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Joan Mott Prize Lecture |
Department of 1 Cardiovascular Medicine, University of Oxford, John Radcliffe Hospital, Oxford OX3 9DU, UK
Abstract
The recent discovery of a NOS1 gene product (i.e. a neuronal-like isoform of nitric oxide synthase or nNOS) in the mammalian left ventricular (LV) myocardium has provided a new key for the interpretation of the complex experimental evidence supporting a role for myocardial constitutive nitric oxide (NO) production in the regulation of basal and β-badrenergic cardiac function. Importantly, nNOS gene deletion has been associated with more severe LV remodelling and functional deterioration in murine models of myocardial infarction, suggesting that nNOS-derived NO may also be involved in the myocardial response to injury. To date, the mechanisms by which nNOS influences myocardial pathophysiology remain incompletely understood. In particular, it seems over simplistic to assume that all aspects of the myocardial phenotype of nNOS knockout (nNOS–/–) mice are a direct consequence of lack of NO production from this source. Emerging data showing co-localisation of xanthine oxidoreductase (XOR) and nNOS in the sarcoplasmic reticulum of rodents, and increased XOR activity in the nNOS–/– myocardium, suggest that nNOS gene deletion may have wider implications on the myocardial redox state. Similarly, the mechanisms regulating the targeting of myocardial nNOS to different subcellular compartments and the functional consequences of intracellular nNOS trafficking have not been fully established. Whether this information could be translated into a better understanding and management of human heart failure remains the most important challenge for future investigations.
(Received 18 August 2006;
accepted after revision 19 September 2006; first published online 21 September 2006)
Corresponding author B. Casadei: Department of Cardiovascular Medicine, University of Oxford, John Radcliffe Hospital, Oxford OX3 9DU, UK. Email: barbara.casadei{at}cardiov.ox.ac.uk
Nitric oxide regulation of myocardial function: an historical prospective
It is now well established that nitric oxide (NO) is produced within the heart, not only by the vascular endothelium, but by the myocytes themselves (Balligand et al. 1993, 1995; Kanai et al. 1997) and that constitutive NO production exerts a significant role in the regulation of cardiac function both under physiological conditions and in disease states (reviewed by Shah & MacCarthy, 2000; Paton et al. 2002; Casadei & Sears, 2003; Massion & Balligand, 2003; Sears et al. 2004). Nevertheless, the complexity of NO downstream signalling and the full range of intracellular targets for NO in the heart are only just beginning to unravel, and several aspects of this complex picture remain unclear.
Until recently, the endothelial isoform of NO synthase (eNOS or NOS3) was the only isoform that was believed to be constitutively expressed in mammalian left ventricular (LV) myocytes, where it was found to localize, at least in part, to plasmalemmal invaginations termed caveolae (Feron et al. 1996). In the early 1990s, experiments using non-isoform-specific inhibitors of nitric oxide synthases (NOS) on isolated rat LV myocytes showed an enhanced inotropic response to mild β-adrenergic receptor stimulation with 2 nmol • l–1 isoproterenol (Balligand et al. 1993), suggesting that constitutive myocardial NO release may act as an endogenous inhibitor of β-adrenergic signalling. Subsequently, intracellular dialysis of NOS inhibitors in guinea-pig LV myocytes was shown to cause a cGMP-dependent increase in the amplitude of the plasmalemma calcium current (ICa; Gallo et al. 1998, 2001), again implying that myocardial eNOS-derived NO may have a tonic inhibitory effect on basal inotropy and calcium handling.
