Reduced urinary excretion of nitric oxide metabolites and increased plasma levels of asymmetric dimethylarginine in men with essential hypertension

The G894-T894 Polymorphism in the Gene for Endothelial Nitric Oxide Synthase and Blood Pressure in Lead-Exposed Workers From Korea

We evaluated whether the G -T polymorphism in exon 7 of the endothelial nitric oxide synthase (eNOS) gene is associated with blood pressure or modifies the relation between lead dose and blood pressure in 803 lead workers in Korea. A total of 84.9% of individuals were homozygous GG, 14.4% heterozygous GT, and 0.8% homozygous TT. The T allele was not significantly associated with systolic or diastolic blood pressure. The prevalence of hypertension did not differ by T status (OR = 0.82; 95% CI = 0.50-1.37). There was no evidence of effect modification by eNOS genotype on relations of lead dose with blood pressure. These data provide no evidence that the T allele is associated with higher blood pressure or modifies the association of lead dose with blood pressure.

Plasma asymmetric dimethylarginine modifies the effect of pravastatin on myocardial blood flow in young adults

Elevated plasma levels of the endogenous nitric oxide synthase inhibitor asymmetric dimethylarginine (ADMA) are related to decreased myocardial vasodilatory capacity and increased risk of acute coronary events. As statin treatment is known to increase nitric oxide bioavailability and enhance myocardial function, we tested whether ADMA concentration modifies the effect of pravastatin on myocardial blood flow in young adults with mild hypercholesterolemia. Fifty-one men (35 +/- 4 years) were randomly assigned to receive either pravastatin (40 mg/day) or placebo for 6 months. Myocardial blood flow was measured at rest and during adenosine-induced hyperemia using positron emission tomography and oxygen-15-labeled water at baseline and after treatment. Plasma ADMA levels were assessed with high performance liquid chromatography. Low baseline plasma ADMA concentration (< median) predicted a significant improvement of adenosine-induced blood flow after statin intervention (baseline to follow-up change +35%, p = 0.004), whereas high baseline ADMA (> or = median) was associated with no increase in adenosine-induced flow.

Asymmetrical dimethylarginine (ADMA) and risk of acute coronary events. Does statin treatment influence plasma ADMA levels?

The purpose of this study was to evaluate the hypothesis that high serum levels of ADMA, an indicator of endothelial dysfunction, are associated with an elevated risk of acute coronary events in middle-aged men. To test the hypothesis that lipid lowering medication with statins lowers circulating ADMA levels, we also investigated the effect of simvastatin and atorvastatin treatment on plasma ADMA concentration. In a prospective nested case-control study in 150 middle-aged non-smoking men from Eastern Finland, those who were in the highest quartile for serum ADMA (>0.62 micromol/l) had a 3.9-fold (95% CI: 1.25-12.3, P=0.02) increase in risk of acute coronary events compared with other quartiles. In an 8-week randomised double-blind placebo-controlled trial, plasma ADMA concentrations remained unchanged in simvastatin 80 mg/day (n=16), atorvastatin 40 mg/day (n=16) and placebo (n=16) groups over the study period. Our findings indicate that high serum levels of ADMA, a potential marker for endothelial dysfunction, may increase the risk of acute coronary syndromes. However, aggressive treatment with either simvastatin or atorvastatin did not reduce plasma ADMA levels.

ADMA and oxidative stress

Elevated plasma concentrations of the endogenous nitric oxide synthase (eNOS) inhibitor asymmetric dimethylarginine (ADMA) represent a novel risk factor for the development of endothelial dysfunction and a predictor for all-cause and cardiovascular mortality. However, it is unknown whether elevated ADMA plasma concentrations may be considered simply as a marker for cardiovascular disease or whether increased ADMA levels per se may predispose to the development of vascular disease. There is experimental and clinical evidence linking endothelial dysfunction to increased production of oxygen-derived free radicals such as superoxide anion. Oxidative stress has been shown to increase the activity of arginine methylating and ADMA degrading enzymes leading to increased ADMA concentrations. Interestingly, the endothelial nitric oxide synthase may become uncoupled in the presence of high ADMA levels further contributing to the vascular oxidative stress burden. It remains to be established to what extent ADMA is able to interact with eNOS in vivo. Possible mechanisms underlying increased oxidative stress in the setting of elevated ADMA concentrations and therapeutic implications will be discussed.

Long-chain polyunsaturated fatty acids interact with nitric oxide, superoxide anion, and transforming growth factor-beta to prevent human essential hypertension

Patients with uncontrolled essential hypertension have elevated concentrations of superoxide anion (O(2)(-*)), hydrogen peroxide (H(2)O(2)), lipid peroxides, endothelin, and transforming growth factor-beta (TGF-beta) with a simultaneous decrease in endothelial nitric oxide (eNO), superoxide dismutase (SOD), vitamin E, and long-chain polyunsaturated fatty acids (LCPUFAs). Physiological concentrations of angiotensin II activate NAD(P)H oxidase and trigger free radical generation (especially that of O(2)(-*)). Normally, angiotensin II-induced oxidative stress is abrogated by adequate production and release of eNO, which quenches O(2)(-*) to restore normotension. Angiotensin II also stimulates the production of endothelin and TGF-beta. TGF-beta enhances NO generation, which in turn suppresses TGF-beta production. Thus, NO has a regulatory role on TGF-beta production and is also a physiological antagonist of endothelin. Antihypertensive drugs suppress the production of O(2)(-*) and TGF-beta and enhance eNO synthesis to bring about their beneficial actions. LCPUFAs suppress angiotensin-converting enzyme (ACE) activity, reduce angiotensin II formation, enhance eNO generation, and suppress TGF-beta expression. Perinatal supplementation of LCPUFAs decreases insulin resistance and prevents the development of hypertension in adult life, whereas deficiency of LCPUFAs in the perinatal period results in raised blood pressure later in life. Patients with essential hypertension have low concentrations of various LCPUFAs in their plasma phospholipid fraction. Based on this, it is proposed that LCPUFAs serve as endogenous regulators of ACE activity, O(2)(-*), eNO generation, and TGF-beta expression. Further, LCPUFAs have actions similar to statins, inhibit (especially omega-3 fatty acids) cyclooxygenase activity and suppress the synthesis of proinflammatory cytokines, and activate the parasympathetic nervous system, all actions that reduce the risk of major vascular events. Hence, it is proposed that availability of adequate amounts of LCPUFAs during the critical periods of growth prevents the development of hypertension in adulthood.

Asymmetrical dimethylarginine: a novel risk factor for coronary artery disease

BACKGROUND

Asymmetrical dimethylarginine (ADMA) is an endogenous competitive inhibitor of nitric oxide synthase and has been associated with systemic atherosclerosis; however, the role of ADMA in patients with coronary artery disease (CAD) has not been investigated.

HYPOTHESIS

The present study was designed to determine whether the plasma ADMA level predicts the presence of CAD independently, and whether the plasma ADMA level correlates with the extent and severity of coronary atherosclerosis.

METHODS

In all, 97 consecutive patients with angina and positive exercise stress test were enrolled prospectively for coronary angiography. According to the result of angiography, the subjects were divided into two groups: Group I (n = 46): patients with normal coronary artery or mild CAD (< 50% stenosis of major coronary arteries); Group 2 (n = 51): patients with significant CAD (> or = 50% stenosis of majorcoronary arteries). Plasma levels of ADMA and L-arginine were determined by high-performance liquid chromatography. In addition, we used coronary atherosclerotic score to assess the extent and severity of CAD.

