Endogenous nitric oxide protects against thromboembolism in venules but not in arterioles

Venous valve hypoxia as a possible mechanism of deep vein thrombosis: a scoping review

INTRODUCTION

The pathogenesis of deep vein thrombosis (DVT) has been explained by an interplay between a changed blood composition, vein wall alteration, and blood flow abnormalities. A comprehensive investigation of these components of DVT pathogenesis has substantially promoted our understanding of thrombogenesis in the venous system. Meanwhile, the process of DVT initiation remains obscure. This systematic review aims to collect, analyze, and synthesize the published evidence to propose hypoxia as a possible trigger of DVT.

EVIDENCE ACQUISITION

An exhaustive literature search was conducted across multiple electronic databased including PubMed, EMBASE, Scopus, and Web of Science to identify studies pertinent to the research hypothesis. The search was aimed at exploring the connection between hypoxia, reoxygenation, and the initiation of deep vein thrombosis (DVT). The following key words were used: "deep vein thrombosis," "venous thrombosis," "venous thromboembolism," "hypoxia," "reoxygenation," "venous valve," and "venous endothelium." Reviews, case reports, editorials, and letters were excluded.

EVIDENCE SYNTHESIS

Based on the systematic search outcome, 156 original papers relevant to the issue were selected for detailed review. These studies encompassed a range of experimental and observational clinical research, focusing on various aspects of DVT, including the anatomical, physiological, and cellular bases of the disease. A number of studies suggested limitations in the traditional understanding of Virchow's triad as an acceptable explanation for DVT initiation. Emerging evidence points to more complex interactions and additional factors that may be critical in the early stages of thrombogenesis. The role of venous valves has been recognized but remains underappreciated, with several studies indicating that these sites may act as primary loci for thrombus formation. A collection of studies describes the effects of hypoxia on venous endothelial cells at the cellular and molecular levels. Hypoxia influences several pathways that regulate endothelial cell permeability, inflammatory response, and procoagulation activity, underpinning the endothelial dysfunction noted in DVT.

CONCLUSIONS

Hypoxia of the venous valve may serve as an independent hypothesis to outline the DVT triggering process. Further research projects in this field may discover new molecular pathways responsible for the disease and suggest new therapeutic targets.

Microvascular thrombosis: experimental and clinical implications

A significant amount of clinical and research interest in thrombosis is focused on large vessels (eg, stroke, myocardial infarction, deep venous thrombosis, etc.); however, thrombosis is often present in the microcirculation in a variety of significant human diseases, such as disseminated intravascular coagulation, thrombotic microangiopathy, sickle cell disease, and others. Further, microvascular thrombosis has recently been demonstrated in patients with COVID-19, and has been proposed to mediate the pathogenesis of organ injury in this disease. In many of these conditions, microvascular thrombosis is accompanied by inflammation, an association referred to as thromboinflammation. In this review, we discuss endogenous regulatory mechanisms that prevent thrombosis in the microcirculation, experimental approaches to induce microvascular thrombi, and clinical conditions associated with microvascular thrombosis. A greater understanding of the links between inflammation and thrombosis in the microcirculation is anticipated to provide optimal therapeutic targets for patients with diseases accompanied by microvascular thrombosis.

Redox Regulation of the Microcirculation

The microcirculation maintains tissue homeostasis through local regulation of blood flow and oxygen delivery. Perturbations in microvascular function are characteristic of several diseases and may be early indicators of pathological changes in the cardiovascular system and in parenchymal tissue function. These changes are often mediated by various reactive oxygen species and linked to disruptions in pathways such as vasodilation or angiogenesis. This overview compiles recent advances relating to redox regulation of the microcirculation by adopting both cellular and functional perspectives. Findings from a variety of vascular beds and models are integrated to describe common effects of different reactive species on microvascular function. Gaps in understanding and areas for further research are outlined. © 2020 American Physiological Society. Compr Physiol 10:229-260, 2020.

Thrombus growth modelling and stenosis prediction in the cerebral microvasculature

Cerebral microvascular occlusions cause restriction of blood supply to the brain, thus potentially severely impacting cognitive abilities. Thus, accurate prediction of thrombus growth in realistic geometries is important. Thrombi growth in an existing 13-generation cerebral microvasculature network is simulated here to study the haemodynamic effects of single and multiple blockages on the occlusion of the network. Compared to a single vessel, in a network, the occlusion probability is found to be different. It is the downstream/smaller arterioles (i.e. the 3rd, 4th, 5th, 6th generation arterioles in this study) that tend to reach occlusion first in a network and thus are the critical vessels. Simulations of simultaneous growth of two independent thrombi in the network (referred to here as the two-block case) show a close coupling between the locations of the various blocks in the network, each influencing the other's growth. The presence of the lead block (LB) slows the growth of the trailing block (TB). In some cases, it stops the TB's growth thereby preventing it from occluding the vessel. Findings in this work thus indicate that, to prevent ischaemia, blocks in the smaller arterioles need to be identified and treated first, and that this is more critical if the number of simultaneous blocks is higher.

