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(Circulation Research. 2004;94:559.)
© 2004 American Heart Association, Inc.

Integrative Physiology

Enhanced S-Nitroso-Albumin Formation From Inhaled NO During Ischemia/Reperfusion

Ella S.M. Ng, David Jourd’heuil, Joe M. McCord, Daniel Hernandez, Mitsukuni Yasui, Derrice Knight, Paul Kubes

From the Department of Physiology and Biophysics (E.S.M.N., D.K., P.K.), Immunology Research Group, University of Calgary, Calgary, Alberta, Canada; Center for Cardiovascular Sciences (D.J.), Albany Medical College, Albany, NY; and Webb-Waring Institute (J.M.M., D.H., M.Y.), University of Colorado Health Sciences Center, Denver, Colo.

Correspondence to Dr Paul Kubes, Immunology Research Group, University of Calgary, Health Sciences Center, 3330 Hospital Dr NW, Calgary, Alberta, Canada, T2N 4N1. E-mail pkubes{at}ucalgary.ca

	Abstract

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In the present study, we investigated whether inhaled nitric oxide (NO) was transported by plasma proteins, such as S-nitroso-albumin (SNO-Alb), in the feline circulation and whether this molecule delivers NO to the periphery under conditions of stress, specifically ischemia/reperfusion (I/R). A flow probe was interposed between the femoral and superior mesenteric artery for blood flow measurements, and a branch of the superior mesenteric vein was cannulated for arterial-venous sampling. In animals breathing room air, SNO-Alb was below detection level in arterial or venous blood. NO inhalation resulted in a significant arterial-venous gradient for SNO-Alb. Concomitant with this loss of SNO-Alb across the intestinal vasculature was an increase in nitrite (NO₂^-). However, this release of NO was not sufficient to alter intestinal blood flow. I/R during NO inhalation caused a very large increase in arterial SNO-Alb that permitted a 5-fold increase in SNO-Alb consumption and significant generation of NO₂^- within the postischemic intestinal vasculature. The increased SNO-Alb consumption was sufficient to dramatically improve intestinal blood flow. The very large burst of arterial SNO-Alb during I/R was completely blocked by the administration of superoxide dismutase, suggesting that oxidative stress contributed to the increased SNO-Alb formation. Our data suggest that inhaled NO can increase nitrosothiol production and these molecules may be a functional NO delivery system during cardiovascular disease.

Key Words: S-nitroso-albumin • oxidative stress • postischemic vasculature

	Introduction

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In the last 10 years, a significant development in the respiratory field has been the use of inhaled nitric oxide (NO), initially in infants with pulmonary hypertension, but more recently with adults who have respiratory distress as one manifestation of multiple-organ dysfunction syndrome.¹ Experimentally, inhaled NO has been shown to prevent lung injury from hemodialysis,² endotoxemia,³ ischemia/reperfusion (I/R) injury,⁴ and even lung allografts.⁵ Clinically, inhaled NO is used as a local vasodilator in the pulmonary vasculature. However, because the prevailing view has been that inhaled NO is rapidly inactivated in the pulmonary capillaries by reaction with oxyhemoglobin,⁶ most research in this area has been restricted to the lung. Recently, work has focused on the potential effects of inhaled NO on peripheral microvessels. Interestingly, inhaled NO has been reported to reduce systemic vascular resistance,⁷ increase kidney filtration rates,⁸ increase aortic cGMP levels,⁹ and improve blood flow in intestine after NO synthase inhibitors.^10,11 The latter work has been extended by Cannon and colleagues¹² who reported that after blockade of forearm NO production followed by forearm exercise, NO inhalation greatly improved blood flow. Clearly, the bulk of evidence would support the view that inhaled NO can be transported to peripheral vasculatures.

