Nitrate tolerance in vivo is not associated with depletion of arterial or venous thiol levels

Nitric oxide-releasing molecules at the interface of inorganic chemistry and biology: a concise overview

The useful aspects of nitric oxide (NO) are nowadays widely known. Due to the need for this molecule in the maintenance of homeostasis, NO-releasing compounds are tested every year to optimize its levels in a patient suffering from low NO production. This manuscript is an update of some important historical concerns about nitrosyl complexes having the ability to act as NO-releasing compounds under the influence of different chemically modified environments. At present, the search for efficient and less harmful NO-releasing molecules at desirable targets and concentrations has gained considerable momentum in nitrosyl chemistry. Iron, ruthenium, and manganese nitrosyls have been investigated elitely to disentangle their electronic transition (excitation) under visible light to act as NO donors without harming the healthy cells of a target. There is much evidence supporting the increase of NO lability if amino acids are used as complexing ligands, the design of a reduction center close to an NO grouping, and the development of porphyrin system-based nitrosyl complexes. From the overall survey, it may be concluded that the desirable properties of such scaffolds need to be evaluated further to complement the biological milieu.

Role of glutaredoxin-mediated protein S-glutathionylation in cellular nitroglycerin tolerance

We hypothesize that nitroglycerin (NTG) causes direct oxidation of multiple cellular sulfhydryl (SH) proteins and that manipulation of SH redox status affects NTG tolerance. In LLC-PK1 cells, we found that nitrate tolerance, as indicated by cGMP accumulation toward NTG, was accompanied by increased protein [(35)S]cysteine incorporation, significant S-glutathionylation of multiple proteins, and decreased metabolic activity of several SH-sensitive enzymes, including creatine kinase, xanthine oxidoreductase, and glutaredoxin (GRX). Cells overexpressing GRX exhibited reduced cellular protein S-glutathionylation (PSSG) and absence of NTG tolerance, whereas those with silenced GRX showed increased extent of NTG-induced tolerance. Incubation of LLC-PK1 cells with oxidized glutathione led to several major observations associated with nitrate tolerance, namely, reduced cGMP accumulation, PSSG formation, superoxide accumulation, and the attenuation of these events by vitamin C. Aortic S-glutathionylated proteins increased approximately 3-fold in rats made tolerant in vivo to NTG and showed significant negative correlation with vascular responsiveness ex vivo. NTG incubation in EA.hy926 endothelial cells and LLC-PK1 cells led to increased S-glutathionylation and activity of p21(ras), a known mediator of cellular signaling. These results indicate that the hallmark events of NTG tolerance, such as reduced bioactivation and redox signaling, are associated with GRX-dependent protein deglutathionylation.

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.

Nitroglycerin-induced S-nitrosylation and desensitization of soluble guanylyl cyclase contribute to nitrate tolerance

Nitrates such as nitroglycerin (GTN) and nitric oxide donors such as S-nitrosothiols are clinically vasoactive through stimulation of soluble guanylyl cyclase (sGC), which produces the second messenger cGMP. Development of nitrate tolerance, after exposure to GTN for several hours, is a major drawback to a widely used cardiovascular therapy. We recently showed that exposure to nitric oxide and to S-nitrosothiols causes S-nitrosylation of sGC, which directly desensitizes sGC to stimulation by nitric oxide. We tested the hypothesis that desensitization of sGC by S-nitrosylation is a mechanism of nitrate tolerance. Our results established that vascular tolerance to nitrates can be recapitulated in vivo by S-nitrosylation through exposure to cell membrane-permeable S-nitrosothiols and that sGC is S-nitrosylated and desensitized in the tolerant, treated tissues. We next determined that (1) GTN treatment of primary aortic smooth muscle cells induces S-nitrosylation of sGC and its desensitization as a function of GTN concentration; (2) S-nitrosylation and desensitization are prevented by treatment with N-acetyl-cysteine, a precursor of glutathione, used clinically to prevent development of nitrate tolerance; and (3) S-nitrosylation and desensitization are reversed by cessation of GTN treatment. Finally, we demonstrated that in vivo development of nitrate tolerance and crosstolerance by 3-day chronic GTN treatment correlates with S-nitrosylation and desensitization of sGC in tolerant tissues. These results suggest that in vivo nitrate tolerance is mediated, in part, by desensitization of sGC through GTN-dependent S-nitrosylation.

Effects of nitric oxide and noradrenergic activation in the posterior hypothalamus on arterial pressure tolerance to nitroglycerin in rats

The effects of nitric oxide (NO) and noradrenergic activation in the posterior hypothalamus on arterial pressure tolerance induced by subcutaneous injection of nitroglycerin (NTG) was investigated in anesthetized Sprague-Dawley rats. Intravenous injections of NTG (3, 10, and 30 microg/kg) and sodium nitroprusside (1, 3, and 10 microg/kg) produced dose-dependant decreases in arterial blood pressure. Tolerance to NTG was produced by subcutaneous administration of 4.0 mg of NTG as 4 separate hourly injections of 1.0 mg each, affecting the dose-dependent response of NTG IV injection. The 4 high-dose NTG pulse injections produced a marked shift in the dose-response curves for arterial pressure depression induced by intravenous injection of the challenge doses of NTG, but did not alter hypotensive responses to sodium nitroprusside. The tolerance responses to arterial pressure depression were enhanced by a bilateral microinjection of NTG (1 nmol) and by diethylamine NONOate (1 nmol), an NO donor, into the posterior hypothalamus. Bilateral microinjection of guanethidine (1.5 nmol), a noradrenergic blocker, into the posterior hypothalamus inhibits NTG tolerance in a period of time within 2 hours. We conclude that exogenous NO and noradrenergic activation in the posterior hypothalamus play an important role in arterial pressure tolerance to systemically administered NTG.

