Myostatin knockout mice increase oxidative muscle phenotype as an adaptive response to exercise

Myostatin-deficient mice (MSTN ^−/−) display excessive muscle mass and this is associated with a profound loss of oxidative metabolic properties. In this study we analysed the effect of two endurance-based exercise regimes, either a forced high-impact swim training or moderate intensity voluntary wheel running on the adaptive properties of the tibialis anterior and plantaris muscle from MSTN ^−/− mice. MSTN ^−/− and wild type (MSTN ^+/+) animals had comparable performances in the wheel running regime in terms of distance, average speed and time, but MSTN ^−/− mice showed a reduced ability to sustain a high-impact activity via swimming. Swim training elicited muscle specific adaptations on fibre type distribution in MSTN ^−/−; the tibialis anterior displaying a partial transformation in contrast to the plantaris which showed no change. Conversely, wheel running induced similar changes in fibre type composition of both muscles, favouring transitions from IIB-to-IIA. Succinate dehydrogenase activity, an indicator of mitochondrial oxidative potential was increased in response to either exercise regime, with wheel running eliciting more robust changes in the MSTN ^−/− muscles. Examination of the cross sectional area of individual fibre types showed genotype-specific responses with MSTN ^−/− mice exhibiting an incapability of fibre enlargement following the wheel running regime, as opposed to MSTN ^+/+ mice and a greater susceptibility to muscle fibre area loss following swimming. In conclusion, the muscle fibre hypertrophy, oxidative capacity and glycolytic phenotype of myostatin deficient muscle can be altered with endurance exercise regimes.