RESULTS

The plasma levels of ADMA in Group 2 patients were significantly higher than those in Group 1 patients (0.66 +/- 0.17 microM vs. 0.44 +/- 0.09 microM, p < 0.001); these were accompanied by significantly lower plasma L-arginine/ADMA ratio in patients with significant CAD (Group 1 vs. 2: 194.0 +/- 55.3 vs. 136.7 +/- 50.3, p < 0.001). In a multivariate stepwise logistic regression analysis, both plasma ADMA level and plasma L-arginine/ADMA ratio were identified as independent predictors for CAD. Moreover, there were significant positive and negative correlations between coronary atherosclerotic score and plasma ADMA level as well as plasma L-arginine/ADMA ratio, respectively (plasma ADMA level: r = 0.518, p < 0.001; L-arginine/ADMA ratio: r = -0.430, p < 0.001).

CONCLUSIONS

Both plasma ADMA level and plasma L-arginine/ADMA ratio were useful in predicting the presence of significant CAD and correlated significantly with the extent and severity of coronary atherosclerosis. Our findings suggest that plasma ADMA level may be a novel marker of CAD.

Demonstrating the dose- and time-related effects of 7-nitroindazole on picrotoxin-induced convulsions, memory formation, brain nitric oxide synthase activity, and nitric oxide concentration in rats

In this study, the dose (50, 100, 150, and 200 mg/kg)- and time (30 and 60 min)- related effects of 7-nitroindazole (7-NI), a neuronal specific inhibitor of nitric oxide synthase (NOS) were tested on picrotoxin (5 mg/kg)-induced convulsions and memory formation in rats. The changes produced by these doses of 7-NI were determined on NOS activity and nitric oxide (NO) concentration in the brain. The effects of 7-NI were tested in animals pretreated (30 min) with L-arginine (500 and 1000 mg/kg). 7-NI, at 50 and 100 mg/kg, did not produce significant changes in NOS activity and NO concentration in the brain and memory formation. However, the convulsant action of picrotoxin was inhibited in a dose-dependent manner in these animals. A time-dependent decrease in the activity of NOS and the concentration of NO, a promotion of picrotoxin-induced convulsions, and an impairment of memory were found in animals treated with 150 and 200 mg/kg of 7-NI. The larger and not the smaller dose of L-arginine raised the concentration of NO, inhibited picrotoxin-induced convulsions and promoted memory process. Either dose of L-arginine failed to prevent 50 and 100 mg/kg of 7-NI from inhibiting convulsions. The effects of the larger doses of 7-NI (150 and 200 mg/kg) were effectively prevented by the increase of NO and not the ineffective dose of L-arginine. These results suggest that 7-NI (50 and 100 mg/kg) decreases convulsions by a nonspecific mechanism and that an inhibition of NOS by the larger doses of it (150 and 200 mg/kg) results in proconvulsant action and memory impairment. The data further show that the margin between the protective and proconvulsant doses of 7-NI is relatively narrow. These results have been taken together with the earlier reports that 7-NI produces learning impairment and fails to increase the anticonvulsant effect of traditional antiepileptic agents on experimentally induced convulsions to conclude that 7-NI can never emerge as an anticonvulsant agent for clinical use.

Vascular endothelium is the organ chiefly responsible for the catabolism of plasma asymmetric dimethylarginine – an explanation for the elevation of plasma ADMA in disorders characterized by endothelial dysfunction

Plasma levels of asymmetric dimethylarginine (ADMA), an endogenously produced competitive inhibitor of nitric oxide synthase (NOS), have been found to be elevated in a large number of disorders characterized by endothelial dysfunction; this remarkable phenomenon has yet to receive a plausible explanation. ADMA arises by proteolysis of methylated proteins throughout the body; the majority of this ADMA is catabolized by the enzyme dimethylarginine dimethylaminohydrolase (DDAH), found in many tissues, including those that express NOS. Since the production of ADMA can be considered constitutive, and little intact ADMA emerges in the urine, impaired catabolism is most likely responsible for elevations of plasma ADMA. The association of elevated ADMA with endotheliopathy is readily explained if we assume that vascular endothelium is the organ chiefly responsible for the catabolism of plasma ADMA--a view that is credible owing to the privileged access of endothelium to plasma, the capacity of endothelium for active transport of arginine (and methylated arginines), and the ample DDAH activity of healthy endothelial cells--and further assume that endothelial dysfunction is often attended by a loss of DDAH activity and/or an impairment of arginine transport, reducing the efficiency of ADMA catabolism. Indeed, there is recent evidence that DDAH is inhibited by endothelial oxidative stress, a typical feature of endotheliopathy; there is also some reason to suspect that arginine transport may be less efficient in dysfunctional endothelium. From this perspective, increased plasma ADMA is not the primary cause of the endothelial dysfunction in various disorders, but rather its effect--though the rise in ADMA can then exacerbate this dysfunction by inhibiting endothelial NOS. Supplemental arginine should be of some clinical benefit in disorders characterized by elevated ADMA, since it can offset that adverse impact of ADMA on NOS activity, and possibly exert other beneficial effects on endothelium--but it cannot be expected to reverse the primary cause of the endothelial dysfunction. Whether or not ADMA plays an important pathogenic role, it seems likely to emerge as a potent risk factor for adverse vascular events, since it may be viewed as a barometer of endothelial health.

Old and new cardiovascular risk factors: from unresolved issues to new opportunities

With the aim of identifying areas that may deserve some further thinking the present review deliberately points out controversial issues in cardiovascular research and risk assessment. In the first part of the review general aspects are addressed regarding the evaluation of risk factors. A first point of concern is the frequent practice of combining different vascular events and effects in different vascular beds into a single endpoint. Furthermore, verification of vascular events in clinical reality may be surprisingly inaccurate. Problems in risk assessment also arise from overlapping properties (shared pathophysiological pathways) of traditional risk factors like hypertension, obesity and diabetes. In the second part of the review unresolved issues concerning selected established and emerging risk factors are discussed. The difficulty of establishing causality in cardiovascular disease is addressed, using modification of LDL cholesterol and accumulating evidence for pleiotropic effects of the LDL cholesterol-lowering statins as an example. As an alternative or supplement to the notion of LDL-related cardiovascular risk it is proposed to distinguish between statin-sensitive and statin-insensitive cardiovascular risk. Finally the future prospects of selected new and emerging risk factors like CRP, homocysteine, asymmetrical dimethylarginine (ADMA), oxLDL, and isoprostanes are evaluated. In summary, imprecise terminology and varying definitions of "cardiovascular risk" may lead to a considerable blurring of our current risk estimates, which may also explain some presently controversial issues. With several new risk factors and substantial changes in lifestyle and treatment patterns on the horizon major changes in the hierarchy of risk factors may be inevitable.

Increased concentrations of homocysteine and asymmetric dimethylarginine and decreased concentrations of nitric oxide in the plasma of patients with Alzheimer's disease

Vascular risk factors increase the risk of developing Alzheimer's disease. Increased concentrations of circulating homocysteine are associated with an increased risk of both vascular disease and Alzheimer's disease. Asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of nitric oxide synthase. There is an increase in the concentration of ADMA in the circulation in vascular disease. We measured the concentrations of homocysteine, ADMA and nitric oxide (as nitrate and nitrite) in the plasma of 25 patients with Alzheimer's disease and 25 control subjects. There was a highly significant increase in the plasma concentration of homocysteine (P<0.001) and ADMA (P<0.0001) and a highly significant decrease in the plasma concentration of nitric oxide (P<0.0001) among the Alzheimer's patients. In the combined patient and control groups a highly significant positive correlation was found between the plasma concentrations of homocysteine and ADMA (r=0.782, P<0.0001). In addition, significant negative correlations were detected between the plasma concentration of nitric oxide and the plasma concentration of homocysteine (r=-0.592, P<0.0001) and ADMA (r=-0.789, P<0.0001). These significant correlations were found to persist, even when they were restricted to the Alzheimer's patients. The inhibition of endothelial nitric oxide synthesis by ADMA impairs cerebral blood flow, which may contribute to the development of Alzheimer's disease. Endothelial dysfunction is also associated with atherosclerosis and stroke, which are important risk factors for Alzheimer's disease. Inflammation plays an important role in Alzheimer's disease and the inhibition of endothelial nitric oxide by ADMA may increase the concentration of inflammatory mediators in the brain. The inhibition of neuronal nitric oxide synthesis by ADMA may cause cognitive dysfunction in Alzheimer's disease.

