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 radiometabolites^2,3. 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 parturition⁵, 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.