Pharmacological classification of adrenergic α receptors in the guinea pig

THE actions of sympathomimetic amines have attracted considerable attention for some time. Ahlquist1–3introduced the concept of adrenergic α and β receptors to explain the differences in the effects of some of these amines on various organ systems and then Lands et al.4,5suggested the existence of two types of β receptors, β1and β2. It has been claimed that there is a pharmacological difference between α receptors at adrenergic neurones and those in smooth muscle6–9. I have previously shown that the inhibitory α receptor of the cholinergic neurone in guinea pig ileum is pharmacologically different from the excitatory α receptor located in the smooth muscle of the guinea pig ileocaecal sphincter and rabbit aorta10–13. Langer14suggested that there are two kinds of α receptors, α1and α2(see also ref. 15). Adopting this nomenclature, it was proposed that α1receptors are present when the relative affinity of phenylephrine for the receptors is stronger than that of clonidine, and α2receptors when the relative affinities are reversed13, α1Receptors have thus been shown to be present in smooth muscle cells of rabbit aorta and pulmonary artery, and in guinea pig ileocaecal sphincter; and inhibitory α2receptors in cholinergic neurones of guinea pig ileum and in adrenergic neurones of rabbit pulmonary artery and rat vas deferens. I report here the results of further studies supporting the theory of two types of α receptors.