Surprisingly, however, selective deletion of the gene encoding for eNOS in mice turned out to have very little effect on the functional and electrophysiological characteristics of LV myocytes. In particular, all published studies (Han et al. 1998; Vandecasteele et al. 1999; Gödecke et al. 2001; Martin et al. 2006) except one (Barouch et al. 2002) reported no difference in basal and β-adrenergic cell shortening and relaxation rate between LV myocytes isolated from eNOS knockout mice (eNOS–/–) and control mice (Table 1; Martin et al. 2006). Similarly, the basal and β-adrenergically stimulated ICa in LV myocytes from eNOS–/– mice was not found to differ from that of control mice (Han et al. 1998; Vandecasteele et al. 1999; Belevych & Harvey, 2000). In contrast, an enhanced inotropic response to β-adrenergic receptor stimulation was consistently observed in isolated heart preparations from eNOS–/– mice or in vivo (Gyurko et al. 2000; Gödecke et al. 2001; Barouch et al. 2002; Table 1), suggesting that most of the physiological effects of eNOS-derived NO on basal and β-adrenergic myocardial inotropy may be paracrine and require the production of NO from endothelial membranes (Pinsky et al. 1997; Shah & MacCarthy, 2000). However, further studies showed that stimulation of eNOS-derived NO in LV myocytes may be involved in mediating the inotropic response to prolonged stretch; i.e. the so-called Anrep effect (Petroff et al. 2001) and, in some species, myocardial eNOS-derived NO has been shown to contribute to the cholinergic inhibition of β-adrenergic responses (i.e. both inotropy and ICa; Table 1); this effect, however, appears to vary with cell type, experimental conditions and bioavailability of eNOS-derived NO (reviewed by Herring et al. 2002; Massion & Balligand, 2003; Sears et al. 2004).
The emerging importance of myocardial neuronal-like NOS
Since constitutive NOS isoforms (named ‘endothelial’ and ‘neuronal’ according to the cell type from which they were first isolated and cloned) are encoded by different genes (Wang & Marsden, 1995), the discrepancy between the results obtained with non-isoform-specific NOS inhibition and selective eNOS gene deletion could be explained by postulating the coexistence of more than one constitutive NOS isoform in LV myocytes. Indeed, Xu et al. (1999) first uncovered the presence of a NOS1 gene product (that is, a neuronal-like NOS or nNOS) in the LV myocardium of several mammalian species, including humans.
In their original paper, Xu and co-workers located the cardiac nNOS isoform to the sarcoplasmic reticulum (SR) membrane and provided preliminary findings suggesting that myocardial nNOS-derived NO may inhibit the activity of the SR calcium pump or SERCA2a (Xu et al. 1999). From their data, it could be extrapolated that disruption of nNOS within the myocardium would result in a phenotype that may be similar to that observed after ablation of the physiological inhibitor of SERCA2a activity, phospholamban (PLB). The PLB–/– mouse has been extensively characterized (Luo et al. 1994) and shown to have a more rapid calcium reuptake into the SR reticulum of LV myocytes, leading to an increase in SR calcium content; functionally, these two actions result in an enhanced myocardial inotropy and a more rapid rate of relaxation.
To test whether nNOS gene deletion resulted in enhanced SERCA2a activity and a myocardial phenotype similar to that observed in the PLB knockout mouse, we compared cell shortening in LV myocytes from nNOS–/– mice and their wild-type littermates and found that the former exhibited an increased SR calcium content and greater contraction (Fig. 1; Sears et al. 2003). Similar findings were obtained in vivo, where we showed that nNOS–/– mice have a greater LV ejection fraction (Sears et al. 2003) and an enhanced LV preload-recruitable stroke work (Dawson et al. 2005).
These findings would be consistent with a tonic inhibition of SERCA2a activity by nNOS-derived NO; however, we also found that nNOS–/– myocytes exhibited a significantly impaired relaxation (Ashley et al. 2002) and a slower decay of the intracellular calcium transient (Sears et al. 2003; Fig. 2), neither of which would be in keeping with the idea that increased SERCA2a activity accounts for the increased inotropy of nNOS–/– mice. Further investigations indicated that ICa density was increased and that the slow time constant of decay of the current was prolonged in nNOS–/– myocytes (Sears et al. 2003; Fig. 3), leading to a significantly increased calcium influx through the sarcolemmal membrane. A larger and prolonged calcium influx would result in an increase in myocardial contraction by acting both as a trigger for calcium-induced calcium release via the ryanodine receptor calcium release channel (RyR-CRC) and, more importantly, as a mechanism for maintaining SR calcium load in the presence of enhanced SR calcium release (Trafford et al. 2001).
