Nitric oxide synthases in mammals

Alpha-NADPH appears to be primarily oxidized by the NADPH-diaphorase activity of nitric oxide synthase (NOS)

Biochemical studies have shown that the NADPH-diaphorase (NADPH-d) activity of nitric oxide synthase (NOS) represents only a part of the total cellular diaphorase pool. Histochemically, NADPH-d activity can be demonstrated in cells expressing no constitutive NOS. Therefore, attempts aimed to improve the specificity of the NADPH-d reaction are currently being undertaken. In this study, the effect of replacing the natural and common diaphorase substrate beta-NADPH with the artificial stereoisomer alpha-NADPH on the extent of NADPH-d staining was examined. When beta-NADPH served as the substrate, discrete populations of central and peripheral neurons as well as numerous non-neural cells in many organs of common laboratory rodents (mouse, rat, gerbil, hamster, guinea pig) and marmosets were found to generate formazan. Substitution of alpha-NADPH for beta-NADPH resulted in reduced staining intensity of nerve cells and muscle fibers. Furthermore, alpha-NADPH-d staining of macula densa cells, enterocytes and granulocytes varied according to the species examined. No reaction was observed in most other cells which stained positively for beta-NADPH-d activity. Examination of adjacent sections, incubated for the demonstration of NOS-immunoreactivity, revealed that alpha-NADPH-d activity and NOS immunostaining are strictly colocalized in neurons, striated muscle fibers and, species-dependently, in macula densa cells. It can thus be concluded that, with the exception of gut granulocytes, alpha-NADPH is primarily metabolized by the reductase activity of NOS.

Nitric oxide synthase activity in human breast cancer

Nitric oxide (NO) is generated by a family of isoenzymes (NO synthases) expressed in a wide range of mammalian cells. We have recently reported NO synthase expression in human gynaecological cancers. In this study we have assessed the activity and distribution of NO synthase in a series of human breast tumours and in normal breast tissue. Calcium-dependent (constitutive) and -independent (inducible) NO synthase activity, as well as NO biosynthesis, was high in invasive tumours compared with benign or normal tissue. Furthermore, for invasive ductal carcinomas, NO biosynthesis was significantly greater for grade III compared with grade II tumours. Immunohistochemical investigations revealed immunolabelling with a monoclonal antibody to murine inducible NO synthase predominantly within tumour-associated macrophages. Immunolabelling with a polyclonal antiserum raised against rat brain NO synthase was also observed in vascular endothelial and myoepithelial cells. Thus NO synthase is expressed in human breast tumours, where its presence correlates with tumour grade.

Glomerular actions of nitric oxide

NO, a simple molecule synthesized from L-arginine by NO synthases, has been identified to play an important role in cell communication, cell defense and cell injury. The half life of NO is very short because NO either reacts with superoxide anion (O2-), and/or binds to heme molecules or Fe-S groups present in proteins. The biological effects of NO depend on both the concentration of NO at the site of action as well as upon the specific location where NO is generated. Small quantities of NO are generated by cNOS such as that present in the vascular endothelium, while large quantities of nitric oxide are synthesized by iNOS in response to cytokines or bacterial products. Within the kidney NO generated by endothelial cNOS participates in the regulation of the glomerular microcirculation by modifying the tone of the afferent arteriole and mesangial cells (Fig. 4). In addition, NO generated by macula densa and the afferent arteriole control glomerular hemodynamics via TGF and by modulating renin release. Therefore NO is important in the physiologic regulation of glomerular capillary blood pressure, glomerular plasma flow and the glomerular ultrafiltration coefficient. Through its actions on glomerular pressures and flows, NO may also regulate the macro- and micromolecular traffic through the mesangium. Chronic NO insufficiency causes hypertension and glomerular damage and may be causally involved in the genesis of salt dependent hypertension. Increased NO production may be involved in the early pathogenic hemodynamic changes in diabetes and in the physiologic hemodynamic responses to normal pregnancy. Maintenance of the antithrombogenic properties of the endothelium is another important action of NO which inhibits platelet aggregation and adhesion. Large quantities of NO such as that synthesized by either glomerular cells or macrophages during glomerular inflammation may lead to glomerular injury. A better understanding of the physiology and pathophysiology of NO in the kidney will lead to the development of new therapeutic avenues.

Nitric oxide-induced nitration of catecholamine neurotransmitters: a key to neuronal degeneration?

Exposure of the neurotransmitters dopamine (1a) and norepinephrine (1b), as well as of other catechol compounds (1c-e), to nitric oxide (NO) in aerated phosphate buffer at room temperature leads to the corresponding 6-nitroderivatives (2a-e) in yields higher than 80%. Formation of nitration products depends on the presence of oxygen and is inhibited by excess ascorbic acid, whereas sulfhydryl compounds, e.g. cysteine, and scavengers of reactive oxygen species, such as catalase and superoxide dismutase, exert no significant inhibitory effect. O-Methylated catechols are poorly or not reactive toward NO. These and other observations are consistent with a mechanism involving coupling of a semiquinone radical with NO or a higher oxide, e.g. nitrogen dioxide (NO2). The observed formation of potentially toxic 6-nitrocatecholamines under physiologically relevant conditions may open new perspectives to an understanding of the biochemical processes underlying NO-induced toxicity and neuronal degeneration.

Modulation of calcium-activated non-selective cation channel activity by nitric oxide in rat brown adipose tissue

1. Single-channel calcium-activated non-selective cation currents from isolated rat brown fat cells were measured using the inside-out patch configuration of the patch-clamp technique. The existence of a possible modulatory effect of nitric oxide on the putative redox-modulatory site located on the intracellular side of the non-selective cation channel was investigated. 2. The nitric oxide-releasing substances nitroglycerin, sodium nitroprusside, S-nitrosocysteine and S-nitroso-N-acetyl-D,L-penicillamine (all at 100 microM) were able to block channel activity almost completely. 3. In each case the blockade was persistent and could not be washed away. Dithiothreitol (DTT, 2 mM) was able to reverse the blockade to a large extent, whereas oxidized DTT (2 mM) was without effect. 4. It was concluded that nitric oxide can modulate non-selective cation channel activity by oxidizing sulfhydryl groups and that this effect can be reversed by reduction.

