Mitochondrial dysfunction, peroxidation damage and changes in glutathione metabolism in PARK6

Oxidative stress and protein aggregation are biochemical hallmarks of Parkinson's disease (PD), a frequent sporadic late-onset degenerative disorder particularly of dopaminergic neurons in the substantia nigra, resulting in impaired spontaneous movement. PARK6 is a rare autosomal-recessively inherited disorder, mimicking the clinical picture of PD with earlier onset and slower progression. Genetic data demonstrated PARK6 to be caused by mutations in the protein PINK1, which is localized to mitochondria and has a serine-threonine kinase domain. To study the effect of PINK1 mutations on oxidative stress, we used primary fibroblasts and immortalized lymphoblasts from three patients homozygous for G309D-PINK1. Oxidative stress was evident from increases in lipid peroxidation and in antioxidant defenses by mitochondrial superoxide dismutase and glutathione. Elevated levels of glutathione reductase and glutathione-S-transferase were also observed. As a putative cause of oxidation, a mild decrease in complex I activity and a trend to superoxide elevation were detectable. These data indicate that PINK1 function is critical to prevent oxidative damage and that peripheral cells may be useful for studies of progression and therapy of PARK6.

Nebivolol inhibits superoxide formation by NADPH oxidase and endothelial dysfunction in angiotensin II-treated rats

Nebivolol is a beta(1)-receptor antagonist with vasodilator and antioxidant properties. Because the vascular NADPH oxidase is an important superoxide source, we studied the effect of nebivolol on endothelial function and NADPH oxidase activity and expression in the well-characterized model of angiotensin II-induced hypertension. Angiotensin II infusion (1 mg/kg per day for 7 days) caused endothelial dysfunction in male Wistar rats and increased vascular superoxide as detected by lucigenin-derived chemiluminescence, as well as dihydroethidine staining. Vascular NADPH oxidase activity, as well as expression at the mRNA and protein level, were markedly upregulated, as well as NOS III uncoupled, as evidenced by NO synthase III inhibitor experiments and dihydroethidine staining and by markedly decreased hemoglobin-NO concentrations. Treatment with the beta-receptor blocker nebivolol but not metoprolol (10 mg/kg per day for each drug) normalized endothelial function, reduced superoxide formation, increased NO bioavailability, and inhibited upregulation of the activity and expression of the vascular NADPH oxidase, as well as membrane association of NADPH oxidase subunits (Rac1 and p67(phox)). In addition, NOS III uncoupling was prevented. In vitro treatment with nebivolol but not atenolol or metoprolol induced a dissociation of p67(phox) and Rac1, as well as an inhibition of NADPH oxidase activity assessed in heart membranes from angiotensin II-infused animals, as well as in homogenates of Nox1 and cytosolic subunit-transfected and phorbol ester-stimulated HEK293 cells. These findings indicate that nebivolol interferes with the assembly of NADPH oxidase. Thus, inhibitory effects of this beta-blocker on vascular NADPH oxidase may explain, at least in part, its beneficial effect on endothelial function in angiotensin II-induced hypertension.

Mechanisms underlying dysfunction of carotid arteries in genetically hyperlipidemic rabbits

In the present study we compared the vascular reactivity and integrity of the nitric oxide (NO)-cyclic 3',5'-guanosine monophopsphate (cGMP) pathway in carotid arteries of hyper- and normolipidemic rabbits. Vasodilation to acetylcholine, nitroglycerin, and sodium nitroprusside was desensitized in hyperlipidemia, but the nitroprusside-induced relaxation was normalized by an NO synthase inhibitor in endothelium-intact and -denuded vessels. Hyperlipidemic carotid arteries exhibited increased basal NO (detected by EPR spin-trapping) and reactive oxygen species formation (detected by chemiluminescence), whereas acetylcholine-induced NO formation was nearly abolished. Hyperlipidemia increased NADPH-dependent superoxide formation in carotid membranes, and carotid cryosections stained with the fluorescent dye dihydroethidium revealed increased endothelial and medial reactive oxygen species formation. Hyperlipidemia elicited macrophage invasion into the carotid wall, as detected by a dot-immunoblot. The basal activity of cGMP-dependent proteinkinase, the nitroprusside-stimulated activity of soluble guanylyl cyclase, and its protein expression were decreased by hyperlipidemia. The cGMP phosphodiesterase activity was marginally increased by hyperlipidemia, such that the ratio of cGMP-forming vs. -degrading capacity was decreased by 2-fold. Hyperlipidemia triggers infiltration of macrophages into the carotid wall and endothelial as well as smooth muscle superoxide formation. Consequently, relaxation of the carotid arteries are impaired due to smooth muscle and endothelial dysfunction.

Biochemistry and pharmacology of novel anthranilic acid derivatives activating heme-oxidized soluble guanylyl cyclase

The heme-enzyme soluble guanylyl cyclase (sGC) is an ubiquitous NO receptor, which mediates NO downstream signaling by the generation of cGMP. We studied the mechanism of action of the anthranilic acid derivatives 5-chloro-2-(5-chloro-thiophene-2-sulfonylamino-N-(4-(morpholine-4-sulfonyl)-phenyl)-benzamide sodium salt (HMR1766) (proposed international nonproprietary name, ataciguat sodium) and 2-(4-chloro-phenylsulfonylamino)-4,5-dimethoxy-N-(4-(thiomorpholine-4-sulfonyl)-phenyl)-benzamide (S3448) as a new class of sGC agonists. Both compounds activated different sGC preparations (purified from bovine lung, or crude from human corpus cavernosum) in a concentration-dependent and quickly reversible fashion (EC50 = 0.5-10 microM), with mixed-type activation kinetics. Activation of sGC by these compounds was additive to activation by NO donors, but instead of being inhibited, it was potentiated by the heme-iron oxidants 1H-[1,2,4]-oxdiazolo[3,4-a]quinoxalin-1-one (ODQ) and 4H-8-bromo-1,2,4-oxadiazolo(3,4-d) benz(b)(1,4)oxazin-1-one (NS2028), suggesting that the new compounds target the ferric heme sGC isoform. Protoporphyrin IX acted as a competitive activator, and zinc-protoporphyrin IX inhibited activation of heme-oxidized sGC by HMR1766 and S3448, whereas heme depletion of sGC by Tween 20 treatment reduced activation. Both compounds increased cGMP levels in cultured rat aortic smooth muscle cells; induced vasorelaxation of isolated endothelium-denuded rat aorta, porcine coronary arteries, and human corpus cavernosum (EC50 1 to 10 microM); and elicited phosphorylation of the cGMP kinase substrate vasodilator-stimulated phosphoprotein at Ser239. HMR1766 intravenous bolus injection decreased arterial blood pressure in anesthetized pigs. All of these pharmacological responses to the new compounds were enhanced by ODQ and NS2028. Our findings suggest that HMR1766 and S3448 preferentially activate the NO-insensitive heme-oxidized form of sGC, which exists to a variable extent in vascular tissues, and is a pharmacological target for these new vasodilator drugs.

Hydralazine is a powerful inhibitor of peroxynitrite formation as a possible explanation for its beneficial effects on prognosis in patients with congestive heart failure

The hemodynamic and anti-ischemic effects of nitroglycerin (GTN) are rapidly blunted as a result of the development of nitrate tolerance. Hydralazine has been shown to prevent tolerance in experimental and clinical studies, all of which may be at least in part secondary to antioxidant properties of this compound. The antioxidant effects of hydralazine were tested in cell free systems, cultured smooth muscle cells, isolated mitochondria, and isolated vessels. Inhibitory effects on the formation of superoxide and/or peroxynitrite formation were tested using lucigenin and L-012 enhanced chemiluminescence as well as DHE-fluorescence. The peroxynitrite scavenging properties were also assessed by inhibition of nitration of phenol. Prevention of impairment of NO downstream signaling and GTN bioactivation was determined by measurement of P-VASP (surrogate parameter for the activity of the cGMP-dependent kinase-I, cGK-I) and mitochondrial aldehyde dehydrogenase (ALDH-2) activity. Hydralazine dose-dependently decreased the chemiluminescence signal induced by peroxynitrite from SIN-1 and by superoxide from HX/XO in a cell free system, by superoxide in smooth muscle cells and mitochondria acutely challenged with GTN. Moreover, hydralazine inhibited the peroxynitrite-mediated nitration of phenols as well as proteins in smooth muscle cells in a dose-dependent fashion. Finally, hydralazine normalized impaired cGK-I activity as well as impaired vascular ALDH-2 activity. Our results indicate that hydralazine is a highly potent radical scavenger. Thus, the combination with isosorbide dinitrate (ISDN) will favorably influence the nitroso-redox balance in the cardiovascular system in patients with congestive heart failure and may explain at least in part the improvement of prognosis in patients with chronic congestive heart failure.

Explaining the phenomenon of nitrate tolerance

During the last century, nitroglycerin has been the most commonly used antiischemic and antianginal agent. Unfortunately, after continuous application, its therapeutic efficacy rapidly vanishes. Neurohormonal activation of vasoconstrictor signals and intravascular volume expansion constitute early counter-regulatory responses (pseudotolerance), whereas long-term treatment induces intrinsic vascular changes, eg, a loss of nitrovasodilator-responsiveness (vascular tolerance). This is caused by increased vascular superoxide production and a supersensitivity to vasoconstrictors secondary to a tonic activation of protein kinase C. NADPH oxidase(s) and uncoupled endothelial nitric oxide synthase have been proposed as superoxide sources. Superoxide and vascular NO rapidly form peroxynitrite, which aggravates tolerance by promoting NO synthase uncoupling and inhibition of soluble guanylyl cyclase and prostacyclin synthase. This oxidative stress concept may explain why radical scavengers and substances, which reduce oxidative stress indirectly, are able to relieve tolerance and endothelial dysfunction. Recent work has defined a new tolerance mechanism, ie, an inhibition of mitochondrial aldehyde dehydrogenase, the enzyme that accomplishes bioactivation of nitroglycerin, and has identified mitochondria as an additional source of reactive oxygen species. Nitroglycerin-induced reactive oxygen species inhibit the bioactivation of nitroglycerin by thiol oxidation of aldehyde dehydrogenase. Both mechanisms, increased oxidative stress and impaired bioactivation of nitroglycerin, can be joined to provide a new concept for nitroglycerin tolerance and cross-tolerance. The consequences of these processes for the nitroglycerin downstream targets soluble guanylyl cyclase, cGMP-dependent protein kinase, cGMP-degrading phosphodiesterases, and toxic side effects contributing to endothelial dysfunction, such as inhibition of prostacyclin synthase, are discussed in this review.