Redox state, oxidative stress and endothelial dysfunction in heart failure: the puzzle of nitrate–thiol interaction

Endothelial dysfunction, a critical component in the progression of heart failure, may result from increased oxidative stress, secondary to activation of the adrenergic and the renin-angiotensin systems and to the production of inflammatory cytokines, which in turn contribute to reduced bioavailability of nitric oxide (NO). Oxidative stress, determined by excess production of reactive oxygen species and impairment in the antioxidant defence, is responsible for both the decline of diffusible NO and the decrease in the concentration of essential co-factors of NO synthases. Reactive oxygen species are formed from NO in the presence of oxidants and are involved in the nitration of protein tyrosine residue that can alter protein function. Recent studies re-addressed the impact of nitrate treatment in heart failure in view of the beneficial vascular and cellular effects of NO, and of the discovery of abnormalities in NO pathways in this disease. Concerns exist, however, on the safety of nitrates in this setting. Nitrates stimulate vascular superoxide anion production via activation of NADPH oxidase, and induction of uncoupling of NO synthase. Furthermore, by using donors of sulfhydryl groups, such as cysteine and glutathione, for NO production, nitrates may favour depletion of the intracellular thiol pool, thus impairing the antioxidant defence mechanisms.

Bioactivation of nitroglycerin to nitric oxide (NO) and S-nitrosothiols in the rat liver and evaluation of the coexisting hypotensive effect

The aim of the present study was to investigate nitroglycerin (NTG) bioactivation pathways in the liver after various periods of its administration. We also attempted to elucidate the relationship between nitric oxide (NO) and S-nitrosothiol (SNT) levels, and concentration of nonprotein thiols (NPSH) and intensity of peroxidative processes. Intravenous injections of NTG cause an increase in NO and SNT levels in the rat liver. The same intravenous NTG injections in the rats pretreated with 5-day i.p. NTG administrations lead to a drop in the levels of the biologically active NO, SNT and NPSH, with no concomitant changes in the rate of lipid peroxidation. This indicates that after such period of nitroglycerin pretreatment, levels of pharmacologically active NO and SNT decrease. However, during longer periods of NTG administration (for 10 and 17 days) NO, SNT and NPSH concentrations remain at the control level in spite of a considerably enhanced lipid peroxidation, which indicates that tolerance did not develop. Effects of NTG bioactivation in the liver, i.e. the levels of NO and SNT released from it, after different periods of drug administration correspond with hypotensive effects, which are known to be dependent on NTG biodegradation in vascular endothelial cells. The changes in NO and SNT levels observed in the rat liver after different periods of NTG administration parallel alterations in the hypotensive effect. In conclusion, NTG treatment for 10 and 17 days does not lead to tolerance, however, a transient loss of its pharmacological activity occurs after 5-day NTG pretreatment.

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.

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.

Enhanced nitric oxide release/synthesis in the posterior hypothalamus during nitroglycerin tolerance in rats

We have recently observed that increasing central noradrenergic transmission and sympathomimetic activity is involved with the complex hemodynamic effects during tolerance to nitroglycerin. The present study was to examine the release of nitric oxide (NO) in the posterior hypothalamus during tolerance to depressor responses to nitroglycerin and determine if, during the tolerance, endogenous NO synthesis is induced in the posterior hypothalamus. A microdialysis probe was implanted in the posterior hypothalamus and perfusion fluid was pumped through the probe at 2 microl/min in conscious rats. Tolerance to nitroglycerin was produced by three intravenous (i.v.) injections of 1.3 mg nitroglycerin each within 40 min compared to the same administrations of low dose of the drug, sodium nitroprusside and papaverine. Dialysate samples were collected 1 h before and 1 h each after injections for 8 h. Concentrations of nitrite (NO(2)(-)), nitrate (NO(3)(-)), and total nitrite plus nitrate (NO(x)(-)) were quantified in the samples by using chemiluminescence. The dose-response curve for arterial depressor induced by intravenous injection of the challenge doses of nitroglycerin was markedly shifted to the right at the first hour after nitroglycerin tolerance, lasted 3 to 5 h and reversed at 7 h. The dialysate NO(3)(-) and NO(x)(-) concentrations in the posterior hypothalamus were significantly increased at the first hour following nitroglycerin tolerance but were not altered by low dose of the drug, sodium nitroprusside, and papaverine. Nitroglycerin tolerance predominantly caused an increase in NO(3)(-) release in the posterior hypothalamus with no or small amount of changes in dialysate NO(2)(-) and the response was partially inhibited by pretreatment with N(G)-Propyl-L-arginine (NPLA) (1.0 mg/kg, i.p.), an inhibitor of neuronal NO synthesis. The increase of NO release in the posterior hypothalamus occurred at the first hour, lasted 2 to 3 h and reversed at 5 to 6 h during nitroglycerin tolerance. The results show that systemically administered high dose of nitroglycerin increases NO release in the posterior hypothalamus which matches the time interval of tolerance to arterial depressor response to the drug. Data suggest that there is an enhanced endogenous NO synthesis in the posterior hypothalamus which may affect central sympathetic functions during nitroglycerin tolerance.

Clinical pharmacokinetics and pharmacodynamics of glyceryl trinitrate and its metabolites

This review discusses the pharmacokinetics and pharmacodynamics of glyceryl trinitrate (nitroglycerin; GTN) pertinent to clinical medicine. The pharmacokinetics of GTN associated with various dose regimens are characterised by prominent intra- and inter-individual variability. It is, nevertheless, important to clearly understand the pharmacokinetics and characteristics of GTN to optimise its use in clinical practice and, in particular, to obviate the development of tolerance. Measurements of plasma concentrations of GTN and of 1,2-glyceryl dinitrate (1,2-GDN), 1,3-glyceryl dinitrate (1,3-GDN), 1-glyceryl mononitrate (1-GMN), and 2-glyceryl mononitrate (2-GMN), its four main metabolites, remain difficult and require meticulous techniques to obtain reliable results. Since GDNs have an effect on haemodynamic function, pharmacokinetic analyses that include the parent drug as well as the metabolites are important. Although the precise mechanisms of GTN metabolism have not been elucidated, two main pathways have been proposed for its biotransformation. The first is a mechanism-based biotransformation pathway that produces nitric oxide (NO) and contributes directly to vasodilation. The second is a clearance-based biotransformation or detoxification pathway that produces inorganic nitrite anions (NO(2) -). NO(2) - has no apparent cardiovascular effect and is not converted to NO in pharmacologically relevant concentrations in vivo. In addition, several non-enzymatic and enzymatic systems are capable of metabolising GTN. This complex metabolism complicates considerably the evaluation of the pharmacokinetics and pharmacodynamics of GTN. Regardless of the route of administration, concentrations of the metabolites exceed those of the parent compound by several orders of magnitude. During continuous steady-state delivery of GTN, for instance by a patch, concentrations of 1,2-GDN are consistently 2-7 times higher than those of 1,3-GDN, and concentrations of 2-GMN are 4-8 times higher than those of 1-GMN. Concentrations of GDNs are approximately 10 times higher, and of GMNs approximately 100 times higher, than those of GTN during sustained administration. The development of tolerance is closely related to the metabolism of GTN, and can be broadly categorised as haemodynamic tolerance versus vascular tolerance. Efforts are warranted to circumvent the development of tolerance and facilitate the use of GTN in clinical practice. Although this remains to be accomplished, it is likely that, in the near future, regimens will be developed based on a full understanding of the pharmacokinetics and pharmacodynamics of GTN and its metabolites.

