GLUCOSE-TRANSPORTER PROTEIN EXPRESSION IN HUMAN PLACENTA THROUGHOUT GESTATION AND IN INTRAUTERINE GROWTH-RETARDATION

Despite the importance of glucose for fetal growth, gestational development of placental glucose transport capacity has not been studied in the human. Furthermore, inadequate glucose transport has been implicated as a pathophysiological mechanism in intrauterine growth retardation (IUGR). We studied glucose transporter (GLUT) protein expression in sections of normal term placental tissue (immunocytochemistry) and in syncytiotrophoblast microvillous (MVM) and basal membranes (BM) isolated from normal term, preterm, and IUGR placentas (immunoblotting). GLUT 1, but not GLUT 3, protein was abundantly present in syncytiotrophoblast membranes. MVM had similar to 3-fold higher GLUT 1 density than BM at term. MVM GLUT 1 density was maintained from 16 weeks of gestation to term. BM GLUT 1 density increased 2-fold in late second trimester and remained unaltered thereafter to term. GLUT 1 densities in term and preterm IUGR placentas were unaltered Net D-glucose uptake rates corresponded to the GLUT 1 densities. These data suggest that 1) GLUT 1 is the main glucose transporter protein isoform in human syncytiotrophoblast; 2) the glucose transport capacity for MVM is potentially similar to 20-fold higher than that of BM; 3) GLUT 1 densities may be regulated independently in MVM and BM; 4) the increase in surface area and the maintenance of a high GLUT 1 density can account for the increase in placental glucose transport in the latter part of pregnancy; and 5) fetal hypoglycemia in IUGR is not due to a decrease in placental glucose transporter density.