Selective pharmacological inhibition of distinct nitric oxide synthase isoforms

Nitric oxide mediates cerebral ischemic tolerance in a neonatal rat model of hypoxic preconditioning

Neuroprotection against cerebral ischemia can be realized if the brain is preconditioned by previous exposure to a brief period of sublethal ischemia. The present study was undertaken to test the hypothesis that nitric oxide (NO) produced from the neuronal isoform of NO synthase (NOS) serves as a necessary signal for establishing an ischemia-tolerant state in brain. A newborn rat model of hypoxic preconditioning was used, wherein exposure to sublethal hypoxia (8% oxygen) for 3 hours renders postnatal day (PND) 6 animals completely resistant to a cerebral hypoxic-ischemic insult imposed 24 hours later. Postnatal day 6 animals were treated 0.5 hour before preconditioning hypoxia with the nonselective NOS inhibitor L-nitroarginine (2 mg/kg intraperitoneally). This treatment, which resulted in a 67 to 81% inhibition of calcium-dependent constitutive NOS activity 0.5 to 3.5 hours after its administration, completely blocked preconditioning-induced protection. However, administration of the neuronal NOS inhibitor 7-nitroindazole (40 mg/kg intraperitoneally) before preconditioning hypoxia, which decreased constitutive brain NOS activity by 58 to 81%, was without effect on preconditioning-induced cerebroprotection, as was pretreatment with the inducible NOS inhibitor aminoguanidine (400 mg/kg intraperitoneally). The protective effects of preconditioning were also not blocked by treating animals with competitive [3-(2-carboxypiperazin-4-yl)propyl-1-phosphonate; 5 mg/kg intraperitoneally] or noncompetitive (MK-801; 1 mg/kg intraperitoneally) N-methyl-D-aspartate receptor antagonists prior to preconditioning hypoxia. These findings indicate that NO production and activity are critical to the induction of ischemic tolerance in this model. However, the results argue against the involvement of the neuronal NOS isoform, activated secondary to a hypoxia-induced stimulation of N-methyl-D-aspartate receptors, and against the involvement of the inducible NOS isoform, but rather suggest that NO produced by the endothelial NOS isoform is required to mediate this profound protective effect.

Review article: the potential role of nitric oxide in chronic inflammatory bowel disorders

The aetiology of the chronic inflammatory bowel diseases-ulcerative colitis and Crohn's disease-as well as 'microscopic colitis'-both collagenous (COC) and lymphocytic colitis (LC)-remains unknown. Autoimmune mechanisms, cytokine polymorphism, commensal bacteria, infectious agents and vascular impairment have all been proposed as playing important roles in the pathogenesis of this spectrum of diseases. A variety of proinflammatory mediators, including tumour necrosis factor alpha, interleukin-1beta, interferon gamma, leukotriene B4 and platelet activating factor, promote the adherence of phagocytes to the venular endothelium and extravasation of these cells into the colonic mucosa. In addition to large amounts of nitric oxide (NO), injurious peroxynitrite may be formed in the epithelium by the inducible nitric oxide synthase (iNOS), which is considered to elicit cytotoxicity by the generation of superoxide with reduced L-arginine availability. In active ulcerative colitis, and to a lesser extent in Crohn's disease, a greatly increased production of NO has been demonstrated by indirect and direct measurements. Surprisingly, even higher rates of production have been observed in COC-a condition which is never associated with injurious inflammation. The latter observation favours the notion that NO promotes mucosal integrity. Further evidence for a protective role of NO in chronic inflammatory bowel disorders is provided by the observation of increased susceptibility to the induction of experi mental colitis in 'knock-out' mice deficient in iNOS. Selective inhibitors of iNOS activity, as well as topical L-arginine, may therefore prove beneficial in inflammatory bowel disease by reducing the production of superoxide by iNOS, while only the former option may be expected to reduce diarrhoea in chronic inflammatory bowel disorders. Clearly, further experimental work needs to be done before testing topical L-arginine in human inflammatory bowel disease.

Effect of inhibition of nitric oxide synthase on chronic tension-type headache: a randomised crossover trial

BACKGROUND

Studies in animals have shown that nitric oxide plays an important part in central sensitisation and that inhibitors of nitric oxide synthase (NOS) decrease sensitisation in models of persistent pain. The efficacy of inhibitors of NOS has not been tested in patients with tension-type chronic headache. We aimed to show whether N(G)-monomethyl-L-arginine hydrochloride (L-NMMA), an inhibitor of NOS, is effective in relieving pain in such patients.

METHODS

We undertook a randomised double-blind, crossover trial of 16 patients with chronic-tension-type headache. Patients were assigned intravenous infusion of 6 mg/kg L-NMMA or placebo on 2 days separated by at least 1 week in a randomised order. Headache intensity was measured on a 100 mm visual analogue scale, and on a verbal rating scale at baseline and at 30 min, 60 min, and 120 min after start of treatment. The primary endpoint was reduction of pain intensity on the visual analogue scale by the active treatment compared with placebo.

FINDINGS

L-NMMA reduced pain intensity on the visual analogue scale significantly more than placebo: 120 min after start of treatment, the mean pain score was decreased from 49 to 33 with L-NMMA and from 44 to 40 with placebo (p=0.01). Pain intensity on the verbal rating scale was also significantly lower for treatment with L-NMMA than for treatment with placebo (p=0.02).

INTERPRETATION

Inhibition of NOS had an analgesic effect in chronic tension-type headache. Further tests are required before clinical application.

Nitric oxide synthase in cardiac sarcoplasmic reticulum

NO. is a free radical that modulates heart function and metabolism. We report that a neuronal-type NO synthase (NOS) is located on cardiac sarcoplasmic reticulum (SR) membrane vesicles and that endogenous NO. produced by SR-associated NOS inhibits SR Ca2+ uptake. Ca2+-dependent biochemical conversion of L-arginine to L-citrulline was observed from isolated rabbit cardiac SR vesicles in the presence of NOS substrates and cofactors. Endogenous NO. was generated from the vesicles and detected by electron paramagnetic resonance spin-trapping measurements. Immunoelectron microscopy demonstrated labeling of cardiac SR vesicles by using anti-neuronal NOS (nNOS), but not anti-endothelial NOS (eNOS) or anti-inducible NOS (iNOS) antibodies, whereas skeletal muscle SR vesicles had no nNOS immunoreactivity. The nNOS immunoreactivity also displayed a pattern consistent with SR localization in confocal micrographs of sections of human myocardium. Western blotting demonstrated that cardiac SR NOS is larger than brain NOS (160 vs. 155 kDa). No immunodetection was observed in cardiac SR vesicles from nNOS knockout mice or with an anti-nNOS mu antibody, suggesting the possibility of a new nNOS-type isoform. 45Ca uptake by cardiac SR vesicles, catalyzed by Ca2+-ATPase, was inhibited by NO. produced endogenously from cardiac SR NOS, and 7-nitroindazole, a selective nNOS inhibitor, completely prevented this inhibition. These results suggest that a cardiac muscle nNOS isoform is located on SR of cardiac myocytes, where it may respond to intracellular Ca2+ concentration and modulate SR Ca2+ ion active transport in the heart.

Regulation of inducible nitric oxide synthase gene expression in vascular smooth muscle cells

1. Formation of nitric oxide (NO) by the constitutive calcium-dependent NO synthase expressed in endothelial cells plays an important role in the control of local blood flow and vascular homeostasis. Expression of the inducible calcium-independent NO synthase (iNOS) in vascular smooth muscle cells (VSMC), on the other hand, is thought to play a potentially detrimental role in the pathogenesis of chronic inflammation or septic shock. In vascular injury, however, iNOS expression in VSMC may be beneficial as a compensatory mechanism for the lack of endothelial NO synthesis, e.g., by preventing restenosis following angioplasty or heart transplant vasculopathy. 2. Because iNOS activity does not seem to be controlled once the enzyme is expressed, regulation of NO release from iNOS-expressing cells predominantly occurs at the transcriptional and/or posttranscriptional level. 3. This review summarizes what is currently known about the regulation of expression of this enzyme in VSMC, details some of the transcription factors involved therein as well as their mode of activation, and highlights some pharmacological strategies based on these findings that may be employed for the control of iNOS expression in VSMC in the clinical arena.

