Hyperoxic vasoconstriction in the brain is mediated by inactivation of nitric oxide by superoxide anions

The hypothesis that decreases in brain blood flow during respiration of hyperbaric oxygen result from inactivation of nitric oxide (NO) by superoxide anions (O2(-)) is proposed. Changes in brain blood flow were assessed in conscious rats during respiration of atmospheric air or oxygen at a pressure of 4 atm after dismutation of O2(-) with superoxide dismutase or suppression of NO synthesis with the NO synthase inhibitor L-NAME. I.v. administration of superoxide dismutase increased brain blood flow in rats breathing air but was ineffective after previous inhibition of NO synthase. Hyperbaric oxygenation at 4 atm induced decreases in brain blood flow, though prior superoxide dismutase prevented hyperoxic vasoconstriction and increased brain blood flow in rats breathing hyperbaric oxygen. The vasodilatory effect of superoxide dismutase in hyperbaric oxygenation was not seen in animals given prior doses of the NO synthase inhibitor. These results provide evidence that one mechanism for hyperoxic vasoconstriction in the brain consists of inactivation of NO by superoxide anions, decreasing its basal vasorelaxing action.

Contributions of endothelial and neuronal nitric oxide synthases to cerebrovascular responses to hyperoxia

Hyperoxia causes a transient decrease in CBF, followed by a later rise. The mediators of these effects are not known. We used mice lacking endothelial or neuronal nitric oxide synthase (NOS) isoforms (eNOS-/- and nNOS-/- mice) to study the roles of the NOS isoforms in mediating changes in cerebral vascular tone in response to hyperoxia. Resting regional cerebral blood flow (rCBF) did not differ between wild type (WT), eNOS-/- mice, and nNOS-/- mice. eNOS-/- mice showed decreased cerebrovascular reactivities to NG-nitro-L-arginine methyl ester (L-NAME), PAPA NONOate, acetylcholine (Ach), and SOD1. In response to hyperbaric oxygen (HBO2) at 5 ATA, WT and nNOS-/- mice showed decreases in rCBF over 30 minutes, but eNOS-/- mice did not. After 60 minutes HBO2, rCBF increased more in WT mice than in eNOS-/- or nNOS-/- mice. Brain NO-metabolites (NOx) decreased in WT and eNOS-/- mice within 30 minutes of HBO2, but after 45 minutes, NOx rose above control levels, whereas they did not change in nNOS-/- mice. Brain 3NT increased during HBO2 in WT and eNOS-/- but did not change in nNOS-/- mice. These results suggest that modulation of eNOS-derived NO by HBO2 is responsible for the early vasoconstriction responses, whereas late HBO2-induced vasodilation depends upon both eNOS and nNOS.

Oxygen seizure latency and peroxynitrite formation in mice lacking neuronal or endothelial nitric oxide synthases

Nitric oxide (NO) from endothelial or neuronal NO synthases (eNOS or nNOS) may contribute both to the cerebrovascular responses to oxygen and potentially to the peroxynitrite-mediated toxic effects of hyperbaric oxygen (HBO(2)) on the central nervous system (CNS O(2) toxicity). In mice lacking eNOS or nNOS (-/-), regional cerebral blood flow (rCBF) and 3-nitrotyrosine (3-NT), a biochemical marker for peroxynitrite (ONOO(-)) formation, were measured in the brain during HBO(2) exposure. These variables were then correlated with EEG spiking activity related to CNS O(2) toxicity. In wild-type (WT) mice, HBO(2) exposure transiently reduced rCBF, but by 60 min rCBF was restored to baseline levels and above, followed by EEG spikes. Mice lacking nNOS also showed initial depression of rCBF followed by hyperemia but the delay in the onset of EEG discharges was greater. In contrast, in eNOS-deficient mice rCBF did not decrease and hyperemia was less pronounced during HBO(2). EEG spike latency was longer in eNOS(-/-) compared to WT or nNOS(-/-) mice. 3-NT gradually increased in all strains during HBO(2) but accumulation was slower in nNOS(-/-) mice, consistent with less ONOO(-) production. These results indicate that NOS-deficient mice have different cerebrovascular responses and tolerance to HBO(2) depending on which enzyme isoform is affected. The data suggest a key role for eNOS-dependent NO production in cerebral vasoconstriction and in the development of hyperoxic hyperemia preceding O(2) seizures, whereas neuronal NO may mediate toxic effects of HBO(2) mainly by its reaction with superoxide to generate the stronger oxidant, peroxynitrite.

