Perivascular adipose tissue and coronary vascular disease

Coronary perivascular adipose tissue is a naturally occurring adipose tissue depot that normally surrounds the major coronary arteries on the surface of the heart. Although originally thought to promote vascular health and integrity, there is a growing body of evidence to support that coronary perivascular adipose tissue displays a distinct phenotype relative to other adipose depots and is capable of producing local factors with the potential to augment coronary vascular tone, inflammation, and the initiation and progression of coronary artery disease. The purpose of the present review is to outline previous findings about the cardiovascular effects of coronary perivascular adipose tissue and the potential mechanisms by which adipose-derived factors may influence coronary vascular function and the progression of atherogenesis.

Differential impact of dilator stimuli on increased myogenic activation of cerebral and skeletal muscle resistance arterioles in obese zucker rats

OBJECTIVE

To use the OZR model of the metabolic syndrome to determine the impact of dilator stimuli on MA of GA and MCA. We tested the hypothesis that increased oxidant stress and TxA2 exacerbate MA, and prevent its blunting with dilator stimuli, in OZR.

METHODS

GA/MCA from OZR and LZR was pressurized ex vivo. MA was determined under control conditions and following challenge with acetylcholine, hypoxia, and adenosine. Responses were also evaluated after pre-treatment with TEMPOL (antioxidant) and SQ-29548 (PGH2 /TxA2 receptor antagonist).

RESULTS

MA was increased (and dilator responses decreased) in GA/MCA from OZR, dependent on the endothelium and ROS. In GA, the impact of ROS on MA and dilator effects was largely via TxA2 , while in MCA, this appeared was more dependent on NO bioavailability. Intrinsic responses of GA/MCA to carbacyclin, U46619, and NO donors were similar between strains.

CONCLUSIONS

A developing ROS-based endothelial dysfunction in MCA and GA of OZR contributes to an enhanced MA of these vessels. Although treatment of GA/MCA with TEMPOL attenuates MA in OZR, the mechanistic contributors to altered MA, distal to ROS, differ between the two resistance vessels.

Contribution of electromechanical coupling between Kv and Ca v1.2 channels to coronary dysfunction in obesity

Previous investigations indicate that diminished functional expression of voltage-dependent K(+) (KV) channels impairs control of coronary blood flow in obesity/metabolic syndrome. The goal of this investigation was to test the hypothesis that KV channels are electromechanically coupled to CaV1.2 channels and that coronary microvascular dysfunction in obesity is related to subsequent increases in CaV1.2 channel activity. Initial studies revealed that inhibition of KV channels with 4-aminopyridine (4AP, 0.3 mM) increased intracellular [Ca(2+)], contracted isolated coronary arterioles and decreased coronary reactive hyperemia. These effects were reversed by blockade of CaV1.2 channels. Further studies in chronically instrumented Ossabaw swine showed that inhibition of CaV1.2 channels with nifedipine (10 μg/kg, iv) had no effect on coronary blood flow at rest or during exercise in lean swine. However, inhibition of CaV1.2 channels significantly increased coronary blood flow, conductance, and the balance between coronary flow and metabolism in obese swine (P < 0.05). These changes were associated with a ~50 % increase in inward CaV1.2 current and elevations in expression of the pore-forming subunit (α1c) of CaV1.2 channels in coronary smooth muscle cells from obese swine. Taken together, these findings indicate that electromechanical coupling between KV and CaV1.2 channels is involved in the regulation of coronary vasomotor tone and that increases in CaV1.2 channel activity contribute to coronary microvascular dysfunction in the setting of obesity.

