Eccentric Training Causes Muscular Hypertrophy, Fiber-type Transformation and Interleuikin-6 Production in the Rat Gastrocnemius Muscle

Abstract

Recent studies have shown that a single bout of muscular exercise causes a transient increase in Interleuikin-6 (IL-6) production within working muscles, though it remains unclear whether similar change of IL-6 occur after repeated bouts of eccentric exercise, i.e. eccentric training. PURPOSE:The purpose of this study was to learn the effects of moderate eccentric training on muscular size and function, and intramuscular levels of cytokines including IL-6, using originally developed isokinetic-exercise system for rats. METHODS: Male Wistar rats (N=39) were randomly assigned into eccentric training (EC) group and sham control (CON) group. After anesthetized with isoflurance, they were firmly fixed on a platform of the isokinetic dynamometer in a prone position. The exercise session consisted of twenty eccentric contractions of the triceps surae muscle, which were elicited by submaximal electric stimulation (100 Hz). Forced lengthening was applied to the muscle by an isokinetic dorsi-flexion of the ankle joint (velocity, 30 deg/s; range of motion, 45 deg). Rats in EC group were trained every second day for 10 days (5 sessions in total) or 20 days (10 sessions in total) after measurements of isometric tetanic force. Only the measurements of isometric tetanic force were made for CON group. After either 5 or 10 sessions of exercise, the muscle specimens were dissected, weighed and subjected to myosin ATPase histochemistry. Tissue concentrations of IL-6 were measured with Western-blot analysis. RESULTS: The wet weight of medial gastrocnemius muscle did not significantly increase after 5 sessions of exercise (EC 724 ± 56.3 mg, CON: 745 ± 44.7 mg), whereas that after 10 sessions significantly increased with concomitant increase in the cross-sectional area (CSA) of muscle fibers(weight, EC 814 ± 62.4 mg, CON 732 ± 46.5 mg, P<0.05; fiber CSA, EC 4000 ± 1331 umz, CON 3331 ± 1321 umz, P<0.001). The isometric tetanic forces on days 10, 16, 18, and 20 were significantly larger in EC group than in CON group (P<0.001). Myosin-ATPase staining showed a decrease in the percentage of type IIB fibers and an increase in the percentage of type IIA fibers (P<0.001).The tissue concentration of IL-6 after 5 sessions but not 10 sessions of exercise was significantly higher than in the CON group. No apparent fiber damage was observed after 10 sessions. CONCLUSIONS: The results showed that a 20-day eccentric training causes increases in muscular size and strength, and a transformation of fiber types towards slower subtypes. These changes were associated with a preceding increase in IL-6 production, suggesting roles played by muscle-derived cytokines in muscular adaptations to eccentric exercise.