Abstract
Dynamic changes in the regional cerebral glucose metabolic rate induced by hypoxia/reoxygenation or ischemia/reperfusion were investigated with a positron autoradiography technique. Fresh rat brain slices were incubated with [18F]2-fluoro-2-deoxy-D-glucose ([18F]FDG) in oxygenated Krebs-Ringer solution at 36 degrees C, and serial two-dimensional time-resolved images of [18F]FDG uptake in the slices were obtained. In the case of loading hypoxia (oxygen deprivation)/pseudoischemia (oxygen and glucose deprivation) for various periods of time, the net influx constant (K) of [18F]FDG at preloading and after reoxygenation/pseudoreperfusion (post-loading) was quantitatively evaluated by applying the Patlak graphical method to the image data. Regardless of the brain region, with hypoxia lasting > or =20 minutes, the postloading K value was decreased compared with the unloaded control, whereas with pseudoischemia of < or =40 minutes, approximately the same level as the unloaded control was maintained. Next, the neuroprotective effect against hypoxia/pseudoischemia loading induced by the addition of a free radical scavenger or an N-methyl-D-aspartate (NMDA) antagonist was assessed by determining whether a decrease in the postloading K value was prevented. Whereas with 20-minute hypoxia, both agents exhibited a neuroprotective effect, in the case of 50-minute pseudoischemia, only the NMDA antagonist did so, with the free radical scavenger being ineffective. These results demonstrate that hypoxia causes irreversible neuronal damage within a shorter period than ischemia, with both free radicals and glutamate suggested to be involved in tandem in the neurotoxicity induced by hypoxia, whereas glutamate alone is involved in ischemic neurotoxicity.
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