High-affinity transport of glutamate in rat brain microvessels.

The maintenance of low extracellular concentrations of glutamate in the brain is a complex process in which the role of capillary transport is poorly understood. We examined the kinetics and substrate specificity of glutamate uptake by isolated rat brain microvessels. We showed that these microvessels take up glutamate by an energy- and temperature-dependent, concentrative, high-affinity active transport system with Km of about 2 microM. The presence of this active transport system, coupled with the known slow inward transport of glutamate across the blood-brain barrier, allows us to suggest that this capillary transport system may function in vivo in the unidirectional outward transport of glutamate from brain to blood.