[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 to 30 nM. Binding is stereoselective with (-) norepinephrine displaying about twenty times greater affinity that (+) 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 to 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.