Rapid cerebral ischemic preconditioning in mice deficient in endothelial and neuronal nitric oxide synthases

BACKGROUND AND PURPOSE

The purpose of this study was to test the hypothesis that nitric oxide is required for preconditioning in an intact animal model of focal ischemia using neuronal and endothelial nitric oxide synthase (nNOS and eNOS) knockout mice.

METHODS

Cerebral blood flow was measured in wild-type, nNOS knockout, and eNOS knockout mice by hydrogen clearance (absolute) and laser Doppler flowmetry (relative). Mice were preconditioned by three 5-minute episodes of transient middle cerebral artery occlusion (MCAO) and subjected to permanent MCAO. Neurological deficit and infarct size were determined 24 hours later.

RESULTS

Although wild-type mice showed protection from ischemic preconditioning, neither eNOS nor nNOS knockout mice showed protection. Laser Doppler measurements indicated that the relative blood flow decreases in core ischemic areas were the same in all groups.

CONCLUSIONS

Neither eNOS nor nNOS knockout mice show protection from rapid ischemic preconditioning, suggesting that nitric oxide may play a role in the molecular mechanisms of protection.

Regulation of the brain's vascular responses to oxygen

The mechanism of oxygen-induced cerebral vasoconstriction has been sought for more than a century. Using genetically altered mice to enhance or disrupt extracellular superoxide dismutase (EC-SOD, SOD3), we tested the hypothesis that this enzyme plays a critical role in the physiological response to oxygen in the brain by regulating nitric oxide (NO*) availability. Cerebral blood flow responses in these genetically altered mice to changes in PO2 demonstrate that SOD3 regulates equilibrium between superoxide (*O2-) and NO*, thereby controlling vascular tone and reactivity in the brain. That SOD3 opposes inactivation of NO* is shown by absence of vasoconstriction in response to PO2 in the hyperbaric range in SOD3+/+ mice, whereas NO-dependent relaxation is attenuated in SOD3-/- mutants. Thus, EC-SOD promotes NO* vasodilation by scavenging *O2- while hyperoxia opposes NO* and promotes constriction by enhancing endogenous *O2- generation and decreasing basal vasodilator effects of NO*.

[Cerebral blood flow modulates hyperbaric oxygen induced neurotoxicity by neuronal and endothelial nitric oxide]

The goal of work was to reveal changes in microcirculation of the rat brain and the role of nitric oxide (NO) in development of seizures at hyperbaric oxygen exposure. The Wistar rats with implanted paired platinum electrodes in left and right striatum were used for experiments. The latency of seizures was defined by the EEG, the cerebral blood flow (CBF) was measured by hydrogen clearance. One group of animals was exposed to a 5-ata oxygen, while the others before oxygen treatment were injected with: Nw-nitro-L-arginine methyl ester (L-NAME), blockator of constitutive NO synthase; 7-nitroindozol (7NI), specific inhibitor of neural NO synthase. The latency of seizures was 41 +/- 1.9 min at 5 ata oxygen exposure. CBF was decreased to 10-14% but before seizures it increased to 23 +/- 9%. L-NAME and 7NI prevented development of hyperoxygen hyperemia and onset of seizures. The results indicate occurrence of hyperbaric oxygen changes of the CBF that modulate neurotoxic effects of NO in neurons as well as in cerebral vessels.

[Hyperoxic vasoconstriction in the brain is realized by inactivation of nitric oxide by superoxide anions]

We tested a hypothesis that the cerebral blood flow (CBF) is reduced at hyperbaric oxygen due to inactivation of nitric oxide (NO) by superoxide anions (O2). In our experiments, the CBF was measured under hyperbaric oxygenation (HBO) 4ATA after inhibition of NO synthesis and inactivation of O2. The CBF was reduced at HBO exposure. Inhibition of NO--synthase type I and III (NOS) by L-NAME in the air caused the same decreasing of the CBF as at 4 ATA HBO. Hyperbaric vasoconstriction was diminished after NOS inhibition. Intravenous injection of superoxide dismutase (CuZn SOD) increased the CBF in the air and HBO exposure. This effect disappeared at preliminary NOS inhibition. These data suggest that inactivation of NO by O2 is a more effective mechanism of HBO vasoconstriction.

