The effect of oxidative stress on endothelium-dependent and nitric oxide donor-induced relaxation: implications for nitrate tolerance

Physiological characteristics associated with increased resistance to decompression sickness in male and female rats

Decompression sickness (DCS) is a complex and poorly understood systemic disease with wide interindividual resistance variability. We selectively bred rats with a threefold greater resistance to DCS than standard ones. To investigate possible physiological mechanisms underlying the resistance to DCS, including sex-related differences in these mechanisms, 15 males and 15 females resistant to DCS were compared with aged-matched standard Wistar males (n = 15) and females (n = 15). None of these individuals had been previously exposed to hyperbaric treatment. Comparison of the allelic frequencies of single nucleotide polymorphisms (SNPs) showed a difference of one SNP located on the X chromosome. Compared with nonresistant rats, the neutrophil-to-lymphocyte ratio and the plasmatic activity of coagulation factor X were significantly higher in DCS-resistant individuals regardless of their sex. The maximal relaxation elicited by sodium nitroprusside was lower in DCS-resistant individuals regardless of their sex. Males but not females resistant to DCS exhibited higher neutrophil and lymphocyte counts and higher prothrombin time but lower mitochondrial basal O₂ consumption and citrate synthase activity. Principal components analysis showed that two principal components discriminate the DCS-resistant males but not females from the nonresistant ones. These components were loaded with activated partial thromboplastin time, monocyte-to-lymphocyte ratio, prothrombin time, factor X, and fibrinogen for PC1 and red blood cells count and neutrophils count for PC2. In conclusion, the mechanisms that drive the resistance to DCS appear different between males and females; lower coagulation tendency and enhanced inflammatory response to decompression stress might be key for resistance in males. The involvement of these physiological adaptations in resistance to DCS must now be confirmed.NEW & NOTEWORTHY By selective breeding of individuals resistant to decompression sickness (DCS) we previously obtained a rat model of inherited resistance to this pathology. Comparison of these individuals with nonresistant animals revealed differences in leukocyte counts, coagulation, and mitochondrial and vascular functions, but not resistance to oxidative stress. This study also reveals sex-related differences in the physiological changes associated with DCS resistance. A principal components analysis of our data allowed us to discriminate DCS-resistant males from standard ones, but not females. These differences represent possible mechanisms driving resistance to DCS. Although still far from the diver, this opens a pathway to future adaptation of personalized decompression procedures for "DCS-prone" individuals.

Nitric oxide generating/releasing materials

Harnessing the impressive therapeutic potential of nitric oxide (NO) remains an ongoing challenge. This paper describes several of the current strategies both with respect to the underlying chemistry and physics and to the applications where they have shown promise. Included in this overview are molecular systems such as NONOates that release NO through chemical reactions and delivery vehicles such as nanoparticles that can generate, store, transport and deliver NO and related bioactive forms of NO such as nitrosothiols. Although there has been much positive movement, it is clear that we are only at the early stages of knowing how to precisely produce, transport and deliver to targeted sites therapeutic levels of NO and related molecules.

Different influences of extracellular and intracellular superoxide on relaxation through the NO/sGC/cGMP pathway in isolated rat iliac arteries

