Hormone Ontogeny in the Ovine Fetus, X. The Effects of β-Endorphin and Naloxone on Circulating Growth Hormone, Prolactin, and Chorionic Somatomammotropin*

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Abstract
The potential role of β-endorphin (β-EP) as a stimulus to fetal GH, PRL, and chorionic somatomammotropin secretion was investigated in the chronically catheterized ovine fetus. β-EP (500 μg/kg, iv) was administered to six fetuses between 84–100 days gestation and to four fetuses between 112–131 days gestation (term, 147 days). The peak incremental GH response of 104.4 ± 21.1 ng/ml in the younger fetuses was greater (P < 0.01) than that of the older fetuses (38.0 ± 11.9 ng/ml). Pretreatment of the fetus with naloxone (100–500 μg/kg, iv) 5 min before β-EP reduced (P < 0.02) the GH response to β-EP, which suggests that the GH stimulating action of β-EP was mediated via opiate receptors. Naloxone alone (400 μg/kg, iv bolus, followed by 400 μg/kg over 1 h of infusion) had no significant effect on fetal plasma GH. There was no significant change in plasma PRL after β-EP; however, fetal PRL rose in the three oldest fetuses studied (122, 126, and 131 days). Fetal ovine chorionic somatomammotropin was not altered by either β-EP or naloxone. The fetal hypothalamic pituitary unit can respond by midgestation to exogenous β-EP with increasing GH secretion. However, in late gestation, the GH responsiveness to β-EP decreases significantly. It is postulated that this decreased responsiveness is a consequence of the maturation of hypothalamic inhibitory mechanisms regulating GH secretion. The effects of β-EP on GH and PRL secretion are dissociated in the ovine fetus.