Hormone Ontogeny in the Ovine Fetus. IX. Luteinizing Hormone and Follicle-Stimulating Hormone Response to Luteinizing Hormone-Releasing Factor in Mid- and Late Gestation and in the Neonate*

Abstract

The responses of chronically catheterized ovine fetuses and neonatal lambs to the administration of an iv bolus of 50 μg LRF were determined. Ovine LH (oLH) and ovine FSH (oFSH) were measured by homologous RIA. Thirty studies were performed in 22 fetal sheep between the gestational ages of 84–141 days, and 12 studies were performed in 7 neonatal lambs, aged 1–20 days. Even in the youngest fetus studied (84 days), LRF evoked a rise in the concentrations of plasma oLH and oFSH. The gonadotropin response to LRF changed with gestational age. The greatest rise in oLH and oFSH occurred in the group of fetuses between 115–132 days. The mean maximal response of plasma oLH was 15.4 ± 1.3 ng/ml in males and 13.8 ± 1.6 ng/ml in females; oFSH rose to 8.7 ± 0.6 ng/ml in males and to 12.5 ± 1.8 ng/ml in females. The highest peak response was preceded by lower values at 84–100 days and followed by a fall in the mean peak response after 135 days of gestation, suggesting an increase in gonadotropin reserve, followed by a decrease in releasable oLH and oFSH by term. At 84–105 days, plasma oLH increased in response to LRF to 6.0 ± 0.5 ng/ml in males and to 10.9 ± 4.0 ng/ml in females; oFSH rose to 5.8 ± 0.4 ng/ml in males and to 8.7 ± 0.7 ng/ml in females. After 135 days, the mean maximal response of oLH was 5.4 ± 1.6 ng/ml in males and 3.9 ± 1.1 ng/ml in females; the response of oFSH was 5.2 ± 0.7 ng/ml in males and 7.2 ± 0.9 ng/ml in females. The pattern of change with advancing gestational age was similar for the maximal incremental rise in plasma LH and FSH above baseline or the integrated incremental response. After 115–118 days, the peak increase in plasma LH and FSH evoked by LRF correlated positively with the basal concentration of plasma gonadotropins. Serial studies were performed on 3 female neonates and 1 male neonatal lamb; in each case, an increase in the response of oLH or oFSH to the administration of LRF was seen with advancing neonatal age. Females had a significantly greater peak FSH response than males at 88–105 and 118–126 days of age. If one uses the LH response to exogenous LRF as an indirect index of endogenous LRF secretion, the data suggest an increase in pulsatile LRF secretion that reaches a peak by about 80% gestation, followed by a fall in late gestation. (Endocrinology108: 881, 1981)