[³H]epinephrine and [³H]norepinephrine binding to α-noradrenergic receptors in calf brain membranes

(±)-[³H]Epinephrine and (-)-[³H]norepinephrine bind saturably to calf cerebral cortex membranes under appropriate incubation conditions in a fashion indicating that they label α-noradrenergic receptors. Binding of the two [³H]catecholamines is saturable with dissociation constants of 20-30 nM. Binding is stereoselective with (-)-norepinephrine displaying about twenty times greater affinity than (+)-norepinephrine. The relative potencies of catecholamines in competing for these binding sites parallels their relative pharmacologic effects at α-noradrenergic receptors in numerous tissues. Thus, (-)-epinephrine is 2-3 times more potent than (-)-norepinephrine and 500 times more potent than (-)-isoproterenol. Binding is inhibited by low concentrations of the α-antagonists phentolamine and phenoxybenzamine but not by the β-antagonist propranolol.