01817naa a2200193 a 450000100080000000500110000800800410001910000220006024501140008226000090019652012890020565000170149465000100151165000190152165000170154065000100155765000170156777300390158415238032023-11-14       1971    bl uuuu     u00u1 u    #d1 aCHALLIS, J. R. G.  aSharp increase in free circulating oestrogens immediately before parturition in sheep.h[electronic resource]  c1971  aAbstract: THE principal oestrogen secreted by the ovary of the non-pregnant ewe is 17?-oestradiol1. After continuous infusion of 3H-17?-oestradiol into both pregnant and non-pregnant sheep, oestrone and 17?-oestradiol have been isolated from blood as the chief unconjugated radiometabolites2,3. In pregnancy, oestrogens are excreted in urine as oestrone and 17?-oestradiol conjugates4, but calculation of blood steroid concentration from these data4 and from the metabolic clearance rate of 17?-oestradiol2 suggested that the circulating concentration of 17?-oestradiol in the pregnant sheep would be extremely small (&lt20 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.  aReproduction  aSheep  aEndocrinologia  aObstetrícia  aOvino  aReprodução  tNaturegv. 229, p. 208, Jan. 1971.