Free radicals in diabetic endothelial cell dysfunction

Effect of vitamin E and allylamine on the proliferation of cultured aortic smooth muscle cells from streptozotocin-induced diabetic rats

To investigate the effect of vitamin E on the proliferation activity of vascular smooth muscle cells (SMCs) in diabetes mellitus, [3H]-thymidine incorporation was measured in cultured SMCs isolated from normal and streptozotocin-induced diabetic rats treated with or without vitamin E and/or allylamine. Untreated diabetic rats demonstrated significantly elevated concentrations of serum total cholesterol, triglycerides and malondialdehyde (MDA). Allylamine caused a further increase in serum MDA. Treatment with vitamin E decreased the serum concentrations of triglycerides and MDA in both allylamine-treated and -untreated diabetic rats. [3H]-Thymidine incorporation in cultured SMCs from diabetic rats was significantly increased compared with that from normal rats. SMCs from allylamine-treated diabetic rats showed an enhanced increase in thymidine incorporation compared with that from untreated diabetic rats. The increase in thymidine incorporation in SMCs from untreated and allylamine-treated diabetic rats was significantly reduced by the treatment with vitamin E. These observations suggest that vitamin E has a preventive effect on the proliferation of vascular SMCs in diabetes, and that this effect may be mediated through an enhancement of free radical scavenging.

Constrictor responses of resistance arterioles during diabetes mellitus

While diabetes mellitus appears to alter nitric oxide synthase-dependent vasodilatation, the effect of diabetes on constrictor responses of resistance arterioles is not clear. Our goal was to examine the effect of diabetes on constrictor responses of cheek pouch arterioles. In vivo diameter of arterioles ( approximately 50 microm) was measured in response to norepinephrine, the thromboxane analogue (U-46619) and endothelin-1 in nondiabetic and diabetic hamsters (4-6 weeks post streptozotocin). Norepinephrine (1.0 and 10 nM) and U-46619 (0.1 and 1.0 nM) produced similar dose-related vasoconstriction in nondiabetic and diabetic hamsters (P > 0.05). In contrast, vasoconstriction to endothelin-1 (0.1 and 1.0 pM) was greater in diabetic than nondiabetic hamsters (P < 0.05). Next, we examined the role of nitric oxide in basal vascular tone and whether enhanced vasoconstriction in diabetic hamsters to endothelin-1 might be related to an alteration in the modulatory role of nitric oxide. N(G)-monomethyl-L-arginine (L-NMMA) (1.0, 10 and 100 microM) produced dose-related vasoconstriction in nondiabetic, but not diabetic hamsters. Further, L-NMMA did not alter vasoconstriction in response to endothelin-1 in nondiabetic and diabetic hamsters. These findings suggest that diabetes alters constriction of cheek pouch resistance arterioles to endothelin-1 which appears to be independent of the synthesis/release of nitric oxide. In addition, based upon findings using L-NMMA, it appears that there is a reduced influence of nitric oxide on basal diameter of resistance arterioles during diabetes.

Endothelium is required in the vascular spasm induced by tetraethylammonium and endothelin-1 in guinea-pig aorta

1. To investigate the role of endothelium in vascular spasm, we studied the influence of endothelin-1 (ET-1) on the contracting and spasmogenic effect of the K+-channel blocker, tetraethylammonium (TEA), in aorta rings of reserpine-treated guinea-pigs, perfused with either control (5.5 mM) or elevated (50 mM) glucose concentration. 2. Endothelium-dependent relaxation induced by acetylcholine was lost in rings contracted by noradrenaline in the presence of elevated glucose. In control medium, TEA (1-20 mM) induced a sustained tonic contraction, followed by a phasic spasm, characterized by rhythmic contractions. Elevated glucose, ET-1 (3 nM), or both, reduced the EC50 of TEA-induced tonic contraction, without modifying the maximum contractile effect. 3. In control medium, ET-1 reduced the time before TEA-induced spasm and increased the rate of rhythmic contractions. TEA-induced spasm was abolished by elevated glucose, and restored by ET-1. The spasm induced by TEA and ET-1 was amplified by the ETA antagonist, EMD94246, and suppressed by the ET(A)-ET(B) antagonist, bosentan. In endothelium-denuded vessels incubated with high glucose and ET-1, TEA evoked only a tonic contraction. 4. In control medium, L-NAME (N(G)-nitro-L-arginine methyl ester) abolished TEA-induced rhythmic contractions. L-arginine, but not D-arginine, prevented the effect of L-NAME. In the presence of elevated glucose and ET-1, TEA-induced spasm was not affected by L-NAME, whereas verapamil, indomethacin, metyrapone, glybenclamide or apamin abolished the phasic spasm, unmasking the tonic contracture. 5. In conclusion, endothelium plays a regulatory role in the genesis and maintenance of TEA-induced rhythmic contractions, through the release endothelium derived relaxing factor and vasodilating eicosanoids.

Impairment of endothelial relaxations by glycosylated human oxyhemoglobin depends on the oxidative state of the heme group

While nanomolar met- or cyanomethemoglobin, either non-glycosylated or glycosylated, did not alter endothelial function, glycosylated oxyhemoglobin induced contractile responses and caused an impairment of endothelium-dependent relaxations in rat aortic segments. The vascular effects induced by glycosylated oxyhemoglobin were prevented by superoxide dismutase. Furthermore, glycosylated oxyhemoglobin produced higher amounts of superoxide anions than other hemoglobin derivatives. These results suggest that glycosylated hemoglobin requires the existence of a functional heme group containing iron in ferrous state to interfere with the endothelial function at nanomolar concentrations. This effect is mediated by generation of superoxide anions.

Relationship of serum N-acetyl-beta-glucosaminidase activity to oxidative stress in diabetes mellitus

Serum N-acetyl-beta-glucosaminidase activity was evaluated in 40 Type 1 and 40 Type 2 diabetic patients and compared with parameters of diabetes control and oxidative stress. Significantly increased mean serum N-acetyl-beta-glucosaminidase activity was found in both groups of diabetic patients as compared with the corresponding group of healthy persons (p < 0.01). Oxidative stress measured by plasma malondialdehyde concentration was significantly higher in Type 2 than in Type 1 diabetic patients (p < 0.01) but in comparison with control subjects it was higher only in Type 2 diabetes. Plasma malondialdehyde concentration positively correlated with body mass index (r=0.77, p<0.001) and with serum N-acetyl-beta-glucosaminidase activities (r=0.57, p <0.001). Treatment of 10 Type 2 diabetic patients with antioxidant alpha-tocopherol caused a significant decrease in malondialdehyde concentration (p < 0.001) which was accompanied by a decrease of N-acetyl-beta-glucosaminidase activity (p < 0.01). We conclude that serum N-acetyl-beta-glucosaminidase activity may be influenced by oxidative stress which is more pronounced in Type 2 than in Type 1 diabetic patients.

Diabetes abolishes the vascular protective effects of estrogen in female rats

Estrogen is known to exert a protective effect against cardiovascular disease. However, women with diabetes have three times the risk as compared with age-matched non-diabetic women. Our previous study on aortic rings of ovariectomized (OVX) female rats treated with 17-beta-estradiol (E2) demonstrated that the beneficial effect of estrogen is related to the basal release of NO from endothelial cells. In the present study, in order to understand why estrogen protection is abolished in diabetes, we tested vascular responses in OVX, streptozotocin-diabetic female rats and their non-diabetic controls receiving or not E2 replacement. Concentration-response curves to norepinephrine (NE) showed attenuation of the contractile response in E2-treated diabetic, with respect to non-diabetic preparations. This response was further impaired in diabetic, E2-deprived rats. The basal release of NO, as evaluated by concentration-related responses to N(G)-methyl-L-arginine acetate in NE-precontracted aortic rings, was found to be impaired in E2-treated diabetic rats, no further effect being induced by E2 deprivation. The endothelium-dependent relaxation produced by carbachol did not change between groups, whereas the relaxation produced by histamine was enhanced by both diabetes and E2 deprivation. However, E2 treatment counteracted the response to histamine only in preparations from non-diabetic animals. Finally, the relaxation induced by sodium nitroprusside, an endothelium-independent relaxant agent, was comparable between groups. These findings suggest that the lack of protective effects of estrogen in diabetes may be mainly ascribed to the failure of estrogen to reverse the impaired basal release of NO and the abnormal relaxation to histamine, which are observed in the aorta of diabetic rats.

Relationship between coronary function by positron emission tomography and temporal changes in morphology by intravascular ultrasound (IVUS) in transplant recipients

BACKGROUND

Transplant coronary vasculopathy is one of the major causes of graft failure and death in cardiac transplant recipients. A non-invasive test of coronary function to predict the course of this disease would be desirable.

METHODS

To determine whether the degree of abnormalities in endothelial dependent coronary vasomotion (cold pressor testing) or endothelial independent vasodilatory capacity (intravenous dipyridamole) as determined by positron emission tomography (PET) one to two years after heart transplantation is correlated with the course of transplant vasculopathy. Nineteen patients had baseline PET and intravascular ultrasound studies (IVUS) at 18 +/- 6 months after cardiac transplantation and a follow up IVUS study 15 +/- 5 months later.

RESULTS

Myocardial blood flow was higher in patients than in healthy controls (p < 0.002) but increased during cold pressor testing only in controls (p < 0.005). Myocardial blood flow normalized to the rate pressure product declined in patients (p < 0.001). Dipyridamole-induced hyperemic blood flow and the flow reserve normalized to the resting rate pressure product were lower in patients than in controls (p < 0.001 and p < 0.01). The normalized flow reserve was correlated with changes in total vessel area (r = 0.55; p = 0.02) and lumen diameter (r = 0.52; p < 0.05).

