L-arginine is the physiological precursor for the formation of nitric oxide in endothelium-dependent relaxation

Threshold concentrations of endothelin-1 potentiate contractions to norepinephrine and serotonin in human arteries. A new mechanism of vasospasm?

Endothelin-1 is an endothelium-derived vasoconstrictor peptide. Its circulating levels are below those known to evoke direct vascular effects. To study whether low concentrations of endothelin-1 potentiate the effects of other vasoconstrictor hormones, we suspended isolated human internal mammary and left anterior descending coronary artery rings in organ chambers for isometric tension recording. In mammary artery rings, the contractions to norepinephrine (3 x 10(-8) M) were potentiated by threshold (3 x 10(-10) M) and low concentrations (10(-9) M) of endothelin-1 (96 +/- 35% and 149 +/- 58% increase from control; p less than 0.01 and 0.001; n = 6). The inhibitor of endothelial nitric oxide formation L-NG-monomethyl arginine did not affect the potentiating effects of the peptide. The calcium antagonist darodipine (10(-7) M) prevented the potentiation of the response to norepinephrine evoked by endothelin-1. Similarly, contractions to serotonin (10(-7) or 3 x 10(-8) M) were amplified by endothelin-1 (3 x 10(-10) M) in the mammary (30 +/- 9%) and in the coronary arteries (59 +/- 25%). Endothelin-1 (10(-9) M) further potentiated the response (57 +/- 23% in mammary and 87 +/- 26% in coronary arteries; p less than 0.05; n = 7 and 3). The sensitivity of mammary arteries to calcium chloride was markedly enhanced in the presence of endothelin-1 (3 x 10(-10) M; concentration shift, eightfold; p less than 0.01; n = 5).(ABSTRACT TRUNCATED AT 250 WORDS)

Endogenous nitric oxide as a modulator of rabbit skeletal muscle microcirculation in vivo

1. Intravital microscopy of rabbit tenuissimus muscle microvasculature was used for in vivo studies of the role of endogenous nitric oxide (NO) in local vascular control. Derivatives of arginine were applied topically in order to modulate the formation of NO from L-arginine. 2. L-NG-monomethylarginine (L-NMMA) (10-100 microM), but not D-NG-monomethylarginine (D-NMMA), dose-dependently reduced microvascular diameters. The vasoconstriction induced by L-NMMA (100 microM) was prevented by pretreatment with L-arginine (1 mM) but not with D-arginine (1 mM). Intravenous infusions of L-arginine (300 mg kg-1) reversed the effect of L-NMMA (100 microM). L-Arginine or D-arginine applied topically at 1 mM per se had no effect on microvascular diameters. 3. Vasodilatation by acetylcholine (0.03-3 microM) was significantly inhibited by L-NMMA (100 microM), whereas vasodilatation by adenosine (0.1-100 microM) or sodium nitroprusside (100 nM) was not affected. 4. The hyperaemic response after tenuissimus muscle contractions induced by motor nerve stimulation was unaffected by the presence of L-NMMA (100 microM). 5. Aggregates of platelets and white blood cells were seen in venules during superfusion with L-NMMA (100 microM), but not with D-NMMA (100 microM). 6. Our results suggest that endogenous NO formed from L-arginine is a modulator of microvascular tone and platelet and white cell-vessel wall interaction in vivo. Nitric oxide does not, however, appear to play a role in the mediation of functional hyperaemia in this tissue.

Impaired vasodilation of forearm resistance vessels in hypercholesterolemic humans

The effect of hypercholesterolemia on vascular function was studied in humans. To eliminate the potential confounding effects of atherosclerosis, vascular reactivity was measured in the forearm resistance vessels of 11 normal subjects (serum LDL cholesterol = 111 +/- 7 mg/dl) and 13 patients with hypercholesterolemia (serum LDL cholesterol = 211 +/- 19 mg/dl, P less than 0.05). Each subject received intrabrachial artery infusions of methacholine, which releases endothelium-derived relaxant factor, and nitroprusside which directly stimulates guanylate cyclase in vascular smooth muscle. Maximal vasodilatory potential was determined during reactive hyperemia. Vasoconstrictive responsiveness was examined during intra-arterial phenylephrine infusion. Forearm blood flow was determined by venous occlusion plethysmography. Basal forearm blood flow in normal and hypercholesterolemic subjects was comparable. Similarly, reactive hyperemic blood flow did not differ between the two groups. In contrast, the maximal forearm blood flow response to methacholine in hypercholesterolemic subjects was less than that observed in normal subjects. In addition, the forearm blood flow response to nitroprusside was less in hypercholesterolemic subjects. There was no difference in the forearm vasoconstrictive response to phenylephrine in the two groups. Thus, the vasodilator responses to methacholine and nitroprusside were blunted in patients with hypercholesterolemia. We conclude that in humans with hypercholesterolemia, there is a decreased effect of nitrovasodilators, including endothelium-derived relaxing factor, on the vascular smooth muscle of resistance vessels.

Influence of NG-monomethyl-L-arginine on endothelium-dependent relaxations in the perfused mesenteric vascular bed of the rat

Endothelium-dependent relaxation mediated by the formation of nitric oxide (NO) from L-arginine, is prevented by the arginine analog NG-monomethyl L-arginine (L-NMMA) (Palmer et al., Biochem. Biophys. Res. Comm. 153:1251-1256 (1988)). In the rat mesenteric arterial bed, incubation with L-NMMA did not prevent acetylcholine-induced relaxation, which, however, was reversed when L-NMMA was added during its maximum effect. A similar profile of action was observed with methylene blue, an inhibitor of guanylate cyclase. Methylene blue, but not L-NMMA, increased basal perfusion pressure. These data indicate that in the mesenteric arterial bed, NO formation via the L-NMMA-sensitive pathway occurs during stimulation with acetylcholine, but not under basal conditions.

Nitric oxide release may be involved in the microcirculatory response to acetylcholine

An image-splitter television microscope for measurement of microvascular dimension changes in the rat exteriorized mesentery was used to investigate the role of endothelium-derived relaxing factor (EDRF)/nitric oxide (NO) in the microcirculation. This was done by studying the effect of NG-monomethyl-L-arginine (L-NMMA), a specific inhibitor of NO formation, on the responses induced by acetylcholine (an endothelium-dependent vasodilator agent) and sodium nitroprusside (an endothelium-independent vasodilator agent). The effect of acetylcholine on mesenteric A2 arterioles was impaired by previous application of L- but not D-NMMA to the preparations whereas the vasodilator response to sodium nitroprusside was not altered. The effect of L-NMMA was slow to disappear, unless accelerated by a 3-fold molar excess of L- but not D-arginine. It is suggested that EDRF/NO might be involved in the vasodilator response to endothelium-dependent agents such as acetylcholine at the microcirculatory level and that L-arginine might be the physiological precursor of NO.

