Evidence for a causal role of the renin-angiotensin system in nitrate tolerance

Sodium nitrite induces tolerance in the mouse aorta: Involvement of the renin-angiotensin system, nitric oxide synthase, and reactive oxygen species

Nitrites have emerged as promising therapeutic agents for cardiovascular diseases, alongside nitrates. While chronic use of organic nitrates is well recognized to lead to vascular tolerance, the tolerance associated with nitrite therapy remains incompletely understood. The aim of the present study was to investigate vascular tolerance to sodium nitrite and the underlying molecular mechanisms. Endothelium-denuded aortic rings isolated from male Balb/C mice were incubated with either the EC50 (10-4 mol/L) or EC100 (10-2 mol/L) concentration of sodium nitrite for 15 min to induce tolerance. The EC100 concentration of sodium nitrite induced vascular tolerance. Pre-incubation with captopril and losartan effectively reversed sodium nitrite-induced tolerance. Similarly, pre-incubation with L-NAME and L-arginine prevented sodium nitrite-induced tolerance. Increased levels of reactive oxidative species (ROS) and reduced bioavailability of nitric oxide (NO) were observed in tolerant aortas. Increased superoxide dismutase (SOD) activity and decreased catalase activity were also verified in tolerant aortas. Both captopril and L-NAME prevented the increased levels of ROS observed in tolerant aortas. Furthermore, pre-incubation with catalase effectively prevented sodium nitrite-induced tolerance. Our findings suggest that sodium nitrite induces vascular tolerance through a signaling pathway involving the renin-angiotensin system, nitric oxide synthase, and ROS. This study contributes to the understanding of the interaction between nitrites and vascular tolerance and highlights potential targets to overcome or prevent this phenomenon.

Vascular redox signaling, eNOS uncoupling and endothelial dysfunction in the setting of transportation noise exposure or chronic treatment with organic nitrates

SIGNIFICANCE

Cardiovascular disease and drug-induced health side effects are frequently associated with - or even caused by - an imbalance between the concentrations of reactive oxygen and nitrogen species (RONS) and antioxidants respectively determining the metabolism of these harmful oxidants.

RECENT ADVANCES

According to the "kindling radical" hypothesis, initial formation of RONS may further trigger the additional activation of RONS formation under certain pathological conditions. The present review will specifically focus on a dysfunctional, uncoupled endothelial nitric oxide synthase (eNOS) caused by RONS in the setting of transportation noise exposure or chronic treatment with organic nitrates, especially nitroglycerin. We will further describe the various "redox switches" that are proposed to be involved in the uncoupling process of eNOS.

CRITICAL ISSUES

In particular, the oxidative depletion of tetrahydrobiopterin (BH4), and S-glutathionylation of the eNOS reductase domain will be highlighted as major pathways for eNOS uncoupling upon noise exposure or nitroglycerin treatment. In addition, oxidative disruption of the eNOS dimer, inhibitory phosphorylation of eNOS at threonine or tyrosine residues, redox-triggered accumulation of asymmetric dimethylarginine (ADMA) and L-arginine deficiency will be discussed as alternative mechanisms of eNOS uncoupling.

FUTURE DIRECTIONS

The clinical consequences of eNOS dysfunction due to uncoupling on cardiovascular disease will be summarized also providing a template for future clinical studies on endothelial dysfunction caused by pharmacological or environmental risk factors.

Nitrites, Nitrates, and Cardiovascular Outcomes: Are We Living "La Vie en Rose" With Pink Processed Meats?

Background Nitrates and nitrites are used as food additives in processed meats. They are also commonly ingested from water and several foods. Several short-term clinical studies suggested beneficial effects of dietary nitrates on blood pressure, while deleterious effects on oxidative damage have been suggested in some experimental studies. However, there is a lack of evidence from longitudinal epidemiological studies linking foods and water-originated and additives-originated nitrites and nitrates, separately, to hypertension and cardiovascular diseases risk. We aimed to study these associations in a large population-based cohort. Methods and Results Overall, 106 288 adults from the French NutriNet-Santé cohort (2009-2022) were included. Associations between nitrites and nitrates intakes and hypertension and cardiovascular disease risks were assessed using multi-adjusted Cox proportional hazard models. During follow-up, 3810 incident cases of hypertension and 2075 cases of cardiovascular diseases were ascertained. Participants with higher intakes of additives-originated nitrites (sodium nitrite in particular [European code e250]) had a higher hypertension risk compared with nonconsumers (hazard ratio, 1.19 [95% CI, 1.08-1.32], P=0.001, and 1.19 [95% CI, 1.08-1.32], P=0.002), respectively. No association was detected between foods and water-originated nitrites, or nitrates with hypertension risk (all P values >0.3). We found no association between nitrites or nitrates and risks of cardiovascular diseases (all P values >0.2). Conclusions These results do not support a protective role of nitrites or nitrates in cardiovascular health. Instead, they suggest a positive association between nitrites from food additives and hypertension risk, which needs confirmation in other large-scale studies. These findings provide new evidence in the context of current discussions about updating regulations on the use of nitrites as food additives.

Organic Nitrate Therapy, Nitrate Tolerance, and Nitrate-Induced Endothelial Dysfunction: Emphasis on Redox Biology and Oxidative Stress

Organic nitrates, such as nitroglycerin (GTN), isosorbide-5-mononitrate and isosorbide dinitrate, and pentaerithrityl tetranitrate (PETN), when given acutely, have potent vasodilator effects improving symptoms in patients with acute and chronic congestive heart failure, stable coronary artery disease, acute coronary syndromes, or arterial hypertension. The mechanisms underlying vasodilation include the release of •NO or a related compound in response to intracellular bioactivation (for GTN, the mitochondrial aldehyde dehydrogenase [ALDH-2]) and activation of the enzyme, soluble guanylyl cyclase. Increasing cyclic guanosine-3',-5'-monophosphate (cGMP) levels lead to an activation of the cGMP-dependent kinase I, thereby causing the relaxation of the vascular smooth muscle by decreasing intracellular calcium concentrations. The hemodynamic and anti-ischemic effects of organic nitrates are rapidly lost upon long-term (low-dose) administration due to the rapid development of tolerance and endothelial dysfunction, which is in most cases linked to increased intracellular oxidative stress. Enzymatic sources of reactive oxygen species under nitrate therapy include mitochondria, NADPH oxidases, and an uncoupled •NO synthase. Acute high-dose challenges with organic nitrates cause a similar loss of potency (tachyphylaxis), but with distinct pathomechanism. The differences among organic nitrates are highlighted regarding their potency to induce oxidative stress and subsequent tolerance and endothelial dysfunction. We also address pleiotropic effects of organic nitrates, for example, their capacity to stimulate antioxidant pathways like those demonstrated for PETN, all of which may prevent adverse effects in response to long-term therapy. Based on these considerations, we will discuss and present some preclinical data on how the nitrate of the future should be designed.

The impact of age-related dysregulation of the angiotensin system on mitochondrial redox balance

Aging is associated with the accumulation of various deleterious changes in cells. According to the free radical and mitochondrial theory of aging, mitochondria initiate most of the deleterious changes in aging and govern life span. The failure of mitochondrial reduction-oxidation (redox) homeostasis and the formation of excessive free radicals are tightly linked to dysregulation in the Renin Angiotensin System (RAS). A main rate-controlling step in RAS is renin, an enzyme that hydrolyzes angiotensinogen to generate angiotensin I. Angiotensin I is further converted to Angiotensin II (Ang II) by angiotensin-converting enzyme (ACE). Ang II binds with equal affinity to two main angiotensin receptors—type 1 (AT₁R) and type 2 (AT₂R). The binding of Ang II to AT₁R activates NADPH oxidase, which leads to increased generation of cytoplasmic reactive oxygen species (ROS). This Ang II-AT₁R–NADPH-ROS signal triggers the opening of mitochondrial K_ATP channels and mitochondrial ROS production in a positive feedback loop. Furthermore, RAS has been implicated in the decrease of many of ROS scavenging enzymes, thereby leading to detrimental levels of free radicals in the cell. AT₂R is less understood, but evidence supports an anti-oxidative and mitochondria-protective function for AT₂R. The overlap between age related changes in RAS and mitochondria, and the consequences of this overlap on age-related diseases are quite complex. RAS dysregulation has been implicated in many pathological conditions due to its contribution to mitochondrial dysfunction. Decreased age-related, renal and cardiac mitochondrial dysfunction was seen in patients treated with angiotensin receptor blockers. The aim of this review is to: (a) report the most recent information elucidating the role of RAS in mitochondrial redox hemostasis and (b) discuss the effect of age-related activation of RAS on generation of free radicals.

