Hypoglycemia activates compensatory mechanism of glucose metabolism of brain

The effect of plasma glucose concentration on the cerebral uptake of [F-18] -fluorodeoxy-D-glucose (FDG) was studied in a broad concentration range in a rabbit brain model using dynamic FDG PET measurements. Hypoglycemic and h yperglycemic conditions were maintained by manipulating plasma glucose appl ying i.v. glucose or insulin load. FDG utilization (K) and cerebral glucose metabolic rate (CGMR) were evaluated in a plasma glucose concentration ran ge between 0.5 mM and 26 mM from the kinetic constant k(1), k(2), k(3) obta ined by the Sokoloff model of FDG accumulation. A decreasing set of standar d FDG uptake values found with increasing blood glucose concentration was e xplained by competition between the plasma glucose and the radiopharmacon F DG. A similar trend was observed for the forward kinetic constants k(1), an d k(3) in the entire concentration range studied. The same decreasing tende ncy of k(2) was of a smaller magnitude and was reverted at the lowest gluco se concentrations where a pronounced decrease of this backward transport ra te constant was detected. Our kinetic data indicate a modulation of the kin etics of carbohydrate metabolism by the blood glucose concentration and rep ort on a special mechanism compensating for the low glucose supply under co nditions of extremely low blood glucose level.