The long-term goal of this project is to determine the role of tissue acid-base and metabolic derangements in the evolution of infarction following the onset of permanent focal ischemia. Experimental focal ischemia is more relevant to most human strokes than are global models. Recent advances in the models of focal ischemia have decreased the variability in the size and location of the ischemic lesions encountered in previous models. We have previously shown that the deterioration of the perifocal region or penumbra (P) cannot be avoided if reflow is initiated after 3 hours of ischemia. The crux of this application is to determine those events involved in the evolution of irreversible damage in the P. Areas of reduced blood flow surrounding the ischemic focus following middle cerebral artery (MCA) occlusion apparently can survive the initial or acute insult, but eventually succumb to some secondary or chronic event. In the first 2 h of ischemia, the ATP levels in the P are near normal despite a time-dependent accumulation of lactate and loss of energy reserves. The loss of the energy reserves suggests that there is an ever increasing energy debt with longer periods of ischemia and if not corrected, energy failure will be inevitable. The perifocal region becomes infarcted within 24 h and the mechanisms of cell death, whether due to energy failure or secondary events, are only a matter of speculation. What is clear is that the death of the P contributes to long-term disability and that effective treatment of focal stroke remains elusive. The problems encountered in the investigation of metabolism in focal stroke have been, in part, resolved. The P and focal regions in brains frozen in situ now can be spatially identified and metabolites in micro-g pieces of tissue can be measured readily using microquantitative histochemistry. Combining these methodologies with MRI and in vivo microdialysis provides an unique approach to the study of metabolic derangements in the P. The immediate goals of this application are to determine if the metabolic conditions of the P trigger a set of secondary potentially pathogenic events, or alternatively, if the P deteriorates due to extrinsic factors which impose a workload which exceeds the capacity of P metabolism. The extrinsic factors to be considered are 1) transient spreading depression and 2) diffusion of neurotransmitters, lactic acid and potassium from the ischemic core. The additional energy demands on the perifocal region, its compromised blood flow, and an increasing energy imbalance would lead to energy failure, loss of cell volume control and infarction. This possibility will be tested by the following aims: 1) to determine in the P and ischemic core the changes in energy state, acid-base balance, water content and neurotransmitter homeostasis at various times after MCA occlusion, 2) to evaluate if extrinsic forces contribute to eventual infarction of the P by increasing the workload in that area and 3) to determine if altered brain temperature, blood glucose levels or hypercapnea, which are known to alter pHi and lactate levels during ischemia and also the characteristics of the extrinsic factors, will alter the rate and size of infarct formation and macromolecular changes. The information gained from these experiments will help provide a basis for developing an intervention which will neutralize the influence of intrinsic and extrinsic factors on the P and thereby improve the outcome following permanent focal ischemia.
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