Exogenous NG-hydroxyl-L-arginine causes nitrite production in vascular smooth muscle cells in the absence of nitric oxide synthase activity

Vasorelaxation induced by a new naphthoquinone-oxime is mediated by NO-sGC-cGMP pathway

It has been established that oximes cause endothelium-independent relaxation in blood vessels. In the present study, the cardiovascular effects of the new oxime 3-hydroxy-4–(hydroxyimino)-2-(3-methylbut-2-enylnaphtalen-1(4H)-one (OximeS1) derived from lapachol were evaluated. In normotensive rats, administration of Oxime S1 (10, 15, 20 and 30 mg/Kg, i.v.) produced dose-dependent reduction in blood pressure. In isolated aorta and superior mesenteric artery rings, Oxime S1 induced endothelium-independent and concentration-dependent relaxations (10⁻⁸ M to 10⁻⁴ M). In addition, Oxime S1-induced vasorelaxations were attenuated by hydroxocobalamin or methylene blue in aorta and by PTIO or ODQ in mesenteric artery rings, suggesting a role for the nitric oxide (NO) pathway. Additionally, Oxime S1 (30 and 100 µM) significantly increased NO concentrations (13.9 ± 1.6 nM and 17.9 ± 4.1 nM, respectively) measured by nitric oxide microsensors. Furthermore, pre-contraction with KCl (80 mM) prevented Oxime S1-derived vasorelaxation in endothelium-denuded aortic rings. Of note, combined treatment with potassium channel inhibitors also reduced Oxime S1-mediated vasorelaxation suggesting a role for potassium channels, more precisely K_ir, K_v and K_ATP channels. We observed the involvement of BK_Ca channels in Oxime S1-induced relaxation in mesenteric artery rings. In conclusion, these data suggest that the Oxime S1 induces hypotension and vasorelaxation via NO pathway by activating soluble guanylate cyclase (sGC) and K⁺ channels.

Participation of nitric oxide pathway in the relaxation response induced by E-cinnamaldehyde oxime in superior mesenteric artery isolated from rats

For many years, nitric oxide (NO) has been studied as an important mediator in the control of vascular tone. Endothelial deficiencies that diminish NO production can result in the development of several future cardiovascular diseases, such as hypertension and arteriosclerosis. In this context, new drugs with potential ability to donate NO have been studied. In this study, 3 aromatic oximes [benzophenone oxime, 4-Cl-benzophenone oxime, and E-cinnamaldehyde oxime (E-CAOx)] induced vasorelaxation in endothelium-denuded and intact superior mesenteric rings precontracted with phenylephrine. E-CAOx demonstrated the most potent effect, and its mechanism of action was evaluated. Vascular reactivity experiments demonstrated that the effect of E-CAOx was reduced by the presence of 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, 1H[1,2,4,]oxadiazolo[4,3-a]quinoxalin-1-one, and (Rp)-8-(para-chlorophenylthio)guanosine-3',5'-cyclic monophosphorothioate, suggesting the participation of NO/sGC/PKG pathway. NO donation seems to be mediated through nicatinamide adenine dinucleotide phosphate-dependent reductases because 7-ethoxyresorufin decreased the effect of E-CAOx on vascular reactivity and reduced NO formation as detected by flow cytometry using the NO indicator diaminofluorescein 4,5-diacetate. Further downstream of NO donation, K+ subtype channels were also shown to be involved in the E-CAOx vasorelaxant effect. The present study showed that E-CAOx acts like an NO donor, activating NO/sGC/PKG pathway and thus K+ channels.

Nitric oxide, inducible nitric oxide synthase and inflammation in veterinary medicine

Inflammation is a process consisting of a complex of cytological and chemical reactions which occur in and around affected blood vessels and adjacent tissues in response to an injury caused by a physical, chemical or biological insult. Much work has been performed in the past several years investigating inducible nitric oxide synthase (NOS, EC 1.14.13.39) and nitric oxide in inflammation. This has resulted in a rapid increase in knowledge about iNOS and nitric oxide. Nitric oxide formation from inducible NOS is regulated by numerous inflammatory mediators, often with contradictory effects, depending upon the type and duration of the inflammatory insult. Equine medicine appears to have benefited the most from the increased interest in this small, inflammatory mediator. Most of the information on nitric oxide in traditional veterinary species has been produced using models or naturally occurring inflammatory diseases of this species.