Nitric oxide down-regulates the expression of the catalytic NADPH oxidase subunit Nox1 in rat renal mesangial cells

Glomerular mesangial cells can produce high amounts of nitric oxide (NO) and reactive oxygen species (ROS). Here we analyzed the impact of NO on the ROS-generating system, particularly on the NADPH oxidase Nox1. Nox1 mRNA and protein levels were markedly decreased by treatment of mesangial cells with the NO-releasing compound DETA-NO in a concentration- and time-dependent fashion. By altering the cGMP signaling system with different inhibitors or activators, we revealed that the effect of NO on Nox1 expression is at least in part mediated by cGMP. Analysis of a reporter construct comprising the 2547 bp of the nox1 promoter region revealed that a stimulatory effect of IL-1beta on nox1 transcription is counteracted by an inhibitory effect of IL-1beta-evoked endogenous NO formation. Moreover, pretreatment of mesangial cells with DETA-NO attenuated platelet-derived growth factor (PDGF)-BB or serum stimulated production of superoxide as assessed by real-time EPR spectroscopy and dichlorofluorescein formation. Transfection of mesangial cells with siRNAs directed against Nox1 and Nox4 revealed that inhibition of Nox1, but not Nox4 expression, is responsible for the reduced ROS formation by NO. Obviously, there exists a fine-tuned crosstalk between NO and ROS generating systems in the course of inflammatory diseases.

The oxidative stress concept of nitrate tolerance and the antioxidant properties of hydralazine

The hemodynamic and anti-ischemic effects of nitroglycerin (NTG) are rapidly blunted as a result of the development of nitrate tolerance. With initiation of NTG therapy, it is possible to detect neurohormonal activation and intravascular volume expansion. These so-called pseudotolerance mechanisms may compromise the vasodilatory effects of NTG. Long-term nitrate treatment also is associated with decreased vascular responsiveness caused by changes in intrinsic mechanisms of the tolerant vasculature itself. According to the oxidative stress concept, increased vascular superoxide (O2-) production and an increased sensitivity to vasoconstrictors secondary to activation of protein kinase C contribute to the development of tolerance. Nicotinamide adenine dinucleotide phosphate oxidase and the uncoupled endothelial nitric oxide synthase may be O2- -producing enzymes. Nitric oxide (NO) and O2-, both derived from NTG and the vessel wall, form peroxynitrite in a diffusion-limited rapid reaction. Peroxynitrite, O2-, or both may be responsible for the development of nitrate tolerance and cross-tolerance to direct NO donors (eg, sodium nitroprusside, sydnonimines) and endothelium-dependent NO synthase-activating vasodilators. Hydralazine is an efficient reactive oxygen species (ROS) scavenger and an inhibitor of O2- generation. When given concomitantly with NTG, hydralazine prevents the development of nitrate tolerance and normalizes endogenous rates of vascular O2- production. Recent experimental work has defined new tolerance mechanisms, including inhibition of the enzyme that bioactivates NTG (ie, mitochondrial aldehyde dehydrogenase isoform 2 [ALDH2]) and mitochondria as potential sources of ROS. NTG-induced ROS inhibit the bioactivation of NTG by ALDH2. Both mechanisms increase oxidative stress and impair NTG bioactivation, and now converge at the level of ALDH2 to support a new theory for NTG tolerance and NTG-induced endothelial dysfunction. The consequences of these processes for NTG downstream targets (eg, soluble guanylyl cyclase, cyclic guanosine monophosphate-dependent protein kinase), toxic effects contributing to endothelial dysfunction (eg, prostacyclin synthase inhibition) and novel applications of the antioxidant properties of hydralazine are discussed.

Differential effects of diabetes on the expression of the gp91phox homologues nox1 and nox4

The nox2-dependent NADPH oxidase was shown to be a major superoxide source in vascular disease, including diabetes. Smooth muscle cells of large arteries lack the phagocytic gp91phox subunit of the enzyme; however, two homologues have been identified in these cells, nox1 and nox4. It remained to be established whether also increases in protein levels of the nonphagocytic NADPH oxidase contribute to increased superoxide formation in diabetic vessels. To investigate changes in the expression of these homologues, we measured their expression in aortic vessels of type I diabetic rats. Eight weeks after streptozotocin treatment, we found a doubling in nox1 protein expression, while the expression of nox4 remained unchanged. This was associated with a significant increase in the NADPH oxidase activity in membrane fractions of diabetic heart and aortic tissue. Furthermore, we observed a decreased sensitivity of diabetic vessels to acetylcholine and nitroglycerin and a decrease in both acetylcholine-stimulated NO production and phosphorylation of VASP, despite an increase in endothelial NO synthase (NOSIII) expression. In addition, xanthine oxidase activity was markedly increased in plasma and 100,000 g supernatant of cardiac tissue of diabetic rats, while myocardial mitochondrial superoxide formation was only weakly enhanced. We conclude that in addition to phagocytic NADPH oxidase, also nonphagocytic, vascular NADPH oxidase subunit nox1, uncoupled NOSIII, and plasma xanthine oxidase contribute to endothelial dysfunction in the setting of diabetes mellitus.

Heterozygous deficiency of manganese superoxide dismutase in mice (Mn-SOD+/-): a novel approach to assess the role of oxidative stress for the development of nitrate tolerance

Nitroglycerin (GTN)-induced tolerance was reported to be associated with increased levels of reactive oxygen species (ROS) in mitochondria. In the present study, we further investigated the role of ROS for the development of nitrate tolerance by using heterozygous manganese superoxide dismutase knock-out mice (Mn-SOD+/-). Mn-SOD is acknowledged as a major sink for mitochondrial superoxide. Vasodilator potency of mouse aorta in response to acetylcholine and GTN was assessed by isometric tension studies. Mitochondrial ROS formation was detected by 8-amino-5-chloro-7-phenylpyrido[3,4-d]pyridazine-1,4-(2H,3H)dione sodium salt (L-012)-enhanced chemiluminescence and mitochondrial aldehyde dehydrogenase (ALDH-2) activity was determined by a high-performance liquid chromatography-based assay. Aortic rings from Mn-SOD+/- mice showed normal endothelial function and vasodilator responses to GTN. In contrast, preincubation of aorta with GTN or long-term GTN infusion caused a marked higher degree of tolerance as well as endothelial dysfunction in Mn-SOD+/- compared with wild type. Basal as well as GTN-stimulated ROS formation was significantly increased in isolated heart mitochondria from Mn-SOD+/- mice, correlating well with a marked decrease in ALDH-2 activity in response to in vitro and in vivo GTN treatment. The data presented indicate that deficiency in Mn-SOD leads to a higher degree of tolerance and endothelial dysfunction associated with increased mitochondrial ROS production in response to in vitro and in vivo GTN challenges. These data further point to a crucial role of ALDH-2 in mediating GTN bioactivation as well as development of GTN tolerance and underline the important contribution of ROS to these processes.

Human-antigen R (HuR) expression in hypertension: downregulation of the mRNA stabilizing protein HuR in genetic hypertension

In aged spontaneously hypertensive rats (SHR), vasorelaxant responses to NO are attenuated compared with normotensive control rats (Wistar-Kyoto [WKY]) because of a decreased expression of the important NO receptor soluble guanylyl cyclase (sGC). Because the expression of sGC subunits alpha1 and beta1 is controlled at the post-transcriptional level by the mRNA-binding protein human-antigen R (HuR), we now assessed whether or not altered expression of HuR could account for downregulation of sGCalpha1 and sGCbeta1 in genetic hypertension. The expression of HuR (and sGCalpha1 and sGCbeta1) in aortas from aged SHR was significantly decreased at the mRNA and protein level compared with age-matched WKY rats, whereas expression of HuR was not different in prehypertensive young (2 months of age) SHR and age-matched WKY rats. The mRNA-binding activity of HuR in native aortic protein extracts from aged SHR was markedly reduced compared with normotensive WKY rats, as detected by RNA electrophoretic mobility shift analysis, using biotin-labeled adenine and uracil (AU)-rich element (ARE)-containing RNA probes from the 3'-untranslated region of sGCalpha1 and sGCbeta1. In contrast, ARE-binding activity was not different between prehypertensive young SHR and young WKY rats. In vitro RNA degradation assays using the same AU-rich sGC mRNA probes revealed an accelerated sGCalpha1 and sGCbeta1 mRNA decay in the presence of native protein extract from hypertensive SHR, which was less rapid with aortic protein from normotensive WKY rats. These findings suggest that in this animal model of genetic hypertension, the reduced expression of sGC subunits is mediated by downregulation of the sGC mRNA-stabilizing protein HuR.

