Diuretic effect of NG-nitro-l-arginine methyl ester in the rat

Abstract— Intravenous infusion of the nitric oxide synthase inhibitor N^G-nitro-l -arginine methyl ester, l -NAME (10 μg kg^?1 min^?1), to anaesthetized rats produced a diuresis and natriuresis. By contrast, infusion of the same dose of N^G-nitro-d -arginine methyl ester had no effect on either urine output or sodium excretion. The effects of l -NAME were first evident 120 min after the start of infusion and by 170 min a fivefold increase in urine volume and sodium excretion was recorded. l -NAME also produced a transient fall in inulin clearance and a persistent decline in renal blood flow. These renal effects of l -NAME were associated with a gradual elevation of mean arterial blood pressure, although this only attained statistical significance, in comparison with saline-infused animals, 170 min after the start of infusion. The findings indicate the diuresis and natriuresis evoked by l -NAME in the rat is a result of a direct tubular action together with a pressure diuresis.     

Nitric Oxide Modulates the Acute Increase of Gastrointestinal Transit Induced by Endotoxin in Rats: a Possible Role for Tachykinins

Because of the evidence that endogenous nitric oxide (NO) plays an essential role in the physiological regulation of gastrointestinal motility we have investigated, by use of the NO synthase inhibitor, N^G-nitro-l -arginine methyl ester (l -NAME), the role of endogenous NO in the acute endotoxin-induced changes of gastrointestinal transit. Pre-treatment with E. coli endotoxin (100 μg kg^?, i.v.) induced a significant increase in the gastrointestinal transit of a charcoal suspension in anaesthetized rats. Previous administration of the NO synthase inhibitor, l -NAME (10 mg kg^?, i.v.) significantly prevented the effects of endotoxin. l -arginine (200 mg kg^?, i.v.) and the substance P antagonist [d -Pro², d -Trp^7,9]-substance P (SPA), significantly reversed the effects of l -NAME on gastrointestinal transit in rats treated with endotoxin. Pre-treatment with dexamethasone (5 mg kg^?, s.c., twice), an inhibitor of the expression of inducible NO synthase, did not affect the increase in the gastrointestinal transit through constitutive NO synthesis. The results suggest that constitutive nitric oxide is involved in the increase of gastrointestinal transit induced by endotoxin and that the reduction in transit induced by l -NAME in endotoxin-treated rats is mediated by endogenous tachykinins.     

EFFECTS OF NITRIC OXIDE SYNTHASE INHIBITION AND ENDOTHELIN ETA RECEPTOR BLOCKADE ON HAEMODYNAMICS IN HYPERTENSIVE RATS

1. The objectives of the present study were to study regional differences in haemodynamics between spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rats induced by the nitric oxide synthase (NOS) inhibitor N^G-monomethyl -l -arginine (l -NMMA) and the endothelin ET_A receptor antagonist BQ123 in vivo in tissues known to be important for blood pressure (BP) regulation (heart, kidney and skeletal muscle). Furthermore, the effect of acetylcholine (ACh) infusion (2 μg/kg per min) was examined after l -NMMA or BQ 123. The microsphere method was used for determinations of cardiac index (CI) and regional haemodynamics. 2. N^G-Monomethyl-l -arginine (20mg/kg) increased BP (26–48%; P < 0.01) and reduced CI in both rat strains. BQ 123 (1 mg/kg) reduced BP slightly (-4 to 11 %; P < 0.05). 3. N^G -Monomethyl-l -arginine significantly increased myocardial and skeletal muscle vascular resistance in SHR only; however, in the kidney, l -NMMA reduced blood flow and increased vascular resistance in both rat strains. 4. BQ 123 induced minor changes in regional haemodynamics that were not significantly different between the two strains. 5. Acetylcholine following BQ123 induced an increase in myocardial blood flow in WKY rats, but decreased blood flow in SHR. Acetylcholine following l -NMMA reduced myocardial blood flow in both strains. 6. Acetylcholine following BQ 123 induced renal vasodilation in WKY rats but, following l -NMMA, ACh did not induce renal vasodilation in either rat strain. In contrast, l -NMMA did not abolish the vasodilation of acetylcholine in skeletal muscle in WKY rats. 7. In conclusion, the contribution of nitric oxide to basal vessel tone was not impaired in the heart, skeletal muscle and kidney in SHR. Antagonism of ET_A receptors caused similar haemodynamic responses in both rat strains in these organs. Furthermore, NOS inhibition, but not ET_A blockade, blunted the expected ACh-induced vasodilation in the heart and kidney in WKY rats, but not in skeletal muscle in both strains.     

