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

The Therapeutic Potential of Angeli's Salt in Mitigating Acute Trypanosoma cruzi Infection in Mice

Chagas disease (CD), caused by Trypanosoma cruzi, is a neglected tropical disease prevalent in Latin America. Infected patients are treated to eliminate the parasite, reduce the cardiomyopathy risk, and interrupt the disease transmission cycle. The World Health Organization recognizes benznidazole (BZ) and nifurtimox as effective drugs for CD treatment. In the chronic phase, both drugs have low cure rates and serious side effects. T. cruzi infection causes intense tissue inflammation that controls parasite proliferation and CD evolution. Compounds that liberate nitric oxide (NO) (NO donors) have been used as anti-T. cruzi therapeutics. Currently, there is no evidence that nitroxyl (HNO) affects T. cruzi infection outcomes. This study investigated the effects of the HNO donor Angeli's salt (AS) on C57BL/6 mice infected with T. cruzi (Y strain, 5 × 103 trypomastigotes, intraperitoneally). AS reduced the number of parasites in the bloodstream and heart nests and increased the protective antioxidant capacity of erythrocytes in infected animals, reducing disease severity. Furthermore, in vitro experiments showed that AS treatment reduced parasite uptake and trypomastigote release by macrophages. Taken together, these findings from the murine model and in vitro testing suggest that AS could be a promising therapy for CD.

Vasoprotective Actions of Nitroxyl (HNO): A Story of Sibling Rivalry

ABSTRACT

Nitroxyl (HNO), the 1 electron-reduced and protonated form of nitric oxide (NO•), has emerged as a nitrogen oxide with a suite of vasoprotective properties and therapeutic advantages over its redox sibling. Although HNO has garnered much attention due to its cardioprotective actions in heart failure, its ability to modulate vascular function, without the limitations of tolerance development and NO• resistance, is desirable in the treatment of vascular disease. HNO serves as a potent vasodilator and antiaggregatory agent and has an ability to limit vascular inflammation and reactive oxygen species generation. In addition, its resistance to scavenging by reactive oxygen species and ability to target distinct vascular signaling pathways (Kv, KATP, and calcitonin gene-related peptide) contribute to its preserved efficacy in hypertension, diabetes, and hypercholesterolemia. In this review, the vasoprotective actions of HNO will be compared with those of NO•, and the therapeutic utility of HNO donors in the treatment of angina, acute cardiovascular emergencies, and chronic vascular disease are discussed.

Haemodynamic effects of the nitroxyl donor cimlanod (BMS-986231) in chronic heart failure: a randomized trial

AIMS

Nitroxyl provokes vasodilatation and inotropic and lusitropic effects in animals via post-translational modification of thiols. We aimed to compare effects of the nitroxyl donor cimlanod (BMS-986231) with those of nitroglycerin (NTG) or placebo on cardiac function in patients with chronic heart failure with reduced ejection fraction (HFrEF).

METHODS AND RESULTS

In a randomized, multicentre, double-blind, crossover trial, 45 patients with stable HFrEF were given a 5 h intravenous infusion of cimlanod, NTG, or placebo on separate days. Echocardiograms were done at the start and end of each infusion period and read in a core laboratory. The primary endpoint was stroke volume index derived from the left ventricular outflow tract at the end of each infusion period. Stroke volume index with placebo was 30 ± 7 mL/m² and was lower with cimlanod (29 ± 9 mL/m² ; P = 0.03) and NTG (28 ± 8 mL/m² ; P = 0.02). Transmitral E-wave Doppler velocity on cimlanod or NTG was lower than on placebo and, consequently, E/e' (P = 0.006) and E/A ratio (P = 0.003) were also lower. NTG had similar effects to cimlanod on these measurements. Blood pressure reduction was similar with cimlanod and NTG and greater than with placebo.

CONCLUSION

In patients with chronic HFrEF, the haemodynamic effects of cimlanod and NTG are similar. The effects of cimlanod may be explained by venodilatation and preload reduction without additional inotropic or lusitropic effects. Ongoing trials of cimlanod will further define its potential role in the treatment of heart failure.

Diabetes Attenuates the Contribution of Endogenous Nitric Oxide but Not Nitroxyl to Endothelium Dependent Relaxation of Rat Carotid Arteries

Endothelial dysfunction is a major risk factor for several of the vascular complications of diabetes, including ischemic stroke. Nitroxyl (HNO), the one electron reduced and protonated form of nitric oxide (NO•), is resistant to scavenging by superoxide, but the role of HNO in diabetes mellitus associated endothelial dysfunction in the carotid artery remains unknown. Aim: To assess how diabetes affects the role of endogenous NO• and HNO in endothelium-dependent relaxation in rat isolated carotid arteries. Methods: Male Sprague Dawley rats were fed a high-fat-diet (HFD) for 2 weeks prior to administration of low dose streptozotocin (STZ; 35 mg/kg i. p./day) for 2 days. The HFD was continued for a further 12 weeks. Sham rats were fed standard chow and administered with citrate vehicle. After 14 weeks total, rats were anesthetized and carotid arteries collected to assess responses to the endothelium-dependent vasodilator, acetylcholine (ACh) by myography. The combination of calcium-activated potassium channel blockers, TRAM-34 (1 μmol/L) and apamin (1 μmol/L) was used to assess the contribution of endothelium-dependent hyperpolarization to relaxation. The corresponding contribution of NOS-derived nitrogen oxide species to relaxation was assessed using the combination of the NO• synthase inhibitor, L-NAME (200 μmol/L) and the soluble guanylate cyclase inhibitor ODQ (10 μmol/L). Lastly, L-cysteine (3 mmol/L), a selective HNO scavenger, and hydroxocobalamin (HXC; 100 μmol/L), a NO• scavenger, were used to distinguish between NO• and HNO-mediated relaxation. Results: At study end, diabetic rats exhibited significantly retarded body weight gain and elevated blood glucose levels compared to sham rats. The sensitivity and the maximal relaxation response to ACh was significantly impaired in carotid arteries from diabetic rats, indicating endothelial dysfunction. The vasorelaxation evoked by ACh was abolished by L-NAME plus ODQ, but not affected by the apamin plus TRAM-34 combination, indicating that NOS-derived nitrogen oxide species are the predominant endothelium-derived vasodilators in sham and diabetic rat carotid arteries. The maximum relaxation to ACh was significantly decreased by L-cysteine in both sham and diabetic rats, whereas HXC attenuated ACh-induced relaxation only in sham rats, suggesting that diabetes impaired the contribution of NO•, whereas HNO-mediated vasorelaxation remained intact. Conclusion: Both NO• and HNO contribute to endothelium-dependent relaxation in carotid arteries. In diabetes, NO•-mediated relaxation is impaired, whereas HNO-mediated relaxation was preserved. The potential for preserved HNO activity under pathological conditions that are associated with oxidative stress indicates that HNO donors may represent a viable therapeutic approach to the treatment of vascular dysfunction.

