Endothelin-1 increases expression and activity of arginase 2 via ETB receptors and is co-expressed with arginase 2 in human atherosclerotic plaques.

BACKGROUND AND AIMS: Endothelin-1 (ET-1) and arginase are both suggested to be involved in the inflammatory processes and development of endothelial dysfunction in atherosclerosis. However, information regarding the roles of ET-1 and arginase, as well as the interactions between the two in human atherosclerosis, is scarce. We investigated the expression of ET-1 and its receptors, ETA and ETB, as well as arginase in human carotid atherosclerotic plaques and determined the functional interactions between ET-1 and arginase in endothelial cells and THP-1-derived macrophages. METHODS: Carotid plaques and blood samples were retreived from patients undergoing surgery for symptomatic or asymptomatic carotid stenosis. Plaque gene and protein expression was determined and related to clinical characteristics. Functional interactions between ET-1 and arginase were investigated in endothelial cells and THP-1 cells. RESULTS: Expression of ET-1 and ETB receptors was increased in plaques from patients with symptomatic carotid artery disease. ET-1 was co-localized with arginase 1 and arginase 2 in the necrotic core, together with macrophage markers CD163 and CD68. Arginase 2, ET-1 and ETB receptors were expressed in endothelial cells as well as in smooth muscle cells in the fibrous cap. ET-1 increased arginase 2 mRNA expression and arginase activity in endothelial cells and arginase activity in macrophages. Moreover, ET-1 stimulated formation of reactive oxygen species (ROS) in THP-1-derived macrophages via an arginase-dependent mechanism. CONCLUSIONS: This is the first study that demonstrates co-localization of ET-1 and arginase 2 in human atherosclerotic plaques. ET-1 stimulated arginase 2 expression and activity in endothelial cells, as well as arginase activity and ROS formation in macrophages via an arginase-dependent mechanism. These results indicate an important interaction between the ET pathway and arginase in human atherosclerotic plaques.

Arginase Inhibition Improves Microvascular Endothelial Function in Patients With Type 2 Diabetes Mellitus.

OBJECTIVES: The development of microvascular complications in diabetes is a complex process in which endothelial dysfunction is important. Emerging evidence suggests that arginase is a key mediator of endothelial dysfunction in type 2 diabetes mellitus by reciprocally regulating nitric oxide bioavailability. The aim of this prospective intervention study was to test the hypothesis that arginase activity is increased and that arginase inhibition improves microvascular endothelial function in patients with type 2 diabetes and microvascular dysfunction. DESIGN: Microvascular endothelium-dependent and -independent dilatation was determined in patients with type 2 diabetes (n = 12) and healthy age-matched control subjects (n = 12) with laser Doppler flowmetry during iontophoretic application of acetylcholine and sodium nitroprusside, respectively, before and after administration of the arginase inhibitor Nω-hydroxy-nor-L-arginine (120 min). Plasma ratios of amino acids involved in arginase and nitric oxide synthase activities were determined. The laser Doppler flowmetry data were the primary outcome variable. RESULTS: Microvascular endothelium-dependent dilatation was impaired in subjects with type 2 diabetes (P < .05). After administration of Nω-hydroxy-nor-L-arginine, microvascular endothelial function improved significantly in patients with type 2 diabetes to the level observed in healthy controls. Endothelium-independent vasodilatation did not change significantly. Subjects with type 2 diabetes had higher levels of ornithine and higher ratios of ornithine/citrulline and ornithine/arginine (P < .05), suggesting increased arginase activity. CONCLUSION: Arginase inhibition improves microvascular endothelial function in patients with type 2 diabetes and microvascular dysfunction. Arginase inhibition may represent a novel therapeutic strategy to improve microvascular endothelial function in patients with type 2 diabetes.

Amino acid metabolism reflecting arginase activity is increased in patients with type 2 diabetes and associated with endothelial dysfunction.