NG-Nitro-l-arginine methyl ester inhibits the effect of an H3-histaminergic receptor agonist on NANC contraction in guinea-pig perfused bronchioles

Abstract— A role for nitric oxide in the H₃-histaminergic agonist-induced inhibition of the non-adrenergic, non-cholinergic (NANC) contraction has been studied in guinea-pig perfused bronchioles. (R)-α-Methylhistamine ((R)-α-MeHA), an agonist for H₃ receptors, inhibited the NANC contraction induced by electrical field stimulation. N^G-Nitro-l -arginine methyl ester (l -NAME) (50 μm ), an inhibitor of nitric oxide synthesis, blocked the effect of (R)-α-MeHA. The effect of l -NAME was reversed by l -arginine (50 μm ). l -NAME, l -arginine or (R)-α-MeHA were without effect on exogenous substance P- or neurokinin A-induced contractile responses of the perfused bronchioles. These results show that an H₃-agonist inhibited the release of neurotransmitters in NANC nerve endings of guinea-pig perfused bronchioles presumably by production of nitric oxide.     

Possible involvement of l-arginine-nitric oxide pathway in modulating regional blood flow to brown adipose tissue of rats

To evaluate whether the l-arginine-nitric oxide (NO) pathway is involved in the regulation of regional blood flow to brown adipose tissue (BAT), the effects of two specific NO synthase inhibitors, N^G-nitro-l-arginine methyl ester (l-NAME) and N^G-monomethyl-l-arginine (l-NMMA), on the blood flow to interscapular brown adipose tissue (IBAT) were studied in urethane-anesthetized rats. Regional blood flow in MAT was measured with laser-Doppler flowmetry.An intravenous injection of l-NAME and l-NMMA, but not of either d-enantiomer, caused a transient and dose-dependent increase in IBAT blood flow. Dose-response curves for these NO synthase inhibitors showed that l-NAME was more potent than l-NMMA in increasing IBAT blood flow. We also observed a concomitant pressor effect accompanied by a slight decrease in heart rate following intravenous injection of l-NAME and l-NMMA. An elevation of IBAT blood flow and blood pressure induced by both l-NAME and l-NMMA was reversed by l-arginine in an enantiomerically specific manner. The increase in IBAT blood flow induced by NO synthase inhibitors was of shorter duration and less sensitive to l-arginine than the increase in blood pressure.Our results show that the WAY blood flow is increased by inhibition of NO synthase and that the response of IBAT vasculature to NO synthase inhibitors is different from that of the resistance vessels which regulate blood pressure. The involvement of l-arginine-NO pathways in modulating microcirculation in IBAT is suggested. Correspondence to: Y. Uchida at the above address     

