Placental transport of retinol in ewes fed high intakes of vitamin A.

Twelve ewes were fed retinyl propionate equivalent to 12,000 micrograms retinol/kg body weight per day for the last two trimesters of pregnancy (group fed high vitamin A). Four ewes received a control level of 120 micrograms/kg body weight per day. Indwelling catheters were implanted surgically in fetal jugular veins and carotid arteries. Ewes or fetal lambs received [3H]retinol intravenously, and blood was sampled until parturition. The ewes fed large amounts of vitamin A maintained viable fetal lambs for 6 d less than did controls. Plasma retinyl ester concentrations were elevated in the ewes fed a high level of vitamin A but not in their fetal lambs. Rates of plasma retinol transport and clearance increased with vitamin A intake in ewes and their fetal lambs. Efficiency of placental retinol transport in the group fed high levels of vitamin A was less than one-quarter that for controls. However, placental transport rate was approximately 100 micrograms/d greater than that of controls with an equivalent amount retained by the fetoplacental unit in the group fed high vitamin A. These data indicate that placental transport of retinol is partially regulated. High maternal vitamin A intake results in high retinol transport to the fetus.