Fetal glucose uptake and utilization as functions of maternal glucose concentration

Seventeen studies were performed in 12 pregnant sheep to examine the relationship among simultaneously measured glucose uptake via the umbilical circulation, fetal glucose utilization (mg.min^-1.kg^-1), and maternal arterial glucose (G(m), mg/dl). Fetal glucose utilization was measured by means of tracer glucose infused into the fetus or both mother and fetus. By fasting the ewe, G(m) was varied in the 62-22 range. A decrease in G(m) was accompanied by a significant (P<0.001) decrease in umbilical uptake (uptake = 0.09 G(m) -0.96, r = 0.82) and in fetal utilization, measured either by [U-¹⁴C]glucose (utilization = 0.062 G(m) + 0.91, r = 0.90) or [6-³H]glucose (utilization = 0.065 G(m) + 0.51, r = 0.91). At uptake greater than 3 mg.min^-1.kg^-1, utilization and uptake were not significantly different. At lower uptakes, utilization did not decline as much as uptake. The results demonstrate that maternal fasting decreases both the umbilical uptake and the fetal utilization of glucose and suggest that fetal glucogenesis increases when the availability of exogenous glucose is markedly reduced.