Homoarginine and 3-nitrotyrosine in patients with takotsubo cardiomyopathy

Homoarginine in the cardiovascular system: Pathophysiology and recent developments

Upcoming experimental and epidemiological data have identified the endogenous non-proteinogenic amino acid L-homoarginine (L-hArg) not only as a novel biomarker for cardiovascular disease but also as being directly involved in the pathogenesis of cardiac dysfunction. The association of low L-hArg levels with adverse cardiovascular events and mortality has proposed the idea of nutritional supplementation to rescue pathways inversely associated with cardiovascular health. Subsequent clinical and experimental studies contributed significantly to our knowledge of potential effects on the cardiorenal axis, acting either as a biomarker or a cardiovascular active agent. In this review article, we provide a comprehensive summary of the L-hArg metabolism, pathophysiological aspects, and current developments in the field of experimental and clinical evidence in favor of protective cardiovascular effects. Establishing a reliable biomarker to identify patients at high risk to die of cardiovascular disease represents one of the main goals for tackling this disease and providing individual therapeutic guidance.

Homoarginine and blood pressure: a 10-year prospective relationship in normotensives

Nitric oxide plays a major role in the regulation of blood pressure, and impaired nitric oxide bioavailability contributes to the development of hypertension (HT). Various factors may contribute to nitric oxide bioavailability-including availability of the substrate for nitric oxide synthesis, L-arginine and its homolog L-homoarginine. We investigated whether blood pressure after 10 years associates with baseline L-homoarginine in participants who remained normotensive (NT) or developed HT, respectively. Data from the South African leg of the Prospective Urban and Rural Epidemiology study, performed in the North-West Province, were used. We investigated participants who either remained NT (N = 166) or who developed HT (N = 166) over 10 years. Blood pressure was measured with validated OMRON devices and serum L-homoarginine was analyzed with liquid chromatography-tandem mass spectrometry. L-homoarginine levels were similar at baseline (p = 0.39) and follow-up (p = 0.93) between NT and hypertensive groups. In the group that remained NT after 10 years, baseline L-homoarginine correlated positively with follow-up brachial systolic blood pressure (adj.R² = 0.13; β = 0.33; p = 0.001), brachial pulse pressure (adj.R² = 0.15 β = 0.40; p = 0.001), and central pulse pressure (adj.R² = 0.20; β = 0.30; p = 0.003). No significant associations were found in the group that developed HT after 10 years. We found a positive, independent association between blood pressure and L-homoarginine in a group that remained NT, but not in a group that developed HT after 10 years. This may suggest a protective role for L-homoarginine to maintain normal blood pressure, but only to a certain level. Once HT develops other factors may overshadow the protective effects of L-homoarginine.

Impact of Homoarginine on Myocardial Function and Remodeling in a Rat Model of Chronic Renal Failure

PURPOSE

Low plasma concentrations of the amino acid homoarginine (HA) have been shown to correlate with adverse cardiovascular outcome, particularly in patients with chronic kidney disease. The present study sought to investigate the effect of HA treatment on cardiac remodeling in rats undergoing artificially induced renal insufficiency by 5/6 nephrectomy (5/6 Nx).

METHODS

A total of 33 male Wistar rats were randomly divided into sham and 5/6 Nx groups, receiving either placebo treatment or 400 mg·kg^-1·day^-1 HA over a 4-week period.

RESULTS

5/6 Nx per se resulted in adverse myocardial remodeling with aggravated cardiac function and associated cardiac overload as the most obvious alteration (-23% ejection fraction, P < 0.0001), as well as increased myocardial fibrosis (+80%, P = 0.0005) compared to placebo treated sham animals. HA treatment of 5/6 Nx rats has led to an improvement of ejection fraction (+24%, P = 0.0003) and fractional shortening (+21%, P = 0.0126), as well as a decrease of collagen deposition (-32%, P = 0.0041), left ventricular weight (-14%, P = 0.0468), and myocyte cross-sectional area (-12%, P < 0.0001). These changes were accompanied by a downregulation of atrial natriuretic factor (-65% P < 0.0001) and collagen type V alpha 1 chain (-44%, P = 0.0006). Sham animals revealed no significant changes in cardiac function, myocardial fibrosis, or any of the aforementioned molecular changes after drug treatment.

