Pharmacological classification of adrenergic α receptors in the guinea pig

Abstract

THE actions of sympathomimetic amines have attracted considerable attention for some time. Ahlquist^1–3 introduced 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.^4,5 suggested the existence of two types of β receptors, β₁ and β₂. It has been claimed that there is a pharmacological difference between α receptors at adrenergic neurones and those in smooth muscle^6–9. 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 aorta^10–13. Langer¹⁴ suggested that there are two kinds of α receptors, α₁ and α₂ (see also ref. 15). Adopting this nomenclature, it was proposed that α₁ receptors are present when the relative affinity of phenylephrine for the receptors is stronger than that of clonidine, and α₂ receptors when the relative affinities are reversed¹³, α₁ Receptors 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 α₂ receptors 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.