Immunovascular communication: activation and deactivation of murine endothelial cell nitric oxide synthase by cytokines

NOS1-, NOS3-, PIK3CA-, and MAPK-pathways in skin following radiation therapy

Background

Essential molecular pathways such as the MAPK pathway, NO system, or the influence of PIK3CA as an oncogene are known to regulate fundamental signalling networks. However, few knowledge about their role in the occurrence of wound healing disorders (WHD) following radiation therapy (RT) exists. This study aims to evaluate the expression profiles of specific molecular pathway marker genes.

Methods

Expression profiles of the genes encoding MAPK, NOS1, NOS3, and PIK3CA were analyzed, by RT-PCR, in specimens from patients with and without a history of RT to the head and neck. Clinical data on the occurrence of cervical WHDs were analyzed.

Results

Expression analysis of patients with postoperative WHDs revealed a significant increase in MAPK expression compared to the control group without occurrence of postoperative WHDs. PIK3CA showed a significantly increased expression in patients with a history of RT. Expression analysis of all other investigated genes did not reveal significant differences.

Conclusions

This current study is able to show the influence of RT on different molecular pathways. This underlines the crucial role of specific molecular networks, responsible for the occurrence of long-term radiation toxicity such as WHDs. Additional studies should be carried out to identify possible starting points for therapeutic interventions.

The eNOS cofactor tetrahydrobiopterin improves endothelial dysfunction in livers of rats with CCl4 cirrhosis

In cirrhosis, intrahepatic endothelial dysfunction is one of the mechanisms involved in the increased resistance to portal blood flow and therefore in the development of portal hypertension. Endothelial nitric oxide synthase (eNOS) uncoupling due to deficiency of tetrahydrobiopterin (BH4) results in decreased production of NO and plays a major role in endothelial dysfunction in other conditions. We examined whether eNOS uncoupling is involved in the pathogenesis of endothelial dysfunction of livers with cirrhosis. Basal levels of tetrahydrobiopterin and guanosine triphosphate (GTP)-cyclohydrolase (BH4 rate-limiting enzyme) expression and activity were determined in liver homogenates of control and rats with CCl4 cirrhosis. Thereafter, rats were treated with tetrahydrobiopterin, and eNOS activity, NO bioavailability, assessed with a functional assay, and the vasodilator response to acetylcholine (endothelial function) were evaluated. Livers with cirrhosis showed reduced BH4 levels and decreased GTP-cyclohydrolase activity and expression, which were associated with impaired vasorelaxation to acetylcholine. Tetrahydrobiopterin supplementation increased BH4 hepatic levels and eNOS activity and significantly improved the vasodilator response to acetylcholine in rats with cirrhosis. In conclusion, the impaired response to acetylcholine of livers with cirrhosis is modulated by a reduced availability of the eNOS cofactor, tetrahydrobiopterin. Tetrahydrobiopterin supplementation improved the endothelial dysfunction of cirrhotic livers.

Suppression effect of Cinnamomum cassia bark-derived component on nitric oxide synthase

The inhibitory effects of Cinnamomum cassia bark-derived material on nitric oxide (NO) production in RAW 264.7 cells was determined through the evaluation of NO production and expression of inducible nitric oxide and compared to the effects of three commercially available compounds, cinnamyl alcohol, cinnamic acid, and eugenol. The biologically active constituents of C. cassia extract were characterized as trans-cinnamaldehyde by spectral analysis. The inhibitory effects varied with both chemical and concentration used. Potent inhibitory effects of cinnamaldehyde against NO production were 81.5 and 71.7% at 1.0 and 0.5 microg/microL, respectively, and a 41.2% inhibitory effect was revealed at 0.1 microg/microL. However, little or no activity was observed for cinnamic acid and eugenol. Suppression effects of cinnamaldehyde on inducible nitric oxide synthase expression were revealed by Western blot analysis. As a naturally occurring therapeutic agent, trans-cinnamaldehyde could be useful for developing new types of NO inhibitors.

Nitric oxide (NO) metabolism in the cerebrospinal fluid of patients with severe head injury. Inflammation as a possible cause of elevated no metabolites

BACKGROUND

This article investigates nitric oxide (NO) metabolism following severe head injury (SHI). We wished to clarify the alterations of NO metabolism end products that is associated with SHI, and to delineate the role of inflammation in this process.

