Physical activity and muscle adaptations in dystrophic mice treated with prednisone

Glucocorticoid administration is standard treatment for individuals with Duchenne’s Muscular Dystrophy. However, the drug effects on physical activity and skeletal muscle are not clearly established. We tested the hypothesis that mdx mice treated with prednisone (Pred) would be more active and that hindlimb muscles would have better contractility and greater injury‐resistance compared with control mdx mice (Con). 6 wk male mdx mice were implanted with time‐release prednisolone pellets, 1.5 – 2.2 mg/kg/d (n=15), or placebo pellets (n=8). Cage activity was monitored from 1 wk pre‐ to 8 wk post‐implantation. At wk 8, mice were sacrificed, muscles harvested and EDL muscles assessed for contractility and susceptibility to eccentric contraction‐induced injury. Daily cage activity, in terms of jumping and total locomotive energies, were 191 and 112% greater in Pred compared with Con mice (P≤0.027). The 8 wk‐change in body mass was greater in Con than Pred mice (P=0.018). Masses of hindlimb extensor muscles (EDL and tibialis anterior) were lower in Pred than Con mice (P≤0.002). EDL specific force was 26% greater (P=0.010) in Pred (12.5 ± 0.6 N/cm 2 ) vs Con mice (9.9 ± 0.9 N/cm 2 ) but susceptibility to an acute injury was not affected by prednisone treatment (P≤0.068). Overall, these data indicate that mdx muscle responds to glucocorticoid treatment in a positive fashion but perhaps indirectly by affecting physical activity.