Control of coronary vascular tone by nitric oxide

A specific difference-spectrophotometric method was used to measure nitric oxide (NO) release into the coronary effluent perfusate of isolated, constant-flow-perfused guinea pig hearts. Authentic NO applied into the coronary circulation decreased vascular resistance dose dependently and enhanced coronary release of cyclic GMP (cGMP) fivefold. Increasing oxygen tension in aqueous solutions from 150 to 700 mm Hg decreased NO half-life (5.6 seconds) by 32%. During single passage through the intact coronary system, 86% of the infused NO was converted to nitrite ions. Oxidation of NO was more than 30 times faster within the heart than in aqueous solution. Endogenously formed NO was constantly released into the coronary effluent perfusate at a rate of 161 +/- 11 pmol/min. The NO scavenger oxyhemoglobin and methylene blue increased coronary resistance and decreased cGMP release (basal release, 342 +/- 4 fmol/min), whereas superoxide dismutase reduced coronary resistance. L-Arginine (10(-5) M) slightly decreased coronary perfusion pressure and enhanced release of cGMP. NG-Monomethyl L-arginine (10(-4) M) reduced basal release of NO and cGMP by 26% and 31%, respectively, paralleled by a coronary vasoconstriction. Bradykinin in the physiological range from 5 x 10(-11) M to 10(-7) M dilated coronary resistance vessels, which was paralleled by the release of NO and cGMP. Onset of NO release preceded onset of coronary vasodilation in all cases. Upon stimulation with bradykinin, amounts of endogenously formed NO were within the same range as the dose-response curves for exogenously applied NO both for changes in coronary resistance and cGMP release. Acetylcholine (10(-5) M), ATP (10(-5) M), and serotonin (10(-8) M) increased the rate of NO and cGMP release, resulting in coronary vasodilation. Our data suggest the following: 1) NO, the most rapidly acting vasodilator presently known, is metabolized within the heart mainly to nitrite and exhibits a half-life of only 0.1 second; 2) in the unstimulated heart, basal formation of NO may play an important role in setting the resting tone of coronary resistance vessels; 3) the kinetics and quantities of NO formation suggest that NO is causally involved in the bradykinin-induced coronary vasodilation; and 4) amounts of NO formed within the heart stimulated with ATP, acetylcholine, and serotonin are effective for vasodilation.

NG-methyl-L-arginine causes endothelium-dependent contraction and inhibition of cyclic GMP formation in artery and vein

The objective of this study was to determine whether the vascular smooth muscle contractile effect of NG-methyl-L-arginine (NMA) is endothelium dependent and attributed to a decline in smooth muscle levels of cyclic GMP. Vascular smooth muscle levels of cyclic GMP are severalfold greater in endothelium-intact than in endothelium-denuded preparations because of the continuous formation and release of a lipophilic endothelium-derived chemical factor that diffuses into the underlying smooth muscle and activates cytosolic guanylate cyclase. This chemical substance, believed to be nitric oxide (NO) or a labile nitroso precursor, appears to account for the biological actions of endothelium-derived relaxing factor. NMA inhibits the formation of NO from endogenous L-arginine in endothelial cells. In the present study, NMA caused marked endothelium-dependent contraction of isolated rings of bovine pulmonary artery and vein, and this was similar to the contraction elicited by hemoglobin, an inhibitor of the relaxant action of NO. Both NMA and hemoglobin caused endothelium-dependent potentiation of contractile responses to phenylephrine in artery and vein. NMA caused endothelium-dependent decreases in the resting or basal levels of cyclic GMP in artery and vein to levels that were characteristic of those in endothelium-denuded vessels. Finally, NMA inhibited endothelium-dependent relaxant responses and cyclic GMP formation stimulated by acetylcholine and bradykinin. These observations reveal that interference with the continuous or basal generation of endothelium-derived NO in artery and vein can cause marked increases in vascular smooth muscle tone as a result of inhibition of cyclic GMP formation.

NG-methyl-L-arginine inhibits tumor necrosis factor-induced hypotension: implications for the involvement of nitric oxide

Clinical assessment of the activity of tumor necrosis factor (TNF) against human cancer has been limited by a dose-dependent cardiovascular toxicity, most frequently hypotension. TNF is also thought to mediate the vascular collapse resulting from bacterial endotoxin. The present studies address the mechanism by which TNF causes hypotension and provide evidence for elevated production of nitric oxide, a potent vasodilator initially characterized as endothelium-derived relaxing factor. Nitric oxide is synthesized by several cell types, including endothelial cells and macrophages, from the guanidino nitrogen of L-arginine; the enzymatic pathway is competitively inhibited by NG-methyl-L-arginine. We found that hypotension induced in pentobarbital-anesthetized dogs by TNF (10 micrograms/kg, i.v., resulting in a fall in mean systemic arterial pressure from 124.7 +/- 7 to 62.0 +/- 22.9 mmHg; 1 mmHg = 133 Pa) was completely reversed within 2 min following administration of NG-methyl-L-arginine (4.4 mg/kg, i.v.). In contrast, NG-methyl-L-arginine failed to reverse the hypotensive response to an equivalent depressor dose of nitroglycerin, a compound that acts by forming nitric oxide by a nonenzymatic, arginine-independent mechanism. The effect of NG-methyl-L-arginine on TNF-induced hypotension was antagonized, and the hypotension restored, by administration of excess L-arginine (100 mg/kg, i.v.). Our findings suggest that excessive nitric oxide production mediates the hypotensive effect of TNF.

Control of regional blood flow by endothelium-derived nitric oxide

The regional hemodynamic consequences of inhibiting vascular endothelial nitric oxide generation with NG-monomethyl-L-arginine (L-NMMA) were studied in conscious Long-Evans rats. Experiments were carried out in groups of chronically instrumented rats with intravascular catheters and pulsed Doppler probes to monitor regional blood flow. L-NMMA (0.3-300 mg/kg) caused a dose-dependent, long-lasting (5-90 minutes), and enantiomerically specific increase in mean blood pressure and also caused bradycardia. The increase in blood pressure was accompanied by a dose-dependent and long-lasting vasoconstriction in the internal carotid, mesenteric, renal, and hindquarters vascular beds that could be attenuated, in a concentration-dependent manner, by L-arginine but not by D-arginine. In contrast, L-arginine did not affect the pressor or vasoconstrictor effects of vasopressin. These results indicate that nitric oxide production by vascular endothelial cells contributes to the maintenance of blood pressure and to the control of the resting tone of different vascular beds in the conscious rat.

