Tolerance to nitrates and simultaneous upregulation of platelet activity prevented by enhancing antioxidant state

Metabolomics-Driven Elucidation of Cellular Nitrate Tolerance Reveals Ascorbic Acid Prevents Nitroglycerin-Induced Inactivation of Xanthine Oxidase

Glyceryl trinitrate (GTN) has found widespread use for the treatment of angina pectoris, a pathological condition manifested by chest pain resulting from insufficient blood supply to the heart. Metabolic conversion of GTN, a nitric oxide (NO) pro-drug, into NO induces vasodilation and improves blood flow. Patients develop tolerance to GTN after several weeks of continuous use, limiting the potential for long-term therapy. The mechanistic cause of nitrate tolerance is relatively unknown. We developed a cell culture model of nitrate tolerance that utilizes stable isotopes to measure metabolism of ¹⁵N₃-GTN into ¹⁵N-nitrite. We performed global metabolomics to identify the mechanism of GTN-induced nitrate tolerance and to elucidate the protective role of vitamin C (ascorbic acid). Metabolomics analyses revealed that GTN impaired purine metabolism and depleted intracellular ATP and GTP. GTN inactivated xanthine oxidase (XO), an enzyme that is critical for the metabolic bioactivation of GTN into NO. Ascorbic acid prevented inactivation of XO, resulting in increased NO production from GTN. Our studies suggest that ascorbic acid has the ability to prevent nitrate tolerance by protecting XO, but not aldehyde dehydrogenase (another GTN bioactivating enzyme), from GTN-induced inactivation. Our findings provide a mechanistic explanation for the previously observed beneficial effects of ascorbic acid in nitrate therapy.

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.

The enigma of nitroglycerin bioactivation and nitrate tolerance: news, views and troubles

Nitroglycerin (glyceryl trinitrate; GTN) is the most prominent representative of the organic nitrates or nitrovasodilators, a class of compounds that have been used clinically since the late nineteenth century for the treatment of coronary artery disease (angina pectoris), congestive heart failure and myocardial infarction. Medline lists more than 15 000 publications on GTN and other organic nitrates, but the mode of action of these drugs is still largely a mystery. In the first part of this article, we give an overview on the molecular mechanisms of GTN biotransformation resulting in vascular cyclic GMP accumulation and vasodilation with focus on the role of mitochondrial aldehyde dehydrogenase (ALDH2) and the link between the ALDH2 reaction and activation of vascular soluble guanylate cyclase (sGC). In particular, we address the identity of the bioactive species that activates sGC and the potential involvement of nitrite as an intermediate, describe our recent findings suggesting that ALDH2 catalyses direct 3-electron reduction of GTN to NO and discuss possible reaction mechanisms. In the second part, we discuss contingent processes leading to markedly reduced sensitivity of blood vessels to GTN, referred to as vascular nitrate tolerance. Again, we focus on ALDH2 and describe the current controversy on the role of ALDH2 inactivation in tolerance development. Finally, we emphasize some of the most intriguing, in our opinion, unresolved puzzles of GTN pharmacology that urgently need to be addressed in future studies.

Bioactivation of nitroglycerin by ascorbate

Bioactivation of nitroglycerin (GTN) into an activator of soluble guanylate cyclase (sGC) is essential for the vasorelaxant effect of the drug. Besides several enzymes that catalyze GTN bioactivation, the reaction with cysteine is the sole nonenzymatic mechanism known so far. Here we show that a reaction with ascorbate results in GTN bioactivation. In the absence of ascorbate, GTN did not affect the activity of purified sGC. However, the enzyme was activated to approximately 20% of maximal NO-stimulated activity by GTN in the presence of 10 mM ascorbate with an EC(50) value of 27.3 +/- 4.9 microM GTN. The EC(50) value of ascorbate was 0.11 +/- 0.011 mM. Activation of sGC was sensitive to oxyhemoglobin, superoxide, and a heme-site enzyme inhibitor. GTN had no effect when ascorbate was replaced by 1000 U of superoxide dismutase per milliliter. Ascorbate is known to reduce inorganic nitrite to NO. In the presence of 10 mM ascorbate, approximately 30 microM nitrite caused the same increase in sGC activity as 0.3 mM GTN. Determination of ascorbate-driven 1,2- and 1,3-glycerol dinitrate formation indicated that a 140 nM concentration of products was generated from 0.3 mM GTN within 10 min, excluding nitrite as a relevant intermediate. Our results suggest that a reaction between GTN and ascorbate or an ascorbate-derived species yields an activator of sGC with NO-like chemical properties. This reaction may contribute to GTN bioactivation in blood vessels under conditions of GTN tolerance and ascorbate supplementation.

The oxidative stress concept of nitrate tolerance and the antioxidant properties of hydralazine

The hemodynamic and anti-ischemic effects of nitroglycerin (NTG) are rapidly blunted as a result of the development of nitrate tolerance. With initiation of NTG therapy, it is possible to detect neurohormonal activation and intravascular volume expansion. These so-called pseudotolerance mechanisms may compromise the vasodilatory effects of NTG. Long-term nitrate treatment also is associated with decreased vascular responsiveness caused by changes in intrinsic mechanisms of the tolerant vasculature itself. According to the oxidative stress concept, increased vascular superoxide (O2-) production and an increased sensitivity to vasoconstrictors secondary to activation of protein kinase C contribute to the development of tolerance. Nicotinamide adenine dinucleotide phosphate oxidase and the uncoupled endothelial nitric oxide synthase may be O2- -producing enzymes. Nitric oxide (NO) and O2-, both derived from NTG and the vessel wall, form peroxynitrite in a diffusion-limited rapid reaction. Peroxynitrite, O2-, or both may be responsible for the development of nitrate tolerance and cross-tolerance to direct NO donors (eg, sodium nitroprusside, sydnonimines) and endothelium-dependent NO synthase-activating vasodilators. Hydralazine is an efficient reactive oxygen species (ROS) scavenger and an inhibitor of O2- generation. When given concomitantly with NTG, hydralazine prevents the development of nitrate tolerance and normalizes endogenous rates of vascular O2- production. Recent experimental work has defined new tolerance mechanisms, including inhibition of the enzyme that bioactivates NTG (ie, mitochondrial aldehyde dehydrogenase isoform 2 [ALDH2]) and mitochondria as potential sources of ROS. NTG-induced ROS inhibit the bioactivation of NTG by ALDH2. Both mechanisms increase oxidative stress and impair NTG bioactivation, and now converge at the level of ALDH2 to support a new theory for NTG tolerance and NTG-induced endothelial dysfunction. The consequences of these processes for NTG downstream targets (eg, soluble guanylyl cyclase, cyclic guanosine monophosphate-dependent protein kinase), toxic effects contributing to endothelial dysfunction (eg, prostacyclin synthase inhibition) and novel applications of the antioxidant properties of hydralazine are discussed.

