Characterization of endothelium-derived hyperpolarizing factor in the human forearm microcirculation

Electronic Cigarette Exposure Causes Vascular Endothelial Dysfunction Due to NADPH Oxidase Activation and eNOS Uncoupling

We recently reported a mouse model of chronic electronic cigarette (e-cig) exposure-induced cardiovascular pathology, where long-term exposure to e-cig vape (ECV) induces cardiac abnormalities, impairment of endothelial function, and systemic hypertension. Here, we delineate the underlying mechanisms of ECV-induced vascular endothelial dysfunction (VED), a central trigger of cardiovascular disease. C57/BL6 male mice were exposed to ECV generated from e-cig liquid containing 0, 6, or 24 mg/ml nicotine for 16 and 60 weeks. Time-dependent elevation in blood pressure and systemic vascular resistance were observed, along with an impairment of acetylcholine-induced aortic relaxation in ECV-exposed mice, compared to air-exposed control. Decreased intravascular nitric oxide (NO) levels and increased superoxide generation with elevated 3-nitrotyrosine levels in the aorta of ECV-exposed mice were observed, indicating that ECV-induced superoxide reacts with NO to generate cytotoxic peroxynitrite. Exposure increased NADPH oxidase expression, supporting its role in ECV-induced superoxide generation. Downregulation of endothelial nitric oxide synthase (eNOS) expression and Akt-dependent eNOS phosphorylation occurred in the aorta of ECV-exposed mice, indicating that exposure inhibited de novo NO synthesis. Following ECV exposure, the critical NOS cofactor tetrahydrobiopterin was decreased, with a concomitant loss of its salvage enzyme, dihydrofolate reductase. NADPH oxidase and NOS inhibitors abrogated ECV-induced superoxide generation in the aorta of ECV exposed mice. Together, our data demonstrate that ECV exposure activates NADPH oxidase and uncouples eNOS, causing a vicious cycle of superoxide generation and vascular oxidant stress that triggers VED and hypertension with predisposition to other cardiovascular disease.

A Novel Role for Cytochrome P450 Epoxygenase Metabolites in Septic Shock

Supplemental Digital Content is available in the text.

This is the largest study describing the role of P450 epoxygenase metabolites in septic shock in humans and suggests a novel therapeutic target.

The importance of endothelial dysfunction in resistance artery remodelling and cardiovascular risk

AIMS

The relationship between resistance artery remodelling and endothelial function remains unknown. In this study, we assessed (i) the capacity of endothelial function and nitric oxide (NO) availability to provide more information on the severity of resistance artery remodelling than common cardiovascular risk factors in subjects at low or high cardiovascular risk; and (ii) differences between patterns of resistance artery remodelling associated with deficit of NO availability and with exposure to cardiovascular risk factors.

METHODS AND RESULTS

All analyses were conducted on the microvascular data set of the Italian Society for Arterial Hypertension (SIIA) that includes 356 patients with measures of small resistance arteries remodelling acquired with pressure or wire myography. Information on endothelial function and NO availability were also available in 116 patients. The European Heart Score (HS) was used to define the total cardiovascular risk of each patient. Endothelial function was inversely related with the severity of the resistance artery remodelling, and this association remained significant after adjustment for the HS. By contrast, the HS lost its significant association with the media-to-lumen (M/L) ratio and the media cross-sectional area after adjustment for endothelial function. The strength of these associations was similar in subjects at high and low cardiovascular risk. The addition of endothelial function and NO availability to the HS significantly improved the identification of subjects at more and less severe resistance artery remodelling. A severe deficit of NO availability was associated with hypertrophic remodelling, while a higher HS was more clearly associated with eutrophic remodelling.

CONCLUSION

Resistance artery endothelial function and NO availability might represent important factors involved in resistance artery remodelling, independently from cardiovascular risk factor exposure.

Peripheral Microvascular Function Reflects Coronary Vascular Function

Objectives- Coronary endothelial dysfunction is a precursor of atherosclerosis and adverse outcomes. Whether endothelial dysfunction is a localized or generalized phenomenon in humans remains uncertain. We simultaneously measured femoral and coronary vascular function with the hypothesis that peripheral vascular endothelial function will be reflective of coronary endothelial function. Approach and Results- Eighty-five subjects underwent coronary angiography for evaluation of chest pain or abnormal stress tests. Endothelium-dependent and -independent vascular function were measured using intracoronary and intrafemoral infusions of acetylcholine and sodium nitroprusside, respectively. Coronary flow reserve was assessed using intracoronary adenosine infusion. Flow velocity was measured in each circulation using a Doppler wire (FloWire, EndoSonics). Coronary vascular resistance and femoral vascular resistance were calculated as mean arterial pressure (mm Hg)/coronary blood flow (mL/min) and mean arterial pressure (mm Hg)/femoral average peak velocity (cm/s), respectively. Mean age was 53±11 years, 37% were female, 44% had hypertension, 12% had diabetes mellitus, and 38% had obstructive coronary artery disease. There was a correlation between the change in femoral vascular resistance with acetylcholine and acetylcholine-mediated changes in both the coronary vascular resistance ( r=0.27; P=0.014) and in the epicardial coronary artery diameter ( r=-0.25; P=0.021), indicating that subjects with normal endothelial function in the femoral circulation had normal endothelial function in the coronary epicardial and microcirculation and vice versa. The coronary vasodilator response to adenosine also correlated with the femoral vasodilatation with acetylcholine ( r=0.4; P=0.0002). There was no correlation between the coronary and femoral responses to sodium nitroprusside. Conclusions- Endothelial functional changes in the peripheral and coronary circulations were modestly correlated. Thus, peripheral microvascular endothelial function reflects endothelium-dependent coronary epicardial and microvascular function and the coronary flow reserve. Visual Overview- An online visual overview is available for this article.

The role of vascular endothelium in nitroglycerin-mediated vasodilation

AIMS

Nitroglycerin (or glyceryl trinitrate, GTN) has been long considered an endothelium-independent vasodilator because GTN vasodilation is intact in the absence of the endothelium and in the presence of endothelial dysfunction. However, in animal and in vitro models, GTN has been shown to stimulate the release of certain endothelium-derived vasodilators such as nitric oxide (NO) and prostacyclin (PGI₂ ). In addition, chronic GTN therapy leads to endothelial dysfunction. In this series of experiments, we explored how GTN might interact with the vascular endothelium in normal humans, without cardiovascular disease or risk factors associated with abnormalities in vascular function.

METHODS

We examined the effect of inhibition of NO, PGI₂ , and epoxyeicosatrienoic acids (EETs, a class of endothelium-derived hyperpolarizing factor) on GTN-mediated vasodilation. We measured arterial blood flow responses to brachial artery infusions of GTN in the absence and presence of L-NMMA (n = 13), ketorolac (n = 14) and fluconazole (n = 16), which are inhibitors of endothelium-derived NO, PGI₂ and EETs, respectively, in healthy volunteers.

RESULTS

Our results demonstrate that inhibition of endothelium-dependent vasodilator mechanisms does not alter forearm resistance vessel responses to GTN.

CONCLUSION

We conclude that GTN-mediated dilation of forearm resistance vessels is largely independent of vascular endothelium.

Possible Role of CYP450 Generated Omega-3/Omega-6 PUFA Metabolites in the Modulation of Blood Pressure and Vascular Function in Obese Children

Obesity is often accompanied by metabolic and haemodynamic disorders such as hypertension, even during childhood. Arachidonic acid (AA) is metabolized by cytochrome P450 (CYP450) enzymes to epoxyeicosatrienoic acids (EETs) and 20-hydroxyeicosatetraenoic acid (20-HETE), vasoactive and natriuretic metabolites that contribute to blood pressure (BP) regulation. Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) omega-3 polyunsaturated fatty acids may compete with AA for CYP450-dependent bioactive lipid mediator formation. We aimed at investigating the role of AA, EPA and DHA and their CYP450-dependent metabolites in BP control and vascular function in 66 overweight/obese children. Fatty acid profile moderately correlated with the corresponding CYP450-derived metabolites but their levels did not differ between children with normal BP (NBP) and high BP (HBP), except for higher EPA-derived epoxyeicosatetraenoic acids (EEQs) and their diols in HBP group, in which also the estimated CYP450-epoxygenase activity was higher. In the HBP group, EPA inversely correlated with BP, EEQs inversely correlated both with systolic BP and carotid Intima-Media Thickness (cIMT). The DHA-derived epoxydocosapentaenoic acids (EDPs) were inversely correlated with diastolic BP. Omega-3 derived epoxymetabolites appeared beneficially associated with BP and vascular structure/function only in obese children with HBP. Further investigations are needed to clarify the role of omega-3/omega-6 epoxymetabolites in children's hemodynamics.