NG-amino-L-arginine: a new potent antagonist of L-arginine-mediated endothelium-dependent relaxation

This study examined the influence of NG-amino-L-arginine, a novel structural analog of L-arginine, on endothelium-dependent relaxation, contraction, and cyclic GMP accumulation in isolated rings of bovine pulmonary artery. NG-Amino-L-arginine caused potent and stereoselective endothelium-dependent contraction that was associated with a marked and endothelium-dependent decline in basal levels of cyclic GMP in smooth muscle. NG-Amino-L-arginine caused concentration-dependent, competitive, and stereoselective antagonism of acetylcholine-elicited relaxation and cyclic GMP accumulation. NG-Amino-L-arginine was 100- to 300- fold more potent than NG-methyl-L-arginine and did not inhibit endothelium-independent relaxation elicited by nitroglycerin. This potent inhibitory analog of L-arginine should be a useful chemical probe for studying the biosynthesis and biological role of L-arginine-derived nitric oxide both in vitro and in vivo.

Regional distribution of EDRF/NO-synthesizing enzyme(s) in rat brain

Stimulation of soluble guanylyl cyclase and increase in cyclic GMP in rat fetal lung fibroblasts (RFL-6 cells) was used as a bioassay to detect EDRF/NO formation. The cytosolic fraction of whole rat brain synthesized an EDRF/NO-like material in a process dependent on L-arginine and NADPH. The enzymatic activity was destroyed by boiling and inhibited by N omega-nitro-L-arginine. Hemoglobin and methylene blue blocked the effect of EDRF/NO. When different brain regions were analyzed in the presence of L-arginine and NADPH, the cytosolic fraction from cerebellum showed the highest EDRF/NO-forming activity (2-3 times higher than whole brain). Activity similar to whole brain was found in hypothalamus and midbrain. Enzymatic activities in striatum, hippocampus and cerebral cortex were about two thirds of whole brain. The lowest activity (less than half of whole brain) was found in the medulla oblongata.

Inhibition of endothelial nitric oxide biosynthesis by N-nitro-L-arginine

1. The actions of N-nitro-L-arginine (NOLA) on the release of nitric oxide (NO) from arterial endothelial cells was studied in rat isolated thoracic aortic rings and by bioassay of NO derived from cultured bovine aortic endothelial cells. 2. NOLA (3-10 mumol/L) caused concentration-dependent inhibition of acetylcholine-induced relaxation of phenylephrine-contracted rat aortic rings, which is dependent on the release of NO from the endothelium. The inhibitory actions of NOLA could be prevented by pre- and co-incubation with L-arginine (1 mmol/L). 3. Endothelium-independent relaxation induced by sodium nitroprusside was not affected by NOLA. 4. The release of NO from bovine aortic endothelial cells, induced by bradykinin (10 nmol/L), was detected by bioassay on pre-contracted rabbit aortic strips. NOLA (1-3 mumol/L, given through the cell column) reduced or abolished the release of NO, but did not affect relaxations of the bioassay tissues induced by glyceryl trinitrate or authentic NO. 5. These data indicate that NOLA potently inhibits the biosynthesis of NO from L-arginine, and thus prevents its release from arterial endothelial cells. It may be a useful pharmacological tool for probing the significance of NO biosynthesis in cardiovascular function.

Endothelial cells metabolize NG-monomethyl-L-arginine to L-citrulline and subsequently to L-arginine

NG-monomethyl-L-arginine (MeArg) inhibits the release of endothelium-derived relaxing factor (EDRF) from endothelial cells (EC) and the formation of nitric oxide (NO) from L-arginine (Arg) in EC and activated macrophages. We have compared the inhibitory potency of MeArg to that of N omega-nitro-L-arginine (NO2Arg), a more potent inhibitor of EDRF synthesis in vitro. NO2Arg (100 microM) was significantly more potent than MeArg in inhibiting the endothelium-dependent relaxation of rabbit aorta induced by acetylcholine. MeArg and NO2Arg (10 and 30 microM) also inhibited the release of EDRF from bovine aortic cultured EC. In the anaesthetized rat in vivo, the pressor effect of NO2Arg (3 and 10 mg kg-1) was significantly larger and longer lasting than that of MeArg. These differences in potency could be due to the extensive metabolism of MeArg but not NO2Arg to L-citrulline (Cit) and subsequently to Arg by EC. The enzyme responsible for the conversion of MeArg to Cit had the characteristics of a novel deiminase, NG,NG-dimethylarginine dimethylaminohydrolase, recently isolated from rat kidney.

L-NG-monomethyl arginine inhibits the vasodilating effects of low dose of endothelin-3 on rat mesenteric arteries

We have reported that low doses of endothelin-3 (ET-3) elicited continuous vasodilation of rat mesenteric arteries, which is possibly related to endothelium-derived relaxing factor (EDRF). In order to clarify whether or not the vasodilating effects of ET-3 are associated with EDRF, we examined the effects of L-NG-monomethyl arginine (L-NMMA), an analog of L-arginine, on low-dose ET-3 induced vasodilation of rat mesente-Hc arteries. Infusion of 50 microM L-NMMA inhibited the vasodilation induced by 10(-13) M ET-3 and rather elicited an increase in perfusion pressure, which itself was decreased by infusion of 150 microM L-arginine. In the presence of 50 microM L-NMMA, 10(-13) M ET-3 did not elicit any vasodilation of the mesenteric arteries preconstricted with NE, in which 150 microM L-arginine, but not D-arginine, caused considerable vasodilation. These data suggest that the vasodilating effects of low doses of ET-3 are associated with EDRF as an endothelium-derived nitric oxide.

Regulation of gastric mucosal integrity by endogenous nitric oxide: interactions with prostanoids and sensory neuropeptides in the rat

1. The interactions between nitric oxide (NO), prostacyclin and sensory neuropeptides in the maintenance of gastric mucosal integrity have been investigated in the anaesthetized rat. 2. Administration of either NG-monomethyl-L-arginine (L-NMMA) to inhibit endothelium-derived NO formation, indomethacin to inhibit prostanoid biosynthesis or chronic capsaicin pretreatment to deplete sensory neuropeptides, did not induce acute mucosal injury. 3. In capsaicin-pretreated rats, however, L-NMMA (12.5-100 mg kg-1 i.v.) dose-dependently induced acute mucosal damage, characterized as vasocongestion and haemorrhagic necrosis. The enatiomer D-NMMA (100 mg kg-1 i.v.) did not induce any detectable mucosal damage. 4. This mucosal injury induced by L-NMMA was inhibited by concurrent administration of L-arginine (300 mg kg-1 i.v.). 5. In indomethacin (5 mg kg-1 i.v.)-pretreated rats, L-NMMA also induced mucosal damage. Furthermore, following indomethacin administration in capsaicin-pretreated rats, L-NMMA induced widespread, severe haemorrhagic necrotic damage. 6. These findings suggest a role for endogenous NO formed from L-arginine, acting in concert with prostacyclin and sensory neuropeptides, in the modulation of gastric mucosal integrity.

