Hydrogen peroxide is an endothelium-derived hyperpolarizing factor in mice

Contributions of nitric oxide, EDHF, and EETs to endothelium-dependent relaxation in renal afferent arterioles

BACKGROUND

Acetylcholine-induced endothelium-dependent relaxation in the renal afferent arteriole has been ascribed to nitric oxide, but the role of endothelium-derived hyperpolarizing factors (EDHFs) and 14,15-epoxyeicosatrienoic acid (14,15-EET) are unclear.

METHODS

Single afferent arterioles were dissected from kidney of normal rabbits and microperfused in vitro at 60 mm Hg. Vessels were preconstricted submaximally with norepinephrine (10(-8) mol/L). Relaxation was assessed following cumulative addition of ACh (10(-9) to 10(-4) mol/L) alone, or in the presence of indomethacin (to inhibit cyclooxygenase), Nw-nitro-L-arginine (L-NNA) (to inhibit nitric oxide synthase), methylene blue (to inhibit soluble guanylate cyclase), or a combination of L-NNA + methylene blue. To assess contributions by EDHF, studies were repeated with either apamin + charybdotoxin [to block Ca2+-activated K+ channels (KCa)] or with 40 mmol/L KCl. To asses the role of 14,15-EET, relaxations were evaluated in the presence of its competitive inhibitor 14,15-epoxyeicosa-5(Z)-enoic acid (14,15-EEZE).

RESULTS

Relaxation by acetylcholine was abolished following endothelial denudation. It was unaffected by indomethacin but was inhibited 54%+/- 5% (P < 0.001) by L-NNA, 57%+/- 5% by methylene blue, and 60%+/- 4% by the combination of L-NNA plus methylene blue. Relaxation was inhibited further by KCl (80%+/- 6%) or by apamin + charybdotoxin (96%+/- 2%). 14,15-EEZE, alone, inhibited acetylcholine-induced relaxation by 29%+/- 3%, and by 80%+/- 5% in the presence of L-NNA.

CONCLUSION

Acetylcholine-induced afferent arteriolar relaxation depends strongly on both nitric oxide, acting via soluble guanylate cyclase, and on an EDHF, likely 14,15-EET, acting via KCa.

Oxidation of tetrahydrobiopterin leads to uncoupling of endothelial cell nitric oxide synthase in hypertension

Tetrahydrobiopterin is a critical cofactor for the NO synthases, and in its absence these enzymes become "uncoupled," producing reactive oxygen species (ROSs) rather than NO. In aortas of mice with deoxycorticosterone acetate-salt (DOCA-salt) hypertension, ROS production from NO synthase is markedly increased, and tetrahydrobiopterin oxidation is evident. Using mice deficient in the NADPH oxidase subunit p47(phox) and mice lacking either the endothelial or neuronal NO synthase, we obtained evidence that hypertension produces a cascade involving production of ROSs from the NADPH oxidase leading to oxidation of tetrahydrobiopterin and uncoupling of endothelial NO synthase (eNOS). This decreases NO production and increases ROS production from eNOS. Treatment of mice with oral tetrahydrobiopterin reduces vascular ROS production, increases NO production as determined by electron spin resonance measurements of nitrosyl hemoglobin, and blunts the increase in blood pressure due to DOCA-salt hypertension. Endothelium-dependent vasodilation is only minimally altered in vessels of mice with DOCA-salt hypertension but seems to be mediated by hydrogen peroxide released from uncoupled eNOS, since it is inhibited by catalase. Tetrahydrobiopterin oxidation may represent an important abnormality in hypertension. Treatment strategies that increase tetrahydrobiopterin or prevent its oxidation may prove useful in preventing vascular complications of this common disease.

Oxidation of tetrahydrobiopterin leads to uncoupling of endothelial cell nitric oxide synthase in hypertension

Tetrahydrobiopterin is a critical cofactor for the NO synthases, and in its absence these enzymes become "uncoupled," producing reactive oxygen species (ROSs) rather than NO. In aortas of mice with deoxycorticosterone acetate-salt (DOCA-salt) hypertension, ROS production from NO synthase is markedly increased, and tetrahydrobiopterin oxidation is evident. Using mice deficient in the NADPH oxidase subunit p47(phox) and mice lacking either the endothelial or neuronal NO synthase, we obtained evidence that hypertension produces a cascade involving production of ROSs from the NADPH oxidase leading to oxidation of tetrahydrobiopterin and uncoupling of endothelial NO synthase (eNOS). This decreases NO production and increases ROS production from eNOS. Treatment of mice with oral tetrahydrobiopterin reduces vascular ROS production, increases NO production as determined by electron spin resonance measurements of nitrosyl hemoglobin, and blunts the increase in blood pressure due to DOCA-salt hypertension. Endothelium-dependent vasodilation is only minimally altered in vessels of mice with DOCA-salt hypertension but seems to be mediated by hydrogen peroxide released from uncoupled eNOS, since it is inhibited by catalase. Tetrahydrobiopterin oxidation may represent an important abnormality in hypertension. Treatment strategies that increase tetrahydrobiopterin or prevent its oxidation may prove useful in preventing vascular complications of this common disease.

Involvement of H2O2 in superoxide-dismutase-induced enhancement of endothelium-dependent relaxation in rabbit mesenteric resistance artery

1 The mechanism underlying the enhancement by superoxide dismutase (SOD) of endothelium-dependent relaxation was investigated in rabbit mesenteric resistance arteries. 2 SOD (200 U ml(-1)) increased the production of H(2)O(2) in smooth muscle cells (as indicated by the use of an H(2)O(2)-sensitive fluorescent dye). 3 Neither SOD nor catalase (400 U ml(-1)) modified either the resting membrane potential or the hyperpolarization induced by acetylcholine (ACh, 1 micro M) in smooth muscle cells. 4 In arteries constricted with noradrenaline, the endothelium-dependent relaxation induced by ACh (0.01-1 micro M) was enhanced by SOD (200 U ml(-1)) (P<0.01). This action of SOD was inhibited by L-N(G)-nitroarginine (nitric oxide (NO)-synthase inhibitor) but not by either charybdotoxin+apamin (Ca(2+)-activated-K(+)-channel blockers) or diclofenac (cyclooxygenase inhibitor). 5 Neither ascorbate (50 micro M) nor tiron (0.3 mM), superoxide scavengers, had any effect on the ACh-induced relaxation, but each attenuated the enhancing effect of SOD on the ACh-induced relaxation. Similarly, catalase (400 U ml(-1)) inhibited the effect of SOD without changing the ACh-induced relaxation. 6 In endothelium-denuded strips constricted with noradrenaline, SOD enhanced the relaxation induced by the NO donor 1-hydroxy-2-oxo-3-(N-methyl-3-aminopropyl)-3-methyl-1-triazene (NOC-7) (P<0.05). Ascorbate and catalase each attenuated this effect of SOD. 7 H(2)O(2) (1 micro M) enhanced the relaxation on the noradrenaline contraction induced by NOC-7 and that induced by 8-bromo-cGMP, a membrane-permeable analogue of guanosine 3',5' cyclic monophosphate (cGMP). 8 SOD had no effect on cGMP production, whether measured in endothelium-intact strips following an application of ACh (0.1 micro M) or in endothelium-denuded strips following an application of NOC-7 (0.1 micro M). 9 It is suggested that in rabbit mesenteric resistance arteries, SOD increases the ACh-induced, endothelium-dependent relaxation by enhancing the action of NO in the smooth muscle via its H(2)O(2)-producing action (rather than via a superoxide-scavenging action).

Hydrogen peroxide is an endothelium-derived hyperpolarizing factor in animals and humans

Vascular endothelium plays an important role in maintaining vascular homeostasis by synthesizing and releasing several vasodilating factors, such as prostacyclin, nitric oxide (NO), and a yet unidentified endothelium-derived hyperpolarizing factor (EDHF). Possible candidates for EDHF include epoxyeicosatrienoic acids, endothelium-derived K(+) ions, and as we have recently identified, hydrogen peroxide (H(2)O(2)). Electrical communication between endothelial and smooth muscle cells through gap junctions has also been suggested to be involved in endothelium-dependent hyperpolarization. Among the above candidates, the H(2)O(2) hypothesis well explains the pathophysiological interactions between NO and EDHF and re-highlights the physiological roles of the reactive oxygen species in endothelium-dependent vascular responses. This brief review summarizes our current knowledge about H(2)O(2) as an EDHF, with special reference to its production by the endothelium, its action on membrane potentials and its pathophysiological roles.

Lack of vascular connexin 40 is associated with hypertension and irregular arteriolar vasomotion

Gap-junctional communication coordinates the behavior of individual cells in arterioles. Gap junctions are formed by connexins 40 (Cx40), Cx43, Cx37, and Cx45 in the vasculature. Previously, we have shown that lack of Cx40 impairs conduction of dilatory signals along arterioles. Herein, we examined whether hypertension is present in conscious animals and whether this is a direct effect or due to secondary mechanisms. Mean arterial pressure was elevated by 20-25 mmHg in conscious Cx40-deficient mice (Cx40(-/-)) compared with wild-type controls in both sexes. Differences in heart rate were not observed. Blockade of NO synthase increased pressure equally in both genotypes. Conversely, the angiotensin AT(1)-receptor antagonist, candesartan, decreased pressure to similar extents in Cx40(-/-) and wild-type mice. Acetylcholine and sodium nitroprusside (0.05-15 nmol) were equally potent and effective in decreasing pressure and inducing dilatory responses in the microcirculation. However, in contrast to wild type, Cx40(-/-) arterioles exhibited spontaneous, irregular vasomotion leading temporarily to complete vessel closure. We conclude that loss of Cx40 is associated with hypertension independent of the action of angiotensin II. It is also not related to an altered efficacy of NO or other endothelial dilators. However, the observed irregular vasomotion suggests that peripheral vascular resistance is affected.

