High-density lipoproteins (HDL) stimulate placental lactogen secretion in pregnant ewes: further evidence for a role of HDL in placental lactogen secretion during pregnancy

ABSTRACT Previous studies from our laboratory showed that high-density lipoproteins (HDL) stimulate the release of human placental lactogen (PL) from cultured trophoblast cells from normal pregnant women. To determine whether HDL stimulates PL secretion in vivo, ovine HDL was infused over 2–5 min into 11 pregnant ewes (22 separate experiments) at 86–130 days of gestation via an indwelling catheter into the maternal jugular vein. The HDL, freshly prepared from the plasma of pregnant ewes by differential flotation ultracentrifugation, was greater than 99% purified as judged by SDS-PAGE. Plasma samples were obtained from the ewes before and at 0·5-h intervals for 6 h following the infusions and were assayed for PL by a specific homologous radioimmunoassay. The maternal infusion of HDL at doses of 302–784 mg (5·3–13·8 mg/kg body weight) stimulated significant increases in maternal plasma PL concentrations in six out of eight experiments (six ewes), and the infusion of 108–264 mg (1·9–4·6 mg/kg) stimulated plasma PL concentrations in two out of six experiments. In contrast, HDL at doses < 100 mg were without effect in eight experiments. The response to the HDL infusions was characterized by a sustained increase in plasma PL concentrations beginning 1·5–2·5 h after the infusions, reaching a maximum 274·2 ± 21·9% of the baseline value (P<0·001). In contrast, the maternal infusion of lipoprotein-free plasma proteins or saline had no effect on maternal plasma PL concentrations. Although the infusion of HDL into pregnant ewes stimulated an increase in maternal plasma PL concentrations, the infusion of HDL (0·8–22·0 mg/kg) into three fetuses in seven separate experiments had no effect on fetal plasma PL concentrations. The demonstration that HDL stimulates an increase in plasma PL concentrations in pregnant ewes strongly supports a novel physiological role for HDL in the regulation of PL secretion. Journal of Endocrinology (1989) 120, 423–427