CONCLUSION

These findings suggest that the degree of abnormalities in endothelial independent myocardial flow as detected by PET one to two years after transplantation is associated with morphological indices of disease progression by IVUS.

Prostaglandin endoperoxide-dependent vasospasm in bovine coronary arteries after nitration of prostacyclin synthase

In the present study we used a bioassay to study the effects of peroxynitrite (ONOO-) on angiotensin II (A-II)-triggered tension in isolated bovine coronary arteries in order to show the consequences of the previously reported PGI2-synthase inhibition by ONOO- in this model. The following results were obtained: 1. 1 micromol L(-1) ONOO- impaired A-II-induced vasorelaxation and caused a second long lasting constriction phase. Indomethacin (10(-5)M) prevented both effects. U51605, a dual blocker of PGI2-synthase and thromboxane (TX)A2-synthase mimicked the effects of ONOO-. 2. The selective TXA2/prostaglandin endoperoxide (PGH2) receptor antagonist SQ29548 antagonized the second vasoconstriction phase after ONOO- -treatment. Since a generation of TXA2 and 8-iso-prostaglandin F2alpha could be excluded a direct action of unmetabolized PGH2 on the TXA2/PGH2 receptor was postulated. 3. ONOO- dose-dependently inhibited the conversion of 14C-PGH2 into 6-keto-PGF1alpha in isolated bovine coronary arteries with an IC50-value of 100 nM. 4. Immunoprecipitation of 3-nitrotyrosine-containing proteins with a monoclonal antibody revealed PGI2-synthase as the only nitrated protein in bovine coronary arteries treated with 1 micromol 1(-1) ONOO-. 5. Using immunohistochemistry a co-localization of PGI2-synthase and nitrotyrosine-containing proteins was clearly visible in both endothelial and vascular smooth muscle cells. We concluded that ONOO- not only eliminated the vasodilatory, growth-inhibiting, antithrombotic and antiadhesive effects of PGI2 but also allowed and promoted an action of the potent vasoconstrictor, prothrombotic agent, growth promoter, and leukocyte adherer, PGH2.

Different effects of the beta-adrenoceptor antagonists celiprolol and metoprolol on vascular structure and function in long-term type I diabetic rats

An intriguing problem of diabetes mellitus is the development of generalized angiopathy and concomitant hypertension. However, there is still a controversy whether beta-adrenoceptor antagonists can be used as antihypertensive agents in diabetes. Four groups of rats were investigated: nondiabetic controls, diabetes mellitus, diabetes + celiprolol (250 mg/kg body weight/day), diabetes + metoprolol (125 mg/kg body weight/day) after 6 months. Diabetes was induced by i.v. streptozotocin injection. We examined vascular structure and function histologically and by an in vitro microvideoangiometry of isolated perfused mesenterium. Additionally, we investigated the effects of hyperglycemia and celiprolol on NO release in cultivated aortic endothelial cells and the effect of celiprolol on transendothelial paracellular permeability. Diabetes resulted in endothelial dysfunction, characterized by a reduced response to acetylcholine and L-N(G)-nitro-arginine and an unchanged response to sodium nitroprusside (SNP). These effects were significantly antagonized by celiprolol but were not influenced by metoprolol treatment. This was supported by the finding of typical vascular changes associated with diabetes like media thickening, reduced cardiac capillary/muscle fiber ratio, and glomerulosclerosis, which were significantly reduced by celiprolol but not influenced by metoprolol treatment. Ketonuria improved after celiprolol treatment, whereas blood glucose, lipids, and body weight were not different between the diabetic groups. In cultured cells, celiprolol did not induce direct NO release but reversed the impairment of stimulated NO release caused by hyperglycemia. Furthermore, celiprolol reduced endothelial paracellular permeability. We conclude that celiprolol can exert antiangiopathic effects in diabetic rats and that both beta-adrenoceptor antagonists did not aggravate diabetic angiopathy and metabolic derangement.

The role of oxidative stress and NF-kappaB activation in late diabetic complications

A common endpoint of hyperglycemia dependent cellular changes is the generation of reactive oxygen intermediates (ROIs) and the presence of elevated oxidative stress. Therefore, oxidative stress is supposed to play an important role in the development of late diabetic complications. Formation of advanced glycation end products (AGE's) due to elevated nonenzymatic glycation of proteins, lipids and nucleic acids is accompanied by oxidative, radical-generating reactions and thus represents a major source for oxygen free radicals under hyperglycemic conditions. Once formed, AGE's can influence cellular function by binding to several binding sites including the receptor for AGE's, RAGE. Binding of AGE's (and other ligands) to RAGE results in generation of intracellular oxidative stress and subsequent activation of the redox-sensitive transcription factor NF-kappaB in vitro and in vivo. Consistently, activation of NF-kappaB in diabetic patients correlates with the quality of glycemic control and can be reduced by treatment with the antioxidant alpha-lipoic acid. The development of techniques allowing for a tissue culture independent measurement of NF-kappaB activation in patients with diabetes mellitus gives insights into the molecular mechanisms linking diabetes mellitus and hyperglycemia with formation of advanced glycated endproducts and generation of oxidative stress finally resulting in oxidative stress mediated cellular activation.

Alterations in enzymatic antioxidant defence in diabetes mellitus--a rational approach

Defence against the reactive oxidants produced during aerobic metabolism is a complex process and is provided by a system of enzymes and antioxidant compounds capable of preventing excess radical production, neutralising free radicals and repairing the damage caused by them. Regulation of the antioxidant system must provide sufficient, properly located, antioxidant compounds and enzymes. Damage to this system has been proved to play a role in various disorders. Long-term complications of diabetes mellitus are supposed to be partially mediated by oxidative stress. The authors summarise experimental and clinical investigations in this field and analyse the possible importance of the changes in the antioxidant system in the development of diabetic vascular complications.

Diabetes mellitus and the clinical and angiographic outcome after coronary stent placement

OBJECTIVES

The objectives of this study were to analyze the clinical and angiographic outcome of diabetic patients with successful coronary stent placement and to compare these results with those achieved after stenting in nondiabetic patients.

BACKGROUND

The outcome of diabetic patients treated with stent placement due to coronary artery disease has not been assessed comprehensively.

METHODS

This study analyzes a consecutive series of patients with successful stent placement comprising 715 patients with diabetes and 2,839 patients without diabetes. Clinical one year follow-up and angiographic control at 6 months were part of the protocol. Death, myocardial infarction and target lesion revascularization were considered as adverse events. An automated edge detection system was used for the angiographic assessment. The primary clinical endpoint was event-free survival at one year. The primary angiographic endpoint was restenosis rate at 6 months (> or = 50% diameter stenosis).

RESULTS

Event-free survival was significantly lower in diabetic than in nondiabetic patients (73.1 vs. 78.5%, p < 0.001). Survival free of myocardial infarction was also significantly reduced in the diabetic group (89.9 vs. 94.4% in nondiabetics, p < 0.001). The incidence of both restenosis (37.5 vs. 28.3%, p < 0.001) and stent vessel occlusion (5.3 vs. 3.4%, p = 0.037) was significantly higher in diabetic patients. Diabetes was identified as an independent risk factor for adverse clinical events and restenosis in multivariate analyses.

CONCLUSIONS

Patients with diabetes mellitus have a less favorable clinical outcome at one year after successful stent placement as compared to the nondiabetic patients. The clinical follow-up was characterized by a higher incidence of death, myocardial infarction and reinterventions. Diabetic patients also demonstrated an increased risk for restenosis.

Methodological considerations and factors affecting 8-hydroxy-2-deoxyguanosine analysis

Oxidative stress is related to a number of diseases due to the formation of reactive oxygen species (ROS). There are also several substances found in the occupational environment or as life style related situations that generates ROS. A stable biomarker for oxidative stress on DNA is 8-hydroxy-2'-deoxyguanosine (8-OH-dG). A potential problem in the work-up and analysis of 8-OH-dG is oxidation of dG with false high levels as a result of analysis. This paper summarizes and discusses some of the critical moments in terms of auto-oxidation. The removal of transition metals, low temperatures, absence of isotopes (or 2'-deoxyguanosine) and incubation times are all important factors. Removal of oxygen is complicated while the problem is reduced if a nitroxide (TEMPO) is added during work-up. Certain reducing agents and enzymes could be critical if added during work-up. The application of the 32p-HPLC method to analyze 8-OH-dG is discussed. The 32P-HPLC method is suitable for 8-OH-dG analysis and avoids several factors that oxidizes dG by removal of dG before addition of isotopes. Factors of crucial importance (columns, eluents, gradients and detection of 32p) for the analysis of 8-OH-dG are commented upon and certain recommendations are made to make it possible to apply the 32P-HPLC methodology for this type of analysis.

