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

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

SIGNIFICANCE

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

RECENT ADVANCES

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

CRITICAL ISSUES

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

FUTURE DIRECTIONS

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

Effect of calcitonin gene-related peptide antagonist on the cardiovascular events, mortality, and prostaglandin E2 production by nitrate-induced tolerant rats with acute myocardial infarction

Background

Anti-ischemic effects of NO releasing by nitroglycerin (NTG) and the release of calcitonin gene-related peptide (CGRP) are involved in the decrease of vascular remodeling in different cardiovascular diseases. Using a nitrate-free period is still generally required to prevent nitrate tolerance and should be used as the first-line option to maintain adequate symptom control and on an individual basis. Personalized anti-ischemic concerns require the urgent change of paradigm from interventional measures to predictive, preventive, and personalized treatment with organic nitrates and its combination with drugs that may improve prognosis and drugs that can be added for patients who remain symptomatic despite therapy with the other classes of agents. The purpose of this study was to evaluate the influence of human calcitonin gene-related peptide antagonist (CGRP_8-37) on cardiohemodynamic events, prostaglandin E₂ (PGE₂) plasma concentration, the severity of ventricular arrhythmias, and mortality occurring during acute myocardial infarction (AMI) in NTG-tolerant and nontolerant rats.

Methods

In the pilot study of efficacy of calcitonin gene-related peptide antagonist (CGRP_8-37), 58 male Wistar rats were included. All procedures were performed according to protocols approved by the General Animal Care and Use Committee. Adult male rats underwent surgery to induce AMI by ligating the left anterior descending coronary artery or SHAM. ECG was used to confirm myocardial ischemia. In each experiment, a rat was maintained under anesthesia for the duration of the experiment. At the end of the experiment, the rat was killed by an overdose of pentobarbital. All animals in accordance with the received pharmacological agent were randomized into three groups: I—received only NTG, 50 mg/kg daily, s.c. injections b.i.d. 3 days prior to AMI; II—received NTG by the same dose, route, and frequency of administration + CGRP antagonist (CGRP_8-37), 10 μg/kg two times daily by a similar period of administration; and III—served as control (C) group without preliminary tolerance to NTG.

Results

Subcutaneous injections of NTG (50 mg/kg) 30 min prior to AMI in NTG-tolerant animals (group I) and in NTG-tolerant rats + CGRP antagonist (group II) caused minor changes in blood pressure and heart period that was accompanied before NTG s.c. administration with blunted baroreflex sensitivity in response to i.v. administration of sodium nitroprusside in these groups of rats (0.66 ± 0.05 and 0.56 ± 0.04 ms/mmHg, P < 0.05, respectively) in comparison to C (group III) animals (0.9 ± 0.1 ms/mmHg). AMI 1 h duration was associated with a high incidence of ventricular arrhythmia and significant mortality in group I (70 %) and especially in group II (90 %) animals at 72 h after reperfusion as compared with group III rats (56 %), that correlated to a decrease of PGE₂ plasma content in group II (2.2 ± 0.4 ng/ml, P < 0.001) and group I (3.6 ± 0.2 ng/ml, P < 0.01) vs. control group of rats (4.8 ± 0.3 ng/ml).

Conclusions

CGRP could be involved in the mechanism of nitrate tolerance via the inhibition of release of the potent vasodilator CGRP leading to exacerbation of acute myocardial ischemia. The influence of CGRP antagonist could enhance this condition.

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

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

Oxidative stress and enhanced sympathetic vasoconstriction in contracting muscles of nitrate-tolerant rats and humans

Sympathetic vasoconstriction is normally attenuated in exercising muscle, but this functional sympatholysis is impaired in rats with hypertension or heart failure due to elevated levels of reactive oxygen species (ROS) in muscle. Whether ROS have a similar effect in the absence of cardiovascular disease or whether these findings extend to humans is not known. We therefore tested the hypothesis that chronic treatment with nitroglycerin (NTG) to induce nitrate tolerance, which is associated with excessive ROS production, impairs functional sympatholysis in healthy rats and humans. NTG treatment increased ethidium fluorescence in rat muscles and urinary F(2)-isoprostanes in humans, demonstrating oxidative stress. In vehicle-treated rats, sympathetic nerve stimulation (1 to 5 Hz) evoked decreases in femoral vascular conductance at rest (range, -30 to -63%) that were attenuated during hindlimb contraction (range, -2 to -31%; P < 0.05). In NTG-treated rats, vasoconstrictor responses were similar at rest, but were enhanced during contraction (range, -17 to -50%; P < 0.05 vs. vehicle). Infusion of the ROS scavenger tempol restored sympatholysis in these rats. In humans, reflex sympathetic activation during lower body negative pressure (LBNP) evoked decreases in muscle oxygenation in resting forearm (-12 ± 1%) that were attenuated during handgrip exercise (-3 ± 1%; P < 0.05). When these subjects became nitrate tolerant, LBNP-induced decreases in muscle oxygenation were unaffected at rest, but were enhanced during exercise (-9 ± 1%; P < 0.05 vs. before NTG). Collectively, these data indicate that functional sympatholysis is impaired in otherwise healthy nitrate-tolerant rats and humans by a mechanism probably involving muscle oxidative stress.

Microcirculatory Hemodynamics and Endothelial Dysfunction in Systemic Lupus Erythematosus

Objective— Impaired flow-mediated dilation (FMD) occurs in disease states associated with atherosclerosis, including SLE. The primary hemodynamic determinant of FMD is wall shear stress, which is critically dependent on the forearm microcirculation. We explored the relationship between FMD, diastolic shear stress (DSS), and the forearm microcirculation in 32 patients with SLE and 19 controls.

