Somlyo A V, Gonzalez-Serratos H G, Shuman H, McClellan G, Somlyo A P
J Cell Biol. 1981 Sep;90(3):577-94. doi: 10.1083/jcb.90.3.577.
Approximately 60-70% of the total fiber calcium was localized in the terminal cisternae (TC) in resting frog muscle as determined by electron-probe analysis of ultrathin cryosections. During a 1.2 s tetanus, 59% (69 mmol/kg dry TC) of the calcium content of the TC was released, enough to raise total cytoplasmic calcium concentration by approximately 1 mM. This is equivalent to the concentration of binding sites on the calcium-binding proteins (troponin and parvalbumin) in frog muscle. Calcium release was associated with a significant uptake of magnesium and potassium into the TC, but the amount of calcium released exceeded the total measured cation accumulation by 62 mEq/kg dry weight. It is suggested that most of the charge deficit is apparent, and charge compensation is achieved by movement of protons into the sarcoplasmic reticulum (SR) and/or by the movement of organic co- or counterions not measured by energy dispersive electron-probe analysis. There was no significant change in the sodium or chlorine content of the TC during tetanus. The unchanged distribution of a permeant anion, chloride, argues against the existence of a large and sustained transSR potential during tetanus, if the chloride permeability of the in situ SR is as high as suggested by measurements on fractionated SR. The calcium content of the longitudinal SR (LSR) during tetanus did not show the LSR to be a major site of calcium storage and delayed return to the TC. The potassium concentration in the LSR was not significantly different from the adjacent cytoplasmic concentration. Analysis of small areas of I-band and large areas, including several sarcomeres, suggested that chloride is anisotropically distributed, with some of it probably bound to myosin. In contrast, the distribution of potassium in the fiber cytoplasm followed the water distribution. The mitochondrial concentration of calcium was low and did not change significantly during a tetanus. The TC of both tetanized and resting freeze-substituted muscles contained electron-lucent circular areas. The appearance of the TC showed no evidence of major volume changes during tetanus, in agreement with the estimates of unchanged (approximately 72%) water content of the TC obtained with electron-probe analysis.
通过对超薄冷冻切片进行电子探针分析确定，在静息蛙肌中，约60 - 70%的纤维钙定位于终池（TC）。在1.2秒的强直收缩期间，终池钙含量的59%（69 mmol/kg干终池）被释放，足以使细胞质总钙浓度升高约1 mM。这相当于蛙肌中钙结合蛋白（肌钙蛋白和小清蛋白）上结合位点的浓度。钙释放与镁和钾大量摄取进入终池相关，但释放的钙量超过测量到的阳离子总蓄积量62 mEq/kg干重。提示大部分电荷亏缺是表观的，电荷补偿通过质子进入肌浆网（SR）和/或通过能量色散电子探针分析未测量的有机共离子或反离子的移动来实现。强直收缩期间终池的钠或氯含量无显著变化。一种渗透性阴离子氯的分布未变，这与强直收缩期间存在大的且持续的跨肌浆网电位的观点相悖，如果原位肌浆网的氯通透性如对分离的肌浆网测量所提示的那样高的话。强直收缩期间纵向肌浆网（LSR）的钙含量未显示其为主要的钙储存位点且钙返回终池延迟。纵向肌浆网中的钾浓度与相邻细胞质浓度无显著差异。对I带小区域和包括几个肌节的大区域的分析表明，氯呈各向异性分布，其中一些可能与肌球蛋白结合。相反，纤维细胞质中钾的分布与水的分布一致。线粒体钙浓度低，在强直收缩期间无显著变化。强直收缩和静息的冷冻替代肌肉的终池均含有电子透明的圆形区域。终池的外观在强直收缩期间未显示出主要体积变化的证据，这与通过电子探针分析得到的终池含水量不变（约72%）的估计一致。
