Characterization of beta-adrenoceptors in myometrium of preparturient rats.

The purpose of this study was to characterize the beta-adrenoceptor subtypes in pregnant rat myometrial membrane fractions and to determine the concentration of beta 2-adrenoceptors in uterus during late pregnancy. Two methods are compared. A non-subtype-selective antagonist radioligand [3H]dihydroalprenolol ([3H]DHA) was used to label all of the beta-adrenoceptors. [3H]DHA bound to both beta 1- and beta 2-adrenoceptors with indistinguishable affinity (KD = 1.31 nM, Bmax = 174 fmol/mg protein). Computer modelling of competition curves of unlabeled selective antagonists or agonists was then required in order to determine reliably beta 1- and beta 2-adrenoceptor affinities and proportions: the beta 1-adrenoceptors represent 35.5% and the beta 2-adrenoceptors 64.5% of the entire beta-adrenoceptor population in rat gravid myometrium at term. The second approach utilized the radioligand [3H]hydroxybenzylisoproterenol ([3H]HBI) which is a very high-affinity beta-adrenoceptor agonist. The characteristics of the [3H]HBI binding sites are essentially those expected of beta 2-adrenoceptors, but the [3H]HBI binding sites represent only 34% of [3H]DHA binding sites and may represent the fraction of beta 2-adrenoceptors that mediate adenylate cyclase stimulation and uterine relaxation. Between 21 d 09 h and 22 d 09 h of gestation, the number of beta 2-adrenoceptors was constant (mean = 225.6 +/- 20.2 fmol/mg protein). At term, the number of [3H]HBI binding sites dropped (-75%) during the last 7 h of pregnancy, suggesting a reduced ability to elicit relaxation through beta-adrenoceptor activation in parturient myometrium of rat.