Nitric oxide production is enhanced in rat brain before oxygen-induced convulsions

Central nervous system oxygen toxicity (CNS O2 toxicity) is preceded by release of hyperoxic vasoconstriction, which increases regional cerebral blood flow (rCBF). These increases in rCBF precede the onset of O2-induced convulsions. We have tested the hypothesis that hyperbaric oxygen (HBO2) stimulates NO* production in the brain that leads to hyperemia and anticipates electrical signs of neurotoxicity. We measured rCBF and EEG responses in rats exposed at 4 to 6 atmospheres (ATA) of HBO2 and correlated them with brain interstitial NO* metabolites (NO(x)) as an index of NO* production. During exposures to hyperbaric oxygen rCBF decreased at 4 ATA, decreased for the initial 30 min at 5 ATA then gradually increased, and increased within 30 min at 6 ATA. Changes in rCBF correlated positively with NO(x) production; increases in rCBF during HBO2 exposure were associated with large increases in NO(x) at 5 and 6 ATA and always preceded EEG discharges as a sign of CNS O2 toxicity. In rats pretreated with L-NAME, rCBF remained maximally decreased throughout 75 min of HBO2 at 4, 5 and 6 ATA. These data provide the first direct evidence that increased NO* production during prolonged HBO2 exposure is responsible for escape from hyperoxic vasoconstriction. The finding suggests that NO* overproduction initiates CNS O2 toxicity by increasing rCBF, which allows excessive O2 to be delivered to the brain.

[Hyperbaric oxygen inhibits neutrophil infiltration and reduces postischemic brain injury in rats]

Reversible occlusion of the middle cerebral artery (MCA) was used to test hypothesis that hyperbaric oxygen inhibits the neutrophile infiltration into the ischemic brain thus reducing the brain injury. Treatment with hyperbaric oxygen prior to ischemia or during MCA occlusion significantly reduced neutrophile infiltration, motor disorders, and cerebral infarction volume.

Hyperbaric oxygen reduces cerebral blood flow by inactivating nitric oxide

Based on recent evidence that nitric oxide (NO(.)) is involved in hyperoxic vasoconstriction, we tested the hypothesis that decreases in NO(.) availability in brain tissue during hyperbaric oxygen (HBO(2)) exposure contribute to decreases in regional cerebral blood flow (rCBF). rCBF was measured in rats exposed to HBO(2) at 5 atmospheres (ATA) and correlated with interstitial brain levels of NO(.) metabolites (NO(X)) and production of hydroxyl radical ((.)OH). Changes in rCBF were also correlated with the effects of NO(.) synthase inhibitor (l-NAME), NO(.) donor PAPANONOate, and intravascular superoxide dismutase (MnSOD) during HBO(2). After 30 min of O(2) exposure at 5 ATA, rCBF had decreased in the substantia nigra, caudate putamen, hippocampus, and parietal cortex by 23 to 37%. These reductions in rCBF were not augmented by exposure to HBO(2) in animals pre-treated with l-NAME. After 30 min at 5 ATA, brain NO(X) levels had decreased by 31 +/- 9% and correlated with the decrease in rCBF, while estimated (.)OH production increased by 56 +/- 8%. The decrease in rCBF at 5 ATA was completely abolished by MnSOD administration into the circulation before HBO(2) exposure. Doses of NO(.) donor that significantly increased rCBF in animals breathing air had no effect at 5 ATA of HBO(2). These results indicate that decreases in rCBF with HBO(2) are associated with a decrease in effective NO(.) concentration and an increase in ROS production in the brain. The data support the hypothesis that inactivation of NO(.) antagonizes basal relaxation of cerebral vessels during HBO(2) exposure, although an effect of HBO(2) on NO(.) synthesis has not been excluded.