Endothelial control of vascular tone in large and small coronary arteries

The endothelium modulates coronary vascular tone by the release of endothelium-derived relaxing or contracting substances. The endothelium-derived relaxing factor has been identified as nitric oxide synthesized in endothelial cells from L-arginine. The endothelium can release other relaxing substances such as prostacyclin and a hyperpolarizing factor. Endothelin-1 is a potent vasoconstrictor peptide formed by endothelial cells, and is likely to be the physiologic antagonist of endothelium-derived relaxing factor. Other putative contracting factors include superoxide anions and products of arachidonic acid metabolism. Endothelium-derived relaxing factor is released spontaneously and in response to flow, platelet-derived products (that is, serotonin, thrombin and adenosine diphosphate) and certain autacoids (that is, acetylcholine, bradykinin, histamine, substance P, vasopressin, alpha-adrenergic agonists). A considerable heterogeneity of responses exists among vessels of different size from different anatomic origin and different species. Hypercholesterolemia, atherosclerosis, hypertension and myocardial ischemia or reperfusion, or both, impair endothelium-dependent relaxation. Under normal conditions, endothelium-derived relaxing factor appears to dominate the control of vascular tone of large and small coronary vessels, whereas in disease states, endothelium-derived contracting factors are released. Impairments of endothelial function may be important in the development of various forms of cardiovascular disease.

Release of endothelium-derived relaxing factor from pig cultured aortic endothelial cells, as assessed by changes in endothelial cell cyclic GMP content, is inhibited by a phorbol ester

1. Cultured aortic endothelial cells of the pig respond to the endothelium-derived relaxing factor (EDRF) they release with an increase in cyclic GMP content. This response is inhibited by haemoglobin or by L-NG-monomethyl-arginine (L-NMMA), and has been used to investigate the effects of phorbol esters on EDRF release. 2. Pretreatment with phorbol-12,13-dibutyrate (PDB) but not the inactive 4 alpha-phorbol-12,13,-didecanoate (PDD), inhibited increases in cyclic GMP induced by substance P (10(-8) M) in a time and concentration-dependent manner. PDB did not affect basal cyclic GMP levels. 3. PDB (3 x 10(-7) M), but not PDD (3 x 10(-7) M), also inhibited ATP (10(-5) M)-induced increases in cyclic GMP, but did not affect those induced by bradykinin (10(-7) M). 4. Increases in cyclic GMP induced by low (10(-7) M) but not high (10(-6) M) concentrations of the calcium ionophore A23187 were inhibited by PDB (3 x 10(-7) M). This inhibitory effect was due to enhanced destruction of EDRF by superoxide anions rather than inhibition of EDRF release, as the inhibition was abolished in the presence of superoxide dismutase (SOD, 30 mu ml-1) and catalase (CAT, 100 mu ml-1). 5. SOD and CAT did not affect the inhibitory action of PDB on substance P or ATP-induced increases in cyclic GMP. 6. Increases in endothelial cell cyclic GMP content induced by sodium nitroprusside (10(-5) M) were unaffected by PDB pretreatment. 7. The inhibitory effects of PDB are probably a result of an action of protein kinase C on the steps between receptor occupation and phospholipase C activation.

N omega-nitro-L-arginine: a potent inhibitor of endothelium-derived relaxing factor formation

Endothelium-derived relaxing factor (EDRF) released from cultured endothelial cells was assayed by examining changes in cyclic GMP levels of rat lung fibroblasts. N omega-nitro-L-arginine and NG-monomethyl-L-arginine inhibited basal and A23187-, ATP- and melittin-induced EDRF release, and the inhibition was prevented with L-arginine. The IC50 values of N omega-nitro-L-arginine and NG-monomethyl-L-arginine for EDRF release evoked with 1 microM A23187 were 230 nM and 16 microM, respectively. N omega-nitro-L-arginine and NG-monomethyl-L-arginine did not affect cyclic GMP accumulation in the fibroblasts with atrial natriuretic factor or sodium nitroprusside. Thus, N omega-nitro-L-arginine is 70 times more potent than NG-monomethyl-L-arginine as a specific inhibitor of EDRF formation/release.

L-NG-nitro arginine (L-NOARG), a novel, L-arginine-reversible inhibitor of endothelium-dependent vasodilatation in vitro

1. The effect of L-NG-nitro arginine (L-NOARG) was compared with that of L-NG-monomethyl arginine (L-NMMA) on vasodilatation of the isolated aorta of the rabbit and perfused mesentery of the rat in response to acetylcholine (ACh) and sodium nitroprusside (NP). 2. L-NOARG (1.5-100 microM) and L-NMMA (3-100 microM) produced concentration-related contraction of the rabbit aorta precontracted with phenylephrine (700-900 nM). Similarly, L-NOARG (10-200 microM) and L-NMMA (30-100 microM) elevated perfusion pressure of the noradrenaline (NA, 0.6-2.5 mM)-preconstricted rat mesentery preparation. 3. L-NOARG (1.5-100 microM) and L-NMMA (3-100 microM) caused concentration-related inhibition of the vasodilator effect of ACh (0.01-1.0 microM) on the rabbit aorta without influencing responses to NP (0.03-0.5 microM). L-NOARG methyl ester (30 microM) also inhibited ACh-induced vasorelaxation with similar potency to NOARG. L-arginine (30-150 microM) but not D-arginine (100 microM) caused graded reversal of the inhibitory effect of both L-NOARG (15 microM) and L-NMMA (30 microM). Complete reversal of the effect of both inhibitors was achieved with 150 microM L-arginine. L-Alanine (50 microM), L-arginosuccinic acid (5 microM), L-citrulline (50 microM), L-methionine (50 microM) and L-ornithine (50 microM) failed to reverse the inhibitory effect of L-NOARG (15 microM). 4. L-NOARG (10-200 microM) and L-NMMA (30-100 microM) inhibited the vasodilator effect of ACh (0.006-18.0 nmol) in the rat mesentery without affecting vasodilatation due to NP (1.1-11.1 nmol). L-Arginine (100 microM) but not D-arginine (100 microM) produced partial reversal of the effect of L-NOARG (30 microM) and L-NMMA (30 microM). 5. L- and D-N'-butyloxycarbonyl No-nitro arginine (100 microM) produced modest (approximately 20%) inhibition of the effect of ACh on the rabbit aorta; this effect was not reversible with L-arginine (100 microM). L-Namonocarbobenzoxy arginine (L-NMCA, 5O microM), L-N-NG-dicarbobenzoxy arginine (L-NDCA, 5 microM) and L-NG-tosyl arginine (50 microM) were inactive. 6. These results identify L-NOARG as a potent, L-arginine reversible inhibitor of endothelium-dependent vasodilatation. The available data suggests that L-NOARG, like L-NMMA, inhibits endothelial nitric oxide (NO) biosynthesis.