Cardiovascular Therapeutic Potential of the Redox Siblings, Nitric Oxide (NO•) and Nitroxyl (HNO), in the Setting of Reactive Oxygen Species Dysregulation

Reactive oxygen species (ROS) dysregulation is a hallmark of cardiovascular disease, characterised by an imbalance in the synthesis and removal of ROS. ROS such as superoxide (•O₂^-), hydrogen peroxide (H₂O₂), hydroxyl (OH•) and peroxynitrite (ONOO^-) have a marked impact on cardiovascular function, contributing to the vascular impairment and cardiac dysfunction associated with diseases such as angina, hypertension, diabetes and heart failure. Central to the vascular dysfunction is a reduction in bioavailability and/or physiological effects of vasoprotective nitric oxide (NO•), leading to vasoconstriction, inflammation and vascular remodelling. In a cardiac context, increased ROS generation can also lead to modification of key proteins involved in cardiac contractility. Whilst playing a key role in the pathogenesis of cardiovascular disease, ROS dysregulation also limits the clinical efficacy of current therapies, such as nitrosovasodilators. As such, alternate therapies are sought. This review will discuss the impact of ROS dysregulation on the therapeutic utility of NO• and its redox sibling, nitroxyl (HNO). Both nitric oxide (NO) and nitroxyl (HNO) donors signal through soluble guanylyl cyclase (sGC). NO binds to the Fe(II) form of sGC and nitroxyl possibly to both sGC heme and thiol groups. In the vasculature, nitroxyl can also signal through voltage-dependent (K_v) and ATP-sensitive (K_ATP) K⁺ channels as well as calcitonin gene-related peptide (CGRP). In the heart, HNO directly targets critical thiols to increase myocardial contractility, an effect not seen with NO. The qualitative effects via elevation of cGMP are similar, i.e. lusitropic in the heart and inhibitory on vasoconstriction, inflammation, aggregation and vascular remodelling. Of pathophysiological significance is the fact the efficacy of NO donors is impaired by ROS, e.g. through chemical scavenging of NO, to generate reactive nitrogen oxide species (RNOS), whilst nitroxyl is apparently not.

Uremic Toxins and Vascular Dysfunction

Vascular dysfunction is an essential element found in many cardiovascular pathologies and in pathologies that have a cardiovascular impact such as chronic kidney disease (CKD). Alteration of vasomotricity is due to an imbalance between the production of relaxing and contracting factors. In addition to becoming a determining factor in pathophysiological alterations, vascular dysfunction constitutes the first step in the development of atherosclerosis plaques or vascular calcifications. In patients with CKD, alteration of vasomotricity tends to emerge as being a new, less conventional, risk factor. CKD is characterized by the accumulation of uremic toxins (UTs) such as phosphate, para-cresyl sulfate, indoxyl sulfate, and FGF23 and, consequently, the deleterious role of UTs on vascular dysfunction has been explored. This accumulation of UTs is associated with systemic alterations including inflammation, oxidative stress, and the decrease of nitric oxide production. The present review proposes to summarize our current knowledge of the mechanisms by which UTs induce vascular dysfunction.

Nitroxyl: A Novel Strategy to Circumvent Diabetes Associated Impairments in Nitric Oxide Signaling

Diabetes is associated with an increased mortality risk due to cardiovascular complications. Hyperglycemia-induced oxidative stress underlies these complications, leading to an impairment in endogenous nitric oxide (NO•) generation, together with reductions in NO• bioavailability and NO• responsiveness in the vasculature, platelets and myocardium. The latter impairment of responsiveness to NO•, termed NO• resistance, compromises the ability of traditional NO•-based therapeutics to improve hemodynamic status during diabetes-associated cardiovascular emergencies, such as acute myocardial infarction. Whilst a number of agents can ameliorate (e.g. angiotensin converting enzyme [ACE] inhibitors, perhexiline, statins and insulin) or circumvent (e.g. nitrite and sGC activators) NO• resistance, nitroxyl (HNO) donors offer a novel opportunity to circumvent NO• resistance in diabetes. With a suite of vasoprotective properties and an ability to enhance cardiac inotropic and lusitropic responses, coupled with preserved efficacy in the setting of oxidative stress, HNO donors have intact therapeutic potential in the face of diminished NO• signaling. This review explores the major mechanisms by which hyperglycemia-induced oxidative stress drives NO• resistance, and the therapeutic potential of HNO donors to circumvent this to treat cardiovascular complications in type 2 diabetes mellitus.

Nitric Oxide Resistance, Induced in the Myocardium by Diabetes, Is Circumvented by the Nitric Oxide Redox Sibling, Nitroxyl

Aim: Impairment of tissue responsiveness to exogenous and endogenous nitric oxide (NO^•), known as NO^• resistance, occurs in many cardiovascular disease states, prominently in diabetes and especially in the presence of marked hyperglycemia. In this study, we sought to determine in moderate and severe diabetes (i) whether NO^• resistance also occurs in the myocardium, and (ii) whether the NO^• redox sibling nitroxyl (HNO) circumvents this. Results: The spectrum of acute NO^• effects (induced by diethylamine-NONOate), including vasodilation, and enhanced myocardial contraction and relaxation were impaired by moderately diabetic rats ([blood glucose] ∼20 mM). In contrast, acute HNO effects (induced by isopropylamine-NONOate) were preserved even in more severe diabetes ([blood glucose] >28 mM). Intriguingly, the positive inotropic effects of HNO were significantly enhanced in diabetic rat hearts. Further, progressive attenuation of soluble guanylyl cyclase (sGC) contribution to myocardial NO^• responses occurred with increasing severity of diabetes. Nevertheless, activation of sGC by HNO remained intact in the myocardium. Innovation: Diabetes is associated with marked attenuation of vascular and myocardial effects of NO and NO donors, and this NO^• resistance is circumvented by HNO, suggesting potential therapeutic utility for HNO donors in cardiovascular emergencies in diabetics. Conclusion: These results provide the first evidence that NO^• resistance occurs in diabetic hearts, and that HNO largely circumvents this problem. Further, the positive inotropic and lusitropic effects of HNO are enhanced in a severely diabetic myocardium, a finding that warrants further mechanistic interrogation. The results support a potential role for therapeutic HNO administration in acute treatment of ischemia and/or heart failure in diabetics.

