Characterization of α-adrenoceptors in myometrium of preparturient rats

Abstract

We describe three methods for the quantitative analysis of the α-adrenoceptor subtypes in preparturient rat myometrial membrane fractions. A non-subtype-selective antagonist radioligand, [ 3H]dihydroergocryptine ([ 3H]DHE), was used to label all of the α-receptors. [ 3H]DHE bound to both α 1- and α 2-receptors with indistinguishable affinity. Computer modelling of competition curves of unlabeled selective antagonists or agonists was then required in order to determine reliably α 1 and α 2 affinities and proportions: the α 1-receptors represent 45% and the α 2-receptors 55% of the entire α-receptor population in rat uterus. The second approach involved the administration of phenoxybenzamine (POB) that irreversibly blocks the α 1-adrenoceptors. Myometrial membranes obtained from rats 1 h after the administration of varying amounts of POB showed a dose-dependent reduction in specific [ 3H]DHE binding. This reduction was accompanied by a progressive increase of the value of the dissociation constant. Our data indicate that a dose of 1 mg of POB left the α 2-receptors intact while entirely blocking the α 1-receptors in rat myometrium. The third approach utilized the selective radioligand antagonists [ 3H]prazosin ([ 3H]PRAZ) and [ 3H]rauwolscine ([ 3H]RAUW). The results obtained with these radioligands confirmed our observations on the α-adrenoceptor subtypes in experiments with [ 3H]DHE. The results obtained with the 3 methods are in good agreement. Each approach appears valid and applicable to the characterization of α 1- and α 2-receptors subtypes in rat uterus, but the method using [ 3H]PRAZ and [ 3H]RAUW demonstrates more directly the presence of the two receptor subtypes.