Active immunization of boars against GnRH does not affect growth hormone but lowers IGF-I in plasma

Active immunization of boars against GnRH inhibits the endocrine testicular function and thus the occurrence of boar taint at slaughter. Some studies indicate that at least a part of the boar-specific growth potential is still retained. To clarify possible mechanisms, key hormones which are involved in growth regulation were measured in four control boars (C), five immunized boars (1) and three barrows (B). Immunization started at an age of 10 weeks and was repeated twice. Growth hormone (GH), IGF-I, LH, estradiol and glucocorticoids were measured radioimmunologically in blood plasma (daily samples or windows) which had been obtained via permanent catheters. GH was in a similar order both for C and 1 (4.19 and 3.88 ng/ml; n.s.) but low in B (2.88 ng/ml). LH was minimal in 1 (130 pg/ml), highest in B (550 pg/ml), and intermediate in C (220 pg/ml). IGF-I was significantly higher in C compared to both, I and B (239 vs. 123 ng/ml; P less than or equal to 0.001). Estradiol was decreased in I compared to C (24.11 vs. 90.21 pg/ml, P less than or equal to 0.001) but glucocorticoids did not differ between C (17.53 ng/ml) and 1 (17.51 ng/ml). It is concluded that 'immunological castration' allows the postnatal imprinting of a male-specific level of GH secretion but no concomitant rise of IGF-I due to the lack of estrogens which couple GH and IGF-I. The remaining anabolic potential thus is due to a late onset of testicular inhibition in some immunization trials. (C) 2002 Elsevier Science B.V. All rights reserved.