Mechanistic insights into the in vitro metal-promoted oxidation of (di)azine hydroxamic acids: evidence of HNO release and N,O-di(di)azinoyl hydroxylamine intermediate

HNO is released through a one-electron oxidation of the hydroxamic acid along with the N,O-di(di)azinoylhydroxylamine intermediate.

A nitroxyl-responsive near-infrared fluorescent chemosensor for visualizing H2S/NO crosstalk in biological systems

We present a near-infrared (NIR) fluorescent probe, NR-HNO, which was successfully applied to visualizing H2S/NO "crosstalk" by the fluorescence detection of nitroxyl with a fast response time (5 min) and a large Stokes shift (131 nm) in living cells and tissue; it was also used to image nitroxyl in live mice.

Reaction-Based Fluorescent Probes for the Imaging of Nitroxyl (HNO) in Biological Systems

Nitroxyl (HNO) has been identified as an important signaling molecule in biological systems and plays critical roles in many physiological processes. Fluorescence imaging could provide a robust approach to explore the biological formation of HNO and its physiological functions. Herein, we summarize the organic reaction types for constructing HNO probes and specifically focus on review of the recent advances in the development of the reaction-based HNO probes and their imaging applications in living systems.

CXL-1020, a Novel Nitroxyl (HNO) Prodrug, Is More Effective than Milrinone in Models of Diastolic Dysfunction-A Cardiovascular Therapeutic: An Efficacy and Safety Study in the Rat

The nitroxyl (HNO) prodrug, CXL-1020, induces vasorelaxation and improves cardiac function in canine models and patients with systolic heart failure (HF). HNO's unique mechanism of action may be applicable to a broader subset of cardiac patients. This study investigated the load-independent safety and efficacy of CXL-1020 in two rodent (rat) models of diastolic heart failure and explored potential drug interactions with common HF background therapies. In vivo left-ventricular hemodynamics/pressure-volume relationships assessed before/during a 30 min IV infusion of CXL-1020 demonstrated acute load-independent positive inotropic, lusitropic, and vasodilatory effects in normal rats. In rats with only diastolic dysfunction due to bilateral renal wrapping (RW) or pronounced diastolic and mild systolic dysfunction due to 4 weeks of chronic isoproterenol exposure (ISO), CXL-1020 attenuated the elevated LV filling pressures, improved the end diastolic pressure volume relationship, and accelerated relaxation. CXL-1020 facilitated Ca²⁺ re-uptake and enhanced myocyte relaxation in isolated cardiomyocytes from ISO rats. Compared to milrinone, CXL-1020 more effectively improved Ca²⁺ reuptake in ISO rats without concomitant chronotropy, and did not enhance Ca²⁺ entry via L-type Ca²⁺ channels nor increase myocardial arrhythmias/ectopic activity. Acute-therapy with CXL-1020 improved ventricular relaxation and Ca²⁺ cycling, in the setting of chronic induced diastolic dysfunction. CXL-1020's lusitropic effects were greater than those seen with the cAMP-dependent agent milrinone, and unlike milrinone it did not produce chronotropy or increased ectopy. HNO is a promising new potential therapy for both systolic and diastolic heart failure.

Angeli's Salt, a nitroxyl anion donor, reverses endothelin-1 mediated vascular dysfunction in murine aorta

Nitroglycerin (Gtn) is a treatment for cardiovascular patients due to its vasodilatory actions, but induces tolerance when given chronically. A proposed mechanism is the superoxide (O₂^-)-oxidative stress hypothesis, which suggests that Gtn increases O₂^- production. Nitric oxide (NO) exists in three different redox states; the protonated, reduced state, nitroxyl anion (HNO) is an emerging candidate in vascular regulation. HNO is resistant to scavenging and of particular interest in conditions where high levels of reactive oxygen species (ROS) exist. We hypothesize that treatment with Gtn will exacerbate endothelin 1 (ET-1) induced vascular dysfunction via an increase in ROS, while treatment with Angeli's Salt (AS), an HNO donor, will not. Aorta from mice were isolated and divided into four groups: vehicle, ET-1 [0.1μM, 1μM], ET-1+Gtn [Gtn 1μM] and ET-1+AS [AS 1μM]. Concentration response curves (CRCs) to acetylcholine (ACh) and phenylephrine (Phe) were performed. Aorta incubated with ET-1 (for 20-22h) exhibited a decreased relaxation response to ACh and an increase in Phe-mediated contraction. Aorta incubated with AS exhibited a reversal in ET-1 induced vascular and endothelial dysfunction. ET-1 increased ROS in aortic vascular smooth muscle cells (VSMCs), visualized by dihydroethidium (DHE) staining. AS incubated reduced this ROS generation, yet maintained with Gtn treatment. These data suggest that aorta incubated with the HNO donor, AS, can reverse ET-1 mediated vascular dysfunction, which may be through a decrease or prevention of ROS generation. We propose that HNO may be vasoprotective and that HNO donors studied as a therapeutic option where other organic nitrates are contraindicative.

