Comparison of specificity of agonist and antagonist radioligand binding to β adrenergic receptors

Abstract

DIRECT radioligand binding methods have been used extensively to identify and characterise a wide range of hormone, drug and neurotransmitter receptors. Among the most intensively studied have been the β adrenergic receptors which mediate stimulation of the enzyme adenylate cyclase by catecholamines¹. Until recently all of the successful binding studies of β adrenergic receptors have been performed with specific high affinity β adrenergic antagonists such as (−) ³H-dihydroalprenolol², (±) ³H-propranolol³ or (±) ¹²⁵I-hydroxybenzylpindolol⁴. Earlier attempts to use radioligand β adrenergic agonists were generally frustrated by a great deal of nonspecific (that is non-receptor) binding⁵. We have developed a high affinity radiolabelled β adrenergic agonist, ³H-hydroxybenzylisoprenaline, which, when used in conjunction with high concentrations of catechol and ascorbic acid which block non-receptor binding sites, seems to be a useful ligand for studying the β-adrenergic receptors⁶. We present here the results of parallel studies of agonist (³H-hydroxybenzylisoprenaline, HBI) and antagonist (³H-dihydroalprenolol, DHA) binding to β-adrenergic receptors in frog erythrocyte membranes. The ability of sixteen β-adrenergic agents (agonists and antagonists) to competitively inhibit binding of the two ligands in identical assay conditions (including catechol and ascorbate) has been compared. The results suggest an impressive coincidence of the binding data obtained with the two radioligands. These results are in contrast with those recently described for several central nervous system neurotransmitter receptors where it seems to be possible to identify discrete ‘agonist’ and ‘antagonist’ conformations of the receptors by performing binding studies with either a radiolabelled agonist or antagonist respectively^7–11.