Important role of endothelium-derived hyperpolarizing factor in shear stress--induced endothelium-dependent relaxations in the rat mesenteric artery

The Natural-Mineral-Based Novel Nanomaterial IFMC Increases Intravascular Nitric Oxide without Its Intake: Implications for COVID-19 and beyond

There are currently no promising therapy strategies for either the treatment or prevention of novel coronavirus disease 2019 (COVID-19), despite the urgent need. In addition to respiratory diseases, vascular complications are rapidly emerging as a key threat of COVID-19. Existing nitric oxide (NO) therapies have been shown to improve the vascular system; however, they have different limitations in terms of safety, usability and availability. In light of this, we hypothesise that a natural-mineral-based novel nanomaterial, which was developed based on NO therapy, might be a viable strategy for the treatment and prevention of COVID-19. The present study examined if it could induce an increase of intravascular NO, vasodilation and the consequent increase of blood flow rate and temperature in a living body. The intravascular NO concentration in the hepatic portal of rats was increased by 0.17 nM over 35.2 s on average after its application. An ultrasonic Doppler flow meter showed significant increases in the blood flow rate and vessel diameter, but no difference in the blood flow velocity. These were corroborated by measurements of human hand surface temperature. To our knowledge, this result is the first evidence where an increase of intravascular NO and vasodilation were induced by bringing a natural-mineral-based nanomaterial into contact with or close to a living body. The precise mechanisms remain a matter for further investigation; however, we may assume that endothelial NO synthase, haemoglobin and endothelium-derived hyperpolarising factor are deeply involved in the increase of intravascular NO.

Age-dependent impact of CaV 3.2 T-type calcium channel deletion on myogenic tone and flow-mediated vasodilatation in small arteries

KEY POINTS

Blood pressure and flow exert mechanical forces on the walls of small arteries, which are detected by the endothelial and smooth muscle cells, and lead to regulation of the diameter (basal tone) of an artery. CaV 3.2 T-type calcium channels are expressed in the wall of small arteries, although their function remains poorly understood because of the low specificity of T-type blockers. We used mice deficient in CaV 3.2 channels to study their role in pressure- and flow-dependent tone regulation and the possible impact of ageing on this role. In young mice, CaV 3.2 channels oppose pressure-induced vasoconstriction and participate in endothelium-dependent, flow-mediated dilatation. These effects were not seen in mature adult mice. The results of the present study demonstrate an age-dependent impact of CaV 3.2 T-type calcium channel deletion in rodents and suggest that the loss of CaV 3.2 channel function leads to more constricted arteries, which is a risk factor for cardiovascular disease.

ABSTRACT

The myogenic response and flow-mediated vasodilatation are important regulators of local blood perfusion and total peripheral resistance, and are known to entail a calcium influx into vascular smooth muscle cells (VSMCs) and endothelial cells (ECs), respectively. CaV 3.2 T-type calcium channels are expressed in both VSMCs and ECs of small arteries. The T-type channels are important drug targets but, as a result of the lack of specific antagonists, our understanding of the role of CaV 3.2 channels in vasomotor tone at various ages is scarce. We evaluated the myogenic response, flow-mediated vasodilatation, structural remodelling and mRNA + protein expression in small mesenteric arteries from CaV 3.2 knockout (CaV 3.2KO) vs. wild-type mice at a young vs. mature adult age. In young mice only, deletion of CaV 3.2 led to an enhanced myogenic response and a ∼50% reduction of flow-mediated vasodilatation. Ni2+ had both CaV 3.2-dependent and independent effects. No changes in mRNA expression of several important K+ and Ca2+ channel genes were induced by CaV 3.2KO However, the expression of the other T-type channel isoform (CaV 3.1) was reduced at the mRNA and protein level in mature adult compared to young wild-type arteries. The results of the present study demonstrate the important roles of the CaV 3.2 T-type calcium channels in myogenic tone and flow-mediated vasodilatation that disappear with ageing. Because increased arterial tone is a risk factor for cardiovascular disease, we conclude that CaV 3.2 channels, by modulating pressure- and flow-mediated vasomotor responses to prevent excess arterial tone, protect against cardiovascular disease.

Novel Pathophysiological Mechanisms in Hypertension

Hypertension is the most common disease affecting humans and imparts a significant cardiovascular and renal risk to patients. Extensive research over the past few decades has enhanced our understanding of the underlying mechanisms in hypertension. However, in most instances, the cause of hypertension in a given patient continues to remain elusive. Nevertheless, achieving aggressive blood pressure goals significantly reduces cardiovascular morbidity and mortality, as demonstrated in the recently concluded SPRINT trial. Since a large proportion of patients still fail to achieve blood pressure goals, knowledge of novel pathophysiologic mechanisms and mechanism based treatment strategies is crucial. The following chapter will review the novel pathophysiological mechanisms in hypertension, with a focus on role of immunity, inflammation and vascular endothelial homeostasis. The therapeutic implications of these mechanisms will be discussed where applicable.

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.

Vascular TRP channels: performing under pressure and going with the flow

Endothelial cells and smooth muscle cells of resistance arteries mediate opposing responses to mechanical forces acting on the vasculature, promoting dilation in response to flow and constriction in response to pressure, respectively. In this review, we explore the role of TRP channels, particularly endothelial TRPV4 and smooth muscle TRPC6 and TRPM4 channels, in vascular mechanosensing circuits, placing their putative mechanosensitivity in context with other proposed upstream and downstream signaling pathways.

2014 Williams Harvey Lecture: importance of coronary vasomotion abnormalities-from bench to bedside

Coronary vasomotion abnormalities play important roles in the pathogenesis of ischaemic heart disease, in which endothelial dysfunction and coronary artery spasm are substantially involved. Endothelial vasodilator functions are heterogeneous depending on the vessel size, with relatively greater role of nitric oxide (NO) in conduit arteries and predominant role of endothelium-derived hyperpolarizing factor (EDHF) in resistance arteries, where endothelium-derived hydrogen peroxide serves as an important EDHF. The functions of NO synthases in the endothelium are also heterogeneous with multiple mechanisms involved, accounting for the diverse functions of the endothelium in vasomotor as well as metabolic modulations. Cardiovascular abnormalities and metabolic phenotypes become evident when all three NO synthases are deleted, suggesting the importance of both NO and EDHF. Coronary artery spasm plays important roles in the pathogenesis of a wide range of ischaemic heart disease. The central mechanism of the spasm is hypercontraction of vascular smooth muscle cells (VSMCs), but not endothelial dysfunction, where activation of Rho-kinase, a molecular switch of VSMC contraction, plays a major role through inhibition of myosin light-chain phosphatase. The Rho-kinase pathway is also involved in the pathogenesis of a wide range of cardiovascular diseases and new Rho-kinase inhibitors are under development for various indications. The registry study by the Japanese Coronary Spasm Association has demonstrated many important aspects of vasospastic angina. The ongoing international registry study of vasospastic angina in six nations should elucidate the unknown aspects of the disorder. Coronary vasomotion abnormalities appear to be an important therapeutic target in cardiovascular medicine.