Long-lasting changes of calcium oscillations in astrocytes. A new form of glutamate-mediated plasticity

Long-term changes of synaptic strength in the central nervous system are mediated by an increase of cytosolic calcium concentration ([Ca2+]i) following activation of excitatory neurotransmitter receptors. These phenomena, which represent a possible cellular basis for learning and memory processes in eukaryotes, are believed to be restricted to neurons. Here we provide evidence for a long-term change of the response elicited by the excitatory neurotransmitter glutamate in a non-neuronal cell population of the central nervous system, i.e. visual cortical astrocytes in culture. Stimulation with glutamate induces in astrocytes a regular pattern of [Ca2+]i oscillations. A second stimulation, after an interval ranging from 2 to 60 min, induces an oscillatory response characterized by an increased frequency. Induction of this change in the astrocyte response is abolished by a specific inhibitor of the nitric oxide synthase and recovers upon exogenous nitric oxide generation or addition of a permeant cGMP analogue. Local brief pulses of glutamate to individual astrocytes, at a rate of 1 Hz, also elicit [Ca2+]i oscillations whose frequency increases following a second series of pulses. The long-lasting modification in the [Ca2+]i oscillatory response induced by glutamate in astrocytes demonstrates that in the central nervous system cellular memory is not a unique feature of neurons.

L-arginine depletion by arginase reduces nitric oxide production in endotoxic shock: an electron paramagnetic resonance study

Nitric oxide (NO) synthesis was measured in the liver, lung, spleen and kidney of lipopolysaccharide-treated male rats using the nitric oxide spin trap, iron (II)-diethyldithiocarbamate (FeDETC2). Nitric oxide formation in vivo was determined by the increase in intensity of the characteristic triplet hyperfine EPR spectrum of [NO-FeDETC2]. Intravenous bovine liver arginase, at a dose which completely depleted circulating arginine, significantly reduced the formation of nitric oxide in these tissues. The general decrease in NO levels was confirmed by the decrease in plasma nitrite levels. These results directly demonstrate that NO formation in endotoxic shock depends on extracellular arginine; depletion of plasma arginine may be a useful therapeutic strategy.

Local nitric oxide production in viral and autoimmune diseases of the central nervous system

Because of the short half-life of NO, previous studies implicating NO in central nervous system pathology during infection had to rely on the demonstration of elevated levels of NO synthase mRNA or enzyme expression or NO metabolites such as nitrate and nitrite in the infected brain. To more definitively investigate the potential causative role of NO in lesions of the central nervous system in animals infected with neurotropic viruses or suffering from experimental allergic encephalitis, we have determined directly the levels of NO present in the central nervous system of such animals. Using spin trapping of NO and electron paramagnetic resonance spectroscopy, we confirm here that copious amounts of NO (up to 30-fold more than control) are elaborated in the brains of rats infected with rabies virus or borna disease virus, as well as in the spinal cords of rats that had received myelin basic protein-specific T cells.

Nitric oxide in inflammation and immune response

This short review deals with the role of a recently found signalling molecule, nitric oxide (NO), in inflammatory and immune responses. NO regulates inflammatory erythema and oedema and has cytotoxic action against micro-organisms. In some instances (such as reperfusion injury) NO has cytoprotective properties. Production of large amounts of NO by activated macrophages accounts for their ability to suppress lymphocyte proliferation. NO synthesis in lymphocytes is questionable but cytokines secreted by activated lymphocytes regulate NO synthesis by macrophages. Constitutive NO synthase is activated in neutrophils in response to inflammatory stimuli and NO has diverse, often biphasic effects on neutrophil functions. Increased concentrations of nitrite and nitrate (metabolites of NO) are present in arthritic joints. NO is synthesized not only by migrated inflammatory cells but also by articular chondrocytes and inflamed synovial membrane. In the inflamed joint, NO regulates the synthesis of several inflammatory mediators and functions of inflammatory cells. In addition, NO seems to mediate some destructive effects of proinflammatory cytokines such as interleukin-1. In conclusion, NO regulates several humoral and cellular responses in inflammation, having both anti-inflammatory and proinflammatory properties depending on the type and phase of the inflammatory reaction.

Pro-inflammatory cytokines in a ruminant model: pathophysiological, pharmacological, and therapeutic aspects

Infection evokes complex changes which are thought to be caused by production and release of pro-inflammatory cytokines such as tumour necrosis factor (TNF-alpha), interferons (INFs), and interleukins (ILs). They regulate local inflammatory reactions, but may also gain access to the circulation and induce systemic effects collectively known as the Acute Phase Response. To improve our understanding of the pathophysiology of pro-inflammatory cytokines in ruminants, studies have been performed with TNF-alpha, IL1-alpha/beta, and IFN-alpha/ gamma as well as with cytokine-inducers in dwarf goats. In relation to therapy, the following aspects may be of interest: a) Cytokine therapy given before or just after microbial challenge induces in vivo antimicrobial activity. Moreover, cytokines potentiate in vivo the antimicrobial activity of antibiotics, b) Cytokines may act as biological response modifiers for enhancing specific immunity to vaccines, and c) Cytokines may affect drug absorption, disposition, and metabolite formation in disease states. Although studies of the actions of corticosteroids, nonsteroidal anti-inflammatory and antipyretic agents, antibodies to endotoxin, TNF-alpha, or IL-1, synthetic E. coli lipid A precursors, hydrazine, isoniazid, chloroquine, polymyxin B, bicyclic imidazoles, hydroxamates, and tyrosine kinase inhibitors in endotoxaemic animals have shed further light on inflammatory processes, clinical studies in this field are urgently required to evaluate their beneficial effect.

Nitric oxide: biological mediator, modulator and effector

Nitric oxide (NO) is synthesized from L-arginine by at least three isoforms of NO synthase enzyme (NOS). Once generated NO can interact with a number of molecular targets including haem proteins, enzymes, DNA, thiols, oxygen and superoxide. These reactions determine the profile of NO as a major biological mediator, modulator and effector molecule.

Alcohol and the brain: what's NO got to do with it?

A potential role for nitric oxide in alcohol-induced changes in brain function is discussed. Chronic alcohol exposure may lead to excitotoxicity partially due to increased levels of nitric oxide (NO). Excessive NO has been linked to cytotoxicity in neurons, glia and myelin. Cytokines produced in response to cell injury may trigger increased production of NO. These events may be involved in alcohol-induced brain damage. Formation of NO has recently been linked to increased preference for and tolerance to alcohol. A hypothesis for prevention and treatment of alcohol-induced brain damage, and craving and alcohol intake by alcoholics is proposed.