Although NO reacts very quickly with heme groups, thereby allowing for rapid clearance of NO from blood, NO can also react with the thiol of cysteine-93 on the ß-globin chain to form S-nitrosohemoglobin (SNO-Hb) within red blood cells allowing them to function as an important transport system. In addition, potent nitrosating and nitrosylating reactions can occur in the presence of plasma constituents that yield NO-carrying molecules that include the S-nitrosothiols (RSNOs).¹³ These molecules were shown to possess endothelium-derived relaxing factor-like properties including vasodilation.¹⁴ Although the responses were 7-fold less potent as a bolus than the classical NO donor nitroprusside, very importantly, they lasted minutes rather than seconds.¹⁴ Moreover, RSNOs including low molecular weight S-nitrosocysteine (CysNO), S-nitrosoglutathione (GSNO), and high molecular weight S-nitroso-albumin (SNO-Alb) have been detected in plasma with the latter molecule being the dominant circulating pool of NO in plasma.^13,15 However, despite the endogenous existence of these molecules, there remains considerable debate regarding physiological levels of these compounds. The variation in levels detected by different groups of researchers is likely due to the analytical techniques used. Moreover, the link between inhaled NO and SNO-Alb formation is tenuous. In fact, to date, inhaled NO has not been shown to increase SNO-Alb, and although only about 0.1% of NO gas infusion could be detected in a form resulting from S-nitrosation,¹⁶ this could still have great importance in certain pathophysiological conditions.

The majority of the aforementioned studies have been performed in healthy individuals under basal physiology. In this study, we measured the production of SNO-Alb in vivo in a pathological condition hallmarked by increased oxidative stress (namely ischemia/reperfusion) and report an unexpected but profound increase in arterial SNO-Alb, the formation of which is entirely dependent on superoxide (O₂^·-). The increased arterial SNO-Alb made it possible for the postischemic vasculature to consume copious amounts of SNO-Alb (large arterial-venous difference). This study also demonstrates that the increased disappearance of SNO-Alb may be linked to improved blood flow during I/R.

	Materials and Methods

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Materials
Sulfanilamide, mercuric chloride, 2,3-diaminonaphthalene (DAN), anhydrous potassium phosphate dibasic (K₂HPO₄), 5-sulfosalicylic acid, sodium chloride, salicylic acid, and benzoic acid were obtained from Sigma. Sodium hydroxide was a product of BDH Inc. Acetonitrile was obtained from VWR Canlab. Econo-Pac 10 DG disposable chromatography columns were purchased from Bio-Rad Laboratories. Millipore Milli-Q ultra-pure deionized water was used throughout the research.

Protocols for Cat Surgery
The surgical procedures used in this study have been described previously.¹⁰ Animal protocols were approved by the University of Calgary Animal Care Committee and met the Canadian Guidelines for Animal Research. Briefly, cats (2.5 to 3 kg; Animal Resources Farm, University of Calgary, Calgary, Alberta, Canada) were fasted for 24 hours and initially anesthetized with ketamine hydrochloride (75 mg intramuscularly). The jugular vein was cannulated, and anesthesia was maintained by the administration of sodium pentobarbital. Systemic arterial pressure was monitored using a pressure transducer (Statham P23A, Gould) connected to a catheter in the left carotid artery. A tracheotomy was performed to support breathing by artificial ventilation. NO at 80 ppm was delivered from a certified grade NO/balance N₂ gas cylinder to the inhalation line of a Harvard ventilator via a high-accuracy Matheson flowmeter (Matheson Gas Products), and NO and NO₂ were measured with a Pulmonox II NO and NO₂ electrochemical analyzer (Pulmonox Research and Development). Throughout the experiments, exhaled NO₂ was read online to ensure that levels were <5 ppm. This NO delivery setup was identical to the one used to deliver NO to newborn infants with respiratory distress in the neonatal intensive care unit of the Foothills Medical Center (University of Calgary), except in our system the cats were not provided with supplemental oxygen but rather ventilated on room air.

A midline abdominal incision was made, and a segment of small intestine was isolated from the ligament of Treitz to the ileocecal valve. The remainder of the small and large intestine was extirpated. Heparin sodium (10 000 U, Leo Pharma Inc) was administered, then an arterial circuit was established between the superior mesenteric artery (SMA) and left femoral artery. The SMA blood flow was continuously monitored using an electromagnetic flowmeter (Carolina Medical Electronics), and blood pressures were recorded with a physiological recorder (Grass Instruments).¹¹ Arterial blood samples were drawn from the circuit emerging from the femoral artery and leading into the SMA, whereas venous blood samples were taken through a catheter placed in a small branch of the superior mesenteric vein (SMV). In some experiments, we tested and observed no noticeable effect of heparin on basal SNO-Alb levels.

