H-Ddihydroergocryptine binding to alpha adrenergic receptors in canine aortic membranes
[3H]Dihydroergocryptine, a potent alpha adrenergic antagonist, was used to label alpha adrenergic receptors in membranes isolated from dog aortas. Binding reached steady state in 4 minutes at 25° C and was completely reversible. At saturation the total number of binding sites was calculated to be 145 fmol/mg of protein. The dissociation constant of dihydroergocryptine binding was 10 nM. The specificity of alpha adrenergic agonists for the binding sites correlated closely with their potencies in inducing an alpha adrenergic response in the aorta. Catecholamines inhibited binding in the following order: (-)-epinephrine > (-)-norepinephrine > (-)-phenylephrine > (±)-cobefrin > (-)-isoproterenol. The binding sites displayed stereospecificity for catecholamines; the (-)-isomers are much more potent than the corresponding (+)-isomers in inhibiting [3H]dihydroergocryptine binding. Serotonin, dopamine, acetylcholine and histamine were weak inhibitors of binding. Neither the metabolic products of catecholamines nor their precursors interacted with the binding sites. Alpha adrenergic antagonists such as phentolamine and ergot alkaloids competed potently with [3H]dihydroergocryptine for the binding sites. These antagonists competed for binding in a manner parallel to their potencies as alpha adrenergic antagonists. In contrast, both beta adrenergic receptor and cholinergic receptor antagonists were poor inhibitors of [3H]dihydroergocryptine binding. These results suggest that [3H]dihydroergocryptine binding sites in dog aortic membrane preparations are identical to the alpha adrenergic receptors.
Journal of Pharmacology and Experimental Therapeutics
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