Myogenin, MyoD, and myosin heavy chain isoform expression following hindlimb suspension

Background: Myogenin expression is associated with a slow myofiber phenotyp e, and MyoD expression is associated with a fast myofiber phenotype. Hindli mb suspension (HS) will induce muscular atrophy, and a transition from a sl ow to fast myofiber phenotype in the rat soleus. Hypothesis: Hindlimb suspe nsion will induce myofiber atrophy, and a slow to fast myofiber type transi tion with corresponding changes in myogenin and MyoD expression. Methods: M yofiber phenotype was evaluated by electrophoretically separating the myosi n heavy chain isoforms. Myogenin expression was evaluated by Northern analy sis, while MyoD expression was evaluated by Northern analysis and semiquant itative RT-PCR. Results: After 28 d of hindlimb suspension, there was signi ficantly (p < 0.05) less myosin heavy chain Type IIA, and more (p < 0.05) m yosin heavy chain Type IIX in the soleus muscles of hindlimb suspended rats compared with soleus muscles from weightbearing (WB) rats. Although there was a shift to a faster myosin heavy chain phenotype in soleus muscles from hindlimb suspended rats, there was no change in myogenin expression, and M yoD expression was undetectable by Northern analysis. Semi-quantitative RT- PCR revealed an up-regulation of MyoD expression following 14 d of hindlimb suspension. Conclusions: Myogenin expression levels do not change during t he slow to fast myofiber phenotypic transition that occurs during hindlimb suspension; MyoD expression appears to increase at the same time as the phe notypic transition. Thus, MyoD expression or the Myogenin: MyoD mRNA ratio may be important in the phenotypic transition. Neither myogenin nor MyoD ap pear to play a critical role in the muscular atrophy that occurs during wei ghtlessness.