Effect of GnRH antagonist-induced prolonged follicular phase on follicularatresia and oocyte developmental competence in vitro in superovulated heifers

A GnRH antagonist (Antarelix) was used to suppress endogenous pulsatile sec retion of LH and delay the preovulatory LH surge in superovulated heifers t o study the effect of a prolonged follicular phase on both follicle and ooc yte quality. Oestrous cycles were synchronized in 12 heifers with progestag en (norgestomet) implants for 10 days. On day 4 (day 0 = day of oestrus), h eifers were stimulated with 24 mg pFSH for 4 days and luteolysis was induce d at day 6 with PGF(2 alpha) (2 ml Estrumate). Animals in the control group (n = 4) were killed 24 h after the last FSH injection. At this time, heife rs in group A36h (n = 4) and group A60h (n = 4) were treated with 1.6 mg of Antarelix every 12 h for 36 and 60 h, respectively, and then killed. After dissection of ovarian follicles, oocytes were collected for individual in vitro maturation, fertilization and culture; follicular fluid was collected for determination of steroid concentrations, and granulosa cells were smea red, fixed and stained for evaluation of pycnosis rates. Granulosa cell sme ars showed that 90% of follicles were healthy in the control group. In cont rast, 36 and 58% of the follicles in group A36h showed signs of early or ad vanced atresia, respectively, while 90% of the follicles in group A60h show ed signs of late atresia. Intrafollicular concentrations of oestradiol decr eased (P < 0.0001) from healthy follicles (799.14 +/- 40.65 ng ml(-1)) to l ate atretic follicles (3.96 +/- 0.59 ng ml(-1)). Progesterone concentration s were higher (P < 0.0001) in healthy follicles compared with atretic folli cles, irrespective of degree of atresia. Oestradiol:progesterone ratios dec reased (P < 0.0001) from healthy (4.58 +/- 0.25) to late atretic follicles (0.07 +/- 0.009). The intrafollicular concentrations of oestradiol and prog esterone were significantly higher (P < 0.0001) in the control than in the treated groups. The oestradiol:progesterone ratio was higher (P < 0.0001) i n the control (4.55 +/- 0.25) than in the A36h (0.40 +/- 0.05) and A60h (0. 07 +/- 0.009) groups. Unexpectedly, the cleavage rate of fertilized oocytes , blastocyst rate and number of cells per blastocyst were not significantly different among control (85%, 41% and 95 +/- 8), A36h (86%, 56% and 93 +/- 5) and A60h (88%, 58% and 79 +/- 4) groups. Ln addition, there were no sig nificant differences in the blastocyst rates from oocytes derived from heal thy (45%), early atretic (54%), advanced atretic (57%) and late atretic fol licles (53%). In conclusion, the maintenance of the preovulatory follicles in superovulated heifers with a GnRH antagonist induced more atresia and a decrease in oestradiol and progesterone concentrations. However, the develo pmental potential in vitro to day 8 of the oocytes recovered from these atr etic follicles was not affected.