Low Sensitivity to Oestrogens in Bovine Placenta at Term

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Abstract
To elucidate the importance of elevated oestrogen levels at term for bovine placental maturation, the sensitivity of placental tissue to oestrogens was investigated by estimating oestrogen receptor levels. Unlabelled oestrogen receptors were bound to heparin sepharose in a small column and the gel was washed to remove coenzymes, cosubstrates and non-binding proteins. After subsequent incubation with 3H-oestradiol and removal of unbound steroids, a valid receptor quantitation was possible. In the maternal compartment of the mature bovine placenta only low oestrogen receptor levels (5-14 fmol/mg protein) were detectable, whereas the fetal compartment presumably contains no receptors (less than 1 fmol/mg). Due to the low oestrogen sensitivity of the bovine placenta we conclude that unconjugated oestrogens are of little or no importance for placental maturation. The final oestrogen peak prior to parturition may be a marker for maturation with a questionable physiological importance for the placentome.