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Nitrite was a therapeutic agent used in the treatment of angina pectoris and hypertension, but was replaced by nitroglycerin. However, nitrite has recently been rediscovered following observations that this anion possesses novel pharmacologic actions such as producing vasodilation, modulating hypoxic vasodilation, and providing cytoprotection in ischemia-reperfusion injury. Moreover, recent observations in animal and human studies have demonstrated that the reduction of nitrite to vasoactive nitric oxide occurs through both enzymatic and non-enzymatic processes. These findings suggest that nitrite may act as a storage form for nitric oxide and provide support for investigating the use of nitrite in the treatment of ischemic disease states including pulmonary hypertension.
The nitrite anion is present in the human body and has been shown to possess a variety of pharmacologic actions in both animals and humans. Until recently, nitrite was considered to be an end-product of nitric oxide metabolism, however, historical and recent investigations have shown that this anion has pharmacologic actions that may be beneficial in the treatment of cardiovascular diseases. The purpose of this review is to discuss historical and present observations about nitrite and possible roles for the use of this inorganic anion in the treatment of cardiovascular diseases.
Cardiology owes a great deal to the work of the Scottish physician, Thomas Lauder Brunton, who in 1870 first used vasodilator agents in the treatment of angina pectoris . As a medical student Brunton performed clinical experiments with digitalis and received a gold medal for this work at the University of Edinburgh in 1866 at the age of 22 . During his medical studies, Brunton also became aware of prior clinical findings with amyl nitrite. In 1859 the British chemist, Frederick Guthrie, published a paper on the cardiac effects of inhaled amyl nitrite and suggested that it may have restorative properties following suffocation, drowning, or protracted fainting [1, 2]. Moreover, in 1864 Benjamin Ward Richardson reported results with amyl nitrite on the heart by demonstrating that the inhaled agent rapidly increased the “action of the organ greater than any other known agent” [1, 3]. Finally, Brunton observed Arthur Gamgee demonstrating that amyl nitrite greatly lessened the arterial tension in both animals and man . From these observations, Brunton administered amyl nitrite to patients with angina pectoris and observed pain relief within a minute, although some patients required repeated administrations [1, 4]. During his postgraduate studies in Carl Ludwig’s laboratory, Brunton demonstrated that the hypotension produced by amyl nitrite was due to vasodilatation [1, 5]. Finally, Brunton also published a paper on the physiologic actions of nitroglycerin, but did not suggest the use of this agent in the treatment of angina [1, 6].
During the same period that Brunton experimented with amyl nitrite, another British physician, William Murrell, began using the organic nitrate, nitroglycerin, in the treatment of angina pectoris [7, 8]. While amyl nitrite was effective, it was difficult to administer due to its volatility. Murrell initially administered placebo for one week before the trial administration of nitroglycerin as positive responses to placebo where well known [7, 8]. With nitroglycerin therapy, all patients would obtain relief from angina, but would experience increasing side effects as the dosage increased [7, 8]. Moreover, some patients reported that their angina could be aborted by taking the drug at the onset of symptoms [7, 8]. From this small but important study, nitroglycerin was established as an effective therapy for angina pectoris .
Research with sodium nitrite was also conducted by Murrell in frogs, cats, and in outpatients at Westminster Hospital with Sidney Ringer . Prior to Murrell’s involvement, Ringer had administered 10 grains (600 mg) of sodium nitrite in which 17 of 18 patients this agent was not well tolerated, due to the associated side-effects of giddiness, throbbing headache, faintness, nervousness, nausea, and vomiting [8, 9]. Following additional clinical experiments with Murrell, the effective dose range was reduced to ½ to 2 grains (30–120 mg) which is still the official dose listed in the British Pharmacopoeia [8, 9].
Murrell also worked with Fancourt Barnes to compare the effects of amyl nitrite and nitroglycerin using sphygmographic analysis where it was found that amyl nitrite and nitroglycerin have similar effects on pulse, as both produced a state of dicrotism and accelerated the heartbeat. However, the actions of these drugs differed in the time of onset and duration . Based upon these experiments, Murrell concluded that nitroglycerin would be more clinically useful than amyl nitrite . His decision to use nitroglycerin instead of amyl nitrite resulted in the successful treatment of angina in hundreds of patients . Murrell was the first physician to use the first synthesized drug, nitroglycerin, in the treatment of angina pectoris that is still in use 150 years later .
