Placental transfer of thiamine in the human subject: in vitro perfusion studies and maternal-cord plasma concentrations.

The transfer of thiamine across human placenta has been studied in an in vitro perfusion system. With the maternal and fetal perfusates not recirculated, the transfer index (ratio of clearance of thiamine to that of L-glucose) toward the fetus was 2.09 +/- 0.36, suggesting a mediated transport system. In the reverse direction, the transfer rate approximates that expected from simple diffusion (transfer index 0.78 +/- 0.25). Stepwise increases in thiamine concentration in the maternal perfusate were associated with parallel increases in transfer rates up to 500 nmol/L. Above that concentration, the rate of increase declined and the transfer index fell. When the maternal perfusate was recirculated and the fetal circulation was kept open, the thiamine concentration rapidly decreased, reaching a plateau in 30 to 40 minutes at a level about 0.1 to 0.2 of the fetal perfusate. In contrast, the concentration within the placenta greatly exceeded those in the perfusates. The observations indicate an effective transport system directed toward the fetus, saturable at low concentrations and capable of establishing a transplacental gradient. Measurement of thiamine concentrations in 20 paired samples of plasma obtained at term demonstrated a mean (+/- SD) cord:maternal ratio of 2.5 +/- 1.4. Differential protein binding was excluded as a cause of the gradient. The in vivo gradient is probably attributable to placental transport.