Superoxide production is increased in diseased blood vessels, which is considered to lead to impairment of the nitric oxide (NO)/soluble guanylate cyclase (sGC)/cGMP pathway. To investigate the respective influence of extracellular and intracellular superoxide on vascular function through the NO/sGC/cGMP pathway, mechanical responses of rat external iliac arteries without endothelium were studied under exposure to a superoxide-generating agent, pyrogallol, or menadione. Exposure to pyrogallol impaired the relaxation induced by acidified NaNO2 (exogenous NO) but not that by nitroglycerin (organic nitrate), BAY 41-2272 (sGC stimulator), BAY 60-2770 (sGC activator), or 8-Br-cGMP (cGMP analog). Superoxide dismutase (SOD) and tempol restored the impaired relaxation by acidified NaNO2. Superoxide production in the bathing solution, but not in artery segments, was significantly increased by exposure to pyrogallol, which was abolished in the presence of SOD or tempol. However, exposure to menadione impaired the relaxant response to acidified NaNO2, nitroglycerin, or BAY 41-2272, whereas it augmented that to BAY 60-2770. Also, this exposure had no effect on the 8-Br-cGMP-induced vasorelxation. Superoxide production in artery segments was dramatically enhanced by exposure to menadione, whereas that in the bathing solution was not affected. This increase in vascular superoxide production was normalized by tempol but not by SOD. These findings suggest that extracellular superoxide reacts with NO only outside the cell, whereas intracellular superoxide not only scavenges NO inside the cell but also shifts the sGC redox equilibrium.

Tolerance to nitroglycerin through proteasomal down-regulation of aldehyde dehydrogenase-2 in a genetic mouse model of ascorbate deficiency

Background and Purpose

L-gulonolactone oxidase-deficient (Gulo^(-/-)) mice were used to study the effects of ascorbate deficiency on aortic relaxation by nitroglycerin (GTN) with focus on changes in the expression and activity of vascular aldehyde dehydrogenase-2 (ALDH2), which catalyses GTN bioactivation.

Experimental Approach

Ascorbate deficiency was induced in Gulo^(-/-) mice by ascorbate deprivation for 4 weeks. Some of the animals were concomitantly treated with the proteasome inhibitor bortezomib and effects compared with ascorbate-supplemented Gulo^(-/-), untreated or nitrate-tolerant wild-type mice. Aortic relaxation of the experimental groups to GTN, ACh and a NO donor was studied. Changes in mRNA and protein expression of vascular ALDH2 were quantified by qPCR and immunoblotting, respectively, and aortic GTN denitration rates determined.

Key Results

Like GTN treatment, ascorbate deprivation induced vascular tolerance to GTN that was associated with markedly decreased rates of GTN denitration. Ascorbate deficiency did not affect ALDH2 mRNA levels, but reduced ALDH2 protein expression and the total amount of ubiquitinated proteins to about 40% of wild-type controls. These effects were largely prevented by ascorbate supplementation or treating Gulo^(-/-) mice with the 26S proteasome inhibitor bortezomib.

Conclusions and Implications

Our data indicate that ascorbate deficiency results in vascular tolerance to GTN via proteasomal degradation of ALDH2. The results support the view that impaired ALDH2-catalysed metabolism of GTN contributes significantly to the development of vascular nitrate tolerance and reveal a hitherto unrecognized protective effect of ascorbate in the vasculature.

Endothelial γ-glutamyltransferase contributes to the vasorelaxant effect of S-nitrosoglutathione in rat aorta

S-nitrosoglutathione (GSNO) involved in storage and transport of nitric oxide (^•NO) plays an important role in vascular homeostasis. Breakdown of GSNO can be catalyzed by γ-glutamyltransferase (GGT). We investigated whether vascular GGT influences the vasorelaxant effect of GSNO in isolated rat aorta. Histochemical localization of GGT and measurement of its activity were performed by using chromogenic substrates in sections and in aorta homogenates, respectively. The role of GGT in GSNO metabolism was evaluated by measuring GSNO consumption rate (absorbance decay at 334 nm), ^•NO release was visualized and quantified with the fluorescent probe 4,5-diaminofluorescein diacetate. The vasorelaxant effect of GSNO was assayed using isolated rat aortic rings (in the presence or absence of endothelium). The role of GGT was assessed by stimulating enzyme activity with cosubstrate glycylglycine, as well as using two independent inhibitors, competitive serine borate complex and non-competitive acivicin. Specific GGT activity was histochemically localized in the endothelium. Consumption of GSNO and release of free ^•NO decreased and increased in presence of serine borate complex and glycylglycine, respectively. In vasorelaxation experiments with endothelium-intact aorta, the half maximal effective concentration of GSNO (EC50 = 3.2±0.5.10⁻⁷ M) increased in the presence of the two distinct GGT inhibitors, serine borate complex (1.6±0.2.10⁻⁶ M) and acivicin (8.3±0.6.10⁻⁷ M), while it decreased with glycylglycine (4.7±0.9.10⁻⁸ M). In endothelium-denuded aorta, EC₅₀ for GSNO alone increased to 2.3±0.3.10⁻⁶ M, with no change in the presence of serine borate complex. These data demonstrate the important role of endothelial GGT activity in mediating the vasorelaxant effect of GSNO in rat aorta under physiological conditions. Because therapeutic treatments based on GSNO are presently under development, this endothelium-dependent mechanism involved in the vascular effects of GSNO should be taken into account in a pharmacological perspective.

