Maternal-fetal glucose exchange: necessity of a three-pool model

Tracer glucose was infused simultaneously into mother ( [6-3H]glucose) and fetus ( [U-14C]glucose) in pregnant sheep in the last month of gestation to study the exchange of glucose molecules between maternal and fetal blood. Net rates of entry and exit of tracer glucose via the uterine and umbilical circulations were measured by application of the Fick principle under steady-state conditions. The net flux of fetal tracer to the uteroplacenta from the fetus was 53.4 +/- 3.1% of the fetal tracer infusion. The net flux of fetal tracer into maternal blood was 43.8 +/- 5.0% of the net flux of fetal tracer to the uteroplacenta from fetal blood. The net flux of maternal tracer to the uterus from the mother was 31.7 +/- 2.8% of the maternal tracer infusion. The net flux of maternal tracer into fetal blood was 40.5 +/- 4.4% of the net flux to pregnant uterus from maternal blood. This evidence demonstrates that the uteroplacental tissue mass constitutes a glucose pool that is interposed between the maternal and fetal glucose pools and rapidly metabolizes glucose molecules derived from both maternal and fetal blood. Calculations based on a three-pool model show that the fetal glucose pool contributes approximately 40% of the glucose that is metabolized by the placenta. The comparison of the three-pool model with a previously proposed two-pool reversible model shows that the latter lumps the placental utilization of fetal glucose with fetal glucose utilization and overestimates the rate of fetal glucose metabolism by approximately 60%.