Gonadotrophin-releasing hormone and the control of ovulation rate by the Fec(B) gene in Booroola ewes

Booroola sheep carry a Fec(B) gene that confers high fecundity. The aims of the present studies were to determine in homozygous carriers (BB) and non-carriers (++) of the Booroola Fec(B) gene whether there are Fec(B) differences in the secretory characteristics of GnRH in hypophyseal-portal blood of ovariectomized ewes (Expt 1) and whether differences in ovulation rate would occur following the administration of PMSG and pulsatile GnRH to ovary-intact Booroola ewes with hypothalamic-pituitary disconnection (HPD) (Expt 2). In Expt 1, no Fec(B) gene-specific differences were noted for GnRH with respect to pulse frequency, pulse amplitude or overall secretion rate. Irrespective of genotype there were 1.9 ± 0.2 GnRH pulses h^-1 (n = 20 ewes; 10 BB and 10 ++ animals) and 1.9 ± 0.1 pulses of immunoreactive (i) LH h^-1. In Expt 2, ovulation was induced in the HPD ovary-intact animals on three occasions (e.g. 56, 393 and 423 days after HPD surgery) using a PMSG (200 or 400 iu)-GnRH pulse (250 ng i.v. for 96 h)-GnRH bolus (10 μg i.v.) regimen after pretreating the animals with GnRH pulses (250 ng i.v. for 14 days). On all occasions the ovulation rates were significantly higher (P < 0.05) in BB ewes (n = 6 or 7) than in ++ animals (n = 7). No differences between the genotypes were noted with respect to the mean concentrations of progesterone in plasma notwithstanding the differences in ovulation rates. No ovulations occurred after treating the HPD ovary-intact animals with GnRH pulses alone, a GnRH bolus alone or PMSG followed 96 h later by a GnRH bolus without the intervening GnRH pulses. During the GnRH pretreatment or ovulation-induction period Fec(B) differences were sometimes noted in the plasma concentrations of either i-FSH or bioactive FSH but not for i-LH (BB > ++; P < 0.05). These results suggest that there are unlikely to be Fec(B) differences with respect to GnRH secretion in ovariectomized BB and ++ ewes. Moreover they show that the Fec(B) differences in ovulation rate remain when an identical ovulation-induction regimen was given to HPD-ovary-intact BB and ++ ewes. However, it is uncertain whether the ovulation rate differences were due to Fec(B) differences in plasma FSH or to intraovarian factors.