Endothelial modulation and tolerance development in the vasorelaxant responses to nitrate of rabbit aorta

We investigated the endothelial modulations in nitrate tolerance in isolated rabbit aorta. Nitrate tolerance was induced by a 72-h treatment with transdermal nitroglycerin (NTG, 0.4 mg/h) in conscious rabbits, which was verified by a 20-fold increase in the EC50 values [NTG tolerance (6.1 +/- 0.8) x 10(-7) M vs control (3.0 +/- 0.6) x 10(-8) M]. The relaxations to NTG in tolerant and nontolerant aortic strips were enhanced when their endothelia were denuded [E(-)]. In the presence of endothelium [E(+)], NTG-tolerant vessels were not tolerant to acetylcholine (ACh), which can release endothelial nitric oxide (NO), exogenous NO or 8-bromo (Br)-cGMP. In NTG-tolerant and nontolerant vessels with endothelium, concentration-response curves for NO were the same as those in endothelium-absent tolerant vessels. In both NTG-tolerant and nontolerant vessels, treatment with superoxide dismutase (SOD, 20 units/ml), an O2-. scavenger, unaffected the responses to NTG reduced in the presence of endothelium, but treatment with NG-nitro-L-arginine methyl ester (L-NAME, 10(-4) M), an NO synthase (NOS) inhibitor, reversed these reductions. Thus, our data did not indicate that an increased endothelial superoxide O2-. production contributes to nitrate tolerance. Our study suggested that (i) an impaired biotransformation process from NTG to NO is responsible for the occurrence of nitrate tolerance and (ii) vascular response to NTG enhanced by endothelial removal is related to blocked endothelial NO release.

Association between vascular tolerance and platelet upregulation: comparison of nonintermittent administration of pentaerithrityltetranitrate and glyceryltrinitrate

Enhanced formation of oxygen-derived radicals O plays a dominant role in the development of nitrate tolerance. In 18 healthy subjects, this study tested the effect of additional vitamin C (Vit-C) administration (1 g three times daily) on glyceryltrinitrate (GTN)-induced hemodynamic changes during 3 days of nonintermittent transdermal administration of GTN (0.4 mg/h) in comparison with administration of pentaerithrityltetranitrate (PETN, 40 mg three times daily, orally). GTN caused an immediate significant rise in arterial conductivity (a/b ratio of dicrotic pulse pressure, from 2.33 +/- 0.06 to 2.52 +/- 0.06). Within 2 days of GTN administration, the a/b ratio progressively decreased and reached pre-GTN control levels, documenting tolerance. However, the administration of GTN along with Vit-C or with PETN alone induced changes in the a/b ratio and in the orthostatic reaction, which were fully maintained for the period of treatment. This vascular tolerance seen after GTN treatment was paralleled by an upregulation of ex vivo platelet activity, which was evident from a rise in aggregation from 29.2 +/- 2.8% at control day to 85.4 +/- 8.5% at day 3, and additionally from thrombin-induced increases of intracellular Ca concentration from 494 +/- 60 nM at control day to 741 +/- 37 nM at day 3. This upregulation was not observed during PETN or GTN; with additional Vit-C administration. Administration of PETN or GTN, the latter supplemented by Vit-C, induced neither vascular tolerance nor the upregulation of washed platelet activity during nonintermittent administration, in contrast to GTN without Vit-C. This is explained by a diminished formation of reactive oxygen species when PETN or when GTN along with Vit-C is used.

Production of 8-epi prostaglandin F(2alpha) in human platelets during administration of organic nitrates

OBJECTIVES

The objective of this study was, using isolated platelets as a surrogate for vascular cells, to examine the effect of nonintermittent organic nitrate administration on 8-epi prostaglandin F(2alpha) (8-epi PGF(2alpha)) content and the effect of concurrent oral ascorbate administration.

BACKGROUND

The long-term efficacy of organic nitrates is hampered by hemodynamic tolerance, which develops during continuous administration. This has been associated with altered production of superoxide and nitric oxide, as well as oxidative stress. This effect may be ameliorated by the co-administration of antioxidants.

METHODS

Ten healthy male subjects received nitroglycerin (NTG) transdermally at a dosage of 0.4 mg/h for 3 days with ascorbate or lactose (1.2 g/day). After two weeks washout, the treatment was repeated with reversed ascorbate/lactose. Platelets were prepared by centrifugation and esterified 8-epi PGF(2alpha) measured at the start and finish of each treatment by immunoassay.

RESULTS

Nitroglycerin, in the absence of supplemental ascorbate, was associated with a significant increase in platelet-esterified 8-epi PGF(2alpha), from 32.9 (95% confidence interval [CI] 11.8 to 54.0) to 51.0 (95% CI 16.3 to 85.7) pg/mg protein (p < 0.05). Co-administration of ascorbate with NTG resulted in a significant decrease in 8-epi PGF(2alpha) production, from 38.8 (95% CI 24.9 to 52.7) to 19.0 (95% CI 13.5 to 24.5) pg/mg protein (p < 0.05).

CONCLUSIONS

Continuous NTG administration results in an increase in platelet-esterified 8-epi PGF(2alpha), a free radical and cyclooxygenase-dependent compound. This is reversed by co-administration of the free radical scavenger ascorbate. Whether this increase is merely a marker for increased oxidative stress or a mediator of oxidative injury contributing to the hemodynamic changes observed in nonintermittent organic nitrate treatment has yet to be resolved.

Platelet nitric oxide and superoxide release during the development of nitrate tolerance: effect of supplemental ascorbate

BACKGROUND

The therapeutic benefits that accompany the continuous administration of organic nitrates are attenuated by the development of tolerance to the compounds. Altered superoxide production and NO bioavailability have been implicated in contributing to the development of tolerance, an effect that may be ameliorated by the administration of antioxidants.

METHODS AND RESULTS

We studied the effect of 3 days of continuous transdermal administration of nitroglycerin (NTG) (10 mg/24 hours) on platelet free radical (NO and superoxide anion [O2*-] activity) with and without coadministration of supplemental ascorbate (2.4 g/24 hours). NAD(P)H oxidase activity, nitric oxide synthase (NOS) activity, and cyclic guanosine monophosphate (cGMP) content were also assessed. Radial artery pressure pulse waveforms were used to track the hemodynamic actions of NTG. Three days of NTG/placebo was associated with a significant increase in platelet NO and O2*- production from 1.0+/-1.17 to 2.52+/-0.88 pmol/10(8) platelets and 13.2+/-4.8 to 72.5+/-34.4 pmol/10(8) platelets, respectively (P<0.01 for both). These changes were accompanied by increased platelet NADH oxidase activity from 47.9+/-11.0 to 65.3+/-13.6 pmol O2*- min/mg protein and cGMP content from 0.60+/-0.10 to 0.89+/-0.16 pmol/10(9) platelets (P<0.05 for both). Administration of NTG/ascorbate attenuated both NO and O2*- release in platelets.