Effect of shear stress on asymmetric dimethylarginine release from vascular endothelial cells

We demonstrated recently that plasma concentrations of asymmetric dimethylarginine (ADMA), an endogenous inhibitor of nitric oxide (NO) synthase, are increased by high salt intake concomitantly with a decrease in plasma levels of NO in human hypertension. We investigated the effect of shear stress on ADMA release in 2 types of cells: transformed human umbilical vein endothelial cells (HUVECs; cell line ECV-304) and HUVECs. Exposure of ECV-304 cells and HUVECs to shear stress with the use of a cone-plate viscometer enhanced gene expression of protein arginine methyltransferase (PRMT-1), ADMA synthase. In HUVECs, the ratio of PRMT-1 to glyceraldehyde 3-phosphate dehydrogenase mRNA was increased by 2-fold by a shear stress of > or =15 dyne/cm2. A dominant-negative mutant of IkappaB kinase alpha and troglitazone at 8 micromol/L, an activator of peroxisome proliferator-activated receptor gamma, abolished the shear stress-induced increase in PRMT-1 gene expression in parallel with the blockade of nuclear factor (NF)-kappaB translocation into the nucleus. The activity of dimethylarginine dimethylaminohydrolase, the degradation enzyme of ADMA, was unchanged after shear stress < or =15 dyne/cm2 and was enhanced by 1.48+/-0.06-fold (P<0.05) by shear stress at 25 dyne/cm2. The release of ADMA was increased by 1.64+/-0.10-fold (P<0.05) by shear stress at 15 dyne/cm2 but was not affected by shear stress at 25 dyne/cm2. These results indicate that shear stress enhances gene expression of PRMT-1 and ADMA release via activation of the NF-kappaB pathway. Shear stress at higher magnitudes facilitates the degradation of ADMA, thus returning ADMA release levels to baseline.

Age-related endothelial dysfunction : potential implications for pharmacotherapy

Aging per se is associated with abnormalities of the vascular wall linked to both structural and functional changes that can take place at the level of the extracellular matrix, the vascular smooth muscle and the endothelium of blood vessels. Endothelial dysfunction is generally defined as a decrease in the capacity of the endothelium to dilate blood vessels in response to physical and chemical stimuli. It is one of the characteristic changes that occur with age, independently of other known cardiovascular risk factors. This may account in part for the increased incidence of cardiovascular events in elderly people that can be reversed by restoring endothelial function. A better understanding of the mechanisms involved and the aetiopathogenesis of this process will help in the search for new therapeutic agents.Age-dependent alteration of endothelium-dependent relaxation seems to be a widespread phenomenon both in conductance and resistance arteries from several species. In the course of aging, there is an alteration in the equilibrium between relaxing and contracting factors released by the endothelium. Hence, there is a progressive reduction in the participation of nitric oxide and endothelium-derived hyperpolarising factor associated with increased participation of oxygen-derived free radicals and cyclo-oxygenase-derived prostanoids. Also, the endothelin-1 and angiotensin II pathways may play a role in age-related endothelial dysfunction. The use of drugs acting at different levels of these signalling cascades, including antioxidant therapy, lipid-lowering drugs and estrogens, seems to be promising.

Elevated levels of the serum endogenous inhibitor of nitric oxide synthase and metabolic control in rats with streptozotocin-induced diabetes

This study was designed to determine the relationship between elevated levels of the endogenous inhibitor of nitric oxide synthase, asymmetric dimethylarginine (ADMA), and metabolic control in rats with streptozotocin-induced diabetes. Serum levels of ADMA were measured by high-performance liquid chromatography at 8 weeks after diabetes was induced. Endothelium-dependent relaxation to acetylcholine was tested in aortic rings from nondiabetic age-matched control, untreated diabetic, and insulin-treated diabetic rats to evaluate endothelial function. Serum concentrations of glucose, glycosylated serum protein, and malondialdehyde were examined to estimate metabolic control. Serum levels of ADMA increased dramatically in untreated diabetic rats compared with control rats. This elevation in ADMA levels was accompanied by impairment of the endothelium-dependent relaxation response to acetylcholine in aortic rings. Long-term insulin treatment not only prevented the elevation of serum ADMA levels, but also improved the impairment of endothelium-dependent relaxation in diabetic rats. Serum levels of glucose, glycosylated serum protein, and malondialdehyde were significantly increased in parallel with the elevation of ADMA in untreated diabetic rats compared with control rats. These parameters were normalized after diabetic rats received insulin treatment for 8 weeks. These results provide the first evidence that an elevation in the concentration of ADMA in rats with streptozotocin-induced diabetes is closely related to metabolic control of the disease.

Insulin resistance, hypertension, and coronary heart disease

The goals of this review are two-fold: to examine the evidence in support of a role for insulin resistance and compensatory hyperinsulinemia in the pathogenesis of essential hypertension, and to evaluate the hypothesis that insulin resistance and its manifestations play major roles in the development of coronary heart disease in patients with essential hypertension. In both instances, only experimental results in human beings will be considered. Although it remains a scientific issue of great importance, the scope of this review precludes a discussion of the mechanistic link between insulin resistance/hyperinsulinemia and essential hypertension.

High-performance liquid chromatographic assay of asymmetric dimethylarginine, symmetric dimethylarginine, and arginine in human plasma by derivatization with naphthalene-2,3-dicarboxaldehyde

Asymmetric dimethylarginine (ADMA) is an emerging cardiovascular risk factor. Its increased levels have been hypothesized to be a cause of endothelial dysfunction in pathological conditions such as hypertension, dyslipidemia, renal failure, hyperglycemia, and hyperhomocysteinemia. It acts as a potent competitive inhibitor of nitric oxide synthase. Methods using ortho-phthaldialdehyde (OPA) as derivatization reagent are widely performed in HPLC determination of ADMA, but they produce derivatives whose fluorescence rapidly decreases during time. Moreover, these methods do not allow a clear separation of ADMA from its stereoisomer symmetric dimethylarginine (SDMA). Our work describes a new method to determine ADMA, SDMA, and arginine that uses, as derivatizing reagent, naphthalene-2,3-dicarboxaldehyde (NDA). Chromatograms with low background, showing a complete separation of ADMA and SDMA, are obtained. NDA derivatives are considerably more stable than the OPA derivatives. The calibration curves of ADMA and SDMA are linear within the range of 0.01-16.0 microM. Coefficients of variation are less than 1.7% for within day and less then 2.3% for day to day. Absolute mean recoveries from supplemented samples are between 100 and 104%. These characteristics make this method reliable and easily manageable for large routine analyses.

Hyperhomocysteinaemia is not associated with increased levels of asymmetric dimethylarginine in patients with ischaemic heart disease

BACKGROUND

Elevated plasma total homocysteine appears to be related to endothelial dysfunction and impaired nitric oxide production. We aimed to investigate [1] whether elevated levels of plasma total homocysteine are associated with high plasma levels of asymmetric dimethylarginine, an endogenous inhibitor of nitric oxide synthase, and [2] whether reduction of plasma total homocysteine levels by folate and vitamin B supplementation lowers plasma concentration of asymmetric dimethylarginine.

