The effects of i.m. injections of 25 mg prostaglandin F2α (PGF2α) on peripheral progesterone concentrations, luteal progesterone concentrations and in vitro progesterone synthesis were determined. Heifers with a history of normal estrous cycles were treated with PGF2α on Day 12 or 13 and corpora lutea were surgically removed 0, 2, 4 or 8 h later (5 animals/time period). Corpora lutea were sliced and aliquots representing 100–200 mg tissue were removed to determine tissue concentrations of progesterone. Dispersed cell preparations were prepared from remaining tissue. Three ml of cells (1 × 106 cells/ml) were incubated for 1 h with 0, 0.1, 1 or 10 ng luteinizing hormone (LH)/ml and 0, 10, 100 or 1000 ng PGF2α/ml in a 4 × 4 factorial design. Plasma concentrations of progesterone were decreased (P<0.05) within 2 h of injection of PGF2α. The progesterone concentration of corpora lutes removed at 0 h was 65 ± 7 μg/g tissue (mean ± SEM) and was greater (P<0.01) than that observed at 2, 4 or 8 h (39 ± 3, 33 ± 3 and 33 ± 2 μg/g, respectively). In the absence of LH or PGF2α in vitro, progesterone synthesis by corpora lutea removed at 0, 2 or 4 h postinjection was similar and was approximately twice that observed by corpora lutea removed at 8 h. Nevertheless, LH increased (P<0.01) progesterone synthesis by corpora lutea removed at all time periods. PGF2α also increased (P<0.01) net progesterone accumulation. However, the response of corpora lutea to PGF2α removed at 8 h was less than that observed at 0, 2 or 4 h indicating a corpus luteum removal time by PGF2α interaction (P<0.01). These data suggest that in vivo administration of PGF2α had effects on plasma and tissue progesterone concentrations before diminution of the in vitro steroidogenic response of corpora lutea to LH or PGF2α.
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Weston et al. (1980) studied this question.