In vitro oestrogen synthesis by bovine placenta during pregnancy and induced parturition

Foetal (cotyledon) placental tissue was collected from 7 pregnant cows and from 8 cows immediately after delivery of the foetus during parturition induced by luteal removal at 250 or 270 days of gestation. These tissues were incubated with [3H]androstenedione (A), [3H]17 alpha-hydroxyprogesterone (17 alpha HP), or [3H]pregnenolone (P5) to examine potential oestrogen precursors in the mid to late pregnant cow. Percentage conversion to unconjugated and conjugated oestrone (Oe1) and oestradiol (Oe2) was measured for each tissue with the various substrates. Unconjugated oestrogens predominated over conjugated (P less than 0.001), and Oe1 over Oe2 (P less than 0.001) in all incubations. There was no difference in conversion of A between pre- and post-partum foetal tissues. There was a significant elevation in conversion of P5 in post-partum foetal tissue (P less than 0.05). These data demonstrate a) placental aromatase activity does not appear to increase as pregnancy progresses towards parturition, b) the bovine placenta is able to convert C21 steroids to oestrogens, and c) induction of enzymes for converting pregnenolone or related substrates to oestrogens may be involved in the pre-partum rise in oestrogen levels.