Inhibition of the gap junctional component of endothelium-dependent relaxations in rabbit iliac artery by 18-alpha glycyrrhetinic acid

Connexin32 ameliorates renal fibrosis in diabetic mice by promoting K48-linked NADPH oxidase 4 polyubiquitination and degradation

BACKGROUND AND PURPOSE

Nox4 is the major isoform of NADPH oxidase found in the kidney and contributes to the pathogenesis of diabetic nephropathy. However, the molecular mechanisms of increased Nox4 expression induced by hyperglycaemia remain to be elucidated. Here, the role of the connexin32-Nox4 signalling axis in diabetic nephropathy and its related mechanisms were investigated.

EXPERIMENTAL APPROACH

Diabetes was induced in mice by low-dose streptozotocin (STZ) combined with a high-fat diet. Effects of connexin32 on Nox4 expression and on renal function and fibrosis in STZ-induced diabetic mice were investigated using adenovirus-overexpressing connexin32 and connexin32-deficient mice. Interactions between connexin32 and Nox4 were analysed by co-immunoprecipitation and immunofluorescence assays.

KEY RESULTS

Connexin32 was down-regulated in the kidneys of STZ-induced diabetic mice. Overexpression of connexin32 reduced expression of Nox4 and improved renal function and fibrosis in diabetic mice, whereas connexin32 deficiency had opposite effects. Down-regulation of fibronectin expression by connexin32 was not dependent on gap junctional intercellular communication involving connexin32. Connexin32 interacted with Nox4 and reduced the generation of hydrogen peroxide, leading to the down-regulation of fibronectin expression. Mechanistically, connexin32 decreased Nox4 expression by promoting its K48-linked polyubiquitination. Interestingly, Smurf1 overexpression inhibited K48-linked polyubiquitination of Nox4. Furthermore, connexin32 interacted with Smurf1 and inhibited its expression.

CONCLUSION AND IMPLICATIONS

Connexin32 ameliorated renal fibrosis in diabetic mice by promoting K48-linked Nox4 polyubiquitination and degradation via inhibition of Smurf1 expression. Targeting the connexin32-Nox4 signalling axis may contribute to the development of novel treatments for diabetic nephropathy.

Arachidonic acid metabolites and endothelial dysfunction of portal hypertension

Increased resistance to portal flow and increased portal inflow due to mesenteric vasodilatation represent the main factors causing portal hypertension in cirrhosis. Endothelial cell dysfunction, defined as an imbalance between the synthesis, release, and effect of endothelial mediators of vascular tone, inflammation, thrombosis, and angiogenesis, plays a major role in the increase of resistance in portal circulation, in the decrease in the mesenteric one, in the development of collateral circulation. Reduced response to vasodilators in liver sinusoids and increased response in the mesenteric arterioles, and, viceversa, increased response to vasoconstrictors in the portal-sinusoidal circulation and decreased response in the mesenteric arterioles are also relevant to the pathophysiology of portal hypertension. Arachidonic acid (AA) metabolites through the three pathways, cyclooxygenase (COX), lipoxygenase, and cytochrome P450 monooxygenase and epoxygenase, are involved in endothelial dysfunction of portal hypertension. Increased thromboxane-A2 production by liver sinusoidal endothelial cells (LSECs) via increased COX-1 activity/expression, increased leukotriens, increased epoxyeicosatrienoic acids (EETs) (dilators of the peripheral arterial circulation, but vasoconstrictors of the portal-sinusoidal circulation), represent a major component in the increased portal resistance, in the decreased portal response to vasodilators and in the hyper-response to vasoconstrictors. Increased prostacyclin (PGI2) via COX-1 and COX-2 overexpression, and increased EETs/heme-oxygenase-1/K channels/gap junctions (endothelial derived hyperpolarizing factor system) play a major role in mesenteric vasodilatation, hyporeactivity to vasoconstrictors, and hyper-response to vasodilators. EETs, mediators of liver regeneration after hepatectomy and of angiogenesis, may play a role in the development of regenerative nodules and collateral circulation, through stimulation of vascular endothelial growth factor (VEGF) inside the liver and in the portal circulation. Pharmacological manipulation of AA metabolites may be beneficial for cirrhotic portal hypertension.

Endothelium-derived Relaxing Factors of Small Resistance Arteries in Hypertension

Endothelium-derived relaxing factors (EDRFs), including nitric oxide (NO), prostacyclin (PGI₂), and endothelium-derived hyperpolarizing factor (EDHF), play pivotal roles in regulating vascular tone. Reduced EDRFs cause impaired endothelium-dependent vasorelaxation, or endothelial dysfunction. Impaired endothelium-dependent vasorelaxation in response to acetylcholine (ACh) is consistently observed in conduit vessels in human patients and experimental animal models of hypertension. Because small resistance arteries are known to produce more than one type of EDRF, the mechanism(s) mediating endothelium-dependent vasorelaxation in small resistance arteries may be different from that observed in conduit vessels under hypertensive conditions, where vasorelaxation is mainly dependent on NO. EDHF has been described as one of the principal mediators of endothelium-dependent vasorelaxation in small resistance arteries in normotensive animals. Furthermore, EDHF appears to become the predominant endothelium-dependent vasorelaxation pathway when the endothelial NO synthase (NOS3)/NO pathway is absent, as in NOS3-knockout mice, whereas some studies have shown that the EDHF pathway is dysfunctional in experimental models of hypertension. This article reviews our current knowledge regarding EDRFs in small arteries under normotensive and hypertensive conditions.

Role of gap junctions modulating hepatic vascular tone in cirrhosis

BACKGROUND & AIMS

Gap junctions are formed by connexins (Cx), a family of proteins that couple endothelial and smooth muscle cells in systemic vessels. In this context, Cx allow the transmission of signals modulating vascular tone. Recently, vascular Cx have been observed in liver cells implicated in liver blood flow regulation. Here, we investigated the role of Cx in the regulation of intrahepatic vascular tone in cirrhosis.

METHODS

Livers of Sprague-Dawley control and cirrhotic (common bile duct ligation-CBDL and CCl4 ) rats were perfused, and concentration-effect curves in response to acetylcholine (ACh) precontracted with methoxamine were obtained in the presence of the specific Cx inhibitor 18-alpha-glycyrrhetinic acid or vehicle. Cx expression was assessed by immunofluorescence, western blot and reverse-transcription polymerase chain reaction in liver tissue, hepatic stellate cells, sinusoidal endothelial cells and hepatocytes isolated from control and cirrhotic rat livers. Cx protein expression was also determined in cirrhotic human tissue.

RESULTS

Gap junction blockade markedly attenuated relaxation of hepatic vasculature in response to ACh in control (maximal relaxation, -55 ± 10.5% vs. -95.3 ± 10% with vehicle; P < 0.01) and CBDL rats (50.9 ± 18.5% vs. -18.7 ± 5.5% with vehicle; P = 0.01). Livers from CBDL rats and patients with cirrhosis exhibited Cx overexpression. By contrast, CCl4 -cirrhotic rats did not show attenuated relaxation of hepatic vasculature after blockade and Cx expression was significantly lower than in controls.

CONCLUSIONS

Gap junctions may contribute to modulating portal pressure and intrahepatic vascular relaxation. Liver gap junctions may represent a new therapeutic target in cirrhotic portal hypertension.

Nitric oxide pretreatment enhances atheroma component highlighting in vivo with intercellular adhesion molecule-1-targeted echogenic liposomes

We present an ultrasound technique for the detection of inflammatory changes in developing atheromas. We used contrast-enhanced ultrasound imaging with (i) microbubbles targeted to intercellular adhesion molecule-1 (ICAM-1), a molecule of adhesion involved in inflammatory processes in lesions of atheromas in New Zealand White rabbits, and (ii) pretreatment with nitric oxide-loaded microbubbles and ultrasound activation at the site of the endothelium to enhance the permeability of the arterial wall and the penetration of ICAM-1-targeted microbubbles. This procedure increases acoustic enhancement 1.2-fold. Pretreatment with nitric oxide-loaded echogenic liposomes and ultrasound activation can potentially facilitate the subsequent penetration of targeted echogenic liposomes into the arterial wall, thus allowing improved detection of inflammatory changes in developing atheromas.