The effect of peroxynitrite decomposition catalyst MnTBAP on aldehyde dehydrogenase-2 nitration by organic nitrates: role in nitrate tolerance

Bioconversion of glyceryl trinitrate (GTN) into nitric oxide (NO) by aldehyde dehydrogenase-2 (ALDH-2) is a crucial mechanism which drives vasodilatory and antiplatelet effect of organic nitrates in vitro and in vivo. Oxidative stress generated by overproduction of free radical species, mostly superoxide anions and NO-derived peroxynitrite, has been suggested to play a pivotal role in the development of nitrate tolerance, though the mechanism still remains unclear. Here we studied the free radical-dependent impairment of ALDH-2 in platelets as well as vascular tissues undergoing organic nitrate ester tolerance and potential benefit when using the selective peroxynitrite decomposition catalyst Mn(III) tetrakis (4-Benzoic acid) porphyrin (MnTBAP). Washed human platelets were made tolerant to nitrates via incubation with GTN for 4h. This was expressed by attenuation of platelet aggregation induced by thrombin (40U/mL), an effect accompanied by GTN-related induction of cGMP levels in platelets undergoing thrombin-induced aggregation. Both effects were associated to attenuated GTN-induced nitrite formation in platelets supernatants and to prominent nitration of ALDH-2, the GTN to NO metabolizing enzyme, suggesting that GTN tolerance was associated to reduced NO formation via impairment of ALDH-2. These effects were all antagonized by co-incubation of platelets with MnTBAP, which restored GTN-induced responses in tolerant platelets. Comparable effect was found under in in vivo settings. Indeed, MnTBAP (10mg/kg, i.p.) significantly restored the hypotensive effect of bolus injection of GTN in rats made tolerants to organic nitrates via chronic administration of isosorbide-5-mononitrate (IS-5-MN), thus confirming the role of peroxynitrite overproduction in the development of tolerance to vascular responses induced by organic nitrates. In conclusion, oxidative stress subsequent to prolonged use of organic nitrates, which occurs via nitration of ALDH-2, represents a key event in GTN tolerance, an effect counteracted both in vitro and in vivo by novel peroxynitrite decomposition catalyst.

Current perspectives on hydralazine and nitrate therapies in heart failure

The origins of the hydralazine/isosorbide dinitrate (H+ISDN) combination therapy are rooted in the first large-scale clinical trial in heart failure: V-HeFT I. Initially utilized for the balanced vasodilatory properties of each drug, we now know there is "more to the story." In fact, the maintenance of the nitroso-redox balance may be the true mechanism of benefit. Since the publication of V-HeFT I 30 years ago, H+ISDN has been the subject of much discussion and debate. Regardless of the many controversies surrounding H+ISDN, one thing is clear: therapy is underutilized and many patients who could benefit never receive the drugs. Ongoing physician and patient education are mandatory to improve the rates of H+ISDN use.

In vitro and in vivo characterization of a new organic nitrate hybrid drug covalently bound to pioglitazone

BACKGROUND/AIMS

Organic nitrates represent a group of nitrovasodilators that are clinically used for the treatment of ischemic heart disease. The new compound CLC-3000 is an aminoethyl nitrate (AEN) derivative of pioglitazone, a thiazolidinedione antidiabetic agent combining the peroxisome proliferator-activated receptor γ agonist activity of pioglitazone with the NO-donating activity of the nitrate moiety.

METHODS

In vitro and in vivo characterization was performed by isometric tension recording, platelet function, bleeding time and detection of oxidative stress.

RESULTS

In vitro, CLC-3000 displayed more potent vasodilation than pioglitazone alone or classical nitrates. In vitro, some effects on oxidative stress parameters were observed. Authentic AEN or the AEN-containing linker CLC-1275 displayed antiaggregatory effects. In vivo treatment with CLC-3000 for 7 days did neither induce endothelial dysfunction nor nitrate tolerance nor oxidative stress. Acute or chronic administration of AEN increased the tail vein bleeding time in mice.

CONCLUSION

In summary, the results of these studies demonstrate that CLC-3000 contains a vasodilative and antithrombotic activity that is not evident with pioglitazone alone, and that 7 days of exposure in vivo showed no typical signs of nitrate tolerance, endothelial dysfunction or other safety concerns in Wistar rats.

Nitrate therapy for heart failure: benefits and strategies to overcome tolerance

Combination therapy with hydralazine and nitrates can improve outcomes in patients with heart failure and low ejection fraction. However, this combination is underused in clinical practice for several reasons, including side effects related to hydralazine and polypharmacy. Some of the benefits seen with hydralazine, including afterload reduction and attenuation of nitrate tolerance, have also been observed with angiotensin-converting enzyme inhibitors. Demonstrating similar clinical benefits with nitrates plus angiotensin-converting enzyme inhibitor therapy alone, in the absence of hydralazine, may represent an opportunity to improve heart failure care by increasing the use of nitrates. In this paper, we summarize data that support studying such an approach.

Effects of telmisartan or amlodipine monotherapy versus telmisartan/amlodipine combination therapy on vascular dysfunction and oxidative stress in diabetic rats

Our previous studies identified potent antioxidant effects and improvement of vascular function by telmisartan therapy in experimental diabetes and nitrate tolerance. The present study compared the beneficial effects of single telmisartan or amlodipine versus telmisartan/amlodipine combination therapy (T+A) in streptozotocin (STZ)-induced type 1 diabetic rats. Male Wistar rats were injected once with STZ (60 mg/kg, i.v.) and 1 week later the drugs (telmisartan, amlodipine, or T+A) were administrated orally by a special diet (2.5-5 mg kg(-1) day(-1)) for another 7 weeks. We only observed a marginal beneficial on-top effect of T+A therapy over the single drug regimen that was most evident in the improvement of endothelial function (acetylcholine response) and less pronounced in the reduction of whole blood, vascular and cardiac oxidative stress (blood leukocyte oxidative burst, aortic dihydroethidine and 3-nitrotyrosine staining, as well as cardiac NADPH oxidase activity and uncoupling of endothelial nitric oxide synthase) in diabetic rats. These effects on oxidative stress parameters were paralleled by those on the expression pattern of NADPH oxidase and nitric oxide synthase isoforms. In addition, development of mild hypotension in the T+A-treated rats was observed. Reasons for this moderate synergistic effect of T+A therapy may be related to the potent beneficial effects of telmisartan alone and the fact that amlodipine and telmisartan share similar pathways to improve endothelial function. Moreover, hypotension in the T+A-treated rats could partially antagonize the beneficial additive effects by counter-regulatory mechanisms (e.g., activation of the renin-angiotensin-aldosterone system).

