Effects of antigravity muscular activity during the lactation period on the development of rat hindlimb muscles

研究了3个月大鼠后肢悬吊或2 g负重对后肢肌肉发育的影响。后肢悬吊能抑制新生大鼠踝关节跖-比目鱼屈肌的生长。虽然不影响生长相关肌纤维数量的增加和纵向生长，但悬浮液抑制了纤维截面积的增加、纤维型向慢抽动型转变以及纤维神经支配由多向单的变化。这些现象与较低水平的核数、细胞核内DNA含量和/或卫星细胞数有关。而悬浮液对其他跖屈肌，腓肠肌和跖肌（快速收缩肌）生长的影响较小，甚至在一定程度上抑制了它们的生长。背屈肌、指长伸肌和胫前肌（TA，快速收缩肌）的生长完全不受悬浮的影响。这表明比目鱼肌纤维类型的生长相关转移不是遗传编程的，而是与肌肉本身的强直活动密切相关。传入输入直接受肌肉活动的影响，对纤维类型的分化有显著影响。在正常生长过程中受到抑制的组织蛋白酶L、热休克蛋白27等基因的表达，甚至在慢性卸荷后增加。相反，热休克蛋白70的表达被卸载抑制。泛素偶联酶E2、线粒体ATP合成酶B亚基前体的表达随生长而增加，卸载后进一步促进。虽然组织蛋白酶H、组织蛋白酶K、胰岛素样生长因子（IGF1）不受影响，但卸载后肌球蛋白轻链1和3、热休克20kDa蛋白、create激酶、α b -晶体蛋白的表达均受到抑制。

Effects of hindlimb suspension or loading at 2-g during the first 3-month of life on the development of hindlimb muscles were studied in rats. The growth of ankle plantar-flexor soleus was inhibited by hindlimb suspension of new-born rats. Although the growth-related increases of muscle fiber number and longitudinal growth were not affected, the increase of fiber cross-sectional area, fiber-type transformation toward slow-twitch type, and the change of fiber innervation from multiple to single were inhibited by suspension. These phenomena were associated with lower levels of myonuclear number, DNA content within a nucleus, and/or satellite cell number. However, the effects of suspension on the growth of other plantar-flexors, gastrocnemius and plantaris (fast twitch muscle), were minor, even their growth was inhibited in some degree. The growth of dorsi-flexors, extensor digitorum longus and tibialis anterior (TA, fast twitch muscle), was not influenced by suspension at all. It was suggested that the growth-associated shift of muscle fiber type in soleus is not programmed genetically but is closely related to the tonic activity of the muscle itself. The significant effect of afferent input, which is directly influenced by muscle activity, on the differentiation of fiber type was also suggested.The expressions of gene, such as cathepsin L, heat shock protein 27, in soleus muscle, which are inhibited during the normal growth, were even increased in response to chronic unloading. The expression of heat shock protein 70 was, on the contrary, inhibited by unloading. The expressions of ubiquitin-conjugating enzyme E2, mitochondrial ATP synthase B subunit precursor were increased following growth and further promoted by unloading. Although cathepsin H, cathepsin K, insulin-like growth factor (IGF1) were not influenced, those of myosin light chain 1 and 3, heat shock 20kDa protein, create kinase, αB-crystallin were inhibited following unload.ing.