METABOLIC PATHWAYS FOR GLUCOSE IN ASTROCYTES

Cultured astroglial cells are able to utilize the monosaccharides gluc ose, mannose, or fructose as well as the sugar alcohol sorbitol as ene rgy fuel. Astroglial uptake of the aldoses is carrier-mediated, wherea s a non-saturable transport mechanism is operating for fructose and so rbitol. The first metabolic step for all sugars, including fructose be ing generated by enzymatic oxidation of sorbitol, is phosphorylation b y 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 localizatio n of glycogen phosphorylase in astrocytes and ependymal cells of centr al nervous tissue correlates well with the occurrence of glycogen in t hese cells. The identification of lactic acid rather than glucose as d egradation product of astroglial glycogen appears to render the presen ce of glucose-6-phosphatase in cultured astrocytes an enigma. The colo calization of pyruvate carboxylase, phosphenol-pyruvate carboxykinase and fructose-1,6-bisphosphatase points to astrocytes as being the gluc oneogenic cell type of the CNS. (C) 1997 Wiley-Liss, Inc.