Sex-specific alterations in NOS regulation of vascular function in aorta and mesenteric arteries from spontaneously hypertensive rats compared to Wistar Kyoto rats

The present study tested the hypothesis that spontaneously hypertensive rats (SHR) have impaired nitric oxide synthase (NOS)‐mediated regulation of vascular function versus Wistar‐Kyoto rats (WKY). Aorta and small mesenteric arteries were studied from male and female SHR (M SHR and F SHR) and WKY (M WKY and F WKY). Phenylephrine (PE)‐induced vasoconstriction was greater in aorta of M SHR versus all others (P < 0.05); there were neither sex nor strain differences in PE contraction in mesenteric arteries. The NOS inhibitor l‐Nitro‐Arginine Methyl Ester (l‐NAME) increased PE‐induced vasoconstriction in all rats, although the increase was the least in male SHR (P < 0.05), revealing a blunted vasoconstrictor buffering capacity of NOS. l‐NAME increased sensitivity to PE‐induced constriction only in mesenteric arteries of SHR, although, the maximal percent increase in contraction was comparable among groups. ACh‐induced relaxation was also less in aorta from M SHR versus all others (P < 0.05). ACh relaxation was comparable among groups in mesenteric arteries, although SHR exhibited a greater NOS component to ACh‐induced relaxation than WKY. To gain mechanistic insight into sex and strain differences in vascular function, NOS activity and NOS3 protein expression were measured. Aortic NOS activity was comparable between groups and M SHR had greater NOS3 expression than M WKY. In contrast, although vascular function was largely maintained in mesenteric arteries of SHR, NOS activity was less in SHR versus WKY. In conclusion, M SHR exhibit a decrease in NOS regulation of vascular function compared to F SHR and WKY, although this is not mediated by decreases in NOS activity and/or expression.

The present study tested the hypothesis that spontaneously hypertensive rats (SHR) have impaired nitric oxide synthase (NOS)‐mediated regulation of vascular function versus Wistar‐Kyoto rats (WKY). Aorta and small mesenteric arteries were studied from male and female SHR and WKY. Male SHR showed a decreased NOS regulation of vascular function compared to F SHR and WKY, although this was not mediated by decreases in NOS activity and/or expression.

Role of endogenous hydrogen peroxide during angiotensin type 1 receptor blockers administration in pacing-induced metabolic coronary vasodilatation in dogs in vivo

BACKGROUND

We have previously demonstrated that endothelium-derived hydrogen peroxide (H2O2) is an endothelium-derived hyperpolarizing factor (EDHF) in canine coronary microcirculation in vivo. However, the role of H2O2/EDHF during angiotensin type 1 receptor blockers (ARB) administration in metabolic coronary dilatation in vivo remains to be examined. We examined whether H2O2 during ARB administration is involved in pacing-induced metabolic coronary vasodilatation in dogs in vivo and if so, whether such beneficial effects of ARB administration acutely improve coronary vasodilatation in diabetes mellitus (DM).

METHODS

Canine subepicardial coronary small arteries (CSA,≥ 100 μm) and arterioles (CA, <100 μm) in left anterior descending artery area were continuously observed by an intravital microscope under cyclooxygenase blockade(ibuprofen, 12.5 mg/kg, intravenous infusion, iv). Experiments were performed during paired right ventricular pacing under the following 4 conditions (n=5 each); (i) control, (ii) DM(alloxan 40 mg/ kg, iv, 1 week prior to study), (iii) DM+ARB(olmesartan, 10 μg/kg/min, 10 min, intracoronary infusion,ic)+L-NMMA (NOS inhibitor, 2 !mol/min, ic) and (iv)DM+ARB+catalase (H2O2 discomposer, 1000 U/ml, 5 min, ic).

RESULTS

Cardiac tachypacing (60 to 120 bpm) caused coronary vasodilatation in both-sized arteries under control conditions. DM significantly decreased the vasodilatation compared with control in CSA and there was a residual vasodilatation for the loss of NO in CA, whereas DM+ARB+L-NMMA improved the vasodilatation compared with DM alone in CA and was significantly decreased by DM+ARB+catalase in CA.

CONCLUSIONS

These results indicate that H2O2 during ARB administration is involved in pacing-induced metabolic coronary vasodilatation in DM in vivo and that there are substantial compensatory interactions between NO and H2O2.

Mesenteric arteries responsiveness to acute variations of wall shear stress is impaired in rats with liver cirrhosis

OBJECTIVE

In liver cirrhosis, excessive splanchnic vasodilation is due to abnormal synthesis of endogenous vasodilators and to decreased sensitivity to vasoconstrictors. The role of mechanical stimuli such as wall shear stress (WSS) on splanchnic circulation remains unclear. The aim of this study was to assess the vasodilation induced by wall shear stress (WSS) and acute changes in blood flow in the mesenteric arteries in an experimental model of liver cirrhosis.

MATERIALS AND METHODS

The effect of acute changes in intraluminal flow (0, 10, and 20 μl/min) and WSS on the diameter of the mesenteric arteries (diameters <500 μm) of control and cirrhotic rats was assessed, at baseline and after the inhibition of nitric oxide synthase, cyclooxygenase and hemeoxygenase. Concentration-response curves to phenylephrine were also obtained.

RESULTS

In controls, the increase in intraluminal flow led to a significant increase in arterial diameter (p < 0.05), while WSS remained stable; the effect was maintained in vessels pre-constricted with phenylephrine, blocked by the exposure to indomethacin and L-NAME and restored by the subsequent addition of chromium mesoporphyrin (p < 0.05). In cirrhotic arteries, arterial diameters did not change in response to acute increase in flow, neither at baseline nor after exposure to indomethacin and L-NAME, while WSS increased (p < 0.01). Responsiveness to flow was partially restored (p < 0.05) after exposure of the arteries to chromium mesoporphyrin in addition to indomethacin and L-NAME.

CONCLUSIONS

Arteries from cirrhotic rats showed an abolished responsiveness to acute variations in flow, which exposes the mesenteric endothelium to sudden variations in WSS.

