The nature of the adrenergic receptors of the trachea of the guinea-pig

The adrenergic receptors of the guinea-pig isolated trachea have been characterised as β-receptors by established criteria. No evidence was obtained that any α-receptors are present. (-)-Isoprenaline was 17 times more potent than (-)-adrenaline which was 10 times more potent than (-)-noradrenaline. High concentrations of piperoxan, thymoxamine and dihydrogenated ergot alkaloids did not antagonise the catecholamines, while phentolamine and phenoxybenzamine potentiated them, isoprenaline almost as much as noradrenaline. Propranolol, pronethalol and the 3, 4-dichloro-analogues of (+)-noradrenaline, adrenaline and isoprenaline each specifically antagonised the catecholamines, isoprenaline moreso than noradrenaline. The characteristics of this blockade by pronethalol and propranolol fulfilled established criteria for competitive antagonism; propranolol (pA₂ against noradrenaline 6·56 + 0·21) was 18·6 (11·4 to 30·5) times more potent than pronethalol, (pA₂ against noradrenaline 5·29 + 0·07).