Hormone Ontogeny in the Ovine Fetus. V. Circulating Prolactin in Mid- and Late Gestation and in the Newborn*

Ovine PRL (oPRL) was determined by homologous RIA on plasma samples drawn from ovine fetuses with chronically implanted vascular catheters from 71 days gestation to term (147 days). Plasma PRL was first detected (>1.0 ng/ml) at 87 days and remained low before 110 days gestation [2.1 ± 0.3 ng/ml (mean ± SE)]. Fetal PRL concentration rose to 14.7 ± 2.5 ng/ml at 111–120 days gestation, to 26.0 ± 2.6 ng/ml at 121–130 days, and to 37.5 ± 2.6 ng/ml at 131–140 days. At term (141–150 days), the mean PRL concentration was 37.8 ± 2.0 ng/ml. Significantly lower PRL concentrations were found in the late gestation fetus during fetal surgery when compared with levels in the chronically catheterized fetus. In samples drawn from the chronically catheterized neonatal lamb, PRL concentrations were lower (15.6 ± 3.7 ng/ml) in the first neonatal week than in the term fetus. However, by the fourth neonatal week, PRL concentrations had risen to levels similar to the late gestation fetus (28.0 ±4.1 ng/ml). No correlation between maternal PRL concentrations and gestational age was observed between 58–148 days gestation. The MCR of oPRL in the ovine fetus was estimated from PRL concentrations achieved by the constant infusion of oPRL into five fetuses between 114–135 days: The calculated clearance (9.27 ± 1.26 ml/min) was similar when corrected for body mass to that previously reported in the infant lamb The pattern of circulating PRL in the ovine fetus is similar to that in the human fetus. In both species, PRL concentrations are low until the third trimester, then rise to high levels, in close temporal relationship to the previously reported increase in fetal estrogen concentrations which have been postulated to have a major influence on the secretion of PRL by the fetal pituitary gland.