Contractile responses elicited by hydrogen peroxide in aorta from normotensive and hypertensive rats. Endothelial modulation and mechanism involved

The present study analyses the influence of hypertension and endothelium on the effect induced by hydrogen peroxide (H2O2) on basal tone in aortic segments from normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR) of 6-month-old, as well as the possible mechanisms involved. Single (1 mM) or cumulative (100 nM-10 mM) concentrations of H2O2 produced a transient contraction or a concentration-dependent increase of basal tone, respectively, in segments from WKY and SHR. In both cases, the contractions were higher in intact segments from hypertensive than from normotensive rats, and increased by endothelium removal in both strains. Catalase (1000 u ml(-1), a H2O2 scavenger) abolished the contraction elicited by 1 mM H2O2 in both strains. Superoxide dismutase (SOD, 150 u ml(-1)) and dimethylsulphoxide (DMSO, 7 mM), scavengers of superoxide anions and hydroxyl radicals, respectively, did not alter H2O2-induced contractions in intact segments from both strains. However, L-NG-nitroarginine methyl ester (L-NAME, 100 microM, a nitric oxide synthase inhibitor) increased the response to H2O2 in normotensive rats, although the increase was less than that produced by endothelium removal. Incubation of segments with 1 mM H2O2 for 15 min and subsequent washout reduced the contractile responses induced by 75 mM KCl in intact segments from SHR and in endothelium-denuded segments from both strains; this effect being prevented by catalase (1000 u ml(-1)). Indomethacin (10 microM, a cyclo-oxygenase inhibitor) and SQ 29,548 (10 microM, a prostaglandin H2/thromboxane A2 receptor antagonist) practically abolished the contractions elicited by H2O2 in normotensive and hypertensive rats. We conclude that: (1) the oxidant stress induced by H2O2 produces contractions mediated by generation of a product of the cyclo-oxygenase pathway, prostaglandin H2 or more probably thromboxane A2, in normotensive and hypertensive rats; (2) oxygen-derived free radicals are not involved in the effect of H2O2; (3) in normotensive rats, endothelium protects against H2O2-mediated injury to contractile machinery, determined by the impairment of KCl-induced contractions; and (4) endothelial nitric oxide has a protective role on the contractile effect induced by H2O2, that is lost in hypertension.

Vascular smooth muscle cell hypertrophy induced by glycosylated human oxyhaemoglobin

1. Nonenzymatic protein glycosylation is a possible mechanism contributing to oxidative stress and vascular disease in diabetes. In this work, the influence of 14%-glycosylated human oxyhaemoglobin (GHHb), compared to the non-glycosylated protein (HHb), was studied on several growth parameters of rat cultured vascular smooth muscle cells (VSMC). A role for reactive oxygen species was also analysed. 2. Treatment of VSMC for 48 h with GHHb, but not with HHb, increased planar cell surface area in a concentration dependent manner. The threshold concentration was 10 nM, which increased cell size from 7965+/-176 to 9411+/-392 microm2. Similarly, only GHHb enhanced protein content per well in VSMC cultures. 3. The planar surface area increase induced by 10 nM GHHb was abolished by superoxide dismutase (SOD; 50 200 u ml(-1)), deferoxamine (100 nM-100 microM), or dimethylthiourea (1 mM), while catalase (50 200 u ml(-1)) or mannitol (1 mM) resulted in a partial inhibition of cell size enhancement. 4. When a known source of oxygen free radicals was administered to VSMC, the xanthine/xanthine oxidase system, the results were analogous to those produced by GHHb. Indeed, enhancements of cell size were observed, which were inhibited by SOD, deferoxamine, or catalase. 5. These results indicate that, at low concentrations, GHHb induces hypertrophy in VSMC, this effect being mediated by superoxide anions, hydrogen peroxide, and/or hydroxyl radicals. Therefore, glycosylated proteins can have a role in the development of the structural vascular alterations associated to diabetes by enhancing oxidative stress.

Effect of glycosylated hemoglobin on endothelium-dependent relaxation of omental arteries from pregnant women

OBJECTIVE

This study was intended to test the hypothesis that glycosylated hemoglobin adversely affects endothelium-dependent vascular relaxation during pregnancy.

STUDY DESIGN

Omental artery rings (3 mm) obtained from patients undergoing cesarean delivery at term were suspended in organ chambers for isometric tension recording in Krebs bicarbonate solution. They were bubbled with 5% carbon dioxide in air (37 degrees C, pH 7.4). After equilibration at 1-g passive tension and contraction with endothelin 1, concentration-relaxation curves in response to bradykinin were determined in the presence or absence of oxyhemoglobin, glycosylated hemoglobin, or nitric oxide synthase inhibitors (N(omega)-nitro-L-arginine methyl ester or N(omega)-nitro-L-arginine).

RESULTS

Oxyhemoglobin and glycosylated hemoglobin attenuated the endothelium-dependent relaxation induced by bradykinin. The inhibition did not differ significantly between the 2 nitric oxide scavengers and was of the same magnitude as that noted with the nitric oxide synthase inhibitors.

CONCLUSION

Glycosylation of hemoglobin does not change its effect on endothelium-dependent relaxation in human omental vessels during pregnancy. These data do not support the hypothesis that glycosylated hemoglobin may be the reason for abnormal endothelial function in diabetes.

Investigation of oxidant stress and vasodepression to glyceryl trinitrate in the obese Zucker rat in vivo

1. We examined the relationship between oxidant stress and the vasodepressor activity of glyceryl trinitrate (GTN) in vivo, including rapid GTN tolerance development, in 13-week old obese and age-matched lean Zucker rats which had been maintained for 4 weeks on either control diet or diets enriched with the lipophilic, chain-breaking antioxidants vitamin E (0.5% w w(-1)) or probucol (0.5% w w(-1)) or the superoxide anion scavenger tiron (1% w v(-1) in drinking water). 2. The basal plasma level of the isoprostane 8-epi-PGF2alpha, an in vivo marker of lipid peroxidation, was elevated by approximately 5 fold in the obese Zucker rat and markedly reduced by dietary lipophilic antioxidants and depressed by dietary tiron. 3. Vasodepression to bolus does GTN (0.1-100 microg kg(-1) i.v.), but not endothelium-dependent vasodepression to bolus dose acetylcholine (ACh, 0.02-2.0 microg kg(-1) i.v.), was impaired in obese animals and completely restored by dietary antioxidants. 4. Nitrate tolerance developed in vivo during a I h infusion of GTN (40 microg kg(-1) min(-1) i.v.) appeared more severe in obese animals. However, rapid nitrate tolerance was not affected by dietary antioxidants in either the obese or lean Zucker rat. 5. We therefore provide evidence that elevated oxidant stress in the obese Zucker rat is associated with an impairment in nitrate vasodepressor activity. However, our data are not consistent with either a role for oxidant stress in rapid nitrate tolerance development in the anaesthetized Zucker rat or the aggravation of this tolerance by pre-existing oxidant stress.

Acute endothelium-mediated vasodilation is not impaired at the onset of diabetes

Vascular injury and impaired vascular function are central to the increased mortality associated with diabetes. Hyperglycemia in diabetes has been suggested to play a role in this process, in part by impairing the function of the vascular endothelium. It has been difficult, however, to isolate the direct effect of glucose in both humans and in animal models of diabetes. This was evaluated in the present study in 7 rats that were chronically instrumented with a Transonic flow probe at the iliac bifurcation of the abdominal aorta, a nonoccluding catheter inserted immediately anterior to the flow probe, and a femoral vein catheter. Acute infusions of acetylcholine and sodium nitroprusside (1 and 10 microg/min IA) increased hindquarter blood flow significantly by approximately 27 and 10 mL/min over baseline, respectively, at the high dose. Streptozotocin (70 mg/kg IV) was administered, but normoglycemia was maintained with continuous intravenous insulin infusion to control for potential streptozotocin side effects. Diabetes was induced 5 to 7 days later by stopping the insulin infusion. Hindlimb blood flow (measured 24 hours per day) decreased during the diabetic period and was accompanied by an increase in mean arterial pressure, suggesting a vasoconstrictor response. However, the responses to acetylcholine and sodium nitroprusside were not altered significantly on either day 2 or day 6 of the diabetic period. This suggests that neither endothelium-mediated vasorelaxation nor responsiveness to nitric oxide is impaired during the initial phase of diabetes and that diabetic hyperglycemia does not have a significant, direct effect to impair endothelium-mediated relaxation in insulin-dependent diabetes mellitus. The mechanism for the change in baseline blood flow and its potential influence on endothelial function, however, are not known.

Stimulation of nerve growth-factor and substance P expression in the iris-trigeminal axis of diabetic rats--involvement of oxidative stress and effects of aldose reductase inhibition

In rats with streptozotocin-induced diabetes, we measured increased (by 61%; P < 0.05) mRNA for nerve growth factor (NGF) in the iris together with increased (by 82%; P < 0.05) mRNA for preprotachykinin (the substance P precursor) in the trigeminal ganglion, suggesting that increased NGF was driving increased substance P gene expression. In other diabetic rats, these changes were prevented by treatment with either an antioxidant (butylated hydroxytoluene; 1% by diet) or an aldose reductase inhibitor (ARI) (sorbinil; 25 mg/kg/day p.o.) and the sorbinil treatment was associated with significant inhibition of polyol pathway intermediates in both lens and sciatic nerve. This suggests that polyol pathway activity in the lens may translate to oxidative stress-driving stimulation of NGF gene expression in the iris. The change is selective for NFG, because expression of the analogous neurotrophin, neurotrophin-3 (NT-3), was unaltered in the same irises. These changes suggest that oxidative stress and/or inflammation can drive up NGF expression in diabetes--a mechanism that might participate in iritis.

Acute hyperglycemia attenuates endothelium-dependent vasodilation in humans in vivo

BACKGROUND

Endothelial function is impaired in patients with diabetes mellitus. However, the factors contributing to this defect are currently unknown. Hyperglycemia attenuates endothelium-dependent relaxation in normal rabbit arteries in vitro and rat arterioles in vivo. Accordingly, this study examined the effect of acute hyperglycemia on endothelium-dependent vasodilation in nondiabetic humans in vivo.