Evidence for a relationship between mitochondrial Complex I activity and mitochondrial aldehyde dehydrogenase during nitroglycerin tolerance: effects of mitochondrial antioxidants

The medical use of nitroglycerin (GTN) is limited by patient tolerance. The present study evaluated the role of mitochondrial Complex I in GTN biotransformation and the therapeutic effect of mitochondrial antioxidants. The development of GTN tolerance (in rat and human vessels) produced a decrease in mitochondrial O(2) consumption. Co-incubation with the mitochondria-targeted antioxidant mitoquinone (MQ, 10(-6)mol/L) or with glutathione ester (GEE, 10(-4)mol/L) blocked GTN tolerance and the effects of GTN on mitochondrial respiration and aldehyde dehydrogenase 2 (ALDH-2) activity. Biotransformation of GTN depended on the mitochondria being functionally active, particularly mitochondrial Complex I. Tolerance induced mitochondrial ROS production and oxidative stress, though these effects were not detected in HUVECρ(0) cells or Complex I mutant cells. Experiments performed to evaluate Complex I-dependent respiration demonstrated that its inhibition by GTN was prevented by the antioxidants in control samples. These results point to a key role for mitochondrial Complex I in the adequate functioning of ALDH-2. In addition, we have identified mitochondrial Complex I as one of the targets at which the initial oxidative stress responsible for GTN tolerance takes place. Our data also suggest a role for mitochondrial-antioxidants as therapeutic tools in the control of the tolerance that accompanies chronic nitrate use.

Polymer-Based Nitric Oxide Therapies: Recent Insights for Biomedical Applications

Since the discovery of nitric oxide (NO) in the 1980s, this cellular messenger has been shown to participate in diverse biological processes such as cardiovascular homeostasis, immune response, wound healing, bone metabolism, and neurotransmission. Its beneficial effects have prompted increased research in the past two decades, with a focus on the development of materials that can locally release NO. However, significant limitations arise when applying these materials to biomedical applications. This Feature Article focuses on the development of NO-releasing and NO-generating polymeric materials (2006-2011) with emphasis on recent in vivo applications. Results are compared and discussed in terms of NO dose, release kinetics, and biological effects, in order to provide a foundation to design and evaluate new NO therapies.

New biomaterials for the sustained release of nitric oxide: past, present and future

Nitric oxide (NO), the 1992 'Molecule of the Year', is the focus of immense medical and scientific exploration. Interest in NO has grown exponentially since the initial and relatively recent discovery that NO is the long sought after endothelial relaxing factor. There is intense research that is continuing to expose the extensive physiologic impact of NO in virtually all organ and tissue systems under both normal and pathological conditions. Both the rate of delivery and the amount of site-specific generated NO modulate a balance between cytoregulatory and cytotoxic activities. This balancing act and the very short lifetime of NO under physiological conditions pose an extreme challenge with respect to harnessing the exceptional therapeutic potential of this molecule. Over the past two decades, the race to translate the therapeutic potential of NO to the bedside has been overwhelmingly through the development of numerous NO delivery devices/vehicles. So far no one product has emerged as a clearcut winner. This review: discusses and evaluates NO-donating platforms that are available at present; attempts to enhance delivery and efficacy through encapsulation in silane-based hydrogel matrices; and discusses and evaluates the future direction of these advances.

