LH AND TESTOSTERONE RESPONSES TO GONADOLIBERIN (LRH) TREATMENT IN YOUNG BULLS PRIOR TO AND DURING PUBERTY

The LH [luteinizing hormone] and testosterone [T] release after a single injection (0.25 mg) of gonadoliberin (LRH) were studied in 23 bulls before and around puberty. Both LH and T were measured by radioimmunoassay of jugular vein blood samples taken at frequent intervals. LRH caused release of LH at all ages, but the magnitude of the LH increase in response to LRH was significantly higher at 3 and 5 mo. (mean total response 4.3 .mu.g/ml .times. 150 min) than at older ages (3.1 .mu.g/ml .times. 150 min). The mean total response of T to LRH increases from very low levels (0.07 .mu.g/ml .times. 150 min) at 3 mo. of age to .apprx. 0.65 .mu.g/ml .times. 150 min between the ages of 7 and 13 mo. The mean LH responses varied significantly among individuals and the intra-class correlation coefficient [r] was 0.3 (.PI. < 0.02). Two distinct periods evidently exist prior to puberty regarding LH and T release in response to LRH: one before 7 mo. of age during which LH release is very sensitive to LRH and the T low, and the other from 7 mo. until after puberty, when no more changes in the magnitude of the responses can be seen. The individual magnitude of the response during the 2nd period does not seem to be independent of that seen in the former period.