Sample Treatments
All blood samples collected from cats were immediately added to culture tubes containing ethylenediaminetetraacetic acid (EDTA) (final concentration 2 mmol/L) obtained from Sigma. They were centrifuged at 1500g for 10 minutes at 4°C. Plasma samples used for measurement of SNO-Alb were transferred to tubes that contained diethylenetriaminepentaacetic acid (DTPA) (final concentration 50 µmol/L) and N-ethylmaleinimide (NEM) (final concentration 5 mmol/L).

Experimental Protocols
In the first set of experiments, baseline arterial and venous plasma levels of SNO-Alb, GSNO, nitrite (NO₂^-), and nitrate (NO₃^-) were obtained in healthy animals. Next, animals were made to breathe 80 ppm NO for 60 minutes, and the levels were again measured. To examine the effect of inhaled NO on intestinal blood flow in healthy animals, SMA blood flow was recorded before and during 60 minutes NO inhalation.

In a second set of experiments, we evaluated whether NO was delivered by SNO-Alb across the intestinal microvasculature during I/R. Experiments were performed in which ischemia was induced by mechanically occluding the SMA blood flow with a Gaskell clamp.¹¹ Blood flow was reduced to 20% of the preischemic value (control) for an hour, and then the clamp was removed to allow reperfusion. At the onset of reperfusion either 0 or 80 ppm NO was given to the animals, and blood samples were collected at 30 and 60 minutes after reperfusion. Arterial and venous plasma SNO-Alb, GSNO, NO₂^-, and NO₃^-, as well as blood pressure and SMA blood flow were obtained.

In the third set of experiments, we examined the effect of superoxide dismutase (SOD) on the levels of RSNOs and NO₂^- during I/R. The SOD used in this study was a chimeric SOD2/3,¹⁷ which binds to vascular endothelial surfaces and to extracellular matrix components. Ischemia was performed for 60 minutes as described above. Animals received intravenous bolus doses (0.5 mg/cat or 0.5 U/gbw) of SOD2/3 5 minutes before reperfusion, and continued until 60 minutes of reperfusion. Inhaled NO was given to animals right at the onset of reperfusion for 60 minutes.

Because of space constraints, all detailed methodologies for RSNOs, NO₂^-, and NO₃^- are found in the expanded Materials and Methods in the online data supplement available at http://circres.ahajournals.org.

Statistical Analysis
All experimental data are expressed as mean±SEM. Probability values were calculated by Mann-Whitney test. Values of P<0.05 were considered statistically significant.

	Results

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Effect of Inhaled NO on Levels of SNO-Alb, Nitrite, and on Intestinal Blood Flow
Figure 1A demonstrates that basal levels of SNO-Alb were below the level of detection in both the arterial and venous vasculature. This is consistent with Rassaf and colleagues¹⁸ who reported basal levels of approximately 38 nmol/L, which is just below our level of detection. By contrast, inhaled NO induced significant amounts of arterial and venous SNO-Alb (Figure 1A). During NO inhalation, the arterial levels of SNO-Alb (382±66 nmol/L) were significantly higher than the venous levels (207±35 nmol/L). When we measured arterial and venous NO₂^- levels, the arterial levels of NO₂^- (489±33 nmol/L) were significantly lower than the venous levels of NO₂^- (664±53 nmol/L) during NO inhalation (Figure 1B). Approximately, 175 nmol/L of SNO-Alb were extracted and 175 nmol/L NO₂^- were produced (Table). Although we also tried to detect GSNO, the levels were never above the level of detection in any of the experiments. In fact, 200 nmol/L GSNO added to blood (but not plasma) degraded very rapidly such that even this sample was below the detection limit of our assay. It is important to note that the 175 nmol/L of SNO-Alb in the plasma phase did not have any physiological effect, because intestinal blood flow of animals breathing NO was identical to those not breathing NO (Figure 1C). Injection of 17.5 nmol/kg (equivalent to 175 nmol/L, assuming a blood volume of 10%) was also not sufficient to affect blood flow. In addition, neither heart rate nor blood pressure changed with the introduction of NO inhalation (data not shown).

