A Comparative Study of Serum Progesterone Levels in Pregnancy and in Various Types of Pseudopregnancy in the Rat¹

Abstract

The changes in the peripheral serum levels of progesterone in the pregnant rat werecompared with those found during ordinarypseudopregnancy, the pseudopregnancy of lactation (large litter), and the pseudopregnancies prolonged by decidualization, or by acute or chronic hysterectomy. The level in the ordinary pseudopregnant rat rose steadily to a peak on day 5 and remained at a plateau of about 70 ng/ml until day 9, after which it fell progressively. Neitheracute hysterectomy nor decidualization increased the level significantly above that found in ordinary pseudopregnancy but both treatments extended the plateau to at least day 16. In the pregnant rat, the progesterone level did not differ significantly from that of these types of pseudopregnancy until after day 11, when it rose to a peak of about 130 ng/ml on day 15; a rapid fall began after day 19. In contrast to the above, both the lactating and the chronically hysterectomized rats' progesterone levels rose steeply to values of over 120 ng/ml by day 5, after which the levels began a slow fall which extended beyond day 17. These results were interpreted as suggestive evidence that the degree of progestational activity of the uterus and the level of circulating LH may affect the rate of progesterone secretion.(Endocrinology95: 275, 1974)