Role of 20-HETE in the pial arteriolar constrictor response to decreased hematocrit after exchange transfusion of cell-free polymeric hemoglobin

Abstract

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 K(ATP) 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 muM 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, WIT-002 (10 microM; -5 +/- 1%), and was blocked by two chemically dissimilar selective inhibitors of the synthesis of 20-HETE, DDMS (50 microM; 0 +/- 4%) and HET-0016 (1 microM; +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 coadministration of the NO synthase inhibitor. We conclude 1) that activation of K(ATP) 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.

Fig. 1

Change in pial arteriolar diameter immediately after completion of an exchange transfusion of the hemoglobin polymer solution expressed as a percentage of the diameter after superfusing the cranial window with 0.25% ethanol vehicle (n = 6), 10 μM 20-hydroxyeicosa-6(Z),15(Z)-dienoic acid (WIT-002; n = 6), 50 μM N-methylsulfonyl-12,12-dibromododec-11-enamide (DDMS; n = 7), or 1 μM N-hydroxy-N′-(4-butyl-2-methylphenyl)-formamidine (HET-0016; n = 6). Values are means ± SD. *P < 0.05 from vehicle group.

Fig. 2

Change in pial arteriolar diameter after superfusion of the cranial window with 0.25% ethanol vehicle (n = 6), 300 μM N^ω-nitro-L-arginine (L-NNA; n = 6), 50 μM DDMS (n = 7), or 300 μM L-NNA plus 50 μM DDMS (n = 6), and at 0, 30, and 60 min after completion of an exchange transfusion of the hemoglobin polymer solution. Values are means ± SD and are expressed as a percentage of the baseline diameter before vehicle or drug superfusion of the window. *P < 0.05 from vehicle group.

Fig. 3

Change in pial arteriolar diameter after superfusion of the cranial window with 0.1% DMSO vehicle (n = 6) or 10 μM glibenclamide (n = 6), and at 0, 30, and 60 min after completion of an exchange transfusion of the albumin solution. Values are means ± SD and are expressed as a percentage of the baseline diameter before vehicle or drug superfusion of the window. *P < 0.05 from vehicle group.

Fig. 4

Percent change in pial arteriolar diameter during superfusion of the thromboxane analog U-46619 (1 μM) 1 h after completion of the exchange transfusion with the albumin (Alb) or hemoglobin (Hb) in the various groups treated with vehicle or drugs. Values are means ± SD. There were no significant differences between the treatment groups and the respective vehicle groups.