View larger version (16K): [in this window] [in a new window]   Figure 1. Arterial and venous levels of SNO-Alb (A) and NO2- (B) under basal conditions and after 60 minutes of NO inhalation. C, Summary of the data for intestinal blood flow before and after 60 minutes of inhaled NO. Data are presented as mean±SEM. *P<0.05 relative to control; +P<0.05 arterial vs venous value.

View this table: [in this window] [in a new window]   Table 1. Differences in Arterial (A) and Venous (V) Levels of SNO-Alb and NO2-

An increase in the arterial and venous plasma NO₃^- was observed in each situation compared with the control (data not shown). However, the arterial and venous NO₃^- levels were not significantly different under all experimental conditions measured, consistent with previous reports that the major product of oxidative breakdown of NO in plasma was NO₂^-, with very little being converted to NO₃^-.¹⁹

Profound Effects of SNO-Alb on Intestinal Blood Flow During Ischemia/Reperfusion
When the intestinal microvasculature was severely stressed by induction of I/R, inhaled NO had very profound effects. Figure 2A shows that SNO-Alb was again not detectable in either arterial or venous plasma in animals ventilated on room air. Significant amounts of arterial and venous SNO-Alb were found by 30 minutes of reperfusion. Interestingly, arterial concentrations of SNO-Alb continued to increase to very high levels by 60 minutes after I/R with inhaled NO (Figure 2A). In fact, the amounts of arterial SNO-Alb generated after 60 minutes of NO inhalation during I/R (927±133 nmol/L; Figure 2A) were much higher than arterial SNO-Alb in cats breathing NO for the same length of time in the absence of I/R (382±66 nmol/L; Figure 1A). However, the same amount of venous SNO-Alb was detected in intact (Figure 1A) and I/R (Figure 2A) animals. Clearly, a much larger arterial-venous SNO-Alb gradient was noted both at 30 and 60 minutes after I/R during NO inhalation. The arterial-venous difference of SNO-Alb was approximately 312 nmol/L at 30 minutes of I/R and 724 nmol/L at 60 minutes of I/R versus 175 nmol/L in the absence of I/R (Table). Consistent with this view is the observation that the arterial to venous gradient of NO₂^- increased dramatically at 30 and 60 minutes of I/R due to a very large increase in venous NO₂^- values (Figure 2B). In fact, the difference in arterial and venous NO₂^- inversely paralleled the degree of arterial to venous SNO-Alb levels (Table). It is interesting however that whereas 724 nmol/L of SNO-Alb were lost across the vasculature, only 520 nmol/L NO₂^- were produced during NO inhalation and I/R (Table). This likely represents sequestration of NO by red blood cells, endothelium, smooth muscle, and other cells. In addition, NO₂^- may be directly taken up by red blood cells.²⁰

View larger version (28K): [in this window] [in a new window]   Figure 2. Arterial and venous levels of SNO-Alb (A) and NO2- (B) under control and at 30 and 60 minutes after reperfusion (Rep) in cats breathing NO. Arterial and venous samples for 60 minutes of inhaled NO (no I/R from Figure 1) are included for comparison. C, Intestinal blood flow under control, during 60 minutes of ischemia, and at 30 and 60 minutes after Rep in animals breathing NO or room air. NO was given to animals at the onset of reperfusion. Data are presented as mean±SEM. *P<0.05 relative to control; +P<0.05 relative to respective arterial samples (in A and B) or to untreated group (in C).

To determine whether this very large increase in SNO-Alb usage translated into physiology, we examined the intestinal blood flow before and after I/R in the presence and absence of NO inhalation in the group of animals described above. In animals that received room air, we observed that intestinal blood flow was 80 mL min^-1 100 g^-1 (Figure 2C). Ischemia was induced by occluding the SMA blood flow to 20% of the control for 1 hour. After removal of the clamp to allow for reperfusion, blood flow increased, but only reached about 60% of the control values at 30 and 60 minutes after reperfusion (Figure 2C). This is thought to be primarily due to a loss of local NO production.²¹ By contrast, NO inhalation during reperfusion resulted in a progressive rise in intestinal blood flow so that by 60 minutes of reperfusion, the blood flow was at control values (Figure 2C). A close correlation between elevated SNO-Alb and improved blood flow during I/R was noted.

