Cellular and Molecular BiologyTM 51, 307-320
Institut für Klinische Pharmakologie, Otto-von-Guericke-Universität, Magdeburg, Germany ✍ Institut für Pharmakologie und Klinische Pharmakologie, Universitätsklinikum Heinrich-Heine-Universität, Fax: +49 211 81 14781; E-mail: kojda@uni-duesseldorf.de Received December 10, 2004, Accepted December 20, 2004, Published September 5, 2005 Abstract - Therapeutic activation of the vascular NO/cGMP pathway is induced by a variety of stimuli/mediators including physical
activity, supplementation with the precursor L-arginine and organic nitrates which generate NO in the vasculature. The necessity of
an enzymatic reduction for NO generation from these drugs as well as differences in the activity of the NO/cGMP pathway within the
vascular tree determine the unique hemodynamic changes elicited by organic nitrates. These changes include preferential venodilation,
vessel-size specific arterial dilation and improvement of the aortic distensibility and Windkessel-function. Some animal experiments
and clinical investigations suggest that nitrates may also be endowed with cardioprotective and/or vasoprotective effects. "Early entry"
therapy with nitrates do not significantly improve survial in myocardial infarction but increases the beneficial effects of the ACE-
inhibitor enalapril by 50%. Furthermore, nitrates have been shown to improve survival in heart failure, but prognostic effects in stable
angina pectoris are unknown. Short-term experimental and clinical investigations suggest that nitrate tolerance induced by
nitroglycerin is associated with toxic effects in the vasculature, but this is not true for pentaerythrityl tetranitrate and isosorbide
mononitrate. The observed endothelial dysfunction induced by a continuous treatment with nitroglycerin may be an additional risk for
patients who receive continuous nitroglycerin to treat conditions such as unstable angina and acute heart failure. In general, nitrates
are remarkably safe drugs and are well tolerated. Appropriate clinical trials are needed to answer the question whether nitrates can do
more than symptomatic relief in cardiovascular disease.
Key words: Nitrates, coronary artery disease, heart failure, L-arginin, nicorandil
intima-media thickness in CAD patients and this wasassociated with a significant reduction of the 3-year event Drugs used in the treatment of CAD fall in two different rate but did not affect mortality (101). In contrast, clinical catagories (Table 1). Some of these drugs exert antianginal data on the effects of nitrates on the prognosis of patients effects, while others delay the progression of CAD (121).
with stable angina pectoris are still lacking (96,122) and Antianginal drugs such as nitrates, β-blockers and calcium although a meta-analysis indicates a small but significant channel antagonists favourably shift the ratio of myocardial decrease in CAD-mortality among nitrate treated patients oxygen demand and supply. They relieve chest pain and (57), some clinical data raised the suspicion that nitrates may reduce the duration and frequency of acute ischemic events.
rather increase mortality in CAD (89). Non-antianginal In addition, β-blockers have been shown to improve survival drugs are used to improve the outcome of CAD patients in acute myocardial infarction, while the various types of (Table 2). During the last 15 years many clinical trials have calcium channel antagonists have a neutral effect on repeatedly shown the value of these drugs. In Germany, the mortality (50,55). The dihydropyridine amlodipine reduced results of the HOPE trial have led to broaden the indications Drugs used in the treatment of coronary artery disease (CAD) Drugs in the treatment of coronary artery disease Antianginal Drugs
Not-antianginal Drugs
inhibitors of platelet aggregation (e.g. aspirin) calcium channel antagonists (e.g. amlodipine) Selected clinical trials showing a significantly positive effect of treatment with non-antianginal drugs on the outcome of CAD- Treatment before
Treatment in
Treatment after
Myocardial Infarction
Acute Myocardial Infarction
Myocardial Infarction
(primary prevention)
("early entry")
("late entry")
of ramipril, which is now also offered to "reduce the risk of mechanism underlying preferential venodilation is not myocardial infarction, stroke or cardiovascular death".
completely understood but there is evidence for aninvolvement of endogenous NO-generation. NO can inhibit HEMODYNAMIC ACTIONS OF NITRATES
enzymatic bioactivation of nitrates and endothelial NOproduction is greater in arteries than in veins (69,75). In vivo Organic nitrates specifically alter hemodynamics in studies in rabbits have shown higher NO-generation from CAD-patients. No other cardiovascular drug is able to nitrates in veins than in arteries (84). Furthermore, the induce similar hemodynamic changes. Venous pooling and steady-state concentration of NO in the vascular wall may vessel size selective coronary vasodilation are of utmost reduce the expression of soluble guanylyl cyclase (35,81), a importance. Other nitrate-like drugs such as molsidomine key enzyme of the NO/cGMP pathway. This mechanism also reduce preload (77), while sodium nitroprusside is a likely reduces arterial vasodilation to nitrates more than balanced vasodilator which lowers both venous and arterial tone and induces a rapid and short lasting reduction of blood The pattern of hemodynamic changes elicited by nitrates is of particular advantage in CAD patients. It redistributes thecirculating blood volume to splanchnic and mesenteric blood vessels, reduces pulmonary arterial pressure, left Venous pooling is evident at low doses of nitrates (Fig. 1) ventricular end diastolic pressure and systolic ventricular- which have no influence on peripheral resistance (8). The wall tension and may increase cardiac output (4).