The Human Microcirculation: Regulation of Flow and Beyond

The microcirculation is responsible for orchestrating adjustments in vascular tone to match local tissue perfusion with oxygen demand. Beyond this metabolic dilation, the microvasculature plays a critical role in modulating vascular tone by endothelial release of an unusually diverse family of compounds including nitric oxide, other reactive oxygen species, and arachidonic acid metabolites. Animal models have provided excellent insight into mechanisms of vasoregulation in health and disease. However, there are unique aspects of the human microcirculation that serve as the focus of this review. The concept is put forth that vasculoparenchymal communication is multimodal, with vascular release of nitric oxide eliciting dilation and preserving normal parenchymal function by inhibiting inflammation and proliferation. Likewise, in disease or stress, endothelial release of reactive oxygen species mediates both dilation and parenchymal inflammation leading to cellular dysfunction, thrombosis, and fibrosis. Some pathways responsible for this stress-induced shift in mediator of vasodilation are proposed. This paradigm may help explain why microvascular dysfunction is such a powerful predictor of cardiovascular events and help identify new approaches to treatment and prevention.

Endothelial nitric oxide synthase in the microcirculation

Endothelial nitric oxide synthase (eNOS, NOS3) is responsible for producing nitric oxide (NO)--a key molecule that can directly (or indirectly) act as a vasodilator and anti-inflammatory mediator. In this review, we examine the structural effects of regulation of the eNOS enzyme, including post-translational modifications and subcellular localization. After production, NO diffuses to surrounding cells with a variety of effects. We focus on the physiological role of NO and NO-derived molecules, including microvascular effects on vessel tone and immune response. Regulation of eNOS and NO action is complicated; we address endogenous and exogenous mechanisms of NO regulation with a discussion of pharmacological agents used in clinical and laboratory settings and a proposed role for eNOS in circulating red blood cells.

The role of asymmetric and symmetric dimethylarginine in acute deep vein thrombosis

BACKGROUND

In this study, we aimed to determine the importance of asymmetric dimethylarginine (ADMA), symmetric dimethyl arginine (SDMA), and l-arginine levels which are influenced by the endothelial dysfunction in acute deep vein thrombosis (DVT), and compare their pretreatment and post-treatment levels.

METHODS

A total of 34 cases (19 men and 15 women, age range 42 ± 14 years) diagnosed with acute DVT and 34 control subjects (22 men and 12 women, age range 45 ± 11 years) without any vascular disorders were included in the study. The patients were administered low-molecular-weight heparin subcutaneously. Blood samples were obtained to measure ADMA, SDMA, and arginine levels before treatment and during the treatment (on 10th day). ADMA, SDMA, and l-arginine levels were measured using high performance liquid chromatography method.

RESULTS

The ADMA and SDMA levels were significantly higher in acute DVT patients when compared with the controls (P = 0.001). Post-treatment decrease of ADMA and SDMA levels when compared with the pretreatment levels were found as statistically significant (P = 0.001). Increase in l-arginine levels were not found significant when compared with the control group (P = 0.12) or post-treatment levels (P = 0.16).

CONCLUSIONS

We concluded that ADMA and SDMA levels can be used as parameters in clinical follow-up for determining the efficacy of treatment in acute DVT patients, and further studies are needed to further clarify the subject.

Transient ischemia elicits a sustained enhancement of thrombus development in the cerebral microvasculature: effects of anti-thrombotic therapy

OBJECTIVE

While transient ischemic attack (TIA) is a well-known harbinger of ischemic stroke, the mechanisms that link TIA to subsequent strokes remain poorly understood. The overall aim of this study was to determine whether: 1) brief periods of transient cerebral ischemia render this tissue more vulnerable to thrombus development and 2) antiplatelet agents used in TIA patients alter ischemia-induced thrombogenesis.