IFN-gamma-activated macrophages: detection by electron paramagnetic resonance of complexes between L-arginine-derived nitric oxide and non-heme iron proteins

Interferon-gamma induces the L-Arginine-dependent pathway that leads to the formation of nitrogen oxides in murine macrophages with subsequent inhibition of mitochondrial non-heme iron-dependent enzymes. To evaluate a possible role of nitric oxide through binding to enzymes containing iron-prosthetic groups, we used Electron Paramagnetic Resonance spectroscopy. In IFN-gamma-activated macrophages, we observed the appearance of a signal in the g = 2.04 region which is consistent with that given by nitrosyl-iron-sulfur complexes. Appearance of this signal was dependent on the presence of L-Arginine in the culture medium. Furthermore, we detected a virtually identical signal in macrophages non stimulated by IFN-gamma, following exposure to nitric oxide (after addition of an excess of nitrite in the presence of ascorbate). These data suggest that L-Arginine-derived nitric oxide may alter the configuration of the catalytic site of certain mitochondrial enzymes by coordinating to iron at their iron-sulfur cluster(s).

Pressor effect of NG-nitro-L-arginine in pentobarbital-anesthetized rats

The pressor effect of NG-nitro-L-arginine (L-NNA), a potent inhibitor of nitric oxide (NO) synthesis, was studied in pentobarbital anesthetized rats. Iv injections of L-NNA from 0.25 to 8 mg/kg caused bradycardia and a dose-dependent increase in mean arterial pressure (MAP) with a maximal response of 43 +/- 5 mmHg and ED50 value of 1.3 +/- 0.2 mg/kg. The time course of the response to the injection of a single dose of L-NNA was also determined. Peak response was reached 60 min after the injection of a single dose (4 mg/kg, iv) and the effect lasted greater than 5 h. The rising phase of the pressor response was accompanied by slight bradycardia while the recovery phase was associated with significant tachycardia. Iv injections of L-arginine (12.5-200 mg/kg) caused transient dose-dependent reductions in MAP. The pressor effect of L-NNA (4 mg/kg, iv bolus) was dose-dependently attenuated by L-arginine. The results show that L-NNA is an efficacious and long-acting pressor agent and are consistent with the hypothesis that endogenous NO plays an important role in the regulation of blood pressure.

Role of endothelium-formed nitric oxide on vascular responses

1. Endothelial cells of blood vessels generate factors which can modulate underlying smooth muscle tone, inducing vasorelaxation, (endothelium-derived relaxing factor, EDRF, and endothelium-derived hyperpolarizing factor) and/or vasoconstriction (endothelium-derived contracting factors, EDCFs, including the peptide endothelin). 2. EDRF is nitric oxide (NO) or a RNO compound from which this oxide is released. Its half-life is very short (6-50 sec), and it produces rapid vasodilations and inhibits platelet aggregation. 3. NO is formed from the terminal guanidino of L-arginine, but not of D-arginine. NO effects and NO formation are inhibited by NG-monomethyl-L-arginine (L-NMMA), but not by D-NMMA. These inhibitory effects are blocked by L-arginine. 4. Removal of endothelium or pathological situations that can induce endothelial dysfunction (atherosclerosis, diabetes, hypertension or subarachnoid hemorrhage) cause increases on the vascular contractility elicited by agonists (noradrenaline, serotonin, EDCFs, etc.). These findings suggest that EDRF produces a physiological inhibitory modulation of vascular smooth muscle tone and its alteration produces or facilitates the development of diseases such as hypertension or coronary and cerebral vasospasm.

Neutrophil-derived relaxing factor relaxes vascular smooth muscle through a cGMP-mediated mechanism

Neutrophils harvested from the peritoneal cavities of rats have been shown to release a factor that relaxes precontracted aorta and has a pharmacologic profile similar to that previously reported for endothelium-derived relaxing factor (EDRF). The present study was designed to determine if this neutrophil-derived relaxing factor (NDRF) relaxes rat aortic smooth muscle by affecting the intracellular cGMP levels. Aortic sheets (endothelium removed) were incubated in organ chambers in a physiological salt solution containing phenylephrine (1 x 10(-7) M) and superoxide dismutase (10 or 100 U/ml). Basal cGMP levels (10-15 pmoles/g tissue) were not affected by the incubation reagents. Neutrophils (3 x 10(6) to 1 x 10(8) cells/10 ml) increased cGMP, but not cAMP, levels in a cell number-dependent manner. Peak induction occurred at 5 min of incubation. Methylene blue (1 x 10(-5) M) inhibited and zaprinast (1 x 10(-5) M) potentiated the neutrophil-induced increases in cGMP. The data thus support the hypothesis that neutrophil-induced vascular smooth muscle relaxation is mediated through a factor, NDRF, which increases intracellular cGMP levels.

Endothelium-derived contractile factors

1. Vascular endothelium releases different substances (endothelium-derived contractile factors, EDCFs), which mediate vasoconstrictor responses induced by several agents. 2. Clear differences have been reported in endothelium-dependent contractions, which suggest at least three distinct EDCFs, named EDCF1, EDCF2 and EDCF3, respectively. 3. EDCF1 is a cyclooxygenase metabolite(s) of arachidonic acid. EDCF2 is a polypeptide released from cultured endothelial cells. It has been isolated and identified as a 21-amino acid peptide called endothelin, which is described as the most potent vasoconstrictor agent known to date. EDCF3 is an unidentified contractile factor(s), which is neither EDCF1 nor EDCF2. 4. The physiological role of these endothelial contractile factors is not yet clear. However, they have been implicated in the local mechanisms involved in blood flow regulation, as well as in some pathological conditions, such as hypertension or cerebral vasospasm.