CONCLUSIONS

Three days of continuous transdermal administration of NTG was accompanied by increased platelet NO and O2*- production and NADH oxidase activity that was suppressed by coadministration of oral ascorbate. Although a significant degree of tolerance would be expected during continuous nitrate administration, a residual hemodynamic action could be identified by arterial pulse contour analysis.

Potentiation of sildenafil-induced hypotension is minimal with nitrates generating a radical intermediate

Recently the new specific phosphodiesterase-5 inhibitor sildenafil was introduced into therapy for erectile dysfunction. Because of the phosphodiesterase-5 inhibitor-induced increases of cyclic GMP in the vasculature, vasodilation in various vascular beds is induced, which in combination with various nitrovasodilators (e.g., when used simultaneously for the treatment of coronary artery disease), may lead to excessive hypotension. Thus nitrovasodilators are contraindicated when sildenafil may be used and reports of a number of accidents have recently been published. We therefore studied the acute interactions of glyceryl trinitrate (GTN), pentaerythritol tetranitrate (PETN), and isosorbide dinitrate (ISDN) with sildenafil in six chronically instrumented conscious dogs for each nitrate to assess the magnitude of blood pressure drops (and compensatory increases in heart rate) during a 24-h nitrate administration (infusion into the pulmonary artery). Sildenafil (3 mg/kg) was given orally (after a 24-h fast) 30 min after start of nitrate infusion. GTN, PETN, or ISDN (which follow different steps of metabolic conversion to nitric oxide) were applied at submaximal dosages leading to 90% of maximal coronary artery dilation at 1.5 microg/kg per min, 0.75 microg/kg per min, or 6 microg/kg per min, respectively. During GTN infusion sildenafil caused a maximum drop in mean blood pressure of 21 +/- 3 mm Hg (rise in heart rate from 117.0 +/- 7.2 to 126.0 +/- 6 .0/min) and during ISDN infusion of 18 +/- 3 mm Hg (rise in heart rate from 115.0 +/- 7.0 to 125 +/- 6/min), which was significantly less (p < 0.01) during PETN (only 6 +/- 1 mm Hg with a rise in heart rate from 107.0 +/- 5.0 to 122.0 +/- 7.0/min). When sildenafil is used during exposure to nitrates (e.g., in coronary artery disease), the PETN-induced drop in blood pressure at equi-effective dosages (with regard to coronary dilation) is substantially smaller compared with that of GTN or ISDN, which is probably because of lesser potentiation of phosphodiesterase-5 inhibitor-induced effects in the arteriolar bed, thus minimizing critical drops in blood pressure.

Ascorbic acid-induced modulation of venous tone in humans

Ascorbic acid appears to have vasodilatory properties, but the underlying mechanisms are not well understood. The aims of this study were to define the acute effects of locally infused ascorbic acid in human veins and to explore underlying mechanisms by using pharmacological tools in vivo. Ascorbic acid was infused in dorsal hand veins submaximally preconstricted with the alpha(1)-adrenoceptor agonist phenylephrine or with prostaglandin F(2alpha) in 23 healthy male nonsmokers, and the venodilator response was measured. Ascorbic acid produced dose-dependent dilation with maximum reversal of constriction of 38+/-4% in phenylephrine-preconstricted veins and of 51+/-13% in prostaglandin F(2alpha)-preconstricted veins. Oral pretreatment with the cyclooxygenase inhibitor acetylsalicylic acid or local coinfusion of ascorbic acid and the nitric oxide synthase inhibitor N:(G)-monomethyl-L-arginine had no effect, but coinfusion of ascorbic acid and methylene blue (to inhibit cGMP generation) abolished venodilation. Coinfusion of ascorbic acid and the nonselective potassium channel blocker quinidine abolished venodilation, whereas the inhibitor of ATP-dependent potassium channels glibenclamide had no effect. In cultured bovine endothelial cells, ascorbic acid did not affect intracellular calcium concentration but blunted the response to ATP or digitonin exposure. Ascorbic acid, in millimolar concentrations, dilates human hand veins, presumably by activation of vascular smooth muscle potassium channels through cGMP. This activation is independent of eNOS-mediated nitric oxide synthesis and cyclooxygenase products and does not involve ATP-dependent potassium channels.

Nitric oxide donors

Nitric oxide (NO) donors are pharmacologically active substances that release NO in vivo or in vitro. NO has a variety of functions such as the release of prostanoids, inhibition of platelet aggregation, effect on angiogenesis, and production of oxygen free radicals. This report discusses the chemical and pharmacological characteristics of NO donors, their effect on platelet function and cyclooxygenase, their cardiac action including myocardial infarction, and release of superoxide anions. This review stresses NO tolerance and the effect of NO donors on angiogenesis in myocardial infarction and in solid tumors.

Inhibition of nitric oxide synthesis increases erythrocyte membrane fluidity and unsaturated fatty acid content

Changes in the lipid composition of the membrane affect its fluidity and function. These variables are altered in various forms of hypertension. Our hypothesis was that the rapid increase in blood pressure (BP) caused by inhibition of nitric oxide production would lead to alterations in membrane fluidity similar to those observed in genetic hypertension. We used Nomega-nitro L-arginine methyl ester (L-NAME) and vehicle-treated (3 weeks) Wistar-Kyoto rats to study the effects of nitric oxide synthase (NOS) inhibition on membrane fluidity and lipid composition. Erythrocyte membrane fluidity was measured by fluorescence anisotropy. Membrane lipids were separated using Sep-Pak and thin-layer chromatography. Fatty acid methyl esters were produced and analyzed by gas chromatography-mass spectrometry. Nomega-nitro L-arginine methyl ester treatment increased BP and erythrocyte membrane fluidity. The phospholipid and unsaturated fatty acid levels in the membranes from the L-NAME-treated rats were consistent with the increase in fluidity (ie, more unsaturated fatty acid, in particular, arachidonic and docosahexaenoic acid) and a reduction in membrane sphingomyelin content. Fatty acid analysis of individual lipid groups suggested the changes in membrane fatty acid composition may be asymmetric, with the majority of the changes occurring in the outer leaflet. Inhibition of NOS results in changes in membrane composition that may explain the concurrent changes in fluidity. The increased membrane fluidity observed here contrasts with the reduced fluidity observed in genetic hypertension or unchanged fluidity in secondary hypertension. The effects could be related to NOS inhibition or may be a direct effect of L-NAME.

