Baker A J, Weiner M W
Magnetic Resonance Unit, Department of Veterans Affairs Medical Center, San Francisco, California, USA.
Am J Physiol. 1997 Jul;273(1 Pt 1):C85-91. doi: 10.1152/ajpcell.1997.273.1.C85.
During relaxation of skeletal muscle, an initial rapid decline of intracellular Ca2+ concentration ([Ca2+]i) (phase 1), is followed by a brief phase in which the decline of [Ca2+]i is markedly slowed or even reversed (phase 2). Phase 2 appears as a prominent "bump" on records of the time course of declining [Ca2+]i during relaxation. The goal of this study was to test the hypothesis that phase 2 represents a release of Ca2+ to the cytosol that occurs with net cross-bridge detachment during relaxation. The experimental approach was to measure [Ca2+]i with indo 1 in stimulated bullfrog semitendinosus muscles and to determine if phase 2 was diminished during relaxation of contractions in which cross-bridge interactions had been reduced by two different methods: 1) stretching muscles to reduce the overlap between actin and myosin filaments or 2) decreasing stimulus duration. The results showed that, when either method was used to reduce cross-bridge interactions during contraction, then the size of phase 2 during relaxation was also decreased. Phase 2 was eliminated during relaxation of contractions in which cross-bridge interactions had been reduced to a lower contraction force approximately 30% of maximum. These findings are consistent with the hypothesis that the phase 2 of [Ca2+]i decline represents a release of Ca2+ to the cytosol that occurs with net cross-bridge detachment during relaxation. This conclusion is consistent with previous studies that suggest that cross-bridge detachment lowers the affinity of troponin for Ca2+.
在骨骼肌舒张过程中，细胞内钙离子浓度（[Ca2+]i）最初会迅速下降（阶段1），随后会进入一个短暂阶段，在此阶段[Ca2+]i的下降明显减缓甚至逆转（阶段2）。在[Ca2+]i舒张过程的时间记录上，阶段2表现为一个明显的“凸起”。本研究的目的是检验这样一个假设，即阶段2代表在舒张过程中随着净横桥解离而发生的钙离子向细胞质的释放。实验方法是用indo 1测量受刺激的牛蛙半腱肌中的[Ca2+]i，并确定在通过两种不同方法减少横桥相互作用的收缩舒张过程中阶段2是否减弱：1）拉伸肌肉以减少肌动蛋白和肌球蛋白丝之间的重叠；或2）缩短刺激持续时间。结果表明，当使用任何一种方法在收缩过程中减少横桥相互作用时，舒张过程中阶段2的大小也会减小。在横桥相互作用已减少到约为最大收缩力30%的较低收缩力的收缩舒张过程中，阶段2被消除。这些发现与以下假设一致，即[Ca2+]i下降的阶段2代表在舒张过程中随着净横桥解离而发生的钙离子向细胞质的释放。这一结论与先前的研究一致，这些研究表明横桥解离会降低肌钙蛋白对钙离子的亲和力。