Substantial regional and hemispheric differences in brain nitric oxide synthase (NOS) inhibition following intracerebroventricular administration of N omega-nitro-L-arginine (L-NA) and its methyl ester (L-NAME)

Nitric oxide synthase (NOS) enzyme activity was determined in a comprehensive selection of regions of the rat brain. The effects of lateral ventricular administration of N omega-nitro-L-arginine (L-NA, 30 micrograms) and its methyl ester (L-NAME, 3-100 micrograms) on NOS activity were examined in the ipsilateral and contralateral areas of 4 of these brain regions and in the cerebellum. NOS activity was determined using a new and rapid ex vivo assay method which ensures minimal dissociation of the enzyme-inhibitor complex. Following infusion of L-NAME, NOS activity was rapidly and dose-dependently inhibited in all brain regions studied (cerebral cortex, striatum, hippocampus, cerebellum and thalamus). However, NOS activity of brain regions within the contralateral hemisphere was inhibited significantly less than in ipsilateral regions, with the exception of the thalamus. The degree of NOS inhibition varied markedly between brain regions within each hemisphere and correlated with their ventricular proximity to the site of NOS inhibitor administration. Therefore, NOS in the thalamus was inhibited most effectively and NOS in the cerebral cortex the least. Within the cerebral cortex further regional differences could be observed, with NOS in the frontal/parietal areas inhibited more effectively than NOS in the temporal/occipital areas. Maximal inhibition of NOS was sustained for approx 6 hr after administration of 30 and 100 micrograms L-NAME. No inhibition of NOS was observed 24 hr after administration. Lateral ventricular administration of the metabolite and active moiety of L-NAME, L-NA, resulted in a similar degree of inhibition and time of inhibitory onset. In contrast, when L-NAME was administered i.p., a significant delay in the onset of NOS inhibition was observed in the above brain regions compared to L-NA. However, no regional or hemispheric differences in NOS inhibition were detected following peripheral administration of these inhibitors. These results indicate that central administration of NOS inhibitors yields a complex pattern of NOS inhibition and that data obtained on brain physiology following the i.c.v. administration of NOS inhibitors, or for that matter any other CNS effector, should therefore be interpreted with extreme caution.

Nitrogen monoxide decreases iron uptake from transferrin but does not mobilise iron from prelabelled neoplastic cells

The effect of congeners of nitrogen monoxide (NO) on iron (Fe) uptake from 59Fe-125I-transferrin (Tf) and release of 59Fe from prelabelled cells have been investigated in SK-MEL-28 human melanoma cells, human K562 cells and mouse MDW-4 cells. These studies have been initiated as it has been suggested that the tumoricidal effects of NO may be mediated by its acting to release Fe from cells (Hibbs et al., 1984 Biochem. Biophys. Res. Commun. 123, 716-723; Hibbs et al., 1988 Biochem. Biophys. Res. Commun. 157, 87-94). The nitrosonium ion (NO+) generator, sodium nitroprusside (SNP), decreased 59Fe uptake by melanoma cells to 57% of the control without decreasing 125I-Tf uptake after a 4-h incubation with 59Fe-125-Tf (1.25 microM). Longer incubations up to 24 h decreased 59Fe uptake and also 125I-Tf uptake. Two breakdown products of SNP, ferricyanide and cyanide, had no effect on 59Fe uptake. In addition, photolysis of the SNP solution prevented the inhibition of 59Fe uptake, suggesting that NO was the active agent. Two nitric oxide (NO.) producing agents, 3-morpholinosydnonimine (SIN), and S-nitroso-N-acetylpenicillamine (SNAP), also decreased 59Fe uptake from 59Fe-125I-Tf. Superoxide dismutase increased the efficacy of SIN, and the NO-scavenger, oxyhaemoglobin, prevented the inhibition of 59Fe uptake mediated by SNAP, again suggesting that NO was the active agent. Furthermore, dialysis studies demonstrated that none of the NO-generating agents could remove 59Fe from 59Fe-125I-Tf, suggesting that the decrease in cellular Fe uptake observed was not due to NO releasing Fe from the Fe-binding sites of Tf. Despite the ability of NO-producing agents at inhibiting 59Fe uptake by cells, they could not remove significant amounts of 59Fe from melanoma cells prelabelled with either 59Fe-citrate or 59Fe-125I-Tf. Similar data were obtained using K562 and MDW-4 cells. Interestingly, the NO+ generating agent, SNP, had no effect on [3H]thymidine uptake. However, when SNP was converted to an NO. generator by the addition of 1 mM ascorbate, its effect was similar to the NO. generator, SNAP, markedly reducing [3H]thymidine incorporation to 33% of the control value. The addition of unlabelled diferric Tf (0.625 microM) to SNAP ameliorated its inhibitory effect on cellular [3H]thymidine uptake, suggesting that the interaction of NO. with Fe was of importance in the inhibition observed. The results are discussed in the context of the cytostatic potential of NO via its binding to Fe.

Roles of nitric oxide in tumor growth

A subclone of the human colon adenocarcinoma cell line DLD-1, which grew reproducibly as subcutaneous tumors in nude mice, was isolated. Such cells, when engineered to generate nitric oxide (NO) continuously, grew more slowly in vitro than the wild-type parental cells. This growth retardation was reversed by the addition of N-iminoethyl-L-ornithine. In nude mice, however, the tumors from these cells grew faster than those derived from wild-type cells and were markedly more vascularized, suggesting that NO may act as part of a signaling cascade for neovascularization. Recent observations that the generation of NO in human breast and gynecological cancers correlates positively with tumor grade are consistent with this hypothesis. We suggest that NO may have a dual pro- and antitumor action, depending on the local concentration of the molecule.

In search of a function for tetrahydrobiopterin in the biosynthesis of nitric oxide

(6R)-5,6,7,8-Tetrahydro-L-biopterin(H4biopterin) is well known as a cofactor of enzymes that hydroxylate aromatic amino acids. More recent work has revealed an essential role of H4biopterin in the biosynthesis of nitric oxide (NO), an intercellular messenger molecule synthesized from L-arginine by different NO synthase isozymes in many species and tissues. While the function of H4biopterin in aromatic amino acid hydroxylation is well established, the role of this pteridine in NO synthesis is, as yet, elusive. Current experimental evidence hints at a dual mode of action of H4biopterin, involving both an allosteric effect on the NO synthase protein and participation as a reactant in L-arginine oxidation. As discussed in detail in the present article, the latter effect of this pteridine may be related to the protection of NO synthase from feedback inhibition by NO.