Nitroglycerin tolerance: different mechanisms in vascular segments with or without intact endothelial function

In vivo tolerance to nitroglycerin seems to be induced by an increase in vascular superoxide anion levels. In rabbits with normal endothelial function, in vivo induced tolerance is functionally reversed by ex vivo removal of the endothelium, probably due to a reduction in superoxide anion levels. However, the impact of in vivo endothelial dysfunction on tolerance development has not been examined. This study investigated how in vivo endothelium denudation affects the development of in vivo nitroglycerin tolerance. The effect of in vivo endothelium denudation was examined ex vivo (myograph experiments) after prolonged continuous nitroglycerin infusion in a conscious rat model. The vascular reactivity to nitroglycerin was studied in vivo in endothelium-denuded and corresponding endothelium-intact arteries. The results show that in vivo endothelium denudation does not affect the degree of tolerance development but significantly alters the effect of interventions targeted to inhibit tolerance development. In endothelium-intact vessels, superoxide dismutase and the angiotensin II receptor blocker losartan significantly inhibited tolerance-inducing properties of the prolonged nitroglycerin infusion (E[max, nitroglycerin] response in % of normal controls: nitroglycerin tolerant 70%, superoxide dismutase 93%, losartan 99%). This effect was absent in in vivo endothelium-denuded segments (nitroglycerin tolerant 57%, superoxide dismutase 72%, losartan 60%). These findings suggest that interventions against in vivo tolerance development, within the same animal, may elicit different results depending on the presence or absence of an in vivo dysfunctional endothelium.

Identification of the enzymatic mechanism of nitroglycerin bioactivation

Nitroglycerin (glyceryl trinitrate, GTN), originally manufactured by Alfred Nobel, has been used to treat angina and heart failure for over 130 years. However, the molecular mechanism of GTN biotransformation has remained a mystery and it is not well understood why "tolerance" (i.e., loss of clinical efficacy) manifests over time. Here we purify a nitrate reductase that specifically catalyzes the formation of 1,2-glyceryl dinitrate and nitrite from GTN, leading to production of cGMP and relaxation of vascular smooth muscle both in vitro and in vivo, and we identify it as mitochondrial aldehyde dehydrogenase (mtALDH). We also show that mtALDH is inhibited in blood vessels made tolerant by GTN. These results demonstrate that the biotransformation of GTN occurs predominantly in mitochondria through a novel reductase action of mtALDH and suggest that nitrite is an obligate intermediate in generation of NO bioactivity. The data also indicate that attenuated biotransformation of GTN by mtALDH underlies the induction of nitrate tolerance. More generally, our studies provide new insights into subcellular processing of NO metabolites and suggest new approaches to generating NO bioactivity and overcoming nitrate tolerance.

Cellular glutathione peroxidase deficiency and endothelial dysfunction

Cellular glutathione peroxidase (GPx-1) is the most abundant intracellular isoform of the GPx antioxidant enzyme family. In this study, we hypothesized that GPx-1 deficiency directly induces an increase in vascular oxidant stress, with resulting endothelial dysfunction. We studied vascular function in a murine model of homozygous deficiency of GPx-1 (GPx-1(-/-)). Mesenteric arterioles of GPx-1(-/-) mice demonstrated paradoxical vasoconstriction to beta-methacholine and bradykinin, whereas wild-type (WT) mice showed dose-dependent vasodilation in response to both agonists. One week of treatment of GPx-1(-/-) mice with L-2-oxothiazolidine-4-carboxylic acid (OTC), which increases intracellular thiol pools, resulted in restoration of normal vascular reactivity in the mesenteric bed of GPx-1(-/-) mice. We observed an increase of the isoprostane iPF(2alpha)-III, a marker of oxidant stress, in the plasma and aortas of GPx-1(-/-) mice compared with WT mice, which returned toward normal after OTC treatment. Aortic sections from GPx-1(-/-) mice showed increased binding of an anti-3-nitrotyrosine antibody in the absence of frank vascular lesions. These findings demonstrate that homozygous deficiency of GPx-1 leads to impaired endothelium-dependent vasodilator function presumably due to a decrease in bioavailable nitric oxide and to increased vascular oxidant stress. These vascular abnormalities can be attenuated by increasing bioavailable intracellular thiol pools.

Tolerance to nitroglycerin induced by isosorbide-5-mononitrate infusion in vivo

We examined the development of tolerance to nitroglycerin (glyceryl trinitrate; GTN) in the rat when isosorbide-5-mononitrate (ISMN) or GTN was continuously infused. Under pentobarbital anesthesia (60 mg/kg, i.p.), mean arterial blood pressure was measured via the left common carotid artery. Bolus injection of ISMN (0.25-250 mg/kg) and GTN (0.25 microg/kg-2.5 mg/kg) was made into the right external jugular vein. ISMN (2.5 mg/h/rat for 7 d), GTN (1.3 microg/h/rat for 7 d), or GTN (0.2 mg/h/rat for 3 d) was infused continuously using an osmotic pump embedded subcutaneously. Bolus injection of ISMN and GTN decreased arterial blood pressure in a dose-dependent manner. The hypotensive effect of ISMN was 2000 times less potent than that of GTN. The GTN-induced hypotensive effect was not affected after continuous infusion of ISMN, whereas it was attenuated after continuous infusion of GTN at either dose. Chronic treatment with ISMN does not induce GTN tolerance as easily as treatment with GTN.