Traumatic brain injury in vivo and in vitro contributes to cerebral vascular dysfunction through impaired gap junction communication between vascular smooth muscle cells

Gap junctions (GJs) contribute to cerebral vasodilation, vasoconstriction, and, perhaps, to vascular compensatory mechanisms, such as autoregulation. To explore the effects of traumatic brain injury (TBI) on vascular GJ communication, we assessed GJ coupling in A7r5 vascular smooth muscle (VSM) cells subjected to rapid stretch injury (RSI) in vitro and VSM in middle cerebral arteries (MCAs) harvested from rats subjected to fluid percussion TBI in vivo. Intercellular communication was evaluated by measuring fluorescence recovery after photobleaching (FRAP). In VSM cells in vitro, FRAP increased significantly (p<0.05 vs. sham RSI) after mild RSI, but decreased significantly (p<0.05 vs. sham RSI) after moderate or severe RSI. FRAP decreased significantly (p<0.05 vs. sham RSI) 30 min and 2 h, but increased significantly (p<0.05 vs. sham RSI) 24 h after RSI. In MCAs harvested from rats 30 min after moderate TBI in vivo, FRAP was reduced significantly (p<0.05), compared to MCAs from rats after sham TBI. In VSM cells in vitro, pretreatment with the peroxynitrite (ONOO(-)) scavenger, 5,10,15,20-tetrakis(4-sulfonatophenyl)prophyrinato iron[III], prevented RSI-induced reductions in FRAP. In isolated MCAs from rats treated with the ONOO(-) scavenger, penicillamine, GJ coupling was not impaired by fluid percussion TBI. In addition, penicillamine treatment improved vasodilatory responses to reduced intravascular pressure in MCAs harvested from rats subjected to moderate fluid percussion TBI. These results indicate that TBI reduced GJ coupling in VSM cells in vitro and in vivo through mechanisms related to generation of the potent oxidant, ONOO(-).

Inhibitory effects of glycyrrhetinic Acid on the delayed rectifier potassium current in Guinea pig ventricular myocytes and HERG channel

Background. Licorice has long been used to treat many ailments including cardiovascular disorders in China. Recent studies have shown that the cardiac actions of licorice can be attributed to its active component, glycyrrhetinic acid (GA). However, the mechanism of action remains poorly understood. Aim. The effects of GA on the delayed rectifier potassium current (I_K), the rapidly activating (I_Kr) and slowly activating (I_Ks) components of I_K, and the HERG K⁺ channel expressed in HEK-293 cells were investigated. Materials and Methods. Single ventricular myocytes were isolated from guinea pig myocardium using enzymolysis. The wild type HERG gene was stably expressed in HEK293 cells. Whole-cell patch clamping was used to record I_K (I_Kr, I_Ks) and the HERG K⁺ current. Results. GA (1, 5, and 10 μM) inhibited I_K (I_Kr, I_Ks) and the HERG K⁺ current in a concentration-dependent manner. Conclusion. GA significantly inhibited the potassium currents in a dose- and voltage-dependent manner, suggesting that it exerts its antiarrhythmic action through the prolongation of APD and ERP owing to the inhibition of I_K (I_Kr, I_Ks) and HERG K⁺ channel.

Ecto-5'-nucleotidase, CD73, is an endothelium-derived hyperpolarizing factor synthase

OBJECTIVE

Adenosine dilates human coronary arteries by activating potassium channels in an endothelial cell-independent manner. Cell surface ecto-5'-nucleotidase (CD73) rapidly dephosphorylates extracellular adenosine 5'-monophosphate to adenosine. We tested the hypothesis that coronary vasodilation to adenine nucleotides is mediated by an endothelial CD73-dependent, extracellular production of adenosine that acts as an endothelium-derived hyperpolarizing factor.

METHODS AND RESULTS

Videomicroscopy showed that adenine nucleotides, but not inosine, potently dilated and hyperpolarized human coronary arteries independent of nitric oxide, prostacyclin, and classical endothelium-derived hyperpolarizing factors, whereas endothelial denudation, adenosine receptor antagonism, adenosine deaminase, or CD73 blockers reduced vasodilations. Liquid chromatography-electrospray ionization-mass spectrometry revealed adenosine accumulation in perfusates from arteries in the presence of adenosine 5'-diphosphate. CD73 was localized on the cell surface of endothelial cells, but not of vascular smooth muscle cells, and its deficiency suppressed vasodilation of mouse coronary arteries to adenine nucleotides and augmented vasodilation to adenosine. Adenosine dose-dependently dilated and hyperpolarized human coronary arteries to a similar extent as adenosine 5'-diphosphate.

CONCLUSIONS

Coronary vasodilation to adenine nucleotides is associated with endothelial CD73-dependent production of extracellular adenosine that acts as an endothelium-derived hyperpolarizing factor by relaxing and hyperpolarizing underlying vascular smooth muscle cells via activating adenosine receptors. Thus, CD73 is a novel endothelium-derived hyperpolarizing factor synthase in human and mouse coronary arteries.

Human endothelial progenitor cells induce extracellular signal-regulated kinase-dependent differentiation of mesenchymal stem cells into smooth muscle cells upon cocultivation

Neovascularization represents an important issue in tissue-engineering applications, since survival of implanted cells strongly relies on sufficient oxygen and nutrient supply. We have recently observed that human bone marrow-derived mesenchymal stem cells (MSCs) support neovessel formation originating from coimplanted endothelial cells (ECs) in vivo, suggesting that MSCs may function as perivascular cells by investing and stabilizing nascent EC-derived neovessels. In this study, we investigated EC-induced mural cell differentiation of MSCs in vitro. For this purpose, endothelial progenitor cells (EPCs) from two different origins, namely adult peripheral blood (pbEPCs) and neonatal cord blood (cbEPCs), or human umbilical vein endothelial cells (HUVECs), were cocultured with human MSCs to analyze the effect on MSC differentiation toward a smooth muscle cell (SMC)/pericyte phenotype. EPCs as well as HUVECs increased alpha-smooth muscle actin expression in MSCs upon cocultivation in a time-dependent manner. This effect was strongly dependent on direct cell-to-cell contact and extracellular signal-regulated kinase (ERK) signaling, but was not mediated by heterotypic gap junction communication. Beyond enhanced SMC marker gene expression in MSCs, EPCs also enhanced the functional characteristics of cocultured MSCs by increasing their ability to attach to EC tubes in vitro. In conclusion, our study has shown that EPCs from adult peripheral blood as well as from cord blood commit cocultivated MSCs toward an SMC/pericyte phenotype in a cell-contact- and ERK-dependent manner.

EDHF: spreading the influence of the endothelium

Our view of the endothelium was transformed around 30 years ago, from one of an inert barrier to that of a key endocrine organ central to cardiovascular function. This dramatic change followed the discoveries that endothelial cells (ECs) elaborate the vasodilators prostacyclin and nitric oxide. The key to these discoveries was the use of the quintessentially pharmacological technique of bioassay. Bioassay also revealed endothelium-derived hyperpolarizing factor (EDHF), particularly important in small arteries and influencing blood pressure and flow distribution. The basic idea of EDHF as a diffusible factor causing smooth muscle hyperpolarization (and thus vasodilatation) has evolved into one of a complex pathway activated by endothelial Ca(2+) opening two Ca(2+) -sensitive K(+) -channels, K(Ca)2.3 and K(Ca)3.1. Combined application of apamin and charybdotoxin blocked EDHF responses, revealing the critical role of these channels as iberiotoxin was unable to substitute for charybdotoxin. We showed these channels are arranged in endothelial microdomains, particularly within projections towards the adjacent smooth muscle, and close to interendothelial gap junctions. Activation of K(Ca) channels hyperpolarizes ECs, and K(+) efflux through them can act as a diffusible 'EDHF' stimulating Na(+) /K(+) -ATPase and inwardly rectifying K-channels. In parallel, hyperpolarizing current can spread from the endothelium to the smooth muscle through myoendothelial gap junctions upon endothelial projections. The resulting radial hyperpolarization mobilized by EDHF is complemented by spread of hyperpolarization along arteries and arterioles, effecting distant dilatation dependent on the endothelium. So the complexity of the endothelium still continues to amaze and, as knowledge evolves, provides considerable potential for novel approaches to modulate blood pressure.

New molecular mechanisms for cardiovascular disease: contribution of endothelium-derived hyperpolarizing factor in the regulation of vasoconstriction in peripheral resistance arteries

Endothelium regulates vascular tone via release of endothelium-derived relaxing factors (EDRF) including nitric oxide (NO), prostaglandin I₂ (PGI₂), and endothelium-derived hyperpolarizing factor (EDHF). The mesenteric vascular bed produces vascular resistance to develop blood pressure and regulate tissue blood flow that plays an important role in maintenance of systemic blood pressure. There is now strong evidence that in these small resistance arteries, EDHF plays a major role in the response to vasoactive substances and regulation of vascular tone. Pharmacological analysis to investigate the role of the vascular endothelium in the regulation of α₁-adrenoceptor agonist (methoxamine)-induced vasoconstriction in rat mesenteric vascular beds showed that vasoconstriction induced by continuous perfusion of methoxamine (7 µM), but not high KCl (60 mM), time-dependently decreased to 20% of the initial constriction. The time-dependent reduction of methoxamine-induced vasoconstriction was inhibited by endothelium removal, inhibitor of EDHF (30 mM KCl, K+-channel blockers), and gap-junction inhibitor, but not NO synthase inhibitor and cyclooxygenase inhibitor and ageing. These results suggest that vascular endothelium counteracts to normalize excess vasoconstriction of the mesenteric resistance arteries by releasing EDHF, which is associated with activation of multiple K+-channels and gap junction involvement and markedly decreases with ageing.