Vascular consequences of endothelial nitric oxide synthase uncoupling for the activity and expression of the soluble guanylyl cyclase and the cGMP-dependent protein kinase

Endothelial dysfunction in the setting of cardiovascular risk factors, such as hypercholesterolemia, hypertension, diabetes mellitus, chronic smoking, as well as in the setting of heart failure, has been shown to be at least partly dependent on the production of reactive oxygen species (ROS), such as the superoxide radical, and the subsequent decrease in vascular bioavailability of nitric oxide (NO). Superoxide-producing enzymes involved in increased oxidative stress within vascular tissue include the NAD(P)H oxidase, the xanthine oxidase, and mitochondrial superoxide-producing enzymes. Superoxide produced by the NADPH oxidase may react with NO released by endothelial nitric oxide synthase (eNOS), thereby generating peroxynitrite. Peroxynitrite in turn has been shown to uncouple eNOS, thereby switching an antiatherosclerotic NO-producing enzyme to an enzyme that may initiate or even accelerate the atherosclerotic process by producing superoxide. Increased oxidative stress in the vasculature, however, is not restricted to the endothelium and has also been demonstrated to occur within the smooth muscle cell layer in the setting of hypercholesterolemia, diabetes mellitus, hypertension, congestive heart failure, and nitrate tolerance. Increased superoxide production by the endothelial and/or smooth muscle cells has important consequences with respect to signaling by the soluble guanylyl cyclase (sGC) and the cGMP-dependent protein kinase I (cGK-I), the activity and expression of which has been shown to be regulated in a redox-sensitive fashion. The present review summarizes current concepts concerning eNOS uncoupling and also focuses on the consequences for downstream signaling with respect to activity and expression of the sGC and cGK-I in various diseases.

Oxidative stress and mitochondrial aldehyde dehydrogenase activity: a comparison of pentaerythritol tetranitrate with other organic nitrates

Mitochondrial aldehyde dehydrogenase (ALDH-2) was recently identified to be essential for the bioactivation of glyceryl trinitrate (GTN). Here we assessed whether other organic nitrates are bioactivated by a similar mechanism. The ALDH-2 inhibitor benomyl reduced the vasodilator potency, but not the efficacy, of GTN, pentaerythritol tetranitrate (PETN), and pentaerythritol trinitrate in phenylephrine-constricted rat aorta, whereas vasodilator responses to isosorbide dinitrate, isosorbide-5-mononitrate, pentaerythritol dinitrate, pentaerythritol mononitrate, and the endothelium-dependent vasodilator acetylcholine were not affected. Likewise, benomyl decreased GTN- and PETN-elicited phosphorylation of the cGMP-activated protein kinase substrate vasodilator-stimulated phosphoprotein (VASP) but not that elicited by other nitrates. The vasodilator potency of organic nitrates correlated with their potency to inhibit ALDH-2 dehydrogenase activity in mitochondria from rat heart and increase mitochondrial superoxide formation, as detected by chemiluminescence. In contrast, mitochondrial ALDH-2 esterase activity was not affected by PETN and its metabolites, whereas it was inhibited by benomyl, GTN applied in vitro and in vivo, and some sulfhydryl oxidants. The bioactivation-related metabolism of GTN to glyceryl-1,2-dinitrate by isolated RAW macrophages was reduced by the ALDH-2 inhibitors benomyl and daidzin, as well as by GTN at concentrations >1 microM. We conclude that mitochondrial ALDH-2, specifically its esterase activity, is required for the bioactivation of the organic nitrates with high vasodilator potency, such as GTN and PETN, but not for the less potent nitrates. It is interesting that ALDH-2 esterase activity was inhibited by GTN only, not by the other nitrates tested. This difference might explain why GTN elicits mitochondrial superoxide formation and nitrate tolerance with the highest potency.

Down-regulation of soluble guanylyl cyclase expression by cyclic AMP is mediated by mRNA-stabilizing protein HuR

We analyzed whether the cyclic AMP induced down-regulation of the nitric oxide (NO) receptor soluble guanylyl cyclase (sGC) is mediated by the mRNA-protecting protein HuR. Exposure (up to 24 h) of isolated rat aortic segments to the activator of adenylyl cyclase, forskolin (10 microM), and to both activators of cAMP-stimulated protein kinase (PKA), 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole-3',5'-cyclic monophosphorothioate, Spisomer (Sp-5,6-DCl-cBIMPS; 400 nM), and N6-phenyl-cAMP (10 microM), strongly reduced sGCalpha1beta1 and HuR protein and mRNA expression in a time-dependent and actinomycin D (10 microM)-sensitive fashion. In vitro degradation of sGCalpha1 and beta1 poly(A)+ mRNA by native rat aortic protein was markedly increased by pretreatment of intact aortas with forskolin. Native protein extract from rat aorta shifted the electrophoretic mobility of biotin-labeled riboprobes from the 3'-untranslated region of sGCalpha1 and beta1 mRNA, and these bands was supershifted by a monoclonal antibody directed against the mRNA-stabilizing protein HuR. Forskolin decreased the HuR-sGCalpha1 and beta1 mRNA interaction and HuR protein expression in rat aorta, and this was prevented by the PKA inhibitory cAMP analog 3',5'-cyclic monophosphorothioate, Rp-isomer (Rp-cAMPS). In cultured smooth muscle cells from rat aorta, forskolin induced a rapid increase in Fos/p-Fos protein levels and activator protein 1 (AP-1) binding activity. Inhibition of this transcription factor by an AP-1 decoy prevented the forskolin-induced down-regulation of HuR. We conclude that forskolin/cAMP decrease the expression of heterodimeric sGC in rat aortic smooth muscle cells via activation of Fos/AP-1, which decreases the expression of HuR and thus destabilizes the sGCalpha1 and beta1 mRNA.

Central role of mitochondrial aldehyde dehydrogenase and reactive oxygen species in nitroglycerin tolerance and cross-tolerance

Recent studies suggest that mitochondrial aldehyde dehydrogenase (ALDH-2) plays a central role in the process of nitroglycerin (glyceryl trinitrate, GTN) biotransformation in vivo and that its inhibition accounts for mechanism-based tolerance in vitro. The extent to which ALDH-2 contributes to GTN tolerance (impaired relaxation to GTN) and cross-tolerance (impaired endothelium-dependent relaxation) in vivo remain to be elucidated. Rats were treated for three days with GTN. Infusions were accompanied by decreases in vascular ALDH-2 activity, GTN biotransformation, and cGMP-dependent kinase (cGK-I) activity. Further, whereas in control vessels, multiple inhibitors and substrates of ALDH-2 reduced both GTN-stimulation of cGKI and GTN-induced vasodilation, these agents had little effect on tolerant vessels. A state of functional tolerance (in the GTN/cGMP pathway) was recapitulated in cultured endothelial cells by knocking down mitochondrial DNA (rho(0) cells). In addition, GTN increased the production of reactive oxygen species (ROS) by mitochondria, and these increases were associated with impaired relaxation to acetylcholine. Finally, antioxidants/reductants decreased mitochondrial ROS production and restored ALDH-2 activity. These observations suggest that nitrate tolerance is mediated, at least in significant part, by inhibition of vascular ALDH-2 and that mitochondrial ROS contribute to this inhibition. Thus, GTN tolerance may be viewed as a metabolic syndrome characterized by mitochondrial dysfunction.

Does nitric oxide mediate the vasodilator activity of nitroglycerin?

Nitroglycerin (glyceryl trinitrate, GTN) relaxes blood vessels primarily via activation of the soluble guanylyl cyclase (sGC)/cGMP/cGMP-dependent protein kinase (cGK-I) pathway. Although the precise mechanism of sGC activation by GTN in the vascular wall is unknown, the mediatory role of nitric oxide (NO) has been postulated. We tested the GTN/NO hypothesis in different types of isolated rat and rabbit blood vessels using two novel approaches: (1) EPR spin trapping using colloid Fe(DETC)2 and (2) analysis of cGK-I-dependent phosphorylation of the vasodilator-stimulated phosphoprotein at Ser239 (P-VASP). For comparison, another organic nitrate, isosorbide dinitrate (ISDN), and endothelium-dependent vasodilator, calcium ionophore A23187, were tested. We found a marked discrepancy between GTN's strong vasoactivity (vasodilation and augmentation of P-VASP) and its poor NO donor properties. In aortas precontracted with phenylephrine, GTN, ISDN, and A23187 induced nearly full relaxations (>80%) and doubling of vascular P-VASP content at concentrations of 100 nmol/L, 100 micromol/L, and 1 micromol/L, respectively. GTN applied in vasorelaxant concentrations (10 to 1000 nmol/L) did not significantly increase the basal vascular NO production, in contrast to ISDN and A23187. The absence of GTN-derived NO was confirmed in rabbit vena cava and renal artery. A significant increase in vascular NO formation was observed only at suprapharmacological GTN concentrations (>10 micromol/L). The concentration dependency of NO formation from GTN was comparable to that of ISDN, although the latter exhibits 100-folds lower vasorelaxant potency. We conclude that GTN activates the sGC/cGMP/cGK-I pathway and induces vasorelaxation without intermediacy of the free radical NO. The full text of this article is available online at http://www.circresaha.org.

Amlodipine activates the endothelial nitric oxide synthase by altering phosphorylation on Ser1177 and Thr495

OBJECTIVE

The Ca2+ antagonist amlodipine increases the generation of nitric oxide (NO) from native and cultured endothelial cells. The aim of this investigation was to determine whether or not the activation of the endothelial NO synthase (eNOS) by this Ca2+ antagonist is related to alterations in eNOS phosphorylation.

METHODS AND RESULTS

In isolated, pre-contracted, endothelium-intact porcine coronary arteries, amlodipine elicited an NO-mediated relaxation and a leftward shift in the concentration-relaxation curve to bradykinin. Moreover, the Ca2+ antagonist increased the generation of NO from native endothelial cells, as detected by electron spin resonance spectroscopy and stimulated an 8-fold increase in cyclic GMP levels in cultured endothelial cells. In unstimulated endothelial cells, eNOS was not phosphorylated on Ser1177 but was phosphorylated on Thr495. Amlodipine elicited the phosphorylation of Ser1177 and attenuated Thr495 phosphorylation, with a time course similar to that of eNOS activation. The amlodipine-induced relaxation of porcine coronary arteries was attenuated by the B2 kinin receptor antagonist, icatibant, but this antagonist did not affect amlodipine-induced changes in eNOS phosphorylation in cultured endothelial cells. Moreover, amlodipine elicited the NO-mediated relaxation of rat aortic rings which do not express the B2 receptor. Amlodipine time-dependently attenuated the phosphorylation of protein kinase C (PKC) in endothelial cells, with a time course similar to the changes in eNOS phosphorylation, and prevented the phorbol-12-myristate-13-acetate-induced activation of PKC. The PKC inhibitor, Ro 31-8220, also elicited the phosphorylation of Ser1177 and the dephosphorylation of Thr495 in cultured cells and induced a leftward shift in the concentration-relaxation curve to bradykinin in rings of porcine coronary artery.

