Direct evidence of two types of β adrenoceptor binding site in lung tissue

Abstract

THERE is much evidence to suggest that β adrenoceptors are not homogeneous—pharmacological data on the actions of β adrenoceptor agonists and antagonists in intact tissues indicate the presence of two broad subclasses β₁ β₂ (refs 1, 2), although there is some controversy regarding this strict subdivision^3,4. The receptors found in heart and adipose tissue have been classified as β₁ whereas those in liver, skeletal muscle, trachea and uterus have been designated β₂. This classification has been supported by biochemical studies examining β adrenoceptor-mediated cyclic AMP formation in various tissues^5–7, although the characterisation of the β adrenoceptor in lung tissue using this approach has been the subject of some debate^6,8. The direct binding of radiolabelled β–adrenoceptor antagonists to tissue membrane preparations has been studied by several workers in an attempt to overcome some of the problems that may arise when examining this receptor indirectly⁹. We have examined the direct binding of the specific β adrenoceptor ligand ³H-dihydroalprenolol (DHA) to rat lung membranes and here we present evidence for the presence of both β₁ and β₂ adrenoceptor subtypes in this tissue in a ratio of approximately 1 : 3.