17 beta-estradiol maintains normal function of corpora lutea throughout pseudopregnancy in hypophysectomized rabbits

The hypothesis was tested that the action of 17 beta-estradiol is sufficient to ensure normal luteal function in the absence of pituitary hormones. Rabbits were mated to vasectomized males to induce a normal pseudopregnancy and the next day (Day 1) hypophysectomized or sham hypophysectomized. At this time a Silastic capsule containing estradiol was inserted s.c. in all rabbits. In the presence of this implant serum estradiol values in hypophysectomized and sham-hypophysectomized animals were not different; the mean concentration for the two groups was 9 pg/ml. Measurements of serum progesterone throughout pseudopregnancy revealed essentially superimposable curves for hypophysectomized, estradiol-treated and sham-hypophysectomized, estradiol-treated animals. Further, pseudopregnancy was of normal duration in the two groups, about 17 days. Progesterone secretion was acutely dependent upon estradiol in the hypophysectomized animals, as shown by removal of the Silastic capsule on Day 10; progesterone declined from 18 ng/ml of serum to 10 ng/ml within 12 h, and by 24 h after removal of the implant was 0.5 ng/ml. Twelve-hour incubations of corpora lutea, and measurement of progesterone and 20 alpha-dihydroprogesterone in medium, revealed essentially superimposable rates of production of these steroids for tissues from the hypophysectomized and sham-hypophysectomized animals. We conclude that 17 beta-estradiol satisfies the criteria for the primary luteotropic hormone in this species; further, regression of the corpora lutea is normal and predictable in the presence of estradiol. Beyond the first day following ovulation, the secretion of luteinizing hormone (LH) or other pituitary hormones is not required for a normal course of development, maintenance and regression of the corpora lutea.