ENDOCRINE AND TESTICULAR RESPONSES OF PREPUBERTAL ZEBU CROSSBRED BULLS TO ACTIVE IMMUNIZATION AGAINST TESTOSTERONE

Sixteen prepubertal Zebu crossbred bulls, aged 378+/-4.7 days, weighin g 189+/-5.3 kg and with a scrotal circumference (SC) of 18.6+/-0.6 cm were used to test the effect of active immunization against testostero ne (T) on SC, liveweight change and luteinizing hormone (LH) and T sec retion, Bulls were randomly allocated to receive one of four treatment s: control (C), no treatment; immunization with 10 mg ovalbumin (OVA); immunization with 5 (T5); or, 10 (T10) mg of a T-OVA conjugate, Antib odies to T were titrated weekly, and expressed as percentage binding o f added [H-3]T at a final serum dilution of 1:700. Concentrations of L H and T were measured in serum samples taken every 10 min for 6 h in W eeks 0, 8 and 12 of the study. Maximal titers obtained for Groups T5 a nd T10, between Weeks 11 and 12 were 76 and 57% (SEM = 4%), respective ly, Bulls in Group T5 had greater SC than those in Groups C and OVA bu t only between Weeks 10 and 16 (P < 0.05), Bulls of Group C had the lo west mean concentrations of LH, Pulse characteristics of LH were simil ar among groups, but several animals immunized against T exhibited LH pulses of greater amplitude. Concentrations of T at Week 0 were below 0.23 ng ml(-1) for bulls in all groups, but at Week 8, T was greater f or bulls in Group T5 (9.68 ng ml(-1); P < 0.01). Similarly, at Week 12 , T for T5 bulls was 27.5 ng ml(-1), compared with 4.9 ng ml(-1) for G roup T10 and less than 0.18 ng ml(-1) for groups control and OVA (P < 0.01). Active immunization against T in prepubertal Zebu crossbred bul ls increased SC, without affecting liveweight change and induced highe r serum concentrations of T, probably due to an increase in steroidoge nesis of Leydig cells in response to stimulation of LH.