Erythropoietin enhances hydrogen peroxide-mediated dilatation of canine coronary collateral arterioles during myocardial ischemia in dogs in vivo

We have previously demonstrated that endothelium-derived hydrogen peroxide (H2O2) plays an important role in the canine coronary microcirculation as an endothelium-derived hyperpolarizing factor in vivo. However, it remains to be examined whether endogenous H2O2 is involved in the dilatation of coronary collaterals during myocardial ischemia in vivo and, if so, whether erythropoietin (EPO) enhances the responses. Canine subepicardial native collateral small arteries (CSA; ≥ 100 μm) and arterioles (CA; <100 μm) were observed using an intravital microscope. Experiments were performed after left anterior descending coronary artery ischemia (90 min) under the following eight conditions ( n = 5 each): control, EPO, EPO+catalase, EPO+ N-monomethyl-l-arginine (l-NMMA), EPO+l-NMMA+catalase, EPO+l-NMMA+iberiotoxin [Ca2+-activated K+ (KCa) channel blocker], EPO+l-NMMA+apamin+charybdotoxin (KCa channel blocker), and EPO+wortmannin (phosphatidylinositol 3-kinase inhibitor). Myocardial ischemia caused significant vasodilatation in CA but not in CSA under control conditions, which was significantly decreased by catalase in CA. After EPO, the vasodilatation was significantly increased in both sizes of arteries and was significantly decreased by catalase. The enhancing effect of EPO was reduced by l-NMMA but not by catalase in CSA and was reduced by l-NMMA+catalase in CA, where the greater inhibitory effects were noted with l-NMMA+catalase, l-NMMA+iberiotoxin, L-NMMA+apamin+charybdotoxin, or wortmannin. EPO significantly ameliorated ischemia-induced impairment of myocardial Akt phosphorylation, which was abolished by l-NMMA+catalase or wortmannin. EPO also ameliorated oxidative stress and myocardial injury, as assessed by plasma 8-hydroxydeoxyguanosine and troponin-T, respectively. These results indicate that EPO enhances H2O2-mediated dilatation of coronary collateral arterioles during myocardial ischemia in dogs in vivo.

Endothelial modulation of agonist-induced vasoconstriction in mesenteric microcirculation

It is widely accepted that vascular endothelium regulates vasoconstriction via release of endothelium-derived relaxing factors (EDRF). The mesenteric circulation, which is the largest vascular bed, influences regulation of systemic blood pressure. However, the role of EDRF in the modulation of vascular tone in peripheral mesenteric circulation has not been extensively studied. Therefore, our recent studies investigated the role of the vascular endothelium in the regulation of methoxamine (alpha(1)-adrenoceptor agonist)-induced vasoconstriction and their age-related changes in rat mesenteric vascular beds. In mesenteric vascular beds with intact endothelium isolated from 8 week-old rats, the initial maximum vasoconstriction induced by continuous perfusion of methoxamine was time-dependently decreased during 3 hour-perfusion. Neither nitric oxide synthase inhibitor nor cyclooxygenase inhibitor altered this time-dependent reduction of methoxamine-induced vasoconstriction. Endothelium removal, K(+)-channel inhibitors and gap junction inhibitor significantly inhibited the time-dependent reduction of methoxamine-induced vasoconstriction. In the preparations with intact endothelium from 16 week-old rats, the time-dependent reduction of methoxamine-induced vasoconstriction disappeared. Furthermore, endothelium removal and treatment with cyclooxygenase inhibitor, thromboxane A(2) receptor antagonist or superoxide dismutase mimetic significantly reduced the methoxamine-induced vasoconstriction in the preparations from 16 week-old rats. These findings suggest that vascular endothelium acts to depress methoxamine-induced vasoconstriction by releasing endothelium-derived hyperpolarizing factor (EDHF), and dysfunction in this endothelial modulation develops with ageing.

Connexins and gap junctions in the EDHF phenomenon and conducted vasomotor responses

It is becoming increasingly evident that electrical signaling via gap junctions plays a central role in the physiological control of vascular tone via two related mechanisms (1) the endothelium-derived hyperpolarizing factor (EDHF) phenomenon, in which radial transmission of hyperpolarization from the endothelium to subjacent smooth muscle promotes relaxation, and (2) responses that propagate longitudinally, in which electrical signaling within the intimal and medial layers of the arteriolar wall orchestrates mechanical behavior over biologically large distances. In the EDHF phenomenon, the transmitted endothelial hyperpolarization is initiated by the activation of Ca(2+)-activated K(+) channels channels by InsP(3)-induced Ca(2+) release from the endoplasmic reticulum and/or store-operated Ca(2+) entry triggered by the depletion of such stores. Pharmacological inhibitors of direct cell-cell coupling may thus attenuate EDHF-type smooth muscle hyperpolarizations and relaxations, confirming the participation of electrotonic signaling via myoendothelial and homocellular smooth muscle gap junctions. In contrast to isolated vessels, surprisingly little experimental evidence argues in favor of myoendothelial coupling acting as the EDHF mechanism in arterioles in vivo. However, it now seems established that the endothelium plays the leading role in the spatial propagation of arteriolar responses and that these involve poorly understood regenerative mechanisms. The present review will focus on the complex interactions between the diverse cellular signaling mechanisms that contribute to these phenomena.

Combination therapy with olmesartan and azelnidipine improves EDHF-mediated responses in diabetic apolipoprotein E-deficient mice

BACKGROUND

The endothelium modulates vascular tone by synthesizing and releasing several vasodilating factors, including vasodilator prostaglandins, nitric oxide (NO) and endothelium-derived hyperpolarizing factor (EDHF). In the present study, we examined whether an angiotensin-receptor blocker, a calcium-channel blocker or their combination improved EDHF-mediated responses in diabetic apolipoprotein E-deficient (ApoE(-/-)) mice.

METHODS AND RESULTS

We used male C57BL/6N (control) and streptozocin-induced diabetic ApoE(-/-) mice. The diabetic ApoE(-/-) mice were administered oral vehicle (untreated), olmesartan (OLM, 30 mgxkg(-1)xday(-1)), azelnidipine (AZL, 10 mgxkg(-1)xday(-1)), their combination (OLM + AZL), or hydralazine (HYD 5 mgxkg(-1)xday(-1)) for 5 weeks. In the untreated group, systolic blood pressure was significantly higher and both EDHF-mediated relaxation and endothelium-dependent hyperpolarization were markedly reduced as compared with the control group. Although EDHF-mediated relaxation was not significantly improved in the HYD, OLM and AZL groups, it was significantly improved in the OLM + AZL group, as was also the case with phosphorylation of Akt and endothelial NO synthase (eNOS). In contrast, the endothelium-independent relaxation response to sodium nitroprusside or NS-1619 (a direct opener of K(Ca) channels) was unaltered in any group.

CONCLUSIONS

OLM + AZL may improve the severely impaired EDHF-mediated responses in diabetic ApoE(-/-) mice, in which activation of the endothelial Akt - eNOS pathway may be involved.

Up-regulation of alkaline phosphatase expression in human primary osteoblasts by cocultivation with primary endothelial cells is mediated by p38 mitogen-activated protein kinase-dependent mRNA stabilization

For the regeneration of bone in tissue engineering applications, it is essential to provide cues that support neovascularization. This can be achieved by cell-based therapies using mature endothelial cells (ECs) or endothelial progenitor cells. In this context, ECs were used in various in vivo studies in combination with primary osteoblasts to enhance neovascularization of bone grafts. In a previous study, we have shown that cocultivation of human primary ECs and human primary osteoblasts (hOBs) leads to a cell contact-dependent up-regulation of alkaline phosphatase (ALP) expression in osteoblasts, indicating that cocultivated ECs may support osteogenic differentiation and osteoblastic cell functions. In the present study, we investigated this effect in more detail, revealing a time and cell number dependency of EC-mediated up-regulation of the early osteoblastic marker ALP, whereas osteocalcin, a late marker of osteogenesis, was down-regulated. The effect on ALP expression was bidirectional specific for both cell types. Functional inhibition of gap junctional communication between ECs and hOBs by 18alpha-glycyrrhetinic acid had only a weak suppressive effect on EC-mediated ALP up-regulation. In contrast, inhibition of p38 mitogen-activated protein kinase nearly completely prevented the EC-mediated stimulation of osteoblastic ALP expression. To investigate the molecular mechanism underlying the ALP up-regulation, we examined the effect of EC cocultivation on osteoblastic ALP promoter activity as well as mRNA stability. Cocultivation of ECs with hOBs significantly elevated the half-life of osteoblastic ALP mRNA without affecting its promoter activity. In summary, our data show that EC-mediated up-regulation of osteoblastic ALP expression is cell-type specific and is posttranscriptionally regulated via p38 mitogen-activated protein kinase-dependent mRNA turn-over.

