EFFECTS OF NEUTRALIZATION OF PREGNANT MARES SERUM GONADOTROPIN (PMSG)SHORTLY BEFORE OR AT THE PREOVULATORY LH SURGE IN PMSG-SUPEROVULATED HEIFERS ON FOLLICULAR FUNCTION AND DEVELOPMENT

Normally cyclic heifers (n = 34) received 2500 iu pregnant mares' seru m gonadotrophin (PMSG) i.m. at day 10 of oestrus, and 15 mg prostaglan din (PG) i.m. at day 12. Thereafter, a monoclonal antibody against PMS G was administered i.v. before (n = 24), at (n = 6) or shortly after ( n = 4) the preovulatory LH surge. Peripheral blood concentrations of L H and oestradiol were compared; follicular development was monitored b y daily ultrasound scanning; and the numbers of preovulatory-sized fol licles and ovulations were counted 96 h after injection of pc followin g death. Anti-PMSG treatment before the LH surge inhibited the LH surg e in 16 heifers (67%). In these heifers, the initial increase in oestr adiol concentration upon PMSG stimulation to 167.5 +/- 35.0 pmol l(-1) was terminated immediately after anti-PMSG treatment and decreased ra pidly to basal values, while the number of preovulatory-sized follicle s remained constant until 68 h after PG injection; on average 0.4 +/- 0.1 ovulations were counted. In the remaining eight heifers, five anim als showed an immediate, but temporary, 20-60% drop in oestradiol conc entration after anti-PMSG treatment. In all eight heifers 25% of the p reovulatory-sized follicles ovulated. Treatment with anti-PMSG at or s hortly after the LH surge did not affect the pattern of oestradiol con centration, but a significantly higher ovulation rate was observed in the animals treated shortly after the LH surge: 20.3 +/- 2.6 versus 6. 3 +/- 2.3 in animals treated at the LH surge, which corresponded to 76 % and 24% of the preovulatory-sized follicles monitored shortly before the period of multiple ovulations. Thus, neutralization of PMSG at an y time before the maximum concentration of the preovulatory LH surge s everely suppresses the functionality of the majority or all of the sti mulated follicles, which is dependent on the time of injection of anti -PMSG. Although the follicles retain their size, they lose the potenti al to ovulate. It is concluded that interfollicular asynchrony of deve lopment is present at the time the LH surge occurs.