Number of nitrate groups determines reactivity and potency of organic nitrates: a proof of concept study in ALDH-2-/- mice

BACKGROUND AND PURPOSE

Mitochondrial aldehyde dehydrogenase (ALDH-2) has been shown to provide a pathway for bioactivation of organic nitrates and to be prone to desensitization in response to highly potent, but not to less potent, nitrates. We therefore sought to support the hypothesis that bioactivation by ALDH-2 critically depends on the number of nitrate groups within the nitrovasodilator.

EXPERIMENTAL APPROACH

Nitrates with one (PEMN), two (PEDN; GDN), three (PETriN; glyceryl trinitrate, GTN) and four (pentaerithrityl tetranitrate, PETN) nitrate groups were investigated. Vasodilatory potency was measured in isometric tension studies using isolated aortic segments of wild type (WT) and ALDH-2-/- mice. Activity of the cGMP-dependent kinase-I (reflected by levels of phosphorylated VAsodilator Stimulated Phosphoprotein, P-VASP) was quantified by Western blot analysis, mitochondrial dehydrogenase activity by HPLC. Following incubation of isolated mitochondria with PETN, PETriN-chromophore and PEDN, metabolites were quantified using chemiluminescence nitrogen detection and mass spectrometry.

KEY RESULTS

Compared to WT, vasorelaxation in response to PETN, PETriN and GTN was attenuated about 10fold in ALDH-2-/- mice, identical to WT vessels preincubated with inhibitors of ALDH-2. Reduced vasodilator potency correlated with reduced P-VASP formation and diminished biotransformation of the tetranitrate- and trinitrate-compounds. None of these findings were observed for PEDN, GDN and PEMN.

CONCLUSIONS AND IMPLICATIONS

Our results support the crucial role of ALDH-2 in bioactivating highly reactive nitrates like GTN, PETN and PETriN. ALDH-2-mediated relaxation by organic nitrates therefore depends mainly on the number of nitrate groups. Less potent nitrates like PEDN, GDN and PEMN are apparently biotransformed by other pathways.

Molecular mechanisms underlying the role of nitric oxide in the cardiovascular system

In the cardiovascular system, nitric oxide (NO) is involved in the short and long-term regulation of haemodynamics, and in a number of their pathological alterations. Investigation into the biochemistry of NO-synthase isoforms has confirmed that they also all produce superoxide anion (O(*)). The free radical NO can interact with many targets on which novel information has been recently obtained. The major results of these interactions are not only the well known activation of guanylyl cyclase, but also the formation of potentially cytotoxic peroxynitrite (ONOO(-)), and the formation of S-nitrosothiols and non-haem iron-dinitrosyl dithiolate complexes. Tissue O(2), O(*), low molecular weight thiols and transition metals (especially FeII) play a pivotal role in directing NO towards targets responsible for biological effects, or storage or release from these stores. In addition, circulating forms of NO have been proposed with S-nitrosation of blood proteins. All these mechanisms provide potential pharmacological targets for future therapeutic strategies.

Novel activation of non-selective cationic channels by dinitrosyl iron-thiosulfate in PC12 cells

Low molecular mass dinitrosyl iron complexes (DNICs) are nitrosating agents and it is known that the dinitrosyl iron moiety can be transferred to proteins. The aim of the present study was to determine if the formation of protein-bound dinitrosyl iron can modulate ionic channel activity. In PC12 cells, dinitrosyl iron-thiosulfate (50 microM) caused irreversible activation of a depolarizing inward current (IDNIC). IDNIC was partially inhibited by the metal chelator diethyldithiocarbamate (DETC, 1 mM), but not by the reducing/denitrosylating agent dithiothreitol (DTT, 5 mM). The activation of IDNIC was not reproduced by application of nitric oxide (NO., 100 microM), S-nitrocysteine (200 microM) or ferrous iron-thiosulfate (50 microM), and was not prevented by the irreversible guanylyl cyclase inhibitor 1H-[1,2,4]oxadiazolo-[4, 3-a]quinoxalin-1-one (ODQ, 1 microM). Similarly, intracellular perfusion of dinitrosyl iron-thiosulfate (100 microM) did not result in activation of IDNIC. Ion replacement experiments show that the DETC-sensitive component of IDNIC is a non-selective cationic current. In accordance, IDNIC was blocked by antagonists of receptor-operated calcium entry, gadolinium (25 microM) and SK&F 96365 (25 microM). Single-channel measurements from outside-out patches reveal that the DETC-sensitive component of IDNIC is an inward current carried by a cationic channel having a conductance of 50 pS. The present observations suggest that the formation of ion channel-bound dinitrosyl iron represents another mechanism of regulation of ion channel activity by NO.-related species, which may be particularly important in pathophysiological processes where NO. is overproduced.