BACKGROUND: Endothelial dysfunction contributes to the development of vascular complication in diabetes. Arginase has emerged as a key mechanism behind endothelial dysfunction by its reciprocal regulation of nitric oxide production by substrate competition. We hypothesized that increased arginase activity in patients with type 2 diabetes shifts the metabolism of l-arginine from nitric oxide synthase to arginase resulting in an increase in the plasma ratio of ornithine/citrulline, and that this ratio is associated with endothelial dysfunction. METHODS: Forearm endothelium-dependent vasodilatation and endothelium-independent vasodilatation were determined in 15 patients with type 2 diabetes and 10 healthy controls and related to amino acids reflecting arginase and nitric oxide synthase activity. RESULTS: Compared to healthy controls, patients with diabetes had impaired endothelium-dependent vasodilatation and endothelium-independent vasodilatation. The ratios of ornithine/citrulline and proline/citrulline were 60% and 95% higher, respectively, in patients with diabetes than in controls (p < 0.001). The plasma ornithine/arginine ratio was 36% higher in patients with diabetes, indicating increased arginase activity. These ratios were inversely correlated to endothelium-dependent vasodilatation and endothelium-independent vasodilatation. CONCLUSION: Patients with diabetes and macrovascular complications have increased amino acid ratios reflecting a shift in arginine metabolism due to arginase activation. These changes are inversely related to endothelial function supporting that arginase activity contributes to endothelial dysfunction.

Increased arginase levels contribute to impaired perfusion after cardiopulmonary resuscitation.

OBJECTIVES: The postcardiac arrest syndrome occurs after global hypoxia leading to microcirculatory impairment. Nitric oxide (NO) is a key molecule regulating microvascular function. The enzyme arginase has been suggested to modulate microvascular function by regulating NO metabolism. Therefore, we investigated whether arginase increases following global hypoxia and resuscitation and tested whether arginase inhibition influences altered microcirculation in resuscitated patients. METHODS: To determine the effect of global hypoxia on circulating arginase levels, fourteen healthy subjects were exposed to hypoxia in a normobaric hypoxia chamber (FiO² = 9·9%). In addition, 31 resuscitated patients were characterized clinically, and arginase 1 was measured on days 1 and 3. In eight resuscitated patients, a microcirculatory analysis was performed using a sidestream darkfield microcirculation camera. Perfused capillary density (PCD) was recorded before and after sublingual incubation of N-omega-hydroxy-nor-l-arginine (nor-NOHA) alone or together with the NOS inhibitor NG-monomethyl-l-arginine (l-NMMA). RESULTS: Circulating arginase 1 levels increased in healthy volunteers following global hypoxia in the hypoxic chamber (P < 0·01). In addition, arginase 1 levels were higher on day 1 (69·1 ± 83·3 ng/mL) and on day 3 (44·2 ± 65·6 ng/mL) after resuscitation than in control subjects (P < 0·001). Incubation of the sublingual mucosa with nor-NOHA increased microcirculatory perfusion (P < 0·001). This effect was inhibited by co-incubation with K-NMMA. CONCLUSIONS: Circulating arginase 1 levels are increased following exposure to global hypoxia and in patients who have been successfully resuscitated after cardiac arrest. Topical arginase inhibition improves microcirculatory perfusion following resuscitation. This is of potential therapeutic importance for the postcardiac arrest syndrome.

Effect of arginase inhibition on ischemia-reperfusion injury in patients with coronary artery disease with and without diabetes mellitus.

BACKGROUND: Arginase competes with nitric oxide synthase for their common substrate L-arginine. Up-regulation of arginase in coronary artery disease (CAD) and diabetes mellitus may reduce nitric oxide bioavailability contributing to endothelial dysfunction and ischemia-reperfusion injury. Arginase inhibition reduces infarct size in animal models. Therefore the aim of the current study was to investigate if arginase inhibition protects from endothelial dysfunction induced by ischemia-reperfusion in patients with CAD with or without type 2 diabetes ( CLINICAL TRIAL REGISTRATION NUMBER: NCT02009527). METHODS: Male patients with CAD (n = 12) or CAD + type 2 diabetes (n = 12), were included in this cross-over study with blinded evaluation. Endothelium-dependent vasodilatation was assessed by flow-mediated dilatation (FMD) of the radial artery before and after 20 min ischemia-reperfusion during intra-arterial infusion of the arginase inhibitor (Nω-hydroxy-nor-L-arginine, 0.1 mg/min) or saline. RESULTS: The forearm ischemia-reperfusion was well tolerated. Endothelium-independent vasodilatation was assessed by sublingual nitroglycerin. Ischemia-reperfusion decreased FMD in patients with CAD from 12.7±5.2% to 7.9±4.0% during saline administration (P<0.05). Nω-hydroxy-nor-L-arginine administration prevented the decrease in FMD in the CAD group (10.3±4.3% at baseline vs. 11.5±3.6% at reperfusion). Ischemia-reperfusion did not significantly reduce FMD in patients with CAD + type 2 diabetes. However, FMD at reperfusion was higher following nor-NOHA than following saline administration in both groups (P<0.01). Endothelium-independent vasodilatation did not differ between the occasions. CONCLUSIONS: Inhibition of arginase protects against endothelial dysfunction caused by ischemia-reperfusion in patients with CAD. Arginase inhibition may thereby be a promising therapeutic strategy in the treatment of ischemia-reperfusion injury.

