Effects of exercise training on the ultrastructural alterations of membrane systems involved in excitation-contraction coupling during contraction in skeletal muscle.

兴奋-收缩（E-C）耦合是从肌纤维表面的电事件（兴奋）到肌纤维收缩的一系列步骤。与E-C偶联过程直接相关的两个主要膜组分：横(T)小管和肌浆网（SR）。T小管是表面膜的内陷，它渗透到肌纤维中，以精确重复的间隔形成横向网络。T小管将电信号传递到肌纤维的中心。SR是一个内部膜系统，相当于其他细胞的内质网，位于肌原纤维之间的空间。SR从肌原纤维中分离钙（Ca^<2+>）。T小管和SR形成特殊的连接，即三联。生理学证据表明，E-C偶联失败与偏心收缩引起的肌肉损伤的可能性是由于骨骼肌纤维中与E-C偶联过程直接相关的膜系统的结构和形态损伤。我们观察了大鼠水平跑和下坡跑后参与E-C耦合的快肌纤维（FT）和慢肌纤维（ST）膜系统的超微结构特征。下坡跑步运动后，大鼠T小管的排列和三联体的分布发生了四种明显的超微结构变化：(1)T小管网络纵段数量增加，(2)三联体的方向和分布发生了变化，(3)出现了腔泡簇，(4)出现了五联体和七联体。这些观察结果也支持了偏心运动引起的E-C偶联失败是由于骨骼肌中参与E-C偶联过程的膜系统的物理和化学破坏。

Excitation-contraction (E-C) coupling is the series of steps that leads from the electrical event at the surface if the muscle fiber (excitation) to contraction of the muscle fibers. Two major membrane components directly concerned with the E-C coupling process : transverse (T) tubules and sarcoplasmic reticulum (SR). T tubules are invaginations of the surface membrane which penetrate into the muscle fibers and form transverse networks at precisely repeated intervals. T tubules transmit the electrical signal to the center of the muscle fiber. The SR is an internal membrane system equivalent to the endoplasmic reticulum of other cells, which is located in the space between the myofibrils. The SR sequesters calcium (Ca^<2+>) from the myofibrils. T tubules and SR form specialized junctions, the triads. Physiological evidence suggests the possibility that the E-C coupling failure with eccentric contraction-induced muscle injury is due to structural and morphological damage to membrane systems directly associated with the E-C coupling processes within skeletal muscle fibres. We observed the ultrastructural features of the membrane systems of rat fast-twitch (FT) and slow-twitch (ST) muscle fibres involved in E-C coupling following level and downhill running exercise. There were four obvious ultrastructural changes in the arrangement of the T tubules and the disposition of triads after the downhill running exercise : (1) an increase in the numbers of longitudinal segments of the T tubule network, (2) changes in the direction and disposition of triads, (3) the appearance of caveolar clusters, and (4) the appearance of pentads and heptads. These observations also support the idea that eccentric-exercise-induced E-C coupling failure is due to physical and chemical disruptions of the membrane systems involved in the E-C coupling process in skeletal muscle.