Adventitia-derived nitric oxide in rat aortas exposed to endotoxin: cell origin and functional consequences

The role of adventitial cells in bacterial lipopolysaccharide (LPS)-induced vascular nitric oxide (NO) overproduction has been largely ignored. In rat aortas exposed to LPS in vitro or in vivo, it was found that adventitia contained the major part of NO synthase (NOS)-2 protein (Western blot and immunohistochemistry) and generated the largest amount of NO (electron paramagnetic resonance spin trapping). NOS-2 immunoreactive cells were mainly resident macrophages at an early stage (5 h, in vitro or in vivo) and fibroblasts at a later stage (20 h, in vitro). Adventitial NOS-2 activity largely accounted for 1) the relaxing effect of L-arginine in rings exposed to LPS in vivo, 2) generation of an "NO store" revealed by N-acetylcysteine-induced relaxation, and 3) formation of protein-bound dinitrosyl iron complexes in the medial layer of aortic rings exposed to LPS in vitro. In conclusion, the adventitia is a powerful source of NO triggered by LPS in the rat aorta. This novel source of NO has an important impact on smooth muscle function and might be implicated in various inflammatory diseases.

Spin trapping of vascular nitric oxide using colloid Fe(II)-diethyldithiocarbamate

Currently available EPR spin-trapping techniques are not sensitive enough for quantification of basal vascular nitric oxide (NO) production from isolated vessels. Here we demonstrate that this goal can be achieved by the use of colloid Fe(DETC)(2). Rabbit aortic or venous strips incubated with 250 microM colloid Fe(DETC)(2) exhibited a linear increase in tissue-associated NO-Fe(DETC)(2) EPR signal during 1 h. Removal of endothelium or addition of 3 mM N(G)-nitro-l-arginine methyl ester (L-NAME) inhibited the signal. The basal NO production was estimated as 5.9 +/- 0.5 and 8.3 +/- 2.1 pmol/min/cm(2) in thoracic aorta and vena cava, respectively. Adding sodium nitrite (10 microM) or xanthine/xanthine oxidase in the incubation medium did not modify the intensity of the basal NO-Fe(DETC)(2) EPR signal. Reducing agents were not required with this method and superoxide dismutase activity was unchanged by the Fe(DETC)(2) complex. We conclude that colloid Fe(DETC)(2) may be a useful tool for direct detection of low amounts of NO in vascular tissue.

Inducible NO synthase activity in blood vessels and heart: new insight into cell origin and consequences

Induction of the inducible form of nitric oxide synthase (iNOS) in the vascular and cardiac tissue by several inflammatory stimuli may result in the production of large amounts of nitric oxide (NO) for a sustained period. Recent data obtained in the rat aorta in which iNOS was induced by lipopolysaccharide (LPS) have demonstrated that adventitial cells represent the main site of NO production. Adventitial-derived NO can exert an immediate down-regulatory effect on smooth muscle contraction (via activation of the cyclic GMP pathway) but may also initiate longer lasting effects through the formation of NO stores within the medial layer. One candidate for such NO stores are dinitrosyl non-heme iron complexes. Low molecular weight thiols interact with preformed NO stores and promote vasorelaxation by a cyclic GMP-independent mechanism involving the activation of potassium channels. In the heart, the induction of iNOS is involved in delayed protection against ischemia-reperfusion-induced functional damages. Recent data obtained with monophosphoryl lipid A, a non-toxin derivative of LPS, strongly suggest that iNOS-derived NO in the rat heart does not act as an immediate mediator of the cardioprotection but rather as a trigger of long-term protective mechanisms. Thus, the present data reveal the important role of adventitial cells as a site of iNOS expression and activity in intact blood vessels. The induction of adaptive mechanisms in the heart and the formation of releasable NO stores in blood vessels are examples of long-term consequences of iNOS induction. These new information are relevant for a better understanding of the circumstances in which NO overproduction by iNOS may play either a beneficial or deleterious role in these tissues.