Oxidative stress and endothelial nitric oxide bioactivity

The endothelium plays an important role in the maintenance of vascular homeostasis. Central to this role is the endothelial production of nitric oxide (NO), synthesized by the constitutively expressed endothelial isoform of nitric oxide synthase. Vascular diseases, including hypertension, diabetes, and atherosclerosis, are characterized by impaired endothelium-derived NO bioactivity that may contribute to clinical cardiovascular events. Growing evidence indicates that impaired endothelium-derived NO bioactivity is due, in part, to excess vascular oxidative stress. This review outlines how different forms of oxidative stress can impact on NO bioactivity and discusses strategies to prevent oxidative stress-induced endothelial dysfunction.

Different modulation by Ca2+-activated K+ channel blockers and herbimycin of acetylcholine- and flow-evoked vasodilatation in rat mesenteric small arteries

1. The present study addressed whether endothelium-dependent vasodilatation evoked by acetylcholine and flow are mediated by the same mechanisms in isolated rat mesenteric small arteries, suspended in a pressure myograph for the measurement of internal diameter. 2. In pressurized arterial segments contracted with U46619 in the presence of indomethacin, shear stress generated by the flow evoked relaxation. Thus, in endothelium-intact segments low (5.1+/-0.6 dyn cm(-2)) and high (19+/-2 dyn cm(-2)) shear stress evoked vasodilatations that were reduced by, respectively, 68+/-11 and 68+/-8% (P<0.05, n=7) by endothelial cell removal. Acetylcholine (0.01-1 microM) evoked concentration-dependent vasodilatation that was abolished by endothelial cell removal. 3. Incubation with indomethacin alone did not change acetylcholine and shear stress-evoked vasodilatation, while the combination of indomethacin with the nitric oxide (NO) synthase inhibitor, N(G),N(G)-asymmetric dimethyl-L-arginine (ADMA 1 mM), reduced low and high shear stress-evoked vasodilatation with, respectively, 52+/-15 and 58+/-10% (P<0.05, n=9), but it did not change acetylcholine-evoked vasodilatation. 4. Inhibition of Ca(2+)-activated K(+) channels with a combination of apamin (0.5 microM) and charybdotoxin (ChTX) (0.1 microM) did not change shear stress- and acetylcholine-evoked vasodilatation. In the presence of indomethacin and ADMA, the combination of apamin (0.5 microM) and ChTx (0.1 microM) increased contraction induced by U46619, but these blockers did not change the vasodilatation evoked by shear stress. In contrast, acetylcholine-evoked vasodilatation was abolished by the combination of apamin and charybdotoxin. 5. In the presence of indomethacin, the tyrosine kinase inhibitor, herbimycin A (1 microM), inhibited low and high shear stress-evoked vasodilatation with, respectively, 32+/-12 and 68+/-14% (P<0.05, n=8), but it did not change vasodilatation induced by acetylcholine. In the presence of indomethacin and ADMA, herbimycin A neither changed shear stress nor acetylcholine-evoked vasodilatation. 6. The present study suggests that Ca(2+)-activated K(+) channels sensitive for the combination of apamin and ChTx are involved in acetylcholine-evoked, mainly non-NO nonprostanoid factor-mediated, vasodilatation, while an Src tyrosine kinase plays a role for flow-evoked NO-mediated vasodilatation in rat mesenteric small arteries.

Endothelium dysfunction in LDL receptor knockout mice: a role for H2O2

1. In this study, the role of endogenous H(2)O(2) as an endothelium-dependent relaxant factor was characterised in aortas from C57BL/6J and LDL receptor-deficient mice (LDLR(-/-)). 2. Aortic rings from LDLR(-/-) mice showed impaired endothelium-dependent relaxation to acetylcholine (ACh; 0.001-100 micro M) and to the Ca(2+) ionophore A23187 (0.001-3 micro M) compared with aortic rings from control mice. Endothelium-independent relaxation produced by the NO donor, 3-morpholino-sydnonimine (SIN-1) was not different between strains. 3. Pretreatment of vessels with L-NNA (100 micro M) or L-NNA (100 micro M) plus L-NAME (300 micro M) plus haemoglobin (10 micro M) markedly decreased, but did not abolish the relaxation to ACh in control mice. In the aortas from LDLR(-/-) mice treated with L-NNA (100 micro M), ACh induced a contractile effect. Catalase (800 and 2400 U ml(-1)) shifted to the right the endothelium-dependent relaxation to ACh in aortas from control but not from LDLR(-/-) mice. Aminotriazole (50 mM), which inhibits catalase, abolished its effect on control mice. Treatment of vessels with L-NNA and catalase abolished vasorelaxation induced by ACh. Indomethacin (10 micro M) did not modify the concentration-response curve to ACh. Superoxide dismutase (300 U ml(-1)) did not change ACh-induced relaxation in both strains. 4. Exogenous H(2)O(2) produced a concentration-dependent relaxation in endothelium-denuded aortic rings, which was not different between strains. 5. It is concluded that H(2)O(2) greatly contributes to relaxation to ACh in aorta from control mice. Endothelial-dependent relaxation to ACh is impaired in LDLR(-/-) mice. Reduced biosynthesis or increased inactivation of H(2)O(2) is the possible mechanism responsible for endothelial dysfunction in aortas of atherosclerosis-susceptible LDLR(-/-) mice.

Oxidation of tetrahydrobiopterin leads to uncoupling of endothelial cell nitric oxide synthase in hypertension

Tetrahydrobiopterin is a critical cofactor for the NO synthases, and in its absence these enzymes become "uncoupled," producing reactive oxygen species (ROSs) rather than NO. In aortas of mice with deoxycorticosterone acetate-salt (DOCA-salt) hypertension, ROS production from NO synthase is markedly increased, and tetrahydrobiopterin oxidation is evident. Using mice deficient in the NADPH oxidase subunit p47(phox) and mice lacking either the endothelial or neuronal NO synthase, we obtained evidence that hypertension produces a cascade involving production of ROSs from the NADPH oxidase leading to oxidation of tetrahydrobiopterin and uncoupling of endothelial NO synthase (eNOS). This decreases NO production and increases ROS production from eNOS. Treatment of mice with oral tetrahydrobiopterin reduces vascular ROS production, increases NO production as determined by electron spin resonance measurements of nitrosyl hemoglobin, and blunts the increase in blood pressure due to DOCA-salt hypertension. Endothelium-dependent vasodilation is only minimally altered in vessels of mice with DOCA-salt hypertension but seems to be mediated by hydrogen peroxide released from uncoupled eNOS, since it is inhibited by catalase. Tetrahydrobiopterin oxidation may represent an important abnormality in hypertension. Treatment strategies that increase tetrahydrobiopterin or prevent its oxidation may prove useful in preventing vascular complications of this common disease.

Mediation of arachidonic acid metabolite(s) produced by endothelial cytochrome P-450 3A4 in monkey arterial relaxation

We investigated mechanisms of endothelium-dependent relaxation by acetylcholine resistant to indomethacin and N(G)-nitro-L-arginine and sensitive to cytochrome P-450 (CYP) inhibitors or charybdotoxin + apamin in the monkey lingual artery. Treatment with quinacrine, an inhibitor of phospholipase A2, abolished the relaxation by acetylcholine. However, treatment with alpha-glycyrrhetinic acid, an inhibitor of gap junctions, or catalase, an enzyme which dismutates hydrogen peroxide to form water and oxygen, did not affect the relaxation by acetylcholine. Immunohistochemistry demonstrated the presence of CYP3A4 in endothelial cells of the artery. Anti-CYP3A4 antibody inhibited relaxations by products of arachidonic acid incubated with human liver microsomes rich in CYPs in the endothelium-denuded artery. Purified CYP3A4 produced epoxyeicosatrienoic acids (EETs) from arachidonic acid, and the production was abolished by a selective CYP3A inhibitor, ketoconazole. It may be concluded that endothelium-derived relaxing substance(s) other than nitric oxide and prostanoids in the monkey lingual artery opens charybdotoxin + apamin-sensitive K+ channels in smooth muscle cells, and arachidonic acid metabolite(s) produced by endothelial CYP3A4 is likely to be the major substance.

