Local metabolic responses to cerebral ischemia.

Abstract

The brain is a highly differentiated organ, exhibiting a variety of local metabolic and hemodynamic responses to ischemia. Several analytical strategies are useful in characterizing these abnormalities: these include the direct assay of tissue metabolites; topographic methods for depicting regional patterns of NADH, ATP, glucose, lactate, and pH; in vivo spectroscopic methods for analyzing mitochondrial redox state over time; autoradiographic approaches to quantitation of local glucose utilization, blood flow, protein synthesis, and pH; and the noninvasive methods of positron emission tomography and NMR spectroscopy, which are applicable as well to human studies. In focal ischemia, "core" regions of severe blood-flow reductions progress to irreversible injury, while the adjacent "penumbral" zone appears to represent an unstable region threatened with possible injury yet potentially amenable to therapeutic intervention. Glucose utilization in focal ischemia is remarkable for its local heterogeneity and, in the postischemic state, tends to be predictive of local tissue injury. The selective vulnerability of particular brain regions to injury following global ischemia has now been extensively correlated with alterations of local metabolism and hemodynamics. Hyperglycemia is generally deleterious to neuronal survival in ischemia--an effect mediated via tissue lactacidosis. Small differences in brain temperature also profoundly influence ischemic outcome. Areas remote from an ischemic focus may also show metabolic and functional abnormalities--so-called "diaschisis," which may be transneuronally and/or humorally mediated. Multiple neurotransmitters are released during ischemia and interact to influence tissue injury. Regional postischemic hypoperfusion may also influence outcome.