Clenbuterol has a greater influence on untrained than on previously trained skeletal muscle in rats.

The effects of clenbuterol, a selective beta 2-adrenergic agonist, and of exercise training on the properties of skeletal muscle were studied in the hindlimb of sedentary and trained rats. A 2-week training programme, consisting of climbing on a grid with a load attached to the tail, did not increase the muscle mass of the soleus, the plantaris and the gastrocnemius muscles or modify the isometric in situ contractile properties of the medial gastrocnemius muscle. The only change observed in a 12-week training regimen was a significant increase in contractile forces (expressed in grams per gram of muscle) of the medial gastrocnemius muscle at sub-tetanic stimulating frequencies (twitch 42%, 25Hz 45% and 50Hz 47%). Both training programmes significantly increased fatigue resistance of the medial gastrocnemius muscle. A 2-week oral treatment with clenbuterol significantly increased the muscle mass of the soleus (19.8%), plantaris (16.9%) and gastrocnemius (15.3%) muscles in all animals treated with the agonist. However, clenbuterol had different effects in animals beginning their training programme than in animals that had been trained for the previous 10 weeks. Specifically, clenbuterol caused a significant increase in gastrocnemius muscle mass in the former group but not in the latter. These results suggest that the responses to the combination of clenbuterol and training in previously trained skeletal muscles are not as marked as those observed in untrained muscles.