Decrease in Ovarian Platelet-Activating Factor during Ovulation in the Gonadotropin-Primed Immature Rat1

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Abstract
Platelet-activating factor (PAF) is a biologically active phospholipid that is released locally during acute inflammatory reactions and tissue injury. Since there is evidence that the biochemical events of mammalian ovulation resemble an inflammatory reaction, the objective of this study was to determine whether ovarian levels of PAF change during ovulation. At 2-h intervals during the ovulatory process in gonadoptropin-primed 25-day-old Wistar rats, the ovaries were extirpated, homogenized, and extracted for lipids. The extracts were subjected to thin-layer chromatography (TLC), and the portion of the silica gel that comigrated with PAF was re-extracted and assayed for PAF activity. The PAF was measured (in fmole equivalents of synthetic PAF) by a bioassay based on the capacity of aliquots of the extracts to release [3H]-serotonin from platelets isolated from while bood of rabbits and prelabeled with [3H]-serotonin. The ovarian level of PAF decreased (p < 0.01) by 36% from 6.67 .+-. 0.77 to 4.27 .+-. 0.45 fmoles/mg ovary by 2 h after treatment with human chorionic gonadotropin (hCG), and it declined another 14% by 4 h after hCG. The ovarian PAF remained at this reduced level for up to 24 h after hCG. The administration of indomethacin (5 mg/rat, s.c.) or epostane (5 mg/rat, s.c.) at 1 h after hCG prevented ovulation, but neither drug affected the decline in ovarian PAF. Preliminary tests showed that the lipid extracts from the ovaries also contained PAF inhibitor(s) that comigrated with PAF on the TLC plates. Similar to PAF, the lipid-soluble inhibitor(s) decreased (p < 0.05) in the ovaries within 4 h after hCG treatment. In conclusion, ovarian PAF decreased at the beginning of the ovulatory process in rats, and the change in this vasoactive phospholipid provides further evidence that the biochemical events of mammalian ovulation resemble an inflammatory reaction.

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