Hexose transport and phosphorylation by capillaries isolated from rat brain

Hexose transport and phosphorylation were studied in capillary segments isolated from rat brain. Uptake of 3-O-methyl-D-glucose (3MG) could be inhibited by cytochalasin B, phloretin, and phlorizin, but not by 2,4-dinitrophenol or ouabain. 2-Deoxy-D-glucose (2DG), D-glucose, galactose, and mannose inhibited 3MG uptake, while L-glucose, fructose, and ribose did not. Accelerative exchange diffusion of 3MG was demonstrated. At equilibrium, the intracellular concentration of hexose did not exceed the external concentration, and transport was, therefore, equilibrative rather than accumulative. Transport of 2DG and D-glucose was not rate limiting for metabolism. When incubated in 5 mM D-glucose, the endothelial cells contained a large pool of free glucose. L-Glucose entered capillaries more slowly than other hexoses and served as a marker for simple diffusion of sugars into the cells. Our results suggest that sugar uptake into isolated brain capillaries occurs by a transport system similar to the one responsible for glucose transport across the blood-brain barrier in vivo.