THE EFFECT OF HYPERGLYCEMIA ON REGIONAL CEREBRAL GLUCOSE-OXIDATION INHUMANS STUDIED WITH [1-C-11]-D-GLUCOSE

The effect of hyperglycaemia on regional cerebral glucose utilization was studied in five healthy males fasted over-night using positron emi ssion tomography. Selectively labelled glucose, [1-C-11]-D-glucose, wa s used as a tracer. After correction for the small loss of [C-11]CO2 f rom the tissue, this tracer measures the rate of glucose oxidation rat her than the total rate of glucose metabolism. Each subject was invest igated twice: during normoglycaemia (plasma glucose 5.3 +/- 0.3 mu mol mL(-1)) and at the end of a 2-h period of hyperglycaemia (plasma gluc ose 13.8 +/- 0.7 mu mol mL(-1)). Assuming unchanged rate constant for loss of labelled CO2 at normo- and hyperglycaemia the oxidative metabo lic rate of glucose was found to be slightly larger at combined hyperg lycaemia and hypersulinemia (0.30 +/- 0.91 mmol mL(-1) min-li than at normal glucose and insulin levels (0.25 +/- 0.01 mmol mL(-1) min(-1)). This suggests that the process of glucose phosphorylation might not b e fully saturated in the human brain or, alternatively, that the glyco gen deposition increases during short-term hyperglycaemia. The relativ e increase of oxidative metabolic rate was considerably larger (approx imate to 50%) in white matter than in the brain as a whole (20%). The brain glucose content was found to increase non-linearly with increasi ng plasma glucose. Together with data from previous studies these resu lts suggest that the free glucose in the human brain is close to zero when the plasma glucose is below 2 mu mol mL(-1).