NITRIC OXIDE INHIBITION IN AN OVINE MODEL OF HEART FAILURE

1. The role of nitric oxide (NO) in congestive heart failure was investigated by studying the acute haemodynamic, hormonal and renal effects of N^G-monomethyl-l -arginine (l -NMMA), a nitric oxide inhibitor, given as incremental bolus doses in six sheep before (normal) and after induction of heart failure (HF) by rapid left ventricular pacing (LVP). 2. l -NMMA caused significant initial dose-dependent rises in left ventricular systolic pressure, mean arterial pressure (MAP), peripheral resistance (PR) and left atrial pressure and declines in cardiac output in both normal and HF states (maximum response in 2–6 min). These responses were all but abolished when l -arginine was given concurrently with l -NMMA. The dose-response curve for the l -NMMA-induced rise in MAP was shifted to the right following LVP (P<0.05), which is consistent with previous observations of blunted NO synthase activity in HF. A subsequent decline in MAP and PR to below prebolus levels was observed 30–60 min after l -NMMA administration in the paced state. No significant hormonal or renal effects were observed. 3. In conclusion, the present study confirms the important haemodynamic role played by endogenous NO in the normal state and demonstrates a blunted pressor response to NO inhibition in this model of heart failure.     

Prevention of water immersion stress-induced gastric lesions through the enhancement of nitric oxide synthase activity in rats

Background: Gastric mucosal microcirculation is an important factor in the protection of gastric mucosa, and nitric oxide (NO) plays a crucial role in the regulation of regional blood flow. This study was designed to evaluate the effect of cetraxate, an anti-ulcer drug, on water immersion stress-induced gastric lesions in relation to the changes in NO synthase activity. Methods: Gastric lesions were induced in rats by water immersion stress. The effects of cetraxate on NO synthase activity with or without stress was determined enzymatically. Changes in gastric mucosal prostaglandin (PG) contents with or without stress were also determined using high-performance liquid chromatography. Gastric mucosal blood flow was measured by hydrogen gas clearance technique. Results: Water immersion stress-induced gastric lesions. Cetraxate significantly mitigated the lesions but N^G-monomethyl-l -arginine (l -NMMA), a specific inhibitor of NO synthase, exacerbated the lesions. The favourable effect of cetraxate was remarkably diminished by administration of l -NMMA. NO synthase activity decreased significantly by 6 h after stress. Cetraxate treatment increased NO synthase activity throughout the experiment in rats with or without stress treatment. Water immersion stress decreased all PGs detected, i.e. 6-keto-PGF_1α, PGF_2α PGE₂ and PGD₂. Cetraxate prevented stress-induced decreases in PG contents. l -NMMA showed no significant effect on PG contents. Cetraxate increased gastric mucosal blood flow significantly and l -NMMA cancelled out cetraxate-induced increase in blood flow. Conclusions: The pharmacological efficacy of anti-ulcer drugs such as cetraxate might be attributable to the enhancement of NO synthase activity resulting in an increase in gastric mucosal blood flow.     

Involvement of nitric oxide in the response to 5-hydroxytryptamine in the rat in-vivo

Abstract— The involvement of nitric oxide (NO) in the effects of 5-HT on intestinal secretion and cardiovascular function in anaesthetized rats was investigated using N^G-nitro-l -arginine methyl ester (l -NAME), a specific NO-synthase antagonist, and its optical isomer d -NAME. l -NAME significantly reduced the prolonged hypotensive response to 5-HT. It also caused a small rightward shift in the colonic 5-HT dose-response curve. This suggests that NO plays a significant role in the prolonged hypotensive response to 5-HT, and may make a small contribution to the secretory response of the colon, but not that of the jejunum, in the rat in-vivo.     

Effect of N-nitro-l-arginine on shock induced by endotoxin and by platelet activating factor in dogs

When N^G-nitro-l-arginine, a nitric oxide synthase inhibitor, administration was started 5 min prior to shock induction in anesthetized dogs, a partial restoration was observed in endotoxin-induced shock and a complete recovery in platelet activating factor (PAF)-induced shock. When N^G-nitro-l-arginine infusion was started 5 min after shock induction, no significant recovery was observed in endotoxin-induced shock and a complete recovery in PAF-induced shock. These data indicate that enhanced production of nitric oxide by vascular endothelial cells may contribute to endotoxin- or PAF-induced shock and also that some mediators including inducible nitric oxide synthase and/or cellular damage might be involved in endotoxin-induced shock.     

