The effect of myoplasmic Mg2+ on Ca2+ release was examined in mechanically skinned skeletal muscle fibres, in which the normal voltage-sensor control of Ca2+ release is preserved. The voltage sensors could be activated by depolarizing the transverse tubular (T-) system by lowering the [K+] in the bathing solution. 2. Fibres spontaneously contracted when the free [Mg2+] was decreased from 1 to 0.05 mM, with no depolarization or change of total ATP, [Ca2+] or pH (pCa 6.7, 50 microM-EGTA). After such a 'low-Mg2+ response' the sarcoplasmic reticulum (SR) was depleted of Ca2+ and neither depolarization nor caffeine (2 mM) could induce a response, unless the [Mg2+] was raised and the SR reloaded with Ca2+. Exposure to 0.05 mM-Mg2+ at low [Ca2+] (2 mM-free EGTA, pCa greater than 8.7) also induced Ca2+ release and depleted the SR. 3. The response to low [Mg2+] was unaffected by inactivation of the voltage sensors, but was completely blocked by 2 microM-Ruthenium Red indicating that it involved Ca2+ efflux through the normal Ca2+ release channels. 4. In the absence of ATP (and creatine phosphate), complete removal of Mg2+ (i.e. no added Mg2+ with 1 mM-EDTA) did not induce Ca2+ release. Depolarization in the absence of Mg2+ and ATP also did not induce Ca2+ release. 5. Depolarization in 10 mM-Mg2+ (pCa 6.7, 50 microM-EGTA, 8 mM-total ATP) did not produce any response. In the presence of 1 mM-EGTA to chelate most of the released Ca2+, depolarizations in 10 mM-Mg2+ did not noticeably deplete the SR of Ca2+, whereas a single depolarization in 1 mM-Mg2+ (and 1 mM-EGTA) resulted in marked depletion. Depolarization in the presence of D600 and 10 mM-Mg2+ produced use-dependent 'paralysis', indicating that depolarization in 10 mM-Mg2+ did indeed activate the voltage sensors. 6. Depolarization in the presence of 10 mM-Mg2+ and 25 microM-ryanodine neither interfered with the normal voltage control of Ca2+ release nor caused depletion of the Ca2+ in the SR even after returning to 1 mM-Mg2+ for 1 min, indicating that few if any of the release channels had been opened by the depolarization.(ABSTRACT TRUNCATED AT 400 WORDS)
摘要
在机械去膜的骨骼肌纤维中研究了肌质网Mg2+对Ca2+释放的影响，在这种纤维中，Ca2+释放的正常电压传感器控制得以保留。通过降低浴液中的[K+]使横管（T管）系统去极化，从而激活电压传感器。2. 当游离[Mg2+]从1 mM降至0.05 mM时，纤维自发收缩，且无去极化现象，总ATP、[Ca2+]或pH也无变化（pCa 6.7，50 μM乙二醇双四乙酸）。在这种“低Mg2+反应”后，肌浆网（SR）中的Ca2+耗尽，去极化或咖啡因（2 mM）均不能诱导反应，除非提高[Mg2+]并使SR重新加载Ca2+。在低[Ca2+]（2 mM游离乙二醇双四乙酸，pCa大于8.7）条件下暴露于0.05 mM - Mg2+也会诱导Ca2+释放并耗尽SR。3. 对低[Mg2+]的反应不受电压传感器失活的影响，但被2 μM钌红完全阻断，表明其涉及通过正常Ca2+释放通道的Ca2+外流。4. 在无ATP（和磷酸肌酸）的情况下，完全去除Mg2+（即加入1 mM乙二胺四乙酸时不添加Mg2+）不会诱导Ca2+释放。在无Mg2+和ATP的情况下进行去极化也不会诱导Ca2+释放。5. 在10 mM - Mg2+（pCa 6.7，50 μM乙二醇双四乙酸，8 mM总ATP）中进行去极化未产生任何反应。在存在1 mM乙二醇双四乙酸以螯合大部分释放的Ca2+的情况下，在10 mM - Mg2+中进行去极化不会明显耗尽SR中的Ca2+，而在1 mM - Mg2+（和1 mM乙二醇双四乙酸）中进行单次去极化会导致明显耗尽。在存在D600和10 mM - Mg2+的情况下进行去极化产生了使用依赖性“麻痹”，表明在10 mM - Mg2+中进行去极化确实激活了电压传感器。6. 在存在10 mM - Mg2+和25 μM兰尼碱的情况下进行去极化，既不干扰Ca2+释放的正常电压控制，也不会导致SR中的Ca2+耗尽，即使在恢复到1 mM - Mg2+ 1分钟后也是如此，这表明去极化几乎没有打开任何释放通道。（摘要截于400字）