Consumption of Plasma SNO-Alb Increases During 60 Minutes of Ischemia/Reperfusion in Cats Breathing NO
Figure 3 summarizes the total amounts of SNO-Alb consumed (or lost across the microvasculature) by the postischemic vasculature after I/R in the presence of NO inhalation (the arterial-venous SNO-Alb difference multiplied by the blood flow). During NO inhalation in intact intestinal microvasculature, 12±3 nmol/min per 100 g intestine of SNO-Alb was lost, whereas 21±4 and 54±11 nmol/min per 100 g SNO-Alb were lost at 30 and 60 minutes after I/R in animals breathing NO (Figure 3). Both postischemic SNO-Alb consumption values were significantly larger than that with NO inhalation alone, suggesting a demand for NO in the postischemic tissues.

View larger version (15K): [in this window] [in a new window]   Figure 3. SNO-Alb consumption during control, 30 and 60 minutes I/R and NO inhalation, or 60 minutes with inhaled NO in the absence of I/R. Data are presented as mean±SEM. *P<0.05 relative to control; +P<0.05 relative to 60 minutes with inhaled NO.

L-NAME Does Not Increase Arterial SNO-Alb Concentrations
To determine whether the increased arterial source of SNO-Alb occurred in response to any vascular stress, animals were treated with 25 mg/kg of N^G-nitro-L-arginine methyl ester (L-NAME) by infusion through the arterial loop at 0.1 mL/min for 60 minutes to inhibit the NO synthesis produced endogenously. This resulted in a dramatic increase in blood pressure and peripheral vascular resistance (data not shown). Under these conditions, inhalation of NO for 1 hour resulted in the same concentration of arterial and venous SNO-Alb regardless of the presence or absence of L-NAME (Figure 4A). Although one could argue that the low arterial levels of SNO-Alb were due to profound ongoing systemic consumption of NO, the NO₂^- levels would then be expected to be greatly elevated. Figure 4B shows no increase in NO₂^- levels, suggesting that the elevated arterial SNO-Alb and increased venous NO₂^- levels were specific to I/R.

View larger version (23K): [in this window] [in a new window]   Figure 4. Arterial and venous levels of SNO-Alb (A) and NO2- (B) in plasma at baseline, during NO inhalation, and during inhaled NO with simultaneous infusion of L-NAME. Data are presented as mean±SEM. *P<0.05 relative to control; +P<0.05 relative to respective arterial samples.

SOD Reduces the Formation of Arterial Plasma SNO-Alb During Ischemia/Reperfusion in NO-Breathing Animals
We report for the first time a dramatic increase in arterial SNO-Alb in an oxidative stress such as I/R. To determine whether the enormous amounts of arterial SNO-Alb during I/R in NO-breathing animals were induced by increased O₂^·- levels on reperfusion, we injected SOD2/3 to cats 5 minutes before reperfusion, and continued until 60 minutes of reperfusion. Figure 5A illustrates that in cats breathing NO, SOD2/3 injection prevented the rise in arterial levels of SNO-Alb during I/R. The levels of SNO-Alb did not exceed those with NO inhalation alone. If the manganese-containing SOD2/3 (MnSOD 2/3) simply degraded SNO-Alb to NO₂^-, this should result in elevated NO₂^- levels. Our data are more consistent with SOD2/3 preventing the production and/or release of a pool of NO containing molecules into the circulation, because when SOD2/3 was administered, plasma NO₂^- concentrations were maintained at the same level as that observed in animals breathing NO alone (Figure 5B). Figure 5C demonstrates that SOD2/3 paradoxically prevented the improvement in intestinal blood flow induced by NO inhalation. Results suggest that the injection of SOD2/3 prevents the formation of SNO-Alb, which in turn decreases the availability of NO from this species, and thus prohibits the improvement in intestinal blood flow. Although SOD2/3 has numerous beneficial effects alone in I/R, it is noteworthy that SOD2/3 per se does not affect blood flow in intestinal I/R in animals not breathing NO (data not shown).

View larger version (28K): [in this window] [in a new window]   Figure 5. Arterial and venous levels of SNO-Alb (A) and NO2- (B) under control and at 30 and 60 minutes after reperfusion (Rep) in cats breathing NO with SOD injected 5 minutes before reperfusion. Arterial and venous values for the inhaled NO group are added for comparison. C, Summary of the intestinal blood flow under control and during 60 minutes of ischemia and at 30 and 60 minutes after Rep in cats breathing NO. NO was given at the start of reperfusion. One group was treated with SOD 5 minutes before reperfusion. Data are presented as mean±SEM. *P<0.05 relative to control; +P<0.05 relative to respective arterial samples (A) or to untreated group (C).

