Membrane Proteins Increase with the Repeated Bout Effect

Purpose The ability of skeletal muscle to adapt to eccentric (ECC) contraction-induced injury is known as the repeated bout effect (RBE). Despite the RBE being a well-established phenomenon observed in skeletal muscle, cellular and molecular events particularly those at the membranes that contribute to the adaptive potential of muscle have yet to be established. Therefore, the purpose of this study was to examine how membrane-associated proteins respond to the RBE. Methods Anterior crural muscles of C57BL/6 female mice (3-5 months) were subjected to repeated bouts of in vivo ECCs, with isometric torque being measured immediately before and after injury. A total of six bouts were completed with 7 d between each bout. Protein content of dystrophin, beta-sarcoglycan, and junctophilin were then assessed via immunoblotting in injured and uninjured muscles. Results When expressed relative to preinjury isometric torque of bout 1, deficits in postinjury isometric torque during bout 2 (38%) did not differ from bout 1 (36%; P = 0.646) and were attenuated during bouts 3 through 6 (range, 24%-15%; P <= 0.014). Contents of dystrophin, beta-sarcoglycan, and junctophilin did not change immediately after a single bout of 50 maximal ECCs (P >= 0.155); however, as a result of repeated bouts, contents of dystrophin, beta-sarcoglycan, and junctophilin all increased compared with muscles that completed one or no bouts of ECC contractions (P <= 0.003). Conclusions The RBE represents a physiological measure of skeletal muscle plasticity. Here, we demonstrate that repeated bouts of ECC contractions increase contents of dystrophin, beta-sarcoglycan, and junctophilin and attenuate postinjury torque deficits. Given our results, accumulation of membrane-associated proteins likely contributes to strength adaptations observed after repeated bouts of ECC contractions.