APPROACH & RESULTS

The middle cerebral artery of C57BL/6 mice was occluded for 2.5-10min, followed by reperfusion periods of 1-28days. Intravital microscopy was used to monitor thrombus development in cerebral microvessels induced by light/dye photoactivation. Thrombosis was quantified as the time to platelet aggregation on the vessel wall and the time for complete blood flow cessation. While brief periods of cerebral ischemia were not associated with neurological deficits or brain infarction (evaluated after 1day), it yielded a pronounced and prolonged (up to 28days) acceleration of thrombus formation, compared to control (sham) mice. This prothrombotic phenotype was not altered by pre- and/or post-treatment of mice with either aspirin (A), clopidogrel (C), dipyridamole (D), or atorvastatin (S), or with A+D+S.

CONCLUSIONS

The increased vulnerability of the cerebral vasculature to thrombus development after a brief period of transient ischemia can be recapitulated in a murine model. Antiplatelet or antithrombotic agents used in patients with TIA show no benefit in this mouse model of brief transient ischemia.

Roles of the Oxidative Stress and ADMA in the Development of Deep Venous Thrombosis

Venous thromboembolism has multifactorial origin and occurs in the context of complex interactions between environmental and genetic predisposing factors. Oxidative stress plays an important role in the physiopathology of venous thrombosis. Current study examined the role of oxidative stress and asymmetric dimethylarginine in the development of DVT with the parameters such as serum malondialdehyde (MDA), glutathione peroxidase (GSH-Px), catalase, ADMA, homocysteine, folic acid, vitamin B₆, and vitamin B₁₂ levels. Serum MDA levels were found significantly (P < 0.005) high in patients with DVT compared with control group. Additionally, serum B₆ levels were found significantly (P < 0.009) low in patients with DVT compared with healthy volunteers. There were no significant differences between the groups in terms of the other parameters (P > 0.05). This study showed that patients with DVT have increased oxidative stress compared with the healthy volunteers whereas there was no significant difference between the groups in terms of serum ADMA levels. Thus serum ADMA levels seemed to be not related with development of DVT.

Blockade of the purinergic P2Y12 receptor greatly increases the platelet inhibitory actions of nitric oxide

Circulating platelets are constantly exposed to nitric oxide (NO) released from the vascular endothelium. This NO acts to reduce platelet reactivity, and in so doing blunts platelet aggregation and thrombus formation. For successful hemostasis, platelet activation and aggregation must occur at sites of vascular injury despite the constant presence of NO. As platelets aggregate, they release secondary mediators that drive further aggregation. Particularly significant among these secondary mediators is ADP, which, acting through platelet P2Y12 receptors, strongly amplifies aggregation. Platelet P2Y12 receptors are the targets of very widely used antithrombotic drugs such as clopidogrel, prasugrel, and ticagrelor. Here we show that blockade of platelet P2Y12 receptors dramatically enhances the antiplatelet potency of NO, causing a 1,000- to 100,000-fold increase in inhibitory activity against platelet aggregation and release reactions in response to activation of receptors for either thrombin or collagen. This powerful synergism is explained by blockade of a P2Y12 receptor-dependent, NO/cGMP-insensitive phosphatidylinositol 3-kinase pathway of platelet activation. These studies demonstrate that activation of the platelet ADP receptor, P2Y12, severely blunts the inhibitory effects of NO. The powerful antithrombotic effects of P2Y12 receptor blockers may, in part, be mediated by profound potentiation of the effects of endogenous NO.

Inhibition of nitric oxide and antiphospholipid antibody-mediated thrombosis

The antiphospholipid syndrome (APS) is characterized by recurrent vascular thrombosis, thrombocytopenia, and fetal loss occurring in the presence of antiphospholipid antibodies (aPL). Along with arterial and venous thrombosis and pregnancy complications, patients with APS have an increased risk of myocardial infarction, stroke, and coronary artery disease, resulting from vascular cell dysfunction induced by aPL. Accumulating evidence to date indicates that interactions between circulating aPL and cell surface molecules of target cells, primarily endothelial cells and platelets, underlie the vascular disease phenotypes of APS. However, the molecular basis of APS is poorly understood. Nitric oxide produced by endothelial cells is a key determinant of vascular health that regulates several physiologic processes, including thrombosis, endothelial-leukocyte interaction, vascular cell migration, and the modulation of vascular tone. This review will discuss recent findings that indicate a novel mechanism by which aPL antagonize endothelial cell production of nitric oxide and thereby promote thrombosis.

