Comparison of regional blood-brain transport kinetics between glucose and fluorodeoxyglucose

The fluorodeoxyglucose (FDG) method for estimating regional cerebral glucose metabolic rate (LCMRglc) requires that a fixed relationship (the "lumped constant") exists between net FDG and glucose (GLC) extraction throughout the brain. In addition to the relative rate of metabolism between FDG and GLC, this assumed constant is affected by the relative rate of blood-to-brain FDG transport compared to that of glucose. However, little data is available regarding the regional stability of the FDG versus GLC transport-rate relationship. We therefore used high resolution, quantitative dual-tracer digital autoradiography to directly compare the blood-to-brain transport rate constants (K1) of radiolabeled GLC and FDG in normal and pharmacologically-stimulated rats. The rats were given 45 sec terminal intravenous infusions of a mixture of 18F-FDG and 14C-GLC. Autoradiograms of the brain representing the FDG and GLC tracer concentrations were produced, digitized, and converted into digital images of K1. We found that the global K1 values of FDG and GLC were not significantly different from each other. However, detailed analysis revealed that some structures in the normal animals, such as the hippocampus and cerebellum, had different quantitative patterns of FDG transport compared to GLC transport. Thus, our results indicate that the relationship between GLC and FDG transport is not uniform throughout the brain as has previously been assumed. This observation suggests that regional variations in the type and distribution of glucose transporters may exist and that the fluorodeoxyglucose "lumped constant" may vary somewhat among different brain regions.