Obesity-induced discrepancy between contractile and metabolic phenotypes in slow- and fast-twitch skeletal muscles of female obese Zucker rats

A clear picture of skeletal muscle adaptations to obesity and related comorbidities remains elusive. This study describes fiber-type characteristics (size, proportions, and oxidative enzyme activity) in two typical hind limb muscles with opposite structure and function in an animal model of genetic obesity. Lesser fiber diameter, fiber type composition, and histochemical succinic dehydrogenase activity (an oxidative marker) of muscle fiber types were assessed in slow- (soleus) and fast-twitch (tibialis cranialis) muscles of obese Zucker rats, and compared with age- (16 weeks) and sex-matched (females) lean Zucker rats (N=16/group). Muscle mass and lesser fiber diameter were lower in both muscle types of obese compared with lean animals, even though body weights were increased in the obese cohort. A faster fiber-type phenotype also occurred in slow- and fast-twitch muscles of obese rats, compared with lean rats. These adaptations were accompanied by a significant increment in histochemical succinic dehydrogenase activity of slow-twitch fibers in the soleus muscle, and fast-twitch fiber-types in the tibialis cranialis muscle. Obesity significantly increased plasma levels of pro-inflammatory cytokines, but did not significantly affect protein levels of peroxisome proliferator activated receptors PPAR and PGC1α in both muscles. These data demonstrate that, in female Zucker rats, obesity induces a reduction of muscle mass in which skeletal muscles show a diminished fiber size and a faster and more oxidative phenotype. It was noteworthy that this discrepancy in muscle's contractile and metabolic features was of comparable nature and extent in muscles with different fiber-type composition and antagonist functions.

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