Hemodynamic actions of glyceryl trinitrate. Reduction of preload is evident at concentrations which have no effect on peripheral Subsequently, myocardial oxygen demand decreases, while nitrates (6). This emphasizes the need to closely monitor oxygen supply, particularly to the endocardium, is improved.
changes of blood pressure in emergency cases such as acute This resolves and prevents ischemic episodes and quickly coronary syndromes or decompensated heart failure.
reduces the typical chest pain in angina. In some instances,when preload is already low, these hemodynamic changes can induce an aggravation of myocardial function mediated The reduction of myocardial oxygen demand by organic by the Frank-Starling mechanism (see contraindications).
nitrates is supported by their effects on aortic function. It has Preferential venodilation also reduces the hepatic venous been shown that organic nitrates increase aortic distensibility pressure gradient and portal flow and these effects are and improve the Windkessel function (130). This reduces the beneficial in liver cirrhosis (37,73,79).
impedance to the ejection of the left ventricle andsubsequently decreases cardiac workload and oxygen Vessel-size selective coronary vasodilation demand. The relative contribution of this activity to the Nitrates redistribute coronary flow to the ischemic regions therapeutic effectiveness of organic nitrates is unclear but of the heart. This was initially observed by measuring the likely important. Organic nitrates and other NO-donors can oxygen saturation in large and small arteries of the coronary also influence myocardial contractility. In isolated circulation in vivo (132). Later investigations in isolated large cardiomyocytes nitrates induce a biphasic effect. Low and small coronary arteries demonstrated that nitrates concentrations of NO (0.1-1 µM) facilitate myocardial potently relax large arteries but not coronary resistance contraction and relaxation, while higher concentrations (>30 vessels (110). The mechanism of this vessel-size specific µM) induce the opposite effect. In contrast, in vivo studies vasodilatory activity may involve a lack or a rapid depletion with nitrates in animal and man showed only positive of free thiols in the vascular wall (111). The selective inotropic effects (102,118). In view of the strong vascular vasodilation of large arteries in the coronary circulation activity of nitrates, their effects on myocardial contractility increases myocardial blood flow to ischemic regions where are rather small and probably of minor importance (67).
resistance vessels are already maximally dilated by adenosinemediated autoregulation. Nitrates induce opposite effects on EFFECTS OF NITRATES
coronary blood flow than the vasodilator dipyridamol. This ON PLATELET AGGREGATION
drug relaxes both large and small coronary vessels whichcounteract autoregulatory mechanisms and redistributes Almost two decades ago it was shown that therapeutic coronary flow to non-ischemic regions ("steal-phenomenon") doses of isosorbide dinitrate can reduce platelet aggregation in which can cause myocardial ischemia in the stetting of patients with stable angina (27). This observation was confimed by Diodati et al. who demonstrated similar effects ofsodium nitroprusside and glyceryl trinitrate in stable angina, unstable angina and myocardial infarction (32,33). Inhibition Organic nitrates reduce blood pressure. At low doses of platelet aggregation by organic nitrates is most likely where preload reduction is almost maximal (Fig. 1) the mediated by activation of the platelet NO/cGMP pathway hypotensive effect likely results from the redistribution of the (115). In addition, it was reported that NO derived from circulating blood volume (8), while higher doses can cause a nitrates induces activation of endothelial cyclooxygenase and direct dilation of larger arterioles resulting in a reduction of release of antiaggregatory prostanoid prostacyclin (109). The peripheral resistance and a substantial drop in blood pressure.
contribution of inhibition of platelet aggregation to the In contrast to organic nitrates, sodium nitroprusside induces a therapeutic efficacy of nitrates is still controversial but it is comparable relaxation of arterial and venous blood vessels likely that these effects might have potential for preventing and reduce preload and afterload to a similar degree. The thrombus formation in unstable angina (4).
rapid onset and short duration of action permits a titration ofblood pressure by adjustment of the infused dose (6).
Reduction of blood pressure is harmful for the ischemic OF ORGANIC NITRATES
heart since it reduces coronary perfusion pressure andsubsequently myocardial oxygen supply. In addition, The use of organic nitrates in the various settings of CAD activation of the baroreceptor reflex increases the sympathetic is largely determined by the pharmacokinetic features.
tone in the heart and leads to enhanced inotropy and Nitrates can be divided in long acting drugs such as chronotropy which increases myocardial oxygen demand.
isosorbide mononitrate, pentaerythritol tetranitrate and Thus, the small effect of organic nitrates on peripheral molsidomine and short acting drugs such as glyceryl resistance is important and valuable in ischemic heart disease.
trinitrate, isosorbide dinitrate and sodium nitroprusside (6).