CONCLUSION

The Ca2+ antagonist, amlodipine, enhances endothelial NO generation by inducing changes in the phosphorylation of eNOS. Although the activation of eNOS was related to the activation of the B2 kinin receptor in the porcine coronary artery, a B2 receptor-independent mechanism involving the inhibition of PKC appears to account for the effects observed in the rat aorta as well as in cultured endothelial cells.

Post-transcriptional regulation of soluble guanylyl cyclase expression in rat aorta

We investigated the molecular mechanism of cyclic GMP-induced down-regulation of soluble guanylyl cyclase expression in rat aorta. 3-(5'-Hydroxymethyl-2'-furyl)-1-benzyl indazole (YC-1), an allosteric activator of this enzyme, decreased the expression of soluble guanylyl cyclase alpha(1) subunit mRNA and protein. This effect was blocked by the enzyme inhibitor 4H-8-bromo-1,2,4-oxadiazolo(3,4-d)benz(b-1,4)oxazin-1-one (NS2028) and by actinomycin D. Guanylyl cyclase alpha(1) mRNA-degrading activity was increased in protein extracts from YC-1-exposed aorta and was attenuated by pretreatment with actinomycin D and NS2028. Gelshift and supershift analyses using an adenylate-uridylate-rich ribonucleotide from the 3'-untranslated region of the alpha(1) mRNA and a monoclonal antibody directed against the mRNA-stabilizing protein HuR revealed HuR mRNA binding activity in aortic extracts, which was absent in extracts from YC-1-stimulated aortas. YC-1 decreased the expression of HuR, and this decrease was prevented by NS2028. Similarly, down-regulation of HuR by RNA interference in cultured rat aortic smooth muscle cells decreased alpha(1) mRNA and protein expression. We conclude that HuR protects the guanylyl cyclase alpha(1) mRNA by binding to the 3'-untranslated region. Activation of guanylyl cyclase decreases HuR expression, inducing a rapid degradation of guanylyl cyclase alpha(1) mRNA and lowering alpha(1) subunit expression as a negative feedback response.

The effect of peroxynitrite on the catalytic activity of soluble guanylyl cyclase

Soluble guanylyl cyclase (sGC) is a key enzyme of the *NO/cGMP pathway. Many cardiovascular disorders are associated with reduced *NO-mediated effects, while vascular superoxide (O(2)*(-)) production is increased. Both radicals rapidly react to peroxynitrite. We investigated whether peroxynitrite affects the activity and protein expression of sGC in intact vascular preparations. Catalytic sGC activity and expression of the sGC-beta(1) subunit was measured by conversion of radiolabeled GTP and western blot, respectively, using cytosolic extracts from rat aorta that had been incubated for 4 h with *NO/O(2)*(-) systems (devoid of free *NO) generating either 0.13 microM or 7.5 microM peroxynitrite/min. Incubation of rat aorta with 0.13 microM peroxynitrite/min had no effect. In striking contrast, incubation with 7.5 microM peroxynitrite/min resulted in a shift of the concentration-response curve obtained with a *NO donor (p =.0004) and a reduction of maximal specific activity from 3579 +/- 495 to 2422 +/- 265 pmol cGMP/mg/min (p =.036). The expression of the sGC-beta(1) subunit was unchanged. Exposure of aorta to the O(2)*(-) component had no effect, while exposure to the *NO-component reduced sGC expression to 58.8 +/- 7% (p <.001) and maximal sGC activity from 4041 +/- 992 to 1429 +/- 491 pmol cGMP/mg/min (p =.031). These data suggest that continuous generation of extracellular peroxynitrite might interfere with the *NO/cGMP signaling in vascular cells.

Effects of in vivo nitroglycerin treatment on activity and expression of the guanylyl cyclase and cGMP-dependent protein kinase and their downstream target vasodilator-stimulated phosphoprotein in aorta

BACKGROUND

Chronic in vivo treatment with nitroglycerin (NTG) induces tolerance to nitrates and cross-tolerance to nitrovasodilators and endothelium-derived nitric oxide (NO). We previously identified increased vascular superoxide formation and reduced NO bioavailability as one causal mechanism. It is still controversial whether intracellular downstream signaling to nitrovasodilator-derived NO is affected as well.

METHODS AND RESULTS

We therefore studied the effects of 3-day NTG treatment of rats and rabbits on activity and expression of the immediate NO target soluble guanylyl cyclase (sGC) and on the cGMP-activated protein kinase I (cGK-I). Tolerance was induced either by chronic NTG infusion via osmotic minipumps (rats) or by NTG patches (rabbits). Western blot analysis, semiquantitative reverse transcription-polymerase chain reaction, and Northern blot analysis revealed significant and comparable increases in the expression of sGC alpha(1) and beta(1) subunit protein and mRNA. Studies with the oxidative fluorescent dye hydroethidine revealed an increase in superoxide in the endothelium and smooth muscle. Stimulation with NADH increased superoxide signals in both layers. Although cGK-I expression in response to low-dose NTG was not changed, a strong reduction in vasodilator-stimulated phosphoprotein (VASP) serine239 phosphorylation (specific substrate of cGK-I) was observed in tolerant tissue from rats and rabbits. Concomitant in vivo and in vitro treatment with vitamin C improved tolerance, reduced oxidative stress, and improved P-VASP.

CONCLUSIONS

We therefore conclude that increased expression of sGC in the setting of tolerance reflects a chronic inhibition rather than an induction of the sGC-cGK-I pathway and may be mediated at least in part by increased vascular superoxide.

MR imaging of nitrosyl-iron complexes: experimental study in rats

PURPOSE

To assess the influence of several nitrosyl-iron complexes on proton nuclear spin relaxation rates to establish a magnetic resonance (MR) imaging technique for nitric oxide.

MATERIALS AND METHODS

The influence of aqueous phantom solutions of nitrosyl-iron complexes on proton relaxation rates was analyzed for signal enhancement at conventional 1.5-T MR imaging. To induce formation of nitrosyl-iron complexes in a biologic tissue, isolated rat liver was perfused with a saline solution of the NO donor sodium nitroprusside (SNP), and the MR signal intensity was examined thereafter.

RESULTS

All investigated nitrosyl-iron complexes shortened the longitudinal, or T1, and transverse, or T2, relaxation times in a concentration-dependent fashion. Relaxivities were highest with a dinitrosyl-iron complex bound to albumin and with a water-soluble mononitrosyl-iron dithiocarbamate complex. The contrast properties of 240 micromol/L of a paramagnetic nitrosyl-iron complex were sufficient to substantially enhance the signal intensity of SNP-perfused rat livers at hydrogen 1 MR imaging.

CONCLUSION

Nitrosyl-iron complexes exhibit a contrast effect at MR imaging that can be exploited for NO imaging in living animals and patients with conventional (1)H MR imaging techniques.

Increased nitrovasodilator sensitivity in endothelial nitric oxide synthase knockout mice: role of soluble guanylyl cyclase

Endogenously produced nitric oxide (NO) modulates nitrovasodilator-induced relaxation. We investigated the underlying mechanism in wild-type (WT) mice and endothelial NO synthase knockout (eNOS(-/-)) mice to determine whether a chronic lack of endothelial NO alters the soluble guanylyl cyclase (sGC) pathway. In aortic segments from eNOS(-/-) mice, the vasodilator sensitivity to sodium nitroprusside (SNP) was significantly greater than that in WT mice. There was no difference in sensitivity to the G-kinase I activator 8-para-chlorophenylthio-cGMP or to cromakalim. N(omega)-Nitro-L-arginine had no effect on the SNP-induced relaxation in eNOS(-/-) but increased the sensitivity in WT mice so it was no longer different than that of eNOS(-/-). Basal cGMP levels in aortic rings were significantly lower in eNOS(-/-) mice than in WT mice. SNP (300 nmol/L) induced a significantly greater cGMP accumulation in eNOS(-/-) mice than in WT mice. The maximal SNP-induced (10 micromol/L) increase in cGMP was similar in both strains. SNP-stimulated sGC activity was significantly greater in eNOS(-/-) mice than in WT mice. Incubation of aortic segments from WT mice with N(omega)-nitro-L-arginine increased sGC activity, an effect prevented by coincubation with SNP (10 micromol/L). The aortic expressions of the sGC alpha1 and beta1 subunits in WT and eNOS(-/-) mice were identical as determined with Western blot analysis. These data suggest that chronic exposure to endothelium-derived NO, as well as acute exposure to nitrovasodilator-derived NO, desensitizes sGC to activation by NO but does not alter sGC expression. Both the acute cessation of endothelial NO formation in WT mice and the chronic deficiency of NO in eNOS(-/-) mice restore the NO sensitivity of sGC and enhance vascular smooth muscle relaxation in response to nitrovasodilator agents.

