[L’inhalation de monoxyde d’azote en 2003：une revue de ses mécanismes et de son action]<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

Tianlong Wang MD，Driss El Kebir PhD，Gilbert Blaise MD

　　Objectif：Revoir les effets pulmonaires et généraux de l’oxyde nitrique （NO） endogène et du NO inhalé，administré aux patients.

　　Source：Une recherche systématique de données expérimentales，d’études de cas humains et dessais cliniques randomisés realizes depuis 1980，année de la découverte de ce facteur relaxant d’origine end othéliale，dénomination qui lui ft attribuée à lepoque.

　　Constatations principales：Le monoxyde dazote a des effets pulmonaires et généraux. Le monoxyde d’azote inhalé ne cause pas seulement une vasodilatation pulmonaire sélective，mais il provoque aussi une vasoconstriction pulmonaire des vaisseaux perfusant les alvéoles non ventilées. Les effets généraux de monoxyde d’azote，ycompris la modulation de la distribution du débit sanguin，l’augmentation du débit rénal，l’interaction avec la coagulation，la capacité fonctionnelle de la fibrinolyse et des plaquettes，la modification de la réponse inflammatoire，sont décrits et les mécanismes du transport de monoxyde d’azote sont expliqués. On discute également de la toxicité de monoxyde d’azote inhalé.

　　Conclusion：Les multiples effets de l’oxyde nitrique inhale expliquent l’intérêt qu’il présente comme médication pulmonaire et extrapulmonaire.

　　IN 1980，Furchgott and Zawadzki1 discovered that endothelial cells stimulated by acetylcholine released a vasodilator. Initially named endothelium－derived relaxing factor，its real nature was established several years later，and the molecule was identified as nitric oxide （NO）.^[2，3] Over the past two decades，many of NO’s biological mechanisms and therapeutic indications have been elucidated. This review summarizes the biological effects of NO.

　　Synthesis of NO<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

　　Endogenous NO

　　Endogenous NO is synthesized by NO synthase （NOS），which combines O₂ with L－arginine to produce NO and L－citrulline.^[4，5] The reaction requires nicotinamide adenine dinucleotide phosphate，flavin adenine dinucleotide，flavin mononucleotide，and tetrahydrobiopterin as cofactors.^[6，7] Three types of NOS have been cloned：type I （neuronal or nNOS），type II （inducible or iNOS），and type III （endothelial or eNOS）. Neuronal NOS and eNOS are constitutive calcium calmodulin－dependent enzymes and produce NO in nanomolar concentrations when activated by different agonists that increase intracellular calcium. Two isoforms of nNOS have been documented by antisense mapping.^[8] Inducible NOS is synthesized in response to several inflammatory mediators and produces NO in micromolar concentrations in a calciumindependent manner.^[9，10]

　　In vivo synthesis of NO occurs in the lungs in the vascular endothelium，epithelial cells， nerve cells， smooth muscle cells，and inflammatory cells such as macrophages. In addition，the cells of the upper airways，especially in the nose and paranasal sinuses，generate large amounts of NO，which is inhaled whenever the patient inspires through the nose. Commercial production of NO In vitro，NO gas can be produced from the reaction of liquid sulfuric acid （H₂SO₄） and liquid sodium nitrite （NaNO₂），and then purified of nitrogen （N₂），nitrous oxide （N₂O） and carbon dioxide （CO₂）：

　　H₂SO₄+2NaNO₂　　NO+NO₂+NaSO4+H₂O18

　　NO gas can be added to the inspiratory gas flow during the inspiratory phase and delivered to the patient at a precise concentration. Several companies have developed very efficient NO delivery systems that match NO administration with inspiratory gas flow.

　　NO is available in industrial and medical grades. Medical grade NO is higher in purity but is more expensive than industrial grade NO. Currently，most Canadian hospitals use industrial NO from different suppliers；however，at some point，the therapeutic standard will require medical grade NO gas. Unfortunately，medical grade NO is only available through the Special Access Program and has not yet received definitive approval from Health Canada.