However, it should be noted that there is a great Short acting nitrates showing a rapid onset of action are used interindividual variability of the blood pressure response to to treat acute ischemic attacks, while long acting nitrates are primarily used to prevent ischemic episodes. However, there myocardial ischemia in silent angina but the use of nitrates are sustained-release oral and transdermal formulations of in this particular setting of CAD is questionable (103,112).
isosorbide dinitrate and glyceryl trinitrate permitting the use of – Unstable angina pectoris and myocardial infarction: A these drugs in preventive therapy (96). In addition, intravenous variety of clinical investigations have shown that nitrates can glycerol trinitrate and isosorbide dinitrate are commonly used reduce the frequency and duration of chest pain in unstable in acute coronary syndromes and heart failure. Furthermore, angina (43,58,60). Most likely, these effects are mediated by many clinical reports have indicated beneficial effects of the their inhibitory activity of nitrates on coronary vasomotor nitrates in diseases characterized by an increased tone of non- tone and platelet aggreability. Furthermore, the cardiovascular smooth muscle tissues (65).
concommitant reduction in left ventricular enddiastolicpressure (LVEDP) enhances the driving pressure for blood flow to the subendocardium. In unstable angina nitrates are Stable angina pectoris: More than 120 years ago
continuously infused at doses adjusted to the disappearance glyceryl trinitrate was reported to have beneficial effects in of pain, while at the same time close hemodynamic stable angina (87) and today this is still the most important monitoring is necessary to avoid a harmful reduction of indication. Three different therapeutic strategies can be blood pressure associated with increased heart rate.
differentiated: a) Treatment of acute anginal attacks; b) short- Furthermore, it cannot be excluded that in unstable angina time prevention of anginal attacks when extraordinay pysical sudden withdrawl or intermittent treatment might precipitate or mental stress is expected, and c) long-term prevention during the day to reduce the occurrence and severity of Nitrate therapy in myocardial infarction is well known anginal attacks. In any case treatment is initiated to provide and frequently used to reduce chest pain, pulmonary symptomatic relief. In contrast, it is not known if this therapy congestion and sustained hypertension (4,6). As discussed is associated with improved survival in CAD (see Section above, these beneficial actions are largely based on the hemodynamic effects of nitrates. During long-term non- –Acute treatment and short-term prevention: intermittent administration for more than a day nitrate Sublingual glyceryl trinitrate and the slower acting tolerance develops. Although increasing the dosage will isosorbide dinitrate are the nitrates of choice to treat acute restore the hemodynamic and antianginal activity, it must be anginal attacks (4,6,96), since they promptly relieve chest considered that severe forms of nitrate tolerance are pain within minutes. Furthermore, sublingual nitrates are associated with an increase of vascular superoxide also useful to prevent anginal attacks if taken a few minutes production (see below) which may worsen coronary artery before activities known to possibly cause an atttack.
function and might predispose to a reinfarction. On the other – Long-term prevention: Long-acting nitrates are hand, postinfarction treatment with nitrates may help to still among the drugs of choise to achieve freedom from preserve diastolic left ventricular function (63). Likewise, anginal attacks when interventional procedures such as low dose transdermal nitroglycerin can improve left PTCA and coronary bypass surgery have failed or are not ventricular function and prevent left ventricular dilation in sufficiently effective anymore (4,96). However, it is not postinfarction patients with a left ventricular ejection fraction possible to provide a 24 hr protection, since nitrates induce tolerance when taken continuously (see section NitrateTolerance). Therefore, it is necessary to follow an therapeutic regimen which includes a daily nitrate pause of Nitrates, for the most part in combination with approximately 10 hr. This can be easily achieved by skipping hydralazine, are an important and often indispensable nitrate application or removal of a nitrate patch in the therapeutic option in the treatment of heart failure (6,23).
evening. Effective management of chronic stable angina Nitrates are considered useful in selected patients, without development of tolerance using this therapeutic particularly in those who cannot take an ACE inhibitor regimen has been proven for glyceryl trinitrate patches (30) because of hypotension or renal insufficiency (54). As and standard- and sustained-formulations of isosorbide already pointed out above, close hemodynamic monitoring mononitrate (22,98,123). Similar effects have been reported is necessary to avoid a harmful reduction of blood pressure with standard- and sustained-release formulations of associated with increased heart rate. Nitrates are known to isosorbide dinitrate, although there is some evidence that the improve left ventricular function and cardiac output and are beneficial effect of isosorbide dinitrate might decline during used to prevent or to treat the development of pulmonary the day (96,97,114). As for sustained-release formulations of edema. Among nitrates and nitrate-like drugs, glyceryl isosorbide mononitrate, clinically evident effects resulting in trinitrate, isosorbide dinitrate and sodium nitroprusside are better exercise tolerance in patients with angina pectoris used in acute and chronic heart failure. In case of concomitant occur one hour after dosing (42). Finally, nitrates have been myocardial ischemia or established CAD, glyceryl trinitrate shown to effectively reduce the frequency of asymptomatic is usually the drug of choice. Another established use of organic nitrates is an add-on therapy in more severe forms of inhibitors and statins is likely involved in the beneficial chronic heart failure to provide symptomatic relief and effects of these interventions in patients with CAD (41). In improve the patients quality of life (23).
