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Cardiovascular Research 2000 45(1):148-155; doi:10.1016/S0008-6363(99)00316-8
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Copyright © 2000, European Society of Cardiology

Inducible nitric oxide synthase and cardiovascular disease

Ajay M Shah*

Department of Cardiology, GKT School of Medicine, King's College London, UK

* Tel.: +44-171-346-3106; fax: +44-171-346-3685

KEYWORDS Adenosine; Arrhythmia (mechanisms); Contractile function; Ischemia; Preconditioning; Reperfusion


   1 Introduction
Top
 1 Introduction
2 Historical account
 3 NO in the...
 4 In vitro expression...
 5 Role of iNOS...
 6 iNOS and endotoxic...
 7 iNOS and endotoxic...
 8 Role of iNOS...
 9 Conclusions
 References
 
The simple gas nitric oxide (NO) has a diverse array of actions in numerous physiological and pathophysiological processes in the cardiovascular system. Three distinct NO synthase (NOS) isoforms, each encoded for by separate genes, have now been identified [1–4]. nNOS (or ‘neuronal’ NOS, NOS1) and eNOS (or ‘endothelial’ NOS, NOS3) are constitutive, Ca2+-regulated isoforms expressed not only in nervous tissue and endothelium respectively, but also in several other cell types. iNOS (or NOS2) can be expressed in almost any cell type upon appropriate stimulation. All NOS isoforms can be transcriptionally and post-transcriptionally regulated [5]. The generation of NO requires L-arginine, O2, NADPH, and tetrahydrobiopterin (BH4). In situations where there is L-arginine and/or BH4 deficiency, all the NOSs can generate superoxide as well as NO [6].


   2 Historical account
Top
 1 Introduction
 2 Historical account
3 NO in the...
 4 In vitro expression...
 5 Role of iNOS...
 6 iNOS and endotoxic...
 7 iNOS and endotoxic...
 8 Role of iNOS...
 9 Conclusions
 References
 
Appreciation of the role of NO in the cardiovascular system dates back to 1980 and the seminal report by Furchgott & Zawadzki of a labile endothelium-derived relaxing factor (EDRF) responsible for acetylcholine-induced vasodilatation [7]. In 1987, Salvador Moncada and colleagues [8] as well as Ignarro and co-workers [9] independently demonstrated that NO accounted for the biological activity of EDRF. Around the same time, it was shown that NO was involved in macrophage-induced cytotoxicity [10,11]. Endothelial NO synthesis was Ca2+-regulated and produced small amounts of NO, whereas the macrophage enzyme was Ca2+-independent and generated much larger amounts of NO. Furthermore, the macrophage enzyme was detectable only after exposure to cytokines or endotoxin (lipopolysaccharide, LPS), ie, was inducible.

In 1989, Patrick Vallance and colleagues had used local intra-arterial infusion of a specific NOS inhibitor, NG-monomethyl L-arginine (L-NMMA), into the forearm to show that endogenous NO contributed to resting arteriolar vasodilator ‘tone’ and mediated acetylcholine-induced vasodilatation in humans [12]. In their Cardiovascular Research paper published that year, they reported that endogenous NO did not affect basal venous tone in normal humans, but did mediate both acetylcholine and bradykinin-induced venodilatation [13]. These were the first studies of the effects of endogenous NO on cardiovascular function in normal humans in vivo.

Wright and colleagues in their paper published in 1992 reported the effects of L-NMMA on endotoxic shock in anaesthetised rabbits [14]. A Ca2+-independent inducible NOS (iNOS) had been implicated in endotoxin- or cytokine-induced vascular hyporesponsiveness and shock, and inhibition of NO synthesis had been suggested to reverse hypotension in this condition [15–19]. Wright et al. [14] reported that intravenous L-NMMA markedly exacerbated endotoxin-induced hypotension and mortality over the 3 h timespan of their experiment, but that co-treatment with an NO donor ameliorated these effects. They concluded that inhibition of constitutive NOS was deleterious, but that selective inhibition of iNOS might be beneficial. They also speculated that iNOS expression in the heart might account for myocardial depression in endotoxic shock. Subsequent studies by these and other workers led to the hypothesis that iNOS expression was deleterious not only with respect to hypotension and vascular hyporeactivity in endotoxic shock, but also in relation to myocardial dysfunction in this condition and others where the enzyme was expressed (e.g. dilated cardiomyopathy) (e.g. [20–23]).

