INDUCTION OF A FAST-OXIDATIVE PHENOTYPE BY CHRONIC MUSCLE STIMULATION- HISTOCHEMICAL AND METABOLIC STUDIES

Chronic electrical stimulation of skeletal muscle at 10 Hz induces fas t-to-slow fiber type transformation. Does a lower aggregate amount of activity lead to a less complete transformation, or does it produce th e same transformation over a longer time course? We examined this ques tion by subjecting adult rabbit tibialis anterior and extensor digitor um longus muscles to continuous stimulation at 2.5 Hz for 2-12 wk. Mos t of the fibers acquired the histochemical and immunocytochemical char acteristics of type 2A, not type 1, fibers. There was a corresponding rise in oxidative activity, but this was accompanied by a marked decli ne in anaerobic glycolysis. The activities of hexokinase and 3-oxoacid CoA-transferase stopped increasing after 2 wk, glutamate oxaloacetate transaminase after 4 wk, and beta-hydroxyacyl-CoA dehydrogenase after 6 wk of stimulation. Succinate dehydrogenase, citrate synthase, lacta te dehydrogenase, and creatine phosphokinase continued to change up to 12 wk of stimulation. Changes in enzyme activity were not as rapid or as marked as those observed for stimulation at 10 Hz, and none showed the typical two-phase response of oxidative enzyme activities to stim ulation at 10 Hz. The latter may therefore be dependent on induction o f type 1 myosin isoforms.