Endothelium-dependent vasorelaxation in inhibited by in vivo depletion of vascular thiol levels: role of endothelial nitric oxide synthase

Thiols like glutathione may serve as reducing cofactors in the production of nitric oxide (NO) and protect NO from inactivation by radical oxygen species. Depletion of thiol compounds reduces NO-mediated vascular effects in vitro and in vivo. The mechanisms underlying these actions are not clear, but may involve decreased synthesis of NO and/or increased degradation of NO. This study investigates the effect of glutathione depletion on the response to NO-mediated vasodilation induced by acetylcholine (Ach, 10 micrograms/kg), endothelial NO synthase (eNOS) activity and potential markers of vascular superoxide anion (O2.-) production in conscious chronically catheterized rats. Thiol depletion induced by buthionine sulfoximine (BSO, 1 g i.p. within 24 h) decreased the hypotensive effect of Ach by 30% (MAP reduction before BSO 27 +/- 3 mmHg, 19 +/- 3 mmHg after BSO, (mean +/- SEM), p < .05, n = 8). The impaired effect of Ach was associated with a significant reduction in eNOS activity (control: 7.7 +/- 0.8, BSO: 3.9 +/- 0.4 pmol/min/mg protein (p < .05), n = 6). In contrast, neither NADH/NADPH driven membrane-associated oxidases nor lucigenin reductase activity were significantly (p < .05) affected by BSO (BSO: 4415 +/- 123, control: 4105 +/- 455 counts/mg; n = 6) in rat aorta. It is concluded that in vivo thiol depletion results in endothelial dysfunction and a reduced receptor-mediated vascular relaxation. This effect is caused by reduced endothelial NO formation.

Tetrahydrobiopterin improves endothelium-dependent vasodilation in nitroglycerin-tolerant rats

Tolerance to nitroglycerin is caused by a nitroglycerin-mediated increase in vascular superoxide anion production. Administration of tetrahydrobiopterin (co-factor for endogenous nitric oxide (NO) formation) may potentially influence nitroglycerin tolerance in at least two different ways. Firstly, tetrahydrobiopterin may act as a scavenger of the nitroglycerin-mediated production of superoxide anions. Secondly, tetrahydrobiopterin may protect endothelial NO synthesis from the deleterious effects of increased oxidative stress. This study investigates whether in vivo nitroglycerin tolerance is affected by tetrahydrobiopterin supplementation and assesses the in vivo role of tetrahydrobiopterin in endogenous NO-mediated vasodilation in normal and nitroglycerin-tolerant rats. The results show that tetrahydrobiopterin does not affect nitroglycerin-derived, NO-mediated vasodilation, but reduces baseline mean arterial blood pressure (by 8 mm Hg, P<0.05) and normalizes endothelium-dependent responses to N(G)-monomethyl-L-arginine (L-NMMA) (from 7+/-1 to 22+/-4 mm Hg, P<0.05) in nitroglycerin-tolerant rats. It is concluded that altered bioavailability of tetrahydrobiopterin is involved in the pathophysiology of endothelial dysfunction seen in nitroglycerin tolerance.

Effect of antioxidant treatments on nitrate tolerance development in normotensive and hypertensive rats

OBJECTIVES

To investigate the effect of chronic antioxidant treatments on the development of nitrate tolerance in spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rats by evaluating (i) coronary vascular reactivity, (ii) lipid peroxidation (malondialdehyde), and (iii) peroxynitrite formation (3-nitrotyrosine).

METHODS

Tolerance was induced in 16-week-old male SHR and WKY, by 4 days of continuous treatment with nitroglycerin patches. Two groups were orally pre-treated (2-weeks) with antioxidants: N-acetyl-L-cysteine (NAC) or melatonin. Effects of serotonin (5-HT) and sodium nitroprusside (SNP) perfusion were tested in isolated Langendorff-perfused hearts. 3-Nitrotyrosine levels were measured in coronary sinus effluent and malondialdehyde in plasma.

RESULTS

Nitrate tolerance reduced SNP-induced dilation in both strains. This alteration was differently improved by antioxidants: melatonin was effective in SHR, whereas NAC was effective in WKY. Tolerance also reduced 5-HT-mediated vasodilation in WKY, which was reversed by both antioxidants. By contrast, nitrate tolerance enhanced the vasoconstriction to 5-HT in SHR and both antioxidants prevented this response. Furthermore, tolerance was associated with higher malondialdehyde levels in both strains and with higher 3-nitrotyrosine levels in SHR. These changes were reversed by both antioxidants.

CONCLUSIONS

A participation of oxidative stress was suggested during nitrate tolerance development, since antioxidants prevented the increase in lipid peroxidation and improved vascular responses to SNP and 5HT. Differential effects of antioxidants on SNP-induced vasodilation in SHR and WKY may suggest distinct mechanisms of tolerance development in hearts from hypertensive and normotensive rats. An increased peroxynitrite generation, expressed by higher 3-nitrotyrosine levels, could contribute to nitrate tolerance in the coronary circulation of SHR.

Synthesis and in vitro anti-HIV activity in human monocyte-derived macrophages of 2-oxothiazolidine-4(R)-carboxylic acid derivatives

Oxidative stress and glutathione (GSH) deficit may play an important role in HIV infection pathogenesis, and oral administration of GSH-replenishing drugs such as N-acetylcysteine (NAC) and 2-oxothiazolidine-4(R)-carboxylic acid (OTC) may be associated with an increased survival rate of HIV-infected patients. Nevertheless, beneficial effects of these molecules are restricted in vivo by the high concentrations that are necessary to obtain biological effects, rapid extracellular metabolization, and low availability and plasma concentrations. We synthesized OTC derivatives that are more lipophilic than OTC and theoretically able to overcome these limitations and to generate, in addition to cysteine, other substrates of the gamma-glutamyl cycle. Their antiviral effects were investigated in human HIV-1/Ba-L-infected monocyte-derived macrophages. In our experimental conditions, OTC exhibited anti-HIV-1 effects and little cytotoxicity at high doses. None of the nine tested derivatives showed higher cytotoxicity than OTC, nor anti-HIV-1/Ba-L activity.

