Clenbuterol-induced fiber type transition in the soleus of adult rats.

This study examined the effects of 6 weeks of treatment with the beta(2)-adrenoceptor agonist, clenbuterol, on the soleus muscle of adult female Sprague-Dawley rats. Animals (4 months old) were divided into two groups: clenbuterol treated (CL, n = 7) (2 mg.kg-1 body mass injected subcutaneously every other day), and control (CON, n = 7) (injected with isotonic saline). Post-treatment body weights were approximately 5% greater in the CL group compared to CON (P < 0.05). Polyacrylamide gel electrophoresis (SDS-PAGE) of soleus myofibrillar protein indicated a clenbuterol-induced decrease (P < 0.05) in the relative percentage of type I myosin heavy chain (MHC) with a concomitant increase (P < 0.05) in type IIdx MHC, while the proportion of type IIa MHC was unaffected. ATPase fiber typing revealed increases (P < 0.05) in the proportion of type II fibers expressed both as a percentage of total fiber number and total cross-sectional area (CSA). Finally, mean type II fiber CSA was approximately 25% greater (P < 0.05) in the CL groups as compared to the CON group. These data indicate that clenbuterol treatment results in alterations in the MHC phenotype and an increased proportion of type II fiber CSA in the soleus of adult rats. These observations were due to an increase in the total number of type II fibers, as well as hypertrophy of these fibers. Thus, the relative increase in the number of histochemically determined type II fibers and the emergence of the normally unexpressed type IIdx MHC isoform in the soleus suggest a clenbuterol-induced transition of muscle fiber phenotype as well as selective hypertrophy of the type II fibers.