Actions of phenylephrine on β-adrenoceptors in guinea-pig trachea

1. Phenylephrine produced relaxation of the isolated guinea-pig tracheal chain preparation, its potency being 1/5 that of noradrenaline on normal tissues.2. The potentiation of phenylephrine by cocaine (10(-5)M) was only slight. Thus on cocaine-treated tissues phenylephrine was 1/45 as potent as noradrenaline.3. The dose-response lines to phenylephrine were shifted in a parallel manner by propranolol 10(-8)M and 10(-7)M, suggesting that the relaxations were mediated through beta-adrenoceptors.4. Phenylephrine had a lower intrinsic activity than the catecholamines and produced multiphasic dose-response lines at the higher doses used in the presence of propranolol (10(-6)M). These observations have been explained by the evidence obtained that phenylephrine is a partial agonist with beta-adrenoceptor blocking activity.5. From experiments using alpha-adrenoceptor blocking drugs, it has been concluded that stimulation of alpha-adrenoceptors has little influence on the beta-adrenoceptor relaxation to phenylephrine on the guinea-pig tracheal chain preparation.