How urine analysis reflects oxidative stress--nitrotyrosine as a potential marker

Enhanced oxidant stress involved in the pathogenesis of cardiovascular (heart failure, atherosclerosis, ischemia-reperfusion injury), neurodegenerative (M. Alzheimer), metabolic (hypercholesterolemia, diabetes) and inflammatory disorders is mimicked by non-intermittent therapy with nitrovasodilators. We used this latter therapy model to study urinary 3-nitrotyrosine (n-tyr) excretion as a potential biomarker that may reflect the enhanced generation of reactive oxygen species. Namely, free or protein-bound n-tyr is formed in the organism by nitration of tyrosine (residues) via peroxynitrite (reaction product of NO&z.ccirf; and O(2)(-)&z. ccirf;). Free n-tyr content was analyzed by gas chromatography in urine obtained from healthy human subjects under a nitrite-limited diet during a two-day non-intermittent transdermal administration of glyceroltrinitrate (GTN; 0.4 mg/h) with or without vitamin C (Vit-C; 55 microg/kg/min) as antioxidant. Concomitant with the development of complete vascular tolerance (loss of dilator action), a progressive increase in urinary n-tyr excretion (up to 186+/-9 microg/day) was demonstrated in volunteers given GTN only. In contrast, when Vit-C was added, the GTN-induced increases in urinary n-tyr content were significantly suppressed (up to 130.20+/-6.91 microg/day), whereas Vit-C alone even decreased urinary n-tyr content (down to 34.00+/-5.66 microg/day), which was below control values (56.0+/-3.4 microg/day). Thus, urinary n-tyr may serve as a biomarker to detect changes in oxidant stress and thereby to evaluate the efficacy of therapeutic interventions aimed at reducing oxidant stress under various pathophysiological conditions.

A new approach for extracellular spin trapping of nitroglycerin-induced superoxide radicals both in vitro and in vivo

Anti-ischemic therapy with nitrates is complicated by the induction of tolerance that potentially results from an unwanted coproduction of superoxide radicals. Therefore, we analyzed the localization of in vitro and in vivo, glyceryl trinitrate (GTN)-induced formation of superoxide radicals and the effect of the antioxidant vitamin C and of superoxide dismutase (SOD). Sterically hindered hydroxylamines 1-hydroxy-3-carboxy-2,2,5,5-tetramethylpyrrolidine (CP-H) and 1-hydroxy-4-phosphonooxy-2,2,6,6-tetramethylpiperidin (PP-H) can be used for in vitro and in vivo quantification of superoxide radical formation. The penetration/incorporation of CP-H or PP-H and of their corresponding nitroxyl radicals was examined by fractionation of the blood and blood cells during a 1-h incubation. For monitoring in vivo, GTN-induced (130 microg/kg) O2*- formation CP-H or PP-H were continuously infused (actual concentration, 800 microM) for 90 to 120 min into rabbits. Formation of superoxide was determined by SOD- or vitamin C-inhibited contents of nitroxide radicals in the blood from A. carotis. The incubation of whole blood with CP-H, PP-H, or corresponding nitroxyl radicals clearly shows that during a 1-h incubation, as much as 8.3% of CP-H but only 0.9% of PP-H is incorporated in cytoplasm. Acute GTN treatment of whole blood and in vivo bolus infusion significantly increased superoxide radical formation as much as 4-fold. Pretreatment with 20 mg/kg vitamin C or 15,000 U/kg superoxide dismutase prevented GTN-induced nitroxide formation. The decrease of trapped radicals after treatment with extracellularly added superoxide dismutase or vitamin C leads to the conclusion that GTN increases the amount of extracellular superoxide radicals both in vitro and in vivo.

Tolerance to nitrates with enhanced radical formation suppressed by carvedilol

Enhanced oxidant stress occurs under many pathophysiologic conditions (e.g., inflammation) and can be induced and mimicked by continuous nitrate therapy, eliciting increases in platelet activity, enhanced formation of reactive oxygen species (ROS), and impaired nitrate-induced vasorelaxation. Analysis was performed of effects of coinfusion of glycerol trinitrate (GTN) either with a carvedilol metabolite with antioxidant properties or with antioxidant vitamin C (Vit-C) on various hemodynamic parameters during enhanced oxidant stress associated with nitrate tolerance. Carvedilol metabolite (BM910228: 4.5 microg/kg/min) or Vit-C (55 microg/kg/min) was coadministered with GTN (1.5 microg/kg/min) for 5 days in chronically instrumented dogs. Changes in coronary diameters (CD) and other hemodynamic parameters were continuously monitored, as well as changes in platelet function. At the beginning of GTN treatment, CD increased by 9.8 +/- 0.4% and progressively declined to basal control values within 3 days. However, with additional antioxidant protection either with BM910228 or with Vit-C, the GTN-induced increase in CD was maintained (8.6 +/- 0.4% or 10.5 +/- 0.6%) and remained elevated for the entire infusion period. The thrombin-stimulated intracellular Ca2+ concentrations of platelets remained nearly unchanged during Vit-C or BM910228 in contrast to the increase with GTN. The basal cyclic guanosine monophosphate (cGMP) contents of platelets after GTN coadministered with BM910228 or with Vit-C increased on day 1 to 233 or to 250% versus control and remained at that level. Additional in vitro tests with xanthine oxidase-induced oxidant stress resulted in a more or less pronounced scavenging of O2- radicals by BM920228, Vit-C, or superoxide dismutase (SOD). Coadministration of carvedilol metabolite BM910228 or of Vit-C along with GTN suppressed noxious effects of GTN-induced oxidant stress such as increased platelet activity and impaired nitrate-induced vasorelaxation.