experimental atherosclerosis organic nitrates such aspentaerythrityl tetranitrate and isosorbide mononitrate can improve endothelial function and reduce intimal lesion Pulmonary arterial hypertension is a serious life- formation and oxidation of LDL (47,82). In addition, an threatening condition (52,107). A treatment-based investigation in LDL-receptor deficient mice showed classification has been suggested that divides the disease into antiatherosclerotic effects of the NO-releasing nitroaspirin five different catagories: a) pulmonary hypertension NCX-4016, while the parent compound acetylsalicylic acid associated with disorders of the respiratory system and/or had no effect on atherosclerosis (90). Finally, the nitrate-like hypoxemia; b) pulmonary venous hypertension; c) chronic drug nicorandil has been shown to improve the prognosis of thromboembolic disease; d) pulmonary arterial patients with stable angina pectoris (124).
hypertension, and e) pulmonary hypertension due to – Myocardial infarction: In contrast to less life-threatening disorders directly affecting the pulmonary vasculature (40).
forms of angina, the effects of short-term treatment with Beside anticoagulation, the condition is treated with nitrates on survival of patients with myocardial infarction vasodilator therapy. According to the recently published have been studied extensively. In 1988, Yusuf et al.
guidelines of the American College of Chest Physicians, published a meta-analysis on the effects of nitrates in patients should undergo vasoreactivity testing and those with myocardial infarction revealing a total reduction of as much a favourable response to a vasodilator should be considered as 35% (134). This result was challenged in two large to receive an oral calcium channel antagonist. Other patients prospective clinical trials including 77,000 post-infarction are candidates for pharmacotherapy with bosentan and patients who received a glyceryl trinitrate patch (46) or an various prostanoids (7,106). Anew and intensively discussed oral sustained-formulation of isosorbide mononitrate (57). In but currently not approved therapeutic option is the use of both studies there was a trend but no significant effect on inhaled nitric oxide (74). It appears that this treatment may survival and similar result emerged from a subsequent meta- fulfill the need of a drug that acts selectively on the analysis (57). However, subgroup analysis in GISSI-3 pulmonary vasculature. In contrast, organic nitrates are showed a significant effect of glyceryl trinitrate on survival considered to be less efficacious and are not included in of elderly patients (>70 years) and women. Furthermore, glyceryl trinitrate significantly improved the beneficial effectof lisinopril on survival (46). It is not known whether these EFFECTS OF NITRATES ON SURVIVAL
favourable results have been adopted to clinical practice of IN CARDIOVASCULAR DISEASES
pharmocotherapy of post-infarction patients.
The reasons for the appearant discrepancy between the results of the initial meta-analysis and the subsequently – Stable angina: There is no prospective randomised trial performed prospective clinical investigations have been on the effects of nitrates on survival in stable angina.
discussed in detail (50,59). One important disadvantage of Therefore, nitrates are usually considered as pure GISSI-3 and ISIS-4 is the frequent use of nitrates (up to symptomatic drugs in the treatment of stable angina.
60%) in the study arms serving as controls for the effects of Nevertheless, many investigations indicate that nitrates may nitrates on survival. This is a crucial point, since the do more than just relieve the symptoms of myocardial beneficial effects of nitrates have been described to occur ischemia. Accumulating experimental and clinical data within the first 24 hr of initiation of treatment (134). Another suggest that late ischemic preconditioning is triggered and important factor is the fact that GISSI-3 and ISIS-4 have mediated by endogenous NO and can be pharmacologically been conducted after the establishment of significant recruited by NO-donors such as glyceryl trinitrate (17).
pharmacotherapeutic proceedings in the management of Furthermore, some of the most important effects that NO myocardial infarction such as lysis. Finally, the duration of exerts in the vascular wall are potentially vasoprotective, treatment (4-6 weeks) might have been too short to show because these effects contribute to and maintain important possible significant benefits of nitrates. For example, in physiological functions such as vasodilation, secondary prevential trials with statins at least 6 months of anticoagulation, leucocyte adhesion, smooth muscle treatment were necessary before the Kaplan-Meier-curves proliferation, apoptosis and the antioxidative capacity (41).
A large body of evidence suggests that NO produced in thevascular endothelium exerts protective effects against the development of atherosclerosis and CAD. Improvement of The results of a randomized controlled clinical trial with endothelial function and vascular NO-bioavailability by mild-to-moderate heart failure has demonstrated that a regular physical activity or pharmacotherapy with ACE- combination of isosorbide dinitrate and the arterial vasodilator hydralazine reduces mortality and morbidity in glyceryl trinitrate was dosed continuously in combination patients receiving a baseline medication with digoxin and with low dose L-arginine (700 mg every 6 hr). This study diuretics (24). The group of patients treated with the shows a 24 hr antianginal effect of continuously isosorbide dinitrate/hydralazine combination had significant administered glyceryl trinitrate. It was concluded that this improvements of ejection fraction, exercise capacity and was the result of prevention of tolerance by L-arginine. A overall mortality as compared with a group of patients similar prevention of tolerance was seen earlier when treated with the vasodilator prazosin (α1-adrenoceptor glyceryl trinitrate and L-arginine was administered antagonist). After inclusion of angiotensin-converting intravenously (3) and when oral treatment with folic acid enzyme inhibitors into the pharmacotherapy of heart failure was added to continuous transdermal glyceryl trinitrate (44).