[Involvement of nitrogen oxide in the cerebral vasoconstriction during respiration with high pressure oxygen]

High pressure oxygen evokes a cerebral vasoconstriction and diminishes cerebral blood flow with the aid of mechanisms which are not yet sufficiently studied. We were checking a hypothesis that the hyperbaric oxygen (HBO2) inactivates cerebral nitrogen oxide (NO), interrupts its basal relaxing effect, and evokes a vasoconstriction. In our experiments, HBO2 decreased cerebral blood flow depending on the pressure. Inhibiting the NO-synthase weakened basal vasorelaxation in breathing with atmosphere air and eliminated the vasoconstriction in exposure to the HBO2. Inactivation of O2 prevented the HBO2-induced vasoconstriction. The data obtained reveal that diminishing of cerebral blood flow in HBO is related to the NO inactivation and weakening of its basal vasorelaxing effect. Possible mechanisms of the NO inactivation may involve its reaction with oxygen and superoxide anion which lead to diminishing of the tissue NO concentration and weakening of its vasorelaxing effect.

Nitric oxide and cerebral blood flow responses to hyperbaric oxygen

We have tested the hypothesis that cerebral nitric oxide (NO) production is involved in hyperbaric O(2) (HBO(2)) neurotoxicity. Regional cerebral blood flow (rCBF) and electroencephalogram (EEG) were measured in anesthetized rats during O(2) exposure to 1, 3, 4, and 5 ATA with or without administration of the NO synthase inhibitor (N(omega)-nitro-L-arginine methyl ester), L-arginine, NO donors, or the N-methyl-D-aspartate receptor inhibitor MK-801. After 30 min of O(2) exposure at 3 and 4 ATA, rCBF decreased by 26-39% and by 37-43%, respectively, and was sustained for 75 min. At 5 ATA, rCBF decreased over 30 min in the substantia nigra by one-third but, thereafter, gradually returned to preexposure levels, preceding the onset of EEG spiking activity. Rats pretreated with N(omega)-nitro-L-arginine methyl ester and exposed to HBO(2) at 5 ATA maintained a low rCBF. MK-801 did not alter the cerebrovascular responses to HBO(2) at 5 ATA but prevented the EEG spikes. NO donors increased rCBF in control rats but were ineffective during HBO(2) exposures. The data provide evidence that relative lack of NO activity contributes to decreased rCBF under HBO(2), but, as exposure time is prolonged, NO production increases and augments rCBF in anticipation of neuronal excitation.

Neutrophil sequestration and the effect of hyperbaric oxygen in a rat model of temporary middle cerebral artery occlusion

A rat model of reversible occlusion of the middle cerebral artery was developed to assess the role of neutrophils and prophylactic hyperbaric oxygen (HBO2) on cerebral injury. Blood flow to the ipsilateral caudate putamen nucleus was reduced by approximately 50% during 2 h of arterial occlusion, but unaffected on the contralateral side. Neutrophil accumulation in brain was documented as myeloperoxidase concentration, which was elevated in both ipsilateral and contralateral cerebral hemispheres at 1 and 46 h after occlusion/reperfusion. HBO2 administered before ischemia at 2.8 atm abs for 45 min, as well as antibody-induced neutropenia, reduced neutrophil accumulation, functional neurologic deficits, and cerebral infarct volume. These data demonstrate that one mechanism for benefit of HBO2 is related to its ability to ameliorate post-ischemic injury by inhibiting neutrophil sequestration. This mechanism should be taken into consideration when choosing partial pressures of oxygen for investigational clinical protocols.

Measurement of cerebral blood flow in rats and mice by hydrogen clearance during hyperbaric oxygen exposure

The hydrogen (H2) clearance method was adapted for the measurement of regional cerebral blood flow (rCBF) in anesthetized rats and mice during hyperbaric oxygen (HBO2) exposure. Polarographic platinum electrodes 0.1 mm in diameter were used to record H2 clearance curves from the parietal cortex (PC), substantia nigra (SN), and caudate putamen nucleus (CPN) after inhalation of 2.5% H2 in air. The system for H2 breathing under hyperbaric conditions was designed for remote operation from outside the chamber. The rCBF values (measured every 10 min) were calculated from the H2 clearance curves using the initial slope method. During air breathing control, rCBF values were similar to values reported using other methods. Considering all control rats together, blood flow (ml.100 g-1.min-1) was 89 +/- 3.6 in the SN, 78 +/- 4.7 in the CPN, and 76 +/- 6.7 in the PC. Blood flow (ml.100 g-1.min-1) for air-breathing mice was 108 +/- 11.4 in the SN and 74 +/- 8.8 in the CPN. During HBO2 exposure to 3 atm abs, rCBF in rats fell within 30 min by 26-39% (P < 0.05) and by 27-29% in mice (P < 0.05). HBO2 exposure to 4 atm abs induced maximal rCBF decreases in rats within 60 min by 37% (P < 0.01) in the SN and by 47% (P < 0.01) in the CPN. Breathing CO2 during HBO2 exposure to 4 atm abs reversed the vasoconstriction and led to a rCBF increase of 80-96% in rats. The H2 clearance method seems to be an accurate and sensitive technique for the repeated measurement of local CBF under hyperbaric conditions.

