Asymmetric dimethylarginine reference intervals determined with liquid chromatography-tandem mass spectrometry: results from the Framingham offspring cohort

Symmetric Dimethylarginine Predicts Previously Undetected Atrial Fibrillation in Patients With Ischemic Stroke

BACKGROUND

Atrial fibrillation (AF) is a stroke risk factor that often remains undetected at hospital admission. Long-term Holter monitoring helps to identify patients with previously unrecognized AF. Asymmetric (ADMA) and symmetric dimethylarginine (SDMA) are elevated in AF in cross-sectional studies. We analyzed ADMA, SDMA, and other L-arginine metabolites to assess their association with AF in the Find-AF trial.

METHODS AND RESULTS

We included 280 patients presenting with acute cerebral ischemia. Patients presenting in sinus rhythm received 7-day Holter-ECG. Biomarkers were quantified by ultra-performance liquid chromatography-tandem mass spectrometry. We also analyzed protein methylation and L-arginine-related metabolites in human induced pluripotent stem cell-derived cardiomyocytes in vitro. ADMA and SDMA were elevated in 44 patients who presented with AF. SDMA, but not ADMA, was significantly elevated in patients newly diagnosed with AF in Holter-ECG as compared with those in sinus rhythm. SDMA plasma concentration >0.571 μmol/L significantly predicted presence of AF in Holter-ECG (area under the curve=0.676 [0.530-0.822]; P=0.029; sensitivity 0.786, specificity 0.572). SDMA levels further increased in patients with AF during the first 24 hours in hospital, and ADMA levels remained stable. In vitro, induced pluripotent stem cell-derived cardiomyocytes showed increased symmetric protein methylation and elevated SDMA during rapid pacing (2.0 Hz versus 0.5 Hz), whereas asymmetric protein methylation and ADMA were unchanged.

CONCLUSIONS

SDMA at admission was significantly elevated in stroke patients presenting with AF and showed a moderate but significant prospective association with previously unrecognized AF. Thus, stroke patients with elevated SDMA concentration at admission may specifically benefit from extended Holter-ECG monitoring.

Comprehensive GC-MS Measurement of Amino Acids, Metabolites, and Malondialdehyde in Metformin-Associated Lactic Acidosis at Admission and during Renal Replacement Treatment

Metformin is the most widely used drug in type 2 diabetes. Regular metformin use has been associated with changes in concentrations of amino acids. In the present study, we used valid stable-isotope labeled GC-MS methods to measure amino acids and metabolites, including creatinine as well as malondialdehyde (MDA), as an oxidative stress biomarker in plasma, urine, and dialysate samples in a patient at admission to the intensive care unit and during renal replacement treatment because of metformin-associated lactic acidosis (MALA, 21 mM lactate, 175 µM metformin). GC-MS revealed lower concentrations of amino acids in plasma, normal concentrations of the nitric oxide (NO) metabolites nitrite and nitrate, and normal concentrations of MDA. Renal tubular reabsorption rates were altered on admission. The patient received renal replacement therapy over 50 to 70 h of normalized plasma amino acid concentrations and their tubular reabsorption, as well as the tubular reabsorption of nitrite and nitrate. This study indicates that GC-MS is a versatile analytical tool to measure different classes of physiological inorganic and organic substances in complex biological samples in clinical settings such as MALA.

Quality Control in Targeted GC-MS for Amino Acid-OMICS

Gas chromatography-mass spectrometry (GC-MS) is suitable for the analysis of non-polar analytes. Free amino acids (AA) are polar, zwitterionic, non-volatile and thermally labile analytes. Chemical derivatization of AA is indispensable for their measurement by GC-MS. Specific conversion of AA to their unlabeled methyl esters (d0Me) using 2 M HCl in methanol (CH3OH) is a suitable derivatization procedure (60 min, 80 °C). Performance of this reaction in 2 M HCl in tetradeutero-methanol (CD3OD) generates deuterated methyl esters (d3Me) of AA, which can be used as internal standards in GC-MS. d0Me-AA and d3Me-AA require subsequent conversion to their pentafluoropropionyl (PFP) derivatives for GC-MS analysis using pentafluoropropionic anhydride (PFPA) in ethyl acetate (30 min, 65 °C). d0Me-AA-PFP and d3Me-AA-PFP derivatives of AA are readily extractable into water-immiscible, GC-compatible organic solvents such as toluene. d0Me-AA-PFP and d3Me-AA-PFP derivatives are stable in toluene extracts for several weeks, thus enabling high throughput quantitative measurement of biological AA by GC-MS using in situ prepared d3Me-AA as internal standards in OMICS format. Here, we describe the development of a novel OMICS-compatible QC system and demonstrate its utility for the quality control of quantitative analysis of 21 free AA and metabolites in human plasma samples by GC-MS as Me-PFP derivatives. The QC system involves cross-standardization of the concentrations of the AA in their aqueous solutions at four concentration levels and a quantitative control of AA at the same four concentration levels in pooled human plasma samples. The retention time (tR)-based isotope effects (IE) and the difference (δ(H/D) of the retention times of the d0Me-AA-PFP derivatives (tR(H)) and the d3Me-AA-PFP derivatives (tR(D)) were determined in study human plasma samples of a nutritional study (n = 353) and in co-processed QC human plasma samples (n = 64). In total, more than 400 plasma samples were measured in eight runs in seven working days performed by a single person. The proposed QC system provides information about the quantitative performance of the GC-MS analysis of AA in human plasma. IE, δ(H/D) and a massive drop of the peak area values of the d3Me-AA-PFP derivatives may be suitable as additional parameters of qualitative analysis in targeted GC-MS amino acid-OMICS.

Plasma levels of Asymmetric Di Methyl Arginine and endothelial dysfunction in diabetic subjects with neuropathic foot ulcer

OBJECTIVE

Asymmetric dimethyl arginine (ADMA) is an amino acid that acts as an endogenous competitive inhibitor of Nitric oxide synthase, leading to endothelial dysfunction (ED). The aim of this study was to evaluate the relationship between plasma ADMA (p-ADMA) level and ED in diabetic subjects with neuropathic foot ulcer (NFU), and the possible predictors of p-ADMA level.

MATERIALS AND METHODS

80 diabetic subjects of matched age, sex and BMI were included; 40 with NFU (G1), 20 with peripheral nerve dysfunction (PND) (G2) and 20 without PND (G3), plus 20 matched healthy subjects (G4). Flow-mediated-dilatation (FMD) of brachial artery and Carotid-intima-media-thickness (CIMT) were measured to evaluate ED and subclinical atherosclerosis, respectively.