Role of reactive oxygen and nitrogen species in the vascular responses to inflammation

Inflammation is a complex and potentially life-threatening condition that involves the participation of a variety of chemical mediators, signaling pathways, and cell types. The microcirculation, which is critical for the initiation and perpetuation of an inflammatory response, exhibits several characteristic functional and structural changes in response to inflammation. These include vasomotor dysfunction (impaired vessel dilation and constriction), the adhesion and transendothelial migration of leukocytes, endothelial barrier dysfunction (increased vascular permeability), blood vessel proliferation (angiogenesis), and enhanced thrombus formation. These diverse responses of the microvasculature largely reflect the endothelial cell dysfunction that accompanies inflammation and the central role of these cells in modulating processes as varied as blood flow regulation, angiogenesis, and thrombogenesis. The importance of endothelial cells in inflammation-induced vascular dysfunction is also predicated on the ability of these cells to produce and respond to reactive oxygen and nitrogen species. Inflammation seems to upset the balance between nitric oxide and superoxide within (and surrounding) endothelial cells, which is necessary for normal vessel function. This review is focused on defining the molecular targets in the vessel wall that interact with reactive oxygen species and nitric oxide to produce the characteristic functional and structural changes that occur in response to inflammation. This analysis of the literature is consistent with the view that reactive oxygen and nitrogen species contribute significantly to the diverse vascular responses in inflammation and supports efforts that are directed at targeting these highly reactive species to maintain normal vascular health in pathological conditions that are associated with acute or chronic inflammation.

Vascular endothelial growth factor: much more than an angiogenesis factor

Vascular endothelial growth factor (VEGF) is best known as a cytokine essential for embryonic vasculogenesis and for the angiogenesis associated with various pathologies including cancer. However, VEGF also serves other functions that are less widely recognized. An early study (Berse et al., 1992) revealed widespread expression of VEGF transcripts in adult tissues devoid of ongoing neovascularization, thereby predicting additional VEGF functions distinct from angiogenesis. More recent studies have confirmed that VEGF does indeed serve multiple additional functions, including normal maintenance of endothelial and neural cell compartments. These findings have important implications for the use of VEGF antagonists and VEGF receptor antagonists in patients for which inhibition of pathological angiogenesis is the therapeutic goal.

Wall shear stress as measured in vivo: consequences for the design of the arterial system

Based upon theory, wall shear stress (WSS), an important determinant of endothelial function and gene expression, has been assumed to be constant along the arterial tree and the same in a particular artery across species. In vivo measurements of WSS, however, have shown that these assumptions are far from valid. In this survey we will discuss the assessment of WSS in the arterial system in vivo and present the results obtained in large arteries and arterioles. In vivo WSS can be estimated from wall shear rate, as derived from non-invasively recorded velocity profiles, and whole blood viscosity in large arteries and plasma viscosity in arterioles, avoiding theoretical assumptions. In large arteries velocity profiles can be recorded by means of a specially designed ultrasound system and in arterioles via optical techniques using fluorescent flow velocity tracers. It is shown that in humans mean WSS is substantially higher in the carotid artery (1.1–1.3 Pa) than in the brachial (0.4–0.5 Pa) and femoral (0.3–0.5 Pa) arteries. Also in animals mean WSS varies substantially along the arterial tree. Mean WSS in arterioles varies between about 1.0 and 5.0 Pa in the various studies and is dependent on the site of measurement in these vessels. Across species mean WSS in a particular artery decreases linearly with body mass, e.g., in the infra-renal aorta from 8.8 Pa in mice to 0.5 Pa in humans. The observation that mean WSS is far from constant along the arterial tree implies that Murray’s cube law on flow-diameter relations cannot be applied to the whole arterial system. Because blood flow velocity is not constant along the arterial tree either, a square law also does not hold. The exponent in the power law likely varies along the arterial system, probably from 2 in large arteries near the heart to 3 in arterioles. The in vivo findings also imply that in in vitro studies no average shear stress value can be taken to study effects on endothelial cells derived from different vascular areas or from the same artery in different species. The cells have to be studied under the shear stress conditions they are exposed to in real life.

