Hyperoxic reperfusion after global ischemia decreases hippocampal energy metabolism

Abstract

Background and purpose: Previous reports indicate that compared with normoxia, 100% ventilatory O(2) during early reperfusion after global cerebral ischemia decreases hippocampal pyruvate dehydrogenase activity and increases neuronal death. However, current standards of care after cardiac arrest encourage the use of 100% O(2) during resuscitation and for an undefined period thereafter. Using a clinically relevant canine cardiac arrest model, in this study we tested the hypothesis that hyperoxic reperfusion decreases hippocampal glucose metabolism and glutamate synthesis.

Methods: After 10 minutes of cardiac arrest, animals were resuscitated and ventilated for 1 hour with 100% O(2) (hyperoxic) or 21% to 30% O(2) (normoxic). At 30 minutes reperfusion, [1-(13)C]glucose was infused, and at 2 hours, brains were rapidly removed and frozen. Extracted metabolites were analyzed by (13)C nuclear magnetic resonance spectroscopy.

Results: Compared with nonischemic controls, the hippocampi from hyperoxic animals had elevated levels of unmetabolized (13)C-glucose and decreased incorporation of (13)C into all isotope isomers of glutamate. These findings indicate impaired neuronal metabolism via the pyruvate dehydrogenase pathway for carbon entry into the tricarboxylic acid cycle and impaired glucose metabolism via the astrocytic pyruvate carboxylase pathway. No differences were observed in the cortex, indicating that the hippocampus is more vulnerable to metabolic changes induced by hyperoxic reperfusion.

Conclusions: These results represent the first direct evidence that hyperoxia after cardiac arrest impairs hippocampal oxidative energy metabolism in the brain and challenge the rationale for using excessively high resuscitative ventilatory O(2).

Figures

Figure 1

Experimental timeline. Chloralose-anesthetized animals underwent 10 minutes of cardiac arrest followed by 2 hours of reperfusion under hyperoxic (100% O2) or normoxic (21% to 30% O2) conditions for the first hour. [1-13C]glucose (0.2 mg/kg) was infused from 30 to 60 minutes of reperfusion. During the second hour of reperfusion, the ventilator settings were adjusted to maintain arterial pO2 at >80 and <120 mm Hg. At the end of the second hour, brains were removed and immediately immersed in liquid N2 (see Methods for details).

Figure 2

Hyperoxic reperfusion leads to increased unmetabolized glucose and increased [3-13C]lactate in the hippocampus and cortex. A, The amount of unmetabolized 13C-glucose was significantly higher in the hippocampus and cortex of animals resuscitated under hyperoxic conditions compared with nonischemic controls. The trend toward increased unmetabolized [1-13C]glucose in animals resuscitated under normoxic conditions was not significant (P=0.1). B, Although not significant (P=0.06), there was a trend toward elevated 13C labeling in the C3 position of lactate in the hippocampus of animals reperfused under hyperoxic, but not normoxic, conditions compared with nonischemic controls. There was no difference in 13C incorporation into lactate in the cortex. Values are mean±SE for n=7 nonischemic controls, 7 hyperoxic animals, and 6 normoxic animals. *Significantly different from nonischemic controls; 1-way ANOVA with Tukey post hoc analysis, P<0.05.

Figure 3

Hyperoxic reperfusion decreases hippocampal but not cortical metabolism of [1-13C]glucose to 13C-glutamate. A, Metabolism of [1-13C]glucose to glutamate but not glutamine is decreased in the hippocampus of animals resuscitated under hyperoxic conditions after a 10-minute cardiac arrest and 2-hour reperfusion paradigm. Decreased incorporation of 13C into glutamate C4 indicates decreased oxidative metabolism in neurons, possibly due to decreased PDHC activity and TCA cycle metabolism. Decreased labeling in the C2 of glutamate indicates impaired metabolism via the pyruvate carboxylase pathway in astrocytes. B, In the cortex, there was no difference in metabolism of 13C-glucose to either glutamate or glutamine after ischemia/reperfusion. Values are mean±SE for n=7 nonischemic controls, 7 hyperoxic animals, and 6 normoxic animals. *Significantly different from nonischemic controls; ANOVA with Tukey test, P<0.05.

Figure 4

Hyperoxic reperfusion decreases 13C percent enrichment of glutamate in the hippocampus. Percent enrichment was calculated as described in Methods. Hyperoxic reperfusion significantly decreased percent enrichment in all isotopomers of glutamate. Values are mean±SE for n=7 nonischemic controls, 7 hyperoxic animals, and 6 normoxic animals. *Significantly different from nonischemic controls; ANOVA with Tukey test, P<0.05.