Human recombinant erythropoietin alters the flow-dependent vasodilatation of in vitro perfused rat mesenteric arteries with unbalanced endothelial endothelin-1 / nitric oxide ratio

Chronic use of human recombinant erythropoietin (r-HuEPO) is accompanied by serious vascular side effects related to the rise in blood viscosity and shear stress. We investigated the direct effects of r-HuEPO on endothelium and nitric oxide (NO)-dependent vasodilatation induced by shear stress of cannulated and pressurized rat mesenteric resistance arteries. Intravascular flow was increased in the presence or absence of the NO synthase inhibitor NG-nitro-l-arginine methyl ester (L-NAME; 10−4 mol/L). In the presence of r-HuEPO, the flow-dependent vasodilatation was attenuated, while L-NAME completely inhibited it. The association of r-HuEPO and L-NAME caused a vasoconstriction in response to the rise in intravascular flow. Bosentan (10−5 mol/L), an inhibitor of endothelin-1 (ET-1) receptors, corrected the attenuated vasodilatation observed with r-HuEPO and inhibited the vasoconstriction induced by flow in the presence of r-HuEPO and L-NAME. r-HuEPO and L-NAME exacerbated ET-1 vasoconstriction. At shear stress values of 2 and 14 dyn/cm2 (1 dyn = 10–5 N), cultured EA.hy926 endothelial cells incubated with r-HuEPO, L-NAME, or both released greater ET-1 than untreated cells. In conclusion, r-HuEPO diminishes flow-induced vasodilatation. This inhibitory effect seems to implicate ET-1 release. NO withdrawal exacerbates the vascular effects of ET-1 in the presence of r-HuEPO. These findings support the importance of a balanced endothelial ET-1:NO ratio to avoid the vasopressor effects of r-HuEPO.

Exercise training enhances flow-mediated dilation in spontaneously hypertensive rats

This study investigated the effect of exercise training on the flow-mediated dilation (FMD) in gastrocnemius muscle arteries from spontaneously hypertensive rats (SHR). SHR and WKY rats were divided into sedentary and exercised groups. After swimming exercise for eight weeks, the isolated arteries were mounted on pressurized myograph and FMD responses examined. The role of nitric oxide (NO), prostaglandins (PGs) and endothelium derived hyperpolarizing factor (EDHF) on FMD were assessed by obtaining dilation responses in the presence and absence of pharmacological antagonists. N(omega)-nitro-L-arginine methyl ester (L-NAME), indomethacin (INDO) and tetraethylamonium (TEA) were used to inhibit nitric oxide synthase, cyclooxygenase and EDHF-mediated responses, respectively. The FMD response was significantly blunted in arteries of SHR compared with WKY rats, and, improved by exercise training in SHR (SHR-ET) group. In SHR arteries, L NAME and TEA did not affect dilation responses to flow, while INDO led to a significant enhancement in this response. Although dilation response was not altered by L-NAME in arteries obtained from trained SHR, TEA caused a significant attenuation and INDO led to significant increases. These results demonstrate that exercise training improves FMD in SHR, and, this enhancement induced by exercise training occurs through EDHF-mediated mechanism(s).

Flow-mediated vasodilation is impaired in adult rat offspring exposed to prenatal hypoxia

There is now a demonstrated association between low birth weight and increased mortality later in life. Changes in fetal development may program the cardiovascular system and lead to an increased risk of cardiovascular diseases later in life. In addition, aging is a risk factor for vascular endothelial-dependent dysfunction. However, the impact of being born intrauterine growth restricted (IUGR) on the normal aging mechanisms of vascular dysfunction is not clear. We hypothesized that IUGR would cause changes in vascular function that would affect the mechanisms of flow-induced vasodilation later in life in an age- or sex-dependent manner. To create an IUGR model, pregnant Sprague-Dawley rats were placed in a hypoxic (11.5% O2) or control (room air, 21% O2) environment from days 15 to 21 of pregnancy. Both male and female offspring were investigated at 4 or 12 mo of age. Vascular function was assessed in small mesenteric arteries using flow-induced vasodilation, a physiological stimuli of vasodilation, in a pressure myograph. Flow-induced vasodilation was unaffected at a young age, but was significantly reduced in aging IUGR compared with aging controls ( P < 0.05). Underlying vasodilator mechanisms were altered such that nitric oxide-mediated vasodilation was abolished in both young adult and aging IUGR males and females and in aging control females ( P > 0.05). Endothelium-derived hyperpolarizing factor-mediated vasodilation was maintained in all groups ( P < 0.01). A change in the mechanisms of vasodilation occurring at an earlier age in IUGR offspring may predispose them to develop cardiovascular diseases as an aging adult.

Oxidant sensing by protein kinases a and g enables integration of cell redox state with phosphoregulation

The control of vascular smooth muscle contractility enables regulation of blood pressure, which is paramount in physiological adaptation to environmental challenges. Maintenance of stable blood pressure is crucial for health as deregulation (caused by high or low blood pressure) leads to disease progression. Vasotone is principally controlled by the cyclic nucleotide dependent protein kinases A and G, which regulate intracellular calcium and contractile protein calcium sensitivity. The classical pathways for activation of these two kinases are well established and involve the formation and activation by specific cyclic nucleotide second messengers. Recently we reported that both PKA and PKG can be regulated independently of their respective cyclic nucleotides via a mechanism whereby the kinases sense cellular oxidant production using redox active thiols. This novel redox regulation of these kinases is potentially of physiological importance, and may synergise with the classical regulatory mechanisms.

Hydrogen peroxide as an endothelium-derived hyperpolarizing factor

The endothelium plays an important role in maintaining cardiovascular homeostasis by synthesizing and releasing several vasodilating substances, including vasodilator prostaglandins, nitric oxide (NO), and endothelium-derived hyperpolarizing factor (EDHF). Since the first report on 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 demonstrated that endothelium-derived hydrogen peroxide (H(2)O(2)) is an EDHF in animals and humans. For the synthesis of H(2)O(2)/EDHF, endothelial NO synthase system that is functionally coupled with Cu,Zn-superoxide dismutase plays a crucial role. Importantly, endothelium-derived H(2)O(2) plays important protective roles in the coronary circulation, including coronary autoregulation, protection against myocardial ischemia/reperfusion injury, and metabolic coronary vasodilatation. 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 important roles as a redox-signaling molecule to cause vasodilatation as well as cardioprotection. In this review, we summarize our current knowledge on H(2)O(2)/EDHF regarding its identification and mechanisms of synthesis and actions.

A sulfaphenazole-sensitive EDHF opposes platelet-endothelium interactions in vitro and in the hamster microcirculation in vivo

AIMS

A CYP2C9-dependent endothelium-derived hyperpolarizing factor (EDHF) controls blood flow in many microvascular beds of various species by targeting vascular smooth muscle potassium channels. Since platelets express the same channels, we tested whether EDHF hyperpolarizes platelets and exerts an antithrombotic function in vivo.

METHODS AND RESULTS

Interaction of injected human platelets with the arteriolar wall (platelet-vessel wall interaction, PVWI) was assessed by intravital microscopy in skin muscle of awake hamsters. To understand the mechanisms of EDHF-induced platelet inhibition, we studied whether cultured human umbilical vein endothelial cells overexpressing CYP2C9-mRNA in vitro released a factor that could hyperpolarize human platelets. Under control conditions, there was no firm adhesion of platelets to the arteriolar wall, but temporary PVWI occurred. Local superfusion of the CYP2C9 inhibitor sulfaphenazole, at doses known to block EDHF-dependent dilations, significantly augmented PVWI, as did inhibition of NO synthase. Inhibition of both factors exerted additive effects on PVWI. Likewise, firm adhesion of a small fraction of platelets was observed. The prothrombotic effects of CYP2C9 inhibition in vivo were reversed by exogenous superfusion with 11,12-epoxyeicosatrienoic acids. Hyperpolarization reduced platelet adhesion to endothelial cells under static conditions in vitro and was dependent on calcium-activated potassium channels. The factor also reduced ADP-induced expression of platelet P-selectin, indicating reduction of platelet activity.

