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.

The types of skeletal single muscle fibers were studied with the distribution of myosin light chains (MLC) and tropomyosin (TM) to make clear the biochemical property and the possibility of the transformation of fiber types by physical training. Since soleus (SOL) muscle is widely used as an example of slow-twich muscle and extensor digitorum longus (EDL) muscle is used as one of fast-twitch muscle, single muscle fibers were prepared from SOL and EDL muscles of 5-6 weeks old male mice (strain DDY) . Total myofibrillar proteins of the muscle bundles or single muscle fibers were homogenized and fractionated with the micro two-dimensional isoelectric focusing/sodium dodecyl sulfate polyacrylamide gel electrophoresis (IEF/SDS-PAGE) . The fractionated proteins were visualized with coomassie brilliant blue stain method or highly sensitive silver stain method.
We found that there were at least two types of fibers in SOL and EDL muscles. In SOL muscle, one contained only slow form of MLC and the other contained both of fast and slow forms of MLC. In EDL muscle, one contained only fast form of MLC and the other contained both of fast and slow forms of MLC. The fiber containing only one form of MLC was classified into typical slow or fast fiber, and the fiber containing both forms of fast and slow types MLC was classified into intermediate fiber. Both of a form and β form of TM were invariably observed in all fibers of SOL and EDL muscles. Relative ratio of a form of TM to β form of TM was, however, larger in the typical fast fiber containing only the fast form of MLC than others. The ratio was smallest in the typical slow fiber. These results suggest the possibility of the continuous transformation in the phenotype of muscular proteins within a single muscle fiber by physical training.