Glucose in brain

Figure 19 Dynamics of carbohydrate in brain and heart of cruising horse-mackerel. O, glycogen in brain A, glucose in brain , glycogen in heart.

In the rare hereditary disease essential pentosuria, considerable quantities of L-xylulose appear in the urine because of absence of the enzyme necessary to reduce L-xylulose to xyhtol. Parenteral administration of xylitol may lead to oxalosis, involving calcium oxalate deposition in brain and kidneys (Figure 20-4). Various drugs markedly increase the rate at which glucose enters the... 

In preparation for a PET scan, a patient is injected with a dose of the tracer, which quickly accumulates in the brain. The patient is placed inside the PET scanner, and the instrument detects the gamma rays emitted by the tracer. The result is an image showing the distribution of glucose in the brain, which indicates where brain activity is greatest. 

ATP certainly fulfils the criteria for a NT. It is mostly synthesised by mitochondrial oxidative phosphorylation using glucose taken up by the nerve terminal. Much of that ATP is, of course, required to help maintain Na+/K+ ATPase activity and the resting membrane potential as well as a Ca +ATPase, protein kinases and the vesicular binding and release of various NTs. But that leaves some for release as a NT. This has been shown in many peripheral tissues and organs with sympathetic and parasympathetic innervation as well as in brain slices, synaptosomes and from in vivo studies with microdialysis and the cortical cup. There is also evidence that in sympathetically innervated tissue some extracellular ATP originates from the activated postsynaptic cell. While most of the released ATP comes from vesicles containing other NTs, some... 

Doege, H., et al. Activity and genomic organization of human glucose transporter 9 (GLUT9), a novel member of the family of sugar-transport facilitators predominantly expressed in brain and leucocytes. Biochem. J. 2000, 350, 771-776. 

FIGURE B1-1 Glucose and monocarboxylic acid transporters in brain. Specific glucose and monocarboxylic acid transporters in brain are localized on different types of brain cells. (Courtesy of Ian Simpson and Susan Vannucci.)... 

Glycogen and its enzymes are compartmentalized. Glycogen granules are only present in astrocytes of adult animals but are found in both astrocytes and neurons of immature animals. Of the enzymes involved in glycogen metabolism, glycogen phosphorylase is found in astrocytes only. Under steady-state conditions, it is probable that less than 10% of phosphorylase in brain is in the unphosphorylated b form (requiring AMP). This form is probably not very active at the low AMP concentrations present when intracellular glucose is sufficient to maintain ATP synthesis. Brain phosphorylase b kinase is activated indirectly by cAMP and by the molar concentrations... 

Brain hexokinase is inhibited by its product glucose-6-phosphate and to a lesser extent by adenosine diphosphate. The isoenzyme of hexokinase found in brain may be soluble in the cytosol or be attached firmly to mitochondria [2 and references therein]. An equilibrium exists between the soluble and the bound enzyme. The binding changes the kinetic properties of hexokinase and its inhibition by Glc-6-P resulting in a more active enzyme. The extent of binding is inversely related to the ATP ADP ratio, i.e. conditions in which energy utilization... 

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