Endothelium-derived hyperpolarizing factor: is there a novel chemical mediator?

Endogenous humoral determinants of vascular endothelial dysfunction as triggers of acute poisoning complications

The vascular endothelium is not only the semipermeable membrane that separates tissue from blood but also an organ that regulates inflammation, vascular tone, blood clotting, angiogenesis and synthesis of connective tissue proteins. It is susceptible to the direct cytotoxic action of numerous xenobiotics and to the acute hypoxia that accompanies acute poisoning. This damage is superimposed on the preformed state of the vascular endothelium, which, in turn, depends on many humoral factors. The probability that an exogenous toxicant will cause life-threatening dysfunction of the vascular endothelium, thereby complicating the course of acute poisoning, increases with an increase in the content of endogenous substances in the blood that disrupt endothelial function. These include ammonia, bacterial endotoxin, indoxyl sulfate, para-cresyl sulfate, trimethylamine N-oxide, asymmetric dimethylarginine, glucose, homocysteine, low-density and very-low-density lipoproteins, free fatty acids and products of intravascular haemolysis. Some other endogenous substances (albumin, haptoglobin, haemopexin, biliverdin, bilirubin, tetrahydrobiopterin) or food-derived compounds (ascorbic acid, rutin, omega-3 polyunsaturated fatty acids, etc.) reduce the risk of lethal vascular endothelial dysfunction. The individual variability of the content of these substances in the blood contributes to the stochasticity of the complications of acute poisoning and is a promising target for the risk reduction measures. Another feasible option may be the repositioning of drugs that affect the function of the vascular endothelium while being currently used for other indications.

Potential role of hydrogen sulfide in diabetes-impaired angiogenesis and ischemic tissue repair

Diabetes is one of the most prevalent metabolic disorders and is estimated to affect 400 million of 4.4% of population worldwide in the next 20 year. In diabetes, risk to develop vascular diseases is two-to four-fold increased. Ischemic tissue injury, such as refractory wounds and critical ischemic limb (CLI) are major ischemic vascular complications in diabetic patients where oxygen supplement is insufficient due to impaired angiogenesis/neovascularization. In spite of intensive studies, the underlying mechanisms of diabetes-impaired ischemic tissue injury remain incompletely understood. Hydrogen sulfide (H₂S) has been considered as a third gasotransmitter regulating angiogenesis under physiological and ischemic conditions. Here, the underlying mechanisms of insufficient H₂S-impaired angiogenesis and ischemic tissue repair in diabetes are discussed. We will primarily focuses on the signaling pathways of H₂S in controlling endothelial function/biology, angiogenesis and ischemic tissue repair in diabetic animal models. We summarized that H₂S plays an important role in maintaining endothelial function/biology and angiogenic property in diabetes. We demonstrated that exogenous H₂S may be a theraputic agent for endothelial dysfunction and impaired ischemic tissue repair in diabetes.

Hyperhomocysteinemia potentiates diabetes-impaired EDHF-induced vascular relaxation: Role of insufficient hydrogen sulfide

Insufficient hydrogen sulfide (H₂S) has been implicated in Type 2 diabetic mellitus (T2DM) and hyperhomocysteinemia (HHcy)-related cardiovascular complications. We investigated the role of H₂S in T2DM and HHcy-induced endothelial dysfunction in small mesenteric artery (SMA) of db/db mice fed a high methionine (HM) diet. HM diet (8 weeks) induced HHcy in both T2DM db/db mice and non-diabetic db/+ mice (total plasma Hcy: 48.4 and 31.3 µM, respectively), and aggravated the impaired endothelium-derived hyperpolarization factor (EDHF)-induced endothelium-dependent relaxation to acetylcholine (ACh), determined by the presence of eNOS inhibitor N(ω)-nitro-L-arginine methyl ester (L-NAME) and prostacyclin (PGI₂) inhibitor indomethacin (INDO), in SMA from db/db mice but not that from db/+ mice. A non-selective Ca²⁺-active potassium channel (K_Ca) opener NS309 rescued T2DM/HHcy-impaired EDHF-mediated vascular relaxation to ACh. EDHF-induced relaxation to ACh was inhibited by a non-selective K_Ca blocker TEA and intermediate-conductance K_Ca blocker (IK_Ca) Tram-34, but not by small-conductance K_Ca (SK_Ca) blocker Apamin. HHcy potentiated the reduction of free sulfide, H₂S and cystathionine γ-lyase protein, which converts L-cysteine to H₂S, in SMA of db/db mice. Importantly, a stable H₂S donor DATS diminished the enhanced O₂^- production in SMAs and lung endothelial cells of T2DM/HHcy mice. Antioxidant PEG-SOD and DATS improved T2DM/HHcy impaired relaxation to ACh. Moreover, HHcy increased hyperglycemia-induced IK_Ca tyrosine nitration in human micro-vascular endothelial cells. EDHF-induced vascular relaxation to L-cysteine was not altered, whereas such relaxation to NaHS was potentiated by HHcy in SMA of db/db mice which was abolished by ATP-sensitive potassium channel blocker Glycolamide but not by K_Ca blockers.

Conclusions

Intermediate HHcy potentiated H₂S reduction via CSE-downregulation in microvasculature of T2DM mice. H₂S is justified as an EDHF. Insufficient H₂S impaired EDHF-induced vascular relaxation via oxidative stress and IK_Ca inactivation in T2DM/HHcy mice. H₂S therapy may be beneficial for prevention and treatment of micro-vascular complications in patients with T2DM and HHcy.