Nitroglycerin headache and nitroglycerin-induced primary headaches from 1846 and onwards: a historical overview and an update

Nitroglycerin (NTG) (glyceryl trinitrate) was synthesized by the Italian chemist Ascanio Sobrero in Paris in 1846. A very unstable explosive, Alfred Nobel while working on explosives, combined it with Kiselguhr and patented it as dynamite in 1867. NTG was introduced in 1879 in medicine in the treatment of angina pectoris by the English doctor William Murrell. NTG-induced headache was quickly recognized as an important adverse event both in the industrial use of NTG, where it was used to produce dynamite, as well as in the use of NTG as drug. This review traces the evolution of our understanding of NTG headache.

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.

Vascular tolerance to nitroglycerin in ascorbate deficiency

AIMS

Nitroglycerin (GTN) acts through release of a nitric oxide (NO)-related activator of soluble guanylate cyclase in vascular smooth muscle. Besides enzymatic GTN bioactivation catalysed by aldehyde dehydrogenase, non-enzymatic reaction of GTN with ascorbate also results in the formation of a bioactive product. Using an established guinea pig model of ascorbate deficiency, we investigated whether endogenous ascorbate contributes to GTN-induced vasodilation.

METHODS AND RESULTS

Guinea pigs were fed either standard or ascorbate-free diet for 2 or 4 weeks prior to measuring the GTN response of aortic rings and isolated hearts. The effects of ascorbate on GTN metabolism were studied with purified mitochondrial aldehyde dehydrogenase (ALDH2) and isolated mitochondria. Ascorbate deprivation led to severe scorbutic symptoms and loss of body weight, but had no (2 weeks) or only slight (4 weeks) effects on aortic relaxations to a direct NO donor. The EC(50) of GTN was increased from 0.058 +/- 0.018 to 0.46 +/- 0.066 and 5.5 +/- 0.9 microM after 2 and 4 weeks of ascorbate-free diet, respectively. Similarly, coronary vasodilation to GTN was severely impaired in ascorbate deficiency. The potency of GTN was reduced to a similar extent by ALDH inhibitors in control and ascorbate-deficient blood vessels. Up to 10 mM ascorbate had no effect on GTN metabolism catalysed by purified ALDH2 or liver mitochondria isolated from ascorbate-deficient guinea pigs.

CONCLUSION

Our results indicate that prolonged ascorbate deficiency causes tolerance to GTN without affecting NO/cyclic GMP-mediated vasorelaxation.

Recent developments in nitric oxide donor drugs

During the 1980s, the free radical, nitric oxide (NO), was discovered to be a crucial signalling molecule, with wide-ranging functions in the cardiovascular, nervous and immune systems. Aside from providing a credible explanation for the actions of organic nitrates and sodium nitroprusside that have long been used in the treatment of angina and hypertensive crises respectively, the discovery generated great hopes for new NO-based treatments for a wide variety of ailments. Decades later, however, we are still awaiting novel licensed agents in this arena, despite an enormous research effort to this end. This review explores some of the most promising recent advances in NO donor drug development and addresses the challenges associated with NO as a therapeutic agent.