METHODS

In a prospective study, we simultaneously measured the concentrations of NO metabolites and interleukin-8 (IL-8) in the ventricular cerebrospinal fluid (CSF) of 11 patients who had suffered SHI. The CSF concentrations of nitrite (NO(-)(2)) and nitrate (NO(-)(3)) combined, and of IL-8 were measured during the following four time periods post-trauma: 6 to 10, 20 to 28, 40 to 56, and 64 to 74 hours. Levels were measured using the corresponding kits.

RESULTS

Compared to the ventricular CSF control values, all of our SHI patients had significantly elevated CSF levels of NO(-)(2) plus NO(-)(3) (NO(-)(2) + NO(-)(3)) and IL-8 during all periods tested. CSF NO(-)(2) + NO(-)(3) and IL-8 concentrations reached their maximums simultaneously at 20 to 28 hours following trauma (Spearman's rank correlation = 0.609, p < 0.05), and NO(-)(2) + NO(-)(3) levels were significantly higher than those measured at 6 to 10, 40 to 56, and 64 to 74 hours. [Nitrite-nitrate concentrations: 6-10 hours: 19.22 +/- 6.75, 20-28 hours: 25 +/- 6.2 micromol/l, 40-56 hours: 19.82 +/- 4.47, and 64-74 hours: 19.72 +/- 4.61 micromol/l, (p < 0.05). IL-8 concentrations: 6-10 hours: 3,232 +/- 2,976.2, 20-28 hours: 3,458.45 +/- 3,048 pg/mL, 40-56 hours: 2,616.41 +/- 2,539.21, 64-74 hours: 1,388.88 +/- 1,216.7 pg/mL, (p < 0.001).]. This simultaneous surge in NO(-)(2) + NO(-)(3) and IL-8 in the initial 24 hours post-traumatic indicated that inflammation secondary to SHI increased the rate of NO metabolism, resulting in higher levels of metabolites in the CSF.

CONCLUSION

In patients with SHI, CSF concentrations of the dominant metabolites of NO are elevated in the first 3 days after trauma. A similar concurrent spike in the CSF level of IL-8, a marker of acute inflammatory response, can also be demonstrated. These data indicate that the predominant cause of the higher CSF NO(-)(2) + NO(-)(3) concentrations observed in SHI is most likely inflammation.

Endobronchial transfection of naked viral interleukin-10 gene in rat lung allotransplantation

BACKGROUND

Recent studies suggest that viral interleukin-10 (vIL-10) suppresses alloimmune response in transplantation. Tissue mRNA expression of inducible nitric oxide synthase (iNOS) and exhaled nitric oxide (NO) levels have been observed to increase in lung allograft rejection. The aims of this study were to examine the feasibility of vIL-10 gene transfer into rat lung allografts and to investigate its effect on subsequent allograft rejection.

METHODS

Male Lewis rats (RT1l) underwent left lung transplantation with allografts from Brown Norway rats (RT1n). The donor rats were endobronchially transfected 2 minutes before harvest with 400 microg (group I, n = 5), 600 microg (group II, n = 5), or 800 microg (group III, n = 5) of naked pCMVievIL-10. Group IV (n = 5) animals, serving as control, received 400 microg of naked pCF1-CAT. All recipients were sacrificed on postoperative day 5. Transgene expression of vIL-10 was assessed by both reverse transcriptase-polymerase chain reaction and immunohistochemistry. Allograft gas exchange, exhaled NO level, histologic rejection score, and mRNA expression of graft cyokines were also assessed.

RESULTS

Transgene expression of lung graft vIL-10 was detected by both reverse transcriptase-polymerase chain reaction and immunohistochemistry. The iNOS mRNA expression in groups I, II, and III was significantly lower than that of group IV (p < 0.05, analysis of variance). Exhaled NO levels in groups I, II, and III were significantly lower than in group IV (p < 0.01, analysis of variance). There was no significant difference between groups with respect to gas exchange, peak airway pressure, or histologic rejection score.

CONCLUSIONS

It appears that endobronchial transfection of naked vIL-10 plasmid in a rat lung allotransplant model is feasible and suppresses lung iNOS mRNA expression and exhaled NO levels. An association between iNOS upregulation and high exhaled NO levels in lung allograft resection was also noted.

