Premature regression of corpora lutea in pseudopregnant rabbits following the removal of polydimethylsiloxane capsules containing 17 beta-estradiol.

17-beta-Estradiol, which is luteotropic in rabbits, was administered during pseudopregnancy via polydimethylsiloxane (Silastic) implants to determine the effects on serum progesterone concentrations. Implants which released estradiol at a rate of approximately 2 mug/day were place beneath the skin the day after sterile mating and ovulation (day 0). Blood (3 ml) was obtained from the marginal ear vein on days 3, 6, 9, 10, 11 and 12. Serum estradiol levels, determined by radioimmunoassay, were 2- to 3-fold higher in estradiol-treated rabbits (11.7 plus or minus 1.2 pg/ml) than in untreated pseudopregnant controls (5.9 plus or minus 1.4 pg/ml). Weights of corpora lutea in treated and control rabbits were not different at the conclusion of the experiment on day 12. Serum progesterone concentrations, also determined by radioimmunoassay, were not significantly different between treated and control animals. However, when estradiol implants were removed from other rabbits on day 10, a rapid decline in serum progesterone occurred, from 14.0 plus or minus 2.4 to 2.6 plus or minus 0.8 ng/ml 24 h later. By comparison, serum progesterone concentrations in rabbits with estradiol implants left in place and in untreated rabbits on day 12 were similar (similar to 12 ng/ml). The premature decline in serum progesterone was accompanied by a decrease in the wet weight of corpora lutea. Other experiments revealed: 1) a precipitous fall in serum estradiol to basal values within 2 h after estradiol implants were removed, preceding the decline in serum progesterone by approximately 6 to 10 h; 2) reduced levels of estradiol in ovarian venous blood, but elevated levels of estradiol in peripheral arterial blood of rabbits with estradiol impants. The inability to elevated estradiol to increase serum progesterone or weights of corpora litea suggests that the luteotropic effect is maximal when estradiol is present at physiological concentrations. Following the continuous administration of estradiol, ovarian secretion of estradiol appears diminished and the corpora lutea become dependent upon the exogenous estradiol for luteotropic support. Although the ovaries continue to release measureable quantities of estradiol, this is inmeasurable quantities of estradiol, this is insufficient to prevent regression of corpora lutea when exogenous estradiol is rapidly withdrawn from the circulation.