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The Proceedings of the American Thoracic Society 3:170-172 (2006)
© 2006 The American Thoracic Society

Summary: Systemic Effects of Inhaled Nitric Oxide

Benjamin Gaston

Pediatric Respiratory Medicine, University of Virginia Children's Hospital, Charlottesville, Virginia

Correspondence and requests for reprints should be addressed to Benjamin Gaston, M.D., Pediatric Respiratory Medicine, University of Virginia Children's Hospital, Box 800386, Charlottesville, VA 22908. E-mail: bmg3g{at}virginia.edu

ABSTRACT

Many effects of inhaled nitric oxide (NO) are not explained by the convention that NO activates pulmonary guanylate cyclase or is inactivated by ferrous deoxy- or oxyheme. Inhaled NO can affect blood flow to a variety of systemic vascular beds, particularly under conditions of ischemia/reperfusion. It affects leukocyte adhesion and rolling in the systemic periphery. Inhaled NO therapy can overcome the systemic effects of NO synthase inhibition. In many cases, these systemic–NO synthase–mimetic effects of inhaled NO seem to involve reactions of NO with circulating proteins followed by transport of NO equivalents from the lung to the systemic periphery. The NO transfer biology associated with inhaled NO therapy is rich with therapeutic possibilities. In this article, many of the whole-animal studies regarding the systemic effects of inhaled NO are reviewed in the context of this emerging understanding of the complexities of NO biochemistry.

Key Words: circulation • nitric oxide • S-nitrosothiol

If biochemically relevant reactions of nitric oxide (NO) were confined to either the activation of guanylate cyclase or the inactivation of NO by hemoglobin, the beneficial effect of inhaled NO on oxygenation should translate into improved survival. In patients with acute lung injury, however, a variety of studies have shown that inhaled NO improves oxygenation but, even at low dose, does not significantly improve survival (1–3). Thus, the beneficial effects of inhaled NO may be balanced by adverse effects. These adverse effects could include reactions with superoxide to form peroxynitrite in the airways and lung parenchyma, causing cytotoxic nitration (4). However, studies like those of Taylor and coworkers (1) do not provide evidence that inhaled NO worsens lung injury or pulmonary mechanics; in fact, in these studies, pulmonary morbidity associated with acute lung injury tended to be worse in the placebo group than in the treatment group. Therefore, adverse systemic effects of inhaled NO must be considered. For example, in the Taylor study (1), although the chance of having pneumonia was lower in the inhaled NO group than in the placebo group, the chance of having a systemic infection was substantially higher.

Nearly a decade ago, Troncy and coworkers (5) showed that inhaled NO dramatically increased porcine glomerular filtration rate in association with a modest increase in renal blood flow (Figure 1). Evidence for a systemic effect of inhaled NO was subsequently confirmed in a series of articles by Fox-Robichaud and coworkers (6, 7) and Kubes (8), who showed that it increased blood flow after ischemia in the reperfused cat mesentery. Fox-Robichaud used intravital microscopy to show that the feline mesenteric artery constriction caused by the NO synthase (NOS) inhibitor N{omega}-nitro-L-arginine methyl ester was completely overcome by 80 ppm inhaled NO (Figure 2) (6).


Figure 1
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  		Figure 1. Variation in glomerular filtration rate (GFR). Control arm: mean (SEM) values for GFR for each sampling time in each stage (I–IV) for experiment A (no intervention during the four stages), experiment B (infusion of phenylephrine 15.75 [0.85] U/kg/min during stages II and III), and experiment C (inhaled NO [iNO] 40 ppm during stages II and III). GFR decreased with time in experiment A. Infusion of phenylephrine had no effect on GFR, whereas iNO increased GFR. * Significant difference for analysis of variance (p < 0.05) and for the Fisher's protected least significant difference test of the stage compared with stage I (p < 0.008). Reproduced by permission from Reference 5.

 

Figure 2
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  		Figure 2. Change in arterial diameter at 2 h from baseline control period in cat mesentery superfused with N{omega}-nitro-L-arginine methyl ester alone or superfused with N{omega}-nitro-L-arginine methyl ester while the cat was treated with 80 ppm inhaled NO. *p < 0.05 relative to untreated group. Reproduced by permission from Reference 6.

 
More recently, Ng and coworkers (9) have gone back to this model of the effect of inhaled NO on mesenteric ischemia to investigate the mechanism by which the systemic effect is caused. They found that inhaled NO dramatically increased arterial levels of S-nitrosylated albumin (SNO-Alb). Half of this SNO-Alb was lost across the mesenteric vascular bed during ischemia/reperfusion (Figure 3). This arteriovenous SNO-Alb gradient was associated with improved mesenteric perfusion in the inhaled NO-treated cats. These data are consistent with a model in which NO can be transferred to circulating proteins in the lung and then transferred to systemic vascular beds to enhance relaxation. The preferential augmentation of blood flow to previously ischemic beds suggests that local tissue conditions (e.g., pH) can favor this kind of NO transfer chemistry. The loss of SNO-Alb was associated with an arteriovenous increase in nitrite across the mesenteric vascular bed, suggesting that the bioactivation of nitrite (through reduction to NO) is not relevant to the mechanism by which inhaled NO causes systemic vascular effects (Figure 3).