Role of inducible nitric oxide synthase in the regulation of neutrophil migration in zymosan-induced inflammation

In the present study, by comparing the responses in wild-type mice and mice lacking the inducible (or type 2) nitric oxide synthase (iNOS), we investigated the role played by iNOS in the regulation of polymorphonuclear granulocyte (PMN) accumulation and chemokine production in the mouse peritoneal cavity in response to administration of zymosan (0.2 mg). Zymosan injection induced the production of nitric oxide, and triggered a time-dependent PMN immigration into the peritoneal cavity. This response was associated with increases in the level of the chemokines macrophage inflammatory protein (MIP)-1alpha, MIP-2, monocyte chemo-attractant protein (MCP)-1 and cytokine-induced neutrophil chemo-attractant (KC), as measured in the peritoneal cavities. Injection of zymosan also induced a time-dependent increase in the production of the anti-inflammatory cytokine interleukin-10 (IL-10) in the peritoneal cavity. When comparing the response between wild-type and iNOS knockout (KO) mice, we observed that the low-level PMN accumulation measured at 1 hr was slightly but significantly increased in the absence of functional iNOS. On the other hand, the delayed response (2-4 hr after zymosan) of PMN accumulation was suppressed in the iNOS KO mice. The early enhancement of PMN infiltration in the iNOS-deficient mice was associated with increased peritoneal levels of MIP-2, KC and IL-10 proteins. The delayed suppression of PMN infiltration was associated with reduced MIP-2 and IL-10 levels in the peritoneal cavity. The lack of iNOS did not affect the release of MIP-1alpha and MCP-1 at any of the time-points studied. The current data demonstrate that iNOS regulates the production of certain CXC (but not CC) proinflammatory chemokines, the production of IL-10 and exerts a biphasic regulatory effect on PMN accumulation in zymosan-induced acute inflammation.

Binding of dynein light chain (PIN) to neuronal nitric oxide synthase in the absence of inhibition

PIN, an 89-amino-acid polypeptide found in a rat hippocampal cDNA library using the yeast two-hybrid system and various neuronal nitric oxide synthase (nNOS) fragments as bait, was reported to be an inhibitor of nNOS (Science 274, 774-778, 1996). PIN reportedly inhibited nNOS selectively and did not interact with either the endothelial or inducible nitric oxide synthase isoforms. Inhibition was attributed to the ability of PIN to dissociate the catalytically active nNOS homodimer. PIN is a dynein light chain (J. Biol. Chem. 271, 19358-19366, 1996), which suggested that PIN may serve as an axonal transport protein for nNOS. We have synthesized a rat PIN cDNA by recursive polymerase chain reaction and have expressed the protein in Escherichia coli. Recombinant PIN is a folded dimeric, mostly alpha-helical protein with a single deeply buried tryptophan residue. We have also expressed and purified the nNOS fragment to which PIN reportedly binds (residues 163-245). This recombinant peptide has a disordered secondary structure. Gel-filtration experiments show that PIN binds to both the full-length nNOS and nNOS fragment. However, PIN neither inhibits nNOS activity nor dissociates the nNOS dimer into monomeric species. PIN thus possibly functions as a dynein light chain involved in nNOS axonal transport but is not an inhibitor of the enzyme. Our results agree with the proposal (Cell 82, 743-752, 1995) that the PIN recognition sequence in nNOS both lies outside the catalytic core and is not part of the monomer-monomer contact region.

The comparative effects of the NOS inhibitor, Nomega-nitro-L-arginine, and the haemoxygenase inhibitor, zinc protoporphyrin IX, on tumour blood flow

PURPOSE

To determine the relative effects of inhibiting nitric oxide synthase (NOS) and haemoxygenase (HO) on blood flow to the rat P22 carcinosarcoma.

METHODS AND MATERIALS

HO is the enzyme responsible for in vivo production of carbon monoxide (CO). The vascular effects of zinc protoporphyrin IX (ZnPP), a competitive inhibitor of HO, were compared with those of copper protoporphyrin IX (CuPP), a poor inhibitor of HO, in isolated ex vivo perfusions of the P22 tumour and in intact tumour-bearing rats. In ex vivo perfusions, tumour vascular resistance was calculated from measurements of perfusion pressure at a known flow rate. In intact animals, blood flow to tumour and normal tissues was calculated using a radiotracer uptake method. The effects of ZnPP were compared with those of the NOS inhibitor, N(omega)-nitro-L-arginine (L-NNA), and the combination of the two drugs.

RESULTS

HO activity in the P22 tumour was reduced by 50% following administration of either ZnPP or CuPP directly to ex vivo perfused tumours, suggesting an indirect effect on the enzyme. Enzyme inhibition was not associated with any significant vasoactive effect. Neither ZnPP nor CuPP, at a dose of 45 micromol x kg(-1) administered i.p., inhibited tumour HO in vivo. However, they did significantly decrease tumour blood flow to 60-70% of control, with similar effects in skin and brain. Skeletal muscle blood flow was increased to 150% of control. L-NNA decreased both tumour and skeletal muscle blood flow to around 40% of control. These differences suggest that the nonspecific effects of ZnPP and CuPP were not mediated by NOS inhibition. The combination of ZnPP and L-NNA improved the selective reduction in tumour blood flow achieved with either agent alone.

CONCLUSION

This suggests that the HO/CO pathway does not play a major vasodilatory role in this tumour. However, ZnPP and CuPP could be useful for inducing a relatively selective decrease in tumour blood flow via mechanisms unrelated to HO inhibition, especially when combined with NOS inhibition.

Synthesis and screening of conformationally restricted and conformationally free N-(tertiary aminoalkyl)dithiocarbamic acids and esters as inhibitors of neuronal nitric oxide synthase

N-(Tertiary aminoalkyl)dithiocarbamic acids and esters were synthesized and evaluated for their ability to inhibit neuronal nitric oxide synthase. Preliminary results show these compounds are able to act at the binding site for L-arginine and the conformationally restricted esters may have a second site of activity involving the cofactor (6R)-5,6,7,8-tetrahydro-L-biopterin.

Altered inotropic response to IGF-I in diabetic rat heart: influence of intracellular Ca2+ and NO

Normally, insulin-like growth factor I (IGF-I) exerts positive effects on cardiac growth and myocardial contractility, but resistance to its action has been reported in diabetes. This study was designed to determine whether IGF-I-induced myocardial contractile action is altered in diabetes as a result of an intrinsic alteration of contractile properties at the cellular level. Contractile responses to IGF-I were examined in left ventricular papillary muscles and ventricular myocytes from normal and short-term (5-7 days) streptozotocin-induced diabetic rats. Mechanical properties of muscles and myocytes were evaluated using a force transducer and an edge detector, respectively. Preparations were electrically stimulated at 0.5 Hz, and contractile properties analyzed include peak tension development (PTD) or peak twitch amplitude (PTA), time to peak contraction/shortening, and time to 90% relaxation/relengthening. Intracellular Ca2+ transients were measured as fura 2 fluorescence intensity changes. IGF-I (1-500 ng/ml) caused a dose-dependent increase in PTD and PTA in preparations from normal but not diabetic animals. IGF-I did not alter time to peak contraction/shortening or time to 90% relaxation/relengthening. Pretreatment with the NO synthase inhibitor Nomega-nitro-L-arginine methyl ester (100 microM) attenuated IGF-I-induced increases in PTD in normal myocardium but unmasked a positive inotropic action in diabetic animals. Pretreatment with Nomega-nitro-L-arginine methyl ester blocked IGF-I-induced increases in PTA in single myocytes. Consistent with its inotropic actions on muscles and myocytes, IGF-I induced a dose-dependent increase in Ca2+ transients in normal but not diabetic myocytes. These results suggest that the IGF-I-induced inotropic response is depressed in diabetes because of an intrinsic alteration at the myocyte level. Mechanisms underlying this alteration in IGF-I-induced myocardial response may be related to changes in intracellular Ca2+ and/or NO production in diabetes.

Effects of intravenous nitric oxide inhibitors on endotoxin-induced fever in prepubertal pigs

1. The ability of nitric oxide inhibitors to antagonize the febrile effects of lipopolysaccharide (LPS) endotoxin (20 microg/animal i.v.) was assessed in prepubertal pigs in which deep body temperature was measured at 10-min intervals for 180 min. 2. In experiment 1, pigs (n=5) were injected with Nomega-nitro-L-arginine-methyl ester (L-NAME) (30 mg/kg i.v.) or aminoguanidine (27 mg/kg i.v.) 30 min before LPS. There was a marked tendency for L-NAME, but not aminoguanidine, to reduce LPS pyrexia. 3. In experiment 2, pigs (n=7) were injected with 2-amino-4-methylpyridine (1 mg/kg i.v.) 30 min before LPS. This drug tended to increase, rather than reduce, core temperature. 4. In experiment 3, pigs (n=5) were injected with S-methylisothiourea (10 or 15 mg/kg i.v.) concomitantly with LPS. Both doses of the drug produced a small, nonsignificant, reduction in the febrile response. 5. The results indicate that the nitric oxide inhibitors used in this study were relatively ineffective in modifying LPS fever in conscious pigs; these findings are in marked contrast with the actions, in this species, of drugs that inhibit prostaglandin production. In addition, the most effective drug, L-NAME, was the one considered to be the least selective for the inducible form of nitric oxide.