Activation and potentiation of the NO/cGMP pathway by NG-hydroxyl-L-arginine in rabbit corpus cavernosum under normoxic and hypoxic conditions and ageing

1 When nitric oxide synthase (NOS) produces NO from N(G)-hydroxy-L-arginine (OH-arginine) instead of L-arginine, the total requirement of molecular oxygen and NADPH to form NO is reduced. The aim of this work was to evaluate the effects of OH-arginine on the contractility of rabbit corpus cavernosum (RCC) and to compare the capacities of L-arginine and OH-arginine to enhance NO-mediated responses under normoxic and hypoxic conditions and in ageing, as models of defective NO production. 2 OH-arginine, but not L-arginine, was able to relax phenylephrine-contracted rabbit trabecular smooth muscle. OH-arginine-induced relaxation was inhibited by the NOS-inhibitor, L-NNA (300 microM), and by the guanylyl cyclase inhibitor, ODQ (20 microM), while it was not affected by the cytochrome P450 oxygenase inhibitor, miconazole (0.1 mM). Administration of OH-arginine, but not L-arginine, produced a significant increment of cGMP accumulation in RCC tissue. 3 Relaxation elicited by OH-arginine (300 microM) was still observed at low oxygen tension. The increase of cGMP levels induced by ACh (30 microM) in RCC was significantly enhanced by addition of OH-arginine (300 microM) in normoxic conditions, as well as under hypoxia, while L-arginine did not alter the effects of ACh on cGMP accumulation. 4 Endothelium-dependent and nitrergic nerve-mediated relaxations were both significantly reduced in RCC from aged animals (>20-months-old) when compared with young adult rabbits (5-months-old). Treatment with OH-arginine (300 microM) significantly potentiated endothelium-dependent and neurogenic relaxation in corpus cavernosum from aged rabbits, while L-arginine (300 microM) did not have significant effects. 5 Results show that OH-arginine promotes NO-mediated relaxation of RCC and potentiates the NO-mediated responses induced by stimulation of endogenous NO generation in hypoxic and aged tissues. We propose that the use of OH-arginine could be of interest in the treatment of erectile dysfunction, at least in those secondary to defective NO production.

Involvement of NO in the endothelium-independent relaxing effects of N(omega)-hydroxy-L-arginine and other compounds bearing a C=NOH function in the rat aorta

The mechanisms of vasorelaxation elicited by N(omega)-hydroxy-L-arginine (L-NOHA) and other compounds bearing a C=NOH function and the structural determinants governing this effect were investigated in rat aorta. L-NOHA, formamidoxime, five aromatic monosubstituted amidoximes, and one aromatic monosubstituted ketoxime elicited relaxation in endothelium-denuded rings. N-Hydroxyguanidine and substituted N-hydroxyguanidines were markedly less active. Relaxations induced by L-NOHA and by the most active studied compound, 4-chlorobenzamidoxime (ClBZA), were unmodified by the presence of endothelium. In endothelium-denuded rings, they were blunted by the NO scavenger 2-phenyl-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide (300 microM) and by the inhibitor of guanylyl-cyclase activation 1H[1,2,4,]oxadiazolo[4,3-a]quinoxalin-1-one (1 microM). In addition, L-NOHA- and ClBZA both caused cGMP accumulation. L-Arginine, but not D-arginine (1 mM), antagonized the effect of L-NOHA but not ClBZA. Both L-NOHA- and ClBZA-induced relaxations were inhibited by the NAD(P)H-dependent enzymes inhibitor diphenyliodonium (30 microM) and the NAD(P)H-dependent reductases inhibitor 7-ethoxyresorufin (10 microM), but they were unmodified by the cytochrome P450 (P450) inhibitor proadifen (10 microM) and by the NO synthase inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME, 300 microM). These results show that L-NOHA and other compounds with a C=NOH function can cause endothelium-independent relaxation in the rat aorta. They suggest that activation of guanylyl cyclase and NO formation is implicated in relaxation and that a 7-ethoxyresorufin-sensitive NAD(P)H-dependent pathway is involved. On one hand, L-NOHA and amidoximes may be useful tools for characterizing this pathway in blood vessels and, on the other, may offer a novel approach for treating vascular diseases with impaired endothelial NO activity.