NG-nitro-L-arginine (N5-[imino(nitroamino)methyl]-L-ornithine) impairs endothelium-dependent dilations by inhibiting cytosolic nitric oxide synthesis from L-arginine

We studied the effects of the L-arginine analogue NG-nitro-arginine (L-NNA), in comparison with its D-isomer [D-NNA), on endothelium-dependent dilations of rabbit femoral arteries (RFA) and on the release of endothelium-derived relaxant factor (EDRF) from native and cultured endothelial cells. In addition, we examined the effects of L- and D-NNA on the L-arginine- and NADPH-dependent synthesis of nitric oxide (NO) in the cytosol of porcine aortic endothelial cells. L-NNA enhanced the noradrenaline-induced contraction of endothelium-intact, but not of endothelium-denuded segments of RFA, indicating an inhibition of basal EDRF release. L-NNA also inhibited significantly the endothelium-dependent dilations to acetylcholine (ACh). Both effects of L-NNA were attenuated by L-arginine. L-NNA rapidly inhibited the release of EDRF from cultured and native endothelial cells stimulated with thimerosal or ACh. L-NNA concentration-dependently and reversibly antagonized the L-arginine- and NADPH-dependent activation of a purified soluble guanylate cyclase (GC) by cytosol from freshly harvested porcine aortic endothelial cells, suggesting a direct competition between L-NNA and L-arginine at the level of endothelial NO-synthesis. D-NNA was ineffective in all instances. These results prove L-NNA to be a stereospecific inhibitor of the cytosolic NO formation from L-arginine in endothelial cells. Therefore, L-NNA will be a useful tool to elucidate the molecular mechanism of mammalian NO synthesis.

L-arginine independent macrophage tumor cytotoxicity

We have investigated the role of L-arginine in macrophage tumor cytotoxicity in coculture. L929, EMT-6, MCA-26, and P815 targets were all susceptible to cytolysis by activated macrophages when cocultured in medium containing L-arginine. When cocultured in arginine-free medium, these targets displayed comparable or even higher levels of lysis. L1210 targets were lytically resistant under either condition. However, 59Fe release from this target did reflect strong dependence on the presence of arginine. The structural analogue, NG-monomethyl-L-arginine, was an effective inhibitor of iron-release from L1210 targets cocultured with activated macrophages, whereas it had minimal inhibitory effects on release of 51Cr from cocultured L929 cells. These results suggest that the L-arginine requiring cytotoxic pathway of activated macrophage is independent of major effector mechanisms involved in tumor cell lysis.

The vasodilator role of endogenous nitric oxide in the rat gastric microcirculation

The role of endogenous nitric oxide (NO) in the gastric microcirculation of the anaesthetised rat was investigated using the selective inhibitor of NO synthesis, NG-monomethyl-L-arginine (L-NMMA). L-NMMA (12.5-50 mg kg-1 i.v.) induced a dose-dependent increase in systemic arterial blood pressure (BP) and fall in resting gastric mucosal blood flow (MBF), as estimated by hydrogen-gas clearance. The effects of L-NMMA on BP and MBF were abolished by concurrent administration of L-arginine. The enantiomer D-NMMA had no effect on resting BP or MBF. These findings indicate that endogenous NO, derived from L-arginine, plays a local vasodilator role in the gastric mucosal microvasculature.

Nitric oxide from L-arginine stimulates the soluble guanylate cyclase in adrenal glands

The formation of nitric oxide (NO) by an L-arginine:NO synthase and its stimulation of the soluble guanylate cyclase was studied in rat whole adrenal and bovine cortex and medulla cytosol. In the presence of L-arginine, the stimulation of soluble guanylate cyclase was accompanied by the formation of citrulline and NO2-, formed from NO. The NO synthase was NADPH- and Ca(2+)-dependent and was inhibited by several L-arginine analogues. These results indicate that rat and bovine adrenal cytosol contains an L-arginine:NO synthase.

The cytosol of N1E-115 neuroblastoma cells synthesizes an EDRF-like substance that relaxes rabbit aorta

The cytosolic fraction of N1E-115 neuroblastoma cells catalysed the L-arginine- and NADPH-dependent formation of a substance that relaxed endothelium-denuded strips of rabbit aorta. Relaxations in response to this substance were enhanced in the presence of superoxide dismutase. N omega-Nitro-L-arginine and NG-monomethyl-L-arginine, two inhibitors of EDRF synthesis, markedly attenuated the relaxations. Hemoglobin, a scavenger of EDRF, and methylene blue, an inhibitor of soluble guanylate cyclase, completely abolished the relaxation to N1E-115 cytosol. In contrast, the cyclo-oxygenase inhibitor indomethacin did not alter the relaxations. These data demonstrate that the cytosol of a neuronally-derived cell line is able to synthesize a substance with pharmacological properties similar to EDRF.

Nitric oxide synthesis in endothelial cytosol: evidence for a calcium-dependent and a calcium-independent mechanism

Release of nitric oxide (NO) from endothelial cells critically depends on a sustained increase in intracellular free calcium maintained by a transmembrane calcium influx into the cells. Therefore, we studied whether the free cytosolic calcium concentration directly affects the activity of the NO-forming enzyme(s) present in the cytosol from freshly harvested porcine aortic endothelial cells. NO was quantified by activation of a purified soluble guanylate cyclase co-incubated with the cytosol. In the presence of 1 mM L-arginine, 0.1 mM NADPH and 0.1 mM EGTA, endothelial cytosol (0.2 mg of cytosolic protein per ml) stimulated the activity of guanylate cyclase 5.0 +/- 0.5-fold (from 31 +/- 9 to 153 +/- 15 nmol cyclic GMP formed per min per mg guanylate cyclase). Calcium chloride increased this stimulation further in a concentration-dependent fashion by up to 136 +/- 15% (with 2 microM free calcium; EC50 0.3 microM). The calcium-dependent and -independent activation of guanylate cyclase was enhanced by superoxide dismutase (0.3 microM) and was inhibited by the stereospecifically acting inhibitor of L-arginine-dependent NO formation NG-nitro-L-arginine (1 mM) and by LY 83583 (1 microM), a generator of superoxide anions. Our findings suggest a calcium-dependent and -independent synthesis of NO from L-arginine by native porcine aortic endothelial cells.