Selective endothelin ETA and dual ET(A)/ET(B) receptor blockade improve endothelium-dependent vasodilatation in patients with type 2 diabetes and coronary artery disease.

AIMS: Endothelin-1 contributes to endothelial dysfunction in patients with atherosclerosis and type 2 diabetes. In healthy arteries the ETA receptor mediates the main part of the vasoconstriction induced by endothelin-1 whilst the ETB receptor mediates vasodilatation. The ETB receptor expression is upregulated on vascular smooth muscle cells in atherosclerosis and may contribute to the increased vasoconstrictor tone and endothelial dysfunction observed in this condition. Due to these opposing effects of the ETB receptor it remains unclear whether ETB blockade together with ETA blockade may be detrimental or beneficial. The aim was therefore to compare the effects of selective ETA and dual ETA/ETB blockade on endothelial function in patients with type 2 diabetes and coronary artery disease. MAIN METHODS: Forearm endothelium-dependent and endothelium-independent vasodilatation was assessed by venous occlusion plethysmography in 12 patients before and after selective ETA or dual ETA/ETB receptor blockade. KEY FINDINGS: Dual ETA/ETB receptor blockade increased baseline forearm blood flow by 30±14% (P<0.01) whereas selective ETA blockade did not (14±8%). Both selective ETA blockade and dual ETA/ETB blockade significantly improved endothelium-dependent vasodilatation. The improvement did not differ between the two treatments. There was also an increase in endothelium-independent vasodilatation with both treatments. Dual ETA/ETB blockade did not significantly increase microvascular flow but improved transcutaneous pO2. SIGNIFICANCE: Both selective ETA and dual ETA/ETB improve endothelium-dependent vasodilatation in patients with type 2 diabetes and coronary artery disease. ETB blockade increases basal blood flow but does not additionally improve endothelium-dependent vasodilatation.

Arginase inhibition reduces infarct size via nitric oxide, protein kinase C epsilon and mitochondrial ATP-dependent K+ channels.

Reduced bioavailability of nitric oxide (NO) contributes to the development of myocardial ischemia-reperfusion (I/R) injury. Increased activity of arginase is a potential factor that reduces NO bioavailability by competing for the substrate L-arginine. The aim of the study was to test the hypothesis that inhibition of arginase after coronary artery occlusion protects from I/R injury and to explore possible mechanisms behind this effect. Male Sprague-Dawley rats subjected to 30 min of coronary artery ligation and 2h reperfusion were given i.v. before the reperfusion: 1) saline; 2) the arginase inhibitor N-omega-hydroxy-nor-L-arginine (nor-NOHA); 3) nor-NOHA with the NO synthase (NOS) inhibitor N(G)-monomethyl-L-arginine (L-NMMA); 4) nor-NOHA with the mitochondrial ATP-dependent K(+) (mitoKATP) channel blocker 5-hydroxydecanoic acid (5-HD); 5) nor-NOHA with the protein kinase C epsilon (PKCε) inhibitor ε-V1-2 or 6) ε-V1-2 alone. Infarct size in the control groups was 61±3% and it was reduced to 47±3% (P<0.01) by nor-NOHA. The cardioprotective effect was blocked by the NOS inhibitor L-NMMA. PKCε expression was reduced by I/R and this reduction was attenuated by nor-NOHA. Furthermore, the PKCε inhibitor ε-V1-2 abolished the protective effect of nor-NOHA (infarct size 69±6%). In addition, the cardioprotective effect of nor-NOHA was also abolished following blockade of the mitoKATP channel (infarct size 62±1%). Inhibition of arginase before reperfusion protects the heart from I/R injury via a NOS-dependent pathway, increased expression of PKCε and activation of mitoKATP channels.

Arginase inhibition improves endothelial function in patients with coronary artery disease and type 2 diabetes mellitus.

