Diminished global arginine bioavailability and increased arginine catabolism as metabolic profile of increased cardiovascular risk

Pulmonary hypertension in thalassemia

Pulmonary hypertension (PH) is common in thalassemia and contributes to mortality. Advancing age and a history of splenectomy are major risk factors in this population. The etiology of PH is multifactorial, involving a complex interaction of platelets, the coagulation system, erythrocytes, and endothelial cells along with inflammatory and vascular mediators. The long-term effect of splenectomy, red cell membrane pathology, coagulation abnormalities, low nitric oxide (NO) bioavailability, excess arginase activity, platelet activation, oxidative stress, iron overload, and chronic hemolysis play a role. The process of hemolysis disables the arginine-NO pathway through the simultaneous release of erythrocyte arginase and cell-free hemoglobin. Both NO and its obligate substrate arginine are rapidly consumed. The biological consequences of hemolysis on NO bioavailability ultimately translate into the clinical manifestations of PH. Guidelines for the management of PH in thalassemia have not yet been established; however, clinical trials are ongoing in an effort to guide future therapy.

Plasma levels of high-density lipoprotein cholesterol and outcomes in pulmonary arterial hypertension

RATIONALE

High-density lipoprotein cholesterol (HDL-C) promotes healthy vascular function, and it is decreased in insulin resistance. Insulin resistance predisposes to pulmonary vascular disease.

OBJECTIVES

We hypothesized that HDL-C is associated with clinical outcomes in pulmonary arterial hypertension (PAH).

METHODS

Plasma HDL-C concentrations were measured in 69 patients with PAH (age, 46.7 +/- 12.9 yr; female, 90%) and 229 control subjects (age, 57 +/- 13 yr; female, 48%). Clinical outcomes of interest included hospitalization for PAH, lung transplantation, and all-cause mortality. Survival and time to clinical worsening curves were derived by the Kaplan-Meier method. Cox regression modeling of outcome versus HDL-C with individual covariate adjustments was performed.

MEASUREMENT AND MAIN RESULTS

HDL-C was low in subjects with PAH compared with control subjects (median, interquartile range: PAH: 36, 29-40 mg/dl; control subjects: 49, 40-60 mg/dl; P < 0.001). An HDL-C level of 35 mg/dl discriminated survivors from nonsurvivors, with a sensitivity of 100% and specificity of 60%. After a median follow-up of 592 days, high HDL-C was associated with decreased mortality (hazard ratio for every 5-mg/dl increase in HDL-C, 0.643; 95% confidence interval, 0.504-0.822; P = 0.001) and less clinical worsening (hazard ratio for every 5-mg/dl increase in HDL-C, 0.798; 95% confidence interval, 0.663-0.960; P = 0.02). HDL-C remained a significant predictor of survival after adjusting for cardiovascular risk factors, C-reactive protein, indices of insulin resistance, and severity of PAH (all P < 0.05).

CONCLUSIONS

Low plasma HDL-C is associated with higher mortality and clinical worsening in PAH. This association does not appear to be explained by underlying cardiovascular risk factors, insulin resistance, or the severity of PAH.

Biomarkers of risk stratification in congestive heart failure: North American view

There is broad adoption of various cardiac and noncardiac biomarkers in clinical practices across North America for the diagnosis and management of heart failure. Like any clinical condition, there are several overall objectives in biomarker testing: to establish or refute a diagnosis of heart failure and/or cardiac dysfunction; to understand the underlying pathophysiologic processes that may warrant specific interventions; to determine the level of disease severity in a manner to triage medical decisions; to detect and potentially avoid adverse consequences as a result of therapeutic interventions; and to monitor responses to treatment. While at present no single biomarker can serve all of these objectives, the growing experience with cardiac-specific biomarkers, such as natriuretic peptide and cardiac troponin testing, has allowed clinicians to better identify those at heightened short- or long-term risk. It is clear that difficulty remains in translating research evidence into clinical practice. While studies demonstrate statistical differences in short- and long-term outcomes, there is still limited information on how such improvements can be achieved solely based on current therapeutic options. Meanwhile, many commonly ordered tests have important prognostic information that can be overlooked, yet their changes may or may not affect prognosis. With healthcare cost on the rise in North America, clinical utility of biomarkers must demonstrate relative safety and potential incremental benefits to standard approaches.