Mechanisms of hydrogen-peroxide-induced biphasic response in rat mesenteric artery

1. In phenylephrine (PHE) (1 micro M)-precontracted superior mesenteric arteries from adult rats, low concentration of hydrogen peroxide (H(2)O(2), 10-100 micro M) caused only contraction, while high concentration of H(2)O(2) (0.3-1 mM) caused a biphasic response: a transient contraction followed by a relaxation response. 2. Endothelium removal did not affect the biphasic response. 7,7-Dimethyl-(5Z,8Z)-eicosadienoic acid, diclofenac, furegrelate, or SQ 29548 greatly inhibited the contraction but did not affect the relaxation. 17-Octadecynoic acid, eicosatriynoic acid, ICI 198615, SQ 22536, or ODQ did not inhibit the biphasic response. 3. KCl at 40 mM inhibited the relaxation response to H(2)O(2) by 98+/-24%. 4-Aminopyridine (4-AP) inhibited while tetraethylammonium chloride (TEA), charybdotoxin, or glibenclamide attenuated the relaxation response. A combination of 4-AP, TEA and glibenclamide mimicked the effects of 40 mM KCl. Iberiotoxin, apamin, or barium chloride did not inhibit the relaxation response. 4. H(2)O(2) at 1 mM hyperpolarized membrane potential and reversibly augmented K(+) current in smooth muscle cells of mesenteric artery. These effects of H(2)O(2) were attenuated significantly by 4-AP. 5. In summary, in PHE-precontracted rat mesenteric artery: (1) the response to H(2)O(2) shifted qualitatively from contraction to a biphasic response as H(2)O(2) increased to 0.3 mM or higher; (2) the relaxation response is caused by the activation of K(+) channels, with voltage-dependent K(+) channels playing a primary role; and the contraction is likely to be mediated by thromboxane A(2); (3) the K(+) channel activation by H(2)O(2) is independent of phospholipase A(2), cyclooxygenase, lipoxygenase, cytochrome P450 monooxygenase, adenylate or guanylate cyclase.

Recent advances in intracellular signalling in hypertension

PURPOSE OF REVIEW

Transmission of external signals from the cell surface to the internal cellular environment occurs via tightly controlled complex transduction pathways. Alterations in these highly regulated signalling cascades in vascular smooth cells may play a fundamental role in the structural, mechanical and functional abnormalities that underlie vascular pathological processes in hypertension. The present review focuses on recent developments relating to two novel signalling pathways: angiotensin II signalling through tyrosine kinases; and oxidative stress and redox-dependent signal transduction. These pathways are emerging as critical mediators of hypertensive vascular disease because they influence multiple cellular responses that are involved in structural remodelling, vascular inflammation and altered tone.

RECENT FINDINGS

A recent advance in the field of angiotensin II signalling was the demonstration that, in addition to its vasoconstrictor properties, angiotensin II has potent mitogenic-like and proinflammatory-like characteristics. These actions are mediated through phosphorylation of both nonreceptor tyrosine kinases and receptor tyrosine kinases. It is also becoming increasingly apparent that many signalling events that underlie abnormal vascular function in hypertension are influenced by changes in intracellular redox status. In particular, increased bioavailability of reactive oxygen species (oxidative stress) stimulates growth-signalling pathways, induces expression of proinflammatory genes, alters contraction-excitation coupling and impairs endothelial function.

SUMMARY

A better understanding of the molecular pathways that regulate vascular smooth muscle cell function will provide further insights into the pathophysiological mechanisms that contribute to vascular changes and end-organ damage associated with high blood pressure, and could permit identification of potential novel therapeutic targets in the prevention and management of hypertension.

Hydrogen peroxide, an endogenous endothelium-derived hyperpolarizing factor, plays an important role in coronary autoregulation in vivo

BACKGROUND

Recent studies in vitro have demonstrated that endothelium-derived hydrogen peroxide (H2O2) is an endothelium-derived hyperpolarizing factor (EDHF) in animals and humans. The aim of this study was to evaluate our hypothesis that endothelium-derived H2O2 is an EDHF in vivo and plays an important role in coronary autoregulation.

METHODS AND RESULTS

To test this hypothesis, we evaluated vasodilator responses of canine (n=41) subepicardial small coronary arteries (> or =100 microm) and arterioles (<100 microm) with an intravital microscope in response to acetylcholine and to a stepwise reduction in coronary perfusion pressure (from 100 to 30 mm Hg) before and after inhibition of NO synthesis with N(G)-monomethyl-L-arginine (L-NMMA). After L-NMMA, the coronary vasodilator responses were attenuated primarily in small arteries, whereas combined infusion of L-NMMA plus catalase (an enzyme that selectively dismutates H2O2 into water and oxygen) or tetraethylammonium (TEA, an inhibitor of large-conductance K(Ca) channels) attenuated the vasodilator responses of coronary arteries of both sizes. Residual arteriolar dilation after L-NMMA plus catalase or TEA was largely attenuated by 8-sulfophenyltheophylline, an adenosine receptor inhibitor.

CONCLUSIONS

These results suggest that H2O2 is an endogenous EDHF in vivo and plays an important role in coronary autoregulation in cooperation with NO and adenosine.

Role for hydrogen peroxide in flow-induced dilation of human coronary arterioles

Flow-induced dilation (FID) is dependent largely on hyperpolarization of vascular smooth muscle cells (VSMCs) in human coronary arterioles (HCA) from patients with coronary disease. Animal studies show that shear stress induces endothelial generation of hydrogen peroxide (H2O2), which is proposed as an endothelium-derived hyperpolarizing factor (EDHF). We tested the hypothesis that H2O2 contributes to FID in HCA. Arterioles (135+/-7 micro m, n=71) were dissected from human right atrial appendages at the time of cardiac surgery and cannulated with glass micropipettes. Changes in internal diameter and membrane potential of VSMCs to shear stress, H2O2, or to papaverine were recorded with videomicroscopy. In some vessels, endothelial H2O2 generation to shear stress was monitored directly using confocal microscopy with 2',7'-dichlorofluorescin diacetate (DCFH) or using electron microscopy with cerium chloride. Catalase inhibited FID (%max dilation; 66+/-8 versus 25+/-7%; P<0.05, n=6), whereas dilation to papaverine was unchanged. Shear stress immediately increased DCFH fluorescence in the endothelial cell layer, whereas treatment with catalase abolished the increase in fluorescence. Electron microscopy with cerium chloride revealed shear stress-induced increase in cerium deposition in intimal area surrounding endothelial cells. Exogenous H2O2 dilated (%max dilation; 97+/-1%, ED50; 3.0+/-0.7x10(-5) mol/L) and hyperpolarized HCA. Dilation to H2O2 was reduced by catalase, 40 mmol/L KCl, or charybdotoxin plus apamin, whereas endothelial denudation, deferoxamine, 1H-(1,2,4)-oxadiazole-[4,3-a]quinoxalin-1-one, or glibenclamide had no effect. These data provide evidence that shear stress induces endothelial release of H2O2 and are consistent with the idea that H2O2 is an EDHF that contributes to FID in HCA from patients with heart disease. The full text of this article is available at http://www.circresaha.org.

[Recent advances in the study of endothelium-dependent hyperpolarizing factor (EDHF)]

Recent advances in the properties and physiological functions of endothelium-dependent hyperpolarizing factor (EDHF) in vascular tissues were reviewed briefly. The EDHF-induced hyperpolarization is inhibited by charybdotoxin, indicating that the potential is produced mainly by activation of intermediate conductance Ca-sensitive K-channels. During generation of EDHF responses, endothelial Ca2+ concentration was elevated, suggesting that the activated K-channels were distributed on the endothelial membrane. This was confirmed by direct recording of membrane potentials from endothelial and smooth muscle cells using double patch electrodes. Measurement of the propagation of potentials applied to endothelial or smooth muscle cells to surrounding cells revealed that there were tight electrical connections between endothelial cells much more than between endothelial and smooth muscle cells or between smooth muscle cells, and these observations yielded a possible spread of electrical signal along the endothelial layer first, and then the signals would be conducted to smooth muscle cell layers. These properties of vascular tissues allow speculating that EDHF is an electrical signal propagated from endothelial cells electrotonically through myoendothelial gap junctions. Several candidates have been proposed as EDHF, and possibilities of individual substances for EDHF were discussed. The cellular mechanism of the hyperpolarization-induced vasodilatation remains unclear, and this should be clarified in the future for further understanding of the EDHF-induced vasodilatation.

Endothelial responses of the aorta from adrenomedullin transgenic mice and knockout mice

Adrenomedullin (AM) is a potent vascular wall-derived vasorelaxing peptide which induces the release of nitric oxide (NO). To explore the role of endogenous AM in vascular function, we examined the effects of acetylcholine (ACh), AM, and AM receptor antagonists [AM (22-52), and calcitonin gene-related peptide (CGRP) (8-37)] on the isometric tension of aortic rings isolated from AM transgenic (TG) and knockout (KO) mice and wild type littermates (WT). ACh and AM caused a dose-dependent reduction of the isometric tension of aortic rings, but the degree of vasodilatation was smaller in TG than in KO or WT (% delta tension [10(-6) mol/l ACh]: KO -69 +/- 10%, WT -39 +/- 8%, TG -29 +/- 1%, p < 0.01). On the other hand, N(G)-nitro-L-arginine methyl ester, an NO synthase inhibitor, induced greater vasoconstriction in TG (% delta tension 10(-5)mol/l: KO +78 +/- 16%, WT +99 +/- 27%, TG +184 +/- 20%, p < 0.01), whereas E-4021, a cyclic guanosine monophosphate (cGMP)-specific phosphodiesterase inhibitor, caused greater vasodilation in TG mice. Both AM antagonists increased tension in TG to a greater extent than in KO or WT mice (% delta tension [10(-6) mol/l CGRP (8-37)]: KO +24 +/- 5%, WT +51 +/- 6%, TG +75 +/- 7%, p < 0.01). Endothelial denudation of the aorta diminished the vasoconstriction caused by the AM antagonists. In conclusion, the amounts of AM expressed in the aortic endothelium influenced baseline NO release. AM antagonists increased vascular tone in WT as well as in TG, suggesting that endogenous AM plays a physiological role in the regulation of aortic tone.

