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An improved model of the distribution and metabolism of progesterone and 20-dihydroprogesterone in sheep

Summary. When [³H]progesterone is infused intravenously into ewes, blood 20-dihydroprogesterone (20-diHP) becomes labelled and the changes in [³H]20-diHP activity with time are clearly related to that of [³H]progesterone. Concentrations of 20-diHP in blood have now been estimated for these experiments. During the infusion, the specific radioactivity of 20-diHP at steady state was only 53% of the specific radioactivity of progesterone, indicating that 20-diHP was produced other than by C-20 reduction of secreted progesterone. The change in blood concentration of 20-diHP during pregnancy in ewes suggests that the placenta is its source.

[³H]20-dihydroprogesterone was infused into pregnant ewes and the specific radioactivity of 20-diHP measured during and after infusion. Together with information from earlier experiments when [³H]progesterone was infused, there is now sufficient data to estimate, without constraint, the parameters of a four-pool model describing the distribution and metabolism of progesterone and 20-dyhydro-progesterone in sheep.