Elevated extracellular potassium prior to muscle contraction reduces onset and steady-state exercise hyperemia in humans

The increase in interstitial potassium (K⁺) during muscle contractions is thought to be a vasodilatory signal that contributes to exercise hyperemia. To determine the role of extracellular K⁺ in exercise hyperemia, we perfused skeletal muscle with K⁺ before contractions, such that the effect of any endogenously-released K⁺ would be minimized. We tested the hypothesis that local, intra-arterial infusion of potassium chloride (KCl) at rest would impair vasodilation in response to subsequent rhythmic handgrip exercise in humans. In 11 young adults, we determined forearm blood flow (FBF) (Doppler ultrasound) and forearm vascular conductance (FVC) (FBF/mean arterial pressure) during 4 min of rhythmic handgrip exercise at 10% of maximal voluntary contraction during 1) control conditions, 2) infusion of KCl before the initiation of exercise, and 3) infusion of sodium nitroprusside (SNP) as a control vasodilator. Infusion of KCl or SNP elevated resting FVC similarly before the onset of exercise (control: 39 ± 6 vs. KCl: 81 ± 12 and SNP: 82 ± 13 ml·min^-1·100 mmHg^-1; both P < 0.05 vs. control). Infusion of KCl at rest diminished the hyperemic (ΔFBF) and vasodilatory (ΔFVC) response to subsequent exercise by 22 ± 5% and 30 ± 5%, respectively (both P < 0.05 vs. control), whereas SNP did not affect the change in FBF ( P = 0.74 vs. control) or FVC ( P = 0.61 vs. control) from rest to steady-state exercise. These findings implicate the K⁺ ion as an essential vasodilator substance contributing to exercise hyperemia in humans. NEW & NOTEWORTHY Our findings support a significant and obligatory role for potassium signaling in the local vasodilatory and hyperemic response to exercise in humans.

Mechanisms and Clinical Significance of Endothelial Dysfunction in High-Risk Pregnancies

The maternal cardiovascular system undergoes critical anatomic and functional adaptations to achieve a successful pregnancy outcome which, if disrupted, can result in complications that significantly affect maternal and fetal health. Complications that involve the maternal cardiovascular system are among the most common disorders of pregnancy, including gestational hypertension, preeclampsia, gestational diabetes, and impaired fetal growth. As a central feature, maternal endothelial dysfunction is hypothesized to play a predominant role in mediating the pathogenesis of these high-risk pregnancies, and as such, might proceed and precipitate the clinical presentation of these pregnancy disorders. Improving or normalizing maternal endothelial function in high-risk pregnancies might be an effective therapeutic strategy to ameliorate maternal and fetal clinical outcomes.

Vascular hyperpolarization in human physiology and cardiovascular risk conditions and disease

Hyperpolarization causing smooth muscle relaxation contributes to the maintenance of vascular homeostasis, particularly in small-calibre arteries and arterioles. It may also become a compensatory vasodilator mechanism upregulated in states with impaired nitric oxide (NO) availability. Bioassay of vascular hyperpolarization in the human circulation has been hampered by the complexity of mechanisms involved and the limited availability of investigational tools. Firm evidence, however, supports the notion that hyperpolarization participates in the regulation of resting vasodilator tone and vascular reactivity in healthy subjects. In addition, an enhanced endothelium-derived hyperpolarization contributes to both resting and agonist-stimulated vasodilation in a variety of cardiovascular risk conditions and disease. Thus, hyperpolarization mediated by epoxyeicosatrienoic acids (EETs) and H₂ O₂ has been observed in coronary arterioles of patients with coronary artery disease. Similarly, ouabain-sensitive and EETs-mediated hyperpolarization has been observed to compensate for NO deficiency in patients with essential hypertension. Moreover, in non-hypertensive patients with multiple cardiovascular risk factors and in hypercholesterolaemia, K_Ca channel-mediated vasodilation appears to be activated. A novel paradigm establishes that perivascular adipose tissue (PVAT) is an additional regulator of vascular tone/function and endothelium is not the only agent in vascular hyperpolarization. Indeed, some PVAT-derived relaxing substances, such as adiponectin and angiotensin 1-7, may exert anticontractile and vasodilator actions by the opening of K_Ca channels in smooth muscle cells. Conversely, PVAT-derived factors impair coronary vasodilation via differential inhibition of some K⁺ channels. In view of adipose tissue abnormalities occurring in human obesity, changes in PVAT-dependent hyperpolarization may be relevant for vascular dysfunction also in this condition.

Impaired Vasodilatory Responses of Omental Arteries to CGRP Family Peptides in Pregnancies Complicated by Fetal Growth Restriction

RATIONALE

Calcitonin gene-related peptide (CGRP), adrenomedullin (ADM), and adrenomedullin2 (ADM2)/intermedin are potent vasorelaxant peptides considered to play a role in the adaptive mechanisms in rat pregnancy through increased vasodilation in mesenteric and uterine artery.

OBJECTIVE

This study was designed to demonstrate the response of omental arteries (OA) to vasoactive peptides CGRP, ADM, and ADM2 in pregnancy complications such as fetal growth restriction (FGR), and assess the changes in the expression of their receptor components in segments of OA from FGR pregnancy compared to the control.

FINDINGS

The findings for this study are: 1) relaxation responses of OA were higher for bradykinin (78.55 ± 3.91 vs 52.67 ± 2.19; P < .05) in pregnancy with FGR compared to the normal, 2) relaxation response of OA segments to CGRP was similar with no change in the expression of G-protein couple receptor-calcitonin receptor-like receptor complex in normal healthy pregnancy and pregnancy complicated by FGR, 3) maximal relaxation response of OA were significantly (P < .05) lower for both ADM (18.2 ± 6.7 vs 38 ± 2.5) and ADM2 (26.9 ± 6.7 vs 48 ± 2.6) along with decreases in their respective ligand-receptor complex in FGR compared to the normal pregnancies, 4) expression of calcitonin receptor-like receptor mRNA was higher but its immunoreactivity was lower in OA from FGR pregnancy compared to the normal, and 5) mRNA and protein levels of RAMP1, RAMP2, and RAMP3 were lower in OA isolated from FGR pregnancies compared to the normal.

CONCLUSION

The current study demonstrates that FGR is associated with an increase in the sensitivity of OA to bradykinin and decreased sensitivity for ADM and ADM2 ligand-receptor system with no change in the response for CGRP compared to the normal healthy pregnancy, and suggests a potential role for ADM and ADM2 in the pathophysiology of maternal vasculature in FGR pregnancy.

Nitric Oxide Contributes to Vasomotor Tone in Hypertensive African Americans Treated With Nebivolol and Metoprolol

Endothelial dysfunction is more prevalent in African Americans (AAs) compared with whites. The authors hypothesized that nebivolol, a selective β1 -antagonist that stimulates nitric oxide (NO), will improve endothelial function in AAs with hypertension when compared with metoprolol. In a double-blind, randomized, crossover study, 19 AA hypertensive patients were randomized to a 12-week treatment period with either nebivolol 10 mg or metoprolol succinate 100 mg daily. Forearm blood flow (FBF) was measured using plethysmography at rest and after intra-arterial infusion of acetylcholine and sodium nitroprusside to estimate endothelium-dependent and independent vasodilation, respectively. Physiologic vasodilation was assessed during hand-grip exercise. Measurements were repeated after NO blockade with L-N(G) -monomethylarginine (L-NMMA) and after inhibition of endothelium-derived hyperpolarizing factor (EDHF) with tetraethylammonium chloride (TEA). NO blockade with L-NMMA produced a trend toward greater vasoconstriction during nebivolol compared with metoprolol treatment (21% vs 12% reduction in FBF, P=.06, respectively). This difference was more significant after combined administration of L-NMMA and TEA (P<.001). Similarly, there was a contribution of NO to exercise-induced vasodilation during nebivolol but not during metoprolol treatment. There were significantly greater contributions of NO and EDHF to resting vasodilator tone and of NO to exercise-induced vasodilation with nebivolol compared with metoprolol in AAs with hypertension.