Comparison of glyceryl trinitrate-induced with pentaerythrityl tetranitrate-induced in vivo formation of superoxide radicals: effect of vitamin C

Glyceryl trinitrate (GTN) and pentaerythrityl tetranitrate (PETN) are among the most known organic nitrates that are used in cardiovascular therapy as vasodilators. However, anti-ischemic therapy with organic nitrates is complicated by the induction of nitrate tolerance. When nitrates are metabolized to release nitric oxide (NO), there is considerable coproduction of superoxide radicals in vessels leading to inactivation of NO. However, nitrate-induced increase of superoxide radical formation in vivo has not been reported. In this work, the authors studied the in vivo formation of superoxide radicals induced by treatment with PETN or GTN and determined the antioxidant effect of vitamin C. The formation of superoxide radicals was determined by the oxidation of 1-hydroxy-3-carboxy-pyrrolidine (CP-H) to paramagnetic 3-carboxy-proxyl (CP) using electron spin resonance spectroscopy. CP-H (9 mg/kg intravenous bolus and 0.225 mg/kg per minute continuous intravenous GTN or PETN 130 microg/kg) were infused into anesthetized rabbits. Every 5 min, blood samples were obtained from Arteria carotis to measure the CP formation. Both PETN and GTN showed similar vasodilator effects. Formation of CP in blood after infusions of GTN and PETN were 2.0+/-0.4 microM and 0.98+/-0.23 microM, respectively. Pretreatment with 30 mg/kg vitamin C led to a significant decrease in CP formation: 0.27+/-0.14 microM (vitamin C plus GTN) and 0.34+/-0.15 microM (vitamin C plus PETN). Pretreatment of animals with superoxide dismutase (15,000 units/kg) significantly inhibited nitrate-induced nitroxide formation. Therefore, in vivo infusion of GTN or PETN in rabbits increased the formation of superoxide radicals in the vasculature. PETN provoked a minimal stimulation of superoxide radical formation without simultaneous development of nitrate tolerance. The data suggest that the formation of superoxide radicals induced by organic nitrate correlates with the development of nitrate tolerance. The effect of vitamin C on CP formation leads to the conclusion that vitamin C can be used as an effective antioxidant for protection against nitrate-induced superoxide radical formation in vivo.

The nitric oxide donor SIN-1 is free of tolerance and maintains its cyclic GMP stimulatory potency in nitrate-tolerant LLC-PK1 cells

PURPOSE

Using an established cell culture model, the present study investigates whether linsidomine (SIN-1), a spontaneous donor of nitric oxide and active metabolite of the antianginal drug molsidomine, induces tolerance to its own cyclic GMP stimulatory action or shows a diminished response after tolerance induction with glyceryl trinitrate.

METHODS

Incubations with nitric oxide donors were carried out in LLC-PK1 kidney epithelial cells. Intracellular levels of cyclic GMP, the vasodilatory second messenger of nitric oxide, were determined by radioimmunoassay.

RESULTS

A 5-h preincubation with glyceryl trinitrate (0.01-100 microM) led to complete inhibition of a subsequent cyclic GMP stimulation by glyceryl trinitrate but left the cyclic GMP response to SIN-1 unaltered. Similarly, cyclic GMP elevations by the spontaneous nitric oxide donors sodium nitroprusside and spermine NONOate were not affected after pretreatment with glyceryl trinitrate. Moreover, pretreatment with SIN-1 (1-1000 microM) had no significant effect on SIN-1-dependent cyclic GMP stimulation.

CONCLUSIONS

Our results show that in LLC-PK1 cells, SIN-1 is free of tolerance induction and not cross-tolerant to glyceryl trinitrate. This may be due to the spontaneous nitric oxide release from SIN-1, which in contrast to nitric acid esters does not require enzymatic bioactivation and may therefore be unaffected by nitrate tolerance.

The role of organic nitrates in the treatment of heart failure

Nitrates have been widely used in the treatment of patients with chronic congestive heart failure. Although the use of these drugs has not been approved by the Food and Drug Administration, multiple studies have shown their favorable effects. Organic nitrates have been shown to have a beneficial effect on ischemia, hemodynamic profile, magnitude of a mitral regurgitation, endothelial function, and cardiac remodeling. These drugs, when used in combination with hydralazine, have improved exercise capacity and survival. Recent studies have shown that the use of nitrates in patients already treated with standard heart failure therapy, including angiotensin converting enzyme (ACE) inhibitors, resulted in hemodynamic improvement, marked enhancement of exercise tolerance, reduction of left ventricular size, and augmentation of systolic function. These data suggest a role for organic nitrates as an adjunctive therapy to ACE inhibitors in patients with chronic heart failure and for nitrates in combination with hydralazine as an alternative treatment in patients who are intolerant to ACE inhibitors.

Randomized, double-blind, placebo-controlled study of carvedilol on the prevention of nitrate tolerance in patients with chronic heart failure

OBJECTIVES

This study was designed to evaluate the effect of carvedilol on nitrate tolerance in patients with chronic heart failure.

BACKGROUND

The attenuation of cyclic guanosine 5'-monophosphate (cGMP) production due to inactivation of guanylate cyclase by increased superoxide has been reported as a mechanism of nitrate tolerance. Carvedilol has been known to combine alpha/beta-blockade with antioxidant properties.

METHODS

To evaluate the effect of carvedilol on nitrate tolerance, 40 patients with chronic heart failure were randomized to four groups that received either carvedilol (2.5 mg once a day [carvedilol group, n=10]), metoprolol (30 mg once a day [metoprolol group, n=10]), doxazosin (0.5 mg once a day [doxazosin group, n=10]) or placebo (placebo group, n=10). Vasodilatory response to nitroglycerin (NTG) was assessed with forearm plethysmography by measuring the change in forearm blood flow (FBF) before and 5 min after sublingual administration of 0.3 mg NTG, and at the same time blood samples were taken from veins on the opposite side to measure platelet cGMP. Plethysmography and blood sampling were obtained serially at baseline (day 0); 3 days after carvedilol, metoprolol, doxazosin or placebo administration (day 3); and 3 days after application of a 10-mg/24-h NTG tape concomitantly with carvedilol, metoprolol, doxazosin or placebo (day 6).

RESULTS

There was no significant difference in the response of FBF (%FBF) and cGMP (%cGMP) to sublingual NTG on day 0 and day 3 among the four groups. On day 6, %FBF and %cGMP were significantly lower in the metoprolol, doxazosin and placebo groups than on day 0 and day 3, but these parameters in the carvedilol group were maintained.

CONCLUSIONS

These results indicated that carvedilol may prevent nitrate tolerance in patients with chronic heart failure during continuous therapy with NTG.

Preventive effects of carvedilol on nitrate tolerance--a randomized, double-blind, placebo-controlled comparative study between carvedilol and arotinolol

OBJECTIVES

This study was designed to compare the preventive efect of nitrate tolerance between carvedilol with antioxidant properties and arotinolol without antioxidant properties.

BACKGROUND

The attenuation of cyclic guanosine monophosphate (cGMP) production due to inactivation of guanylate cyclase by increased superoxide has been reported as a mechanism of nitrate tolerance. Carvedilol has been known to combine alpha- and beta-blockade with antioxidant properties.