Akt-dependent phosphorylation of serine 1179 and mitogen-activated protein kinase kinase/extracellular signal-regulated kinase 1/2 cooperatively mediate activation of the endothelial nitric-oxide synthase by hydrogen peroxide

Hydrogen peroxide mediates vasodilation, but the mechanisms responsible for this process remain undefined. We examined the effect of H(2)O(2) on nitric oxide (NO*) production and the signaling events involved. NO* release from bovine aortic endothelial cells was detected with an NO*-specific microelectrode. The addition of H(2)O(2) caused a potent dose-dependent increase in NO* production. This was partially Ca(2+)-dependent because BAPTA/AM reduced NO* production at low (<50 microM) but not high (>100 microM) concentrations of H(2)O(2). Phosphatidylinositol (PI) 3-kinase inhibition [with wortmannin or 2-(4-morpholinyl)-8-phenyl-1(4H)-benzopyran-4-one hydrochloride], infection with a dominant-negative mutant of Akt, or mitogen-activated protein kinase kinase/extracellular signal-regulated kinase 1/2 (MEK/ERK1/2) inhibition (with PD98059 or U0126) partially attenuated, whereas inhibition of both PI 3-kinase and MEK1/2 abolished H(2)O(2)-dependent NO* production. ERK1/2 seemed necessary for NO* production early (<5 min) after H(2)O(2) addition, whereas PI 3-kinase/Akt was more important at later time points. Phosphorylation of endothelial nitric-oxide synthase (eNOS) at serine 1179 was observed >10 min after the addition of H(2)O(2), and this was prevented by wortmannin but not by PD98059. c-Src family tyrosine kinase(s) was found to be upstream of H(2)O(2)-dependent Akt and eNOS serine 1179 phosphorylation and subsequent NO* production. In summary, H(2)O(2) causes endothelial NO* release mediated by cooperative effects between PI 3-kinase/Akt-dependent eNOS serine 1179 phosphorylation and activation of MEK/ERK1/2. This may represent an acute cellular adaptation to an increase in oxidant stress.

Development of novel fluorescence probes that can reliably detect reactive oxygen species and distinguish specific species

We designed and synthesized 2-[6-(4'-hydroxy)phenoxy-3H-xanthen-3-on-9-yl]benzoic acid (HPF) and 2- [6-(4'-amino)phenoxy-3H-xanthen-3-on-9-yl]benzoic acid (APF) as novel fluorescence probes to detect selectively highly reactive oxygen species (hROS) such as hydroxyl radical (*OH) and reactive intermediates of peroxidase. Although HPF and APF themselves scarcely fluoresced, APF selectively and dose-dependently afforded a strongly fluorescent compound, fluorescein, upon reaction with hROS and hypochlorite ((-)OCl), but not other reactive oxygen species (ROS). HPF similarly afforded fluorescein upon reaction with hROS only. Therefore, not only can hROS be differentiated from hydrogen peroxide (H(2)O(2)), nitric oxide (NO), and superoxide (O2*-) by using HPF or APF alone, but (-)OCl can also be specifically detected by using HPF and APF together. Furthermore, we applied HPF and APF to living cells and found that HPF and APF were resistant to light-induced autoxidation, unlike 2',7'-dichlorodihydrofluorescein, and for the first time we could visualize (-)OCl generated in stimulated neutrophils. HPF and APF should be useful as tools to study the roles of hROS and (-)OCl in many biological and chemical applications.

Mechanisms underlying the hydrogen peroxide-induced, endothelium-independent relaxation of the norepinephrine-contraction in guinea-pig aorta

The mechanisms underlying the hydrogen peroxide-induced relaxation of the norepinephrine-contraction were studied by measuring isometric force, myosin light chain (MLC(20)) phosphorylation and cyclic GMP in endothelium-denuded muscle from the guinea-pig aorta. Norepinephrine (5.2+/-1.3 microM) produced a phasic, followed by a tonic contraction. Hydrogen peroxide (10 and 100 microM), glyceryl trinitrate (30 and 300 nM) and 8-bromo cyclic GMP (30 and 100 microM) did not change the basal tone, but reduced the norepinephrine-induced contraction. Phosphorylation of MLC(20) (percentage of phosphorylated to total MLC(20)) was increased 1 min (5.9+/-1.0% vs. 35.9+/-4.9%) and, to a lesser extent, 20 min (3.7+/-1.7% vs. 13.9+/-1.6%) after the addition of norepinephrine. Hydrogen peroxide (100 microM) did not modify basal MLC(20) phosphorylation, but reduced the increase in MLC(20) phosphorylation induced by 1-min exposure to norepinephrine (20.9+/-4.1%). Its effect was abolished by catalase. When the tissue was incubated for 20 min with norepinephrine in the presence of hydrogen peroxide, norepinephrine-induced MLC(20) phosphorylation was not changed (13.6+/-1.5%), as compared to that in the absence of hydrogen peroxide. Hydrogen peroxide relaxed norepinephrine-stimulated aortas in a concentration-dependent fashion with EC(50) values of 5.9+/-0.2 microM. The relaxation was inhibited by soluble guanylate cyclase inhibitors and increased by an inhibitor of cyclic GMP-selective phosphodiesterase. In aorta precontracted with norepinephrine, hydrogen peroxide (100 microM) relaxed the tissue by 89+/-11% and almost doubled tissue concentrations of cyclic GMP, whereas sodium nitroprusside (1 microM) relaxed the tissue by 100% and increased cyclic GMP concentrations 30-fold. It is suggested that the inhibitory effects of hydrogen peroxide on the norepinephrine-induced phasic and sustained contractions are explained by a decrease in MLC(20) phosphorylation and by an alteration in MLC(20) phosphorylation-independent mechanisms, respectively. The effects of hydrogen peroxide were in part mediated by cyclic GMP.

Acetylcholine-induced vasodilation may depend entirely upon NO in the femoral artery of young piglets

1 To characterize agonist-induced relaxation in femoral artery rings from young piglets, we compared the effect of a NOS-inhibitor N(omega)-nitro-L-arginine (L-NOARG), an NO-inactivator oxyhaemoglobin (HbO) and a soluble guanyl cyclase(sGC)-inhibitor 1H-[1,2,4]Oxadiazolo-[4,3,-alpha]quinoxalin-1-one (ODQ) on acetylcholine(ACh)-induced relaxation. The involvement of K(+) channel activation was studied on relaxations induced by ACh, the two NO donors sodium nitroprusside (SNP) and diethylamine (DEA) NONOate, and the cell membrane permeable guanosine 3'5' cyclic monophosphate (cGMP) analogue 8-Br-cGMP. 2 Full reversal of phenylephrine-mediated precontraction was induced by ACh (1 nM-1 microM) (pD(2) 8.2+/-0.01 and R(max) 98.7+/-0.3%). L-NOARG (100 microM) partly inhibited relaxation (pD(2) 7.4+/-0.02 and R(max) 49.6+/-0.8%). The L-NOARG/indomethacin(IM)-resistant response displayed characteristics typical for endothelium-derived hyperpolarizing factor (EDHF), being sensitive to a combination of the K(+) channel blockers charybdotoxin (CTX) (0.1 microM) and apamin (0.3 microM). 3 ODQ (10 microM) abolished relaxations induced by ACh and SNP. L-NOARG/IM-resistant relaxations to ACh were abolished by HbO (20 microM). 4 Ouabain (1 microM) significantly inhibited ACh-induced L-NOARG/IM-resistant relaxations and relaxations induced by SNP (10 microM) and 8-Br-cGMP (0.1 mM). A combination of ouabain and Ba(2+) (30 microM) almost abolished L-NOARG/IM-resistant ACh-induced relaxation (R(max) 7.7+/-2.5% vs 23.4+/-6.4%, with and without Ba(2+), respectively, P<0.05). 5 The present study demonstrates that in femoral artery rings from young piglets, despite an L-NOARG/IM-resistant component sensitive to K(+) channel blockade with CTX and apamin, ACh-induced relaxation is abolished by sGC-inhibition or a combination of L-NOARG and HbO. These findings suggest that relaxation can be fully explained by the NO/cGMP pathway.

