Demonstration of electrogenic Na+-dependent D-glucose transport in intestinal brush border membranes

Na(+)-coupled D-glucose transport was studied in isolated membrane vesicles from intestinal brush borders. Concentration gradients of SCN(-), K(+), and H(+) were established between the intravesicular solution and the incubation medium and their influence on D-glucose uptake from the medium was measured. A gradient (medium > vesicle) of NaSCN, but not of KSCN, produced a transient overshoot of D-glucose uptake above the equilibrium level. Similarly, an increase of the membrane conductance with valinomycin (K(+)-conductance) or with uncoupling agents of oxidative phosphorylation (H(+)-conductance) induced an overshooting D-glucose uptake, provided a (vesicle > medium) K(+)-gradient or a H(+)-gradient, respectively, was present in each case. The transient overshoot is evidence that D-glucose was taken up against its concentration gradient (up to 10-fold). The gradients of SCN(-), K(+) (in the presence of valinomycin), and H(+) (in the presence of uncouplers) are thought to contribute to the "driving" force for this "active" D-glucose transport by changing the electrical potential across the vesicle membrane and thus making the inside more negative (with respect to the medium). These experiments, therefore, provide evidence that the Na(+)-coupled D-glucose translocation across the brush border membrane is an electrogenic process, i.e., the positive charge associated with Na(+) is not compensated by the co-movement of an anion or the counter-movement of a cation via the glucose "carrier". The results imply that an electrical potential across the brush border membrane may play an important role in determining the transport of D-glucose by intact cells.