock et al., 1995; Cheng et al., 1996). Spark sites are therefore separated by 1.8 m longitudinally in resting cardiac cells. In the transverse direction, calcium spark sites have a more variable spacing and sites that are closer to each other are more likely to coactivate (Parker et al., 1996). The whole-cell calcium transient can be explained by the spatio-temporal summation of calcium sparks (Cannell et al., 1994, 1995). Hence, the whole-cell sarcoplasmic reticulum (SR) calcium release flux should be the average probability of SR release (PSpark) multiplied by the average local SR release flux (JRel), but these variables are not separable in conventional whole-cell photometric measurements. However, recording calcium sparks overcomes this problem because this method provides a direct measure of PSpark (which is proportional to the number of sparks detected in the confocal imaging volume per unit time) as well as JRel (which can be estimated from the spatio-temporal properties of the calcium spark). An initial estimate of JRel by Cheng et al.(1993) suggested that the calcium spark could be explained by a calcium flux of 4 pA for 10 ms. Subsequent detailed mathematical modeling of the calcium removal processes suggests that the average flux of calcium associated with a spark is 3 pA (Blatter et al., 1997). Relating these fluxes to the number of SR release channels involved is problematic because there is considerable uncertainty in the value of the open probability of the SR calcium release channels during E–C coupling as well as the calcium flux passed by the channel under physiological conditions. Nevertheless, such small fluxes immediately suggested that the calcium spark was due to either a single channel or a small number of channels gating in concert (Cheng et al., 1993; Blatter et al., 1997). More recently, it has been proposed that a single SR release channel may conduct 0.5 pA under near physiological conditions (Mejia-Alvarez et al., 1998), indicating that the spark is almost certainly due to a cluster of SR release channels gating in concert. Such a measurement of the single SR release channel current only places a lower bound on the number of channels involved because we do not know the time course of the open probability of the SR calcium release channels during the rise of the calcium spark. The time course of the SR calcium release channel gating should be reflected in …