Reactive oxygen and nitrogen species in pulmonary hypertension

Pulmonary vascular disease can be defined as either a disease affecting the pulmonary capillaries and pulmonary arterioles, termed pulmonary arterial hypertension, or a disease affecting the left ventricle, called pulmonary venous hypertension. Pulmonary arterial hypertension (PAH) is a disorder of the pulmonary circulation characterized by endothelial dysfunction, as well as intimal and smooth muscle proliferation. Progressive increases in pulmonary vascular resistance and pressure impair the performance of the right ventricle, resulting in declining cardiac output, reduced exercise capacity, right-heart failure, and ultimately death. While the primary and heritable forms of the disease are thought to affect over 5000 patients in the United States, the disease can occur secondary to congenital heart disease, most advanced lung diseases, and many systemic diseases. Multiple studies implicate oxidative stress in the development of PAH. Further, this oxidative stress has been shown to be associated with alterations in reactive oxygen species (ROS), reactive nitrogen species (RNS), and nitric oxide (NO) signaling pathways, whereby bioavailable NO is decreased and ROS and RNS production are increased. Many canonical ROS and NO signaling pathways are simultaneously disrupted in PAH, with increased expression of nicotinamide adenine dinucleotide phosphate (NADPH) oxidases and xanthine oxidoreductase, uncoupling of endothelial NO synthase (eNOS), and reduction in mitochondrial number, as well as impaired mitochondrial function. Upstream dysregulation of ROS/NO redox homeostasis impairs vascular tone and contributes to the pathological activation of antiapoptotic and mitogenic pathways, leading to cell proliferation and obliteration of the vasculature. This paper will review the available data regarding the role of oxidative and nitrosative stress and endothelial dysfunction in the pathophysiology of pulmonary hypertension, and provide a description of targeted therapies for this disease.

Organic nitrates and nitrate resistance in diabetes: the role of vascular dysfunction and oxidative stress with emphasis on antioxidant properties of pentaerithrityl tetranitrate

Organic nitrates represent a class of drugs which are clinically used for treatment of ischemic symptoms of angina as well as for congestive heart failure based on the idea to overcome the impaired NO bioavailability by “NO” replacement therapy. The present paper is focused on parallels between diabetes mellitus and nitrate tolerance, and aims to discuss the mechanisms underlying nitrate resistance in the setting of diabetes. Since oxidative stress was identified as an important factor in the development of tolerance to organic nitrates, but also represents a hallmark of diabetic complications, this may represent a common principle for both disorders where therapeutic intervention should start. This paper examines the evidence supporting the hypothesis that pentaerithrityl tetranitrate may represent a nitrate for treatment of ischemia in diabetic patients. This evidence is based on the considerations of parallels between diabetes mellitus and nitrate tolerance as well as on preliminary data from experimental diabetes studies.

Resveratrol improves cardiovascular function and reduces oxidative organ damage in the renal, cardiovascular and cerebral tissues of two-kidney, one-clip hypertensive rats

OBJECTIVES

The putative protective effects of resveratrol against oxidative injury in the heart, kidney and brain tissues of rats induced with the two-kidney, one-clip (2K1C) hypertension model were investigated.

METHODS

Wistar albino rats were divided into sham-operated (n = 8) or 2K1C groups, in which rats received either resveratrol (10 mg/kg per day, i.p., n = 8), or saline (n = 8) starting at Week 3 after the surgery and continuing for the following 6 weeks. Indirect blood pressure recordings and echocardiographic images were made to evaluate cardiac function. At the end of Week 9 the animals were decapitated and plasma, heart, kidney and brain were taken for biochemical assays, while aortic rings were prepared for vascular reactivity studies.

KEY FINDINGS

2K1C hypertension resulted in increased blood pressure, aortic hypercontractility and reduced left ventricular function, leading to increased lipid peroxidation and myeloperoxidase activity, concomitant with significant reductions in tissue glutathione, superoxide dismutase, Na+/K+-ATPase and catalase activities in the cardiac, renal and brain tissues, indicating the presence of oxidative tissue damage in peripheral target organs. Elevated plasma levels of lactate dehydrogenase, creatine kinase, as well as reduced plasma levels of antioxidant capacity and nitric oxide further verified the severity of oxidative injury. A 6-week treatment with resveratrol reversed all the measured parameters, ameliorated hypertension-induced oxidative injury in the target organs and improved cardiovascular function.

CONCLUSIONS

Resveratrol improved cardiovascular function through the augmentation of endogenous antioxidants and the inhibition of lipid peroxidation by maintaining a balance in oxidant/antioxidant status, which also ameliorated hypertension-induced oxidative injury in the cardiac, renal and cerebral tissues.

Effects of green tea, black tea and Rooibos tea on angiotensin-converting enzyme and nitric oxide in healthy volunteers

OBJECTIVE

Tea has been reported to reduce cardiovascular mortality, but the underlying mechanisms are largely unknown. The aim of the current project was to investigate the effect of green tea (Japanese Sencha), black tea (Indian Assam B.O.P.) and Rooibos tea (South Africa) on angiotensin-converting enzyme (ACE) and nitric oxide (NO).

DESIGN

Seventeen healthy volunteers received a single oral dose of 400 ml green tea, black tea or Rooibos tea in a randomized, three-phase, crossover study. ACE activity and NO concentration were measured (at 0, 30, 60 and 180 min) in all phases. ACE activity was analysed by means of a commercial radioenzymatic assay. Nitrite was analysed as a marker of NO concentration. In addition, ACE genotype was determined using a PCR method.

RESULTS

Oral intake of a single dose of Rooibos tea significantly inhibited ACE activity after 30 min (P < 0.01) and after 60 min (P < 0.05). A significant inhibition of ACE activity was seen with green tea for the ACE II genotype 30 min after intake of the tea (P < 0.05) and for the ACE ID genotype 60 min after intake (P < 0.05). A significant inhibition of ACE activity was also seen with Rooibos tea for the ACE II genotype 60 min after intake (P < 0.05). No significant effect on NO concentration was seen.

CONCLUSIONS

These results suggest that green tea and Rooibos tea may have cardiovascular effects through inhibition of ACE activity.

Redox signaling (cross-talk) from and to mitochondria involves mitochondrial pores and reactive oxygen species

This review highlights the important role of redox signaling between mitochondria and NADPH oxidases. Besides the definition and general importance of redox signaling, the cross-talk between mitochondrial and Nox-derived reactive oxygen species (ROS) is discussed on the basis of 4 different examples. In the first model, angiotensin-II is discussed as a trigger for NADPH oxidase activation with subsequent ROS-dependent opening of mitochondrial ATP-sensitive potassium channels leading to depolarization of mitochondrial membrane potential followed by mitochondrial ROS formation and respiratory dysfunction. This concept was supported by observations that ethidium bromide-induced mitochondrial damage suppressed angiotensin-II-dependent increase in Nox1 and oxidative stress. In another example hypoxia was used as a stimulator of mitochondrial ROS formation and by using pharmacological and genetic inhibitors, a role of mitochondrial ROS for the induction of NADPH oxidase via PKCvarepsilon was demonstrated. The third model was based on cell death by serum withdrawal that promotes the production of ROS in human 293T cells by stimulating both the mitochondria and Nox1. By superior molecular biological methods the authors showed that mitochondria were responsible for the fast onset of ROS formation followed by a slower but long-lasting oxidative stress condition based on the activation of an NADPH oxidase (Nox1) in response to the fast mitochondrial ROS formation. Finally, a cross-talk between mitochondria and NADPH oxidases (Nox2) was shown in nitroglycerin-induced tolerance involving the mitochondrial permeability transition pore and ATP-sensitive potassium channels. The use of these redox signaling pathways as pharmacological targets is briefly discussed.

