Goldberg A L, Etlinger J D, Goldspink D F, Jablecki C
Med Sci Sports. 1975 Fall;7(3):185-98.
Skeletal muscle can undergo rapid growth in response to a sudden increase in work load. For example, the rat soleus muscle increases in weight by 40% within six days after the tendon of the synergistic gastrocnemius is sectioned. Such growth of the overworked muscle involves an enlargement of muscle fibers and occasional longitudinal splitting. Hypertrophy leads to greater maximal tension development, although decreased contraction time and reduced contractility have also been reported. Unlike normal developmental growth, work-induced hypertrophy can be induced in hypophysectomized or diabetic animals. This process thus appears independent of growth hormone and insulin as well as testosterone and thyroid hormones. Hypertrophy of the soleus can also be induced in fasting animals, in which there is a generalized muscle wasting. Thus muscular activity takes precedence over endocrine influences on muscle size. The increase in muscle weight reflects an increase in protein, especially sarcoplasmic protein, and results from greater protein synthesis and reduced protein breakdown. Within several hours after operation, the hypertrophying soleus shows more rapid uptake of certain amino acids and synthesis of phosphatidyl-inositol. By 8 hours, protein synthesis is enhanced. RNA synthesis also increases, and hypertrophy can be prevented with actinomycin D. Nuclear DNA synthesis also increases on the second day after operation and leads to a greater DNA content. The significance of the increased RNA and DNA synthesis is not clear, since most of it occurs in interstitial and satellite cells. The proliferation of the non-muscle cells seems linked to the growth of the muscle fibers; in addition, factors causing muscle atrophy (e.g. denervation) decrease DNA synthesis by such cells. In order to define more precisely the early events in hypertrophy, the effects of contractile activity were studied in rat muscles in vitro. Electrical stimulation enhanced active transport of certain amino acids within an hour, and the magnitude of this effect depended on the amount of contractile activity. Stimulation or passive stretch of the soleus or diaphragm also retarded protein degradation. Presumably these effects of mechanical activity contribute to the changes occuring during hypertrophy in vivo. However, under the same conditions, or even after more prolonged stimulation, no change in rates of protein synthesis was detected. These findings with passive tension in vitro are particularly interesting, since passive stretch has been reported to retard atrophy or to induce hypertrophy of denervated muscle in vivo. It is suggested that increased tension development (either passive or active) is the critical event in initiating compensatory growth.
骨骼肌能够对工作负荷的突然增加做出快速生长反应。例如，在协同的腓肠肌腱被切断后的六天内，大鼠比目鱼肌的重量增加40%。这种过度劳累肌肉的生长涉及肌纤维的增大以及偶尔的纵向分裂。肥大导致更大的最大张力发展，尽管也有报道称收缩时间缩短和收缩性降低。与正常发育生长不同，工作诱导的肥大可以在垂体切除或糖尿病动物中诱导产生。因此，这个过程似乎独立于生长激素、胰岛素以及睾酮和甲状腺激素。比目鱼肌的肥大也可以在禁食动物中诱导产生，在这些动物中存在全身性肌肉消瘦。因此，肌肉活动在对肌肉大小的影响上优先于内分泌因素。肌肉重量的增加反映了蛋白质的增加，尤其是肌浆蛋白，这是由更大的蛋白质合成和减少的蛋白质分解导致的。手术后数小时内，肥大的比目鱼肌对某些氨基酸的摄取更快，并且磷脂酰肌醇的合成增加。到8小时时，蛋白质合成增强。RNA合成也增加，并且放线菌素D可以阻止肥大。核DNA合成在手术后第二天也增加，并导致DNA含量增加。RNA和DNA合成增加的意义尚不清楚，因为大部分合成发生在间质细胞和卫星细胞中。非肌肉细胞的增殖似乎与肌纤维的生长有关；此外，导致肌肉萎缩的因素（如去神经支配）会减少这些细胞的DNA合成。为了更精确地确定肥大早期的事件，在体外研究了大鼠肌肉中收缩活动的影响。电刺激在一小时内增强了某些氨基酸的主动转运，这种效应的大小取决于收缩活动的量。比目鱼肌或膈肌的刺激或被动拉伸也会延缓蛋白质降解。推测机械活动的这些效应有助于体内肥大过程中发生的变化。然而，在相同条件下，甚至在更长时间的刺激后，未检测到蛋白质合成速率的变化。体外被动张力的这些发现特别有趣，因为据报道被动拉伸可延缓萎缩或在体内诱导去神经支配肌肉的肥大。有人认为，增加的张力发展（被动或主动）是启动代偿性生长的关键事件。
