[The effect of the beta-adrenergic agonist clenbuterol on the growth of skeletal muscles of rats].

The effects of clenbuterol (CB) on the weights of 6 hindleg muscles as well as on structural and biochemical characteristics of extensor digitorum longus muscle (EDL) were studied on Wistar rats. Different CB-doses (5 vs. 18 mg/kg diet) were tested and rats of different sex were used. The 12 days treatment accelerated the growth of the 6 hindleg muscles by 13 to 24% without predominant response of fast twitch or slow twitch muscles. The higher muscle weights were achieved by fibre hypertrophy as demonstrated by increased fibre diameters (+13 to +16%), and not by fibre multiplication. There was no evidence for a selective fibre hypertrophy of one of the metabolic fibre types. Fibre type composition shifted to the fast glycolytic fibres (FTG +0.7 to +4.9%-units). There were no significant differences in responses between the sexes of CB-doses. Clenbuterol decreased the nucleus/cytoplasm ratio, measured as nuclei number/mm2 fibre area, between 24 and 28% and the DNA/protein ratio by 17%. RNA content and RNA/DNA ratio were enhanced by 25 and 22%, respectively; the protein/RNA ratio remained unchanged. The results suggest that clenbuterol changes fibre type composition and stimulates fibre growth in a pretranslational stage of protein synthesis without addition of nuclei by satellite cell proliferation.