To establish whether the observed changes in ICa were sufficient to explain the myocardial phenotype of the nNOS–/– mice, we measured ICa and cell shortening after disrupting SR function with thapsigargin (Sears et al. 2003). Under these conditions, inotropy is controlled by the calcium entering into the cell via the L-type calcium channels and by the calcium sensitivity of the contractile myofilaments. In unloaded LV myocytes, we demonstrated that myofilament calcium sensitivity did not differ between nNOS–/– mice and their wild-type littermates; nevertheless, after thapsigargin, both ICa and contraction remained elevated in nNOS–/– myocytes, suggesting that the increase in calcium entry was indeed sufficient to account for the increase in myocardial contraction in these animals (Sears et al. 2003). This was further confirmed by data indicating that modelling the changes in ICa observed in the nNOS–/– myocytes was sufficient to mimic the increase in the calcium transient amplitude that we observed experimentally.
Taken together, our data suggest that a greater and prolonged sarcolemmal calcium influx may account for increasing both the SR calcium content and the amplitude of the calcium transient in nNOS–/– myocytes, leading to an enhanced calcium-induced calcium release and (in the absence of changes in myofilament calcium sensitivity) in an increased contraction. From these findings, we inferred that nNOS-derived NO may exert a negative feedback on calcium entry via the L-type calcium channels, since a rise in intracellular free calcium may stimulate nNOS to produce NO, which in turn may inhibit ICa. Consistent with this hypothesis, adenoviral gene-mediated overexpression of nNOS has been shown to reduce ICa density in sinoatrial node cells of spontaneously hypertensive rats (Heaton et al. 2006). Interestingly, calcium entry in neurones through the N-methyl-D-aspartate subtype of glutamate receptors has been reported to be regulated by nNOS-derived NO in a very similar fashion (reviewed by Baranano & Snyder, 2001).
An increased basal LV systolic function in nNOS–/– mice in vivo has been reported by other investigators (Barouch et al. 2003); however, cell contraction data (Ashley et al. 2002; Barouch et al. 2003; Khan et al. 2003, 2004; Sears et al. 2003) have been more variable and difficult to compare because of differences in experimental conditions (e.g. temperature, field stimulation frequency and choice of control animals; previously reviewed by Massion & Balligand, 2003; Sears et al. 2004; Martin et al. 2006).
Co-immunoprecipitation and immunofluorescence studies have suggested that in normal hearts, nNOS may partly colocalize and physically interact with RyRs in the SR (Barouch et al. 2002; Damy et al. 2003, 2004; Bendall et al. 2004). Other investigators have reported that application of NO donors can dose-dependently and reversibly increase the open probability of purified RyR by S-nitrosylating free thiols in the channel protein (i.e. via a cGMP-independent effect; Stoyanovsky et al. 1997; Xu et al. 1998), and it has been suggested that this mechanism may subtend at least part of the functional correlates of nNOS disruption in the myocardium (Barouch et al. 2002; Hare & Stamler, 2005). However, there is at present no direct experimental evidence to support the idea that nNOS-derived NO regulates RyR function in LV myocytes or that this putative effect is nNOS specific. Indeed, eNOS has also been reported to copurify with RyR in cardiac cells (Zahradnikova et al. 1997; Martinez-Moreno et al. 2005) and, under certain conditions (e.g. myocyte stretch), eNOS-derived NO has been shown to enhance RyR function and SR calcium release, independent of cGMP signalling (Petroff et al. 2001). Furthermore, others have shown that stimulation of endogenous NO production in the cardiac or skeletal muscle inactivates calcium release from cardiac SR and reduces the open probability of RyR channels fused into planar lipid bilayers (Meszaros et al. 1996; Zahradnikova et al. 1997).