The role of epoxyeicosatrienoic acids in the cardiovascular system

There is increasing evidence suggesting that epoxyeicosatrienoic acids (EETs) play an important role in cardioprotective mechanisms. These include regulating vascular tone, modulating inflammatory responses, improving cardiomyocyte function and reducing ischaemic damage, resulting in attenuation of animal models of cardiovascular risk factors. This review discusses the current knowledge on the role of EETs in endothelium-dependent control of vascular tone in the healthy and in subjects with cardiovascular risk factors, and considers the pharmacological potential of targeting this pathway.

Palmitoylethanolamide treatment reduces blood pressure in spontaneously hypertensive rats: involvement of cytochrome p450-derived eicosanoids and renin angiotensin system

Palmitoylethanolamide (PEA), a peroxisome proliferator-activated receptor-α agonist, has been demonstrated to reduce blood pressure and kidney damage secondary to hypertension in spontaneously hypertensive rat (SHR). Currently, no information is available concerning the putative effect of PEA on modulating vascular tone. Here, we investigate the mechanisms underpinning PEA blood pressure lowering effect, exploring the contribution of epoxyeicosatrienoic acids, CYP-dependent arachidonic acid metabolites, as endothelium-derived hyperpolarizing factors (EDHF), and renin angiotensin system (RAS) modulation. To achieve this aim SHR and Wistar-Kyoto rats were treated with PEA (30 mg/kg/day) for five weeks. Functional evaluations on mesenteric bed were performed to analyze EDHF-mediated vasodilation. Moreover, mesenteric bed and carotid were harvested to measure CYP2C23 and CYP2J2, the key isoenzymes in the formation of epoxyeicosatrienoic acids, and the soluble epoxide hydrolase, which is responsible for their degradation in the corresponding diols. Effect of PEA on RAS modulation was investigated by analyzing angiotensin converting enzyme and angiotensin receptor 1 expression. Here, we showed that EDHF-mediated dilation in response to acetylcholine was increased in mesenteric beds of PEA-treated SHR. Western blot analysis revealed that the increase in CYP2C23 and CYP2J2 observed in SHR was significantly attenuated in mesenteric beds of PEA-treated SHR, but unchanged in the carotids. Interestingly, in both vascular tissues, PEA significantly decreased the soluble epoxide hydrolase protein level, accompanied by a reduced serum concentration of its metabolite 14-15 dihydroxyeicosatrienoic acid, implying a reduction in epoxyeicosatrienoic acid hydrolisis. Moreover, PEA treatment down-regulated angiotensin receptor 1 and angiotensin converting enzyme expression, indicating a reduction in angiotensin II-mediated effects. Consistently, a damping of the activation of angiotensin receptor 1 underlying pathways in mesenteric beds was shown in basal conditions in PEA-treated SHR. In conclusion, our data demonstrate the involvement of epoxyeicosatrienoic acids and renin angiotensin system in the blood pressure lowering effect of PEA.

Contribution of endothelium-derived hyperpolarizing factor to exercise-induced vasodilation in health and hypercholesterolemia

The role of endothelium-derived hyperpolarizing factor (EDHF) in either the healthy circulation or in those with hypercholesterolemia is unknown. In healthy and hypercholesterolemic subjects, we measured forearm blood flow (FBF) using strain-gauge plethysmography at rest, during graded handgrip exercise, and after sodium nitroprusside infusion. Measurements were repeated after l-NMMA, tetraethylammonium (TEA), and combined infusions. At rest, l-NMMA infusion reduced FBF in healthy but not hypercholesterolemic subjects. At peak exercise, vasodilation was lower in hypercholesterolemic compared to healthy subjects (274% vs 438% increase in FBF, p=0.017). TEA infusion reduced exercise-induced vasodilation in both healthy and hypercholesterolemic subjects (27%, p<0.0001 and -20%, p<0.0001, respectively). The addition of l-NMMA to TEA further reduced FBF in healthy (-14%, p=0.012) but not in hypercholesterolemic subjects, indicating a reduced nitric oxide and greater EDHF-mediated contribution to exercise-induced vasodilation in hypercholesterolemia. In conclusion, exercise-induced vasodilation is impaired and predominantly mediated by EDHF in hypercholesterolemic subjects. CLINICAL TRIAL REGISTRATION IDENTIFIER NCT00166166:

Endothelium-derived hyperpolarizing factor mediates bradykinin-stimulated tissue plasminogen activator release in humans

AIMS

Bradykinin (BK) stimulates tissue plasminogen activator (t-PA) release from human endothelium. Although BK stimulates both nitric oxide and endothelium-derived hyperpolarizing factor (EDHF) release, the role of EDHF in t-PA release remains unexplored. This study sought to determine the mechanisms of BK-stimulated t-PA release in the forearm vasculature of healthy human subjects.

METHODS

In 33 healthy subjects (age 40.3 ± 1.9 years), forearm blood flow (FBF) and t-PA release were measured at rest and after intra-arterial infusions of BK (400 ng/min) and sodium nitroprusside (3.2 mg/min). Measurements were repeated after intra-arterial infusion of tetraethylammonium chloride (TEA; 1 µmol/min), fluconazole (0.4 µmol·min(-1)·l(-1)), and N(G)-monomethyl-L-arginine (L-NMMA, 8 µmol/min) to block nitric oxide, and their combination in separate studies.

RESULTS

BK significantly increased net t-PA release across the forearm (p < 0.0001). Fluconazole attenuated both BK-mediated vasodilation (-23.3 ± 2.7% FBF, p < 0.0001) and t-PA release (from 50.9 ± 9.0 to 21.3 ± 8.9 ng/min/100 ml, p = 0.02). TEA attenuated FBF (-14.7 ± 3.2%, p = 0.002) and abolished BK-stimulated t-PA release (from 22.9 ± 5.7 to -0.8 ± 3.6 ng/min/100 ml, p = 0.0002). L-NMMA attenuated FBF (p < 0.0001), but did not inhibit BK-induced t-PA release (nonsignificant).

CONCLUSION

BK-stimulated t-PA release is partly due to cytochrome P450-derived epoxides and is inhibited by K(+)Ca channel blockade. Thus, BK stimulates both EDHF-dependent vasodilation and t-PA release.

Endothelial dysfunction in metabolic and vascular disorders

Vascular endothelium has important regulatory functions in the cardiovascular system and a pivotal role in the maintenance of vascular health and metabolic homeostasis. It has long been recognized that endothelial dysfunction participates in the pathogenesis of atherosclerosis from early, preclinical lesions to advanced, thrombotic complications. In addition, endothelial dysfunction has been recently implicated in the development of insulin resistance and type 2 diabetes mellitus (T2DM). Considering that states of insulin resistance (eg, metabolic syndrome, impaired fasting glucose, impaired glucose tolerance, and T2DM) represent the most prevalent metabolic disorders and risk factors for atherosclerosis, it is of considerable scientific and clinical interest that both metabolic and vascular disorders have endothelial dysfunction as a common background. Importantly, endothelial dysfunction has been associated with adverse outcomes in patients with established cardiovascular disease, and a growing body of evidence indicates that endothelial dysfunction also imparts adverse prognosis in states of insulin resistance. In this review, we discuss the association of insulin resistance and T2DM with endothelial dysfunction and vascular disease, with a focus on the underlying mechanisms and prognostic implications of the endothelial dysfunction in metabolic and vascular disorders. We also address current therapeutic strategies for the improvement of endothelial dysfunction.

Differences in vascular nitric oxide and endothelium-derived hyperpolarizing factor bioavailability in blacks and whites

OBJECTIVE

Abnormalities in nitric oxide (NO) bioavailability have been reported in blacks. Whether there are differences in endothelium-derived hyperpolarizing factor (EDHF) in addition to NO between blacks and whites and how these affect physiological vasodilation remain unknown. We hypothesized that the bioavailability of vascular NO and EDHF, at rest and with pharmacological and physiological vasodilation, varies between whites and blacks.

