Placental transport of riboflavin: differential rates of uptake at the maternal and fetal surfaces of the perfused human placenta.

The kinetics of riboflavin transfer across the perfused human placenta has been studied. With both circulations open, the transfer index (clearance of riboflavin: clearance of antipyrine) toward the fetus was 3.3 +/- 0.08 (+/- SD) versus 0.87 +/- 0.02 in the reverse direction (p less than 0.01). Galactoflavin reduced the materno-fetal transfer rate. The uptake of riboflavin and L-glucose after bolus injection into the maternal perfusate was 35% +/- 9.6% versus 3% +/- 16%, p = 0.005, which indicates an uptake rate in excess of diffusion. There was no significant difference in uptake at the fetal surface. With recirculation of either the maternal or fetal perfusate, the transplacental gradient that was established was lower at high concentrations of riboflavin, which indicates the saturability of this function. Riboflavin was concentrated in the intracellular space and partly metabolized to flavin mononucleotide and flavin adenine dinucleotide. An equilibrium was approximated when both perfusates were recirculated, with the concentration lowest in the maternal perfusate, highest in the placenta, and intermediate in the fetal perfusate. It is concluded that the major factor directing riboflavin toward the fetus is the difference in rates of uptake at the two surfaces of the placenta.