Sharp increase in free circulating oestrogens immediately before parturition in sheep

Abstract

THE principal oestrogen secreted by the ovary of the non-pregnant ewe is 17β-oestradiol1. After continuous infusion of 3H-17β-oestradiol into both pregnant and non-pregnant sheep, oestrone and 17α-oestradiol have been isolated from blood as the chief unconjugated radiometabolites2,3. In pregnancy, oestrogens are excreted in urine as oestrone and 17α-oestradiol conjugates4, but calculation of blood steroid concentration from these data4 and from the metabolic clearance rate of 17β-oestradiol2 suggested that the circulating concentration of 17β-oestradiol in the pregnant sheep would be extremely small (<20 pg/ml. of plasma). Measurements, therefore, of this steroid in circulating blood of the sheep have not been reported, and the role of oestrogens in late pregnancy and parturition has not been defined clearly. Because a decreasing progesterone block on the myometrial contractile response to oxytocin cannot always account for the onset of parturition 5, it has become increasingly important to establish any significant changes in oestrogen biosynthesis in late pregnancy. I wish to report a rapid ten-fold increase in the maternal circulating oestrogen on the day before parturition in the sheep, which could provide a positive trigger for the onset of labour. © 1971 Nature Publishing Group.