Reversal of indomethacin blockade of ovulation in gilts by prostaglandins.

Prepubertal gilts given 750 IU pregnant mares' serum gonadotropin (PMSG) followed 72 h later by 500 IU human chorionic gonadotropin (hCG) to induce follicular growth and ovulation fail to ovulate when 10 mg/kg indomethacin (INDO) is injected 24 h after hCG administration. This study examines the effects of administration of exogenous prostaglandins F2 alpha and E2 (PGF2 alpha and PGE2) alone or in combination, and at various times prior to the expected time of ovulation, on the INDO blockade of ovulation in PMSG/hCG-treated gilts. Occurrence of ovulation was determined by visual observation at laparotomy 48 h after hCG. When 5 mg or 10 mg PGF2 alpha was injected at each of 38, 40 and 42 h after hCG injection, 63% and 79%, respectively, of preovulatory follicles ovulated. In contrast, injection of 5 mg PGE2 or 5 mg PGE2 plus 5 mg PGF2 alpha induced ovulation in 0% and 24% of preovulatory follicles, respectively. In control groups, 100% of follicles in PMSG/hCG-treated gilts ovulated whereas none did so in PMSG/hCG/INDO-treated animals. These results indicate that administration of PGF2 alpha can induce ovulation in the PMSG/hCG/INDO-treated prepubertal gilt and suggest that PGE2 is ineffective and may be antagonistic to PGF2 alpha in overcoming the ovulation blocking effect of INDO.