Autocrine inhibition of Na+/K(+)-ATPase by nitric oxide in mouse proximal tubule epithelial cells

An inducible nitric oxide synthase has recently been described in proximal tubule epithelium. To investigate the effects of proximal tubule NO on Na+/K(+)-ATPase, we induced NO production in mouse proximal tubule epithelial cells by treatment with lipopolysaccharide (LPS) and interferon-gamma (IFN gamma) followed by determinations of ouabain-sensitive ATPase activity. Na+/K(+)-ATPase activity decreased after 4 h of LPS/IFN gamma treatment, reaching maximal inhibition after 24 h (34% reduction in activity). The inhibition of Na+/K(+)-ATPase activity by LPS/IFN gamma was prevented by simultaneous incubation with N omega-nitro L-arginine and markedly blunted by removal of L-arginine from the medium. The NO donors sodium nitroprusside and SIN-1 also inhibited Na+/K(+)-ATPase activity to a similar extent than LPS/IFN gamma. However, treatment with 8-pCPT-cGMP only modestly reduced Na+/K(+)-ATPase activity. Interestingly, superoxide dismutase prevented the inhibitory effects of NO on Na+/K(+)-ATPase activity, suggesting a role for peroxynitrite in this inhibition. We conclude that NO generated by mouse proximal tubule epithelial cell iNOS inhibits Na/K ATPase activity in an autocrine fashion and that this inhibition is accompanied by a reduction in Na-dependent solute transport.

Cholinergic signaling in the rat pineal gland

1. Innervation of the mammalian pineal gland is mainly sympathetic. Pineal synthesis of melatonin and its levels in the circulation are thought to be under strict adrenergic control of serotonin N-acetyltransferase (NAT). In addition, several putative pineal neurotransmitters modulate melatonin synthesis and secretion. 2. In this review, we summarize what is currently known on the pineal cholinergic system. Cholinergic signaling in the rat pineal gland is suggested based on the localization of choline acetyltransferase (ChAT) and acetylcholinesterase (AChE), as well as muscarinic and nicotinic ACh binding sites in the gland. 3. A functional role of ACh may be regulation of pineal synaptic ribbon numbers and modulation of melatonin secretion, events possibly mediated by phosphoinositide (PI) hydrolysis and activation of protein kinase C via muscarinic ACh receptors (mAChRs). 4. We also present previously unpublished data obtained using primary cultures of rat pinealocytes in an attempt to get more direct information on the effects of cholinergic stimulus on pinealocyte melatonin secretion. These studies revealed that the cholinergic effects on melatonin release are restricted mainly to intact pineal glands since they were not readily detected in primary pinealocyte cultures.

Acetylcholine elicits a rebound stimulation of Ca2+ current mediated by pertussis toxin-sensitive G protein and cAMP-dependent protein kinase A in atrial myocytes

Cholinergic inhibition of atrial contraction is typically followed by a rebound positive inotropic response. In the present study, we used a nystatin-perforated patch whole-cell recording method to determine whether acetylcholine (ACh) elicits a rebound stimulation of L-type Ca2+ current (ICa,L) in cat atrial myocytes. ACh (1 mumol/L) decreased basal ICa,L (-19 +/- 2%). Within approximately 30 s of returning to ACh-free solution, basal ICa,L exhibited a rebound increase above the control level (+61 +/- 7%) that returned to the control level within 4 to 5 minutes. ACh elicited concomitant changes in cell shortening, ie, a decrease followed by a rebound increase. The EC50 and maximal response of ACh-induced inhibition and rebound stimulation of ICa,L were 1.9 x 10(-9) mol/L and -30%, respectively, and 2.9 x 10(-8) mol/L and +64%, respectively. All effects of ACh on ICa,L were blocked by prior exposure to 1 mumol/L atropine or 100 mumol/L AFDX116 and unaffected by 0.2 mumol/L pirenzepine or 1 mumol/L propranolol. In the presence of ACh, exposure to atropine elicited stimulation of ICa,L.ACh-induced inhibition and rebound stimulation of current were independent of external Ca2+. Rebound stimulation of ICa,L was associated with a negative shift in the voltage dependence of ICa,L activation. Inhibition of protein kinase A by 50 mumol/L Rp-cAMPs decreased basal ICa,L by 36 +/- 1% and abolished the rebound stimulation of ICa,L. Forskolin (0.01 mumol/L) or isoproterenol (0.01 mumol/L) had no effect on basal ICa,L, but each accentuated the rebound increase in ICa,L. When adenylate cyclase was maximally stimulated with 1 mumol/L isoproterenol plus 2 mumol/L forskolin, ACh decreased ICa,L but failed to elicit rebound stimulation of ICa,L. Milrinone (10 mumol/L) increased basal ICa,L by 70 +/- 7% and significantly attenuated the rebound stimulation of ICa,L. Exposure to 1 mmol/L 8-bromo-cGMP elicited a small decrease in basal ICa,L, attenuated ACh-induced inhibition, and enhanced the rebound stimulation of ICa,L. Incubation in pertussis toxin prevented all ACh-induced changes in ICa,L. Inhibition of nitric oxide synthase by 100 mumol/L NG-monomethyl-L-arginine (L-NMMA) decreased basal ICa,L by -20 +/- 5%, prevented ACh-induced inhibition, and markedly attenuated the rebound stimulation of ICa,L. We conclude that in cat atrial myocytes ACh acts via M2 muscarinic receptors and pertussis toxin-sensitive G protein to inhibit basal ICa,L and that on withdrawal ACh elicits a rebound stimulation of ICa,L. Rebound stimulation of ICa,L is mediated via cAMP-dependent protein kinase A enhanced by ACh-induced inhibition of phosphodiesterase.(ABSTRACT TRUNCATED AT 400 WORDS)

Up-regulation of nitric oxide synthase by estradiol in human aortic endothelial cells

We have examined the effects of sex hormones on calcium-dependent NO production and protein levels of NO synthase in cultured human aortic endothelial cells, which were treated with various doses of 17 beta-estradiol and testosterone for 8-48 h. Treatment with 17 beta-estradiol enhanced calcium-dependent NO production, but testosterone had exerted no effect. Western blot using monoclonal anti-human endothelial NO synthase antibody clarified that increased NO production by 17 beta-estradiol treatment was accompanied by increased NO synthase protein. Our results provide evidence that human endothelial NO synthase can be regulated by estrogens.