CONCLUSION

Dietary HA supplementation appears to have the potential of preventing cardiac remodeling and improving heart function in the setting of chronic kidney disease. Our findings shed new light on HA as a possible new therapeutic agent for patients at high cardiovascular risk.

Takotsubo Syndrome: Impact of endothelial dysfunction and oxidative stress

Takotsubo Syndrome (TTS) is characterized by a transient left ventricular dysfunction recovering spontaneously within days or weeks. Although the pathophysiology of TTS remains obscure, there is growing evidence suggesting TTS to be associated with increased production of reactive oxygen species (ROS), which may be involved in causing transient coronary and peripheral endothelial dysfunction leading to a transient impairment of myocardial contraction due to stunning (apical ballooning). Endothelial dysfunction is mainly caused by decreased vascular and myocardial nitric oxide bioavailability in response to increased ROS production. Accordingly, studies in humans and animal models demonstrated increased myocardial dihydroethidium staining of the myocardium in endomyocardial biopsy specimens, increased levels of hydrogen peroxide and malondialdehyde as well as reduced glutathione levels compatible with increased oxidative stress. As significant superoxide sources the mitochondria and the NADPH oxidase isoform NOX-4 and the NOX-2 regulating cytosolic subunit p67phox have been identified. Treatment with antioxidants such as sodium hydrosulfide reduced superoxide production in mitochondria and reduced expression of NOX-4 and p67phox, respectively. The presence of superoxide and nitric oxide also provides the basis for the concept of nitro-oxidative as well as nitrosative stress in TTS.

L-homoarginine is associated with decreased cardiovascular- and all-cause mortality

BACKGROUND

Increasing evidence suggests that L-homoarginine, an endogenous analogue of the amino acid L-arginine, may have beneficial effects on vascular homeostasis. We examined whether L-homoarginine is associated with 10-year risk of all-cause and cardiovascular mortality in a black South African population.

METHODS

We included 669 black South African participants (mean age 59.5 years), 143 of whom died during the 10-year follow-up period. Mortality data were acquired via verbal autopsy. Plasma L-homoarginine (and other related markers) were analysed with liquid chromatography-tandem mass spectrometry.

RESULTS

Survivors had higher L-homoarginine levels compared with nonsurvivors (1.25 µM vs. 0.89 µM; P < .001). Multivariable Cox regression analyses revealed that higher plasma L-homoarginine predicted a reduction in 10-year cardiovascular (hazard ratio [HR] per SD increment, 0.61; 95% CI 0.50 to 0.75) and all-cause (hazard ratio [HR] per SD increment, 0.59; 95% CI 0.41 to 0.84) mortality risk.

CONCLUSION

Higher L-homoarginine levels are associated with reduced risk of 10-year cardiovascular and all-cause mortality. Regulation of L-homoarginine levels as a therapeutic target in the management of cardiovascular disease should be investigated.

NO Synthesis Markers are Not Significantly Associated with Blood Pressure and Endothelial Dysfunction in Patients with Arterial Hypertension: A Cross-Sectional Study