Vasorelaxation to the Nitroxyl Donor Isopropylamine NONOate in Resistance Arteries Does Not Require Perivascular Calcitonin Gene-Related Peptide

Nitroxyl (HNO) donors offer considerable therapeutic potential for the treatment of hypertension-related cardiovascular disorders, particularly heart failure, as they combine an inotropic action with peripheral vasodilation. Angeli's salt is the only HNO donor whose mechanism has been studied in depth, and recently, Angeli's salt vasodilation was suggested to be indirect and caused by calcitonin gene-related peptide (CGRP) released from perivascular nerves after HNO activates TRPA1 (transient receptor potential cation channel subfamily A member 1) channels. We investigated resistance artery vasorelaxation to the HNO donor, isopropylamine NONOate (IPA/NO), one of the structures providing a template for therapeutic development. Wire myography in combination with measurements of smooth muscle membrane potential was used to characterize the effect of IPA/NO in mesenteric resistance arteries. Immunohistochemistry was assessed in pressurized arteries. IPA/NO concentration dependently hyperpolarized and relaxed arteries precontracted with the α1-adrenoreceptor agonist, phenylephrine. These effects were blocked by the soluble guanylyl cyclase inhibitor, ODQ (1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one) but not by the KATP channel inhibitor, glibenclamide. Vasorelaxation persisted in the presence of raised [K+]o, used to block hyperpolarization, capsaicin to deplete perivascular CGRP, or HC030031 (2-(1,3-dimethyl-2,6-dioxo-1,2,3,6-tetrahydro-7H-purin-7-yl)-N-(4 isopropylphenyl) acetamide) to block TRPA1 receptors. Without preconstriction, hyperpolarization to IPA/NO was suppressed by glibenclamide, capsaicin, or HC030031. Hyperpolarization but not vasorelaxation to exogenous CGRP was inhibited with glibenclamide. Thus, vascular hyperpolarization is not necessary for vasorelaxation to the HNO donor IPA/NO, even though both effects are cGMP dependent. The reduced hyperpolarization after depletion of perivascular CGRP or block of TRPA1 receptors indicates some release of CGRP, but this does not contribute to HNO vasorelaxation. Therefore, HNO-TRPA1-CGRP signaling does not seem important for vasodilation to IPA/NO in resistance arteries.

Vasoactive actions of nitroxyl (HNO) are preserved in resistance arteries in diabetes

Endothelial dysfunction is a major risk factor for the vascular complications of diabetes. Increased reactive oxygen species (ROS) generation, a hallmark of diabetes, reduces the bioavailability of endothelial vasodilators, including nitric oxide (NO·). The vascular endothelium also produces the one electron reduced and protonated form of NO·, nitroxyl (HNO). Unlike NO·, HNO is resistant to scavenging by superoxide anions (·O₂^─). The fate of HNO in resistance arteries in diabetes is unknown. We tested the hypothesis that the vasodilator actions of endogenous and exogenous HNO are preserved in resistance arteries in diabetes. We investigated the actions of HNO in small arteries from the mesenteric and femoral beds as they exhibit marked differences in endothelial vasodilator function following 8 weeks of streptozotocin (STZ)-induced diabetes mellitus. Vascular reactivity was assessed using wire myography and ·O₂^─ generation using lucigenin-enhanced chemiluminescence. The HNO donor, Angeli's salt, and the NO· donor, DEA/NO, evoked relaxations in both arteries of control rats, and these responses were unaffected by diabetes. Nox2 oxidase expression and ·O₂^─ generation were upregulated in mesenteric, but unchanged, in femoral arteries of diabetic rats. Acetylcholine-induced endothelium-dependent relaxation was impaired in mesenteric but not femoral arteries in diabetes. The HNO scavenger, L-cysteine, reduced this endothelium-dependent relaxation to a similar extent in femoral and mesenteric arteries from control and diabetic animals. In conclusion, HNO and NO· contribute to the NO synthase (NOS)-sensitive component of endothelium-dependent relaxation in mesenteric and femoral arteries. The role of HNO is sustained in diabetes, serving to maintain endothelium-dependent relaxation.

A reductant-resistant ratiometric, colorimetric and far-red fluorescent probe for rapid and ultrasensitive detection of nitroxyl

A reductant-resistant ratiometric, colorimetric and far-red fluorescent probe for rapid and ultrasensitive detection of nitroxyl was developed.

A targetable fluorescent probe for imaging exogenous and intracellularly formed nitroxyl in mitochondria in living cells

We have developed a new mitochondrial-targeted turn-on fluorescent HNO probe (Mito-HNO). Fluorescence imaging shows that Mito-HNO is suitable for ratiometric visualization of HNO within mitochondria in living cells.

The opposing roles of NO and oxidative stress in cardiovascular disease

Nitric oxide (NO) plays a pivotal role in the maintenance of cardiovascular homeostasis. A reduction in the bioavailability of endogenous NO, manifest as a decrease in the production and/or impaired signaling, is associated with many cardiovascular diseases including hypertension, atherosclerosis, stroke and heart failure. There is substantial evidence that reactive oxygen species (ROS), generated predominantly from NADPH oxidases (Nox), are responsible for the reduced NO bioavailability in vascular and cardiac pathologies. ROS can compromise NO function via a direct inactivation of NO, together with a reduction in NO synthesis and oxidation of its receptor, soluble guanylyl cyclase. Whilst nitrovasodilators are administered to compensate for the ROS-mediated loss in NO bioactivity, their clinical utility is limited due to the development of tolerance and resistance and systemic hypotension. Moreover, efforts to directly scavenge ROS with antioxidants has had limited clinical efficacy. This review outlines the therapeutic utility of NO-based therapeutics in cardiovascular diseases and describes the source and impact of ROS in these pathologies, with particular focus on the interaction with NO. Future therapeutic approaches in the treatment of cardiovascular diseases are highlighted with a focus on nitroxyl (HNO) donors as an alternative to traditional NO donors and the development of novel Nox inhibitors.

Nitroxyl (HNO) reduces endothelial and monocyte activation and promotes M2 macrophage polarization

In the present study, the effect of nitroxyl anion (HNO) donors on factors that precede atherosclerosis was examined. They reduced endothelial cell inflammation and monocyte activation and as such may be an effective treatment for coronary artery disease.

HNO suppresses LPS-induced inflammation in BV-2 microglial cells via inhibition of NF-κB and p38 MAPK pathways

Both hydrogen sulfide (H2S) and nitric oxide (NO) are important gaseous mediators. We and others previously reported that these two gases react with each other to generate a new mediator, nitroxyl (HNO), and regulate cardiovascular functions. In this study, we demonstrated for the first time that the interaction between the two gases also existed in microglia. The biological functions of HNO in microglial cells were further studied with Angeli's salt (AS), an HNO donor. We found that AS attenuated lipopolysaccharide (LPS)-evoked production of reactive oxygen species (ROS) and pro-inflammatory cytokines (e.g. IL-1β and TNFα) through downregulating the expression of inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2). HNO significantly reduced the phosphorylation of p38 mitogen-activated protein kinase (MAPK) and the activation of nuclear factor-κB (NF-κB) through suppression of phosphorylation p65 and IκBα. The above effects were abolished by l-cysteine, an HNO scavenger, but were not mimicked by nitrite, another product of AS during generating HNO. A Cys-179-to-Ala mutation in inhibitory κB kinase β (IKKβ) mimicked the effect of HNO on LPS-induced NF-κB activation. Interestingly, AS abolished the inflammation in cells overexpressing WT-IKKβ, but had no significant effect in cells overexpressing C179A-IKKβ. These data suggest that HNO may act on C179 to prevent IKKβ-dependent inflammation. Taken together, our data demonstrated for the first time that H2S interacts with NO to generate HNO in microglial cells. HNO produces anti-inflammatory effects through suppressing the IKKβ dependent NF-κB activation and p38 MAPK pathways.

