Abstract/Summary

Aim: We assessed whether muscle fibres in Myostatin knockout (MSTN-/-) mice are just larger, or also exhibit morphological, metabolic and functional differences from MSTN+/+ mice. Methods: We compared single fibre contractile properties and histological fibre properties in muscles from MSTN-/- and MSTN+/+ mice. Results: Even though in permeabilised muscle fibres from the extensor digitorum longus and soleus muscle maximal force was higher (P < 0.001) there were no significant differences in specific power (power per unit volume), specific tension (force per cross-sectional area), maximal shortening velocity, or curvature of the force-velocity relationship between MSTN-/- and MSTN+/+ mice. In histological sections of the soleus muscle, fibres were larger (P < 0.001), but the succinate dehydrogenase staining intensity and capillary density did not differ significantly between MSTN-/- and MSTN+/+ mice, which was explicable by the larger number of capillaries around a fibre (P < 0.001). A model showed no significant differences in soleus muscle oxygenation. Discussion: In conclusion, the larger force generating capacity of fibres from MSTN-/- mice is explicable by the larger fibre cross-sectional area. Combined with the similar fibre oxidative capacity, and the relationship between capillary supply and the size of a fibre in the soleus muscle, this indicates that muscle fibres from MSTN-/- mice are quantitatively, but not qualitatively different from muscle fibres from MSTN+/+ mice. This suggests that myostatin inhibition may help increase muscle mass in conditions accompanied by muscle weakness without detrimental impact on muscle quality, but systemic side effects need to be considered.