RESULTS

G1&2 had a significantly lower FMD than G3&4 [-5.09 (-22.5 to 22.92), 4.67 (-15 to 23.91) vs. 15.74 (8.33-36.59) and 20.1 (10.0-46.15)%, respectively] (p < 0.001), and higher CIMT [0.9 (0.6-1.5), 0.9 (0.6-1.3) vs. 0.6 (0.5-0.8) and 0.7 (0.5-0.9) cm, respectively] (p < 0.001, r = 0.237, p = 0.034, r = 0.330, p = 0.003, respectively), with no significant correlation with FMD (r = -0.176, p = 0.118). FMD was inversely and strongly related to CIMT (r = -0.520, p < 0.001). p-ADMA levels were significantly higher in uncontrolled hypertensive patients in comparison to controlled and normotensive subjects [717 (286-3611) vs. 648 (335-874) and 686 (526-857) ng/L, respectively] (p = 0.026). Metformin users and hypertensive subjects on ACEIs or ARBs had the lowest p-ADMA levels than the non-users (p < 0.001, p = 0.007, respectively).

CONCLUSION

The remarkable ED in diabetic subjects with NFU is unlikely to be due to alteration in p-ADMA. Further studies are needed in order to conclude a causal association between p-ADMA and ED in this group of patients.

Asymmetric dimethylarginine predicts perioperative cardiovascular complications in patients undergoing medium-to-high risk non-cardiac surgery

Objectives

Perioperative cardiovascular events remain an important factor that affects surgery outcome. We assessed if asymmetric dimethylarginine (ADMA), an endogenous inhibitor of nitric oxide synthesis, predicts perioperative risk, and if pre-operative supplementation with L-arginine/L-citrulline improves the plasma L-arginine/ADMA ratio.

Methods

In this prospective study, planned thoracic and/or abdominal surgery patients were randomized to receive L-arginine/L-citrulline (5 g/day) or placebo 1 to 5 days before surgery. We measured perioperative plasma ADMA and L-arginine levels. The primary outcome was a 30-day combined cardiovascular endpoint.

Results

Among 269 patients, 23 (8.6%) experienced a major adverse cardiovascular event. ADMA and C-reactive protein were significantly associated with the incidence of cardiovascular complications in the multivariable-adjusted analysis. The L-arginine plasma concentration was significantly higher on the day of surgery with L-arginine/L-citrulline supplementation compared with placebo. In patients with high pre-operative ADMA, there was a non-significant trend towards reduced incidence of the primary endpoint with L-arginine/L-citrulline supplementation (six vs. nine events).

Conclusions

ADMA is a predictor of major adverse cardiovascular complications in the perioperative period for patients who are undergoing major abdominal and/or thoracic surgery. Supplementation with L-arginine/L-citrulline increased the L-arginine plasma concentration, enhanced the L-arginine/ADMA ratio, and induced a trend towards fewer perioperative events.

Homocysteine in Neurology: A Possible Contributing Factor to Small Vessel Disease

Homocysteine (Hcy) is a sulfur-containing amino acid generated during methionine metabolism, accumulation of which may be caused by genetic defects or the deficit of vitamin B12 and folate. A serum level greater than 15 micro-mols/L is defined as hyperhomocysteinemia (HHcy). Hcy has many roles, the most important being the active participation in the transmethylation reactions, fundamental for the brain. Many studies focused on the role of homocysteine accumulation in vascular or degenerative neurological diseases, but the results are still undefined. More is known in cardiovascular disease. HHcy is a determinant for the development and progression of inflammation, atherosclerotic plaque formation, endothelium, arteriolar damage, smooth muscle cell proliferation, and altered-oxidative stress response. Conversely, few studies focused on the relationship between HHcy and small vessel disease (SVD), despite the evidence that mice with HHcy showed a significant end-feet disruption of astrocytes with a diffuse SVD. A severe reduction of vascular aquaporin-4-water channels, lower levels of high-functioning potassium channels, and higher metalloproteinases are also observed. HHcy modulates the N-homocysteinylation process, promoting a pro-coagulative state and damage of the cellular protein integrity. This altered process could be directly involved in the altered endothelium activation, typical of SVD and protein quality, inhibiting the ubiquitin-proteasome system control. HHcy also promotes a constant enhancement of microglia activation, inducing the sustained pro-inflammatory status observed in SVD. This review article addresses the possible role of HHcy in small-vessel disease and understands its pathogenic impact.

Increased Serum Levels of Asymmetric Dimethylarginine and Symmetric Dimethylarginine and Decreased Levels of Arginine in Sudanese Patients with Essential Hypertension

INTRODUCTION

Essential hypertension (EH) is a disease caused by various environmental and genetic factors. Nitric oxide (NO) is important for the functional integrity of the endothelium. It is produced in endothelial cells by endothelial NO synthase (eNOS) that mediates the conversion of the amino acid arginine into NO and citrulline. Asymmetric dimethylarginine (ADMA) acts as an inhibitor of eNOS. In contrast, symmetric dimethylarginine (SDMA) has no direct effect on eNOS but plays an important role competing with arginine for transport across the amino acid transporter. ADMA and SDMA have been found to play a central role in the development of cardiovascular diseases. Serum ADMA levels may serve as a future diagnostic marker and a target of therapy in hypertensive patients in the Sudanese population. This study aimed to investigate the relation between serum arginine, ADMA, and SDMA levels with EH in the Sudanese population.

METHODS

Patients (n = 260) with established hypertension and controls (n = 144) with normal blood pressure were included in this case-control study. Serum blood samples were analyzed for arginine, ADMA, and SDMA, using high-performance liquid chromatography-tandem mass spectrometry. Other laboratory data were measured using routine methods. Mann-Whitney's U test and χ2 tests were used for continuous and categorical data, respectively. A multivariate logistic regression analysis was conducted to investigate the independent effect of multiple variables on the development of hypertension.

RESULTS

Serum arginine levels were significantly lower in the patient group than in the control group (p < 0.001). ADMA and SDMA levels were significantly higher in the patient group than the control group (p < 0.001, p = 0.001, respectively). Multivariate logistic regression analysis showed that only older age, being a male, and arginine levels are independent factors controlling the development of hypertension (p < 0.001, p < 0.001, and p = 0.046, respectively). ADMA and SDMA levels were not independent factors for the development of hypertension.