METHODS

To evaluate the preventive effect of nitrate tolerance, 24 patients with untreated hypertension were randomized to receive either carvedilol (10 mg twice a day [carvedilol group, n=8]), arotinolol (10 mg twice a day [arotinolol group, n=8]), or placebo (placebo group, n=8). Vasodilatory response to nitroglycerin (NTG) was assessed with forearm plethysmography by measuring the change in forearm blood flow (FBF) before and 5 min after sublingual administration of 0.3 mg NTG, and at the same time blood samples were taken from veins on the opposite side to measure platelet cGMP. Plethysmography and blood sampling were obtained serially at baseline (day 0), 3 days after carvedilol, arotinolol or placebo administration (day 3) and 3 days after application of a 20 mg/24 h NTG tape concomitantly with carvedilol, arotinolol or placebo (day 6).

RESULTS

There was no significant difference in the response of FBF (%FBF) and cGMP (%cGMP) to sublingual administration of NTG on days 0 and 3 among the three groups. On day 6, %FBF and %cGMP were significantly lower in the arotinolol group and the placebo group than days 0 and 3, but these parameters in the carvedilol group were maintained.

CONCLUSIONS

The results indicated that carvedilol with antioxidant properties may prevent the development of nitrate tolerance during continuous therapy with NTG compared with arotinolol without antioxidant properties.

Detection of superoxide radicals and peroxynitrite by 1-hydroxy-4-phosphonooxy-2,2,6,6-tetramethylpiperidine: quantification of extracellular superoxide radicals formation

The reactions of the new sterically hindered hydroxylamine 1-hydroxy-4-phosphonooxy-2,2,6,6-tetramethylpiperidine (PP-H) with superoxide radical and peroxynitrite have been studied. These reactions produce the nitroxide 4-phosphonooxy-2,2,6, 6-tetramethyl-piperidinyloxy. The rate constant for reaction of superoxide with PP-H is determined as (8.4+/-0.6).10(2) M-1s-1. It was found that PP-H provides almost the same spin trapping efficacy as 1-hydroxy-3-carboxy-pyrrolidine (CP-H). The background oxidation of PP-H in blood is much less than for CP-H. The extremely slow PP-H penetration into the cells makes possible the study of extracellular formation of superoxide radical. The acute treatment of blood with nitroglycerin is shown to induce an extracellular superoxide radical formation. PP-H is more sensitive for detection of reactive oxygen species as compared with CP-H. PP-H is an effective scavenger of superoxide radical and of peroxynitrite, and can be used to quantify the extracellular formation of these reactive oxygen species.

Dietary supplement with vitamin C prevents nitrate tolerance

Enhanced formation of superoxide radicals has been proposed to play a major role in the development of nitrate tolerance in humans. We tested the effects of vitamin C (Vit-C) supplementation on glyceroltrinitrate (GTN)-induced hemodynamic effects during 3-d nonintermittent transdermal administration of GTN (0.4 mg/h) in nine healthy subjects. Tolerance development was monitored by changes in arterial pressure, dicrotic digital pulse pressure, and heart rate. Studies with GTN, Vit-C, or GTN/Vit-C were successively carried out at random in three different series in the same subjects. GTN treatment caused an immediate rise in arterial conductivity (a/b ratio of dicrotic pulse), but within 2 d of initiating GTN, the a/b ratio progressively decreased and reached basal levels. In addition, there was a progressive loss of the orthostatic decrease in blood pressure. However, coadministration of Vit-C and GTN fully maintained the GTN-induced changes in the orthostatic blood pressure, and the rise of a/b ratio was augmented by 310% for the duration of the test period. Changes in vascular tolerance in GTN-treated subjects were paralleled by upregulation of the activity of isolated platelets, which was also reversed by Vit-C administration. These findings demonstrate that dietary supplementation with Vit-C eliminates vascular tolerance and concomitant upregulation of ex vivo-washed platelet activity during long-term nonintermittent administration of GTN in humans.

Impaired modulation of sympathetic excitability by nitric oxide after long-term administration of organic nitrates in pigs

BACKGROUND

Endogenous nitric oxide (NO) reduces sympathetic vasoconstriction by attenuating neuronal excitability in the brain stem and inhibition of postganglionic neurotransmission. We studied whether this modulation of sympathetic circulatory control by NO may be altered during chronic administration of NO donor drugs in pigs.

METHODS AND RESULTS

Nitrate tolerance was induced by oral administration of isosorbide dinitrate (ISDN, 4 mg/kg per day for 4 weeks) in eight pigs. Four of them were chronically instrumented for the measurement of mean arterial blood pressure and cardiac output in the conscious state. ISDN treatment caused hemodynamic tolerance to NO donors and significantly increased the hypotensive responses to pharmacologic ganglionic blockade in conscious pigs. In general anesthesia, ISDN-treated animals and age-matched controls (n=5) had similar baseline renal sympathetic nerve activity and in both groups neither inhibition of NO synthases (NOS) nor administration of NO donors to the brain stem by intracerebroventricular (i.c.v.) infusions caused significant changes in baseline renal sympathetic nerve activity. However, whereas sympathoexcitatory responses to glutamate (0.5 mL, 0.1 mol/L, i.c.v.) or electrical stimulation of somatic nerve afferents were significantly potentiated by central NOS inhibition and attenuated by NO donors in controls, these treatments no longer had significant effects in ISDN-treated pigs. Furthermore, reflex sympathetic activation in response to intravenous NO donor treatment was more pronounced in nitrate tolerant animals, which suggests loss of central sympathoinhibitory effects of NO. Subsequent histology on brain stem slices with NADPH-diaphorase as NOS marker revealed significant reduction of NOS density in ISDN-treated pigs.

CONCLUSIONS

Long-term administration of organic nitrates reduces the number of NO-producing neurons in the brain stem and causes loss of inhibitory effects of NO on sympathetic excitability. This component of tolerance to organic nitrates may be important in patients confronted frequently with sympathetic activation caused by mental and/or physical stressors.

Vitamin C attenuates nitrate tolerance independently of its antioxidant effect

In LLC-PK1 kidney epithelial cells, a 5-h pretreatment with glyceryl trinitrate (GTN) resulted in substantial desensitization of the intracellular cyclic GMP response to a subsequent 10-min challenge with GTN (1 microM). GTN-tolerant cells were fully sensitive to the spontaneous nitric oxide (NO) donor spermine NONOate, which does not require enzymatic bioactivation. Cyclic GMP stimulation by GTN was up to 3.1-fold higher when vitamin C (1-10 mM) was present during the pretreatment period. In contrast, other oxygen radical scavengers such as tiron or dimethylsulfoxide and the NO scavenger PTIO left tolerance induction unaltered. Together, our results suggest that reactive oxygen species or NO do not contribute to the development of nitrate tolerance. Tolerance reduction by vitamin C may be due to a stabilizing effect on enzymes involved in the bioconversion of GTN to NO.