it was shown that treatment with enalapril is superior to thenitrate combination (25). In the enalapril arm mortality was Decreased generation of vascular NO by organic nitrates 28% lower than in the nitrate arm. This favorable effect was Many efforts have been untertaken to elucidate the attributable to a reduction in the incidence of sudden death molecular mechanism underlying nitrate tolerance and and was more prominent in patients with less severe several different mechanisms have been shown to be symptoms (New York Heart Association class I or II). In involved. The most extensively discussed hypothesis is that contrast, only the nitrate combination increased body sulfhydryl depletion of the vascular smooth muscle inhibits oxygen consumption at peak exercise and it induced in a generation of NO from organic nitrates (92). However, it stronger increase of ejection fraction during the first 3 was demonstrated that restoration of thiol levels in the months of treatment. Nevertheless, there are patients vascular wall does not reverse in vitro tolerance to organic showing an insufficient response, a poor tolerability or nitrates (45). Likewise, it is possible to strongly reduce the contraindications to ACE-inhibitors so that the nitrate thiol content of vascular smooth muscle without affecting combination remains a valuable therapeutic approach (23).
the vasodilator activity of organic nitrates (68). There is also Despite the considerable debate on raced-based therapeutics in vivo evidence speaking against the "sulfhydryl-depletion"- (10), afroamericans (blacks) with heart failure appear to hypothesis (15). Thus, depletion of vascular free thiols might represent a particular group of patients with a significant be associated with nitrate tolerance but is probably not the reponse to the isosorbide dinitrate/hydralazine combination. A very recent study with 1050 black patients with severe heart In contrast, in vitro and in vivo studies provided evidence for an involvement of impaired vascular nitrate bioactivation pharmacotherapy therapy for heart failure such as angiotensin- to NO in nitrate tolerance. Almost two decades ago in vitro converting-enzyme inhibitors, angiotensin-receptor-blockers, studies demonstrated that nitrate tolerance is associated with β-blockers for at least three months before randomization, reduced vascular bioactivation of nitrates as indicated by a digoxin, spironolactone, and diuretics showed, that the lower concentration of the dinitrate metabolites found in addition of isosorbide dinitrate/hydralazine is efficacious and tolerant vascular preparations or isolated cells (9,18). As increases survival these black patients (120).
shown by studies in isolated coronary arteries, the reductionof nitrate bioactivation might be initiated by NO and thus represent a form of feed-back inhibition (69). These data are When nitrates are continuously applied, their in vivo consistant with the results of a recent cDNA mircroarray vasodilator activity ceases within 17-24 hr. This can be analysis on the influence of nitrate tolerance on vascular demonstrated by a reduction of exercise capacity and an gene expression (127). There was a significantly reduced increased frequency of anginal attacks (6). In general, nitrate expression of enzymes such as glutathion-S-transferase or tolerance is a multifactorial phenomenon (Fig. 2) which also cytochrome P450 which are known to be involved in includes a so-called pseudo-tolerance that involves increased vascular bioactivation of organic nitrates to NO.
circulating levels of vasoconstrictors (65,94). Earlier studies Although there was no association between reduced have shown that an eccentric dosing regimen including a bioconversion of high-dose glyceryl trinitrate in an in vivo nitrate-free interval of 10-12 hr is a useful therapeutic study in the rat (72), this well known hypothesis has been approach to prevent the development of nitrate tolerance.
recently reinforced by a clinical investigation which provided This has been proven for glyceryl trinitrate (30), and evidence that a reduced bioactivation of glyceryl trinitrate different formulations of isosorbide mononitrate occurs in nitrate tolerance induced by short-term i.v. glyceryl (22,98,123). Nevertheless, nitrate tolerance is still considered trinitrate in man (108). In vivo studies in rats and rabbits have disadvantageous since the eccentric dosing regimen cannot shown the development of a moderate form of nitrate provide a 24 hr lasting protection from anginal attacks and is tolerance characterized by a specific inhibition of vasodilation associated with a decreased anginal threshold during the by organic nitrates such as glyceryl trinitrate and isosorbide nitrate-free interval (95). A potential solution to this problem mononitrate, while the vasodilator activity of NO itself may be seen in a recent study (99) in which transdermal remained (28,83). A recent study proposed the mitochondrial aldehyde dehydrogenase as an important enzyme for nitrate Decreased bioavailability of vascular NO bioactivation and demonstrated its inhibition in the setting of Recent investigations in rabbits treated with GTN patches in vitro nitrate tolerance (20). Likewise, the results of a continuously releasing the drug for 3 days showed the another report suggest that nitrate tolerance is partially development of a severe form of nitrate tolerance where both mediated by inhibition of vascular mitochondrial aldehyde endothelium-dependent and NO-induced vasorelaxations dehydrogenase and that mitochondrial reactive oxygen were markedly impaired. This was associated with other species contribute to this inhibition (119).
changes of vascular reactivity including increased vascular However, it should be noted that there are important but superoxide production induced by angiotensin II stimulated unexplained steps in this newly proposed mechanism of activation of NADH/NADPH oxidases and an increased nitrate bioactivation in mitochondria. For example, sensitivity to vasoconstrictors such as phenylephrine (85,86).