Hyperhomocysteinemia impairs endothelium-derived hyperpolarizing factor-mediated vasorelaxation in transgenic cystathionine beta synthase-deficient mice

Hyperhomocysteinemia (HHcy) is associated with endothelial dysfunction (ED), but the mechanism is largely unknown. In this study, we investigated the role and mechanism of HHcy-induced ED in microvasculature in our newly established mouse model of severe HHcy (plasma total homocysteine, 169.5 μM). We found that severe HHcy impaired nitric oxide (NO)- and endothelium-derived hyperpolarizing factor (EDHF)-mediated, endothelium-dependent relaxations of small mesenteric arteries (SMAs). Endothelium-independent and prostacyclin-mediated endothelium-dependent relaxations were not changed. A nonselective Ca(2+)-activated potassium channel (K(Ca)) inhibitor completely blocked EDHF-mediated relaxation. Selective blockers for small-conductance K(Ca) (SK) or intermediate-conductance K(Ca) (IK) failed to inhibit EDHF-mediated relaxation in HHcy mice. HHcy increased the levels of SK3 and IK1 protein, superoxide (O(2)(-)), and 3-nitrotyrosine in the endothelium of SMAs. Preincubation with antioxidants and peroxynitrite (ONOO(-)) inhibitors improved endothelium-dependent and EDHF-mediated relaxations and decreased O(2)(-) production in SMAs from HHcy mice. Further, EDHF-mediated relaxation was inhibited by ONOO(-) and prevented by catalase in the control mice. Finally, L-homocysteine stimulated O(2)(-) production, which was reversed by antioxidants, and increased SK/IK protein levels and tyrosine nitration in cultured human cardiac microvascular endothelial cells. Our results suggest that HHcy impairs EDHF relaxation in SMAs by inhibiting SK/IK activities via oxidation- and tyrosine nitration-related mechanisms.

P2X purinoceptors mediate an endothelium-dependent hyperpolarization in longitudinal smooth muscle of anterior mesenteric artery in young chickens

BACKGROUND AND PURPOSE

The chicken anterior mesenteric artery contains an outer longitudinal smooth muscle layer, whose neural regulation remains to be elucidated. ATP evokes a depolarization in the smooth muscle through P2Y purinoceptors. However, there may be an additional inhibitory regulation because blockade of P2Y purinoceptors converts the depolarization to hyperpolarization. The objective of the present study was to examine the mechanism underlying this hyperpolarization.

EXPERIMENTAL APPROACH

Membrane potentials of longitudinal smooth muscle of the chicken mesenteric artery were recorded with a microelectrode technique. Perivascular nerves were stimulated by applying electrical field stimulation (EFS).

KEY RESULTS

EFS induced a hyperpolarization in preparations obtained from 5-week-old chickens, whereas it evoked a depolarization in those from 12-week-old chickens. The EFS-evoked hyperpolarization in 5-week-old chickens was blocked by a non-specific purinoceptor antagonist, suramin, and by a specific P2X purinoceptor antagonist, pyridoxal phosphate-6-azophenyl-2',4'-disulphonic acid. Desensitization of the P2X purinoceptor with its agonist alpha,beta-MeATP significantly suppressed EFS-evoked hyperpolarization. Blockade of the P2Y purinoceptor did not affect EFS-evoked hyperpolarization. The application of the NOS inhibitor Nomega-nitro-L-arginine methyl ester or the removal of the endothelium inhibited the hyperpolarization. The application of the nitric oxide (NO) donor sodium nitroprusside mimicked the hyperpolarization. Reverse transcriptase-PCR showed that P2X purinoceptors are expressed in the endothelium of the anterior mesenteric artery.

CONCLUSIONS AND IMPLICATIONS

Hyperpolarization in the longitudinal smooth muscle of the chicken anterior mesenteric artery was induced by ATP. ATP released from perivascular nerves may act on P2X purinoceptors in the endothelium and thereby stimulate NO production.

Dietary Silica Modifies the Characteristics of Endothelial Dilation in Rat Aorta

Considering the importance of nitric oxide generation in the regulation of vessel tone, reduced endothelial nitric oxide synthase (eNOS) expression in alveolar macrophages exposed to short-term silica (Si) suggests the possibility of Si-induced changes in endothelial functions. In this experimental study, the functional changes of the endothelial cells were investigated in the aortic rings of rats subjected to 50 mg Si/kg body weight in their drinking water for 8 days. Norepinephrine elicited contractility and dilation response to acetylcholine (ACh) was significantly high in the aortic rings of Si-treated group. Alteration in receptor-independent endothelial response to A23187 in the aortic rings of Si-exposed rats was less obvious, but sodium nitroprusside (SNP)-elicited dilation was reduced significantly. A23187-induced relaxation was fully eliminated with N-nitro-L-arginine methyl ester (L-NAME) pretreatment, whereas 19.24 +/- 4.36% of ACh response was L-NAME resistant and eliminated with 10-5 M tetraethylammonium (TEA). Despite a significant reduction in the share of NO, the contribution of indomethacine (IND)-sensitive relaxation to ACh response remained unchanged in Si group. As a result, our findings demonstrated that Si both modifies the characteristics of endothelial relaxants and attenuates smooth muscle cell responsiveness to NO. Si-induced reduced NO association with elevated endothelium-derived hyperpolarizing factor (EDHF) in response to ACh, together with reduced NO sensitization, might have clinical importance in cardiovascular pathology.