APPROACH AND RESULTS

In 74 white and 86 black subjects without known cardiovascular disease risk factors, forearm blood flow was measured using plethysmography at rest and during inhibition of NO with N(G)-monomethyl-L-arginine and of K(+) Ca channels (EDHF) with tetraethylammonium. The reduction in resting forearm blood flow was greater with N(G)-monomethyl-L-arginine (P=0.019) and similar with tetraethylammonium in whites compared with blacks. Vasodilation with bradykinin, acetylcholine, and sodium nitroprusside was lower in blacks compared with whites (all P<0.0001). Inhibition with N(G)-monomethyl-L-arginine was greater in whites compared with blacks with bradykinin, acetylcholine, and exercise. Inhibition with tetraethylammonium was lower in blacks with bradykinin, but greater during exercise and with acetylcholine.

CONCLUSIONS

The contribution to both resting and stimulus-mediated vasodilator tone of NO is greater in whites compared with blacks. EDHF partly compensates for the reduced NO release in exercise and acetylcholine-mediated vasodilation in blacks. Preserved EDHF but reduced NO bioavailability and sensitivity characterizes the vasculature in healthy blacks.

CLINICAL TRIAL REGISTRATION URL

http://clinicaltrials.gov/. Unique identifier: NCT00166166.

Vasodilator interactions in skeletal muscle blood flow regulation

During exercise, oxygen delivery to skeletal muscle is elevated to meet the increased oxygen demand. The increase in blood flow to skeletal muscle is achieved by vasodilators formed locally in the muscle tissue, either on the intraluminal or on the extraluminal side of the blood vessels. A number of vasodilators have been shown to bring about this increase in blood flow and, importantly, interactions between these compounds seem to be essential for the precise regulation of blood flow. Two compounds stand out as central in these vasodilator interactions: nitric oxide (NO) and prostacyclin. These two vasodilators are both stimulated by several compounds, e.g. adenosine, ATP, acetylcholine and bradykinin, and are affected by mechanically induced signals, such as shear stress. NO and prostacyclin have also been shown to interact in a redundant manner where one system can take over when formation of the other is compromised. Although numerous studies have examined the role of single and multiple pharmacological inhibition of different vasodilator systems, and important vasodilators and interactions have been identified, a large part of the exercise hyperaemic response remains unexplained. It is plausible that this remaining hyperaemia may be explained by cAMP- and cGMP-independent smooth muscle relaxation, such as effects of endothelial derived hyperpolarization factors (EDHFs) or through metabolic modulation of sympathetic effects. The nature and role of EDHF as well as potential novel mechanisms in muscle blood flow regulation remain to be further explored to fully elucidate the regulation of exercise hyperaemia.

Endothelium-derived hyperpolarizing factor and vascular function

Endothelial function refers to a multitude of physiological processes that maintain healthy homeostasis of the vascular wall. Exposure of the endothelium to cardiac risk factors results in endothelial dysfunction and is associated with an alteration in the balance of vasoactive substances produced by endothelial cells. These include a reduction in nitric oxide (NO), an increase in generation of potential vasoconstrictor substances and a potential compensatory increase in other mediators of vasodilation. The latter has been surmised from data demonstrating persistent endothelium-dependent vasodilatation despite complete inhibition of NO and prostaglandins. This remaining non-NO, non-prostaglandin mediated endothelium-dependent vasodilator response has been attributed to endothelium-derived hyperpolarizing factor/s (EDHF). Endothelial hyperpolarization is likely due to several factors that appear to be site and species specific. Experimental studies suggest that the contribution of the EDHFs increase as the vessel size decreases, with a predominance of EDHF activity in the resistance vessels, and a compensatory up-regulation of hyperpolarization in states characterized by reduced NO availability. Since endothelial dysfunction is a precursor for atherosclerosis development and its magnitude is a reflection of future risk, then the mechanisms underlying endothelial dysfunction need to be fully understood, so that adequate therapeutic interventions can be designed.

Endothelium-derived hyperpolarizing factor determines resting and stimulated forearm vasodilator tone in health and in disease

BACKGROUND

We assessed the contribution of endothelium-derived hyperpolarizing factors to resting and agonist-stimulated vasodilator tone in health and disease. Tetraethylammonium chloride (TEA) was used to inhibit K(+)(Ca) channel activation and fluconazole was used to inhibit cytochrome P450 2C9-mediated epoxyeicosatrienoic acid synthesis. We hypothesized that endothelium-derived hyperpolarizing factors contribute to resting vascular tone by K(+)(Ca) channel activation and epoxyeicosatrienoic acid release and that endothelium-derived hyperpolarizing factors compensate for reduced nitric oxide bioavailability at rest and with endothelium-dependent vasodilators.

METHODS AND RESULTS

In 103 healthy subjects and 71 nonhypertensive subjects with multiple risk factors, we measured resting forearm blood flow (FBF) using venous occlusion plethysmography before and after intra-arterial infusions of N(G)-monomethyl-l-arginine (L-NMMA), TEA, fluconazole, and their combination. The effects of these antagonists on resting FBF and on bradykinin- and acetylcholine-mediated vasodilation were studied. Resting FBF decreased with TEA and L-NMMA in all subjects (P<0.001); however, the vasoconstrictor response to L-NMMA was greater (P=0.04) and to TEA was lower (P=0.04) in healthy subjects compared with those with risk factors. Fluconazole decreased resting FBF in all subjects, and the addition of TEA further reduced FBF after fluconazole, suggesting that cytochrome P450 metabolites and other hyperpolarizing factor(s) activate K(+)(Ca) channels. Both L-NMMA and TEA attenuated bradykinin-mediated vasodilation in healthy and hypercholesterolemic subjects (P<0.001). In contrast, acetylcholine-mediated vasodilation remained unchanged with TEA in healthy subjects but was significantly attenuated in hypercholesterolemia (P<0.04).

CONCLUSIONS

First, by activating TEA-inhibitable K(+)(Ca) channels, endothelium-derived hyperpolarizing factors, together with nitric oxide, contribute to resting microvascular dilator tone. The contribution of K(+)(Ca) channel activation compared with nitric oxide is greater in those with multiple risk factors compared with healthy subjects. Second, activation of K(+)(Ca) channels is only partly through epoxyeicosatrienoic acid release, indicating the presence of other hyperpolarizing mechanisms. Third, bradykinin, but not acetylcholine, stimulates K(+)(Ca) channel-mediated vasodilation in healthy subjects, whereas in hypercholesterolemia, K(+)(Ca) channel-mediated vasodilation compensates for the reduced nitric oxide activity. Thus, enhanced endothelium-derived hyperpolarizing factor activity in conditions of nitric oxide deficiency contributes to maintenance of resting and agonist-stimulated vasodilation. Clinical Trial Registration- URL: http://www.clinicaltrials.gov. Unique identifier: NCT00166166.

Age-related change in endothelial and microvessel function and therapeutic consequences

As the absolute numbers and proportion of older adults increases across most of the developed world, a greater understanding of the aetiopathogenic mechanisms of the increased vascular risk and their therapeutic implications becomes essential to all clinicians assessing and managing the geriatric patient. The role of endothelial function and the microcirculation is increasingly recognized in the maintenance of adequate perfusion, and their dysfunction is thought to be an early and potentially reversible mechanism by which age acts to increase cardiovascular risk.

Here we review evidence that altered microvascular function appears before other recognized predictors of vascular disease, and progresses from childhood to late adult life, preceding fulminant atherosclerotic or arteriosclerotic disease. Low birth-weight babies have reduced endothelial function in skin microvessels at 3 months, and by age ten brachial artery endothelial function is reduced in comparison with normal birth-weight babies. In overweight/obese adolescent children with clustering of traditional cardiovascular disease risk factors, endothelial function is lower compared with normal weight children and this appears to persist into early adulthood. Adult ageing is associated with impaired microvessel endothelial function and an increase in capillary blood pressure, independent of brachial artery blood pressure. Biological and lifestyle factors that influence microvessel function include body fat and visceral adiposity, sex hormone status, diet and physical activity.

Exploration of the therapeutic implications for management of endothelial dysfunction remains in embryonic state. The use of ACE-inhibitors, angiotensin receptor blockers and direct renin inhibitors in patients with evidence of microvascular damage such as retinopathy and microalbuminuria has been established; however, in the general older population the benefit has yet to be established. Therefore current recommendations are to screen for microvascular damage and if present target treatments after control of other vascular risk factors such as hypertension.

