Effect of Hypophysectomy, Prolactin, and Prostaglandin F2αon Gonadotropin Binding in Vivo and in Vitro in the Corpus Luteum*

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Abstract
The present studies focus on the mechanism of the early action of prostaglandin F2.alpha. (PGF2.alpha.) on the corpus luteum. In pseudopregnant rats with an intact pituitary, PGF2.alpha. produced a significant decrease in accumulation of [125I]iodo-CG [chorionic gonadotropin] by corpora lutea (-40%; P < 0.01) and plasma progesterone (-56%; P < 0.05) within 2 h. These effects of PGF2.alpha. were prevented in animals treated with PRL [prolactin] 6 h before PGF2.alpha. injection. Luteal membrane-binding capacity for LH [luteinizing hormone] was unchanged within 2 h of PGF2.alpha. treatment. Hypophysectomy markedly reduced luteal membrane-binding capacity for LH, accumulation of [125I]iodo-CG by corpora lutea in vivo, and plasma progesterone within 48 h; these effects were prevented by PRL replacement injections. In hypophysectomized control animals, PGF2.alpha. did not significantly change [125I]iodo-CG and [125]iodo-PRL accumulation by corpora lutea in vivo, or plasma progesterone levels. In hypophysectomized PRL-treated animals, PGF2.alpha. caused a significant decrease in both [125I]iodo-CG (-69%; P < 0.001) and [125I]iodo-PRL (-38%; P < 0.05) accumulation by corpora lutea as well as a significant decrease in the concentration of plasma progesterone (-60%; P < 0.01). It seems that the rapid drop in plasma progesterone levels produced by PGF2.alpha. was associated with a decrease in labeled gonadotropin uptake by corpora lutea but not from a loss of gonadotropin receptors.