BACKGROUND: Endothelial dysfunction plays an important role in the early development of atherosclerosis and vascular complications in type 2 diabetes mellitus. Increased expression and activity of arginase, metabolizing the nitric oxide substrate l-arginine, may result in reduced production of nitric oxide and thereby endothelial dysfunction. We hypothesized that inhibition of arginase activity improves endothelial function in patients with coronary artery disease (CAD) and type 2 diabetes mellitus. METHODS AND RESULTS: Three groups of subjects were included: 16 patients with CAD, 16 patients with CAD and type 2 diabetes mellitus (CAD+Diabetes), and 16 age-matched healthy control subjects. Forearm endothelium-dependent and endothelium-independent vasodilatation were assessed with venous occlusion plethysmography before and during intra-arterial infusion of the arginase inhibitor N(ω)-hydroxy-nor-l-arginine (nor-NOHA; 0.1 mg/min). Nor-NOHA was also coinfused with the nitric oxide synthase inhibitor (N(G)-monomethyl L-arginine). The expression of arginase was determined in the internal mammary artery of patients undergoing bypass surgery. Nor-NOHA markedly increased endothelium-dependent vasodilatation (up to 2-fold) in patients with CAD+Diabetes and CAD (P<0.001) but not in the control group. N(G)-monomethyl L-arginine completely inhibited the increase in endothelium-dependent vasodilatation induced by nor-NOHA. Endothelium-independent vasodilatation was slightly improved by nor-NOHA in the CAD+Diabetes group. Arginase I was expressed in vascular smooth muscle cells and endothelial cells, and arginase II was expressed in endothelial cells of patients with and without diabetes mellitus. CONCLUSIONS: Arginase inhibition markedly improves endothelial function in patients with CAD and type 2 diabetes mellitus suggesting that increased arginase activity is a key factor in the development of endothelial dysfunction.

Local arginase inhibition during early reperfusion mediates cardioprotection via increased nitric oxide production.

Consumption of L-arginine contributes to reduced bioavailability of nitric oxide (NO) that is critical for the development of ischemia-reperfusion injury. The aim of the study was to determine myocardial arginase expression and activity in ischemic-reperfusion myocardium and whether local inhibition of arginase within the ischemic myocardium results in increased NO production and protection against myocardial ischemia-reperfusion. Anesthetized pigs were subjected to coronary artery occlusion for 40 min followed by 4 h reperfusion. The pigs were randomized to intracoronary infusion of vehicle (n = 7), the arginase inhibitor N-hydroxy-nor-L-arginine (nor-NOHA, 2 mg/min, n = 7), the combination of nor-NOHA and the NO synthase inhibitor N(G)-monomethyl-L-arginine (L-NMMA, 0.35 mg/min, n = 6) into the jeopardized myocardial area or systemic intravenous infusion of nor-NOHA (2 mg/min, n = 5) at the end of ischemia and start of reperfusion. The infarct size of the vehicle group was 80 ± 4% of the area at risk. Intracoronary nor-NOHA reduced infarct size to 46 ± 5% (P<0.01). Co-administration of L-NMMA abrogated the cardioprotective effect mediated by nor-NOHA (infarct size 72 ± 6%). Intravenous nor-NOHA did not reduce infarct size. Arginase I and II were expressed in cardiomyocytes, endothelial, smooth muscle and poylmorphonuclear cells. There was no difference in cytosolic arginase I or mitochondrial arginase II expression between ischemic-reperfused and non-ischemic myocardium. Arginase activity increased 2-fold in the ischemic-reperfused myocardium in comparison with non-ischemic myocardium. In conclusion, ischemia-reperfusion increases arginase activity without affecting cytosolic arginase I or mitochondrial arginase II expression. Local arginase inhibition during early reperfusion reduces infarct size via a mechanism that is dependent on increased bioavailability of NO.

New perspectives on endothelin-1 in atherosclerosis and diabetes mellitus.

Endothelin-1 (ET-1) is a vasoconstrictor, proinflammatory and proliferative endothelial cell-derived peptide that is of significant importance in the regulation of vascular function. It is involved in the development of endothelial dysfunction including important interactions with nitric oxide. The expression and functional effects of ET-1 and its receptors are markedly altered during development of cardiovascular disease. Increased production of ET-1 and its receptors mediate many pathophysiological events contributing to the development of atherosclerosis and vascular complications in diabetes mellitus. The present review focuses on the pathophysiological role of ET-1 and the potential importance of ET receptors as a therapeutic target for treatment of these conditions.

Endothelin-1 reduces glucose uptake in human skeletal muscle in vivo and in vitro.

