Stephenson E W, Podolsky R J
J Gen Physiol. 1977 Jan;69(1):1-16. doi: 10.1085/jgp.69.1.1.
The effect of Mg on Ca movement between the sarcoplasmic reticulum (SR) and myofilament space (MFS) was studied in skinned muscle fibers by using isometric force as an indicator of MFS Ca. In Ca-loaded fibers at 20 degrees C, the large force spike induced by Ca in 1 mM Mg (5 mM ATP) was strongly inhibited in 3 mM Mg, and force development was extremely slow. After a brief Ca stimulus in 1 mM Mg, relaxation in Ca-free solution was significantly faster in 3 mM Mg. These changes were due to altered Ca movements, since the effect of 3 mM Mg on steady force in CaEGTA solutions was small. Changes in Mg alone induced force transients apparently due to altered Ca movement. In relaxed fibers, decreasing the Mg to 0.25 mM caused phasic force development. In contracting fibers in Ca solutions, increasing the Mg caused a large transient relaxation. The effects of increased Mg were antagonized by 0.5 mM Cd, an inhibitor of the SR Ca transport system. The results indicate that active Ca uptake by the SR in situ is stimulated by Mg, and that it can affect local MFS [Ca++] in the presence of a substantial Ca source. These results provide evidence that an increased rate of Ca uptake in 3 mM Mg could account for inhibition of the large force spike associated with Ca-induced Ca release in skinned fibers.
通过使用等长力作为肌丝间隙（MFS）钙的指标，在剥除肌膜的肌纤维中研究了镁对肌浆网（SR）和肌丝间隙之间钙移动的影响。在20℃下的钙负载纤维中，1 mM镁（5 mM ATP）中由钙诱导的大力峰值在3 mM镁中受到强烈抑制，并且力的发展极其缓慢。在1 mM镁中短暂的钙刺激后，在3 mM镁中无钙溶液中的松弛明显更快。这些变化是由于钙移动的改变，因为3 mM镁对CaEGTA溶液中稳定力的影响很小。单独镁的变化显然由于钙移动的改变而引起力瞬变。在松弛的纤维中，将镁降低到0.25 mM会导致阶段性力的发展。在钙溶液中的收缩纤维中，增加镁会导致大幅度的瞬时松弛。0.5 mM镉（SR钙转运系统的抑制剂）可拮抗镁增加的作用。结果表明，原位SR的主动钙摄取受到镁的刺激，并且在存在大量钙源的情况下它可以影响局部MFS[Ca++]。这些结果提供了证据，表明在3 mM镁中钙摄取速率的增加可以解释与剥除肌膜纤维中钙诱导的钙释放相关的大力峰值的抑制。