Redox control of endothelial function and dysfunction: molecular mechanisms and therapeutic opportunities

The endothelium is essential for the maintenance of vascular homeostasis. Central to this role is the production of endothelium-derived nitric oxide (EDNO), synthesized by the endothelial isoform of nitric oxide synthase (eNOS). Endothelial dysfunction, manifested as impaired EDNO bioactivity, is an important early event in the development of various vascular diseases, including hypertension, diabetes, and atherosclerosis. The degree of impairment of EDNO bioactivity is a determinant of future vascular complications. Accordingly, growing interest exists in defining the pathologic mechanisms involved. Considerable evidence supports a causal role for the enhanced production of reactive oxygen species (ROS) by vascular cells. ROS directly inactivate EDNO, act as cell-signaling molecules, and promote protein dysfunction, events that contribute to the initiation and progression of endothelial dysfunction. Increasing data indicate that strategies designed to limit vascular ROS production can restore endothelial function in humans with vascular complications. The purpose of this review is to outline the various ways in which ROS can influence endothelial function and dysfunction, describe the redox mechanisms involved, and discuss approaches for preventing endothelial dysfunction that may highlight future therapeutic opportunities in the treatment of cardiovascular disease.

In vivo regulation of endothelium-dependent vasodilation in the rat renal circulation and the effect of streptozotocin-induced diabetes

We assessed the relative contributions of endothelium-derived relaxing factors to renal vasodilation in vivo and determined whether these are altered in established streptozotocin-induced diabetes. In nondiabetic rats, stimulation of the endothelium by locally administered ACh or bradykinin-induced transient renal hyperemia. Neither basal renal blood flow (RBF) nor renal hyperemic responses to ACh or bradykinin were altered by blockade of prostanoid production (indomethacin) or by administration of charybdotoxin (ChTx) plus apamin to block endothelium-derived hyperpolarizing factor (EDHF). In contrast, combined blockade of nitric oxide (NO) synthase, Nω-nitro-l-arginine methyl ester (l-NAME), and prostanoid production reduced basal RBF and the duration of the hyperemic responses to ACh and bradykinin and revealed a delayed ischemic response to ACh. Accordingly, l-NAME and indomethacin markedly reduced integrated (area under the curve) hyperemic responses to ACh and bradykinin. Peak increases in RBF in response to ACh and bradykinin were not reduced by l-NAME and indomethacin but were reduced by subsequent blockade of EDHF. l-NAME plus indomethacin and ChTx plus apamin altered RBF responses to endothelium stimulation in a qualitatively similar fashion in diabetic and nondiabetic rats. The integrated renal hyperemic responses to ACh and bradykinin were blunted in diabetes, due to a diminished contribution of the component abolished by l-NAME plus indomethacin. We conclude that NO dominates integrated hyperemic responses to ACh and bradykinin in the rat kidney in vivo. After prior inhibition of NO synthase, EDHF mediates transient renal vasodilation in vivo. Renal endothelium-dependent vasodilation is diminished in diabetes due to impaired NO function.

Identification of 15-hydroxy-11,12-epoxyeicosatrienoic acid as a vasoactive 15-lipoxygenase metabolite in rabbit aorta

Arachidonic acid (AA) causes endothelium-dependent smooth muscle hyperpolarizations and relaxations that are mediated by a 15-lipoxygenase-I (15-LO-I) metabolite, 11,12,15-trihydroxyeicosatrienoic acid (11,12,15-THETA). We propose that AA is metabolized sequentially by 15-LO-I and hydroperoxide isomerase to an unidentified hydroxyepoxyeicosatrienoic acid (HEETA), which is hydrolyzed by a soluble epoxide hydrolase (sEH) to 11,12,15-THETA. After incubation of aorta with 14C-labeled AA, metabolites were extracted and the HEETAs were resolved by performing HPLC. Mass spectrometric analyses identified 15-Hydroxy-11,12-epoxyeicosatrienoic acid (15-H-11,12-EETA). Incubation of aortic incubates with methanol and acetic acid trapped the acid-sensitive 15-H-11,12-EETA as methoxydihydroxyeicosatrienoic acids (MDHEs) (367 m/z, M-H). Pretreatment of the aortic tissue with the sEH inhibitor 12-(3-adamantan-1-yl-ureido)-dodecanoic acid (AUDA; 10(-6) M) increased the formation of 15-H-11,12-EETA, measured as MDHEs. Thus 15-H-11,12-EETA is an acid- and sEH-sensitive precursor of 11,12,15-THETA. Aortic homogenates and endothelial cells contain a 57-kDa protein corresponding to the rabbit sEH. In preconstricted aortic rings, AA (10(-7)-10(-4) M) and acetylcholine (10(-9)-10(-6) M) caused concentration-related relaxations that were enhanced by pretreatment with AUDA. These enhanced relaxations were inhibited by increasing extracellular [K(+)] from 4.8 to 20 mM. AA (3 x 10(-6) M) induced cell membrane hyperpolarization (from -31.0 +/- 1 to -46.8 +/- 2 mV) in aortic strips with an intact endothelium, which was enhanced by AUDA. These results indicate that 15-H-11,12-EETA is produced by the aorta, hydrolyzed by sEH to 11,12,15-THETA, and mediates relaxations by membrane hyperpolarization. 15-H-11,12-EETA represents an endothelium-derived hyperpolarizing factor.

