BETA-ADRENOCEPTOR-MEDIATED EFFECTS OF CLENBUTEROL (NAB 365) ON TRACHEA, HEART AND SKELETAL MUSCLE IN ANAESTHETIZED GUINEA-PIGS

1. Intravenous injection of clenbuterol (NAB 365) in anaesthetized guinea-pigs produced decreases in tracheal segment pressure and in soleus muscle contractions and a slight increase in heart rate. These responses were reduced by propranolol. 2. Clenbuterol was less potent than isoprenaline on the trachea and soleus muscle and even less potent on the heart. The maximum response to clenbuterol was the same as that to isoprenaline on the trachea and the soleus muscle but it was not more than 30% of that to isoprenaline on the heart. There was no significant difference between the potencies of clenbuterol on the trachea and the soleus muscle. 3. The onset of the responses to clenbuterol were slower and the duration of action longer than those to isoprenaline. 4. High doses of clenbuterol blocked isoprenaline responses on heart rate. 5. Clenbuterol showed selectivity for the trachea and the soleus muscle (β₂-adrenoceptors) as compared with the heart (β₁-adrenoceptors). This could result, at least in part, from its partial agonist activity on the heart.