Fetal utilization of maternally derived ketone bodies for lipogenesis in the rat

When D-beta-[3-14C]hydroxybutyrate was injected via the femoral vein into pregnant Sprague-Dawley rats at 21 days of gestation, D-beta-[3-14C]hydroxybutyrate was enzymatically detected in fetal plasma within 5 min. The time course of the incorporation of DL-beta-[3-14C]hydroxybutyrate into fetal lipids was studied. Lipid extracts of brown adipose tissue exhibited the greatest relative incorporation followed by pancreas, liver and lung. Less radioactivity was incorporated into brain and placenta. The incorporation into fetal lipids was several-fold greater than into maternal lipids. The labelling of the individual phospholipids was similar in the different tissues with phosphatidylcholine accounting for more than 50%. 75% of the radioactivity in brown adipose tissue was in the triacylglycerol fraction. In brain, liver and placenta, approximately half of the neutral lipid radioactivity was in cholesterol. Experiments in which D-beta-[3-14C]hydroxybutyrate was directly injected into fetuses in utero confirmed that this substrate was directly used by the fetuses without maternal intervention. These studies demonstrate that the rat placenta is permeable to beta-hydroxybutyrate and suggest that this ketone body is rapidly used by the fetus for the synthesis of fatty acids and cholesterol.