Activity of endothelium-derived hyperpolarizing factor is augmented in monocrotaline-induced pulmonary hypertension of rat lungs

The mechanism of endothelium-dependent vasodilator signaling involves three components such as nitric oxide, prostacyclin, and endothelium-derived hyperpolarizing factor (EDHF). Although EDHF is distinct from nitric oxide and prostacyclin, it requires activation of Ca(2+)-sensitive K(+) channels (K(Ca)) and cytochrome P(450) metabolites. However, the physiological role of EDHF in the pulmonary circulation is unclear. Thus, we tested if EDHF would regulate vascular tone in rat lungs of control and monocrotaline (MCT)-induced pulmonary hypertension. Inhibition of EDHF with a combination of K(Ca) blockers, charybdotoxin (50 nM) plus apamin (50 nM), increased baseline vascular tone in MCT-induced hypertensive lungs. Thapsigargin (TG; 100 nM), an inhibitor of Ca-ATPase, caused greater EDHF-mediated vasodilation in MCT-induced hypertensive lungs. TG-induced vasodilation was abolished with the charybdotoxin-apamin combination. Sulfaphenazole (10 muM), a cytochrome P(450) inhibitor, reduced the TG-induced vasodilation in MCT-induced hypertensive lungs. RT-PCR analysis exhibited an increase in K(Ca) mRNA in MCT-treated lungs. These results indicate the augmentation of tonic EDHF activity, at least in part, through the alteration in cytochrome P(450) metabolites and the upregulation of K(Ca) expression in MCT-induced pulmonary hypertension.

The role of myoendothelial gap junctions in the formation of arterial aneurysms: the hypothesis of "connexin 43:40 stoichiometry"

Heterocellular myoendothelial gap junctions (MEGJs) are essential in coordinating and regulating vasomotion. Little is known about their potential role in disease states. We discuss how alteration in the Cx 43:40 expression ratio at the level of MEGJs may begin a chain of reactions in the arterial wall resulting in an aneurysm formation. In this model, we assumed that aneurysm is a chronic arterial disease associated with medial degeneration and intimal hyperplasia. It also was assumed that MEGJs are composed of Cx43 and Cx40 in different stoichiometry and that the characteristic of a given junction is in the favor of its most abundantly expressed constituent. The hypothesis of Cx 43:40 stoichiometry indicates that impaired MEGJs may play a role in the pathogenesis of arterial aneurysms. Cx43 upregulation and Cx40 downregulation (increased Cx 43:40 stoichiometry) may induce a cascade of inflammatory, electrical, metabolic and proliferative derangements in the arterial wall, which finally lead to the matrix degradation, intimal hyperplasia, endothelial-medial dissociation and loss of endothelium-dependent hyperpolarizing currents, irregular vasomotion, impaired growth factor activation, and arterial sympathetic deprivation. The final consequence of these alterations is aneurysm formation.

Interaction of Endothelial Nitric Oxide and Angiotensin in the Circulation

Discovery of the unexpected intercellular messenger and transmitter nitric oxide (NO) was the highlight of highly competitive investigations to identify the nature of endothelium-derived relaxing factor. This labile, gaseous molecule plays obligatory roles as one of the most promising physiological regulators in cardiovascular function. Its biological effects include vasodilatation, increased regional blood perfusion, lowering of systemic blood pressure, and antithrombosis and anti-atherosclerosis effects, which counteract the vascular actions of endogenous angiotensin (ANG) II. Interactions of these vasodilator and vasoconstrictor substances in the circulation have been a topic that has drawn the special interest of both cardiovascular researchers and clinicians. Therapeutic agents that inhibit the synthesis and action of ANG II are widely accepted to be essential in treating circulatory and metabolic dysfunctions, including hypertension and diabetes mellitus, and increased availability of NO is one of the most important pharmacological mechanisms underlying their beneficial actions. ANG II provokes vascular actions through various receptor subtypes (AT1, AT2, and AT4), which are differently involved in NO synthesis and actions. ANG II and its derivatives, ANG III, ANG IV, and ANG-(1-7), alter vascular contractility with different mechanisms of action in relation to NO. This review article summarizes information concerning advances in research on interactions between NO and ANG in reference to ANG receptor subtypes, radical oxygen species, particularly superoxide anions, ANG-converting enzyme inhibitors, and ANG receptor blockers in patients with cardiovascular disease, healthy individuals, and experimental animals. Interactions of ANG and endothelium-derived relaxing factor other than NO, such as prostaglandin I2 and endothelium-derived hyperpolarizing factor, are also described.

Airway smooth muscle excitation-contraction coupling and airway hyperresponsiveness

The primary complaints from patients with asthma pertain to function of airway smooth muscle (ASM) function including shortness of breath, wheezing, and coughing. Thus, it is imperative to better understand the mechanisms underlying excitation-contraction coupling in ASM. Here, we review the various signaling pathways underlying contraction in ASM, and then examine how these are altered in asthma and airway hyperresponsiveness (a hallmark feature of asthma). Throughout, we highlight how studies of vascular smooth muscle have helped or hindered progress in understanding ASM physiology and pathophysiology.