Triggering role of nitric oxide in the delayed protective effect of monophosphoryl lipid A in rat heart

1. The main objective of the present study was to further evaluate the role of nitric oxide (NO) in delayed cardiac protection against ischaemia-reperfusion injury induced by monophosphoryl lipid A (MLA). 2. For this purpose, rats were administered with either 0.5 or 2.5 mg kg(-1) MLA (i.p.). Eight or 24 h later, in vivo NO production in the heart was analysed by electron paramagnetic resonance (EPR) spin trapping technique. In parallel experiments, hearts were removed and perfused according to Langendorff. Functional ventricular parameters and incidence of ventricular fibrillation (VF) were determined after 30 min global ischaemic insult (37 degrees C) followed by 30 min reperfusion. Vascular reactivity of aortic rings was also assessed. 3. Hearts from rats pretreated with 2.5 mg kg(-1) MLA for 24 h (but not those from rats treated with 0.5 mg kg(-1) MLA for 8 and 24 h, or with 2.5 mg kg(-1) MLA for 8 h) exhibited preservation of ventricular function (LVDP, +/-dP/dtmax) and a reduced incidence of VF (25% vs 87.5% in vehicle control) during reperfusion. At the cardioprotective dose of 2.5 mg kg(-1) (for 8 or 24 h), MLA did not produce alterations of the contractile response of aortic rings to noradrenaline. 4. An increased formation of NO was detected in hearts removed from rats pretreated with 2.5 mg kg(-1) MLA for 8 h, but not in those from rats treated for 24 h (or with 0.5 mg kg(-1) MLA). 5. Pretreatment of the animals with the inhibitors of inducible NO-synthase, aminoguanidine (2x300 mg kg(-1)) or L-N6-(1-Iminoethyl)-lysine (L-NIL, 10 mg kg(-1)) abolished both MLA (2. 5 mg kg(-1))-induced rise of NO production (observed 8 h after MLA) and cardioprotection (observed 24 h after MLA). However MLA-induced cardioprotection was not attenuated when the hearts were perfused with aminoguanidine (150 microM) for 30 min before the ischaemic insult. 6. Altogether, the present data suggest that NO acts as a trigger rather then a direct mediator of the delayed cardioprotective effect of MLA in rat heart.

Dinitrosyl iron complexes with thiol-containing ligands and S-nitroso-D,L-penicillamine as inductors of heat shock protein synthesis in H35 hepatoma cells

The concentration-dependent effect of various nitric oxide donors on synthesis of different heat shock proteins was evaluated in Reuber H35 hepatoma cells and their heat shock protein-inducing ability was compared with the effect of a heat shock. A 6 h incubation of H35 cells with the dimeric (diamagnetic) form of dinitrosyl iron complex with glutathione or N-acetyl-L-cysteine activated synthesis of various heat shock proteins, heat shock protein 28, 32, 60, 70, 90 and 100. Synthesis of these proteins was evaluated by [35S]methionine and [35S]cysteine labelling with subsequent separation of proteins by polyacrylamide gel electrophoresis. The dinitrosyl iron complex with glutathione appeared to be the most efficient inductor of heat shock protein synthesis and initiated the synthesis of heat shock protein 28 even more efficiently than a 30 min heating of cells. In the same experiments, S-nitroso-D,L-penicillamine exerted a considerably lesser effect on the synthesis of heat shock proteins. It was suggested that the active moiety of dinitrosyl iron complexes as inductors of heat shock protein synthesis is represented by their Fe+(NO+)2 groups which move to thiol groups of the proteins participating in the initiation of heat shock protein synthesis.

Induction of nitric oxide synthase and dual effects of nitric oxide and cyclooxygenase products in regulation of arterial contraction in human septic shock

BACKGROUND

The role of endogenous nitric oxide (NO) and cyclooxygenase metabolites was investigated in contractile responses of small omental arteries from patients with hyperdynamic septic shock.

