Utilization of mannose by astroglial cells.
Uptake and metabolism of mannose were studied in astroglia-rich primary cultures derived from neonatal rat brains. A saturable component of mannose uptake was found with half-maximal uptake at 6.7 +/- 1.0 mM mannose. In addition, a non-saturable component dominated the uptake at high concentrations of mannose. Glucose, cytochalasin B, or phloretin in the incubation buffer inhibited the carrier-mediated uptake of mannose. Within the astroglial cells mannose is phosphorylated to mannose-6-phosphate. In cell homogenates, the KM value of mannose-phosphorylating activity was determined to be 24 +/- 7 microM. The Vmax value of this activity is only 40% that of glucose-phosphorylating activity. Mannose-6-phosphate was converted to fructose-6-phosphate by mannose-6-phosphate isomerase. The specific activity of this enzyme in homogenates of astroglial cultures was higher than that of hexokinase. Two products of mannose utilization in astroglial cells are glycogen and lactate. The amounts of each of these products increased with increasing concentrations of mannose. In contrast to the generation of lactate, that of glycogen from mannose was enhanced in the presence of insulin. In conclusion, we suggest that mannose is taken up into the cells of astroglia-rich primary cultures by the glial glucose transporter and is metabolized to fructose-6-phosphate within the astroglial cells.