Nitric oxide synthase is the mediator of late preconditioning against myocardial infarction in conscious rabbits

BACKGROUND

Despite intense investigation, the effector of the infarct-limiting protection observed during the late phase of ischemic preconditioning (PC) remains unknown. The goal of this study was to test the hypothesis that late PC against myocardial infarction is mediated by the activity of nitric oxide synthase (NOS).

METHODS AND RESULTS

Conscious rabbits underwent a 30-minute coronary occlusion followed by 3 days of reperfusion. In group I (control group, n= 10), infarct size (tetrazolium staining) averaged 56.8+/-5.3% of the risk region, which was decreased to 27.6+/-2.5% (P<0.05) in rabbits preconditioned 24 hours earlier with a sequence of six 4-minute occlusion/4-minute reperfusion cycles (group II, n= 10). When preconditioned rabbits were given the nonselective NOS inhibitor N(omega)-nitro-L-arginine (L-NA, 13 mg/kg i.v. [group III, n=8]) or the selective iNOS inhibitor aminoguanidine (AG, 150 mg/kg SC [group V, n=7]) before the 30-minute occlusion, the protective effect of late PC was completely abrogated; that is, infarct size (59.9+/-4.5% and 65.8+/-3.3%, respectively) was similar to that measured in the control group. Measurements of systolic wall thickening (sonomicrometry) demonstrated that L-NA and AG also abolished the improved recovery of myocardial function effected by late PC in group II. When rabbits were given L-NA or AG without prior PC (group IV [n=8] and group VI [n=6], respectively), infarct size did not differ from that observed in controls (53.8+/-4.3% and 59.8+/-4.3%, respectively), demonstrating that L-NA and AG do not increase the extent of cell death in nonpreconditioned myocardium.

CONCLUSIONS

Taken together, these results indicate that in the conscious rabbit, the infarct-sparing effect of the late phase of ischemic PC is mediated by the activity of NOS and suggest that the specific isoform primarily responsible for this cardioprotective phenomenon is iNOS. Thus, NO appears to be a pivotal component of the pathophysiological cascade of late PC.

Design of isoform-selective inhibitors of nitric oxide synthase

Nitric oxide synthase, the mammalian enzyme catalyzing the oxidation of L-arginine to L-citrulline and nitric oxide, is present in three isoforms that have distinct physiological roles. Overstimulation or overexpression of individual nitric oxide synthase isoforms plays a role in a wide range of disorders including septic shock, arthritis, diabetes, ischemia-reperfusion injury, pain and various neurodegenerative diseases. Animal studies and early clinical trials suggest that nitric oxide synthase inhibitors could be therapeutic in many of these disorders, but preservation of physiologically important nitric oxide synthase functions might require use of isoform-selective inhibitors. Within the past few years both amino acid and nonamino acid nitric oxide synthase inhibitors with pharmacologically useful isoform selectivity have been reported. Selectivity has been achieved on the basis of initial binding affinity and, for mechanism-based inactivators, on the basis of isoform-dependent catalytic activation; particularly interesting are N5-(1-imino-3-butenyl)-L-ornithine, ARL 17477, 1400W and S-(2-aminoethyl)isothiourea.

Vascular effects of LPS in mice deficient in expression of the gene for inducible nitric oxide synthase

The inducible isoform of nitric oxide synthase (iNOS) is expressed after systemic administration of lipopolysaccharide (LPS). The importance of expression of iNOS in blood vessels is poorly defined. Because nitric oxide from iNOS may alter vasomotor function, we examined effects of LPS on vasomotor function in carotid arteries from iNOS-deficient mice. We studied contraction of the carotid artery from wild-type and iNOS-deficient mice in vitro 12 h after injection of LPS (20 mg/kg ip). Contractile responses to PGF2alpha (3-30 microM) and thromboxane A2 analog (U-46619; 3-100 nM) were evaluated using vascular rings from mice treated with vehicle or LPS. Maximum force of contraction generated by rings in response to PGF2alpha was 0.39 +/- 0.02 and 0.25 +/- 0.01 (SE) g (n = 14) in vehicle and LPS-treated wild-type mice, respectively (P < 0.001 vs. vehicle). Thus LPS reduced constrictor responses in wild-type mice. Thiocitrulline and aminoguanidine (inhibitors of iNOS) improved contractile responses from LPS-treated wild-type vessels. Indomethacin also improved constrictor responses in arteries from wild-type mice injected with LPS. In contrast, contraction of the carotid arteries in response to PGF2alpha and U-46619 was not impaired in LPS-treated iNOS-deficient mice, and contraction was not altered by inhibitors of iNOS. Expression of iNOS mRNA was confirmed using RT-PCR in carotid arteries from wild-type mice after injection of LPS but not vehicle. PCR products for iNOS were not observed in iNOS-deficient mice. These findings provide the first direct evidence that iNOS mediates impairment of vascular contraction after treatment with LPS.

NADPH diaphorase histochemistry detects inducible nitric oxide synthetase activity in the thymus of naive and staphylococcal enterotoxin B-stimulated mice

Here we examined the changes in NADPH diaphorase (NADPHd) and inducible nitric oxide synthetase (iNOS) positivity in the medulla of the mouse thymus in response to treatment with the superantigen, Staphylococcal enterotoxin B (SEB). A few NADPHd+ and iNOS+ cells scattered in the medulla were detected in the thymi of naive mice. SEB induced the appearance of a large number of NADPHd+- and iNOS-immunoreactive cells in the thymic medulla. In the thymus of iNOS-deficient mice, a total absence of these NADPHd+ and iNOS+ medullary cells was found both under basal conditions and after SEB stimulation. With the NADPHd reaction, only endothelial staining was detected in the thymi of iNOS-deficient mice. Our data indicate that NADPHd+ cells in the thymic medulla express iNOS and that SEB induces iNOS expression in the mouse thymus.

Melatonin inhibits expression of the inducible isoform of nitric oxide synthase in murine macrophages: role of inhibition of NFkappaB activation

The role of melatonin as an immunomodulator is well established. Recent reports showed that melatonin exerts protective effects in septic and hemorrhagic shock and in inflammation. The expression of the inducible isoform of nitric oxide synthase (iNOS) makes an important contribution to the pathophysiology of shock and inflammation. We studied, in cultured murine macrophages, the role of melatonin in the regulation of the expression of iNOS and defined the mode of melatonin's action. Our results show that melatonin, at 1 microM-1 mM, decreased the production of nitrite/nitrate (the breakdown products of NO) as well as the production of 6-keto-prostaglandin F1alpha (the major stable breakdown product of prostacyclin) in macrophages stimulated with bacterial lipopolysaccharide (10 microg/ml). We observed that melatonin reduces iNOS steady-state mRNA levels and iNOS protein expression in the same concentration range (1 microM-1 mM). Melatonin, up to 10 mM, exerted only a slight direct inhibitory effect on iNOS activity. Using iNOS promoter-luciferase constructs, we found that melatonin inhibits iNOS promoter activation. Inhibition of iNOS expression was associated with inhibition of activation of the transcription factor nuclear factor kappa B (NFkappaB). We conclude that melatonin inhibits NO production in immunostimulated macrophages mainly by inhibiting the expression of iNOS. This is due to inhibition of iNOS transcription, in part through inhibition of NFkappaB activation. Inhibition of iNOS-derived NO production by melatonin may contribute to the anti-inflammatory effects of this pineal secretory product.

Role of poly(ADP-ribose)synthetase in inflammation

Peroxynitrite and hydroxyl radicals are potent initiators of DNA single strand breakage, which is an obligatory stimulus for the activation of the nuclear enzyme poly(ADP-ribose)synthetase (PARS). Rapid activation of PARS depletes the intracellular concentration of its substrate, NAD+, slowing the rate of glycolysis, electron transport and ATP formation. This process can result in acute cell dysfunction and cell necrosis. Accordingly, inhibitors of PARS protect against cell death under these conditions. In addition to the direct cytotoxic pathway regulated by DNA injury and PARS activation, PARS also appears to modulate the course of inflammation by regulating the expression of a number of genes, including the gene for intercellular adhesion molecule 1, collagenase and the inducible nitric oxide synthase. The research into the role of PARS in inflammatory conditions is now supported by novel tools, such as novel, potent inhibitors of PARS, and genetically engineered animals lacking the gene for PARS. In vivo data demonstrate that inhibition of PARS protects against various forms of inflammation, including zymosan or endotoxin induced multiple organ failure, arthritis, allergic encephalomyelitis, and diabetic islet cell destruction. Pharmacological inhibition of PARS may be a promising novel approach for the experimental therapy of various forms of inflammation.