Roles of endothelial oxidases in endothelium-derived hyperpolarizing factor responses in mice

The endothelium synthesizes and releases several vasodilator substances, including prostacyclin, nitric oxide (NO), and endothelium-derived hyperpolarizing factor (EDHF). We have demonstrated that endothelium-derived hydrogen peroxide (H2O2) is an EDHF in animals and humans and that superoxide anions derived from endothelial nitric oxide synthases (NOSs) system are an important precursor for EDHF/H2O2 in mice. There are several intracellular sources of superoxide anions other than NOSs, including NAD(P)H oxidase, xanthine oxidase, lipoxygenase, and mitochondrial electron transport chain. In this study, we examined the possible role of endothelial oxidases other than NOSs in the EDHF-mediated responses. In angiotensin II-infused mice, both EDHF-mediated relaxations and hyperpolarizations to acetylcholine were significantly reduced, nitric oxide-mediated relaxations were rather enhanced, and vascular smooth muscle responses were preserved. Antihypertensive treatment normalized blood pressure but failed to improve EDHF-mediated responses in those mice. Acute inhibition of endothelial oxidases other than NOSs, including NAD(P)H oxidase, xanthine oxidase, lipoxygenase, or mitochondrial electron transport chain, had no inhibitory effects on EDHF-mediated responses. Furthermore, in p47phox-knockout mice, EDHF-mediated responses were unaltered. These results suggest that endothelial oxidases other than NOSs are not involved in EDHF/H2O2 responses in mice, suggesting a specific link between endothelial NOSs system and EDHF responses under physiological conditions.

Crucial role of nitric oxide synthases system in endothelium-dependent hyperpolarization in mice

The endothelium plays an important role in maintaining vascular homeostasis by synthesizing and releasing several relaxing factors, such as prostacyclin, nitric oxide (NO), and endothelium-derived hyperpolarizing factor (EDHF). We have previously demonstrated in animals and humans that endothelium-derived hydrogen peroxide (H(2)O(2)) is an EDHF that is produced in part by endothelial NO synthase (eNOS). In this study, we show that genetic disruption of all three NOS isoforms (neuronal [nNOS], inducible [iNOS], and endothelial [eNOS]) abolishes EDHF responses in mice. The contribution of the NOS system to EDHF-mediated responses was examined in eNOS(-/-), n/eNOS(-/-), and n/i/eNOS(-/-) mice. EDHF-mediated relaxation and hyperpolarization in response to acetylcholine of mesenteric arteries were progressively reduced as the number of disrupted NOS genes increased, whereas vascular smooth muscle function was preserved. Loss of eNOS expression alone was compensated for by other NOS genes, and endothelial cell production of H(2)O(2) and EDHF-mediated responses were completely absent in n/i/eNOS(-/-) mice, even after antihypertensive treatment with hydralazine. NOS uncoupling was not involved, as modulation of tetrahydrobiopterin (BH(4)) synthesis had no effect on EDHF-mediated relaxation, and the BH(4)/dihydrobiopterin (BH(2)) ratio was comparable in mesenteric arteries and the aorta. These results provide the first evidence that EDHF-mediated responses are dependent on the NOSs system in mouse mesenteric arteries.

Functional role of gap junctions in cytokine-induced leukocyte adhesion to endothelium in vivo

To assess the hypothesis that gap junctions (GJs) participate on leukocyte-endothelium interactions in the inflammatory response, we compared leukocyte adhesion and transmigration elicited by cytokine stimulation in the presence or absence of GJ blockers in the hamster cheek pouch and also in the cremaster muscle of wild-type (WT) and endothelium-specific connexin 43 (Cx43) null mice (Cx43e(-/-)). In the cheek pouch, topical tumor necrosis factor-alpha (TNF-alpha; 150 ng/ml, 15 min) caused a sustained increment in the number of leukocytes adhered to venular endothelium (LAV) and located at perivenular regions (LPV). Superfusion with the GJ blockers 18-alpha-glycyrrhetinic acid (AGA; 75 microM) or 18-beta-glycyrrhetinic acid (50 microM) abolished the TNF-alpha-induced increase in LAV and LPV; carbenoxolone (75 microM) or oleamide (100 microM) reduced LAV by 50 and 75%, respectively, and LPV to a lesser extent. None of these GJ blockers modified venular diameter, blood flow, or leukocyte rolling. In contrast, glycyrrhizin (75 microM), a non-GJ blocker analog of AGA, was devoid of effect. Interestingly, when AGA was removed 90 min after TNF-alpha stimulation, LAV started to rise at a similar rate as in control. Conversely, application of AGA 90 min after TNF-alpha reduced the number of previously adhered cells. In WT mice, intrascrotal injection of TNF-alpha (0.5 microg/0.3 ml) increased LAV (fourfold) and LPV (threefold) compared with saline-injected controls. In contrast to the observations in WT animals, TNF-alpha stimulation did not increase LAV or LPV in Cx43e(-/-) mice. These results demonstrate an important role for GJ communication in leukocyte adhesion and transmigration during acute inflammation in vivo and further suggest that endothelial Cx43 is key in these processes.

Reduction of NO- and EDHF-mediated vasodilatation in hypertension: role of asymmetric dimethylarginine

Asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of nitric oxide (NO) synthase (NOS), and endothelial dysfunction is related to the elevation of ADMA level in hypertension. Besides the NO-mediated pathway, the endothelium-derived hyperpolarizing factor (EDHF)-mediated pathway is involved in endothelial dysfunction. The aims of the present study were to evaluate the changes of endothelium-dependent dilatation of arteries in hypertension and the role of ADMA in NO- and EDHF-mediated vasodilatation. The great omental arteries were isolated from essential hypertensive and normotensive patients, and mesenteric arteries were isolated from spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats. NO-, EDHF-, and prostaglandin (PGI(2))-mediated endothelium-dependent vasodilatation were measured, and plasma concentrations of ADMA were determined in rats. Cultured endothelial cells were treated with ADMA (1-10 microM) for 48 h, and the mRNA and protein level of small-conductance Ca(2+)-activated K(+) channel 3 (SK3), which has been thought to be a key mediator of EDHF, was determined. Both NO- and EDHF-mediated endothelium-dependent responses were decreased in the great omental arteries of hypertensive patients and mesenteric arteries of SHR. Plasma levels of ADMA were significantly increased in SHR. In cultured endothelial cells, the expressions of SK3 mRNA and protein were concentration-dependently down-regulated in the presence of ADMA. The present study suggests that the inhibitory effect of ADMA on endothelial function not only involves NO-mediated endothelium-dependent vasodilatation but also the EDHF-mediated pathways in hypertensive animals and humans, and that ADMA can down-regulate the expression of SK3 channels in endothelial cells.

Role of Cu,Zn-SOD in the synthesis of endogenous vasodilator hydrogen peroxide during reactive hyperemia in mouse mesenteric microcirculation in vivo

We have recently demonstrated that endothelium-derived hydrogen peroxide (H2O2) is an endothelium-derived hyperpolarizing factor and that endothelial Cu/Zn-superoxide dismutase (SOD) plays an important role in the synthesis of endogenous H2O2 in both animals and humans. We examined whether SOD plays a role in the synthesis of endogenous H2O2 during in vivo reactive hyperemia (RH), an important regulatory mechanism. Mesenteric arterioles from wild-type and Cu,Zn-SOD(-/-) mice were continuously observed by a pencil-type charge-coupled device (CCD) intravital microscope during RH (reperfusion after 20 and 60 s of mesenteric artery occlusion) in the cyclooxygenase blockade under the following four conditions: control, catalase alone, N(G)-monomethyl-L-arginine (L-NMMA) alone, and L-NMMA + catalase. Vasodilatation during RH was significantly decreased by catalase or L-NMMA alone and was almost completely inhibited by L-NMMA + catalase in wild-type mice, whereas it was inhibited by L-NMMA and L-NMMA + catalase in the Cu,Zn-SOD(-/-) mice. RH-induced increase in blood flow after L-NMMA was significantly increased in the wild-type mice, whereas it was significantly reduced in the Cu,Zn-SOD(-/-) mice. In mesenteric arterioles of the Cu,Zn-SOD(-/-) mice, Tempol, an SOD mimetic, significantly increased the ACh-induced vasodilatation, and the enhancing effect of Tempol was decreased by catalase. Vascular H(2)O(2) production by fluorescent microscopy in mesenteric arterioles after RH was significantly increased in response to ACh in wild-type mice but markedly impaired in Cu,Zn-SOD(-/-) mice. Endothelial Cu,Zn-SOD plays an important role in the synthesis of endogenous H(2)O(2) that contributes to RH in mouse mesenteric smaller arterioles.