By the 1930s, the nitrites were an established treatment for angina and hypertension . However, detailed clinical studies of the cardiovascular effects of sodium nitrite were performed in 1933 by Weiss and coworkers in normal individuals, in patients with hypertension, and in patients with renal disorders . In these studies, sodium nitrite produced significant vasodilation and it was suggested that this action was probably the result of direct action on the arterial smooth muscle. Moreover, the degree of systolic pressure decrease was dependent on the level of arterial tone. Only in patients with arterial hypertension did cardiac output change, with observed decreases of 15–30%. In patients with renal insufficiency, sodium nitrite did not improve renal function. Finally, Weiss determined that the use of sodium nitrite for treatment of hypertension would not be efficacious and in fact could be dangerous .
It is now widely accepted that nitrite has vasodilator activity in experimental animals and in human subjects [13–15]. Although nitrite is no longer used for treatment of ischemic heart disease or hypertension , it still has benefit in the treatment of cyanide toxicity , along with the recently discovered actions in mediating or modulating hypoxic vasodilation [15, 18, 19] and the ability to produce cytoprotection in ischemia-reperfusion injury in a number of organ systems [20–22].
Nitric oxide is produced by nitric oxide synthases as well as by nitric oxide synthase-independent mechanisms [15, 23, 24]. Nitrite can be produced by oxidation of nitric oxide under physiologic conditions [25–27]. However, nitric oxide generation by nitric oxide synthase may be rapidly depleted in ischemic conditions, as nitric oxide synthase is dependent upon the availability of oxygen [28, 29]. Earlier studies in rat myocardium have demonstrated the presence of nitric oxide synthase-independent generation of nitric oxide as detected by electron paramagnetic resonance spectroscopy, suggesting that nitric oxide generation was due to chemical reduction of endogenous nitrite under ischemic conditions . As nitric oxide donors can protect the heart during ischemia-reperfusion in in vitro models [30–32], Webb, Bond et al, 2004 investigated the role of nitrite metabolism under ischemic conditions and nitric oxide was observed to be generated from nitrite by the enzyme, xanthine oxidoreductase . Moreover, in the isolated perfused rat heart, nitrite could reduce infarct size by 30% and improve the recovery of left ventricular function . This protective effect of nitrite, however, could be blocked by a nitric oxide scavenger (2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide; carboxy-PTIO) . These findings suggest that nitric oxide can be generated from nitrite by xanthine oxidoreductase in ischemic myocardium and that the addition of nitric oxide can reduce infarct size and improve ventricular function . Since nitrite has a protective effect in a number of ischemic–reperfusion models; it has been hypothesized that nitrite represents a storage form of nitric oxide that has important biological activities [21, 34–38]. Several mechanisms for the reduction of nitrite to nitric oxide have been proposed including nonenzymatic disproportionation at low pH [23, 37, 39, 40] and enzymatically by deoxyhemoglobin, xanthine oxidoreductase, and other heme–containing enzymes [15, 21, 39, 41–45].
Nitric oxide formation by endothelial nitric oxide synthase plays an important role in the regulation of vasomotor tone in the pulmonary and systemic vascular beds [46–49]. The importance of nitric oxide in the regulation of vasomotor tone has been demonstrated in experimental animals and in human subjects by the use of nitric oxide synthase inhibitors [46, 48–50]. Once released from the endothelium, nitric oxide diffuses into vascular smooth muscle cells inducing vasodilatation and into the blood stream where it inhibits platelet aggregation (Fig. 1) [51–53]. Nitric oxide can be rapidly scavenged by reacting with hemoglobin in red blood cells [37, 54–56]. However, nitric oxide that escapes hemoglobin scavenging can be oxidized to nitrite (Fig. 1) [57, 58]. Although mammalian sources of nitrite have been considered by-products of nitric oxide synthase metabolism, or consumed as a food additive, or a result of nitrite reduction by commensal bacteria in the digestive tract , recent studies have shown that plasma nitrite concentrations reflect constitutive nitric oxide synthase activity and correlate with endothelial function [25, 27].