CONCLUSIONS

This study demonstrated increased serum levels of ADMA and SDMA and decreased arginine levels in Sudanese patients with EH. Lowering serum ADMA levels or increasing the arginine levels might be a novel therapeutic target in these individuals.

Enhanced dimethylarginine degradation improves coronary flow reserve and exercise tolerance in Duchenne muscular dystrophy carrier mice

Duchenne muscular dystrophy (DMD) is an X-linked disease caused by null mutations in dystrophin and characterized by muscle degeneration. Cardiomyopathy is common and often prevalent at similar frequency in female DMD carriers irrespective of whether they manifest skeletal muscle disease. Impaired muscle nitric oxide (NO) production in DMD disrupts muscle blood flow regulation and exaggerates postexercise fatigue. We show that circulating levels of endogenous methylated arginines including asymmetric dimethylarginine (ADMA), which act as NO synthase inhibitors, are elevated by acute necrotic muscle damage and in chronically necrotic dystrophin-deficient mice. We therefore hypothesized that excessive ADMA impairs muscle NO production and diminishes exercise tolerance in DMD. We used transgenic expression of dimethylarginine dimethylaminohydrolase 1 (DDAH), which degrades methylated arginines, to investigate their contribution to exercise-induced fatigue in DMD. Although infusion of exogenous ADMA was sufficient to impair exercise performance in wild-type mice, transgenic DDAH expression did not rescue exercise-induced fatigue in dystrophin-deficient male mdx mice. Surprisingly, DDAH transgene expression did attenuate exercise-induced fatigue in dystrophin-heterozygous female mdx carrier mice. Improved exercise tolerance was associated with reduced heart weight and improved cardiac β-adrenergic responsiveness in DDAH-transgenic mdx carriers. We conclude that DDAH overexpression increases exercise tolerance in female DMD carriers, possibly by limiting cardiac pathology and preserving the heart's responses to changes in physiological demand. Methylated arginine metabolism may be a new target to improve exercise tolerance and cardiac function in DMD carriers or act as an adjuvant to promote NO signaling alongside therapies that partially restore dystrophin expression in patients with DMD.NEW & NOTEWORTHY Duchenne muscular dystrophy (DMD) carriers are at risk for cardiomyopathy. The nitric oxide synthase inhibitor asymmetric dimethylarginine (ADMA) is released from damaged muscle in DMD and impairs exercise performance. Transgenic expression of dimethylarginine dimethylaminohydrolase to degrade ADMA prevents cardiac hypertrophy, improves cardiac function, and improves exercise tolerance in DMD carrier mice. These findings highlight the relevance of ADMA to muscular dystrophy and have important implications for therapies targeting nitric oxide in patients with DMD and DMD carriers.

Arginine derivatives in atrial fibrillation progression phenotypes

Arginine, homoarginine (hArg), asymmetric dimethylarginine (ADMA), and symmetric dimethylarginine (SDMA) affect nitric oxide metabolism and altered concentrations are associated with cardiovascular morbidity and mortality. We analyzed these metabolites using liquid chromatography-tandem mass spectrometry in patients with atrial fibrillation (AF) (n = 241) with a focus on heart rhythm at blood withdrawal, AF progression phenotypes, and successful sinus rhythm (SR) restoration (n = 22). AF progression phenotypes were defined as paroxysmal AF with/without low voltage areas (LVA) and persistent AF with/without LVA. While arginine, ADMA, and hArg were within reference limits for healthy controls, SDMA was higher in the AF cohort (0.57 ± 0.12 vs. 0.53 μmol/L (97.5th percentile in reference cohort)). SR restoration in AF patients resulted in normalization of SDMA concentrations (0.465 ± 0.082 vs. 0.570 ± 0.134 μmol/L at baseline, p < 0.001). Patients with AF at the time of blood sampling had significantly lower hArg (1.65 ± 0.51 vs. 1.85 ± 0.60 μmol/L, p = 0.006) and higher ADMA concentrations (0.526 ± 0.08 vs. 0.477 ± 0.08 μmol/L, p < 0.001) compared with AF patients in SR. hArg concentrations were lower in patients with advanced AF progression phenotypes (persistent AF with LVA (p = 0.046)) independent of heart rhythm at blood sampling. Summarizing, arginine metabolism imbalance is associated with AF in general and AF progression and may contribute to associated risk. KEY MESSAGES: • Heart rhythm at blood withdrawal affects ADMA and hArg level in AF patients. • SDMA is higher in AF patients. • SDMA levels normalize after sinus rhythm restoration. • hArg levels decrease in advanced AF progression phenotypes.

Asymmetric Dimethylarginine at Sea Level Is a Predictive Marker of Hypoxic Pulmonary Arterial Hypertension at High Altitude

Background: Prolonged exposure to altitude-associated chronic hypoxia (CH) may cause high-altitude pulmonary hypertension (HAPH). Chronic intermittent hypobaric hypoxia (CIH) occurs in individuals who commute between sea level and high altitude. CIH is associated with repetitive acute hypoxic acclimatization and conveys the long-term risk of HAPH. As nitric oxide (NO) regulates pulmonary vascular tone and asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of NO synthesis, we investigated whether ADMA concentration at sea level predicts HAPH among Chilean frontiers personnel exposed to 6 months of CIH.

Methods: In this prospective study, 123 healthy army draftees were subjected to CIH (5 days at 3,550 m, 2 days at sea level) for 6 months. In 100 study participants with complete data, ADMA, symmetric dimethylarginine (SDMA), L-arginine, arterial oxygen saturation (SaO₂), systemic blood pressure, and hematocrit were assessed at months 0 (sea level), 1, 4, and 6. Acclimatization to altitude was determined using the Lake Louise Score (LLS) and the presence of acute mountain sickness (AMS). Echocardiography was performed after 6 months of CIH in 43 individuals with either good (n = 23) or poor (n = 20) acclimatization.

Results: SaO₂ acutely decreased at altitude and plateaued at 90% thereafter. ADMA increased and SDMA decreased during the study course. The incidence of AMS and the LLS was high after the first ascent (53 and 3.1 ± 2.4) and at 1 month of CIH (47 and 3.0 ± 2.6), but decreased to 20 and 1.4 ± 2.0 at month 6 (both p < 0.001). Eighteen participants (42%) showed a mean pulmonary arterial pressure (mPAP) >25 mm Hg, out of which 9 (21%) were classified as HAPH (mPAP ≥ 30 mm Hg). ADMA at sea level was significantly associated with mPAP at high altitude in month 6 (R = 0.413; p = 0.007). In ROC analysis, a cutoff for baseline ADMA of 0.665 μmol/L was determined to predict HAPH (mPAP > 30 mm Hg) with a sensitivity of 100% and a specificity of 63.6%.

