The effect of beta-agonists and antagonists on muscle growth and body composition of young rats (Rattus sp.).

1. The addition of the beta-selective adrenergic agonist clenbuterol to the diet was associated with an increase in the protein and RNA of skeletal and cardiac muscle, a reduction in fat deposition and an increase in energy expenditure. 2. Neither propranolol nor atenolol blocked the effect of clenbuterol on muscle protein but both reduced its effect on cardiac and fat mass and energy expenditure. 3. Five other beta-agonists were tested. All increased the interscapular brown fat mass and lowered body fat but only two increased skeletal muscle protein. 4. It is concluded that the anabolic and anti-lipogenic actions of certain beta-agonists are mechanistically distinct.