Effect of immunization against synthetic peptide sequences of bovine inhibin α-subunit on gonadotrophin concentrations in heifers

Three peptide sequences from the bovine inhibin (bI) α_c- subunit (P1: bI-α_c-Tyr¹⁶Gly¹⁷(18-30); P2: bI-α_c(63-72)Gly⁷³Tyr⁷⁴; P3: bI-α_cCys¹⁰⁵Gly¹⁰⁶(107-122)) were synthesized and conjugated to human serum albumin (HSA). Hereford-cross heifers (n=5 per group) were injected subcutaneously into four sites in the brisket with 3 mg of one of the peptide conjugates at primary. Control heifers (n= 5) were injected with HSA only. A total of three booster injections consisting of 1.5 mg of conjugate were given at intervals of 11 weeks. Following the second boost, four heifers from each group were blood sampled once every hour around oestrus and again during the mid-luteal phase. Four time periods relative to the peak of the luteinizing hormone (LH) surge (0 h) were analysed: (1) presurge (-16 to - 5 h) ; (2) during the surge (- 4 to 4 h); (3) after the surge (5 to 16 h) ; (4) mid-luteal phase (Days 10-12). Follicle stimulating hormone (FSH) concentration was increased in immunized heifers by up to 94% above control values; however, these increases were not statistically significant (P>0.05). There was a positive correlation (r=0.79, d.f. = 5, P < 0.05), independent of treatment, between mean FSH concentration during Period 1 and inhibin binding in immunized heifers. LH concentration was also increased in immunized heifers by up to 83% above control values; however, these increases were not statistically significant (P > 0.05). This study demonstrates that the increase in FSH recorded during Period 1 in inhibin peptide immunized heifers is associated with inhibin binding at this time and that the increase in ovulation rate in this study is probably a result of immunoneutralization of the negative feedback effect of inhibin on FSH secretion.