The effects of L-arginine and NG-monomethyl L-arginine on the response of the rat anococcygeus muscle to NANC nerve stimulation

The effect of the competitive inhibitor of L-arginine, NG-monomethyl L-arginine (L-NMMA) on the response of the rat anococcygeus muscle to non-adrenergic, non-cholinergic (NANC) inhibitory nerve stimulation has been examined. L-NMMA causes a rise in muscle tone and inhibition of the response to nerve stimulation. The stereoisomer D-NMMA is without effect. The rise in tone and inhibition of the nerve response is reversed by L-arginine. Another analogue, L-canavanine, which is effective against L-arginine utilization in the macrophage, was without effect on the rat anococcygeus. These results provide indirect evidence for nitric oxide (NO) or a substance releasing NO as the transmitter of the NANC nerves in this tissue.

Depletion of arterial L-arginine causes reversible tolerance to endothelium-dependent relaxation

This study examined the influence of lowered arterial levels of L-arginine on endothelium-dependent relaxation of isolated rings of bovine pulmonary artery. Incubation of arterial rings under tension for 24 hr in oxygenated Krebs bicarbonate solution at 37 degrees C resulted in the development of marked or complete tolerance to A23187 (calcium ionophore)- and acetylcholine-elicited relaxation. Relaxant responses to nitric oxide were unaffected. Addition of L-arginine did not relax control rings but did elicit marked endothelium-dependent relaxation of tolerant rings that was inhibited by oxyhemoglobin or methylene blue. L-Arginine also restored acetylcholine-elicited relaxation. Inclusion of L-canavanine in the 24 hr incubations protected against the development of tolerance. The tissue concentration of arginine was 3-fold lower in tolerant than control arterial rings and L-canavanine restored arterial arginine levels to control values. Therefore, depletion of arterial L-arginine causes reversible tolerance to endothelium-dependent relaxation.

Effects of endothelium-derived nitric oxide on peripheral arteriolar tone in man

NG monomethyl-L-arginine (L-NMMA), a specific inhibitor of the synthesis of endothelium-derived nitric oxide (NO), was infused into the brachial arteries of healthy volunteers to study the role of NO in the control of forearm blood flow. L-NMMA caused a 50% fall in basal blood flow and attenuated the dilator response to infused acetylcholine but not that to glyceryl trinitrate. These results indicate that the dilator action of endothelium-derived NO contributes to the control of basal and stimulated regional blood flow in man. Impairment of production of NO might account for the abnormalities in vascular reactivity that characterise a wide variety of disease states.

NMDA receptor activation induces nitric oxide synthesis from arginine in rat brain slices

Activation of N-methyl-D-aspartate (NMDA) receptors in rat cerebellum leads to the release of endothelium-derived relaxing factor, now identified as nitric oxide (NO), a stimulator of soluble guanylate cyclase. L-NG-monomethylarginine (L-NMMA), which blocks NO synthesis from L-arginine in several tissues, including a crude synaptosomal preparation from brain, inhibited the elevation of cyclic GMP induced by NMDA in rat cerebellar slices. D-NMMA was ineffective. L-Arginine, but not its D enantiomer, augmented the response to NMDA and reversed the inhibition by L-NMMA. The results indicate that stimulation of NMDA receptors results in the activation of the enzyme which catalyzes the formation of NO from L-arginine.

Cyclic GMP stimulation by vasopressin in LLC-PK1 kidney epithelial cells is L-arginine-dependent

The L-arginine antagonist NG-monomethyl-L-arginine has been shown to inhibit nitric oxide formation from L-arginine in endothelial cells. In the present study NG-monomethyl-L-arginine was used to assess the role of L-arginine for cyclic GMP stimulation by vasopressin in a kidney epithelial cell line (LLC-PK1). Preincubation of cells with 1 mumol/l, 10 mumol/l and 100 mumol/l NG-monomethyl-L-arginine decreased cyclic GMP stimulation at 1 mumol/l vasopressin by 25%, 71% and 90%, respectively. This inhibition by NG-monomethyl-L-arginine was markedly reduced by L-arginine (2 mmol/l) but not D-arginine (2 mmol/l). Cyclic GMP stimulation by the calcium ionophore A23187 was also inhibited by NG-monomethyl-L-arginine and enantioselectively restored by L-arginine. However, NG-monomethyl-L-arginine did not affect cyclic GMP stimulation by sodium nitroprusside that spontaneously releases nitric oxide. These results suggest that, in kidney epithelial cells, vasopressin induces nitric oxide formation from L-arginine leading to activation of soluble guanylate cyclase. It is concluded that nitric oxide formation from L-arginine is not only responsible for endothelium-dependent relaxation but may be a more general pathway with regulatory function for intracellular guanylate cyclase activity.

Modulation of the vasodepressor actions of acetylcholine, bradykinin, substance P and endothelin in the rat by a specific inhibitor of nitric oxide formation

1. The effects of the specific inhibitor of nitric oxide (NO) formation, NG-monomethyl-L-arginine (L-NMMA), on resting systemic arterial blood pressure (BP) and on the actions of both endothelium-dependent and endothelium-independent vasodilators were investigated in the anaesthetized, normotensive rat. 2. Intravenous administration of L-NMMA (12.5-50 mg kg-1; 47-188 mumol kg-1) but not its enantiomer, D-NMMA, induced a dose-related increase in BP, which was reversed by the intravenous administration of L-arginine (150-600 mumol kg-1), but not D-arginine. 3. The vasodepressor responses to intravenous administration of the endothelium-dependent vasodilators, acetylcholine, bradykinin and substance P were significantly inhibited by L-NMMA (94 and 188 mumol kg-1 i.v.), but not by D-NMMA. 4. The inhibition by L-NMMA of these vasodepressor responses was reversed by administration of L-arginine, but not D-arginine. 5. Endothelin (ET-1) induced dose-related vasodepressor responses following bolus intravenous administration, which were significantly inhibited by L-NMMA but not by D-NMMA. This inhibition was reversed by administration of L-arginine. 6. The vasodepressor effects of the endothelium-independent vasodilators, glyceryl trinitrate or prostacyclin, were not significantly inhibited by L-NMMA. 7. These findings with L-NMMA suggest that resting blood pressure in the rat is modulated by endogenous NO biosynthesis and that endothelium-dependent vasodilators act through the formation of endogenous NO to exert their actions in vivo.

L-arginine availability determines the duration of acetylcholine-induced systemic vasodilation in vivo

In vitro studies have shown that acetylcholine-induced vasorelaxation is mediated by endothelium-derived relaxing factor/nitric oxide (EDRF/NO). EDRF/NO is synthesized from L-arginine by an enzymatic pathway that is inhibited by L-NG-methylarginine. To assess whether EDRF/NO also mediates the vasodilating action of acetylcholine in vivo, we have investigated the effect of L-arginine and L-NG-methylarginine on the hypotensive response to acetylcholine in the anesthetized guinea pig. L-arginine prolonged the duration of the depressor response to acetylcholine and L-NG-methylarginine decreased it. However, neither L-arginine nor L-NG-methylarginine modified the magnitude of acetylcholine's hypotensive effect unless the blood pressure was previously elevated by infusion with norepinephrine. Thus, de novo synthesis of nitric oxide from L-arginine contributes importantly, but not exclusively, to acetylcholine's hypotensive effect in the guinea pig. Furthermore, the concentration of circulating L-arginine may influence the duration and magnitude of acetylcholine-induced depressor responses under normotensive and hypertensive conditions.