Thus, although regulation of cardiac RyR activity by nNOS-derived NO remains an intriguing possibility, direct evidence that this mechanism may contribute to the myocardial phenotype of nNOS–/– mice is still lacking.
Constitutive NO production and myocardial β-adrenergic responses
Nitric oxide produced by endothelial or neuronal tissue has consistently been shown to inhibit myocardial sympathetic/β-adrenergic responses (Shah & MacCarthy, 2000; Paton et al. 2002). In contrast, evidence for a role of myocardial constitutive NO production in the modulation of β-adrenergic inotropy has proved elusive (Balligand, 1999; Massion & Balligand, 2003; Sears et al. 2004).
In isolated rat LV myocytes, NOS inhibition has been reported to increase the inotropic response to low concentrations of isoproterenol (2 nmol • l–1; Balligand et al. 1993), suggesting that constitutive myocardial NO production may suppress β-adrenergic inotropic responses. However, the increase in contraction or calcium influx via the L-type calcium channels in response to β-adrenergic receptor stimulation was found to be largely unaltered in LV myocytes from eNOS–/– mice (reviewed by Martin et al. 2006, Table 1), despite the fact that transgenic or adenoviral-mediated overexpression of eNOS in the myocardium results in an attenuation of β-adrenergic contraction (Champion et al. 2004; Janssens et al. 2004; Massion et al. 2004; Danson et al. 2005), in agreement with findings obtained by using high concentrations of NO donors (Flesch et al. 1997; Abi-Gerges et al. 2001).
We observed an enhanced inotropic response to 2 nmol • l–1 isoproterenol in LV myocytes from nNOS–/– mice or in wild-type myocytes after pharmacological nNOS inhibition (Ashley et al. 2002; Fig. 4). These data suggested that the myocardial source of NO involved in the autocrine regulation of β-adrenergic responses may be nNOS. However, findings obtained by Barouch et al. (2002) indicated that the inotropic response to β-adrenergic stimulation in LV myocytes from nNOS–/– mice may be biphasic, i.e. enhanced at low concentrations of isoproterenol (i.e. < 10 nmol l–1) but greatly attenuated at higher concentrations. By comparing sarcomere shortening in LV myocytes from nNOS–/– mice and their wild-type littermates, we have recently observed that contraction remains greater in nNOS–/– myocytes in the presence of 100 nmol l–1 of isoproterenol (Martin et al. 2006; Fig. 4); however, the absolute increase in sarcomere shortening in response to isoproterenol does not differ significantly between groups, confirming that potentiation of the β-adrenergic responses in nNOS–/– myocytes may only occur at low (< 10 nmol • l–1) concentrations of isoproterenol.
In contrast, pharmacological β-adrenergic stimulation in vivo (i.e. over and above the neurohumoral activation already present in anaesthetized and instrumented/open-chested mice) has consistently been shown to elicit a smaller LV inotropic response in nNOS–/– mice compared with wild-type mice (Barouch et al. 2002; Dawson et al. 2005). These data do not tally with our findings in isolated LV myocytes, and perhaps this is not surprising if one considers that disruption of the nNOS gene is not myocardial-specific in these mice and that nNOS-derived NO has also been shown to modulate autonomic transmission both in the central nervous system and in peripheral nerves (reviewed by Paton et al. 2002). In particular, nNOS–/– mice have been shown to have an impaired vagal control of heart rate and an elevated basal sympathetic nerve activity (Jumrussirikul et al. 1998; Choate et al. 2001; Wang et al. 2003, 2005). It is possible, therefore, that the latter may contribute to the enhanced basal LV inotropy and the reduced β-adrenergic reserve we and others observed in nNOS–/– mice in vivo (Barouch et al. 2002; Dawson et al. 2005). Similar discrepancies between data obtained in LV myocytes and in vivo have been reported for eNOS–/– mice, as outlined earlier (Table 1).