Aging and chronic hypertension decrease expression of rat aortic soluble guanylyl cyclase

We analyzed the influence of aging and genetic hypertension on the function and expression of soluble guanylyl cyclase (sGC) in the aortas of prehypertensive and old spontaneously hypertensive rats (SHR) as well as in age-matched normotensive Wistar-Kyoto rats (WKY). The expression of heterodimeric sGC (alpha(1) and beta(1)) was assessed at the mRNA and protein level, and its function was assessed by the relaxant responses of phenylephrine-contracted endothelium-denuded aortic rings to the nitric oxide (NO) donor sodium nitroprusside. The vasodilator potency of sodium nitroprusside was significantly reduced (P<0.05) with age (3- to 6-fold increase in the EC(50) in old WKY and SHR compared with their young counterparts) as well as with hypertension (3-fold increase in old SHR compared with age-matched WKY), whereas the vasodilator potency of sodium nitroprusside did not differ between young SHR and WKY. A similar influence of aging and hypertension on NO-stimulated GC activity was revealed at the GC expression level: Whereas the beta(1) protein content was similar in young rats of both strains, old WKY exhibited 60% lower and old SHR exhibited 80% lower beta(1) subunit protein compared with young rats (P<0.05). Moreover, the abundance of alpha(1) and beta(1) mRNA (assessed by reverse transcriptase-polymerase chain reaction) was similar in young rats but was 2.5-fold (alpha(1)) and 4.3-fold (beta(1)) lower in old SHR compared with old WKY. In conclusion, our findings show that both aging and hypertension decrease sGC expression and its NO-dependent activation in aortic tissue. Downregulation of sGC may therefore contribute to arterial dysfunction in senescence and chronic hypertension.

Detection of nitrosyl-iron complexes by proton-electron-double-resonance imaging

The nitrogen monoxide radical (NO*) forms paramagnetic mono- and dinitrosyl-iron complexes in biologic tissues. To establish a noninvasive technique for in vivo NO* imaging, we evaluated the suitability of these complexes as magnetic resonance (MR) contrast agents, making use of the ability of the unpaired electrons of the complexes to enter into dynamic nuclear polarization with water protons and hence produce enhancement on images generated by the technique of proton-electron-double-resonance imaging (PEDRI). Phantom solutions of synthetic nitrosyl-iron complexes (NICs) altered the signal intensity of PEDRI images. The dinitrosyl-iron complex (DNIC) with serum albumin induced a significantly larger signal alteration than the mononitrosyl-iron complex (MNIC) with dithiocarbamate. Exposure of rat liver to sodium nitroprusside (SNP) by ex vivo and in situ perfusion induced a composite X-band electron spin resonance (ESR) spectrum of the isolated liver characteristic of a MNIC and DNIC. On storage of the tissue, the MNIC signal disappeared and the DNIC signal intensity increased. Correspondingly, in cross-sectional PEDRI images taken at room temperature, the SNP-exposed livers initially exhibited a weak signal that strongly increased with time. In conclusion, NICs can be detected using PEDRI and could be exploited for in vivo NO* imaging.

Role of superoxide dismutase in in vivo and in vitro nitrate tolerance

We assessed whether pharmacological inhibition of CuZn-superoxide dismutase (SOD) mimics the molecular mechanism of either in vitro or in vivo nitrovasodilator tolerance. In endothelium-intact aortic rings from in vivo tolerant rabbits the GTN- and acetylcholine (ACh)-induced maximal relaxation was attenuated by 36 and 23%, respectively. In vitro treatment of control rings with GTN (1 h 10 microM) similarly attenuated the vasorelaxant response to GTN, but not to ACh. Formation of superoxide radicals (*O2-) in endothelium-intact rings (lucigenin-chemiluminescence) increased 2.5 fold in in vivo tolerance, but significantly decreased in in vitro tolerance. The membrane associated NADH oxidase activity was increased 2.5 fold in homogenates of in vivo tolerant aortae, but was not changed in in vitro tolerant aorta. Conversely, SOD activity and protein expression was halved in in vivo tolerance, but SOD activity was not altered by in vitro tolerance. The *O2- scavenger tiron (10 mM) effectively restored the vasorelaxant response to GTN in in vivo tolerant aortic rings, but not the reduced response to GTN in in vitro tolerant rings. Pretreatment (1 h) of vessels with diethyldithiocarbamate (DETC; 10 mM) attenuated vasorelaxant responses to GTN and ACh, increased vascular *O2- production, and inhibited SOD activity in vessel homogenates to a similar degree as observed in in vivo tolerance. DETC-treatment of in vivo-tolerant vessels induced an additional increase in *O2- production. Increased *O2- production in in vivo nitrate tolerant aorta is associated with activation of vascular NADH oxidase and inactivation of CuZnSOD. Therefore, in vivo tolerance can be mimicked by in vitro inhibition of CuZnSOD, but not by in vitro exposure to GTN, which does not affect vascular *O2- production, NADH oxidase and CuZnSOD.

Nitric oxide inhibits caspase-3 by S-nitrosation in vivo

In cultured human endothelial cells, physiological levels of NO prevent apoptosis and interfere with the activation of the caspase cascade. In vitro data have demonstrated that NO inhibits the activity of caspase-3 by S-nitrosation of the enzyme. Here we present evidence for the in vivo occurrence and functional relevance of this novel antiapoptotic mechanism. To demonstrate that the cysteine residue Cys-163 of caspase-3 is S-nitrosated, cells were transfected with the Myc-tagged p17 subunit of caspase-3. After incubation of the transfected cells with different NO donors, Myc-tagged p17 was immunoprecipitated with anti-Myc antibody. S-Nitrosothiol was detected in the immunoprecipitate by electron spin resonance spectroscopy after liberation and spin trapping of NO by N-methyl-D-glucamine-dithiocarbamate-iron complex. Transfection of cells with a p17 mutant, where the essential Cys-163 was mutated into alanine, completely prevented S-nitrosation of the enzyme. As a functional correlate, in human umbilical vein endothelial cells the NO donors sodium nitroprusside or PAPA NONOate (50 microM) significantly reduced the increase in caspase-3-like activity induced by overexpressing caspase-3 by 75 and 70%, respectively. When human umbilical vein endothelial cells were cotransfected with beta-galactosidase, morphological analysis of stained cells revealed that cell death induction by overexpression of caspase-3 was completely suppressed in the presence of sodium nitroprusside, PAPA NONOate, or S-nitroso-L-cysteine (50 microM). Thus, NO supplied by exogenous NO donors serves in vivo as an antiapoptotic regulator of caspase activity via S-nitrosation of the Cys-163 residue of caspase-3.

Validation of lucigenin as a chemiluminescent probe to monitor vascular superoxide as well as basal vascular nitric oxide production

Lucigenin has been widely used as a chemiluminescent substrate to monitor vascular superoxide (O*-2) formation. The validity of lucigenin for detection of O*-2 has been questioned because O*-2 is generated by lucigenin itself. It has been shown that the concentration of lucigenin is a critical parameter affecting the validity of this assay. In the present studies we evaluated a reduced concentration of lucigenin (5 microM) as a tool to quantify O*-2 production in vascular tissue. Lucigenin-induced effects on endothelial function were assessed by isometric tension recording of isolated aortic rings suspended in organ baths. The effects of lucigenin on O*-2 production were studied using spin trapping and electron spin resonance spectroscopy. Lucigenin at 250 microM but not at 5 microM caused a significant attenuation of endothelium-dependent relaxations to acetylcholine, which was prevented by pretreatment with superoxide dismutase. Spin-trapping studies revealed that lucigenin at 250 microM increased vascular O*-2 production several fold while 5 microM lucigenin did not stimulate O*-2 production. Inhibition of NO synthase by NG-momomethyl-l-arginine as well as the removal of the endothelium almost doubled lucigenin-derived chemiluminescence (LDCL), indicating that basal production of endothelium-derived NO depresses the baseline chemiluminescence signal. Thus, lucigenin at a concentration of 5 microM seems to be a sensitive and valid probe for assessing O*-2 in vascular tissue. It can also be used as an indirect probe to estimate basal vascular NO release.

Normoxic cardiopulmonary bypass reduces oxidative myocardial damage and nitric oxide during cardiac operations in the adult

OBJECTIVE

Hyperoxic cardiopulmonary bypass is widely used during cardiac operations in the adult. This management may cause oxygenation injury induced by oxygen-derived free radicals and nitric oxide. Oxidative damage may be significantly limited by maintaining a more physiologic oxygen tension strategy (normoxic cardiopulmonary bypass).

METHODS

During elective coronary artery bypass grafting, 40 consecutive patients underwent either hyperoxic (oxygen tension = 400 mm Hg) or normoxic (oxygen tension = 140 mm Hg) cardiopulmonary bypass. At the beginning and the end of bypass this study assessed polymorphonuclear leukocyte elastase, nitrate, creatine kinase, and lactic dehydrogenase, antioxidant levels, and malondialdehyde in coronary sinus blood. Cardiac index was measured before and after cardiopulmonary bypass.

RESULTS

There was no difference between groups with regard to age, sex, severity of disease, ejection fraction, number of grafts, duration of cardiopulmonary bypass, or ischemic time. Hyperoxic bypass resulted in higher levels of polymorphonuclear leukocyte elastase (377 +/- 34 vs 171 +/- 32 ng/ml, p = 0.0001), creatine kinase 672 +/- 130 vs 293 +/- 21 U/L, p = 0.002), lactic dehydrogenase (553 +/- 48 vs 301 +/- 12 U/L, p = 0.003), antioxidants (1.97 +/- 0.10 vs 1.41 +/- 0.11 mmol/L, p = 0.01), malondialdehyde (1.36 +/- 0.1 micromol/L,p = 0.005), and nitrate (19.3 +/- 2.9 vs 10.1 +/- 2.1 micromol/L, p = 0.002), as well as reduction in lung vital capacity (66% +/- 2% vs 81% +/- 1%,p = 0.01) and forced 1-second expiratory volume (63% +/- 10% vs 93% +/- 4%, p = 0.005) compared with normoxic management. Cardiac index after cardiopulmonary bypass at low filling pressure was similar between groups (3.1 +/- 0.2 vs 3.3 +/- 0.3 L/min per square meter). [Data are mean +/- standard error (analysis of variance), with p values compared with an oxygen tension of 400 mm Hg.]

CONCLUSIONS

Hyperoxic cardiopulmonary bypass during cardiac operations in adults results in oxidative myocardial damage related to oxygen-derived free radicals and nitric oxide. These adverse effects can be markedly limited by reduced oxygen tension management. The concept of normoxic cardiopulmonary bypass may be applied to surgical advantage during cardiac operations.