Nitroxyl (HNO): A Reduced Form of Nitric Oxide with Distinct Chemical, Pharmacological, and Therapeutic Properties

Nitroxyl (HNO), the one-electron reduced form of nitric oxide (NO), shows a distinct chemical and biological profile from that of NO. HNO is currently being viewed as a vasodilator and positive inotropic agent that can be used as a potential treatment for heart failure. The ability of HNO to react with thiols and thiol containing proteins is largely used to explain the possible biological actions of HNO. Herein, we summarize different aspects related to HNO including HNO donors, chemistry, biology, and methods used for its detection.

Design and Synthesis of Near-infrared Fluorescent Probes for Imaging of Biological Nitroxyl

Nitroxyl (HNO), the reduced and protonated form of nitric oxide (NO), has recently been identified as an interesting and important signaling molecule in biological systems. However, research on its biosynthesis and bioactivities are hampered by the lack of versatile HNO detection methods applicable to living cells. In this report, two new near-infrared (NIR) probes were designed and synthesized for HNO imaging in living cells. One of the probes was found to display high sensitivity towards HNO, with up to 67-fold of fluorescence increment after reaction with HNO. The detection limit was determined to be as low as 0.043 μM. The probe displayed high selectivity towards HNO over other biologically related species including metal ions, reactive oxygen species, reactive nitrogen species and reactive sulfur species. Furthermore, the probe was shown to be suitable for imaging of exogenous and endogenous HNO in living cells. Interestingly, the probe was found to be mainly localized in lysosomes. We envision that the new NIR probe described here will serve as a useful tool for further elucidation of the intricate roles of HNO in living cells.

Ring expansions of acyloxy nitroso compounds

Treatment of cyclopentanone and cyclobutanone-derived oximes with lead (IV) tetraacetate gives the bright blue acyloxy nitroso compounds, which upon basic hydrolysis yields the ring expansion product cyclic hydroxamic acids in 12-81% yield. Reactions of substituted cyclopentanones provide ring expanded products where the -NOH group regioselectively inserts to the more substituted position and gives a better yield compared to the treatment of the same ketone with a basic solution of Piloty's acid. Reaction of phosphines with acyloxy nitroso compounds generally generates a ring-expanded Beckmann rearrangement product that can be hydrolyzed to the corresponding lactam. Acyloxy nitroso compounds that undergo rapid hydrolysis to HNO do not show this ring expansion reactivity. These results further demonstrate the versatility of acyloxy nitroso compound to yield structurally complex materials.

Nitroxyl: a vasodilator of human vessels that is not susceptible to tolerance

Pre-clinical studies have identified nitroxyl (HNO), the reduced congener of nitric oxide (NO•), as a potent vasodilator which is resistant to tolerance development. The present study explores the efficacy of HNO in human blood vessels and describes, for the first time, a vasodilator for humans that is not susceptible to tolerance. Human radial arteries and saphenous veins were obtained from patients undergoing coronary artery graft surgery and mounted in organ baths. Repeated vasodilator responses to the HNO donor Angeli's salt (AS) and NO• donor glyceryl trinitrate (GTN) were determined. AS- and GTN-induced concentration-dependent vasorelaxation of both human radial arteries (AS pEC50: 6.5 ± 0.2; -log M) and saphenous veins (pEC50: 6.7 ± 0.1) with similar potency. In human radial arteries, GTN-induced relaxation was reduced by the NO• scavenger hydroxocobalamin (HXC; P<0.05) but was unaffected by the HNO scavenger L-cysteine. Alternately, AS was unaffected by HXC but was reduced by L-cysteine (5-fold shift, P<0.05). The sGC (soluble guanylate cyclase) inhibitor ODQ abolished responses to both AS and GTN in arteries and veins (P<0.05). Inhibition of voltage-dependent potassium channels (Kv channels) with 4-AP also significantly reduced responses to AS (pEC50: 5.5) and GTN, suggesting that the relaxation to both redox congeners is cGMP- and Kv channel-dependent. Critically, a concentration-dependent development of tolerance to GTN (1 and 10 μM; P<0.05), but not to AS, was observed in both saphenous veins and radial arteries. Like GTN, the HNO donor AS causes vasorelaxation of human blood vessels via activation of a cGMP-dependent pathway. Unlike GTN, however, it does not develop tolerance in human blood vessels.

Role of aldehyde dehydrogenase in hypoxic vasodilator effects of nitrite in rats and humans

BACKGROUND AND PURPOSE

Hypoxic conditions favour the reduction of nitrite to nitric oxide (NO) to elicit vasodilatation, but the mechanism(s) responsible for bioconversion remains ill defined. In the present study, we assess the role of aldehyde dehydrogenase 2 (ALDH2) in nitrite bioactivation under normoxia and hypoxia in the rat and human vasculature.

EXPERIMENTAL APPROACH

The role of ALDH2 in vascular responses to nitrite was studied using rat thoracic aorta and gluteal subcutaneous fat resistance vessels from patients with heart failure (HF; 16 patients) in vitro and by measurement of changes in forearm blood flow (FBF) during intra-arterial nitrite infusion (21 patients) in vivo. Specifically, we investigated the effects of (i) ALDH2 inhibition by cyanamide or propionaldehyde and the (ii) tolerance-independent inactivation of ALDH2 by glyceryl trinitrate (GTN) on the vasodilator activity of nitrite. In each setting, nitrite effects were measured via evaluation of the concentration-response relationship under normoxic and hypoxic conditions in the absence or presence of ALDH2 inhibitors.