Constitutive NO production and myocardial relaxation
It is well established that stimulation of NO production by the coronary endothelium increases LV compliance in humans and in animal models (Paulus & Shah, 1999). Similarly, application of NO donors or cGMP analogues to LV myocytes increases resting cell length and hastens relaxation without changing the rate of decay of the intracellular calcium transient (Shah et al. 1994; Layland et al. 2002), suggesting that endothelium-derived NO may facilitate relaxation via a cyclic GMP-mediated reduction in myofilament calcium sensitivity. In isolated mouse hearts, non-isoform-specific NOS inhibition prolongs the time constant of LV isovolumic relaxation (Gyurko et al. 2000), whereas eNOS gene deletion does not affect basal relaxation; indeed, the LV lusitropic response to β-adrenergic stimulation was found to be enhanced in eNOS–/– mice in vivo (Gyurko et al. 2000). These data suggest that the impairment in myocardial lusitropy observed after the application of non-isoform-specific NOS blockers in mice may result predominantly from inhibition of myocardial nNOS activity. Indeed, we and others have reported that relaxation was impaired in nNOS–/– mice compared with wild-type littermates, both in LV myocytes (Ashley et al. 2002; Khan et al. 2003; Sears et al. 2003; Martin et al. 2006) and in vivo (Dawson et al. 2005). We also found that disabling SR function with thapsigargin or caffeine abolished the differences in myocardial relaxation and rate of decay of the intracellular calcium transient between nNOS–/– mice and their wild-type littermates (Sears et al. 2003; Zhang et al. 2005), suggesting that either slowing of the calcium reuptake via SERCA2a or, possibly, an increased leak of calcium from the RyR calcium release channels may be responsible for these findings. It is difficult to reconcile either of these putative mechanisms with the fact that we have consistently found an increase in SR calcium content in the LV myocardium of nNOS–/– mice (Sears et al. 2003; Zhang et al. 2005). A possible explanation may be that the increase in ICa observed in nNOS–/– mice overcompensates for a reduction in the rate of SR calcium reuptake that is nevertheless sufficient to cause a significant impairment in myocardial relaxation. This situation would have precedent in the work by Brittsan et al. (2003), who demonstrated that tonic inhibition of SERCA2a in a transgenic mouse model overexpressing a non-phosphorylatable mutant of PLB was associated with impaired relaxation but a trend towards an increased basal contraction. Similarly, although the effect of isoproterenol on the rate of decline of the intracellular calcium transients was abolished in these mice, the β-adrenergic inotropic response remained unaffected. Interestingly (similar to the nNOS–/– mice), LV myocytes from transgenic mice with non-phosphorylatable PLB showed a 25% increase in ICa density.
These findings are intriguing; however, it remains to be established whether a similar increase in ICa density would be equally effective in compensating for an inhibition of SERCA2a activity under different experimental conditions or in diseased hearts. For instance, Khan et al. (2003) showed that the increase in SR calcium stores in response to increasing stimulation frequencies was suppressed in nNOS–/– myocytes, which also lacked a frequency-related lusitropic effect. Similarly, the balance between the effects of nNOS gene deletion on calcium influx and SR calcium reuptake may be altered in the failing myocardium, where other factors (e.g. an increased SR calcium leak via the RyR, see Lehnart et al. 2004; or increased calcium efflux via the sodium–calcium exchanger, see Hasenfuss & Pieske, 2002) may uncover the detrimental effect of impaired SERCA2A activity on SR calcium stores and the β-adrenergic inotropic reserve.
