Abstract/Summary

Obesity is a worldwide nutritional disorder affecting body performance including 29 skeletal muscle. Inhibition of myostatin not only increases the muscle mass but also it reduces body fat accumulation. We examined the effect of high-fat diet on the phenotypic properties of forelimb muscles from myostatin null mice. Male wild-type and myostatin null mice were fed on either normal diet or high-fat diet (45 % fat) for ten weeks. M. triceps brachii Caput longum; M. triceps brachii Caput laterale; M. triceps brachii Caput mediale; M. extensor carpi ulnaris and M. flexor carpi ulnaris were processed for fiber type composition using immunohistochemistry and morphometric analysis. Although the muscle mass revealed no change under high-fat diet, there were morphometric alterations in the absence of myostatin. We show that high-fat diet reduces the cross-sectional area of the fast (IIB and IIX) fibers in M. triceps brachii Caput longum and M. triceps brachii Caput laterale of both genotypes. In contrast, increases of fast fibers area were observed in both M. extensor carpi ulnaris of wild- type and M. flexor carpi ulnaris of myostatin null. Meanwhile, a high-fat diet increases the area of the fast IIA fibers in wild-type, myostatin null displays a muscle-dependent alteration in the area of the same fiber type. The combined high-fat diet and myostatin deletion shows no effect on the area of slow type I fibers. Despite, a high-fat diet causes a reduction in the area of the peripheral IIB fibers in both genotypes, only myostatin null shows an increase in the area of the central IIB fibers. We provide evidence that a high-fat diet induces a muscle-dependent fast to slow myofiber shift in the absence of myostatin. Taken together, the data suggest that the morphological alterations of muscle fibers under combined high-fat diet and myostatin deletion reflect a functional adaptation of the muscle to utilize the high energy intake.