Impaired cardiometabolic responses to glucagon-like peptide 1 in obesity and type 2 diabetes mellitus

Glucagon-like peptide 1 (GLP-1) has insulin-like effects on myocardial glucose uptake which may contribute to its beneficial effects in the setting of myocardial ischemia. Whether these effects are different in the setting of obesity or type 2 diabetes (T2DM) requires investigation. We examined the cardiometabolic actions of GLP-1 (7-36) in lean and obese/T2DM humans, and in lean and obese Ossabaw swine. GLP-1 significantly augmented myocardial glucose uptake under resting conditions in lean humans, but this effect was impaired in T2DM. This observation was confirmed and extended in swine, where GLP-1 effects to augment myocardial glucose uptake during exercise were seen in lean but not in obese swine. GLP-1 did not increase myocardial oxygen consumption or blood flow in humans or in swine. Impaired myocardial responsiveness to GLP-1 in obesity was not associated with any apparent alterations in myocardial or coronary GLP1-R expression. No evidence for GLP-1-mediated activation of cAMP/PKA or AMPK signaling in lean or obese hearts was observed. GLP-1 treatment augmented p38-MAPK activity in lean, but not obese cardiac tissue. Taken together, these data provide novel evidence indicating that the cardiometabolic effects of GLP-1 are attenuated in obesity and T2DM, via mechanisms that may involve impaired p38-MAPK signaling.

Blunted temporal activity of microvascular perfusion heterogeneity in metabolic syndrome: a new attractor for peripheral vascular disease?

A key clinical outcome for peripheral vascular disease (PVD) in patients is a progressive decay in skeletal muscle performance and its ability to resist fatigue with elevated metabolic demand. We have demonstrated that PVD in obese Zucker rats (OZR) is partially due to increased perfusion distribution heterogeneity at successive microvascular bifurcations within skeletal muscle. As this increased heterogeneity (γ) is longitudinally present in the network, its cumulative impact is a more heterogeneous distribution of perfusion between terminal arterioles than normal, causing greater regional tissue ischemia. To minimize this negative outcome, a likely compensatory mechanism against an increased γ should be an increased temporal switching at arteriolar bifurcations to minimize downstream perfusion deficits. Using in situ cremaster muscle, we determined that temporal activity (the cumulative sum of absolute differences between successive values of γ, taken every 20 s) was lower in OZR than in control animals, and this difference was present in both proximal (1A-2A) and distal (3A-4A) arteriolar bifurcations. Although adrenoreceptor blockade (phentolamine) improved temporal activity in 1A-2A arteriolar bifurcations in OZR, this was without impact in the distal microcirculation, where only interventions against oxidant stress (Tempol) and thromboxane A(2) activity (SQ-29548) were effective. Analysis of the attractor for γ indicated that it was not only elevated in OZR but also exhibited severe reductions in range, suggesting that the ability of the microcirculation to respond to any challenge is highly restricted and may represent the major contributor to the manifestation of poor muscle performance at this age in OZR.

Dynamic micro- and macrovascular remodeling in coronary circulation of obese Ossabaw pigs with metabolic syndrome

Previous studies from our laboratory showed that coronary arterioles from type 2 diabetic mice undergo inward hypertrophic remodeling and reduced stiffness. The aim of the current study was to determine if coronary resistance microvessels (CRMs) in Ossabaw swine with metabolic syndrome (MetS) undergo remodeling distinct from coronary conduit arteries. Male Ossabaw swine were fed normal (n = 7, Lean) or hypercaloric high-fat (n = 7, MetS) diets for 6 mo, and then CRMs were isolated and mounted on a pressure myograph. CRMs isolated from MetS swine exhibited decreased luminal diameters (126 ± 5 and 105 ± 9 μm in Lean and MetS, respectively, P < 0.05) with thicker walls (18 ± 3 and 31 ± 3 μm in Lean and MetS, respectively, P < 0.05), which doubled the wall-to-lumen ratio (14 ± 2 and 30 ± 2 in Lean and MetS, respectively, P < 0.01). Incremental modulus of elasticity (IME) and beta stiffness index (BSI) were reduced in CRMs isolated from MetS pigs (IME: 3.6 × 10(6) ± 0.7 × 10(6) and 1.1 × 10(6) ± 0.2 × 10(6) dyn/cm(2) in Lean and MetS, respectively, P < 0.001; BSI: 10.3 ± 0.4 and 7.3 ± 1.8 in Lean and MetS, respectively, P < 0.001). BSI in the left anterior descending coronary artery was augmented in pigs with MetS. Structural changes were associated with capillary rarefaction, decreased hyperemic-to-basal coronary flow velocity ratio, and augmented myogenic tone. MetS CRMs showed a reduced collagen-to-elastin ratio, while immunostaining for the receptor for advanced glycation end products was selectively increased in the left anterior descending coronary artery. These data suggest that MetS causes hypertrophic inward remodeling of CRMs and capillary rarefaction, which contribute to decreased coronary flow and myocardial ischemia. Moreover, our data demonstrate novel differential remodeling between coronary micro- and macrovessels in a clinically relevant model of MetS.