Sustained pharmacological inhibition of nitric oxide synthase does not affect the survival of intrastriatal rat fetal mesencephalic transplants

The objective of the present study was to investigate the potential role of the free radical nitric oxide (NO) in the development of fetal rat mesencephalic neurons grafted in a 6-hydroxydopamine (6-OHDA) lesioned rat model of Parkinson's disease. First, using nitric oxide synthase (NOS)-immunocytochemistry and reduced nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry, we investigated the presence of the neuronal isoform of NOS (nNOS) in intrastriatal mesencephalic grafts. During the course of the experiment (16 weeks) an increase in the staining intensity and the number of nNOS/NADPH-d positive cells within the grafts was observed, as well as a gradual maturation of dopaminergic neurons. In addition, within both the host striatal and grafted mesencephalic tissue, a NO-dependent accumulation of cyclic guanosine monophosphate (cGMP) was detected, indicating the presence of guanylate cyclase, i.e., the target-enzyme for NO. Secondly, to determine the impact of NO on the survival of grafted dopaminergic neurons, 6-OHDA lesioned rats received mesencephalic grafts and were subsequently treated with the competitive NOS-inhibitor Nomega-nitro-l-arginine methylester (l-NAME). After chronic treatment for 4 weeks, tyrosine hydroxylase immunocytochemistry revealed no apparent differences between the survival of grafted dopaminergic neurons in control- or l-NAME treated animals, respectively. As the maturation of grafted dopaminergic neurons coincides with a gradual increase in the expression of nNOS within the graft and since dopaminergic cell numbers are not changed upon administration of l-NAME, it is concluded that endogenously produced and potentially toxic NO does not affect the survival of grafted fetal dopaminergic neurons.

Endogenous nitric oxide and the pulmonary microvasculature in healthy sheep and during systemic inflammation

Nitric oxide (NO) influences microvascular integrity. NO synthase inhibitors are regarded as therapeutic options, but their impact on the pulmonary microvasculature is not well defined. We studied the microvascular effects of the nonselective NO synthase inhibitor N(omega)-nitro L-arginine methylester (L-NAME) in healthy sheep and during systemic inflammation. Permeability analysis was performed in 30 adult ewes with chronic lung lymph fistulas and pulmonary venous occluders. Experiment 1: 20 sheep received Escherichia coli endotoxin (lipopolysaccharide, 10 ng/kg/min) for 32 h. After 24 h of endotoxemia, 10 sheep were given L-NAME (25 mg/kg), and 10 sheep received NaCl 0.9%. Experiment 2: six sheep were treated with L-NAME (25 mg/kg), and four animals received NaCl 0.9%. Endotoxin induced a phasic pulmonary microvascular response with early transiently increased endothelial permeability at 4 h and late normalization of microvascular integrity to large molecules after 24 h. At that time systemic vasodilation had occurred. L-NAME raised pulmonary artery pressure and pulmonary vascular resistance index without signs of increased permeability in either experiment. NO is involved in vascular tone in healthy sheep and during systemic inflammation, but it does not seem to play a role in the integrity of the pulmonary microvascular barrier function to large molecules.

Cytokines and their roles in pancreatic islet beta-cell destruction and insulin-dependent diabetes mellitus

Insulin-dependent diabetes mellitus (IDDM) is a disease that results from autoimmune destruction of the insulin-producing beta-cells in the pancreatic islets of Langerhans. The autoimmune response against islet beta-cells is believed to result from a disorder of immunoregulation. According to this concept, a T helper 1 (Th1) subset of T cells and their cytokine products, i.e. Type 1 cytokines--interleukin 2 (IL-2), interferon gamma (IFNgamma), and tumor necrosis factor beta (TNFbeta), dominate over an immunoregulatory (suppressor) Th2 subset of T cells and their cytokine products, i.e. Type 2 cytokines--IL-4 and IL-10. This allows Type 1 cytokines to initiate a cascade of immune/inflammatory processes in the islet (insulitis), culminating in beta-cell destruction. Type 1 cytokines activate (1) cytotoxic T cells that interact specifically with beta-cells and destroy them, and (2) macrophages to produce proinflammatory cytokines (IL-1 and TNFalpha), and oxygen and nitrogen free radicals that are highly toxic to islet beta-cells. Furthermore, the cytokines IL-1, TNFalpha, and IFNgamma are cytotoxic to beta-cells, in large part by inducing the formation of oxygen free radicals, nitric oxide, and peroxynitrite in the beta-cells themselves. Therefore, it would appear that prevention of islet beta-cell destruction and IDDM should be aimed at stimulating the production and/or action of Type 2 cytokines, inhibiting the production and/or action of Type 1 cytokines, and inhibiting the production and/or action of oxygen and nitrogen free radicals in the pancreatic islets.

Contribution of NO synthases to neutrophil infiltration in the gastric mucosal lesions in rats with water immersion restraint stress

A decrease in constitutive NO synthase (cNOS) activity and an increase in inducible NO synthase (iNOS) activity occurred with an increase in myeloperoxidase (MPO) activity, an index of neutrophil infiltration, in the gastric mucosa of rats with water immersion restraint (WIR) stress. This increase in gastric mucosal MPO activity was enhanced by pretreatment with NG-monomethyl L-arginine, a non-selective NOS inhibitor, but was prevented with maintenance of gastric mucosal cNOS activity by pretreatment with aminoguanidine, a selective iNOS inhibitor. The MPO activity was negatively correlated with the cNOS activity in all WIR-stressed rats used (r=-0.723). These results suggest that a decrease in cNOS activity could contribute to an increase in neutrophil infiltration in the gastric mucosa of WIR-stressed rats.

Nitric oxide synthase activity and L-arginine metabolism in the retinas from streptozotocin-induced diabetic rats

1. Nitric oxide synthase (NOS) activity was studied in the retinas from normal rats and in the retinas from two groups of streptozotocin-induced (8 days and 4 months) diabetic rats. In each animal group, the NOS activity was correlated to the concentration of amino acids related to L-arginine metabolism and to L-arginine uptake. 2. Retinas from both groups of streptozotocin-induced diabetes (8 days and 4 months) showed an increased NOS activity compared with the NOS activity in retinas from normal rats. In retinas lysate from normal rats, the NOS activity was most potently inhibited by NO-Arg (1 mM), whereas, in both groups of streptozotocin-induced diabetes, the NOS activity was most potently inhibited by the NOS inhibitor aminoguanidine (0.5 mM). 3. The basal levels of the amino acids related to L-arginine metabolism-namely, L-arginine, L-citrulline, L-ornithine and L-glutamine-in retinas from both groups of rats with streptozotocin-induced diabetes were decreased compared with the amino acid levels in retinas from normal rats. 4. The uptake of L-[3H]arginine in retinas from both groups of rats with streptozotocin-induced diabetes was increased compared with the uptake of of L-[3H]arginine in retinas from normal rats.

Pharmacological characterization of endothelial cell nitric oxide synthase inhibitors in isolated rabbit aorta

Different receptors mediating the release of endothelium-derived nitric oxide (EDNO) have been identified at endothelial level. In the present study we aimed to characterise, on rabbit aorta by means of pharmacological tools, the generation of EDNO by receptors located on endothelial cell membrane (M3, P2u, P2y) and by direct activation of Ca2+ entry into the endothelial cell. Four vasodilating drugs were tested (acetylcholine, UTP, A23187 and 2-methyl-thio-ATP); they were active only if the endothelial layer was intact, suggesting that they act through endothelial receptors. The effect of different nitric oxide synthase (NOS) inhibitors (0.1 mM: L- and D-NAME, L-NMMA, L-NIO and 7-NI) was investigated on NO-mediated relaxation induced by the relaxants in vessels with intact endothelium. NOS inhibitors differently affected relaxation mediated by the vasoactive drugs in isolated rabbit aorta. Reversibility of the inhibition by using a fixed concentration of L-arginine (0.1 mM) was different depending on the relaxing drug and NOS-inhibitor. The data obtained support the coexistence in aortic vessel of more than one endothelial cell NOS isoform, each provided with different receptor coupling.

