Maternal-fetal transfer of ascorbic acid in the guinea pig

Placental transfer of ascorbic acid was studied in the guinea pig using an in situ placental perfusion technique. A total of 14 animals were studied during their last week of gestation. The fetus was shown to have higher plasma total ascorbic acid (TAA) levels than the mother prior to maternal ascorbic acid infusion. However, following maternal venous infusion this normal gradient for TAA (fetal greater than maternal) disappeared and a new gradient was established (maternal greater than fetal). As maternal plasma TAA levels rose, the level of TAA in the placental perfusate steadily increased to a maximal concentration of approximately 0.85 mg/dl. This occurred when maternal plasma TAA levels were above 3.0 mg/dl. These results demonstrate that a saturable or carrier-mediated transport mechanism is involved in the placental transfer of ascorbic acid in this species. The apparent Vmax (the maximal transport rate by the saturable system per gram placenta) and the Kt (or one-half saturation concentration) were estimated to be 8.3 nmoles/minute and 0.12 mM, respectively. Additional results are presented to suggest that dehydroascorbic acid (DHA) may be the predominant form of the vitamin crossing the guinea pig placenta.