Exercise-induced morphological and biochemical changes in skeletal muscles of the rat.

Morphological and biochemical changes have been studied in two forelimb (i.e., brachialis and extensor carpi radialis) and two hindlimb (i.e., soleus and extensor digitorum longus) muscles of rats subjected to short bursts of high-intensity exercise over 2 wk. Regardless of muscle type, all four muscles grew significantly, accumulating protein, RNA, and DNA at faster rates than in the growing control tissues. Of the intrinsic fiber types within the individual muscles all increased their cross-sectional areas, but the fast-oxidative, glycolytic fibers (type IIa) showed marginally more hypertrophy than the slow-oxidative (I) or fast-glycolytic fibers (IIb). Induced changes in protein turnover were consistent with the additional growth of the exercised muscles. However, the precise alterations in the rates of protein synthesis and protein breakdown varied according to the fiber type composition of the muscle. The increased growth rate of the two principally fast-twitch muscles (i.e., brachialis and extensor digitorum longus) correlated solely with an enhancement of protein synthesis (measured in vivo). In contrast, the hypertrophy of the slow-twitch soleus appeared to relate only to a decrease in protein breakdown (a calculated value). In a more intermediate type of muscle (i.e., extensor carpi radialis) a complementary combination of an increase in synthesis and a decrease in breakdown was found.