A comparison of the effects of hypothermia, pentobarbital, and isoflurane on cerebral energy stores at the time of ischemic depolarization

Background: In an accompanying article, we report that hypothermia (27- 28°C) delayed postischemic cortical depolarization longer than did large- dose pentobarbital or isoflurane anesthesia, even though preischemic cerebral metabolic rates for glucose were similar in the three groups. To examine the mechanism that may underlie these differences, we measured the cerebral concentrations of high-energy phosphates (including adenosine triphosphate [ATP] and adenosine diphosphate) in normal conditions and at the moment of depolarization. Methods: Rats were anesthetized with 0.8% halothane/50% N₂O and prepared for measurement of the cortical direct-current potential by glass microelectrodes. Animals were assigned to one of four groups: (1) halothane/nitrous oxide anesthesia, pericranial temperature approximately 38°C; (2) halothane/nitrous oxide, approximately 28°C; (3) halothane/nitrous oxide anesthesia with pentobarbital added to achieve electroencephalographic isoelectricity, approximately 38°C; or (4) 2.4% isoflurane/50% N₂O anesthesia (with electroencephalographic isoelectricity), approximately 38°C. The latter three groups were chosen on the basis of earlier work showing similar cerebral metabolic rates for glucose. In a subgroup of each, circulatory arrest was induced with KCl and the brain was frozen tn situ (with liquid nitrogen) at the moment of cortical depolarization. In remaining animals, the brain was frozen without any ischemia. Tissue ATP, adenosine diphosphate, adenosine monophosphate, and phosphocreatine concentrations were measured by high-performance liquid chromatography. Results: High-energy phosphate concentrations in nonischemic brain tissue were similar in all groups (e.g., ATP concentration 2.47-2.79 μmol/g brain). With ischemia, depolarization occurred when ATP concentrations had decreased to 13-18% of normal. There were no significant differences in the concentration of any compound or in the energy charge among the groups, even though the time until depolarization was much longer in hypothermic animals (242 s) than in animals receiving large doses of anesthesia (119 and 132 s) or in normothermic halothane/nitrous oxide animals (73 s). Conclusions: The ATP/energy charge threshold for cortical depolarization was similar in all groups despite differing temperature or anesthetic conditions. Because hypothermia increased the time until depolarization, the rate of decrease in ATP concentration must have been slower in these animals than in the two groups receiving large-dose anesthetics, despite similar preischemic cerebral metabolic rates for glucose. This finding is similar to that of earlier studies and indicates that factors other than preischemic metabolic rate are responsible for controlling energy utilization after ischemia.