In the present update, we focus on the role of iNOS in endotoxic shock and other cardiovascular disorders.


   3 NO in the cardiovascular system
Top
 1 Introduction
 2 Historical account
 3 NO in the...
4 In vitro expression...
 5 Role of iNOS...
 6 iNOS and endotoxic...
 7 iNOS and endotoxic...
 8 Role of iNOS...
 9 Conclusions
 References
 
NO is now known to be a potent, locally acting vasodilator that has a central role in the regulation of vascular smooth muscle tone. In addition, it inhibits leukocyte and platelet adhesion to the endothelium, leukocyte activation and platelet aggregation, and endothelial permeability [6,24]. It thus optimises blood flow regulation in the microcirculation. In the heart, endothelium-derived NO modulates myocardial relaxation and diastolic function and the Frank-Starling response [25], and reduces oxygen consumption independent of effects on contractile function [26]. NO generated within cardiac myocytes by eNOS and possibly also nNOS may modulate excitation-contraction coupling via effects on sarcolemmal Ca2+ channels and sarcoplasmic reticular function [27,28]. It can also modulate heart rate and β-adrenergic inotropic responses [27]. NO has vascular and myocardial anti-proliferative potential, and can act as a bifunctional regulator of cell apoptosis [27,28].

NO-triggered downstream signal transduction may be cGMP-dependent or -independent [6,24]. The latter usually involves direct reactions of NO with amino, thiol, or diazo groups in proteins, and with haem and Fe2+ or sulphur centres. Under conditions where both NO and superoxide are generated, a diffusion-limited, essentially irreversible reaction between these molecules leads to the formation of peroxynitrite [29]. Low levels of peroxynitrite can be beneficial via stimulation of guanylyl cyclase, but higher levels generate highly reactive hydroxyl-like species that induce toxic effects secondary to protein oxidation [29].

A reduction in endothelial NO production or bioavailability contributes to ‘endothelial dysfunction’, which is a feature of many cardiovascular pathologies – e.g., hypertension, hypercholesterolaemia, atherosclerosis, diabetes, heart failure [30]. Endothelial dysfunction contributes to disease pathophysiology and, in at least some cases, may even have a primary pathogenetic role. In conditions such as ischaemia-reperfusion, the generation of peroxynitrite from reaction between superoxide and eNOS-derived NO may have damaging effects. The induction of iNOS in cardiovascular tissues has been suggested to be involved in the pathophysiology of several disorders.


   4 In vitro expression and effects of iNOS
Top
 1 Introduction
 2 Historical account
 3 NO in the...
 4 In vitro expression...
5 Role of iNOS...
 6 iNOS and endotoxic...
 7 iNOS and endotoxic...
 8 Role of iNOS...
 9 Conclusions
 References
 
Upon exposure to cytokines or LPS, iNOS can be expressed in most cardiovascular tissues, e.g., vascular smooth muscle, endothelial cells, and cardiac myocytes [20,31–35]. However, in many in vivo settings (and especially in the heart), a major proportion of iNOS expression and activity may in fact be in infiltrating inflammatory cells (e.g. [36]). Induction of iNOS mRNA is inhibited by glucocorticoids (e.g. dexamethasone), transforming growth factor β(TGFβ), and osteopontin – a multifunctional extracellular matrix phosphoprotein that may itself be induced by cytokines [37]. On the other hand, iNOS expression is augmented by cAMP, angiotensin II, vasopressin and adrenomedullin. Cytokine-induced expression of iNOS is often accompanied by co-induction of GTP cyclohydrolase I, which regulates BH4 production [31], and cationic amino acid transporters, which regulate L-arginine transport [38]; co-induction of these proteins may be required for optimal NO production by iNOS.