METHODS AND RESULTS

Endothelium-dependent vasodilation was assessed through brachial artery infusion of methacholine chloride both before and during 6 hours of local hyperglycemia (300 mg/dL) achieved by intra-arterial infusion of 50% dextrose. Forearm blood flow was determined by plethysmography. In a group of 10 subjects, there was a trend toward attenuated methacholine-mediated vasodilation during hyperglycemia compared with euglycemia (P=.07 by ANOVA; maximal response, 13.3+/-2.8 versus 14.7+/-1.5 mL x min(-1) x 100 mL(-1), respectively). In these subjects, the systemic serum insulin levels increased significantly during the dextrose infusion (P<.001). To eliminate the confounding vasoactive effects of insulin, the protocol was repeated during systemic infusion of octreotide (30 ng x kg(-1) x min(-1)) to inhibit pancreatic secretion of insulin. In these subjects (n=10), hyperglycemia significantly attenuated the forearm blood flow response to methacholine (P<.01 by ANOVA; maximal response, 16.9+/-2.5 before versus 12.7+/-1.8 mL x min(-1) x 100 mL(-1) during hyperglycemia). Methacholine-mediated vasodilation was not attenuated by an equimolar infusion of mannitol (P>.40), nor did hyperglycemia reduce endothelium-independent vasodilation to verapamil (P>.50).

CONCLUSIONS

Acute hyperglycemia impairs endothelium-dependent vasodilation in healthy humans in vivo. This finding suggests that elevated glucose may contribute to the endothelial dysfunction observed in patients with diabetes mellitus.

Glyc-oxidized LDL impair endothelial function more potently than oxidized LDL: role of enhanced oxidative stress

Hypercholesterolemia is associated with impairment of endothelial function due to increased levels of LDL. In diabetic patients, however, attenuation of endothelial function occurs even under normocholesterolemic conditions. Here we assessed whether glycation of LDL potentiates their influence on endothelial function, with particular emphasis on the oxidizability of LDL and the role of O2-. Human LDL was glycated by dialyzation for 7 days against buffer containing 200 mmol/l glucose, or sham-treated without glucose, and oxidized by incubation with Cu2+. Glycation significantly enhanced the oxidizability of LDL, as detected by diene formation and by electrophoretic mobility (27.5 mm for oxidized LDL vs. 34 mm for oxidized glycated LDL at 20 h of oxidation). Isolated rings of rabbit aorta were superfused with physiological salt solution, and isometric tension was recorded. Incubation of the aortic rings with sham-treated or with glycated LDL, not oxidized, had no influence on acetylcholine-induced, endothelium-dependent relaxation. Exposure of the aortic rings to oxidized non-glycated LDL caused a significant inhibition (30% at 1 microM acetylcholine) of the endothelium-dependent relaxation only in the presence of diethyl-dithiocarbamate (DDC), an inhibitor of the endogenous superoxide dismutase (SOD). Incubation of aortic rings with oxidized glycated LDL attenuated endothelium-dependent relaxation even in the absence of DDC (by 31% at 1 microM acetylcholine). The presence of DDC potentiated the inhibition of relaxation (65% inhibition at 1 microM acetylcholine), and co-incubation with exogenous SOD and catalase prevented the inhibition of relaxation, indicating a mediator role of O2-. Endothelium-independent relaxation induced by forskolin was unaffected by any of the lipoproteins. Using a chemiluminescence assay, significantly increased O2- production of aortic rings pretreated with oxidized glycated LDL (4101 +/- 360 counts/s) in comparison to control rings (753 +/- 81 counts/s) or arteries pretreated with oxidized non-glycated LDL (2358 +/- 169 counts/s) could be detected, suggesting that enhanced NO-inactivation by O2- could be the underlying mechanism for the stronger impairment of endothelium-dependent dilations by oxidized glycated LDL. Glycation increases the oxidizability of LDL and potentiates its endothelium-damaging influence. The likely mechanism for attenuation of endothelium-dependent dilations is increased formation of O2-, resulting in inactivation of nitric oxide. This mechanism may play an important role in diabetic patients and may contribute to disturbed organ perfusion.

A novel mechanism for vasoconstrictor action of 8-isoprostaglandin F2 alpha on retinal vessels

Using a video-imaging technique, we characterized the effects of 8-isoprostaglandin F2 alpha (8-iso-PGF2 alpha) on retinal vasculature from piglets. 8-Iso-PGF2 alpha potently contracted (EC50 = 5.9 +/- 0.5 nM) retinal vessels. These effects were completely antagonized by the cyclooxygenase inhibitor indomethacin, the thromboxane synthase blocker CGS-12970, the thromboxane receptor antagonist L-670596, and the putative inhibitor of the non-voltage-dependent receptor-operated Ca2+ pathway SKF-96365; constrictor effects of 8-iso-PGF2 alpha were also partly attenuated by the ETA-receptor blocker BQ-123 and an inhibitor of endothelin-converting enzyme, phosphoramidon, but was negligibly affected by the L-type voltage-gated Ca2+ channel blocker nifedipine. Correspondingly, 8-iso-PGF2 alpha elicited endothelin release from retinal preparations, which was markedly reduced by SKF-96365. 8-Iso-PGF2 alpha also increased thromboxane production in the retina and cultured endothelial cells, but not on retinovascular smooth muscle cells; these effects of 8-iso-PGF2 alpha were blocked by indomethacin, CGS-12970, SKF-96365, and EGTA, but not by nifedipine. 8-Iso-PGF2 alpha also increased Ca2+ transients in retinal endothelial cells, which were inhibited by SKF-96365 and EGTA, but not by nifedipine, whereas in smooth muscle cells U-46619, but not 8-iso-PGF2 alpha, stimulated a rise in Ca2+ transients. Finally, H2O2 + FeCl2 (in vitro) and anoxia followed by reoxygenation (in vivo) stimulated formation of 8-iso-PGF2 alpha in the retina. In conclusion, 8-iso-PGF2 alpha-induced retinal vasoconstriction is mediated by cyclooxygenase-generated formation of thromboxane and, to a lesser extent, by endothelin after Ca2+ entry into cells, possibly through receptor-operated channels. Retinal vasoconstriction to 8-isoprostanes might play a role in the genesis of ischemic retinopathies.

Acute glucose overload abolishes Ca2+ oscillation in cultured endothelial cells from bovine aorta: a possible role of superoxide anion

Effects of acute glucose overload on [Ca2+]i were investigated in cultured endothelial cells from bovine aorta. Application of 0.1 micromol/L ATP elicited an oscillatory increase in [Ca2+]i (Ca2+ oscillation) in Krebs solution containing 11.5 mmol/L glucose. The frequency of Ca2+ oscillation induced by ATP increased in a concentration-dependent manner, ranging between 0.03 and 1 micromol/L. When cells were preincubated with 23 mmol/L glucose-containing Krebs solution (high glucose solution) for 3 hours, 0.1 micromol/L ATP failed to induce Ca2+ oscillation but evoked only a phasic followed by sustained increase in [Ca2+]i. Application of a higher concentration of ATP (10 micromol/L) evoked a transient increase in [Ca2+]i both in control and high glucose-treated cells. However, the falling phase of [Ca2+]i was prolonged in high glucose-treated cells. Thapsigargin (1 micromol/L), an inhibitor of endoplasmic Ca2+-ATPase, induced a transient followed by a sustained increase in [Ca2+]i in control cells. Preincubation with high glucose solution increased the rate of rise of the thapsigargin-induced increase in [Ca2+]i and abolished the sustained increase, suggesting that glucose overload accelerates Ca2+ leak from intracellular store sites and impairs Ca2+ release-activated Ca2+ entry. We found that all of the glucose overload-induced changes in Ca2+ mobilization could be mimicked by xanthine with xanthine oxidase and abolished by superoxide dismutase. These results indicate that acute glucose overload accumulates superoxide anion in bovine aortic endothelial cells, thereby diminishing ATP-induced Ca2+ oscillation through the impairment of Ca2+ homeostasis.

Free radical generators cause changes in endothelial and inducible nitric oxide synthases and endothelin-1 immunoreactivity in endothelial cells from hyperlipidemic rabbits

Reactive oxygen species (ROS) play an important role in the damage of vascular endothelium during atherogenesis and impaired endothelium-dependent vasorelaxation. We have studied the effect of two ROS generators (H2O2 and menadione) and one of the most potent antioxidants (morin) on the double immunofluorescent staining of endothelial cells (EC) from both Watanabe Heritable Hyperlipidemic (WHHL) and New Zealand White (NZW) rabbits in primary cultures using antibodies against endothelin-1 (ET-1), endothelial (eNOS), and inducible nitric oxide synthase (iNOS). In aortic EC from normal rabbits, ROS decreased the immunoreactivity of eNOS and ET-1 and this effect was significantly reversed by morin. In atherosclerotic rabbits, ROS had the same effect on the immunoreactivity of eNOS and ET-1 but also induced the expression of iNOS immunoreactivity. In general, the cells from WHHL rabbits were less sensitive to the protective effects of morin and more sensitive to the effects of ROS. It thus appears that the protective effect of morin may be due to neutralization of ROS and may be considered for the treatment of early stages of atherosclerosis, before macroscopic lesions have occurred.