Effect of inhibiting NO synthesis on hippocampal extracellular glutamate concentration in seizures induced by kainic acid

It has been suggested that nitric oxide (NO) interferes with both glutamatergic neurotransmission and the regulation of cerebral blood flow in epileptic seizures. This study examines the effect of an inhibitor of NO synthesis, NG-nitro-L-arginine methyl ester (L-NAME, 20 mg/kg), on the extracellular concentration of glutamate during seizures induced by kainic acid (KA; 10 mg/kg), both drugs being administered systemically. L-NAME was injected 40 min before KA. The extracellular glutamate concentration was measured in the hippocampus of awake, spontaneously breathing rats using microdialysis combined with HPLC. The arterial blood gases and glycemia were periodically checked. The arterial blood pressure, the electrocorticogram and the body temperature were continuously monitored. In basal conditions, the systemic injection of L-NAME increased arterial blood pressure but did not significantly change the hippocampal glutamate level. In seizure conditions, the hippocampal glutamate concentration was either slightly increased or not significantly changed in saline-treated rats (n = 6) but it was decreased in L-NAME-treated rats (n = 6). At all times after KA injection, the hippocampal glutamate concentration was significantly lower in L-NAME-treated rats than in saline-treated rats. Unlike saline-treated rats, L-NAME-treated rats died during status epilepticus. This study shows that acute systemic injection of L-NAME reduces the extracellular concentration of glutamate in the rat hippocampus during seizures induced by KA.

Delayed circulatory failure due to the induction of nitric oxide synthase by lipoteichoic acid from Staphylococcus aureus in anaesthetized rats

1. This study investigates the effect of lipoteichoic acid (LTA) from the cell wall of Staphylococcus aureus, a micro-organism without endotoxin, on haemodynamics and induction of nitric oxide synthase (iNOS) in the anaesthetized rat. 2. Intravenous injection of LTA (10 mg kg-1) resulted in a decrease in blood pressure from 123 +/- 1 mmHg to 83 +/- 7 mmHg after 270 min (P < 0.001) and a reduction of the pressor response to noradrenaline (1 microgram kg-1) from 33 +/- 1 mmHg.min to 23 +/- 3 mmHg.min after 270 min (P < 0.05). 3. The delayed circulatory failure (hypotension and vascular hyporeactivity) caused by LTA was prevented by pretreatment of rats with dexamethasone (10 mg kg-1, 60 min prior to LTA) or the nitric oxide synthase inhibitor NG-monomethyl-L-arginine (L-NMMA, 10 mg kg-1 h-1, i.v. infusion starting 30 min prior to LTA). 4. In contrast, treatment of rats with polymyxin B (0.05 mg kg-1), an agent which binds endotoxin (lipopolysaccharides, LPS), did not affect the delayed circulatory failure caused by LTA. Polymyxin B, however, attenuated the hypotension and vascular hyporeactivity to noradrenaline afforded by endotoxaemia (2 mg kg-1 LPS, i.v.) for 270 min. 5. The delayed circulatory failure caused by LTA was associated with a time-dependent increase in (i) the expression of iNOS protein in the lung (Western blot analysis), and (ii) iNOS activity. This increase in iNOS protein and activity was prevented by pretreatment of LTA-rats with dexamethasone (10 mg kg-1). 6. Intravenous injection of LTA resulted in an increase in serum tumour necrosis factor (TNF)-alpha(maximum at 90 min after LTA), which was attenuated by pretreatment of rats with dexamethasone(10 mg kg-1, 60 min prior to LTA). The magnitude of the rise in TNF-alpha caused by LTA was similar to the one elicited by LPS (10mgkg-', i.v.).7. Thus, an enhanced formation of nitric oxide following the induction of iNOS contributes importantly to the delayed vascular failure (hypotension and vascular hyporeactivity) caused by LTA in the anaesthetized rat. We suggest that the endogenous release of TNF-alpha contributes to the induction ofiNOS caused by LTA in vivo.

Determination of brain nitric oxide synthase inhibition in vivo: ex vivo assays of nitric oxide synthase can give incorrect results

The in vivo potencies of N omega-nitro-L-arginine (L-NA), N omega-monomethyl-L-arginine (L-NMMA) and N omega-iminoethyl-L-ornithine (L-NIO) against brain nitric oxide synthase (NOS) were determined by assessing their ability to inhibit harmaline-induced increases in rat cerebellar cGMP. L-NA, L-NIO and L-NMMA were all able to completely prevent the harmaline-induced increase in cGMP with ID50s of 0.5, 30 and 55 mg/kg, respectively, and with the same order of potency as that seen for inhibition of cerebellar NOS in vitro. The inhibitory effects of low doses of L-NA on cerebellar cGMP were maintained for at least 8 hr. The ID50 of L-NA for inhibition of cerebellar cGMP in vivo was similar to its ID50 for inhibition of cerebellar NOS ex vivo but only when NOS activity was assayed as an initial rate. However, doses of L-NMMA and L-NIO that inhibited harmaline-induced increases in cerebellar cGMP in vivo by 50% failed to inhibit NOS ex vivo. The methyl ester of L-NA, L-NAME, produced substantial inhibition of cerebellar NOS ex vivo when given either orally, intraperitoneally or intravenously but with a slower onset of action than L-NA. These results demonstrate that measurement of NOS activity ex vivo can accurately reflect the degree of inhibition of NOS in vivo with inhibitors that dissociate slowly from the enzyme such as L-NA, but only when the initial rate of NOS activity is measured.

