Effect of cell-free hemoglobin transfusion and 20-hete synthesis inhibition on pial arteriolar diameter during middle cerebral artery occlusion

Background and aims: Hemodilution can increase cerebral blood flow during middle cerebral artery occlusion (MCAO), but neuroprotection is limited by the decrease in oxygen carrying capacity. Exchange transfusion of cell-free hemoglobin polymers (Hb) during MCAO can decrease infarct volume. Without MCAO, exchange transfusion of a 5% albumin solution produces pial arteriolar dilation, whereas exchange transfusion with a Hb solution produces pial arteriolar constriction that is blocked by the 20-HETE synthesis inhibitor HET0016 (Qin et al. J Appl Physiol 2006; 100:336-42). Because 20-HETE synthesis is oxygen dependent, the constriction after Hb transfusion may represent an autoregulatory response to limit over-oxygenation at decreased blood viscosity. We tested whether HET0016 would augment pial arteriolar dilation selectively after Hb exchange transfusion compared to albumin transfusion during MCAO. Methods: In isoflurane-anesthetized and mechanically ventilated rats, cranial windows were constructed over the MCA-ACA border region for measurement of pial arteriolar diameter. MCAO was produced for 2 h by the intraluminal filament technique. At 15 min of MCAO, the window was superfused with 0.1% ethanol vehicle or 1 uM HET0016 for the remainder of the protocol. At 35-50 min of MCAO, an exchange transfusion with a 5% albumin solution or 5% albumin + 6% Hb solution was performed. The Hb polymer had a nominal molecular weight of ~20 MDa and a P50 of 4 mmHg. Results: MCAO initially produced 36±10% (±SD) dilation which subsided to 19±6% dilation at 30 min of MCAO in the MCA border region. Subsequent exchange transfusion with the albumin solution resulted in no further change in diameter (22±16% of preischemia at 2h MCAO; n=8). In a group with initial dilation of 36±16%, HET0016 superfusion did not attenuate the loss of dilation at 30 min (22±14%) or the level of dilation after albumin transfusion (27±22% at 2 h MCAO; n=8). In a third group in which initial dilation after MCAO (25±9%) subsided to 12±12% at 30 min with superfusion of vehicle, subsequent exchange transfusion of Hb resulted in a loss of sustained dilation (4±19% of preischemia at 2h MCAO; n=8). In a fourth group with initial dilation of 35±12%, superfusion of HET0016 failed to block this loss of dilation (6±17%; n=9) after Hb transfusion. Conclusion: Initial vasodilation after MCAO is not fully sustained by either albumin or Hb transfusion. However, loss of vasodilation is greater after Hb transfusion than after albumin transfusion and may limit efficacy of this oxygen carrier during MCAO. Blunted vasodilation after albumin or Hb transfusion is not dependent on 20-HETE synthesis, possibly because low oxygenation during MCAO limits 20-HETE synthesis. (Supported by NIH NS38684).