MATERIALS AND METHODS

Sixty patients with ischaemic heart disease and with plasma total homocysteine levels of 15.0 micromol L-1 were randomized to open therapy with folic acid, pyridoxine and cyancobalamin for 3 months (n = 30) or to no treatment (n = 30). Samples were also obtained from 34 patients with plasma total homocysteine levels of 8.0 micromol L-1 on admission.

RESULTS

Plasma asymmetric dimethylarginine concentrations in patients with elevated total homocysteine levels were not significantly higher (0.68 +/- 0.19 micromol L-1) than in patients with low total homocysteine levels (0.61 +/- 0.10 micromol L-1; P = 0.08). Plasma asymmetric dimethylarginine level in the vitamin supplemented group was 0.65 +/- 0.12 micromol L-1 before, and 0.64 +/- 0.12 micromol L-1 after 3 months of vitamin supplementation (NS). Plasma asymmetric dimethylarginine levels were correlated with serum cystatin C levels (P < 0.001).

CONCLUSION

A nonsignificant trend to increased plasma levels of asymmetric dimethylarginine in patients with high plasma total homocysteine levels may be explained by concomitant subtle renal dysfunction. Substantial reduction of plasma total homocysteine did not affect the level of plasma asymmetric dimethylarginine.

The pharmacology of nitric oxide in the peripheral nervous system of blood vessels

Unanticipated, novel hypothesis on nitric oxide (NO) radical, an inorganic, labile, gaseous molecule, as a neurotransmitter first appeared in late 1989 and into the early 1990s, and solid evidences supporting this idea have been accumulated during the last decade of the 20th century. The discovery of nitrergic innervation of vascular smooth muscle has led to a new understanding of the neurogenic control of vascular function. Physiological roles of the nitrergic nerve in vascular smooth muscle include the dominant vasodilator control of cerebral and ocular arteries, the reciprocal regulation with the adrenergic vasoconstrictor nerve in other arteries and veins, and in the initiation and maintenance of penile erection in association with smooth muscle relaxation of the corpus cavernosum. The discovery of autonomic efferent nerves in which NO plays key roles as a neurotransmitter in blood vessels, the physiological roles of this nerve in the control of smooth muscle tone of the artery, vein, and corpus cavernosum, and pharmacological and pathological implications of neurogenic NO have been reviewed. This nerve is a postganglionic parasympathetic nerve. Mechanical responses to stimulation of the nerve, mainly mediated by NO, clearly differ from those to cholinergic nerve stimulation. The naming "nitrergic or nitroxidergic" is therefore proposed to avoid confusion of the term "cholinergic nerve", from which acetylcholine is released as a major neurotransmitter. By establishing functional roles of nitrergic, cholinergic, adrenergic, and other autonomic efferent nerves in the regulation of vascular tone and the interactions of these nerves in vivo, especially in humans, progress in the understanding of cardiovascular dysfunctions and the development of pharmacotherapeutic strategies would be expected in the future.

Plasma concentrations of asymmetric-dimethyl-arginine in type 2 diabetes associate with glycemic control and glomerular filtration rate but not with risk factors of vasculopathy

Asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of nitric oxide synthase (NOS). Increased plasma levels of ADMA may indicate endothelial dysfunction and increased risk of angiopathy. The relation of ADMA to diabetes, glycemic control, and renal function, especially early diabetic hyperfiltration, remains unknown. We tried to evaluate whether there is an association between ADMA and glycosylated hemoglobin (GHbA(1c)) on the one hand and glomerular filtration rate (GFR) on the other hand in diabetic subjects with normal or slightly increased GFR. We also studied whether plasma ADMA is associated with some risk factors of vasculopathy (hypercholesterolemia and hypertension). The study subjects consisted of 86 patients with type 2 diabetes and 65 control subjects. Plasma ADMA levels were measured by high-pressure liquid chromatography as o-pthalaldehyde (OPA) derivatives and GFR was determined by the plasma clearance of chromium 51-EDTA. The diabetic patients had lower plasma ADMA levels than the nondiabetic control subjects (0.29 +/- 0.15 v 0.34 +/- 0.16 micromol/L, P <.03). In the diabetic subjects, plasma ADMA concentrations were inversely correlated with GHbA(1c) (R = -0.28, P =.01). In a multivariate linear model, significant predictors of ADMA were GFR (R = -0.32, P =.008) in diabetic subjects and GHbA(1c) (R = -0.19, P =.03) and GFR (R = -0.19, P =.02) in all subjects. Plasma ADMA was not associated with risk factors of vasculopathy. We conclude that diabetic patients with a normal or slightly increased GFR have lower circulating ADMA concentrations than nondiabetic control subjects. In type 2 diabetic patients high GFR and poor glycemic control were related to low plasma ADMA concentrations.

Effect of high-dose statin treatment on plasma concentrations of endogenous nitric oxide synthase inhibitors

Asymmetric dimethylarginine (ADMA) and monomethylarginine (LMMA) are endogenous inhibitors of nitric oxide synthase. A high level of ADMA in plasma has shown to be a significant risk factor for acute coronary syndromes and elevated plasma ADMA levels are prevalent in patients with hypercholesterolemia. It was therefore hypothesized that lowering plasma cholesterol levels with statin treatment would also lower ADMA concentrations. This double-blind study addressed the effect of high-dose statin treatment on plasma levels of ADMA and LMMA. Forty-eight subjects with mild hypercholesterolemia were randomly assigned to receive simvastatin 80 mg/d, atorvastatin 40 mg/d, or placebo for 8 weeks. Both statins decreased low-density lipoprotein cholesterol effectively (simvastatin 54% and atorvastatin 49%). However, concentrations of arginine derivatives remained unchanged during statin treatment and did not correlate with cholesterol levels. This study indicates that statin treatment has no clear influence on plasma ADMA or LMMA concentrations.

Endothelium-dependent vasorelaxation and the expression of calcitonin gene-related peptide in aged rats

To explore the age-related change in endothelium-dependent vasorelaxation, plasma concentrations and the expression of calcitonin gene-related peptide (CGRP) as well as serum concentrations of asymmetry dimethylarginine (ADMA), male Sprague-Dawley rats, aged 6 and 24 months were tested. The level of ADMA and CGRP in blood, and CGRP mRNA in dorsal root ganglia (DRG) were determined by high-performance liquid chromatography (HPLC), radioimmunoassay or semi-quantitative reverse-transcription polymerase chain reaction, respectively. Vasodilator responses to cumulative concentrations of acetylcholine (ACh) (3 x 10(-9) - 3 x 10(-7)M) and CGRP (3 x 10(-10) - 3 x 10(-8)M) were tested at the plateau of contraction in aortas rings. Vasodilator responses to ACh or CGRP at 24 months of age were decreased compared with 6 months. The serum concentration of ADMA at 24 months of age was significantly increased compared with 6 months, while both plasma concentrations of CGRP and the expressions of alpha- and beta-CGRP mRNA were unchanged at two age groups. The results suggest that endothelium-dependent relaxation, but not concentrations and the expressions of CGRP, is decreased in aged rats, and the decrease in endothelium dependence of vasodilator response to CGRP may be related to the elevation of endogenous ADMA.

Hypertension in relation to nitric oxide, asymmetric dimethylarginine, and inflammation: different patterns in heart transplant recipients and individuals with essential hypertension

BACKGROUND

Most heart transplant (HTx) recipients develop hypertension, characterized by increased peripheral vascular tone and endothelial dysfunction. Reduced levels of nitric oxide (NO) have been found in essential hypertension, and herein we investigated the possible role of altered concentrations of NO in posttransplant hypertension.