Numerous studies have shown that iNOS induction in vessels in vitro leads to vasodilatation and vascular hyporeactivity to vasoconstrictors. Many studies have also documented myocardial dysfunction after iNOS induction. In some studies, this was manifest as a decrease in baseline myocardial contractile function [21,34,35,39–42], whereas others reported just a reduction in β-adrenergic inotropic responsiveness [22,31,43]. iNOS-induced myocyte apoptosis and death have also been documented [44,45]. It should be noted that cytokine-induced changes in vascular or myocardial function may involve several NO-independent pathways. In addition, cytokines can activate pre-existing eNOS, so that NO-mediated dysfunction may not necessarily be due to induction of iNOS [46,47].


   5 Role of iNOS in vivo: General considerations
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 1 Introduction
 2 Historical account
 3 NO in the...
 4 In vitro expression...
 5 Role of iNOS...
6 iNOS and endotoxic...
 7 iNOS and endotoxic...
 8 Role of iNOS...
 9 Conclusions
 References
 
In vivo expression of iNOS mRNA and protein in cardiac and/or vascular tissues has been demonstrated in several conditions, e.g., endotoxic shock, cardiac allograft rejection, dilated cardiomyopathy, myocarditis, and heart failure (see below). A role of iNOS in the vascular hyporeactivity of endotoxic shock has been suggested by the results of many studies that have employed pharmacological inhibitors of NOS (both non-selective and iNOS-selective) in vivo. However, in the case of cardiac iNOS expression, investigation of functional consequences in vivo has been very limited. Nevertheless, based on the detection of iNOS and on the effects of cytokine-induced iNOS expression in vitro, it has been widely speculated that intra-cardiac iNOS expression leads to contractile depression and other harmful effects.

In determining the relationship between evidence of iNOS expression and potential functional consequences, several factors need to borne in mind. First, the potential involvement of iNOS should ideally be based on the direct detection of iNOS mRNA, protein and activity; the use of glucocorticoid-inhibitable responses or Ca2+-independent NOS activity alone as markers of iNOS can be misleading. Second, the level of expression of iNOS mRNA or protein does not necessarily reflect functional activity, which may be impaired because of substrate or co-factor deficiency, or even be dysfunctional with the production of superoxide [6,48]. In this regard, the results of in vitro biochemical NOS activity assays performed in the presence of non-limiting concentrations of substrate and co-factors do not necessarily reflect the true activity of the enzyme in vivo. Third, the temporal and spatial expression/activity of iNOS may vary according to disease stage and severity. For example, the time course of iNOS expression and activity in endotoxic shock does not always correlate with functional vascular dilatation [49]. Fourth, cytokine-independent pathways and cytokine-dependent but NOS-independent pathways may be involved in many conditions. Finally, only relatively recently has it been appreciated that iNOS may have beneficial as well as harmful effects on cardiovascular function.

A beneficial role of iNOS is not surprising given that its induction by cytokines usually occurs as a component of a host defence response. iNOS expression in macrophages has anti-viral [50] and anti-bacterial effects [51]. Other beneficial actions include cytoprotection, decreased leukocyte adhesion, anti-platelet activity, reduced vascular permeability, anti-oxidant activity, and improved cardiac diastolic function. On the other hand, the widespread expression of iNOS, particularly in non-inflammatory cells, may be harmful. As discussed above, this is especially likely in situations of concurrent oxidative stress, when high levels of peroxynitrite may be formed [29]. In the heart, high concentrations of peroxynitrite are reported to potently inhibit both contractility and respiration [52], reduce cardiac efficiency [53], and promote Ca2+ overload [54].