Effect of anger provocation on endothelium-dependent and -independent vasodilation

Anger is an independent predictor of coronary heart disease events, although the mechanisms for this relation are unclear. The effects of an anger-provoking interview compared with a neutral interview on endothelium-dependent and -independent vasodilation assessed by brachial artery ultrasound were examined in 14 healthy subjects without coronary heart disease risk factors. The anger provocation condition, but not the neutral condition, caused a significant impairment in endothelium-dependent vasodilation at 90 minutes compared with baseline (p = 0.004) and 30 minutes (p = 0.013). Similarly, endothelium-independent vasodilation was significantly impaired at 90 minutes after the angry interview compared with baseline (p = 0.003) and 30 minutes (p = 0.001). The decreases in endothelium-dependent and -independent vasodilation were greater after the anger-provoking interview than after the neutral interview, especially between 30 and 90 minutes. In conclusion, preliminary results suggest that an episode of anger is associated with a dysregulation in endothelium-dependent and -independent pathways, suggesting that these mechanisms might contribute to the link between anger and coronary heart disease events.

Nitroxyl anion donor, Angeli's salt, does not develop tolerance in rat isolated aortae

The nitroxyl anion (HNO) is emerging as a novel regulator of cardiovascular function with therapeutic potential in the treatment of diseases such as heart failure. It remains unknown whether tolerance develops to HNO donors, a limitation of currently used nitrovasodilators. The susceptibility of the HNO donor, Angeli's salt (AS), to the development of vascular tolerance was compared with the NO donors, glyceryl trinitrate (GTN) and diethylamine/NONOate (DEA/NO) in rat isolated aortae. Vasorelaxation to AS was attenuated (P<0.01) by the HNO scavenger l-cysteine, whereas the sensitivity to GTN and DEA/NO was decreased (P<0.01) by the NO. scavenger carboxy-[2-(4-carboxyphenyl)-4,4,5,5-tetramethyl-imidozoline-1-oxy-3-oxide]. The soluble guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxaline-1-one impaired responses to GTN>or=AS>>DEA/NO. Pretreatment with 10, 30, and 100 micromol/L of GTN for 60 minutes induced a 4- (P<0.05), 13- (P<0.01), and 48-fold (P<0.01) decrease in sensitivity to GTN, demonstrating tolerance development. In contrast, pretreatment with AS or DEA/NO (10, 30, and 100 micromol/L) did not alter their subsequent vasorelaxation. All of the nitrovasodilators (30 micromol/L) displayed a similar time course of vasorelaxation and cGMP accumulation over a 60-minute period. Unlike vasorelaxation, the magnitude of peak cGMP accumulation differed substantially: DEA/NO>>AS>GTN. GTN did not induce cross-tolerance to either AS or DEA/NO. In contrast, pre-exposure to DEA/NO, but not AS, caused a concentration-dependent attenuation (P<0.01) of GTN-mediated relaxation, which was negated by the protein kinase G inhibitor guanosine 3',5'-cyclic monophosphorothioate, 8-(4-chlorophenylthio)-,Rp-isomer, triethylammonium salt. In conclusion, vascular tolerance does not develop to HNO, nor does cross-tolerance between HNO and GTN occur. Thus, HNO donors may have therapeutic advantages over traditional nitrovasodilators.

Complex I dysfunction and tolerance to nitroglycerin: an approach based on mitochondrial-targeted antioxidants

Nitroglycerin (GTN) tolerance was induced in vivo (rats) and in vitro (rat and human vessels). Electrochemical detection revealed that the incubation dose of GTN (5x10(-6) mol/L) did not release NO or modify O(2) consumption when administered acutely. However, development of tolerance produced a decrease in both mitochondrial O(2) consumption and the K(m) for O(2) in animal and human vessels and endothelial cells in a noncompetitive action. GTN tolerance has been associated with impairment of GTN biotransformation through inhibition of aldehyde dehydrogenase (ALDH)-2, and with uncoupling of mitochondrial respiration. Feeding rats with mitochondrial-targeted antioxidants (mitoquinone [MQ]) and in vitro coincubation with MQ (10(-6) mol/L) or glutathione (GSH) ester (10(-4) mol/L) prevented tolerance and the effects of GTN on mitochondrial respiration and ALDH-2 activity. Biotransformation of GTN requires functionally active mitochondria and induces reactive oxygen species production and oxidative stress within this organelle, as it is inhibited by mitochondrial-targeted antioxidants and is absent in HUVECrho(0) cells. Experiments analyzing complex I-dependent respiration demonstrate that its inhibition by GTN is prevented by mitochondrial-targeted antioxidants. Furthermore, in presence of succinate (10x10(-3) mol/L), a complex II electron donor added to bypass complex I-dependent respiration, GTN-treated cells exhibited O(2) consumption rates similar to those of controls, thus suggesting that complex I was affected by GTN. We propose that, following prolonged treatment with GTN in addition to ALDH-2, complex I is a target for mitochondrially generated reactive oxygen species. Our data also suggest a role for mitochondrial-targeted antioxidants as therapeutic tools in the control of the tolerance that accompanies chronic nitrate use.