METHODS

Plasma levels of the NO-derived end products NO2(-) + NO3(-), the endogenous inhibitor of NO synthase asymmetric dimethylarginine (ADMA), and the inflammatory cytokine tumor necrosis factor (TNF)-alpha were examined in 65 stable hypertensive long-term (6 years [range 1-13]) survivors of HTx. HTx recipients were compared with 39 individuals with essential hypertension and 25 normotensive controls. RESULTS Hypertensive HTx recipients had raised NO2(-) + NO3(-) levels in plasma, positively correlated with 24-hour mean blood pressure (BP). In contrast, individuals with essential hypertension had decreased NO2(-) + NO3(-) concentration comparing controls, inversely correlated with 24-hour mean BP. Moreover, although TNF-alpha levels were significantly raised in HTx recipients compared with both healthy controls and individuals with essential hypertension, it was positively correlated to 24-hour BP and NO2(-) + NO3(-). Although only a slight increase was found in essential hypertension, no correlations were found in these nontransplant individuals. Finally, although asymmetric dimethylarginine (ADMA) tended to be raised in essential hypertension, this endogenous nitric oxide synthase (NOS) inhibitor was significantly decreased in HTx recipients compared with normotensive controls.

CONCLUSION

Our findings suggest that different mechanisms may be operating in the pathogenesis of posttransplant compared with essential hypertension, with persistent inflammation, raised NO2(-) + NO3(-), and decreased ADMA levels characterizing the former group.

Plasma asymmetric dimethylarginine and hyperemic myocardial blood flow in young subjects with borderline hypertension or familial hypercholesterolemia

OBJECTIVE

The goal of this study was to examine the relationship between plasma asymmetric dimethylarginine (ADMA) level and hyperemic myocardial blood flow (MBF) in subjects with borderline hypertension (BHT) and familial hypercholesterolemia (FH).

METHODS

Asymmetric dimethylarginine is an endogenous competitive inhibitor of nitric oxide synthase that may modulate vascular function. We measured plasma ADMA levels and myocardial flow in 77 young men (mean age 35 +/- 5 years), including 47 healthy controls, 16 men with BHT, and 14 men with FH. Basal and dipyridamole-induced myocardial flow was measured using positron emission tomography. Plasma ADMA levels were measured using high-pressure liquid chromatography.

RESULTS

Asymmetric dimethylarginine levels were significantly elevated in the BHT group compared with controls (0.59 +/- 0.13 micromol/l vs. 0.43 +/- 0.12 micromol/l, p < 0.001), and they had significantly lower dipyridamole flow (2.85 +/- 1.20 ml/min/g vs. 3.69 +/- 1.68 ml/min/g, p < 0.05). In a multivariate regression model adjusted for the study group, dipyridamole flow was inversely associated with ADMA (p < 0.05), age (p < 0.05), and apolipoprotein B concentration (p < 0.05).

CONCLUSIONS

We conclude that plasma ADMA concentration is related to dipyridamole-induced vasodilatory function in young men, independently of blood pressure elevation and hypercholesterolemia. Subjects with BHT have significantly increased plasma ADMA levels, which may partly explain the impaired hyperemic MBF in this condition.

The nitric oxide pathway in the cardiovascular system

The present review analyzes the role nitric oxide (NO) plays in the homeostasis of the cardiovascular system. By regulating vascular smooth muscle cell and myocyte contractility, myocardial oxygen consumption and renal tubular transport, this simple molecule plays a central role in the control of vascular tone, cardiac contractility and short and long term regulation of arterial pressure. Fifteen years ago, all we knew about NO is that it had very similar properties as those of endothelium-derived relaxing factor and that its action was probably mediated by cGMP. An enormous amount of knowledge has since been amassed on the biochemical pathways that NO follows from the moment it is synthesized from L-arginine until the physiological or pathological actions take place in the effector cells. This review intends to organize this knowledge in a fashion that is easy to understand. We will dissect the NO pathway in different steps, focusing on the physiological and pathophysiological actions of the isoenzymes which synthesize NO, the molecules involved in this synthesis such as caveolins, protein kinases and cofactors, the situations in which endogenous inhibitors of NO synthase are formed from L-arginine instead of NO, the way in which NO exerts its physiological actions through cGMP-dependent protein kinases and finally, the pathological routes NO may follow when the oxidative status of the cell is high.

Angiotensin-converting enzyme activity is involved in the mechanism of increased endogenous nitric oxide synthase inhibitor in patients with type 2 diabetes mellitus

The renin-angiotensin system plays an important role in the elevation of asymmetric dimethylarginine (ADMA), an endogenous inhibitor of nitric oxide synthase, in hypertensive patients, so the present study was designed to examine whether angiotensin-converting enzyme (ACE) activity is also involved in the mechanism of ADMA elevation in type 2 diabetes mellitus (NIDDM). A crossover study was performed to determine if ACE inhibition with perindopril (4 mg/day) for 4 weeks decreases serum ADMA concentration and plasma von Willebrand factor (vWF) level (a marker of endothelial injury) in 11 patients with NIDDM. None of the patients was treated with insulin or oral hypoglycemic drugs, and none had major diabetic complications. Before the protocol began, serum ADMA and plasma vWF were significantly higher in the 11 NIDDM patients, when compared with 8 control subjects without diabetes. Perindopril did not affect blood pressure or glucose metabolism, but did significantly decrease serum ADMA and plasma vWF. These results suggest that endothelial injury associated with ADMA elevation may be present even in patients with non-complicated NIDDM, and that increased activity of ACE may be involved in such endothelial dysfunction.

The pathophysiology of erectile dysfunction related to endothelial dysfunction and mediators of vascular function

The incidence of erectile dysfunction increases with diabetes, hypertension, hypercholesterolaemia, cardiovascular disease and renal failure. All these conditions are associated with endothelial dysfunction. This review addresses the pathophysiology of erectile dysfunction with a special focus on new insights into nitric oxide (NO)-mediated pathways, oxidative stress and parallels to endothelial dysfunction. NO appears to be the key mediator promoting endothelium-derived vasodilation and penile erection. The possibility is discussed that elevated plasma concentrations of asymmetrical dimethylarginine (ADMA), an endogenous NO synthase inhibitor, may provide an additional pathomechanism for various forms of erectile dysfunction associated with cardiovascular risk factors and disease. Likewise, the role of endothelium-derived factors mediating NO-independent pathways is evaluated.

Determination of arginine, asymmetric dimethylarginine, and symmetric dimethylarginine in human plasma and other biological samples by high-performance liquid chromatography

Asymmetric dimethylarginine (ADMA), an endogenous inhibitor of nitric oxide synthase (NOS), may be related to reduced biosynthesis of nitric oxide in diseases associated with accelerated atherosclerosis. The closely related compound symmetric dimethylarginine (SDMA) does not inhibit NOS, but may compete with arginine for cellular uptake, thereby limiting substrate availability for NOS. We report on a method for the simultaneous measurement of arginine, ADMA, and SDMA as a tool to gain insight in the role of these compounds in the regulation of NOS activity. Sample cleanup was performed by solid-phase extraction on polymeric cation-exchange columns using monomethylarginine as internal standard. After derivatization with ortho-phthaldialdehyde reagent containing 3-mercaptopropionic acid, analytes were separated by isocratic reversed-phase HPLC with fluorescence detection. The stable derivatives were separated with near baseline resolution. Using a sample volume of 0.2 ml, linear calibration curves were obtained with limits of quantification of 0.08 microM for arginine and 0.01 microM for ADMA and SDMA. Analytical recovery was 98-102%, and interassay CV was better than 3%. Plasma from healthy volunteers (n = 53) contained 94 +/- 26 microM arginine, 0.42 +/- 0.06 microM ADMA, and 0.47 +/- 0.08 microM SDMA. Due to its high precision and sensitivity this method is a valuable tool in research on the metabolism of dimethylated arginines and their role in the regulation of NOS activity.

