Model of placental glucose consumption and glucose transfer.

Net ovine uteroplacental glucose consumption (Ro,up) and transfer rates to the fetus (Rf,up) were measured at different concentrations of maternal (GA) and fetal (Ga) arterial plasma glucose that were set and maintained independently by a glucose clamp procedure. Five GA/Ga combinations were studied: 70/15, 70/20, 70/30, 50/14, and 50/24 mg/dl. Rf,up was inversely related to Ga both at GA = 70 and GA = 50. Linear regression analysis of Rf,up vs. Ga for the GA = 70 and GA = 50 groups of observations revealed similar slopes (-0.286 +/- 0.012 vs. -0.217 +/- 0.028 dl.min-1.kg fetus-1) but a significantly higher intercept for the GA = 70 group (10.3 +/- 0.12 vs. 5.5 +/- 0.47 mg.min-1.kg fetus-1). In contrast, Ro,up increased significantly in response to an increase of Ga and had no significant dependence on GA. These results indicate that uteroplacental glucose metabolism occurs primarily in tissues that have direct access to glucose molecules carried by the umbilical circulation and that the glucose transport capacity of the placental barrier is greater on its fetal than its maternal surface. Uteroplacental glucose metabolic rate and its dependence on fetal glucose concentration are major factors that determine the magnitude and variability of the glucose concentration gradient (and thus the rate of net glucose transfer) between maternal and fetal plasma.