Glucose transport across the basal plasma membrane of human placental syncytiotrophoblast.

Transfer of glucose from maternal to fetal circulations requires transport across both the microvillous (maternal-facing) and basal (fetal-facing) plasma membranes of the placental syncytium. We have previously reported transport properties of the microvillous membrane and we now report those of the basal membrane. Basal plasma membrane vesicles were prepared by selective sonication and density gradient centrifugation. Glucose or glucose analogues were rapidly transported across these membranes by facilitated diffusion. Transport was inhibited by cytochalasin B, phloretin and phloridzin. L-Glucose at 1 mM was transferred at only 1/700 of the rate of D-glucose, which indicated an insignificant nonspecific diffusion component. Transport was independent of sodium gradients, and kinetic studies under equilibrium-exchange conditions demonstrated a Km of 23 mM. Competition studies demonstrated that aldohexoses in the C-1 chair conformation were the preferred substrates. Placental steroids estriol and progesterone inhibited transport. In contrast to other polarized epithelia, the basal and microvillous membranes of the human placental syncytium possess transport systems with similar properties. Thus, the directionality and rate of transfer of glucose across the intact syncytium are likely to be direct functions of the materno-fetal concentration gradient and the total transport capacities of the two plasma membranes.