Probucol preserves endothelial function by reduction of the endogenous nitric oxide synthase inhibitor level

1. Oxide low-density lipoprotein (ox-LDL) is believed to play an important role in early events of atherogenesis, and asymmetric dimethylarginine (ADMA) is associated with the development of endothelial dysfunction. The present study examined the effect of a single injection of native low-density lipoprotein (LDL) on endothelium function and the serum level of ADMA and the effect of probucol on endothelium function and ADMA level in rats. 2. Endothelial injury was induced by intravenous injection of LDL at the dose of 2, 4, or 6 mg kg(-1) for 24, 48, or 72 h, and vasodilator responses to acetylcholine in the aortic rings and serum levels of ADMA, nitrite/nitrate (NO) and malondialdehyde (MDA) were determined. 3. Pretreatment with LDL markedly reduced endothelium-dependent relaxation in a concentration-dependent manner. Inhibition of vasodilator responses to acetylcholine by LDL was abolished in the presence of L-arginine (3 x 10(-4) M). Serum levels of ADMA and MDA were significantly elevated in the rats pretreated with LDL, while serum level of nitrite/nitrate was markedly decreased. 4. Pretreatment with probucol significantly improved endothelium-dependent relaxation, decreased concentrations of ADMA and MDA and increased nitrite/nitrate level in the rats treated with LDL. A similar effect was seen in the rats pretreated with an antioxidant vitamin E. 5. These results suggest that a single injection of native LDL causes endothelial dysfunction by elevation of ADMA levels and that the protective effect of probucol on endothelial cells is related to reduction of ADMA concentration.

An integrated proposal to explain the epidemic of cardiovascular disease in a developing country. From socioeconomic factors to free radicals

An increase of coronary artery disease has been observed in developing countries during the last years. Various factors may explain this accelerated increase. We propose that inappropriate diet and inadequate sanitary infrastructure may act as triggers to create an imbalance between nitric oxide (NO) and superoxide (O2-). An increase in the concentrations of oxidized LDL produces both decreased NO and increased O2- endothelial synthesis, by accumulation of asymmetrical NG-NG-dimethyl-L-arginine, the endogenous inhibitor of NO, and by activation of NAD(P)H oxidase. On the other hand, high rates of chronic infection-inflammation, due to inappropriate sanitary environment stimulate higher circulating levels of proinflammatory cytokines. These cytokines also contribute to reduced NO and increased O2- endothelial production through the same mechanisms of oxidized LDL. The net result of this imbalance is an increased generation of peroxynitrate that injures the endothelium in a proatherogenic, prothrombotic and vasoconstrictive manner.

[The arginine paradox]

L-Arginine has attracted major interest because it has been identified as the natural substrate of nitric oxide synthase and is now recognized as a major player in the regulation of biological function. The arginine paradox refers to the phenomenon that exogenous L-arginine causes NO-mediated biological effects despite the fact that nitric oxide synthases (NOS) are theoretically saturated with the substrate L-arginine. There have been several explanations for this phenomenon, although none of them can explain the arginine paradox fully: (1) L-arginine-induced insulin, which has vasodilatory actions. (2) Neither extracellular nor intracellular concentration determines the NOS activity but rather the L-arginine amount transported across the plasma membrane may do so. (3) Endogenous NOS inhibitors reduce the enzyme sensitivity to L-arginine. These inhibitors include, NG, NG-dimethyl-L-arginine, L-citrulline, argininosuccinic acid and agmatine. (4) Intracellular L-citrulline, an NOS product, is a potent inhibitor of NOS so that the cells may need extra L-arginine to compete with L-citrulline inhibition.

Plasma concentration of asymmetrical dimethylarginine and mortality in patients with end-stage renal disease: a prospective study

BACKGROUND

The plasma concentration of asymmetrical dimethylarginine (ADMA), an inhibitor of nitric-oxide synthase, which has been linked to endothelial dysfunction and atherosclerosis in the general population, is raised in patients with end-stage renal disease and could contribute to the high cardiovascular risk in patients with chronic renal failure. We investigated the relation between cardiovascular risk factors and plasma ADMA concentration in a cohort of haemodialysis patients (n=225), and tested the predictive power of ADMA for mortality and cardiovascular outcomes.

METHODS

Patients had standard dialysis three times a week. We accurately recorded cardiovascular events over a mean follow-up of 33.4 months (SD 14.6); these events were reviewed by a panel of physicians. We identified correlates of plasma ADMA by univariate and multivariate analyses.

FINDINGS

On univariate analysis, ADMA concentration in plasma was directly related to concentrations of fibrinogen and L-arginine in plasma, duration of dialysis treatment, and serum cholesterol concentration, and was inversely related to serum albumin concentration. On multivariate analysis, only plasma fibrinogen (p=0.0001) and serum albumin (p=0.04) concentrations were independently related to plasma ADMA concentration (multiple r=0.44, p=0.0001). 83 patients died, 53 (64%) by cardiovascular causes. In a Cox's proportional-hazards model, plasma ADMA ranked as the second factor predicting overall mortality (hazard ratio 1.26, 95% Cl 1.11-1.41, p=0.0001) and cardiovascular events (1.17, 1.04-1.33, p=0.008).

INTERPRETATION

In haemodialysis patients, plasma ADMA is a strong and independent predictor of overall mortality and cardiovascular outcome. These findings lend support to the hypothesis that accumulation of ADMA is an important risk factor for cardiovascular disease in chronic renal failure.

An enzyme hydrolyzing methylated inhibitors of nitric oxide synthase is present in circulating human red blood cells

N(G),N(G)-dimethyl-L-arginine (asymmetric dimethylarginine or ADMA) and N(G)-monomethyl-L-arginine (L-NMMA) are post-translationally synthesized amino acids of nuclear proteins. Upon release during protein turnover, they are not used in protein synthesis, but are excreted or metabolized by dimethylarginine dimethylaminohydrolase (DDAH) found in many tissues. DDAH is present in monocytic and polynuclear cells of blood, but no report has appeared of its presence in red blood cells (RBCs). Because methylated arginines can inhibit nitric oxide synthase (NOS) and elevations are reported in several diseases, we explored whether RBCs express this enzyme. DDAH is present in RBCs as supported by hydrolysis of both ADMA and L-NMMA, but not symmetric dimethylarginine, and by immunoprecipitation/Westem blot using a specific monoclonal antibody to human DDAH. In a pilot study of end-stage renal disease (ESRD) patients, RBC DDAH activity with ADMA as substrate correlated inversely with age (p = 0.005) and enzyme activities were higher in patients with greater diastolic blood pressure drops during hemodialysis (p = 0.02). Similar correlations were found with white cell DDAH activity. Thus, human RBCs can hydrolyze methylated arginines. These findings indicate the RBC could be used to assess the status of DDAH in various disease states.

Elevated serum endogenous inhibitor of nitric oxide synthase and endothelial dysfunction in aged rats

1. The purpose of the present study was to determine the relationship between endothelial dysfunction and the endogenous inhibitor of nitric oxide synthase NG,NG'-asymmetric dimethylarginine (ADMA) in aged rats. 2. Studies were performed in male adult Sprague-Dawley rats (6 months old; n = 8) and in aged rats (20 months old; n = 8). Serum levels of ADMA and L-arginine were measured by high-performance liquid chromatography and responses of endothelium-intact aortic rings to acetylcholine (ACh) were tested. Nitric oxide synthase activity in kidney tissue and serum concentrations of nitrite, a stable end-product of nitric oxide, were assayed and serum contents of malondialdehyde, derived from lipid peroxidation and serum lipid and creatinine level were determined. 3. Serum levels of ADMA increased significantly in aged rats compared with adult rats (P < 0.01), whereas serum levels of L-arginine were similar in both groups (P = NS). Accordingly, the ratio of L-arginine/ADMA in old rats was lower than that in young rats (P < 0.01). Endothelium-dependent relaxation responses to ACh in aortic rings from aged rats were impaired and these impaired responses were improved by pre-incubation of aortic rings with L-arginine. 4. Nitric oxide synthase activity in the kidney, together with serum concentration of nitrite, was significantly decreased and serum contents of malondialdehyde, cholesterol and triglycerides were increased in old compared with young rats. However, the serum creatinine level was not significantly different between adult and aged rats. 5. Endogenous ADMA may be a contributor to age-related endothelial dysfunction and increases in endogenous ADMA may be linked to lipid peroxidation in aged rats.