Oxidant stress and skeletal muscle microvasculopathy in the metabolic syndrome

The evolution of the metabolic syndrome in afflicted individuals is, in part, characterized by the development of a severely pro-oxidant state within the vasculature. It has been previously demonstrated by many investigators that this increasingly pro-oxidant state can have severe negative implications for many relevant processes within the vasculature, including the coordination of dilator/constrictor tone or reactivity, the structural adaptations of the vascular wall or distal networks, as well as the integrated regulation of perfusion resistance across and throughout the vascular networks. The purpose of this review article is to present the different sources of oxidant stress within the setting of the metabolic syndrome, the available mechanism for attempts at regulation and the vascular outcomes associated with this condition. It is anticipated that this overview will help readers and investigators to more effectively design experiments and interpret their results within the extremely complicated setting of metabolic syndrome.

The ex vivo isolated skeletal microvessel preparation for investigation of vascular reactivity

The isolated microvessel preparation is an ex vivo preparation that allows for examination of the different contributions of factors that control vessel diameter, and thus, perfusion resistance(1-5). This is a classic experimental preparation that was, in large measure, initially described by Uchida et al.(15) several decades ago. This initial description provided the basis for the techniques that was extensively modified and enhanced, primarily in the laboratory of Dr. Brian Duling at the University of Virginia(6-8), and we present a current approach in the following pages. This preparation will specifically refer to the gracilis arteriole in a rat as the microvessel of choice, but the basic preparation can readily be applied to vessels isolated from nearly any other tissue or organ across species(9-13). Mechanical (i.e., dimensional) changes in the isolated microvessels can easily be evaluated in response to a broad array of physiological (e.g., hypoxia, intravascular pressure, or shear) or pharmacological challenges, and can provide insight into mechanistic elements comprising integrated responses in an intact, although ex vivo, tissue. The significance of this method is that it allows for facile manipulation of the influences on the integrated regulation of microvessel diameter, while also allowing for the control of many of the contributions from other sources, including intravascular pressure (myogenic), autonomic innervation, hemodynamic (e.g., shear stress), endothelial dependent or independent stimuli, hormonal, and parenchymal influences, to provide a partial list. Under appropriate experimental conditions and with appropriate goals, this can serve as an advantage over in vivo or in situ tissue/organ preparations, which do not readily allow for the facile control of broader systemic variables. The major limitation of this preparation is essentially the consequence of its strengths. By definition, the behavior of these vessels is being studied under conditions where many of the most significant contributors to the regulation of vascular resistance have been removed, including neural, humoral, metabolic, etc. As such, the investigator is cautioned to avoid over-interpretation and extrapolation of the data that are collected utilizing this preparation. The other significant area of concern with regard to this preparation is that it can be very easy to damage cellular components such as the endothelial lining or the vascular smooth muscle, such that variable source of error can be introduced. It is strongly recommended that the individual investigator utilize appropriate measurements to ensure the quality of the preparation, both at the initiation of the experiment and periodically throughout the course of a protocol.

