Role of Decidual Luteotropin and Prolactin in the Control of Luteal Cell Receptors for Estradiol 1

In pseudopregnant rats, the luteotropic effect of estradiol depends on the presence of prolactin or decidual luteotropin. To determine if prolactin and decidual luteotropin regulate the functions of estradiol by maintaining luteal cell availability of estradiol receptors, decidual tissue was induced on Day 5 of pseudopregnancy. Control rats were hysterectomized on the same day. On Day 8, the rats were injected with 0.4 mg of ergocryptine (ECO) to inhibit prolactin secretion. Control rats were administered the vehicle only. On Day 9, in ECO-treated hysterectomized rats, serum progesterone declined from a control value of 50 +/- 7 to 7 +/- 2 ng/ml. In addition, luteal cell content of cytosolic and nuclear estradiol receptors was reduced by 95% and 48%, respectively. However, serum estradiol levels remained unchanged (vehicle treated: 23 +/- 7 pg/ml; ECO treated: 23 +/- 6 pg/ml). Administration of prolactin (250 micrograms/day, Days 7-9) to hysterectomized rats completely reversed the effect of ECO on serum progesterone levels and estradiol receptor content in both luteal compartments and had no effect on serum estradiol levels. In contrast, suppression of prolactin secretion in decidua-bearing rats had no effect on progesterone synthesis (vehicle treated: 54 +/- 6 ng/ml; ECO treated: 66 +/- 5 ng/ml) nor did it significantly decrease cytoplasmic and nuclear estradiol receptor content in luteal cells. These results suggest that 1) the availability of cytoplasmic receptors limits the biological responses of estradiol and 2) the synergistic luteotropic effect of prolactin or decidual luteotropin and estradiol involves the maintenance of luteal cell estradiol receptors by either prolactin or decidual luteotropin.