Limited myogenic response to a single bout of weight-lifting exercise in old rats

The purpose of the present study was to compare themyogenic response of hindlimb muscles in young (14-20 wk of age)and old (>120 wk of age) rats with a single exhaustive bout of heavyresistance weight lifting. ³H]thymidine and¹⁴C]leucine labeling were monitored for up to2 wk after the exercise bout to estimate serial changes in mitoticactivity and the level of amino acid uptake and myosin synthesis.Histological, histochemical, and immunohistochemicalanti-5-bromo-2'-deoxyuridine and myogenic determinationgenes (MyoD)] analyses of whole muscles and analysis ofmuscle-specific gene expression (MyoD) using Western blotting andRT-PCR were performed. Old rats showed significant muscle atrophy and alower exercise capacity than young rats. Exercise-induced muscledamage, as assessed in histological sections, and increases in serumcreatine kinase activity were evident in both young and old exercisedgroups. Mitotic activity was increased in young, but not old, rats 2 days after exercise. There was a biphasic increase in¹⁴C]leucine uptake during the 14 dayspostexercise (peaks at 1-4 and 10 days) in young rats: only thefirst peak was observed in old rats. There was a lower uptake of¹⁴C]leucine in the myosin fraction and animpaired expression of MyoD at the protein (immunohistochemistry andWestern blotting) and mRNA (RT-PCR) levels in old rats throughout thepostexercise period. These results demonstrate a reduced reparativecapability of muscle in response to a single bout of exercise in oldcompared with young rats.