Protective effects of mercaptoethylguanidine, a selective inhibitor of inducible nitric oxide synthase, in ligature-induced periodontitis in the rat

1. Excessive production of nitric oxide (NO), and the generation of peroxynitrite have been implicated in various proinflammatory conditions. In the present study, using mercaptoethylguanidine (MEG), a selective inhibitor of iNOS and a peroxynitrite scavenger, we investigated the role of iNOS and peroxynitrite in a rat model of periodontitis. 2. Periodontitis was produced in rat by a ligature of 2/0 braided silk placed around the cervix of the lower left 1st molar. Animals were then divided into two groups: one group of rats was treated with MEG (30 mg kg(-1), i.p., 4 times per day for 8 days), animals in the other group received vehicle. At day 8, the gingivomucosal tissue encircling the mandibular 1st molars was removed on both sides from ligated and sham operated animals for inducible nitric oxide synthase (iNOS) activity assay and for immunocytochemistry with anti-iNOS serum. Plasma extravasation was measured with the Evans blue technique. Alveolar bone loss was measured with a videomicroscopy. 3. Ligation caused a significant, more than 3 fold increase in the gingival iNOS activity, whereas it did not affect iNOS activity on the contralateral side, when compared to sham-operated animals. Immunohistochemical analysis revealed iNOS-positive macrophages, lymphocytes and PMNs in the connective tissue and immunoreactive layers of epithelium on side of the ligature, and only a few iNOS reactive connective tissue cells on the contralateral side [corrected]. Ligation significantly increased Evans blue extravasation in gingivomucosal tissue and alveolar bone destruction compared to the contralateral side. MEG treatment significantly reduced the plasma extravasation and bone destruction. 4. The present results demonstrated that ligature-induced periodontitis increases local NO production and that MEG treatment protects against the associated extravasation and bone destruction. Based on the present data, we propose that enhanced formation of NO and peroxynitrite plays a significant role in the pathogenesis of periodontitis.

Responses of carotid artery in mice deficient in expression of the gene for endothelial NO synthase

We examined the hypotheses that responses to acetylcholine are impaired and responses to NO are enhanced in carotid artery from mice made deficient in endothelial nitric oxide synthase (eNOS) by gene targeting (eNOS-deficient mice). We also tested the hypothesis that deletion of one copy of the eNOS gene is sufficient to alter vascular responses. Vessels were studied in vitro from heterozygous (+/-) and homozygous (-/-) eNOS-deficient mice as well as wild-type [eNOS(+/+)] littermates. After precontraction with prostaglandin F2 alpha, acetylcholine produced marked relaxation of carotid arteries in eNOS(+/+) mice, with impaired vasorelaxation in eNOS(+/-) mice. For example, 1 microM acetylcholine relaxed carotid arteries by 55 +/- 5% (mean +/- SE) in eNOS(+/-) mice (n = 13) compared with 83 +/- 3% in eNOS(+/+) mice (n = 14, P < 0.001 vs. +/-). In contrast, acetylcholine caused no relaxation in carotid arteries from eNOS(-/-) mice (P < 0.001 vs. +/+ and +/-). Relaxation of the carotid artery in response to nitroprusside [a nitric oxide (NO) donor] was enhanced (P < 0.001) in eNOS-deficient mice. For example, in response to 10 nM nitroprusside, the carotid artery relaxed by 18 +/- 2% in eNOS(+/+) mice (n = 14), 33 +/- 2% in eNOS(+/-) mice (n = 13), and 47 +/- 4% in eNOS(-/-) mice (n = 5). Thus relaxation of the carotid artery is impaired with acetylcholine and enhanced with the NO donor nitroprusside in eNOS-deficient mice. Enhanced responses to NO may represent a compensatory response expressed in the absence of eNOS. The findings that vascular responses to acetylcholine and NO are altered in eNOS(+/-) mice compared with those observed in eNOS(+/+) mice suggest a "gene-dosing" effect.

Antiinflammatory effects of mercaptoethylguanidine, a combined inhibitor of nitric oxide synthase and peroxynitrite scavenger, in carrageenan-induced models of inflammation

In vitro studies have demonstrated that mercaptoethylguanidine (MEG), a selective inhibitor of the inducible NO synthase (iNOS), is also effective as a scavenger of peroxynitrite (a potent cytotoxic oxidant produced by the reaction of NO and superoxide). In the present study, we evaluated the antiinflammatory potential of MEG treatment in two models of acute inflammation (carrageenan-induced paw edema and pleurisy), where oxyradicals, NO, and peroxynitrite play a crucial role in the inflammatory process. Our data show that MEG (given at 25 microg/paw in the paw edema model or 10 mg/kg in the pleurisy model) inhibits the inflammatory response (paw swelling, pleural exudate formation, mononuclear cell infiltration, histological injury) in both models. Furthermore, MEG reduced nitrite/nitrate concentrations in the exudate and reduced the activity of the inducible isoform of NO synthase in the lung ex vivo. MEG also reduced the appearance of nitrotyrosine immunoreactivity in the inflamed tissues. Taken together, the present results demonstrate that MEG exerts potent antiinflammatory effects. Part of these antiinflammatory effects may be related to an inhibition of the expression/activity of the inducible NO synthase, another part may be related to oxyradical and peroxynitrite scavenging.

Increase in nitric oxide in the hypoxic-ischemic neonatal rat brain and suppression by 7-nitroindazole and aminoguanidine

We measured the changes in nitric oxide (NO) metabolites in the brains of neonatal rats with hypoxic-ischemic damage. There were two peaks of NO metabolites in the lesioned side of the cortex without treatment: one during hypoxia and the other during the re-oxygenation period. Prehypoxic treatment with 7-nitroindazole, a selective neuronal NO synthase inhibitor, suppressed both peaks of NO metabolites, whereas prehypoxic treatment with aminoguanidine, a selective inducible NO synthase inhibitor, partially suppressed only the peak in the re-oxygenation period. These data suggest different roles of neuronal and inducible NO synthases in the pathogenesis of hypoxic-ischemic encephalopathy.

Regulation of the cerebral circulation: role of endothelium and potassium channels

Several new concepts have emerged in relation to mechanisms that contribute to regulation of the cerebral circulation. This review focuses on some physiological mechanisms of cerebral vasodilatation and alteration of these mechanisms by disease states. One mechanism involves release of vasoactive factors by the endothelium that affect underlying vascular muscle. These factors include endothelium-derived relaxing factor (nitric oxide), prostacyclin, and endothelium-derived hyperpolarizing factor(s). The normal vasodilator influence of endothelium is impaired by some disease states. Under pathophysiological conditions, endothelium may produce potent contracting factors such as endothelin. Another major mechanism of regulation of cerebral vascular tone relates to potassium channels. Activation of potassium channels appears to mediate relaxation of cerebral vessels to diverse stimuli including receptor-mediated agonists, intracellular second messenger, and hypoxia. Endothelial- and potassium channel-based mechanisms are related because several endothelium-derived factors produce relaxation by activation of potassium channels. The influence of potassium channels may be altered by disease states including chronic hypertension, subarachnoid hemorrhage, and diabetes.

Nitric oxide synthase inhibition: a new principle in the treatment of migraine attacks

Glyceryl trinitrate, an exogenous nitric oxide (NO) donor, and histamine, which causes NO formation in vascular endothelium, have been shown to trigger migraine attacks. However, it remains uncertain whether NO is involved in the subsequent phase of migraine attacks. To answer this question we studied the effect of L-NGmethylarginine hydrochloride (546C88), a NO-synthase inhibitor, on spontaneous migraine attacks. In a double-blind study design, 18 patients with migraine without aura randomly received 546C88 (6 mg/kg) or placebo (5% dextrose) i.v. given over 15 min for a single migraine attack (546C88:placebo, 15:3). Furthermore, 11 placebo-treated patients from previous double-blind trials with almost identical design were added to the placebo group in the statistical evaluation. Two hours after the infusion, 10 of 15 L-NGmethylarginine hydrochloride-treated patients experienced headache relief compared to 2 of 14 placebo-treated patients (p = 0.01). Symptoms such as phono- and photophobia were also significantly improved. A similar trend for nausea was not significant. We conclude that NO may be involved in the pain mechanisms throughout the course of spontaneous migraine attacks.

Role of inducible nitric oxide synthase in regulation of pulmonary vascular tone in the late gestation ovine fetus

Nitric oxide (NO) produced by NO synthase (NOS) modulates fetal pulmonary vascular tone and contributes to the fall in pulmonary vascular resistance (PVR) at birth. Although the inducible (type II) NOS isoform is present in human and rat fetal lungs, it is uncertain whether type II NOS activity contributes to vascular NO production in the fetal lung. To determine whether type II NOS is present in the ovine fetal lung and to study the potential contribution of type II NOS on the regulation of basal PVR in the fetus, we measured the hemodynamic effects of three selective type II NOS antagonists: aminoguanidine (AG), 2-amino-5,6-dihydro-6-methyl-4H-1,3 thiazine (AMT), and S-ethylisothiourea (EIT). Studies were performed after at least 72 h of recovery from surgery in 19 chronically prepared fetal lambs (133+/-3 d; 147 d, term). Brief intrapulmonary infusions of AG (140 mg), AMT (0.12 mg), and EIT (0.12 mg) increased basal PVR by 82, 69, and 77%, respectively (P < 0.05). The maximum increase in PVR occurred within 20 min, but often persisted up to 80 min. These agents also increased mean aortic pressure but did not alter the pressure gradient between the pulmonary artery and aorta, suggesting little effect on tone of the ductus arteriosus. Acetylcholine-induced pulmonary vasodilation remained intact after treatment with selective type II NOS antagonists, but not after treatment with the nonselective NOS blocker, nitro-L-arginine. Using Northern blot analysis with poly(A)+ RNA, we demonstrated the presence of two mRNA transcripts for type II NOS (4.1 and 2.6 kb) in the fetal lung. We conclude that the type II NOS isoform is present in the ovine fetal lung, and that selective type II NOS antagonists increase PVR and systemic arterial pressure in the late-gestation fetus. We speculate that type II NOS may play a physiological role in the modulation of vascular tone in the developing fetal lung.

