| Research Abstract |
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.
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