LH and testosterone release in developing bulls following LH-RH treatment. Effect of gonadectomy and chronic testosterone propionate pre-treatment.

LH and testosterone release after LH-RH treatment was studied in male calves from 1 week to 8 months of age. Total LH release, LH peak values and duration of the discharge increased from 1 week to 4 months of age by 406, 453 and 110%, respectively. Significant decreases of 54, 48 and 24% were observed at 6 months of age compared to 4 months of age. This was followed by steep increases in those variables of 344, 129 and 69% at 8 months compared to 4 months. A testosterone response following an LH increase was first seen at 4 months of age when a weak discharge occurred in 2 out of 5 calves. In older calves strong and LH-correlated testosterone responses were always observed. In a second experiment the LH response to LH-RH was studied in 6 months old calves castrated 2 months previously. The total LH response was 61% greater than in intact controls. When animals were castrated and pre-treated briefly with testosterone propionate the response was increased by 183% and the time to reach the LH peak was increased by 360%. From these and other results a synthesis of endocrine events during the first year of life is proposed. In particular it seems that the period between 4 and 8 months of age is crucial since interactions between steroid feedback and pituitary sensitivity to LH-RH change and eventually reach an equilibrium.