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.
|Original language||English (US)|
|Number of pages||14|
|Journal||Journal of Biological Chemistry|
|State||Published - 1977|
ASJC Scopus subject areas
- Molecular Biology
- Cell Biology