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Excerpt
In mammals, serial dynamic growth of skeletal muscles appear during postnatal “growing phase”. When we investigate an effect of exercise for muscle development, it is important to distinguish from growing effects to exercise effects. The purpose of this study is to examine a changes in mitotic activity and amino acid uptake of rat lower hindlimb muscles during postnatal growth. Wistar male rats 6-15 week-old were used for this study.[3H]thymidine and amino acid ([14C]leucine) labeling were used to determine the serial changes in cellular mitotic activity and the level of amino acid uptake and myosin synthesis. The mitotic activity (mainly reflects the number of activated satellite cells) was gradually decreased from 6 to 10 week-old (66.2±9.5 to 23.4±1.7 dpm/mg protein (pt)). After 10 week-old, the mitotic activity showed almost constant value (16-19 dpm/mg pt). Amino acid uptake also gradually decreased from 6 to 10 week-old(161.5±7.3 to 123.9±25.3 dpm/mg pt), however, it increased again in 12 week-old (138.3±13.7 dpm/mg pt), then decreased in 15 week-old(108.2±3.0 dpm/mg pt). Mitotic activity and amino acid uptake of flexor muscles tended to show higher level than that of extensors. Further, the amino acid uptake for myosin (the fraction of myosin uptake relative to the total) showed higher percentage during 6 to 12 week (33-41%) as compared with 15 week-old (27.0±1.5%). In particular, highest level was shown in 12 week-old (41.6±1.7%). These results suggest that the increase of amino acid uptake in 12 week-old reflect a critical point of the change from“length growth” to “width growth” just before the“steady phase”(maturation). Thus, Wistar male rat would mature at 15 week-old, and it should be used to examine the effects of exercise and/or various overloads on skeletal muscle after 15 week-old.