The Endothelium in Health and Disease-A Target for Therapeutic Intervention

In this review we discuss the contribution of NO, prostacyclin and endothelium-derived relaxing factor--endothelium-derived hyperpolarizing factor, or EDHF, to vascular function. We also explore the hypotheses (1): that tissues can store NO as nitrosothiols (RSNOs) and (2) that such RSNO stores can be modulated by physiological and pathophysiological processes. Notably in the microcirculation, EDHF appears to play an important role in the regulation of vascular tone. Leading candidates for EDHF include extracellular potassium (K+), an epoxygenase product, hydrogen peroxide and/or a contribution from myoendothelial gap junctions. Data from our laboratory indicate that in mouse vessels, different endothelium-dependent vasodilators, such as acetylcholine and protease-activated receptor (PAR) agonists, release different endothelium-derived relaxing factors. The combination of two K-channel toxins, apamin and charybdotoxin, inhibits EDHF activity in most protocols. Endothelial dysfunction is considered as the major risk factor and a very early indicator of cardiovascular disease including the cardiovascular complications of type I & types II diabetes. Impaired endothelium-dependent vasodilatation results primarily from a decreased synthesis of endothelium-derived nitric oxide (NO) and/or an increase in the production of reactive oxygen species such as superoxide. We have shown that the administration of tetrahydrobiopterin, an important co-factor for nitric oxide synthase (NOS) partially restores endothelial function (1) in leptin-deficient mice (db/db) with spontaneous type II diabetes, as well as (2) in human vascular tissue harvested for coronary artery bypass grafting (CABG). These data suggest that a deficiency in the availability of tetrahydrobiopterin plays an important role in vascular dysfunction associated with Type II diabetes. In addition, changes in the contribution of EDHF occur in vascular tissue from the db/db mice suggesting a compensatory increase in EDHF production; whether this alteration in EDHF production is physiological or pathophysiological remains controversial.

Estrogen-Induced Augmentation of Endothelium-Dependent Nitric Oxide-Mediated Vasodilation in Isolated Rat Cerebral Small Arteries

We examined chronic effects of 17beta-estradiol (E(2)beta) on the responses of isolated rat anterior cerebral small arteries to vasoactive substances with special reference to endothelial function. Female Sprague-Dawley rats were separated into four groups: (1) sham-operated group (Sham), (2) sham-operated plus E(2)beta treated group (Sham+E), (3) ovariectomized group (OVX), (4) ovariectomized plus E(2)beta treated group (OVX+E). 5-Hydroxytryptamine (5-HT) (10(-10)-10(-3) M) and U46619 (10(-15)-10(-8) M) induced concentration-dependent contractions in the cerebral small arteries. The 5-HT- and U46619-induced contractions were not affected by pretreatment with 3 x 10(-5) M N(omega)-nitro-L-arginine methyl ester (L-NAME). No significant difference in high potassium (80 mM)- and the agonists-mediated contractions was observed among the four groups. Administration of acetylcholine (ACh) (10(-9)-10(-3) M) and sodium nitroprusside (SNP) (10(-8)-10(-3) M) caused dose-related relaxations in the cerebral small arteries precontracted by 10(-8) M U46619. Chronic treatment with E(2)beta caused a significant potentiation of the ACh-induced relaxations in the Sham+E and OVX+E groups. The dose-response curve for ACh in the OVX group was quite similar to that obtained with the Sham group. The ACh-induced relaxation was reduced significantly by pretreatment with 3 x 10(-5) M L-NAME, and an additional treatment with 10(-3) M L-arginine reversed significantly the L-NAME-induced inhibition. The removal of endothelial cells produced a significant reduction of the ACh-induced relaxation. Indomethacin (10(-5) M) did not alter the ACh-induced relaxation. The findings suggest that E(2)beta potentiates ACh-induced endothelium-dependent relaxation in rat anterior cerebral arteries and that the potentiation may be, in part, mediated by increasing production and release of endogenous NO from the endothelial cells.

Comparison of vasodilatory properties of 14,15-EET analogs: structural requirements for dilation

Epoxyeicosatrienoic acids (EETs) are endothelium-derived eicosanoids that activate potassium channels, hyperpolarize the membrane, and cause relaxation. We tested 19 analogs of 14,15-EET on vascular tone to determine the structural features required for activity. 14,15-EET relaxed bovine coronary arterial rings in a concentration-related manner (ED(50) = 10(-6) M). Changing the carboxyl to an alcohol eliminated dilator activity, whereas 14,15-EET-methyl ester and 14,15-EET-methylsulfonimide retained full activity. Shortening the distance between the carboxyl and epoxy groups reduced the agonist potency and activity. Removal of all three double bonds decreased potency. An analog with a Delta8 double bond had full activity and potency. However, the analogs with only a Delta5 or Delta11 double bond had reduced potency. Conversion of the epoxy oxygen to a sulfur or nitrogen resulted in loss of activity. 14(S),15(R)-EET was more potent than 14(R),15(S)-EET, and 14,15-(cis)-EET was more potent than 14,15-(trans)-EET. These studies indicate that the structural features of 14,15-EET required for relaxation of the bovine coronary artery include a carbon-1 acidic group, a Delta8 double bond, and a 14(S),15(R)-(cis)-epoxy group.

Adenoviral-mediated overexpression of catalase inhibits endothelial cell proliferation

Although hydrogen peroxide (H(2)O(2)) induces proliferation of vascular smooth muscle cells, its role in endothelial cell proliferation is unclear. Our aim was to study the role of hydrogen peroxide in endothelial cell proliferation by overexpressing catalase. Human aortic endothelial cells were transduced with adenoviral vectors encoding beta-galactosidase (Adbetagal) or catalase (AdCat) or were exposed to diluent alone (control). Transgene expression was demonstrated by beta-galactosidase staining, Western analysis, and significantly increased enzyme activity in AdCat-transduced cells. Overexpression of catalase decreased DNA synthesis in AdCat compared with control and Adbetagal-transduced cells (536.8 +/- 31 vs. 1,875.1 +/- 132.9 vs. 1,347.5 +/- 93.7 dpm/well, respectively; P < 0.05 vs. control and Adbetagal). Six days after transduction with AdCat (multiplicity of infection = 50), cell numbers were significantly reduced (AdCat: 38 +/- 1.8% of cell counts in control, P < 0.05; and 45 +/- 2% of cell count in Adbetagal, P < 0.05). Incubation with aminotriazole 10 mmol/l, an inhibitor of catalase, prevented this effect. The number of apoptotic cells was increased one- and threefold 2 and 4 days, respectively, after transduction with AdCat. Exogenous administration of low concentrations of H(2)O(2) (50 microM) significantly increased cell proliferation, whereas it was inhibited by higher concentrations. These results suggest that H(2)O(2) is an important modulator of endothelial cell proliferation.

The nitration of proteins in platelets: significance in platelet function

Exogenous peroxynitrite has been shown to inhibit or activate platelets according to the concentration added and, at the same time, nitrate platelet proteins. Here, recent evidence is discussed which indicates that nitration of proteins may also occur during normal platelet activation by collagen, by mechanical stimulation during isolation and by exposure to low levels of hydrogen peroxide. Furthermore, this nitration appears to be transient. The implications of these findings are discussed in terms of platelet biology and cell signaling processes.

Critical role of NADPH oxidase-derived reactive oxygen species in generating Ca2+ oscillations in human aortic endothelial cells stimulated by histamine

There is increasing evidence that intracellular reactive oxygen species (ROS) play a role in cell signaling and that the NADPH oxidase is a major source of ROS in endothelial cells. At low concentrations, agonist stimulation of membrane receptors generates intracellular ROS and repetitive oscillations of intracellular Ca(2+) concentration ([Ca(2+)](i)) in human endothelial cells. The present study was performed to examine whether ROS are important in the generation or maintenance of [Ca(2+)](i) oscillations in human aortic endothelial cells (HAEC) stimulated by histamine. Histamine (1 microm) increased the fluorescence of 2',7'-dihydrodichlorofluorescin diacetate in HAEC, an indicator of ROS production. This was partially inhibited by the NADPH oxidase inhibitor diphenyleneiodonium (DPI, 10 microm), by the farnesyltransferase inhibitor H-Ampamb-Phe-Met-OH (2 microm), and in HAEC transiently expressing Rac1(N17), a dominant negative allele of the protein Rac1, which is essential for NADPH oxidase activity. In indo 1-loaded HAEC, 1 microm histamine triggered [Ca(2+)](i) oscillations that were blocked by DPI or H-Ampamb-Phe-Met-OH. Histamine-stimulated [Ca(2+)](i) oscillations were not observed in HAEC lacking functional Rac1 protein but were observed when transfected cells were simultaneously exposed to a low concentration of hydrogen peroxide (10 microm), which by itself did not alter either [Ca(2+)](i) or levels of inositol 1,4,5-trisphosphate (Ins-1,4,5-P(3)). Thus, histamine generates ROS in HAEC at least partially via NADPH oxidase activation. NADPH oxidase-derived ROS are critical to the generation of [Ca(2+)](i) oscillations in HAEC during histamine stimulation, perhaps by increasing the sensitivity of the endoplasmic reticulum to Ins-1,4,5-P(3).

Inhibition of renin-angiotensin system ameliorates endothelial dysfunction associated with aging in rats

OBJECTIVE

Endothelial vasodilator functions are progressively impaired with aging, which may account in part for the increased incidence of cardiovascular events in elderly people. We examined what treatment could ameliorate the endothelial dysfunction associated with aging in rats.

