Ontogeny of secretory patterns of LH release and effects of gonadectomy in the chronically catheterized pig fetus and neonate.

Two experiments were conducted to determine the ontogeny of secretory patterns of luteinizing hormone (LH) release and effects of gonadectomy on the characteristics of LH secretion in the chronically catheterized pig fetus and neonate. To study secretory patterns in intact animals, blood samples were collected from 44 pig fetuses and their mothers (Days 81, 99, 109 and 113 of gestation) as well as from 25 neonates (Days 4 and 8) every 15 min for 3 h (2 h on Day 81). The results indicate that the fetal adenohypophysis secretes occasional pulses of LH as early as Day 81 of fetal life. Fetal and maternal mean LH levels are low (0.25-0.50 ng/ml) at all gestational ages, with lowest values just before birth (Day 113 post coitum). Four-day-old neonates show a significant increase in pulse frequency (male and female) as well as pulse amplitude (female), relative to fetal values, leading to significant augmentations in mean LH levels. This is associated with reductions in both 17 beta-estradiol and progesterone. By 8 days of age significant sex differences in mean LH levels (males greater than females) appear. Testosterone/5 alpha-dihydrotestosterone levels (males) are low prenatally but are significantly increased after birth, possibly due to the stimulating effects of increasing LH levels. To study the gonadal control of LH secretion, forty-one 105-day-old fetuses and thirty-eight 4-day-old neonates were chronically catheterized and were either gonadectomized or remained as sham or control animals. Forty-eight and 96 h after surgery, blood samples were taken every 15 min for 3 h. No significant changes are detectable at 96 h in mean LH, pulse frequency and amplitude in female or male fetuses or in neonates. While significant reductions in testosterone levels are observed at 96 h in the male fetus and neonate, progesterone concentration is reduced only in the neonate. In the castrated female, on the other hand, neither fetus nor neonate display significant changes in circulating levels of progesterone and 17 beta-estradiol at 96 h. It is concluded that the pituitary of the pig is able to discharge LH with occasional pulses as early as Day 81 of fetal life; however, the pituitary remains suppressed until after birth, probably due to high circulating nongonadal steroids in the fetal compartment.