Relationships between excitation-contraction coupling deficiency and ultrastructural features of internal membrane systems in skeletal muscle fibers with aging

肌纤维的分化、纤维类型特定结构和功能特性的维持依赖于神经支配、活动模式、激素影响和衰老。在兴奋-收缩（E-C）耦合和弛豫过程中负责Ca^<2+>运动的膜系统也受到相同因素的影响。然而，目前还不完全清楚，但对于他们的反应是什么，没有普遍的共识。例如，据报道，随着年龄的增长，横向（T-）小管和sacoplasmic网（SR）的体积引力增加，Ca^<2+>-泵密度降低。本研究的具体目的是确定衰老对骨骼肌纤维中E-C偶联直接相关的膜系统的影响，以阐明衰老肌纤维中膜系统超微结构变化与E-C偶联缺乏之间的关系。随着年龄的增长，膜系统排列紊乱。横小管网络明显增加；三联体明显多于年轻纤维，五元体和七元体结构（即SR末端池的三或四个元素与两个或三个t小管元素接近）在老化后频繁出现。目前还不清楚为什么五边形和七边形会随着年龄的增长而迅速出现。另一方面，跑步运动训练有抑制这些膜系统随衰老而发生的超微结构变化的趋势。这种下降可能是由于老化的骨骼肌纤维的自发活动，这表明五边形和七轴体比肌原纤维对少量活动更敏感。因此，在这些原肌纤维中，参与E-C偶联的膜系统组分与旧骨骼肌纤维中系统的其余部分之间存在定量不平衡。

Maintrnance of differentiated, fiber-type specific structural and functional properties of muscle fibers is dependent on innervation, pattern of activity, hormonal influences, and aging. The membrane systems responsible for Ca^<2+> movements during excitation-contraction (E-C) coupling and relaxation are also affected by the same factors. However, it is not entirely clear but there is no general agreement on what there response is. For example, both an increase in the volume gractions of transverse (T-) tubules and sacoplasmic reticulum (SR) and reduced Ca^<2+>-pump density following aging have been reported. The specific aim of thes tudy is to determine the effect of ageing on the membrane systems directly concerned with the E-C coupling in skeletal muscle fibers to clarify the relationship between the ultrastructural changes of membrane systems and the E-C coupling deficiency in aged muscle fibers. The arrangement of the membrane systems was disordered with aging. Increases in transverse tubules network were apparent ; there were clearly more triads than in younger fibers, and pentadic and heptadic structures (i.e., a close approximation of two or three T-tubule elements with three or four elements of terminal cisternae of SR) were frewuently appeared following aging. It is not clear why pentads and heptads appear promptly with aging. On the other hand, the running exercise traning tends to inhibit ultrastructural changes of these membrane systems depend on aging. It is possible that decline is due to the spontaneous activity of the aged skeletal muscle fibers, indicating the pentads and heptads are more sensitive than myofibrils to a small amount of activity. Thus in these former muscle fibers there is a quantitative unbalance between the components of the membrane systems involved in E-C coupling and the rest of the system in old skeletal muscle fibers.