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

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Abstract
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 (10−5M) 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 10−8M and 10−7M, suggesting that the relaxations were mediated through β-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 (10−6M). These observations have been explained by the evidence obtained that phenylephrine is a partial agonist with β-adrenoceptor blocking activity. 5 . From experiments using α-adrenoceptor blocking drugs, it has been concluded that stimulation of α-adrenoceptors has little influence on the β-adrenoceptor relaxation to phenylephrine on the guinea-pig tracheal chain preparation.