Nitric oxide synthase inhibition blocks phencyclidine-induced behavioural effects on prepulse inhibition and locomotor activity in the rat

The ability of the nitric oxide synthase inhibitor, N ^G-nitro-L-arginine methyl ester (L-NAME), to block the behavioural effects of the potent psychotomimetic, phencyclidine, was tested in rats using two different behavioural models. L-NAME was found to block both phencyclidine-induced disruption of prepulse inhibition of acoustic startle and phencyclidine-induced stimulation of locomotor activity. A selective action of L-NAME on the effects of phencyclidine was indicated, since L-NAME did not alter the effects of amphetamine, another potent psychotomimetic, in these behavioural models. These observations suggest that a nitric oxide-dependent mechanism may be involved in the effects of phencyclidine in the central nervous system. Received: 2 September 1996/Final version: 21 December 1996     

Differential sensitivity of basal and acetylcholine-induced activity of nitric oxide to blockade by asymmetric dimethylarginine in the rat aorta

Background and purpose:

Previous work has shown that N^G-monomethyl-l-arginine (l-NMMA) paradoxically inhibits basal, but not ACh-stimulated activity of nitric oxide in rat aorta. The aim of this study was to determine if the endogenously produced agent, asymmetric N^G, N^G-dimethyl-l-arginine (ADMA), also exhibits this unusual selective blocking action.

Experimental approach:

The effect of ADMA on basal nitric oxide activity was assessed by examining its ability to enhance phenylephrine (PE)-induced tone in endothelium-containing rings. Its effect on ACh-induced relaxation was assessed both in conditions where ADMA greatly enhanced PE tone and where tone was carefully matched with control tissues at a range of different levels.

Key results:

ADMA (100 µM) potentiated PE-induced contraction, consistent with inhibition of basal nitric oxide activity. Higher concentrations (300–1000 µM) had no greater effect. Although ADMA (100 µM) also appeared to block ACh-induced relaxation when it enhanced PE tone to maximal levels, virtually no block was seen at intermediate levels of tone in the presence of ADMA. Even ADMA at 1000 µM had no effect on the maximal relaxation to ACh, although it produced a small (two- to threefold) reduction in sensitivity. ADMA and l-NMMA, like l-arginine (all at 1000 µM), protected ACh-induced relaxation against blockade by l-NAME (30 µM).

Conclusions and implications:

In the rat aorta, ADMA, like l-NMMA, blocks basal activity of nitric oxide, but has little effect on that stimulated by ACh. Further studies are required to explain these seemingly anomalous actions of ADMA and l-NMMA.     

N-Nitro-L-arginine protects against ischaemia-induced increases in nitric oxide and hippocampal neuro-degeneration in the gerbil

To assess the effects of the nitric oxide synthase inhibitor N^G-Nitro-L-arginine on behavioural, biochemical and histological changes following global ischaemia, the Mongolian gerbil was used. Ischaemia was induced by bilateral carotid occlusion for 5 min. N^G-Nitro-L-arginine was administered i.p. at either 1 or 10 mg/kg 30 min, 6, 24, and 48 h after surgery. 5 min bilateral carotid occluded animals were hyperactive 24, 48 and 72 h after surgery. N^G-Nitro-L-arginine caused some attenuation in this hyperactivity. The activity of nitric oxide synthase was increased in the cerebellum, brain stem, striatum, cerebral cortex and hippocampus of 5 min bilateral carotoid occluded animals. N^G-Nitro-L-arginine reversed the increase in nitric oxide synthase activity in all brain regions. Extensive neuronal death was observed in the CA₁ layer of the hippocampus in 5 min bilateral carotid occluded animals 96 h after surgery. N^G-Nitro-L-arginine significantly protected against the neuronal death of cells in the CA₁ layer.     

