Ontogeny of Pulsatile Luteinizing Hormone and Testosterone Secretion in Male Lambs*

Abstract

The ontogeny of pulsatile LH [luteinizing hormone] and testosterone (T) secretion was studied in the developing male sheep. Three variables of LH secretion (frequency of LH pulses, magnitude of LH pulses, and levels between pulses) were determined during a 6 h period at each of 1, 2, 3, 5, 7, 8, 12 and 16 wk of age in 6 male lambs. Pulsatile LH secretion was evident by 1-2 wk of age in rapidly growing lambs and by 3-7 wk of age in slowly growing lambs. Although mean height of LH discharges increased 2-fold after the onset of pulsatile release, levels between discharges remained constant throughout the entire study. The most striking change, however, occurred in the frequency of LH discharges. The frequency increased 20-fold between weeks 1 and 8 and then decreased 3-fold between weeks 8 and 16. In each lamb, a brief (< 3h), but marked rise (> 2 ng/ml) in levels of circulating T occurred after the first observed pulse of LH. Thereafter, as the lambs older, increased concentrations of serum T were observed only during periods of pulsatile LH secretion, the highest mean level of T occurring about 1 h after each LH peak. Administration of human chorionic gonadotropin (hCG) i.v. to 2 day old males resulted in a marked increase in circulating T. A greater increment in concentrations of serum T occurred after the same absolute dose (50 IU) of hCG at 8 wk of age than at 2 days of age. These results indicate that in the male lamb pulsatile secretion of LH is initiated shortly after birth and changes in mean levels of LH are primarily a reflection of changes in frequency of LH discharges; before the onset of pulsatile LH secretion, the testes are capable of responding to an exogenous LH-like gonadotropin with increased secretion of T; after the onset of pulsatile LH secretion, a rise in circulating T follows each pulse of LH; and an increase in capacity of the testes to secrete T occurs during the 1st 8 wk of postnatal life. The pattern of LH secretion, coupled with the capacity of the testes to secrete T, determines the pattern and magnitude of T secretion in the developing male sheep.