Augmentation of nitric oxide production by gamma interferon in a mouse vascular endothelial cell line and its modulation by tumor necrosis factor alpha and lipopolysaccharide

The effect of gamma interferon (IFN-gamma), tumor necrosis factor alpha (TNF-alpha), and lipopolysaccharide (LPS) on nitric oxide (NO) production in the mouse vascular aortic endothelial cell line END-D was examined. LPS, TNF-alpha, and a low concentration of IFN-gamma inhibited NO production in END-D cells, while a high concentration of IFN-gamma definitely enhanced it. The NO production induced by a high concentration of IFN-gamma was further augmented by using IFN-gamma in combination with LPS or TNF-alpha. In sequential incubations of LPS and IFN-gamma, the enhancement of NO production required prior treatment with IFN-gamma. Stimulation of END-D cells with a high concentration of IFN-gamma led to the expression of inducible NO synthase (iNOS). The augmentation of NO production by IFN-gamma alone or in combination with LPS or TNF-alpha was completely blocked by several inhibitors of iNOS. It was strongly suggested that a high concentration of IFN-gamma itself enhanced NO production in END-D cells through inducing the expression of iNOS. LPS and TNF-alpha exclusively modulated the activity of iNOS once its expression was triggered by IFN-gamma. On the other hand, a low concentration of IFN-gamma, LPS, and TNF-alpha reduced NO production through down-regulating constitutive NOS (cNOS). The differential regulation of cNOS- and iNOS-mediated NO production by IFN-gamma, TNF-alpha, and LPS is discussed.

Spinal plasticity of acute opioid tolerance

Spinal acute opioid tolerance remains mechanistically undercharacterized. Expanded clinical use of direct spinal administration of opioids and other analgesics indicates that studies to further understand spinal mechanisms of analgesic tolerance are warranted. Rodent models of spinal administration facilitate this objective. Specifically, acute spinal opioid tolerance in mice presents a plasticity-dependent, rapid, and efficient opportunity for evaluation of novel clinical agents. Similarities between the pharmacology of acute and chronic spinal opioid tolerance, neuropathic pain, and learning and memory suggest that this model may serve as a high through-put predictor of bioactivity of novel plasticity-modifying compounds.

TGF-beta and the endothelium during immune injury

During immune injury, activation of endothelial cells by inflammatory cytokines stimulates leukocyte adhesion to the endothelium, turns the endothelium from an anticoagulant surface to one that is frankly procoagulant, and results in the release of vasoactive mediators and growth factors. Cytokine activation of endothelial cells also results in increased endothelial cell TGF-beta 1 synthesis and enhanced activation of latent TGF-beta, the latter involving a shift of plasmin production from the apical to subendothelial surface. In cytokine-stimulated endothelial cells, TGF-beta hinders leukocyte adhesion and transmigration via inhibition of IL-8 and E-selectin expression. TGF-beta also profoundly diminishes cytokine-stimulated inducible nitric oxide synthase production and instead augments endothelial nitric oxide synthase expression. Thus, some of the TGF-beta actions on endothelium during immune activation can viewed as immunosuppressive. TGF-beta also influences mechanisms of vascular remodeling during the healing phase of immune injury. It stimulates PDGF-B synthesis by endothelial cells, causes bFGF release from subendothelial matrix, and promotes VEGF synthesis by non-endothelial cells. Together these mediators control angiogenesis, a critical component of the vascular repair phenomenon. Further, endothelial cell derived PDGF-B and bFGF influence the proliferation and migration of neighboring cells. Thus, endothelial cells and TGF-beta actions on the endothelium play important roles both during the initial phase of immune injury and during the later remodeling phase.

Nitric oxide synthase expression and nitric oxide production are reduced in hypertrophic scar tissue and fibroblasts

We have recently demonstrated that human dermal fibroblasts express both constitutive and inducible nitric oxide synthases (NOS) and produce nitric oxide (NO). In this study, NOS expression and NO production were compared in human fibroblasts derived from hypertrophic scar (HSc) and site-matched normal dermis. NOS expression and NO production in HSc fibroblasts are significantly reduced when compared with normal fibroblasts. Without stimulation, the HSc fibroblasts produced 7.37 +/- 1.17 nmol and the normal fibroblasts produced 11.15 +/- 0.79 nmol/10(6) cells/96 h (paired t test, p < 0.01, n = 6) NO as determined by the Griess reaction. After stimulation with 200 units interferon-gamma per ml and 40 microg lipopolysaccharide per ml, both HSc and normal fibroblasts produced significantly higher NO, indicating that the HSc fibroblasts retain the capacity to express inducible NOS (iNOS). The Ca2+-dependent NOS (cNOS) activity in the cytosol of HSc fibroblasts (1.43 pmol/min/g of protein) was significantly lower than that in normal fibroblasts (2.60 pmol/min/g of protein), as determined by citrulline assay (p < 0.01, n = 4). The mRNAs for endothelial cNOS and iNOS in both HSc and normal fibroblasts were detectable by RT-PCR. Flow cytometry confirmed that the untreated HSc fibroblasts expressed less endothelial cNOS protein than untreated normal fibroblasts. Because NO markedly inhibits cell proliferation, our results suggest that after thermal injury, fibroblasts in the scar tissue may undergo phenotypic alteration leading to reduced endothelial cNOS expression. The low levels of endogenous NO in HSc fibroblasts might be responsible for the cellularity of post-burn HSc, a characteristic feature of this fibrotic condition.