Endothelial dysfunction in diabetes: multiple targets for treatment

Robert Furchgott's discovery of the obligatory role that the endothelium plays in the regulation of vascular tone has proved to be a major advance in terms of our understanding of the cellular basis of diabetic vascular disease. Endothelial dysfunction, as defined by a reduction in the vasodilatation response to an endothelium-dependent vasodilator (such as acetylcholine) or to flow-mediated vasodilatation, is an early indicator for the development of the micro- and macroangipathy that is associated with diabetes. In diabetes, hyperglycaemia plays a key role in the initiation and development of endothelial dysfunction; however, the cellular mechanisms involved as well as the importance of dyslipidaemia and co-morbidities such as hypertension and obesity remain incompletely understood. In this review, we discuss the mechanisms whereby hyperglycaemia, oxidative stress and dyslipidaemia can alter endothelial function and highlight their effects on endothelial nitric oxide synthase (eNOS), the endothelium-dependent hyperpolarising factor (EDHF) pathway(s), as well as on the role of endothelium-derived contracting factors (EDCFs) and adipocyte-derived vasoactive factors such as adipose-derived relaxing factor (ADRF).

Epoxyeicosatrienoic acids and endothelium-dependent responses

Epoxyeicosatrienoic acids (EETs) are cytochrome P450 metabolites of arachidonic acid that are produced by the vascular endothelium in response to agonists such as bradykinin and acetylcholine or physical stimuli such as shear stress or cyclic stretch. In the vasculature, the EETs have biological actions that are involved in the regulation of vascular tone, hemostasis, and inflammation. In preconstricted arteries in vitro, EETs activate calcium-activated potassium channels on vascular smooth muscle and the endothelium causing membrane hyperpolarization and relaxation. These effects are observed in a variety of arteries from experimental animals and humans; however, this is not a universal finding in all arteries. The mechanism of EET action may vary. In some arteries, EETs are released from the endothelium and are transferred to the smooth muscle where they cause potassium channel activation, hyperpolarization, and relaxation through a guanine nucleotide binding protein-coupled mechanism or transient receptor potential (TRP) channel activation. In other arteries, EETs activate TRP channels on the endothelium to cause endothelial hyperpolarization that is transferred to the smooth muscle by gap junctions or potassium ion. Some arteries use a combination of mechanisms. Acetylcholine and bradykinin increase blood flow in dogs and humans that is inhibited by potassium channel blockers and cytochrome P450 inhibitors. Thus, the EETs are endothelium-derived hyperpolarizing factors mediating a portion of the relaxations to acetylcholine, bradykinin, shear stress, and cyclic stretch and regulate vascular tone in vitro and in vivo.

Vascular pharmacology of epoxyeicosatrienoic acids

Epoxyeicosatrienoic acids (EETs) are cytochrome P450 metabolites of arachidonic acid that are produced by the vascular endothelium in responses to various stimuli such as the agonists acetylcholine (ACH) or bradykinin or by shear stress which activates phospholipase A(2) to release arachidonic acid. EETs are important regulators of vascular tone and homeostasis. In the modulation of vascular tone, EETs function as endothelium-derived hyperpolarizing factors (EDHFs). In models of vascular inflammation, EETs attenuate inflammatory signaling pathways in both the endothelium and vascular smooth muscle. Likewise, EETs regulate blood vessel formation or angiogenesis by mechanisms that are still not completely understood. Soluble epoxide hydrolase (sEH) converts EETs to dihydroxyeicosatrienoic acids (DHETs) and this metabolism limits many of the biological actions of EETs. The recent development of inhibitors of sEH provides an emerging target for pharmacological manipulation of EETs. Additionally, EETs may initiate their biological effects by interacting with a cell surface protein that is a G protein-coupled receptor (GPCR). Since GPCRs represent a common target of most drugs, further characterization of the EET receptor and synthesis of specific EET agonists and antagonist can be used to exploit many of the beneficial effects of EETs in vascular diseases, such as hypertension and atherosclerosis. This review will focus on the current understanding of the contribution of EETs to the regulation of vascular tone, inflammation, and angiogenesis. Furthermore, the therapeutic potential of targeting the EET pathway in vascular disease will be highlighted.

Endothelial dysfunction and cardiovascular risk factors

The endothelium plays an integral role in the regulation of vascular tone, platelet activity, leukocyte adhesion, and thrombosis and is intimately involved in the development of atherosclerosis. Endothelial dysfunction has been observed in patients with established coronary artery disease or coronary risk factors, both in the coronary and peripheral vasculature. Therapeutic interventions with lipid-lowering drugs, ACE inhibitors, physical activity, and antioxidant agents have been shown to improve endothelial function in coronary and peripheral vessels. This systemic manifestation and improvement of endothelial function suggests that a common mechanism may contribute to endothelial dysfunction in the coronary and peripheral circulation.

TARGET AUDIENCE

Internist, Cardiologists, Family physicians.

LEARNING OBJECTIVES

After completion of this article, the reader should be able to define the participation of cardiovascular risk factors in the various complications associated with endothelial dysfunction.

Angiotensin-converting enzyme in skeletal muscle: sentinel of blood pressure control and glucose homeostasis

Recent evidence suggests a coordinated regulation by the local renin-angiotensin system (RAS) and tissue kallikrein-kinin system (TKKS) of blood flow and substrate supply in oxidative red myofibres of skeletal muscle tissue during endurance exercise. The performance of these myofibres is dependent on the increased oxidation of substrates facilitated by augmenting nutritive blood flow and glucose uptake. Humoral factors released by the contracting fibres, such as adenosine and kinins, are suggested to be responsible for this metabolic adjustment. The considerable drain of blood volume and the enormous consumption of glucose during endurance exercise require a control mechanism for the maintenance of blood pressure (BP) and glucose homeostasis. This is achieved by the sympathetic nervous system and its subordinate RAS, which is located in the nutritive vessels and parenchyma of the red myofibres. The angiotensin-converting enzyme (ACE) is the primary enzyme responsible for kinin degradation during exercise, underscoring the important interrelationship between the RAS and the TKKS in the critical role of kinins in the multifactorial regulation of muscle bioenergetics and glucose and BP homeostasis. Importantly, overactivity of the ACE, as occurs in individuals displaying risk factors such as overweight, causes exaggerated BP response and reduced glucose disposal. If they persist over years, compensatory responses to this ACE overactivity, such as hypersecretion of insulin and compliance of the vessel walls, will inevitably be exhausted, leading ultimately to the manifestation of type 2 diabetes and hypertension. This concept also provides a unifying explanation for the beneficial effects of ACE-inhibitors and Angiotensin II receptor antagonists in the treatment of hypertension and insulin resistance.

15-Lipoxygenase metabolites contribute to age-related reduction in acetylcholine-induced hypotension in rabbits

Arachidonic acid (AA) metabolites from the 15-lipoxygenase-1 (15-LO-1) pathway, trihydroxyeicosatrienoic acids (THETAs) and hydroxy-epoxyeicosatrienoic acids (HEETAs), are endothelium-derived hyperpolarizing factors (EDHFs) and relax rabbit arteries. Rabbit vascular 15-LO-1 expression, THETA and HEETA synthesis, and nitric oxide and prostaglandin-independent relaxations to acetylcholine (ACh) and AA decreased with age (neonates to 16-wk-old). We characterized age-dependent ACh-hypotensive responses in vivo in 1-, 4-, 8-, and 16-wk-old rabbits and the contribution of THETAs and HEETAs to these responses. In anesthetized rabbits, blood pressure responses to ACh (4-4,000 ng/kg) were determined in the presence of vehicle or various inhibitors. ACh responses decreased with age (P > 0.001). In the absence or presence of N(omega)-nitro-l-arginine methyl ester (l-NAME) and indomethacin (Indo), maximum responses in 1 (-54.7 +/- 7.4 and -37.9 +/- 3.9%)- and 4 (-48.8 +/- 2.4 and -35.5 +/- 7.8%)-wk-old rabbits were higher than 8 (-30.0 +/- 2.8 and -26.6 +/- 4.4%)- and 16 (-36.7 +/- 3.5 and -27.3 +/- 10%)-wk-old rabbits. A lipoxygenase inhibitor, BW755C, reduced THETA and HEETA synthesis in mesenteric arteries. In the presence of Indo and N(omega)-nitro-l-arginine, ACh relaxations were reduced by BW755C to a greater extent in the mesenteric arteries from the younger rabbits. In 4-wk-old rabbits treated with l-NAME and Indo, the maximum ACh hypotension was reduced by the potassium channel inhibitors apamin and charybdotoxin to -6.9 +/- 0.9%, by apamin alone to -19.5 +/- 1.4%, and by BW755C to -18.8 +/- 3.5%. The present study indicates that the age-related decrease in ACh-induced hypotension is mediated by the decreased synthesis of the 15-LO-1 metabolites THETAs and HEETAs.

