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

Abstract

After synchronization of oestrus, normally cyclic heifers (n = 31) received 2500 iu pregnant mares' serum gonadotrophin (PMSG) i.m. and had a progesterone-releasing intravaginal device (PRID) without the oestradiol capsule inserted on day 10 of the oestrous cycle and received 15 mg prostaglandin (PG) i.m. 48 h later. PRIDs were removed 96 h after insertion and 16 heifers received 1.0 mg GnRH i.m. while the controls (n = 15) received 10 ml saline i.m. All heifers were injected with anti-PMSG i.v. 10 h later. Peripheral blood concentrations of PMSG, progesterone, oestradiol and LH were compared. Ovaries were collected on death 7 days after the GnRH or saline injection and the number of corpora lutea counted. Heifers were considered to have responded well ( > 60 pmol l-1) or poorly ( < 60 pmol l-1) to superovulation on the basis of the oestradiol concentration 24 h after PG administration. During PRID treatment, LH concentrations remained at basal values. In the heifers treated with GnRH, a single LH surge occurred 2.3 ± 0.1 h (SD) after the GnRH injection with a maximum concentration of 14.6 ± 2.3 (SEM) μg l-1 and a duration of 6-8 h. In 12 of the 15 control heifers, LH concentrations remained low (range 0.10-1.94 μg l-1) during the 72 h following the saline injection; three controls showed a spontaneous LH surge at 18, 23 and 23 h after the saline injection, respectively, with a maximum concentration of 6.0-12.5 μg l-1 and a duration of 10-12 h. The oestradiol concentration increased continuously during PRID treatment until the injection of GnRH or saline, when it was four times higher in the heifers that responded well than in the heifers that responded poorly. It decreased sharply 6 h after GnRH indicating that the follicles still responded normally to a preovulatory LH signal, whereas in control heifers a similar decrease took place 4 h later following anti-PMSG treatment. In the GnRH-treated heifers, the heifers responding well showed a significantly higher number of corpora lutea 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 heifers without an immediate LH surge showed a single corpus luteum, and seven heifers did not have a corpus luteum. For the three controls with an LH surge, 33.0 ± 8.5 corpora lutea were observed. In conclusion, the preovulatory LH signal can be effectively postponed in PMSG/PG-superovulated heifers using a PRID. However, the PRID treatment has to be followed by GnRH to obtain the LH surge at a defined time of preovulatory follicular development. Follicular function with regard to oestradiol secretion and the potential to ovulate remains unchanged.