Growing evidence indicates that the subcellular localization of NOSs may be instrumental in targeting NO to specific effector proteins; this in turn would make it possible for a highly reactive molecule like NO to exert diverse and specific actions within the same cell type (Barouch et al. 2002). Furthermore, the myoglobin-rich environment of LV myocytes may provide an effective barrier to the free diffusion of NO (Flogel et al. 2001), facilitating the subcellular targeting of its actions. In the light of these data, the recently emerged evidence of a ‘translocation’ of myocardial nNOS from the SR to the sarcolemmal membrane in failing hearts (Damy et al. 2003, 2004; Bendall et al. 2004) has potentially interesting implications.
Myocardial nNOS and myocardial remodelling
In humans, heart failure has been associated with both an increase (Fukuchi et al. 1998; Stein et al. 1998) and a reduction (Damy et al. 2004) in myocardial eNOS expression and activity, suggesting that variables such as treatment and stage of disease may influence the ‘remodelling’ of constitutive NOS isoforms in the LV myocardium.
The functional significance of these changes in human heart failure remains uncertain (Harding et al. 1998); however, transgenic upregulation of myocardial eNOS expression in mice has been shown to have a beneficial effect on LV remodelling (Janssens et al. 2004), whereas eNOS gene deletion has been found to be either detrimental or to have no significant impact on the development of LV failure post myocardial infarction (Scherrer-Crosbie et al. 2001; Liu et al. 2002).
The protein level and the activity of nNOS and have been shown to be enhanced in the LV myocardium of chronically infarcted animals and in failing human hearts (Damy et al. 2003, 2004; Bendall et al. 2004; Dawson et al. 2005), suggesting that nNOS may play a part in the myocardial response to stress. Furthermore, under these conditions, nNOS seems to be preferentially located to the sarcolemmal membrane (rather then to the SR), where it colocalizes with caveolin-3. Bendall et al. (2004) have shed some light on the functional significance of these findings by showing that under basal conditions, nNOS inhibition increased basal LV inotropy and prolonged the time constant of isovolumic relaxation in sham-operated rat hearts, whereas in failing hearts these effects were significantly reduced. In contrast, inhibition of nNOS enhanced the inotropic and lusitropic response to β-adrenergic stimulation in failing hearts but had no significant effect in sham-operated rats.
From these findings, it is difficult to extrapolate whether upregulation and preferential sarcolemmal localization of nNOS in failing hearts is a beneficial adaptation aimed at shielding the diseased myocardium from the detrimental effect of catecholamine toxicity or a maladaptive mechanism contributing to the depressed β-adrenergic reserve of failing hearts. To answer this question, we and others have compared LV size and β-adrenergic reserve in infarcted nNOS–/– mice and control animals and found that nNOS gene deletion exacerbates LV adverse remodelling after myocardial infarction (Dawson et al. 2005; Saraiva et al. 2005). By using serial 3-D echocardiography (Dawson et al. 2004), we showed a more pronounced LV dilatation over time in infarcted nNOS mice compared with wild-type littermates prospectively matched for infarct size (Dawson et al. 2005; Fig. 5). Evaluation of myocardial β-inotropic reserve by dobutamine infusion showed that although basal LV inotropy remained significantly greater in infarcted nNOS–/– mice than in their wild-type littermates, the β-adrenergic reserve was significantly reduced in the former (Fig. 6; Dawson et al. 2005).
