The activity of electroencephalogram (EEG) and cortical somatosensory evoked potential (SEP) was suppressed during cerebral Ischemia in rats subjected to the 4-vessel occlusion. Considerable variations were demonstrated in the decrease of phosphocreatine and ATP concentration during ischemia among the rats measured with 31 P-NMR, accompanied with cerebral acidification. Hypercapnia, induced in the rats studied by the inhalation of a gas mixture of 3O-4O° C02, suppressed the activity of EEG and cortical SEP. The cerebral acidification observed during the ischemia was more severe than that under the hypercapnia, implying that cerebral acidification is one of the possible causes for the decrease in the electrical activity of the brain during Ischemia.Key words : cerebral pH, tial, 31 P-NMR.cerebral Ischemia, hypercapnia, evoked potenCerebral ischemia is well known to depress the activity of the brain as demonstrated by electroencephalogram (EEG). NARUSE et al. (1984) demonstrated clearly that cerebral ischemia reduced ATP and phosphocreatine concentration accompanied with cerebral acidification during Ischemia in rats using 31P-NMR. In this study, we examined the effect of ischemia on the activity of cortex and brainstem, monitoring the somatosensory evoked potentials (SEP) and auditory brainstem responses (ABR) together with energy metabolism in rats with 31P-
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