Inhaled ENO improved neurologic deficits attributable to experimental SAH. This was associated with greater vessel diameter, decreased brain edema, and improved LDF and tPO2, but ENO had no effect on MAP. ENO increases tissue oxygenation selectively in the ischemic brain and suggests that SNO-Hb provides a route to regulate microvascular blood flow.
The release of NO bioactivity from SNO-Hb is regulated allosterically by O2
saturation: NO bioactivity is liberated preferentially in environments that favor O2
occurs in clinical SAH.27
After SAH, tPO2
values observed in this experiment were increased to a viable range by ENO (for example, 32±10 mm Hg). A likely basis for this response was an ENO-induced increase in circulating SNO-Hb concentration. MAP remained unaffected by ENO, as did LDF in the unaffected cortex, consistent with the normal tPO2
values in sham-operated animals. This hypoxic selectivity provides perhaps the most clinically relevant attribute of pharmacologically increased SNO-Hb concentrations.
A target for S
-nitrosylation is the β
93 cysteine thiol in Hb. The extent to which Hb is S
-nitrosylated is dependent on the Hb oxygenation state.1,28
Oxyhemoglobin is readily nitrosylated and causes the S
-nitrosothiol to face inward, protecting the NO moiety from solvent.1
In the deoxy state, the cysteine residue is allosterically rotated outward into the blood phase, thereby enabling SNO-Hb to transnitrosylate other moieties. Thus, SNO-Hb provides a hypoxia-activated source of NO bioactivity that constitutes a basis for increased delivery of O2
to a hypoxic region.
There have been brief human exposures to ENO. Treatment of chronic pulmonary hypertension with ENO (0 to 70 ppm) by facemask increased SNO-Hb, decreased pulmonary artery pressure and resistance, and increased arterial PO2
without affecting systemic vascular resistance or MAP.13
Critically ill newborns with persistent pulmonary hypertension were exposed to ENO concentrations of up to 80 ppm, which dose-dependently increased arterial O2
Methemoglobin levels remained in a clinically acceptable range.
Humans have not undergone sustained ENO exposure. Although we saw no adverse effects in mice exposed to ENO for 72 hours, the clinical course of delayed arteriopathy after SAH is many days. It remains to be determined whether sustained ENO exposure is required to provide benefit. It is noteworthy that prolonged ENO exposure served to abate vasospasm onset and that acute ENO exposure 72 hours after SAH provided beneficial effects on tPO2 and LDF once vasospasm had become established.
-nitrosylation of proteins may play other roles in SAH.29,30
-nitrosylation inhibits both caspase-3 and nuclear factor-κ
B activation, thereby attenuating inflammation and cell death.31,32
Treatment with ENO blocks nuclear factor-κ
B activation in acute lung injury.18
-methyl-D-aspartate receptor undergoes S
-nitrosylation during hyperexcitation associated with hypoxia, with S
-nitrosylation inhibiting N
-methyl-D-aspartate– evoked currents.33
and elevated glutamate36
are known SAH consequences. Although we have provided direct evidence of improved tissue perfusion and tPO2
from ENO, favorable effects on cellular events may have also contributed to improved neurologic function.
ENO improved acute recovery from murine SAH. Adverse effects were not observed. Pharmacologic enhancement of SNO-Hb provides a novel strategy for SAH therapeutic intervention. Optimal ENO dosing strategies, the presence of sustained efficacy in long-term outcome analyses, direct comparison with established treatments, and confirmation of purported molecular mechanisms of action require continued investigation.