Effect of inhibin immunisation using different synthetic peptide fragments of the bovine alpha(c)-subunit on plasma anti-inhibin titres, plasma FSH concentrations and the incidence of multiple ovulation in heifers

In an effort to improve the effectiveness of inhibin immunisation in promoting multiple ovulation in cattle and to clarify the mechanism(s) involved, heifers (n = 5 per group) were immunised against ovalbumin conjugates of different synthetic peptide sequences of the alpha(c)-subunit of bovine inhibin (bI alpha) selected using antigenic prediction methods, Plasma inhibin antibody titre (percentage binding of I-125-labelled M(r) 32 000 native bovine inhibin), plasma follicle-stimulating hormone (FSH) concentration and ovulatory response (number of corpora lutea observed by transrectal ovarian ultrasonography) were recorded over a 16 week period. Heifers immunised against the bI alpha 1-29 and 63-72 peptides (alone or in combination), had relatively high anti-inhibin titres (over 7.5% binding) and showed a significantly (P < 0.05) increased incidence of multiple ovulations (18-65%) compared with ovalbumin-immunised controls. However, immunisation against the 1-16 and 108-123 peptides was relatively ineffective in generating antibodies reactive with native inhibin (less than 7.5% binding) and gave little or no increase in incidence of multiple ovulations (0-10%). Analysis of results for all 33 heifers revealed a significant linear relationship between mean inhibin antibody titre and mean plasma FSH concentration (r = 0.42; P < 0.02) and between mean inhibin :antibody titre and incidence of multiple ovulation (r = 0.89; P < 0.0001). A significant quadratic relationship existed between mean inhibin antibody titre and the mean number of ovulations per cycle (r = 0.88; P < 0.0001). However, partial correlation analysis showed a highly significant association between anti-inhibin titre and ovulatory response which was independent of changes in mean plasma FSH concentrations. These results extend previous studies involving inhibin peptide-immunised cattle by showing that the magnitude of the ovulatory response is directly related to the prevailing titre of antibodies reactive with native inhibin. However, they do not support the hypothesis that the ovulatory response is mediated solely by a rise in FSH secretion.