OBJECTIVE: Endothelin (ET)-1 is a vasoconstrictor and proinflammatory peptide that may interfere with glucose uptake. Our objective was to investigate whether exogenous ET-1 affects glucose uptake in the forearm of individuals with insulin resistance and in cultured human skeletal muscle cells. RESEARCH DESIGN AND METHODS: Nine male subjects (aged 61 ± 3 years) with insulin resistance (M value <5.5 mg/kg/min or a homeostasis model assessment of insulin resistance index >2.5) participated in a protocol using saline infusion followed by ET-1 infusion (20 pmol/min) for 2 h into the brachial artery. Forearm blood flow (FBF), endothelium-dependent vasodilatation, and endothelium-independent vasodilatation were assessed. Molecular signaling and glucose uptake were determined in cultured skeletal muscle cells. RESULTS: ET-1 decreased forearm glucose uptake (FGU) by 39% (P < 0.05) after the 2-h infusion. ET-1 reduced basal FBF by 36% after the 2-h infusion (P < 0.05) and impaired both endothelium-dependent vasodilatation (P < 0.01) and endothelium-independent vasodilatation (P < 0.05). ET(A) and ET(B) receptor expression was detected on cultured skeletal muscle cells. One-hour ET-1 incubation increased glucose uptake in cells from healthy control subjects but not from type 2 diabetic patients. Incubation with ET-1 for 24 h reduced glucose uptake in cells from healthy subjects. ET-1 decreased insulin-stimulated Akt phosphorylation and increased phosphorylation of insulin receptor substrate-1 serine 636. CONCLUSIONS: ET-1 not only induces vascular dysfunction but also acutely impairs FGU in individuals with insulin resistance and in skeletal muscle cells from type 2 diabetic subjects. These findings suggest that ET-1 may contribute to the development of insulin resistance in skeletal muscle in humans.

Regulation of glucose uptake by endothelin-1 in human skeletal muscle in vivo and in vitro.

CONTEXT: Expression of the vasoconstrictor and proinflammatory peptide endothelin (ET)-1 is increased in insulin-resistant (IR) subjects. OBJECTIVE: The aim of this study was to investigate whether ET-1 regulates skeletal muscle glucose uptake in IR subjects in vivo and in cultured human skeletal muscle cells. DESIGN AND PARTICIPANTS: Eleven subjects participated in three protocols using brachial artery infusion of: A) BQ123 (10 nmol/min) and BQ788 (10 nmol/min) (ET(A) and ET(B) receptor antagonist, respectively), followed by coinfusion with insulin (0.05 mU/kg/min); B) insulin alone; and C) insulin followed by coinfusion with ET-1 (20 pmol/min). MAIN OUTCOME MEASURES: Forearm blood flow (FBF) and forearm glucose uptake (FGU) were determined. Glucose uptake and molecular signaling were determined in cultured skeletal muscle cells. RESULTS: ET(A)/ET(B) receptor blockade increased FGU by 63% (P < 0.05). Coadministration of insulin caused a further 2-fold increase in FGU (P < 0.001). ET(A)/ET(B) receptor blockade combined with insulin resulted in greater FGU than insulin infusion alone (P < 0.005). ET(A)/ET(B) receptor blockade increased FBF by 30% (P < 0.05), with a further 16% increase (P < 0.01) during insulin coinfusion. ET-1 decreased basal FBF by 35% without affecting FGU. ET-1 impaired basal and insulin-stimulated glucose uptake in cultured muscle cells (P < 0.01) via an effect that was prevented by ET(A)/ET(B) receptor blockade. CONCLUSION: ET(A)/ET(B) receptor blockade enhances basal and insulin-stimulated glucose uptake in IR subjects. ET-1 directly impairs glucose uptake in skeletal muscle cells via a receptor-dependent mechanism. These data suggest that ET-1 regulates glucose metabolism via receptor-dependent mechanisms in IR subjects.

Different subpopulations of endothelial progenitor cells and circulating apoptotic progenitor cells in patients with vascular disease and diabetes.