   6 iNOS and endotoxic shock – vascular dysfunction and mortality
Top
 1 Introduction
 2 Historical account
 3 NO in the...
 4 In vitro expression...
 5 Role of iNOS...
 6 iNOS and endotoxic...
7 iNOS and endotoxic...
 8 Role of iNOS...
 9 Conclusions
 References
 
Septic shock resulting from gram negative bacterial infection is a major cause of morbidity and mortality worldwide. It is a systemic inflammatory process triggered by LPS and other microbial products [55,56]. Multiple signal transduction cascades are activated in the condition, including the production of cytokines (TNF{alpha}, IL-1, IL-2, interferon {gamma}, leukaemia inhibitory factor), platelet activating factor, endothelin, kinins, eicosanoids, reactive oxygen species, and adhesion molecules, and the activation of the coagulation and complement pathways. Cardinal clinical features of the syndrome are hypotension, vascular hyporeactivity to vasoconstrictors, intrinsic myocardial depression (independent of changes in cardiac loading), blood flow maldistribution to organs, impaired oxygen extraction, and eventually multi-organ failure.

It has been suggested that the induction of iNOS in multiple tissues is a central, even obligatory, component of the pathophysiology of evolving septic shock and of the presumed final common pathway leading to death. Overproduction of NO could, at least in theory, account for many of the clinical features described above since it (a) is a potent vasodilator and is involved in blood flow regulation, (b) can depress myocardial function, (c) can impair cellular respiration, and (d) is generated in large amounts in septic shock. Indeed, NOS inhibitors have been documented to improve or reverse hypotension in experimental models of septic shock (e.g. [17,57,58]) and in preliminary small clinical studies [59]. However, in many studies NOS inhibitors worsened overall outcome (e.g. [14]). Earlier studies, such as that by Wright et al. [14], suggested that the inhibition of eNOS was the reason for these deleterious effects, and that selective iNOS inhibition alone may be the preferred therapeutic option [21]. However, as discussed above, iNOS itself may also have beneficial effects. Indeed, some studies have shown that organ dysfunction in endotoxic shock is not prevented by selective inhibition of iNOS [60].

Recent studies of iNOS knockout mice have provided significant new information about the role of iNOS in endotoxic shock. As expected, LPS does not induce iNOS in these mice [61–63]. MacMicking et al. reported that hypotension and mortality in response to low-dose LPS were significantly lower in anaesthetized iNOS knockout mice [61]. However, in conscious mice injected higher doses of LPS, iNOS knockout animals were not protected against tissue damage. Furthermore, in iNOS knockout mice treated with LPS after priming with Propionobacterium, there was no difference in mortality compared to wild type mice [61]. Laubach et al. [62] independently generated iNOS-knockout mice, and found no difference in mortality between wild-type and knockout animals administered either low-dose or high-dose LPS. These investigators also reported that female knockout mice had a higher LPS-induced mortality than wild-type female mice [64]. A third independent group [63] reported that iNOS knockout animals were more resistant to LPS-induced death. This group also found that LPS-induced hypotension in conscious, instrumented knockout mice was significantly reduced [58]. Despite the somewhat contradictory nature of these data, it is clear that while a role for iNOS-derived NO in the vascular hyporeactivity of septic shock is supported by some studies in iNOS knockout mice [58,61,65], iNOS induction is not obligatory for LPS-induced shock and death [61,62,64]. At least part of the reason for this may be that iNOS has some beneficial effects in septic shock. In addition to the beneficial effects discussed earlier, in iNOS knockout mice given LPS, it has recently been reported that leukocyte adhesion and rolling in the microcirculation are markedly increased [66]; thus, iNOS-derived NO may be especially important for microvascular integrity and function.