Organic nitrates and nitrate tolerance--state of the art and future developments

The hemodynamic and antiischemic effects of nitroglycerin (GTN) are lost upon chronic administration due to the rapid development of nitrate tolerance. The mechanism of this phenomenon has puzzled several generations of scientists, but recent findings have led to novel hypotheses. The formation of reactive oxygen and nitrogen species in the mitochondria and the subsequent inhibition of the nitrate-bioactivating enzyme mitochondrial aldehyde dehydrogenase (ALDH-2) appear to play a central role, at least for GTN, that is, bioactivated by ALDH-2. Importantly, these findings provide the opportunity to reconcile the two "traditional" hypotheses of nitrate tolerance, that is, the one postulating a decreased bioactivation and the concurrent one suggesting a role of oxidative stress. Furthermore, recent animal and human experimental studies suggest that the organic nitrates are not a homogeneous group but demonstrate a broad diversity with regard to induction of vascular dysfunction, oxidative stress, and other side effects. In the past, attempts to avoid nitrate-induced side effects have focused on administration schedules that would allow a "nitrate-free interval"; in the future, the role of co-therapies with antioxidant compounds and of activation of endogeneous protective pathways such as the heme oxygenase 1 (HO-1) will need to be explored. However, the development of new nitrates, for example, tolerance-free aminoalkyl nitrates or combination of nitrate groups with established cardiovascular drugs like ACE inhibitors or AT(1)-receptor blockers (hybrid molecules) may be of great clinical interest.

Non-hemodynamic effects of organic nitrates and the distinctive characteristics of pentaerithrityl tetranitrate

Organic nitrates are among the oldest and yet most commonly employed drugs in the long-term therapy of coronary artery disease and congestive heart failure. While they have long been used in clinical practice, our understanding of their mechanism of action and side effects remains incomplete. For instance, recent findings provide evidence of previously unanticipated, non-hemodynamic properties that include potentially beneficial mechanisms (such as the induction of a protective phenotype that mimics ischemic preconditioning), but also toxic effects (such as endothelial and autonomic dysfunction, rebound angina, tolerance). To date, the most commonly employed organic nitrates are isosorbide mononitrate, isosorbide dinitrate, and nitroglycerin (glyceryl trinitrate). Another organic nitrate, pentaerithrityl tetranitrate (PETN), has long been employed in eastern European countries and is currently being reintroduced in Western countries. In light of their wide use, and of the (re)introduction of PETN in Western markets, the present review focuses on the novel effects of organic nitrates, describing their potential clinical implications and discussing differences among different compounds. We believe that these recent findings have important clinical implications. Since the side effects of organic nitrates such as nitroglycerin and isosorbides appear to be mediated by reactive oxygen species, care should be taken that drugs with antioxidant properties are co-administered. On the other hand, efforts should be made to clinically exploit the preconditioning effects of these drugs.

First evidence for a crosstalk between mitochondrial and NADPH oxidase-derived reactive oxygen species in nitroglycerin-triggered vascular dysfunction

Chronic nitroglycerin treatment results in development of nitrate tolerance associated with endothelial dysfunction (ED). We sought to clarify how mitochondria- and NADPH oxidase (Nox)-derived reactive oxygen species (ROS) contribute to nitrate tolerance and nitroglycerin-induced ED. Nitrate tolerance was induced by nitroglycerin infusion in male Wistar rats (100 microg/h/4 day) and in C57/Bl6, p47(phox/) and gp91(phox/) mice (50 microg/h/4 day). Protein and mRNA expression of Nox subunits were unaltered by chronic nitroglycerin treatment. Oxidative stress was determined in vascular rings and mitochondrial fractions of nitroglycerin-treated animals by L-012 enhanced chemiluminescence, revealing a dominant role of mitochondria for nitrate tolerance development. Isometric tension studies revealed that genetic deletion or inhibition (apocynin, 0.35 mg/h/4 day) of Nox improved ED, whereas nitrate tolerance was unaltered. Vice versa, nitrate tolerance was attenuated by co-treatment with the respiratory chain complex I inhibitor rotenone (100 microg/h/4 day) or the mitochondrial permeability transition pore blocker cyclosporine A (50 microg/h/4 day). Both compounds improved ED, suggesting a link between mitochondrial and Nox-derived ROS. Mitochondrial respiratory chain-derived ROS are critical for the development of nitrate tolerance, whereas Nox-derived ROS mediate nitrate tolerance-associated ED. This suggests a crosstalk between mitochondrial and Nox-derived ROS with distinct mechanistic effects and sites for pharmacological intervention.

Dissociation between superoxide accumulation and nitroglycerin-induced tolerance

We hypothesize that superoxide (O(2)(*-)) accumulation is not a crucial causative factor in inducing nitroglycerin (NTG) tolerance. In LLC-PK1 cells, pre-exposure to NTG resulted in increased O(2)(*-) accumulation and reduced cGMP response to NTG versus vehicle control. O(2)(*-) stimulated by NTG was reduced by oxypurinol (100 microM), a xanthine oxidase inhibitor. Exposure to angiotensin II (Ang II) increased O(2)(*-) but did not reduce cGMP response. The O(2)(*-) scavenger tiron reduced Ang II-induced O(2)(*-) production but did not increase NTG-stimulated cGMP production. Using p47(phox-/-) and gp91(phox-/-) mice versus their respective wild-type controls (WT), we showed that aorta from mice null of these critical NADPH oxidase subunits exhibited similar vascular tolerance after NTG dosing (20 mg/kg s.c., t.i.d. for 3 days), as indicated by their ex vivo pEC(50) and cGMP accumulation upon NTG challenge. In vitro aorta O(2)(*-) production was enhanced by NTG incubation in both p47(phox) null and WT mice. Pre-exposure of isolated mice aorta to 100 microM NTG for 1 h resulted in vascular tolerance toward NTG and increased O(2)(*-) accumulation. Oxypurinol (1 mM) reduced O(2)(*-) but did not attenuate vascular tolerance. These results suggest that O(2)(*-) does not initiate either in vitro and in vivo NTG tolerance, and that the p47(phox) and gp91(phox) subunits of NADPH oxidase are not critically required. Increased O(2)(*-) accumulation may be an effect, rather than an initiating cause, of NTG tolerance.

Xanthine oxidase and mitochondria contribute to vascular superoxide anion generation in DOCA-salt hypertensive rats

Vascular superoxide anion (O(2)(*-)) levels are increased in DOCA-salt hypertensive rats. We hypothesized that the endothelin (ET)-1-induced generation of ROS in the aorta and resistance arteries of DOCA-salt rats originates partly from xanthine oxidase (XO) and mitochondria. Accordingly, we blocked XO and the mitochondrial oxidative phosphorylation chain to investigate their contribution to ROS production in mesenteric resistance arteries and the aorta from DOCA-salt rats. Systolic blood pressure rose in DOCA-salt rats and was reduced after 3 wk by apocynin [NAD(P)H oxidase inhibitor and/or radical scavenger], allopurinol (XO inhibitor), bosentan (ET(A/B) receptor antagonist), BMS-182874 (BMS; ET(A) receptor antagonist), and hydralazine. Plasma uric acid levels in DOCA-salt rats were similar to control unilaterally nephrectomized (UniNx) rats, reduced with allopurinol and bosentan, and increased with BMS. Levels of thiobarbituric acid-reacting substances were increased in DOCA-salt rats versus UniNx rats, and BMS, bosentan, and hydralazine prevented their increase. Dihydroethidium staining showed reduced O(2)(*-) production in mesenteric arteries and the aorta from BMS- and bosentan-treated DOCA-salt rats compared with untreated DOCA-salt rats. Increased O(2)(*-) derived from XO was reduced or prevented by all treatments in mesenteric arteries, whereas bosentan and BMS had no effect on aortas from DOCA-salt rats. O(2)(*-) generation decreased with in situ treatment by tenoyltrifluoroacetone and CCCP, inhibitors of mitochondrial electron transport complexes II and IV, respectively, whereas rotenone (mitochondrial complex I inhibitor) had no effect. Our findings demonstrate the involvement of ET(A) receptor-modulated O(2)(*-) derived from XO and from mitochondrial oxidative enzymes in arteries from DOCA-salt rats.

Effect of pioglitazone on nitroglycerin-induced impairment of nitric oxide bioavailability by a catheter-type nitric oxide sensor

BACKGROUND

We examined whether nitroglycerin (NTG)-induced impairment of nitric oxide (NO) bioavailability could be modified by a peroxisome proliferator-activated receptor (PPAR) gammaagonist.

