Metabolic pathways for glucose in astrocytes.

Cultured astroglial cells are able to utilize the monosaccharides glucose, mannose, or fructose as well as the sugar alcohol sorbitol as energy fuel. Astroglial uptake of the aldoses is carrier-mediated, whereas a non-saturable transport mechanism is operating for fructose and sorbitol. The first metabolic step for all sugars, including fructose being generated by enzymatic oxidation of sorbitol, is phosphorylation by hexokinase. Besides glucose only mannose may serve as substrate for build-up of astroglial glycogen. Whereas glycogen synthase appears to be present in astrocytes as well as neurons, the exclusive localization of glycogen phosphorylase in astrocytes and ependymal cells of central nervous tissue correlates well with the occurrence of glycogen in these cells. The identification of lactic acid rather than glucose as degradation product of astroglial glycogen appears to render the presence of glucose-6-phosphatase in cultured astrocytes an enigma. The colocalization of pyruvate carboxylase, phosphenolpyruvate carboxykinase and fructose-1,6-bisphosphatase points to astrocytes as being the gluconeogenic cell type of the CNS.