PROGESTERONE IN BLOOD

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Abstract
SUMMARY: Progesterone has been identified in the placenta of intact and ovariectomized ewes in concentrations of 4–9 μg/kg tissue. In addition to progesterone, a closely related metabolite, 20-αhydroxypregn-4-en-3-one, has also been found, in concentrations of 9–12 μg/kg tissue. The concentrations of both these steroids were highest in the placenta from ovariectomized animals, suggesting that the placenta might be able to compensate for the removal of the ovaries. Progesterone and 20α-hydroxypregn-4-en-3-one levels were also measured in the peripheral blood of pregnant, non-pregnant and post-partum ewes; the mean concentrations of both hormones during pregnancy were 0·41 and 0·37 μg/100 ml. plasma respectively. These levels were not significantly higher than those found in non-pregnant animals, and there was no significant difference (progesterone: P > 0·02; 20α-hydroxypregn-4-en-3-one: P > 0·05) between the concentrations in ewes with single and multiple pregnancies. The ratio of the two hormones did not appear to change during the course of pregnancy, and there was no sign of a fall in the blood levels prior to parturition. After birth, however, the levels seemed to fall rapidly. It is concluded that the sheep placenta is capable of producing small amounts of progesterone, and that some homeostatic mechanism exists for the maintenance of a relatively constant blood progesterone level throughout pregnancy. However, it is still not possible to assess the relative importance of the adrenals and the placenta as extra-ovarian sources of progesterone in the ewe.