Characterization of agonist-induced endothelium-dependent vasodilatory responses in the vascular bed of the equine digit

The role of endothelium-derived relaxing factors was studied in the regulation of vascular responses in the Krebs perfused equine isolated digit. Perfusion pressure was recorded in response to bolus doses of 5-hydroxytryptamine (6 nmol) alone or co-administered with carbachol (CCh; 0.2 micromol), bradykinin (BK; 0.2 nmol), substance P (SP; 0.2 nmol) or sodium nitroprusside (SNP; 0.2 micromol). N(omega)-Nitro-L-Arginine methyl ester hydrochloride (L-NAME; 300 microm) caused partial but significant inhibition of CCh-induced vasodilatory response, whereas BK and SP-induced responses were resistant to L-NAME. High potassium (K(+), 30 mm) and the cytochrome P-450 (CYP) epoxygenase inhibitor, clotrimazole (10 microm) plus L-NAME (100 microm), completely abolished the CCh, BK and SP-induced vasodilatory responses, whereas the response to SNP was unaffected. In contrast, the L-NAME-resistant proportion of CCh, BK and SP-induced vasodilatory response was not inhibited by the highly selective CYP2C9 inhibitor, sulphaphenazole (10 microm). The cyclo-oxygenase inhibitor, ibuprofen (10 microm) did not affect the CCh, BK and SP-induced responses. These data demonstrate that CCh, BK and SP-induced relaxation in the equine digit involve a combination of the NO and endothelium-derived hyperpolarizing factor (EDHF) pathways. These results do not support the evidence for the involvement of CYP-derived epoxyeicosatrienoic acids and the exact nature of EDHF in the equine digit remains to be established.

Endothelial 15-lipoxygenase-1 overexpression increases acetylcholine-induced hypotension and vasorelaxation in rabbits

Arachidonic acid is metabolized by the 15-lipoxygenase-1 pathway to the vasodilatory eicosanoids hydroxy-epoxyeicosatrienoic acid and trihydroxyeicosatrienoic acid. We determined the in vitro and in vivo effects of the 15-lipoxygenase-1 metabolites in rabbits infected with adenovirus containing cDNA for human 15-lipoxygenase-1 (Ad-15-LO-1). Forty hours after intravenous adenoviral injection, 15-lipoxygenase-1 expression increased in liver and mesenteric arteries 10-fold and 3-fold, respectively. Expression of 15-LO-1 was limited to the endothelium of mesenteric arteries. Overexpression did not occur in tissues from rabbits infected with a beta-galactosidase containing adenovirus. Trihydroxyeicosatrienoic acid and hydroxy-epoxyeicosatrienoic acid synthesis per milligram of tissue increased by 2.1- and 1.5-fold, respectively, in mesenteric arteries from Ad-15-LO-1-infected rabbits compared with normal rabbits. Pretreatment with a 15-lipoxygenase inhibitor BW755C inhibited the synthesis. NO and prostaglandin-independent, maximal relaxations to acetylcholine were greater in mesenteric arteries from Ad-15-LO-1-infected rabbits (98.3+/-1.7%) compared with normal (60.93+/-10.5%) and beta-galactosidase containing adenovirus-infected rabbits (68.3+/-7.7%). Pretreatment with BW755C decreased these relaxations. Mean arterial pressure and heart rate did not differ in Ad-15-LO-1-infected rabbits compared with normal or beta-galactosidase containing adenovirus-infected rabbits. The hypotensive responses to acetylcholine in the presence and absence of inhibition of NO and prostaglandins were greater in Ad-15-LO-1-infected rabbits (-52+/-2% and -47+/-2%) compared with normal (-31+/-3% and -25+/-5%) or beta-galactosidase containing adenovirus-infected rabbits (-38+/-2% and -30+/-3%). Therefore, increased expression of 15-LO-1 increases acetylcholine relaxation in arteries and hypotensive responses in rabbits because of increased hydroxy-epoxyeicosatrienoic acid and trihydroxyeicosatrienoic acid synthesis.

Role of cytochrome P450 metabolites of arachidonic acid in regulation of corporal smooth muscle tone in diabetic and older rats

This study examined the role of cytochrome P450 (CYP) metabolites of arachidonic acid (AA) to rat corporal smooth muscle tone. 11, 12-Epoxyeicosatrienoic acid (EET) (10(-11)-10(-6 )M) produced dose-dependent relaxation of rat (control; 10 weeks old) corpus cavernosum with a pD(2) value of 8.8+/-0.2 and a maximal relaxation of 80+/-9%, whereas 20-hydroxyeicosatetraenoic (20-HETE) did not have an effect. EET-mediated relaxation of corpus cavernosum was attenuated by 71+/-3%, 55+/-2%, 53+/-5% and 84+/-3% in the presence of nitro-L-arginine methyl ester (L-NAME) (10(-4) M), an inhibitor of nitric oxide (NO) synthase, iberiotoxin (5 x 10(-8) M), an inhibitor of calcium-activated potassium (BK) channels, glibenclamide (10(-5) M), an inhibitor of ATP-sensitive K(+) channels or 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) (10(-5) M), an inhibitor of soluble guanylyl cyclase, respectively. EET-mediated relaxation of rat corpus cavernosum was significantly less in the streptozotocin (STZ)-treated (diabetic) and 30 weeks old (older) animals compared to control. Carbachol (10(-9)-10(-4) M)-induced relaxation was significantly reduced whereas phenylephrine (PE) (10(-9)-5 x 10(-3) M)-induced contraction was significantly increased in the cavernosum strips from old and diabetic rats compared to the control. Pre-incubation of the cavernosum strips obtained from control, older or diabetic rats with N-hydroxy-N'-(4-butyl-2-methyl-phenyl)-formamidine (HET0016), a selective inhibitor of 20-HETE synthesis, or 1-cyclohexyl-3-dodecyl urea (CDU), a specific inhibitor of soluble epoxide hydrolase (sEH) resulted in a significant attenuation of PE-induced contraction and improvement in carbachol-induced relaxation. We conclude that 11, 12-EET-induced relaxation of the rat corpus cavernosum involves activation of cGMP/NO pathway as well as activation of ATP-sensitive K(+) channels and BK channels. These results also suggest that inhibition of 20-HETE production or reduction of EET inactivation may have therapeutic potential to prevent erectile dysfunction associated with diabetes and aging.

Contributions of endothelium-derived relaxing factors to control of hindlimb blood flow in the mouse in vivo

We determined the contributions of various endothelium-derived relaxing factors to control of basal vascular tone and endothelium-dependent vasodilation in the mouse hindlimb in vivo. Under anesthesia, catheters were placed in a carotid artery, jugular vein, and femoral artery (for local hindlimb circulation injections). Hindlimb blood flow (HBF) was measured by transit-time ultrasound flowmetry. N(omega)-nitro-L-arginine methyl ester (L-NAME, 50 mg/kg plus 10 mg x kg(-1) x h(-1)), to block nitric oxide (NO) production, altered basal hemodynamics, increasing mean arterial pressure (30 +/- 3%) and reducing HBF (-30 +/- 12%). Basal hemodynamics were not significantly altered by indomethacin (10 mg x kg(-1) x h(-1)), charybdotoxin (ChTx, 3 x 10(-8) mol/l), apamin (2.5 x 10(-7) mol/l), or ChTx plus apamin (to block endothelium-derived hyperpolarizing factor; EDHF). Hyperemic responses to local injection of acetylcholine (2.4 microg/kg) were reproducible in vehicle-treated mice and were not significantly attenuated by L-NAME alone, indomethacin alone, L-NAME plus indomethacin with or without co-infusion of diethlyamine NONOate to restore resting NO levels, ChTx alone, or apamin alone. Hyperemic responses evoked by acetylcholine were reduced by 29 +/- 11% after combined treatment with apamin plus charybdotoxin, and the remainder was virtually abolished by additional treatment with L-NAME but not indomethacin. None of the treatments altered the hyperemic response to sodium nitroprusside (5 microg/kg). We conclude that endothelium-dependent vasodilation in the mouse hindlimb in vivo is mediated by both NO and EDHF. EDHF can fully compensate for the loss of NO, but this cannot be explained by tonic inhibition of EDHF by NO. Control of basal vasodilator tone in the mouse hindlimb is dominated by NO.