Possible mediators involved in decreasing peripheral vascular resistance with blackcurrant concentrate (BC) in hind-limb perfusion model of the rat

We analyzed mechanisms decreasing hind-limb perfusion pressure (PP) with blackcurrant concentrate (BC) in the rat. The decrease in PP with BC was abolished by endothelial removal, nitroarginine plus tetraethylammonium, nitroarginine plus catalase or 1H-[1,2,4]oxadiazolo[4,3-a]quinoxaline-1-one as an inhibitor of guanylyl cyclase and potassium channel(s), and accompanied by the increased cyclic GMP level. Partial but significant inhibition caused by KCl was observed during contraction. Authentic H2O2 decreased the PP in a sensitive manner to catalase and tetraethylammonium. The decrease in PP with BC in the presence of nitroarginine was significantly attenuated by diverse potassium channel blockers. Two delphinidins of 4 anthocyanins purified from BC definitely decreased the PP through similar mechanisms to BC. These results suggest that the decreased PP with BC is possibly mediated by endothelial NO and H2O2, and partially through the activation of diverse potassium channels, and that 2 delphinidins play a major role as active components of BC.

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.

Mediators involved in decreasing peripheral vascular resistance with carbachol in the rat hind limb perfusion model

We examined the involvement of nitric oxide (NO) and/or endothelium-derived hyperpolarizing factor (EDHF) in decreasing peripheral vascular resistance in the rat hind limb perfusion model and analyzed the identity of EDHF in this model. The potency of carbachol (CCh) to produce relaxation was quantitatively similar to sodium nitroprusside (SNP). CCh-induced relaxation was abolished after endothelial denudation, but resistant to nitroarginine and indomethacin. The relaxation was inhibited by tetraethylammonium, ouabain, charybdotoxin plus apamin, and under depolarization. SNP-induced relaxation was accompanied by increased cGMP production, which was inhibited by ODQ (1H-[1,2,4]oxadiazolo[4,3-a]quinoxaline-l-one). Although CCh produced a similar extent of relaxation to SNP, the cGMP level was 24 times lower than that with SNP. Low KCl produced a definite relaxation, which was inhibited by ouabain, but independent of NO, prostacyclin, and endothelium. 1-EBIO (1-ethyl-2-benzimidazolinone) as an activator of IK(Ca) channel also produced a concentration-dependent relaxation, which was inhibited by charybdotoxin, ouabain, and depolarization, but independent of NO and prostacyclin. Clotrimazole and 17-octadecynoic acid as inhibitors of P(450) monooxygenase inhibited the CCh-induced relaxation. Meanwhile, catalase at a concentration sufficient to inhibit H(2)O(2)-induced relaxation did not exert definite inhibition of the CCh-induced relaxation. These results suggest that CCh produces an endothelium-dependent, EDHF-dependent, and NO-cGMP-independent relaxation and that K(+) and metabolite(s) of P(450) monooxygenase possibly play an important role for this relaxation.

The relaxant effect of vasoactive intestinal polypeptide in the isolated canine uterine artery: the role of endothelium

The purpose of this study was to examine the effect of vasoactive intestinal polypeptide (VIP) on the uterine artery obtained from non-pregnant dogs. VIP (3 x 10(-9)-3 x 10(-7) M) induced concentration-dependent relaxation in canine uterine arteries with intact endothelium, pre-contracted with 10(-5) M phenylephrine (pEC(50) = 7.52 +/- 0.02, maximal response was 82.19 +/- 2.15%, n = 36). The administration of the cyclooxygenase inhibitor indomethacin (10(-5) M) or 4-aminopyridine (4-AP), a blocker of potassium channels (10(-5) M), did not modify the relaxation induced by VIP. Contrary to this, N(G)-nitro-L-arginine (L-NOARG) (10(-5) M) inhibited relaxation is evoked by VIP. Indomethacin applied with L-NOARG did not provoke further inhibition of VIP-induced relaxation. In the presence of both L-NOARG and L-NOARG + indomethacin, 4-AP led to the further inhibition of VIP-induced relaxation of canine uterine artery. It is concluded that VIP induces endothelium-dependent relaxation of uterine arteries of non-pregnant dogs, which can be entirely explained by the production of nitric oxide (NO) from the endothelial cells. We proposed that when NO synthesis is inhibited, VIP induces further relaxation, independent of the edothelium-derived relaxing factors, probably through activation of K(+) channels.

Non-esterified fatty acids impair endothelium-dependent vasodilation in rat mesenteric resistance vessels

Elevated circulating levels of NEFAs (non-esterified fatty acids) are associated with states of insulin resistance and increased risk of vascular disease. Previous animal and human studies have demonstrated NEFA-induced endothelial dysfunction of large conduit arteries, reversible by the antioxidant ascorbic acid. We therefore investigated the effect of NEFAs on carbachol-induced endothelium-dependent vasodilation of rat resistance arteries in vitro using the technique of wire myography. In addition, we investigated the effect of co-incubation of NEFAs and ascorbic acid. Cumulative concentration-response curves to carbachol (endothelium-dependent vasodilation) and SNAP (S-nitroso-N-acetyl-DL-penicillamine; endothelium-independent vasodilation) were constructed. Those to carbachol were repeated following a 30 min incubation with either oleic acid (10(-4) M) or palmitic acid (10(-4) M), demonstrating significant impairment of endothelium-dependent vasodilation with both [P<0.05, comparison of pD2 values (the negative log concentration of agonist required to effect a 50% response)]. A cumulative concentration-response curve to carbachol was repeated following co-incubation with palmitic acid (10(-4) M) and the antioxidant ascorbic acid (10(-5) M), demonstrating an abolition of the previously observed endothelial dysfunction induced by palmitic acid. There was no impairment of vasodilation to SNAP following NEFA incubation. We conclude that NEFAs directly impair endothelial function in rat resistance arteries via an increase in oxidative stress at the vascular endothelium.