METHODS AND RESULTS

Aged (12-month-old) Wistar-Kyoto rats were treated with vehicle, temocapril, CS-866 (an angiotensin II type 1 receptor antagonist), cerivastatin, or hydralazine for 2 weeks. Endothelium-dependent relaxations (EDRs) of aortas from aged rats were markedly impaired compared with EDRs of aortas from young (3-month-old) rats. Indomethacin, NS-398 (a cyclooxygenase [COX]-2 inhibitor), and SQ-29548 (a thromboxane A2/prostaglandin H2 receptor antagonist) acutely restored EDRs in aged rats, suggesting an involvement of COX-2-derived vasoconstricting eicosanoids. Tiron, a superoxide scavenger, also partially improved EDRs, suggesting an involvement of superoxide. EDRs were significantly ameliorated in aged rats after long-term treatment with temocapril or CS-866 but not after treatment with cerivastatin or hydralazine. Indomethacin induced no further improvement of EDRs after treatment with temocapril or CS-866. COX-2 protein expression and superoxide production were increased in the aortas of aged rats and were also attenuated by treatment with temocapril or CS-866.

CONCLUSIONS

These results demonstrate that long-term inhibition of the renin-angiotensin system ameliorates endothelial dysfunction associated with aging through the inhibition of the synthesis of COX-2-derived vasoconstricting factors and superoxide anions.

Renal damage mediated by oxidative stress: a hypothesis of protective effects of red wine

Over the last decade, oxidative stress has been implicated in the pathogenesis of a wide variety of seemingly unrelated renal diseases. Epidemiological studies have documented an association of moderate wine consumption with a decreased risk of cardiovascular and neurological diseases; however, similar studies in the kidney are still lacking. The kidney is an organ highly vulnerable to damage caused by reactive oxygen species (ROS), likely due to the abundance of polyunsaturated fatty acids in the composition of renal lipids. ROS are involved in the pathogenic mechanism of conditions such as glomerulosclerosis and tubulointerstitial fibrosis. The health benefits of moderate consumption of red wine can be partly attributed to its antioxidant properties. Indeed, the kidney antioxidant defense system is enhanced after chronic exposure to moderate amounts of wine, a response arising from the combined effects of ethanol and the nonalcoholic components, mainly polyphenols. Polyphenols behave as potent ROS scavengers and metal chelators; ethanol, in turn, modulates the activity of antioxidant enzymes. Therefore, a hypothesis that red wine causes a decreased vulnerability of the kidney to the oxidative challenges could be proposed. This view is partly supported by direct evidences indicating that wine and antioxidants isolated from red wine, as well as other antioxidants, significantly attenuate or prevent the oxidative damage to the kidney. The present hypothesis paper provides a collective body of evidence suggesting a protective role of moderate wine consumption against the production and progression of renal diseases, based on the existing concepts on the pathophysiology of kidney injury mediated by oxidative stress.

Critical role of gap junctions in endothelium-dependent hyperpolarization in rat mesenteric arteries

1. Acetylcholine (ACh) evokes endothelium-dependent hyperpolarization in arterial cells, presumably through endothelium-derived hyperpolarizing factor (EDHF). The identity of EDHF is still elusive; however, several recent studies suggest the possible involvement of myoendothelial gap junctions in the EDHF response. 2. To elucidate the role of gap junctions in endothelium-dependent hyperpolarization, we examined the effects of the gap junction inhibitors 18 alpha-glycyrrhetinic acid (18 alpha-GA; 10(-4) mol/L) and carbenoxolone (3 x 10(-4) mol/L), a water-soluble form of 18 beta-GA, on hyperpolarization and relaxation to ACh in rat proximal and distal mesenteric arteries. Experiments were performed in the presence of indomethacin (10(-5) mol/L) and N(G)-nitro-L-arginine (10(-4) mol/L). 3. In both proximal and distal mesenteric arteries, ACh-induced hyperpolarization and relaxation were partially inhibited by 18 alpha-GA and abolished by carbenoxolone. 4. Endothelium-independent hyperpolarization to levcromakalim, an ATP-sensitive K+ channel opener, were unaffected by 18 alpha-GA or carbenoxolone in both arteries. 5. Relaxations to levcromakalim were unaffected by 18 alpha-GA, but were inhibited somewhat by carbenoxolone in proximal mesenteric arteries. 6. These findings suggest that myoendothelial gap junctions play a critical role in EDHF-mediated responses in both proximal and distal mesenteric arteries of the rat.

Hydrogen peroxide acts as an EDHF in the piglet pial vasculature in response to bradykinin

We investigated the mechanism of EDHF-mediated dilation to bradykinin (BK) in piglet pial arteries. Topically applied BK (3 micromol/l) induced vasodilation (62 +/- 12%) after the administration of N(omega)-nitro-L-arginine methyl ester (L-NAME) and indomethacin, which was inhibited by endothelial impairment or by the BK(2) receptor antagonist HOE-140 (0.3 micromol/l). Western blotting showed the presence of BK(2) receptors in brain cortex and pial vascular tissue samples. The cytochrome P-450 antagonist miconazole (20 micromol/l) and the lipoxygenase inhibitors baicalein (10 micromol/l) and cinnamyl-3,4-dyhydroxy-alpha-cyanocinnamate (1 micromol/l) failed to reduce the BK-induced dilation. However, the H(2)O(2) scavenger catalase (400 U/ml) abolished the response (from 54 +/- 11 to 0 +/- 2 microm; P < 0.01). The ATP-dependent K(+) (K(ATP)) channel inhibitor glibenclamide (10 micromol/l) had a similar effect as well (from 54 +/- 11 to 16 +/- 5 microm; P < 0.05). Coapplication of the Ca(2+)-dependent K(+) channel inhibitors charybdotoxin (0.1 micromol/l) and apamin (0.5 micromol/l) failed to reduce the response. We conclude that H(2)O(2) mediates the non-nitric oxide-, non-prostanoid-dependent vasorelaxation to BK in the piglet pial vasculature. The response is mediated via BK(2) receptors and the opening of K(ATP) channels.

Leukemia inhibitory factor relaxes arteries through endothelium-dependent mechanism

Leukemia inhibitory factor (LIF) is a cytokine, which inhibits angiogenesis and decreases endothelial cell proliferation and migration, suggesting that LIF may modulate vascular tone. In this study, we examined the effects of LIF on the tone of rat arteries. The isometric tension of ring preparations from rat superior mesenteric arteries was continuously measured. LIF relaxed the mesenteric arteries in a dose-dependent manner, when the arterial rings were precontracted with phenylephrine. The relaxation was totally inhibited by mechanical removal of endothelium. N(G)-nitro-L-arginine methyl ester did not affect the relaxation by LIF. Ca(2+)-dependent K channel (KCa) blockers, apamin with charybdotoxin, inhibited the relaxation by LIF. Catalase, an enzyme which scavenges hydrogen peroxide, also inhibited the relaxation by LIF. Endothelium-derived hyperpolarizing factor relaxes smooth muscle cells and the effect is blocked by KCa and catalase. Our results suggest that LIF regulates vascular tone through the effect of this factor.

Role of calcium-activated potassium channels in impaired acetylcholine vasodilatory responses in diabetic rats

Muscarinic agonists produce endothelium-dependent vasodilatation in the presence of nitric oxide synthase (NOS) inhibition. The importance of this mechanism was assessed in the methoxamine-preconstricted perfused mesenteric vascular bed (MVB) of streptozotocin diabetic Sprague-Dawley rats. At 9 weeks of age, male rats were treated with streptozotocin (55 mg/kg in citrate buffer) or with citrate buffer alone. The superior mesenteric artery was cannulated and the MVB was detached from its intestinal borders. Concentration-response curves to acetylcholine were determined in the presence and in the absence of indomethacin, tetrabutylammonium (a calcium-activated potassium channel blocker), high extracellular potassium, or NOS inhibition with Nomega-nitro-l-arginine and l-NG-nitro-l-arginine. There was a rightward shift in the concentration-response curve with an increase in median inhibitory concentration (p < 0.05) and a reduction in acetylcholine IMAX (p < 0.05) values in 14-week streptozotocin rats. The ability of NOS inhibition to attenuate vasodilatation was reduced in the 14-week streptozotocin group relative to the 2-week streptozotocin treatment group (p < 0.05). However, the ability of tetrabutylammonium to block acetylcholine-mediated vasodilatation remained consistent in streptozotocin rats at both stages. The results demonstrate that an alternate pathway involving calcium-activated potassium channels may compensate for diminished nitric oxide bioactivity. This effect is contingent on the duration of diabetes. This study provides insight into the development and progression of altered diabetic vascular responses.

14,15-Dihydroxyeicosatrienoic acid relaxes bovine coronary arteries by activation of K(Ca) channels

Epoxyeicosatrienoic acids (EETs) cause vascular relaxation by activating smooth muscle large conductance Ca(2+)-activated K(+) (K(Ca)) channels. EETs are metabolized to dihydroxyeicosatrienoic acids (DHETs) by epoxide hydrolase. We examined the contribution of 14,15-DHET to 14,15-EET-induced relaxations and characterized its mechanism of action. 14,15-DHET relaxed U-46619-precontracted bovine coronary artery rings but was approximately fivefold less potent than 14,15-EET. The relaxations were inhibited by charybdotoxin, iberiotoxin, and increasing extracellular K(+) to 20 mM. In isolated smooth muscle cells, 14,15-DHET increased an iberiotoxin-sensitive, outward K(+) current and increased K(Ca) channel activity in cell-attached patches and inside-out patches only when GTP was present. 14,15-[(14)C]EET methyl ester (Me) was converted to 14,15-[(14)C]DHET-Me, 14,15-[(14)C]DHET, and 14,15-[(14)C]EET by coronary arterial rings and endothelial cells but not by smooth muscle cells. The metabolism to 14,15-DHET was inhibited by the epoxide hydrolase inhibitors 4-phenylchalcone oxide (4-PCO) and BIRD-0826. Neither inhibitor altered relaxations to acetylcholine, whereas relaxations to 14,15-EET-Me were increased slightly by BIRD-0826 but not by 4-PCO. 14,15-DHET relaxes coronary arteries through activation of K(Ca) channels. Endothelial cells, but not smooth muscle cells, convert EETs to DHETs, and this conversion results in a loss of vasodilator activity.

