~Pergamon 0197-0186(95)00095-X Neurochem, Int. Vol. 28, No. 3, pp. 243 250, 1996 Copyright (c) 1996Elsevier Science Ltd Printedin Great Britain.All rights reserved 0197-0186/96$15.00+0.00 DEPHOSPHORYLATION OF 2-DEOXYGLUCOSE 6-PHOSPHATE AND 2-DEOXYGLUCOSE EXPORT FROM CULTURED ASTROCYTES R. J. FORSYTH,'* K. BARTLETT2 and J. EYRE' ~Paediatric Neuroscience Group, Department of Child Health, and 2Department of Child Health, Sir James Spence Institute of Child Health, Royal Victoria Infirmary, Queen Victoria Road, Newcastle upon Tyne NEI 4LP, U.K. (Received 16 March 1995 ; accepted 17 July 1995) Abstract~Neurotransmitter-stimulated mobilization of astrocyte glycogen has been proposed as a basis for local energy homeostasis in brain. However, uncertainty remains over the fate of astrocyte glycogen. Upon transfer of cultured astrocytes pre-loaded with [2-3H]2-deoxyglucose6-phosphate at non-tracer concentrations to a glucose-free, 2-deoxyglucose-freemedium, rapid dephosphorylation of a proportion of the intracellular 2-deoxyglucose 6-phosphate pool and export of 2-deoxyglucose to the extracellular fluid occurs. Astrocytes show very low, basal rates of gluconeogenesis from pyruvate (approx. 1 nmol mg protein ~h ~).Astr~cytesinviv~maybecapab~e~fphysi~gica~ysigni~cantg~uc~seexp~rtfr~mg~uc~se- 6-phosphate. The low gluconeogenicactivity in astrocytes suggests that the most likely source of glucose- 6-phosphate may be glycogen. These findings support the hypothesis that export, as glucose, to adjacent neurons may be one of the possible fate(s) of astrocytic glycogen. Such export of glycogen as glucose occurring in response to increasesin neuronal activity could contribute to energy homeostasis on a paracrine scale within brain. We have recently demonstrated transient excesses of the glucose content of blood sampled from the superior jugular venous bulb over that of arterial blood in children undergoing cardiopulmonary bypass (Eyre et al., 1994). We have also recently become aware of reports in Russian literature of tran- sients of glucose export in adults during neuro- intensive care (Guseinov, 1991).t These are clearly not maintained, steady-state phenomena, but are probably generated during state-transitions in glu- cose-transporting and -metabolizing processes under a combination of conditions including depressed neu- ronal metabolism and hypothermia. As such, they suggest the presence of glucose fluxes within brain that are not detectable under steady-state conditions. We have proposed that one such flux is a capacity for glucose export from the astrocyte intracellular com- partment to the brain extracellular fluid (ECF). This, however, would require that astrocyte glycogen is exportable as glucose (via glucose 6-phosphate), requiring, in turn, that astrocytes express glucose 6- phosphatase (E.C.3.1.3.9) activity. We suggest that these transient exports are due to brief periods when the sum of delivery rates to the brain ECF of glucose derived from blood and from astrocyte glycogen exceeds glucose extraction rates (including that due to neuronal metabolism) from the ECF, resulting in brain ECF glucose concentrations transiently increas- ing above plasma concentrations, and consequent brief reversal of glucose flux between blood and brain (Eyre et al., 1994). The necessity, in general terms, for a third compartment of glucose behind the blood- brain barrier, capable of glucose delivery to the ECF, for the generation of transients of glucose export is examined in the accompanying paper (Forsyth, 1996). Export of astrocyte glycogen as glucose would require astrocytes to express glucose 6-phosphatase. The degree (if any) of expression of glucose 6-phos- phatase in brain has been controversial (see for exam- ple discussion in Forsyth et al., 1993). Nevertheless, there is now immunohistochemical and biochemical evidence of expression of glucose 6-phosphatase by * Author to whom all correspondence should be addressed. t Our translation of this paper is available upon request. 243