EFFECT OF IMMUNIZATION OF RAMS AGAINST BOVINE INHIBIN ALPHA-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 con centrations were determined by immunizing adult rams against bovine in hibin alpha 1-26-Gly-Tyr conjugated to human serum albumin (n = 16) in non-ulcerative Freund's adjuvant and DEAE:dextran (1:1) or adjuvant a lone (n = 16) on days 0 (29 June), 30, 60, 191, 303 and 394. Blood sam ples were collected and bovine inhibin alpha 1-26-Gly-Tyr antibody tit res and serum testosterone concentrations were determined Each month, between days 174 and 417, semen was collected every 30 min to a maximu m of 15 ejaculates over 7 h and scrotal circumference was measured. Ra m fertility was recorded during natural service. FSH, LH and testoster one concentrations and GnRH-induced FSH and LH release were measured i n a subgroup of immunized (n = 5) and control (n = 5) rams at frequent intervals. Antibody, titres were variable among immunized rams (0-46% I-125-labelled bovine inhibin alpha 1-26-Gly-Tyr at 1:1600 serum dilu tion) but mean titres were consistently higher than in control rams (P less than or equal to 0.001). Immunization did not alter the semen vo lume, output or quality of spermatozoa or ram fertility, but increased the mean scrotal circumference (37.6 +/- 0.8 cm versus 34.4 +/- 0.7 c m, P < 0.001). Mean FSH concentrations were higher in immunized rams d uring 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 (r(2) = 0.3, P < 0.05 in J une; and r(2) = 0.8, P < 0.001 in August). Discrete FSH pulses were no t detected. Immunization did not alter mean or basal testosterone or L H concentrations, or LH pulse frequency; LH pulse amplitude was increa sed (1.6 +/- 0.2 ng ml(-1) versus 0.8 +/- 0.2 ng ml(-1), P < 0.02) and was correlated with antibody titres (r(2) = 0.6, P < 0.01). Immunizat ion enhanced GnRH-induced FSH (P < 0.05) but not LH release. In conclu sion, immunization of adult rams against bovine inhibin alpha 1-26 Gly -Tyr increased scrotal circumference, mean FSH concentrations and LH p ulse amplitude, without altering semen characteristics, fertility, mea n LH concentrations, LH pulse frequency or mean testosterone concentra tions.