The pathogenesis of diabetic retinopathy: old concepts and new questions

Hyperglycaemia appears to be a critical factor in the aetiology of diabetic retinopathy and initiates downstream events including: basement membrane thickening, pericyte drop out and retinal capillary non-perfusion. More recently, focus has been directed to the molecular basis of the disease process in diabetic retinopathy. Of particular importance in the development and progression of diabetic retinopathy is the role of growth factors (eg vascular endothelial growth factor, placenta growth factor and pigment epithelium-derived factor) together with specific receptors and obligate components of the signal transduction pathway needed to support them. Despite these advances there are still a number of important questions that remain to be answered before we can confidently target pathological signals. How does hyperglycaemia regulate retinal vessels? Which growth factors are most important and at what stage of retinopathy do they operate? What is the preferred point in the growth factor signalling cascade for therapeutic intervention? Answers to these questions will provide the basis for new therapeutic interventions in a debilitating ocular condition.

Ascorbate blocks endothelium-derived hyperpolarizing factor (EDHF)-mediated vasodilatation in the bovine ciliary vascular bed and rat mesentery

1. The effects of ascorbate were assessed on vasodilatation mediated by endothelium-derived hyperpolarizing factor (EDHF) in the ciliary vascular bed of the bovine isolated perfused eye and in the rat isolated perfused mesenteric arterial bed. 2. In the bovine eye, EDHF-mediated vasodilator responses induced by acetylcholine or bradykinin were powerfully blocked when ascorbate (50 microM) was included in the perfusion medium for at least 120 min; with acetylcholine a normally-masked muscarinic vasoconstrictor response was also uncovered. 3. The blockade of EDHF-mediated vasodilatation by ascorbate was time-dependent (maximum blockade at 120 min) and concentration-dependent (10 - 150 microM). 4. Ascorbate (50 microM) also blocked acetylcholine-induced, EDHF-mediated vasodilator responses in the rat mesenteric arterial bed in a time-dependent manner (maximum blockade at 180 min). 5. The ability of ascorbate to block EDHF-mediated vasodilatation is likely to result from its reducing properties, since this action was mimicked in the bovine eye by two other reducing agents, namely, N-acetyl-L-cysteine (1 mM) and dithiothreitol (100 microM), but not by the redox-inactive analogue, dehydroascorbate (50 microM). 6. In conclusion, concentrations of ascorbate present in normal plasma block EDHF-mediated vasodilator responses in the bovine eye and rat mesentery. The mechanism and physiological consequences of this blockade remain to be determined.

PECAM-1 (CD31) regulates a hydrogen peroxide-activated nonselective cation channel in endothelial cells

Hydrogen peroxide (H2O2) released by neutrophils is an important mediator of endothelial cell (EC) injury and vascular inflammation via its effect on EC-free Ca2+, [Ca2+]i. Although the underlying mechanisms are not well understood, platelet endothelial cell adhesion molecule (PECAM)-1/CD-31 is a critical modulator of neutrophil-EC transmigration. PECAM-1 is also known to regulate EC calcium signals and to undergo selective tyrosine phosphorylation. Here, we report that PECAM-1 molecules transduce EC responses to hydrogen peroxide. In human umbilical vein EC and REN cells (a PECAM-1-negative EC-like cell line) stably transfected with PECAM-1 (RHP), noncytolytic H2O2 exposure (100-200 microM H2O2) activated a calcium-permeant, nonselective cation current, and a transient rise in [Ca2+]i of similar time course. Neither response was observed in untransfected REN cells, and H2O2-evoked cation current was ablated in REN cells transfected with PECAM-1 constructs mutated in the cytoplasmic tyrosine-containing domain. The PECAM-dependent H2O2 current was inhibited by dialysis of anti-PECAM-1 cytoplasmic domain antibodies, required Src family tyrosine kinase activity, was independent of inositol trisphosphate receptor activation, and required only an intact PECAM-1 cytoplasmic domain. PECAM-1-dependent H2O2 currents and associated [Ca2+]i transients may play a significant role in regulating neutrophil-endothelial interaction, as well as in oxidant-mediated endothelial response and injury.

Estradiol alters nitric oxide production in the mouse aorta through the alpha-, but not beta-, estrogen receptor

Although estradiol (E(2)) has been recognized to exert several vasculoprotective effects in several species, its effects in mouse vasomotion are unknown, and consequently, so is the estrogen receptor subtype mediating these effects. We investigated the effect of E(2) (80 microg/kg/day for 15 days) on NO production in the thoracic aorta of ovariectomized C57Bl/6 mice compared with those given placebo. E(2) increased basal NO production. In contrast, the relaxation in response to ATP, to the calcium ionophore A23187, and to sodium nitroprusside was unaltered by E(2), whereas acetylcholine-elicited relaxation was decreased. The abundance of NO synthase I, II, and III immunoreactive proteins (using Western blot) in thoracic aorta homogenates was unchanged by E(2). To determine the estrogen receptor (ER) subtype involved in these effects, transgenic mice in which either the ERalpha or ERbeta has been disrupted were ovariectomized and treated, or not, with E(2). Basal NO production was increased and the sensitivity to acetylcholine decreased in ERbeta knockout mice in response to E(2), whereas this effect was abolished in ERalpha knockout mice. Finally, these effects of E(2) on vasomotion required long-term and/or in vivo exposure, as short-term incubation of aortic rings with 10 nmol/L E(2) in the isolated organ chamber did not elicit any vasoactive effects. In conclusion, this study demonstrates that ERalpha, but not ERbeta, mediates the beneficial effect of E(2) on basal NO production.

Iron chelation and hydroxyl radical scavenging reduce the inflammatory response of endothelial cells after infection with Chlamydia pneumoniae or influenza A

BACKGROUND

Chronic low-grade inflammation is associated with increased risk of vascular diseases. The source of inflammation is unknown but may well be chronic and/or repetitive infections with microorganisms. Direct infection of endothelial cells (ECs) may also be a starting point for atherogenesis by initiating endothelial procoagulant activity, increased monocyte adherence and increased cytokine production. We hypothesized that iron-mediated intracellular hydroxyl radical formation after infection is a key event in triggering the production of interleukin-6 (IL-6) by ECs in vitro.

METHODS

Cultured ECs were incubated with Fe(II) and Fe(III) or infected with Chlamydia pneumoniae or influenza A/H1N1/Taiwan/1/81 for 48 and 24 h, respectively. To determine the role of iron and reactive oxygen species, cells were coincubated with the H2O2 scavenger N-acetyl-l-cysteine, with the iron chelator deferoxamine (DFO) or with the intracellular hydroxyl radical scavenger dimethylthiourea (DMTU). After the incubation periods, supernatants were harvested for IL-6 determination.

RESULTS

Incubating ECs with Fe(II) and Fe(III) resulted in increased IL-6 production. Similarly, infection with C. pneumoniae and influenza A also induced an IL-6 response. Coincubating ECs with DFO or DMTU blocked this response. Nuclear factor-kappaB activity was increased after infection and blocked by coincubation with DFO or DMTU.

CONCLUSION

Cultured ECs respond to infection and iron incubation with increased production of IL-6. Iron, the generation of intracellular hydroxyl radical and NF-kappaB activity are essential in cellular activation, suggesting that reactive oxygen species generated in the Haber-Weiss reaction are essential in invoking an immunological response to infection by ECs.

What is new in endothelium-derived hyperpolarizing factors?

The chemical identification and functional characterization of endothelium-derived hyperpolarizing factors varies depending on vascular size, vascular bed and species. Three major candidates are the epoxyeicosatrienoic acids, cytochrome P450 metabolites of arachidonic acid, potassium ion and hydrogen peroxide. Additionally, electrical coupling through myoendothelial gap junctions serves to conduct electrical changes from the endothelium to the smooth muscle and may mediate or propagate hyperpolarization. Endothelium-derived hyperpolarizing factors are important mediators of vascular relaxation most specifically in resistance sized arteries where they regulate tissue blood flow. The release of the factors is modulated by a number of influences including agonist stimulation, shear stress, estrogen and disease. This article reviews the latest studies concerning the characterization of endothelium-derived hyperpolarizing factors, the mechanisms of factor release and alterations of the factors.