The results of iNOS knockout and other studies indicate that NO-independent pathways (either instead of or in parallel to iNOS) may contribute significantly to many aspects of septic shock pathophysiology. In normal humans, Vallance and colleagues addressed this question by developing a technique of local instillation of endotoxin into dorsal hand veins [67]. They found that endotoxin-induced venous hyporesponsiveness was glucocorticoid-inhibitable but not mediated by NO [67]. In experimental endotoxic shock in rats, widespread vascular dilatation was observed at a time when iNOS activity had returned to normal [49]. Haem oxygenase-1 (HO-1), an enzyme that generates carbon monoxide (CO) in the process of haem catabolism, has recently been found to be markedly induced in systemic vessels and other tissues (e.g., liver, lung, heart) in experimental endotoxic shock [68–70]. CO generated either by a constitutive haem oxygenase-2 (HO-2) or by HO-1 activates guanylyl cyclase and is capable of causing vasodilatation [71,72]. In rat endotoxic shock, an inhibitor of HO, zinc protoporphyrin IX, was found to abrogate endotoxin-induced hypotension [69]. Consistent with these findings, an NO-independent activation of guanylyl cyclase in the vasculature was reported in rat endotoxic shock [73]. Interestingly, it has been suggested that HO-1 may be inducible by NO [74]. As is the case with iNOS, it is likely that HO-1 induction may have beneficial (e.g., antioxidant) as well as deleterious effects [75]. The relative roles of iNOS, HO-1, and other induced proteins in septic shock remain to be worked out in detail.


   7 iNOS and endotoxic shock – myocardial dysfunction
Top
 1 Introduction
 2 Historical account
 3 NO in the...
 4 In vitro expression...
 5 Role of iNOS...
 6 iNOS and endotoxic...
 7 iNOS and endotoxic...
8 Role of iNOS...
 9 Conclusions
 References
 
Intrinsic myocardial dysfunction plays a major part in the morbidity and mortality of septic shock [55]. iNOS mRNA is expressed within <3 h in inflammatory cells, cardiac myocytes, and coronary vascular smooth muscle in experimental endotoxic shock [76,77], while Ca2+-independent NOS activity peaks at about 6 h after LPS injection in rats [20]. Cardiac iNOS protein expression has been demonstrated in autopsy tissue of human patients who died of septicaemia [78,79].

In 1992, Brady et al. [21] published a paper suggesting that iNOS expression in cardiac myocytes contributed to myocardial depression in septic shock. These authors reported that the depressed basal contractile function of myocytes isolated 4 h after LPS injection in guinea pigs was partially restored by acute exposure to a NOS inhibitor, and was fully prevented by pre-treatment with dexamethasone. However, no direct evidence of iNOS expression or activity was provided [21]. Several subsequent studies have failed to substantiate this hypothesis. In a study ostensibly performed in an identical model (ie, LPS-injected guinea pigs), Decking et al. [80] could not detect Ca2+-independent NOS activity in myocardium, and found that in vitro cardiac depression was not altered by NOS inhibitors. Likewise, Keller et al. [81] found no effect of non-selective NOS inhibitors or iNOS-selective inhibitors (aminoguanidine) on depressed isolated atrial or myocyte function 4 or 16 h after LPS injection in guinea pigs. A similar lack of effect of NOS inhibitors on depressed in vitro cardiac function in LPS-injected rats has been reported by several groups [82–85]. In vivo load-independent cardiac depression caused by TNF{alpha} in awake dogs was also suggested to be independent of NO [86].

The clear conclusion from the above studies is that iNOS induction is not an obligatory event in the development of intrinsic myocardial depression in endotoxaemia or endotoxic shock. Indeed, it is known that LPS treatment results in the altered expression of many proteins in the heart, e.g., decreased expression of eNOS and cyclooxygenase-1 [87,88], and increased expression of cyclooxygenase-2 [88], TNF{alpha} [89], and HO-1 [70].

Thus, although there is considerable evidence supporting a role for iNOS in the vascular hyporeactivity and hypotension of septic shock, it is clear that (a) iNOS is not involved in all aspects of septic shock pathophysiology, notably myocardial depression, (b) even where iNOS is involved, other parallel pathways may exist that become important when iNOS is inhibited, and (c) iNOS may exert beneficial effects (e.g., anti-bacterial and anti-leukocytactivity, cytoprotection). It therefore seems unlikely that NOS inhibition alone will prove to be an effective strategy in the treatment of septic shock. Appropriate therapy of endotoxic shock may instead require the simultaneous targetting of more than one final common pathway, as well as the use of organ-specific treatment.