CONCLUSION

The arteriolar endothelium in vivo continuously releases a CYP2C9-derived EDHF. This EDHF exerts its effects by hyperpolarization of platelets through activation of K(Ca) channels and reduction of platelet adhesion molecule expression, indicating that hyperpolarization reduces platelet activation. This demonstrates that EDHF is part of the antithrombotic properties of healthy endothelium in vivo.

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.

Important role of endogenous hydrogen peroxide in pacing-induced metabolic coronary vasodilation in dogs in vivo

OBJECTIVES

We examined whether endogenous hydrogen peroxide (H2O2) is involved in pacing-induced metabolic vasodilation in vivo.

BACKGROUND

We have previously demonstrated that endothelium-derived H2O2 is an endothelium-derived hyperpolarizing factor in canine coronary microcirculation in vivo. However, the role of endogenous H2O2 in metabolic coronary vasodilation in vivo remains to be examined.

METHODS

Canine subepicardial small coronary arteries (> or =100 microm) and arterioles (<100 microm) were continuously observed by a microscope under cyclooxygenase blockade (ibuprofen, 12.5 mg/kg intravenous [IV]) (n = 60). Experiments were performed during paired right ventricular pacing under the following 7 conditions: control, nitric oxide (NO) synthase inhibitor (N(G)-monomethyl-L-arginine [L-NMMA], 2 micromol/min for 20 min intracoronary [IC]), catalase (a decomposer of H2O2, 40,000 U/kg IV and 240,000 U/kg/min for 10 min IC), 8-sulfophenyltheophylline (SPT) (an adenosine receptor blocker, 25 mug/kg/min for 5 min IC), L-NMMA+catalase, L-NMMA+tetraethylammonium (TEA) (K(Ca)-channel blocker, 10 microg/kg/min for 10 min IC), and L-NMMA+catalase+8-SPT.

RESULTS

Cardiac tachypacing (60 to 120 beats/min) caused coronary vasodilation in both-sized arteries under control conditions in response to the increase in myocardial oxygen consumption. The metabolic coronary vasodilation was decreased after L-NMMA in subepicardial small arteries with an increased fluorescent H2O2 production compared with catalase group, whereas catalase decreased the vasodilation of arterioles with an increased fluorescent NO production compared with the L-NMMA group, and 8-SPT also decreased the vasodilation of arterioles. Furthermore, the metabolic coronary vasodilation was markedly attenuated after L-NMMA+catalase, L-NMMA+TEA, and L-NMMA+catalase+8-SPT in both-sized arteries.

CONCLUSIONS

These results indicate that endogenous H2O2 plays an important role in pacing-induced metabolic coronary vasodilation in vivo.

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

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

Depletion of intracellular Ca2+ stores enhances flow-induced vascular dilatation in rat small mesenteric artery

The effect of depleting intracellular Ca2+ stores on flow-induced vascular dilatation and the mechanism responsible for the vasodilatation were examined in rat isolated small mesenteric arteries. The arteries were pressurized to 50 mmHg and preconstricted with phenylephrine. Intraluminal flow reversed the effect of phenylephrine, resulting in vasodilatation. Flow dilatation consisted of an initial transient peak followed by a sustained plateau phase. The magnitude of dilatation was markedly reduced by removing Ca2+ from the intraluminal flow medium. Depletion of intracellular Ca2+ stores with either cyclopiazonic acid (CPA, 2 microM) or 1,4-dihydroxy-2,5-di-tert-butylbenzene (BHQ, 10 microM) significantly augmented the magnitude of flow dilatation. Flow-induced endothelial cell Ca2+ influx was also markedly enhanced in arteries pretreated with CPA or BHQ.Flow-induced dilatation was insensitive to Nw-nitro-L-arginine methyl ester (100 microM) plus indomethacin (3 microM) or to oxyhemoglobin (3 microM), but was markedly reduced by 30 mM extracellular K+ or 2 mM tetrabutylammonium (TBA), suggesting an involvement of EDHF. Catalase at 1200 U ml-1 abolished the flow-induced dilatation, while the application of exogenous H2O2 (90-220 microM) induced relaxation in phenylephrine-preconstricted arteries. Relaxation to exogenous H2O2 was blocked in the presence of 30 mM extracellular K+, and H2O2 (90 microM) hyperpolarized the smooth muscle cells, indicating that H2O2 can act as an EDHF. In conclusion, flow-induced dilatation in rat mesenteric arteries can be markedly enhanced by prior depletion of intracellular Ca2+ stores. Furthermore, these data are consistent with a role for H2O2 as the vasodilator involved.

Direct stimulation of K(ATP) channels by exogenous and endogenous hydrogen sulfide in vascular smooth muscle cells

ATP-sensitive K+ (K(ATP)) channels in vascular smooth muscle cells (VSMC) are important targets for endogenous metabolic regulation and exogenous drug therapy. H2S, as a novel gasotransmitter, has been shown to relax rat aortic tissues via opening of K(ATP) channels. However, interaction of H2S, exogenous-applied or endogenous-produced, with K(ATP) channels in resistance artery VSMC has not been delineated. In the present study, using the whole-cell and single-channel patch-clamp technique, we demonstrated that exogenous H2S activated K(ATP) channels and hyperpolarized cell membrane in rat mesenteric artery VSMC. H2S enhanced the amplitude of whole-cell K(ATP) currents with an EC50 value of 116 +/- 8.3 microM and increased the open probability of single K(ATP) channels. H2S hyperpolarized membrane potentials by -12 mV in nystatin-perforated VSMC. Furthermore, inhibition of endogenous H2S production with D,L-propargylglycine (PPG) reduced whole-cell K(ATP) currents. PPG alone had no effect on unitary K(ATP) channel currents in cell-free membrane patches. In addition, effects of H2S on K(ATP) channels and membrane potentials were independent of cGMP-mediated phosphorylation. This study demonstrated modulation of K(ATP) channel activity by exogenous and endogenous H2S in resistance artery VSMC, thus helping elucidate cardiovascular functions of this endogenous gas.

