Alpha Adrenergic Receptors in Beef Aortic Membranes Characterised by [3H] Dihydroergocryptine Binding

Alpha adrenergic receptors of beef vascular smooth muscle membranes were identified and characterised using [3H] dihydroergocryptine as a radioligand. Under our experimental conditions [3H] dihydroergocryptine binds rapidly and in saturable fashion to isolated beef aortic membranes, specific binding reaching equilibrium in 5 min at 37 degrees C. Scatchard analysis of the binding data indicates a single population of binding sites with the dissociation constant of 10.4 nM and maximum number of binding sites equaling 156 fmol/mg of membrane protein. The identity of [3H] dihydroergocryptine binding sites with the alpha receptor was established by the competition studies. Catecholamines inhibited binding in the following order: (-) epinephrine > (-) norepinephrine > (+/-) norepinephrine > (-) isoproterenol. Alpha adrenergic antagonists (prazosin, phentolamine, ergotamine, dihydroergocryptine, yohimbine) are potent inhibitors of binding whereas the beta adrenergic antagonist propranolol showed a weak affinity for the binding sites. These results suggest that the high affinity [3H] dihydroergocryptine binding sites are identical to the vascular (alpha 1) adrenergic receptor.