Effects of passive immunization against inhibin-peptide on secretion of follicle-stimulating hormone and ovulation rate in ewes carrying the Booroola fecundity gene

Objectives of the study were to determine whether 1) inhibin negative feedback regulation of FSH secretion is diminished in ewes carrying a copy of the Booroola fecundity (Fec(B)) gene and 2) differential FSH secretion is obligatory for expression of gene-specific differences in ovulation rate (OR). The approach was to compare FSH and ovulatory responses to passive immunoneutralization of inhibin in ewes with and without a copy of the Fec(B) gene. Twenty-eight 2- to 3-yr-old ewes were assigned within genotype to antibody (alpha-IF-Ab) or control groups. Genotypes consisted of 3/4 Rambouillet x 1/4 Booroola ewes with one copy of the Fec(B) gene (Fec(B)+; 57 kg) and 3/4 Rambouillet x 1/4 Booroola ewes without the Fec(B) gene (++; 59 kg). Estrus was synchronized during the breeding season using progesterone-releasing pessaries (CIDR-G). Pessaries were removed at 0 h. A single injection of alpha-IF-Ab or control solution was given at -48 h. Alpha-IF-Ab had been developed against a synthetic inhibin fragment matching the N-terminal region of ovine inhibin's or subunit. For injection, alpha-IF-Ab had been precipitated from ovine immune sera and concentrated. Blood samples were collected at 6-h intervals from -48 to 48 h, and laparoscopy was performed 14 days after CIDR-G withdrawal. All ewes exhibited estrus and ovulated. Genotype and alpha-IF-Ab treatment were without effect on intervals to estrus. Both factors affected OR (p less than or equal to 0.001). Mean OR in control ++ and Fec(B)+ ewes were 1.6 and 2.7, respectively; mean OR in alpha-IF-Ab-treated ++ and Fec(B) ewes were 2.5 and 4.6, respectively. Following injection of alpha-IF-Ab, FSH concentrations increased within 6 h, peaked 12-18 h later, and then declined. Magnitude of FSH increases was similar in ++ and Fec(B)+ ewes (70% and 85% over control values, respectively). Results demonstrate that 1) inhibin negative feedback regulation of FSH secretion is not a site of Fec(B) gene action and 2) the mechanism by which the Fec(B) gene increases OR does not necessarily involve increased FSH secretion during the period of preovulatory follicular development.