Selective iNOS inhibition is superior to norepinephrine in the treatment of rat endotoxic shock

S-methyl-isothiourea (SMT) is a potent inhibitor of NO synthase (NOS) with relative selectivity towards the inducible isoform (iNOS). We compared SMT and norepinephrine for the treatment of experimental endotoxic shock. Anesthetized rats challenged intravenously with lipopolysaccharide (LPS), 10 mg/kg, were treated after 1 h with a 4-h infusion of norepinephrine (titrated to maintain blood pressure within baseline values), SMT at low dose (0.1 mg x kg-1 x h-1), or at high dose (1 mg x kg-1 x h-1), or an equivalent volume of saline (2 ml x kg-1 x h-1). In saline-treated animals, LPS increased plasma nitrate and produced hypotension, low cardiac output (CO), lactic acidosis, and signs of liver and kidney dysfunction. Norepinephrine maintained blood pressure (BP) and reduced the fall in CO, without affecting lactic acidosis, organ dysfunction, and nitrate accumulation. The latter was dose-dependently blunted by SMT. Treatment with this agent prevented hypotension, through systemic vasoconstriction with the high dose and a maintained CO with the low dose. Low, but not high, dose SMT blunted lactic acidosis. Both doses reduced the signs of renal, but not liver, dysfunction. In additional studies, we obtained evidence that, in contrast with the high dose, SMT at low dose did not interfere with the function of constitutive NOS. These findings suggest a potential advantage of selective iNOS inhibition over standard adrenergic support in the therapy of septic shock.

Pharmacological and immunohistochemical evidence for a functional nitric oxide synthase system in rat peritoneal eosinophils

Eosinophil migration in vivo is markedly attenuated in rats treated chronically with the NO synthase (NOS) inhibitor Nomega-nitro-L-arginine methyl ester (L-NAME). In this study, we investigated the existence of a NOS system in eosinophils. Our results demonstrated that rat peritoneal eosinophils strongly express both type II (30.2 +/- 11.6% of counted cells) and type III (24.7 +/- 7.4% of counted cells) NOS, as detected by immunohistochemistry using affinity purified mouse mAbs. Eosinophil migration in vitro was evaluated by using 48-well microchemotaxis chambers and the chemotactic agents used were N-formyl-methionyl-leucyl-phenylalanine (fMLP, 5 x 10(-8) M) and leukotriene B4 (LTB4, 10(-8) M). L-NAME (but not D-NAME) significantly inhibited the eosinophil migration induced by both fMLP (54% reduction for 1.0 mM; P < 0.05) and LTB4 (61% reduction for 1.0 mM; P < 0.05). In addition, the type II NOS inhibitor 2-amino-5,6-dihydro-6-methyl-4H-1,3-thiazine and the type I/II NOS inhibitor 1-(2-trifluoromethylphenyl) imidazole also markedly (P < 0. 05) attenuated fMLP- (52% and 38% reduction for 1.0 mM, respectively) and LTB4- (52% and 51% reduction for 1.0 mM, respectively) induced migration. The inhibition of eosinophil migration by L-NAME was mimicked by the soluble guanylate cyclase inhibitor 1H-[1,2,4] oxadiazolo [4,3,-a] quinoxalin-1-one (0.01 and 0.1 mM) and reversed by either sodium nitroprusside (0.1 mM) or dibutyryl cyclic GMP (1 mM). We conclude that eosinophils do express NO synthase(s) and that nitric oxide plays an essential role in eosinophil locomotion by acting through a cyclic GMP transduction mechanism.

The protective effect of late preconditioning against myocardial stunning in conscious rabbits is mediated by nitric oxide synthase. Evidence that nitric oxide acts both as a trigger and as a mediator of the late phase of ischemic preconditioning

Seventy-four conscious rabbits undergoing a sequence of six 4-minute coronary occlusion/4-minute reperfusion cycles for 3 consecutive days (days 1, 2, and 3) were assigned to nine groups. In group I (controls, n = 8), the recovery of systolic wall thickening (WTh) after the sixth reperfusion was markedly improved on days 2 and 3 compared with day 1, indicating late preconditioning (PC) against myocardial stunning; the total deficit of WTh after the sixth reperfusion was reduced by 56% on day 2 and 50% on day 3 compared with day 1 (P < .01). Administration on day 2 of the nonselective NO synthase (NOS) inhibitor N omega-nitro-L-arginine (L-NA) (group II, n = 8) or of the selective inducible NOS inhibitors aminoguanidine (AG) (group IV, n = 8) and S-methylisothiourea sulfate (SMT) (group VI, n = 6) completely abrogated late PC against stunning on day 2. On day 3, the expected PC effect became manifest in all groups. Administration of L-NA, AG, or SMT on day 1 (groups III [n = 7], V [n = 6], and VII [n = 5], respectively) had no discernible effect on the deficit of WTh on day 1, indicating that these agents do not augment the severity of myocardial stunning in nonpreconditioned myocardium. In group VIII (n = 7), the abrogation of late PC by SMT on day 2 was completely reversed by the concomitant administration of L-arginine (595 mg/kg IV), indicating that it was not due to nonspecific NOS-unrelated actions. Administration of L-arginine alone on day 2 (group IX [n = 5]) had no effect on the deficit of WTh. Furthermore, administration of L-NA on day 1 (group III) prevented the appearance of the PC effect on day 2, whereas AG (group V) and SMT (group VI) did not, suggesting that the development of late PC on day 1 is triggered by the endothelial (type III) isoform of NOS. This study demonstrates that three structurally different NOS inhibitors (L-NA, AG, and SMT), given 24 hours after the PC ischemia, consistently abrogate late PC against myocardial stunning in conscious rabbits, indicating that this cardioprotective effect is mediated by the activity of NOS. The results obtained with AG and SMT specifically implicate the inducible (type II) isoform as the mediator of the protection on day 2. Previous studies have shown that NO triggers the development of late PC. The present results indicate that NO plays a dual role in late PC against stunning, acting initially as the trigger and subsequently as the mediator of the protection.

Comparative aspects on nitric oxide in brain and its role as a cerebral vasodilator

Histological studies have detected nitric oxide (NO) synthase in the central nervous system of all vertebrates examined, from lampreys to mammals. However, there are still very few comparative physiological studies on the function of NO synthase in the brain of non-mammalian vertebrates. So far, we know that acetylcholine can cause an NO-dependent increase in brain blood flow in turtles and some fish species (crucian carp and rainbow trout), whereas some other fishes appear to lack such a mechanism. Hypercapnia can induce NO-dependent cerebral vasodilation in mammals, but such a mechanism appears to be lacking in the ectothermic vertebrates examined. The number of species studied needs to be expanded before we can draw any firm conclusions about the origin of NO-dependent brain blood flow regulation: if it has evolved more than once or if it has been occasionally lost during evolution. We conclude that NO synthase may be present in all vertebrate brains but that its functions can vary, as judged from its role in cerebral blood flow regulation. The diversity of functions that NO has proven to have within the mammalian brain is likely to be paralleled by the same degree of diversity of function between vertebrate groups.

Nitric oxide synthase activity in tissues of the bovine eye

BACKGROUND

Nitric oxide synthase (NOS) is present in many ocular tissues where it may have different physiological functions. This warrants a thorough characterization of NOS activity in the eye.

METHODS

NOS distribution and its biochemical properties were determined in the retina, choroid, ciliary processes (CP), and trabecular meshwork (TM).

RESULTS

Retinal NOS required NADPH (diphenylene-iodonium, a flavoprotein inhibitor, which inhibited enzyme activity with an IC50 of 0.36 microM, FAD (40 microM), FMN (40 microM), and BH4 (4 microM) as cofactors for optimal activity. Ocular NOS appeared to be regulated by free divalent cations, since its activity was inhibited by EDTA (slopes > 3.0 and IC50 values of 12.8, 19.7, and 53 microM, respectively). Ocular NOS required calmodulin, since NOS activity was inhibited by trifluoperazine (calmodulin inhibitor, IC50 = 41 microM). NOS activity is widely distributed in the eye, (choroid > retina > CP > TM) and is mainly cytosolic (70-95%). L-Arginine analogs inhibited NOS in the retina, choroid, and TM. In all three tissues, NG-methyl-L-arginine displayed the highest affinity for inhibition (IC50 = 0.2-0.7 microM) followed by canavanine (IC50 = 13-33 microM), while aminoguanidine only weakly inhibited NOS (IC50 = 93-179 microM).