   8 Role of iNOS in other cardiovascular disorders
Top
 1 Introduction
 2 Historical account
 3 NO in the...
 4 In vitro expression...
 5 Role of iNOS...
 6 iNOS and endotoxic...
 7 iNOS and endotoxic...
 8 Role of iNOS...
9 Conclusions
 References
 
8.1 Myocarditis
In experimental viral myocarditis, intra-cardiac iNOS expression appears to have beneficial effects, probably because of its anti-viral activity [90] and its restricted expression predominantly in infiltrating inflammatory cells [91]. Administration of intravenous non-selective NOS inhibitors in experimental Coxsackie B3 viral myocarditis, either early or late, increased cardiac damage and mortality [90,92].

On the other hand, it has been suggested that cardiac iNOS induction may be harmful in autoimmune myocarditis, perhaps because of its widespread expression in cardiac myocytes and coronary microvessels as well as macrophages [93]. Indeed, in experimental rat models of myosin-induced myocarditis, an iNOS-selective inhibitor, aminoguanidine, attenuated cardiac necrosis and nitrotyrosine staining [93] and improved haemodynamic parameters [94]. However, a recent comparison of myosin-induced myocarditis in IRF-1 knockout and wild type mice indicated that iNOS expression was not essential for development of disease [95]. Although the IRF-1 knockout mice failed to induce iNOS and had no nitrotyrosine staining, the prevalence and severity of myocarditis were unaffected.

8.2 Dilated cardiomyopathy and heart failure
Several studies have demonstrated the presence of Ca2+-independent NOS activity [23,96] or of iNOS mRNA and protein [96–98] in the cardiac tissue of patients with severe or end-stage heart failure. iNOS expression does not seem to be specific for idiopathic dilated cardiomyopathy as was initially suggested [23], but may be associated with heart failure per se [96,97,99]. Immunocytochemical evidence for cytoplasmic iNOS protein expression in cardiac myocytes of failing hearts was found by Haywood et al. [97] and Habib et al. [98]. However, Vejlstrup et al. [99] in a study of 22 explanted hearts from patients with end-stage dilated or ischaemic cardiomyopathy, found that iNOS mRNA and protein were invariably located to vascular endothelial and smooth muscle cells, with some myocyte expression only in a minority of patients. Likewise, Fukuchi et al. [36] found that Ca2+-independent NOS activity in end-stage failing human myocardium correlated with the density of infiltrating macrophages but not with apparent iNOS protein expression in cardiac myocytes. Stein et al. [100] could detect iNOS mRNA expression in only 2 of 30 failing human hearts, and even in these only at a low level.

The precise functional role of cardiac iNOS expression in heart failure remains uncertain. Direct evidence to support a deleterious effect is weak. In patients with severe dilated cardiomyopathy, intracoronary L-NMMA had no significant effects on baseline LV contractility (LV dP/dtmax), but did increase the response to intracoronary or intravenous dobutamine [101]. This was consistent with a lack of effect of endogenous NO on basal function, but an inhibition of β-adrenergic inotropic response. However, no assessment of eNOS or iNOS expression/activity was made in this study, and it remains unclear what NOS isoform was responsible for the observed effects. Drexler et al. [96] found no effect of L-NMMA on baseline isometric force of left ventricular muscle strip preparations from end-stage failing hearts with high iNOS activity. However, these investigators reported that high iNOS mRNA expression was associated with early myocardial relaxation and a reduced inotropic response to isoproterenol. No comparison with the response of normal myocardium was performed. In contrast, Harding et al. [102] reported that L-NMMA had no effect either on baseline function or the response to isoproterenol in cardiac myocytes isolated from end-stage failing human myocardium. Heymes et al. [103] recently reported a study in which parameters of LV contractile function in dilated cardiomyopathy patients were correlated with iNOS and eNOS mRNA expression, measured by competitive PCR in simultaneously procured LV endomyocardial biopsies. A good linear correlation was found between LV stroke work and the expression level of eNOS or iNOS mRNA, and these authors suggested that this involved a beneficial effect of NO on LV diastolic function.

