EFFECTS OF SUPPRESSION BY A PROGESTERONE-RELEASING INTRAVAGINAL DEVICE AND SUBSEQUENT INDUCTION BY GNRH OF THE PREOVULATORY LH SURGE ON FOLLICULAR FUNCTION IN PMSG PG-TREATED HEIFERS

After synchronization of oestrus, normally cyclic heifers (n = 31) rec eived 2500 iu pregnant mares' serum gonadotrophin (PMSG) i.m. and had a progesterone-releasing intravaginal device (PRID) without the oestra diol capsule inserted on day 10 of the oestrous cycle and received 15 mg prostaglandin (PG) i.m. 4sh later. PRIDs were removed 96 h after in sertion and 16 heifers received 1.0 mg GnRH i.m. while the controls (n = 15) received 10 mi saline i.m. All heifers were injected with anti- PMSG i.v. 10 h later. Peripheral blood concentrations of PMSG, progest erone, oestradiol and LH were compared. Ovaries were collected on deat h 7 days after the GnRH or saline injection and the number of corpora lutea counted. Heifers were considered to have responded well (> 60 pm ol l(-1)) or poorly (< 60 pmol l(-1)) to superovulation on the basis o f the oestradiol concentration 24 h after PG administration. During PR ID treatment, LH concentrations remained at basal values. In the heife rs treated with GnRH, a single LH surge occurred 2.3 +/- 0.1 h (SD) af ter the GnRH injection with a maximum concentration of 14.6 +/- 2.3 (S EM) mu g l(-1) and a duration of 6-8 h. In 12 of the 15 control heifer s, LH concentrations remained low (range 0.10-1.94 mu g l(-1)) during the 72h following the saline injection; three controls showed a sponta neous LH surge at 18, 23 and 23 h after the saline injection, respecti vely, with a maximum concentration of 6.0-12.5 mu g l(-1) and a durati on of 10-12 h. The oestradiol concentration increased continuously dur ing PRID treatment until the injection of GnRH or saline, when it was four times higher in the heifers that responded well than in the heife rs that responded poorly. It decreased sharply 6 h after GnRH indicati ng that the follicles still responded normally to a preovulatory LH si gnal, whereas in control heifers a similar decrease took place 4 h lat er following anti-PMSG treatment. In the GnRH-treated heifers, the hei fers responding well showed a significantly higher number of corpora l utea than did the animals showing a poor response, 16.4 +/- 2.2 (n = 9 ) and 5.4 +/- 1.4 (n = 7), respectively. Five of the 12 control heifer s without an immediate LH surge showed a single corpus luteum, and sev en heifers did not have a corpus luteum. For the three controls with a n LH surge, 33.0 +/- 8.5 corpora lutea were observed. In conclusion, t he preovulatory LH signal can be effectively postponed in PMSG/PG-supe rovulated heifers using a PRID. However, the PRID treatment has to be followed by GnRH to obtain the LH surge at a defined time of preovulat ory follicular development. Follicular function with regard to oestrad iol secretion and the potential to ovulate remains unchanged.