Conclusions: ADMA concentration increases during CIH. ADMA at sea level is an independent predictive biomarker of HAPH. SDMA concentration decreases during CIH and shows no association with HAPH. Our data support a role of impaired NO-mediated pulmonary vasodilation in the pathogenesis of HAPH.

The Controversial Role of Homocysteine in Neurology: From Labs to Clinical Practice

Homocysteine (Hcy) is a sulfur-containing amino acid that is generated during methionine metabolism. Physiologic Hcy levels are determined primarily by dietary intake and vitamin status. Elevated plasma levels of Hcy can be caused by deficiency of either vitamin B12 or folate. Hyperhomocysteinemia (HHcy) can be responsible of different systemic and neurological disease. Actually, HHcy has been considered as a risk factor for systemic atherosclerosis and cardiovascular disease (CVD) and HHcy has been reported in many neurologic disorders including cognitive impairment and stroke, independent of long-recognized factors such as hyperlipidemia, hypertension, diabetes mellitus, and smoking. HHcy is typically defined as levels >15 micromol/L. Treatment of hyperhomocysteinemia with folic acid and B vitamins seems to be effective in the prevention of the development of atherosclerosis, CVD, and strokes. However, data from literature show controversial results regarding the significance of homocysteine as a risk factor for CVD and stroke and whether patients should be routinely screened for homocysteine. HHcy-induced oxidative stress, endothelial dysfunction, inflammation, smooth muscle cell proliferation, and endoplasmic reticulum (ER) stress have been considered to play an important role in the pathogenesis of several diseases including atherosclerosis and stroke. The aim of our research is to review the possible role of HHcy in neurodegenerative disease and stroke and to understand its pathogenesis.

Age-dependent effects of homocysteine and dimethylarginines on cardiovascular mortality in claudicant patients with lower extremity arterial disease

The association among serum homocysteine (HCY), symmetric dimethylarginine (SDMA), and asymmetric dimethylarginine (ADMA) is of interest in endothelial dysfunction, although the underlying pathology is not fully elucidated. We investigated the relationship of HCY with SDMA and ADMA regarding their long-time outcome and the age dependency of HCY, SDMA, and ADMA values in claudicant patients with lower extremity arterial disease. 120 patients were included in a prospective observational study (observation time 7.96 ± 1.3 years) with cardiovascular mortality as the main outcome parameter. Patients with intermittent claudication prior to their first endovascular procedure were included. HCY, SDMA, and ADMA were measured by high-performance liquid chromatography. Cutoff values for HCY (≤/>15 µmol/l), SDMA (≤/>0.75 µmol/l), and ADMA (≤/>0.8 µmol/l) differed significantly regarding cardiovascular mortality (p < 0.001, p < 0.001, p = 0.017, respectively). Age correlated significantly with HCY (r = 0.393; p < 0.001), SDMA (r = 0.363; p < 0.001), and ADMA (r = 0.210; p = 0.021). HCY and SDMA (r = 0.295; p = 0.001) as well as SDMA and ADMA (r = 0.380; p < 0.001) correlated with each other, while HCY and ADMA did not correlate (r = 0.139; p = 0.130). Patients older than 65 years had higher values of HCY (p < 0.001) and SDMA (p = 0.01), but not of ADMA (p = 0.133). In multivariable linear regression, age was the only significant independent risk factor for cardiovascular death (beta coefficient 0.413; 95% CI 0.007-0.028; p = 0.001). Age correlated significantly with HCY, SDMA, and ADMA. However, only age was an independent predictor for cardiovascular death. Older patients have higher values of HCY and SDMA than younger subjects suggesting age-adjusted cutoff values of HCY and SDMA due to strong age dependency.

Long-Term Intermittent Work at High Altitude: Right Heart Functional and Morphological Status and Associated Cardiometabolic Factors

Background: Living at high altitude or with chronic hypoxia implies functional and morphological changes in the right ventricle and pulmonary vasculature with a 10% prevalence of high-altitude pulmonary hypertension (HAPH). The implications of working intermittently (day shifts) at high altitude (hypobaric hypoxia) over the long term are still not well-defined. The aim of this study was to evaluate the right cardiac circuit status along with potentially contributory metabolic variables and distinctive responses after long exposure to the latter condition. Methods: A cross-sectional study of 120 healthy miners working at an altitude of 4,400-4,800 m for over 5 years in 7-day commuting shifts was designed. Echocardiography was performed on day 2 at sea level. Additionally, biomedical and biochemical variables, Lake Louise scores (LLSs), sleep disturbances and physiological variables were measured at altitude and at sea level. Results: The population was 41.8 ± 0.7 years old, with an average of 14 ± 0.5 (range 5-29) years spent at altitude. Most subjects still suffered from mild to moderate symptoms of acute mountain sickness (mild was an LLS of 3-5 points, including cephalea; moderate was LLS of 6-10 points) (38.3%) at the end of day 1 of the shift. Echocardiography showed a 23% mean pulmonary artery pressure (mPAP) >25 mmHg, 9% HAPH (≥30 mmHg), 85% mild increase in right ventricle wall thickness (≥5 mm), 64% mild right ventricle dilation, low pulmonary vascular resistance (PVR) and fairly good ventricle performance. Asymmetric dimethylarginine (ADMA) (OR 8.84 (1.18-66.39); p < 0.05) and insulin (OR: 1.11 (1.02-1.20); p < 0.05) were associated with elevated mPAP and were defined as a cut-off. Interestingly, the correspondence analysis identified association patterns of several other variables (metabolic, labor, and biomedical) with higher mPAP. Conclusions: Working intermittently at high altitude involves a distinctive pattern. The most relevant and novel characteristics are a greater prevalence of elevated mPAP and HAPH than previously reported at chronic intermittent hypobaric hypoxia (CIHH), which is accompanied by subsequent morphological characteristics. These findings are associated with cardiometabolic factors (insulin and ADMA). However, the functional repercussions seem to be minor or negligible. This research contributes to our understanding and surveillance of this unique model of chronic intermittent high-altitude exposure.

