Effect of High vs. Low Arterial Blood Oxygen Content on Cerebral: Energy Metabolite Levels during Hypoxia with: Normothermia and Hypothermia in the Rat

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Abstract
Effects of different levels of arterial blood O2 content (CaO2) on brain tissue ATP phosphocreatine (PCr), lactate and reduced NADH were studied during cerebral hypoxia in normothermic and hypothermic male Wistar rats with unilateral carotid ligation. Animals were exposed to hypoxia (PaO2 [arterial O2 pressure] 19-26 torr) for 25 min, and brain tissue metabolite values measured microfluorometrically were compared with those of normothermic normoxic controls. CaO2 was 4.0 .+-. 0.2 ml/dl (mean .+-. SEM [standard error of the mean]) at PaO2 26 torr in normothermic animals. CaO2 was increased to 8.2 .+-. 0.3 ml/dl at PaO2 26 torr by bicarbonate infusion producing a leftward shift of the HbO2 dissociation curve in 1 normothermic hypoxic group. In all normothermic hypoxic groups ATP and PCr decreased, and lactate and NADH increased significantly compared to control values. There was no significant difference in brain tissue metabolite values among these groups despite a 2-fold increase in CaO2 in 1 group. Hypothermia (32.degree. C) resulted in CaO2 8.4 .+-. 0.2 ml/dl at PaO2 26 torr. This was decreased to 4.0 .+-. 0.2 ml/dl by decreasing PaO2 to 19 torr in another group at the same temperature. ATP and PCr were well preserved in both groups despite the difference in CaO2. Although the lactate and NADH levels were increased in the hypothermic group with CaO2 4.0 .+-. 0.2 ml/dl, they were significantly lower than those values in normothermic hypoxic groups. Apparently the increase in CaO2 produced by hypothermia is not a major determinant in hypothermic protection during cerebral hypoxia.