Deep hypothermia and rewarming alters glutamate levels and glycogen content in cultured astrocytes

Background: Deep hypothermia has been associated with an increased incidence of postoperative neurologic dysfunction after cardiac surgery in children. Recent studies suggest an excitotoxic mechanism involving overstimulation of glutamate receptors. Extracellular glutamate uptake occurs primarily by astrocytes. Astrocytes also store glycogen, which may be used to sustain the energy-consuming glutamate uptake. Extracellular glutamate and glycogen content were studied during temperature changes mimicking cardiopulmonary bypass in vivo. Methods: Primary cultures of cerebral cortical astrocytes were used in a specially designed incubator allowing continuous changes of temperature and ambient gas concentrations. The sequence of events was as follows: normothermia, rapid cooling (2.8 °C/min) followed by 60 min of deep hypothermia (15 °C), followed by rewarming (3.0 °C/min) and subsequent 5 h of mild hyperthermia (38.5 °C). Two different conditions of oxygenation were studied: (1) normoxia (25% O₂, 70% N₂, 5% CO₂); or (2) hyperoxia (95% O₂, 5% CO₂). The extracellular glutamate concentrations and intracellular glycogen levels were measured at nine time points. Results: One hundred sixty-two cultures were studied in four independent experiments. The extracellular concentration of glutamate in the normoxic group increased significantly from 35 ± 10 nm/mg protein at baseline up to 100 ± 15 nM/mg protein at the end of 5 h of mild hyperthermia (P < 0.05). In contrast, extracellular glutamate levels did not vary from control in the hyperoxic group. Glycogen levels decreased significantly from 260 ± 85 nm/mg protein at baseline to < 25 ± 5 nM/mg protein at the end of 5 h in the normoxic group (P < 0.05) but returned to control levels after rewarming in the hyperoxic group. No morphologic changes were observed in either group. Conclusion: The extracellular concentration of glutamate increases, whereas the intracellular glycogen content decreases when astrocytes are exposed to a sequence of deep hypothermia and rewarming. This effect of hypothermia is prevented when astrocytes are exposed to hyperoxic conditions.