Pulmonary particulate matter and systemic microvascular dysfunction

Pulmonary particulate matter (PM) exposure has been epidemiologically associated with an increased risk of cardiovascular morbidity and mortality, but the mechanistic foundations for this association are unclear. Exposure to certain types of PM causes changes in the vascular reactivity of several macrovascular segments. However, no studies have focused upon the systemic microcirculation, which is the primary site for the development of peripheral resistance and, typically, the site of origin for numerous pathologies. Ultrafine PM--also referred to as nanoparticles, which are defined as ambient and engineered particles with at least one physical dimension less than 100 nm (Oberdorster et al. 2005)--has been suggested to be more toxic than its larger counterparts by virtue of a larger surface area per unit mass. The purpose of this study was fourfold: (1) determine whether particle size affects the severity of postexposure microvascular dysfunction; (2) characterize alterations in microvascular nitric oxide (NO) production after PM exposure; (3) determine whether alterations in microvascular oxidative stress are associated with NO production, arteriolar dysfunction, or both; and (4) determine whether circulating inflammatory mediators, leukocytes, neurologic mechanisms, or a combination of these play a fundamental role in mediating pulmonary PM exposure and peripheral microvascular dysfunction. To achieve these goals, we created an inhalation chamber that generates stable titanium dioxide (TiO2) aerosols at concentrations up to 20 mg/m3. TiO2 is a well-characterized particle devoid of soluble metals. Sprague Dawley and Fischer 344 (F-344) rats were exposed to fine or nano-TiO2 PM (primary count modes of approximately 710 nm and approximately 100 nm in diameter, respectively) at concentrations of 1.5 to 16 mg/m3 for 4 to 12 hours to produce pulmonary loads of 7 to 150 microg in each rat. Twenty-four hours after pulmonary exposure, the following procedures were performed: the spinotrapezius muscle was prepared for in vivo microscopy, blood samples were taken from an arterial line, and various tissues were harvested for histologic and immunohistochemical analyses. Some rats received a bolus dose of cyclophosphamide 3 days prior to PM exposure to deplete circulating neutrophils and bronchoalveolar lavage (BAL) was performed in separate groups of rats exposed to identical TiO2 loads. No significant differences in BAL fluid composition based on PM size or load were found in these rats. Plasma levels of interleukin (IL)-2, IL-18, IL-13, and growth-related oncogene (GRO) (also known as keratinocyte-derived-chemokine [KC]) were altered after PM exposure. In rats exposed to fine TiO2, endothelium-dependent arteriolar dilation was significantly decreased, and this dysfunction was robustly augmented in rats exposed to nano-TiO2. This effect was not related to an altered smooth-muscle responsiveness to NO because arterioles in both groups dilated comparably in response to the NO donor sodium nitroprusside (SNP). Endogenous microvascular NO production was similarly decreased after inhalation of either fine or nano-TiO2 in a dose-dependent manner. Microvascular oxidative stress was significantly increased among both exposure groups. Furthermore, treatment with antioxidants (2,2,6,6-tetramethylpiperdine-N-oxyl [TEMPOL] plus catalase), the myeloperoxidase (MPO) inhibitor 4-aminobenzoic hydrazide (ABAH), or the nicotinamide adenine dinucleotide phosphate oxidase (NADPH oxidase) inhibitor apocynin partially restored NO production and normalized arteriolar function in both groups. Neutrophil depletion restored dilation in PM-exposed rats by as much as 42%. Coincubation of the spinotrapezius muscle with the fast sodium (Na+) channel antagonist tetrodotoxin (TTX) restored arteriolar dilation by as much as 54%, suggesting that sympathetic neural input may be affected by PM exposure. The results of these experiments indicate that (1) the size of inhaled PM dictates the intensity of systemic microvascular dysfunction; (2) this arteriolar dysfunction is characterized by a decreased bioavailability of endogenous NO; (3) the loss of bioavailable NO after PM exposure is at least partially caused by elevations in local oxidative stress, MPO activity, NADPH oxidase activity, or a combination of these responses; and (4) circulating neutrophils and sympathetic neurogenic mechanisms also appear to be involved in the systemic microvascular dysfunction that follows PM exposure. Taken together, these mechanistic studies support prominent hypotheses that suggest peripheral vascular effects associated with PM exposure are due to the activation of inflammatory mechanisms, neurogenic mechanisms, or both.

