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 youn g calves for 50 days. After this period the animals were slaughtered a nd beta-adrenoceptor density, ligand affinity, and basal and stimulate d adenylate cyclase activities were studied in smooth muscle and epith elium 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 desensitizati on of beta-adrenoceptors in the trachea was not observed. This might b e a reason for the effectiveness of long-term treatment with beta-agon ists.