Studies on the metabolism of neutral steroids by preimplantation rabbit blastocysts in vitro and the origin of blastocyst oestrogen

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Abstract
Day-6 rabbit blastocysts were incubated with 3H-labeled neutral steroids. Pregnenolone metabolism was insignificant, but progesterone was estensively metabolized to 5.beta.-pregnanedione (5.beta.-pregnane-3,20-dione) and 5.beta.-pregnanolone (5.beta.-pregnan-3.beta.-ol-20-one). Some dehydroepiandrosterone (DHA) was converted to 5-androstene-3.beta.,17.beta.-diol; androstenedione and testosterone were interconverted and both 4-ene steroids were metabolized to 5.alpha.-reduced compounds. There was no evidence for estrogen synthesis from progesterone or androgens. Exogenous cofactors had no significant effect on the course of steroid metabolism. Estradiol and estrone were determined in blastocysts, and high levels of estrogen were found in blaastocysts from superovulating rabbits. Injection of [3H]estradiol into pregnant rabbits gave rise to radioactivity associated primarily with estradiol in uterine tissue, uterine flushings and blastocysts. The capacity of the day-6 rabbit blastocyst to metabolize progesterone and androgen apparently may be utilized in vivo to protect blastocyst from excess maternal progesterone and to involve androgens in an anabolic role; blastocyst estrogen may be partly of maternal origin.