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.

Decreased plasma concentrations of L-hydroxy-arginine as a marker of reduced NO formation in patients with combined cardiovascular risk factors

Patients with metabolic syndrome represent a group with extensive cardiovascular risk factors for the development of atherosclerosis, which may be preceded by an impairment of endothelial function. Endothelial dysfunction is characterized by a reduced availability of bioactive nitric oxide, the principal mediator of endothelium-dependent vasodilation. In the present study we assessed NO synthesis in vivo by measuring the NO-related amino acids L-arginine and L-citrulline and in particular the stable intermediate compound N(omega)-hydroxy-L-arginine (L-NHA) in patients with metabolic syndrome by using high-performance liquid chromatography (HPLC) analysis. As a prerequisite to our study, we measured the amino acid concentrations in 31 healthy volunteers to investigate gender and age differences. To prove whether blood drawn from peripheral veins reflects plasma concentrations of the whole vessel system, several blood samples from different regions were obtained from patients undergoing elective left and right heart catheterization. In the latter group, no significant differences were noted among the plasma concentrations between the different sample sites. In healthy volunteers, there were no significant differences in plasma concentrations of any one specific amino acid between males and females or age groups. The main finding of the study is that the intermediate product of NO synthesis, L-NHA, is significantly reduced in the plasma samples of patients with a metabolic syndrome as compared with samples from healthy control subjects. The plasma concentrations of the NO precursor L-arginine and the end product of NO synthesis, L-citrulline, were unchanged. In conclusion, our results suggest that plasma levels of L-NHA are independent of age and gender and are not different at various locations within the vascular system. In a group of patients at high risk for the development of atherosclerosis, we found reduced plasma concentrations of L-NHA, either caused by a decreased endothelial NO synthase activity or caused by an increased breakdown of L-NHA by pathways independent of NO synthase, resulting in a reduced availability of L-NHA for NO synthesis.

The inducible nitric oxide synthase in vascular and cardiac tissue

Expression of the inducible form of nitric oxide synthase (iNOS) has been reported in a variety of cardiovascular diseases. The resulting high output nitric oxide (NO) formation, besides the level of iNOS expression, depends also on the expression of the metabolic pathways providing the enzyme with substrate and cofactor. NO may trigger short and long term effects which are either beneficial or deleterious, depending on the molecular targets with which it interacts. These interactions are governed by local factors (like the redox state). In the cardiovascular system, the major targets involve not only guanylyl cyclase, but also other haem proteins, protein thiols, iron-non-haem complexes, and superoxide anion (forming peroxynitrite). The latter has several intracellular targets and may be cytotoxic, despite the existence of endogenous defence mechanisms. These interactions may either trigger NO effects or represent releasable NO stores, able to buffer NO and prolong its effects in blood vessels and in the heart. Besides selectively inhibiting iNOS, a number of other therapeutic strategies are conceivable to alleviate deleterious effects of excessive NO formation, including peroxynitrite (ONOO-) scavenging and inhibition of metabolic pathways triggered by ONOO-. When available, these approaches might have the advantage to preserve beneficial effects of iNOS induction. Counteracting vascular hyper-responsiveness to endogenous vasoconstrictor agonists in septic shock, or inducing cardiac protection against ischaemia-reperfusion injury are examples of such beneficial effects of iNOS induction.