Hydroxylamine is a vasorelaxant and a possible intermediate in the oxidative conversion of L-arginine to nitric oxide

Our objective was to determine whether hydroxylamine is a possible intermediate in the oxidative conversion of L-arginine to nitric oxide. Vasorelaxation by hydroxylamine is known to be mediated by nitric oxide. The vasorelaxant properties of hydroxylamine were examined using rat aortic rings and an isolated rat lung perfusion model. Hydroxylamine and acetylcholine were equally effective in relaxing norepinephrine-contracted intact aortic rings, whereas only hydroxylamine relaxed aortic rings with endothelium removed. This endothelium-independent vasorelaxation by hydroxylamine indicated that the hydroxylamine-converting enzyme is not localized solely within endothelial cells. Catalase, an enzyme known to oxidize hydroxylamine to nitric oxide, was present in homogenates of intact and endothelium-denuded rings. Cyanamide, another catalase substrate and a known precursor of nitroxyl (HNO), was not a vasorelaxant of aortic rings or of isolated, hypoxia-constricted lungs. These results suggest that free nitroxyl is not an intermediate in the oxidation of hydroxylamine to nitric oxide. An overall pathway for the oxidative conversion of L-arginine through an hydroxylamine intermediate to nitric oxide is proposed.

Human neutrophils and mononuclear cells inhibit platelet aggregation by releasing a nitric oxide-like factor

Incubation of neutrophils or mononuclear cells with washed platelets (all prepared from human venous blood) resulted in an inhibition of thrombin-induced platelet aggregation that was dependent on the number of nucleated cells added. The inhibition was potentiated by superoxide dismutase and reversed by oxyhemoglobin. In the case of neutrophils the inhibition was associated with an increase in cGMP, whereas with mononuclear cells both cAMP and cGMP were increased. The inhibitory activity of neutrophils or mononuclear cells was prevented by their preincubation with NG-monomethyl-L-arginine methyl ester. L-Arginine reversed the action of NG-monomethyl-L-arginine methyl ester, whereas D-arginine was ineffective. Preincubation of the cells with catalase or mannitol did not prevent their inhibitory action on platelet aggregation. The inhibition of platelet aggregation was not due to platelet damage or to uptake of thrombin by neutrophils or mononuclear cells. It was overcome by increasing the concentration of thrombin and was absent in cell-free supernatants obtained from a suspension of neutrophils or mononuclear cells or from mixtures of platelets with neutrophils or platelets with mononuclear cells. These data provide evidence for the release of a nitric oxide-like factor from human neutrophils and mononuclear cells. In addition, evidence is provided that, as in stimulated murine macrophages and endothelial cells, the precursor of this factor is L-arginine.

Nitric oxide synthesized from L-arginine regulates vascular tone in the coronary circulation of the rabbit

1. The role of nitric oxide (NO) in the regulation of the vascular tone of the coronary circulation of the Langendorff-perfused rabbit heart was investigated. 2. NG-monomethyl-L-arginine (L-NMMA; 10-100 microM), a specific inhibitor of NO formation from L-arginine (L-Arg), but not its D-enantiomer (D-NMMA; 100 microM) produced a dose-related, sustained increase in the coronary perfusion pressure (CPP). In addition, L-NMMA inhibited the vasodilator responses of acetylcholine (ACh), unmasking in some instances its direct vasoconstrictor effect. These effects of L-NMMA were attenuated by L-Arg. 3. L-NMMA (10 and 30 microM), but not D-NMMA (30 microM), caused a long-lasting inhibition of NO formation which was reversed by L-Arg (30 and 100 microM), but not by D-Arg (100 microM). 4. This study indicates that the formation of NO from L-Arg in the coronary circulation of the rabbit plays a role both as a regulator of vascular tone and as a mediator of the vasodilatation induced by ACh.

L-arginine causes whereas L-argininosuccinic acid inhibits endothelium-dependent vascular smooth muscle relaxation

This study examined the actions of L-arginine, a putative precursor of endothelium-derived nitric oxide, and arginine analogs on endothelium-dependent relaxation of isolated rings of bovine pulmonary artery. L-Arginine did not consistently relax arterial rings unless rings were first rendered refractory to endothelium-dependent relaxation by pretreatment with 1 microM A23187 for 45 min. L-Arginine-elicited relaxation was endothelium-dependent, antagonized by oxyhemoglobin or methylene blue, and unaffected by indomethacin. L-Argininosuccinic acid caused endothelium-dependent contractions and irreversible inhibition of endothelium-dependent but not nitroglycerin-elicited relaxation, which was not overcome by addition of L-arginine. Inhibition of endothelium-dependent relaxation by L-NG-monomethyl arginine, however, was reversible and overcome by L-arginine. Therefore, endothelium-dependent relaxants may cause arginine depletion in endothelial cells and endogenous argininosuccinic acid may modulate the biosynthesis of endothelium-derived nitric oxide from arginine.

Role of endothelium-derived nitric oxide in the regulation of blood pressure

The role of endothelium-derived nitric oxide in the regulation of blood pressure in the anesthetized rabbit was studied with N omega-monomethyl-L-arginine (L-NMMA), a specific inhibitor of its formation from L-arginine. L-NMMA (3-100 mg.kg-1), but not its D-enantiomer, induced a dose-dependent long-lasting (15-90 min) increase in mean systemic arterial blood pressure. L-NMMA (100 mg.kg-1) also inhibited significantly the hypotensive action of acetylcholine, without affecting that of glyceryl trinitrate. Both these actions of L-NMMA were reversed by L-arginine (300 mg.kg-1), but not by D-arginine (300 mg.kg-1), indomethacin (1 mg.kg-1), prazosin (0.3 mg.kg-1), or by vagotomy. The effects of L-NMMA in vivo were associated with a significant inhibition of the release of nitric oxide from perfused aortic segments ex vivo. This inhibition was reversed by infusing L-arginine through the aortic segments. These results indicate that nitric oxide formation from L-arginine by the vascular endothelium plays a role in the regulation of blood pressure and in the hypotensive actions of acetylcholine.