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1 Department of Anesthesiology and Critical Care Medicine, The Johns Hopkins University, Baltimore, Maryland, USA
2 Department of Biochemistry and Molecular Biology, University of Maryland, Baltimore, Maryland, USA
3 Department of Physiology, Medical College of Wisconsin, Milwaukee, Wisconsin, USA
* To whom correspondence should be addressed. E-mail: rkoehler{at}jhmi.edu.
The cerebrovascular response to decreases in hematocrit and viscosity depends on accompanying changes in arterial O2 content. This study examines whether: 1) the arteriolar dilation seen after exchange transfusion with a 5% albumin solution can be reduced by the KATP channel antagonist, glibenclamide (known to inhibit hypoxic dilation), and 2) the arteriolar constriction seen after exchange transfusion with a cell-free hemoglobin polymer to improve O2 carrying capacity can be blocked by inhibitors of the synthesis or vasoconstrictor actions of 20-HETE. In anesthetized rats, decreasing hematocrit by one-third with albumin exchange transfusion dilated pial arterioles (14 ± 2%; SD), whereas superfusion of the surface of the brain with 10 µM glibenclamide blocked this response (-10 ± 7%). Exchange transfusion with polymeric hemoglobin decreased the diameter of pial arterioles by 20 ± 3% without altering arterial pressure. This constrictor response was attenuated by superfusing the surface of the brain with a 20-HETE antagonist, WIT002 (10 µM; -5 ± 1%) and was blocked by two chemically dissimilar selective inhibitors of the synthesis of 20-HETE, DDMS (50 µM; 0 ± 4%) and HET0016 (1 µM; +6 ± 4%). The constrictor response to hemoglobin transfusion was not blocked by an inhibitor of nitric oxide (NO) synthase and the inhibition of the constrictor response by DDMS was not altered by co-administration of the NO synthase inhibitor. We conclude 1) that activation of KATP channels contributes to pial arteriolar dilation during anemia, whereas 2) constriction to polymeric hemoglobin transfusion at reduced hematocrit represents a regulatory response that limits increased O2 transport and that is mediated by increased formation of 20-HETE, rather than by NO scavenging.
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