Role of Ena/VASP proteins in homeostasis and disease

The actin cytoskeleton is required for many important processes during embryonic development. In later stages of life, important homeostatic processes depend on the actin cytoskeleton, such as immune response, haemostasis and blood vessel preservation. Therefore, the function of the actin cytoskeleton must be tightly regulated, and aberrant regulation may cause disease. A growing number of proteins have been described to bind and regulate the actin cytoskeleton. Amongst them, Ena/VASP proteins function as anti-capping proteins, thereby directly modulating the actin ultrastructure. Ena/VASP function is regulated by their recruitment into protein complexes downstream of plasma membrane receptors and by phosphorylation. As regulators of the actin ultrastructure, Ena/VASP proteins are involved in crucial cellular functions, such as shape change, adhesion, migration and cell-cell interaction and hence are important targets for therapeutic intervention. In this chapter, we will first describe the structure, function and regulation of Ena/VASP proteins. Then, we will review the involvement of Ena/VASP proteins in the development of human diseases. Growing evidence links Ena/VASP proteins to important human diseases, such as thrombosis, cancer, arteriosclerosis, cardiomyopathy and nephritis. Finally, present and future perspectives for the development of therapeutic molecules interfering with Ena/VASP-mediated protein-protein interactions are presented.

Redox-dependent impairment of vascular function in sickle cell disease

The vascular pathophysiology of sickle cell disease (SCD) is influenced by many factors, including adhesiveness of red and white blood cells to endothelium, increased coagulation, and homeostatic perturbation. The vascular endothelium is central to disease pathogenesis because it displays adhesion molecules for blood cells, balances procoagulant and anticoagulant properties of the vessel wall, and regulates vascular homeostasis by synthesizing vasoconstricting and vasodilating substances. The occurrence of intermittent vascular occlusion in SCD leads to reperfusion injury associated with granulocyte accumulation and enhanced production of reactive oxygen species. The participation of nitric oxide (NO) in oxidative reactions causes a reduction in NO bioavailability and contributes to vascular dysfunction in SCD. Therapeutic strategies designed to counteract endothelial, inflammatory, and oxidative abnormalities may reduce the frequency of hospitalization and blood transfusion, the incidence of pain, and the occurrence of acute chest syndrome and pulmonary hypertension in patients with SCD.

Effect of a low-dose oral contraceptive on venous endothelial function in healthy young women: preliminary results

BACKGROUND

A possible increase in the incidence of venous thromboembolic events has been reported among users of third generation oral contraceptives. The objective of this study was to evaluate the effect of a low dose oral contraceptive (15 microg ethinyl estradiol/60 microg gestodene) on the venous endothelial function of healthy young women.

METHODS

Prospective case control study using the dorsal hand vein technique. Venous endothelial function was evaluated at baseline and after 4 months in the oral contraceptive users group (11 women) and in a control group (9 women). After preconstriction of the vein with phenylephrine, dose-response curves for acetylcholine and sodium nitroprusside were constructed.

RESULTS

In the contraceptive users group, a reduction occurred in the maximum venodilation response to acetylcholine and sodium nitroprusside after 4 months of oral contraceptive use, but this difference was not statistically significant (P > 0.05). No significant changes were detected in maximum venodilation responses to acetylcholine and sodium nitroprusside at the 4-month time point in the control group.

CONCLUSION

This study found no significant impairment of endothelium-dependent or independent venodilation in healthy young women following oral contraceptive use. Further studies are necessary using the same methodology in a larger sample over a longer follow-up period.

Wall shear stress--an important determinant of endothelial cell function and structure--in the arterial system in vivo. Discrepancies with theory

It has been well established that wall shear stress is an important determinant of endothelial cell function and gene expression as well as of its structure. There is increasing evidence that low wall shear stress, as present in artery bifurcations opposite to the flow divider where atherosclerotic lesions preferentially originate, expresses an atherogenic endothelial gene profile. Besides, wall shear stress regulates arterial diameter by modifying the release of vasoactive mediators by endothelial cells. Most of the studies on the influence of wall shear stress on endothelial cell function and structure have been performed in vitro, generally exposing endothelial cells from different vascular regions to an average wall shear stress level calculated according to Poiseuille's law, which does not hold for the in vivo situation, assuming wall shear stress to be constant along the arterial tree. Also in vivo wall shear stress has been determined based upon theory, assuming the velocity profile in arteries to be parabolic, which is generally not the case. Wall shear stress has been calculated, because of the lack of techniques to assess wall shear stress in vivo. In recent years, techniques have been developed to accurately assess velocity profiles in arterioles, using fluorescently labeled particles as flow tracers, and non-invasively in large arteries by means of ultrasound or magnetic resonance imaging. Wall shear rate is derived from the in vivo recorded velocity profiles and wall shear stress is estimated as the product of wall shear rate and plasma viscosity in arterioles and whole blood viscosity in large arteries. In this review, we will discuss wall shear stress in vivo, paying attention to its assessment and especially to the results obtained in both arterioles and large arteries. The limitations of the methods currently in use are discussed as well. The data obtained in the arterial system in vivo are compared with the theoretically predicted ones, and the consequences of values deviating from theory for in vitro studies are considered. Applications of wall shear stress as in flow-mediated arterial dilation, clinically in use to assess endothelial cell (dys)function, are also addressed. This review starts with some background considerations and some theoretical aspects.