Moreover, recent research suggests that the nitrite anion represents an important storage form of nitric oxide that can have important physiologic actions [15, 37, 60, 61]. Forearm blood flow and arterial and venous plasma levels of nitrite were assessed at rest and during forearm exercise and also during regional inhibition of nitric oxide synthesis . Significant arterial-venous plasma nitrite gradients and increased nitrite consumption with exercise were observed during the inhibition of regional endothelial synthesis of nitric oxide . These findings suggest that nitrite provides a novel delivery source for intravascular nitric oxide and that circulating nitrite is bioactive . In subsequent studies, the vasodilator properties and mechanisms for bioactivation of nitrite were studied in the human forearm . The infusion of sodium nitrite into the forearm brachial artery resulted in an increase in forearm blood flow before and during exercise, with or without nitric oxide synthase inhibition . These findings suggest that nitrite represents a novel source of nitric oxide, that hemoglobin acts as a nitrite reductase, and that nitric oxide release from nitrite contributes to hypoxic vasodilation .
There is increasing evidence that nitrite reduction to nitric oxide is catalyzed by deoxyhemoglobin and that pulmonary vasodilator responses to inhaled and to injected sodium nitrite are enhanced by hypoxia . In contrast, it has also been reported that in the presence of red blood cells, sodium nitrite had no effect on the hypoxic pulmonary vasoconstrictor response, suggesting that insufficient nitric oxide escapes red cell scavenging to cause pulmonary vasodilation .
Responses to sodium nitrite were recently determined in the pulmonary vascular bed of the intact chest rat under normal and elevated tone conditions. It was observed that that sodium nitrite had significant pulmonary and systemic vasodilator activity under normoxic conditions, and that pulmonary vasodilator responses to sodium nitrite are enhanced when baseline tone is increased regardless of the mechanism used to increase vasoconstrictor tone . Moreover, pulmonary and systemic vasodilator responses to sodium nitrite could be attenuated by xanthine oxidoreductase inhibition, and that pulmonary vasodilator responses were not increased by ventilatory hypoxia. Therefore the results of studies with sodium nitrite were different in the neonatal lamb , and intact chest rat . Moreover, the studies in the rat show that xanthine oxidoreductase is nitrite reductase converting nitrite to vasoactive nitric oxide suggesting that the mechanism of nitrite bioactivation is different in the rat and human circulation where oxypurinol did not inhibit responses to infused nitrite [39, 63].
Xanthine oxidoreductase is a ubiquitous enzyme that can reduce nitrite to nitric oxide and under severe conditions in the rat heart, large amounts of nitric oxide can be generated [23, 66, 67]. Xanthine oxidoreductase is widely distributed in mammalian tissues and is a key enzyme in purine metabolism. Xanthine oxidoreductase also catalyses the oxidation of a wide range of substrates by passing electrons to molecular oxygen that generates reactive oxygen species. While xanthine oxidoreductase has been implicated in ischemia-reperfusion injury [23, 66, 67], its involvement in normal physiological processes is uncertain. Although xanthine oxidoreductase can reduce nitrite to nitric oxide, the effects of xanthine oxidoreductase inhibitors on vasodilator responses to sodium nitrite are uncertain, and studies in the literature show no inhibitory effect [39, 68]. Moreover, it is uncertain whether nitric oxide formation from nitrite occurs primarily in tissues or in blood (Fig. 1) . The addition of nitrite triggered a large amount of nitric oxide generation in tissues such as heart and liver, but only trace nitric oxide production in blood was observed . Moreover, the addition of carbon monoxide to the blood could increase nitric oxide release, and these findings suggested that hemoglobin acts to scavenge nitric oxide but not to generate nitric oxide . The administration of the xanthine oxidoreductase inhibitor, oxypurinol, or the aldehyde oxidase inhibitor, raloxifene, decreased nitric oxide generation from nitrite was observed in the heart or liver . The formation of nitric oxide was markedly increased with decreasing pH or with decreased oxygen tension . These findings suggest that xanthine oxidoreductase or aldehyde oxidase, but not hemoglobin, are important nitrite reductases  and that nitric oxide generation from nitrite occurs mainly in tissues and not in blood .
