Regulation of oxytocin receptor concentration in rat uterine explants by estrogen and progesterone

Incubation of uterine explants from immature rats with 0.01–100 ng of 17β-estradiol/mL resulted in approximately a fivefold increase in the number of oxytocin receptors per milligram of protein in 48 h. This increase was maintained for at least an additional 48 h in the presence of estrogen. When the explants were incubated with 1 μg progesterone/mL from the outset, the concentration of oxytocin receptors was the same as initial (0 time) levels. The estrogen-induced increase in oxytocin receptor concentration was blocked by incubation with cycloheximide, an inhibitor of protein synthesis. Once increased, however, the concentration of oxytocin receptors exhibited no turnover for at least a 48-h period in the presence of estrogen. The addition of progesterone and estrogen to explants with elevated receptor levels resulted in almost a 60% reduction in oxytocin receptor concentration by 24 h, with no change in affinity of the receptor for oxytocin. The reduction in receptor concentration by progesterone was not prevented by cycloheximide. The progesterone effect may involve inactivation or degradation of oxytocin receptors or activation of substances that are inhibitory to oxytocin binding. The effects of estradiol and progesterone on oxytocin receptor concentration in uterine explants are similar to those seen when the steroids are administered in vivo. The explant system, therefore, should prove useful in clarifying factors and processes that are involved in regulation of oxytocin receptor concentration in the uterus and in the initiation of parturition in the rat.