Steroidogenic Effect of 170-Estradiol in the Rabbit: Stimulation of Progesterone Synthesis in Prematurely Regressing Corpora Lutea*

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Abstract
The possibility that estradiol might have a steroidogenic effect on the rabbit corpus luteum was investigated, 1st by rapid withdrawal of estradiol to allow a partial decline in serum progesterone, followed by replacement of estradiol to determine whether luteal progesterone synthesis could be stimulated. Estradiol-filled Silastic capsules, which cause rapid, 5- to 6-fold elevations in serum estradiol and which release estradiol at constant rates, were implanted s.c. on day 1 (the day after sterile mating) of pseudopregnancy. The estradiol implant was left in place until day 10, when it was removed. On day 11 the implant was either replaced or sham replaced. Serum progesterone on day 10 was 14 ng/ml and, after removal of estradiol implants, decreased to 3.9 ng/ml within 24 h. After replacement of estradiol on day 11, serum progesterone increased to 6.5 ng/ml within 12 h and continued to rise to mean values of 12-13 ng/ml on days 15-17. In animals without estradiol replacement, serum progesterone continued to decrease to basal values, the corpora lutea regressed, and these rabbits returned to estrus by day 17. Measurements of accumulations of steroid in incubations of corpora lutea removed before and 12 h after in vivo replacement of estradiol revealed a significant increase in progesterone and no significant change in 20.alpha.-dihydroprogesterone. Incubations of corpora lutea from rabbits without estradiol replacement, progesterone in incubation medium decreased and, again, 20.alpha.-dihydroprogesterone was unchanged. During the 12-h interval after replacement or sham replacement of estradiol, the wet weight of corpora lutea did not change. Luteal progesterone synthesis is tightly coupled to estrogen action; decreasing progesterone synthesis and incipient regression of corpora lutea can be reversed by estradiol administration; estradiol has a sustained, steroidogenic effect on the corpus luteum that can be dissociated from luteal growth.