Blood flow regulation by S-nitrosohemoglobin in the physiological oxygen gradient

The binding of oxygen to heme irons in hemoglobin promotes the binding of nitric oxide (NO) to cysteinebeta93, forming S-nitrosohemoglobin. Deoxygenation is accompanied by an allosteric transition in S-nitrosohemoglobin [from the R (oxygenated) to the T (deoxygenated) structure] that releases the NO group. S-nitrosohemoglobin contracts blood vessels and decreases cerebral perfusion in the R structure and relaxes vessels to improve blood flow in the T structure. By thus sensing the physiological oxygen gradient in tissues, hemoglobin exploits conformation-associated changes in the position of cysteinebeta93 SNO to bring local blood flow into line with oxygen requirements.

Production of hydroxyl radical in the hippocampus after CO hypoxia or hypoxic hypoxia in the rat

Carbon monoxide poisoning produces both immediate and delayed neuronal injury in selective regions of the brain that is not readily explained on the basis of tissue hypoxia. One possibility is that cellular injury during and after CO poisoning is related to the production of reactive oxygen species (ROS) by the brain. In this study, we hypothesized that the extent of ROS generation in the brain would be greater after CO than after hypoxic hypoxia due to intracellular uptake of CO. We assessed hydroxyl radical (OH.) production by comparing the nonenzymatic hydroxylation of salicylic acid to 2,3-dihydroxybenzoic acid (2,3-DHBA) in the hippocampus of the rat by microdialysis during either CO hypoxia or an exposure to hypoxic hypoxia that produced similar PO2 and cerebral blood flow (CBF) values in the region of microdialysis. We found neither control animals nor animals exposed to 30 min of hypoxic hypoxia at a mean tissue PO2 of 15 mmHg demonstrated significant increases in 2,3-DHBA production in the hippocampus over the 2-h the exposure. In contrast, CO exposed rats which also developed brain PO2 values in the range of 15 mmHg showed highly significant increases in 2,3-DHBA production. We conclude that cerebral oxidative stress in the hippocampus of the rat during CO hypoxia in vivo is not a direct effect of decreased tissue oxygen concentration.

Dynamics of ion currents in the membrane of the isolated mollusc neuron under high pressure

The action of helium (up to 101 abs. atm.) under high pressure on ionic currents through the cellular membrane was studied in experiments on isolated neurons of a gastropod. A method of intracellular dialysis which is new for physiological investigations under hyperbaric conditions was used. A substantial decrease in the inward (sodium) current with increased pressure was found. Its amplitude was decreased by 10-15% at 25 abs. atm. and by 64% at 101 abs. atm. as compared with the control. At the same time, a shift in the currentvoltage characteristics along the abscissa was not observed. Significant changes in the outward (potassium) current were not appreciable. The inhibition of the sodium current observed in these experiments with increased pressure is associated with a possible change in the structure of the cell membrane under the influence of pressure; this leads in its turn to shifts in the functioning of the membrane components participating in the generation of the nerve impulse.

[Monitoring brain PO2 as a means of predicting brain PN2 changes under hyperbaric oxygenation simulating free-floating conditions]

In rabbits, rats and mice, the delays of maximum of the brain oxygenation curves in respect to the maximum pressure curves were found to be 47.4 +/- 5.5 s; 32.6 +/- 2.9 s; and 26.4 +/- 2.0 s, resp. The data obtained shows the brain oxygenation monitoring to help to predict the nitrogen tension dynamics in the "fast" tissues during simulation of free surfacing.

[The dynamics of the ion currents in the membrane of an isolated mollusk neuron under the action of high pressure]

The effect of high ambient pressure on the inward and outward currents in the membrane of the snail isolated neuron was studied. A significant inhibition of the inward (sodium) current occurred at 101 ata. No significant modification of the outward (potassium) current was found. The data obtained corroborates the hypothesis of inhibitory effect of high pressure on some membrane structures involved in generation of neuronal activity.