8.3 Cardiac allograft rejection
Quite good evidence implicates iNOS in the pathophysiology of cardiac transplant rejection. iNOS induction was documented in cardiac myocytes, microvascular endothelial cells, and inflammatory cells in rejecting heterotopic abdominal cardiac transplants in rats [104]. This was paralleled by nitrotyrosine immunostaining of myocytes and macrophages (suggestive of peroxynitrite-mediated protein nitration), and by apoptosis of myocytes, macrophages and endothelial cells [105]. Similar iNOS expression, positive nitrotyrosine staining and apoptosis were found in human cardiac transplant rejection [106]. In a rat transplantation model, the iNOS-selective inhibitor, aminoguanidine, attenuated histological features of rejection although not as well as with steroids [107]. The shortening of cardiac myocytes isolated from rat heart transplants could also be augmented by aminoguanidine [108]. In human cardiac transplant recipients studied in the first year after surgery, an association was reported between the presence of iNOS mRNA in surveillance right ventricular endomyocardial biopsies and systolic and diastolic LV dysfunction assessed by echocardiography [109]. iNOS has also been implicated in barrier dysfunction (ie, increased microvascular permeability) during early rejection of rat cardiac allografts [107].

Later during chronic transplant rejection in rats, iNOS is expressed in the coronary vessels, which become arteriosclerotic – leading to suggestions that it may be involved in this process [110]. However, iNOS expression in coronary vessels may in fact be protective, based on the finding that transplant arteriosclerosis was worse in iNOS knockout mice [111]. Recent studies suggest that the induction of HO-1 may contribute to protection against transplant ateriosclerosis [75].


   9 Conclusions
Top
 1 Introduction
 2 Historical account
 3 NO in the...
 4 In vitro expression...
 5 Role of iNOS...
 6 iNOS and endotoxic...
 7 iNOS and endotoxic...
 8 Role of iNOS...
 9 Conclusions
References
 
Tremendous strides in our knowledge about NO have been made in the last 8–10 years since the papers by Vallance and colleagues [13] and Wright et al. [14] that this update relates to were published. In particular, advances in the molecular biology of the NOSs have led to the development of powerful new experimental approaches for dissecting out the physiological and pathophysiological roles of NO, e.g., the use of gene-modified mice. Likewise, clinical investigation has also moved apace, and many of the data from experimental animal studies have been confirmed (or refuted) in humans. As far as iNOS is concerned, it is now abundantly clear that NO derived from this source may have beneficial as well as deleterious effects. This is not surprising given that the induction of iNOS is an extremely well conserved host defence response. The balance between beneficial and deleterious effects may be particularly influenced by the spatial and temporal restriction of iNOS expression, and the presence or absence of concurrent oxidative stress. With respect to endotoxic shock, iNOS plays a significant role in the vascular hyporeactivity characteristic of this condition, but is not the only factor responsible for this abnormality. Furthermore, some aspects of the pathophysiology of endotoxic shock (e.g., intrinsic myocardial depression) may to a large extent be NO-independent, while iNOS-derived NO also has beneficial effects (e.g., on microcirculatory function). The use of iNOS-selective or non-selective NOS inhibitors may therefore well prove inefficacious in treatment of this condition. Among the other cardiovascular disorders in which iNOS expression has been documented, the best evidence for a role for iNOS is in viral myocarditis (where it seems to be generally beneficial) and in acute allograft rejection (where it appears to be harmful). The role of iNOS in the myocardial dysfunction of heart failure remains to be clarified.


   Acknowledgements
 
AMS holds the British Heart Foundation Chair of Cardiovascular Medicine at King's College London.


   References
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 1 Introduction
 2 Historical account
 3 NO in the...
 4 In vitro expression...
 5 Role of iNOS...
 6 iNOS and endotoxic...
 7 iNOS and endotoxic...
 8 Role of iNOS...
 9 Conclusions
 References
 

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