mitochondrial bioactivation of nitrates yield nitrite anion and Interestingly, it was suggested that both effects result from it is unclear how nitrite is reduced to NO. Recently published activation of protein kinase C by angiotensin II. In experiments using purified mitochondrial aldehyde accordance, the angiotensin II receptor antagonist losartan dehydrogenase showed generation of an activator of soluble abolished vascular superoxide production and normalized guanylyl cyclase by glyceryl trinitrate, but the contribution of endothelium-dependent vasodilation in severe nitrate this enzyme in mitochiondrial preparations to the tolerance (71). However, losartan did not completely restore bioactivation of glyceryl trinitrate appeared rather small (31).
the vascular sensitivity to GTN suggesting that other Likewise, immunoblot analysis indicate a that the majority of mechanisms of nitrate tolerance were also involved. This is vascular mitochondrial aldehyde dehydrogenase is present in consistant with a study in humans where losartan had no the cytoplasm and inhibitors of this enzyme such as effect on the development of nitrate tolerance (80).
cyanamide and propionaldehyde induce a comparable It has been reported that a damaged form of endothelial rightwardshift of the concentration-response curves for nitric oxide synthase, the so-called "uncoupled eNOS", may nitrates in both tolerant and non-tolerant arteries (31).
also contribute to vascular generation of superoxide in nitrate Although the identity of the enzyme leading to bioactivation tolerance (2,3), and there are other observations supporting of nitrates is still a matter of debate, the findings above further this hypothesis as recently reviewed (94). In contrast, studies reinforce the view that nitrate tolerance is indeed at least in eNOS–/– and eNOS+/+ mice showed no difference in nitrate partially mediated by impairment of nitrate bioactivation.
tolerance after continuous infusion of glyceryl trinitrate as Severe Nitrate Tolerance
activated renin-angiotensin system
strongly impaired vasodilation by nitrates
activated vascular protein kinase C
impaired vasodilation by NO
activated endothelial NADPH-oxidase
impaired endothelial function
modulated NO-signal transduction
increased vascular superoxide production
Moderate Nitrate Tolerance
vasodilation by nitrates
selective reduction of
enzymatic nitrate
No Nitrate Tolerance
normal vasodilation by nitrates
Illustration of the hypothesis that nitrate tolerance is a dynamic event where increasing severity (left side) is mediated by strikingly different molecular mechanisms (right side). Severe nitrate tolerance increases vascular oxidant stress which and induces endothelialdysfunction, while moderate nitrate tolerance seems to be restricted to a specific impairment of bioactivation of nitrates to NO. Beside NADPH-oxidase uncoupled eNOS might serve as a source of increased vascular oxidant stress (please see chapter Nitrate tolerance).
evidenced by identical reductions of vasodilator responses nitrate tolerance seem to be restricted to a specific impairment and vascular cGMP accumulation (128). It should be noted of bioactivation of nitrates to NO. Thus, nitrate tolerance is however that this apparent contradiction might be the result most likely not a simple on-off phenomen but a rather of compensatory action of vascular neuronal NOS (nNOS) in dynamic event where increasing severity is mediated by eNOS–/– mice (53). Another mechanism of nitrate tolerance strikingly different molecular mechanisms.
that is associated with a decreased bioavailability of NO wassuggested recently (62). This study provided evidence for an Side effects, contraindications and interactions with other increased expression of the cGMP-hydrolizing phospho- diesterase 1A1 in nitrate tolerance, while the cGMP In general, organic nitrates are remarkably safe and well hydrolizing phosphodiesterase V was not changed.
tolerated drugs. Important side effects include headache, In summary, nitrate tolerance is a well documented and hypotension, flush and nausea (6,65,96). These side effects clinically important phenomenon in pharmacotherapy with are directly related to the vasodilator activity of nitrates.
nitrates. It is characterized by a loss of vasodilator activity and Table 4 lists the side effects of the typical organic nitrate can be avoided by a nitrate pause of at least 8 h, preferably isosorbide dinitrate. Headache is the most prominent side during the night. The molecular mechanism of nitrate effect and is caused by cerebral vasodilation (93). However, tolerance is multifactorial and seems to depend on its severity the mechanism of cerebral vasodilation by organic nitrates (Fig. 2). In severe nitrate tolerance oxidative stress decreases seems to differ from that of coronary vasodilation. It has nitrate bioactivation, the bioavailability of vascular NO and been reported that the vasodilator efficacy of organic induces endothelial dysfunction, while moderate forms of nitrates in isolated cerebral arteries is 100-fold lower than Side Effect
at first days of therapy, when dosage is increased,associated with dizziness, drowsiness,reflex tachycardia, feeling of weakness vomiting, flush, allergic scin reaction (e.g. rash),marked hypotension with increased angina symptoms,collapse (sometimes accompanied by bradyarrhythmia and syncope) (Please note: not listed is the possiblility that during the treatment with sustained release formulation of isosorbide dinitrate, a temporaryhypoxemia may occur due to a relative redistribution of the blood flow in hypoventilated alveolar areas. Particularly in patients with coronaryartery disease this may lead to a myocardial hypoxia.) that in isolated coronary arteries, while the reponse of both cholesterolemia and atherosclerosis (26,126). Therefore, a lot artery types to NO is essentially the same (78). These data of studies were conducted to elucidate whether dietary L- suggest a specific lack of enzymatic bioactivation of nitrates arginine supplementation can augment NO production in man to NO in cerebral arteries. One interesting alternative and thereby improve vascular health (13,16).