Effects of activating and deactivating cytokines on the functionally linked tetrahydrobiopterin. No pathways in vascular smooth muscle cells

The functional relationship of nitric oxide (NO) production and synthesis of tetrahydrobiopterin (BH4), the requisite cofactor for NO synthase, was investigated in rat aortic smooth muscles cells (SMC). Inflammatory cytokines induced BH4 and NO synthesis in different ratios, IL-1 beta induced mainly NO synthesis with concomitant but limiting amounts of BH4 for maximal NO production. TNF alpha did not induce NO synthesis but induced BH4 synthesis. IFN gamma was ineffective on both the induction of NO and BH4 synthesis. TGF beta downregulated NO production but did not affect BH4 biosynthesis. IL-4 and IL-10 had no effect on both BH4 and NO synthesis. Activating cytokines strongly synergized in induction of NO production, whereas endogenous BH4 production became insufficient for maximal NO synthesis. Exogenous cofactor in the form of sepiapterin or authentic BH4, but not the natural isomer 7-BH4, enhanced NO production twofold. Inhibition of BH4 synthesis with dicumarol abolished NO production that could be restored in the presence of BH4.

Proinflammatory agents, IL-8 and IL-10, upregulate inducible nitric oxide synthase expression and nitric oxide production in avian osteoclast-like cells

Nitric oxide synthase (NOS) isoenzymes generate nitric oxide (NO), a sensitive multifunctional intercellular signal molecule. High NO levels are produced by an inducible NOS (iNOS) in activated macrophages in response to proinflammatory agents, many of which also regulate local bone metabolism. NO is a potent inhibitor of osteoclast bone resorption, whereas inhibitors of NOS promote bone resorption both in vitro and in vivo. The possibility that osteoclasts, like macrophages, express a regulated iNOS and produce NO as a potential autocrine signal following inflammatory stimulation was investigated in well-characterized avian marrow-derived osteoclast-like cells. NO production (reflected by medium nitrite levels) was markedly elevated in these cells by the proinflammatory agents lipopolysaccharide (LPS) and the synergistic action of IL-1 alpha, TNF alpha, and IFN gama. inhibitors of NOS activity (aminoguanidine, L-NAME) or iNOS induction (dexamethasone, TGF beta) reduced LPS-stimulated nitrite production. LPS also increased the NOS-associated diaphorase activity of these cells and their reactivity with anti-iNOS antibodies. RT-PCR cloning, using avian osteoclast-like cell RNA and human iNOS primers, yielded a novel 900 bp cDNA with high sequence homology (76%) to human, rat, and mouse iNOS genes. In probing osteoclast-like cell RNA with the PCR-derived iNOS cDNA, a 4.8 kb mRNA species was detected whose levels were greatly increased by LPS. Induction of iNOS mRNA by LPS, or by proinflammatory cytokines, occurred prior to the rise of medium nitrite in time course studies and was diminished by dexamethasone. Moreover, osteoclast-like cells demonstrated an upregulation of NO production and iNOS mRNA by IL-8 and IL-10, regulatory mechanism's not previously described. It is concluded that osteoclast-like cells express a novel iNOS that is upregulated by inflammatory mediators, leading to NO production. Therefore, NO may serve as both a paracrine and autocrine signal for modulating osteoclast bone resorption.