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

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

Aldehyde dehydrogenase, nitric oxide synthase and superoxide in ex vivo nitrate tolerance in rat aorta

The role of aldehyde dehydrogenase (ALDH) in ex vivo tolerance to transdermal glyceryl trinitrate was explored in rat aorta. ALDH activity, measured by aldehyde-induced NADH formation, was strongly depressed in the tolerant arteries. ALDH inhibitors, chloral hydrate (0.3 mM) and cyanamide (0.1-1 mM) inhibited relaxation to glyceryl trinitrate in non-tolerant and tolerant arteries. The inhibition differed from tolerance in that (a) the glyceryl trinitrate concentration-response curve was sigmoidal cf. biphasic in tolerance, (b) the potentiating effect of nitric oxide synthase (eNOS) inhibition was unchanged cf. increased in tolerance and (c) superoxide inhibited the response cf. no significant effect in tolerant or non-tolerant arteries. Hence, reduced ALDH activity does not account fully for ex vivo tolerance. The discrepancies are consistent with evidence that (a) organic nitrates, unlike chloral and cyanamide, irreversibly inactivate ALDH (hence reduced enzyme saturability can explain the biphasic curve) and (b) eNOS contributes to tolerance by a mechanism independent of glyceryl trinitrate metabolism.

Vasomotor responses in MnSOD-deficient mice

MnSOD is the only mammalian isoform of SOD that is necessary for life. MnSOD(-/-) mice die soon after birth, and MnSOD(+/-) mice are more susceptible to oxidative stress than wild-type (WT) mice. In this study, we examined vasomotor function responses in aortas of MnSOD(+/-) mice under normal conditions and during oxidative stress. Under normal conditions, contractions to serotonin (5-HT) and prostaglandin F2alpha (PGF2alpha), relaxation to ACh, and superoxide levels were similar in aortas of WT and MnSOD(+/-) mice. The mitochondrial inhibitor antimycin A reduced contraction to PGF2alpha and impaired relaxation to ACh to a similar extent in aortas of WT and MnSOD(+/-) mice. The Cu/ZnSOD and extracellular SOD inhibitor diethyldithiocarbamate (DDC) paradoxically enhanced contraction to 5-HT and superoxide more in aortas of WT mice than in MnSOD(+/-) mice. DDC impaired relaxation to ACh and reduced total SOD activity similarly in aortas of both genotypes. Tiron, a scavenger of superoxide, normalized contraction to 5-HT, relaxation to ACh, and superoxide levels in DDC-treated aortas of WT and MnSOD(+/-) mice. Hypoxia, which reportedly increases superoxide, reduced contractions to 5-HT and PGF2alpha similarly in aortas of WT and MnSOD(+/-) mice. The vasomotor response to acute hypoxia was similar in both genotypes. In summary, under normal conditions and during acute oxidative stress, vasomotor function is similar in WT and MnSOD(+/-) mice. We speculate that decreased mitochondrial superoxide production may preserve nitric oxide bioavailability during oxidative stress.