Methods and Results— DSS was calculated using (mean diastolic velocity×8×blood viscosity)/baseline brachial artery diameter. Doppler velocity envelopes from the first 15 seconds of reactive hyperemia were analyzed for resistive index (RI), and interrogated in the frequency domain to assess forearm microvascular hemodynamics. FMD was significantly impaired in SLE patients (median, 2.4%; range, −2.1% to 10.7% versus median 5.8%; range, 1.9% to 14%; P <0.001). DSS (dyne/cm 2 ) was significantly reduced in SLE patients (median, 18.5; range, 3.9 to 34.0 versus median 21.8; range, 14.1 to 58.7; P =0.037). A strong correlation between FMD and DSS, r s =0.65, P =0.01 was found. Postischemic RI was not significantly different between the 2 groups; however, there were significant differences in the power-frequency spectrums of the Doppler velocity envelopes ( P <0.05).

Conclusions— These data suggest that in SLE, altered structure and function of the forearm microcirculation contributes to impaired FMD through a reduction in shear stress stimulus.

Explaining the phenomenon of nitrate tolerance

During the last century, nitroglycerin has been the most commonly used antiischemic and antianginal agent. Unfortunately, after continuous application, its therapeutic efficacy rapidly vanishes. Neurohormonal activation of vasoconstrictor signals and intravascular volume expansion constitute early counter-regulatory responses (pseudotolerance), whereas long-term treatment induces intrinsic vascular changes, eg, a loss of nitrovasodilator-responsiveness (vascular tolerance). This is caused by increased vascular superoxide production and a supersensitivity to vasoconstrictors secondary to a tonic activation of protein kinase C. NADPH oxidase(s) and uncoupled endothelial nitric oxide synthase have been proposed as superoxide sources. Superoxide and vascular NO rapidly form peroxynitrite, which aggravates tolerance by promoting NO synthase uncoupling and inhibition of soluble guanylyl cyclase and prostacyclin synthase. This oxidative stress concept may explain why radical scavengers and substances, which reduce oxidative stress indirectly, are able to relieve tolerance and endothelial dysfunction. Recent work has defined a new tolerance mechanism, ie, an inhibition of mitochondrial aldehyde dehydrogenase, the enzyme that accomplishes bioactivation of nitroglycerin, and has identified mitochondria as an additional source of reactive oxygen species. Nitroglycerin-induced reactive oxygen species inhibit the bioactivation of nitroglycerin by thiol oxidation of aldehyde dehydrogenase. Both mechanisms, increased oxidative stress and impaired bioactivation of nitroglycerin, can be joined to provide a new concept for nitroglycerin tolerance and cross-tolerance. The consequences of these processes for the nitroglycerin downstream targets soluble guanylyl cyclase, cGMP-dependent protein kinase, cGMP-degrading phosphodiesterases, and toxic side effects contributing to endothelial dysfunction, such as inhibition of prostacyclin synthase, are discussed in this review.

Reactive Oxygen Species

Platelets participate not only in thrombus formation but also in the regulation of vessel tone, the development of atherosclerosis, angiogenesis, and in neointima formation after vessel wall injury. It is not surprising, therefore, that the platelet activation cascade (including receptor-mediated tethering to the endothelium, rolling, firm adhesion, aggregation, and thrombus formation) is tightly regulated. In addition to already well-defined platelet regulatory factors, such as nitric oxide (NO), prostacyclin (PGI2), and adenosine, reactive oxygen species (ROS) participate in the regulation of platelet activation. Although exogenously derived ROS are known to affect the regulation of platelet activation, recent data suggest that the platelets themselves generate ROS. Intracellular ROS signaling in activated platelets could be of significant relevance after transient platelet contact with the vessel wall, during the recruitment of additional platelets, and in thrombus formation. This review discusses the potential cellular and enzymatic sources of ROS in platelets, their molecular mechanisms of action in platelet activation, and summarizes in vitro and in vivo evidence for their physiological and potential therapeutic relevance.

Functional consequences of endothelial nitric oxide synthase uncoupling in congestive cardiac failure

BACKGROUND

Impaired endothelium-mediated vasodilatation (EMVD) in congestive cardiac failure (CCF) has been linked to decreased nitric oxide (NO) bioavailability because of its interaction with vascular superoxide (O2*-), derived predominantly from NAD(P)H-dependent oxidases. When uncoupled from essential cofactors, endothelial nitric oxide synthase (eNOS) produces O2*-. We studied the functional consequences of eNOS uncoupling in relation to EMVD in patients with CCF.

METHODS AND RESULTS

We employed the platelet as a compartmentalized ex-vivo model to examine O2*- and NO production. When eNOS is functioning normally, incorporation of Nomega-Nitro-L-Arginine methyl ester (L-NAME, 1 mmol/L), results in increased O2*- detection, as inhibition of NO production prevents NO scavenging of O2*-. This was observed in controls and 9 of the CCF patients, in whom O2*- detection increased by 63% and 101%, respectively. In the remaining 9 CCF patients, incorporation of L-NAME reduced O2*- production by 39%, indicating O2*- production by eNOS uncoupling. Detection of platelet-derived NO was significantly greater in eNOS-coupled platelets compared with the uncoupled group (2.8+/-1.4 versus 0.9+/-0.4 pmol/108 platelets, P=0.04). Endothelium-dependent and -independent vasodilator responses to acetylcholine and sodium nitroprusside recorded using venous occlusion plethysmography were significantly impaired in patients exhibiting eNOS uncoupling.

CONCLUSIONS

This study provides first evidence that platelet eNOS can become uncoupled in human CCF. Impaired endothelium-dependent and -independent vasodilator responses and diminished platelet-derived NO production occurred in association with enzyme uncoupling.