EFFECT OF CASTRATION ON PLASMA LUTEINIZING HORMONE CONCENTRATIONS IN PREPUBERTAL BOARS

Mean concentrations and the occurrence of pulsatile release of luteinizing hormone (LH) were determined in 14-wk-old crossbred boars (50.5 ± 1.5 kg) after bilateral or unilateral castration at 10 wk of age. Blood was collected at 10-min intervals for 5 h. Then gonadotropin releasing hormone (GnRH; 40 / μg) was given and sampling was continued at 5-min intervals for 1 h. Compared with intact boars, bilateral castration increased (P\u3c.001) mean LH (982 ± 56 vs 389 ± 56 pg/ml), pulsatile releases of LH (7.0 ± .6 vs 2.0 ± .6 pulses/5 h) and LH pulse amplitude (617 ± 29 vs 360 ± 58 pg/ml). Unilaterally castrated boars did not differ from intact boars in any of the above measures of LH secretion. Testis weight increased more between 10 and 14 wk of age in the unilateral castrates than in the intact boars (432 ± 42 vs 245 ± 34%; P\u3c.05). Thus, compensatory hypertrophy occurred within 4 wk of castration. Plasma testosterone was lower for bilateral castrates than for intact animals (.1 ± .8 vs 3.6 ± .9 ng/ml; P\u3c.05) while unilateral castrates (3.8 -+ 1.0 ng/ml) and intact boars did not differ. Plasma estradiol concentrations in bilateral and unilateral castrates were not different from levels found in intact boars (1.8 ± 1.8, 8.8 ± 2.1 and 6.0 ± 1.8 pg/ml, respectively). While mean LH after GnRH was higher in bilateral and unilateral castrates than in intact boars (2,588 ± 147 and 2,693 ± 164 vs 1,928 ± 147 pg/ml; P\u3c.01), castration did not affect the highest LH level after GnRH, time of highest level, magnitude of LH change or rate of LH response during the first 15 rain after GnRH. The rate of change of LH from 0 min to time of maximum LH of the bilateral castrates tended to be greater than that in intact boars (b=202 vs 94; P\u3c.10). These data indicate that the negative feedback of gonadal steroids affects LH secretion