The purpose of this study was to test the hypothesis that calcineurin, a calcium-dependent protein phosphatase recently implicated in the signaling of skeletal muscle hypertrophy and fiber type conversion, is required to induce some mitochondrial enzyme adaptations to endurance exercise training in skeletal muscle. Three- to four-week-old male Sprague-Dawley rats with an initial body weight ranging from 45 to 55 g were used in this study. The rats were randomly assigned to groups injected with either a specific calcineurin inhibitor, cyclosporin A (CsA), (group CI) or vehicle (group VI). CsA was subcutaneously injected into the rats at a rate of 50 mg·kg-1 body weight per day for 10 days. The CI and VI groups were further assigned to sedentary (SED) or exercise training (EX groups. In the EX group, the rats were trained for 10 days (90 min·day-1, ∼14-20 m·min-1, 10% grade). The citrate synthase (CS) activities in the soleus and plantaris muscles of the EX group rats were significantly higher than those of the SED group rats (p<0.001). Furthermore, 3-β-hydroxy-acyl-CoA dehydrogenase (3-HAD) activities in the soleus and plantaris muscles were significantly higher in the EX group rats than in the SED group rats (p<0.001). However, there were no significant differences in CS and 3-HAD activities between the VI and CI groups. The interactions between CsA injection and exercise training were not statistically significant in any of the parameters. These results may suggest that calcineurin is not involved in some mitochondrial enzyme adaptations to endurance exercise training.
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