METHODS AND RESULTS

Expression of inducible NO synthase (immunostaining) and a high but variable level of NO production (NO spin trapping) was detected in arteries from patients with septic shock. In these vessels, ex vivo contractile responses to the thromboxane A2 analogue U46619 and to low concentrations of norepinephrine (NE) (up to 10 micromol/L) were not significantly different from controls. However, higher concentrations of NE caused pronounced fading of contraction in septic but not in nonseptic arteries. Exposure to either the NO synthase inhibitor NG-nitro-L-arginine methyl ester or the cyclooxygenase inhibitor indomethacin had no effect in control vessels. However, both inhibitors increased the response to the contractile effects of the 2 agonists only in patients with septic shock. In contrast to NG-nitro-L-arginine methyl ester, which decreased the threshold concentration of the fading effect of NE, indomethacin abolished this effect in arteries from septic patients.

CONCLUSIONS

These results provide direct evidence for the induction of NO synthase in small arteries from patients with septic shock. They suggest that in these arteries, increased production of NO, in conjunction with vasodilatory cyclooxygenase metabolites, contributes to counteract hyperreactivity to agonists and decreases the cyclooxygenase product-mediated pronounced fading of contraction caused by a high concentration of NE.

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.

Role of adventitial nitric oxide in vascular hyporeactivity induced by lipopolysaccharide in rat aorta

This study was designed to elucidate the role of the adventitia in NO-mediated vascular effects of lipopolysaccharide (LPS). After incubation of rat aorta with LPS, the adventitia generated 3.5 times more nitrite plus nitrate than a corresponding segment of media. Control media covered by adventitia from LPS-treated aortic rings exhibited a 4 fold elevated level of cyclic GMP. Medial layers from LPS-treated aortic rings (like LPS-treated adventitia-intact rings) exhibited a decrease in sensitivity to noradrenaline (NA) that was reversed by 1H-(1,2,4)oxadiazolo(4,3-a)quinoxalin-1-one (1 microM) or N omega-nitro-L-arginine methylester (0.3 mM). However, in contrast to LPS-treated adventitia-intact rings, medial layers showed no reduction in maximal contraction to NA and virtually no relaxation to L-arginine. These data indicate that in blood vessels exposed to LPS, the adventitia is a more powerful source of NO than the media. The adventitia-derived NO can reach soluble guanylyl cyclase in the medial layer and contribute greatly to vascular hyporeactivity and L-arginine-induced relaxation observed in blood vessels exposed to LPS.

Nitric oxide-related cyclic GMP-independent relaxing effect of N-acetylcysteine in lipopolysaccharide-treated rat aorta

1. We have recently demonstrated the formation of protein-bound dinitrosyl-iron complexes (DNIC) in rat aortic rings exposed to lipopolysaccharide (LPS) and shown that N-acetylcysteine (NAC) can promote vasorelaxation in these arteries, possibly via the release of nitric oxide (NO) as low molecular weight DNIC from these storage sites. The aim of the present study was to investigate further the mechanism of the relaxation induced by NAC in LPS-treated vessels. 2. In rings incubated with LPS (10 microg ml(-1) for 18 h) and precontracted with noradrenaline (NA, 3 microM) plus N(omega)-nitro-L-arginine methylester (L-NAME, 3 mM), the relaxation evoked by NAC (0.1 to 10 mM) was abolished by 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, 1 microM, a selective inhibitor of soluble guanylyl cyclase) but not affected by Rp-8-bromoguanosine 3'5'-cyclic monophosphorothioate (Rp-8BrcGMPS, 60 microM a selective inhibitor of cyclic GMP-dependent protein kinase). Tetrabutylammonium (TBA, 3 mM, as a non selective K+ channels blocker) or elevated concentration of external KCl (25 or 50 mM) significantly attenuated the NAC-induced relaxation. Selective K+ channels blockers (10 microM glibenclamide, 0.1 microM charybdotoxin, 0.5 microM apamin or 3 mM 4-aminopyridine) did not affect the NAC-induced relaxation. The relaxing effect of NAC (10 mM) was not associated with an elevation of guanosine 3':5' cyclic monophosphate (cyclic GMP) in LPS-treated rings. 3. In aortic rings precontracted with NA (0.1 microM), low molecular weight DNIC (with thiosulphate as ligand, 1 nM to 10 microM) evoked a concentration-dependent relaxation which was antagonized by ODQ (1 microM) and Rp-8BrcGMPS (150 microM) but not significantly affected by TBA (3 mM) or by the use of KCl (50 mM) as preconstricting agent. The relaxation produced by DNIC (0.1 microM) was associated with an 11 fold increase in aortic cyclic GMP content, which was completely abolished by ODQ (1 microM). 4. Taken together with our previous data, the main finding of the present study is that the vascular relaxation induced by NAC in LPS-treated aorta, although probably related to NO through an interaction via preformed NO stores, was not mediated by activation of the cyclic GMP pathway. It may involve the activation of TBA-sensitive K+ channels. The differences in the mechanism of relaxation induced by NAC and by exogenous DNIC suggest that the generation of low molecular weight DNIC from protein-bound species does not play a major role in the NAC-induced relaxation observed in LPS-treated rat aorta. In addition, it is suggested that ODQ may display other properties than the inhibition of soluble guanylyl cyclase.