Endothelial cell dysfunction and the vascular complications associated with type 2 diabetes: assessing the health of the endothelium

Diabetes-associated vascular complications are collectively the major clinical problems facing patients with diabetes and lead to the considerably higher mortality rate than that of the general population. People with diabetes have a much higher incidence of coronary artery disease as well as peripheral vascular diseases in part because of accelerated atherogenesis. Despite the introduction of new therapies, it has not been possible to effectively reduce the high cardiovascular morbidity and mortality associated with diabetes. Of additional concern is the recognition by the World Health Organization that we are facing a global epidemic of type 2 diabetes. Endothelial dysfunction is an early indicator of cardiovascular disease, including that seen in type 2 diabetes. A healthy endothelium, as defined in terms of the vasodilator/blood flow response to an endothelium-dependent vasodilator, is an important indicator of cardiovascular health and, therefore, a goal for corrective interventions. In this review we explore the cellular basis for endothelial dysfunction in an attempt to identify appropriate new targets and strategies for the treatment of diabetes. In addition, we consider the question of biomarkers for vascular disease and evaluate their usefulness for the early detection of and their role as contributors to vascular dysfunction.

Role of 20-hydroxyeicosatetraenoic acid and epoxyeicosatrienoic acids in hypertension

PURPOSE OF REVIEW

Cytochrome P-450 metabolites of arachidonic acid have been reported to play an important role in the control of renal function and vascular tone, and in the long-term control of arterial pressure. In this regard, 20-hydroxyeicosatetraenoic acid is a potent vasoconstrictor that inhibits sodium reabsorption in the kidney. Epoxyeicosatrienoic acids are endothelium-derived relaxing factors that hyperpolarize vascular smooth muscle cells and also promote sodium excretion in the kidney.

RECENT FINDINGS

Studies have demonstrated that the expression of cytochrome P-450 enzymes and the synthesis of 20-hydroxyeicosatetraenoic acid and epoxyeicosatrienoic acids in the kidney and peripheral vasculature are altered in many genetic and experimental models of hypertension. The production of these compounds is altered following exposure to high-salt or high-fat diets, in hepatorenal syndrome, in diabetes and in patients with toxemia of pregnancy. However, the functional significance of changes in the formation of 20-hydroxyeicosatetraenoic acid and epoxyeicosatrienoic acids in the pathogenesis of hypertension are just being uncovered.

SUMMARY

This review summarizes recent findings that address the issue of whether cytochrome P-450 metabolites of arachidonic acid play an important role in the regulation of renal tubular and peripheral vascular function and contribute to the pathogenesis of hypertension.

Nascent EDHF-mediated cerebral vasodilation in ovariectomized rats is not induced by eNOS dysfunction

In estrogen-depleted [i.e., ovariectomized (Ovx)] animals, an endothelium-derived hyperpolarizing factor (EDHF)-like mechanism may arise to, at least partially, replace endothelial nitric oxide (NO) synthase (eNOS)-derived NO in modulating cerebral arteriolar tone. Additional findings show that eNOS expression and function is restored in estrogen-treated Ovx female rats, while the nascent EDHF-like activity disappears. Because NO has been linked to repression of EDHF activity in the periphery, the current study was undertaken to examine whether the nascent EDHF role in cerebral vessels of Ovx females relates to a chronically repressed eNOS-derived NO-generating function. We compared the effects of chronic NOS inhibition with Nomega-nitro-L-arginine-methyl ester (L-NAME; 100 mg. kg-1. day-1 for 3 wk) on EDHF-mediated pial arteriolar vasodilation in anesthetized intact, Ovx, and 17beta-estradiol-treated (0.1 mg. kg-1. day-1 ip, 1 wk) Ovx (OVE) female rats as well as in male rats that were prepared with closed cranial windows. In the chronic NOS inhibition groups, pial arteriolar responses were monitored in the absence (all groups) and presence (females only) of indomethacin (Indo; 10 mg/kg iv). Finally, the gap junction inhibitory peptide Gap 27 (300 muM) was applied to block EDHF-related vasodilation. NO donor (S-nitroso-N-acetyl-penicillamine) responses were similar in all rats studied. Acetylcholine (ACh) reactivity was virtually absent in control Ovx rats and chronically NOS-inhibited intact female, OVE, and male rats. However, a partial recovery of ACh reactivity was seen in L-NAME-treated Ovx females. In addition, in the presence of L-NAME, a normal CO2 reactivity was observed in all females, whereas a 50% reduction in CO2 reactivity was seen in males. In intact and OVE rats, both chronic and acute (NG-nitro-L-arginine suffusion) NOS inhibition, combined with Indo, depressed ADP-induced dilation by > or =50%, and subsequent application of Gap 27 had no further effect on ADP-induced vasodilation. ADP reactivity was retained in Ovx rats after combined chronic NOS inhibition and acute Indo, but was attenuated significantly by Gap 27. In males, Gap 27 had no effect on arteriolar reactivity. Taken together, our data demonstrate that in the cerebral microcirculation, NO does not have an inhibitory effect on EDHF production or action. The increased EDHF-like function in chronic estrogen-depleted animals is not due to eNOS deficiency, suggesting a more direct effect of estrogen in modulating EDHF-mediated cerebral vasodilation.