CONCLUSION

In all tissues, the order of potency of inhibition points to the presence of constitutive rather than inducible NOS. Moreover, it is possible that TM contains more than a single form of NOS.

Influence of endotoxin on contractility in the rat gastric fundus

The influence of in vivo treatment with E. coli lipopolysaccharide endotoxin on the contractility of the rat gastric fundus was studied. Four h after lipopolysaccharide treatment (20 mg/kg i.p.), the contractile responses to prostaglandin F2alpha in longitudinal muscle strips from the gastric fundus were not different from those in control animals, while the well-known decreased response to noradrenaline in rings of the thoracic aorta was confirmed. Incubation of the tissues with L-arginine did not depress the response to prostaglandin F2alpha in fundus strips of lipopolysaccharide-treated rats. Twelve h after lipopolysaccharide treatment (6.7 mg/kg i.p.), the prostaglandin F2alpha-induced contractions were consistently depressed. The impairment of the prostaglandin F2alpha-induced responses by lipopolysaccharide treatment was not reversed by the nitric oxide synthase inhibitors NG-nitro-L-arginine (L-NNA, 10(-4) M), NG-nitro-L-arginine methyl ester (L-NAME, 3 x 10(-4) M), aminoguanidine (10(-4) M) and L-N6-l-iminoethyl-lysine (L-NIL, 10(-4) M) nor by the cyclooxygenase inhibitor indomethacin (10(-5) M). The impairment was prevented by pretreating the animals with dexamethasone (5 mg/kg i.p.), which had no effect per se on the contractile response to prostaglandin F2alpha. Lipopolysaccharide treatment did not influence the contractile responses to KCl and serotonin. The nonadrenergic noncholinergic relaxant responses to transmural electrical stimulation were not influenced 4 h after lipopolysaccharide treatment but were moderately reduced after 12 h. The results illustrate that the selective impairment of prostaglandin F2alpha-induced contractions in the rat gastric fundus by lipopolysaccharide treatment is not mediated via generation of nitric oxide; downregulation of the prostaglandin F2alpha-receptor by lipopolysaccharide treatment might be involved.

Endogenous nitric oxide facilitates striatal dopamine and glutamate efflux in vivo: role of ionotropic glutamate receptor-dependent mechanisms

We have investigated the influence of the nitric oxide synthase (NOS) substrate, NG-hydroxy-L-arginine (H-ARG) on dopamine (DA) and glutamate (GLU) efflux in vivo using concentric microdialysis probes implanted in the anterior-medial striatum of chloral hydrate-anesthetized rats. Intrastriatal infusion of H-ARG (100 microM, 200 microM, or 1 mM for 120 min) increased DA efflux in a dose-dependent fashion. The facilitatory effect of H-ARG (1 mM) on DA efflux was abolished following pretreatment (80 min) with the constitutive NOS inhibitor 7-nitroindazole (7-NI, 10 microM) but unaffected by L-NG(1-iminoethyl) lysine (100 microM) infusion. As both H-ARG (1 mM) and the NO-generator (+/-)-S-nitroso-N-acetylpenicillamine (1 mM) were observed to increase GLU efflux concurrently with the effect on DA efflux, we evaluated the potential intermediary role of GLU in NO-facilitated DA efflux using ionotropic GLU receptor antagonists. Local infusion of dizocilpine maleate (10 microM) or (+/-)-2-amino-3-[3-(carboxymethoxy)-5-methyl-isoxazol-4-yl] propionic acid (100 microM), attenuated the H-ARG (1 mM)-induced elevation of extracellular DA levels. Conversely, similar treatment with the kainate receptor antagonist d-gamma-glutamyl-aminomethanesulfonic acid did not alter H-ARG-induced DA efflux. To evaluate the regulatory influence of striatal NO on NMDA receptor activation, NMDA (100 microM) was co-perfused with either H-ARG (2 mM) or 7-NI (10 microM). While co-perfusion with 7-NI potentiated NMDA-induced DA efflux, similar treatment with H-ARG (2 mM) abolished the effect. These results demonstrate that endogenous NO production, stimulated via H-ARG-dependent activation of type 1 NOS, enhances striatal DA efflux via an increase in glutamatergic tone on ionotropic GLU-receptors. At higher levels of NOS activation (following H-ARG (2 mM) or NMDA infusion), NO may block glutamatergic neurotransmission via inhibition of NMDA receptor function.

Emerging therapies for acute renal failure

Ischemia is the most common cause of acute renal failure (ARF). In the last decade, several new and important pathophysiological mechanisms that underlie the renal dysfunction have been discovered. These pathophysiological mechanisms include the role of both calcium and calcium-dependent enzymes, oxidant stress, loss of polarity of the tubular cell, tubular obstruction and arginine-glycine-aspartic acid (RGD) peptides, neutrophils, intracellular adhesion molecules (ICAM), and growth factors. A better understanding of tubular and vascular mechanisms has led to therapeutic studies in animals and clinical trials in humans. In this review, the pathophysiology of ischemic ARF will be correlated with the rationale for both current and future therapies.

Nitric oxide (NO): in vivo electrochemical monitoring in the dorsal horn of the spinal cord of the rat

NO synthase (NOS) is largely distributed in the superficial and deep laminae of the dorsal horn as well as in dorsal root ganglion cells. It has been proposed that nitric oxide (NO) participates in the transmission of sustained, and possibly brief, nociceptive, inputs at the spinal level. The aim of this study was to check the ability of in vivo electrochemical monitoring of NO within the dorsal horn of the lumbar spinal cord (L3-L4 level) of chloral hydrate anesthetized or decerebrated spinalized rats. 30 microm diameter and 450 microm length treated carbon fiber electrodes coated with nickel(II) tetrakis (3-methoxy-4-hydroxy-phenyl) porphyrine and NafionR, and associated with differential normal pulse voltammetry, gave a peak of oxidation current around 650 mV (vs. Ag-AgCl) in vitro in NO solutions between 0.125 and 1.25 microM. In vivo, a 650 mV peak appeared which was stable (recording interval 2 min) for up to 3 h (+/-6%). Comparison between in vitro calibration and in vivo voltammograms gave an estimated in vivo extracellular concentration of 0.50 microM. In vivo, peaks decreased by 95% at 90 min and for up to 3 h after an i.p. injection of 100 mg/kg of the NOS inhibitor (NOSI) L-arginine-p-nitroanilide (L-ANA). At the same dose i.p., N(G)-nitro-L-arginine methyl ester (L-NAME) was almost ineffective after 90 min in animals paralyzed with pancuronium bromate or gallamine trethiodide. However, in non-curarized decerebrated spinalized animals, L-NAME depressed the voltammograms by 36% at 90 min. S-Ethylthiourea (80 mg/kg i.p.), also decreased the voltammograms by 45% at 140 min, and finally, 7-nitroindazole (7-NI, 90 mg/kg i.p), induced a important decrease of the 650 mV peak (23% of control) at 120 min. These results are in agreement with biochemical data showing the decrease of NOS activity within the lumbar spinal cord by L-NAME (45% of control at 90 min) and 7-NI (20% of control at 90 min). The NO donor hydroxylamine (30 mg/kg i.p.) significantly increased the peaks (140% at 90 min), and sodium nitroprusside (SNP, 20 mM) when directly superfused upon the spinal cord (200-300 microl min(-1)) induced a large increase in the peak (300% at 90 min). Moreover, SNP 60 min after L-ANA, or 90 min after L-NAME, rapidly restored the 650 mV peak up to control values. These results demonstrate the validity of electrochemical monitoring of NO within the dorsal horn of the spinal cord. The in vivo electrochemical detection of NO is in progress to study the implication of this messenger in the transmission of nociceptive messages at the spinal level.

The structure of nitric oxide synthase oxygenase domain and inhibitor complexes

The nitric oxide synthase oxygenase domain (NOSox) oxidizes arginine to synthesize the cellular signal and defensive cytotoxin nitric oxide (NO). Crystal structures determined for cytokine-inducible NOSox reveal an unusual fold and heme environment for stabilization of activated oxygen intermediates key for catalysis. A winged beta sheet engenders a curved alpha-beta domain resembling a baseball catcher's mitt with heme clasped in the palm. The location of exposed hydrophobic residues and the results of mutational analysis place the dimer interface adjacent to the heme-binding pocket. Juxtaposed hydrophobic O2- and polar L-arginine-binding sites occupied by imidazole and aminoguanidine, respectively, provide a template for designing dual-function inhibitors and imply substrate-assisted catalysis.