Human dermal fibroblasts produce nitric oxide and express both constitutive and inducible nitric oxide synthase isoforms

Nitric oxide (NO) is produced by a variety of human and animal cells and is involved in a broad array of physiological and pathophysiological processes. It can cause vasodilation, serve as a neurotransmitter, and have anti-neoplastic, anti-microbial, and anti-proliferative effects. In this study, we have demonstrated that fibroblasts derived from human skin spontaneously produce NO and that this production can be enhanced by stimulating the cells with interferon-gamma and lipopolysaccharide. The production of NO by human dermal fibroblasts can be blocked by NG-monomethyl-L-arginine (L-NMMA). The inhibitory effect of L-NMMA on NO production was restored by addition of L-arginine but not D-arginine. By measuring the rate of conversion of [14C]L-arginine to [14C]L-citrulline, we show that unstimulated cells expressed only Ca2+-dependent NO synthase (NOS) activity (1.36 +/- 0.57 pmol/mg/min; n = 4) whereas stimulated cells expressed both Ca2+-dependent (2.60 +/- 0.54 pmol/mg/min; n = 4) and -independent (1.59 +/- 0.14 pmol/mg/min; n = 4) NOS activities. With reverse transcription polymerase chain reaction (RT-PCR), the 422-bp RT-PCR product for human endothelial constitutive NOS and the 462-bp RT-PCR product for human hepatocyte inducible NOS were detected in proportion to the amount of mRNA-related RT-cDNA added to the reaction mixture. Further evidence by immunocytochemistry demonstrated that human dermal fibroblasts express both constitutive and inducible NOS proteins. These data collectively suggest that in addition to macrophages and other inflammatory cells, nitric oxide production by dermal fibroblasts could be important during the inflammatory stages of wound healing and possibly also in the later stages of proliferation and tissue remodeling after skin injury in humans.

Effect of interleukin-4 and interleukin-10 on leucocyte migration and nitric oxide production in the mouse

1. The effect of systemic treatment of mice with murine recombinant interleukin-4 (IL-4) or interleukin-10 (IL-10) on neutrophil infiltration into a specific tissue site and nitric oxide (NO) production from peritoneal macrophages was investigated. 2. Intravenously (i.v.) administered IL-4 (0.01-10 micrograms per mouse, approximately 0.3-300 micrograms kg-1, i.v.) and IL-10 (0.01-1 micrograms per mouse, approximately 0.3-30 micrograms kg-1, i.v.) dose-dependently inhibited neutrophil accumulation into a 6-day-old murine air-pouch induced by local application of interleukin-1 beta (IL-1 beta, 5 ng), with approximate ED50s of 0.35 and 0.90 micrograms, respectively. Neither IL-4 (1 micrograms, 30 micrograms kg-1, i.v.) nor IL-10 (1 micrograms, 30 micrograms kg-1, i.v.) prevented leucocyte accumulation in the mouse air-pouches when interleukin-8 (IL-8, 1 micrograms) was used as chemoattractant. Similarly, neither cytokine had any effect on the in vitro up-regulation of CD11b antigen on the surface of murine circulating neutrophils. 3. Treatment of mice with lipopolysaccharide (LPS, 0.3 mg kg-1, i.p.) caused an increase in the formation of NO (measured as nitrite accumulation) in the supernatant of peritoneal macrophages ex vivo. Pretreatment of mice with IL-4 (0.01-1 micrograms i.v., 20 min before LPS), but not with IL-10 (1 micrograms i.v., 20 min before LPS), caused a dose-dependent reduction in this LPS-stimulated formation of nitrite by peritoneal macrophages ex vivo. 4. Activation of murine macrophages with LPS (1 microgram ml-1 for 24 h) in vitro caused a significant increase in nitrite release in the supernatant of these cells. Pretreatment of either J774.2 or peritoneal macrophages with IL-4 (0.1-1 microg ml-1, 20 min before LPS), but not with IL-1O (1 microg ml', 20 min before LPS) caused a concentration-related attenuation of this LPS-stimulated nitrite formation.5 Thus, both IL-4 and IL-10 inhibit the migration of leucocytes (stimulated by IL-1beta>) in vivo; IL-4 (but not IL-10) inhibits the induction of NO synthase caused by LPS in murine macrophages in vitro and ex vivo.

Tumor cells suppress cytokine-induced nitric-oxide (NO) production in cerebral endothelial cells

Nitric oxide (NO) produced by endothelial cells (EC) has been shown to exert cytotoxic activity on tumor cells. In order to analyze events involved in brain metastasis, the modulation of NO production in rat-brain-derived EC was investigated. NO release was increased by tumor necrosis factor-alpha (TNF-alpha), interferon-gamma (IFN-gamma), interleukin-1 beta, lipopolysaccharide or forskolin in EC219 cells, a rat-brain-microvessel-derived EC line. Dexamethasone decreased NO release by cytokine-activated EC219 cells. Tumor cells (DHD/K12/PROb, a rat colon-carcinoma cell line) were highly adherent to EC219 cells, and adhesion was not modified by TNF-alpha plus IFN-gamma, or by dexamethasone. Addition of tumor cells or tumor-cell-conditioned medium significantly inhibited NO release induced by any of the stimuli examined, but only if added during the initial phase of endothelial-cell activation. Tumor-derived suppression of NO release was also observed in primary cultures of cerebral EC. NO synthase (NOS) activity in cytosol extracts of the cerebral EC line was Ca(2+)-independent and required both NADPH and tetrahydrobiopterin. NOS activity was increased by TNF-alpha and IFN-gamma, and significantly reduced by tumor-cell-conditioned medium. These results suggest that rat colon-carcinoma cells may have developed a protective mechanism involving the release of (a) soluble factor(s) which inhibit(s) NO production by cerebral EC.

