Control of corpus luteum function in the pregnant rabbit: role of the placenta ("placental luteotropin") in regulating responsiveness of corpora lutea to estrogen

Experiments were performed to examine the interaction between estrogen and "placental luteotropin," the two luteotropins thought to be required for corpus luteum maintenance in the pregnant rabbit. Experiment 1 was designed to determine whether estrogen alone was luteotropic in the absence of "placental luteotropin." Pregnant rabbits were assigned to the following groups: Group A, sham hysterectomy; Group B, hysterectomy; Group C, hysterectomy plus estradiol on Day 20; and Group D, hysterectomy plus estradiol on Day 22. "Placental luteotropin" was removed on Day 21 of pregnancy by hysterectomy and estrogen was administered via an estradiol-filled Silastic implant which was placed s.c. before (Day 20, Group C) or after (Day 22, Group D) hysterectomy. Daily blood samples were taken for radioimmunoassay of serum progesterone and estradiol concentrations. Corpus luteum weights were measured at autopsy on Day 27. Hysterectomy caused serum progesterone concentrations to fall rapidly from 13 +/- 1 ng/ml on Day 21 to 2 +/- 1 ng/ml on Day 23, and to 1 ng/ml or less on Days 24-27. In sham hysterectomized rabbits (Group A), pregnancy was maintained and serum progesterone concentrations remained elevated at 9-15 ng/ml throughout (Days 20-27). Estradiol treatment (Groups C and D) did not prevent or reverse luteal regression induced by hysterectomy and serum progesterone concentrations declined in a similar fashion to Group B. Serum estradiol concentrations were 4-8 pg/ml in all groups and did not vary substantially with stage of pregnancy or treatment.