Regulation of neuronal nitric oxide synthase in rat adrenal medulla

Neuronal nitric oxide synthase (nNOS) has been suggested to be involved in cardiovascular homeostasis. We studied the regulation of nNOS expression, determining nNOS mRNA expression levels in various tissues in spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY). We also investigated the effects of antihypertensive treatment with the angiotensin II antagonist hydralazine or reserpine on nNOS mRNA expression. The expression levels of nNOS mRNA and nNOS protein were determined by Northern and Western blot analysis, respectively. NADPH-diaphorase histochemistry was used to identify cells in the adrenal medulla that expressed nNOS. No significant differences in expression levels in SHR and WKY were observed in the cerebellum and brain stem. nNOS mRNA expression levels in the decapsular portion of the adrenal gland were developmentally modulated and in a 24-week-old WKY were 2.5 times higher than in an age-matched SHR. This reduced expression of nNOS mRNA in the decapsular portion of the adrenal gland of SHR seemed to be a result of hypertension in the SHR, because administration of either an angiotensin II antagonist (TCV-116) or hydralazine upregulated nNOS mRNA expression in both SHR and WKY. Marked augmentation of nNOS mRNA expression in the decapsular portion of the adrenal gland by reserpine treatment suggested an intimate relation between nNOS in the decapsular portion of the adrenal gland and the sympathoadrenal system. Reserpine treatment also increased the expression of nNOS protein; however, reserpine treatment did not affect the distribution pattern of nNOS-positive cells (NADPH-diaphorase-positive cells) in the adrenal medulla.(ABSTRACT TRUNCATED AT 250 WORDS)

Bone cell function, regulation, and communication: a role for nitric oxide

A large array of factors serve as vital communication links between cells and the characterization, regulation, and mechanisms of action of such factors are topics of intense research efforts. Most intercellular messenger molecules which have been described over the years are represented by proteins, small peptides, amino acids or their derivatives, ions, lipid metabolites, or steroids. However, a small uncharged free radical, nitric oxide, has recently garnered much attention as a potent multifunctional signal molecule with widespread actions within and between diverse tissues. Biochemical, molecular, and regulatory studies of the family of enzymes responsible for nitric oxide synthesis, nitric oxide synthases, have established that there are at least three distinct isoforms of this enzyme which are differentially expressed and regulated in various cells or tissues. Modulation of these isoenzyme levels or activities by diverse signals is mediated via transcriptional, translational, and/or post-translational mechanisms, and consequently, alterations in such control may influence normal or pathological processes. Nitric oxide appears to exert pronounced effects on skeletal physiology and its production by various bone cells, elicited target cell responses, modulation by other signalling molecules (e.g., cytokines, hormones, fatty acid derivatives), and chemical interactions with other free radicals (e.g., superoxide anions, hydroxyl radicals) may form one important facet of the many complicated communication pathways controlling bone cell physiology and remodeling. Further cell and molecular studies are needed to address the precise roles that nitric oxide plays in bone development and in the formation and degradation of bone during ordinary bone metabolism. In addition, alterations in the regulation and action of the bone nitric oxide system as a function of certain bone disorders may be manifested by perturbations in bone integrity or mineral homeostasis. In this article, we review the current evidence implicating nitric oxide as an important messenger molecule in bone intercellular communication, speculate on potential roles for this radical in bone biology, and discuss possible future directions for advanced research into the function of nitric oxide in skeletal physiology.

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

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

Low levels of reactive oxygen species as modulators of cell function

In this paper, we present various arguments supporting the hypothesis that reactive oxygen species (ROS) could be responsible for the modulation of various cellular functions, besides their well known toxic effects. We first review the recent evidence indicating that ROS are able to modulate genome expression through specific and precise mechanisms during cell activation. The role of the nitrogen reactive radicals such as nitric oxide is separately analyzed because of its specific role in the nervous and vascular systems. The action of the other ROS on gene activation will then be reviewed by first looking at their possible involvement in the activation of transcription factors like NF-kappa B. Arguments will then be developed in favor of the implication of the ROS in the cellular effects of PMA, TNF-alpha and other cytokines on the modulation of the genetic expression. Possible mechanisms will be presented for linking the production of the ROS with cell activation. In a general way we postulate that ROS can play a role of secondary messengers in several cell responses to external stimuli. In the second part of the paper, we will examine the long term influence of ROS and their possible roles in cellular aging. Different links exist between ROS and aging and the relationship between them is probably indirect. We propose to consider the effect of ROS as one of the multiple challenges that cells have to face, the cell being considered as a global system which must optimize its energy expenditure for carrying out its basic functions such as turnover, differentiated phenotype functions, multiplication, defense and repair processes. This thermodynamic point of view will help to understand the effect of low ROS stresses, among others, on accelerated aging.

Inhibition of nitric oxide synthase--potential for a novel class of therapeutic agent?

The free-radical gas nitric oxide (NO) plays an important role in a wide and diverse range of physiological processes. As progress is made in understanding the biological function of NO, there is growing interest in the possibility that inhibitors of NO synthase (NOS) may be of clinical use in the therapy of certain disease states. The search for novel and clinically relevant inhibitors of this enzyme represents a truly multidisciplinary approach to drug screening and will no doubt benefit from the application of recombinant DNA (rDNA) technology.

Characteristics of the nitric oxide synthase-catalyzed conversion of arginine to N-hydroxyarginine, the first oxygenation step in the enzymic synthesis of nitric oxide

The nitric oxide synthase-catalyzed conversion of L-arginine to L-citrulline and nitric oxide is known to be the sum of two partial reactions: oxygenation of arginine to N-hydroxyarginine, followed by oxygenation of N-hydroxyarginine to citrulline and nitric oxide. Whereas the conversion of N-hydroxyarginine to citrulline and nitric oxide has been the subject of a number of studies, the oxygenation of arginine to N-hydroxyarginine has received little attention. Here we show that substrate amounts of rat cerebellar nitric oxide synthase, in the absence of added NADPH, catalyze the conversion of arginine to N-hydroxyarginine as the dominant product. The product appears not to be tightly bound to the enzyme. A maximum of 0.16 mol of N-hydroxyarginine/mol of nitric oxide synthase subunit was formed. The reaction requires oxygen and the addition of Ca2+/calmodulin and is stimulated 3-fold by tetrahydrobiopterin. Upon addition of NADPH, citrulline is formed exclusively. Conversion of N-hydroxyarginine to citrulline, like the first partial reaction, requires Ca2+/calmodulin and is stimulated by tetrahydrobiopterin but differs from the first partial reaction in being completely dependent upon addition of NADPH. These results indicate that brain nitric oxide synthase contains an endogenous reductant that can support oxygenation of arginine but not of N-hydroxyarginine. The reductant is not NADPH, since the amount of nitric oxide synthase-bound NADPH is appreciably less than the amount required for N-hydroxyarginine synthesis. Possible candidates for this role are discussed in relation to proposed mechanisms of action of nitric oxide synthase.