Techniques: Intravital microscopy--a method for investigating disseminated intravascular coagulation?

Intravital microscopy (IVM) enables the study of cellular and molecular events in living organisms. Confocal microscopy permits images to be collected from narrow focal planes without interference from out-of-focus regions, and multi-photon microscopy produces high-resolution images from deep (several hundred micrometers) within opaque organs and tissues. Lasers that are targeted through microscope objectives can injure individual microvessels and induce thrombi that can be studied in detail. The marriage of these technologies provides exciting possibilities for investigating the inflammation and coagulation that is associated with disseminated intravascular coagulation (DIC). In this review, I consider some of the new technology associated with microscopy, give examples of discoveries that have been made using this technology and speculate on how the study of DIC might be advanced using IVM.

L-arginine improves endothelial vasoreactivity and reduces thrombogenicity after thrombolysis in experimental deep venous thrombosis

PURPOSE

Nitric oxide (NO) is important in regulation of platelet aggregation, endothelial function, and intravascular thrombosis. The purposes of this study were to assess the effect of thrombolysis on endothelial function in a porcine model of deep venous thrombosis (DVT) and to evaluate the effect of NO precursor l-arginine on endothelial function after thrombolytic therapy.

METHODS

DVT was created in bilateral iliac veins by deploying a self-expanding stent-graft that incorporated an intraluminal stenosis, from a groin approach. Five pigs underwent sham operation. After 7 days of DVT, animals were randomized to three groups: saline pulse-spray (saline group, n = 5), thrombolytic pulse-spray with tissue plasminogen activator (alteplase, 8 mg; t-PA group, n = 5), and thrombolytic pulse-spray plus intravenous l-arginine (20 mmol/L; arginine group, n = 5). At 2 weeks iliac vein patency was evaluated at venography and intravascular ultrasound scanning. NO level was determined with a chemiluminescent assay of the nitrite and nitrate metabolites (NO(x)). Thrombogenicity was evaluated with radiolabeled platelet and fibrin deposition. Veins were harvested and evaluated with light microscopy and scanning electron microscopy. Endothelial function was evaluated with organ chamber analysis.

RESULTS

All iliac veins remained patent at 2 weeks. The luminal areas in the sham, saline, t-PA, and arginine groups were 53 +/- 23 mm(2), 14 +/- 11 mm(2), 34 +/- 19 mm(2), and 42 +/- 21 mm(2), respectively. No difference in endothelial cell structure was observed between the three treatment groups at light microscopy or scanning electron microscopy. Although no difference in fibrin deposition was noted among the three treatment groups, decreased platelet deposition occurred in the arginine group compared with the saline or t-PA groups (P <.05). The arginine group showed greater endothelial-dependent relaxation compared with the t-PA or saline groups (73% +/- 23% vs 49% +/- 18% and 32% +/- 21%; P <.05). Local NO(x) level in the arginine group was correspondingly higher compared with the saline or t-PA groups (1.8 +/- 0.3 micromol/L vs 0.3 +/- 0.05 micromol/L and 0.2 +/- 0.04 micromol/L; P <.05).

CONCLUSIONS

NO precursor l-arginine supplementation enhances NO production at sites of venous thrombosis. Moreover, l-arginine preserves endothelial vasoreactivity and reduces platelet deposition after thrombolysis in iliac DVT. These data suggest that l-arginine may preserve endothelial function after thrombolysis and may reduce the likelihood of postthrombotic syndrome.

Nitric oxide and its relationship to thrombotic disorders

Nitric oxide (NO) is released by the endothelium preventing platelet adhesion to the vessel wall. When released by platelets, NO inhibits further recruitment of platelets to a growing thrombus. Modulation of endogenous NO release may be a mechanism by which the thrombotic response can be regulated as suggested by several clinical diseases associated with impaired bioactive NO. Diseases including atrial fibrillation and coronary atherothrombotic disease have been associated with impaired NO release or decrease in NO bioavailability.