Randomized, double-blind, placebo-controlled study of the preventive effect of supplemental oral vitamin C on attenuation of development of nitrate tolerance

OBJECTIVES

This study sought to evaluate the preventive effect of vitamin C, an antioxidant, on the development of nitrate tolerance.

BACKGROUND

Decreased intracellular production of cyclic guanosine monophosphate (cGMP) is a mechanism of nitrate tolerance, and increased superoxide levels and reduced activation of guanylate cyclase have been observed in vitro.

METHODS

In this double-blind, placebo-controlled study, 24 normal volunteers and 24 patients with ischemic heart disease (IHD) were randomized to receive either vitamin C (2 g three times daily [vitamin C group, n=12]) or placebo (placebo group, n=12). The vasodilator response to nitroglycerin was assessed with forearm plethysmography by measuring the change in FBF before and 5 min after sublingual administration of 0.3 mg of nitroglycerin. Blood samples were simultaneously obtained to measure platelet cGMP levels. FBF was measured, and blood sampling was performed serially at baseline (day 0), 3 days after administration of vitamin C or placebo (day 3) and 3 days after application of a 10-mg/24-h nitroglycerin tape concomitantly with oral vitamin C or placebo (day 6).

RESULTS

There were no differences between the vitamin C and placebo groups in percent increases in FBF (%FBF) or platelet cGMP levels (%cGMP) after administration of sublingual nitroglycerin on day O (%FBF: normal volunteers 31+/-8 vs. 32+/-10; patients with IHD 32+/-9 vs. 32+/-8; %cGMP: normal volunteers 37+/-9 vs. 39+/-10; patients with IHD 38+/-10 vs. 39+/-10 [vitamin C group vs. placebo group]) or day 3 (%FBF: normal volunteers 32+/-9 vs. 33+/-9; patients with IHD 31+/-10 vs. 31+/-10; %cGMP: normal volunteers 36+/-8 vs. 37+/-9; patients with IHD 39+/-11 vs. 38+/-10 [vitamin C group vs. placebo group]). The %FBF and %cGMP in the placebo group were significantly lower on day 6 than in the vitamin C group (%FBF: normal volunteers 30+/-8 vs. 19 4, p < 0.01; patients with IHD 29+/-9 vs. 17+/-6, p < 0.01; %cGMP: normal volunteers 36 10 vs. 17+/-6, p < 0.01; patients with IHD 37+/-11 vs. 15+/-5, p < 0.01 [vitamin C group vs. placebo group]).

CONCLUSIONS

These results indicate that combination therapy with vitamin C is potentially useful for preventing the development of nitrate tolerance.

Randomized, double-blind, placebo-controlled study of ascorbate on the preventive effect of nitrate tolerance in patients with congestive heart failure

BACKGROUND

Reduced cGMP production caused by increased superoxide has been proposed as a mechanism of nitrate tolerance during continuous nitrate therapy. This study was designed to evaluate the effects of ascorbate, an antioxidant, on the development of nitrate tolerance during continuous nitrate therapy in patients with congestive heart failure.

METHODS AND RESULTS

Twenty patients with congestive heart failure were randomized to receive intravenous infusion of nitroglycerin concomitantly with placebo (placebo group, n=10) or intravenous ascorbate (vitamin C group, n=10). After baseline measurements were obtained, dose titration was started by the infusion of nitroglycerin at a rate of 0.5 microg/kg per minute (titration period). Measurements of hemodynamic parameters and blood sampling were performed serially at 0, 6, 12, 18, and 24 hours after the titration period. At baseline, mean pulmonary artery pressure (MPAP, mm Hg), mean pulmonary capillary wedge pressure (PCWP, mm Hg), plasma vitamin E level (micromol/L), and platelet cGMP level (pmol/10[9] platelets) were comparable in the two groups (placebo group: MPAP, 48+/-6; PCWP, 24+/-4; cGMP, 0.76+/-0.12; vitamin E, 18.2+/-1.2; vitamin C: MPAP, 49+/-7; PCWP, 24+/-4; cGMP, 0.71+/-0.16; vitamin E, 18.6+/-1.3). In both groups, at 6 hours after the titration period, MPAP and PCWP were significantly decreased (placebo group: MPAP, 26+/-5; PCWP, 15+/-4; vitamin C: MPAP, 26+/-4; PCWP, 16+/-4), and platelet cGMP was significantly increased (placebo group: 2.42+/-0.24; vitamin C: 2.26+/-0.26). However, at 18 hours after titration, in the placebo group, MPAP (44+/-5) and PCWP (23+/-4) were increased, and platelet cGMP (0.85+/-0.20) and plasma vitamin E levels (12.4+/-1.4) were significantly decreased. In contrast, in the vitamin C group, MPAP (31+/-6), PCWP (17+/-5), platelet cGMP (2.49+/-0.23), and plasma vitamin E levels (17.6+/-1.4) were maintained for 18 hours after the titration period.

CONCLUSIONS

These findings indicate that ascorbate, an antioxidant, may prevent the development of nitrate tolerance during continuous nitrate therapy in patients with congestive heart failure.

Formation of Reactive Oxygen Species in Various Vascular Cells During Glyceryltrinitrate Metabolism

BACKGROUND: Anti-ischemic therapy with organic nitrates as nitric oxide (NO) donors is complicated by the induction of tolerance. When nitrates are metabolized to release NO, there is a considerable coproduction of reactive oxygen species (superoxide radical and peroxynitrite) in vessels leading to inactivation of NO, to diminished cyclic quanosine monophosphate production in smooth muscle cells (SMC), to impaired vasomotor responses to the endothelium-derived relaxation factor (EDRF), and to formation of nitrotyrosine as a marker of glyceryltrinitrate (GTN)-induced formation of peroxynitrite. The aim of the study was to analyze in vitro the formation of superoxide radicals and of peroxynitrite in GTN-treated endothelial and smooth muscle cells and in washed ex vivo platelets using electron spin resonance and spin-trapping techniques. METHODS AND RESULTS: Using 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as a spin trap, it was shown that in platelets, smooth muscle, and endothelial cells incubated acutely for 15 minutes with 0.5 mM GTN, the rate of generation of reactive oxygen species (ROS) was twice as high as under control conditions. Using the new spin-trap 2H-imidazole-1-oxide (TMIO), a GTN-induced peroxynitrite formation was detected in SMC and in platelets incubated with 0.5 mM GTN for 15 minutes. Spin-trap 1-hydroxy-3-carboxy-pyrrolidine (CP-H) was used to estimate the rate of ROS formation in platelets incubated for 15 minutes with 0.5 mM GTN; the rate amounted to 14.6 +/- 1.1 nM/min/mg protein compared with 4.0 +/- 0.4 nM/min/mg protein in controls. The rate of ROS formation in SMCs was substantially increased (240 +/- 16%) after initiation of GTN tolerance by treatment of the cells in culture with 100 µM GTN for 24 hours. CONCLUSIONS: GTN increases the formation of superoxide radicals in endothelial cells, SMCs, and platelets. Peroxynitrite is formed during GTN metabolism in vascular cells and may contribute to the development of tolerance. A decrease in the nitrate-induced inhibition of platelet aggregation during GTN tolerance is associated with oxidative actions of ROS formed in platelets during GTN metabolism.

