Binding of ³H-catecholamines to α-noradrenergic receptor sites in calf brain

The binding of (±)-[³H]epinephrine and (-)-[³H]norepinephrine to calf cortical membranes in the presence of 1.0 mM pyrocatechol has properties indicating an association with α-noradrenergic receptors. Binding of the ³H-catecholamines associated with β-noradrenergic receptors has not been detected, even at low concentrations of pyrocatechol. Chromatographic studies reveal that radioactivity bound to membranes is authentic catecholamine. Binding of both ³H-catecholamines is saturable with dissociation constants at 37° of 18 nM for (±))-[³H]epinephrine and 26 nM for (-)-[³H]norepinephrine. The Hill coefficients for binding of both ³H-ligands are 1.0. At 37°, both ³H-catecholamines associate rapidly to equilibrium, and binding is completely reversible. Kinetically determined dissociation constants correspond well with values obtained from equilibrium studies. Binding is stereoselective for both ligands, with (-)-norepinephrine displaying about 40 times greater affinity than (+)-norepinephrine. The structure-activity relationship of a series of phenylethanolamine derivatives in competing for ³H-catecholamine binding sites parallels the known relative potencies of these compounds at peripheral α-receptors. α-Antagonists and partial agonists are potent competitors, whereas β-antagonists and catechol derivatives are extremely weak. The structurally dissimilar α-agonist, [³H]clonidine, seems to bind to the same calf cortical membrane sites as the ³H-catecholamines, whereas the α-antagonist, ³H-labeled WB-4101, binds to sites with the same α-receptor characteristics, but which have greater affinity for antagonists and lesser affinity for agonists. At 25° and 4°, the ³H-catecholamines and agonists competitors have 2 to 4 times greater affinity for the binding sites than at 37°, whereas antagonists are 2 to 10 times weaker, and partial agonists have approximately the same affinity. At 4° and 25°, ³H-catecholamines dissociate in a biphasic manner which is time-dependent.