Implications for repetitive application of acetylcholine in the determination of the mechanisms of endothelium-dependent relaxation

The influence of repetitive vasodilator concentration response curve determination was investigated in rat mesenteric and femoral small arteries. Arteries were precontracted with noradrenaline and relaxed with acetylcholine (ACh) or K+. Mesenteric arteries exhibited attenuation of ACh-stimulated relaxations during repetitive precontraction/relaxation cycles that was not prevented by SQ29548. Apamin, but not charybdotoxin, prevented the attenuation of this relaxation response. Borderline (p=0.064) statistical differences in the relaxations of mesenteric arteries in response to ACh remained in the presence of the nitric oxide synthase inhibitor L-NNA. In contrast, femoral arterial relaxations increased with repeated acetylcholine challenges. The enhanced responsiveness was prevented with L-NNA or Ba2+. In one experimental series, Ba2+ appeared to be without influence upon K+-stimulated relaxation of femoral arteries but a significant inhibitory effect was revealed when appropriate time control experiments were considered. These experiments reveal that impairment of SK(Ca) function and, to a lesser extent, an impaired NO signalling account for the attenuation of the relaxation responses of rat mesenteric arteries. In contrast, alterations of nitric oxide signalling and inward rectifier K channel activity contribute to enhanced relaxation responses in rat femoral arteries. These experiments highlight the importance of appropriate time control experiments for the proper interpretation of results derived from pharmacological experiments directed toward the elucidation of mechanisms of arterial vasorelaxation.

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.

ENHANCED RELEASE OF ENDOTHELIUM-DERIVED HYPERPOLARIZING FACTOR IN SMALL CORONARY ARTERIES FROM RATS WITH CONGESTIVE HEART FAILURE

1. Previous studies have suggested that the production of nitric oxide (NO) is reduced in coronary vessels of animals with congestive heart failure (CHF). However, the response to endothelium-derived hyperpolarizing factor (EDHF) in small coronary resistance arteries from CHF rats has not been investigated. The aim of the present study was to determine whether flow-induced dilation (FID) is altered in small coronary arteries from CHF rats and to characterize the role of EDHF in this process. 2. Small coronary arteries (97 +/- 6 microm) were isolated from control rats and from rats in which CHF was induced by left coronary artery ligation. The arteries were cannulated at 60 mmHg with flow. Changes in internal diameter were examined using videomicroscopy. 3. There was no significant difference in FID in small coronary arteries between control and CHF rats (68 +/- 6 vs 61 +/- 4% (expressed as a percentage of maximal dilation induced by nitroprusside (%MaxD(NP))), respectively). Flow-induced dilation in control rat vessels showed greater attenuation by N(G)-monomethyl-L-arginine (L-NMMA) than vessels from CHF rats (%NO-mediated FID 32 +/- 5 vs 16 +/- 3% (%MaxD(NP)), respectively). Pretreatment with indomethacin had no significant effect on FID in vessels from either rat group. Flow-induced dilation was attenuated by KCl (40 mmol/L) to a greater degree in vessels from CHF rats in the presence of L-NMMA and indomethacin compared with vessels from control rats (%EDHF-mediated FID: 36 +/- 4 vs 25 +/- 5% (%MaxD(NP)), respectively). Flow-induced dilation was abolished by removal of the endothelium and was significantly decreased in vessels from CHF rats in response to charybdotoxin plus apamin or tetrabutylammonium compared with control rat vessels. 17-Octadecynoic acid had no significant effect on FID in vessels from either control or CHF rats. 4. In conclusion, the FID of small coronary arteries is mediated by K+ channels, including the K(Ca) channels. Endothelium-derived hyperpolarizing factor-mediated dilation may compensate for the loss of NO-mediated dilation in CHF.

ORIGINAL RESEARCH—BASIC SCIENCE: Enhancement of Both EDHF and NO/cGMP Pathways Is Necessary to Reverse Erectile Dysfunction in Diabetic Rats

AIMS AND METHODS

Phosphodiesterase 5 (PDE5) inhibitors are less effective in the treatment of erectile dysfunction (ED) in diabetic men than in nondiabetic patients. We have evaluated the effects of sildenafil, a PDE5 inhibitor that enhances the nitric oxide (NO)/cGMP pathway, calcium dobesilate (DOBE), which potentiates endothelium-derived hyperpolarizing factor (EDHF)-mediated responses and the combination of both on erectile responses elicited by cavernosal nerve electrical stimulation (CNES) in a rat model of ED after 8 weeks of streptozotocin-induced diabetes.

RESULTS

After 8 weeks of diabetes, erectile responses to CNES were significantly decreased in diabetic animals compared with nondiabetic time controls. While intravenous administration of sildenafil (0.3 mg/kg) or DOBE (10 mg/kg), individually, enhanced erectile responses in nondiabetic rats (214.7 +/- 34.1% and 268.5 +/- 30.1% of control response at 1 Hz, respectively), each failed to significantly enhance erectile responses in diabetic rats. Only when administered in combination did DOBE and sildenafil markedly potentiate erectile responses in these animals (380.1 +/- 88.6% of control response at 1 Hz), completely restoring erectile function.

CONCLUSIONS

These findings emphasize the importance of NO/cGMP and EDHF pathways for normal erectile function. They also give support to the in vitro observation that diabetes impairs NO and EDHF-dependent responses, precluding the complete recovery of erectile function with PDE5 inhibitors and explaining the relatively poor clinical response of diabetic men with ED to PDE5 inhibition. Finally, our study suggests that a pharmacological approach that combines enhancement of NO/cGMP and EDHF pathways could be necessary to treat ED in many diabetic men.

Involvement of NO and EDHF in Flow-Induced Vasodilation in Isolated Hamster Cremasteric Arterioles

Flow-induced vasodilation in hamster cremasteric arterioles was investigated with special reference to the roles of nitric oxide (NO) and endothelium-derived hyperpolarizing factor (EDHF). Arterioles (approximately 60 microm resting diameter) were cannulated, and suffused with MOPS solution at 37 degrees C (mean intraluminal pressure: 80 cm H(2)O). Step increases in the perfusate flow elicited a dose-dependent vasodilation, which was almost proportional to the increases in calculated wall shear stress (WSS). N(omega)-nitro L-arginine methyl ester (L-NAME, 100 microM) reduced the flow-induced vasodilation by approximately 50%, whereas indomethacin (10 microM) produced no significant effect. In the presence of L-NAME, the residual vasodilation was eliminated by treatment with the cytochrome P-450 monooxygenase inhibitor 17-octadecynoic acid (17-ODYA, 50 microM), sulfaphenazol (10 microM), tetraethylammonium (TEA, 3 mM; a nonselective Ca(2+)-activated K(+) channel inhibitor), or charybdotoxin (ChTX, 0.1 microM; intermediate or large conductance Ca(2+)-activated K(+) channel inhibitor). In the absence of L-NAME, the dilation was also reduced by approximately 50% by treatment with 17-ODYA, TEA, or ChTX. The residual vasodilation was eliminated by additional treatment with L-NAME. The inhibitor of ATP-sensitive K(+) channels (K(ATP)), glibenclamide, also caused a significant, but partial, reduction of the flow-induced vasodilation. The residual vasodilation was completely reduced by additional treatment with 17-ODYA, but not L-NAME. These findings suggest that in hamster cremaster, higher flow rate produces NO, K(ATP), and EDHF vasodilation of the arterioles under physiological conditions.