Effect of lipopolysaccharide on nitric oxide synthase activity in rat proximal tubules

Renal proximal tubules isolated from the rat possess nitric oxide synthase (NOS) activity that is calcium/calmodulin dependent and stereoselectively inhibited by NG-monomethyl-arginine (NMMA). In the absence of added Ca2+ and calmodulin, activity was reduced 84 +/- 13% compared with the activity in the presence of 2 mM Ca2+ and 25 micrograms/mL calmodulin. Inhibition by EGTA (10 mM) was 95 +/- 5% and by calmidazolium (R24571, 250 microM) was 99 +/- 1%. Inhibition by L-NMMA (100 microM) was 78 +/- 13% and by D-NMMA (100 microM) was 7 +/- 7%. The majority of NOS activity was found in the soluble fraction. NOS activity in isolated proximal tubules was also examined 6 hr after a single i.v. injection of lipopolysaccharide. Activity was increased significantly (P < 0.05) in the soluble fraction by 2-fold [from 0.320 +/- 0.052 to 0.648 +/- 0.046 (nmol/mg protein/30 min)] and in the particulate fraction by 3-fold [from 0.081 +/- 0.030 to 0.256 +/- 0.034 (nmol/mg protein/30 min)]. All activities were inhibited by EGTA. These data demonstrate that proximal tubules express a calcium/calmodulin-dependent NOS activity that is increased in vivo by lipopolysaccharide.

Protein thiol modification and apoptotic cell death as cGMP-independent nitric oxide (NO) signaling pathways

Nitric oxide signaling is achieved through both cGMP-dependent and cGMP-independent mechanisms. The latter are exemplified by protein thiol modification followed by subsequent NAD(+)-dependent automodification of the glycolytic enzyme GAPDH, or by mechanisms inducing accumulation of the tumor suppressor gene p53 and causing apoptotic cell death. Both cGMP-independent actions are initiated using NO-releasing compounds and an active LPS/cytokine-inducible NO synthase. NO-synthase inhibitors block the release of NO and hinder downstream signaling mechanisms; they are therefore valuable pharmacological tools linking a defined cellular response to various NO actions. Signal transducing mechanisms elicited by NO can be studied using GAPDH as a representative example of NO-induced protein modification and are grouped as follows: --S-Nitrosylation reactions initiated by NO+ --NAD(+)-dependent, post-translational covalent automodification of GAPDH --Oxidative modification (thiol oxidation) and inhibition of GAPDH by NO-related agents, probably ONOO- GAPDH and several other protein targets may serve as molecular sensors of elevated NO concentrations and may transmit this message through posttranslational modification and oxidation-induced conformational changes as cGMP-independent NO signaling pathways. Toxicity of NO seems to be linked to both apoptosis and necrosis, depending on the chemistry of NO it undergoes in a given biological milieu. Toxicity manifests as a relative excess of NOx, metal-NO interactions, and ONOO- formation in relation to cellular defense systems. Although accumulation of the tumor-suppressor gene product p53 in response to NO opens a regulatory mechanism known to be involved in apoptotic cell death, cGMP-independent signaling pathways remain to be elucidated. As NO-dependent modification of GAPDH would imply down-regulation of glycolysis and concomitant energy production followed by cell death, our data so far do not support this assumption. In recent years, NO has proved to be a beneficial messenger with a potentially toxic activity. It will be challenging to investigate NO biochemistry in closer detail and to elucidate how NO targets biological systems, especially in relation to its pathophysiological role.

Plasma nitrate clearance in mice: modeling of the systemic production of nitrate following the induction of nitric oxide synthesis

Nitric oxide (NO) is produced in mammals by the enzyme NO synthase (NOS) in response to a number of agents, including the experimental antitumour agent flavone acetic acid (FAA) and the cytokine tumour necrosis factor-alpha (TNF). NO is converted rapidly in the presence of oxygen, water and haemoglobin to oxidation products, largely nitrate. To quantitate the production of nitric oxide it is necessary to know the clearance of nitrate. The concentration of nitrite and nitrate ion in the plasma of C3H and BDF1 (C57BL6 x DBA2) mice was assessed before and after injection of sodium nitrate and sodium nitrite. Nitrite was covered rapidly to nitrate and the kinetics of elimination of nitrate were determined. There was no significant difference between results obtained with different mouse strains, between levels of nitrite and nitrate, or between i.p. and i.v. administration, and the observations were therefore combined. The volume of distribution of nitrate was 0.71 +/- 0.04 l/kg and the clearance was 0.32 +/- 0.02 l/h-1/kg-1 (plasma half-life, 1.54 h). Using previously published data, we developed a pharmacokinetic-pharmacodynamic model that relates the production of TNF in response to administration of FAA, the enhancement of NOS activity in response to TNF, and the elevation of plasma nitrate in response to NO production. This information permits the prediction from observed plasma nitrate values of the amount of NOS induced in vivo.

Tetrahydrobiopterin deficiency and brain nitric oxide synthase in the hph1 mouse

Tetrahydrobiopterin (BH4) is the cofactor for the aromatic amino acid monoxygenase group of enzymes and for all known isoforms of nitric oxide synthase (NOS). Inborn errors of BH4 metabolism lead to hyperphenylalaninaemia and impaired catecholamine and serotonin turnover. The effects of BH4 deficiency on brain nitric oxide (NO) metabolism are not known. In this study we have used the hph-1 mouse, which displays GTP cyclohydrolase deficiency, to study the effects of BH4 deficiency on brain NOS. In the presence of exogenous BH4, NOS specific activity was virtually identical in the control and hph-1 preparations. However, omission of BH4 from the reaction buffer led to a significant 20% loss of activity in the hph-1 preparations only. The Km for arginine was virtually identical for the control and hph-1 NOS when BH4 was present in the reaction buffer. In the absence of cofactor, the Km for arginine was 3-fold greater for control and 5-fold greater for hph-1 preparations. It is concluded that (a) BH4 does not regulate the intracellular concentration of brain NOS; (b) less binding of BH4 to NOS occurs in BH4 deficiency states; (c) BH4 has a potent effect on the affinity of NOS for arginine; and (d) the availability of arginine for NOS activity may become severely limiting in BH4 deficiency states. Since, in the presence of suboptimal concentrations of BH4 or arginine, NOS may additionally form oxygen free-radicals, it is postulated that in severe BH4 deficiency states NO formation is impaired and the central nervous system is subjected to increased oxidative stress.

