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Effects of re-immunization of heifers against inhibin on hormonal profiles and ovulation rate

¹ Laboratory of Veterinary Physiology, Department of Veterinary Medicine, Faculty of Agriculture, Tokyo University of Agriculture and Technology, Tokyo 183-8509, Japan, ² Department of Theriogenology, Faculty of Veterinary Medicine, Suez Canal University, Ismailia, Egypt, ³ Reproductive Cell Biology, Department of Animal Breeding and Reproduction, National Institute of Livestock and Grassland Science, Ibaraki 305-0901, Japan, ⁴ Department of Genetic Resources II, National Institute of Agrobiological Resources, Tsukuba, Ibaraki 305, Japan, ⁵ Department of Basic Veterinary Science, The United Graduate School of Veterinary Science, Gifu University, Gifu, Japan and ⁶ Biotech Australia Pty Ltd, PO Box 20, East Roseville, NSW 2460, Australia

Correspondence should be addressed to Kazuyoshi Taya; Email: taya{at}cc.tuat.ac.jp

To study the effect of re-immunization against inhibin on ovarian response and hormonal profiles, Japanese beef heifers (n = 5) were re-immunized three times with inhibin vaccine (recombinant ovine inhibin -subunit in oil emulsion, 125 µg ml^–1) one year after the primary immunization. Control heifers (n = 5) were injected with placebo (Montanide: Marcol adjuvant alone). Oestrous cycles were synchronized by using prostaglandin F₂ (PGF₂) and ovarian response was monitored daily by ultrasonography. Blood samples were collected by jugular venipuncture for assessment of hormonal levels and inhibin antibody titres. In contrast to controls, inhibin re-immunized heifers generated antibodies against inhibin rapidly reaching a peak level 9 days after the first booster injection. The mean concentrations of FSH in re-immunized cows increased significantly in comparison with controls. In addition, there was a significant increase in oestradiol-17ß and progesterone levels in re-immunized cows compared with controls. Inhibin re-immunized heifers had a significant increase in small (4 < 7 mm), medium (7 < 10 mm) and large (10 mm in diameter) sized follicles. Moreover, the mean ovulation rate was 5.0 ± 1.1 after the third booster injection in re-immunized heifers compared with control heifers (single ovulation). These results clearly demonstrate that re-immunization of inhibin can be used to enhance ovarian follicular development and ovulation rate. Furthermore, the great number of follicles is a potential source of oocytes that could be harvested for in vitro fertilization and embryo transfer programmes.