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

Abstract

Corpora lutea from pregnant rats were incubated to determine their ability to produce 17β-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. 17β-Estradiol in tissue and medium were quantified by a highly specific radioimmunoassay. The estradiol content invariably increased in non-luteal tissues during incubation, while it decreased or remained the same in incubated corpora lutea. The synthesis of 17β-estradiol by corpora lutea was approximately 4 pg/mg tissue/4 h compared with estradiol synthesis in non-luteal tissues, which was 18 to 400-fold greater. 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 17β-estradiol in medium of 2.5 × 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 [7α-³H]-testosterone resulted in ∼15% conversion to presumptive [7α-³H]17β-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 17β-estradiol in amounts detectable by radioimmunoassay. In conclusion, the synthesis of 17β-estradiol by rat corpora lutea in vitro is virtually negligible compared with the synthesis of 17β-estradiol by non-luteal ovarian tissues. However, the corpora lutea show a striking capacity to aromatize testosterone, which might explain the high estradiol content of rat corpora lutea during pregnancy. The physiological significance of this aromatizing system and of 17β-estradiol in the corpus luteum is unknown but may be related to the luteotropic action of estradiol in the pregnant rat.