Elucidation of the temporal relationship between endothelial-derived NO and EDHF in mesenteric vessels

Although the endothelium co-generates both nitric oxide (NO) and endothelium-derived hyperpolarizing factor (EDHF), the relative contribution from each vasodilator is not clear. In studies where the endothelium is stimulated acutely, EDHF responses predominate in small arteries. However, the temporal relationship between endothelial-derived NO and EDHF over more prolonged periods is unclear but of major physiological importance. Here we have used a classical pharmacological approach to show that EDHF is released transiently compared with NO. Acetylcholine (3 x 10(-6) mol/l) dilated second- and/or third-order mesenteric arteries for prolonged periods of up to 1 h, an effect that was reversed fully and immediately by the subsequent addition of L-NAME (10(-3) mol/l) but not TRAM-34 (10(-6) mol/l) plus apamin (5 x 10(-7) mol/l). When vessels were pretreated with L-NAME, acetylcholine induced relatively transient dilator responses (declining over approximately 5 min), and vessels were sensitive to TRAM-34 plus apamin. When measured in parallel, the dilator effects of acetylcholine outlasted the smooth muscle hyperpolarization. However, in the presence of L-NAME, vasodilatation and hyperpolarization followed an identical time course. In vessels from NOSIII(-/-) mice, acetylcholine induced small but detectable dilator responses that were transient in duration and blocked by TRAM-34 plus apamin. EDHF responses in these mouse arteries were inhibited by an intracellular calcium blocker, TMB-8, and the phospholipase A(2) inhibitor AACOCF(3), suggesting a role for lipid metabolites. These data show for the first time that EDHF is released transiently, whereas endothelial-derived NO is released in a sustained manner.

C-type natriuretic peptide: a new endothelium-derived hyperpolarizing factor?

Vascular relaxation mediated by endothelium-derived hyperpolarizing factor (EDHF) is important for resistance artery function and is underpinned by hyperpolarization of the smooth muscle cells of the blood vessel wall. Debate surrounds the identity of EDHF and its mechanism of action, with the consensus being that there is no universal EDHF. Regional differences in vascular function reflect the complex mechanisms of EDHF. Two primary mechanistic pathways are implicated: (i) myoendothelial gap junctions mediating the spread of endothelial cell hyperpolarization or small signaling molecules (or both) to the smooth muscle; and (ii) diffusible mediators released from the endothelium, including K+ and epoxyeicosatrienoic acids. Here, we discuss the evidence for and against C-type natriuretic peptide (CNP), the latest candidate for a diffusible mediator.

Both endothelium and afferent nerve endings play a role in acetylcholine-induced renal vasodilation

We investigated the nature and signaling pathways of endothelium- and sensory-nerve ending-derived substances involved in acetylcholine-induced vasodilation in rat isolated perfused kidney. Endothelial denudation by Triton X-100 (0.2%, 0.1 ml) or depletion of afferent nerve endings by capsaicin (10(-6) mol/l) attenuated acetylcholine-induced vasodilation. When these two agents were administered together, the response to acetylcholine was completely inhibited. CGRP1 receptor blocker CGRP 8-37 (10(-7) mol/l) and adenosine A(2) receptor antagonist ZM 241 385 (10(-7) mol/l) inhibited acetylcholine-induced dilation. When indomethacin (10(-5) mol/l), a cyclooxygenase inhibitor, l-NOARG (10(-4) mol/l), a nitric oxide (NO) synthase inhibitor, and potassium chloride (30 mmol/l), to test EDHF response, were perfused simultaneously, the inhibition was greater than that was observed with each agent alone. Guanylate cyclase inhibitor ODQ (10(-5) mol/l) or protein kinase A inhibitor KT 5720 (5x10(-7) mol/l) inhibited acetylcholine-induced dilation. Gap junction uncoupler 18alpha-glycyrrhetinic acid (10(-4) mol/l) caused an uncontrollable increase in basal perfusion pressure making it impossible to test against acetylcholine-induced dilation. Our data suggest that NO, prostanoids, EDHF, and CGRP released from vascular endothelium and afferent nerve endings participate in acetylcholine-induced vasodilation and their signal transduction molecules include protein kinase A and guanylate cyclase.

Cardioprotective role of endogenous hydrogen peroxide during ischemia-reperfusion injury in canine coronary microcirculation in vivo

We have recently demonstrated that endogenous H2O2 plays an important role in coronary autoregulation in vivo. However, the role of H2O2 during coronary ischemia-reperfusion (I/R) injury remains to be examined. In this study, we examined whether endogenous H2O2 also plays a protective role in coronary I/R injury in dogs in vivo. Canine subepicardial small coronary arteries (>or=100 microm) and arterioles (<100 microm) were continuously observed by an intravital microscope during coronary I/R (90/60 min) under cyclooxygenase blockade (n=50). Coronary vascular responses to endothelium-dependent vasodilators (ACh) were examined before and after I/R under the following seven conditions: control, nitric oxide (NO) synthase (NOS) inhibitor NG-monomethyl-L-arginine (L-NMMA), catalase (a decomposer of H2O2), 8-sulfophenyltheophylline (8-SPT, an adenosine receptor blocker), L-NMMA+catalase, L-NMMA+tetraethylammonium (TEA, an inhibitor of large-conductance Ca2+-sensitive potassium channels), and L-NMMA+catalase+8-SPT. Coronary I/R significantly impaired the coronary vasodilatation to ACh in both sized arteries (both P<0.01); L-NMMA reduced the small arterial vasodilatation (both P<0.01), whereas it increased (P<0.05) the ACh-induced coronary arteriolar vasodilatation associated with fluorescent H2O2 production after I/R. Catalase increased the small arterial vasodilatation (P<0.01) associated with fluorescent NO production and increased endothelial NOS expression, whereas it decreased the arteriolar response after I/R (P<0.01). L-NMMA+catalase, L-NMMA+TEA, or L-NMMA+catalase+8-SPT further decreased the coronary vasodilatation in both sized arteries (both, P<0.01). L-NMMA+catalase, L-NMMA+TEA, and L-NMMA+catalase+8-SPT significantly increased myocardial infarct area compared with the other four groups (control, L-NMMA, catalase, and 8-SPT; all, P<0.01). These results indicate that endogenous H2O2, in cooperation with NO, plays an important cardioprotective role in coronary I/R injury in vivo.

Endothelium-derived hyperpolarizing factor: where are we now?

The endothelium controls vascular tone not only by releasing nitric oxide (NO) and prostacyclin but also by other pathways causing hyperpolarization of the underlying smooth muscle cells. This characteristic was at the origin of the denomination endothelium-derived hyperpolarizing factor (EDHF). We know now that this acronym includes different mechanisms. In general, EDHF-mediated responses involve an increase in the intracellular calcium concentration, the opening of calcium-activated potassium channels of small and intermediate conductance and the hyperpolarization of the endothelial cells. This results in an endothelium-dependent hyperpolarization of the smooth muscle cells, which can be evoked by direct electrical coupling through myo-endothelial junctions and/or the accumulation of potassium ions in the intercellular space. Potassium ions hyperpolarize the smooth muscle cells by activating inward rectifying potassium channels and/or Na+/K(+)-ATPase. In some blood vessels, including large and small coronary arteries, the endothelium releases arachidonic acid metabolites derived from cytochrome P450 monooxygenases. The epoxyeicosatrienoic acids (EET) generated are not only intracellular messengers but also can diffuse and hyperpolarize the smooth muscle cells by activating large conductance calcium-activated potassium channels. Additionally, the endothelium can produce other factors such as lipoxygenases derivatives or hydrogen peroxide (H2O2). These different mechanisms are not necessarily exclusive and can occur simultaneously.

