Endocrine and testicular responses of prepubertal Zebu crossbred bulls to active immunization against testosterone

Sixteen prepubertal Zebu crossbred bulls, aged 378 ± 4.7 days, weighing 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 testosterone (T) on SC, liveweight change and luteinizing hormone (LH) and T secretion. Bulls were randomly allocated to receive one of four treatments: control (C), no treatment; immunization with 10 mg ovalbumin (OVA); immunization with 5 (T5); or, 10 (T10) mg of a T-OVA conjugate. Antibodies to T were titrated weekly, and expressed as percentage binding of added [3H]T at a final serum dilution of 1:700. Concentrations of LH and T were measured in serum samples taken every 10 min for 6 h in Weeks 0, 8 and 12 of the study. Maximal titers obtained for Groups T5 and T10, between Weeks 11 and 12 were 76 and 57% (SEM = 4%), respectively. Bulls in Group T5 had greater SC than those in Groups C and OVA but only between Weeks 10 and 16 (P < 0.05). Bulls of Group C had the lowest mean concentrations of LH. Pulse characteristics of LH were similar 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 for 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 Group 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 bulls increased SC, without affecting liveweight change and induced higher serum concentrations of T, probably due to an increase in steroidogenesis of Leydig cells in response to stimulation of LH.