Characteristics of glucose transport across the microvillous membranes of human term placenta.

Transport characteristics of D-glucose were studied in the microvillous vesicles isolated from the human term placenta. Transport occurred by selective and rapid facilitated diffusion system which was inhibitable by phloretin and HgCl2. The transport was dependent on a transmembrane. Na+-gradient indicating a "secondary active transport" system operating. The transport influx was saturable and the kinetic analysis based on Hanes-Woolf plot produced a kt and Jmax value of 1.2 mM and 34 nmoles.mgprotein(-1).min(-1), respectively. The efflux of D-glucose from the membrane vesicles in a pre-equilibrated assay conditions showed a distinct biphasic pattern differing significantly in the half time efflux. The t1/2 of the fast and slow components was found to be 15 sec and 660 sec, respectively. The transport showed distinct sensitivity to temperature and the Ea values both below and above the transition temperature of 37 degrees C, as calculated from the Arrhenius plot were found to be 7600 and 5472 kCal.mol(-1), respectively. Inhibition studies with a number of sugars for hexose transport pathway showed that the glucose epimers, phosphorylated sugars, and even the disaccharides and the pentose sugars competed effectively with D-glucose. The influx was also inhibited by a number of steroids such as progesterone, 17alpha-hydroxyprogesterone, testosterone and estrogen. Insulin was found to increase glucose transport in a dose- dependent fashion at a concentration of 0.2-1 unit.ml(-1). Ouabain, dinitrophenol and nicotine strongly inhibited D-glucose uptake in the membrane vesicles.