Spatial heterogeneity in skeletal muscle microvascular blood flow distribution is increased in the metabolic syndrome

Previous studies have demonstrated that the metabolic syndrome is associated with impaired skeletal muscle arteriolar function, although integrating observations into a conceptual framework for impaired perfusion in peripheral vascular disease (PVD) has been limited. This study builds on previous work to evaluate in situ arteriolar hemodynamics in cremaster muscle of obese Zucker rats (OZR) to integrate existing knowledge into a greater understanding of impaired skeletal muscle perfusion. In OZR cremaster muscle, perfusion distribution at microvascular bifurcations (γ) was consistently more heterogeneous than in controls. However, while consistent, the underlying mechanistic contributors were spatially divergent as altered adrenergic constriction was the major contributor to altered γ at proximal microvascular bifurcations, with a steady decay with distance, while endothelial dysfunction was a stronger contributor in distal bifurcations with no discernible role proximally. Using measured values of γ, we found that simulations predict that successive alterations to γ in OZR caused more heterogeneous perfusion distribution in distal arterioles than in controls, an effect that could only be rectified by combined adrenoreceptor blockade and improvements to endothelial dysfunction. Intravascular (125)I-labeled albumin tracer washout from in situ gastrocnemius muscle of OZR provided independent support for these observations, indicating increased perfusion heterogeneity that was corrected only by combined adrenoreceptor blockade and improved endothelial function. These results suggest that a defining element of PVD in the metabolic syndrome may be an altered γ at microvascular bifurcations, that its contributors are heterogeneous and spatially distinct, and that interventions to rectify this negative outcome must take a new conceptual framework into account.

Divergence between arterial perfusion and fatigue resistance in skeletal muscle in the metabolic syndrome

The metabolic syndrome is associated with elevated peripheral vascular disease risk, characterized by mismatched blood flow delivery/distribution and local metabolism. The obese Zucker rat (OZR) model of the metabolic syndrome exhibits myriad vascular impairments, although their integrated impact on functional hyperaemia remains unclear. In this study, arterial pressor responses and skeletal muscle perfusion were assessed in lean Zucker rats (LZRs) and OZRs during adrenergic stimulation (phenylephrine), challenge with thromboxane (U46619) and endothelium-dependent dilatation (methacholine). The OZRs were hypertensive compared with the LZRs, but this was abolished by adrenoreceptor blockade (phentolamine); pressor responses to U46619 were similar between strains and were abolished by blockade with the prostaglandin H(2)/thromboxane A(2) receptor antagonist, SQ-29548. Depressor reactivity to methacholine was impaired in OZRs, but was improved by antioxidant treatment (TEMPOL). Across levels of metabolic demand, blood flow to in situ gastrocnemius muscle was restrained by adrenergic constriction in OZRs, although this diminished with increased demand. Oxygen extraction, reduced in OZRs compared with LZRs across levels of metabolic demand, was improved by TEMPOL or SQ-29548; treatment with phentolamine did not impact extraction, and neither TEMPOL nor SQ-29548 improved muscle blood flow in OZRs. While oxygen uptake and muscle performance were consistently reduced in OZRs versus LZRs, treatment with all three agents improved outcomes, while treatment with individual agents was less effective. These results suggest that contributions of vascular dysfunction to perfusion, oxygen uptake and muscle performance are spatially distinct, with adrenergic constriction impacting proximal resistance and endothelial dysfunction impacting distal microvessel-tissue exchange. Further, these data suggest that increasing skeletal muscle blood flow in OZRs is not sufficient to improve performance, unless distal perfusion inhomogeneities are rectified.