Increased nitric oxide concentrations in posterior hypothalamus and central sympathetic function on nitrate tolerance following subcutaneous nitroglycerin

The present study was to examine the distributions of nitric oxide (NO) in the brain regions and peripheral vessels following subcutaneously administered nitroglycerin (NTG) and determine the noradrenergic activity and the role of central sympathetic function in acute nitrate tolerance. Tolerance to NTG was produced by subcutaneous (sc) administration of 4.0 mg NTG as four separate hourly pulse injections of 1.0 mg each in male (5-8 months) Sprague-Dawley rats. Rats in sham-treated group received sc injections of saline. Rats were killed by sodium pentobarbital (150 mg/kg, ip) at 10 min after last sc injection. The brain, gracilis muscle, aorta, superior mesenteric artery, coronary artery, and pulmonary vessels were quickly removed. Concentrations of nitrite (NO2-), nitrate (NO3-), and total NO2- plus NO3- (NOx-) were quantified in the micropunches of the anterior hypothalamus, the posterior hypothalamus (PH), the nucleus tractus solitarius, the lateral reticular nucleus, and the vessels in a blinded fashion. The central actions of acute tolerance to NTG were also determined using blockades of sympathetic functions in conscious rats. Four separate hourly pulse sc injections of 1.0 mg NTG produced a marked shift of the dose-response curve for arterial pressure depression induced by intravenous injection of the challenge doses of NTG. The same doses of sc NTG caused increases in NOx- [92+/-16% (mean +/- SE)] and NO3- productions (77+/-15%) in the PH, but did not significantly change in other brain regions (n = 6). NOx- and NO3- productions were significantly enhanced in the superior mesenteric artery, aorta, coronary artery, and pulmonary vessels following sc NTG, but were not altered in gracilis muscle by the treatment. The tolerance responses to arterial pressure depression were attenuated by intravenous administration of either prazosin (300 microg/kg), an alpha1-adrenoceptor antagonist, or chlorisondamine (10 mg/kg), a sympathetic ganglion blockading agent (n = 5-6). The results suggest that acute NTG tolerance predominately increases NO production in the PH. NO production was also markedly enhanced in the large and middle vessels but not in small vessels during acute NTG tolerance. The arterial pressure tolerance to NTG was reversed by blockade of central sympathetic function. We conclude that NO formation is increased in the PH following systemically administered NTG and NO in the PH may facilitate central sympathetic functions which contribute to nitrate tolerance.

New developments in nitrosovasodilator therapy

Under basal conditions, nitric oxide (NO) modulates vascular tone, serves as an antithrombotic agent, and inhibits vascular smooth muscle cell proliferation. NO deficiency has been implicated in the pathophysiology of several vascular disorders, including hypertension, atherosclerosis, and restenosis, and provides a plausible biologic basis for the use of NO replacement therapy in these conditions. Treatment with conventional nitrate preparations is limited by a short therapeutic half-life, systemic absorption with potentially adverse hemodynamic effects, and drug tolerance. To overcome these limitations, novel delivery systems and novel NO donors have been developed that offer selective effects, a prolonged half-life, and a reduced incidence of tolerance.

L-2-Oxothiazolidine-4-carboxylic acid reverses endothelial dysfunction in patients with coronary artery disease

The effective action of endothelium-derived nitric oxide (EDNO) is impaired in patients with atherosclerosis. This impairment has been attributed in part to increased vascular oxidative stress. EDNO action is improved by administration of ascorbic acid, a water-soluble antioxidant. Ascorbic acid is a potent free-radical scavenger in plasma, and also regulates intracellular redox state in part by sparing cellular glutathione. We specifically investigated the role of intracellular redox state in EDNO action by examining the effect of L-2-oxo-4-thiazolidine carboxylate (OTC) on EDNO-dependent, flow-mediated dilation in a randomized double-blind placebo-controlled study of patients with angiographically proven coronary artery disease. OTC augments intracellular glutathione by providing substrate cysteine for glutathione synthesis. Brachial artery flow-mediated dilation was examined with high-resolution ultrasound before and after oral administration of 4.5 g of OTC or placebo in 48 subjects with angiographically documented coronary artery disease. Placebo treatment produced no change in flow-mediated dilation (7.0+/-3.9% vs. 7.2+/-3.7%), whereas OTC treatment was associated with a significant improvement in flow-mediated dilation (6.6+/-4.4% vs. 11.0+/-6.3%; P = 0.005). OTC had no effect on arterial dilation to nitroglycerin, systemic blood pressure, heart rate, or reactive hyperemia. These data suggest that augmenting cellular glutathione levels improves EDNO action in human atherosclerosis. Cellular redox state may be an important regulator of EDNO action, and is a potential target for therapy in patients with coronary artery disease.

Neurodegenerative disorders in humans: the role of glutathione in oxidative stress-mediated neuronal death

Oxidative stress has been implicated in both normal aging and in various neurodegenerative disorders and may be a common mechanism underlying various forms of cell death including necrosis, apoptosis, and excitotoxicity. In this review, we develop the hypothesis that oxidative stress-mediated neuronal loss may be initiated by a decline in the antioxidant molecule glutathione (GSH). GSH plays multiple roles in the nervous system including free radical scavenger, redox modulator of ionotropic receptor activity, and possible neurotransmitter. GSH depletion can enhance oxidative stress and may also increase the levels of excitotoxic molecules; both types of action can initiate cell death in distinct neuronal populations. Evidence for a role of oxidative stress and diminished GSH status is presented for Lou Gehrig's disease (ALS), Parkinson's disease, and Alzheimer's disease. Potential links to the Guamanian variant of these diseases (ALS-PD complex) are discussed. In context to the above, we provide a GSH-depletion model of neurodegenerative disorders, suggest experimental verifications of this model, and propose potential therapeutic approaches for preventing or halting these diseases.

Smooth muscle cell expression of type I cyclic GMP-dependent protein kinase is suppressed by continuous exposure to nitrovasodilators, theophylline, cyclic GMP, and cyclic AMP

