Identification of alpha-adrenergic receptors in uterine smooth muscle membranes by [3H]dihydroergocryptine binding.

Abstract

[3H]Dihydroergocryptine, a potent alpha-adrenergic antagonist, was used to label smooth muscle membrane binding sites which have the characteristics expected of alpha-adrenergic receptors. Binding of [3H]dihydroergocryptine to rabbit uterine membranes was rapid and reversible with rate constants of 1.26 X 10(7) M-1 min-1 and 0.034 min-1 for the forward and reverse reactions, respectively. [3H]Dihydroergocryptine binding was of high affinity, with an equilibrium dissociation constant (KD) of 8 to 10 nM. Binding was saturable with 0.14 to 0.17 pmol of [3H]dihydroergocryptine bound/mg of protein at maximal occupancy of the sites. No cooperative interactions among the sites were detected. The specificity of the binding sites for a large number of adrenergic agonists and antagonists was identical with the specificity of alpha-adrenergic responses to these agents. The alpha-adrenergic agonist (-)-epinephrine competed for binding with a KD of 0.23 muM. The order of potencies for several adrenergic agonists in competing for the binding sites was (-)-epinephrine greater than (-)-norepinephrine greater than (-)-phenylephrine greater than (-)-isoproterenol in agreement with their alpha-adrenergic potencies. A series of 19 phenylethylamine adrenergic agonists competed for binding in a manner paralleling their potencies as alpha-adrenergic agonists. alpha-Adrenergic antagonists such as phentolamine (KD = 15 nM) and phenoxybenzamine (KD = 18 nM) potently competed for the binding sites. In contrast, beta-adrenergic antagonists such as propranolol (KD = 27,000 nM) and practolol (KD greater than 10(6) nM) did not have high affinity for the binding sites. A series of ergot alkaloids competed for [3H]dihydroergocryptine binding in a manner which paralleled their potencies as alpha-adrenergic agents. Competition for binding sites by alpha-adrenergic agonists and antagonists was a stereospecific process. The (-)-stereoi somers of epinephrine, norepinephrine, and ergotamine were at least 20- to 50-fold more potent than the corresponding (+)-stereoisomers. Compounds devoid of significant alpha-adrenergic activity, such as pyrocatechol, 3,4-dihydroxymandelic acid, normetanephrine, and D-lysergic acid, did not effectively compete for [3H]dihydroergocryptine binding sites. These rabbit uterine binding sites for [3H]dihydroergocryptine appear to have characteristics indistinguishable from those of the physiologically active alpha-adrenergic receptors.