　　In Canada，medical grade NO is available as ViaNOx^TM－H （Summit Technology Inc.） in C－size cylinders （2.¹⁸ L of 800 ppm NO） at $2，728 Canadian. In the United States and Europe，INO Therapeutics sells a complete package that comprises a delivery apparatus，medical grade NO，staff training，and paramedical and technical support. The company charges for NO treatment on an hourly basis （US $150　hr^－1 or Euro 150　hr^－1 for a maximum of 96 hr）.

　　Biological mechanisms of NO

　　NO stimulates soluble guanylate cyclase （sGC） and increases cyclic guanylate monophosphate （cGMP）. The latter activates cGMP－dependent protein kinases that are abundant in the cerebellum，smooth and cardiac myocytes，platelets，and leukocytes.^[20] In turn，the kinases mediate the cGMP－induced decrease in intracellular calcium concentration in vascular smooth muscles and produce relaxation and vasodilatation.

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　　Pulmonary effects of NO

　　NO has several potentially beneficial effects on pulmonary function by maintaining low pulmonary arterial pressures and sustaining normal vascular permeability. In addition to its vasodilatory effects，recent work in pigs suggests that in vivo NO production is inhibited by a blood－borne inhibitor of NOS （such as asymmetric dextromethylarginine or by direct inhibition of eNOS） in the presence of inhaled NO （iNO）. The effect is greater in hypoxic regions. Thus，iNO may have a dual effect：pulmonary vasodilatation in ventilated regions and pulmonary vasoconstriction in poorly ventilated or hypoxic regions. The overall effect is a decrease in pulmonary arterial pressures，attenuation of ventilation－perfusion mismatch，and improvement in oxygenation. Animal studies show that iNO is particularly effective in reversing hypoxia－induced pulmonary hypertension but is less potent in vasoconstrictorinduced pulmonary hypertension from thromboxane A₂ or protamine－heparin reaction.

　　In addition to its effect on the pulmonary vasculature，NO has some antibacterial actions provided through formation of reactive nitrogen oxides like peroxynitrite.  NO also modulates ciliary beat frequency and can inhibit or stimulate mucus secretion.

　　Inhaled NO has been considered for a long time to be a selective pulmonary vasodilator that has no clinically significant effect on blood pressure and cardiac output. Its selective action results from the fixation of iNO to the heme moiety of the hemoglobin molecule after passing through the pulmonary vessel wall. NO is then oxidized to NO₂ and NO₃. Hemoglobin is transformed to methemoglobin，which is secondarily reduced to hemoglobin by methemoglobin reductase.  Although iNO has no systemic hemodynamic effects，it does have extra－pulmonary activity.

　　Effects of NO on coagulation

　　NO interferes with platelet and leukocyte functions， fibrinolysis， restenosis， and reperfusion injury by inhibiting expression of adhesion molecules at leukocyte surfaces and by activating sGC，which lead to rapid increase in platelet cGMP and inhibition of platelet aggregation. NO also inhibits vascular smooth muscle cell proliferation，leading to decreased neointimal hyperplasia.

　　Effects on inflammation

　　The mechanisms of action of NO on the inflammatoryprocess are complex. Conflicting data on the effect of iNO on the pulmonary inflammatory response have been published，as both increases and decreases in inflammatory mediators have been observed in patients suffering from severe acute respiratory distress syndrome treated by iNO. The different effects depend on the concentration of NO，the local oxidation－reduction potential，and the presence of other inflammatory mediators and oxygen－derived free radicals. Low amounts of NO derived from eNOS and nNOS are believed to be beneficial，whereas large quantities produced by iNOS contribute to the injury observed in different experimental models of inflammation. However，inhibition of iNOS may exacerbate injury in certain situations，suggesting that iNOS－derived NO may be protective as well.

References<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

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6. Tiefenbacher CP. Tetrahydrobiopterin：a critical cofactor for eNOS and a strategy in the treatment of endothelial dysfunction? Am J Physiol Heart Circ Physiol 2001；280：H2484－8.

7. Blaise G. The endothelium at rest. In：Spiess （Ed.）. The Relationship Between Coagulation，Inflammation，and Endothelium － A Pyramid Towards Outcome. Baltimore：Lippincott Williams & Wilkins；2000：31－78.

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9. Liu HW，Anand A，Bloch K，Christiani D，Kradin R. Expression of inducible nitric oxide synthase by macrophages in rat lung. Am J Respir Crit Care Med 1997；156：223－8.

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