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In this article, many of the whole-animal studies regarding the systemic effects …
Biology Articles » Biochemistry » Summary: Systemic Effects of Inhaled Nitric Oxide
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: [email protected]
The Proceedings of the American Thoracic Society 3:170-172 (2006).
Many effects of inhaled nitric oxide (NO) are not explainedby the convention that NO activates pulmonary guanylate cyclaseor is inactivated by ferrous deoxy- or oxyheme. Inhaled NO canaffect blood flow to a variety of systemic vascular beds, particularlyunder conditions of ischemia/reperfusion. It affects leukocyteadhesion and rolling in the systemic periphery. Inhaled NO therapycan overcome the systemic effects of NO synthase inhibition.In many cases, these systemic–NO synthase–mimeticeffects of inhaled NO seem to involve reactions of NO with circulatingproteins followed by transport of NO equivalents from the lungto the systemic periphery. The NO transfer biology associatedwith inhaled NO therapy is rich with therapeutic possibilities.In this article, many of the whole-animal studies regardingthe systemic effects of inhaled NO are reviewed in the contextof this emerging understanding of the complexities of NO biochemistry.
Key Words: circulation • nitric oxide • S-nitrosothiol
If biochemically relevant reactions of nitric oxide (NO) wereconfined to either the activation of guanylate cyclase or theinactivation of NO by hemoglobin, the beneficial effect of inhaledNO on oxygenation should translate into improved survival. Inpatients with acute lung injury, however, a variety of studieshave shown that inhaled NO improves oxygenation but, even atlow dose, does not significantly improve survival (1–3).Thus, the beneficial effects of inhaled NO may be balanced byadverse effects. These adverse effects could include reactionswith superoxide to form peroxynitrite in the airways and lungparenchyma, causing cytotoxic nitration (4). However, studieslike those of Taylor and coworkers (1) do not provide evidencethat inhaled NO worsens lung injury or pulmonary mechanics;in fact, in these studies, pulmonary morbidity associated withacute lung injury tended to be worse in the placebo group thanin the treatment group. Therefore, adverse systemic effectsof inhaled NO must be considered. For example, in the Taylorstudy (1), although the chance of having pneumonia was lowerin the inhaled NO group than in the placebo group, the chanceof having a systemic infection was substantially higher.
Nearly a decade ago, Troncy and coworkers (5) showed that inhaledNO dramatically increased porcine glomerular filtration ratein association with a modest increase in renal blood flow (Figure 1).Evidence for a systemic effect of inhaled NO was subsequentlyconfirmed in a series of articles by Fox-Robichaud and coworkers(6, 7) and Kubes (8), who showed that it increased blood flowafter ischemia in the reperfused cat mesentery. Fox-Robichaudused intravital microscopy to show that the feline mesentericartery constriction caused by the NO synthase (NOS) inhibitorNω -nitro-L-arginine methyl ester was completely overcome by 80ppm inhaled NO (Figure 2) (6).
More recently, Ng and coworkers (9) have gone back to this modelof the effect of inhaled NO on mesenteric ischemia to investigatethe mechanism by which the systemic effect is caused. They foundthat inhaled NO dramatically increased arterial levels of S-nitrosylatedalbumin (SNO-Alb). Half of this SNO-Alb was lost across themesenteric vascular bed during ischemia/reperfusion (Figure 3).This arteriovenous SNO-Alb gradient was associated with improvedmesenteric perfusion in the inhaled NO-treated cats. These dataare consistent with a model in which NO can be transferred tocirculating proteins in the lung and then transferred to systemicvascular beds to enhance relaxation. The preferential augmentationof blood flow to previously ischemic beds suggests that localtissue conditions (e.g., pH) can favor this kind of NO transferchemistry. The loss of SNO-Alb was associated with an arteriovenousincrease in nitrite across the mesenteric vascular bed, suggestingthat the bioactivation of nitrite (through reduction to NO)is not relevant to the mechanism by which inhaled NO causessystemic vascular effects (Figure 3).