Hypercholesterolemia and microvascular dysfunction: interventional strategies

Hypercholesterolemia is defined as excessively high plasma cholesterol levels, and is a strong risk factor for many negative cardiovascular events. Total cholesterol levels above 200 mg/dl have repeatedly been correlated as an independent risk factor for development of peripheral vascular (PVD) and coronary artery disease (CAD), and considerable attention has been directed toward evaluating mechanisms by which hypercholesterolemia may impact vascular outcomes; these include both results of direct cholesterol lowering therapies and alternative interventions for improving vascular function. With specific relevance to the microcirculation, it has been clearly demonstrated that evolution of hypercholesterolemia is associated with endothelial cell dysfunction, a near-complete abrogation in vascular nitric oxide bioavailability, elevated oxidant stress, and the creation of a strongly pro-inflammatory condition; symptoms which can culminate in profound impairments/alterations to vascular reactivity. Effective interventional treatments can be challenging as certain genetic risk factors simply cannot be ignored. However, some hypercholesterolemia treatment options that have become widely used, including pharmaceutical therapies which can decrease circulating cholesterol by preventing either its formation in the liver or its absorption in the intestine, also have pleiotropic effects with can directly improve peripheral vascular outcomes. While physical activity is known to decrease PVD/CAD risk factors, including obesity, psychological stress, impaired glycemic control, and hypertension, this will also increase circulating levels of high density lipoprotein and improving both cardiac and vascular function. This review will provide an overview of the mechanistic consequences of the predominant pharmaceutical interventions and chronic exercise to treat hypercholesterolemia through their impacts on chronic sub-acute inflammation, oxidative stress, and microvascular structure/function relationships.

Association between dental hygiene, cardiovascular disease risk factors and systemic inflammation in rural adults

PURPOSE

A growing body of epidemiologic evidence links oral health, periodontal disease and cardiovascular health. While underlying pathophysiologic mechanisms are unclear, several studies have suggested a sub-acute inflammatory state, also implicated in the etiology of cardiovascular disease. The objective of the current study was to investigate associations between self-reported dental hygiene (brushing, flossing, preventive care and overall dental health), cardiovascular disease risk factors and systemic inflammation.

METHODS

128 adults from 5 different rural counties in West Virginia participated in a comprehensive, community-based health screening that included anthropometric assessments, collection of a blood specimen and completion of a questionnaire about dental hygiene practices and oral health.

RESULTS

Univariate analysis demonstrated multiple statistically significant associations between self-reported dental hygiene and cardiovascular disease risk factors and markers of systemic inflammation. In regression analysis, after controlling for demographic and cardiovascular disease risk factor covariates, self-reported dental hygiene demonstrated statistically significant and independent associations with adiponectin, fibrinogen, C-reactive protein (CRP) and cellular adhesion molecule-1 (sICAM-1).

CONCLUSION

This study demonstrated associations between dental hygiene and systemic inflammation, independent from BMI and blood cholesterol. Future studies should investigate whether periodontal-related systemic inflammation begins before the onset of clinical disease. Results from this and other studies highlight the importance of dental hygiene in overall systemic health, and are beginning to collectively suggest that regular dental hygiene care is an integral part of comprehensive health care.

Self-reported dental hygiene, obesity, and systemic inflammation in a pediatric rural community cohort

BACKGROUND

A growing body of epidemiologic evidence links oral health, obesity, and cardiovascular health, though few studies have reported on these relationships in children. While underlying mechanisms are unclear, adult studies have suggested sub-acute systemic inflammation, also implicated in the etiology of both obesity and cardiovascular disease. This study investigated associations between self-reported dental hygiene, obesity, and systemic inflammation in children.

