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Circulation of red blood cells having high levels of 2,3-bisphosphoglycerate protects rat brain from ischemic metabolic changes during hemodilution.

BACKGROUND AND PURPOSE
We designed the present study to examine the effects of red blood cell oxygen-delivering capacity on ischemic brain metabolism during hemodilution with respect to red blood cell 2,3-bisphosphoglycerate content.


METHODS
A modification of red blood cell 2,3-bisphosphoglycerate content was achieved by an exchange transfusion of blood in which red blood cells were treated with either phospho(enol)pyruvate or inorganic phosphate in spontaneously hypertensive rats. Hematocrit values of circulating blood were varied from 30% to 20% during transfusion. Brain ischemia was produced in rats by bilateral carotid artery occlusion lasting 60 minutes. The concentrations of ATP and 2,3-bisphosphoglycerate in the blood and the ATP, phosphocreatine, and lactate concentrations in the brain were estimated by an enzymatic method.


RESULTS
Red blood cell 2,3-bisphosphoglycerate concentration increased to 200% of the pretransfusion level after the transfusion in which red blood cells were treated with phospho(enol)pyruvate, whereas the concentration decreased to 80% after the transfusion in which red blood cells were treated with phosphate. Red blood cell ATP content did not differ significantly between the phospho(enol)pyruvate- and phosphate-treated groups after transfusion. When hematocrit was approximately 30%, the ischemic brain ATP and lactate contents did not differ between the nonischemic and ischemic groups. However, as hematocrit was reduced to less than 25% the ischemic brain ATP content remarkably decreased and the lactate content substantially increased in the 2,3-bisphosphoglycerate-subnormal red blood cell group. In contrast, the ischemic brain ATP and phosphocreatine contents in the 2,3-bisphosphoglycerate-enriched red blood cell group were preserved and as high as those in the nonischemic group under the same conditions.


CONCLUSIONS
Cerebral ischemia was compensated with the increment of cerebral blood flow as a result of the reduction of hematocrit to optimal levels, but the extreme hemodilution induced insufficient oxygen supply to the brain tissue, resulting in a more marked impairment of brain metabolism despite an increase in cerebral blood flow. However, even in extreme hemodilution conditions the 2,3-bisphosphoglycerate-enriched red blood cells in circulating blood protected the brain from ischemic metabolic changes. These results suggest that the 2,3-bisphosphoglycerate-enriched red blood cells in the circulating blood may thus compensate for the insufficient oxygen supply in extremely anemic conditions by providing a sufficient supply of oxygen in the face of ischemic insult.