Simultaneous determination of l-citrulline and l-arginine in plasma by high performance liquid chromatography

OBJECTIVE

To develop a method for simultaneously determining l-citrulline and L-arginine levels in plasma using RP-HPLC with ultraviolet detection.

DESIGN AND METHODS

Plasma samples were deproteinized by trichloroacetic acid and heat. Phenyl-isothiocyanate (PITC) solution was used as derivatization reagent and a gradient elution was carried out.

RESULTS

The linearity for L-citrulline and L-arginine ranged from 0 to at least 1000 micromol/L. R(2) values were above 0.9999 for both. LODs for L-citrulline and L-arginine were 0.0201 micromol/L and 0.0476 micromol/L, respectively, while LOQs were 0.240 micromol/L and 0.448 micromol/L, respectively. Intra- and inter-day CVs were less than 3.40% and 7.2%, respectively. The average recovery was from 86.22% to 118.9%. L-citrulline and L-arginine concentrations in healthy controls were 60.77+/-9.18 micromol/L and 58.19+/-16.43 micromol/L, respectively.

CONCLUSION

This approach offers a reliable, efficient analytical platform for the simultaneous determination of citrulline and arginine levels in plasma.

Targeted metabolomic evaluation of arginine methylation and cardiovascular risks: potential mechanisms beyond nitric oxide synthase inhibition

BACKGROUND

We examine the relationship of related posttranslational modification products of arginine methylation and coronary artery disease (CAD) phenotypes.

METHODS AND RESULTS

Plasma was isolated from 1011 consecutive consenting subjects undergoing elective diagnostic cardiac catheterization, and future major adverse cardiac events (MACE, including myocardial infarction, stroke, and death) at 3 years were investigated. Plasma levels of asymmetrical dimethylarginine (ADMA, endogenous nitric oxide synthase [NOS] inhibitor), symmetrical dimethylarginine (SDMA, which lacks NOS inhibitory activity), N-mono-methylarginine (MMA, a potent NOS inhibitor), methyl-lysine (Methyl-Lys, an unrelated methylated amino acid), arginine, and its major catabolites (citrulline and ornithine) were quantified simultaneously by stable isotope dilution HPLC with online electrospray ionization tandem mass spectrometry and adjusted for traditional risk factors, C-reactive protein, and estimated creatinine clearance. High SDMA levels (adjusted odds ratio [OR] 1.6, 95%CI, 1.1 to 2.6, P<0.001), low MMA (adjusted OR 0.5, 95%CI 0.4 to 0.8, P=0.007), but not ADMA (adjusted OR 1.3, 95%CI 0.88 to 2.0, P=0.177) were associated with increased prevalence of significantly obstructive CAD. Elevated levels of SDMA (adjusted Hazard Ratio [HR] 2.4, 95%CI 1.2 to 4.6, P=0.009), ADMA (adjusted HR 2.2, 95%CI 1.2 to 4.0, P=0.015), as well as an integrated index of arginine methylation [ArgMI=(ADMA+SDMA)/MMA] (adjusted HR 2.4, 95%CI 1.3 to 4.5, P=0.006) were significant independent predictors of incident MACE. ArgMI was predictive of incident MACE even following adjustments for global arginine bioavailability, particularly within secondary prevention patients.

CONCLUSIONS

ADMA, SDMA, and the integrated quantification of arginine methylation (in the form of a methylation index) provided independent risk prediction for both significantly obstructive CAD and incident MACE in stable patients undergoing cardiac evaluation. These results suggest that factors beyond direct NOS inhibition contribute to the clinical associations between methylarginines and CAD outcomes.