Regulation of the cerebral circulation: role of endothelium and potassium channels

Several new concepts have emerged in relation to mechanisms that contribute to regulation of the cerebral circulation. This review focuses on some physiological mechanisms of cerebral vasodilatation and alteration of these mechanisms by disease states. One mechanism involves release of vasoactive factors by the endothelium that affect underlying vascular muscle. These factors include endothelium-derived relaxing factor (nitric oxide), prostacyclin, and endothelium-derived hyperpolarizing factor(s). The normal vasodilator influence of endothelium is impaired by some disease states. Under pathophysiological conditions, endothelium may produce potent contracting factors such as endothelin. Another major mechanism of regulation of cerebral vascular tone relates to potassium channels. Activation of potassium channels appears to mediate relaxation of cerebral vessels to diverse stimuli including receptor-mediated agonists, intracellular second messenger, and hypoxia. Endothelial- and potassium channel-based mechanisms are related because several endothelium-derived factors produce relaxation by activation of potassium channels. The influence of potassium channels may be altered by disease states including chronic hypertension, subarachnoid hemorrhage, and diabetes.

Increased NADH oxidase activity in the retina of the BBZ/Wor diabetic rat

This morphological study demonstrates a role for endothelial cells in generating reactive oxygen species in early stages of retinopathy in the BBZ/Wor rat, an obese, noninsulin dependent model of diabetes. Hyperglycemia induced pseudohypoxia results in an imbalance in cytosolic NADH/NAD+. In the oxygen-rich environment of the retina, NADH oxidase generates superoxide radical which is dismutated to hydrogen peroxide. Localization of hydrogen peroxide by the cerium NADH oxidase enzyme activity cytochemical localization technique shows a statistically significant increase of peroxide localization in the central retina of diabetic rats as compared to age-matched, nondiabetic controls. Endothelial cell dysfunction, indicated by leakage of endogenous serum albumin, coincided with areas of NADH oxidase activity localization. In diabetic rats there are increased levels of fibronectin in areas of hydrogen peroxide localization. This in vivo, morphological study is the first demonstration of oxidative injury and endothelial cell dysfunction in the retina of a spontaneous, noninsulin dependent model of diabetes.

Evidence for altered sensitivity of the nitric oxide/cGMP signalling cascade in insulin-resistant skeletal muscle

Nitric oxide activates guanylate cyclase to form cGMP, comprising a signalling system that is believed to be a distinct mechanism for increasing glucose transport and metabolism in skeletal muscle. The effects of a selective cGMP phosphodiesterase inhibitor, zaprinast, on basal glucose utilization was investigated in incubated rat soleus muscle preparations isolated from both insulin-sensitive (lean Zucker; Fa/?) and insulin-resistant (obese Zucker; fa/fa) rats. Zaprinast at 27 microM significantly increased cGMP levels in incubated soleus muscle isolated from lean, but not obese, Zucker rats. Muscles were incubated with 14C-labelled glucose and various concentrations of zaprinast (3, 27 and 243 microM). Zaprinast (at 27 and 243 microM) significantly increased rates of net and 14C-labelled lactate release and of glycogen synthesis in lean Zucker rat soleus muscle; glucose oxidation was also increased by 27 microM zaprinast. In addition, regardless of concentration, the phosphodiesterase inhibitor failed to increase any aspect of 14C-labelled glucose utilization in soleus muscles isolated from obese Zucker rats. The maximal activity of nitric oxide synthase (NOS) was significantly decreased in insulin-resistant obese Zucker muscles. Thus the lack of effect of zaprinast in insulin-resistant skeletal muscle is consistent with decreased NOS activity. To test whether there is a defect in insulin-resistant skeletal muscle for endogenous activation of guanylate cyclase, soleus muscles were isolated from both insulin-sensitive and insulin-resistant Zucker rats and incubated with various concentrations of the NO donor sodium nitroprusside (SNP; 0.1, 1, 5 and 15 mM). SNP significantly increased rates of net and 14C-labelled lactate release, as well as glucose oxidation in muscles isolated from both insulin-sensitive and insulin-resistant rats. A decreased response to SNP was observed in the dose-dependent generation of cGMP within isolated soleus muscles from insulin-resistant rats. A possible link between impaired NO/cGMP signalling and abnormal glucose utilization by skeletal muscle is discussed.

Free radical scavenging enzymes in fetal dysmorphogenesis among offspring of diabetic rats

Recent studies have suggested that the fetal dysmorphogenesis in diabetic pregnancies is associated with an increase in embryonic oxygen-free radicals. This excess of oxygen-free radicals may result from either overproduction or decreased clearance by the enzymatic scavenging mechanism. However, there are no in vivo data on the activity of embryonic oxygen-free radical scavenging enzymes. The purpose of the current study is to investigate whether this increase in embryonic oxygen-free radicals is the result of a change in the activity of the fetal oxygen-free radical scavenging/antioxidant enzymes during pregnancy complicated by maternal diabetes in an in vivo rat model. Thirty-six Sprague-Dawley rats were randomly assigned to one of two study groups: nondiabetic control and an untreated diabetic group. On day 12, fetuses were examined for crown-rump lengths, somite numbers, and external anomalies. The activity of fetal oxygen-free radical scavenging enzymes, including superoxide dismutase (SOD), glutathione peroxidase (GPX), and catalase (CAT), were determined. The untreated diabetic group of rats had a significantly higher mean blood glucose level than that of the nondiabetic controls and also a significantly lower weight gain, higher resorption rate, smaller embryonic size with lower total protein content, and a approximately 6-fold increase in the rate of fetal neural tube defects compared to the nondiabetic controls. Superoxide dismutase activity was significantly reduced in the embryos with neural tube defects regardless of maternal diabetic status (2.25 +/- 0.83 vs. 1.17 +/- 0.04 u/mg protein; P < 0.05). Glutathione peroxidase and catalase activity were significantly reduced in malformed versus normal-formed embryos of nondiabetic mothers (GPX-2.68 +/- 1.15 vs. 4.46 +/- 1.12 mu/mg protein, CAT -1.67 +/- 0.53 vs 2.49 +/- 0.61 u/mg protein respectively; P < 0.01). However, overall catalase activity was increased in embryos of diabetic mothers as compared to controls. Two-way analysis of variance identified fetal malformations as the variance associated with reduced fetal SOD activity, whereas maternal diabetes was associated with the increase in fetal catalase activity. Neither neural tube defect nor maternal diabetes was found to be the variable affecting fetal GPX activity, Fetal oxygen-free radical scavenging enzymes respond differently to the adverse environment created by maternal diabetes during pregnancy. Defects in embryonic SOD and catalase activity, regardless of maternal diabetic status, may reduce the ability of the fetus to clear free oxygen radicals, thereby exposing it to an increased oxidative load that may cause fetal dysmorphogenesis. The diabetic state of the mothers did not decrease embryonic activity of any of the scavenging enzymes. Therefore, although excess oxidative load, as observed in diabetes, may cause tissue injury and embryopathy, the mechanism does not appear to be a diabetes-induced reduction in the action of the scavenging enzymes.

Basal nitric oxide production is not reduced in patients with noninsulin-dependent diabetes mellitus

Vascular disease is the leading cause of morbidity, disability and death in patients with noninsulin-dependent diabetes mellitus. Abnormalities in endothelium-derived nitric oxide (NO) have been demonstrated to be involved in the pathogenesis of vascular disease. By measuring hemodynamic responses to a NO synthase agonist or antagonist, previous studies have shown the presence of NO deficiency in patients with noninsulin-dependent diabetes mellitus, a method of assessing bioactive NO formation. However, direct biochemical evidence that this is the case, has not been produced. In vivo NO is metabolized into nitrate, an end breakdown product of NO, which can be used as an index of endogenous NO formation. To investigate further whether decreased basal synthesis of NO may be a major cause of endothelium-mediated vascular dysfunction in patients with noninsulin-dependent diabetes mellitus, the plasma nitrite/nitrate levels of 15 patients were examined and compared with 13 normal controls. The results showed that in basal conditions plasma nitrite/nitrate levels were not reduced in diabetic patients compared with normal controls (37.3 +/- 14.7 versus 29.4 +/- 8.6 mumol/l). It was concluded that in noninsulin-dependent diabetes mellitus patients, endothelium-derived basal NO formation is not impaired. This study, taken with previous observations, suggests that factors other than diminished basal NO production, such as reduced bioavailability of NO probably due to the augmented production of superoxide anion with subsequently increased inactivation of NO, contribute to the high incidence of vascular disease in patients with noninsulin-dependent diabetes mellitus.

Changes in isoprenaline-induced endothelium-dependent and -independent relaxations of aorta in long-term STZ-diabetic rats: reversal effect of dietary vitamin E

1. The present study concerns in vitro isoprenaline (ISO)-induced relaxation of aortic rings of long-term streptozotocin (STZ)-diabetic and nondiabetic rats, both with and without dietary vitamin E supplementation. 2. Incubation with propranolol, NG-nitro-L-arginine methyl ester and methylene blue, as well as absence of endothelium, all negatively affect the ISO-induced relaxations. 3. Thiobarbituric acid reactivity levels used as an index of lipid peroxidation are elevated in the aorta by diabetes. Four months of STZ-diabetes results in a significant increase in the ISO-induced relaxations together with endothelial dysfunction in the rat aorta. Diabetes also causes the loss of vascular integrity. 4. Dietary vitamin E supplementation during the last 2 months of diabetes allows normalization of the levels of lipid peroxides. This vitamin also completely reverses the increased sensitivity (pD2 value) of the aorta to ISO, whereas the maximum ISO-induced relaxations are partially restored after the treatment in diabetic rats. 5. The results suggest that ISO-induced relaxation in the aorta partially depends on the intact endothelium and that the endothelium-dependent relaxant effect of ISO is mediated by endothelium-derived relaxing factor. Results also indicate that abnormal vascular reactivity and structure of the diabetic rat aorta may be related to the increased lipid peroxidation. In conclusion, vitamin E can protect the arterial wall from oxidative stress-induced injury associated with chronic STZ-diabetes and allows normalization of the response to ISO and the structure of the aorta in diabetic rats.

