Serum Progesterone Levels in Ovariectomized Rats Injected with Progesterone and Estrone: Relation to Pregnancy Maintenance and Growth of Decidual Tissue1

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Abstract
Serum progesterone levels (ng/ml) were measured by competitive protein binding in ovariectomized pregnant or pseudopregnant rats with or without deciduomata, treated daily with 2 or 4 mg progesterone, with (E/P2 or E/P4) or without (P2 or P4) 1 Mg estrone. Treatment with E/P2 or E/P4 markedly enhanced fetal survival and deciduoma growth over the results obtained with P2 or P4, but serum progesterone levels were not different from those animals receiving progesterone alone. The serum progesterone concentrations were directly related to the dose of progesterone administered, were not significantly affected by duration of treatment, the presence of developing fetuses or decidual tissue, and were significantly lower than the values found in intact pregnant or pseudopregnant rats. These findings plus the lack of effect of pregnancy or deciduoma formation on the metabolic clearance rate of progesterone (Pepe and Rothchild, submitted with this paper) indicated that in the rat, changes in the level of progesterone in the peripheral circulation accurately reflect changes in the rate of its secretion. (Endocrinology93: 1193, 1973)

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