Brum Gustavo, Piriz Nazira, DeArmas Rafael, Rios Eduardo, Stern Michael, Pizarro Gonzalo
Departamento de Biofísica, Facultad de Medicina, Montevideo, Uruguay.
Biophys J. 2003 Jul;85(1):245-54. doi: 10.1016/S0006-3495(03)74470-1.
In voltage-clamped frog skeletal muscle fibers, Ca(2+) release rises rapidly to a peak, then decays to a nearly steady state. The voltage dependence of the ratio of amplitudes of these two phases (p/s) shows a maximum at low voltages and declines with further depolarization. The peak phase has been attributed to a component of Ca(2+) release induced by Ca(2+), which is proportionally greater at low voltages. We compared the effects of two interventions that inhibit Ca(2+) release: inactivation of voltage sensors, and local anesthetics reputed to block Ca(2+) release induced by Ca(2+). Holding the cells partially depolarized strongly reduced the peak and steady levels of Ca(2+) release elicited by a test pulse and suppressed the maximum of the p/s ratio at low voltages. The p/s ratio increased monotonically with test voltage, eventually reaching a value similar to the maximum found in noninactivated fibers. This implies that the marked peak of Ca(2+) release is a property of a cooperating collection of voltage sensors rather than individual ones. Local anesthetics reduced the peak of release flux at every test voltage, and the steady phase to a lesser degree. At variance with sustained depolarization, they made p/s low at all voltages. These observations were well-reproduced by the "couplon" model of dual control, which assumes that depolarization and anesthetics respectively, and selectively, disable its Ca(2+)-dependent or its voltage-operated channels. This duality of effects and their simulation under such hypotheses are consistent with the operation of a dual, two-stage control of Ca(2+) release in muscle, whereby Ca(2+) released through multiple directly voltage-activated channels builds up at junctions to secondarily open Ca(2+)-operated channels.
在电压钳制的青蛙骨骼肌纤维中，Ca(2+)释放迅速上升至峰值，然后衰减至近乎稳定状态。这两个阶段（峰值/稳态）振幅比值的电压依赖性在低电压时显示出最大值，并随着进一步去极化而下降。峰值阶段归因于由Ca(2+)诱导的Ca(2+)释放成分，在低电压下该成分比例更大。我们比较了两种抑制Ca(2+)释放的干预措施的效果：电压传感器失活和据称能阻断由Ca(2+)诱导的Ca(2+)释放的局部麻醉剂。使细胞部分去极化会强烈降低测试脉冲引发的Ca(2+)释放的峰值和稳态水平，并抑制低电压下峰值/稳态比值的最大值。峰值/稳态比值随测试电压单调增加，最终达到与未失活纤维中发现的最大值相似的值。这意味着Ca(2+)释放的明显峰值是电压传感器协同集合的特性，而非单个传感器的特性。局部麻醉剂在每个测试电压下都降低了释放通量的峰值，对稳态阶段的降低程度较小。与持续去极化不同的是，它们在所有电压下都使峰值/稳态比值较低。这些观察结果在双控“偶联子”模型中得到了很好的重现，该模型假设去极化和麻醉剂分别且选择性地使Ca(2+)依赖性通道或电压门控通道失活。这种效应的双重性及其在这些假设下的模拟与肌肉中Ca(2+)释放的双重、两阶段控制操作一致，即通过多个直接电压激活通道释放的Ca(2+)在连接处积累，从而继发性地打开Ca(2+)门控通道。
