EFFECT OF GLUCOCORTICOID EXCESS ON SKELETAL-MUSCLE AND HEART PROTEIN-SYNTHESIS IN ADULT AND OLD RATS

This study was carried out to analyse glucocorticoid-induced muscle wa sting and subsequent recovery in adult (6-8 months) and old (18-24 mon ths) rats because the increased incidence of various disease states re sults in hypersecretion of glucocorticoids in ageing. Adult and old ra ts received dexamethasone in their drinking water for 5 or 6 d and wer e then allowed to recover for 3 or 7 d. As dexamethasone decreased foo d intake, all groups were pair-fed to dexamethasone-treated old rats ( i.e. the group that had the lowest food intake). At the end of the tre atment, adult and old rats showed significant increases in blood gluco se and plasma insulin concentrations. This increase disappeared during the recovery period. Protein synthesis of different muscles was asses sed in viva by a flooding dose of [C-13]valine injected subcutaneously 50 min before slaughter. Dexamethasone induced a significant decrease in protein synthesis in fast-twitch glycolytic and oxidative glycolyt ic muscles (gastrocnemius, tibialis anterior, extensor digitorum longu s). The treatment affected mostly ribosomal efficiency. Adult dexameth asone-treated rats showed an increase in protein synthesis compared wi th their pair-fed controls during the recovery period whereas old rats did not. Dexamethasone also significantly decreased protein synthesis in the predominantly oxidative soleus muscle but only in old rats, an d increased protein synthesis in the heart of adult but not of old rat s. Thus, in skeletal muscle, the catabolic effect of dexamethasone is maintained or amplified during ageing whereas the anabolic effect in h eart is depressed. These results are consistent with muscle atrophy oc curring with ageing.