The issue of plasma asymmetric dimethylarginine reference range - A systematic review and meta-analysis

Background

Asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of nitric oxide synthase, marker and mediator of endothelial dysfunction. Several studies have demonstrated its value in cardiovascular risk stratification and all-cause mortality prediction. The aim was to determine the reference range of plasma ADMA in healthy adults.

Methods and results

Taking into account the most widely used ADMA measurement methods, only studies using either high performance liquid chromatography (HPLC) -with fluorescence or mass spectrometric detection-, or enzyme-linked immunosorbent assay (ELISA) to quantify plasma ADMA concentrations were enrolled. 66 studies were included in the quantitative analysis (24 using ELISA and 42 using HPLC) reporting a total number of 5528 non-diabetic, non-hypertensive, non-obese adults without any medication (3178 men and 2350 women, 41.6 ± 16.9 years old). The reference range of ADMA (in μmol/l with 95% confidence interval in parenthesis) was 0.34 (0.29–0.38)– 1.10 (0.85–1.35) with a mean of 0.71 (0.57–0.85) (n = 4093) measured by HPLC and 0.25 (0.18–0.31)– 0.92 (0.76–1.09) with a mean of 0.57 (0.48–0.66) (n = 1435) by ELISA.

Conclusions

Numerous publications suggested that asymmetric dimethylarginine is not only an outstanding tool of disease outcome prediction but also a new potential therapeutic target substance; the reference range provided by this meta-analysis can become of great importance and aid to further investigations. However, developing a standard measurement method would be beneficial to facilitate the clinical usage of ADMA.

Long-Term Intermittent Exposure to High Altitude Elevates Asymmetric Dimethylarginine in First Exposed Young Adults

Lüneburg, Nicole, Patricia Siques, Julio Brito, Juan José De La Cruz, Fabiola León-Velarde, Juliane Hannemann, Cristian Ibanez, and Rainer Böger. Long-term intermittent exposure to high altitude elevates asymmetric dimethylarginine in first exposed young adults. High Alt Med Biol. 18:226–233, 2017.—Hypoxia-induced dysregulation of pulmonary and cerebral circulation may be related to an impaired nitric oxide (NO) pathway. We investigated the effect of chronic intermittent hypobaric hypoxia (CIH) on metabolites of the NO pathway. We measured asymmetric and symmetric dimethylarginine (ADMA and SDMA) and monomethyl-L-arginine (L-NMMA) and assessed their associations with acclimatization in male draftees (n = 72) undergoing CIH shifts at altitude (3550 m) during 3 months. Sixteen Andean natives living at altitude (3675 m) (chronic hypobaric hypoxia [CH]) were included for comparison. In CIH, ADMA and L-NMMA plasma concentrations increased from 1.14 ± 0.04 to 1.95 ± 0.09 μmol/L (mean ± SE) and from 0.22 ± 0.07 to 0.39 ± 0.03 μmol/L, respectively, (p < 0.001 for both) after 3 months, whereas SDMA did not change. The concentrations of ADMA and L-NMMA were higher in CH (3.48 ± 0.07, 0.53 ± 0.08 μmol/L; p < 0.001) as compared with CIH. In both CIH and CH, ADMA correlated with hematocrit (r² = 0.07, p < 0.05; r² = 0.26; p < 0.01). In CIH, an association of ADMA levels with poor acclimatization status was observed. We conclude that the endogenous NO synthase inhibitors, ADMA and L-NMMA, are elevated in hypoxia. This may contribute to impaired NO production at altitude and may also be predictive of altitude-associated health impairment.

Asymmetric and Symmetric Dimethylarginine as Risk Markers for Total Mortality and Cardiovascular Outcomes: A Systematic Review and Meta-Analysis of Prospective Studies

Background

A growing number of studies linked elevated concentrations of circulating asymmetric (ADMA) and symmetric (SDMA) dimethylarginine to mortality and cardiovascular disease (CVD) events. To summarize the evidence, we conducted a systematic review and quantified associations of ADMA and SDMA with the risks of all-cause mortality and incident CVD in meta-analyses accounting for different populations and methodological approaches of the studies.

Methods

Relevant studies were identified in PubMed until February 2015. We used random effect models to obtain summary relative risks (RR) and 95% confidence intervals (95%CIs), comparing top versus bottom tertiles. Dose-response relations were assessed by restricted cubic spline regression models and potential non-linearity was evaluated using a likelihood ratio test. Heterogeneity between subgroups was assessed by meta-regression analysis.

Results

For ADMA, 34 studies (total n = 32,428) investigating associations with all-cause mortality (events = 5,035) and 30 studies (total n = 30,624) investigating the association with incident CVD (events = 3,396) were included. The summary RRs (95%CI) for all-cause mortality were 1.52 (1.37–1.68) and for CVD 1.33 (1.22–1.45), comparing high versus low ADMA concentrations. Slight differences were observed across study populations and methodological approaches, with the strongest association of ADMA being reported with all-cause mortality in critically ill patients. For SDMA, 17 studies (total n = 18,163) were included for all-cause mortality (events = 2,903), and 13 studies (total n = 16,807) for CVD (events = 1,534). High vs. low levels of SDMA, were associated with increased risk of all-cause mortality [summary RR (95%CI): 1.31 (1.18–1.46)] and CVD [summary RR (95%CI): 1.36 (1.10–1.68) Strongest associations were observed in general population samples.

Conclusions

The dimethylarginines ADMA and SDMA are independent risk markers for all-cause mortality and CVD across different populations and methodological approaches.

Measurement of Reactive Oxygen Species, Reactive Nitrogen Species, and Redox-Dependent Signaling in the Cardiovascular System: A Scientific Statement From the American Heart Association

Reactive oxygen species and reactive nitrogen species are biological molecules that play important roles in cardiovascular physiology and contribute to disease initiation, progression, and severity. Because of their ephemeral nature and rapid reactivity, these species are difficult to measure directly with high accuracy and precision. In this statement, we review current methods for measuring these species and the secondary products they generate and suggest approaches for measuring redox status, oxidative stress, and the production of individual reactive oxygen and nitrogen species. We discuss the strengths and limitations of different methods and the relative specificity and suitability of these methods for measuring the concentrations of reactive oxygen and reactive nitrogen species in cells, tissues, and biological fluids. We provide specific guidelines, through expert opinion, for choosing reliable and reproducible assays for different experimental and clinical situations. These guidelines are intended to help investigators and clinical researchers avoid experimental error and ensure high-quality measurements of these important biological species.