Possible Involvement of Nitric Oxide Synthase in Oxidative Stress-Induced Endothelial Cell Injury

Abstract: The purpose of this study was to characterize the protective effect of N^G-nitro-L-arginine methyl ester (L-NAME), an inhibitor of nitric oxide synthase, on oxidative stress-induced endothelial cell injury. Intracellular oxidative stress was induced by 1-chloro-2,4-dinitrobenzene, a glutathione (GSH) depleting agent, and the leakage of intracellular lactate dehydrogenase was measured as a marker of cell injury. Addition of 1-chloro-2,4-dinitrobenzene (100-500 μM) induced leakage of lactate dehydrogenase from endothelial cells, and the leakage of lactate dehydrogenase was strongly attenuated by L-NAME, but not by N^G-methyl-L-arginine, also an inhibitor of nitric oxide synthase. However, cell injury induced by the Ca²⁺ ionophore ionomycin was not affected by L-NAME or N^G-methyl-L-arginine. Moreover, neither L-NAME nor N^G-methyl-L-arginine affected GSH depleting agent-induced or H₂O₂-induced cell injury in a rat foetal lung fibroblast cell line which lacks nitric oxide synthase. These results suggest that the protective effect of L-NAME is likely to be related to nitric oxide synthase, while the inhibition of nitric oxide production may not be involved in the protective effect of L-NAME, since N^G-methyl-L-arginine did not affect endothelial cell injury.     

Nitric oxide is involved in the memory facilitation induced by spermidine in rats

Rationale Spermidine (SPD) is an endogenous polyamine that modulates N-methyl-d-aspartate receptor functions, which has been reported to facilitate memory formation.Objectives In the current study, we investigated the involvement of nitric oxide in the facilitatory effect of SPD on the memory of adult male Wistar rats in the inhibitory avoidance task.Results The coadministration of the nonspecific NOS inhibitor N ^G nitro-l-arginine methyl ester (l-NAME) (0.1 nmol, intrahippocampus) with spermidine (0.2 nmol), immediately after training, prevented the memory improvement caused by spermidine in the avoidance inhibitory task. Spermidine increased nitrite and nitrate levels (NO_X) in the hippocampus 30 min after its administration, and l-NAME coinjection prevented the stimulatory effect of spermidine on NO_X levels. The systemic injection of 7-nitroindazole (30 mg/kg, i.p.), 30 min before training, impaired memory and did not prevent spermidine-induced increase of NO_X levels in the hippocampus.Conclusions These results suggest that memory enhancement by spermidine is prevented by the nonspecific nitric oxide synthase inhibitor l-NAME.     

Attenuation of some signs of opioid withdrawal by inhibitors of nitric oxide synthase

Effects of nitric oxide synthase (NOS) inhibitors (l-N ^G-nitroarginine,l-N ^G-nitroarginine methyl ester) on precipitated opioid withdrawal were studied in morphine-dependent rats given naloxone, in order to assess the involvement of nitric oxide (NO) in opioid dependence.l-N ^G-Nitroarginine (7.5 mg/kg, IP, 1 h before naloxone or b.i.d. on days 4–7 of an 8-day morphine treatment) reduced wet dog shakes and weight loss; when given by osmotic pumps (15 mg/kg per day), the drug reduced wet dog shakes but not weight loss.l-N ^G-Nitroarginine methyl ester (60 mg/kg, 1 h before naloxone) also reduced wet dog shakes and weight loss. The results indicate that NOS inhibitors warrant further study as potential treatment of the opioid withdrawal syndrome.Abstracts were presented at meetings of the Society for Neuroscience in New Orleans, La., November 10–15, 1991 and of the American Society for Pharmacology and Experimental Therapeutics in Orlando, Fla., August 10–18, 1992     

A STUDY OF ANGIOTENSIN II RECEPTORS AFTER CHRONIC INHIBITION OF NITRIC OXIDE SYNTHASE IN THE SPONTANEOUSLY HYPERTENSIVE RAT