Endothelium-dependent Relaxation of Canine Uterine Artery in Response to Acetylcholine: the Possible Involvement of Alternative Pathways

The effect of acetylcholine on the isolated, pre-contracted, uterine artery of non-pregnant dog was investigated. Acetylcholine-induced concentration-dependent relaxation of isolated canine uterine artery with endothelium (pEC50 = 6.48 +/-0.01, n = 37) and was without effect on arterial segments denuded of endothelium. Indomethacin, 4-aminopyridine (10-5 m) and pre-contraction with K+-rich Krebs-Ringer bicarbonate solution had no effect on acetylcholine-induced relaxation. NG-nitro-l-arginine (l-NOARG) (10-5 m) inhibited relaxation evoked by acetylcholine. Indomethacin applied with l-NOARG led to further inhibition of acetylcholine-induced relaxation. In the presence of both l-NOARG and indomethacin, 4-aminopiridine did not provoke further inhibition of acetylcholine-induced relaxation of canine uterine artery. It is concluded that the acetylcholine-induced relaxation of canine uterine artery is probably mediated by endothelial production of nitric oxide (NO). However, if NO-synthase is inhibited, acetylcholine-induced vasorelaxation may be, in part, mediated through activation of cyclooxygenase pathway.

Role of cyclooxygenase and haemoxygenase products in nitric oxide-independent vasodilatation in the porcine ciliary artery

PURPOSE

Vascular endothelial cell dysfunction has been noted in patients with normal pressure glaucoma. Although nitric oxide (NO) accounts for a large proportion of vasorelaxation in the posterior ciliary artery, considerable relaxation remains unexplained. We investigated the roles of haemoxygenase (HO) and cyclooxygenase (COX), which produce the vasodilators carbon monoxide (CO) and prostacyclin, respectively, in NO-independent endothelium-dependent vasodilatation in porcine posterior ciliary arteries.

METHODS

Isolated vascular rings were mounted in a Mulvaney-Halpern small vessel myograph for the measurement of isometric tension development. Vasodilator responses to bradykinin (BK) were elicited in each ring on three separate occasions following preconstriction with prostaglandin F(2alpha): first in the absence of inhibitors, second in the presence of the NO synthase inhibitor N-nitro-L-arginine methyl ester (L-NAME, 10(-3) M), and third in the presence of L-NAME and either a COX (indomethacin, 10(-6) M) or an HO inhibitor (tin protoporphyrin-IX 10(-5) M). Results were expressed as a percentage of the maximal relaxation in the presence of L-NAME alone.

RESULTS

Incubation with indomethacin (n=6), in the presence of L-NAME, significantly reduced (P<0.01) maximum BK-induced relaxation (-103.5+/-8.8%) compared to paired rings in the presence of L-NAME alone (-130.8+/-8.8%). HO inhibition did not reduce NO-independent, BK-induced relaxation when compared to paired control vessels.

CONCLUSIONS

These data suggest that in the presence of L-NAME, a COX product accounts for a significant proportion of NO-independent vasodilatation. In contrast, endogenous CO production does not have a functionally significant role in the porcine ciliary artery. Eye (2003) 17, 628-636. doi:10.1038/sj.eye.6700437

Endothelium-dependent relaxation and endothelial hyperpolarization by P2Y receptor agonists in rat-isolated mesenteric artery

(1) Vasorelaxation and hyperpolarization of endothelial cells by adenosine 5'-[beta-thio]diphosphate (ADPbetaS) and adenosine 5'-[gamma-thio]triphosphate (ATPgammaS) were studied in rat-isolated mesenteric artery. Effects from stimulation of P2X receptors were avoided by desensitization with alpha,beta-methylene adenosine triphosphate. (2) ADPbetaS caused concentration- and endothelium-dependent relaxations of methoxamine-precontracted small (third generation) and main mesenteric artery. These were inhibited by N(omega)-nitro-L-arginine methyl ester (L-NAME) or a combination of apamin plus charybdotoxin (inhibitors of Ca(2+)-activated K(+) channels); L-NAME, apamin and charybdotoxin applied together abolished the response. (3) ATPgammaS induced limited relaxation (35% of methoxamine-induced tone at 10 micro M) of small mesenteric artery, which was sensitive to L-NAME or endothelium denudation. However, it almost completely relaxed the main mesenteric artery over an extended concentration range (>6 orders of magnitude) in an endothelium-dependent manner. This relaxation was inhibited by either L-NAME or a combination of apamin with charybdotoxin, and abolished by a combination of all the three inhibitors. (4) The P2Y(1) receptor antagonist MRS 2179 (2'-deoxy-N(6)-methyladenosine 3',5'-bisphosphate; 0.3-3 micro M) caused parallel rightward shifts of the concentration/relaxation curve to ADPbetaS (pA(2)=7.1). However, MRS 2179 did not inhibit, but potentiated, relaxant responses to ATPgammaS. MRS 2179 did not affect the contractile responses ATPgammaS in small mesenteric artery; ATPgammaS did not contract the main mesenteric artery. (5) ADPbetaS hyperpolarized the endothelium of the main mesenteric artery in a concentration-dependent manner. This was unaffected by L-NAME but antagonized by MRS 2179. ATPgammaS also hyperpolarized the mesenteric artery endothelium in a concentration-dependent manner but, when ATPgammaS was applied at 10 micro M, its effect was potentiated by MRS 2179 (3 micro M). (6) It is concluded that both relaxation and hyperpolarization to ADPbetaS are mediated by P2Y(1) receptors and that the endothelial hyperpolarization is related to the L-NAME-resistant relaxation. Relaxation to the P2Y(2) agonist ATPgammaS shows regional variation along the mesenteric vasculature. The mechanisms for potentiation of relaxation and hyperpolarization by ATPgammaS are unknown, but may indicate interactions between P2Y receptor subtypes.