Effects of hard metal on nitric oxide pathways and airway reactivity to methacholine in rat lungs

To examine whether the development of hard metal (HM)-induced occupational asthma and interstitial lung disease involves alterations in nitric oxide (NO) pathways, we examined the effects of an industrial HM mixture on NO production, interactions between HM and lipopolysaccharide (LPS) on NO pathways, and alterations in airway reactivity to methacholine in rat lungs. HM (2.5 to 5 mg/100 g intratracheal) increased NO synthase (NOS; EC 1.14.23) activity of rat lungs at 24 h without increasing inducible NOS (iNOS) or endothelial NOS (eNOS) mRNA abundance or iNOS, eNOS, or brain NOS (bNOS) proteins. The increase in NOS activity correlated with the appearance histologically of nitrotyrosine immunofluorescence in polymorphonuclear leukocytes (PMN) and macrophages. Intraperitoneal injection of LPS (1 mg/kg) caused up-regulation of iNOS activity, mRNA, and protein at 8 h but not at 24 h. HM at 2.5 mg/100 g, but not at 5 mg/100 g, potentiated the LPS-induced increase in NOS activity, iNOS mRNA, and protein. However, HM decreased eNOS activity at 8 h and eNOS protein at 24 h. Whole body plethysmography on conscious animals revealed that HM caused basal airway obstruction and a marked hyporeactivity to inhaled methacholine by 6-8 h, which intensified over 30-32 h. HM-treatment caused protein leakage into the alveolar space, and edema, fibrin formation, and an increase in the number of inflammatory cells in the lungs and in the bronchoalveolar lavage. These results suggest that a HM-induced increase in NO production by pulmonary inflammatory cells is associated with pulmonary airflow abnormalities in rat lungs.

Hypocholesterolemic properties of nitric oxide. In vivo and in vitro studies using nitric oxide donors

Previous results suggested that changes in the activity of nitric oxide (NO) can influence metabolism of apo B-containing lipoproteins. Therefore, we studied effects of exogenous NO donors and physiological NO precursors on metabolism of these lipoproteins. In rabbits, addition of 0.03% sodium nitroprusside (NaNP) to a semipurified, cholesterol-free, casein diet counteracted the elevation of LDL cholesterol induced by this diet but did not alter liver lipids after 4 weeks of feeding. In HepG2 cells, addition of nontoxic concentrations of another NO donor, S-nitroso-N-acetylpenicillamine (SNAP) to culture medium caused a dose-dependent reduction of medium apo B after 24 h. At the concentration 0.5 mM, SNAP significantly decreased medium apo B by 50% without altering total synthesis and secretion of proteins and without altering rates of cellular sterol synthesis. In cells incubated with L-arginine, reduction of medium apo B was not associated with increased NO production whereas in those exposed to N-OH-Arg medium apo B levels were not altered. We concluded that synthetic NO donors can reduce hypercholesterolemia by affecting apo B metabolism directly in the liver, via the sterol-independent mechanism.

Oxidative denitrification of the antitumour drug hydroxyguanidine

The oxidative denitrification of the antitumour agent hydroxyguanidine (HOG) has been investigated by radiolysis methods and EPR spectroscopy. The azide radical (N3.), a model one-electron oxidant, reacts with HOG with the rate constant 5.1 x 10(9) dm3 mol(-1) s(-1) to yield the guanidino carbon-centred radical (HOG.) which rapidly eliminates nitric oxide (k = 3.1 x 10[3] s[-1]) with the concomitant formation of urea. The HOG. undergoes conjugation with molecular oxygen to form a peroxyl radical (HOGOO.) with a rate constant 8.8 x 10(8) dm3 mol(-1) s(-1). The HOGOO. radical also eliminates nitric oxide but may act as a precursor to the peroxynitrite (ONOO-) ion. The oxidation of HOG by the dibromide radical (Br2.-) was found to release nitric oxide with a yield of 95% relative to Br2.- as determined from the combined yields of inorganic nitrite, nitrate and a HOG/nitric oxide-adduct. This study provides a possible mechanistic basis for the oxidative denitrification of HOG which may contribute to the observed toxicity of the drug both in vitro and in vivo and for the oxidation of nonphysiological hydroxyguanidines to NO. via nitric oxide synthase-independent pathways.

Role of vascular nitric oxide in physiological and pathological conditions

This review describes the ability of certain diseases, such as essential hypertension, atherosclerosis, angina, and vasospasm, to reduce vascular nitric oxide (NO) formation or to increase its metabolism. In contrast, others, such as hypotension, sepsis, stroke, myocardial depression, and inflammatory responses, increase NO synthesis. The mechanism implicated in the changes in the formation and metabolism of NO are described. To prevent or treat these pathological processes, in which a deficiency in vascular NO formation plays a causative role, NO may be provided through methods such as direct NO administration or indirect NO supply through either NO donors or L-arginine, which facilitates NO formation.

