GLUCOCORTICOID EFFECTS ON INSULIN-REGULATED AND IGF-I-REGULATED MUSCLE PROTEIN-METABOLISM DURING AGING

This study was performed to assess the effect of glucocorticoids (dexa methasone) on insulin-and IGF-I-regulated muscle protein metabolism in adult and old rats. Muscle atrophy occurred more rapidly in old rats, and recovery of muscle mass was impaired when compared with adults. M uscle wasting resulted mainly from increased protein breakdown in adul t rat but from depressed protein synthesis in the aged animal. Glucoco rticoid treatment significantly decreased the stimulatory effect of in sulin and IGF-I on muscle protein synthesis in adult rats by 25.9 and 58.1% respectively. In old rats, this effect was even greater, being 4 9.3 and 100% respectively. With regard to muscle proteolysis, glucocor ticoids blunted the anti-proteolytic action of insulin and IGF-I in bo th age groups. During the recovery period, adult rats reversed the glu cocorticoid-induced resistance of muscle protein metabolism within 3 d ays, at which time old rats still exhibited the decrease in insulin-re gulated proteolysis. In conclusion, the higher sensitivity of old rat muscle to glucocorticoids may in part result from the greater modifica tion of the effects of insulin and ICF-I on muscle protein metabolism. These responses to glucocorticoids in old rats may be associated with the emergence of muscle atrophy with advancing age.