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Dirk Pette and Dejan Škorjanc

We compared responses of the fast extensor digitorum longus (EDL) and tibialis anterior (TA) muscles in young (15-week) and aging (101-week) male Brown Norwegian rats to 50 days of chronic low-frequency stimulation (CLFS, 10 Hz, 10 hours/day). After 50 days of CLFS, the EDL muscles of the young (22-week) and aging (108-week) rats displayed similar increases in type IIA fibers, relative concentration of myosin heavy chain MHCIIa, elevations in mitochondrial citrate synthase and 3-hydroxyacyl-CoA dehydrogenase activities, and similar decreases in glycolytic enzyme activities (glyceraldehydephosphate dehydrogenase, lactate dehydrogenase). TA muscle in young rats contained a few cytochrome c oxidase negative (COX) type I fibers. Their number was ~2-fold elevated by CLFS. Conversely, aging muscle, which contained a slightly higher amount of COX fibers than young TA muscle, responded to CLFS with a significant decrease in COX− fibers. The appearance of small COX-positive type I fibers in stimulated aging muscle indicated that regenerating type I fibers “diluted” the COX-deficient fiber population.