mechanism was reported by Wei et al. who found that Interestingly, the intracellular levels of L-arginine in the organic nitrates can activate sensory nerve fibres to release millimolar range far exceed the Km of the NOS enzyme, calcitonin gene related peptide, which then induces cerebral which is in the micromolar range, and therefore it is unlikely vasodilation via the NO/cGMP pathway (129). It is well that application of L-arginine affects NO production (16).
known that nitrate-induced headache ceases during the first This phenomenon was called the "L-arginine paradox". The days of therapy, while other vasodilator actions remain. This most plausible explanation to solve this paradox may be the clinical observation also indicates different mechanisms of presence of the endogenous competitive inhibitor of the action underlying vasodilation of cerebral and non-cerebral NOS, asymmetrical dimethylarginine (ADMA). Plasma blood vessels. A recent report suggests that isosorbide dinitrate, isosorbide mononitrate and slow release glyceryl hypercholesterolemia, atherosclerosis, hypertension, chronic trinitrate induce cluster headache which may be caused by renal failure, and chronic heart failure. In several studies central mechanisms involving a decrease of sympathetic ADMA evolved as a novel cardiovascular risk factor and administration of L-arginine has been shown to improve Nitrates are contraindicated in severe hypotension, endothelium-dependent vascular functions in subjects with shock, obstructive cardiomyopathy and toxic pulmonary high ADMA levels (16). Oral L-arginine improves edema, because their vasodilatory actions would worsen endothelial function in healthy individuals older than 70 these conditions. Close cardiovascular monitoring is years (14). The pharmacokinetics of L-arginine have been recommended if nitrates are administered in acute investigated, side effects are rare and mostly mild and dose- myocardial infarction with low filling pressures or in patients suffering from orthostatic dysregulation.
Today, we have reached phase 2 of the human testing: Nitrates show just a few interactions with other drugs and about 500 subjects and patients got oral L-arginine and we this is of considerable advantage in treatment of more have results about the effectiveness and safety aspects. The severly ill cardiovascular patients who need a variety of problem is the heterogenity of the published studies drugs. The most important interactions are those resulting in concerning doses and outcome parameters. Table 3 a magnification of hypotensive effects. This holds true for summarizes published studies in which L-arginine was any kind of vasodilatory active drugs such as β-blockers, applicated in patients with coronary artery disease and diuretics and calcium antagonists but also for alcohol, chronic heart failure. For coronary artery disease we had one neuroleptics and tricyclic antidepressants. For the same negative: Blum et al. (11) conducted a randomized, double- reason, patients receiving nitrates cannot take inhibitors of blind crossover study with 9 g QD for 1 month and assessed phosphodiesterase V such as sildenafil or vardenafil. This flow-mediated brachial artery dilation and cell adhesion latter interaction is of particular importance since the rate of molecule expression. In this study it was unclear if NO erectile dysfunction is comparably high in patients with production was increased. The study subjects were on full cardiovascular medication with statins and ACE inhibitors,both have been shown to improve endothelial function. In Other drugs activating the NO/CGMP pathway addition, β-blockers and aspirin as an antioxidant were used.
During the last decade there have been many reports on This may have confounded possible effects of L-arginine activators of the NO/cGMP pathway including L-arginine itself. In chronic heart failure Chin-Dusting (21) described a and derivatives, phosphodiesterase inhibitors, NOS- negative result with 20 g L-arginine for 28 days, whereas activating β-blockers, NO-independent activators of soluble Rector et al. (104) showed a significant improvement in guanylyl cyclase and nitroaspirins (41). In many aspects, the functional status. It may be that L-arginine may not be useful pharmacology of the latter two classes of new drugs closely in later stages of atherosclerosis in which the NOS resemble that of organic nitrates and their relations to nitrates expression was decreased by other factors.
will be discussed below. In addition, this paragraph sheds – Nicorandil: Nicorandil is a nicotinamid ester that activates some light on clinical data about L-arginine and the nitrate- two different cellular pathways to induce vasodilation. Early studies on the effect of nicorandil on coronary cGMP-levels – L-arginine: L-arginine is the substrate for the enzyme clearly demonstrated a predominant nitrate-like activity that nitric oxide synthase (NOS), which is responsible for the is most likely initiated by an enzymatic generation of NO production of nitric oxide (NO). Acute and chronic from the nitrate nitrogen of nicorandil (70). In addition, administration of L-arginine has been shown to improve nicorandil was shown to be a potassium channel opener, endothelial function in animal models of hyper- although with a much lower potency than cromakalim (131).