METHODS AND RESULTS

Male New Zealand White rabbits were treated for 7 days with NTG patches, either alone or in combination with pioglitazone. Plasma NO concentration was measured with the catheter-type NO sensor located in the aorta. N(G)-methyl-L-arginine and acetylcholine (ACh) were infused into the aortic arch to measure the basal and ACh-induced plasma NO concentrations. Vascular nitrotyrosine and tetrahydrobiopterin (BH(4)) concentrations were measured by enzyme-linked immunosorbent assay and high-performance liquid chromatography with fluorescence detection, respectively. The negative effects of NTG, that is, the decrease in basal and ACh-induced NO production, were significantly suppressed by co-treatment with pioglitazone. NTG-induced increases in vascular nitrotyrosine and BH(4) concentrations were significantly decreased with co-treatment with pioglitazone.

CONCLUSIONS

NTG-induced impairment of basal and ACh-stimulated NO production might be prevented by the co-treatment with a PPAR gamma agonist, pioglitazone through suppressions of nitrosative stress.

Prevention of nitroglycerin tolerance in vitro by T0156, a selective phosphodiesterase type 5 inhibitor

The efficacy of nitroglycerin as a vasodilator is limited by tolerance, which develops shortly after treatment begins. The present study aims to examine whether T0156, a newly developed potent and selective inhibitor of phosphodiesterase type 5 (PDE5), could attenuate the tolerance to nitroglycerin on rat aortas. Rat aortic rings were suspended in organ bath for the measurement of changes in isometric tension and nitrate tolerance was acutely induced by preceding exposure for 90 min to 30 microM nitroglycerin. Concentration-response curves to nitroglycerin were obtained on aortic rings pre-contracted with phenylephrine. Pre-exposure of rings with or without endothelium to nitroglycerin reduced the relaxations to nitroglycerin. The tissue levels of cyclic GMP were measured by enzyme immunoassay kit. Treatment with T0156 inhibited and prevented the reduced relaxation and cyclic GMP levels in response to nitroglycerin in tolerant rings. In contrast, nitroglycerin-induced tolerance was unaffected by cilostazol (PDE3 inhibitor) and rolipram (PDE4 inhibitor). Finally, incubation of aortic rings with thromboxane prostanoid receptor antagonist, cyclooxygenase inhibitor, or endothelin ET(A) receptor antagonist did not inhibit the development of tolerance. The present results suggest that nitroglycerin tolerance may involve an increased activity of PDE5 but not PDE3 or PDE4 isoforms in vascular smooth muscle cells since T0156 prevents the development of tolerance. Thromboxane A(2), cyclooxygenase (COX)-dependent prostaglandins and endothelin 1 play little role in the acute induction of nitroglycerin tolerance.

Possible usefulness of apocynin, an NADPH oxidase inhibitor, for nitrate tolerance: prevention of NO donor-induced endothelial cell abnormalities

The long-term benefits of nitroglycerin therapy are limited by tolerance development. Understanding the precise nature of mechanisms underlying nitroglycerin-induced endothelial cell dysfunction may provide new strategies to prevent tolerance development. In this line, we tested interventions to prevent endothelial dysfunction in the setting of nitrate tolerance. When bovine aortic endothelial cells (BAECs) were continuously treated with nitric oxide (NO) donors, including nitroglycerin, over 2-3 days, basal production of nitrite and nitrate (NO(x)) was diminished. The diminished basal NO(x) levels were mitigated by intermittent treatment allowing an 8-h daily nitrate-free interval during the 2- to 3-day treatment period. Addition of the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase inhibitor apocynin restored the basal levels of NO(x) that were decreased by continuous nitroglycerin treatment of BAECs. Apocynin caused significant improvement of increased mRNA and protein levels of endothelial nitric oxide synthase (eNOS) in BAECs given nitroglycerin continuously over the treatment period. Apocynin also reduced endothelial production of reactive oxygen species (ROS) after continuous nitroglycerin treatment. These results showed an essential similarity to the effects of a nitrate-free interval. Application of the NOS inhibitor N(omega)-nitro- l-arginine methyl ester caused a recovery effect on basal NO(x) and eNOS expression but was without effect on ROS levels in continuously NO donor-treated BAECs. In conclusion, the present study characterized abnormal features and functions of endothelial cells following continuous NO donor application. We suggest that inhibition of NADPH oxidase, by preventing NO donor-induced endothelial dysfunction, may represent a potential therapeutic strategy that confers protection from nitrate tolerance development.

Targeting Nitric Oxide With Drug Therapy

Increasing knowledge of the role of nitric oxide (NO) in physiology and disease has stimulated efforts to target the NO pathway pharmacologically. These therapeutic strategies include NO donors that directly or indirectly release NO and agents that increase NO bioactivity. Traditional organic nitrates such as nitroglycerin, which indirectly release NO, were believed to have limited long-term efficacy and tolerability, chiefly because of nitrate tolerance. Recent studies, however, suggest more effective ways of using these agents and new applications for them. Nicorandil, a hybrid organic nitrate that also activates potassium channels, has demonstrated significant benefits in acute coronary syndromes. Other nitrates are being investigated for use in neurodegenerative diseases. Direct NO donors include NO gas, which is useful in respiratory disorders, and the more recent classes of diazeniumdiolates, sydnonimines, and S-nitrosothiols. Preliminary data suggest that these agents may be effective as antiatherosclerotic agents as well as in other disease states. In addition, hybrid agents that consist of an NO donor coupled with a parent anti-inflammatory drug, including nonsteroidal anti-inflammatory drugs, have demonstrated enhanced efficacy and tolerability compared with the anti-inflammatory parent drug alone in diverse experimental models. Established drugs that enhance NO bioactivity include antihypertensive agents, particularly angiotensin-converting enzyme inhibitors, calcium channel blockers, and newer vasodilating beta-blockers. In addition, 3-methylglutaryl coenzyme A reductase inhibitors (statins) promote NO bioactivity, both through and independent of lipid lowering. The NO-promoting actions of these established drugs provide some insight into their known benefits and suggest possible therapeutic potential.

Chronic nitroglycerine administration reduces endothelial nitric oxide production in rabbit mesenteric resistance artery

We investigated whether 10 days' in vivo treatment with nitroglycerine (NTG) would inhibit nitric oxide production by the endothelial cells of resistance arteries ex vivo and, if so, what the underlying mechanism might be. ACh increased the intracellular nitric oxide concentration ([NO]i; estimated using the nitric oxide-sensitive fluorescent dye diaminofluorescein-2) within the endothelial cells of rabbit mesenteric resistance arteries. This effect was significantly smaller in arteries isolated from NTG-treated rabbits than in those from control rabbits. The reduction in endothelial [NO]i in NTG-treated rabbits was prevented when olmesartan (blocker of type 1 angiotensin II receptors (AT1Rs)) was coadministered in vivo with NTG and also when the superoxide scavenger manganese (III) tetrakis-(4-benzoic acid) porphyrin (Mn-TBAP), the protein kinase C (PKC) inhibitor GF109203X or L-arginine (with or without the active form of folate (5-methyltetrahydrofolate)) was incubated with the arteries in vitro. Endothelial cell superoxide production (estimated by ethidium fluorescence) was greatly increased in arteries from NTG-treated rabbits. This was normalized by in vivo coadministration of olmesartan with NTG and also by in vitro application of Mn-TBAP or GF109203X (but not of 5-methyltetrahydrofolate+L-arginine). ACh increased the intracellular Ca2+ concentration (estimated using the Ca2+-sensitive dye Fura 2) within endothelial cells, the increase being not significantly different between NTG-treated rabbits and control rabbits. We conclude that in NTG-treated rabbits, endothelial nitric oxide production in mesenteric resistance arteries is reduced, possibly through a reduction in the bioavailability of L-arginine via an action mediated by superoxide. Activation of the AT1R-PKC pathway may be involved in increasing superoxide production.

