RESUMO
A study was conducted to investigate the effects of the beta-2 receptor agonist, clenbuterol, on the slow-twitch soleus (SOL) and the fast-twitch extensor digitorum longus (EDL) muscles. Twelve male 5-wk-old mice (ddY ) were divided into two groups ; control ( CANT, n=6 ) and clenbuterol-treated (CLEB, n=6) . Clenbuterol was given in the drinking water (0.02 mg/mL) for 5 weeks. Post-treatment body weights were approximately 10% greater in the CLEB group compared to CANT (P<0.05) . In the SQL muscles of the CLEB group the wet weight and the ratio of muscle weight to body weight was significantly higher than the CANT (P<0.01), but not in the EDL. In the SOL muscles of the CLEB group the muscle type LDH isozyme distribution and enzymatic activity was significantly increased (P<0.01) . However, there was no metabolic change in the EDL muscles. Polyacrylamide gel electrophoresis (SDS-PAGE) of myosin heavy chain (Myosin-HC) indicated a clenbuterol-induced decrease (P<0.05) in the relative percentage of type I Myosin-HC with a concomitant increase (P<0.05) in Type II Myosin-HC in the SQL of the CLEB group. The Myosin-HC composition in the EDL was not altered. These findings suggest that clenbuterol may have induced selectively changes of muscle hypertrophy, the predominant anerobic glycolysis, and the transition of Myosin-HC in the SOL. The differences in the two muscles can be explained by the higher number of beta-2 receptors in slow-twitch muscles.