The nitrate-like action activates cGMP-dependent protein one has to await the results of clinical investigations. It is kinase I and leads to phosphorylation of several proteins such likely that these drugs will be markted for the treatment of as phospholamban, 1,4,5-inositoltrisphosphate receptor hypertension, heart failure and/or CAD. It is not known associated cGMP kinase substrate (IRAG) and calcium- whether NO-independent activation of soluble guanylyl dependent potassium channels (41) which reduces the cyclase can improve endothelial function and reduce the intracellular calcium-concentration. In addition, the ATP- intimal lesion area in experimental atherosclerosis as has been dependent potassium channel opening activity induces a reported for organic nitrates (see above). NO was shown to membrane hyperpolarization that impairs the activation of L- elicit cGMP-independent and potentially vasoprotective effects such as a reduction of lipid oxidation, activation of It is still a matter of debate which of these mechanisms are potassium channels and antiproliferative activity in vascular predominant in vivo. However, the nitrate-like hemodynamic smooth muscle cells (41) and these effects might not be changes induced by nicorandil together with the knowledge mimicked by NO-independent activators of soluble guanylyl that the primarily arteriolar dilator effect initiated by cyclase. Another important difference to organic nitrates is the potassium channel openers are rather contraproductive in strong effect of BAY412272 on platelet aggregation and CAD strongly speak in favour of a predominant nitrate bleeding time. Again, it is not known whether this component (61). Nicorandil induces an antianginal activity pharmacologic effect is of advantage in CAD. Although comparable to that of nitrates, β-blockers and specific inhibition of platelet activation is a useful pharmacologic calcium channel blockers and does not precipitate a coronary intervention, increased bleeding time may become a problem, steal phenomenon. In addition, a large randomized clinical particularly in combination with aspirin.
trial recently demonstrated beneficial effects of nicorandil on – Nitroaspirins: Nitroaspirins such as NCX-4016 (2- the prognosis of patients with stable CAD and additional (acetoxy)-3-[(nitroxy)methyl]phenyl ester) (Fig. 3) were cardiovascular risk factors (124). The primary composite designed to combine pharmacologic effects of organic endpoint was coronary heart disease death, nonfatal nitrates and acetylsalicylic acid in one drug. Basically, these myocardial infarction, or unplanned hospital admission for drugs resemble the above mentioned direct activators of cardiac chest pain. Treatment with nicorandil for a mean of soluble guanylyl cyclase with respect to vasodilation and 1.6 years resulted in a 17% reduction of this endpoint.
inhibition of platelet aggregation (88). Furthermore, it was Although no firm conclusion can be drawn, it seems possible anticipated that liberation of NO from these drugs inhibits the that the nitrate-like action of has contributed to the beneficial initiation of peptic ulcers by aspirin and improve its effect of nicorandil in the IONA-study (66).
therapeutic safety and this seems indeed to be the case (36).
The liberation of NO from NCX-4016 is a metabolic and not NO-independent activators of soluble guanylyl cyclase a spontaneous process (29). This is not surprising since the In 1994, researchers from Taiwan reported on generation of NO from a nitrate moiety requires a transfer of pharmacological activites of YC-1, a new benzylindazol 3 electrons to the nitrate nitrogen. Presumably, NCX-4016 is derivative. They found that this drug activate the enzyme metabolized by the same enzymes than organic nitrates, soluble guanylyl cyclase by a NO-independent mechanism namely cytochrome P 450 enzymes glutathion-S-transferase and exerts antiplatelet effects in vivo (64,133). Further studies and mtALDH, but further studies are needed to answer this showed that YC-1 also potentiates the effects of NO and that question. However, the need for enzymatic bioactivation this is mediated by a stabilization of the active configuration implies that the NO-generation from NCX-4016 might be of soluble guanylyl cyclase, presumably via prologation of reduced by a tolerance phenomenon when the drug is given the dissociation of NO (38,39). Although the potency of YC- in a continuous fashion (see above).
1 is too low to consider this compound for clinical The effects of NCX-4016 on the development of development, it served as a lead compound in the search of experimental atherogenesis have also been investigated.
new activators of soluble guanylyl cyclase. This new class of Cholesterol-fed LDL-receptor deficient mice were given drugs is much more potent than YC-1 and display useful NCX-4016 by oral administration. After 12 weeks mice pharmacologic actions such as vasodilation and inhibition of treated with NCX-4012 showed a significant reduction of platelet aggregation (117). A more detailed investigation of plasma LDL-oxidation, oxidative stress and atherogenesis, the mechanism of action of these drugs resulted in the while treatment with aspirin itself had no effect. Thus, these discovery of a new regulatory site located in the region of the favourable effects of NCX-4016 can be most likely attributed cysteins 238 and 243 in the α1-subunit of soluble guanylyl to the nitrate moiety of NCX-4016. Other studies showed cyclase (116). The effects of one of the new drugs, namely protective effects of NCX-4016 in animal models of BAY412272, has been investigated in hypertensive rats and restenosis, ischemia-reperfusion and myocardial infarction was shown to reduce blood pressure and to prolong bleeding (91,105,125). In summary, nitroaspirins are a new class of time. Today, it is too early to estimate the possible therapeutic drugs which may used for the treatment of cardiovascular value of the new activators of soluble guanylyl cyclase and L-arginine in patients with coronary artery disease on medical management. Circulation 2000, 101: 2160-2164.
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