Evidence for the presence of an unusual nitric oxide- and citrulline-producing enzyme in rat kidney

We have found an enzymatic activity obtained from rat kidney capable of producing citrulline and NOx. (nitrate and nitrite) which was resistant to inhibition by conventional arginine analogues. This enzyme activity does not require any calcium or calmodulin and was found to be induced during pregnancy. This unique enzyme was found to be tissue and species specific. Another unique feature of this enzyme is that it did not bind to 2'5'-ADP-sepharose under standard conditions. Western blot analysis of the 100,000 g kidney supernatant using monoclonal antibody for macrophage inducible nitric oxide synthase failed to produce a band for inducible nitric oxide synthase. HPLC and capillary ion analysis for nitrate and nitrite (NOx) showed clear peaks for [3H] L-citrulline and NOx, respectively, which were not changed either in the absence of calcium and calmodulin or in the presence of 300 microM S-ethylisothiourea, which has been shown to be a very potent and selective inhibitor of inducible nitric oxide synthase with a Ki of about 14.7 nM. These results suggest the possible existence of another isoform of nitric oxide synthase with very distinct properties from the known isoforms.

Nitric oxide generation from extracellularly applied NG-hydroxy-L-arginine in LPS-activated RAW 264 macrophages

Lipopolysaccharide (LPS)-activated but not control RAW 264 macrophages produced nitric oxide (NO) from extracellularly-applied NG-hydroxy-L-arginine (L-NOHA) in a concentration-dependent manner, as measured by EPR spin trapping and assays for NO2- and NO3-. This production was inhibited by NG-nitro-L-arginine methyl ester and NG-monomethyl-L-arginine, NO-synthase inhibitors, as well as by L-lysine, a competitor for the y+ amino acid carrier system. No significant differences were found between L-NOHA and L-arginine with respect to the rate of NO production and the effects of inhibitors. These results provide evidence that extracellular L-NOHA can enter LPS-activated RAW 264 macrophages via a cationic amino acid carrier system and be metabolized to NO by NO-synthase. The data also suggest that no alternative pathway exists for NO production from L-NOHA in non-activated RAW 264 macrophages.

Possible mechanism of nitric oxide production from N(G)-hydroxy-L-arginine or hydroxylamine by superoxide ion

It has been speculated the N(G)-hydroxy-L-arginine (OH-L-Arg), which is an intermediate in NO production from L-arginine, may be converted to NO by superoxide ion. However, there is still no direct evidence for this conversion. In the present study this was investigated using superoxide ion generated either in acellular or cellular systems. It was found that OH-L-Arg and hydroxylamine were converted to nitrite and nitrate apparently via NO by superoxide ion in aqueous solution. Arginine remained unaffected. These changes were observed during reaction of chemical substances as well as in a biological system (zymosan-activated macrophages in culture). Superoxide dismutase prevented this transformation. OH-L-Arg was also spontaneously hydrolysed to hydroxylamine and L-citrulline, however this occurred at pH > 9 only. Activated microsomes (containing different isoforms of cytochrome P450) were unable to replace NO-synthase in its ability to produce OH-L-Arg from L-arginine. These data support the hypothesis that a pathway alterative to the well-known synthesis of NO by NO-synthase via OH-L-Arg exists. This pathway may involve the production of OH-L-Arg by NO-synthase and decomposition of OH-L-Arg to NO by the action of superoxide ion. Alternatively, hydrolysis of OH-L-Arg to hydroxylamine may occur followed by its oxidation to NO, again by superoxide ion.