METHODS

128 children < 19 years of age from rural counties in West Virginia participated in a community-based health screening that included anthropometric assessments, blood collection, and a questionnaire about dental hygiene and self-assessed oral health.

RESULTS

Participants ranged from 3.0-18.7 years. Univariate analysis demonstrated an association between parent-reported dental hygiene, including frequency of preventive dental care and parent-assessed overall dental health, and markers of systemic inflammation but not obesity. In multivariable regression, parent-assessed overall dental health and obesity were independent predictors of systemic inflammation, after adjustment for age, gender, and parent education.

CONCLUSIONS

This is the first known study of the association between dental hygiene, obesity, and systemic inflammation in children. These results highlight the importance of preventive dental care in overall, systemic health in children and are consistent with previous reports in adults.

Differential impact of familial hypercholesterolemia and combined hyperlipidemia on vascular wall and network remodeling in mice

Genetic familial hypercholesterolemia (FH) and combined hyperlipidemia (FCH) are characterized by elevated plasma low-density lipoprotein (LDL) (FH) and LDL/triglycerides (FCH), with mouse models represented by LDL receptor (LDLR) and apolipoprotein E (ApoE) gene deletion mice, respectively. Given the impact of FH and FCH on health outcomes, we determined the impact of FH/FCH on vascular structure in LDLR and ApoE mice. LDLR, ApoE and control mice were utilized at 12-13 and 22-23 weeks when gracilis arteries were studied for wall mechanics and gastrocnemius muscles were harvested for microvessel density measurements. Conduit arteries and plasma samples were harvested for biochemical analyses. Arteries from ApoE and LDLR exhibited blunted expansion versus control, reduced distensibility and left-shifted stress versus strain relation (LDLR > ApoE). Microvessel density was reduced in ApoE and LDLR (ApoE > LDLR). Secondary analyses suggested that wall remodeling in LDLR was associated with cholesterol and MCP-1, while rarefaction in ApoE was associated with tumor necrosis factors-alpha, triglycerides and vascular production of TxA(2). Remodeling in ApoE and LDLR appears distinct; as that in LDLR is preferential for vascular walls, while that for ApoE is stronger for rarefaction. Remodeling in LDLR may be associated with cellular adhesion, while that in ApoE may be associated with pro-apoptotsis and constrictor prostanoid generation.

Aspirin resistance with genetic dyslipidemia: contribution of vascular thromboxane generation

One clinical intervention against the negative outcomes associated with atherothrombotic vascular disease (AVD) is low-dose, chronic aspirin therapy. However, epidemiological studies suggest that recurrence of adverse vascular events with aspirin therapy is growing and associated with therapy duration. The contributors to this outcome are unclear and include poor patient compliance and aspirin-resistant platelet thromboxane A(2) (TxA(2)) production. Based on previous results in hypercholesterolemic mice, we hypothesized that elevated aspirin-insensitive arachidonic acid (AA)-induced TxA(2) production by the vascular endothelium contributes to aspirin resistance in AVD independent of platelet behavior. AA-induced dilation was blunted in aortic rings and in arterioles from apolipoprotein E (ApoE) and low-density lipoprotein receptor (LDLR) gene deletion mice (vs. C57/Bl6/J), partially due to elevated TxA(2) production. Acute inhibition of cyclooxygenases or TxA(2) synthase attenuated the increased TxA(2) production in ApoE and LDLR and improved AA-induced dilation, responses that were mirrored by chronic treatment with low-dose aspirin of 16 wk duration. However, this effect was not temporally stable, and, with longer-duration therapy, the beneficial impact of aspirin on outcomes diminished. A similar, though less robust, pattern to the impact of chronic aspirin therapy on vascular outcomes was identified with chronic antioxidant treatment (TEMPOL). These results suggest that in dyslipidemic mice, the beneficial impact of chronic aspirin therapy on improving vascular outcomes decay with time and that a contributing element to subsequent negative vascular events may be the development of aspirin-resistant TxA(2) production by the vasculature itself.