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.

Effect of renal perfusion pressure on responses of intrarenal blood flow to renal nerve stimulation in rabbits

1. We investigated how sympathetic nerve activity and renal perfusion pressure (RPP) interact in controlling renal haemodynamics in pentobarbitone-anaesthetized rabbits. 2. Renal blood flow (RBF) was reduced by electrical renal nerve stimulation (0.5-8 Hz), with RPP set using an extracorporeal circuit to 65, 100 and 135 mmHg. 3. Responses of RBF and cortical laser Doppler flux to renal nerve stimulation were blunted by increased RPP. For example, 4 Hz stimulation reduced RBF by 68 +/- 7% with baseline perfusion pressure approximately 65 mmHg, but only by 22 +/- 3% at approximately 135 mmHg. Medullary laser Doppler flux was less responsive than cortical laser Doppler flux to renal nerve stimulation and its response was not dependent on perfusion pressure. 4. When perfusion pressure was clamped at its baseline level during renal nerve stimulation, responses of RBF and cortical laser Doppler flux, but not medullary laser Doppler flux, were still blunted with increased baseline perfusion pressure. 5. A frequency rich stimulus was applied to assess the effects of perfusion pressure on dynamic neural control of RBF. Renal blood flow responded similarly at each level of perfusion pressure, as a low-pass filter with a pure time delay. 6. Our results suggest that, in the rabbit extracorporeal circuit model, increased RPP blunts the ability of steady state renal nerve stimulation to reduce cortical, but not medullary perfusion. However, in this model the level of RPP appears to have little impact on dynamic neural control of RBF.

Role of tissue kallikrein in response to flow in mouse resistance arteries

BACKGROUND

Tissue kallikrein, an essential enzyme in the formation of vascular kinins, contributes to flow-dependent dilatation (FDD) in large arteries. We hypothesized that the vascular kinin-kallikrein system may be involved in shear stress signalling in small resistance arteries, which have a key role in the systemic regulation of blood pressure.

OBJECTIVE

To investigate the role of the vascular kallikrein-kinin system in mesenteric resistance arteries of mice during acute changes in blood flow.

DESIGN

Arteries from wild-type mice (TK) and mice lacking tissue kallikrein (TK) were mounted in an arteriograph for the recording of changes in outer diameter during step increases in flow rate.

RESULTS

Responses to phenylephrine, acetylcholine or sodium nitroprusside were not different between the two strains. FDD was significantly reduced in arteries of TK mice compared with that in mesenteric arteries of TK mice exposed to phenylephrine (P = 0.04). FDD was no longer different between TK and TK mice when experiments were performed in the presence of the nitric oxide synthase (NOS) inhibitor N-nitro-l-arginine methyl ester (l-NAME; P = 0.26), l-NAME plus diclofenac (P = 0.73), or l-NAME plus diclofenac plus potassium chloride (P = 0.31), indicating that inactivation of tissue kallikrein preferentially affects the contribution of nitric oxide to flow response. However, expression of endothelial NOS was comparable between TK and TK mesenteric arteries. Finally, the bradykinin B2 receptor antagonist, HOE-140, significantly decreased FDD in TK but not in TK arteries (P = 0.03 and P = 0.82, respectively).

CONCLUSION

These results demonstrate the specific role of the tissue kallikrein in flow-induced dilatation, which is mediated by nitric oxide and bradykinin B2 receptor activation in resistance arteries.

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.

Flow-induced endothelium-dependent vasoreactivity in rat mesenteric arterial bed

We studied rat mesenteric arterial beds to determine the relationship between the effects of flow-induced shear stress and agonists on mesenteric vasoreactivity. When beds were perfused at gradually increasing flow rates, perfusion pressure was flow rate-dependently increased. The flow rate-mediated increase in perfusion pressure was significantly enhanced by N(G)-nitro-L-arginine (L-NOARG) plus methylene blue (MB) and slightly enhanced by treatment with tetraethylammonium (TEA). In the presence of L-NOARG, MB, TEA, and indomethacin, the flow rate-induced increase in perfusion pressure was significantly enhanced, but this enhancement was significantly inhibited by combined treatment with BQ-123 plus BQ-788 (ET(A)- and ET(B)- receptor antagonists, respectively). The ET-1 content of the perfusate was significantly increased following combined pretreatment with L-NOARG, MB, TEA, and indomethacin at a high flow rate. The methoxamine-induced contraction was significantly enhanced by NOS inhibition in both high- and low- flow-treated groups. The released nitrite level was significantly greater in high-flow-loaded than in the low-flow-loaded beds. We conclude that in this model, the response of vascular tone to flow stimulation is subtly regulated by endothelium-derived factors (especially, NO, endothelium-derived hyperpolarizing factor, and ET-1), and that these factors interact with each other.

Structural response of microcirculatory networks to changes in demand: information transfer by shear stress

Matching blood flow to metabolic demand in terminal vascular beds involves coordinated changes in diameters of vessels along flow pathways, requiring upstream and downstream transfer of information on local conditions. Here, the role of information transfer mechanisms in structural adaptation of microvascular networks after a small change in capillary oxygen demand was studied using a theoretical model. The model includes diameter adaptation and information transfer via vascular reactions to wall shear stress, transmural pressure, and oxygen levels. Information transfer is additionally effected by conduction along vessel walls and by convection of metabolites. The model permits selective blocking of information transfer mechanisms. Six networks, based on in vivo data, were considered. With information transfer, increases in network conductance and capillary oxygen supply were amplified by factors of 4.9 +/- 0.2 and 9.4 +/- 1.1 (means +/- SE), relative to increases when information transfer was blocked. Information transfer by flow coupling alone, in which increased shear stress triggers vascular enlargement, gave amplifications of 4.0 +/- 0.3 and 4.9 +/- 0.5. Other information transfer mechanisms acting alone gave amplifications below 1.6. Thus shear-stress-mediated flow coupling is the main mechanism for the structural adjustment of feeding and draining vessel diameters to small changes in capillary oxygen demand.

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.

Chronic hypoxia opposes pregnancy-induced increase in uterine artery vasodilator response to flow

We tested the hypotheses that pregnancy increases the uterine artery (UA) vasodilator response to flow and that this increase is impaired under conditions of chronic hypoxia (30 days, simulated elevation 3,960 m). UA were isolated from 24 normoxic or chronically hypoxic midpregnant guinea pigs and studied with the use of pressure myography. Normoxic pregnancy increased UA flow vasodilator response and protected against a rise in wall shear stress (WSS). Chronic hypoxia opposed these effects, prompting vasoconstriction at high flow and increasing WSS above levels seen in normoxic pregnant UA. The nitric oxide synthase inhibitor N(G)-nitro-l-arginine (l-NNA) eliminated the pregnancy-associated increase in flow vasodilation in normoxic UA, suggesting that increased nitric oxide production was responsible. The considerable residual vasodilation after nitric oxide synthase and cyclooxygenase inhibition implicated endothelial-derived hyperpolarizing factor (EDHF) as an additional contributor to flow vasodilation. l-NNA increased flow vasodilation in UA from chronically hypoxic animals, suggesting that chronic hypoxia may have lowered EDHF or elevated peroxynitrite production. In conclusion, flow is an important physiological vasodilator for the acute and more chronic UA dimensional changes required to increase uteroplacental blood flow during normal pregnancy. Chronic hypoxia may be a mechanism that opposes the pregnancy-associated rise in UA flow vasodilation, thereby increasing the incidence of preeclampsia and intrauterine growth restriction at a high altitude.

