THE LATE INDUCTION OF PROSTAGLANDIN G H SYNTHASE-2 IN EQUINE PREOVULATORY FOLLICLES SUPPORTS ITS ROLE AS A DETERMINANT OF THE OVULATORY PROCESS/

PGs are important mediators of the ovulatory process and prostaglandin G/H synthase-2 (PGHS-2) is a key rate-limiting enzyme in the PG biosy nthetic pathway. To determine whether PGHS-2 is regulated in equine fo llicles before ovulation and, if so, to characterize its time course o f induction, preovulatory follicles were isolated during estrus, 0, 12 , 24, 30, 33, 36, and 39 h after an ovulatory dose of hCG (n = 5 folli cles/time point). Cellular extracts were obtained from preparations of follicle wall (theca interna with attached granulosa cells), isolated granulosa cells, and theca interna and were analyzed by Western blot using specific anti-PGHS antibodies. Immunohistochemistry was used to characterize the in situ localization of PGHS-2 protein in preovulator y follicles, and follicular fluid concentrations of PGE(2) and PGF wer e determined. The results showed the induction of PGHS-2, but not PGHS -1, in equine follicles before ovulation. The PGHS-2 protein (72,000 m ol wt) was undetectable 0, 12, and 24 h post-hCG, first became apparen t at 30 h, and reached maximal levels 39 h after hCG treatment. The in duction of follicular PGHS-2 was localized exclusively in granulosa ce lls, and a pronounced staining was observed in the perinuclear region. Follicular fluid concentrations of PGE(2) and PGF were low and not di fferent between 0-33 h, but levels were increased at 36 and 39 h post- hCG (P < 0.01). Thus, the time course of PGHS-2 induction in equine fo llicles (30 h post-hCG) is clearly distinct from those previously obse rved in rat (4 h post-hCG) and bovine (18 h post-hCG) preovulatory fol licles. Interestingly, in all three species, the interval from PGHS-2 induction to follicular rupture is highly conserved (similar to 10 h). Therefore, the progressively delayed expression of PGHS-2 in species with longer ovulatory processes supports its role as a molecular deter minant of the species-specific length of the ovulatory process.