KEY RESULTS

Both in rat aorta and human resistance vessels, dilatation to nitrite was diminished following ALDH2 inhibition, in particular under hypoxia. In humans there was a non-significant trend towards attenuation of nitrite-mediated increases in FBF.

CONCLUSIONS AND IMPLICATIONS

In human and rat vascular tissue in vitro, hypoxic nitrite-mediated vasodilatation involves ALDH2. In patients with HF in vivo, the role of this enzyme in nitrite bioactivation is at the most, modest, suggesting the involvement of other more important mechanisms.

New acyloxy nitroso compounds with improved water solubility and nitroxyl (HNO) release kinetics and inhibitors of platelet aggregation

New acyloxy nitroso compounds, 4-nitrosotetrahydro-2H-pyran-4-yl 2,2,2-trichloroacetate and 4-nitrosotetrahydro-2H-pyran-4-yl 2,2-dichloropropanoate were prepared. These compounds release HNO under neutral conditions with half-lives between 50 and 120min, identifying these HNO donors as kinetically intermediate to the much slower acetate derivative and the faster trifluoroacetic acid derivative. These compounds or HNO-derived from these compounds react with thiols, including glutathione, thiol-containing enzymes and heme-containing proteins in a similar fashion to other acyloxy nitroso compounds. HNO released from these acyloxy nitroso compounds inhibits activated platelet aggregation. These acyloxy nitroso compounds augment the range of release for this group of HNO donors and should be valuable tools in the further study of HNO biology.

Sulfur as a signaling nutrient through hydrogen sulfide

Hydrogen sulfide (H₂S) has emerged as an important signaling molecule with beneficial effects on various cellular processes affecting, for example, cardiovascular and neurological functions. The physiological importance of H₂S is motivating efforts to develop strategies for modulating its levels. However, advancement in the field of H₂S-based therapeutics is hampered by fundamental gaps in our knowledge of how H₂S is regulated, its mechanism of action, and its molecular targets. This review provides an overview of sulfur metabolism; describes recent progress that has shed light on the mechanism of H₂S as a signaling molecule; and examines nutritional regulation of sulfur metabolism, which pertains to health and disease.

A FRET-based ratiometric fluorescent probe for nitroxyl detection in living cells

HNO has recently been found to possess distinct biological functions from NO. Studying the biological functions of HNO calls for the development of sensitive and selective fluorescent probes. Herein, we designed and synthesized a FRET-based ratiometric probe to detect HNO in living cells. Our studies revealed that the probe is capable of detecting HNO in a rapid and ratiometric manner under physiological conditions. In bioimaging studies, the probe displayed a clear color change from blue to green when treated with HNO.

Chronic administration of the nitroxyl donor 1-nitrosocyclo hexyl acetate limits left ventricular diastolic dysfunction in a mouse model of diabetes mellitus in vivo

BACKGROUND

Nitroxyl (HNO), a redox congener of nitric oxide (NO·), is a novel regulator of cardiovascular function, combining concomitant positive inotropic, lusitropic, and vasodilator properties. Moreover, HNO exhibits myocardial antihypertrophic and superoxide-suppressing actions. Despite these favorable actions, the impact of chronic HNO administration has yet to be reported in the context of cardiomyopathy. Diabetic cardiomyopathy is characterized by early diastolic dysfunction and adverse left ventricular (LV) structural remodeling, with LV superoxide generation playing a major causal role. We tested the hypothesis that the HNO donor 1-nitrosocyclohexylacetate (1-NCA) limits cardiomyocyte hypertrophy and LV diastolic dysfunction in a mouse model of diabetes mellitus in vivo.

METHODS AND RESULTS

Diabetes mellitus was induced in male FVB/N mice using streptozotocin. After 4 weeks, diabetic and nondiabetic mice were allocated to 1-NCA therapy (83 mg/kg per day IP) or vehicle and followed up for a further 4 weeks. Diabetes mellitus-induced LV diastolic dysfunction was evident on echocardiography-derived E and A wave velocities, E:A ratio, deceleration, and isovolumic relaxation times; LV systolic function was preserved. Increased LV cardiomyocyte size, hypertrophic and profibrotic gene expression, and upregulation of LV superoxide were also evident. These characteristics of diabetic cardiomyopathy were largely prevented by 1-NCA treatment. Selectivity of 1-NCA as an HNO donor was demonstrated by sensitivity of acute 1-NCA to l-cysteine but not to hydroxocobalamin in the normal rat heart ex vivo.

CONCLUSIONS

Our studies provide the first evidence that HNO donors may represent a promising strategy for treatment of diabetic cardiomyopathy and implies therapeutic efficacy in settings of chronic heart failure.

A near-infrared fluorescent probe for the selective detection of HNO in living cells and in vivo

We present a near-infrared fluorescent probe for the detection of nitroxyl (HNO) in living cells and in mice.

A selective phosphine-based fluorescent probe for nitroxyl in living cells

A novel fluorescein-based fluorescent probe for nitroxyl (HNO) based on the reductive Staudinger ligation of HNO with an aromatic phosphine was prepared. This probe reacts with HNO derived from Angeli's salt and 4-bromo Piloty's acid under physiological conditions without interference by other biological redox species. Confocal microscopy demonstrates this probe detects HNO by fluorescence in HeLa cells and mass spectrometric analysis of cell lysates confirms this probe detects HNO following the proposed mechanism.

Resorufin based fluorescence ‘turn-on’ chemodosimeter probe for nitroxyl (HNO)

A cellular responsive, highly selective fluorogenic and chromogenic chemodosimeter probe for HNO is developed.

Light-controlled release of nitric oxide from solid polymer composite materials using visible and near infra-red light

Composite materials were prepared using biocompatible polymers, upconverting nanoparticles, and a nitric oxide (NO) donor complex. We have demonstrated NO release from the solid composites after visible and near infra-red light irradiation.