Ramipril treatment protects against nitrate-induced oxidative stress in eNOS-/- mice: An implication of the NADPH oxidase pathway

The development of nitrate tolerance has been found to be associated with vascular production of superoxide anion (O2-*), generated mainly by the eNOS and NADPH oxidase pathways. The aim of our study was to investigate whether long-term angiotensin-converting enzyme inhibition by ramipril is able to protect against nitrate tolerance in the aortas of eNOS-deficient (eNOS-/-) mice and to assess the implication of the NADPH oxidase pathway. Therefore, 3 types of treatment were given to wild-type (WT) and eNOS-/- mice: group 1 received ramipril for 5 weeks and a co-treatment with ramirpil plus nitroglycerine (NTG) during the last 4 days, group 2 received only NTG, and group 3 served as control. Relaxations to NTG (0.1 nmol/L to 0.1 mmol/L) were determined on U44619, a thromboxane analogue, precontracted rings, and O2-* production were assessed on aorta homogenates with the lucigenin-enhanced chemiluminescence technique. Cyclic guanosine monophosphate and reverse-transcriptase-polymerase chain reaction analyses were performed on whole mouse aortas. In WT group 2, the concentration-effect curves to NTG were significantly shifted to the right: the pD2 was 6.16 +/- 0.17 (n = 6) vs 6.81 +/- 0.10 (n = 6) in WT group 3 (not exposed to NTG; P < 0.05) and O2-* production was enhanced from 100% +/- 11% (n = 9) to 191% +/- 21% (n = 6; P < 0.01). In contrast, in WT group 1, the rightward shift was abolished: the pD2 value was 6.73 +/- 0.13 (n = 6; NS vs group 3 WT) and O2-* production was 117% +/- 6% (n = 7; NS vs group 3 WT). In eNOS groups 1 and 3, similar data were observed: the pD2 values were 7.58 +/- 0.08 and 7.38 +/- 0.11 (NS) vs 6.89 +/- 0.20 in eNOS group 2 (n = 6; P < 0.01). In the WT mice aortas, ramipril treatment significantly increased the cyclic guanosine monophosphate levels (reflecting nitric oxide availability), which returned to control values after in vivo co-treatment with a bradykinin BK2 antagonist (Icatibant). In both strains, candesartan, an AT1 blocker, was also able to protect against the development of nitrate tolerance. Moreover, before NTG exposure, ramipril treatment decreased p22phox and gp91phox (essential NADPH oxidase subunits) mRNA expression in aortas from both mice strains. In conclusion, long-term ramipril treatment in mice protects against the development of nitrate tolerance by counteracting NTG-induced increase in O2 production, which involves a direct interaction with the NADPH oxidase pathway and seems to be completely independent of the eNOS pathway.

Effect of resveratrol on nitrate tolerance in isolated human internal mammary artery

The present study aims to examine whether resveratrol, a natural antioxidant present in red wine, restores the tolerance to nitroglycerin (GTN) on isolated human internal mammary artery (IMA), using an in vitro model of nitrate tolerance. IMA rings were obtained from 53 male patients undergoing coronary bypass operation. Nitrate tolerance was induced by incubating the artery ring with 100 microM GTN for 90 minutes. Concentration-response curves to GTN (10(-9) to 10(-4) M) were obtained on IMA rings precontracted with noradrenaline. A low concentration (5 microM) of lucigenin was used as a tool to measure superoxide production in IMA segments. GTN produced concentration-dependent relaxation in isolated human IMA rings. Preexposure of artery rings to GTN reduced the relaxations to GTN [E(max) values: 105 +/- 2% and 76 +/- 3%, n = 10 to 12, P < 0.05; EC(50) values (-log M): 6.72 +/- 0.05 and 4.95 +/- 0.06, P < 0.05, respectively]. Relaxation to sodium nitroprusside remained unchanged. Diminished relaxation to GTN is partially restored after removing endothelium or L(G)-nitro-L-arginine (L-NOARG, 10 M) or superoxide dismutase (20 and 200 U/mL) or catalase (200 U/mL) pretreatments. Pretreatments with resveratrol (1, 10, and 20 microM) for 20 minutes relatively improved the reduced relaxation to GTN in tolerant IMA rings. Coadministration of L-NOARG with resveratrol did not abolish the beneficial effect of resveratrol on nitrate tolerance. The inhibitory effect of resveratrol on GTN-induced tolerance was not abolished in arterial rings without endothelium. Exposure to GTN increased superoxide production in IMA segments with endothelium. Endothelium denudation, L-NOARG, or superoxide dismutase pretreatments markedly inhibited the increased superoxide production in tolerant arteries. Resveratrol (1 and 10 microM) almost completely abolished basal or NAD(P)H-stimulated superoxide production in tolerant and nontolerant arteries. Vascular tolerance to GTN, in in vitro tolerant human IMA rings, can be induced by endothelial superoxide anions. Resveratrol partially restored the reduced relaxation to GTN by inhibiting NAD(P)-derived superoxide production in endothelium.

Glyceryl trinitrate-induced angiotensin-converting enzyme (ACE) inhibition in healthy volunteers is dependent on ACE genotype

Evidence concerning the importance of angiotensin-converting enzyme (ACE) genotype in cardiovascular diseases is accumulating. The aim of this study was to investigate if nitric oxide (NO), generated from glyceryl trinitrate (GTN), affects human serum ACE activity in vivo, and if so, whether this effect was dependent on ACE genotype and (or) reflected in blood pressure reduction. A tablet containing 5 mg GTN was bucally administered for 5 minutes to 17 healthy volunteers. Blood pressure (BP) was recorded, and serum ACE activity, ACE genotype, and plasma cGMP was analyzed. GTN administration significantly reduced BP only in individuals with the deletion/deletion (DD) genotype. Sixty minutes after GTN administration, serum ACE activity was reduced in individuals with the insertion/insertion (II) and insertion/deletion (ID) genotypes, but not the DD genotype. Comparing the change in ACE activity over time between the genotypes resulted in the following: II vs. DD, p < 0.01; II vs. ID, p < 0.05; and ID vs. DD, p < 0.05. There was no significant difference in plasma cGMP content neither between the ACE genotypes nor before and after GTN administration. In conclusion, GTN inhibits serum ACE in vivo in individuals with the II and ID, but not the DD genotype.

Nitroglycerin reduces myocardial oxygen consumption during exercise despite vascular tolerance

The long-term benefits of nitroglycerin (NTG) therapy are limited by the development of vascular tolerance and endothelial dysfunction in conductance coronary arteries. We have determined whether nitrate tolerance extends to NTG effects on myocardial O2 consumption (MV̇o2) and the ability of endogenous nitric oxide (NO) to modulate MV̇o2 during exercise. In chronically instrumented dogs ( n = 8), hemodynamic and MV̇o2 responses to treadmill exercise were measured before, during tolerance (3 and 7 days of NTG delivery), and 7 days after NTG withdrawal. Acute NTG delivery caused a parallel downward shift of the MV̇o2-triple product (TP) relations and reversed the disproportionate increases in MV̇o2 caused by the blockade of NO formation. After 7 days of continuous transdermal NTG delivery, vascular tolerance was displayed as a >75% reduction of coronary blood flow (CBF) responses to NTG boluses. Despite vascular nitrate tolerance, MV̇o2-TP relations were shifted downward compared with pre-NTG exercise. Seven days after NTG withdrawal, vascular responses to boluses of NTG had recovered from tolerance, and MV̇o2-TP relations during exercise were back to pre-NTG level. At that time, blockade of NO formation failed to alter MV̇o2-TP relations. Thus NTG caused a sustained reduction of cardiac MV̇o2, independent of metabolic demand during exercise, despite tolerance of the coronary microcirculation. NTG-induced vascular tolerance and MV̇o2 reductions were reversible by NTG withdrawal, but endogenous NO-dependent modulation of O2 consumption was severely impaired.

