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Gonadotropin-Induced Expression of Messenger Ribonucleic Acid for Cyclooxygenase-2 and Production of Prostaglandins E and F₂ in Bovine Preovulatory Follicles Are Regulated by the Progesterone Receptor

Department of Biomedical Sciences, College of Veterinary Medicine, Cornell University, Ithaca New York 14853

Address all correspondence and requests for reprints to: Dr. J. E. Fortune, Department of Biomedical Sciences, Cornell University, Ithaca, New York 14853. E-mail: JF11{at}cornell.edu.

Follicular production of prostaglandins (PGs) is essential for ovulation, but the factors mediating gonadotropin-induced secretion of PGE and PGF₂ remain largely unknown. We tested the hypothesis that gonadotropin-induced changes in progesterone and its receptor (PR) mediate the increase in periovulatory PGs. Heifers were treated with PGF₂ and GnRH to induce luteolysis and the LH/FSH surge (ovulation occurs 30 h after GnRH). Because there are two increases in intrafollicular progesterone/PR mRNA during the bovine periovulatory period, we first examined the temporal pattern of PG production by follicles collected at 0, 3.5, 6, 12, 18, and 24 h after GnRH. Although PGs did not increase in the follicular fluid until 24 h after GnRH, acute secretion of PGs by follicle wall (theca + granulosa cells) was initiated by 18 h and had increased manyfold by 24 h after GnRH. In vitro, FSH and LH induced dramatic transient increases in PG production by follicle wall and granulosa, but not theca, cells isolated from preovulatory follicles (0 h after GnRH). PG accumulation peaked on d 2 of culture, mimicking the secretion pattern after a gonadotropin surge in vivo. In cultures of follicle wall and granulosa cells, the PR antagonist mifepristone (MIFE, 1 µM) inhibited LH-induced PG secretion and the progestin medroxyprogesterone acetate (1 or 10 µM), but not the glucocorticoid dexamethasone (1 or 10 µM), overcame the effect of MIFE on PGs. Semiquantitative RT-PCR revealed that MIFE inhibited LH-induced expression of cyclooxygenase-2 mRNA in granulosa cells in vitro. Again, treatment with medroxyprogesterone acetate overcame the effect of MIFE. Together these results provide strong evidence that periovulatory increases in cyclooxygenase-2 mRNA, PGE, and PGF₂ are mediated by gonadotropin-induced increases in progesterone/PR, indicating that in some species there is an important functional relationship between these pathways in the ovulatory cascade.

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