Oxidative denitrification of N omega-hydroxy-L-arginine by the superoxide radical anion

The superoxide radical anion (O2-.) produced during the catalytic activity of nitric oxide synthase (NOS) and cytochrome P-450 has been implicated in the oxidative denitrification of hydroxyguanidines ( > C = NOH). The reactivity of the radiolytically generated O2-. radical with N omega-hydroxy-L-arginine (NHA) is pH dependent and appears to parallel the prototropic equilibrium of the hydroxyguanidino group ( > C = NOH reversible > C = NO(-)+H+; pK = 8). The N omega-hydroxyguanidino group is more reactive towards O2-. when deprotonated but exhibits negligible reactivity when protonated. Based on a model, the rate constant for the reaction of the O2-. with NHA was estimated as kappa (O2-.+ > C = NO-) approximately 200-500 M-1.s-1, which is probably too low to compete with O2-. reactions with NO- or superoxide dismutase, which occur many orders of magnitude faster. The oxidative elimination of NO from NHA by O2-. was not accompanied by the formation of L-citrulline. Since only 21% of NHA will exist in the deprotonated > C = NO- form at physiological pH, it is unlikely that oxidative denitrification of NHA by cytochrome P-450 or NOS-derived O2-. radicals will prove a major free-radical pathway to NO. and L-citrulline.

Mechanisms mediating the vasodilatory effects of N-hydroxy-L-arginine in coronary arteries

We assessed the mechanisms of action of NG-hydroxy-L-arginine in isolated porcine large coronary arterial rings. Increasing (1, 10 and 100 microM) concentrations of NG-hydroxy-L-arginine evoked endothelium-dependent dilation which was eliminated by 100 microM of NG-nitro-L-arginine methyl ester, but not affected by a cytochrome P450 inhibitors (miconazole or 7-ethoxyresorufin). At a given concentration, the dilatory response to NG-hydroxy-L-arginine was stronger than that elicited by L-arginine. NG-Hydroxy-L-arginine (100 microM), but not NG-hydroxy-D-arginine, potentiated the endothelium-dependent dilation of calcium ionophore A23187 but had no effect on endothelium-independent dilation evoked by an NO donor. NO release by endothelium-intact porcine coronary arterial rings was measured with a chemiluminescence analyser. A23187 (10 microM), NG-Hydroxy-L-arginine (100 microM), and to a lesser extent NG-hydroxy-D-arginine (100 microM), significantly increased NO concentration over 15 min observation period. When A23187 and NG-hydroxy-L-arginine were combined, NO concentration increased in an additive fashion. Enhanced NO release by either A23187, NG-hydroxy-L-arginine or NG-hydroxy-D-arginine was attenuated by NG-nitro-L-arginine methyl ester. We conclude that NG-hydroxy-L-arginine exerts its effects on the contractility of coronary arteries by acting as a substrate for the endothelial nitric oxide synthase leading to enhanced NO production. Cytochrome P450 were not involved the dilatory response to NG-hydroxy-L-arginine. In this respect, porcine coronary arteries differ significantly from cultured smooth muscle cells in metabolising NG-hydroxy-L-arginine.

Interactions between endothelial mediators

Prostacyclin, nitric oxide and tissue plasminogen activator constitute a prominent triad of endothelial mediators. Prostacyclin is responsible mainly for maintaining vascular thromboresistance against platelet clumps, inhibits proliferation of vascular smooth muscle and modulates cholesterol turnover, tissue plasminogen activator is a fibrinolytic agent and nitric oxide controls vascular tone and structure. Receptor agonists such as acetylcholine, kinins, endothelins or adenosine diphosphate evoke a coupled release of mediators from endothelial cells. Prostacyclin and nitric oxide synergize in their antiplatelet, fibrinolytic and cardioprotective, but not in their hypotensive actions. Prostacyclin, but not nitric oxide, prevents paradox thrombogenic effects of tissue plasminogen activator. Filogenetically, prostacyclin and tissue plasminogen activator are younger brothers of nitric oxide from which they take over and perfect regulatory properties in circulation. Further studies on interactions of endothelial mediators may lead to a better understanding of mechanisms of thrombosis, atherogenesis, diabetic angiopathies, endotoxic shock and arterial hypertension.

