(±)-[³H]epinephrine and (-)-[³H]dihydroalprenolol binding to β₁- and β₂-noradrenergic receptors in brain, heart, and lung membranes

(±)-[³H]Epinephrine binds to β-receptors in calf cerebellar and rat lung membranes in the presence of 1.0 mM pyrocatechol and 1.0 μM phentolamine, with dissociation constants at 4° of 11 nM and 24 nM, respectively. (±)-[³H]Epinephrine associates to equilibrium within 20 min in both tissues, and over 50% of the binding is rapidly dissociable. Inhibition of binding by agonists and antagonists is highly stereoselective, and the structure-activity relationships of adrenergic agents in inhibiting (±)-[³H]epinephrine binding suggest an interaction with β₂ type noradrenergic receptors. (-)-Isoproterenol has an apparent K(i) of 2 nM, (-)-epinephrine is 1.5 to 3 times weaker and (-)-norepinephrine is 30 to 60 times weaker. Salbutamol and terbutaline, selective β₂-agonists, are potent inhibitors of binding, as are several nonspecific antagonists. Properties of the sites labeled by (±)-[³H]epinephrine in calf cerebellum and rat are closely similar. (-)-[³H]Dihydroalprenolol binding in calf cerebellum and rat lung also shows β₂ characteristics. Antagonists have similar potencies in inhibiting (-)-[³H]dihydroalprenolol and (±)-[³H]epinephrine binding in both tissues, but agonists are in general more potent inhibitors of (±)-[³H]epinephrine. Sodium and lithium selectively lower 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 suggests a β₁-receptor. A physiological significance of (±)-[³H]epinephrine binding is suggested by the strong correlation for agonists and antagonists between affinities in inhibiting binding, and in stimulating or inhibiting a β-receptor-coupled adenylate cyclase in frog erythrocytes.