Synthesis of 17β-Estradiol by Isolated Ovarian Tissues of the Pregnant Rat: Aromatization in the Corpus Luteum

Corpora lutea from pregnant rats were incubated to determine their ability to produce 17beta-estradiol and to aromatize testosterone in vitro. Corpora lutea and non-luteal ovarian tissues were removed from rats on days 7, 15, and 22 of pregnancy, and these tissues were immediately frozen or incubated separately in medium 199 at 37 C in an atmosphere of 95% O2-5% CO2 for 4 h. 17Beta-Estradiol in tissue and medium were quantified by a highly specific radioimmunoassay. The estradiol content ivnariably increased in non-luteal tissues during incubation, while it decreased or remained the same in incubated corpora lutea. The synthesis in non-luteal tissues, which was 18 to 400-fold greter. The incubation of corpora lutea (5 to 25 mg of tissue) with testosterone (200 ng) on days 7, 15, and 22 of pregnancy resulted in a mean accumulation of 17beta-estradiol in medium of 2.5 x 103 pg/mg tissue, compared with a mean value of 6 pg/mg for luteal tissue removed from the same ovaries and incubated without testosterone. The incubation of corpora lutea from 15-day pregnant rats with (7alpha-3H)-testosterone resulted in 15% conversion to presumptive (7alpha-3H)17beta-estradiol, which was isolated identically to estradiol isolated for radioimmunoassay. Recrystallization to constant specific activity revealed a high degree of radiochemical purity (75%) of the isolated (3H)estradiol. Rat diaphragm muscle and rabbit corpora lutea did not aromatize testosterone to 17beta-estradiol in amounts detectable by radioirpora lutea in vitro is virtually diol by non-luteal ovarian tissues. However,the corpora lutea show a striking capacity to aromatize testosterone, which might explain the high estradiol content of the rat corpora lutea during pregnancy. The physiological significance of this aromatizing system and of 17beta-estradiol in the corpus luteum is unknown but may be related to the luteotropic action of estradiol in the pregnant rat.