Unexpected, tolerance-devoid vasomotor and platelet actions of pentaerythrityl tetranitrate

Efficacy of nitrate therapy is limited by tolerance. A surprising upregulation of ex vivo platelet activity, a decrease in platelet thiol levels, and an enhanced release of vasoconstrictors from platelets is associated with enhanced superoxide-mediated oxidant stress leading to vascular tolerance to nitrates. We tested the NO-donor pentaerythrityl tetranitrate (PETN), which to date had not been precisely tested either with regard to the induction of tolerance or to a potential development of changes in platelet activity in comparison with glycerol trinitrate (GTN). Long-term instrumented dogs nonintermittently received: 1.5 microg/kg/min GTN, i.v., with or without vitamin C (55 microg/kg/min, i.v.) or PETN 4 x 60 mg/day orally for 5 days. Tested daily were (a) the dilation of the epicardial arteries, (b) thrombin-induced (0.5 U/ml) increases of the intracellular Ca2+ concentration and aggregability of platelets, (c) concentrations of reduced low-molecular-weight thiols (LMTs) in plasma and platelets, and (d) formation of reactive oxygen species (ROSs). During nonintermittent PETN and during GTN with additional vitamin C, a 9.8 +/- 0.4% coronary artery dilation was observed in contrast to that with GTN alone, which resulted in complete tolerance at day 4. This vascular tolerance was associated with enhanced platelet activity and formation of ROSs (incubated platelets) and a 38 +/- 3% reduction in LMT. These unfavorable changes were absent in the presence of PETN or with additional vitamin C as an antioxidant. Vascular tolerance associated with platelet upregulation is avoided either by nonintermittent nitroglycerin (5 days) when vitamin C is coadministered or by pentaerythrityl tetranitrate without the coadministration of vitamin C.

Quantification of superoxide radicals and peroxynitrite in vascular cells using oxidation of sterically hindered hydroxylamines and electron spin resonance

The reactions of two hydroxylamines, 1-hydroxy-3-carboxy-pyrrolidine (CP-H) and 1-hydroxy-2,2,6,6-tetramethyl-4-oxo-piperidine (TEMPONE-H), with superoxide radicals and peroxynitrite were studied. In these reactions corresponding stable nitroxyl radicals 3-carboxy-proxyl (CP) and 1-hydroxy-2,2,6,6-tetramethyl-4-oxopiperidinoxyl (TEMPONE) are formed and the amount of them can be quantified by electron spin resonance (ESR). It was found that CP-H and TEMPONE-H provide almost the same efficacy in assaying peroxynitrite by ESR in vitro at pH 7.4. The formation of superoxide radicals in suspensions of cells was discriminated from that of peroxynitrite using superoxide dismutase or dimethyl sulfoxide as competitive reagents. The stability of the radicals CP and TEMPONE in the presence of ascorbate or thiols was studied in vitro. The reduction rate of CP by ascorbate was 66-fold slower than the rate of reduction of TEMPONE. Therefore, the quantification of the formation of superoxide radicals and of peroxynitrite is much less affected by ascorbic acid when CP-H, but not TEMPONE-H, is used. Both TEMPONE-H and CP-H were used to determine the formation rates of superoxide radicals and peroxynitrite in suspensions of cultured aortic smooth muscle cells and endothelial cells, in washed ex vivo platelets, and in blood treated with glycerol trinitrate (GTN) as an NO donor. It was shown that both the acute addition of GTN (0.5 mM) to vascular cells and the incubation of smooth muscle or endothelial cells in culture with 0.1 mM GTN for 24 h enhance significantly the formation of reactive oxygen species in cells. The rates of of superoxide radical formation were increased at least in two times and peroxynitrite was detected. Hydroxylamines TEMPONE-H and CP-H can be used as nontoxic compounds in ESR assay capable of quantifying the formation of superoxide radicals and peroxynitrite in suspensions of cells and in the whole blood with high sensitivity.

Urinary NItrotyrosine Content as a Marker of Peroxynitrite-induced Tolerance to Organic NItrates

BACKGROUND: Anti-ischemic therapy with nitrovaasodilators as NO-donors is complicated by the induction of tolerance. When nitrovasodilators are metabolized to release NO there is a considerable coproduction of oxygen-derived radicals leading to a diminished cyclic GMP production and to impaired vasomotory responses. We analyzed in vivo the glyceroltrinitrate-induced generation of strong oxidative/nitrating compounds contributing to development of tolerance. METHODS AND RESULTS: In 16 patients we studied the urinary nitrotyrosine excretion during either (1) placebo control conditions, (2) 2-day nonintermittent transdermal nitroglycerin administration (0.4 mg/h), (3) 2-day nonintermittent glyceroltrinitrate administration (0.4 mg/h) along with a continuous infusion of vitamin C (55 µg/kg/min) as an antioxidant, or (4) with vitamin C but without glyceroltrinitrate (diminished urinary nitrotyrosine content of 34 +/- 18 µg/day observed). Glyceroltrinitrate administration augmented urinary nitrotyrosine from 56 +/- 24 (basal) to 186 +/- 32 µg/day (glyceroltrinitrate tolerance). Coadministration of vitamin C caused complete elimination of tolerance and a decrease in urinary nitrotyrosine to 130 +/- 28 µg/day. Glyceroltrinitrate-induced formation of oxidants was confirmed in vitro comparing glyceroltrinitrate-induced and peroxynitrite-induced tachyphylaxis in isolated perfused rabbit hearts and analyzing tolerance-induced inactivation of solbule guanylyl cyclase in cultured aortic smooth muscle cells. CONCLUSIONS: Augmented urinary nitrotyrosine excretion during glyceroltrinitrate administration reflects enhanced formation of peroxynitrite and of nitrotyrosine. Glyceroltrinitrate-induced tolerance is the result of oxidative stress and can be suppressed by additional antioxidant therapy aimed to prevent glyceroltrinitrate-induced formation and/or actions of peroxynitrite.