| Abstract: | To examine the relative roles of placental lactogen (PL) and GH in fetal metabolism, we have examined the effects of ovine PL (oPL), ovine GH (oGH), and ovine PRL (oPRL) on glycogen metabolism in cultured ovine fetal hepatocytes and have examined the binding of these hormones to hepatic membranes from fetal and neonatal lambs. In ovine fetal hepatocytes, oPL (150 ng/ml-20 micrograms/ml) stimulated dose-dependent increases in [14C]glucose incorporation into glycogen (18-167%) and total cellular glycogen content (10-69%). oGH and oPRL also stimulated glycogen synthesis in fetal hepatocytes, but the potencies of these hormones were only 12% and 4% that of oPL. The dose-response curves of the three hormones were parallel, and their maximal effects were identical, suggesting a common mechanism of action. In hepatic membranes from fetal lambs, the maximal specific binding of [125I]oPL was 26.3% while the maximal specific binding of [125I]oGH was only 0.9-1.5%. The binding of [125I]oPL was saturable and reversible and varied with incubation time and temperature. Unlabeled oPL (1 ng/ml-5 micrograms/ml) caused a dose-dependent inhibition of the binding of [125I]oPL to fetal hepatic membranes, with half-maximal displacement of [125I]oPL by 5-7 ng unlabeled oPL/ml. oGH and oPRL caused parallel displacement of [125I]oPL, but with potencies only 2% and 0.1% that of oPL. Scatchard analysis of oPL dose-response curves indicated that the hormone bound to a single class of receptors with a dissociation constant of 1.1 X 10(-10) M. The maximal specific binding of [125I]oGH to hepatic membranes of neonatal lambs (20.1%) greatly exceeded the binding of oGH to fetal hepatic membranes. In addition, the potency of oGH in competing for [125I]oPL binding sites in neonatal liver greatly exceeded the potency of oGH in competing for [125I]oPL binding sites in fetal liver. Although the biological effects of both oPL and oGH in postnatal subprimate tissues may be mediated through binding to nonprimate GH receptors, the results of these studies suggest that the glycogenic effects of oPL in ovine fetal liver are mediated through binding to specific fetal oPL receptors. The relatively weak biological effects of oGH and oPRL in ovine fetal liver appear to be mediated through the binding of the hormones to fetal oPL receptors. The presence of specific, high affinity PL receptors in ovine fetal tissues provides a mechanism whereby oPL may function as a GH in the ovine fetus. |
