Effect of immunization against synthetic peptide sequences of the α(N)-subunit of bovine inhibin on ovulation rate, gonadotrophin concentrations and fertility in heifers

The effects of immunizing cattle against either of two peptides from the amino terminal peptide (α(N)) of the α_4.3-subunit of bovine inhibin on ovulation rate, gonadotrophin concentration and fertility were investigated. Two peptide sequences from the α_4.3-subunit of bovine inhibin (P1N, bIα-(8-20) and P2N, bIα-(153-167)) were synthesized and conjugated to human serum albumin (HSA). Hereford-cross heifers (n = 5 per group) were given an initial injection of 3 mg of one of the peptide conjugates, followed by three booster injections (1.5 mg) at intervals of 11 weeks. Control heifers (n = 5) were injected with HSA only. Blood samples were taken once a week to measure antibody titre and every hour at about the time of the first oestrus and during the mid-luteal phase after the second booster injection, to measure FSH and LH concentrations. Ovulation rate was measured by ultrasonography. Gonadotrophin concentrations were analysed for four periods relative to the peak (time = 0 h) of the preovulatory LH surge as follows: pre-surge: -16 to -5 h; surge: -4 to 4 h; post-surge: 5 to 16 h and a period of 12 h during the mid-luteal (days 10-12) phase. Antibodies that bound to the individual peptides were generated and the ovulation rate increased (P < 0.05) in immunized heifers. Control heifers had one ovulation at all ovulatory cycles monitored. In group P1N, one heifer had two ovulations at each of the six cycles monitored, while another heifer had two ovulations at one cycle. Three heifers in group P2N had more than one ovulation; one heifer had a superovulation response on seven occasions and another heifer on three occasions while the third heifer had two ovulations at one cycle. The superovulation responses ranged from three to eight ovulations. There was no evidence of a significant correlation between peptide antibody titre, ovulation rate and FSH at any of the periods studied. After mating following the third booster injection, nine of the ten immunized and all five control heifers calved. One heifer from group P1N and two heifers from group P2N gave birth to twin calves. These data show that in cattle immunization against the α_4.3-subunit of bovine inhibin significantly disrupts the mechanism(s) controlling ovulation rate but does not impair fertility.