Morphological and biochemical alterations of skeletal muscles from the genetically obese (ob/ob) mouse

Background: Knowledge of the morphological and biochemical alterations occurring in skeletal muscles of obese animals is relatively limited, particularly with respect to non-limb muscles and relationship to fibre type. Objective: Sternomastoid (SM; fast-twitch), extensor digitorum longus (EDL; fast-twitch), and soleus (SOL; mixed) muscles of ob/ob mouse (18–22 weeks) were examined with respect to size (mass, muscle mass-to-body mass ratio, cross-sectional area (CSA)), fibre CSA, protein content, myosin heavy chain (MHC) content, MHC isoform (MHCi) composition, MHCi-based fibre type composition, and lactate dehydrogenase isoenzyme (LDHiso) composition. Results: Compared with (control) muscles from lean mice, all the three muscles from ob/ob mice were smaller in size (by 13–30%), with SM and EDL being the most affected. The CSA of IIB and IIB+IID fibres (the predominant fibre types in SM and EDL muscles) was markedly smaller (by ~30%) in ob/ob mice, consistent with differences in muscle size. Total protein content (normalised to muscle mass) was significantly lower in EDL (−9.7%) and SOL (−14.1%) muscles of ob/ob mice, but there were no differences between SM, EDL, and SOL muscles from the two animal groups with respect to MHC content (also normalised to muscle mass). Electrophoretic analyses of MHCi composition in whole muscle homogenates and single muscle fibres showed a shift towards slower MHCi content, slower MHCi containing fibres, and a greater proportion of hybrid fibres in all the three muscles of ob/ob mice, with a shift towards a more aerobic-oxidative phenotype also observed with respect to LDHiso composition. Conclusion: This study showed that SM, EDL, and SOL muscles of ob/ob mice display size reductions to an extent that seems to be largely related to fibre type composition, and a shift in fibre type composition that may result from a process of structural remodelling, as suggested by the increased proportion of hybrid fibres in muscles of ob/ob mice.