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

研究了出生后3个月内后肢悬吊或负重2-g对大鼠后肢肌肉发育的影响。新生鼠后肢悬吊可抑制比目鱼肌的生长。虽然不影响肌纤维数量和纵向生长的生长相关增加，但悬吊抑制了纤维横截面积的增加，纤维类型向慢抽动类型的转变，以及纤维神经支配从多个向单一的转变。这些现象与肌核数量、细胞核内DNA含量和/或卫星细胞数量减少有关。然而，悬吊对其他足底屈肌--腓肠肌和足底(快速抽动肌)的生长影响很小，甚至受到一定程度的抑制。悬吊对背屈肌指长伸肌和胫骨前伸肌(TA，快速抽动肌)的生长没有影响。这表明，比目鱼肌中肌纤维类型的生长相关变化不是遗传的，而是与肌肉本身的紧张性活动密切相关。肌肉的传入直接影响比目鱼肌纤维类型的分化，组织蛋白酶L、热休克蛋白27等基因在比目鱼肌中的表达在正常生长过程中受到抑制，但在慢性去负荷时表达增加。反之，热休克蛋白70的表达被卸载所抑制。泛素结合酶E2、线粒体三磷酸腺苷合成酶B亚单位前体的表达随生长发育而增加，且随去负荷的增加而进一步增强。组织蛋白酶H、组织蛋白酶K、胰岛素样生长因子(IGF1)无明显变化，但肌球蛋白轻链1、3、热休克蛋白20 kDa蛋白、创造酶、αB-晶状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.