The reaction products of sulfide and S-nitrosoglutathione are potent vasorelaxants

The chemical interaction of sodium sulfide (Na2S) with the NO-donor S-nitrosoglutathione (GSNO) has been described to generate new reaction products, including polysulfides and nitrosopersulfide (SSNO(-)) via intermediacy of thionitrous acid (HSNO). The aim of the present work was to investigate the vascular effects of the longer-lived products of the Sulfide/GSNO interaction. Here we show that the products of this reaction relax precontracted isolated rings of rat thoracic aorta and mesenteric artery (but to a lesser degree rat uterus) with a >2-fold potency compared with the starting material, GSNO (50 nM), whereas Na2S and polysulfides have little effect at 1-5 µM. The onset of vasorelaxation of the reaction products was 7-10 times faster in aorta and mesenteric arteries compared with GSNO. Relaxation to GSNO (100-500 nM) was blocked by an inhibitor of soluble guanylyl cyclase, ODQ (0.1 and 10 µM), and by the NO scavenger cPTIO (100 µM), but less affected by prior acidification (pH 2-4), and unaffected by N-acetylcysteine (1 mM) or methemoglobin (20 µM heme). By contrast, relaxation to the Sulfide/GSNO reaction products (100-500 nM based on the starting material) was inhibited to a lesser extent by ODQ, only slightly decreased by cPTIO, more markedly inhibited by methemoglobin and N-acetylcysteine, and abolished by acidification before addition to the organ bath. The reaction mixture was found to generate NO as detected by EPR spectroscopy using N-(dithiocarboxy)-N-methyl-D-glucamine (MGD2)-Fe(2+) as spin trap. In conclusion, the Sufide/GSNO reaction products are faster and more pronounced vasorelaxants than GSNO itself. We conclude that in addition to NO formation from SSNO(-), reaction products other than polysulfides may give rise to nitroxyl (HNO) and be involved in the pronounced relaxation induced by the Sulfide/GSNO cross-talk.

Soluble guanylate cyclase is required for systemic vasodilation but not positive inotropy induced by nitroxyl in the mouse

Nitroxyl (HNO), the reduced and protonated form of nitric oxide (NO·), confers unique physiological effects including vasorelaxation and enhanced cardiac contractility. These features have spawned current pharmaceutical development of HNO donors as heart failure therapeutics. HNO interacts with selective redox sensitive cysteines to effect signaling but is also proposed to activate soluble guanylate cyclase (sGC) in vitro to induce vasodilation and potentially enhance contractility. Here, we tested whether sGC stimulation is required for these HNO effects in vivo and if HNO also modifies a redox-sensitive cysteine (C42) in protein kinase G-1α to control vasorelaxation. Intact mice and isolated arteries lacking the sGC-β subunit (sGCKO, results in full sGC deficiency) or expressing solely a redox-dead C42S mutant protein kinase G-1α were exposed to the pure HNO donor, CXL-1020. CXL-1020 induced dose-dependent systemic vasodilation while increasing contractility in controls; however, vasodilator effects were absent in sGCKO mice whereas contractility response remained. The CXL-1020 dose reversing 50% of preconstricted force in aortic rings was ≈400-fold greater in sGCKO than controls. Cyclic-GMP and cAMP levels were unaltered in myocardium exposed to CXL-1020, despite its inotropic-vasodilator activity. In protein kinase G-1α(C42S) mice, CXL-1020 induced identical vasorelaxation in vivo and in isolated aortic and mesenteric vessels as in littermate controls. In both groups, dilation was near fully blocked by pharmacologically inhibiting sGC. Thus, sGC and cGMP-dependent signaling are necessary and sufficient for HNO-induced vasodilation in vivo but are not required for positive inotropic action. Redox modulation of protein kinase G-1α is not a mechanism for HNO-mediated vasodilation.

The concomitant coronary vasodilator and positive inotropic actions of the nitroxyl donor Angeli's salt in the intact rat heart: contribution of soluble guanylyl cyclase-dependent and -independent mechanisms

BACKGROUND AND PURPOSE

The NO redox sibling nitroxyl (HNO) elicits soluble guanylyl cyclase (sGC)-dependent vasodilatation. HNO has high reactivity with thiols, which is attributed with HNO-enhanced left ventricular (LV) function. Here, we tested the hypothesis that the concomitant vasodilatation and inotropic actions induced by a HNO donor, Angeli's salt (sodium trioxodinitrate), were sGC-dependent and sGC-independent respectively.

EXPERIMENTAL APPROACH

Haemodynamic responses to Angeli's salt (10 pmol-10 μmol), alone and in the presence of scavengers of HNO (L-cysteine, 4 mM) or of NO [hydroxocobalamin (HXC), 100 μM] or a selective inhibitor of sGC [1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), 10 μM], a CGRP receptor antagonist (CGRP8-37 , 0.1 μM) or a blocker of voltage-dependent potassium channels [4-aminopyridine (4-AP), 1 mM] were determined in isolated hearts from male rats.

KEY RESULTS

Angeli's salt elicited concomitant, dose-dependent increases in coronary flow and LV systolic and diastolic function. Both L-cysteine and ODQ shifted (but did not abolish) the dose-response curve of each of these effects to the right, implying contributions from HNO and sGC in both the vasodilator and inotropic actions. In contrast, neither HXC, CGRP8-37 nor 4-AP affected these actions.

CONCLUSIONS AND IMPLICATIONS

Both vasodilator and inotropic actions of the HNO donor Angeli's salt were mediated in part by sGC-dependent mechanisms, representing the first evidence that sGC contributes to the inotropic and lusitropic action of HNO in the intact heart. Thus, HNO acutely enhances LV contraction and relaxation, while concomitantly unloading the heart, potentially beneficial actions in failing hearts.

Nitroxyl (HNO) reacts with molecular oxygen and forms peroxynitrite at physiological pH. Biological Implications

Nitroxyl (HNO), the protonated one-electron reduction product of NO, remains an enigmatic reactive nitrogen species. Its chemical reactivity and biological activity are still not completely understood. HNO donors show biological effects different from NO donors. Although HNO reactivity with molecular oxygen is described in the literature, the product of this reaction has not yet been unambiguously identified. Here we report that the decomposition of HNO donors under aerobic conditions in aqueous solutions at physiological pH leads to the formation of peroxynitrite (ONOO(-)) as a major intermediate. We have specifically detected and quantified ONOO(-) with the aid of boronate probes, e.g. coumarin-7-boronic acid or 4-boronobenzyl derivative of fluorescein methyl ester. In addition to the major phenolic products, peroxynitrite-specific minor products of oxidation of boronate probes were detected under these conditions. Using the competition kinetics method and a set of HNO scavengers, the value of the second order rate constant of the HNO reaction with oxygen (k = 1.8 × 10(4) m(-1) s(-1)) was determined. The rate constant (k = 2 × 10(4) m(-1) s(-1)) was also determined using kinetic simulations. The kinetic parameters of the reactions of HNO with selected thiols, including cysteine, dithiothreitol, N-acetylcysteine, captopril, bovine and human serum albumins, and hydrogen sulfide, are reported. Biological and cardiovascular implications of nitroxyl reactions are discussed.