Antioxidant and triglyceride-lowering effects of vitamin E associated with the prevention of abnormalities in the reactivity and morphology of aorta from streptozotocin-diabetic rats. Antioxidants in Diabetes-Induced Complications (ADIC) Study Group

In this study, we evaluated the effects of vitamin E on the vascular reactivity and structure of thoracic aorta from streptozotocin (STZ)-diabetic rats. Plasma glucose, cholesterol, and triglyceride concentrations in rats were increased markedly by STZ-diabetes. The thiobarbituric acid (TBA) reactivity level as an index of lipid peroxidation was higher in both plasma and aorta of STZ-diabetic rats compared with controls. The rings of thoracic aorta with or without endothelium were mounted in organ chambers for measurement of isometric tension and were contracted by a single dose (10-5 mol/L) and then cumulative doses of noradrenaline ([NA] 10(-9) to 10(-5) mol/L). Pretreatment with methylene blue (MB) or removal of the endothelium resulted in a similar degree of enhancement in NA-induced contraction of control rings. STZ-diabetes increased the fast and slow components of NA-induced contraction in all experiments. The maximal contractile response of aorta to NA was also augmented by STZ-diabetes, whereas the sensitivity (pD2) remained unaltered. STZ-diabetes resulted in significant increases in the maximum contractile response and sensitivity of aorta to KCl. STZ-diabetic rats showed a significant reduction in the percentage of endothelial response (PER). A group of diabetic rats was treated from the time of diabetes induction with a 0.5% dietary supplement of vitamin E. Vitamin E supplementation of STZ-diabetic rats eliminated accumulation of lipid peroxides and returned plasma triglycerides toward normal levels. Diabetes-induced abnormal contractility and endothelial dysfunction were significantly but not completely prevented by vitamin E treatment. The endothelium-independent relaxation response to sodium nitroprusside (SNP) was not affected by diabetes or vitamin E treatment. Electron microscopic examination of thoracic aorta revealed that normal tissue organization was disrupted in STZ-diabetic rats, and that vitamin E treatment can protect the morphological integrity of aorta against STZ-diabetes. The results suggest the following: (1) The increased triglycerides/lipid peroxides may be an important reason for morphological or functional disruption of endothelium and enhanced activation of contractile mechanisms of vascular smooth muscle in STZ-diabetic rats. Both contribute to an increased responsiveness of diabetic aorta to vasoconstrictor agents. (2) Vitamin E treatment of STZ-diabetic rats can prevent the development of abnormal contractility and structure and endothelial dysfunction in aorta. (3) The triglyceride- and/or lipid peroxidation-lowering effect of vitamin E may be crucial for the protective effect of this vitamin on the vasculature.

High glucose increases nitric oxide synthase expression and superoxide anion generation in human aortic endothelial cells

BACKGROUND

Hyperglycemia is a primary cause of premature vascular disease. Endothelial cell dysfunction characterized by diminished endothelium-dependent relaxations is likely to be involved. Little is known about the molecular mechanisms of hyperglycemia-induced endothelial dysfunction.

METHODS AND RESULTS

This study was designed to determine the effect of hyperglycemia on the L-arginine/nitric oxide (NO) pathway. Expression of endothelial nitric oxide synthase (eNOS) mRNA and production of NO were studied in human aortic endothelial cells exposed to control levels (5.5 mmol/L) and high levels (22.2 mmol/L) of glucose for 5 days. We examined the effect of glucose on NO release by measuring changes in nitrite (NO2-) levels by Griess reaction. Superoxide anion (O2-) production was also examined by the ferrocytochrome c assay. NOS mRNA and protein expression, which were evaluated by reverse transcription-polymerase chain reaction and Western blotting, were approximately twofold greater in endothelial cells exposed to high glucose. Elevated glucose levels increased NO2- production by only 40% but increased the release of O2- by more than threefold.

CONCLUSIONS

The present study demonstrates that prolonged exposure to high glucose increases eNOS gene expression, protein expression, and NO release. However, upregulation of eNOS and NO release is associated with a marked concomitant increase of O2- production. These results provide the molecular basis for understanding how chronic exposure to elevated glucose leads to an imbalance between NO and O2-. This may explain impaired endothelial function and be important for diabetic vascular disease.

Impairment of endothelium-dependent but not of endothelium-independent dilatation in guinea-pig aorta rings incubated in the presence of elevated glucose

1. Purine compounds such as ATP and adenosine, respectively endothelium-dependent and- independent vasodilators, are largely involved in the control of vascular tone and vascular reactivity to contracting stimuli. We investigated the relaxing activity of ATP and adenosine in guinea-pig aorta rings exposed for 6 h to elevated glucose concentration (50 mM), in order to mimic hyperglycaemic conditions. Guinea-pigs were reserpine-treated (2 mg kg-1, i.p., 48 and 24 h before death). 2. Rings of aortae incubated in 50 mM glucose, contracted submaximally by 1 microM noradrenaline, lost endothelium-dependent relaxation in response to acetylcholine (10 nM to 10 microM). Aortae incubated with 50 mM mannose, as a hyperosmotic control, relaxed to acetylcholine normally. Rings of aortae incubated in 50 mM glucose, contracted submaximally by 3 mM 4-aminopyridine, lost endothelium-dependent relaxation in response to ATP (30 microM) whereas endothelium-independent relaxation in response to adenosine (0.3 mM) was well preserved. 4. The relaxation induced by A23187 or sodium nitroprusside (10 nM to 0.1 microM) did not differ between rings exposed to control (5.5 mM) or elevated glucose (50 mM) and contracted submaximally by 3 mM 4-aminopyridine. 5. When incubated with aortic tissue in the presence of elevated glucose, the cyclo-oxygenase inhibitors, indomethacin (10 microM) and mefenamic acid (30 microM), or the scavenger of superoxide anions, superoxide dismutase (150 u ml-1), prevented the impairment of ATP-mediated relaxation. 6. The present results indicate that endothelium-dependent, receptor-induced relaxation in response to acetylcholine and ATP is impaired in guinea-pig aorta rings exposed to elevated glucose. The endothelial dysfunction caused by glucose might be located at a step between receptor activation and intracellular calcium increase, and might be related to an increased metabolism of arachidonic acid coupled to an increased production, or to a reduced inactivation of superoxide anions.

Superoxide dismutase partially restores impaired dilatation of the basilar artery during diabetes mellitus

The goal of this study was to test the hypothesis that administration of superoxide dismutase restores nitric oxide synthase-mediated dilatation of the basilar artery during diabetes mellitus. We measured the diameter of the basilar artery in vivo in nondiabetic and diabetic rats (streptozotocin; 50-60 mg/kg i.p.) in response to nitric oxide synthase-dependent agonists (acetylcholine and bradykinin) and a nitric oxide synthase-independent agonist (nitroglycerin) before and during application of superoxide dismutase. Topical application of acetylcholine (1.0 and 10 microM) and bradykinin (1.0 and 10 microM) produced dose-related dilatation of the basilar artery in nondiabetic and diabetic rats. However, the magnitude of vasodilation produced by acetylcholine and bradykinin was significantly less in diabetic rats. Topical application of nitroglycerin (0.1 and 1.0 microM) produced similar dose-related dilatation of the basilar artery in nondiabetic and diabetic rats. Treatment with superoxide dismutase (150 U/ml) did not alter baseline diameter of the basilar artery in nondiabetic and diabetic rats. However, topical application of superoxide dismutase partially restored nitric oxide synthase-dependent dilatation of the basilar artery in diabetic rats towards that observed in nondiabetic rats. Superoxide dismutase did not alter dilatation of the basilar artery in nondiabetic rats. These findings suggest that impaired nitric oxide synthase-dependent dilatation of the basilar artery during diabetes mellitus may be related, in part, to enhanced release of oxygen-derived free radicals.

Reactive oxygen species produced by macrophage-derived foam cells regulate the activity of vascular matrix metalloproteinases in vitro. Implications for atherosclerotic plaque stability

Vulnerable areas of atherosclerotic plaques often contain lipid-laden macrophages and display matrix metalloproteinase activity. We hypothesized that reactive oxygen species released by macrophage-derived foam cells could trigger activation of latent proforms of metalloproteinases in the vascular interstitium. We showed that in vivo generated macrophage foam cells produce superoxide, nitric oxide, and hydrogen peroxide after isolation from hypercholesterolemic rabbits. Effects of these reactive oxygens and that of peroxynitrite, likely to result from simultaneous production of nitric oxide and superoxide, were tested in vitro using metalloproteinases secreted by cultured human vascular smooth muscle cells. Enzymes in culture media or affinity-purified (pro-MMP-2 and MMP-9) were examined by SDS-PAGE zymography, Western blotting, and enzymatic assays. Under the conditions used, incubation with xanthine/xanthine oxidase increased the amount of active gelatinases, while nitric oxide donors had no noticeable effect. Incubation with peroxynitrite resulted in nitration of MMP-2 and endowed it with collagenolytic activity. Hydrogen peroxide treatment showed a catalase-reversible biphasic effect (gelatinase activation at concentrations of 4 microM, inhibition at > or = 10-50 microM). Thus, reactive oxygen species can modulate matrix degradation in areas of high oxidant stress and could therefore contribute to instability of atherosclerotic plaques.

