Klein M G, Lacampagne A, Schneider M F
Department of Biochemistry and Molecular Biology, University of Maryland School of Medicine, 108 North Greene Street, Baltimore, MD 21201, USA.
Proc Natl Acad Sci U S A. 1997 Sep 30;94(20):11061-6. doi: 10.1073/pnas.94.20.11061.
Applying a brief repolarizing pre-pulse to a depolarized frog skeletal muscle fiber restores a small fraction of the transverse tubule membrane voltage sensors from the inactivated state. During a subsequent depolarizing test pulse we detected brief, highly localized elevations of myoplasmic Ca2+ concentration (Ca2+ "sparks") initiated by restored voltage sensors in individual triads at all test pulse voltages. The latency histogram of these events gives the gating pattern of the sarcoplasmic reticulum (SR) calcium release channels controlled by the restored voltage sensors. Both event frequency and clustering of events near the start of the test pulse increase with test pulse depolarization. The macroscopic SR calcium release waveform, obtained from the spark latency histogram and the estimated open time of the channel or channels underlying a spark, exhibits an early peak and rapid marked decline during large depolarizations. For smaller depolarizations, the release waveform exhibits a smaller peak and a slower decline. However, the mean use time and mean amplitude of the individual sparks are quite similar at all test depolarizations and at all times during a given depolarization, indicating that the channel open times and conductances underlying sparks are essentially independent of voltage. Thus, the voltage dependence of SR Ca2+ release is due to changes in the frequency and pattern of occurrence of individual, voltage-independent, discrete release events.
对去极化的青蛙骨骼肌纤维施加一个短暂的复极化预脉冲，可使一小部分横管膜电压传感器从失活状态恢复。在随后的去极化测试脉冲期间，我们在所有测试脉冲电压下，检测到由单个三联体中恢复的电压传感器引发的肌浆Ca2+浓度的短暂、高度局部性升高（Ca2+“火花”）。这些事件的潜伏期直方图给出了由恢复的电压传感器控制的肌浆网（SR）钙释放通道的门控模式。事件频率以及测试脉冲开始附近事件的聚集都随着测试脉冲去极化而增加。从火花潜伏期直方图以及一个火花或多个火花所对应的通道的估计开放时间获得的宏观SR钙释放波形，在大去极化期间呈现出一个早期峰值和快速显著下降。对于较小的去极化，释放波形呈现出较小的峰值和较慢的下降。然而，在所有测试去极化情况下以及在给定去极化期间的所有时间，单个火花的平均使用时间和平均幅度都非常相似，这表明火花所对应的通道开放时间和电导基本上与电压无关。因此，SR Ca2+释放的电压依赖性是由于单个、电压无关的离散释放事件的频率和发生模式的变化所致。