Vasodilator dysfunction in aged spontaneously hypertensive rats: changes in NO synthase III and soluble guanylyl cyclase expression, and in superoxide anion production

OBJECTIVE/METHODS

Genetic hypertension is associated with an apparent endothelial dysfunction and impaired endothelium-dependent vasodilatation in response to increased flow and receptor-dependent agonists. However, the link between impaired vasodilatation and nitric oxide (NO) synthase expression is still unclear. In the present study, dilator responses were determined in the aorta and coronary circulation of 16 month old spontaneously hypertensive (SHR) and Wistar Kyoto rats (WKY). Changes in vascular reactivity were compared with alterations in superoxide anion production as well as endothelial NO synthase (NOS III) and soluble guanylyl cyclase expression.

RESULTS

In the isolated perfused heart both the bradykinin- and sodium nitroprusside-induced vasodilator responses were attenuated in SHR compared to WKY. Western blot analysis revealed a parallel reduction in NOS III expression in coronary microvascular endothelial cells from SHR. Superoxide anion production in aortae from SHR was markedly elevated over that of aortae from WKY, and was almost completely abolished by pretreatment with superoxide dismutase. Superoxide dismutase induced similar relaxations in phenylephrine-preconstricted aortic rings from both SHR and WKY, but failed to restore the attenuated acetylcholine- and sodium nitroprusside-induced relaxations in SHR. No difference in NOS III expression was detected in the aortae from either strain whereas soluble guanylyl cyclase expression was markedly decreased in SHR.

CONCLUSIONS

These results demonstrate that NOS III expression in different tissues is differentially affected by hypertension. Moreover, although an elevated superoxide anion production is apparent in the aorta, a reduced soluble guanylyl cyclase expression appears to account for the observed vasodilator dysfunction in SHR.

Characterization of NS 2028 as a specific inhibitor of soluble guanylyl cyclase

1 The haeme-containing soluble guanylyl cyclase (alpha1beta1-heterodimer) is a major intracellular receptor and effector for nitric oxide (NO) and carbon monoxide (CO) and mediates many of their biological actions by increasing cyclic GMP. We have synthesized new oxadiazolo-benz-oxazins and have assessed their inhibitory actions on guanylyl cyclase activity in vitro, on the formation of cyclic GMP in cultured cells and on the NO-dependent relaxation of vascular and non-vascular smooth muscle. 2 Soluble guanylyl cyclase, purified to homogeneity from bovine lung, was inhibited by 4H-8-bromo-1,2,4-oxadiazolo(3,4-d)benz(b)(1,4)oxazin-1-one (NS 2028) in a concentration-dependent and irreversible manner (IC50 30 nM for basal and 200 nM for NO-stimulated enzyme activity). Evaluation of the inhibition kinetics according to Kitz & Wilson yielded a value of 8 nM for Ki, the equilibrium constant describing the initial reversible reaction between inhibitor and enzyme, and 0.2 min(-1) for the rate constant k3 of the subsequent irreversible inhibition. Inhibition was accompanied by a shift in the soret absorption maximum of the enzyme's haem cofactor from 430 to 390 nm. 3 S-nitroso-glutathione-enhanced soluble guanylyl cyclase activity in homogenates of mouse cerebellum was inhibited by NS 2028 (IC50 17 nM) and by 17 structural analogues in a similar manner, albeit with different potency, depending on the type of substitution at positions 1, 7 and 8 of the benzoxazin structure. Small electronegative ligands such as Br and Cl at position 7 or 8 increased and substitution of the oxygen at position 1 by -S-,- NH- or -CH2- decreased the inhibition. 4 In tissue slices prepared from mouse cerebellum, neuronal NO synthase-dependent activation of soluble guanylyl cyclase by the glutamate receptor agonist N-methyl-D-aspartate was inhibited by NS 2028 (IC50 20 nM) and by two of its analogues. Similarly, 3-morpholino-sydnonimine (SIN-1)-elicited formation of cyclic GMP in human cultured umbilical vein endothelial cells was inhibited by NS 2028 (IC50 30 nM). 5 In prostaglandin F2alpha-constricted, endothelium-intact porcine coronary arteries NS 2028 elicited a concentration-dependent increase (65%) in contractile tone (EC50 170 nM), which was abolished by removal of the endothelium. NS 2028 (1 microM) suppressed the relaxant response to nitroglycerin from 88.3+/-2.1 to 26.8+/-6.4% and induced a 9 fold rightward shift (EC50 15 microM) of the concentration-relaxation response curve to nitroglycerin. It abolished the relaxation to sodium nitroprusside (1 microM), but did not affect the vasorelaxation to the KATP channel opener cromakalim. Approximately 50% of the relaxant response to sodium nitroprusside was recovered after 2 h washout of NS 2028. 6 In phenylephrine-preconstricted, endothelium-denuded aorta of the rabbit NS 2028 (1 microM) did not affect relaxant responses to atrial natriuretic factor, an activator of particulate guanylyl cyclase, or forskolin, an activator of adenylyl cyclase. 7 NO-dependent relaxant responses in non-vascular smooth muscle were also inhibited by NS 2028. The nitroglycerin-induced relaxation of guinea-pig trachea preconstricted by histamine was fully inhibited by NS 2028 (1 microM), whereas the relaxations to terbutaline, theophylline and vasoactive intestinal polypeptide (VIP) were not affected. The relaxant responses to electrical field stimulation of non-adrenergic, non-cholinergic nerves in the same tissue were attenuated by 50% in the presence of NS 2028 (1 microM). 8 NS 2028 and its analogues, one of which is the previously characterized 1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one (ODQ), appear to be potent and specific inhibitors of soluble guanylyl cyclase present in various cell types. Oxidation and/or a change in the coordination of the haeme-iron of guanylyl cyclase is a likely inhibitory mechanism.

Dinitrosyl-dithiol-iron complexes, nitric oxide (NO) carriers in vivo, as potent inhibitors of human glutathione reductase and glutathione-S-transferase

Human glutathione reductase (GR) and rat liver glutathione-S-transferases (GSTs) had been shown to be inhibited by the nitric oxide (NO) carrier S-nitroso-glutathione (GSNO). We have now extended these studies by measuring the effects of dinitrosyl-iron complexed thiols (DNIC-[RSH]2) on human GR, GST and glutathione peroxidase. DNIC-[RSH]2 represent important transport forms of NO but also of iron ions and glutathione in vivo. Human GR was found to be inhibited by dinitrosyl-iron-di-glutathione (DNIC-[GSH]2) and dinitrosyl-iron-di-L-cysteine (DNIC-Cys2) in two ways: both compounds were competitive with glutathione disulfide (GSSG), the inhibition constant (Ki) for reversible competition of DNIC-[GSH]2 with GSSG being approximately 5 microM; preincubating GR for 10 min with 4 microM DNIC-[GSH]2 and 40 microM DNIC-Cys2, respectively, led to 50% irreversible enzyme inactivation. More than 95% GR inactivation was achieved by incubation with 36 microM DNIC-[GSH]2 for 30 min. This inhibition depended on the presence of NADPH. Absorption spectra of inhibited GR showed that the charge-transfer interaction between the isoalloxazine moiety of the prosthetic group flavin adenine dinucleotide (FAD) and the active site thiol Cys63 is disturbed by the modification. Cys2 and FAD could be ruled out as sites of the modification. Isolated human placenta glutathione-S-transferase and GST activity measured in hemolysates were also inhibited by DNIC-[GSH]2. This inhibition, however, was reversible and competitive with reduced glutathione, the Ki being 20 nM. The inhibition of GST induced by GSNO was competitive with reduced glutathione (GSH) (Ki = 180 microM) and with the second substrate of the reaction, 1-chloro-2,4,-dinitrobenzene (Ki = 170 microM). An inhibition of human glutathione peroxidase by GSNO or DNIC-[RSH]2 was not detectable. Inactivation of GR by DNIC-[GSH]2 is by two orders of magnitude more effective than modification by GSNO; this result and the very efficient inhibition of GST point to a role of DNIC-[RSH]2 in glutathione metabolism.

Inhibition of glutathione reductase by dinitrosyl-iron-dithiolate complex

The biological signal molecule nitric oxide (NO) exists in a free and carrier-bound form. Since the structure of the carrier is likely to influence the interaction of NO with macromolecular targets, we assessed the interaction of a dinitrosyl-iron-dithiolate complex carrying different thiol ligands with glutathione reductase. The enzyme was irreversibly inhibited by dinitrosyl-iron-di-L-cysteine and dinitrosyl-iron-di-glutathione in a concentration- and time-dependent manner (IC50 30 and 3 microM, respectively). Evaluation of the inhibition kinetics according to Kitz-Wilson yielded a Ki of 14 microM, and a k3 of 1.3 x 10(-3) s-1. A participation of catalytic site thiols in the inhibitory mechanism was indicated by the findings that only the NADPH-reduced enzyme was inhibited by dinitrosyl-iron complex and that blockade of these thiols by Hg2+ afforded protection against irreversible inhibition. This inhibition was not accompanied by formation of a protein-bound dinitrosyl-iron complex and/or S-nitrosation of active site thiols (Cys-58 and Cys-63). However, one NO moiety exhibiting an acid lability similar to a secondary N-nitrosamine was present per mol of inhibited monomeric enzyme. These findings suggest specifically N-nitrosation of glutathione reductase as a likely mechanism of inhibition elicited by dinitrosyl-iron complex and demonstrate in general that structural resemblance of an NO carrier with a natural ligand enhances NO+ transfer to the ligand-binding protein.

