MODIFICATION OF HYPOXIA-INDUCED INJURY IN CULTURED RAT ASTROCYTES BY HIGH-LEVELS OF GLUCOSE

Background and Purpose: Preexisting hyperglycemia exacerbates central nervous system injury after transient global and focal cerebral ischem ia. Increased anaerobic metabolism with resultant lactic acidosis has been shown to cause the hyperglycemic, neuronal injury. The contributi on of astrocytes in producing lactic acidosis under hyperglycemic/isch emic conditions is unclear, whereas the protective role of astrocytes in ischemic-induced neuronal injury has been documented. The ability o f astrocytes to maintain energy status and ion homeostasis under hyper glycemic conditions could ultimately reduce neuronal injury. Therefore , we determined the effects of increased glucose concentrations on glu cose utilization, lactate production, extracellular pH, and adenosine triphosphate concentrations in hypoxia-treated astrocyte cultures. Met hods. Primary astrocytes were prepared from neonatal rat cerebral cort ices. After 35 days in vitro, cultures were incubated with 0-60 mmol/L glucose and subjected to hypoxic conditions at 95% N2/5% CO2 for 24 h ours. In addition, under high-glucose conditions (30 mmol/L), astrocyt es were exposed to up to 72 hours of hypoxia. Determination of lactate dehydrogenase efflux, adenosine triphosphate concentrations, and extr acellular lactate concentrations defined astrocyte status. Equiosmolar levels of mannitol were added in place of high glucose concentrations to distinguish hyperosmotic effect. Results. When physiological conce ntrations of glucose (7.5 mmol/L) or lower concentrations were used, s ignificant cell damage occurred with 24 hours of hypoxia, as determine d by increased efflux of lactate dehydrogenase and loss of cell protei n. When higher glucose concentrations (15-60 mmol/L) were used, efflux of lactate dehydrogenase was similar to that observed in normoxic cul tures, despite an increased utilization of glucose. Lactate concentrat ions in the media at low or normal glucose concentrations exceeded nor moxic levels, but higher glucose concentrations (15-30 mmol/L) failed to increase lactate levels further. Values of adenosine triphosphate f or hypoxic astrocytes treated with high glucose concentrations were si gnificantly higher than those of astrocytes with zero or low glucose l evels. In cultures exposed to hypoxia and high glucose levels (30 mmol /L), no cellular injury was observed before 48 hours of hypoxia. Lacta te concentrations in the media increased during the first 24 hours of hypoxia and reached steady state. The pH of the media decreased to 6.4 after 24 hours and to 5.5 at 48 hours. The latter pH was concomitant with a marked increase in extracellular lactate dehydrogenase activity . Hyperosmotic mannitol failed to protect cultured astrocytes against hypoxia. Conclusions: Hypoxic injury to mature astrocytes was reduced by the presence of 15-60 mmol/L glucose in the medium during 24-30 hou rs of hypoxia. Injury occurred when the pH of the medium was <5.5. Thi s protection was not afforded by the hyperosmotic effect of high gluco se concentrations, nor was the hypoxic injury at later time periods wi th 30 mmol/L glucose mediated solely by lactate accumulation.