MYOGENIC REGULATORY FACTORS DURING REGENERATION OF SKELETAL-MUSCLE INYOUNG, ADULT, AND OLD RATS

Myogenic factor mRNA expression was examined during muscle regeneratio n after bupivacaine injection in Fischer 344/Brown Norway F1 rats aged 3, 18, and 31 mo of age (young, adult, and old, respectively). Mass o f the tibialis anterior muscle in the young rats had recovered to cont rol values by 21 days postbupivacaine injection but in adult and old r ats remained 40% less than that of contralateral controls at 21 and 28 days of recovery. During muscle regeneration, myogenin mRNA was signi ficantly increased in muscles of young, adult, and old rats 5 days aft er bupivacaine injection. Subsequently, myogenin mRNA levels in young rat muscle decreased to postinjection control values by day 21 but did not return to control values in 28-day regenerating muscles of adult and old rats. The expression of MyoD mRNA was also increased in muscle s at day 5 of regeneration in young, adult, and old rats, decreased to control levels by day 14 in young and adult rats, and remained elevat ed in the old rats for 28 days. In summary, either a diminished abilit y to downregulate myogenin and MyoD mRNAs in regenerating muscle occur s in old rat muscles, or the continuing myogenic effort includes eleva ted expression of these mRNAs.