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Effect of immunization of rams against bovine inhibin 1–26 on semen characteristics, scrotal size, FSH, LH and testosterone concentrations

The effects of inhibin immunization on inhibin antibody titres, semen characteristics, scrotal size, fertility, FSH, LH and testosterone concentrations were determined by immunizing adult rams against bovine inhibin 1–26-Gly-Tyr conjugated to human serum albumin (n = 16) in non-ulcerative Freund's adjuvant and DEAE:dextran (1:1) or adjuvant alone (n = 16) on days 0 (29 June), 30, 60, 191, 303 and 394. Blood samples were collected and bovine inhibin 1–26-Gly-Tyr antibody titres and serum testosterone concentrations were determined. Each month, between days 174 and 417, semen was collected every 30 min to a maximum of 15 ejaculates over 7 h and scrotal circumference was measured. Ram fertility was recorded during natural service. FSH, LH and testosterone concentrations and GnRH-induced FSH and LH release were measured in a subgroup of immunized (n = 5) and control (n = 5) rams at frequent intervals. Antibody titres were variable among immunized rams (0–46% I-labelled bovine inhibin 1–26-Gly-Tyr at 1:1600 serum dilution) but mean titres were consistently higher than in control rams (P 0.001). Immunization did not alter the semen volume, output or quality of spermatozoa or ram fertility, but increased the mean scrotal circumference (37.6 ±0.8 cm versus 34.4 ± 0.7 cm, P < 0.001). Mean FSH concentrations were higher in immunized rams during two intensive blood sampling periods (in June and August) (5.8 ± 0.7 ng ml^–1 versus 3.0 ± 0.3 ng ml^–1, P < 0.001 in June; and 4.8 ± 0.9 ng ml^–1 versus 2.0 ± 0.3 ng ml^–1, P < 0.02 in August), and were correlated with antibody titres (r² = 0.3, P < 0.05 in June; and r² = 0.8, P < 0.001 in August). Discrete FSH pulses were not detected. Immunization did not alter mean or basal testosterone or LH concentrations, or LH pulse frequency; LH pulse amplitude was increased (1.6 ± 0.2 ng ml^–1 versus 0.8 ± 0.2 ng ml^–1, P < 0.02) and was correlated with antibody titres (r²=0.6, P < 0.01). Immunization enhanced GnRH-induced FSH (P <0.05) but not LH release. In conclusion, immunization of adult rams against bovine inhibin 1–26 Gly-Tyr increased scrotal circumference, mean FSH concentrations and LH pulse amplitude, without altering semen characteristics, fertility, mean LH concentrations, LH pulse frequency or mean testosterone concentrations.

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