Contrasting effects of oestradiol-17 and human chorionic gonadotrophin on steroidogenesis in the rabbit corpus luteum

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Abstract
On Day 10 of pseudopregnancy, rabbits were given an i.v. injection of hCG [human chorionic gonadotropin] (10-20 IU) that was sufficient to cause new ovulations and the loss of follicular estradiol secretion. There was an immediate 3- to 4-fold rise in serum progesterone which returned to near prestimulation values (.apprx. 27 ng/ml) within 12 h in the presence of an implant containing estradiol-17.beta.. In the absence of estradiol, serum progesterone continued to decline to reach low values (.apprx. 4 ng/ml) within 24 h and the original corpora lutea subsequently regressed. The administration of estradiol 24 h after injection of hCG, when progesterone secretion was low, arrested any further decline in progesterone and then restored serum progesterone to normal values. This steroidogenic effect of estradiol in vivo was a function of enhanced luteal steroidogenesis; corpora lutea removed and incubated for 12 h produced progesterone at high, linear rates, whereas the corpora lutea from animals that did not receive estradiol produced low or insignificant quantities of progesterone in vitro. hCG at these doses apparently is compatible with continued responsiveness of the corpora lutea to estrogen; hCG produces in luteolytic effect primarily by ovulating follicles, thus stopping the secretion of the luteotrophic hormone, estradiol.