Disposition of plasma cholesterol in the pregnant hypocholesterolemic rabbit.

Processes that might explain the hypocholesterolemia observed in pregnant rabbits were studied in animals fed a cholesterol-free chow diet. Excretion of endogenous steroids, measured after administration of a single intravenous dose of radioactive cholesterol, averaged 75 mg/day in controls and 96 mg/day in pregnant hypocholesterolemic rabbits. The mass of maternal plasma cholesterol transferred to the fetuses was measured in animals in which a constant plasma cholesterol specific activity was maintained. Placental transfer during the first 28 days of gestation ranged from 2 to 12 mg/day for mothers carrying litters of one to eight fetuses. It is concluded that loss of maternal plasma cholesterol due to placental transfer is small compared to loss by fecal steroid excretion. Because large decreases in plasma cholesterol concentration during pregnancy and increases at parturition were not accompanied by changes in the slopes of the specific activity-time curves, it appears most likely that cholesterol in isotopic equilibrium with plasma cholesterol is temporarily and reversibly sequestered in maternal tissues of the pregnant rabbit.