Reference intervals for plasma-free amino acid in a Japanese population

BACKGROUND

Plasma amino acid concentrations vary with various diseases. Although reference intervals are useful in daily clinical practice, no reference intervals have been reported for plasma amino acids in a large Japanese population.

METHODS

Reference individuals were selected from 7685 subjects examined with the Japanese Ningen Dock in 2008. A total of 1890 individuals were selected based on exclusion criteria, and the reference samples were selected after the outlier samples for each amino acid concentration were excluded. The lower limit of the reference intervals for the plasma amino acid concentrations was set at the 2.5th percentile and the upper limit at the 97.5th percentile.

RESULTS

By use of the nested analysis of variance, we analysed a large dataset of plasma samples and the effects of background factors (sex, age and body mass index [BMI]) on the plasma amino acid concentrations. Most amino acid concentrations were related to sex, especially those of branched-chained amino acid. The citrulline, glutamine, ornithine and lysine concentrations were related to age. The glutamate concentration was related to body mass index.

CONCLUSIONS

The concentrations of most amino acids are more strongly related to sex than to age or body mass index. Our results indicate that the reference intervals for plasma amino acid concentrations should be stratified by sex when the background factors of age and body mass index are considered.

Supplementation of folic acid and vitamin B₁₂ reduces plasma levels of asymmetric dimethylarginine in patients with acute ischemic stroke

Increased levels of asymmetric dimethylarginine (ADMA) have been observed in patients with acute ischemic stroke. We aimed to investigate the correlation between ADMA and ischemic stroke, and evaluate the effect of supplementation of folic acid and vitamin B12 on concentrations of ADMA. Patients were randomized into intervention and non-intervention groups within 3 days after symptom onset. Intervention group patients were treated with folic acid (5mg daily) and vitamin B12 (500 μg twice daily) for 12 weeks. ADMA and homocysteine (Hcy) concentrations were measured before treatment (baseline) and 2 and 12 weeks after treatment. The laboratory measures were also collected from healthy controls. Eighty five subjects were enrolled in this study, from whom 72 with complete baseline and follow-up laboratory data were included in the present analysis. Thirty four patients were assigned to the intervention group and 38 patients to the non-intervention group. Sixty people were enrolled as healthy controls. Levels of ADMA and Hcy were raised (p<0.05) in patients with acute ischemic stroke. With supplementation of both folic acid and vitamin B12, the levels of ADMA and Hcy decreased significantly at 2 and 12 weeks (p<0.05). The present study reconfirmed that ADMA can be regarded as a risk biomarker for acute ischemic stroke. We observed that with supplementation of folic acid and vitamin B12, levels of ADMA were decreased in patients with acute ischemic stroke.

Serum reference intervals of homoarginine, ADMA, and SDMA in the Study of Health in Pomerania

Low circulating homoarginine as well as high levels of asymmetric dimethylarginine (ADMA) and symmetric dimethylarginine (SDMA) have been associated with impaired cardiovascular (CV) outcome and mortality in patients at risk and in the general population. The present study aimed to formulate reference intervals for serum homoarginine, ADMA, and SDMA to improve risk stratification between healthy individuals and individuals at risk.

We determined age- and sex-specific reference intervals for homoarginine, ADMA, and SDMA in a subgroup of 1359 healthy participants (no diabetes mellitus, CV disease, increased blood pressure, elevated blood lipids, chronic kidney disease stadium III or IV, or a body mass index >30) of the Study of Health in Pomerania (SHIP) using quantile regression analyses. Homoarginine, ADMA, and SDMA serum concentrations were measured using liquid chromatography-tandem mass spectrometry.

Median age of the investigated cohort was 36 (25th; 75th percentile 28; 47) years, with 62% women. Median serum concentrations of homoarginine, ADMA, and SDMA were 2.63 (2.08; 3.32) μmol/L, 0.64 (0.57; 0.73) μmol/L, and 0.43 (0.37; 0.49) μmol/L, respectively. Serum levels of homoarginine, ADMA, and SDMA showed material age- and sex-related differences (p<0.05 for all). Overall reference ranges were 1.41–5.00 and 1.20–5.53 μmol/L (2.5th; 97.5th percentile; for men and women, respectively) for homoarginine, 0.41–0.95 and 0.43–0.96 μmol/L for ADMA, and 0.30–0.67 and 0.27–0.63 μmol/L for SDMA.

We formulated for the first time homoarginine, ADMA, and SDMA reference intervals in serum. These reference intervals might be useful for individual CV risk stratification.

The unique role of dietary L-arginine in the acceleration of peritoneal macrophage sensitivity to bacterial endotoxin

It is known that cells and organisms can indirectly "sense" changes in L-arginine availability via changes in the activity of various metabolic pathways. However, the mechanism(s) by which genes can be directly regulated by L-arginine in mammalian cells have not yet been elucidated. We investigated the effect of L-arginine in the in vivo model of peritoneal inflammation in mice and in vitro in RAW 264.7 macrophages. A detailed analysis of basic physiological functions and selected intracellular signaling cascades revealed that L-arginine is crucial for the acceleration of macrophage activation by bacterial lipopolysaccharide. L-arginine increased the production of reactive oxygen species, nitric oxide, release of Ca(2+), as well as inducible nitric oxide synthase expression. Interestingly, the effect of L-arginine on macrophage activation was dependent on the phosphorylation of mitogen-activated protein kinases and activity of phospholipase C. In RAW 264.7 cells, L-arginine was shown to modulate the response of macrophages toward lipopolysaccharide via the activation of G-protein-coupled receptors. According to our data, we concluded that L-arginine availability plays a key role in the initiation of intracellular signaling pathways that trigger the lipopolysaccharide-induced inflammatory responses in murine macrophages. Although macrophages are partially stimulated in the absence of extracellular L-arginine, the presence of this amino acid significantly accelerates the sensitivity of macrophages to bacterial endotoxin.

Elevated levels of asymmetric dimethylarginine are associated with lower CD4+ count and higher viral load in HIV-infected individuals

OBJECTIVES

To compare asymmetric dimethylarginine (ADMA) among HIV-infected and uninfected individuals and to evaluate predictors of ADMA in HIV infection.

BACKGROUND

HIV-infected individuals have high rates of atherosclerosis. Endothelial dysfunction is central to atherogenesis and is one possible mechanism underlying this increased cardiovascular risk. ADMA is an endogenous inhibitor of endothelial nitric oxide synthase. Among uninfected individuals, higher ADMA levels predict cardiovascular events and mortality. The association between HIV infection, HIV-related factors, and ADMA has not been well described.