A key component of the nitric oxide-cyclic guanosine monophosphate (cGMP) pathway in smooth muscle cells (SMC) is the type I GMP-dependent protein kinase (PK-G I). Activation of PK-G I mediates the reduction of cytoplasmic calcium concentrations and vasorelaxation. In this manuscript, we demonstrate that continuous exposure of SMC in culture to the nitrovasodilators S-nitroso-N-acetylpenicillamine (SNAP) or sodium nitroprusside (SNP) results in approximately 75% suppression of PK-G I mRNA by 48 h. PK-G I mRNA and protein were also suppressed by continuous exposure to cGMP analogues 8-bromo- and 8-(4-chlorophenylthio) guanosine-3,5-monophosphate or the cAMP analogue dibutyryl cAMP. These results suggest that activation of one or both of the cyclic nucleotide-dependent protein kinases mediates PK-G I mRNA suppression. Using isoform-specific cDNA probes, only the PK-G I alpha was detected in SMC, either at baseline or after suppression, while PK-G I beta was not detected, indicating that isoform switch was not contributing to the gene regulation. Using the transcription inhibitor actinomycin D, the PK-G I mRNA half-life in bovine SMC was observed to be 5 h. The half-life was not affected by the addition of SNAP to actinomycin D, indicating no effect on PK-G I mRNA stability. Nuclear runoff studies indicated a suppression of PK-G I gene transcription by SNAP. PK-G I suppression was also observed in vivo in rats given isosorbide dinitrate in the drinking water, with a dose-dependent suppression of PK-G I protein in the aorta. PK-G I antigen in whole rat lung extract was also suppressed by administration of isosorbide or theophylline in the drinking water. These data may contribute to our understanding of nitrovasodilator resistance, a phenomenon resulting from continuous exposure to nitroglycerin or other nitrovasodilators.

Investigation of role for oxidant stress in vascular tolerance development to glyceryl trinitrate in vitro

1. The role of reactive oxygen species (ROS) during the development of vascular cellular tolerance to glyceryl trinitrate (GTN), was studied in the rat isolated aorta. 2. Nitrate tolerance induced by a 30 min incubation with GTN (30 or 100 microM) in vitro, was not affected by pretreatment with the intracellular superoxide anion scavenger, tiron (10 mM), or the intracellular scavenger of peroxynitrite anion and hydroxyl radical, dimethylsulphoxide (DMSO, 0.2% v v-1). In contrast, pretreatment with the intracellular sulphydryl donor, N-acetyl-L-cysteine (NAC, 1 mM), significantly attenuated GTN-induced tolerance. 3. Pretreatment with a putative inhibitor of oxidant stress-mediated, transcription factor NF-kappa B activation, pyrrolidine dithiocarbamate (PDTC, 50 microM), an inhibitor of gene activation by NF-kappa B, dexamethasone (1 microM) or an inhibitor of protein synthesis, cycloheximide (10 microM), failed to affect tolerance development to GTN. 4. Pretreatment with DMSO (0.2% v v-1) or PDTC (50 microM) depressed non-tolerant vasorelaxation to GTN (1 nM 1 microM) per se. 5. Tiron (10 mM) abolished the reduction of ferricytochrome c by a superoxide anion generating system, assessed photometrically in vitro. In contrast, DMSO (0.2% v v-1), NAC (1 mM) and PDTC (50 microM) were without effect. 6. Our data suggests that neither oxidant stress nor nuclear activation, is important in the development of cellular tolerance to GTN in rat isolated aortic smooth muscle.

N-acetylcysteine attenuates nitroglycerin tolerance in patients with angina pectoris and normal left ventricular function

The aim of this study was to assess whether N-acetylcysteine (NAC) is able to prevent tolerance to a 48-hour infusion of nitroglycerin (NTG) in the setting of normal left ventricular function. In 16 patients, the hemodynamic response to 0.8 mg sublingual (s.l.) NTG was assessed by measuring mean arterial, pulmonary artery, pulmonary capillary wedge and right atrial pressures, cardiac output, and calculation of the systemic and pulmonary vascular resistances. The parameters were obtained at baseline and 1 to 10 minutes after the s.l. NTG application (day 1). NTG was started at 1.5 microg/kg/min; concomitantly, a bolus of 2,000 mg of NAC was administered, followed by an infusion of 5 mg/kg/hour. Both infusions were continued for 48 hours, and the hemodynamic study was repeated (day 3). The same measurements were obtained in a matched control group of 15 patients with NTG infusion alone. Plasma renin activity, aldosterone, and norepinephrine were measured before and after the infusion period. The first s.l. NTG infusion (day 1) caused a significant decrease in mean arterial (p <0.01), pulmonary artery (p <0.001), and right atrial pressures (p <0.001), and in systemic (p <0.01) and pulmonary vascular resistances (p <0.001) in both groups. After the 48-hour infusion (day 3), there was a total loss of nitrate-mediated vasodilation (pressure values and vascular resistances day 3 > day 1) in 5 of 16 patients (NAC nonresponders), whereas in the other 11 of 16 patients (NAC responders), there was significant vasodilation throughout the infusion period. Tolerance had developed in 14 of 15 patients with NTG infusion alone. The same difference (responder vs nonresponder vs NTG alone) held true regarding the response to the second s.l. NTG infusion after 48 hours. The neurohormonal counter-regulation and intravascular volume expansion (increase in plasma renin activity, p <0.001, and norepinephrine, p <0.05; decrease in aldosterone, p <0.01) did not differ between responders and nonresponders. We conclude that NAC attenuates tolerance development to a continuous NTG infusion in a specific patient subgroup and that this occurs despite the same amount of neurohormonal counter-regulation and intravascular volume expansion compared with patients with tolerance development.

Prevention of vascular nitroglycerin tolerance by inhibition of protein kinase C

We investigated whether in vivo inhibition of protein kinase C (PKC) can prevent the development of vascular tolerance and restore the sensitivity of isolated vessels to nitroglycerin (NTG). Tolerance was induced in male Wistar rats by a constant i.v. infusion of NTG 1 mg kg-1 h-1, a dose which did not alter blood pressure. After 72 h, the aorta was removed and the sensitivity of aortic rings to NTG tested. Chronic NTG infusion resulted in a 5.5 fold decrease in NTG-sensitivity as compared with controls (vehicle), indicating the development of vascular tolerance. The simultaneous in vivo administration of the specific PKC inhibitor N-benzoyl-staurosporine (30 mg kg-1 day-1) prevented this decrease in NTG sensitivity. These results suggest a role for PKC activation in the development of vascular NTG tolerance.

