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This study investigated the sequence of hormonal changes within the preovulatory follicles of mares. Mares were injected i.v. with 2500 IU human chorionic gonadotrophin (hCG) when a preovulatory follicle of 35 mm in diameter was detected. Fluid was aspirated from preovulatory follicles before (0 h), and 12, 24 and 36 h after administration of hCG. Concentrations of progesterone, prostaglandin (PG) E2, PGF, 6-keto-PGF1
and thromboxane B2 in follicular fluid increased significantly (P<0·01) between 0 and 36 h. At 36 h, PGE2 was present in highest concentrations, followed by PGF and 6-keto-PGF1; thromboxane B2 was present at lower concentrations than other prostanoids. Concentrations of 13,14-dihydro-15-keto-PGF2 increased significantly (P<0·05) between 24 and 36 h. Leukotriene B4, leukotriene C4 and histamine were present in follicular fluid at all sampling periods and did not change significantly over time.
In another experiment, buffered saline or indomethacin (either 100 or 500 µg) was injected into preovulatory follicles on the day that they reached 35 mm in diameter to determine whether blocking intrafollicular PG synthesis would affect ovulation. The interval between intrafollicular injection and ultrasonographic detection of luteinization was significantly longer (P<0·05) in mares treated with 500 µg indomethacin. Plasma progesterone concentrations were significantly (P<0·05) lower in indomethacin-treated mares than in control mares on the first 5 days after injection.
These results indicate that intrafollicular concentrations of PGs increase significantly before ovulation in mares and may be involved in the ovulatory process.
Journal of Endocrinology (1991) 129, 131–139
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