EDHF-mediated vasodilation involves different mechanisms in normotensive and hypertensive rat lungs

The role of endothelium-derived hyperpolarizing factor (EDHF) in regulating the pulmonary circulation and the participation of cytochrome P-450 (CYP450) activity and gap junction intercellular communication in EDHF-mediated pulmonary vasodilation are unclear. We tested whether tonic EDHF activity regulated pulmonary vascular tone and examined the mechanism of EDHF-mediated pulmonary vasodilation induced by thapsigargin in salt solution-perfused normotensive and hypoxia-induced hypertensive rat lungs. After blockade of both cyclooxygenase and nitric oxide synthase, inhibition of EDHF with charybdotoxin plus apamin did not affect either normotensive or hypertensive vascular tone or acute hypoxic vasoconstriction but abolished thapsigargin vasodilation in both groups of lungs. The CYP450 inhibitors 7-ethoxyresorufin and sulfaphenazole and the gap junction inhibitor palmitoleic acid, but not 18alpha-glycyrrhetinic acid, inhibited thapsigargin vasodilation in normotensive lungs. None of these agents inhibited the vasodilation in hypertensive lungs. Thus tonic EDHF activity does not regulate either normotensive or hypertensive pulmonary vascular tone or acute hypoxic vasoconstriction. Whereas thapsigargin-induced EDHF-mediated vasodilation in normotensive rat lungs involves CYP450 activity and might act through gap junctions, the mechanism of vasodilation is apparently different in hypertensive lungs.

Effect of tumour necrosis factor-alpha and interleukin 1beta on endothelium-dependent relaxation in rat mesenteric resistance arteries in vitro

1. Pre-eclampsia is associated with elevated proinflammatory cytokine levels and endothelial dysfunction. This study examined the effect of two cytokines, tumour necrosis factor-alpha (TNF) and interleukin-1beta (IL-1) on endothelium-dependent relaxation in response to acetylcholine (ACH), bradykinin (BK) and histamine (HIS) in rat mesenteric small arteries in vitro. 2. Rat mesenteric arteries were mounted in an isometric myograph. Tone was induced with phenylephrine (PE) or a depolarizing solution containing 80 mM KCl (K(80)). Relaxation was measured in response to ACH, BK, HIS and sodium nitroprusside (SNP), an endothelium-independent relaxant. Inhibition of NO synthase by a combination of N(omega)-monomethyl-L-arginine (L-NMMA) and N(omega)-nitro-L-arginine methyl ester (L-NAME) significantly inhibited relaxation in response to ACH and BK. Addition of an inhibitor of cyclooxygenase, indomethacin, had no additional effect when added to L-NMMA and L-NAME. Inhibition of endothelium-derived hyperpolarizing factor (EDHF) by K(80) partially reduced responses to ACH and BK. Inhibition of HIS-induced relaxation was more marked with K(80). L-NMMA and L-NAME largely abolished the remaining relaxation to ACH, BK and HIS in arteries contracted with K(80). 3. Preincubation with TNF for 30 min caused an inhibition of relaxation in response to ACH and BK in arteries contracted with PE. Responses to HIS and SNP were not affected by TNF under these conditions. TNF also inhibited ACH-induced relaxation in arteries contracted with K(80). IL-1 had no effect on responses to ACH and the combination of TNF and IL-1 was not more effective than TNF alone. 4. The inhibitory effect of TNF on ACH-induced relaxation was abolished by coincubation with superoxide dismutase (SOD) and was not seen if NO synthase was inhibited by L-NMMA and L-NAME. 5. TNF inhibits the NO-dependent component of endothelium-dependent relaxation in response to ACH and BK, but does not inhibit the EDHF-dependent component. This effect may be attributable to the ability of TNF to increase levels of superoxide anions (O(2)(-)) and the ability of O(2)(-) to inactivate NO. This mechanism could contribute to the endothelial dysfunction seen in situations where TNF is elevated, such as pre-eclampsia.

20-Hydroxyeicosatetraenoic acid potentiates stretch-induced contraction of canine basilar artery via PKC alpha-mediated inhibition of KCa channel

1. The present study was undertaken to elucidate whether PKCalpha plays a role in the mechanism of the stretch-induced contraction potentiated by 20-hydroxyeicosatetraenoic acid (20-HETE). The effects of 20-HETE on the canine basilar artery were compared with those of iberiotoxin, a blocker of large conductance Ca(2+)-activated K(+) channels (K(Ca) channels), as this blocker was shown earlier to sensitize these arteries to mechanical stretch. 2. Slow stretch at rates of 0.1 to 3 mm s(-1) did not produce any contraction in normal physiological solution. 3. In the presence of 20-HETE, the slow stretch could produce contraction, which was inhibited by nicardipine, a 1,4-dihydropyridine Ca(2+) channel blocker, and gadolinium, a blocker of stretch-activated cation channels. 4. 20-HETE inhibited whole-cell K(+) current and depolarized the membrane by approximately 10 mV. These effects of 20-HETE were similar to those of iberiotoxin. 5. Calphostin C, an inhibitor of protein kinase C (PKC), inhibited the action of 20-HETE, but not that of iberiotoxin. 6. In response to 20-HETE PKCalpha isoform was translocated from the cytosol to the membrane fraction, which translocation was inhibited by calphostin C. 7. These results suggest that 20-HETE induced sensitization of the canine basilar artery to stretch was caused by PKCalpha-mediated inhibition of K(Ca) channel activity.