Characteristics of attenuated endothelium-dependent relaxation seen in rabbit intrapulmonary vein following chronic nitroglycerine administration

1 This study was undertaken to determine whether long-term in vivo administration of nitroglycerine (NTG) downregulates the endothelium-dependent relaxation induced by acetylcholine (ACh) in the rabbit intrapulmonary vein and, if so, whether the type 1 angiotensin II receptor (AT(1)R) blocker valsartan normalizes this downregulated relaxation. 2 In strips treated with the cyclooxygenase inhibitor diclofenac, ACh induced a relaxation only when the endothelium was intact. A small part of this ACh-induced relaxation was inhibited by coapplication of two Ca(2+)-activated K(+)-channel blockers (charybdotoxin (CTX)+apamin) and the greater part of the response was inhibited by the nitric-oxide-synthase inhibitor N(omega)-nitro-L-arginine (L-NNA). 3 The endothelium-dependent relaxation induced by ACh, but not the endothelium-independent relaxation induced by the nitric oxide donor NOC-7, was significantly reduced in NTG-treated rabbits (versus those in NTG-nontreated control rabbits). The attenuated relaxation was normalized by coapplication of valsartan with the NTG. 4 In the vascular wall, both the amount of localized angiotensin II and the production of superoxide anion were increased by in vivo NTG treatment. These variables were normalized by coapplication of valsartan with the NTG. 5 It is suggested that long-term in vivo administration of NTG downregulates the ACh-induced endothelium-dependent relaxation, mainly through an inhibition of endothelial nitric oxide production in the rabbit intrapulmonary vein. A possible role for AT(1)R is proposed in the mechanism underlying this effect.

Angiotensin II-Receptor Antagonist Losartan Does not Prevent Nitroglycerin Tolerance in Patients with Coronary Artery Disease

The study evaluated the effect of Losartan in preventing nitrate tolerance during continuous transdermal nitroglycerin (TD-GTN) therapy in patients with coronary disease. Fifteen subjects with chronic stable ischemia evaluated by exercise test, were randomized to 28 days of TD-GTN 20 mg once a day without free interval plus Losartan 100 mg or Losartan-placebo with a double blind crossover design. Myocardial ischemic parameters during stress test were evaluated after each test period and results of Losartan therapy were compared to those with placebo. Time to onset 1 mm ST-depression was significantly higher after acute TD-GTN 20 mg with respect to placebo run-in, sustained TD-GTN 20 mg plus Losartan 100 mg or Losartan-placebo (p < 0.001). ST-depression at peak exercise and time to recovery of ST segment were markedly lower after acute TD-GTN 20 mg compared to placebo run-in (p < 0.05), sustained TD-GTN 20 mg plus Losartan 100 mg (p < 0.001) or Losartan-placebo (p < 0.05). At 1 mm-ST depression and at peak exercise, systolic blood pressure and rate-pressure product significantly decreased after sustained TD-GTN 20 mg plus Losartan 100 mg (p < 0.001, p < 0.05 respectively) with respect to placebo run-in, acute and sustained TD-GTN 20 mg plus Losartan-placebo. Moreover at peak exercise, these data were also observed after acute TD-GTN 20 mg compared to placebo run-in and sustained TD-GTN 20 mg plus Losartan-placebo (p < 0.001). The AT(1) antagonist Losartan administration does not prevent the development of nitrate tolerance during continuous TD-GTN therapy.

Endothelin ETA receptor antagonist did not affect development of tolerance to glyceryl trinitrate in rat

Glyceryl trinitrate (GTN), extensively used for the treatment of cardiovascular disorders, is associated with rapid development of tolerance. The exact mechanism responsible for tolerance development to GTN is still not known. Recently, it has been demonstrated that GTN tolerance is associated with increased expression of endothelin (ET). This study was carried out to determine the effect of ET(A) receptor antagonist, BMS182874, on the development of tolerance to GTN in urethane-anaesthetized rats. Diastolic blood pressure (DBP), systolic blood pressure (SBP) and heart rate (HR) were continuously recorded in vehicle- and BMS182874 (3 mg kg(-1), i.v.)-treated rats. GTN was infused at the rate of 10 microg min(-1), intravenously for 4 h. Tolerance to GTN was determined using challenge doses of GTN (10, 30 and 90 microg, i.v.). GTN produced a fall in DBP, SBP and an increase in HR. In vehicle-treated rats, the fall in SBP before induction of GTN tolerance was 28 +/- 2, 43 +/- 4 and 52 +/- 4 mmHg with 10, 30 and 90 microg GTN, respectively. However, following GTN infusion (10 microg min(-1), i.v. for 4 h) a rapid development of tolerance was observed and the fall in SBP was 1 +/- 1, 9 +/- 4 and 15 +/- 4 mmHg with 10, 30 and 90 microg GTN, respectively. Similarly, in BMS182874-treated rats the fall in SBP in non-tolerant rats was 28 +/- 4, 42 +/- 4 and 48 +/- 5 mmHg with 10, 30 and 90 microg GTN, respectively. In BMS182874-treated rats following GTN infusion (10 microg min(-1), i.v. for 4 h) a rapid development of tolerance was observed and the fall in SBP was 4 +/- 3, 10 +/- 2 and 13 +/- 4 mmHg with 10, 30 and 90 microg GTN, respectively. The decrease in DBP and SBP in vehicle- and BMS182874-treated GTN-tolerant rats was statistically similar. These results suggest that ET(A) receptor antagonist BMS182874 did not affect development of tolerance to GTN in rats.

Nitrate tolerance and the links with endothelial dysfunction and oxidative stress

Identification of nitric oxide as the molecule responsible for endothelial dependant vasodilatation has led to an explosion of interest in endothelial function. Oxidative stress has been identified as an important factor in the development of tolerance to organic nitrates. This review examines the evidence supporting this recently developed theory and how mechanisms of nitrate tolerance may link with the wider picture of primary nitric oxide resistance.

BIOCHEMICAL MECHANISM OF NITROGLYCERIN ACTION AND TOLERANCE: Is This Old Mystery Solved?

Organic nitrates such as nitroglycerin (NTG) have been used as potent vasodilators in medicine for more than a century, but their biochemical mechanisms of action, particularly in relation to tolerance development, are still incompletely defined. Numerous candidate enzymes for NTG metabolism, as well as a multiplicity of tolerance mechanisms, have been proposed in the literature, but a consolidating hypothesis that links these phenomena together has not appeared. Here, we outline a "thionitrate oxidation hypothesis," which attempts to link nitrate bioactivation and tolerance development in an overall mechanism. We also attempt to compare and contrast the proposed mechanism against existing theories of nitrate action and tolerance. Interactions between organic nitrates, which have been thought of as endothelium-independent agents, and the vascular endothelium and endothelial nitric oxide synthase (eNOS) are also discussed.

Role of PKC in the attenuation of the cGMP-mediated relaxation of skinned resistance artery smooth muscle seen in glyceryl-trinitrate-tolerant rabbit

We examined whether 10 days' in vivo treatment with glyceryl trinitrate (GTN) might reduce cGMP-induced relaxation in the smooth muscle of rabbit mesenteric resistance arteries and, if so, whether protein kinase C (PKC) plays a role in this downregulation. The relaxation responses to GTN and the nitric oxide donor NOC-7 were significantly reduced in endothelium-denuded strips from GTN-treated rabbits. In beta-escin-skinned smooth muscle, the ability of 8-bromoguanosine 3',5' cyclic monophosphate (8-Br-cGMP, a phosphodiesterase-resistant cGMP analogue) to relax the contraction induced by 0.3 microM Ca2+ was significantly reduced in GTN-treated rabbits. In beta-escin-skinned smooth muscle, an inhibitor of conventional and/or novel PKCs, GF109203X (0.6 microM), inhibited the Ca2+ -induced contraction and enhanced the 8-Br-cGMP-induced relaxation. However, since the relaxing ability of 8-Br-cGMP was found to be unchanged by GF109203X when contractions were amplitude-matched (0.2 microM Ca2+ alone vs 0.3 microm Ca2+ + GF109203X), the increase in the 8-Br-cGMP-response seen with GF109203X was probably due to its inhibitory action on the Ca2+ -induced contraction. Furthermore, although the PKC activator phorbol 12,13-dibutyrate (PDBu, 0.1 microM) decreased the 8-Br-cGMP-induced relaxation of the Ca2+ (0.3 microM) contraction, this was probably due to its enhancement of the Ca2+ -induced contraction since no such effect of PDBu was seen when the Ca2+ -induced contractions were amplitude-matched (0.2 microM Ca2+ + PDBu vs 0.3 microM Ca2+ alone). These results suggest that the relaxing response to cGMP is reduced in the smooth muscle of mesenteric resistance arteries in GTN-treated rabbits but that conventional and/or novel PKCs do not play a major role in maintaining this downregulation. British Journal of Pharmacology (2004) 141, 391-398. doi:10.1038/sj.bjp.0705625