The nitric oxide pathway is amplified in venular vs arteriolar cultured rat mesenteric endothelial cells

To determine if there are differences in nitric oxide activity between pre- and postcapillary microvessels, we studied cultured rat mesenteric arteriolar and venular endothelial cells (RMAEC, RMVEC). We measured expression of endothelial nitric oxide synthase (eNOS), the activity of eNOS, and L-arginine transport in live RMAEC and RMVEC and the L-arginine content of RMAEC and RMVEC lysates. The abundance of eNOS was significantly greater in RMVEC vs RMAEC; this was also true for freshly harvested, pooled microvessels. Baseline NOS activity was higher in RMVEC than in RMAEC. NG-monomethyl-L-arginine (L-NMA; 5 mM) inhibited NOS activity by approximately 70-80% in both RMAEC and RMVEC, indicating that metabolism of l-arginine is largely via NOS. Intracellular L-arginine levels were higher in RMVEC vs RMAEC and well above the eNOS Km in both cell types. L-arginine levels increased with L-NMA in both RMAEC and RMVEC, presumably due to reduced substrate utilization. Since L-arginine transport was not higher in RMVEC vs RMAEC, this may reflect higher intracellular arginine synthesis. A higher intrinsic level of baseline NO production in the postcapillary microvascular endothelium may reflect both the contribution of venular derived NO to control of arteriolar tone and a key role of venular-derived NO in local thrombosis control.

Effects of Nitric Oxide (NO) and Soluble Nucleoside Triphosphate Diphosphohydrolase (NTPDase) on Inhibition of Platelet Deposition In Vitro

Vascular thrombosis is regulated via the release of several constituents from the vascular endothelium, including nucleoside triphosphate diphosphohydrolases (NTPDases or ectonucleotidases), nitric oxide (NO), and eicosanoids. Currently, it is unknown how these constituents interact in the inhibition of platelet aggregation and adhesion. To investigate the combined effects of NO and NTPDase on platelet deposition sequestration, an in vitro study was performed to compare inhibition of platelet deposition to a biomaterial by NO in the absence or presence of soluble NTPDase. Results of the platelet inhibition studies with NO and NTPDase conclusively show that the inhibitory effects of NTPDase and NO are additive. The platelet inhibitory potency in the presence of NO was enhanced by NTPDase in a dose-dependent manner, for a given NO exposure. This augmentation is independent of aspirin; the ability of NTPDase or NO alone to inhibit platelet deposition is also independent of aspirin. Clearly, NO and NTPDase independently contribute to platelet inhibition via different mechanisms. The inaction of NO on the activity of NTPDase confirmed that NO or reaction products in the presence of O(2) do not interact with NTPDase directly.

Antiplatelet effects of sodium nitroprusside in flowing human blood: studies under normoxic and hypoxic conditions

We explored the ability of sodium nitroprusside to modify adhesive and cohesive function of platelets in flowing blood, under normoxic and hypoxic conditions. Aliquots of both untreated and sodium nitroprusside-treated blood were prepared for studies of: (1) platelet aggregation in plasma; (2) erythrocyte deformability; (3) platelet interaction with damaged subendothelium, by using a well-defined perfusion system; and (4) blood gasometry in the perfused samples. Results showed that sodium nitroprusside-treated blood always showed a totally inhibited arachidonic acid-induced platelet aggregation in plasma, as well as significantly increased erythrocyte deformability (0.44+/-0.09 up to 0.66+/-0.05; p<0.05). However, treatment with sodium nitroprusside did not modify the pattern of platelet interaction with subendothelium (percentage of contact, adhesion, thrombus, and covered surface) with respect to untreated blood, under any of the shear rates used (300, 800, and 1800 seconds(-1)), although it significantly reduced the height of thrombi (9.8+/-0.4 vs. 8.3+/-0.4 microm; p<0.05). Hypoxic conditions did not have a noticeable effect in modifying antiplatelet effects of sodium nitroprusside. Additionally, the presence of sodium nitroprusside impaired the normal oxygenation of the blood during perfusion. pO2 in control untreated samples rose from 40.3+/-5.0 mm Hg perfusions to 100.4+/-12.5 mm Hg but remained at 66.3+/-6.3 mm Hg in sodium nitroprusside-treated blood (p<0.05). Our results did not show a significant effect of sodium nitroprusside in the modulation of platelet interaction with subendothelium. The marginal reduction in the thrombi height could be related to rheological interference of increased erythrocyte deformability.