METHODS

We compared ADMA in 248 HIV-infected individuals and 50 uninfected controls. We performed multivariable analysis using traditional cardiovascular and HIV-specific factors as covariates to identify factors associated with ADMA.

RESULTS

HIV-infected men were older, less often Caucasian, more hypertensive, and had lower HDL than uninfected men. The median duration of HIV infection was 13 years, median CD4+ count was 592 cells/μL, 76% had an undetectable viral load, and 76% were on antiretroviral therapy. ADMA levels were modestly higher in HIV-infected individuals than controls [median (IQR): 0.46 μM (0.41-0.52) vs. 0.44 μM (0.38-0.46), p = 0.019], but the association lost statistical significance after controlling for cardiovascular risk factors (+0.028 μM, p = 0.054). Lower CD4+ count and both detectable and higher viral load were independently associated with increased ADMA.

CONCLUSIONS

ADMA levels were modestly elevated in the setting of HIV infection. Notably, a greater HIV-associated inflammatory burden, as evidenced by lower CD4+ counts and higher viral loads, was associated with increased ADMA levels. Our findings suggest that HIV infection impairs endothelial function and predisposes to atherosclerosis through chronic inflammation and subsequent accumulation of ADMA.

Protection of DDAH2 overexpression against homocysteine-induced impairments of DDAH/ADMA/NOS/NO pathway in endothelial cells

BACKGROUND/AIMS

Homocysteine-induced endothelial dysfunction favors the development of cardiovascular diseases through accumulation of endogenous nitric oxide (NO) synthase (NOS) inhibitor asymmetric dimethylarginine (ADMA). Dimethylarginine dimethylaminohydrolase 2 (DDAH2) is the major enzyme for the degradation of ADMA in endothelial cells. The purpose of this study was to determine whether suppressed DDAH2 expression contributed to impairments of DDAH/ADMA/NOS/NO pathway induced by homocysteine in endothelial cells and whether DDAH2 overexpression could prevent endothelial cell dysfunction caused by homocysteine.

METHODS

Liposome-mediated transfection of endothelial cells was performed to establish the cell line of DDAH2 overexpression. After treatment of cells with 1 mmol/L homocysteine for 24 h, the transcription and expression of DDAH1 and DDAH2, DDAH and NOS activities as well as ADMA and NO concentrations were measured.

RESULTS

Treatment of endothelial cells with homocysteine significantly suppressed the transcription and expression of DDAH2 but not DDAH1. This suppression was associated with the declined DDAH activity, increased ADMA accumulation, inhibited NOS activity and decreased NO production in endothelial cells. DDAH2 overexpression not only resisted homocysteine-induced decline of DDAH activity, but also decreased the accumulation of endogenous ADMA, subsequently attenuated the reductions of NOS activity and NO production induced by homocysteine.

CONCLUSIONS

These results indicate that suppression of DDAH2 expression is a culprit for homocysteine-induced impairments of DDAH/ADMA/NOS/NO pathway in endothelial cells, and therapeutic manipulation of DDAH2 expression may be a promising strategy for preventing endothelial dysfunction and cardiovascular diseases associated with hyperhomocysteinemia.

Influence of dietary fat ingestion on asymmetrical dimethylarginine in lean and obese human subjects

BACKGROUND AND AIMS

Asymmetrical dimethylarginine (ADMA) may contribute to hypertension and cardiovascular disease by decreasing NO formation. In diabetic patients, a high fat meal acutely increased plasma ADMA while impairing endothelial function. We hypothesized that chronic and acute increases in dietary fat intake augment ADMA also in lean and in obese subjects without diabetes.

METHODS AND RESULTS

Seventeen lean and twelve obese volunteers were randomized to two weeks of isocaloric diets with approximately 20% or >40% calories from fat in a cross-over fashion. At the end of the high and low fat periods, volunteers received corresponding test meals. ADMA was measured by GC-MS/MS using a deuterated standard. Mean fasting plasma ADMA concentration was 0.52 (0.49-0.54; 95% CI) μmol/l in lean and 0.53 (0.50-0.55) μmol/l in obese subjects (p = 0.55). The two week high fat diet did not influence ADMA. Both test meals elicited a 6%increase in circulating ADMA in lean subjects. In obese subjects, plasma ADMA concentration did not change with the low fat meal, and decreased by approximately 4% with the high fat meal.

CONCLUSION

Our findings challenge the idea that obesity and dietary fat intake have a major effect on plasma ADMA, at least in subjects without overt cardiovascular and metabolic disease. This finding is important with regard to dietary recommendations for weight loss. Overestimation of the influence of dietary fat intake and obesity on circulating ADMA in previous reports was most likely due to methodological issues concerning ADMA measurements.

Quantification of L-arginine, asymmetric dimethylarginine and symmetric dimethylarginine in human plasma: a step improvement in precision by stable isotope dilution mass spectrometry

The amino acid L-arginine and its metabolites ADMA and SDMA are important markers for a range of diseases in humans. Increased levels of ADMA and SDMA in plasma point to endothelial dysfunction, hypertension, renal impairment and other pathological states. We present here a method to quantify L-arginine, ADMA and SDMA in human plasma, which is suitable to support clinical research in this field. Sample preparation consisted only of protein precipitation and the analytes were separated using a silica based HILIC column. The analytes were detected by ESI MS/MS, providing high selectivity and sensitivity. The calibration functions were linear in the ranges of 7.5-150 μmol/l for l-arginine, 0.15-3 μmol/l for ADMA and 0.2-4 μmol/l for SDMA. These ranges cover the concentrations encountered in healthy and pathological human plasma. The method employs (13)C(6)-arginine, D(7)-ADMA and, for the first time in LC-MS/MS, D(6)-SDMA as internal standards for L-arginine, ADMA and SDMA. Therefore, matrix independency and a high intra-day precision of 0.82% for L-arginine, 2.12% for ADMA and 2.83% for SDMA, were achieved at basal plasma concentrations. The respective inter-day precision values were 4.01% for l-arginine, 3.77% for ADMA and 3.86% for SDMA.

HIV replication, inflammation, and the effect of starting antiretroviral therapy on plasma asymmetric dimethylarginine, a novel marker of endothelial dysfunction

BACKGROUND

HIV infection is associated with premature development of cardiovascular disease. Understanding the effects of HIV replication on endothelial dysfunction and platelet activation may identify treatment targets to reduce cardiovascular disease risk.