Data showing that SNO-Alb depletion is associated with a systemicvascular effect of inhaled NO are consistent with those of Palmerand 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, withprofound vascular effects (11) and with data showing that glutathione(GSH) can deplete SNO-Hb, forming bioactive S-nitrosoglutathionewith systemic effects on respiratory control (12). Indeed, SNO-Hbcan augment vascular steal from nonmuscularized tumor vascularbeds by deoxygenation-augmented NO delivery to systemic arterioles(10). These observations suggest that investigators might wantto include measurement of the complete blood gas, which includesmeasurements of PCO2, PO2, oxygen saturation, and Hb-SNO (13),in studies of the systemic effects of inhaled NO therapy. Asdescribed by Gow and by McMahon and Doctor elsewhere in thisissue (pages 150–152, 153–160), SNO-Hb is lost exponentiallywith decreasing oxyhemoglobin saturation, reflecting NO transferfrom erythrocytes to bioactive thiols (12–14). This NOtransfer from deoxyhemoglobin can signal a variety of effectsin response to hypoxia, ranging from dilation of vascular bedsto increased minute ventilation, and likely represents an importantmechanism by which inhaled NO can cause systemic effects (11–14).Recent data suggest that erythrocyte SNO-Hb might be most accuratelymeasured by reduction in CuCl, cysteine, and CO to avoid lossof allosteric signal and to improve accuracy (13, 15).
The tissue and cellular localization to which circulating NOis delivered in the systemic vascular bed is critical. InhaledNO therapy has been proposed to have cardioprotective (16) andadverse cardiac effects (17). These effects may reflect increasedleft ventricular filling (18) and/or may result from systemicplatelet inhibition (16), but direct effects on cardiac myocyteshave also been proposed. At the cellular level, NO can increaseand decrease cardiac myocyte contractility (18, 19). NOS 1 localizationin the sarcoplasmic reticulum can activate the ryanodine receptor(sarcoplasm reticulum Ca2+ release channel) through S-nitrosylationof a specific cysteine, whereas activation of NOS 3 localizednear the L-type calcium channel on the cell membrane is inhibitory;activation of the former increases cardiac contractility, whereasactivation of the latter inhibits contractility (19). Deliveryof NO from circulating proteins to different regions of thecardiac myocyte could therefore have opposing effects on inotropy.
Little is known about the mechanisms by which NO delivered bycirculating proteins is targeted to specific cells and subcellularcompartments, although recent evidence suggest that, after transferfrom hemoglobin and/or albumin (11–14), SNO targetingis regulated by stereoselective mechanisms (permitting the L-isomer,but not the D-isomer of S-nitrosocysteine to be actively transportedinto cells and/or active in causing specific SNO-stimulatedeffects) (20, 21), and SNO bioactivities are locally regulatedby regional SNO metabolism in the cell (22).
As described Mannick elsewhere in this issue, the systemic effectsof inhaled NO therapy are not limited to myocytes. For example,80 ppm inhaled NO dramatically inhibits leukocyte adhesion inthe 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 effectis associated with decreased mesentery vascular leak. In addition,inhaled NO therapy can prolong bleeding time, likely throughthe inhibition of platelet aggregation (23).
In summary, the classical model by which NO diffuses out ofthe airway and into vascular smooth muscle exclusively to activateguanylate cyclase may explain the pulmonary vasodilator effectsof inhaled NO therapy but is inadequate to explain a varietyof other effects. Inhaled NO therapy results in the formationof circulating SNO proteins. These SNO proteins appear to transferNO to systemic vascular beds to varying degrees, depending onlocal redox conditions. These systemic effects include vascularsmooth muscle relaxation (particularly after ischemia), impairedleukocyte adhesion, impaired inflammatory response, increasedvascular leak, and impaired platelet adhesion. Recent evidencesuggests that inhaled NO can also improve long-term neurodevelopmentaloutcome in human infants (24). The availability of new assaytechniques for SNO protein detection (13, 25) and experimentsusing bait reactants (11) will likely help to clarify the mechanismsunderlying these systemic effects. It is likely that the wonderfulintricacy of this NO/SNO biochemistry will have important therapeuticimplications in the intensive care unit and elsewhere.
Supported by an unrestricted grant from INO Therapeutics andby the Ivy Foundation.
Conflict of Interest Statement: B.G. has consulted for and hasminor equity in Nitrox, LLC.
(Received in original form June 3, 2005; accepted in final form July 15, 2005)
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