Dynamics of the fetal distribution and transfer of Vitamin A between rat fetuses and their mother.

During the final trimester of pregnancy (15-21 days) in well-nourished dams, the body weight of rat fetuses increased over 30-fold whereas fetal vitamin A concentrations increased about twice as fast. The average vitamin A content of each newborn pup was 8 microgram, or 1.45 microgram/g without suckling, but increased to 12 microgram with 1 day of suckling. Only about one-third of total fetal vitamin A, however, was found in the liver (5-8 microgram/g). The total content and concentration of vitamin A in the pooled placentas increased about 6-fold (1.2 to 8 microgram) and 2-fold (1 to 2 microgram/g), respectively, during this period. The placental vitamin A concentration was significantly correlated (R2 = 0.72, P less than 0.025) with total litter vitamin A content. The total vitamin A content of different litters varied up to 5-fold and of littermates up to 2-fold. A significant portion of the radioactivity recovered after the intraperitoneal injection of 3H-retinyl acetate into fetuses was found in the placenta (2.7-3.8%) and in the mother's liver and serum (1.5-2.4%) within 1-5 hours. Thus, although the total content of vitamin A in the rat fetus is low, its concentration increases more rapidly than growth during the third trimester, the liver is its preferred albeit not its sole storage site, and its fetal metabolism and excretion through the placenta to the maternal circulation are appreciable. The fetal-maternal interaction relative to vitamin A should therefore be considered as a dynamic steady-state relationship rather than as a one-way flow constrained by a so-called "placental barrier".