Tissue-specific differences in 2-fluoro-2-deoxyglucose metabolism beyond FDG-6-P: a 19F NMR spectroscopy study in the rat. 
2-Fluoro-[(18)F]-2-deoxy-glucose (FDG) is a positron-emitting analogue of glucose used clinically in positron emission tomography (PET) to assess glucose utilization in diseased and healthy tissue.  Originally developed to measure local cerebral glucose utilization rates, it has now found applications in tumour diagnosis and in the study of myocardial glucose uptake.  Once taken up into the cell, FDG is phosphorylated to FDG-6-phosphate (FDG-6-P) by hexokinase and was originally believed to be trapped as a terminal metabolite.  This 'metabolic trapping' of FDG-6-P forms the basis of the analysis of PET data.  In this study, we have used (19)F NMR spectroscopy to investigate FDG metabolism following the injection of a bolus of the glucose tracer into the rat (n=6).  Ninety minutes after the (19)FDG injection, the brain, heart, liver and kidneys were removed and the (19)FDG metabolites in each were extracted and quantified.  We report that significant metabolism of FDG occurs beyond FDG-6-P in all organs examined and that the extent of this metabolism varies from tissue to tissue (degree of metabolism beyond FDG-6-P, expressed as percentage of total organ FDG content, was brain 45 +/- 3%; heart 29 +/- 2%; liver 22+/-3% and kidney 17 +/- 3%, mean +/- SEM n=6).  Furthermore, we demonstrate that the relative accumulation of each metabolite was tissue-dependent and reflected the metabolic and regulatory characteristics of each organ.  Such inter-tissue differences may have implications for the mathematical modelling of glucose uptake and phosphorylation using FDG as a glucose tracer.