Implications for the role of endogenous nitric oxide inhibitors in hemodialysis hypotension

Hypotensive episodes during hemodialysis in patients with end-stage renal disease in the absence of inadequate maintenance of the plasma volume, pre-existence of cardiovascular disease, or autonomic nervous system dysfunction is accompanied by increase in the plasma concentrations of the end-products of nitric oxide metabolism, above the levels expected based on the reduction of urea. Factors that can influence the synthesis of nitric oxide or the regulation of the effects of this free radical in patients with chronic renal failure are reviewed. Convergence of these factors and their interactions during the hemodialysis procedure are discussed as the basis for the generation of excessive amounts of nitric oxide that serves as an important contributing factor in the development of symptomatic hypotension.

Renin-angiotensin system is involved in the mechanism of increased serum asymmetric dimethylarginine in essential hypertension

Endothelium-dependent/nitric oxide (NO)-mediated vasodilation is impaired in hypertensive individuals. Asymmetric dimethylarginine (ADMA), an endogenous inhibitor of NO synthase, is synthesized by many types of cells including vascular endothelial cells. The serum level of ADMA is elevated in patients with essential hypertension, but the mechanism for this increase is unknown. Therefore, the present study examined whether the renin-angiotensin system (RAS) is involved. Patients with essential hypertension [systolic blood pressure (BP) > 160 mmHg and/or diastolic BP > 95 mmHg] were randomized to an angiotensin-converting enzyme (ACE) inhibitor treatment group (perindopril, 4mg/day for 4 weeks, n = 7), an angiotensin II type 1 (AT1) receptor antagonist treatment group (losartan, 50 mg/day for 4 weeks, n = 7) or a beta-blocker treatment group (bisoprolol, 5 mg/day for 4 weeks, n = 7). Before and after the treatment, BP, serum concentration of ADMA and plasma concentration of von Willebrand factor (vWF, a biological marker of endothelial injury) were measured. Perindopril, losartan and bisoprolol decreased BP to a similar extent, and either perindopril or losartan, but not bisoprolol, significantly decreased serum ADMA and plasma vWF. These findings suggest that the RAS may contribute to the mechanism of increased serum ADMA as well as to the endothelial injury observed in hypertensive patients. The vasculoprotective actions of ACE inhibitors or AT1 receptor antagonists may be explained at least in part by amelioration of the endothelial injury through a decrease in the serum ADMA concentration.

The clinical pharmacology of L-arginine

L-Arginine (2-amino-5-guanidinovaleric acid) is the precursor of nitric oxide, an endogenous messenger molecule involved in a variety of endothelium-mediated physiological effects in the vascular system. Acute and chronic administration of L-arginine has been shown to improve endothelial function in animal models of hypercholesterolemia and atherosclerosis. L-Arginine also improves endothelium-dependent vasodilation in humans with hypercholesterolemia and atherosclerosis. The responsiveness to L-arginine depends on the specific cardiovascular disease studied, the vessel segment, and morphology of the artery. The pharmacokinetics of L-arginine have recently been investigated. Side effects are rare and mostly mild and dose dependent. The mechanism of action of L-arginine may involve nitric oxide synthase substrate provision, especially in patients with elevated levels of the endogenous NO synthase inhibitor asymmetric dimethylarginine. Endocrine effects and unspecific reactions may contribute to L-arginine-induced vasodilation after higher doses. Several long-term studies have been performed that show that chronic oral administration of L-arginine or intermittent infusion therapy with L-arginine can improve clinical symptoms of cardiovascular disease in man.

Atherogenesis and the arginine hypothesis

In patients who have elevated levels of plasma ADMA, a relative deficiency of L-arginine has been found to contribute to the pathophysiology of athersclerosis, causing vasoconstriction, and accelerating atherogenesis. This finding--that there is a relative deficiency of L-arginine in atherosclerotic disease--is a breakthrough that will open new avenues of therapy.

Relationship of asymmetric dimethylarginine to dialysis treatment and atherosclerotic disease

Asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of endothelial nitric oxide (NO) synthase. Its concentration is elevated in patients with end-stage renal disease (ESRD), in part because it is excreted via the kidneys. In addition, ADMA is degraded by the enzyme dimethylarginine dimethylaminohydrolase (DDAH), which hydrolyzes ADMA to L-citrulline and dimethylamine. Activity of DDAH is decreased by oxidized low density lipoprotein (LDL) or tumor necrosis factor-alpha (TNF-alpha) in vitro yielding increased levels of ADMA. Furthermore, plasma levels of ADMA are elevated in hyperhomocyst(e)inemia and in hypertensive patients on a high salt diet. Data from several experimental studies suggest that ADMA concentrations in a pathophysiologically high range (3 to 10 micromol/L) significantly inhibit vascular NO formation by NO synthase in the presence of L-arginine in isolated human blood vessels, cultured macrophages, and in cultured endothelial cells. It has been well demonstrated that ADMA accumulates in chronic renal failure. Although there is controversy concerning the absolute concentration of ADMA, all authors found a two- to sixfold increase in ADMA levels in patients in chronic renal failure as compared to controls. Different dialysis treatment strategies differentially affect ADMA levels. The presence of atherosclerosis is associated with higher ADMA levels in patients with normal renal function as well as in dialysis patients, but this phenomenon may be unrelated to renal handling of ADMA. Reduced NO elaboration secondary to accumulation of ADMA may be an important pathogenic factor for atherosclerosis in chronic renal failure and ADMA may be a new uremic toxin. Clinical studies on the effect of ADMA are needed to further elucidate its pathophysiological role in atherosclerosis and uremia.

Serum concentrations of asymmetric (ADMA) and symmetric (SDMA) dimethylarginine in renal failure patients

BACKGROUND

Nitric oxide (NO) synthesis is inhibited by the ADMA that accumulates in the plasma of patients with renal failure; however, the concentration of SDMA also is enhanced. Therefore, it has been hypothesized that ADMA and SDMA may contribute to hypertension in these patients.

METHODS

We measured the concentrations of ADMA, SDMA and 21 endogenous amino acids in 257 persons by high pressure liquid chromatography (HPLC).

RESULTS

The plasma concentrations of both ADMA and SDMA were significantly elevated in patients with chronic renal failure (CRF). The increase was more pronounced for SDMA (2.05 +/- 0.1 micromol/L vs. 0.5 +/- 0.04 micromol/L), whereas it was only moderate for ADMA (0.85 +/- 0.03 micromol/L vs. 0.73 +/- 0.06 micromol/L). In dialysis patients, the concentrations were further increased (ADMA, 1.05 +/- 0.04 micromol/L; SDMA, 2.68 +/- 0.13 micromol/L). After kidney transplantation, the concentration of SDMA returned to the baseline value (1.15 +/- 0.11 micromol/L), but that of ADMA remained enhanced (0.99 +/- 0.06 micromol/L).

CONCLUSIONS

In CRF, especially the concentration of SDMA is significantly increased. Not only ADMA, but also SDMA are likely to be responsible for hypertension. Competition for reabsorption between SDMA and arginine within the kidney has to be considered for the interpretation of changes in the ratio between dimethylarginines and arginine in renal failure. Hemodialysis is not suitable for a long-lasting removal of methylarginines. Whether the administration of arginine could have promising effects on hypertension and complications of CRF needs to be studied in prospective trials.

