EFFECTS OF ISOFLURANE AND HYPOTHERMIA ON GLUTAMATE RECEPTOR-MEDIATED CALCIUM INFLUX IN BRAIN-SLICES

Background: To understand how volatile anesthetics protect neurons dur ing cerebral ischemia, we studied the effects of isoflurane on cerebra l glutamate receptor-mediated calcium influx. Calcium influx via these key excitatory receptors may mediate pain transmission, memory, and t he pathophysiologic sequelae of cerebral anoxia or ischemia. Because c erebral protection by hypothermia may involve a decrease in glutamate receptor activity, we also examined the interaction of temperature and isoflurane on glutamate receptor inhibition. Methods: We measured glu tamate receptor-mediated changes in cytosolic calcium in 300-mu m-thic k rat cortical brain slices. Temperature was varied to 28, 34, 37, or 39 degrees C and isoflurane partial pressure to 0.016-0.019 atm (equiv alent to 1.16 minimum alveolar concentration [MAC], adjusted for tempe rature and age). Brain slices were loaded with fura-2 to permit measur ement of cytosolic free calcium. Calcium changes due to the glutamate receptor agonist N-methyl-D-aspartate (NMDA) (50 mu M), to ischemia le vels of L-glutamate (1.0 mM) or to simulated ischemia (1.0 mM glutamat e, 100 mu M NaCN, and 3.5 mM iodoacetate) was then measured. Slice lac tate dehydrogenase leakage and adenosine triphosphate were measured as indices of cellular integrity. Results: Isoflurane reduced both L-glu tamate and NMDA-mediated calcium fluxes by approximately 60%. Neither the activity of the NMDA receptor nor its inhibition by isoflurane was altered by temperature. The rate of calcium influx during ischemia wa s significantly reduced both by temperature and by isoflurane (P < 0.0 5). Adenosine triphosphate loss and lactate dehydrogenase leakage were reduced by isoflurane during simulated ischemia by 37% and 73% (P < 0 .05), respectively. Conclusions: (1) At 1.16 MAC, isoflurane potently inhibits glutamate receptors and delays cellular injury induced by sim ulated ischemia, and (2) hypothermia does not reduce the intrinsic act ivity of cortical glutamate receptors but delays calcium accumulation during simulated ischemia. Isoflurane reduces the severity of key path ophysiologic events in an in vitro model of simulated cerebral ischemi a.