N-alpha-tosyl-L-lysine chloromethylketone prevents expression of iNOS in vascular smooth muscle by blocking activation of NF-kappa B

Certain cytokines and lipopolysaccharide stimulate expression of inducible nitric oxide synthase (iNOS) in vascular smooth muscle, an event that is regulated at the transcriptional level and appears to involve several transcription factors, including nuclear factor kappa B (NF-kappa B). Since proteases play an essential role in NF-kappa B activation, experiments were designed to clarify, in both cultured rat aortic smooth muscle cells (SMCs) and isolated rat aortas, whether protease inhibitors affect the interleukin-1 beta (IL-1 beta)-elicited expression of iNOS. The formation of NO was assessed by nitrite release in cultured SMCs and the attenuation of phenylephrine-induced contraction in aortic rings, the expression of iNOS by Western blot analysis and reverse transcription-polymerase chain reaction, and NF-kappa B activity in nuclear extracts by gel electrophoretic mobility shift assya. Exposure of cultured SMCs to IL-1 beta increased NF-kappa B binding activity within 30 minutes and was associated with nitrite accumulation and the appearance of iNOS protein 24 hours later. These responses were abolished in cells that had been exposed to the cytokine in the presence of the protease inhibitor N-alpha-tosyl-L-lysine chloromethylketone. Aprotinin and p-toluenesulfonyl-L-arginine methyl ester, two other protease inhibitors, also reduced the cytokine-stimulated release of nitrite and the level of iNOS protein. Exposure of rat aortic segments without endothelium to IL-1 beta activated NF-kappa B within 30 minutes and was associated with the appearance of iNOS mRNA and an attenuation of phenylephrine-induced contraction 6 hours later. These responses were blunted when the segments were incubated with the cytokine and N-alpha-tosyl-L-lysine chloromethyl ketone. The present observations indicate that protease inhibitors prevent iNOS expression in both cultured and native vascular SMCs by blocking the activation of NF-kappa B.

Regional measurements of NO formed in vivo during brain ischemia

Nitric oxide formed in vivo in the rat brain regions of hippocampus, striatum, neocortex and cerebellum was spin trapped and measured ex vivo by cryogenic electron paramagnetic resonance spectroscopy. In non-ischemic control animals the rate of nitric oxide (NO) formation in the individual brain regions ranged from 15 to 42 pmol.g-1.min-1. During exposure to global ischemia for 7 min the generation of NO increased in all parts of the brain. In the hippocampus the rate of NO formation during ischemia increased by 6-fold from a control rate of 19 pmol.g-1.min-1. This increase was attenuated 47% by pretreatment with the NO synthase antagonist 7-nitroindazole, whereas pretreatment with the non-NMDA receptor anatogonist NBQX and the Ca2+ channel blocker NS638 did not influence the NO formation. The data show that short-duration ischemia elicits a significant, NO-synthase-dependent formation of NO in all brain regions.

Effect of YC-1, an NO-independent, superoxide-sensitive stimulator of soluble guanylyl cyclase, on smooth muscle responsiveness to nitrovasodilators

1. We studied the effects of 3-(5'-hydroxymethyl-2'furyl)-1-benzyl indazole (YC-1) on the activity of purified soluble guanylyl cyclase (sGC), the formation of guanosine-3':5' cyclic monophosphate (cyclic GMP) in vascular smooth muscle cells (VSMC), and on the tone of rabbit isolated aortic rings preconstricted by phenylephrine (PE). In addition, we assessed the combined effect of YC-1, and either NO donors, or superoxide anions on these parameters. 2. YC-1 elicited a direct concentration-dependent activation of sGC (EC50 18.6 +/- 2.0 microM), which was rapid in onset and quickly reversible upon dilution. YC-1 altered the enzyme kinetics with respect to GTP by decreasing KM and increasing Vmax. Activation of sGC by a combination of sodium nitroprusside (SNP) and YC-1 was superadditive at low and less than additive at high concentrations, indicating a synergistic activation of the enzyme by both agents. A specific inhibitor of sGC, 1H-(1,2,4)-oxdiazolo-(4,3-a)-6-bromo-quinoxazin-1-one (NS 2028), abolished activation of the enzyme by either compound. 3. YC-1 induced a concentration-dependent increase in intracellular cyclic GMP levels in rat cultured aortic VSMC, which was completely inhibited by NS 2028. YC-1 applied at the same concentration as SNP elicited 2.5 fold higher cyclic GMP formation. Cyclic GMP-increases in response to SNP and YC-1 were additive. 4. YC-1 relaxed preconstricted endothelium-denuded rabbit aortic rings in a concentration-dependent manner (50% at 20 microM) and markedly increased cyclic GMP levels. Relaxations were inhibited by NS 2028. A concentration of YC-1 (3 microM), which elicited only minor effects on relaxation and cyclic GMP, increased the vasodilator potency of SNP and nitroglycerin (NTG) by 10 fold and markedly enhanced SNP- and NTG-induced cyclic GMP formation. 5. Basal and YC-1-stimulated sGC activity was sensitive to inhibition by superoxide (O-2) generated by xanthine/xanthine oxidase, and was protected from this inhibition by superoxide dismutase (SOD). YC-1-stimulated sGC was also sensitive to inhibition by endogenously generated (O-2 in rat preconstricted endothelium-denuded aortic rings. Relaxation to YC-1 was significantly attenuated in aortae from spontaneously hypertensive rats (SHR), which generated O-2 at a higher rate than aortae from normotensive Wistar Kyoto rats (WKY). SOD restored the vasodilator responsiveness of SHR rings to YC-1. 6. In conclusion, these results indicate that YC-1 is an NO-independent, O-2-sensitive, direct activator of sGC in VSMC and exerts vasorelaxation by increasing intracellular cyclic GMP levels. The additive or even synergistic responses to NO-donors and YC-1 in cultured VSMC and isolated aortic rings apparently reflect the direct synergistic action of YC-1 and NO on the sGC. The synergism revealed in this in vitro study suggests that low doses of YC-1 may be of therapeutic value by permitting the reduction of nitrovasodilator dosage.

In vivo nitrate tolerance is not associated with reduced bioconversion of nitroglycerin to nitric oxide

BACKGROUND

In vitro data suggest that reduced bioconversion of nitroglycerin (NTG) to nitric oxide (NO) contributes to the development of vascular and hemodynamic tolerance to NTG. We examined the in vivo validity of this hypothesis by measuring NTG-derived NO formation by in vivo spin-trapping of NO in vascular tissues from nitrate-tolerant and -nontolerant rats.

METHODS AND RESULTS

Five groups (n = 6 to 8 each) of conscious chronically catheterized rats received NTG (0.2 or 1 mg/h IV) for 72 hours (nitrate-tolerant groups). Four other groups received either NTG vehicle (placebo, for 72 hours) or were left untreated (control). Nitrate tolerance was substantiated by a reduced (55% to 85%) hypotensive response to NTG in vivo and a reduced relaxation to NTG in isolated aortic rings. NTG-derived NO formation in aorta, vena cava, heart, and liver was measured as NOFe(DETC)2 and NO-heme complexes formed in vivo during 35 minutes combined with ex vivo cryogenic electron spin resonance spectroscopy. NO formation was significantly (P < .05) increased in all tissues in nitrate-tolerant rats in an NTG dose-dependent manner. Furthermore, the amount of NO formed from a bolus dose of NTG (6.5 mg/kg over 20 minutes) was similar in nitrate-tolerant and -nontolerant rats.

CONCLUSIONS

The results suggest that vascular and hemodynamic NTG tolerance occurs despite high and similar rates of NO formation by NTG in tolerant and nontolerant target tissues. This finding is compatible with the assumption that reduced biological activity of NO, rather than reduced bioconversion of NTG to NO, contributes to in vivo development of nitrate tolerance.

Effect of cyclic GMP-dependent vasodilators on the expression of inducible nitric oxide synthase in vascular smooth muscle cells: role of cyclic AMP

1. In the present study we examined whether interleukin-1 beta (IL-1 beta) increases the activity of adenylyl cyclase in vascular smooth muscle cells and determined its role in the cytokine-induced expression of the inducible nitric oxide synthase (iNOS) and activation of nuclear transcription factor-kappa B (NF-kappa B). In addition the interaction between cyclic AMP- and cyclic GMP-elevating agonists on the IL-1 beta-stimulated expression of iNOS was examined. 2. Exposure of vascular smooth muscle cells to IL-1 beta stimulated the formation of cyclic AMP but not of cyclic GMP. The intracellular level of cyclic AMP reached a maximum within 1 h and then gradually declined over the next 5 h. This IL-1 beta (60 u ml-1)-stimulated formation of cyclic AMP was modest (about 3 fold at 60 u ml-1 for 1 h) compared to that evoked by isoprenaline (about 9 fold at 3 x 10(-6) M for 2 min). 3. The IL-1 beta (60 u ml-1 for 24 h)-stimulated accumulation of nitrite, which was taken as an index of NO production, was concentration-dependently increased by preferential inhibitors of cyclic AMP-dependent phosphodiesterases (rolipram and trequinsin). This effect was reproduced by a specific activator of the cyclic AMP-dependent protein kinase(s) A, Sp-8-CPT-cAMPS (10(-4) M) but was prevented by a specific inhibitor of cyclic AMP-dependent protein kinase(s) A, Rp-8-CPT-cAMPS (10(-4) M). These compounds alone [rolipram (10(-6) M), trequinsin (3 x 10(-6) M) and Sp-8-CPT-cAMPS (10(-4) M)] slightly but significantly increased the release of nitric oxide while Rp-8-CPT-cAMPS elicited no such effect. 4. Inducible NOS protein was expressed in IL-1 beta (30 u ml-1, 24 h)-stimulated smooth muscle cells as assessed by Western blot analysis. The level of iNOS protein was markedly increased in smooth muscle cells which had been exposed to IL-1 beta in combination with either rolipram (3 x 10(-6) M) or Sp-8-CPT-cAMPS (10(-4) M) but was reduced in those exposed to IL-1 beta and Rp-8-CPT-cAMPS (10(-4) M). A weak expression of iNOS protein was found in smooth muscle cells which had been exposed to either Sp-8-CPT-cAMPS or rolipram alone for 24 h while Rp-8-CPT-cAMPS elicited no such effect. 5. Exposure of smooth muscle cells to IL-1 beta (30 u ml-1) for 30 min increased the level of NF-kappa B-DNA complexes in nuclear extracts as detected by electrophoretic mobility shift assay. Similar levels of NF-kappa B-DNA complexes were found in cells which had been exposed to IL-1 beta in combination with either Sp-8-CPT-cAMPS (10(-4) M), trequinsin (10(-6) M) or rolipram (10(-6) M). None of the modulators alone affected the basal level of NF-kappa B binding activity. 6. NO-donors [sodium nitroprusside (SNP) 10(-4) M; dinitrosyl-iron-di-L-cysteine-complex (DNIC), 10(-4) M; 3-morpholino-sydnonimine (SIN-1), 10(-4) M] and atrial natriuretic factor (10(-6) M) significantly increased the IL-1 beta (30 or 60 u ml-1, 24 h)-stimulated expression of iNOS protein and activity as assessed indirectly by the conversion of oxyhaemoglobin to methaemoglobin. In the absence of IL-1 beta, SNP (10(-4) M, 24 h) but not the other cyclic GMP-dependent vasodilators caused a modest expression of iNOS protein. No such effect was found in smooth muscle cells exposed to SNP in combination with Rp-8-CPT-cAMPS (10(-4) M) while an increased level of iNOS protein was found in those exposed to SNP in combination with either Sp-8-CPT-cAMPS (10(-4) M) or rolipram (3 x 10(-6) M). 7. Exposure of vascular smooth muscle cells to either S-nitroso-L-cysteine (Cys-SNO, 10(-4) M), SNP (10(-4) M) or SIN-1 (10(-4) M) for 35 min affected minimally the basal activation of NF-kappa B but abolished that evoked by IL-1 beta (30 u ml-1 added during the last 30 min). However, addition of Cys-SNO following the stimulation with IL-1 beta (during the last 5 min of the 30 min exposure period) reduced the level of NF-kappa B-DNA complexes only slightly. 8. These data indicate that the cyclic AMP-dependent pathway plays a decisi