Up-regulation of endothelial nitric oxide activity as a central strategy for prevention of ischemic stroke - just say NO to stroke!

Nitric oxide (NO) produced by the endothelium of cerebral arterioles is an important mediator of endothelium-dependent vasodilation (EDV), and also helps to prevent thrombosis and vascular remodeling. A number of risk factors for ischemic stroke are associated with impaired EDV, and this defect is usually at least partially attributable to a decrease in the production and/or stability of NO. These risk factors include hypertension, high-sodium diets, homocysteine, diabetes, visceral obesity, and aging. Conversely, many measures which may provide protection from ischemic stroke - such as ample dietary intakes of potassium, arginine, fish oil, and selenium - can have a favorable impact on EDV. Protection afforded by exercise training, estrogen replacement, statin drugs, green tea polyphenols, and cruciferous vegetables may reflect increased expression of the endothelial NO synthase. IGF-I activity stimulates endothelial NO production, and conceivably is a mediator of the protection associated with higher-protein diets in Japanese epidemiology and in hypertensive rats. These considerations prompt the conclusion that modulation of NO availability is a crucial determinant of risk for ischemic stroke. Multifactorial strategies for promoting effective cerebrovascular NO activity, complemented by measures that stabilize platelets and moderate blood viscosity, should minimize risk for ischemic stroke and help maintain vigorous cerebral perfusion into ripe old age. The possibility that such measures will also diminish risk for Alzheimer's disease, and slow the normal age-related decline in mental acuity, merits consideration. A limited amount of ecologic epidemiology suggests that both stroke and senile dementia may be extremely rare in cultures still consuming traditional unsalted whole-food diets. Other lines of evidence suggest that promotion of endothelial NO activity may decrease risk for age-related macular degeneration.

Does ADMA cause endothelial dysfunction?

Asymmetric dimethylarginine (ADMA) is an endogenous and competitive inhibitor of nitric oxide synthase. Plasma levels of this inhibitor are elevated in patients with atherosclerosis and in those with risk factors for atherosclerosis. In these patients, plasma ADMA levels are correlated with the severity of endothelial dysfunction and atherosclerosis. By inhibiting the production of nitric oxide, ADMA may impair blood flow, accelerate atherogenesis, and interfere with angiogenesis. ADMA may be a novel risk factor for vascular disease.

Enhanced NO inactivation and hypertension induced by a high-fat, refined-carbohydrate diet

We have recently demonstrated that long-term consumption of a high-fat, refined-carbohydrate (HFS) diet induces hypertension (HTN) in normal rats compared with a low-fat, complex-carbohydrate (LFCC) diet. Limited evidence suggests that high-fat or high-sugar diets cause enhanced generation of reactive oxygen species (ROS). We therefore hypothesized that by inducing oxidative stress, the HFS diet may promote nitric oxide (NO) inactivation and HTN. To test this hypothesis, female Fischer rats were placed on either the HFS or the LFCC diet starting at 2 months of age. Blood pressure, urinary NO metabolites (NO(x)), and total renal NO synthase activity were monitored, and the tissue abundance of nitrotyrosine (NT), which is the stable "footprint" of NO oxidation by ROS, was determined. The HFS diet group exhibited a gradual rise in arterial blood pressure and were hypertensive by 18 months. This trend was accompanied by a marked accumulation of NT in all tested tissues, an initial rise and a subsequent fall in NO synthase activity, and a fall in urinary NO(x) excretion. The HFS diet-fed animals had a blunted blood pressure response to the NO synthase inhibitor N:(omega)-nitro-L-arginine methyl ester (L-NAME) compared with the LFCC diet group, which showed a marked hypertensive response to L-NAME. L-NAME-induced HTN was reversible with L-arginine in the LFCC diet group; however, HTN was not corrected by L-arginine supplementation in the HFS diet group. These findings point to enhanced ROS-mediated inactivation and sequestration of NO, which may contribute to the reduction of bioactive NO and HTN in the HFS diet-fed animals.

Endogenous nitric oxide synthase inhibitors are responsible for the L-arginine paradox

L-Arginine, the substrate of nitric oxide (NO) synthases (NOSs), is found in the mammalian organism at concentrations by far exceeding K(M) values of these enzymes. Therefore, additional L-arginine should not enhance NO formation. In vivo, however, increasing L-arginine concentration in plasma has been shown repeatedly to increase NO production. This phenomenon has been named the L-arginine paradox; it has found no satisfactory explanation so far. In the present work, evidence for the hypothesis that the endogenous NOS inhibitors methylarginines, asymmetric dimethylarginine being the most powerful (IC(50) 1.5 microM), are responsible for the L-arginine paradox is presented.

LDL cholesterol upregulates synthesis of asymmetrical dimethylarginine in human endothelial cells: involvement of S-adenosylmethionine-dependent methyltransferases

Asymmetrical dimethylarginine (ADMA) is an endogenous nitric oxide synthase inhibitor. It is formed by protein arginine N-methyltransferases (PRMTs), which utilize S-adenosylmethionine as methyl group donor. ADMA plasma concentration is elevated in hypercholesterolemia, leading to endothelial dysfunction and producing proatherogenic changes of endothelial cell function. Four different isoforms of human PRMTs have been identified. Because the release of ADMA from human endothelial cells is increased in the presence of native or oxidized LDL cholesterol, we investigated the potential involvement of PRMT activity and gene expression in this effect. We found that the production of ADMA by human endothelial cells is upregulated in the presence of methionine or homocysteine and inhibited by either of the methyltransferase inhibitors S-adenosylhomocysteine, adenosine dialdehyde, or cycloleucine. This effect is specific for ADMA but not symmetrical dimethylarginine. The upregulation of ADMA release by native and oxidized LDL is abolished by S-adenosylhomocysteine and by the antioxidant pyrrollidine dithiocarbamate. Furthermore, a methyl-(14)C label is transferred from S-adenosylmethionine to ADMA but not symmetrical dimethylarginine, in human endothelial cells. The expression of PRMTs is upregulated in the presence of native or oxidized LDL. Our data suggest that the production of ADMA by human endothelial cells is regulated by S-adenosylmethionine-dependent methyltransferases. This activity is upregulated by LDL cholesterol, which may be due in part to the enhanced gene expression of PRMTs. In concentrations reached by stimulation of methyltransferases (5 to 50 micromol/L), ADMA significantly inhibited the formation of (15)N-nitrite from L-[guanidino-(15)N(2)]arginine. These findings suggest a novel mechanism by which ADMA concentration is elevated in hypercholesterolemia, leading to endothelial dysfunction and atherosclerosis.

Assessment of nitric oxide synthase activity in vitro and in vivo by gas chromatography-mass spectrometry

A gas chromatographic-mass spectrometric method for the determination of nitric oxide synthase activity is described. The method is based on the gas chromatographic-mass spectrometric measurement of L-[15N2]arginine-derived [15N]nitrite as its pentafluorobenzyl derivative in the negative-ion chemical ionization mode. Application of the method to the analysis of [15N]nitrite formation by purified neuronal nitric oxide synthase revealed K(M) values of 3.1 microM by Hanes and 4.6 microM by Lineweaver-Burk for L-[15N2]arginine. The corresponding Vmax values were 0.204 and 0.228 micromol [15N]nitrite min(-1) mg(-1) NOS, respectively. N(G)-Nitro-L-arginine and N(G),N(G)-dimethylarginine (asymmetric dimethylarginine) were identified by this method as the most potent enzyme inhibitors. Nitric oxide synthase activity was also assessed in vivo by i.v. injection of L-[15N2]arginine in a rat and determination of plasma [15N]nitrite and [15N]nitrate. The assay described in this work allows for accurate, specific and highly sensitive determination of nitric oxide synthase activity in vitro and in vivo.