Arachidonic acid metabolites as endothelium-derived hyperpolarizing factors

The endothelium regulates vascular tone through the release of a number of soluble mediators, including NO, prostaglandin I2, and endothelium-derived hyperpolarizing factor. Epoxyeicosatrienoic acids are cytochrome P450 epoxygenase metabolites of arachidonic acid. They are synthesized by the vascular endothelium and open calcium-activated potassium channels, hyperpolarize the membrane, and relax vascular smooth muscle. Endothelium-dependent relaxations to acetylcholine, bradykinin, and shear stress that are not inhibited by cyclooxygenase and NO synthase inhibitors are mediated by the endothelium-derived hyperpolarizing factor. In arteries from experimental animals and humans, the non-NO, non-prostaglandin-mediated relaxations and endothelium-dependent hyperpolarizations are blocked by cytochrome P450 inhibitors, calcium-activated potassium channel blockers, and epoxyeicosatrienoic acid antagonists. Acetylcholine and bradykinin stimulate epoxyeicosatrienoic acid release from endothelial cells and arteries. These findings indicate that epoxyeicosatrienoic acids act as endothelium-derived hyperpolarizing factors and regulate arterial tone.

Crucial Role of NO and Endothelium-Derived Hyperpolarizing Factor in Human Sustained Conduit Artery Flow-Mediated Dilatation

Whether NO is involved or not in sustained conduit artery flow-mediated dilatation in humans remains unclear. Moreover, the role of endothelium-derived hyperpolarizing factor (EDHF), synthesized by cytochrome epoxygenases and acting through calcium-activated potassium channels, and its relationship with NO during flow-mediated dilatation have never been investigated previously. In 12 healthy subjects we measured radial artery diameter (echotracking) and blood flow (Doppler) during flow-mediated dilatation induced by gradual distal hand skin heating (34 to 44 degrees C), during the local infusion of saline and inhibitors of NO synthase (N(G)-monomethyl-l-arginine [l-NMMA]: 8 to 20 micromol/min per liter), calcium-activated potassium channels (tetraethylammonium chloride: 9 micromol/min per liter), and cytochrome epoxygenases (fluconazole: 0.4 to 1.6 micromol/min per liter), alone and in combination. Mean wall shear stress, the flow-mediated dilatation stimulus, was calculated at each level of flow, and the diameter-wall shear stress relationship was constructed. During heating, compared with saline, the diameter-shear stress relationship was shifted downward by l-NMMA, tetraethylammonium, fluconazole, and, in a more pronounced manner, by the combinations of l-NMMA with tetraethylammonium or with fluconazole. Therefore, maximal radial artery flow-mediated dilatation, compared with saline (0.62+/-0.03 mm), was decreased under our experimental conditions by l-NMMA (-39+/-4%), tetraethylammonium chloride (-14+/-4%), fluconazole (-18+/-6%), and to a greater extent, by the combinations of l-NMMA with tetraethylammonium (-64+/-4%) or with fluconazole (-71+/-3%). This study demonstrates that NO and a cytochrome-related EDHF are involved in peripheral conduit artery flow-mediated dilatation in humans during sustained flow conditions. Moreover, the synergistic effects of the inhibitors strongly suggest a functional interaction between NO and EDHF pathways.

Low-frequency oscillations of the laser Doppler perfusion signal in human skin

Spectral analysis of the laser Doppler flow (LDF) signal in the frequency interval from 0.0095-2.0 Hz reveals blood flow oscillations with frequencies around 1.0, 0.3, 0.1, 0.04 and 0.01 Hz. The heartbeat, the respiration, the intrinsic myogenic activity of vascular smooth muscle, the neurogenic activity of the vessel wall and the vascular endothelium influence these oscillations, respectively. The first aim of this study was to investigate if a slow oscillatory component could be detected in the frequency area below 0.0095 Hz of the human cutaneous blood perfusion signal. Unstimulated basal blood skin perfusion and enhanced perfusion during iontophoresis with the endothelium-dependent vasodilator acetylcholine (ACh) and the endothelium-independent vasodilator sodium nitroprusside (SNP) were measured in healthy male volunteers and the wavelet transform was computed. A low-frequency oscillation between 0.005 and 0.0095 Hz was found both during basal conditions and during iontophoresis with ACh and SNP. Iontophoresis with ACh increased the normalized amplitude to a greater extent than SNP (P = 0.001) indicating modulation by the vascular endothelium. To gain further insight into the mechanisms for this endothelium dependency, we inhibited nitric oxide (NO) synthesis with N(G)-monomethyl-L-arginine (L-NMMA) and prostaglandin (PG) synthesis by aspirin. L-NMMA did not affect the increased response to ACh vs. SNP iontophoresis in the 0.005-0.0095-Hz interval (P = 0.006) but abolished the difference in the 0.0095-0.021-Hz interval (P = 0.97). Aspirin did not affect the difference in response to ACh and SNP in either of the two frequency intervals. Thus, other endothelial mechanisms, such as endothelium-derived hyperpolarizing factor (EDHF), might be involved in the regulation of this sixth frequency interval (0.005-0.0095 Hz).

Identification of a cytochrome P450 2C9-derived endothelium-derived hyperpolarizing factor in essential hypertensive patients

OBJECTIVES

We assessed the role of cytochrome P450 2C9 (CYP 2C9)-derived endothelium-derived hyperpolarizing factor (EDHF) in the forearm microcirculation of essential hypertensive patients (EH) by utilizing sulfaphenazole (SUL), a selective CYP 2C9 inhibitor.

BACKGROUND

In EH patients, EDHF acts as a compensatory pathway when nitric oxide (NO) availability is reduced. Cytochrome P450 2C9 is a possible source of EDHF.

METHODS

In 36 healthy subjects (normotensive [NT]) and 32 hypertensive patients (HT), we studied forearm blood flow (strain-gauge plethysmography) changes induced by intraarterial acetylcholine (ACH) and bradykinin (BDK), repeated during N(G)-monomethyl-L-arginine (L-NMMA) (100 mug/100 ml/min) or SUL (0.03 mg/100 ml/min). In HT, the effect of SUL on ACH and BDK was repeated during vitamin C (8 mg/100 ml/min). Sodium nitroprusside (SNP) was utilized as control.

RESULTS

In NT, vasodilation to ACH and BDK was blunted by L-NMMA and not changed by SUL. In contrast, in HT responses to ACH and BDK, reduced compared with NT, were resistant to L-NMMA. In these patients, SUL blunted vasodilation to ACH and to a greater extent the response to BDK. When retested with vitamin C, SUL was no longer effective on both endothelial agonists. In 2 final groups of normotensive control subjects, vasodilation to ACH or BDK residual to cyclooxygenase and L-NMMA blockade was further inhibited by simultaneous SUL infusion. Response to SNP, similar between NT and HT, was unaffected by SUL.

CONCLUSIONS

Cytochrome P450 epoxygenase-derived EDHF acts as a partial compensatory mechanism to sustain endothelium-dependent vasodilation in HT, particularly the BDK-mediated response, when NO activity is impaired because of oxidative stress.

