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 t wo groups: clenbuterol treated (CL, n = 7) (2 mg . kg(-1) body mass in jected subcutaneously every other day), and control(CON, n = 7) (injec ted with isotonic saline). Post-treatment body weights were approximat e to 5% greater in the CL group compared to CON (P < 0.05). Polyacryla mide ,eel electrophoresis (SDS-PAGE) of soleus myofibrillar protein in dicated a clenbuterol-induced decrease (P < 0.05) in the relative perc entage 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 p roportion of type II fibers expressed both as a percentage of total fi ber number and total cross-sectional area (CSA). Finally, mean type II fiber CSA was approximate to 25% greater (P < 0.05) in the CL groups as compared to the CON group, These data indicate that clenbuterol tre atment results in alterations in the MHC phenotype and an increased pr oportion of type II fiber CSA in the soleus of adult rats. These obser vations were due to an increase in the total number of type II fibers, as well as hypertrophy of these fibers, Thus, the relative increase i n the number of histochemically determined type II fibers and the emer gence 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.