Extracellular nitric oxide release mediates soluble guanylate cyclase-independent vasodilator action of spermine NONOate: comparison with other nitric oxide donors in isolated rat femoral arteries

Nitric oxide (NO) and NO donors exhibit actions that are not entirely mediated by soluble guanylate cyclase (sGC). The site of NO release may influence the involvement of sGC-independent effects. Here we use spermine NONOate (SPER/NO) to release NO extracellularly, compared with other NO donors. Isolated rat femoral arteries were perfused luminally and perfusion pressure monitored. Vessels were contracted with phenylephrine (2-14 microM) in the presence of an NO synthase inhibitor (N(omega)-nitro-L-arginine methyl ester; 20 microM). Vasodilator responses to NO donors were assessed before and after perfusion of an sGC inhibitor (1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one; ODQ; 20 microM), NO scavengers (hemoglobin; Hb & hydroquinone; HQ), and a superoxide generator (duroquinone; DQ). ODQ (20 microM) abolished the vasodilator responses to glyceryl trinitrate (10(-8) - 10(-3) M), and sodium nitroprusside (10(-8) - 10(-4) M), which release NO intracellularly. ODQ (20 microM) attenuated, but failed to abolish, the vasodilator responses to SPER/NO (10(-6) - 10(-3) M). ODQ abolished responses to S-nitrosoglutathione and S-nitroso-N-valeryl-D-penicillamine (10(-8) - 10(-4) M), but a small residual vasodilatation remained in response to 10(-3) M. In the presence of ODQ, the remaining vasodilatation to SPER/NO was all but abolished by scavengers of extracellular NO (Hb; 10 microM, HQ; 100 microM). Superoxide generation (DQ; 100 microM) also attenuated ODQ-resistant vasodilatation. The data suggest that, in rat femoral arteries, NO donors that are capable of releasing extracellular NO cause vasodilatation that is only partially mediated by sGC. Lack of augmentation of sGC-independent effects by superoxide suggests that they are not mediated by peroxynitrite.

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

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

Interaction of regular exercise and chronic nitroglycerin treatment on blood pressure and rat aortic antioxidants

Many cardiac patients undergo exercise conditioning with or without medication. Therefore, we investigated the interaction of exercise training and chronic nitroglycerin treatment on blood pressure (BP), aortic nitric oxide (NO), oxidants and antioxidants in rats. Fisher 344 rats were divided into four groups and treated as follows: (1) sedentary control, (2) exercise training (ET) for 8 weeks, (3) nitroglycerin (15 mg/kg, s.c. for 8 weeks) and (4) ET+nitroglycerin. BP was monitored with tail-cuff method. The animals were sacrificed 24 h after the last treatments and thoracic aorta was isolated and analyzed. Exercise training on treadmill for 8 weeks significantly increased respiratory exchange ratio (RER), aortic NO levels, and endothelial nitric oxide synthase (eNOS) protein expression. Training significantly enhanced aortic glutathione (GSH), reduced to oxidized glutathione (GSH/GSSG) ratio, copper/zinc-superoxide dismutase (CuZn-SOD), Mn-SOD, catalase (CAT), glutathione peroxidase (GSH-Px) glutathione disulfide reductase (GR) activities and protein expressions. Training significantly depleted aortic malondialdehyde (MDA) and protein carbonyls without change in BP. Nitroglycerin administration for 8 weeks significantly increased aortic NO levels and eNOS protein expression. Nitroglycerin significantly enhanced aortic Mn-SOD, CAT, GR and glutathione-S-transferase (GST) activities and protein expressions with decreased MDA levels, protein carbonyls and BP. Interaction of training and nitroglycerin treatment significantly increased aortic NO levels, eNOS protein expression, GSH/GSSG ratio, antioxidant enzymes and normalized BP. The data suggest that the interaction of training and nitroglycerin maintained BP by up-regulating the aortic NO and antioxidants and reducing the oxidative stress in rats.