Astrocytes and the delivery of glucose from plasma to neurons

doi:10.1016/0197-0186(95)00094-1

Neurochemistry International

Volume 28, Issue 3, March 1996, Pages 231-241

https://doi.org/10.1016/0197-0186(95)00094-1 Get rights and content

Abstract

Conventional kinetic models of brain glucose uptake and metabolism that visualize brain glucose as being in a single pool in equilibrium with plasma, are unable to account for some recently described experimental findings. These include microdialysis demonstrations of a brain extracellular fluid glucose concentration that is both low, and significantly affected by changes in neuronal activity; and observations of transient glucose export (transient negative whole-brain arteriovenous differences) in certain neuro-intensive care settings. A kinetic model that treats brain glucose as divided into more than one, kinetically distinct, compartment, implying the presence of a glucose “reservoir” behind the blood-brain barrier, and with plasma glucose initially entering a compartment other than the brain extracellular fluid, is more consistent with these experimental observations. Neuroanatomical considerations suggest that plasma glucose may initially exchange with an intracellular astrocytic glucose pool, rather than the brain extracellular fluid. Astrocyte glycogen, mobilized at times of increased neuronal activity, could form the reservoir whose presence is inferred from demonstrations of transient glucose export, but only if glycogenolytic products can be exported from astrocytes as glucose. This hypothesis is considered in the light of the frequently suggested concept of a “nutritional” role for perivascular astrocytes and invertebrate glia, taking up blood-borne glucose and passing on metabolic substrates to neurons. The implications of this model for 2-deoxyglucose-based methods for regional cerebral metabolic rate estimation are discussed. In general, errors due to the approximations inherent in the conventional three compartment kinetic model, may be expected to become less significant as metabolism is averaged over space and time. Thus the three-compartment model is probably acceptable for the description of metabolism at the relatively low spatial and temporal resolutions of these techniques.

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Glycogen shunt represents the fraction of metabolized glucose that passes through glycogen molecules, prior to entering the glycolytic pathway, and may be as large as 40% of overall glucose metabolism [17]. The presence of glucose-6-phosphatase (G6Pase) has been shown in astrocytes [18] but the role of astrocytes as glucose releasing cells has to be further investigated [19,20]. ATP is a major factor mediating intercellular communication and triggers a variety of different biological effects (reviewed by [21]).
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Our patients exhibited no significant increase in glutamate during the physiologic increases in plasma glucose immediately after EN. Glucose accumulates chiefly in astrocytes, and is released as glucose or lactate for further metabolism in neurons when the metabolic demand rises [12–14]. The blood flow has remarkable influences on these concentrations [15, 16].
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CommentaryAstrocytes and the delivery of glucose from plasma to neurons

Abstract

Brain Res.

Brain Res.

Brain Res.

Neurochem. Int.

Brain Res.

Brain Res.

Neurosci. Lett.

Facilitated transport of glucose from blood to brain in man and the effect of moderate hypoglycaemia on cerebral glucose utilization

Eur. J. Nucl. Med.

Kinetics of blood-brain barrier transport of pyruvate, lactate and glucose in suckling, weaning and adult rats

J. Neurochem.

Monitoring the timecourse of cerebral deoxyglucose metabolism by 31P nuclear magnetic resonance spectroscopy

Science

Physiological stimulation increases nonoxidative glucose metabolism in the brain of the freely moving rat

J. Neurochem.

Extracellular brain glucose levels reflect local neuronal activity: a microdialysis study in awake, freely moving rats

J. Neurochem.

Astrocytic glucose-6-phosphatase and the permeability of brain microsomes to glucose-6-phosphate

Biochem. J.

Focal physiological uncoupling of cerebral blood flow and oxidative metabolism during somatosensory stimulation in human subjects

Non-oxidative glucose consumption during focal physiologic neural activity

Science

Neurons and microvessels express the brain glucose transporter protein GLUT3

Carbohydrate metabolism of the brain in hypoxia [in Russian]

Anesteziol. Reanimat.

Noninvasive determination of local cerebral metabolic rate of glucose in man

Am. J. Physiol.

Three-dimensional structure of astrocytes in the rat dentate gyrus

J. Comp. Neurol.

Commentary
Astrocytes and the delivery of glucose from plasma to neurons

Monitoring the timecourse of cerebral deoxyglucose metabolism by ³¹P nuclear magnetic resonance spectroscopy