Placental transport of vitamin B12 in the pregnant rat

Placental transport of vitamin B(12) was studied in the pregnant rat in two series of experiments. In the first series animals were given cyanocobalamin-(57)Co intravenously at various stages of gestation. High specific activity tracer was used and doses of B(12) were 1-2 ng per animal. The rats were killed from 15 min to 24 hr after injection and the fetuses, placentas, and serum were assayed for radioactivity. In the second series using uninjected animals, absolute amounts of vitamin B(12) in fetuses and placentas were measured at stages of gestation from day 12 through day 20. There was a progressive increase in B(12) transferred to the fetus during gestation. Although the quantity of vitamin B(12) transported per 24 hr was proportional to fetal weight, the amount transported per gram of placenta increased tenfold from day 10 through day 19. Uptake of tracer B(12) by placenta was initially rapid; however, no radioactivity appeared in the fetus until 2 hr after injection. The actual amount of B(12) in placenta increased throughout gestation, and the placental concentration of B(12) was greater than maternal plasma and fetal tissue concentrations at all times measured. These data suggest that the ability of placenta to transport B(12) increased throughout gestation, and that the rate-limiting step in the transport process was either the passage of B(12) from the maternal to the fetal side of placenta or the transfer from placenta into fetal plasma.