Impairment of endothelium-dependent relaxation by increasing percentages of glycosylated human hemoglobin. Possible mechanisms involved

High levels of glycosylated human hemoglobin impair nitric oxide-mediated responses. However, the percentage of glycosylation for which this effect is observed and the mechanisms involved are unknown. We tested endothelium-dependent relaxations caused by acetylcholine in rat aortic segments either in control conditions or after preincubation with increasing percentages of glycosylated human hemoglobin. Human hemoglobin (1 and 10 nmol/L) inhibited endothelium-dependent relaxations only when glycosylated at 9% or higher. We evaluated the effect of 14% glycosylated human hemoglobin on acetylcholine-evoked responses in vessels preincubated with scavengers of superoxide anions, hydroxyl radical, or hydrogen peroxide (superoxide dismutase, deferoxamine, and catalase, respectively); with inhibitors of xanthine oxidase, cyclooxygenase, or thromboxane synthase (allopurinol, indomethacin, and dazoxiben, respectively); with blockers of thromboxane A2/prostaglandin H2 or endothelin receptors (SQ 30741 and BQ-123); and with the precursor of nitric oxide synthesis L-arginine. Superoxide dismutase abolished the effect of glycosylated hemoglobin, and the other substances did not have any effect. Glycosylated hemoglobin at 14% did not modify either the vasoconstrictions induced by the blocker of nitric oxide synthase NG-nitro-L-arginine methyl ester or the relaxations evoked in deendothelialized vessels by sodium nitroprusside and 8-bromo-cGMP. However, it inhibited the vasodilations evoked by exogenous nitric oxide. Superoxide dismutase abolished this latter effect. We conclude that the threshold for glycosylated human hemoglobin (Hb A1) to inhibit endothelium-dependent relaxation is 9%. This effect is due to interference with endothelial nitric oxide by means of superoxide anion production.

Cerebral blood flow in the diabetic patient

Diabetes is a major risk factor for cardiovascular disease. Coronary revascularization utilizing cardiopulmonary bypass (CPB) is frequently required for the diabetic patient. Nondiabetic individuals can autoregulate cerebral blood flow (CBF) through metabolic and perfusion pressure mechanisms during CPB. However, it has been reported that diabetic patients have impaired CBF autoregulation during CPB. It is possible, therefore, that impaired CBF autoregulation may contribute to postoperative neuropsychologic dysfunction. The mechanisms for this defect may reside in impaired endothelial-dependent responses in the diabetic that are related to morphological and functional changes linking the vascular endothelium and the vascular smooth muscle. The morphological changes occurring in the diabetic include microangiopathy and macroangiopathy which are characterized by endothelial cell (EC) hyperplasia and basement membrane thickening. Also, significant functional changes in local control of vascular tone, such as an imbalance in the synthesis and secretion of vasoactive factors by the EC and abnormal reactivity of the vascular smooth muscle, are seen in the diabetic when compared to the nondiabetic. More specifically, vascular responses to both calcium-dependent pathways of vasoconstriction and nitric oxide pathways of vasorelaxation have been shown to significantly differ between the diabetic and nondiabetic. The emphasis of this discussion is to examine the molecular mechanisms by which diabetes alters vascular function, with emphasis placed on regulation of cerebral artery blood flow during CPB.

Immunotargeting of antioxidant enzyme to the pulmonary endothelium

Oxidative injury to the pulmonary endothelium has pathological significance for a spectrum of diseases. Administration of antioxidant enzymes, superoxide dismutase (SOD) and catalase (Cat), has been proposed as a method to protect endothelium. However, neither these enzymes nor their derivatives possess specific affinity to endothelium and do not accumulate in the lung. Previously we have described a monoclonal antibody to angiotensin-converting enzyme (ACE) that accumulates selectively in the lung after systemic injection in rats, hamsters, cats, monkeys, and humans. In the present work we describe a system for selective intrapulmonary delivery of CuZn-SOD and Cat conjugated with biotinylated anti-ACE antibody mAb 9B9 (b-mAb 9B9) by a streptavidin (SA)-biotin bridge. Both enzymes biotinylated with biotin ester at biotin/enzyme ratio 20 retain enzymatic activity and bind SA without loss of activity. We have constructed tri-molecular heteropolymer complexes consisting of b-mAb 9B9, SA, and biotinylated SOD or biotinylated Cat and have studied biodistribution and pulmonary uptake of these complexes in the rat after i.v. injection. Biodistribution of biotinylated enzymes was similar to that of nonmodified enzymes. Binding of SA markedly prolonged lifetime of biotinylated enzymes in the circulation. In contrast to enzymes conjugated with nonspecific IgG, other enzyme derivatives, and nonmodified enzymes, biotinylated enzymes conjugated with b-mAb 9B9 accumulated specifically in the rat lung (9% of injected SOD/g of lung tissue and 7.5% of injected Cat/g of lung tissue). Pulmonary uptake of nonmodified enzymes or derivatives with nonspecific IgG did not exceed 0.5% of injected dose/g. Both SOD and Cat conjugated with b-mAb 9B9 were retained in the rat lung for at least several hours. Trichloracetic acid-precipitable radiolabeled Cat was associated with microsomal and plasma membrane fractions of the lung tissue homogenate. Thus, modification of antioxidant enzymes with biotin and SA-mediated conjugation with b-mAb 9B9 prolongs the circulation of enzymes resulting in selective accumulation in the lung and intracellular delivery of enzymes to the pulmonary endothelium. These results provide the background for an approach to provide protection of pulmonary endothelium against oxidative insults.

Relationship between glutathione and sorbitol concentrations in erythrocytes from diabetic patients

Red blood cell (RBC) concentrations of sorbitol and reduced glutathione (GSH) were evaluated in 29 type 11 diabetic subjects and eight normal controls. In erythrocytes from diabetic subjects, sorbitol levels were higher (18.7 +/- 1.33 v 11.2 +/- 0.7 nmol/g hemoglobin [Hb], P < .001) and GSH levels were lower (5.48 +/- 0.19 v8.33 +/- 0.24 micromol/g Hb, P < .01) than in nondiabetics. RBC sorbitol levels were positively correlated with fasting blood glucose (r =.57, P < .001) but not with HbAlc (r =.16, P < .05). RBC GSH levels showed a negative correlation with fasting blood glucose (r = -.35, P <.05) and with HbA1c (r = -.34, P < .05) and a significant negative correlation with RBC sorbitol levels (r = -.62, P < .001). Stepwise regression analysis highlighted the fact that the hyperglycemia-dependent increase in RBC sorbitol was significantly influenced by GSH concentrations (partial F = 14.6, P < .001). These data suggest the hypothesis that the hyperglycemia-induced enhanced activity of the polyol pathway leads to GSH depletion and, in turn, GSH depletion, reducing the glycolytic flux to pyruvate, enhances the rate of glucose metabolism through the polyol pathway. The overall effect is a progressive worsening of metabolic pseudohypoxia and depletion of GSH, resulting in lower defense against oxidative stress.

L-Arginine does not restore dilatation of the basilar artery during diabetes mellitus

The goal of this study was to test the hypothesis that administration of L-arginine, a substrate for the synthesis of nitric oxide, restores endothelium-dependent dilatation of the basilar artery during diabetes mellitus. We measured the diameter, of the basilar artery in vivo in nondiabetic and diabetic (streptozotocin; 50-60 mg/kg i.p.) rats in response to endothelium-dependent agonists (acetylcholine and bradykinin) and an endothelium-independent agonist (nitroglycerin) before and during application of L-arginine. Topical application of acetylcholine (1.0 and 10 muM) and bradykinin (1.0 and 10 microM) produced dilatation in nondiabetic rats of the basilar artery which was impaired in diabetic rats. Topical application of nitroglycerin (0.1 and 1.0 microM) produced similar dilatation of the basilar artery in nondiabetic and diabetic rats. Topical application of L-arginine (0.1 and 3 mM) did not enhance dilatation of the basilar artery in response to acetylcholine and bradykinin in diabetic rats. Thus, impairment of dilatation of the basilar artery in diabetic rats in response to acetylcholine and bradykinin appears to be related to a mechanism unrelated to the availability of L-arginine for nitric oxide synthase.

Failure of nimodipine to prevent or correct the long-term nerve conduction defect and increased neuronal Ca(2+)-currents in the diabetic BB/W-rat

It has been suggested that L-type Ca2+ channel antagonists exert a beneficial effect on nerve conduction velocity (NCV) slowing in short-term experimental diabetic neuropathy. We examined the effects of long-term treatment with the L-channel blocker, nimodipine, on two aspects of neuronal function previously documented to be abnormal in the spontaneously diabetic BB/W-rat: nerve conduction velocity and calcium influx in dorsal root ganglion (DRG) neurons. Treatment with 20 mg/kg nimodipine i.p. every 48 h from onset of diabetes for 6 months led to a transient, non-significant (30%) improvement in NCV. Intervention with the same regimen from 3 to 6 months of diabetes had no corrective effect on the already established NCV defect. Voltage activated calcium currents were recorded in isolated DRG neurons from nimodipine-treated and untreated diabetic and non-diabetic age-matched BB/W control rats. The peak high-threshold calcium current density (IDCa, pA/pF) was significantly larger in non-treated diabetic rats compared with control rats (157 +/- 12 vs. 66 +/- 5.5 (P < or = 0.05)). Long-term treatment with nimodipine was associated with a non-significant (28%) decrease (112 +/- 29) in the IDCa compared with non-treated diabetic rats. We conclude that L-channel mediated perturbations of cytosolic Ca2+ levels are only of minor pathophysiologic significance in the development of chronic diabetic neuropathy.