Nitric oxide (NO) synthesis markers, comprising L-homoarginine, asymmetric dimethylarginine (ADMA) and symmetric dimethylarginine (SDMA), are significantly associated with cardiovascular events and mortality. Being involved in NO pathways, they may be of high importance regulating vascular tone and arterial hypertension, but data on this topic are sparse and controversial. In this study, we evaluated whether these NO synthesis markers are associated with blood pressure values and pulse wave velocity (PWV). This analysis was based on the data of the Styrian Vitamin D Hypertension Trial, which included adults with arterial hypertension. We analyzed correlations of NO synthesis markers with 24 h ambulatory blood pressure values and PWV (primary outcomes), as well as with anthropometric and laboratory data. A total of 509 patients were included in the present analysis. The mean age was 61.2 ± 10.5 years, mean PWV was 8.6 ± 2.4 m/s, mean 24 h systolic blood pressure was 127.5 ± 13.8 mmHg and mean 24 h diastolic blood pressure was 76.4 ± 9.5 mmHg. In bivariate analyses, there was a significant positive correlation between homoarginine and 24 h diastolic blood pressure (r = 0.1; p = 0.02), which was revealed to be no longer significant after adjustment for age, gender and glomerular filtration rate (GFR) in multivariate regression analysis. No other significant correlations of any NO synthesis markers with blood pressure or PWV were observed. In line with previous studies, there were inverse associations between homoarginine and age and between ADMA or SDMA and GFR (p < 0.05 for all). This study did not reveal a significant association between homoarginine, ADMA or SDMA and blood pressure or PWV in hypertensive adults. These results suggested that the associations of these parameters with adverse outcome may not be mediated by hypertension and/or endothelial dysfunction.

Plasma asymmetric dimethylarginine concentrations are not related to differences in maximal oxygen uptake in endurance trained and untrained men

NEW FINDINGS

What is the central question of this study? Is there an association of plasma concentration of asymmetric dimethylarginine, which is related to exercise capacity in patients with cardiovascular diseases, with oxygen delivery and subsequently exercise capacity in healthy subjects in the absence of the potentially confounding influence of inflammation and oxidative stress? What is the main finding and its importance? Plasma asymmetric dimethylarginine concentrations are not related to exercise capacity in healthy subjects, while O₂ delivery in the working skeletal muscle during the maximal graded-exercise test is not associated with any of the l-arginine analogues. ADMA alone does not play a crucial role in local muscle perfusion and in maintaining exercise capacity.

ABSTRACT

Asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of nitric oxide (NO) synthesis that could limit oxygen (O₂ ) delivery in the working skeletal muscles by altering endothelium-dependent vasodilatation. Exercise capacity is associated with plasma ADMA concentrations in patients with cardiovascular diseases, but this issue has still not been investigated in healthy subjects. We aimed to determine whether plasma ADMA concentrations were negatively associated with exercise capacity in young healthy male subjects. Ten men with maximal oxygen uptake ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub><mml:mover><mml:mi>V</mml:mi> <mml:mo>̇</mml:mo></mml:mover> <mml:mrow><mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> <mml:mi>max</mml:mi> </mml:mrow> </mml:msub> </mml:math> ) > 65 mL kg^-1  min^-1 were included in the high exercise capacity group (HI-FIT), and 10 men with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub><mml:mover><mml:mi>V</mml:mi> <mml:mo>̇</mml:mo></mml:mover> <mml:mrow><mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> <mml:mi>max</mml:mi> </mml:mrow> </mml:msub> </mml:math>  < 45 mL kg^-1  min^-1 were included in the low exercise capacity group (LO-FIT). Plasma ADMA and other l-arginine analogue concentrations were measured before and after a maximal graded-exercise test by liquid chromatography-tandem mass spectrometry. Microvascular O₂ delivery during exercise was estimated through the pattern from the sigmoid model of muscle deoxygenation in the vastus lateralis measured by near infrared spectroscopy. <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub><mml:mover><mml:mi>V</mml:mi> <mml:mo>̇</mml:mo></mml:mover> <mml:mrow><mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> <mml:mi>max</mml:mi> </mml:mrow> </mml:msub> </mml:math> was 60% higher in the HI-FIT group (median: 70.2 mL kg^-1  min^-1 ; IQR: 68.0-71.9 mL kg^-1  min^-1 ) than in the LO-FIT group (median: 43.8 mL kg^-1  min^-1 ; IQR: 34.8-45.3 mL kg^-1  min^-1 ). Plasma ADMA concentrations did not differ between the LO-FIT and HI-FIT groups before (0.50 ± 0.06 vs. 0.54 ± 0.07 μmol L^-1 , respectively) and after the maximal incremental exercise test (0.49 ± 0.08 vs. 0.55 ± 0.03 μmol L^-1 , respectively). There was no significant association of plasma ADMA concentrations with the pattern of local muscle deoxygenation and exercise capacity. Exercise capacity and microvascular O₂ delivery are not related to plasma ADMA concentrations in young healthy male subjects. Our findings show that ADMA does not play a crucial role in local muscle perfusion and in maintaining exercise capacity without pathological conditions.