Figure 3
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  		Figure 3. Arterial and venous levels of S-nitrosylated albumin (SNO-Alb; A) and Formula (B) under control and at 30 and 60 min after reperfusion (Rep) in cats breathing NO. Arterial and venous samples for 60 min of inhaled NO are included for comparison. (C) Intestinal blood flow under control, during 60 min of ischemia, and at 30 and 60 min 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). Reproduced by permission from Reference 9.

 
Data showing that SNO-Alb depletion is associated with a systemic vascular effect of inhaled NO are consistent with those of Palmer and colleagues (11), who have shown that N-acetyl cysteine, serving as a bait reactant, can deplete SNO-Alb and S-nitrosohemoglobin (SNO-Hb), forming bioactive S-nitroso-N-acetylcysteine, with profound vascular effects (11) and with data showing that glutathione (GSH) can deplete SNO-Hb, forming bioactive S-nitrosoglutathione with systemic effects on respiratory control (12). Indeed, SNO-Hb can augment vascular steal from nonmuscularized tumor vascular beds by deoxygenation-augmented NO delivery to systemic arterioles (10). These observations suggest that investigators might want to include measurement of the complete blood gas, which includes measurements of PCO2, PO2, oxygen saturation, and Hb-SNO (13), in studies of the systemic effects of inhaled NO therapy. As described by Gow and by McMahon and Doctor elsewhere in this issue (pages 150–152, 153–160), SNO-Hb is lost exponentially with decreasing oxyhemoglobin saturation, reflecting NO transfer from erythrocytes to bioactive thiols (12–14). This NO transfer from deoxyhemoglobin can signal a variety of effects in response to hypoxia, ranging from dilation of vascular beds to increased minute ventilation, and likely represents an important mechanism by which inhaled NO can cause systemic effects (11–14). Recent data suggest that erythrocyte SNO-Hb might be most accurately measured by reduction in CuCl, cysteine, and CO to avoid loss of allosteric signal and to improve accuracy (13, 15).

The tissue and cellular localization to which circulating NO is delivered in the systemic vascular bed is critical. Inhaled NO therapy has been proposed to have cardioprotective (16) and adverse cardiac effects (17). These effects may reflect increased left ventricular filling (18) and/or may result from systemic platelet inhibition (16), but direct effects on cardiac myocytes have also been proposed. At the cellular level, NO can increase and decrease cardiac myocyte contractility (18, 19). NOS 1 localization in the sarcoplasmic reticulum can activate the ryanodine receptor (sarcoplasm reticulum Ca2+ release channel) through S-nitrosylation of a specific cysteine, whereas activation of NOS 3 localized near the L-type calcium channel on the cell membrane is inhibitory; activation of the former increases cardiac contractility, whereas activation of the latter inhibits contractility (19). Delivery of NO from circulating proteins to different regions of the cardiac myocyte could therefore have opposing effects on inotropy.

Little is known about the mechanisms by which NO delivered by circulating proteins is targeted to specific cells and subcellular compartments, although recent evidence suggest that, after transfer from hemoglobin and/or albumin (11–14), SNO targeting is regulated by stereoselective mechanisms (permitting the L-isomer, but not the D-isomer of S-nitrosocysteine to be actively transported into cells and/or active in causing specific SNO-stimulated effects) (20, 21), and SNO bioactivities are locally regulated by regional SNO metabolism in the cell (22).

As described Mannick elsewhere in this issue, the systemic effects of inhaled NO therapy are not limited to myocytes. For example, 80 ppm inhaled NO dramatically inhibits leukocyte adhesion in the reperfused feline mesentery (6). Fox-Robichaud and colleagues (6) have shown that inhaled NO overcomes the effect of NOS inhibition, mitigating leukocyte flux in the feline mesentery. This effect is associated with decreased mesentery vascular leak. In addition, inhaled NO therapy can prolong bleeding time, likely through the inhibition of platelet aggregation (23).

In summary, the classical model by which NO diffuses out of the airway and into vascular smooth muscle exclusively to activate guanylate cyclase may explain the pulmonary vasodilator effects of inhaled NO therapy but is inadequate to explain a variety of other effects. Inhaled NO therapy results in the formation of circulating SNO proteins. These SNO proteins appear to transfer NO to systemic vascular beds to varying degrees, depending on local redox conditions. These systemic effects include vascular smooth muscle relaxation (particularly after ischemia), impaired leukocyte adhesion, impaired inflammatory response, increased vascular leak, and impaired platelet adhesion. Recent evidence suggests that inhaled NO can also improve long-term neurodevelopmental outcome in human infants (24). The availability of new assay techniques for SNO protein detection (13, 25) and experiments using bait reactants (11) will likely help to clarify the mechanisms underlying these systemic effects. It is likely that the wonderful intricacy of this NO/SNO biochemistry will have important therapeutic implications in the intensive care unit and elsewhere.

FOOTNOTES

Supported by an unrestricted grant from INO Therapeutics and by the Ivy Foundation.

Conflict of Interest Statement: B.G. has consulted for and has minor equity in Nitrox, LLC.

(Received in original form June 3, 2005; accepted in final form July 15, 2005)

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