Regional studies of blood-brain barrier transport of glucose and leucine in awake and anesthetized rats.

D-Glucose and L-leucine are transported across the blood-brain barrier (BBB) by two separate carrier-mediated facilitated diffusion mechanisms. In the awake rat there are regional differences in blood-to-brain glucose transport among the cerebral cortex, cerebellum, hippocampus, and striatum. To determine whether these are due to variations in the regional density or affinity of the glucose transporter moiety of brain capillaries or are secondary to regional tissue perfusion and capillary arrangement characteristics, we studied regional blood-to-brain transport of L-leucine in awake rats; regional blood-to-brain transport of both glucose and leucine under chloral hydrate anesthesia, a condition associated with altered regional brain blood flow (BF) and metabolism; and regional brain vascular volume, derived from the L-glucose and insulin spaces, in both awake and anesthetized rats. We found the same regional differences in blood-to-brain leucine transport in awake rats as we previously described for D-glucose transport. These regional differences in glucose and leucine transport disappear under chloral hydrate anesthesia, as regional differences in BF are abolished. However, we found regional differences in the brain vascular volumes, which are evident in wakefulness and persist during anesthesia. These results suggest that the regional differences in blood-to-brain transport are due mainly to local tissue perfusion and capillary arrangement characteristics rather than to intrinsic regional differences in the transport systems of the BBB.