Angiotensin II-mediated hypertension in the rat increases vascular superoxide production via membrane NADH/NADPH oxidase activation. Contribution to alterations of vasomotor tone

We tested the hypothesis that angiotensin II-induced hypertension is associated with an increase in vascular .O2- production, and characterized the oxidase involved in this process. Infusion of angiotensin II (0.7 mg/kg per d) increased systolic blood pressure and doubled vascular .O2- production (assessed by lucigenin chemiluminescence), predominantly from the vascular media. NE infusion (2.75 mg/kg per d) produced a similar degree of hypertension, but did not increase vascular .O2- production. Studies using various enzyme inhibitors and vascular homogenates suggested that the predominant source of .O2- activated by angiotensin II infusion is an NADH/NADPH-dependent, membrane-bound oxidase. Angiotensin II-, but not NE-, induced hypertension was associated with impaired relaxations to acetylcholine, the calcium ionophore A23187, and nitroglycerin. These relaxations were variably corrected by treatment of vessels with liposome-encapsulated superoxide dismutase. When Losartan was administered concomitantly with angiotensin II, vascular .O2- production and relaxations were normalized, demonstrating a role for the angiotensin type-1 receptor in these processes. We conclude that forms of hypertension associated with elevated circulating levels of angiotensin II may have unique vascular effects not shared by other forms of hypertension because they increase vascular smooth muscle .O2- production via NADH/NADPH oxidase activation.

Alpha-tocopherol pretreatment improves endothelium-dependent vasodilation in aortic strips of young and aging rats exposed to oxidative stress

Acetylcholine-induced, endothelium-dependent relaxation of norepinephrine-precontracted aortic strips, was severely impaired after exposure to a hypoxanthine/xanthine oxidase reaction generating oxygen radicals. This effect was more evident in aortic strips of aging rats (24 months old) in comparison to young rats (3 months old). The addition of authentic .NO (1 microM) completely relaxed aortic strips exposed to oxidative stress both in young and aging rats. In vitro EPR measurements showed that the .NO signal was reduced by enzymatic O2.- generating reaction. The activity of a partial purified preparation of constitutive NO synthase from rat cerebellum was significantly decreased after exposure to exogenous oxygen radicals. Pretreatment of aortic strips with 100 microM alpha-tocopherol-phosphate, produced a significant improvement of acetylcholine-dependent relaxation in the aortic strips exposed to oxidative stress, particularly in the aged vessel. The content of malondialdehyde in aortic tissue did not change after oxidative stress or alpha-tocopherol pretreatment. Alpha-tocopherol was unable to recover the NO synthase activity depressed in vitro by hypoxanthine/xanthine oxidase reaction. This study confirms that an oxidative stress impairs the endothelium-mediated vasodilation. Alpha-tocopherol pretreatment protects the vessel against this damage. The mechanism of action of alpha-tocopherol is unknown, but seems unrelated to an antioxidant activity.

Increased circulating intercellular adhesion molecule-1 levels in type II diabetic patients: the possible role of metabolic control and oxidative stress

Blood levels of the circulating form of the integrin intercellular adhesion molecule-1 (ICAM-1), malondialdehyde (MDA), and hemoglobin A1c (HbA1c) were studied at baseline and 3 months after improved metabolic control in 25 type II diabetic patients without signs of macroangiopathy, and were compared with those in 15 matched healthy normal controls. Circulating ICAM-1 and MDA levels were increased in diabetic patients, both at baseline and 3 months later. However, with improving metabolic control HbA1c, circulating ICAM-1, and MDA significantly decreased. A significant correlation between circulating ICAM-1, HbA1c, and MDA was found in diabetic patients at each time. Multiple regression analysis considering circulating ICAM-1 as the dependent variable and HbA1c and MDA as independent variables, showed a significant correlation between the three variable at each time. Similar correlations were found in control subjects. These data show increased levels of circulating ICAM-1 in type II diabetic patients, independent of the presence of macroangiopathy. Moreover, these results suggest that oxidative stress and metabolic control might participate in determining increased circulating ICAM-1 levels in both type II diabetic patients and normal subjects.

Potential mechanisms promoting restenosis in diabetic patients

Diabetes is associated with greater restenosis rates after successful balloon angioplasty. The metabolic alterations that occur as a result of hyperglycemia or hyperinsulinemia can accelerate many of the pathophysiologic processes that lead to restenosis. Diabetes results in endothelial dysfunction and accelerated platelet deposition, which increase the propensity to thrombosis. Several growth factors known to promote the restenosis process are overexpressed in the presence of hyperglycemia. Advanced glycosylation promotes inflammatory cell recruitment and smooth muscle cell proliferation. Many of the potential mechanisms promoting restenosis in diabetic patients can be ameliorated by improved metabolic control.

Impaired nitric oxide-mediated vasodilation in patients with non-insulin-dependent diabetes mellitus

OBJECTIVES

This study sought to determine whether nitric oxide-mediated vasodilation is abnormal in patients with non-insulin-dependent diabetes mellitus.

BACKGROUND

Multiple investigations, both in experimental models and in patients with insulin-dependent diabetes mellitus, demonstrate impaired endothelium-dependent vasodilation. Decreased availability of endothelium-derived nitric oxide may contribute to the high prevalence of vascular disease in diabetes.

METHODS

Vascular reactivity was measured in the forearm resistance vessels of 21 patients with non-insulin-dependent diabetes mellitus and 23 matched healthy control subjects. No patient had hypertension or hypercholesterolemia. Each subject was pretreated with aspirin to inhibit endogenous production of vasoactive prostanoids. Methacholine chloride (0.3 to 10 microg/min) was administered through a brachial artery cannula to assess vasodilation to endothelium-derived nitric oxide. Sodium nitroprusside (0.3 to 10 microg/min) was infused to evaluate vasodilation to an exogenous nitric oxide donor. Verapamil (10 to 300 microg/min) was administered to distinguish impaired nitric-oxide-mediated vasodilation from general dysfunction of vascular smooth muscle. Forearm blood flow was determined by venous occlusion plethysmography, and dose-response curves were generated for each agent. To assess the role of vasoconstrictor prostanoids, a subset of eight diabetic subjects were reexamined in the absence of aspirin treatment.

RESULTS

Basal forearm blood flow in diabetic and nondiabetic subjects was comparable. The forearm blood flow responses to both methacholine chloride and nitroprusside were significantly attenuated in diabetic compared with nondiabetic subjects (p < 0.005 by analysis of variance for both agents). In contrast, the response to verapamil was not significantly different between the groups (p > 0.50). The forearm blood flow responses to these agents were not significantly affected by cyclooxygenase inhibition.

CONCLUSIONS

Nitric oxide-mediated vasodilation is impaired in non-insulin-dependent diabetes mellitus. Vasoconstrictor prostanoids do not contribute significantly to vascular dysfunction. The attenuated response to exogenous as well as endogenous nitric oxide donors suggests that the abnormality is due to increased inactivation of nitric oxide or to decreased reactivity of the vascular smooth muscle to nitric oxide.

Activation of L-arginine transport (system y+) and nitric oxide synthase by elevated glucose and insulin in human endothelial cells

1. Modulation of L-arginine transport (system y+) and release of nitric oxide (NO) and prostacyclin (PGI2) by elevated glucose and insulin were investigated in human cultured umbilical vein endothelial cells. 2. Elevated glucose induced a time- (6-12 h) and concentration-dependent stimulation of L-arginine transport, which was reversible and associated with a 3-fold increase in intracellular cGMP accumulation (index of NO synthesis) and 75% decrease in PGI2 production. 3. Elevated glucose had no effect on the initial transport rates for L-serine, L-citrulline, L-leucine, L-cystine or 2-deoxyglucose. 4. Resting membrane potential was unaffected by elevated glucose whereas basal intracellular [Ca2+] increased from 65 +/- 5 nM to 136 +/- 16 nM. 5. Insulin induced a protein synthesis-dependent stimulation of L-arginine transport and increased NO and PGI2 production in cells exposed to 5 mM glucose. 6. In cells exposed to high glucose, insulin downregulated elevated rates of L-arginine transport and cGMP accumulation but had no effect on the depressed PGI2 production. 7. Our findings suggest that insulin's normal stimulatory action on human endothelial cell vasodilator pathways may be impaired under conditions of sustained hyperglycaemia.

Superoxide anion and endothelial regulation of arterial tone

Evidence continues to accumulate on the importance of paracrine substances formed in the endothelium in regulation of the vascular system. Mechanisms that govern the balance between relaxing and contracting factors are important for understanding the regulation of blood vessel tone in health and disease. Chemical antagonism between superoxide anions and nitric oxide has been recognized as a potentially important modulator of vascular reactivity as well as being a source of peroxynitrite, a potent oxidant. In several models of vascular diseases, impairment of endothelium-dependent relaxations and promotion of endothelium-dependent contractions has been ascribed to increased production of superoxide anions. In pathologic conditions, increased production of superoxide anions may be responsible for an impairment of balance between relaxing and contracting factors favoring an increase in arterial tone. In this review the role of superoxide anions in modulation of endothelial mechanisms responsible for regulation of arterial tone will be discussed.