1. Nitric oxide (NO) synthase inhibition, induces a sustained increase in blood pressure and amplifies the pressor response to infused angiotensin II (AngII). This study was designed to investigate the contribution of AngII receptors to the elevated blood pressure and enhanced pressor response to AngII in the spontaneously hypertensive rat (SHR) chronically treated with N^G-nitro-l -arginine-methyl ester (l -NAME). 2. Two groups of 13 week old female SHR were housed four to a box. Group I rats received l -NAME for 7 days (2.5 mg/kg per day) in their drinking water. Group II rats received water only. Blood pressure was monitored daily by tail-cuff plethysmography. Plasma AngII was measured by radioimmunoassay. Aortic and uterine receptor binding was determined by saturation analysis using [¹²⁵I]-Sar⁸, Ile¹)AngII. Data was analysed using the computer program ligand. 3. Mean systolic blood pressure was significantly elevated in rats treated with l -NAME compared with the control group. Plasma AngII concentration was slightly decreased in rats treated with l -NAME compared with control. Densities of both aortic and uterine AngII receptors increased significantly following NO synthase inhibition. Receptor affinity in the aorta decreased in the l -NAME group compared with control. However, uterine AngII receptor affinity was unchanged. 4. We conclude that the increased blood pressure and enhanced pressor responsiveness that occurs with chronic inhibition of NO synthesis may result partly from increased vascular AngII receptor expression.     

Asymmetric dimethylarginine does not inhibit arginase activity and is pro‐proliferative in pulmonary endothelial cells

Asymmetric dimethylarginine (ADMA) is an endogenously produced nitric oxide synthase (NOS) inhibitor. l ‐Arginine can be metabolised by NOS and arginase, and arginase is the first step in polyamine production necessary for cellular proliferation. We tested the hypothesis that ADMA would inhibit NOS but not arginase activity and that this pattern of inhibition would result in greater l ‐arginine bioavailability to arginase, thereby increasing viable cell number. Bovine arginase was used in in vitro activity assays with various concentrations of substrate (l ‐arginine, ADMA, N^G‐monomethyl‐l ‐arginine (L‐NMMA) and N^G‐nitro‐l ‐arginine methyl ester (l ‐NAME)). Only l ‐arginine resulted in measurable urea production (K_m = 6.9 ± 0.8 mmol/L; V_max = 6.6 ± 0.3 μmol/mg protein per min). We then incubated bovine arginase with increasing concentrations of ADMA, l ‐NMMA and l ‐NAME in the presence of 1 mmol/L l ‐arginine and found no effect of any of the tested compounds on arginase activity. Using bovine pulmonary arterial endothelial cells (bPAEC) we determined the effects of ADMA on nitric oxide (NO) and urea production and found significantly lower NO production and greater urea production (P < 0.003) with ADMA, without changes in arginase protein levels. In addition, ADMA treatment resulted in an approximately 30% greater number of viable cells after 48 h than in control bPAEC. These results demonstrate that ADMA is neither a substrate nor an inhibitor of arginase activity and that in bPAEC ADMA inhibits NO production and enhances urea production, leading to more viable cells. These results may have pathophysiological implications in disorders associated with higher ADMA levels, such as pulmonary hypertension.     

N-nitro- -arginine methylester is protective against ethanol-induced gastric damage in cholestatic rats

In this study the effect of nitric oxide (NO) synthesis inhibition on ethanol-induced gastric damage was evaluated in bile duct-ligated, sham-operated and unoperated rats. The animals were injected intraperitoneally with saline, -arginine (200 mg/kg) or N^G-nitro- -arginine methylester ( -NAME) in doses of 5, 15 and 30 mg/kg, 30 min before ethanol administration. The animals were killed 1 h after ethanol administration and their stomachs were removed for measurement of gastric mucosal damage. The results showed that -NAME significantly enhanced the development of gastric mucosal lesions in sham-operated and unoperated rats, while in bile duct-ligated animals, -NAME decreased and -arginine enhanced the potentiation of ethanol-induced gastric mucosal damage. The plasma level of nitrite and nitrate was also measured and was significantly higher in bile duct-ligated rats than in control groups. The results suggest that inhibition of NO synthase with -NAME has different effects on ethanol-induced gastric damage in cholestatic groups and in normal rats and that these effects can be explained by overproduction of NO in bile duct-ligated animals.     

