The present study investigated the time-of-day effects on acute response and chronic adaptations to resistance exercise (RE) in rat skeletal muscle. Male rats were divided into Early and Late training groups and performed climbing RE during the first or last hour of the active (dark) period, respectively. The first experiment measured muscle mass and strength after a 10-week climbing training program. The second experiment examined inflammatory signaling response and satellite cell (SC) numbers following an acute bout of RE. The results showed no significant differences between rats training at early and late active periods in relative muscle weight (muscle-to-weight ratio), cross sectional area (CSA) and strength. The acute study observed increased STAT1 phosphorylation, oxidative stress (2-thiobarbituric acid reacting substances, TBARS), SCs (Pax7+), neutrophils (His48+) and macrophages (CD68+), and decreased interleukin 6 (IL-6) protein expression of skeletal muscle relative to non-exercise control after an acute bout of RE. Interestingly, higher plasma IL-6 and STAT3 phosphorylation response was observed in the late training group when compared to the early training group after an acute bout of RE. The results of this study suggest that animals can adapt to resistance training at different time-of-day, by modulating inflammatory signaling of skeletal muscle.
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