Effect of L-lysine on nitric oxide overproduction in endotoxic shock

1. An enhanced production of nitric oxide (NO) from L-arginine, related to the diffuse expression of an inducible NO synthase (iNOS), contributes to the pathogenesis of endotoxic shock. Since iNOS activity depends on extracellular L-arginine, we hypothesized that limiting cellular L-arginine uptake would reduce NO production in endotoxic shock. We investigated the effects of L-lysine, an inhibitor of L-arginine uptake through system y+, on NO production, multiple organ dysfunction and lactate levels, in normal and endotoxaemic rats. 2. Anaesthetized rats challenged with intravenous lipopolysaccharide (LPS, 10 mg kg[-1]) received a 5 h infusion of either L-lysine (500 micromol kg(-1) h(-1), n = 12) or isotonic saline (2 ml kg(-1) h(-1), n = 11). In rats treated with saline, LPS produced a large increase in plasma nitrate and L-citrulline concentrations at 5 h, both markers of enhanced NO production. LPS also caused severe hypotension, low cardiac output and marked hyperlactataemia. All these changes were significantly reduced by L-lysine administration. 3. Endotoxaemia also caused a significant rise in the plasma levels of alanine aminotransferase (ALAT), lipase, urea and creatinine, and hence, liver, pancreatic and renal dysfunction. These changes tended to be less pronounced in rats treated with L-lysine, although the differences did not reach statistical significance. 4. Similar experiments were conducted in 10 rats challenged with LPS vehicle in place of LPS and then treated with L-lysine (500 micromol kg(-1) h(-1), n = 5) or saline (2 ml kg(-1) h(-1), n = 5) for 5 h. In these animals, all the haemodynamic and metabolic variables remained stable and not statistically different between both treatment groups, except for a slight rise in ALAT, which was comparable in L-lysine and saline-treated rats. 5. In conclusion, L-lysine, an inhibitor of cellular L-arginine uptake, reduces NO production and exerts beneficial haemodynamic effects in endotoxaemic rats. L-lysine also reduces hyperlactataemia and tends to blunt the development of organ injury in these animals. Contrastingly, L-lysine has no effects in the absence of endotoxin and thus appears to act as a selective modulator of iNOS activity.

In vivo inhibition of nitric oxide synthase by bisisothiouronium and bisguanidinium salts

The ability of two S,S'-(alkane-1,omega-diyl) bisisothiouronium dibromides, three N,N'-(alkane-1, omega-diyl) bis guanidinium dinitrates and N,N'-bis (3-guanidinopropyl)piperazine dinitrate to inhibit constitutive (i.e. endothelial and neuronal forms) and inducible forms of nitric oxide synthases has been evaluated in vivo. These compounds, synthesized by two of us (J. C. L. and C. S.), have been tested in vivo; they were administered simultaneously with an irritant (carrageenan lambda) into the pleural cavity. The amount of nitrites collected 0.5 and 7 hours after this injection can be considered as an indicator of nitric oxide (NO) production. According to previous data, the first harvesting time can be related to activation of constitutive NO synthases and the second to activation of inducible NO synthases. These substances significantly inhibited nitrite production as did 2-methyl-2-thiopseudourea sulphate, previously described as a potent inhibitor of NO synthases and considered as the reference compound. The inhibiting effect varied according to the chemical structure of the compounds. Results were significantly different from controls at 0.5 h only with the S,S'-(octane-1,8-diyl) bisisothiouronium dibromide and the S,S'-(nonane-1,9-diyl) bisisothiouronium dibromide at the highest concentration, N,N'-(heptane-1,7-diyl) bisguanidinium dinitrate and N,N'-bis (3-guanidinopropyl)piperazine dinitrate. At 7 h, all the results were significantly different from controls, with a major effect observed with N,N'-(heptane-1,7-diyl) bisguanidinium dinitrate. The most active substances exerted similar effects to the reference substance.

Circulating methemoglobin and nitrite/nitrate concentrations as indicators of nitric oxide overproduction in critically ill children with septic shock

OBJECTIVES

To examine the relationship between circulating methemoglobin and nitrite/nitrate concentrations and to compare these markers of nitric oxide overproduction with clinical variables in children diagnosed with septic shock.

DESIGN

Prospective, controlled, clinical study.

SETTING

Pediatric intensive care unit and outpatient clinic in a children's hospital.

PATIENTS

Twenty-two children diagnosed with septic shock and ten age-matched healthy control patients.

INTERVENTIONS

Patients diagnosed with septic shock had blood specimens taken on study entry and every 6 hrs for 72 hrs for methemoglobin and nitrite/nitrate determinations. Single blood specimens were obtained from controls.

MEASUREMENTS AND MAIN RESULTS

Circulating methemoglobin and nitrite/nitrate concentrations were significantly higher in children diagnosed with septic shock in comparison with healthy control children (p = .01 and .05, respectively). Peak nitrite/nitrate concentrations correlated with serum creatinine (r2 = .19; p = .04) and were inversely correlated with arterial pH (r2 = .28; p = .01) and urine output (r2 = .21; p = .03) when analyzed by log-linear regression. There were no significant relationships between methemoglobin and nitrite/nitrate or between methemoglobin and any other clinical variable.

CONCLUSIONS

Circulating methemoglobin and nitrite/nitrate concentrations are increased in children diagnosed with septic shock. Plasma nitrite/nitrate values correlate with selected clinical variables in these children. Circulating methemoglobin measurements are not superior to plasma nitrite/nitrate concentrations as an indicator of endogenous overproduction of nitric oxide in children diagnosed with septic shock. A need remains to develop markers of endogenous nitric oxide activity that have greater accuracy and reliability.

Clinical and serologic manifestations of autoimmune disease in MRL-lpr/lpr mice lacking nitric oxide synthase type 2

Nitric oxide (NO) is an important mediator of the inflammatory response. MRL-lpr/lpr mice overexpress inducible nitric oxide synthase (NOS2) and overproduce NO in parallel with the development of an autoimmune syndrome with a variety of inflammatory manifestations. In previous studies, we showed that inhibiting NO production with the nonselective nitric oxide synthase (NOS) inhibitor NG-monomethyl-arginine reduced glomerulonephritis, arthritis, and vasculitis in MRL-lpr/lpr mice. To define further the role of NO and NOS2 in disease in MRL-lpr/lpr mice, mice with targeted disruption of NOS2 were produced by homologous recombination and bred to MRL-lpr/lpr mice to the N4 generation. MRL-lpr/lpr littermates homozygous for disrupted NOS2 (-/-), heterozygous for disrupted NOS2 (+/-), or wildtype (+/+) were derived for this study. Measures of NO production were markedly decreased in the MRL-lpr/lpr (-/-) mice compared with MRL-lpr/lpr (+/+) mice, with intermediate production by the MRL-lpr/lpr (+/-) mice. There was no detectable NOS2 protein by immunoblot analysis of the spleen, liver, kidney, and peritoneal macrophages of the (-/-) animals, whereas that of (+/+) was high and (+/-) intermediate. The (-/-) mice developed glomerular and synovial pathology similar to that of the (+/-) and (+/+) mice. However, (-/-) mice and (+/-) mice had significantly less vasculitis of medium-sized renal vessels than (+/+) mice. IgG rheumatoid factor levels were significantly lower in the (-/-) mice as compared with (+/+) mice, but levels of anti-DNA antibodies were comparable in all groups. Our findings show that NO derived from NOS2 has a variable impact on disease manifestations in MRL-lpr/lpr mice, suggesting heterogeneity in disease mechanisms.

Nitric oxide and septic shock

1. Nitric oxide (NO) is generated by three different isoforms of NO synthase, two of which are expressed constitutively (in endothelium: eNOS, brain: nNOS), while one is induced by endotoxin (LPS) or cytokines (iNOS). 2. Expression of iNOS in many organs or tissues in septic shock (caused by Gram-negative or Gram-positive bacteria) results in an enhanced formation of NO that contribute to hypotension, vascular hyporeactivity to vasoconstrictors, organ injury, and dysfunction as well as host defense. 3. Inhibition of either the expression of iNOS protein (e.g., with dexamethasone) or of NOS activity (e.g., with selective inhibitors of iNOS activity) exerts beneficial effects in animal models of shock. In contrast, inhibition of eNOS activity may lead to excessive vasoconstriction (adverse effects). 4. There is limited evidence regarding the degree of iNOS induction in human cells or tissues with septic shock. Preliminary data from ongoing clinical trials indicate that nonselective inhibitors of NOS activity (e.g., NG-methyl-L-arginine [L-NMMA]) exert beneficial hemodynamic effects.