Regulation of Myometrial β2-Adrenergic Receptors by Progesterone and Estradiol-17β in Late Pregnant Rats1

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Abstract
Rat myometrium exhibited a marked decrease in the concentrations of .beta.2-adrenergic receptors immediately before parturition, i.e., in the last 6 h of pregnancy. This phenomenon continued until the withdrawal of myometrial progesterone (-94% from Day 18 of pregnancy to term) and coincided with the sharp increase (+200%) of the myometrial concentrations of estradiol. A linear positive correlation was found (r2 = 0.654) between the concentration of .BETA.2-adrenergic receptors and the log ratio of myometrial concentration of progesterone/myometrial concentration of estradiol (P/E2), suggesting a modulation of .beta.2-adrenergic receptors by steroids. In rats with estrogen-dominated uteri (intact or ovariectomized late pregnant rats injected with estradiol), there was no change either in concentration or affinity of .beta.2-adrenergic receptors relative to untreated control pregnant rats. In contrast, rats with progesterone-dominated uteri (intact or ovariectomized late pregnant rats treated with progesterone or ovariectomized rats) have an increased number of .beta.2-adrenergic receptors, with a decreased affinity of these receptors compared to untreated control pregnant rats or to estrogen-treated rats. These results suggest that progesterone regulates the number of .beta.2-adrenergic receptors in myometrium of late pregnant rats. The mechanisms by which progesterone exerts this regulation remains to be elucidated.