Influence of Past Injurious Exercise on Fiber Type Specific Acute Anabolic Response to Resistance Exercise in Skeletal Muscle

We investigated influence of past injurious exercise on anabolic response of skeletal muscle fibers to resistance exercise (RE). Wistar rats were divided into exercise (E) and exercise after injury (I-E) groups. At age 10 wk, the right gastrocnemius muscle in each rat in the I-E group was subjected to strenuous eccentric contractions. Subsequently, RE was imposed on the same muscle of each rat at 14 wk of age in both groups. Peak joint torque and total force generation per body mass during RE were similar between the groups. Muscle protein synthe-sis (MPS) in the I-E group was higher than that in the E group 6 h after RE. Furthermore, lev-els of phospho-p70S6K (Thr³⁸⁹) and phospho-ribosomal protein S6 (phospho-rpS6) (Ser^240/244), a downstream target of p70S6K, were higher in the I-E group than in the E group. For the anabolic response in each fiber type, the I-E group showed a higher MPS response in IIb, IIa, and I and a higher phospho-rpS6 response in IIx, IIa, and I than the E group. In the I-E group, the relative content of myosin heavy chain (MHC) IIa was higher and that of MHC IIb was lower than those in the E group. In addition, type IIa fibers showed a lower MPS response to RE than type IIb fibers in the I-E group. In conclusion, the past injurious exercise enhanced the MPS and phospho-rpS6 responses in type IIb, IIa, and I fibers and type IIx, IIa, and I fibers, respectively.

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