Rabbit myometrial oxytocin and alpha 2-adrenergic receptors are increased by estrogen but are differentially regulated by progesterone.

Both myometrial oxytocin and alpha 2-adrenergic receptors are induced by estrogen. To compare the regulation of these two receptor populations by progesterone, we measured myometrial receptor concentration in ovariectomized steroid-treated and in pregnant rabbits. To control for the effects of estrogen withdrawal, we used concomitant rather than sequential presentation of estrogen and progesterone in ovariectomized rabbits. Estradiol increased both myometrial oxytocin and alpha 2-adrenergic receptor concentrations in ovariectomized rabbits after 8 days of treatment. Simultaneous progesterone administration during the last 4 days of estradiol treatment reversed the induction of oxytocin, but not alpha 2-adrenergic, receptors. Similarly, administration of the antiprogestin RU 38486 to pregnant rabbits on day 27 of gestation resulted in premature delivery and evoked an increase in myometrial oxytocin receptor concentration mimicking that observed at term (day 31). However, RU 38486 did not significantly affect alpha 2-adrenergic receptor concentration. Our data provide further support for involvement of oxytocin receptors in parturition, but do not indicate a comparable function for myometrial alpha 2-adrenergic receptors.