Invalidity of criticisms of the deoxyglucose method based on alleged glucose-6-phosphatase activity in brain

Abstract

The observations made by Sacks et al. [Neurochem. Res. 8, 661-685 (1983)] on which they based their criticisms of the deoxyglucose method have been examined and found to have no relationship to the conclusions drawn by them. (1) The observations of Sacks et al. (1983) of constant concentrations of [14C]deoxyglucose and [14C]deoxyglucose-6-phosphate, predominantly in the form of product, reflects only the postmortem phosphorylation of the precursor during the dissection of the brain in their experiments. When the brains are removed by freeze-blowing, the time courses of the [14C]deoxyglucose and [14C]deoxyglucose-6-phosphate concentrations in brain during the 45 min after the intravenous pulse are close to those predicted by the model of the deoxyglucose method. (2) Their observation of a reversal of the cerebral arteriovenous difference from positive to negative for [14C]deoxyglucose and not for [14C]glucose after an intravenous infusion of either tracer is, contrary to their conclusions, not a reflection of glucose-6-phosphatase activity in brain but the consequence of the different proportions of the rate constants for efflux and phosphorylation for these two hexoses in brain and is fully predicted by the model of the deoxyglucose method. (3) It is experimentally demonstrated that there is no significant arteriovenous difference for glucose-6-phosphate in brain, that infusion of [32P]glucose-6-phosphate results in no labeling of brain, and that the blood-brain barrier is impermeable to glucose-6-phosphate. Glucose-6-phosphate cannot, therefore, cross the blood-brain barrier, and the observation by Sacks and co-workers [J. Appl. Physiol. 24, 817-827 (1968); Neurochem. Res. 8, 661-685 (1983)] of a positive cerebral arteriovenous difference for [14C]glucose-6-phosphate and a negative arteriovenous difference for [14C]glucose cannot possibly reflect glucose-6-phosphatase activity in brain as concluded by them. Each of the criticisms raised by Sacks et al. has been demonstrated to be devoid of validity.