METHODS

A subgroup of HIV-infected participants in the Strategies for Management of Antiretroviral Therapy study off antiretroviral therapy (ART) at entry enabled a randomized comparison of immediate versus deferred ART initiation of changes in asymmetric dimethylarginine (ADMA), soluble CD40 ligand (sCD40L), and P-selectin levels.

RESULTS

At study entry, median (interquartile range) levels of ADMA, sCD40L, and P-selectin were 0.57 (0.49-0.66) μg/mL, 251 (135-696) μmol/L, and 34 (28-44) pg/mL. Compared to those randomized to deferral of ART (n = 114), participants randomized to immediate ART (n = 134) had 10.3% lower ADMA levels (P = 0.003) at 12 months; treatment differences in sCD40L (95% confidence interval: -17% to 44%; P = 0.53) and P-selectin (95% confidence interval: -10% to 10%; P = 0.95) were not significant. The difference in ADMA for those assigned immediate ART compared with those assigned ART deferral was greater among younger patients and those with higher levels of high-sensitivity C-reactive protein and D-dimer (P ≤ 0.05 for interaction for both) but not HIV RNA level at baseline (P = 0.51).

DISCUSSION

ART initiation leads to declines in ADMA levels, a marker of nitric oxide-mediated endothelial dysfunction. Improvement in ADMA levels was related to the degree of inflammation and coagulation, suggesting that upregulation of these pathways contributes to premature vascular disease among individuals with HIV infection. Whether declines in ADMA levels impact risk of disease requires further research.

New role for L-arginine in regulation of inducible nitric-oxide-synthase-derived superoxide anion production in raw 264.7 macrophages

Dietary supplementation with L-arginine was shown to improve immune responses in various inflammatory models. However, the molecular mechanisms underlying L-arginine effects on immune cells remain unrecognized. Herein, we tested the hypothesis that a limitation of L-arginine could lead to the uncoupled state of murine macrophage inducible nitric oxide synthase and, therefore, increase inducible nitric-oxide-synthase-derived superoxide anion formation. Importantly, we demonstrated that L-arginine dose- and time dependently potentiated superoxide anion production in bacterial endotoxin-stimulated macrophages, although it did not influence NADPH oxidase expression and activity. Detailed analysis of macrophage activation showed the time dependence between LPS-induced iNOS expression and increased O(2)(∙-) formation. Moreover, downregulation of macrophage iNOS expression, as well as the inhibition of iNOS activity by NOS inhibitors, unveiled an important role of this enzyme in controlling O(2)(∙-) and peroxynitrite formation during macrophage stimulation. In conclusion, our data demonstrated that simultaneous induction of NADPH oxidase, together with the iNOS enzyme, can result in the uncoupled state of iNOS resulting in the production of functionally important levels of O(2)(∙-) soon after macrophage activation with LPS. Moreover, we demonstrated, for the first time that increased concentrations of L-arginine further potentiate iNOS-dependent O(2) (∙-) formation in inflammatory macrophages.

Reference intervals for plasma L-arginine and the L-arginine:asymmetric dimethylarginine ratio in the Framingham Offspring Cohort

L-arginine, as a precursor of NO synthesis, has attracted much scientific attention in recent years. Experimental mouse models suggest that L-arginine supplementation can retard, halt, or even reverse atherogenesis. In human studies, supplementation with L-arginine improved endothelium-dependent vasodilation. However, L-arginine levels are best interpreted in the context of levels of asymmetric dimethylarginine (ADMA), a competitive inhibitor of NO synthase. Thus, reference limits for circulating L-arginine and the L-arginine:ADMA ratio may help to determine the nutritional state of individuals at high cardiovascular risk in light of increased ADMA levels. We defined reference limits for plasma L-arginine in 1141 people and for the L-arginine:ADMA ratio in 1138 relatively healthy individuals from the Framingham Offspring Cohort. Plasma L-arginine and ADMA concentrations were determined by using a stable isotope-based LC-MS/MS method. The reference limits (2.5th and 97.5th percentiles) for plasma L-arginine were 41.0 μmol/L (95% CI = 39.5-42.5 μmol/L) and 114 μmol/L (95% CI = 112-115 μmol/L), whereas corresponding reference limits (2.5th and 97.5th percentiles) for the L-arginine:ADMA ratio were 74.3 μmol/L (95% CI = 71.1-77.3 μmol/L) and 225 μmol/L (95% CI = 222-228 μmol/L). Plasma L-arginine was positively associated with the estimated glomerular filtration rate (eGFR) and blood glucose levels, whereas the L-arginine:ADMA ratio was positively associated with eGFR and diastolic blood pressure but inversely associated with homocysteine and (log)C-reactive protein. We report reference levels for plasma L-arginine and for the L-arginine:ADMA ratio that may be helpful for evaluation of the effects of L-arginine supplementation in participants with an impaired L-arginine/NO pathway.

Plasma symmetric dimethylarginine reference limits from the Framingham offspring cohort

BACKGROUND

Symmetric dimethylarginine (SDMA) is a by-product of protein methylation. Once released from proteins, SDMA is eliminated by the kidneys; consequently, plasma concentration has been suggested as a sensitive marker of renal function. Furthermore, recent work implicates SDMA in the pathogenesis of cardiovascular disease. To date, reference limits for SDMA plasma concentrations in healthy individuals are lacking.

METHODS

This study defined reference limits for plasma SDMA concentrations in 840 relatively healthy individuals of the Offspring Cohort from Framingham Heart Study (mean age 56 years, 61% women). Plasma SDMA concentrations were determined by LC-MS/MS using a stable isotope dilution assay.

RESULTS

The median SDMA concentration in the reference sample was 0.37 μmol/L (Q1, Q3:0.32, 0.43 μmol/L) and the reference limits were 0.225 and 0.533 (2.5th and 97.5th percentile). In a multivariable regression model, serum creatinine, age and total homocysteine were positively associated with SDMA (p<0.001 for all), whereas the body mass index and diastolic blood pressure were inversely related to SDMA (p-values<0.01 and 0.03, respectively).

CONCLUSIONS

This study reports plasma SDMA reference limits from the community-based Framingham Heart Study. Plasma SDMA concentration was related positively to advancing age, but inversely to renal function. These reference limits may allow the identification of individuals with raised plasma SDMA concentrations.