1. A single subcutaneous injection of clenbuterol hydrochloride (0.125 mg/kg body wt.) to female Wistar rats produced a rapid increase in muscle cyclic AMP and lactate concentrations and a decrease in muscle glycogen concentrations. These changes are characteristic of muscle beta-adrenoceptor stimulation and were abolished by intraperitoneal injection of propranolol (12.5 mg/kg) 15 min before clenbuterol administration. 2. When this dose of clenbuterol was injected twice daily, the changes in muscle metabolite concentrations which followed its acute administration persisted until day 7 of treatment, and were accompanied by increases in muscle mass, body weight and muscle protein synthesis rate (ks). When the clenbuterol injections were preceded by propranolol injections (12.5 mg/kg administered according to the protocol described above), or if animals were treated with propranolol only, the values of these variables were not significantly different from those of sham-injected controls. 3. In rats fed on a semi-synthetic diet (PW3) supplemented with 2 mg of clenbuterol/kg of diet for 7 days, the muscle mass was greater than that of rats fed on unsupplemented PW3. The increased muscle mass was accompanied by increased muscle lactate and decreased muscle glycogen concentrations. When PW3 was supplemented with 2 mg of clenbuterol/kg and 200 mg of propranolol/kg, the increase in muscle mass remained, but decreased muscle glycogen concentrations and increased muscle lactate concentrations were also observed. 4. These data are consistent with the hypothesis that clenbuterol influences muscle growth via beta-adrenoceptor stimulation.

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