There is substantial evidence that oxidative stress plays a role in the pathophysiology of cardiovascular disease and that xanthine oxidoreductase can produce superoxide . Xanthine oxidoreductase has been shown to reduce nitrite to nitric oxide and that nitrite reduction is greatly enhanced under hypoxic and ischemic conditions in the rat heart . It has been reported that allopurinol inhibits responses to sodium nitrite in the rat [15, 60, 61, 70]. The findings that the responses to the nitrite anion were inhibited by allopurinol in a dose that did not alter responses to sodium nitroprusside support the hypothesis that xanthine oxidoreductase contributes to the activation of nitrite in the rat and that the nitrite anion represents a storage form of nitric oxide . These findings are consistent with the hypothesis that pulmonary and systemic vasodilator responses are mediated in part by the reduction of nitrite to nitric oxide by xanthine oxidoreductase . Moreover, in a recent study presented at the Second International Meeting on the Role of Nitrite in Physiology, Pathophysiology and Therapeutics, National Institutes of Health, Bethesda, Maryland, allopurinol was shown to attenuate decreases in systemic arterial pressure in response to systemic administration of sodium nitrite in nitro-L-arginine methyl ester treated rats  and support the findings in our study  and are consistent with the hypothesis that in the intact rat under normoxic conditions, xanthine oxidoreductase plays an important role in mediating pulmonary and systemic vasodilator responses to sodium nitrite [72–74].
Studies in the literature provide evidence that vasorelaxant responses to nitroglycerin are mediated by the formation of nitric oxide or a closely related molecule [74–77]. However, the mechanism of this vasorelaxant response to nitroglycerin is uncertain. Although studies in the literature indicate that nitric oxide contributes to the activation of guanylate cyclase and vascular smooth muscle relaxation [78, 79], other studies suggest that vasorelaxant responses to nitroglycerin may be independent of nitric oxide release and cyclic guanosine monophosphate formation . It has been reported that mitochondrial aldehyde dehydrogenase catalyzes the formation of glyceryl dinitrate and nitrite from nitroglycerin leading to the production of cyclic guanosine monophosphate and vasorelaxation [78, 80, 81]. Moreover, it has been suggested that the nitrite formed from nitroglycerin metabolism may be further metabolized to nitric oxide and/or converted to a S-nitrosothiol . Although it has been reported that mitochondrial aldehyde dehydrogenase plays an important role in the bioactivation of nitroglycerin, the role of this enzyme in the reduction of nitrite to vasoactive nitric oxide has only been recently determined. In rats treated with nitro-L-arginine methyl ester to block nitric oxide synthase, the decreases in systemic arterial pressure in response to iv injections of sodium nitrite were attenuated by cyanamide (Fig. 2). These data are consistent with the results of Ohtake and colleagues [63, 71]. The inhibitory effects of cyanamide and of allopurinol on responses to sodium nitrite were not additive (Fig. 2). These data suggest that xanthine oxidoreductase and mitochondrial aldehyde dehydrogenase can act in a parallel manner to reduce nitrite to vasoactive nitric oxide in the systemic vascular bed of the rat.
Although the nitrite anion was believed to be an inactive end product of nitric oxide oxidation, results in the literature show that it has significant biologic activity in a variety of species, including man. In this review we discussed the history of sodium nitrite in clinical medicine and its use in the early management of angina pectoris. Studies in the literature report when sodium nitrite is administered in pharmacologic doses, nitric oxide derived from injected nitrite can, in part, escape inactivation and produce a vasodilator response. Therefore it appears that nitrovasodilators such as nitroglycerin and nitrite can act by a common molecular mechanism, formation of nitric oxide or a nitric oxide-derivative and activation of guanylyl cyclase. These agents can be considered prodrugs or carriers of nitric oxide, which mediates endothelial dependent vasodilation, inhibition of platelet aggregation and inhibition of inflammation. These nitrovasodilators can be therapeutic substitutes for endogenous nitric oxide, however the pathway of bioactivation substantially differs. In the case of organic nitrates such as nitroglycerin, nitric oxide is only formed if thiols are present as a cofactor. Inorganic nitrites such as amyl nitrite and sodium nitrite require reduction of nitrite to vasoactive nitric oxide that can be mediated by xanthine oxidoreductase or mitochondrial aldehyde dehydrogenase. In addition recent studies demonstrate that nitrite has a novel cytoprotective action in organ preservation for transplantation and during ischemic injury from circulatory stress and in pulmonary hypertension. Recent studies show that xanthine oxidoreductase and mitochondrial aldehyde dehydrogenase can act as reductases to convert nitrite to vasoactive nitric oxide in the rat under physiologic conditions.
The authors gratefully acknowledge the graphic assistance of Reade B. Nossaman.