	Discussion

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In 1992, Stamler and colleagues¹⁵ proposed that NO undergoes S-nitrosylation with protein-bound thiol groups, forming stable S-nitrosoproteins, including SNO-Alb. This molecule could conceivably stabilize NO, thereby establishing an NO delivery system in vivo. Since that time, it has been demonstrated that multiple biochemical pathways exist in vitro that cause the formation of RSNOs from NO and thiol-containing plasma proteins. Unfortunately, much less evidence exists that these biochemical pathways and their end products play any role as either homeostatic regulators of biological function or as mediators of pathophysiology in vivo. Recently, in vivo work demonstrated that inhaled NO extends far beyond the pulmonary vasculature to affect peripheral microvascular beds devoid of NO.^10,11 Although these in vivo studies did not address the mechanisms of action of NO transport, herein, we hypothesized that SNO-Alb may be one transporter.

In this study, we focused our attention on SNO-Alb, because it is much more stable than other plasma RSNOs and accounts for approximately 95% of all RSNOs in vivo.¹³ Under basal physiological conditions, NO inhalation induced a significant increase in SNO-Alb (380 nmol/L) as well as the end product of NO, NO₂^-. These results are in line with those of Rassaf and colleagues,¹⁶ who injected pure NO gas into humans and were able to detect approximately the same increase in SNO-Alb as we did with inhaled NO. By contrast, recent work by Cannon and colleagues has demonstrated no increase in SNO-Alb during NO inhalation.¹² It must be understood that different assays were used to measure SNO-Alb. In addition, SNO-Alb decomposes rapidly due to the interaction of this species with low molecular weight thiol groups.¹² To circumvent this problem, we added sulfanilamide, DTPA, and NEM during sampling. Such treatment reduces the background concentration of NO₂^- in biological samples, and prevents SNO-Alb degradation and trans-nitrosation reactions. Indeed, in the absence of sulfanilamide, DTPA, and NEM, it was very difficult to obtain consistent detectable levels of SNO-Alb (unpublished results, 2002).

A very unexpected but exciting finding in this study was the augmented formation of arterial SNO-Alb in the circulation during I/R when inhaled NO was simultaneously given to animals. During I/R, the arterial levels of SNO-Alb were almost 3-fold higher and SNO-Alb consumption was 5-fold greater than arterial SNO-Alb levels in NO-breathing animals without I/R. The mechanism of RSNO formation in vivo is unclear at this time but likely involves multiple reaction pathways. The S-nitrosylation of thiols by the NO autoxidation intermediate dinitrogen trioxide (N₂O₃) is well documented,²² but increased oxidative stress during I/R could also induce an increase in one electron oxidation intermediates of thiols, which may provide an important source of thyil radicals (RS^·) available for direct radical-radical combination with NO to form RSNOs.²³ An important alternative pathway is the reaction of thiols with peroxynitrite (ONOO^-/ONOOH), the diffusion-limited reaction product of NO with O₂^·-.²⁴ Several investigators reported that a number of molecules (albumin, uric acid, and glutathione) present in blood could combine with ONOO^- to generate NO donor compounds.²⁵ I/R would be an extremely conducive environment for the latter reaction to take place inasmuch as this pathology is associated with a burst of O₂^·- formation and a reduction in antioxidants such as glutathione peroxidase. Because this antioxidant has been shown to reduce S-nitrosylation by peroxynitrite,²⁶ a reduction of glutathione peroxidase would potentially enhance these reactions.

