Luminal and abluminal endothelial plasma membrane vesicles were isolated from bovine cerebral microvessels, the site of the blood-brain barrier. Glucose transport across each membrane was measured using a rapid-filtration technique. Glucose transport into luminal vesicles occurred by a stereospecific energy-independent transporter [Michaelis-Menten constant (K(m)) = 10.3 +/- 2.8 (SE) mM and maximal velocity (Vmax) = 8.6 +/- 2.0 nmol.mg protein(-1).min-1]. Kinetic analysis of abluminal vesicles also showed a transport system with characteristics similar to the luminal transporter (K(m) = 12.5 +/- 2.3 mM and Vmax = 10.0 +/- 1.0 nmol.mg protein-1.min-1). These functional, facilitative glucose transporters were symmetrically distributed between the luminal and abluminal membrane domains, providing a mechanism for glucose movement between blood and brain. The studies also revealed a Na-dependent transporter on the abluminal membrane with a higher affinity and lower capacity than the facilitative transporters (K(m) = 130 +/- 20 microM and Vmax = 1.59 +/- 0.44 nmol.mg protein-1.min-1. The abluminal Na-dependent glucose transporter is in a position to transport glucose from the brain extracellular fluid into the endothelial cells of the blood-brain barrier. The functional significance of its presence there remains to be determined.
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