Fetomaternal transfer of glucose analogues by sheep placenta

The steady-state permeability of the placenta to radiolabeled 3-O-methyl--D-glucopyranose (3MeG), alpha-methyl-D-glucopyranoside (AMG), and D-xylose was measured in chronically catheterized conscious sheep near term. Fetomaternal flux was calculated by application of Fick's principle to the uterine circulation after injection to the fetus. At a fetal glucose concentration of 10.2 +/- 1.25 mM, the permeability of 3MeG was half the maximal value of 59.5 +/- 11.2 ml/min found by extrapolation to zero glucose concentration. The permeability to 3MeG is considerably greater than is compatible with restricted diffusion, which together with the competitive effect of D-glucose suggests a carrier-mediated transfer mechanism. The permeability to AMG was less than 0.1 ml/min. The specificity pattern of sugar transport from the fetal side of the sheep placenta is different from that reported for gut lumen or kidney tubule in other species. D-Xylose is not metabolically inert after its injection into the fetal lamb.