Author + information
- Tienush Rassaf, MD* (, )
- Christian Heiss, MD,
- Sarah Mangold, MD,
- Thorsten Leyendecker, MD,
- Eva S. Kehmeier, MD,
- Malte Kelm, MD and
- Thomas Lauer, MD
- ↵*Department of Medicine, Division of Cardiology, Pulmonology and Vascular Medicine, University Hospital Düsseldorf, Moorenstrasse 5, 40225 Dusseldorf, Germany
To the Editor:
Endothelial dysfunction with an impaired bioactivity of nitric oxide (NO) is associated with coronary artery disease (CAD). Plasma nitrite reflects endothelial NO-synthase (eNOS) activity under fasting conditions (1). Using plasma nitrite as a marker for endothelial dysfunction in individual patients, however, was hampered because baseline nitrite levels show great variability. We tested the hypothesis that the level of plasma nitrite after exercise identifies patients with major cardiovascular risk factors (CVRF) and CAD, unmasking endothelial dysfunction.
Twenty-five patients with CVRF and CAD as determined by coronary angiography and 25 elderly healthy control subjects without major cardiovascular risk factors were enrolled (age 58 ± 1 years and age 61 ± 7 years, respectively; p = 0.07). Individuals were studied in the morning after a 12-h fasting period. Endothelial function was assessed as flow-mediated dilation (FMD) of the brachial artery. All subjects underwent an ergometric exercise test with a stepwise increase in force (2). Blood (1 ml each) was taken before (basal) and 10 min after exercise termination (peak) from the antecubital vein. Time point of peak was determined in a subset of 15 subjects, where blood was drawn consecutively for 60 min. Nitrite levels were measured as described (3). Differences were assessed by repeated measurements analysis of variance, with p values for multiple comparisons adjusted by the Bonferroni criterion. Comparisons between 2 groups were performed with unpaired t test. A multivariate regression analysis was performed to determine the independent factors of the changes in nitrite. Our study was approved by the ethics board of the medical faculty of the local university, and all participants gave written informed consent.
Patients with CVRF and CAD had an impaired FMD (3.9 ± 0.4% vs. 5.4 ± 0.4%, CVRF and CAD vs. control; p = 0.01), suggesting endothelial dysfunction. Baseline nitrite did not differ (80 ± 8 nmol/l vs. 82 ± 10 nmol/l, CVRF and CAD vs. control; p = 0.33). Ergometric exercise stress measured as rate pressure product was similar in both groups (16,353 ± 689 mm Hg/min vs. 16,760 ± 751 mm Hg/min; p = 0.46). In healthy subjects, exercise increased plasma nitrite by 22 ± 8% (p < 0.001). No increase but rather a decrease in plasma nitrite was seen in patients with CAD (−7.0 ± 4%; p = 0.52). With a cutoff point of post-exercise increase in nitrite of −4.5%, a sensitivity of 72%, a specificity of 88% with a positive predictive value of 0.86, and a negative predictive value of 0.76 was calculated. In a multivariate regression model only FMD (p = 0.047) and total cholesterol (p = 0.038) remained independent predictors of nitrite increase.
For a long time the nitrite anion has been considered as a meta-stable intermediate in the oxidation of NO to nitrate. However, experimental and clinical studies over the past years challenged this dogma. There is emerging evidence that, along the physiological and pathophysiological oxygen gradient, nitrite plays a crucial role in hypoxic signaling, including the regulation of perfusion to metabolically active tissues (4). The amount of circulating nitrite is determined by the rate of production and by its consumption. Nitrite is mainly produced by eNOS. Deoxygenated hemoglobin acts as a nitrite reductase that consumes nitrite and whose activity is controlled by the ambient oxygen level. Mechanistically, a local drop in oxygen levels favors the reduction of nitrite to nitric oxide, which in turn leads to physiologically required vasodilation. During exercise, systemic shear stress leads to activation of eNOS with consecutive increase in plasma nitrite levels. In parallel, oxygen levels in the circulation and in the muscle drop, leading to nitrite consumption. The reduced eNOS activity in patients with cardiovascular risk factors and CAD (5) leads to an imbalance in nitrite homeostasis and thus to a drop in circulating nitrite levels in these subjects.
The assessment of plasma nitrite after ergometric exercise might offer a noninvasive, nonradioactive, and cost-effective approach to screen patients for the presence of vascular dysfunction. This might allow early risk stratification in clinical practice. Further evaluation and studies are necessary to determine the value of this test as an additional screening test in the diagnosis of CAD.
Please note: Drs. Rassaf and Kelm were supported by grants from the Deutsche Forschungsgemeinschaft (DFG RA 969/5-1 to Dr. Rassaf and DFG KE 405/5-1 to Dr. Kelm). Dr. Rassaf is a Heisenberg scholar of the DFG.
- American College of Cardiology Foundation