SERUM HORMONAL CHANGES AND TESTICULAR RESPONSE TO LH DURING PUBERTY IN BULLS

Concentrations of luteinizing hormone (LH), follicle stimulating hormone (FSH), testosterone, androstenedione and prolactin were measured in serum collected at 20 min intervals during the same 24 h period from bulls 1, 2, 4, 5, 7, or 9 months of age. This experimental design was adopted to allow measurement of endocrine changes due to age, independent of changes in temperature and photoperiod. Testicular tissue from these bulls was incubated in vitro with 0, 5 and 50 ng of LH/ml of media to measure testosterone and androstenedione production. Concentrations of LH in serum and frequency of episodic peaks of LH were greater in 4-month-old bulls than in bulls 1, 5, 7 or 9 months of age. Concentrations of testosterone were greatest at 5 months, 1 month after LH concentrations had increased, whereas concentrations of androstenedione were greatest in 4-month-old bulls. Serum FSH concentrations did not change with age and no episodic peaks were detected. However, prolactin increased in serum from bulls 1-5 months of age whereas frequency of prolactin peaks increased in serum from bulls 1-4 months of age. When averaged over all doses (0, 5, 50 ng LH/ml), testicular tissue from bulls 4, 5 or 7 months of age produced more testosterone than tissue from 1 or 3-month-old bulls. In addition, androstenedione secretion was greater in bulls 1, 3 or 4 months of age than in bulls 5, 7 or 9 months of age. Inclusion of LH in the culture medium stimulated testosterone and androstenedione secretion. However, stimulation of androgen secretion by LH was similar among age groups. Increased frequency of episodic peaks of LH at 4 months of age may be involved in increased testosterone production in 5-month-old bulls.