Studies of vascular tolerance to nitroglycerin: effects of N-acetylcysteine, NG-monomethyl L-arginine, and endothelin-1

Development of vascular tolerance to nitroglycerin (NTG) has been attributed to sulfhydryl (SH) depletion, guanylate cyclase desensitization, or both. Controversy regarding the precise contribution of these mechanisms may be due to variations in experimental design. To examine further the biochemical basis of NTG tolerance, norepinephrine (NE)-precontracted rat aortic rings were exposed to NTG (10(-5)M), which resulted in 84 +/- 6% relaxation. Other rings were first superfused with NTG (10(-6)M) and then contracted with NE. These rings showed a marked tolerance to the vasorelaxant effects of NTG (maximal relaxation 20 +/- 5%, n = 15, p < 0.001 vs. control rings). Similar tolerance to NTG was observed when the vascular rings were first superfused with acetylcholine (ACh 10(-6)M), indicating cross-tolerance between ACh and NTG. Treatment of NTG-tolerant rings with N-acetylcysteine (NAC) (10(-5)M) did not restore vascular smooth muscle (VSM) relaxation in response to NTG (maximal relaxation 23 +/- 5%, n = 8), suggesting that SH depletion may not be the basis of NTG tolerance in these experiments. Parallel sets of NTG-tolerant aortic rings were contracted with endothelin-1 (ET-1, n = 5) or the endothelium-derived relaxing factor (EDRF) synthase inhibitor NG-monomethyl L-arginine (L-NMMA, 10(-4)M, n = 8). In both ET-1- and L-NMMA-contracted rings, vascular relaxation in response to NTG was preserved (80 +/- 6 and 88 +/- 8% relaxation, respectively). Measurement of cyclic GMP in aortic rings showed marked accumulation on initial exposure of tissues to NTG (310 +/- 10 fmol/mg), whereas the NTG-tolerant rings showed much less cyclic GMP accumulation (48 +/- 29 fmol/mg). Rings contracted with L-NMMA or ET-1, but not NE, accumulated cyclic GMP when exposed to NTG (280 +/- 20 fmol/mg). These data indicate that NTG tolerance develops on exposure of vascular rings superfused with NTG or ACh and is probably not related to tissue SH depletion. Contraction of NTG-tolerant rings with ET-1 or L-NMMA restores NTG-mediated relaxation.

Nitrates in the treatment of congestive heart failure

Nitrates have been widely used for the treatment of patients with chronic congestive heart failure. Although the use of these drugs has not been evaluated by large-scale studies traditionally used for evaluation of new therapy, multiple studies over the years have demonstrated their favorable effects. Organic nitrates have been shown to have a beneficial effect on ischemia, hemodynamic profile, magnitude of mitral regurgitation, endothelial function, and cardiac remodeling. These drugs alone or in combination with hydralazine have improved exercise capacity, maximal oxygen consumption, cardiac function, and survival. The use of nitrates in patients with heart failure has been limited by reduced responsiveness (resistance) and early development of tolerance. Nitrate resistance is due to reduced vascular response and results in the need to use a larger dose of any nitrate preparation when used for the treatment of patients with heart failure compared to patients without heart failure. Recent information suggests that nitrate tolerance is caused by increased levels of superoxide at the vascular wall, which leads to reduced nitric oxide level and to increased sensitivity to vasoconstrictive mechanisms, such as endothelin and angiotensin II. Intermittent dosing of nitrates allowing a 12-hour nitrate-free interval is effective in preventing nitrate tolerance and is, therefore, recommended. Recent information suggests that augmentation of nitrate dose by the use of an escalating dose regimen and a concomitant use of hydralazine can prevent or overcome the effect of nitrate tolerance in patients with heart failure.

New insights into mechanisms underlying nitrate tolerance

The hemodynamic and anti-ischemic efficacy of organic nitrates is rapidly blunted due to the development of nitrate tolerance. The mechanisms underlying this phenomenon remain poorly understood and likely involve several independent factors. More recent experimental observations suggest that tolerance may be the consequence of intrinsic abnormalities of the vasculature, including enhanced vascular superoxide and endothelin production. Superoxide anions degrade nitric oxide derived from nitroglycerin, whereas autocrine-produced endothelin within vascular smooth muscle sensitizes the vasculature to circulating neurohormones, such as catecholamines and angiotensin II, all of which may compromise the vasodilator potency of nitroglycerin. Interestingly, these vascular consequences of in vivo nitroglycerin treatment can be mimicked by incubating cultured endothelial and smooth muscle cells with angiotensin II. Further, nitrate tolerance and rebound following sudden cessation of prolonged nitroglycerin therapy can be prevented by concomitant treatment with high-dose angiotensin-converting enzyme inhibition or angiotensin-I receptor blockade. These data strongly suggest that increased circulating levels of angiotensin II, which are encountered during in vivo nitroglycerin treatment, initiate cellular events that ultimately attenuate the nitroglycerin vasodilator effects during prolonged treatment periods.

Baroreflex resetting but no vascular tolerance in response to transdermal glyceryl trinitrate in conscious rabbits

1. We investigated whether acute (5 h) and chronic (3 days) transdermal glyceryl trinitrate (GTN) patches could cause the development of tolerance in terms of haemodynamics and vascular reactivity in the conscious rabbit. The effects of haemodynamic tolerance were assessed on arterial pressure, heart rate and the baroreflex control of heart rate, while hindquarter vascular reactivity in response to dilator and constrictor drugs and reactive hyperaemia were used to assess vascular tolerance. 2. Seven days prior to experiments, an inflatable cuff, a pulsed Doppler flow probe and an indwelling intra-aortic catheter (for i.a. agonist infusions) were implanted around the lower abdominal aorta. 3. In acute experiments, the effects of 0-5 h treatment with transdermal GTN (0 Sham), 10 or 20 mg 24 h-1) on MAP, HR and the baroreflex were examined. Chronic experiments were performed on three separated days (days 0 - before, 4 - with GTN patch and 8 - recovery). On each day, the baroreflex, reactive hyperaemic responses and hindquarter vascular dose-response curves to i.a. GTN, adenosine, acetylcholine, S-nitroso-N-acetylpenicillamine (SNAP) and methoxamine were assessed. On days 1-4, GTN was administered transdermally via a patch(es) (10 mg 24 h-1 (low dose) or 20 mg 24 h-1 (high dose); renewed every 24 h). 4. Acute treatment with 20 mg GTN 24 h-1, but not with 0 (n = 4) or 10 mg GTN 24 h-1 (n = 4), caused a significant fall in MAP (8 +/- 1 mmHg; n = 4) and resetting of the baroreflex by 5 h. Chronic GTN caused a significant fall in MAP of 8 +/- 2 and 8 +/- 2 mmHg on day 4 with low (n = 8) and high dose (n = 8), respectively, with no change in HR. There was no significant change to hindquarter vascular reactivity to i.a. infusion of GTN, nor were there any significant differences in the reactivity to i.a. adenosine, acetylcholine, SNAP or methoxamine with either low or high doses of GTN. 5. Chronic GTN treatment with low and high dose patches caused a parallel leftward shift ('resetting') of the baroreflex on day 4. By day 8, the baroreflex had still not recovered from this leftward shift 6. In the rabbit, chronic exposure to clinical nitrate patches caused haemodynamic compensation and baroreflex resetting but no evidence of vascular reactivity tolerance. Novel NO donor drugs and delivery regimens which provide intermittent dosing may prevent the development of haemodynamic resetting rather then preventing vascular tolerance, a commonly perceived difficulty in chronic nitrate therapy.