Endothelial mediators of the acetylcholine-induced relaxation of the rat femoral artery

This study examined endothelium-derived mediators of acetylcholine-induced relaxation in male rat femoral arteries. Arterial rings were suspended in a myograph for the measurement of isometric force. The generation of hydrogen peroxide (H2O2) in endothelial cells was detected using the fluorescent probe, 5-(and-6)-chloromethyl-2',7'-dichlorodihydrofluorescein diacetate acetyl ester. N(G)-nitro-L-arginine methyl ester (L-NAME, NOS inhibitor) and 1H-[1,2,4]oxadiazolo[4,2-alpha]quinoxalin-1-one (ODQ, guanylate cyclase inhibitor) alone or in combination with indomethacin (cycloxygenase inhibitor) diminished acetylcholine-induced endothelium-dependent relaxation to a similar extent. A small relaxation to acetylcholine in 60 mM KCl-constricted rings was abolished by L-NAME. Acetylcholine-induced relaxation was reduced by charybdotoxin plus apamin (intermediate- and small-conductance Ca2+-activated K+ channel blockers, respectively) or by 30 mM KCl. Both ouabain (Na+/K+ ATPase inhibitor) and BaCl2 (K(IR) channel blocker) also inhibited the relaxation albeit to a lesser degree. In the presence of L-NAME, ODQ plus indomethacin, charybdotoxin plus apamin or ouabain plus BaCl2 produced further inhibition. Catalase attenuated acetylcholine-induced relaxations and this attenuation was prevented by 3-amino-1,2,4-triazole (catalase inhibitor). Catalase did not affect acetylcholine-induced relaxations in rings treated with L-NAME or ODQ. Acetylcholine increased the dichlorofluorescein fluorescence intensity in native endothelial cells and this effect was abolished by catalase and by L-NAME. Exogenous H2O2 caused endothelium-independent relaxation that was slightly inhibited by iberiotoxin, ODQ or significantly reduced by elevated KCl, and abolished by catalase. The present results indicate that in addition to nitric oxide (NO) and endothelium-derived hyperpolarizing factor (EDHF, sensitive to charybdotoxin plus apamin, ouabain, and BaCl2), the endothelium of rat femoral artery can release H2O2 in response to acetylcholine, which was sensitive to L-NAME. Thus, the eNOS-dependent H2O2 is likely to be the third mediator of acetylcholine-mediated relaxations in rat femoral arteries.

K+ potentiates hyperosmolarity-induced vasorelaxations in rat skeletal muscle arterioles

Several regulatory mechanisms have been proposed for the exercise hyperemia in skeletal muscles. Since different vasoactive factors might interact during the hyperemic response, we investigated the influence of elevated K(+) concentrations on hyperosmolarity (HO)-induced vasorelaxations. Small gluteal rat arteries were isolated and mounted in an organ bath for isometric tension recording. After precontraction with norepinephrine, 20 (S20), 40 (S40) or 60 mM (S60) sucrose was added in control conditions (5 mM K(+); K5) or in the presence of additional 3 (K8) or 5 mM (K10) K(+). Removal of the endothelium and the addition of ouabain, Ba(2+), iberiotoxin or 18-alpha glycyrrhetinic acid (alphaGA) were used to study the underlying mechanisms. Sucrose evoked significant concentration-dependent vasorelaxations (S20 15.62+/-1.61%; S40 26.47+/-1.71%; S60 43.66+/-2.50%), which were significantly increased on addition of 3 and 5 mM. After removal of the endothelium and in the presence of 5 x 10(-5) M alphaGA, the influence of K(+) was significantly blocked but not in the presence of 5 x 10(-5) M ouabain. The K(IR) channel inhibitor Ba(2+) and BK(Ca) channel inhibitor iberiotoxin totally abolished the potentiating effect. We conclude that K(+) significantly enhances the relaxing effect of HO in gluteal blood vessels. We hypothesize that K(+) may stimulate the endothelial K(IR) channels which elicits the release of a mediator of the BK(Ca) channels. This factor may be transferred through myo-endothelial gap-junctions to the smooth muscle cells where modulation of the BK(Ca) channels sensitizes the arteries for hyperosmolarity-induced relaxations.

Effects of Endothelium-Derived Hyperpolarizing Factor and Nitric Oxide on Endothelial Function in Femoral Resistance Arteries of Spontaneously Hypertensive Rats

In hypertension, endothelium-dependent relaxation is attenuated and this attenuation contributes to the increased peripheral resistance. However, the role of endothelium-derived hyperpolarizing factor (EDHF) in the arteries of hypertensive rats remains unclear. Therefore, the aim of this study was to evaluate the role of EDHF in the femoral resistance arteries of hypertensive rats. The femoral resistance arteries were isolated from 5-, 15- and 25-week-old spontaneously hypertensive rats (SHR) and age-matched Wistar Kyoto rats (WKY). Changes in internal diameter were examined with videomicroscopy. EDHF-mediated dilatation was determined by differences between the degree of acetylcholine (ACh)-induced dilatation in the presence of NG-monomethy-L-arginine (L-NMMA) plus a prostaglandin I2 inhibitor (indomethacin) and the degree of such dilatation in the presence of L-NMMA, indomethacin and KCl. Charybdotoxin (CTx) and apamin (a Ca2+-activated K+ channel [KCa] inhibitor)-sensitive EDHF dilatation was also compared between in 5-, 15- and 25-week-old SHR and WKY. ACh-induced vasodilatation was not different between 5-week-old SHR and WKY. There was no difference between NO- and EDHF-mediated vasodilatation in 5-week-old rats. ACh-induced vasodilatation was weaker in 15-week-old SHR than in WKY. NO-mediated vasodilatation did not differ between the two groups. EDHF-mediated dilatation was attenuated in SHR but not in WKY. ACh-induced dilatation was weaker in 25-week-old SHR than in WKY. NO- and EDHF-mediated vasodilatation were attenuated in SHR but not WKY. EDHF-mediated vasodilatation was attenuated before the loss of NO-mediated vasodilatation in the femoral resistance arteries of SHR. The attenuation of this vasodilatation was mediated by the CTx plus apamin-sensitive EDHF.

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

The endothelium plays an important role in maintaining vascular homeostasis by synthesizing and releasing several vasodilating substances, including vasodilator prostaglandins, nitric oxide (NO), and endothelium-derived hyperpolarizing factor (EDHF). Since the first report for the existence of EDHF, several substances/mechanisms have been proposed for the nature of EDHF, including epoxyeicosatrienoic acids (metabolites of arachidonic P450 epoxygenase pathway), K ions, and electrical communications through myoendothelial gap junctions. We have recently demonstrated that endothelium-derived hydrogen peroxide (H(2)O(2)) is an EDHF in mouse and human mesenteric arteries and in porcine coronary microvessels. For the synthesis of H(2)O(2) as an EDHF, endothelial Cu,Zn-superoxide dismutase plays an important role in mesenteric arteries of mice and humans. We also have demonstrated that EDHF-mediated responses are attenuated by several arteriosclerotic risk factors, including diabetes mellitus and hyperlipidemia and their combination in particular. Recent studies have indicated that endothelium-derived H(2)O(2) plays an important protective role in coronary autoregulation and myocardial ischemia/reperfusion injury in vivo. Indeed, our H(2)O(2)/EDHF theory demonstrates that endothelium-derived H(2)O(2), another reactive oxygen species in addition to NO, plays an important role as a redox signaling molecule to cause vasodilatation as well as cardioprotection. In this review, we summarize our knowledge on H(2)O(2)/EDHF regarding its identification, mechanisms of synthesis, and clinical implications.

Angiotensin-converting enzyme inhibitors reveal non-NO-, non-prostacycline-mediated endothelium-dependent relaxation in internal thoracic artery of hypertensive patients

BACKGROUND

We have shown that treatment of hypertension with ACE inhibitors (ACE-I) enhances relaxation to acetylcholine in human internal thoracic artery (ITA) above this in nonhypertensive patients receiving no ACE-I. Present study assesses the endothelium-dependent responses mediated by neither NO nor prostacyclin in human ITA.

METHODS

We compared isolated ITA rings from hypertensive patients treated with ACE-I (ACE-I group) with those from normotensive patients on no ACE-I (control group). Relaxation to acetylcholine was assessed before and after inhibition of NO synthase and cyclooxygenase with L-NMMA and indomethacin, respectively.

RESULTS

The maximal relaxation in ACE-I group was 79+/-3.3% and was depressed by incubation with L-NMMA and indomethacin to 41+/-2.7% (p<0.001); pD(2)=7.7+/-0.1 vs. 7.4+/-0.8 (p=0.265). The maximal relaxation to acetylcholine was lower in the control group: 65+/-3.3% (p=0.01); pD(2)=7.5+/-0.1 (p=0.07). Incubation with L-NMMA and indomethacin produced contraction to acetylcholine with a maximum of 43+/-7% (p<0.001); pD(2)=5.3+/-0.3 (p<0.001). The area under the concentration-response curve for acetylcholine-induced relaxation in ACE-I group equaled [arbitrary units] 596+/-71 and after incubation with L-NMMA and indomethacin 281+/-40 (p=0.002). Estimated LNMMA- and indomethacin-resistant relaxation, absent in control group, accounted for 47+/-4% of relaxation to acetylcholine in ACE-I group. Estimated NO- and prostacyclin-mediated relaxation was higher in control group than ACE-I group: 628+/-74 vs. 315+/-47 (p=0.009).