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.

Early endothelial dysfunction following renal mass reduction in rats

BACKGROUND

Endothelial dysfunction has been previously described in severely hypertensive rats with renal mass reduction (RMR) receiving large dietary Na loads. Because hypertension and Na loading reduce endothelium-dependent vasodilation, the effect of renal failure per se is unclear.

METHODS

Responses to acetylcholine in noradrenaline-contracted isolated perfused mesenteric arteries were studied. Vessels were obtained from RMR rats kept on a normal diet, 3 and 10 days after surgery, and the results were compared with those from sham-operated rats (SN). The role of three putative mediators of endothelium-dependent vasodilation was assessed using: L-NAME (10(-4) mol L(-1)); indomethacin (INDO, 10(-5) mol L(-1)); and a mixture of charybdotoxin and apamin (C/A, both 10(-7) mol L(-1)), inhibitors of Ca-activated K-channels to mediate the effects of endothelium-derived hyperpolarizing factor (EDHF).

RESULTS

Response to acetylcholine but not that to nitroprusside (endothelium-independent) was decreased in RMR. L-NAME reduced further acetylcholine relaxations in SN but not in RMR. By contrary, INDO decreased acetylcholine vasodilation in RMR but had no effect in SN. C/A had similar effects in the SN and RMR rats. The levels of 6-keto prostaglandin F1alpha were elevated in the urine of the RMR rats and were perfusate from the RMR vessels.

CONCLUSION

Endothelial dysfunction occurs early after RMR, even when systolic blood pressure is only minimally elevated and Na intake is normal. This alteration may be because of decreased availability of nitric oxide, partially compensated by increased prostacyclin production.

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.

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.

Propionate-induced relaxation in rat mesenteric arteries: a role for endothelium-derived hyperpolarising factor

Short chain fatty acids, including propionate, are generated in the caecum and large intestine, and when absorbed may elicit localised increases in intestinal blood flow. We sought to assess the mechanism by which propionate caused vasorelaxation. Propionate-mediated relaxation of noradrenaline-preconstricted rat mesenteric small arteries (RMSAs, i.d. 200-300 microm) was studied using small vessel myography. Propionate (1-30 mM) produced a concentration-dependent relaxation. Relaxation induced by 10 mM propionate (the approximate EC50) was almost abolished by endothelial denudation, although a marked relaxation to a very high concentration of propionate (50 mM) persisted in the absence of the endothelium. In endothelium-intact RMSAs, relaxation to 10 mM propionate was almost abolished by elevating [K+]o to 25 mM, but was unaffected by 100 microM N(omega)-nitro-L-arginine methyl ester (L-NAME) (68 +/- 4 vs. 66 +/- 3% in controls, n = 35), or by 1 microM indomethacin (60 +/- 4 vs. 61 +/- 7 % in controls, n = 15). In the presence of L-NAME, relaxation to 10 mM propionate was significantly and markedly (i.e. > 50 %) inhibited by 50 microM Ba2+ and by the combination of 100 nM charybdotoxin and 100 nM apamin. A similar effect on propionate-mediated relaxation was also exerted by 100 microM ouabain, and by the combination of 50 microM barium with ouabain. Relaxation was also significantly and markedly inhibited by pre-treatment of RMSAs with 100 nM thapsigargin or 10 microM cyclopiazonic acid (CPA). The results demonstrate that 10 mM propionate relaxes RMSAs via endothelium-derived hyperpolarising factor (EDHF). The observation that relaxation by propionate is inhibited by thapsigargin and CPA suggests that this action of propionate involves the release of endothelial cell Ca2+ stores.

Role of small-conductance Ca2+-dependent K+ channels in in vitro nitric oxide-mediated aortic hyporeactivity to alpha-adrenergic vasoconstriction in rats with cirrhosis

BACKGROUND/AIMS

In vitro studies have shown that cirrhotic aortas are hyporeactive to the contractile effect of vasoconstrictors because upregulated endothelial nitric oxide-synthase (NOS) overproduces nitric oxide (NO). Although stimulation of endothelial small-conductance Ca2+-dependent K+ (SK(Ca)) channels may elicit vasorelaxation in normal arteries, the role of these channels in cirrhosis-induced hyporeactivity is unknown. Thus, the aim of the present study was to investigate the role of endothelial SK(Ca) channels in cirrhosis-induced, NO-mediated, in vitro aortic hyporeactivity to alpha1-adrenergic vasoconstrictors.

METHODS

Isolated thoracic aortas from cirrhotic and normal rats were used. The effects of apamin, a selective SK(Ca) channel blocker, were measured on the vascular reactivity to phenylephrine. In addition, SK(Ca) channel protein expression was studied. The effects of iberiotoxin and charybdotoxin, blockers of other K(Ca) channels, were also studied in cirrhotic aortas.

RESULTS

Apamin suppressed cirrhosis-induced aortic hyporeactivity to phenylephrine in an endothelium-dependent, NOS-inhibitor-sensitive manner. SK(Ca) channel protein was overexpressed in cirrhotic aortic walls. Iberiotoxin abolished cirrhosis-induced aortic hyporeactivity to phenylephrine in an endothelium-dependent but NOS-inhibitor-resistant manner. Charybdotoxin did not induce any significant increase in phenylephrine-elicited contraction.

CONCLUSIONS

In cirrhotic aortas, the overexpression and overactivity of endothelial SK(Ca) channels contributes to in vitro NO-mediated hyporeactivity to the contractile action of alpha1-adrenergic agonists.

