THE TRANSPORT OF VITAMIN-C IN THE ISOLATED HUMAN NEAR-TERM PLACENTA

We investigated the specific uptake (reference: [H-3]-L-glucose) as a measure of membrane transfer of [C-14]-labeled L-ascorbic acid (AA), L-dehydroascorbic acid (DHA) and diketogulonic acid using the single injection, double tracer dilution method in the artificially perfused lobe of the near-term human placenta. The uptake of DHA (40-60%) on both the fetal and the maternal side was 3-6 times higher than the uptake of AA, whereas an uptake of diketogulonic acid was not detected. AA transport was slightly higher on the maternal side. Uptake of DHA was suppressed by phloretin and cytocholasin B, whereas AA transfer was not affected. Low sodium concentrations inhibited the uptake of DHA. D-glucose (>20 mmol/l) inhibited the DHA uptake, and DHA inhibited D-glucose uptake but not L-alanine uptake. The K-m value (self inhibition) for DHA was 6-14 mmol/l. Vitamin C enters the trophoblast predominantly as L-dehydroascorbic acid. Its transfer through the microvillous and basal membrane might use the glucose transporter, however, a specific sodium-dependent pathway is not ruled out. Our transfer data suggest an intracellular pool of vitamin C which fills up with increasing plasma DHA-concentrations.