Low plasma homoarginine concentration is associated with high rates of all-cause mortality in renal transplant recipients

In renal transplant recipients (RTR), we recently found that low urinary excretion of homoarginine (hArg) is associated with mortality and graft failure. However, it is not known whether such prospective associations also hold true for plasma concentrations of hArg. In the present study, we therefore determined plasma concentrations of hArg in the same cohort, i.e. in 687 RTR (functioning graft ≥1 year), and in 140 healthy donors, before and after kidney donation. Plasma hArg concentrations were significantly lower in RTR compared to healthy controls [1.24 (0.95-1.63) µM vs. 1.58 (1.31-2.03) µM, P < 0.001], and kidney donation resulted in a decrease in plasma hArg concentration to 1.41 (1.10-1.81) µM (P < 0.001). In RTR, multivariable linear regression analysis revealed BMI (β = 0.124), heart rate (β = -0.091), pre-emptive transplantation (β = 0.078), antidiabetic medication (β = -0.091), eGFR (β = 0.272), plasma PTH (β = -0.098), uric acid (β = 0.137), alkaline phosphatase (β = -0.100), HDL (β = -0.111), NT-pro-BNP (β = -0.166), and urinary urea excretion (β = 0.139) as main determinants of plasma hArg (all P < 0.05). In RTR, plasma hArg concentration was inversely associated with all-cause [hazard ratio (HR) 0.59 (95% CI 0.50-0.70), P < 0.001] and cardiovascular mortality [HR 0.50 (0.39-0.66), P < 0.001], both expressed per standard deviation change in log-transformed hArg, independent of potential confounders. To conclude, our results suggest that the kidney is a major hArg production site and an important modulator of hArg homeostasis in the renal and cardiovascular systems. Moreover, low plasma hArg is independently associated with increased risk of cardiovascular mortality in RTR, which corroborates the cardiovascular importance of preserving kidney function after transplantation.

The role of L-arginine/L-homoarginine/nitric oxide pathway for aortic distensibility and intima-media thickness in stroke patients

Asymmetric dimethylarginine (ADMA) and L-homoarginine (hArg) are L-arginine (Arg) metabolites derived from different pathways. Protein arginine N-methyltransferase (PRMT) and subsequent proteolysis of proteins containing methylarginine residues release ADMA. Arginine:glycine amidinotransferase (AGAT) converts Arg to hArg and guanidinoacetate (GAA). While high concentrations of ADMA and low concentrations of hArg in the blood have been established as cardiovascular risk markers, the cardiovascular relevance of GAA is still unexplored. Arg and hArg are substrates and ADMA is an inhibitor of nitric oxide (NO) synthase (NOS). The cardiovascular effects of ADMA and hArg have been related to NO, a potent endogenous vasodilator. ADMA and hArg are considered to exert additional, not yet explored, presumably NO-unrelated effects and to act antagonistically in the renal and cardiovascular systems. Although the physiological role of Arg, ADMA, hArg and NO for endothelial function in small- and medium-sized arteries has been intensively studied in the past, the clinical relevance of aortic wall remodeling still remains unclear. Here, we evaluated potential relation between aortic distensibility (AD) or aortic intima-media thickness (aIMT) and circulating ADMA, hArg, GAA, and the NO metabolites nitrite and nitrate in the plasma of 78 patients (24 females, 54 males; aged 59 ± 14 years) with recent ischemic stroke or transient ischemic attack (TIA). All biochemical parameters were determined by stable-isotope dilution gas chromatography-mass spectrometry. AD and aIMT were measured by transesophageal echocardiography. Arg, hArg, ADMA and GAA median plasma concentrations (µM) were determined to be 61, 1.43, 0.50 and 2.16, respectively. hArg, ADMA and GAA correlated closely with Arg. Nitrite, nitrate and creatinine median plasma concentrations (µM) were 2.49, 48.7, and 84.1, respectively. Neither AD (2.61 vs. 1.85 10^-6 × cm² × dyn^-1, P = 0.064) nor aIMT (1.25 vs. 1.13 mm, P = 0.596) differed between females and males. The hArg/ADMA molar ratio (r = -0.351, P = 0.009), nitrate (r = 0.364, P = 0.007) and nitrite (r = 0.329, P = 0.015) correlated with aIMT but not with AD. Arg, hArg, ADMA and GAA correlated with aIMT but not with AD. The results demonstrate a strong relation between the Arg/NO pathway and aortic atherosclerosis but not with AD suggesting different mechanisms underlying the two aspects of aortic wall remodeling.

