Negligible glucose-6-phosphatase activity in cultured astroglia

2-Deoxy[C-14]glucose-6-phosphate (2-[C-14]DG-6-P) dephosphorylation and glucose-6-phosphatase (G-6-Pase) activity were examined in cultured rat astrocytes under conditions similar to those generally used in assays of glucose utilization. Astrocytes were loaded with 2-[C-14]DG-6-P by preincubation for 15 min in medium containing 2 mM glucose and 50 mu M 2-deoxy[C-14]glucose (2-[C-14]DG). The medium was then replaced with identical medium including 2 mM glucose but lacking 2-[C-14]DG, and incubation was resumed for 5 min to diminish residual free 2-[C-14]DG levels in the cells by either efflux or phosphorylation. The medium was again replaced with fresh 2-[C-14]DG-free medium, and the incubation was continued for 5, 15, or 30 min. Intracellular and extracellular C-14 contents were measured at each time point, and the distribution of C-14 between 2-[C-14]DG and 2-[C-14]DG-6-P was characterized by paper chromatography. The results showed little if any hydrolysis of 2-[C-14]DG-6-P or export of free 2-[C-14]DG from cells to medium; there were slightly increasing losses of 2-[C-14]DG and 2-[C-14]DG-6-P into the medium with increasing incubation time, but they were in the same proportions found in the cells, suggesting they were derived from nonadherent or broken cells. Experiments carried out with medium lacking glucose during the assay for 2-deoxyglucose-6-phosphatase activity yielded similar results. Evidence for G-6-Pase activity was also sought by following the selective detritiation of glucose from the 2-C position when astrocytes were incubated with [2-H-3]glucose and [U-C-14]glucose in the medium. No change in the H-3/C-14 ratio was found in incubations for as long as 15 min. These results indicate negligible G-6-Pase activity in cultured astrocytes.