Endothelium-derived relaxing factors: A focus on endothelium-derived hyperpolarizing factor(s)

Endothelium-derived hyperpolarizing factor (EDHF) is defined as the non-nitric oxide (NO) and non-prostacyclin (PGI2) substance that mediates endothelium-dependent hyperpolarization (EDH) of vascular smooth muscle cells (VSMC). Although both NO and PGI2 have been demonstrated to hyperpolarize VSMC by cGMP- and cAMP-dependent mechanisms, respectively, and in the case of NO by cGMP-independent mechanisms, a considerable body of evidence suggests that an additional cellular mechanism must exist that mediates EDH. Despite intensive investigation, there is no agreement as to the nature of the cellular processes that mediates the non-NO/PGI2 mediated hyperpolarization. Epoxyeicosatrienoic acids (EET), an endogenous anandamide, a small increase in the extracellular concentration of K+, and electronic coupling via myoendothelial cell gap junctions have all been hypothesized as contributors to EDH. An attractive hypothesis is that EDH is mediated via both chemical and electrical transmissions, however, the contribution from chemical mediators versus electrical transmission varies in a tissue- and species-dependent manner, suggesting vessel-specific specialization. If this hypothesis proves to be correct then the potential exists for the development of vessel and organ-selective vasodilators. Because endothelium-dependent vasodilatation is dysfunctional in disease states (i.e., atherosclerosis), selective vasodilators may prove to be important therapeutic agents.Key words: endothelium, nitric oxide, potassium channels, hyperpolarization, gap junctions.

Endothelial dysfunction in cirrhosis and portal hypertension

Portal hypertension (PHT) is a common clinical syndrome associated with chronic liver diseases; it is characterized by a pathological increase in portal pressure. Pharmacotherapy for PHT is aimed at reducing both intrahepatic vascular tone and elevated splanchnic blood flow. Due to the altered hemodynamic profile in PHT, dramatic changes in mechanical forces, both pressure and flow, may play a pivotal role in controlling endothelial and vascular smooth muscle cell signaling, structure, and function in cirrhotics. Nitric oxide, prostacyclin, endothelial-derived contracting factors, and endothelial-derived hyperpolarizing factor are powerful vasoactive substances released from the endothelium in response to both humoral and mechanical stimuli that can profoundly affect both the function and structure of the underlying vascular smooth muscle. This review will examine the contributory role of hormonal- and mechanical force-induced changes in endothelial function and signaling and the consequence of these changes on the structural and functional response of the underlying vascular smooth muscle. It will focus on the pivotal role of hormonal and mechanical force-induced endothelial release of vasoactive substances in dictating the reactivity of the underlying vascular smooth muscle, i.e., whether hyporeactive or hyperreactive, and will examine the extent to which these substances may exert a protective and/or detrimental influence on the structure of the underlying vascular smooth muscle in both a normal hemodynamic environment and following hemodynamic perturbations typical of PHT and cirrhosis. Finally, it will discuss the intracellular processes that regulate the release/expression of these vasoactive substances and that control the transformation of this normally protective cell to one that may promote the development of vasculopathy in PHT.

Recovery of microvascular responses during streptozotocin-induced diabetes

Microvascular reactivity of cannulated and pressurised rat cremaster arterioles was studied during the progress of diabetes using mechanical (intraluminal pressure) and chemical (acetylcholine, sodium nitroprusside) stimulation. Microvessels were studied in controls and at 2, 4 and 8 weeks following induction of diabetes by streptozotocin. Mechanical responses were stable at the test pressure (70 mmHg) used for pharmacological investigations during the period of diabetes. Acetylcholine application could induce maximal dilatation in control vessels and in vessels exposed to 8 weeks of diabetes. However, acetylcholine administration failed to generate maximal dilatation at 2 and 4 weeks of diabetes. During the period of diabetes, loss of nitric oxide (NO) pathway effectiveness was revealed by diminished response to sodium nitroprusside and by reduced capacity of Nomega-nitro-L-arginine methyl ester (L-NAME) to decrease resting diameter and acetylcholine-evoked dilatation. L-NAME and indomethacin application revealed a significant non-NO, non-prostaglandin contribution to the acetylcholine response at 4 and 8 weeks of diabetes. Recovery of responsiveness to acetylcholine and stabilisation of resting vessel diameter during diabetes may, in part, be due to increasing effectiveness of non-NO, non-prostaglandin pathways.

Role of calcium-sensitive K(+) channels and nitric oxide in in vivo coronary vasodilation from enhanced perfusion pulsatility

BACKGROUND

In vitro studies support K(+)(Ca) channel-induced smooth muscle hyperpolarization as underlying acetylcholine-mediated (or bradykinin-mediated) vasodilation that persists despite combined nitric oxide (NO) and PGI(2) inhibition. We tested the hypothesis that these channels are activated by enhanced pulsatile perfusion in vivo and contribute substantially to vasodilation from this stimulus.

METHODS AND RESULTS

The canine left descending coronary artery was perfused with whole blood at constant mean pressure, and physiological flow pulsatility was set at 40 or 100 mm Hg by computer servo-pump. Cyclooxygenase was inhibited by indomethacin. Mean flow increased +18+/-2% (P:<0.0001) with enhanced pulsatility. This response declined approximately 50% by blocking NO synthase (L-NMMA) or K(+)(Ca) [charybdotoxin (CbTX)+apamin (AP)]. Combining both inhibitors virtually eliminated the flow rise. Inhibiting either or both pathways minimally altered basal coronary flow, whereas agonist-stimulated flow was blocked. Bradykinin-induced dilation declined more with CbTX+AP than with L-NMMA (-66% versus -46%, P:=0.03) and was fully blocked by their combination. In contrast, acetylcholine-induced dilation was more blunted by L-NMMA than by CbTX+AP (-71% versus -44%, P:<0.002) and was not fully prevented by the combination. Substituting iberiotoxin (IbTX) for CbTX greatly diminished inhibition of pulse pressure and agonist flow responses (with or without NOS inhibition). Furthermore, blockade by IbTX+AP was identical to that by AP alone, supporting a minimal role of IbTX-sensitive large-conductance K(+)(Ca) channels.

CONCLUSIONS

K(+)(Ca) activation and NO comodulate in vivo pulsatility-stimulated coronary flow, supporting an important role of a hyperpolarization pathway in enhanced mechanovascular signaling. Small- and intermediate-conductance K(+)(Ca) channels are the dominant species involved in modulating both pulse pressure- and bradykinin-induced in vivo coronary dilation.

Heterogeneous control of blood flow amongst different vascular beds

The control and maintenance of vascular tone is due to a balance between vasoconstrictor and vasodilator pathways. Vasomotor responses to neural, metabolic and physical factors vary between vessels in different vascular beds, as well as along the same bed, particularly as vessels become smaller. These differences result from variation in the composition of neurotransmitters released by perivascular nerves, variation in the array and activation of receptor subtypes expressed in different vascular beds and variation in the signal transduction pathways activated in either the vascular smooth muscle or endothelial cells. As the study of vasomotor responses often requires pre-existing tone, some of the reported heterogeneity in the relative contributions of different vasodilator mechanisms may be compounded by different experimental conditions. Biochemical variations, such as the expression of ion channels, connexin subtypes and other important components of second messenger cascades, have been documented in the smooth muscle and endothelial cells in different parts of the body. Anatomical variations, in the presence and prevalence of gap junctions between smooth muscle cells, between endothelial cells and at myoendothelial gap junctions, between the two cell layers, have also been described. These factors will contribute further to the heterogeneity in local and conducted responses.