Effect of clenbuterol on beta-adrenoceptors and adenylate cyclase activity in smooth muscle and epithelium of the trachea of calves.

A potent beta-agonist (clenbuterol) was administered perorally to young calves for 50 days. After this period the animals were slaughtered and beta-adrenoceptor density, ligand affinity, and basal and stimulated adenylate cyclase activities were studied in smooth muscle and epithelium of the trachea. Although the density of lung beta-adrenoceptors was down regulated by clenbuterol, cAMP production remained constant (epithelium) or even increased (smooth muscle). Therefore desensitization of beta-adrenoceptors in the trachea was not observed. This might be a reason for the effectiveness of long-term treatment with beta-agonists.