Role of reactive oxygen species in vascular remodeling associated with pulmonary hypertension

Several manifestations of neonatal pulmonary hypertension are associated with vascular remodeling, resulting in increased muscularity of the small pulmonary arteries. Abnormal structural development of the pulmonary vasculature has been implicated in persistent pulmonary hypertension of the newborn (PPHN). Increased plasma levels of the vasoconstrictor endothelin-1 (ET-1) have been demonstrated in patients with PPHN, which is likely to contribute to hypertension. In addition, several studies have identified a role for ET-1 in the proliferation of vascular smooth muscle cells (SMCs), suggesting that ET-1 may also be involved in the vascular remodeling characteristic of this disease. However, the mechanisms of ET-1-induced SMC proliferation are unclear and appear to differ between cells from different origins within the vasculature. In SMCs isolated from fetal pulmonary arterial cells, ET-1 stimulated proliferation via an induction of reactive species (ROS). Furthermore, other lines of evidence have demonstrated the involvement of ROS in ET-1-stimulated SMC growth, suggesting that ROS may be a common factor in the mechanisms involved. This review discusses the potential roles for ROS in the abnormal pulmonary vascular development characteristic of PPHN, and the treatment strategies arising from our increasing knowledge of the molecular mechanisms involved.

Mechanisms of nitrate tolerance: potential roles of folate

More than 100 years since their introduction in cardiovascular therapy, nitrates continue to be widely used in ischaemic heart disease despite incomplete knowledge of their intimate mechanism of action. Particularly, the development of a progressive attenuation of their efficacy over prolonged use (tolerance) continues to be the subject of current investigation. Newer findings point to the role of increased intracellular oxidative stress as a mechanism for tolerance and to folic acid derivatives as pharmacologic means to attenuate its development. This paper reviews nitrate mechanism of action, the history of nitrate tolerance and newer findings related to the use of folate to prevent this phenomenon.

Management of Acute Decompensated Heart Failure in the Emergency Department

Heart failure, a disease of epidemic proportions, has a tremendous clinical and financial impact on the US health care system. With more than 5 million Americans diagnosed with heart failure and 5-year mortality approaching 50%, it is the most common cause of hospitalization in patients older than 65 years and is the single most expensive diagnosis in the US health care system. Because the average US hospital loses more than 1000 dollars per heart failure admission, effective therapies that decrease length of stay, reduce hospital costs, and prevent 30-day readmissions are needed. This article reviews the relevant pathophysiology of heart failure, discusses the newest diagnostic strategies for emergency department diagnoses, evaluates recent advances and effects of early aggressive therapies, and presents a suggested algorithm for the treatment of acutely decompensated heart failure in emergency departments.

The vascular NAD(P)H oxidases as therapeutic targets in cardiovascular diseases

Activation of vascular NAD(P)H oxidases and the production of reactive oxygen species (ROS) by these enzyme systems are common in cardiovascular disease. In the past several years, a new family of NAD(P)H oxidase subunits, known as the non-phagocytic NAD(P)H oxidase (NOX) proteins, have been discovered and shown to play a role in vascular tissues. Recent studies make clearer the mechanisms of activation of the endothelial and vascular smooth muscle NAD(P)H oxidases. ROS produced following angiotensin II-mediated stimulation of NAD(P)H oxidases signal through pathways such as mitogen-activated protein kinases, tyrosine kinases and transcription factors, and lead to events such as inflammation, hypertrophy, remodeling and angiogenesis. Studies in mice that are deficient in p47(phox) and gp91(phox) (also known as NOX2) NAD(P)H oxidase subunits show that ROS produced by these oxidases contribute to cardiovascular diseases including atherosclerosis and hypertension. Recently, efforts have been devoted to developing inhibitors of NAD(P)H oxidases that will provide useful experimental tools and might have therapeutic potential in the treatment of human diseases.

Preserved endothelial function after long-term eccentric isosorbide mononitrate despite moderate nitrate tolerance

OBJECTIVES

We sought to investigate the effects of orally administered, long-term, eccentric isosorbide mononitrate (ISMN) on endothelial function.

BACKGROUND

Previous studies have shown that nitrate tolerance induced by continuous transdermal glyceryl trinitrate (GTN) is associated with increased vascular superoxide production and endothelial dysfunction. In contrast, it is unclear whether vascular superoxide increases during eccentric administration of oral nitrates, which is a widely used therapeutic dosing regimen.

METHODS

New Zealand White rabbits were randomly classified into three groups (n = 10, each) that received either placebo, ISMN at 2 mg/kg body weight per day (ISMN-2), or ISMN at 200 mg/kg body weight per day (ISMN-200) in an eccentric, twice-daily scheme for four months. Animals were sacrificed 3 h after application of the last ISMN dose.

RESULTS

The continuously present, lowest ISMN plasma levels (ng/ml) were 4.8 +/- 0.2 in ISMN-2 and 14.5 +/- 4 in ISMN-200 (p = 0.026). Treatment with ISMN had no effect on aortic reactivity to phenylephrine, acetylcholine, or the nitric oxide (NO) donor S-nitroso-N-acetyl-D,L-penicillamine, while the half-maximal effective concentration of ISMN (EC(50)-value in -logM) was shifted from 5.23 +/- 0.03 (placebo) to 4.69 +/- 0.04 (ISMN-200) (p < 0.0001 by analysis of variance). This moderate in vivo nitrate tolerance was not associated with increased aortic superoxide production (5 micromol/l lucigenin). The cumulative (20-min) lucigenin signals (cpm/mg) were 211 +/- 34 (ISMN-200) and 230 +/- 22 (placebo) (p = 0.415).

CONCLUSIONS

Long-term treatment with high-dose, eccentric ISMN does not increase vascular superoxide production and/or impair endothelium-dependent vasorelaxation, despite the development of moderate nitrate tolerance. Thus, it is unlikely that long-term anti-ischemic treatment with ISMN aggravates endothelial dysfunction in coronary artery disease.

Interactions of angiotensin II with NAD(P)H oxidase, oxidant stress and cardiovascular disease

An elevation in angiotensin II (Ang II) levels is a common occurrence in a diverse number of cardiovascular diseases including hypertension, hypercholesterolaemia, atherosclerotic coronary artery disease, left ventricular hypertrophy (LVH), heart failure and diabetes. An important effect of Ang II is activation of the NAD(P)H oxidase, a major source of reactive oxygen species (ROS) production by vascular cells. This increase in cellular ROS contributes to the pathogenesis of vascular disease by altering endothelial cell function, enhancing smooth muscle cell growth and proliferation, stimulating inflammatory proteins, including macrophage chemoattractant agents, growth factors and cytokines, and modulating matrix remodelling. Studies of genetically-altered mice have unequivocally shown that activation of the NAD(P)H oxidase by Ang II contributes to hypertension, LVH and atherosclerosis. Furthermore, increasing evidence suggest that the NAD(P)H oxidase contributes to human disease, suggesting that it is a potential target for future therapeutic intervention.