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The purpose of the present study was to compare the myogenic response of hindlimb muscles in young (14-20 wk of age) and old (>120 wk of age) rats with a single exhaustive bout of heavy resistance weight lifting. [(3)H]thymidine and [(14)C]leucine labeling were monitored for up to 2 wk after the exercise bout to estimate serial changes in mitotic activity and the level of amino acid uptake and myosin synthesis. Histological, histochemical, and immunohistochemical [anti-5-bromo-2'-deoxyuridine and myogenic determination genes (MyoD)] analyses of whole muscles and analysis of muscle-specific gene expression (MyoD) using Western blotting and RT-PCR were performed. Old rats showed significant muscle atrophy and a lower exercise capacity than young rats. Exercise-induced muscle damage, as assessed in histological sections, and increases in serum creatine kinase activity were evident in both young and old exercised groups. Mitotic activity was increased in young, but not old, rats 2 days after exercise. There was a biphasic increase in [(14)C]leucine uptake during the 14 days postexercise (peaks at 1-4 and 10 days) in young rats: only the first peak was observed in old rats. There was a lower uptake of [(14)C]leucine in the myosin fraction and an impaired expression of MyoD at the protein (immunohistochemistry and Western blotting) and mRNA (RT-PCR) levels in old rats throughout the postexercise period. These results demonstrate a reduced reparative capability of muscle in response to a single bout of exercise in old compared with young rats.

The influence of an anabolic androgenic steroid (AAS) on thymidine and amino acid uptake in rat hindlimb skeletal muscles during 14 days after a single exhaustive bout of weight lifting was determined. Adult male rats were divided randomly into Control or Steroid groups. Nandrolone decanoate was administered to the Steroid group 1 wk before the exercise bout. [3H]thymidine and [14C]leucine labeling were used to determine the serial changes in cellular mitotic activity, amino acid uptake, and myosin synthesis. Serum creatine kinase (CK) activity, used as a measure of muscle damage, increased 30 and 60 min after exercise in both groups. The total amount of weight lifted was higher, whereas CK levels were lower in Steroid than in Control rats. [3H]thymidine uptake peaked 2 days after exercise in both groups and was 90% higher in Control than in Steroid rats, reflecting a higher level of muscle damage. [14C]leucine uptake was approximately 80% higher at rest and recovered 33% faster postexercise in Steroid than in Control rats. In a separate group of rats, the in situ isometric mechanical properties of the plantaris muscle were determined. The only significant difference was a higher fatigue resistance in the Steroid compared with the Control group. Combined, these results indicate that AAS treatment 1) ameliorates CK efflux and the uptake of [3H]thymidine and enhances the rate of protein synthesis during recovery after a bout of weight lifting, all being consistent with there being less muscle damage, and 2) enhances in vivo work capacity and the in situ fatigue resistance of a primary plantarflexor muscle.