IDENTICAL RESPONSES OF FAST MUSCLE TO SUSTAINED ACTIVITY BY LOW-FREQUENCY STIMULATION IN YOUNG AND AGING RATS

To investigate effects of sustained activity on major phenotypic prope rties, the left extensor digitorum longus muscle of young (15 wk) and aging (101 wk) male Brown Norway rats was subjected to 50 days of chro nic low-frequency stimulation (CLFS; 10 Hz, 10 h/day). The contralater al muscle served as control. Changes in metabolic enzymes were analyze d by using glyceraldehyde-3-phosphate dehydrogenase and lactate dehydr ogenase as reference enzymes of glycolysis and by using citrate syntha se and 3-hydroxyacyl-CoA dehydrogenase as mitochondrial enzymes repres entative of aerobic-oxidative metabolism. Myosin heavy chain (MHC) iso forms were analyzed by SDS-PAGE. No differences existed between the en zyme activity profiles of control muscles from young and aging rats. C LFS induced similar increases in mitochondrial enzymes, as well as sim ilar decreases in glycolytic enzymes. Although the MHC composition of the control muscles in the aging rats displayed a shift toward slower isoforms, the ultimate changes induced by CLFS led to nearly identical MHC phenotypes in both young and aging rats. These results demonstrat e an unaltered adaptability of skeletal muscle to increased neuromuscu lar activity in the aging rat.