Acute Dependence of Ovarian Progesterone Output on the Presence of Placentas in 21-Day Pregnant Rabbits1

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Abstract
The role of the fetuses, placentas and uterus in sustaining the endocrine status of preg-nancy beyond the duration of pseudopregnancy has been examined in the rabbit. Systemic plasma concentrations and ovarian release of progesterone and 20±-hydroxypregn-4-en-3-one remained high when placentas were left in situ 24 hr after fetectomy at day 21 of gestation. However, when the placentas also were extirpated, systemic plasma concentrations and ovarian release of progesterone declined within 24 hr, while systemic plasma concentrations and ovarian release of 20±-hy-droxypregn- 4-en-3-one did not decline. Removal of the entire gravid uterus and hence also pos-sible uterine luteolysin, resulted in decreased systemic plasma concentrations and ovarian release of progesterone 24 hr postoperatively, but not at 6 hr. These observations on systemic plasma concentrations and ovarian release of progestins establish that at day 21 of pregnancy in the rabbit, production of progesterone, by the maternal ovary, is acutely dependent upon tropic influences emanating from the placentas. (Endocrinology94: 1438, 1974)

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