Changes in tissue blood flow and beta-receptor density of skeletal muscle in rats treated with the beta2-adrenoceptor agonist clenbuterol.
Rats injected with the beta2-adrenoceptor agonist clenbuterol (2 mg kg-1 per day) for 18 days gained significantly more weight than controls. Tissue blood flow assessed 24 h after the last injection from the distribution of radiolabelled microspheres was increased in white (5 fold) and brown (3 fold) adipose tissue of clenbuterol-treated rats but was unaffected in kidney, brain and diaphragm, and was reduced by about 80% in skeletal muscle. Acute injection of clenbuterol one hour before measuring blood flow, increased blood flow to brown fat (20 fold) in both treated and control groups. Blood flow to skeletal muscle increased more in the rats treated chronically with clenbuterol (6 fold increase) than in control rats (2 fold increase), but absolute flow rates were still significantly lower in the rats treated chronically with clenbuterol. Skeletal muscle beta-adrenoceptor density and subtype were assessed from ligand binding and displacement studies using [3H]-dihydroalprenolol. Rats treated with clenbuterol for 18 days showed a 50% reduction in beta-receptor density, but the ratio of beta 1/beta 2-receptors was unaffected (15% beta 1/85% beta 2). The results indicate that, although clenbuterol produces acute increases in muscle blood flow, chronic treatment results in lower flow rates immediately (1 h) and 24 h after the previous injection. The attenuated response following chronic treatment is associated with a marked reduction in skeletal muscle beta-adrenoceptor density. The data suggest that any anabolic effects of clenbuterol on muscle which may require, or may be mediated by increases in blood supply, cannot be sustained by chronic treatment. Conversely, blood flow to white and brown adipose tissue would appear to be potentiated by chronic treatment, possibly reflecting increases in lipolytic and/or thermogenic activity.