Glucose transfer across the intact guinea-pig placenta

Experiments were carried out in anaesthetized pregnant guinea-pigs. Following the maternal injection of a bolus containing ¹⁴C-hexose and ³H₂O, blood was sampled from the fetal umbilical vein during a single circulatory transit. A placental transfer index was calculated from the ratio of the tracers in the fetal whole blood divided by that in maternal plasma. The transfer index for D-glucose, 0.66±0.03 (SEM), greatly exceeded that for L-glucose, 0.013±0.004. Elevation of the maternal plasma D-glucose concentration, with unlabelled D-glucose, resulted in saturation of D-glucose transfer with an apparent K of 1.2x10^-2 mol/l mean maternal plasma D-glucose. Phlorizin at maternal plasma concentrations of approximately 10^-3 mol/l inhibited D-glucose transfer by 40%. Phloretin did not affect D-glucose transfer at levels estimated to be 10^-4 mol/l. Specificity studies with substituted D-glucose analogues showed that α-methyl-D-glucoside is not transported by a facilitated pathway; 2-deoxy-D-glucose and 3-O-methyl-D-glucose share the D-glucose carrier and D-galactose has a partial affinity for the D-glucose carrier.