CONCLUSIONS

The results suggest that therapy with ACE-I improves endothelial function of hypertensive patients mainly by enhancing the endothelium-derived hyperpolarizing factor (EDHF) (and not NO)-mediated responses. It seems that it reveals measurable non-NO- non-PGI-mediated endothelium-dependent relaxation otherwise absent in conduit arteries.

Connexin-mimetic peptides dissociate electrotonic EDHF-type signalling via myoendothelial and smooth muscle gap junctions in the rabbit iliac artery

Synthetic peptides corresponding to the Gap 26 and Gap 27 domains of the first and second extracellular loops of the major vascular connexins (Cx37, Cx40 and Cx43), designated as (43)Gap 26, (40)Gap 27, (37,40)Gap 26 and (37,43)Gap 27 according to Cx homology, were used to investigate the role of gap junctions in the spread of endothelial hyperpolarizations evoked by cyclopiazonic acid (CPA) through the wall of the rabbit iliac artery. Immunostaining and confocal microscopy demonstrated that gap junction plaques constructed from Cx37 and Cx40 were abundant in the endothelium, whereas Cx43 was the dominant Cx visualized in the media. None of the Cx-mimetic peptides affected endothelial hyperpolarizations evoked by CPA directly. When administered individually, (40)Gap 27, (37,40)Gap 26 and (37,43)Gap 27, but not (43)Gap 26, attenuated endothelium-dependent subintimal smooth muscle hyperpolarization. By contrast, only (43)Gap 26 and (37,43)Gap 27 reduced the spread of subintimal hyperpolarization through the media of the rabbit iliac artery. The site of action of the peptides therefore correlated closely with the expression of their target Cxs in detectable gap junction plaques. The findings provide further evidence that the EDHF phenomenon is electrotonic in nature, and highlight the contribution of myoendothelial and homocellular smooth muscle communication via gap junctions to arterial function.

Influence of Diabetes Mellitus, Hypercholesterolemia, and Their Combination on EDHF-Mediated Responses in Mice

The endothelium synthesizes and releases several vasodilator substances, including vasodilator prostaglandins, NO, and EDHF. NO-mediated relaxations are reduced by various risk factors, such as diabetes mellitus and hypercholesterolemia. However, it remains to be elucidated whether EDHF-mediated relaxations also are reduced by those factors and their combination. In this study, we addressed this point in mice. We used small mesenteric arteries from control, diabetic (streptozotocin-induced), apolipoprotein-E-deficient (ApoE-/-), and diabetic ApoE-/- mice. In control mice, endothelium-dependent relaxations to acetylcholine were largely mediated by EDHF. This EDHF-mediated component was slightly reduced in diabetic mice, preserved in ApoE-/- mice, and markedly reduced in diabetic ApoE-/- mice with an increase in NO-mediated component and a negative contribution of indomethacin-sensitive endothelium-derived contracting factor (EDCF). Endothelium-independent relaxations to sodium nitroprusside or NS1619, a direct opener of calcium-activated K channels, were attenuated in ApoE-/- and diabetic ApoE-/- mice. Endothelium-dependent hyperpolarizations were significantly reduced in diabetic mice, preserved in ApoE-/- mice, and again markedly reduced in diabetic ApoE-/- mice. These results indicate that hypercholesterolemia alone minimally affects the EDHF-mediated relaxations, and diabetes mellitus significantly attenuated the responses, whereas their combination markedly attenuates the responses with a compensatory involvement of NO and a negative contribution of EDCF.

Endothelium-dependent smooth muscle hyperpolarization: do gap junctions provide a unifying hypothesis?

An endothelium-derived hyperpolarizing factor (EDHF) that is distinct from nitric oxide (NO) and prostanoids has been widely hypothesized to hyperpolarize and relax vascular smooth muscle following stimulation of the endothelium by agonists. Candidates as diverse as K(+) ions, eicosanoids, hydrogen peroxide and C-type natriuretic peptide have been implicated as the putative mediator, but none has emerged as a 'universal EDHF'. An alternative explanation for the EDHF phenomenon is that direct intercellular communication via gap junctions allows passive spread of agonist-induced endothelial hyperpolarization through the vessel wall. In some arteries, eicosanoids and K(+) ions may themselves initiate a conducted endothelial hyperpolarization, thus suggesting that electrotonic signalling may represent a general mechanism through which the endothelium participates in the regulation of vascular tone.

The Nicorandil-Induced Vasodilation in Humans Is Inhibited by Miconazole

Nicorandil, N-(2-hydroxyethyl)-nicotinamide nitrate, exerts its vasodilatory effects by opening ATP-sensitive potassium (K-ATP) channels and by acting as the exogenous nitric oxide (NO). It is not clear, however, whether the actions of other endothelium-dependent vasodilators, such as NO, endothelium-derived hyperpolarizing factor (EDHF), and prostaglandins, contribute to nicorandil-induced vasodilation in the vasculature in humans. We evaluated forearm blood flow (FBF) response to intraarterial infusion of nicorandil alone and in the presence of glibenclamide, a K-ATP channel inhibitor, N(G)-monomethyl-L-arginine, an NO synthase inhibitor, indomethacin, a cyclooxygenase inhibitor, or miconazol, a cytochrome P-450 inhibitor, in 24 healthy male subjects. FBF was measured using strain-gauge plethysmography. Infusion of nicorandil significantly increased the FBF response in a dose-dependent manner. Intraarterial infusion of glibenclamide attenuated nicorandil-induced vasodilation (160.9 +/- 21.2% versus 90.2 +/- 19.4%, P < 0.01), and miconazole also attenuated the FBF response to nicorandil (160.9 +/- 21.2% versus 66.1 +/- 9.2%, P < 0.001). N-monomethyl-L-arginine or indomethacin did not alter the FBF response to nicorandil. These findings suggest that nicorandil causes vasodilation in forearm circulation in humans, at least in part through a pathway that is dependent on K-ATP channels and cytochrome P-450, but not on endogenous NO and prostaglandins. EDHF may contribute to nicorandil-induced vasodilation in humans.

Effects of the gap junction blocker glycyrrhetinic acid on gastrointestinal smooth muscle cells

In the tunica muscularis of the gastrointestinal (GI) tract, gap junctions form low-resistance pathways between pacemaker cells known as interstitial cells of Cajal (ICCs) and between ICC and smooth muscle cells. Coupling via these junctions facilitates electrical slow-wave propagation and responses of smooth muscle to enteric motor nerves. Glycyrrhetinic acid (GA) has been shown to uncouple gap junctions, but previous studies have shown apparent nonspecific effects of GA in a variety of tissues. We tested the effects of GA using isometric force measurements, intracellular microelectrode recordings, the patch-clamp technique, and the spread of Lucifer yellow within cultured ICC networks. In murine small intestinal muscles, beta-GA (10 muM) decreased phasic contractions and depolarized resting membrane potential. Preincubation of GA inhibited the spread of Lucifer yellow, increased input resistance, and decreased cell capacitance in ICC networks, suggesting that GA uncoupled ICCs. In patch-clamp experiments of isolated jejunal myocytes, GA significantly decreased L-type Ca(2+) current in a dose-dependent manner without affecting the voltage dependence of this current. The IC(50) for Ca(2+) currents was 1.9 muM, which is lower than the concentrations used to block gap junctions. GA also significantly increased large-conductance Ca(2+)-activated K(+) currents but decreased net delayed rectifier K(+) currents, including 4-aminopyridine and tetraethylammonium-resistant currents. In conclusion, the reduction of phasic contractile activity of GI muscles by GA is likely a consequence of its inhibitory effects on gap junctions and voltage-dependent Ca(2+) currents. Membrane depolarization may be a consequence of uncoupling effects of GA on gap junctions between ICCs and smooth muscles and inhibition of K(+) conductances in smooth muscle cells.

