(+/-)-[H-3]EPINEPHRINE AND (-)-[H-3]DIHYDROALPRENOLOL BINDING TO BETA-1-NORADRENERGIC AND BETA-2-NORADRENERGIC RECEPTORS IN BRAIN, HEART, AND LUNG MEMBRANES

Abstract

(.+-.)-[3H]Epinephrine bound to .beta.-receptors in calf cerebellar and rat lung membranes in the presence of 1.0 mM pyrocatechol and 1.0 .mu.M phentolamine, with dissociation constants at 4.degree. C of 11 nM and 24 nM, respectively. (.+-.)-[3H]Epinephrine associated to equilibrium within 20 min in both tissues, and over 50% of the binding was rapidly dissociable. Inhibition of binding by agonists and antagonists was highly stereoselective, and the structure-activity relationships of adrenergic agents in inhibiting (.+-.)-[3H]epinephrine binding suggested an interaction with .beta.2 type noradrenergic receptors. (-)-Isoproterenol had an apparent Ki of 2 nM, (-)-epinephrine was 1.5-3 times weaker, and (-)-norepinephrine was 30-60 times weaker. Salbutamol and terbutaline, selective .beta.2-agonists, were potent inhibitors of binding, as were several nonspecific antagonists. Properties of the sites labeled by (.+-.)-[3H]epinephrine in calf cerebellum and rat lung were closely similar. (-)-[3H]Dihydroalprenolol binding in calf cerebellum and rat lung showed .beta.2 characteristics. Antagonists had similar potencies in inhibiting (-)-[3H]dihydroalprenolol and (.+-.)-[3H]epinephrine binding in both tissues, but agonists were in general more potent inhibitors of (.+-.)-[3H]epinephrine. Na and Li selectively lowered the affinity of (.+-.)-[3H]epinephrine at its binding sites and the affinities of agonists, but not antagonists, at the (-)-[3H]dihydroalprenolol site. Specific (.+-.)-[3H]epinephrine binding was not detectable in calf cortex and rat heart, where (-)-[3H]dihydroalprenolol binding suggested a .beta.1-receptor. A physiological significance of (.+-.)-[3H]epinephrine binding was suggested by the strong correlation for agonists and antagonists between affinities in inhibiting binding, and in stimulating or inhibiting a .beta.-receptor-coupled adenylate cyclase in frog erythrocytes.