Inhibition of PGI2 signaling by miconazole in vascular smooth muscle cells

Miconazole is widely used clinically as an anti-fungal agent and experimentally as a cytochrome P450 (CYP) inhibitor. In rat coronary arteries that produce PGI(2) as the major arachidonic acid (AA) metabolite, activation of the large-conductance K(+) (BK) channels in coronary arterial smooth muscle cells by AA was inhibited by miconazole but not by the CYP inhibitor SKF525A. Activation of BK currents in coronary smooth muscle cells by carbacyclin or iloprost also was inhibited by miconazole but not by SKF525A, suggesting that miconazole might have properties other than those of CYP inhibition. In addition, carbacyclin-induced dilation of isolated mesenteric arteries was inhibited by treatment with miconazole (51.9+/-4.2% dilation in control, n=7 versus 30.1+/-4.0% with miconazole, n=4, p<0.005) but not SKF525A (52.8+/-3.6%, n=8). In contrast, miconazole did not affect BK channel activation and vasodilation produced by the phosphodiesterase inhibitor RO-201724. In cultured coronary smooth muscle cells, carbacyclin (1microM) stimulated cAMP production by 22-fold (183+/-29pmol/mg at baseline, 4062+/-212pmol/mg with carbacyclin, n=3, p<0.001). The carbacyclin effect was significantly attenuated by treatment with miconazole (1542+/-201pmol/mg, n=3, p<0.001 versus carbacyclin alone), but not by SKF525A (3460+/-406pmol/mg, n=3, p=NS versus carbacyclin alone). These results indicate that in addition to its CYP inhibition properties, miconazole inhibits PGI(2) signaling. Hence, experiments using miconazole as a CYP inhibitor should be interpreted with caution.

Evidence for a basal release of a cytochrome-related endothelium-derived hyperpolarizing factor in the radial artery in humans

Whether a cytochrome P-450 (CYP)-related endothelium-derived hyperpolarizing factor (EDHF), acting through calcium-activated potassium (KCa) channels, interacts with nitric oxide (NO) to regulate the basal diameter of human peripheral conduit arteries is unexplored in vivo. Radial artery diameter (echo tracking) and blood flow (Doppler) were measured, after oral aspirin (500 mg), in eight healthy volunteers during local infusion for 8 min of tetraethylammonium chloride (TEA; 9 μmol/min), as KCachannel inhibitor, and fluconazole (0.4 μmol/min), as CYP inhibitor, alone and in combination with NG-monomethyl-l-arginine (l-NMMA; 8 μmol/min), as endothelial NO synthase inhibitor. Endothelium-independent dilatation was assessed by using sodium nitroprusside (SNP). Radial diameter was unaffected by l-NMMA (0.4 ± 0.9%) and fluconazole (−1.6 ± 0.8%) but was decreased by TEA (−5.0 ± 1.0%), l-NMMA + fluconazole (−5.3 ± 0.5%), and l-NMMA + TEA (−9.9 ± 1.3%). These effects are still significant even when the concomitant decreases in blood flow induced by l-NMMA (−24 ± 4%), TEA (−21 ± 3%), l-NMMA + fluconazole (−26 ± 5%), and l-NMMA + TEA (−35 ± 4%) were taken as covariate into analysis. Conversely, fluconazole alone slightly but not significantly increased radial flow (13 ± 6%). l-NMMA alone or with TEA and with fluconazole enhanced radial artery dilatation to SNP, whereas TEA and fluconazole alone did not modify this response. These results confirm in humans the involvement of NO and KCachannels in the regulation of basal conduit artery diameter. Moreover, the synergistic effect of combined inhibition of NO synthesis and CYP on the decrease in radial diameter in the absence of such effect after l-NMMA and fluconazole alone unmasks the role of CYP in this regulation and shows the presence of an interaction between NO and a CYP-related EDHF to maintain peripheral conduit artery diameter in vivo. Furthermore, the higher vasoconstrictor effect of TEA compared with fluconazole suggests that different KCachannel-dependent hyperpolarizing mechanisms could exist in conduit arteries.

From endothelium-derived hyperpolarizing factor (EDHF) to angiogenesis: Epoxyeicosatrienoic acids (EETs) and cell signaling

Epoxyeicosatrienoic acids (EETs) are generated from arachidonic acid by cytochrome P450 (CYP) epoxygenases. The expression of CYP epoxygenases in endothelial cells is determined by a number of physical (fluid shear stress and cyclic stretch) and pharmacological stimuli as well as by hypoxia. The activation of CYP epoxygenases in endothelial cells is an important step in the nitric oxide and prostacyclin (PGI2)-independent vasodilatation of several vascular beds and EETs have been identified as endothelium-derived hyperpolarizing factors (EDHFs). However, in addition to regulating vascular tone, EETs modulate several signaling cascades and affect cell proliferation, cell migration, and angiogenesis. Signaling molecules modulated by EETs include tyrosine kinases and phosphatases, mitogen-activated protein kinases, protein kinase A (PKA), cyclooxygenase (COX)-2, and several transcription factors. This review summarizes the role of CYP-derived EETs in cell signaling and focuses particularly on their role as intracellular amplifiers of endothelial cell hyperpolarization as well as in cell proliferation and angiogenesis. The angiogenic properties of CYP epoxygenases and CYP-derived EETs implicate that these enzymes may well be accessible targets for anti-angiogenic as well as angiogenic therapies.

Endothelial nitric oxide synthase Glu298Asp gene polymorphism, blood pressure and hypertension in a general population sample

OBJECTIVE

The Glu298Asp (E/D) polymorphism of the endothelial nitric oxide synthase (eNOS) gene has been related to hypertension. Since several studies have produced contradictory results, this issue is still subject to ongoing debate. We investigated the association of the eNOS E298D polymorphism with hypertension and with blood pressure (BP) in a large population-based sample of Caucasian ethnicity.

DESIGN

Cross-sectional study in a random sample of the general population.

METHODS

The eNOS E298D polymorphism was determined by 5'-exonuclease assay among 4219 participants aged 20-79 years of the Study of Health in Pomerania (SHIP).

RESULTS

The percentages of the EE298, ED298 and DD298 genotypes were 49.2, 42.0 and 8.8%, respectively. The D allele frequencies did not differ between the groups of normotensive and hypertensive subjects (29.7 versus 29.9%, P = 0.812). Similarly, no association could be established between E298D genotype and prevalent hypertension, neither for D allele carriership (multivariate odds ratio 0.97, 95% confidence interval 0.83-1.12) nor for DD homozygosity (multivariate odds ratio 1.10, 95% confidence interval 0.84-1.43). Likewise, genotype groups did not differ as to the distribution of systolic (ANCOVA P = 0.917) or diastolic BP values (ANCOVA P = 0.657). Nearly identical results were obtained if the analyses were repeated sex-specifically or if subjects on antihypertensive medication were excluded.

CONCLUSION

In a population-based cohort of Caucasians covering a broad age range, the eNOS E298D polymorphism is neither associated with prevalent hypertension nor with systolic or diastolic BP. These results do not support the hypothesis that the E298D polymorphism contributes to the genetic susceptibility to hypertension.

The endothelin-1 receptor antagonist bosentan protects against ischaemia/reperfusion-induced endothelial dysfunction in humans

Endothelial dysfunction may contribute to the extent of ischaemia/reperfusion injury. ET (endothelin)-1 receptor antagonism protects against myocardial ischaemia/reperfusion injury in animal models. The present study investigated whether oral administration of an ET(A)/ET(B) receptor antagonist protects against ischaemia/reperfusion-induced endothelial dysfunction in humans. FBF (forearm blood flow) was measured with venous occlusion plethysmography in 13 healthy male subjects. Forearm ischaemia was induced for 20 min followed by 60 min of reperfusion. Using a cross-over protocol, the subjects were randomized to oral administration of 500 mg of bosentan or placebo 2 h before ischaemia. Endothelium-dependent and -independent vasodilatation were determined by intra-brachial infusion of acetylcholine (1-10 microg/min) and nitroprusside (0.3-3 microg/min) respectively, before and after ischaemia. Compared with pre-ischaemia, the endothelium-dependent increase in FBF was significantly impaired at 15 and 30 min of reperfusion when the subjects received placebo (P<0.01). When the subjects received bosentan, the endothelium-dependent increase in FBF was not affected by ischaemia/reperfusion. Endothelium-independent vasodilatation was not affected during reperfusion compared with pre-ischaemia. The vaso-constrictor response induced by intra-arterial infusion of ET-1 was attenuated significantly by bosentan (P<0.001). The results suggest that the dual ET(A)/ET(B) receptor antagonist bosentan attenuates ischaemia/reperfusion-induced endothelial dysfunction in humans in vivo. Bosentan may thus be a feasible therapeutic agent in the treatment of ischaemia/reperfusion injury in humans.