From these findings, one could infer that nNOS-derived NO may delay the development of heart failure after myocardial infarction; however, whether all aspects of the myocardial phenotype of nNOS–/– mice can be directly attributed to lack of NO production from this particular source remains to be established. For instance, recent evidence indicates that the activity of xanthine oxidoreductase (XOR) is increased in the LV myocardium of nNOS–/– mice (Khan et al. 2004; Kinugawa et al. 2005), suggesting that increased production of reactive oxygen species (ROS) may contribute to adverse LV remodelling in infarcted nNOS–/– mice. Furthermore, Kinugawa et al. (2005) have shown that stimulation of XOR activity may lead to a reduction in the bioavailability of eNOS-derived NO in the myocardium of nNOS–/– mice (by increasing ROS-mediated NO scavenging), implying that aspects of the myocardial phenotype of nNOS–/– mice may result from the combined disruption of eNOS and nNOS activity. Nevertheless, since the contribution of cardiac XOR to oxidative stress in the human LV myocardium appears to be small (Eddy et al. 1987; De Jong et al. 1990; Janssen et al. 1993), the relevance of these findings to human pathophysiology is uncertain. Species differences in the contribution of different oxidases to myocardial ROS formation may explain why trials of XOR inhibitors (e.g. allopurinol or oxypurinol) have been very successful in preventing or treating LV failure in rodents (Engberding et al. 2004; Stull et al. 2004; Minhas et al. 2006; Naumova et al. 2006) but not in humans (reviewed by Kass & Solaro, 2006).
Summary and future directions
Recent investigations have suggested that a myocardial nNOS-like isoform may play a significant role in the regulation of LV function and calcium fluxes both through NO and, indirectly, through the inhibition of myocardial ROS production and bioavailability. Upregulation of nNOS in the LV myocardium of chronically infarcted animals appears to attenuate adverse LV remodelling and preserve β-adrenergic inotropic reserve. Nevertheless, many questions remain unanswered. In particular, the downstream signalling pathway mediating the effect of nNOS-derived NO in the myocardium is only partly understood. It has recently become apparent that nNOS may decrease sarcolemmal calcium influx by S-nytrosylating the L-type calcium channel (Sun et al. 2006), suggesting that cyclic GMP-independent effects may play a major role in translating the effect of nNOS-derived NO. Furthermore, there is evidence indicating that classic cyclic nucleotide-mediated signalling in the heart can be modulated by ROS (Brennan et al. 2006), adding another layer of complexity to the interaction between NO and ROS formation in the myocardium.
In vivo, understanding of the interplay between presynaptic and postsynaptic effects of nNOS-derived NO in the regulation of β-adrenergic responses and the cardiac adaptation to stress will require more sophisticated approaches (e.g. tissue-specific, conditional knockout of nNOS).
Finally, a recurrent motif in cardiovascular patho-physiology is that compensatory mechanisms that are effective in counterbalancing the adverse effects of acute myocardial injury become pathogenic when sustained over the long term. In the vascular endothelium of hypertensive or diabetic animals, NOS activity can be profoundly altered by ROS-induced oxidation of the NOS critical cofactor, tetrahydrobiopterin (BH4; Alp et al. 2003; Landmesser et al. 2003). Under conditions of BH4 deficiency, all NOS isoforms can become enzymatically ‘uncoupled’ and synthesize ROS rather than NO, further increasing oxidative stress. Emerging findings indicate that this phenomenon can also occur in the myocardium (Kim et al. 2005; Takimoto et al. 2005), where it may act as a switch regulating the progression from adaptive to adverse LV remodelling and ultimately heart failure.
The last 5 years have seen a revival of interest for the possibility that constitutive myocardial NO production may play an important role in the physiological regulation of cardiac function and excitability. An important task for the future would be to test how much of the knowledge accrued so far is relevant to human physiology and to the understanding and treatment of common disease processes such as heart failure.
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This is the 2004 Joan Mott Prize Lecture, which was given by Dr Barbara Casadei at The Physiological Society Meeting, University of Cork on Thursday 2 September 2004.
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P < 0.01). Left ventricular dilatation was significantly greater in infarcted nNOS–/– mice compared with their WT littermates, as indicated by their greater ESVI (*P < 0.02) and EDVI (**P < 0.05). The lower panels show the relative increase over time in ESV and EDV in infarcted mice. Left ventricular remodelling was more accentuated in infarcted nNOS–/– mice at all time points (*P < 0.01 for ESV and **P < 0.05 for EDV) compared with infarcted WT mice with the same infarct size. Modified from 