Duration of expression of inducible nitric oxide synthase in glial cells

Lipopolysaccharide (LPS) or a combination of interleukin (IL)-1 beta and interferon (IFN)-gamma cause transcriptional induction of a calcium-independent nitric oxide synthase (NOS) in astrocytes and C6 glioma cells. LPS induction of NOS in C6 cells was evidenced by a small amount of nitrite accumulation as compared with cells exposed to IL-1 beta/IFN-gamma, but the maximal NOS activity achieved (as revealed by cGMP formation) was the same. The NOS activity induced by LPS in C6 cells was maximal at 4 to 8 hr and then rapidly decreased, while NOS activity induced by IL-1 beta/IFN-gamma slowly decreased after 4 hr. In addition, the effects of re-presenting IL-1 beta/IFN-gamma to both astrocytes and C6 cells after maximal induction of activity of the inducible form of NOS were studied. The re-addition of cytokines prolonged both NOS mRNA expression and also enzyme activity, suggesting effects at either the transcriptional (further induction) or translational level (mRNA stability). These results imply that the time course of NO production by induced astrocytes depends both upon the nature of the inducing stimulus and the frequency of the cells' exposure to it.

Evidence that nitric oxide is an endogenous antiangiogenic mediator

1. The involvement of nitric oxide (NO) in the regulation of angiogenesis was examined in the in vivo system of the chorioallantoic membrane (CAM) of the chick embryo and in the matrigel tube formation assay. 2. Sodium nitroprusside (SNP) (0.37-28 nmol/disc), which releases NO spontaneously, caused a dose-dependent inhibition of angiogenesis in the CAM in vivo and reversed completely the angiogenic effects of alpha-thrombin (6.7 nmol/disc) and the protein kinase C (PKC) activator 4-beta-phorbol-12-myristate-13-acetate (PMA) (0.97 nmol/disc). In addition, SNP (28 x 10(-6) M) stimulated the release of guanosine 3'-5'-cyclic monophosphate (cyclic GMP) from the CAM in vitro. 3. In the matrigel tube formation assay, an in vitro assay of angiogenesis, both SNP (1-3 x 10(-6) M) and the cell permeable cyclic GMP analogue, Br-cGMP (0.3-1.0 x 10(-3) M) reduced tube formation. 4. The inhibitors of NO synthase, NG-monomethyl-L-arginine (L-NMMA) (3.8-102 nmol/disc) and NG-nitro-L-arginine methylester (L-NAME) (1.3-34.2 nmol/disc) stimulated angiogenesis in the CAM in vivo, in a dose-dependent fashion. D-NMMA and D-NAME on the other hand had no effect on angiogenesis in this system. 5. L-Arginine (10.9 nmol/disc), although it had a modest antiangiogenic effect by itself, was capable of abolishing the angiogenic effects of L-NMMA (34.2 nmol/disc) and of L-NAME (3.8 nmol/disc). 6. Dexamethasone, an inhibitor of the induction of NO synthase, at 0.2-116.1 nmol/disc, stimulated angiogenesis in the CAM, whereas at 348.4-1161 nmol/disc it inhibited this process. Combination of 38.7 nmol/disc dexamethasone with L-NAME (9.3 nmol/disc) resulted in a potentiation of the angiogenic effect of the former. It appears therefore that both the constitutive and the inducible NO synthase may contribute to the NO-mediated inhibition of angiogenesis. 7. Superoxide dismutase (SOD), which prevents the destruction of NO, at 300 i.u./disc had a modest antiangiogenic effect in the CAM, by itself. In addition, SOD, prevented alpha-thrombin (6.7 nmol/disc) and PMA (0.97 nmol/disc) from stimulating angiogenesis in the CAM.8. These results suggest that NO may be an endogenous antiangiogenic molecule of pathophysiological importance.