Important Role of Superoxide Dismutase in EDHF-Mediated Responses of Human Mesenteric Arteries

The endothelium synthesizes and releases several vasodilator substances, including prostacyclin, nitric oxide (NO), and endothelium-derived hyperpolarizing factor (EDHF). We have identified hydrogen peroxide (H2O2) as an EDHF in mouse and human mesenteric arteries and porcine coronary microvessels. We also have recently demonstrated that Cu,Zn-SOD plays an important role in EDHF synthesis in mouse mesenteric arteries. However, it remains to be determined whether SOD also plays an important role in EDHF-mediated responses of human arteries. In this study, we addressed this point in human mesenteric arteries. We used small mesenteric arteries of patients who underwent gastrectomy operations. Isometric tensions and membrane potentials were recorded in the presence of indomethacin and N-nitro-L-arginine to inhibit the synthesis of prostacyclin and NO, respectively. Pretreatment with Tiron, a cell-permeable SOD-mimetic, significantly enhanced the EDHF-mediated relaxations and hyperpolarizations to bradykinin, and this effect was abolished by catalase, indicating that this enhancing effect was achieved by H2O2. By contrast, Tiron did not affect endothelium-independent relaxations, indicating that the enhancing effect of Tiron is not caused by the enhancement of vascular smooth muscle responses. These results indicate that SOD plays an important role in EDHF-mediated relaxations and hyperpolarizations of human mesenteric arteries.

Receptor-activating peptides for PAR-1 and PAR-2 relax rat gastric artery via multiple mechanisms

Receptor-activating peptides for protease-activated receptors (PARs) 1 or 2 enhance gastric mucosal blood flow (GMBF) and protect against gastric mucosal injury in rats. We thus examined and characterized the effects of PAR-1 and PAR-2 agonists on the isometric tension in isolated rat gastric artery. The agonists for PAR-2 or PAR-1 produced vasodilation in the endothelium-intact arterial rings, which was abolished by removal of the endothelium. The mechanisms underlying the PAR-2- and PAR-1-mediated relaxation involved NO, endothelium-derived hyperpolarizing factor (EDHF) and prostanoids, to distinct extent, as evaluated by use of inhibitors of NO synthase, cyclo-oxygenase and Ca2+-activated K+ channels. The EDHF-dependent relaxation responses were significantly attenuated by gap junction inhibitors. These findings demonstrate that endothelial PAR-1 and PAR-2, upon activation, dilate the gastric artery via NO and prostanoid formation and also EDHF mechanisms including gap junctions, which would enhance GMBF.

Junctional and nonjunctional effects of heptanol and glycyrrhetinic acid derivates in rat mesenteric small arteries

1 Heptanol, 18alpha-glycyrrhetinic acid (18alphaGA) and 18beta-glycyrrhetinic acid (18betaGA) are known blockers of gap junctions, and are often used in vascular studies. However, actions unrelated to gap junction block have been repeatedly suggested in the literature for these compounds. We report here the findings from a comprehensive study of these compounds in the arterial wall. 2 Rat isolated mesenteric small arteries were studied with respect to isometric tension (myography), [Ca2+]i (Ca(2+)-sensitive dyes), membrane potential and--as a measure of intercellular coupling--input resistance (sharp intracellular glass electrodes). Also, membrane currents (patch-clamp) were measured in isolated smooth muscle cells (SMCs). Confocal imaging was used for visualisation of [Ca2+]i events in single SMCs in the arterial wall. 3 Heptanol (150 microm) activated potassium currents, hyperpolarised the membrane, inhibited the Ca2+ current, and reduced [Ca2+]i and tension, but had little effect on input resistance. Only at concentrations above 200 microm did heptanol elevate input resistance, desynchronise SMCs and abolish vasomotion. 4 18betaGA (30 microm) not only increased input resistance and desynchronised SMCs but also had nonjunctional effects on membrane currents. 18alphaGA (100 microm) had no significant effects on tension, [Ca2+]i, total membrane current and synchronisation in vascular smooth muscle. 5 We conclude that in mesenteric small arteries, heptanol and 18betaGA have important nonjunctional effects at concentrations where they have little or no effect on intercellular communication. Thus, the effects of heptanol and 18betaGA on vascular function cannot be interpreted as being caused only by effects on gap junctions. 18alphaGA apparently does not block communication between SMCs in these arteries, although an effect on myoendothelial gap junctions cannot be excluded.

Mechanisms underlying endothelium-dependent flow increase in perfused rat mesenteric vascular bed

The isolated rat mesenteric vasculature was perfused at constant pressures of 40, 80 or 120 mm Hg and the change in flow rate was measured. In the presence of phenylephrine, treatment with 3-[(3-cholamidopropyl) dimethylammonio]-1-propane sulfonate (CHAPS) or N(G)-nitro-L-arginine (L-NA) significantly inhibited the pressure-dependent flow rate increase, but treatment with indomethacin or charybdotoxin plus apamin did not. Acetylcholine, bradykinin and ADP increased the flow rate, which had been markedly suppressed by CHAPS. At 80 mm Hg, the flow rate increase induced by these agonists was not affected by indomethacin plus L-NA, but was suppressed by subsequent treatment with charybdotoxin plus apamin. Changes in the perfusion pressure did not significantly affect the flow rate increases induced by the agonists. In conclusion, the opening of charybdotoxin plus apamin-sensitive Ca(2+)-dependent K(+) channels may be mainly involved in the endothelium-dependent flow rate increase induced by the agonists, whereas nitric oxide (NO) may be responsible for the endothelium-dependent, pressure-induced flow rate increase.

Pivotal role of Cu,Zn-superoxide dismutase in endothelium-dependent hyperpolarization

The endothelium plays an important role in maintaining vascular homeostasis by synthesizing and releasing several vasodilating factors, including prostacyclin, NO, and endothelium-derived hyperpolarizing factor (EDHF). We have recently identified that endothelium-derived H2O2 is an EDHF in mesenteric arteries of mice and humans and in porcine coronary microvessels. However, the mechanism for the endothelial production of H2O2 as an EDHF remains to be elucidated. In this study, we tested our hypothesis that Cu,Zn-superoxide dismutase (Cu,Zn-SOD) plays a pivotal role in endothelium-dependent hyperpolarization, using control and Cu,Zn-SOD-/- mice. In mesenteric arteries, EDHF-mediated relaxations and hyperpolarizations were significantly reduced in Cu,Zn-SOD-/- mice with no inhibitory effect of catalase, while endothelium-independent relaxations and hyperpolarizations were preserved. Endothelial H2O2 production also was significantly reduced in Cu,Zn-SOD-/- mice. In Langendorff isolated heart, bradykinin-induced increase in coronary flow was significantly reduced in Cu,Zn-SOD-/- mice, again with no inhibitory effect of catalase. The exogenous SOD mimetic tempol significantly improved EDHF-mediated relaxations and hyperpolarizations and coronary flow response in Cu,Zn-SOD-/- mice. These results prove the novel concept that endothelial Cu,Zn-SOD plays an important role as an "EDHF synthase" in mice, in addition to its classical role to scavenge superoxide anions.

Ouabain exerts biphasic effects on connexin functionality and expression in vascular smooth muscle cells

1. We have compared the effects of ouabain on the maintenance of gap junctional communication in rat aortic A7r5 smooth muscle cells, monkey COS-1 fibroblasts and human HeLa epithelial cells. 2. Ouabain (1 mM) interrupted dye coupling between confluent A7r5 cells within approximately 1 h, and high concentrations of ouabain were similarly required to reduce coupling between COS-1 cells selected to express the rat alpha1 Na+/K+-ATPase subunit, which is ouabain resistant. By contrast, low concentrations of ouabain (1-10 microM) attenuated dye transfer in wild-type COS-1 and HeLa cells, whose endogenous alpha1 subunits possess relatively high affinity for the glycoside (Ki approximately 0.3 vs approximately 100 microM) Ouabain-induced reductions in dye transfer therefore correlated with the ability of the glycoside to bind to the Na+/K+-ATPase isoenzymes expressed in these different cell lines. 3. No consistent relationship between inhibition of intercellular dye transfer and secondary changes in [Ca2+]i or pHi could be identified following incubation with ouabain. 4. In separate experiments, the effects of ouabain on real-time trafficking of connexin (Cx) protein were monitored by time-lapse microscopy of A7r5 cells transfected to express a fluorescent Cx43-green fluorescent protein (GFP) and the ability of the glycoside to modulate endogenous expression of Cx40 and Cx43 evaluated in A7r5 cells by immunochemical and Western blot analysis. 5. Ouabain (1 mM) depressed vesicular trafficking of Cx43-GFP after approximately 1 h, and caused a time-dependent loss of endogenous Cx40 and Cx43 protein that was first evident at 2 h and almost complete after 4 h. These effects of ouabain on Cx expression were reversed 90 min following washout of the glycoside. 6. We conclude that ouabain exerts biphasic effects on intercellular communication that involve an initial decrease in gap junctional permeability followed by a global reduction in the expression of Cx protein. Further studies are necessary to establish to what extent these actions of ouabain reflect inversion of the normal [Na+]i/[K+]i ratio and/or conversion of the Na+/K+-ATPase into a general signal transducer that regulates downstream protein synthesis.