Protective effect of oral -arginine administration on gentamicin-induced renal failure in rats

We investigated the effects of orally supplemented -arginine, the substrate of nitric oxide (NO) and N^ω-nitro- -arginine methyl ester ( -NAME), a nitric oxide-synthase inhibitor in gentamicin-induced renal failure. Rats were given gentamicin (100 mg/kg/day s.c.), gentamicin and -arginine (2 g/l, drinking water), gentamicin and -NAME (100 mg/l, drinking water) or gentamicin plus -arginine and -NAME. After 8 days, the gentamicin group developed marked renal failure, characterized by a significantly decreased creatinine clearance and increased blood creatinine, fractional excretion of sodium, fractional excretion of lithium, urine gamma glutamyl transferase, systolic blood pressure and daily urine volume when compared to controls. Renal histological analysis confirmed tubular necrosis. -arginine administration caused normalization of these parameters, whereas -NAME led to aggravation of the failure. Concomittant administration of -NAME and -arginine to gentamicin-treated rats caused no significant changes when compared to the rats receiving gentamicin alone. We conclude that -arginine supplementation has beneficial effects in gentamicin-induced renal failure in rats and that these effects are reversed by the NO-synthase inhibitor, -NAME.     

The effects of thiol compounds and ebselen on nitric oxide activity in rat aortic vascular responses

1 Thiols have been implicated to play a role in a variety of aspects of nitric oxide (NO) generation and activity. Thiol dependence of nitric oxide synthase (NOS) has remained controversial and its mechanism is not clear. This study investigates possible mechanisms between thiol (SH group) and NOS activation, through thiol compounds (glutathione, dithiothreitol, N‐acetyl‐L ‐cysteine) and Ebselen [2‐phenyl‐1,2‐benzisoselenazole‐3(2H)‐one] on rat aortic vascular responses. 2 In rat thoracic aorta, acetylcholine (10^–6–10^–9 M) induced a relaxation of phenylephrine (PE) (10^–7 M )‐induced tone, which was inhibited dose dependently by increasing concentration of ebselen (1–10 μM ). 3 In rings of rat thoracic aorta, ebselen and NOS inhibitors (N^G‐monomethyl‐L ‐arginine, N^G‐nitro‐L ‐arginine methyl ester) produced an augmentation of phenylephrine (10^–7 M )‐ induced tone and acetylcholine induced a relaxation of PE (10^–7 M )‐induced tone in rat thoracic aorta, which was inhibited by ebselen (10 μM ) like NOS inhibitor. 4 The thiol compounds (glutathione, dithiothreitol, and N‐acetyl‐L ‐cysteine) alone did not change vascular tone in rat thoracic aorta. Pretreatment with thiol compounds before ebselen treatment, however, reversed the inhibitory effect of ebselen which acts like the NOS inhibitor in rat thoracic aorta. Posttreatment with thiol compounds after ebselen treatment did not reverse the inhibitory effect of ebselen by as much as pretreatment. 5 Calcium ionophore A23187 (10^–7 M )‐induced vasodilation was inhibited in ebselen pretreated rat thoracic aorta, but sodium nitroprusside (SNP, 10^–7 M )‐induced relaxation was not inhibited by ebselen. This suggests that NOS is involved in the inhibitory effect of ebselen on rat thoracic aorta relaxation. 6 These results suggest that ebselen exerts an inhibitory action on the nitric oxide synthesis in rat thoracic aorta by interacting with thiol groups.