Placental transport of retinol in sheep

Six ewes in the last trimester of pregnancy were maintained on adequate intakes of vitamin A. Indwelling catheters were implanted surgically in fetal jugular veins and carotid arteries. Three ewes and three fatal lambs received intravenous [15-3H] retinol 19 days prior to parturition, followed by sequential sampling of maternal and fetal blood. Fetal plasma retinol was found to be complexed with retinol-binding protein and another protein other than prealbumin, with a total molecular weight of 30,000 to 35,000. Transfer of labeled retinol from ewe to fetus peaked at 24 hours after maternal injection. Calculation of plasma retinol transport rate from the areas under the radioactivity-time curves showed that transport in the fetus averaged 5% of that in the ewe. Approximately 250 microgram of retinol was transferred daily from ewes to fetal lambs and 50 microgram/day carried back to mothers. The study suggested that the carrier system of retinol in blood may not be the same in the fetal lamb as in adults, and demonstrated that retinol transfer is appreciable from fetus to ewe.