S-nitrosation of serum albumin by dinitrosyl-iron complex

The objective of this study was to identify a potential mechanism for S-nitrosation of proteins. Therefore, we assessed S-nitrosation of bovine serum albumin by dinitrosyl-iron-di-L-cysteine complex [(NO)2Fe(L-cysteine)2], a compound similar to naturally occurring iron-nitrosyls. Within 5-10 min, (NO)2Fe(L-cysteine)2 generated paramagnetic albumin-bound dinitrosyl-iron complex and S-nitrosoalbumin in a ratio of 4:1. Although S-nitroso-L-cysteine was concomitantly formed in low amounts, its concentration was not sufficient to account for formation of S-nitrosoalbumin via a trans-S-nitrosation reaction. Low oxygen tension did not affect S-nitrosation by the dinitrosyl-iron complex thus excluding the involvement of oxygenated NOx-species in the nitrosation reaction. Blockade of albumin histidine residues by pyrocarbonate, which prevented formation of dinitrosyl-iron-albumin complex, did not inhibit S-nitrosation of albumin. Thus, S-nitrosation of albumin by (NO)2Fe(L-cysteine)2 can proceed by direct attack of a nitrosyl moiety on the protein thiolate, without previous binding of the iron. We conclude that protein-bound dinitrosyl-iron complexes detected in high concentrations in certain tissues provide a reservoir of S-nitrosating species, e.g. low molecular dinitrosyl iron complexes.

Specificity of different organic nitrates to elicit NO formation in rabbit vascular tissues and organs in vivo

1. In the present study we assessed the formation of nitric oxide (NO) from classical and thiol-containing organic nitrates in vascular tissues and organs of anaesthetized rabbits, and established a relationship between the relaxant response elicited by nitroglycerin (NTG) and NO formation in the rabbit isolated aorta. Furthermore, the effect of isolated cytochrome P450 on NO formation from organic nitrates was investigated. 2. Rabbits received diethyldithiocarbamate (DETC; 200 mg kg-1 initial bolus i.p. and 200 mg kg-1 during 20 min, i.v.) and either saline, or one of the following organic nitrates: nitroglycerin (NTG, 0.5 mg kg-1), isosorbide dinitrate (ISDN), N-(3-nitratopivaloyl)-L-cysteine ethylester (SPM 3672), S-carboxyethyl-N-(3-nitratopivaloyl)-L-cysteine ethylester (SPM 5185), at 10 mg kg-1 each. After 20 min the animals were killed, blood vessels and organs were removed, and subsequently analyzed for spin-trapped NO by cryogenic electron spin resonance (e.s.r.) spectroscopy. 3. In the saline-treated control group, NO remained below the detection limit in all vessels and organs. In contrast, all of the nitrates tested elicited measurable NO formation, which was higher in organs (liver, kidney, heart, lung, spleen) (up to 4.8 nmol g-1 20 min-1) than in blood vessels (vena cava, mesenteric bed, femoral artery, aorta) (up to 0.7 nmol g-1 20 min-1). Classical organic nitrates (NTG, ISDN) formed NO preferentially in the mesenteric bed and the vena cava, while the SPM compounds elicited comparable NO formation in veins and arteries. 4. Using a similar spin trapping technique, NO formation was assessed in vitro in phenylephrine-precontracted rabbit aortic rings. The maximal relaxation elicited by a first exposure (10 min) to NTG (0.3 to 10 microM) was positively correlated (r = 0.8) with the net increase (NTG minus basal) of NO spin-trapped during a second exposure to the same concentration of NTG in the presence of DETC. 5. Cytochrome P450 purified from rabbit liver enhanced NO formation in a NADPH-dependent fashion from NTG, but not from the other nitrates, as assessed by activation of purified soluble guanylyl cyclase. 6. We conclude that the vessel selective action of different organic nitrates in vivo reflects differences in vascular NO formation. Thus, efficient preload reduction by classical organic nitrates can be accounted for by higher NO formation in venous capacitance as compared to arterial conductance and resistance vessels. In contrast, NO is released from cysteine-containing nitrates (SPMs) to a similar extent in arteries and veins, presumably independently of an organic nitrate-specific biotransformation. Limited tissue bioavailability of NTG and ISDN might account for low NO formation in the aorta, while true differences in biotransformation seem to account for differences in NO formation in the other vascular tissues.

In vivo spin trapping of glyceryl trinitrate-derived nitric oxide in rabbit blood vessels and organs

BACKGROUND

The objectives of this study were (1) to assess glyceryl trinitrate (GTN)-derived nitric oxide (NO) formation in vascular tissues and organs of anesthetized rabbits in vivo, (2) to establish a correlation between tissue NO levels and a biological response, and (3) to verify biotransformation of GTN to NO by cytochrome P-450.

METHODS AND RESULTS

NO was trapped in tissues in vivo as a stable paramagnetic mononitrosyl-iron-diethyldithiocarbamate complex [NOFe(DETC)2]. After removal of the tissues, NO was determined by cryogenic electron spin resonance spectroscopy. NO formation in vitro was assessed by spin trapping and by activation of soluble guanylyl cyclase. The GTN-elicited decrease in coronary perfusion pressure was monitored in isolated, constant-flow perfused rabbit hearts. NO was not detected in control tissues. In GTN-treated rabbits, NO formation was higher in organs than in vascular tissues and higher in venous than in arterial vessels. In isolated hearts, ventricular NO levels and decreases in coronary perfusion pressure achieved by GTN were closely correlated. Purified cytochrome P-450 catalyzed NO formation from GTN in a P-450-NADPH reductase- and NADPH-dependent fashion.

CONCLUSIONS

Since GTN-derived NO formation in myocardial tissue correlates to the GTN-elicited vasodilator response, we conclude that GTN-derived NO detected in vivo correlates with the systemic effects of GTN. Therefore, the higher rate of NO formation detected in veins compared with arteries explains the preferential venodilator activity of GTN. High NO formation in cytochrome P-450-rich organs in vivo and efficient NO formation from GTN by cytochrome P-450 in vitro highlights the importance of this pathway for NO formation from GTN in the intact organism.

Coordinate up- and down-modulation of inducible nitric oxide synthase, nitric oxide production, and tumoricidal activity in rat bone-marrow-derived mononuclear phagocytes by lipopolysaccharide and gram-negative bacteria

Simultaneous incubation of primary rat bone-marrow-derived mononuclear phagocytes (BMMo) and tumor cells with gram-negative agents triggers within 24 h interferon gamma (IFN gamma)- and tumor necrosis factor (TNF alpha)-independent tumoricidal activity. On the other hand, BMMo that had been incubated for 24 h with gram-negative agents prior to re-exposure to the same agent had largely lost their ability to generate tumoricidal activity, although their ability to bind lipopolysaccharide (LPS) was not diminished. Parallel measurements of the kinetics of inducible nitric oxide synthase (iNOS), nitrite secretion, and tumoricidal activity triggered in primary BMMo by LPS revealed that these parameters take a coordinate course, reaching a peak within 24 h and then rapidly decaying. Down-regulation of expression of NOS protein and iNOS activity could be attributed neither to down-regulation of LPS receptors nor to L-arginine depletion.

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.

Synthesis of 15N omega-hydroxy-L-arginine and ESR and 15N-NMR studies for the elucidation of the molecular mechanism of enzymic nitric oxide formation from L-arginine

N omega-Hydroxy-L-arginine (2) was prepared by a multi-stage synthesis; the key step was the addition of hydroxylamine to the protected cyanamide 8. The presence of N-hydroxyguanidines was confirmed, above all, by 15N-NMR investigations. 15N omega-Hydroxy-L-arginine (2) was converted quantitatively to 15NO by NO synthases from macrophages. 15NO was identified by ESR-spectroscopy. These experiments confirm that 15N omega-hydroxy-L-arginine (2) is an intermediate in the biosynthesis of NO from arginine (1) and that the N-hydroxylated N-atom is present in the NO formed.