Characterization of nitroglycerine-induced relaxation in human corpus cavernosum smooth muscle: implications to erectile physiology and dysfunction

The importance of the nitric oxide--guanylate cyclase--cGMP system in modulating corporal smooth muscle tone and penile erection has been amply demonstrated. The goal of these studies was to evaluate the possibility that age- or disease-related alterations in human corporal smooth muscle responsivity to activation of this pathway might play a role in the etiology of erectile dysfunction. Thus, we utilized a previously described heuristic model to assess the kinetic and steady-state characteristics of relaxation of precontracted isolated corporal tissue strips elicited by nitroglycerine (NTG). Studies were conducted on corporal tissue strips excised from 26 patients with organic erectile dysfunction, and 7 patients with documented erections. For the purposes of statistical analysis the impotent patient population was stratified into two age groups (A, < or = 59 years; B, > or = 60 years) and further subdivided into two diagnostic categories, diabetic and nondiabetic patients, respectively. In approximately 75% of precontracted corporal tissue strips derived from impotent patients (contracted to approximately 75% of maximum with phenylephrine), the NTG-induced response was biphasic, consisting of a rapid relaxation response that reached steady state before onset of a more slowly developing regaining of tension, termed the desensitization response. In contrast, a biphasic response was observed much less frequently (approximately 30%) in corporal tissue strips derived from a potent patient population (p < 0.0001). Statistical analysis revealed significant heterogeneity among corporal tissue strips derived from patients with organic erectile dysfunction, with respect to both the kinetic and steady-state characteristics of the NTG-induced relaxation and desensitization responses. In particular, the maximal rate constant for both NTG-induced relaxation (krelmax; p < 0.01) and desensitization (kdes; p < 0.03) responses was significantly greater in corporal tissue strips excised from diabetic than nondiabetic patients. Furthermore, the EC50 for NTG-induced relaxation of precontracted corporal smooth muscle strips from potent patients (approximately 25 nM) was 0.90 log unit less than that for equivalently contracted corporal smooth muscle strips derived from impotent patients (approximately 180 nM; p < 0.03). Such observations suggest that alterations in corporal smooth muscle responsivity to activation of the guanylate cyclase--cGMP pathway, per se, may be a characteristic of organic erectile dysfunction. In the absence of compensatory changes in other vasodilatory mechanisms, this may contribute to incomplete corporal smooth muscle relaxation and the etiology of erectile dysfunction in some patients.

Tolerance to nitroglycerin in vascular smooth muscle cells is not affected by the level of intracellular glutathione or L-cysteine

A major hypothesis for the mechanism of tolerance to nitroglycerin (NTG) is that continued use causes a decrease in thiol donors within the vascular smooth muscle cell that are essential for the effect of NTG. We tested this idea directly in the target cell. NTG tolerance, measured as reduced formation of intracellular cyclic guanosine monophosphate (cGMP), was induced in pig coronary smooth muscle cells. The consequence of altering intracellular levels of the thiol donors, glutathione (GSH) and L-cysteine (L-cys), was determined. Incubating cells with 100 microM NTG for 1 h caused an 83% reduction in cGMP formation in response to acute readministration of 200 microM NTG for 2 min but was not associated with a reduction in intracellular GSH or L-cys. This result was not altered when intracellular GSH levels were increased three-fold by including 1 mM GSH in the incubation buffer. Also, recovery from tolerance was not affected by supplementation with GSH. Further, the response of cGMP to NTG was not altered by inhibiting the synthesis of GSH and lowering intracellular levels of GSH by 77%. Similar findings were made with supplemental L-cys or N-acetyl-L-cysteine. These results do not support the hypothesis that tolerance to NTG is the result of a reduction of the thiol donors GSH and L-cys within vascular smooth muscle cells.

Absence of vascular tolerance in conductance vessels after 48 hours of intravenous nitroglycerin in patients with coronary artery disease

OBJECTIVES

We examined whether reflex neurohormonal constrictor forces attenuate the vasodilator action of nitroglycerin on large peripheral conductance vessels.

BACKGROUND

Continuous nitroglycerin therapy is associated with the development of early tolerance with respect to its hemodynamic effects. It remains to be demonstrated whether vascular tolerance of large conductance vessels is an important contributory factor.

METHODS

Radial artery diameter and forearm blood flow velocity were measured before and 24 and 48 h after continuous intravenous nitroglycerin infusion (0.5 microgram/kg body weight per min) in 10 patients with coronary artery disease (mean age +/- SEM 59 +/- 4 years) by using a high resolution ultrasound device. Blood flow (ml/min) was calculated from mean blood flow velocity and cross-sectional area.

RESULTS

Increasing concentrations of nitroglycerin led to a dose-dependent increase in radial artery diameter (maximal +24 +/- 2%) and heart rate. Forearm vascular resistance and forearm blood flow were unchanged. After 24 and 48 h of treatment, additional nitroglycerin did not further increase radial artery diameter, indicating that the nitroglycerin-induced dilation of the radial artery was maintained and was still maximal. In addition, radial artery diameter measured before and after 48 h of nitroglycerin infusion and after withdrawal of nitroglycerin in five additional patients showed that, after withdrawal, arterial diameter returned to baseline values within 35 min. Plasma renin activity and serum aldosterone and vasopressin levels increased significantly at 24 and 48 h, accompanied by a decrease in hematocrit.

CONCLUSIONS

Continuous intravenous administration of nitroglycerin exerts a sustained vasodilator effect for 48 h in large conductance vessels. Neurohormonal activation and compensatory intravascular volume expansion do not attenuate the vasodilator effects of nitroglycerin on peripheral conductance vessels during the 1st 48 h of treatment.