Biosynthesis of homoarginine (hArg) and asymmetric dimethylarginine (ADMA) from acutely and chronically administered free L-arginine in humans

Asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of nitric oxide (NO) synthesis, whereas L-arginine (Arg) and L-homoarginine (hArg) serve as substrates for NO synthesis. ADMA and other methylated arginines are generally believed to exclusively derive from guanidine (N (G))-methylated arginine residues in proteins by protein arginine methyltransferases (PRMTs) that use S-adenosylmethionine (SAM) as the methyl donor. L-Lysine is known for decades as a precursor for hArg, but only recent studies indicate that arginine:glycine amidinotransferase (AGAT) is responsible for the synthesis of hArg. AGAT catalyzes the formation of guanidinoacetate (GAA) that is methylated to creatine by guanidinoacetate methyltransferase (GAMT) which also uses SAM. The aim of the present study was to learn more about the mechanisms of ADMA and hArg formation in humans. Especially, we hypothesized that ADMA is produced by N (G)-methylation of free Arg in addition to the known PRMTs-involving mechanism. In knockout mouse models of AGAT- and GAMT-deficiency, we investigated the contribution of these enzymes to hArg synthesis. Arg infusion (0.5 g/kg, 30 min) in children (n = 11) and ingestion of high-fat protein meals by overweight men (n = 10) were used to study acute effects on ADMA and hArg synthesis. Daily Arg ingestion (10 g) or placebo for 3 or 6 months by patients suffering from peripheral arterial occlusive disease (PAOD, n = 20) or coronary artery disease (CAD, n = 30) was used to study chronic effects of Arg on ADMA synthesis. Mass spectrometric methods were used to measure all biochemical parameters in plasma and urine samples. In mice, AGAT but not GAMT was found to contribute to plasma hArg, while ADMA synthesis was independent of AGAT and GAMT. Arg infusion acutely increased plasma Arg, hArg and ADMA concentrations, but decreased the plasma hArg/ADMA ratio. High-fat protein meals acutely increased plasma Arg, hArg, ADMA concentrations, as well as the plasma hArg/ADMA ratio. In the PAOD and CAD studies, plasma Arg concentration increased in the verum compared to the placebo groups. Plasma ADMA concentration increased only in the PAOD patients who received Arg. Our study suggests that in humans a minor fraction of free Arg is rapidly metabolized to ADMA and hArg. In mice, GAMT and N (G)-methyltransferases contribute to ADMA and hArg synthesis from Arg, whereas AGAT is involved in the synthesis of hArg but not of ADMA. The underlying biochemical mechanisms remain still elusive.