NO-synthase: what can research on invertebrates add to what is already known?

The present study attempts to review presently known data regarding the distribution of nitric oxide (NO) synthase and the function of NO in invertebrate species. NO is synthesized from L-arginine by the enzyme NO-synthase, and activates guanylate cyclase which in turn leads to an increase in levels of cGMP in target cells. Major contributions to the knowledge of NO as a messenger molecule in invertebrates have been made by NADPH-diaphorase histochemistry and biochemical assays. These techniques suggest the presence of a L-arginine/NO pathway in a variety of tissues, thus implicating multiple roles for NO in invertebrates.

Mechanisms controlling nitric oxide synthesis in osteoblasts

Nitric oxide (NO) modulates the activity of a number of cell types, but little is known about its possible role in bone metabolism. In the present study we demonstrate that freshly isolated murine osteoblasts and an osteoblastic cell line express NO-synthase mRNA and release NO when stimulated with IL-1 or LPS, thus confirming the results of some recent reports using human and rat osteoblast-like cells. Synergistic effects were found between IL-1 and LPS or TNF. Enzyme induction was blocked by dexamethasone and IL-4. 1,25-dihydroxyvitamin D3 did not modify basal NO synthesis, but it markedly increased the cytokine-induced NO release. M-CSF, GM-CSF, IL-3, LIF, PTH, estradiol and calcitonin did not show significant effects on NO synthesis. NOS induction was blocked by various tyrosine-kinase inhibitors, geldanamycin and herbimycin A being the most potent. These results suggest that endogenous NO might participate in the regulation of bone remodeling at the local level, and may mediate some effects of vitamin D on bone. NO has recently been reported to inhibit osteoclastic bone resorption. The release of NO induced by bone-stimulating factors such as IL-1 may represent a protective mechanism helping to avoid excess resorption and preserve bone integrity in inflammatory conditions.

Nitric oxide: what role does it play in inflammation and tissue destruction?

Large amount of nitric oxide (NO) are produced at sites of inflammation through the action of inducible nitric oxide synthase (iNOS) present in both infiltrating leucocytes and activated, resident tissue cells. However, the role of NO in inflammation remains unclear. NO is a vasodilator, which inhibits the adhesion of neutrophils to the vascular endothelium; it reduces the production of IL-6 by Kupffer cells and chondrocytes, and the production of gamma-IFN and TNF-alpha by splenocytes. The literature provides contradictory information on the effect of NO on vascular leakiness, chemotaxis, prostaglandin production and tissue damage. Increasingly, data suggest that NO is immunosuppressive. Inhibitors of NOS have potent prophylactic activity in several but not all, animal models of inflammatory disease. However, in rat adjuvant arthritis, therapeutic activity is weak. Whether inhibitors of iNOS will be therapeutically useful in human inflammatory diseases cannot be predicted on the basis of present information.

Activated T cells enhance nitric oxide production by murine splenic macrophages through gp39 and LFA-1

Macrophages can be stimulated to produce a relatively large amount of nitric oxide, which is an important component in macrophage-mediated defense mechanisms and regulation of T cell activities. It has been known that T helper (Th) cell activation requires intimate physical interaction between T helper cells and macrophages and that cytokines from activated Th cells regulate macrophage activities including nitric oxide production. The current study indicates that surface molecules on activated Th cells also can synergize with cytokines to substantially enhance nitric oxide production by macrophages through cell-cell contact. The CD40 ligand (gp39) and LFA-1 appear to be two major contributors for T cell dependent nitric oxide production.

Nitric oxide: an ancestral immunocyte effector molecule

The presence and the role of nitric oxide synthase (NOS) were investigated in the molluscan hemocytes by immunocytochemical, biochemical and functional approaches. Using an anti-NOS polyclonal antibody, immunoreactivity was observed in the hemocytes, and this reactivity increased after stimulation of the animals with Escherichia coli, indicating that this enzyme is inducible. The NOS inducibility was also histochemically demonstrated by detection of NADPH-diaphorase activity. Biochemical studies show that the enzyme is 70% cytoplasmatic and 30% membrane bound and that the inducible form is mainly cytoplasmatic. The nitrite + nitrate and citrulline formation, the inhibition by N omega-nitro-L-arginine, the Km value for arginine, the calcium and co-enzyme dependence show that the molluscan NOS shares the same properties as the NOS isoenzymes so far studied. However, it cannot be identified with any of these enzymes. It appears to be in some way similar to an inducible form of human hepatocyte NOS. Also cytokines are able to induce NOS. In vitro studies have shown that hemocytes produce nitric oxide (NO), a bactericide substance, and that there is a relationship between the NO system and phagocytosis. The presence of NO in the invertebrate hemocyte demonstrates that critical molecules have been conserved over the course of evolution.

Middle T antigen-transformed endothelial cells exhibit an increased activity of nitric oxide synthase

Endothelioma cell lines transformed by polyoma virus middle T antigen (mTa) cause cavernous hemangiomas in syngeneic mice by recruitment of host cells. The production of nitric oxide (NO), as measured by nitrite and citrulline production, was significantly higher in mTa-transformed endothelial cells in comparison with nontransformed control cells. The maximal activity of NO synthase (NOS) was about 200-fold higher in cell lysates from the tEnd.1 endothelioma cell line than in lysates from nontransformed controls, whereas the affinity for arginine did not differ. The biochemical characterization of NOS and the study of mRNA transcripts indicate that tEnd.1 cells express both the inducible and the constitutive isoforms. NOS hyperactivity is not a simple consequence of cell transformation but needs a tissue-specific mTa expression. Since tEnd.1-conditioned medium induces NOS activity in normal endothelial cells, most likely NOS hyperactivity in endothelioma cells is attributable to the release of a soluble factor. This NOS-activating factor, which seems to be an anionic protein, could stimulate tEnd.1 cells to express NOS by an autocrine way. By the same mechanism, tEnd.1 cells could induce NOS in the neighboring endothelial cells, and NO release could play a role in the hemangioma development. Such hypothesis is confirmed by our in vivo experiments, showing that the administration of the NOS inhibitor L-canavanine to endothelioma-bearing mice significantly reduced both the volume and the relapse time of the tumor.