Anemia results in increased cerebral blood flow attributable to decreased blood viscosity and O2 carrying capacity. Attenuating the decrease in O2 carrying capacity by exchange transfusion with cell-free tetrameric crosslinked hemoglobin attenuates the increase in blood flow at reduced hematocrit independent of potential nitric oxide scavenging. During focal cerebral ischemia, in contrast, blood flow is promoted in the ischemic regions. In the present proposal, cell-free polymeric hemoglobin, which has a higher O2 carrying capacity than tetrameric hemoglobin, will be used as a physiologic tool to dissociate effects of viscosity from O2 carrying capacity on cerebrovascular regulation. Further, experimental studies of anemia are largely based on acute reductions in hematocrit, yet clinical anemia is usually a chronic condition. The overall goals of the proposal are to determine a) the mechanisms of changes in cerebral blood flow, baseline arteriolar diameter and vascular reactivity during acute and chronic reductions in hematocrit with and without reductions in O2 carrying capacity, and b) the role of heme oxygenase in ameliorating focal ischemic injury when plasma-based hemoglobin is exchanged for red cell- based hemoglobin. Specifically, the role of cytochrome p450 omega-hydroxylase activity, ATP-sensitive potassium channels and endothelin in the differential pial arteriolar diameter responses to albumin versus hemoglobin exchange transfusions will be determined pharmacologically. P450 omega-hydroxylase activity is O2 dependent in the physiological range and produces 20-HETE, a potent constrictor, whereas K-ATP channels are involved in hypoxic vasodilation. Endothelin has been reported to increase after tetrameric hemoglobin transfusion and may contribue to cerebral vasoconstriction. The role of P450 metabolites and endothelin in depressed vascular CO2 reactivity two days after hemoglobin transfusion will be studied. The contribution of heme oxygenase to endothelial dependent dilation and potential upregulation of this contribution during chronic anemia will be investigated both pharmacologically and in transgenic animals deficient in heme oxygenase -2. Reduction of infarct volume by hemoglobin transfusion at reduced hematocrit during focal cerebral ischemia may depend on adequate amounts of heme oxygenase to metabolize extravasated hemoglobin into antioxidant bilirubin. This mechanism will be investigated by using transgenic animlas deficient constitutive heme oxygenase -2 and inducible heme oxygenase -1, by infusion or bilirubin, and by prior hemoglobin transfusion as a preconditioning stimulus to upregulate heme oxygenase -1 before ischemia. These studies will render new insights into cerebrovascular regulation during anemia and into novel methodologies for hemodynamically ameliorating injury from stroke.
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