AIMS: Endothelial progenitor cells (EPCs) represent a repair mechanism involving reendothelialization and neoangiogenesis. Patients with both diabetes and known vascular disease have low numbers of circulating EPCs. To assess the role of diabetes in vascular disease we investigated the number and viability of circulating EPCs and related this to endothelial function. METHODS: Different EPC subpopulations were enumerated by flow cytometry using triple staining (CD34, CD133, KDR). Viability was assessed by 7AAD and Annexin-V-staining. Endothelial function was evaluated by Endo-PAT2000 in 19 patients with known vascular disease without diabetes and 20 patients with vascular disease and diabetes mellitus. RESULTS: CD34+, CD133+, CD34+/CD133+, CD34+/KDR+, CD34+/CD133+/KDR+ cell counts did not differ between patients with and without diabetes. CD34-/CD133+/KDR+ cells were lower, whereas staining for apoptosis in progenitor cells was higher in patients with diabetes. Progenitor cell counts did not correlate to peripheral arterial function test. DISCUSSION: Patients having diabetes and vascular disease have lower CD34-/CD133+/KDR+ EPC counts, a recently discovered subpopulation of EPCs, and larger proportion of apoptotic EPCs. Higher apoptotic rates of progenitor cells in diabetes could represent an important mechanism explaining lower functional capability of these cells. These observations may be of importance for the complications associated with diabetes mellitus.

Dual endothelin receptor blockade acutely improves insulin sensitivity in obese patients with insulin resistance and coronary artery disease.

OBJECTIVE: Endothelin (ET)-1 is a vasoconstrictor and proinflammatory peptide that may inhibit glucose uptake. The objective of the study was to investigate if ET (selective ET(A) and dual ET(A)+ET(B)) receptor blockade improves insulin sensitivity in patients with insulin resistance and coronary artery disease. RESEARCH DESIGN AND METHODS: Seven patients (aged 58 +/- 2 years) with insulin resistance and coronary artery disease completed three hyperinsulinemic-euglycemic clamp protocols: a control clamp (saline infusion), during ET(A) receptor blockade (BQ123), and during combined ET(A) (BQ123) and ET(B) receptor blockade (BQ788). Splanchnic blood flow (SBF) and renal blood flow (RBF) were determined by infusions of cardiogreen and p-aminohippurate. RESULTS: Total-body glucose uptake (M) differed between the clamp protocols with the highest value in the BQ123+BQ788 clamp (P < 0.05). The M value corrected by insulin was higher in the BQ123+BQ788 than in the control clamp (P < 0.01) or the BQ123 clamp (P < 0.05). There was no difference between the control clamp and the BQ123 clamp. Mean arterial pressure did not change during the control clamp, whereas it decreased during both the BQ123 (P < 0.01) and BQ123+BQ788 (P < 0.05) clamps. RBF increased and renal vascular resistance decreased in the BQ123+BQ788 clamp (P < 0.05) but not in the BQ123 clamp. There was no change in SBF in either clamp. CONCLUSIONS: Dual ET(A)+ET(B) receptor blockade acutely enhances insulin sensitivity in patients with insulin resistance and coronary artery disease, indicating an important role for endogenous ET-1.

Enhanced endothelium-dependent vasodilatation by dual endothelin receptor blockade in individuals with insulin resistance.

Insulin resistance is associated with endothelial dysfunction and increased production of the pro-inflammatory vasoconstrictor peptide endothelin-1 (ET-1). The aim of this study was to test the hypothesis that blockade of ET receptors results in enhanced endothelium-dependent vasodilatation (EDV) in individuals with insulin resistance. Twelve individuals with insulin resistance without any history of diabetes or cardiovascular disease and 8 age-matched controls with high insulin sensitivity, as determined by hyperinsulinemic-euglycemic clamp, were investigated on 2 separate occasions using forearm venous occlusion plethysmography. Endothelium-dependent and endothelium-independent vasodilatation was determined before and after selective ET(A) and dual ET(A)/ET(B) receptor blockade. A 60 minute intraarterial infusion of the ET(A) receptor antagonist BQ123 (10 nmol/min) combined with the ET(B) receptor antagonist BQ788 (5 nmol/min) evoked a significant increase in acetylcholine-mediated EDV (P < 0.01) in individuals with insulin resistance. The endothelium-independent vasodilator response to nitroprusside was not changed by dual ET(A)/ET(B) receptor blockade. Dual ET(A)/ET(B) receptor blockade did not affect the response to acetylcholine or nitroprusside in the insulin-sensitive group. Selective ET(A) receptor blockade did not evoke any changes in endothelium-dependent or endothelium-independent vasodilatation in either group. This study demonstrates that dual ET(A)/ET(B) receptor blockade, but not selective ET(A) blockade, enhances EDV in subjects with insulin resistance, suggesting that ET-1 is involved in the regulation of endothelial function in individuals with insulin resistance.