The influence of prostaglandins on leukocyte-endothelium interactions in rabbit mesenteric venules

Contradictory results have been reported concerning the effects of prostaglandins (PGs) on leukocyte-endothelium interactions. Therefore, we investigated the in vivo effects of PGE1, PGE2, Iloprost (a stable PGI2-analogue), and also of a combination of these PGs on leukocyte rolling and FMLP-induced leukocyte adhesion in venules of rabbit mesentery. This preparation was used because of its low level of vasoactivity, eliminating hemodynamic effects on leukocyte-endothelium interactions. The mesentery was superfused with PGs or vehicle. After 30 min FMLP was added to the PG-solution for 15 min, whereupon the tissue was superfused with the PG-solution alone for another 30 min. Neither the PGs nor the cocktail influenced leukocyte rolling. During FMLP administration leukocyte adhesion increased and leukocyte rolling decreased; adhesion was highest in the presence of PGE2. The FMLP-induced decrease in leukocyte rolling was similar in all groups. After FMLP administration had been stopped the number of adherent cells almost returned to baseline and the level of leukocyte rolling increased, the baseline level being reached only in the presence of PGE2. In conclusion, these findings indicate that the effects of PGs on leukocyte-endothelium interactions are limited.

The relationship between polymorphisms in the endothelial cell nitric oxide synthase gene and the platelet GPIIIa gene with myocardial infarction and venous thromboembolism in African Americans

STUDY OBJECTIVES

To determine whether the polymorphic dinucleotide repeats found in intron 4 of the endothelial cell nitric oxide synthase (ecNOS) gene and the platelet GPIIIa PLA(1)/A(2) polymorphism are associated with myocardial infarction (MI) and venous thromboembolism (VTE) in African Americans. Because these two genes may interact physiologically, the third objective was to determine if there was a relationship between the polymorphisms with respect to MI and VTE.

DESIGN

A hospital-based case-control study. After informed consent was obtained, blood used for DNA extraction was drawn from the subjects.

SETTING

The study was conducted in the Anticoagulant Clinic and the Cardiology Clinic at Grady Memorial Hospital in Atlanta Georgia.

PATIENTS

Subjects were recruited from African-American patients with a reported history of MI (n = 110) or VTE (n = 91). Control subjects (n = 185) without a history of cardiovascular or venous disease were recruited from an outpatient clinic.

MEASUREMENTS AND RESULTS

The 393 ecNOS allele was more common among MI cases (36%; p = 0.01) and VTE cases (35%; p = 0.04) than among control subjects (26%). There was no association between the GPIIIa genotypes and either MI or VTE. However, among the MI subjects, there was a strong association between the ecNOS 393/393 genotype and the Pl(A2) allele. It was also found that the frequency of the 393 allele was higher in African-American persons (0.26) compared with what has been reported for Australian Caucasians (0. 14) and Japanese (0.10).

CONCLUSIONS

The 393 allele but not the Pl(A2) allele was significantly associated with both MI and VTE in African Americans. Homozygosity for the 393 allele was significantly associated to the diagnosis of MI prior to the age of 45. The combination of the 393 allele and a Pl(A2) allele was also highly associated with MI. The frequency of the 393 allele was significantly higher in African Americans than what has been reported for other populations. This study furthers not only extends the association of the 393 allele to VTE but has demonstrated an interaction with the Pl(A2) allele with respect to MI.

Impaired antiplatelet effects of aspirin associated with hypoxia and ATP release from erythrocytes. Studies in a system with flowing human blood

BACKGROUND

We have explored how hypoxic conditions may affect the antiplatelet effects of aspirin.

MATERIALS AND METHODS

For this purpose, a perfusion system containing a damaged vessel segment was modified in order to induce hypoxia (low Po2) in flowing blood. Blood samples were incubated with 50 mumol L-1 aspirin and divided into two aliquots, one being perfused under standard conditions (normoxic) and the other under hypoxic conditions. The interaction of platelets with the subendothelium was morphometrically evaluated.

RESULTS

In studies with untreated blood under normoxic conditions, platelet interaction with the subendothelium was 0.3 +/- 0.1% of contact, 5.3 +/- 1.6% of adhesion, 24.3 +/- 3.3% of thrombus and 29.9 +/- 2.7% of total covered surface. Aspirin-treated blood perfused under normoxic conditions showed a marked decrease in thrombus with a concomitant increase in both platelet adhesion and covered surface percentages. However, when aspirin-treated blood was perfused under hypoxic conditions, platelet interaction was not significantly different from that observed in untreated blood. Hypoxia induced a 10-fold increase in ATP release from erythrocytes in the perfusates. If apyrase was added to the perfusates, ATP release was prevented and aspirin effects were evident again.

CONCLUSION

Our results suggest that, under hypoxic conditions, the presence of aspirin would not help to inhibit further platelet activation.