H2S and its role in redox signaling

Hydrogen sulfide (H2S) has emerged as an important gaseous signaling molecule that is produced endogenously by enzymes in the sulfur metabolic network. H2S exerts its effects on multiple physiological processes important under both normal and pathological conditions. These functions include neuromodulation, regulation of blood pressure and cardiac function, inflammation, cellular energetics and apoptosis. Despite the recognition of its biological importance and its beneficial effects, the mechanism of H2S action and the regulation of its tissue levels remain unclear in part owing to its chemical and physical properties that render handling and analysis challenging. Furthermore, the multitude of potential H2S effects has made it difficult to dissect its signaling mechanism and to identify specific targets. In this review, we focus on H2S metabolism and provide an overview of the recent literature that sheds some light on its mechanism of action in cellular redox signaling in health and disease. This article is part of a Special Issue entitled: Thiol-Based Redox Processes.

H2S and NO cooperatively regulate vascular tone by activating a neuroendocrine HNO-TRPA1-CGRP signalling pathway

Nitroxyl (HNO) is a redox sibling of nitric oxide (NO) that targets distinct signalling pathways with pharmacological endpoints of high significance in the treatment of heart failure. Beneficial HNO effects depend, in part, on its ability to release calcitonin gene-related peptide (CGRP) through an unidentified mechanism. Here we propose that HNO is generated as a result of the reaction of the two gasotransmitters NO and H₂S. We show that H₂S and NO production colocalizes with transient receptor potential channel A1 (TRPA1), and that HNO activates the sensory chemoreceptor channel TRPA1 via formation of amino-terminal disulphide bonds, which results in sustained calcium influx. As a consequence, CGRP is released, which induces local and systemic vasodilation. H₂S-evoked vasodilatatory effects largely depend on NO production and activation of HNO–TRPA1–CGRP pathway. We propose that this neuroendocrine HNO–TRPA1–CGRP signalling pathway constitutes an essential element for the control of vascular tone throughout the cardiovascular system.

Nitric oxide (NO) and hydrogen sulphide (H₂S) are two gaseous signalling molecules produced in tissues. Here the authors propose that NO and H₂S react with each other to form nitroxyl (HNO), which activates the TRPA1 channel in nerve cells and triggers the release of the vasoactive peptide CGRP.

A highly sensitive and reductant-resistant fluorescent probe for nitroxyl in aqueous solution and serum

A novel coumarin-based fluorescent probe, P-CM, for quantitative detection of nitroxyl (HNO) was developed. P-CM exhibits a selective response to HNO over other biological reductants and was also applied for quantitative detection of HNO in bovine serum with satisfactory results.

Nitroxyl inhibits overt pain-like behavior in mice: role of cGMP/PKG/ATP-sensitive potassium channel signaling pathway

BACKGROUND

Several lines of evidence have indicated that nitric oxide (NO) plays complex and diverse roles in modulation of pain/analgesia. However, the roles of charged and uncharged congeners of NO are less well understood. In the present study, the antinociceptive effect of the nitroxyl (HNO) donor, Angeli's salt (Na2N2O3; AS) was investigated in models of overt pain-like behavior. Moreover, whether the antinociceptive effect of nitroxyl was dependent on the activation of cGMP (cyclic guanosine monophosphate)/PKG (protein kinase G)/ATP-sensitive potassium channels was addressed.

METHODS

The antinociceptive effect of AS was evaluated on phenyl-p-benzoquinone (PBQ)- and acetic acid-induced writhings and via the formalin test. In addition, pharmacological treatments targeting guanylate cyclase (ODQ), PKG (KT5923) and ATP-sensitive potassium channel (glybenclamide) were used.

RESULTS

PBQ and acetic acid induced significant writhing responses over 20min. The nociceptive response in these models were significantly reduced in a dose-dependent manner by subcutaneous pre-treatment with AS. Furthermore, AS also inhibited both phases of the formalin test. Subsequently, the inhibitory effect of AS in writhing and flinching responses were prevented by ODQ, KT5823 and glybenclamide, although these inhibitors alone did not alter the writhing score. Furthermore, pretreatment with L-cysteine, an HNO scavenger, confirmed that the antinociceptive effect of AS depends on HNO.

CONCLUSION

The present study demonstrates the efficacy of a nitroxyl donor and its analgesic mechanisms in overt pain-like behavior by activating the cGMP/PKG/ATP-sensitive potassium channel (K(+)) signaling pathway.

Participation of nitric oxide pathway in the relaxation response induced by E-cinnamaldehyde oxime in superior mesenteric artery isolated from rats

For many years, nitric oxide (NO) has been studied as an important mediator in the control of vascular tone. Endothelial deficiencies that diminish NO production can result in the development of several future cardiovascular diseases, such as hypertension and arteriosclerosis. In this context, new drugs with potential ability to donate NO have been studied. In this study, 3 aromatic oximes [benzophenone oxime, 4-Cl-benzophenone oxime, and E-cinnamaldehyde oxime (E-CAOx)] induced vasorelaxation in endothelium-denuded and intact superior mesenteric rings precontracted with phenylephrine. E-CAOx demonstrated the most potent effect, and its mechanism of action was evaluated. Vascular reactivity experiments demonstrated that the effect of E-CAOx was reduced by the presence of 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, 1H[1,2,4,]oxadiazolo[4,3-a]quinoxalin-1-one, and (Rp)-8-(para-chlorophenylthio)guanosine-3',5'-cyclic monophosphorothioate, suggesting the participation of NO/sGC/PKG pathway. NO donation seems to be mediated through nicatinamide adenine dinucleotide phosphate-dependent reductases because 7-ethoxyresorufin decreased the effect of E-CAOx on vascular reactivity and reduced NO formation as detected by flow cytometry using the NO indicator diaminofluorescein 4,5-diacetate. Further downstream of NO donation, K+ subtype channels were also shown to be involved in the E-CAOx vasorelaxant effect. The present study showed that E-CAOx acts like an NO donor, activating NO/sGC/PKG pathway and thus K+ channels.