Hydrogen peroxide activates endothelial nitric-oxide synthase through coordinated phosphorylation and dephosphorylation via a phosphoinositide 3-kinase-dependent signaling pathway

Endothelial nitric-oxide synthase (eNOS) is an important component of vascular homeostasis. During vascular disease, endothelial cells are exposed to excess reactive oxygen species that can alter cellular phenotype by inducing various signaling pathways. In the current study, we examined the implications of H(2)O(2)-induced signaling for eNOS phosphorylation status and activity in porcine aortic endothelial cells. We found that H(2)O(2) treatment enhanced eNOS activity and NO bioactivity as determined by the conversion of l-[(3)H]arginine to l-[(3)H]citrulline and cellular cGMP content. Concomitant with eNOS activation, H(2)O(2) also activated Akt, increased eNOS phosphorylation at Ser-1177, and decreased eNOS phosphorylation at Thr-495. H(2)O(2)-induced promotion of eNOS activity and modulation of the eNOS phosphorylation status at Ser-1177 and Thr-495 were significantly attenuated by selective inhibitors of Src kinase, the ErbB receptor family, and phosphoinositide 3-kinase (PI 3-K). We found that Akt activation, eNOS Ser-1177 phosphorylation, and eNOS activation by H(2)O(2) were calcium-dependent, whereas eNOS dephosphorylation at Thr-495 was not, suggesting a branch point in the signaling cascade downstream from PI 3-K. Consistent with this, overexpression of a dominant negative isoform of Akt inhibited H(2)O(2)-induced phosphorylation of eNOS at Ser-1177 but not dephosphorylation of eNOS at Thr-495. Together, these data indicate that H(2)O(2) promotes calcium-dependent eNOS activity through a coordinated change in the phosphorylation status of the enzyme mediated by Src- and ErbB receptor-dependent PI 3-K activation. In turn, PI 3-K mediates eNOS Ser-1177 phosphorylation via a calcium- and Akt-dependent pathway, whereas eNOS Thr-495 dephosphorylation does not involve calcium or Akt. This response may represent an attempt by endothelial cells to maintain NO bioactivity under conditions of enhanced oxidative stress.

Reactive oxygen species as mediators in asthma

This review describes production and effects of reactive oxygen species (ROS) on airway function. ROS are important in many physiological processes but can also have detrimental effects on airway cells and tissues when produced in high quantities or during the absence of sufficient amounts of anti-oxidants. Therefore, these mediators play a prominent role in the pathogenesis of various inflammatory airway disorders, including asthma. Effects of ROS on airway function in asthma have been studied with isolated airway cells and tissues and with animal models and patients. With the use of inhibitors, transgenic animals and measurements of the release of ROS within the airways, it became clear that oxidative stress contributes to the initiation and worsening of inflammatory respiratory disorders.

Oxygen species in the microvascular environment: regulation of vascular tone and the development of hypertension

Derangements of the three endothelium-related vasodilator systems (prostaglandins, endothelium-derived hyperpolarizing factor(s) and nitric oxide) cause the endothelial dysfunction observed in hypertension. Free radical-induced nitric oxide degradation plays a crucial role in hypertension. An increase in superoxide producing enzymes such as NAD(P)H oxidase and xanthine oxidase has been demonstrated. Superoxide dismutase may correct endothelial dysfunction in vitro and superoxide dismutase mimetics can lower blood pressure in experimental animals. Antioxidant agents and xanthine oxidase-inhibiting compounds have been used in humans. In addition, the synthesis of vasoconstrictor peroxides derived from the activity of cyclooxygenase in the endothelium and the vascular smooth muscle is stimulated by the OH. radical. Hydrogen peroxide levels are augmented in hypertension, but its role is unclear because recent investigations have shown that this substance may act as a hyperpolarizing factor. It is thought that the therapeutic benefit of anti-hypertensive drugs, such as calcium antagonists and angiotensin-converting enzyme inhibitors, could be in part due to an inhibition of free radical production. A role of superoxide in the endothelial dysfunction and hypertension of chronic renal failure has also been suggested by recent animal experiments.

Are endothelium-derived hyperpolarizing and contracting factors isoprostanes?

The endothelium releases many vasoactive substances, including prostacyclin, nitric oxide and endothelin, in addition to several other factors about which little is known. The latter are referred to as 'endothelium-derived hyperpolarizing factors' (EDHFs) and 'endothelium-derived contracting factors' (EDCFs). Although there is much debate about the identities of EDHFs and EDCFs, a prevailing hypothesis is that they are cyclooxygenase-independent metabolites of arachidonic acid and many researchers associate them with free radicals. These properties are shared with isoprostanes. In this article, I compare the properties of EDHFs and EDCFs with those of the isoprostanes and propose novel experiments that might identify isoprostanes as candidate molecules for EDHFs and EDCFs.

Hydrogen peroxide is an endothelium-derived hyperpolarizing factor in human mesenteric arteries

The endothelium plays an important role in maintaining vascular homeostasis by synthesizing and releasing several vasodilating factors, including prostacyclin, nitric oxide, and endothelium-derived hyperpolarizing factor (EDHF). We have recently identified that endothelium-derived hydrogen peroxide (H(2)O(2)) is an EDHF in mice. The present study was designed to examine whether this is also the case in humans. Bradykinin elicited endothelium-dependent relaxations and hyperpolarizations in the presence of indomethacin and N(omega)-nitro-l-arginine, which thus were attributed to EDHF, in human mesenteric arteries. The EDHF-mediated relaxations were significantly inhibited by catalase, an enzyme that specifically decomposes H(2)O(2), whereas catalase did not affect endothelium-independent hyperpolarizations to levcromakalim. Exogenous H(2)O(2) elicited relaxations and hyperpolarizations in endothelium-stripped arteries. Gap junction inhibitor 18alpha-glycyrrhetinic acid partially inhibited, whereas inhibitors of cytochrome P450 did not affect the EDHF-mediated relaxations. These results indicate that H(2)O(2) is also a primary EDHF in human mesenteric arteries with some contribution of gap junctions.

Hydrogen peroxide induces a greater contraction in mesenteric arteries of spontaneously hypertensive rats through thromboxane A(2) production

1. Hydrogen peroxide (H(2)O(2)) caused a transient contraction in endothelium-intact (E+) and -denuded (E-) mesenteric arteries (MA) from 8 - 10-month-old spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY) in a concentration-dependent manner (10(-5) M to 10(-3) M). 2. The contraction to H(2)O(2) in MA (E+ or E-) was greater in SHR than in WKY. Removal of endothelium potentiated the contraction to H(2)O(2) in WKY but not in SHR. Tachyphylaxis to H(2)O(2) was less prominent in SHR than in WKY. 3. The contraction of aorta to H(2)O(2) (5 x 10(-4) M), expressed as a percentage of 80 mM KCl-induced contraction, was approximately half of that found in the MA. A greater contraction was found in E+ but not E- SHR aortic rings. 4. The contraction of MA to H(2)O(2) (5 x 10(-4) M) was greatly inhibited by SQ 29548 and ICI 192605 (thromboxane A(2) (TXA(2))/prostaglandin H(2) receptor antagonists), quinacrine (a phospholipase A(2) (PLA(2)) inhibitor), indomethacin and diclofenac (cyclooxygenase (COX) inhibitors), and furegrelate (a TXA(2) synthase inhibitor). 5. Production of thromboxane B(2) induced by H(2)O(2) (5 x 10(-4) M) was greater in SHR MA than in WKY, and was inhibited by quinacrine, indomethacin and diclofenac, and furegrelate, but not by SQ 29584 and ICI 192605. 6. These results suggested (1) that SHR MA exhibits a higher contraction involving an increased smooth muscle reactivity and less tachyphylaxis to H(2)O(2) than WKY; (2) that a greater production of TXA(2) through activation of PLA(2)-COX-TXA(2) synthase pathway appeared to be responsible for the enhanced contraction in SHR MA. The enhanced vascular response to H(2)O(2) may be related to hypertension in SHR.

Interactions of nitric oxide-derived reactive nitrogen species with peroxidases and lipoxygenases

Nitric oxide (NO) is a major free radical modulator of smooth muscle tone, which under basal conditions acts to preserve vascular homeostasis through its anti-inflammatory properties. The biochemistry of NO, in particular, its rapid conversion in vivo into secondary reactive nitrogen species (RNS), its chemical nature as a free radical and its high diffusibility and hydrophobicity dictate that this species will interact with numerous biomolecules and enzymes. In this review, we consider the interactions of a number of enzymes found in the vasculature with NO and NO-derived RNS. All these enzymes are either homeostatic or promote the development of atherosclerosis and hypertension. Therefore their interactions with NO and NO-derived RNS will be of central importance in the initiation and progression of vascular disease. In some examples, (e.g. lipoxygenase, LOX), such interactions provide catalytic 'sinks' for NO, but for others, in particular peroxidases and prostaglandin H synthase (PGHS), reactions with NO may be detrimental. Nitric oxide and NO-derived RNS directly modulate the activity of vascular peroxidases and LOXs through a combination of effects, including transcriptional regulation, altering substrate availability, and direct reaction with enzyme turnover intermediates. Therefore, these interactions will have two major consequences: (i) depletion of NO levels available to cause vasorelaxation and prevent leukocyte/platelet adhesion and (ii) modulation of activity of the target enzymes, thereby altering the generation of bioactive signaling molecules involved in maintenance of vascular homeostasis, including prostaglandins and leukotrienes.