Association of homoarginine and methylarginines with liver dysfunction and mortality in chronic liver disease

Previous studies on arginine metabolites reported an association of asymmetric dimethylarginine (ADMA) and symmetric dimethylarginine (SDMA) with liver dysfunction and an inverse relation of homoarginine (hArg) with cardiovascular risk. The aim of the present study was to investigate the relationships between hArg, ADMA, SDMA, and the dimethylarginine score (DAS, i.e., ADMA + SDMA) and liver dysfunction and survival in chronic liver disease. In 94 consecutive cirrhotic patients admitted to our outpatient liver clinic, serum levels of hArg, ADMA, and SDMA were measured by HPLC at baseline. Patients were followed with respect to mortality. In the entire study cohort (age 58.5 ± 11.2 years; 31 % females), the serum concentrations were 1.94 ± 0.90 µM for homoarginine, 0.90 ± 0.22 µM for ADMA, and 0.70 (0.60-0.93) µM for SDMA. ADMA correlated with both Child-Pugh and MELD scores, while SDMA, DAS, and hArg correlated with MELD score only. Thirty patients (32 %) died during a median follow-up of 3.5 years. Age- and sex-adjusted Cox proportional hazard ratios (HR) per µM (with 95 % confidence intervals) showed that hArg was associated with decreased mortality [HR 0.59 (0.37-0.96)], whereas mortality was increased in patients with higher ADMA [HR 3.78 (0.98-14.60)], SDMA [HR 6.54 (3.15-13.59)] and DAS [HR 4.13 (2.26-7.56)]. Only SDMA and DAS remained significantly associated with mortality after additional adjustments for either Child-Pugh stage or MELD score. In conclusion, in cirrhotic patients seen in an outpatient liver clinic, hArg as well as the dimethylarginines ADMA and SDMA was related to long-term mortality. In particular, SDMA predicts mortality independently of both Child-Pugh stage and MELD score.

Plasma and tissue homoarginine concentrations in healthy and obese humans

Increased cardiovascular risk associated with obesity cannot be fully explained by traditional risk markers. We therefore assessed plasma and interstitial concentrations of the novel cardiovascular risk biomarker homoarginine (hArg) in 18 individuals without signs of cardiovascular disease, including 4 morbidly obese subjects before and after bariatric surgery and subsequent weight reduction of 36 ± 7 kg. hArg concentrations were greater in skeletal muscle compared with adipose tissue. Plasma and tissue hArg concentrations did not correlate with BMI. Adipose tissue interstitial hArg concentrations were not affected by obesity, an oral glucose load, or dramatic weight loss. In conclusion, obesity seems not to have a major effect on hArg homeostasis, and hArg may not explain the added cardiovascular risk associated with obesity. Yet, given the small sample size of the study, the significance of hArg in obesity should be investigated in a larger population.

Plasma homoarginine, arginine, asymmetric dimethylarginine and total homocysteine interrelationships in rheumatoid arthritis, coronary artery disease and peripheral artery occlusion disease

Elevated circulating concentrations of total L-homocysteine (thCys) and free asymmetric dimethylarginine (ADMA) are long-established cardiovascular risk factors. Low circulating L-homoarginine (hArg) concentrations were recently found to be associated with increased cardiovascular morbidity and mortality. The biochemical pathways of these amino acids overlap and share the same cofactor S-adenosylmethionine (SAM). In the present study, we investigated potential associations between hArg, L-arginine (Arg), ADMA and thCys in plasma of patients suffering from rheumatoid arthritis (RA), coronary artery disease (CAD) or peripheral artery occlusive disease (PAOD). In RA, we did not find any correlation between ADMA or hArg and thCys at baseline (n = 100) and after (n = 83) combined add-on supplementation of omega-3 fatty acids, vitamin E, vitamin A, copper, and selenium, or placebo (soy oil). ADMA correlated with Arg at baseline (r = 0.446, P < 0.001) and after treatment (r = 0.246, P = 0.03). hArg did not correlate with ADMA, but correlated with Arg before (r = 0.240, P = 0.02) and after treatment (r = 0.233, P = 0.03). These results suggest that hArg, ADMA and Arg are biochemically familiar with each other, but unrelated to hCys in RA. In PAOD and CAD, ADMA and thCys did not correlate.