Induction of nitric oxide production by polyosides from the cell walls of Streptococcus mutans OMZ 175, a gram-positive bacterium, in the rat aorta

The cardiovascular dysfunctions associated with septic shock induced by gram-negative or gram-positive bacteria (gram-positive or gram-negative septic shock) are comparable. In gram-negative septic shock, lipopolysaccharide (LPS) induces nitric oxide (NO) synthase, which contributes to the vascular hypotension and hyporeactivity to vasoconstrictors. The role of NO in gram-positive septic shock and the nature of the bacterial wall components responsible for the vascular effects of gram-positive bacteria are not well known. This study investigated the vascular effects of cell wall serotype polyosides, rhamnose glucose polymers (RGPs), from Streptococcus mutans, in comparison with lipoteichoic acid (LTA) from Staphylococcus aureus, on the induction of NO synthase activity in the rat aorta. We show that 10 microg of both RGPs and LTA per ml induced hyporeactivity to noradrenaline, L-arginine-induced relaxation, increases of 2.2- and 7.8-fold, respectively, of cyclic GMP production, and increases of 7- and 12-fold in nitrite release. All of these effects appeared after several hours of incubation and were inhibited by N(omega)-nitro-L-arginine methyl ester (L-NAME), an inhibitor of NO synthase. Electron paramagnetic resonance spin trapping experiments demonstrated directly that RGPs and LTA induced NO overproduction (four- to eightfold, respectively) in rat aortic rings; this production was inhibited by L-NAME and prevented by dexamethasone. These results demonstrate directly the induction of NO production in vascular tissue by LTA and show that another, chemically different component of gram-positive bacteria can also have these properties. This result suggests that different components of the gram-positive bacterial wall could be implicated in the genesis of cardiovascular dysfunctions observed in gram-positive septic shock.

Nitric oxide production and endothelium-dependent vasorelaxation induced by wine polyphenols in rat aorta

1. The aim of this work was to investigate the mechanism of vasorelaxation induced by red wine polyphenolic compounds (RWPC) and two defined polyphenols contained in wine, leucocyanidol and catechin. The role of the endothelium, especially endothelium-derived nitric oxide (NO), was also investigated. 2. Relaxation produced by polyphenols was studied in rat aortic rings with and without functional endothelium, pre-contracted to the same extent with noradrenaline (0.3 and 0.1 microM, respectively). RWPC and leucocyanidol, but not catechin, produced complete relaxation of vessels with and without endothelium. However, 1000 fold higher concentrations were needed to relax endothelium-denuded rings compared to those with functional endothelium. 3. High concentrations of catechin (in the range of 10(-1) gl-1) only produced partial relaxation (maximum 30%) and had the same potency in rings with and without endothelium. 4. The NO synthase inhibitor, N omega-nitro-L-arginine-methyl-ester (L-NAME, 300 microM) completely abolished the endothelium-dependent but not the endothelium-independent relaxations produced by all of the polyphenolic compounds. 5. In contrast to superoxide dismutase (SOD, 100 u ml-1), neither RWPC nor leucocyanidol affected the concentration-response curve for the NO donor, SIN-1 (3-morpholino-sydnonimine) which also produces superoxide anion (O2-). 6. In aortic rings with endothelium, RWPC (10(-2) gl-1) produced, a 7 fold increase in the basal production of guanosine 3':5'-cyclic monophosphate (cyclic GMP) which was prevented by L-NAME (300 microM). 7. Electron paramagnetic resonance (e.p.r.) spectroscopy studies with Fe(2+)-diethyldithiocarbamate as an NO spin trap demonstrated that RWPC and leucocyanidol increased NO levels in rat thoracic aorta about 2 fold. This NO production was entirely dependent on the presence of the endothelium and was abolished by L-NAME (300 microM). 8. These results show that RWPC and leucocyanidol, but not the structurally closely related polyphenol catechin, induced endothelium-dependent relaxation in the rat aorta. They indicate that this effect results from enhanced synthesis of NO rather than enhanced biological activity of NO or protection against breakdown by O2. It is concluded that some polyphenols, with specific structure, contained in wine possess potent endothelium-dependent vasorelaxing activity.