The elevated levels of SNO-Alb in arterial blood suggest that this molecule was made before reaching the intestine. Moreover, the levels of SNO-Alb leaving the intestine were lower than arterial blood, suggesting that net loss of SNO-Alb occurred in the postischemic intestine. Although the source of the increased SNO-Alb formation remains unknown, studies have shown that SNO-Alb can be formed in pulmonary tissues.^27,28 Moreover, very recent work has suggested that tissues can release NO from stored RSNOs.²⁷ Although O₂^·- has never been tested for its ability to induce the release of NO from RSNOs in tissues, our data would certainly support this contention. Sources of O₂^·- in the lung after intestinal I/R would include activated neutrophils and macrophages,²⁹ as well as perhaps circulating xanthine oxidase.²⁹ Consistent with this hypothesis is the observation that during I/R of various peripheral tissues including intestine, a dramatic increase in neutrophil recruitment into lungs occurs.³⁰ Moreover, during intestinal I/R increased xanthine oxidase has been demonstrated in both the circulation and bound to pulmonary endothelium.³¹ The possibility therefore exists that oxidative stress stimulates the release of NO from RSNOs in tissues by either circulating xanthine oxidase and/or inflammatory leukocytes in the pulmonary circulation. Finally, it should be noted that red blood cells may also contribute to the superoxide-induced increase in RSNOs. Indeed, Gow et al³² suggested that the intramolecular transfer from heme to thiols involves O₂^·- production.

In this study, we focused primarily on plasma SNO-Alb. However, it is very important to note that NO will directly or indirectly react with the heme and cysteine residues on the ß-globin chains of hemoglobin forming nitrosylhemoglobin (HbNO) and SNO-Hb, respectively.^32,33 Indeed, during NO inhalation (80 ppm) in humans, Cannon and colleagues¹² observed a 10-fold increase in HbNO levels with a significant artery to vein gradient, suggesting NO metabolism and/or delivery. The authors concluded that HbNO directly or via a SNO-Hb intermediate was stabilized and transported in blood peripherally to modulate blood flow. Finally, one can envisage both NO carrying molecules in plasma (SNO-Alb) and in RBCs transporting NO from lung to periphery in a cooperative manner. Clearly, examination of the RBC compartment in our model system is warranted.

Study Limitations
It is clear that the absolute concentration of these nitrosylated proteins in plasma range widely according to the method used and the way in which samples are prepared. Many investigators have detected higher levels of SNO-Alb. For example, Tyurin and colleagues³⁴ recently used photolysis/DAN fluorescence to measure micromolar amounts of SNO-Alb, which has been confirmed by others.^35,36 By contrast, Rassaf and colleagues¹⁸ reported much lower levels (38 nmol/L), which in our own assay was below the level of detection. We added sulfanilamide and NEM to eliminate nitrite and thiols, respectively. This could quite likely reduce the amount of SNO-Alb detected in plasma due to disruption of SNO homeostasis. Along the same lines, we could not detect GSNO in blood after NO inhalation, but it should be stressed that this was the case even when blood was spiked with 200 nmol/L GSNO. This could reflect the lack of survival of GSNO in whole blood, but it may also reflect an inability to measure this thiol using our approach. Finally, we used 80 ppm to observe significant functional increases in SNO-Alb. Although a common criticism has been that this is much higher than the standard 20 ppm or less used in clinical situations, it should be noted that in those clinical scenarios the lung is being targeted. Nevertheless, in the human neonatal clinical trial, 80 ppm was used with no adverse effects.³⁷ A recent study administered 80 ppm of NO to adults¹² with no harmful results. These studies underscore the plausibility of therapeutic use of 80 ppm NO to affect peripheral microvasculatures.

In conclusion, there is a growing body of evidence that NO depletion from peripheral tissues is a major complication of various cardiovascular pathologies including I/R. In our study, we demonstrated an increase in SNO-Alb associated with inhaled NO. Surprisingly, during I/R and NO inhalation, the body has the capacity to synthesize and/or release additional SNO-Alb, which can reverse some of the detrimental effects including reduced peripheral blood flow. Our data that inhaled NO can repair regional deficiencies in NO production without impacting on intact vascular beds strongly supports the idea that inhaled NO may be an effective form of therapeutic intervention in postischemic injury in the periphery.

	Acknowledgments

 
This study was supported by a group grant from the Canadian Institutes of Health Research (CIHR) (P.K.) and NIH grant CA89366 (D.J.). P. Kubes is a Canada Research Chair and an Alberta Heritage Foundation for Medical Research (AHFMR) scientist.

	Footnotes

 
Original received June 6, 2003; resubmission received November 12, 2003; revised resubmission received January 6, 2004; accepted January 9, 2004.

	References

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