Effects of clenbuterol and ICI118551, a selective beta 2-antagonist, on the growth of skeletal muscle of suckling rats.

The β2-adrenergic agonist, clenbuterol, was administered to lactating rats (4 mg/kg diet) from post-partum day 1 to day 19, or directly injected into neonate rats (0.1 and 1.0 mg/kg body weight) from post-partum day 3 until day 15. Changes in body weight and the skeletal muscles soleus (SOL) and extensor digitorum longus (EDL) were studied in both dams and suckling offspring. Drug treatment consistently increased body weight in dams whilst significantly reducing the growth of their suckling pups. In dams treated with clenbuterol (4 mg/kg of diet) muscle weights and protein contents were significantly increased. Total protein content increased by 16% in SOL and 47% in EDL after 19 days of treatment. In contrast, in their suckling pups, there was a 22% and 26% reduction in protein content of SOL and EDL respectively. Administration of the β2-antagonist ICI1 18551 to these pups failed to prevent these reductions in body and muscle weights. Hence, if clenbuterol did reach the pups via the milk from treated mothers it did not act via conventional β2-receptors. Injection of pups with clenbuterol (1.0 mg/kg every 12 h) from litters suckling from untreated dams also resulted in significant reductions in muscle weights and protein contents. Protein content was reduced by 10% in SOL and 13% in EDL after 12 days of treatment. No alteration in fibre type proportions in SOL or EDL resulted from this treatment. Further work is required to determine whether the growth suppression in the two situations occurs via the same mechanism.