FOLLICLE SIZE-DEPENDENT INDUCTION OF PROSTAGLANDIN G H SYNTHASE-2 DURING SUPEROVULATION IN CATTLE/

Under physiological conditions, prostaglandin G/H synthase-2 (PGHS-2) is induced in bovine preovulatory follicles by the endogenous surge of gonadotropins. To characterize the pattern of follicular PGHS-2 expre ssion during superovulation in cattle, heifers were treated with exoge nous FSH and ovulation was induced with hCG. Animals were ovariectomiz ed 0, 18, and 24 h post-hCG, and extracts of follicles greater than or equal to 6 mm were analyzed by Western blotting. Follicular fluid con centrations of prostaglandin (PG) E-2, PGF(2 alpha), progesterone, and estradiol-17 beta were determined by RIAs, and the morphology of the cumulus oocyte complex was examined. Results showed that PGHS-2 protei n was absent in all follicles isolated at 0 h post-hCG (n = 119) and i n small follicles (6 to < 8 mm) isolated between 0 and 24 h post-hCG ( n = 27 follicles). In contrast, 12.3% of medium (8 to < 10 mm) and 43. 7% of large (greater than or equal to 10 mm) follicles were PGHS-2-pos itive at 18 h post-hCG, and these percentages rose at 24 h to 45.9% an d 91.0% in medium and large follicles, respectively (p < 0.05). Follic ular fluid concentrations of PGE(2) and PGF(2 alpha) were low in folli cles isolated at 0 h and increased only in PGHS-2-positive follicles i solated 24 h post-hCG (p < 0.05), Concentrations of progesterone and e stradiol-17 beta at 0 h were 28.2 +/- 5.8 and 291.8 +/- 13.0 ng/ml, re spectively, and a shift from estradiol-17 beta to progesterone dominan ce (luteinization) occurred at 24 h post-hCG only in PGHS-2-positive f ollicles. Also, expansion of the cumulus oocyte complex was detected a t 24 h post-hCG only in PGHS-2-positive follicles. Lack of PGHS-2 indu ction in follicles of ovulatory size (> 8 mm) was associated with an a pparent failure to respond to hCG (absence of luteinization and cumulu s expansion). Collectively, these results demonstrate the presence of a time-and follicle size-dependent induction of PGHS-2 in bovine folli cles during superovulatory treatment and suggest that PGHS-2 expressio n can be used as a marker for follicular commitment to ovulation durin g ovarian hyperstimulation protocols.