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.

Evidence for N-acetylcysteine-sensitive nitric oxide storage as dinitrosyl-iron complexes in lipopolysaccharide-treated rat aorta

1. The aim of this study was to assess whether or not vasoactive nitric oxide (NO) stores exist within vascular tissue after lipopolysaccharide (LPS)-treatment. 2. Rat thoracic aortic rings (for contraction experiments) or whole thoracic aortae (for electron paramagnetic resonance (e.p.r.) spectroscopy) were incubated for 18 h at 37 degrees C in the absence (control) or in the presence of LPS (10 micrograms ml-1), with or without L-arginine (L-Arg, 1 mM), the substrate of NO synthase (NOS) or N omega-nitro-L-arginine methyl ester (L-NAME, 1 mM), an inhibitor of NOS. 3. Incubation of rat aortic rings with LPS and L-Arg resulted in a significant decrease of the maximum contractile response to noradrenaline (NA, 3 microM). Addition of L-NAME (3 mM) enhanced contraction towards control values. After precontraction with NA and L-NAME, addition of N-acetyl-L-cysteine (NAC, 0.1 to 10 mM) evoked a concentration-dependent relaxation in rings incubated with LPS and L-Arg, but not in control rings, rings incubated with LPS in the absence of L-Arg or rings incubated with LPS in the presence of L-Arg and L-NAME. Removal of the endothelium did not significantly modify the relaxation induced by NAC. Methylene blue (3 microM), an inhibitor of the activation of guanylyl cyclase by NO, completely abolished the relaxing effect of NAC. 4. The presence of protein-bound dinitrosyl non-haem iron complexes (DNIC) was detected by e.p.r. spectroscopy in aortae incubated with LPS and L-Arg, but not in control aortae. Furthermore in LPS-treated aortae, addition of NAC (20 mM) gave rise to the appearance of an e.p.r. signal characteristic of low molecular weight DNIC. 5. These results provide evidence that, within vascular tissue, NO generated from L-Arg by LPS-induced NOS activity can be stored as protein-bound DNIC in non-endothelial cells. Upon addition of NAC, low molecular weight DNIC are released from these storage sites and induce vascular relaxation probably through guanylyl cyclase activation.

Biotransformation of sodium nitroprusside into dinitrosyl iron complexes in tissue of ascites tumors of mice

We have found that treatment of various types of murine ascites tumors with sodium nitroprusside (SNP) both in vitro (1.5 mM) and in vivo (25 mg/kg; i.p.) results in formation of EPR-detectable dinitrosyl nonheme iron complexes (DNIC) with RS- groups of proteins (g perpendicular = 2,037; g parallel = 2,012). The DNIC were mainly localized in the ascitic extracellular fluid. The appearance of DNIC was unaffected by an inhibitor of nitric oxide synthase, N omega-nitro-L-arginine (1 mM), but abolished with a SH-blockator, p-chloromercuribenzoate (0.1 mM). No DNIC formation was observed when ascitic fluid (after tumor cells separation) was incubated with SNP alone, but with SNP and L-cysteine (10 mM). Thus, ascites tumor cells contribute to the transformation of SNP into DNIC and thiols are essential in this process.