EFFECTS OF CLENBUTEROL ON CONTRACTILE AND BIOCHEMICAL-PROPERTIES OF SKELETAL-MUSCLE

We investigated the effects of clenbuterol on the muscle mass, contrac tile properties, myosin phenotype, and bioenergetic enzyme activity in the gastrocnemius (GS)-plantaris (PL)-soleus (SO) muscle complex. Rat s were sham-injected or treated with clenbuterol (2 mg . kg(-1), subcu taneously) for 14 d. Clenbuterol increased (P < 0.05) body weight and muscle complex weight. Also, clenbuterol treatment resulted in an incr ease in total muscle force production and maximal shortening velocity (P < 0.05). No difference (P > 0.05) in relative force production (for ce . g(-1) muscle)existed between experimental groups. However, muscle fatigue increased with clenbuterol treatment. Myosin heavy chain (MHC ) composition was not altered in the GS or PL muscles, but shifted tow ard the fast Type II MHC in the SO. Myosin light chain (MLC) compositi on was not altered in any of the muscles. Clenbuterol caused a decreas e in oxidative and glycolytic enzyme activity in the GS and FL, but no t the SO. These data suggest that the clenbuterol-induced increase in muscle mass and maximal force generation is due to hypertrophy of both fast and slow fibers. Furthermore, these findings support the notion that beta-agonists may be beneficial in combating conditions that resu lt in muscle wasting and dysfunction.