Characterization of the stable L-arginine-derived relaxing factor released from cytokine-stimulated vascular smooth muscle cells as an NG-hydroxyl-L-arginine-nitric oxide adduct

The nature of an L-arginine-derived relaxing factor released from vascular smooth muscle cells cultured on microcarrier beads and stimulated for 20 h with interleukin 1 beta was investigated. Unlike the unstable relaxation elicited by authentic nitric oxide (NO) in a cascade superfusion bioassay system, the effluate from vascular smooth muscle cells induced a stable relaxation that was susceptible to inhibition by oxyhemoglobin. Three putative endogenous NO carriers mimicked this stable relaxing effect: S-nitroso-L-cysteine, low molecular weight dinitrosyl-iron complexes (DNICs), and the adduct of NG-hydroxy-L-arginine (HOArg) with NO. Inactivation of S-nitroso-L-cysteine by Hg2+ ions or trapping of DNICs with agarose-bound bovine serum albumin abolished their relaxing effects, whereas that of the vascular smooth muscle cell effluate remained unaffected. In addition, neither S-nitrosothiols nor DNICs were detectable in the effluate from these cells, as judged by UV and electron spin resonance (ESR) spectroscopy. The HOArg-NO adduct was instantaneously generated upon reaction of HOArg with authentic NO under bioassay conditions. Its pharmacological profile was indistinguishable from that of the vascular smooth muscle cell effluate, as judged by comparative bioassay with different vascular and nonvascular smooth muscle preparations. Moreover, up to 100 nM HOArg was detected in the effluate from interleukin 1 beta-stimulated vascular smooth muscle cells, suggesting that sufficient amounts of HOArg are released from these cells to spontaneously generate the HOArg-NO adduct. This intercellular NO carrier probably accounts for the stable L-arginine-derived relaxing factor released from cytokine-stimulated vascular smooth muscle cells and also from other NO-producing cells, such as macrophages and neutrophils.

Increase in serum NG-hydroxy-L-arginine in rats treated with bacterial lipopolysaccharide

Aortic rings isolated from rats 4 h after an injection i.p. of 30 mg/kg Escherichia coli lipopolysaccharide showed a marked hyporeactivity to noradrenaline. This effect was paralleled by an increase in the level of nitrite/nitrate in the serum of lipopolysaccharide-treated rats, indicative of an enhanced nitric oxide (NO) synthase activity. Most important, however, the serum concentration of the NO synthase intermediate, NG-hydroxy-L-arginine, was also markedly elevated from 3.7 to 15.8 microM. Circulating NG-hydroxy-L-arginine may thus represent a sensitive and specific marker of NO synthase activity in vivo.

Inhibition of arginase by NG-hydroxy-L-arginine in alveolar macrophages: implications for the utilization of L-arginine for nitric oxide synthesis

The hypothesis was investigated that the nitric oxide (NO) synthase intermediate, NG-hydroxy-L-arginine (HOArg), is an arginase inhibitor in rabbit or rat alveolar macrophages. Exogenously applied HOArg strongly inhibited the arginase activity present in these cells (IC50 > or = 15 microM), and attenuated L-[3H]arginine transport (IC50 > or = 500 microM) in rabbit alveolar macrophages. Moreover, up to 37 microM HOArg were detected in the conditioned medium, but not in the lysate, of rat alveolar macrophages exposed to bacterial lipopolysaccharide for 18 h. HOArg may thus be a potent endogenous arginase inhibitor in these cells which increases the availability of L-arginine for NO biosynthesis.

Endothelium-dependent hyperpolarization: a role in the control of vascular tone

Endothelial-dependent relaxation of vascular smooth muscle cells evoked by a number of agonists, including cholinomimetics and substance P, is often accompanied by an increase (repolarization and/or hyperpolarization) in the membrane potential. This change in membrane potential appears predominantly to reflect the action of an endothelial-derived hyperpolarizing factor (EDHF), which is distinct from NO (or endothelial-derived relaxing factor), and is discussed in this article by Chris Garland and colleagues. In large conducting arteries, EDHF may provide a secondary system to NO, which assumes primary importance in some disease states such as pulmonary hypertension and atherosclerosis. However, in small resistance arteries (100-300 microns), EDHF appears to be a major determinant of vascular calibre under normal conditions, and may therefore be of primary importance in the regulation of vascular resistance.