Plasma α-Tocopherol and Coronary Endothelium-Dependent Vasodilator Function
Background—In the presence of atherosclerosis, the coronary endothelial vasomotor response to acetylcholine is frequently abnormal but is variable between patients. We tested the hypothesis that the plasma concentration of α-tocopherol is associated with the preservation of nitric oxide–mediated endothelium-dependent vasomotion.
Methods and Results—We studied 15 men and 6 women (mean age 61±10 years) at coronary angiography who were not taking vitamin supplements. Coronary endothelium-dependent and -independent vasomotion was assessed by intracoronary infusions of acetylcholine and nitroglycerin. The vasomotor responses were compared with the plasma concentration of α-tocopherol and the plasma α-tocopherol concentration relative to total lipid (total cholesterol plus triglycerides). The mean plasma α-tocopherol was 25.6±6.1 μmol/L, total cholesterol 193±27 mg/dL, triglycerides 115±66 mg/dL, and α-tocopherol to total lipid 4.2±0.9 μmol · L−1 · (mmol/L)−1. The mean vasomotor response to acetylcholine was −1% (range −33% to 28%) and to nitroglycerin 22% (range 0% to 54%). Plasma α-tocopherol was significantly correlated with the acetylcholine response (r=0.49, P<0.05) but not the nitroglycerin response (r=0.13, P>0.05). The acetylcholine response remained significant after adjustment for other potential sources of oxidant stress (total cholesterol, diabetes mellitus, smoking, angina class) (P<0.01). The relative concentration of α-tocopherol to total lipid was not related to endothelial function (r=0.24, P=0.3, n=20).
Conclusions—α-Tocopherol may preserve endothelial vasomotor function in patients with coronary atherosclerosis. This effect may be related primarily to the action of α-tocopherol in the vascular wall. Further studies that assess the impact of α-tocopherol supplementation as therapy of endothelial dysfunction are justified.
Nitric oxide plays an essential role in the regulation of arterial tone.1 Nitric oxide also inhibits local inflammation, coagulation, and cell proliferation.1 Loss of nitric oxide impairs these protective mechanisms throughout the development of atherosclerosis.
Evidence is accumulating that oxidant stress contributes to endothelial dysfunction in coronary atherosclerosis by enhancing the degradation and inhibiting the synthesis of nitric oxide.1 2 Antioxidants can restore endothelium-mediated vasodilator function in patients with atherosclerosis or with coronary risk factors.3 4 5 6
In the presence of coronary atherosclerosis or its risk factors, coronary endothelial vasomotor function is abnormal on average but is variable from patient to patient when assessed with acetylcholine or other stimuli.7 8 Endogenous mechanisms may preserve the availability of nitric oxide in some patients.
α-Tocopherol is a potent lipid-soluble antioxidant and the most abundant isomer of vitamin E found in humans.9 It circulates in plasma within LDL particles and is also taken up by arterial wall cells. We tested the hypothesis that plasma concentrations of α-tocopherol account for some of the variability in the coronary endothelium-dependent vasomotor function in patients with coronary atherosclerosis.
We studied 21 patients referred for coronary angiography for evaluation of chest pain. Eligibility criteria included age 18 to 80 years, total cholesterol <280 mg/dL, and a diameter stenosis in ≥1 coronary artery >30%. Subjects were excluded because of rest angina within 24 hours of catheterization; consumption of any vitamin supplements; inability to safely withhold vasoactive medications, including nitrates, ACE inhibitors, or calcium-channel antagonists for ≥18 hours before catheterization; or if angiography revealed extensive 3-vessel or left main coronary artery disease (diameter stenoses >50%).
The study was approved by the Human Research Committee of the Brigham and Women’s Hospital. Written informed consent was obtained from each patient.
Assessment of Coronary Vasoreactivity
Coronary endothelium-dependent responses were assessed according to a protocol established in our laboratory7 by graded infusions of acetylcholine (Miochol-E, OMJ Pharmaceuticals) into the left anterior descending or circumflex arteries over a period of 2 minutes, with estimated final concentrations of 10−8, 10−7, and 10−6 mol/L. Endothelium-independent vasodilation was assessed by an infusion of nitroglycerin at 25 μg/min over a period of 2 minutes.
Quantitative angiography was used to assess the diameters of 2 coronary segments in response to the drug infusions according to a previously established protocol,3 and the measurements were averaged.
Vitamin E Assay
Blood was drawn at the time of coronary angiography after an overnight fast into tubes with heparin anticoagulation. Plasma was stored at −80°C and shipped on dry ice for the measurement of α-tocopherol by high-performance liquid chromatography with electrochemical detection.10 Total, HDL, and LDL cholesterol and triglycerides were measured at the time of coronary angiography. Because the protection of lipoprotein lipids from oxidation depends on the relative concentration of α-tocopherol to total lipid, plasma α-tocopherol levels were expressed both in absolute μmol/L concentration and as the ratio of α-tocopherol (μmol/L)/(total cholesterol+triglycerides) (mmol/L), as described.11
Definitions of Clinical Variables
Diabetes was defined by a medical history of treatment with diet, oral hypoglycemic agents, or insulin; hypertension by the use of antihypertensive therapy; and current smoking as ≥1 cigarette per day in the previous 30 days. Anginal classes were defined according to Braunwald criteria12 as stable angina, unstable angina without rest pain, and unstable angina with rest pain from 24 hours to 1 month before the study.
Of the 21 patients, 7 (33%) had stable angina, 10 (48%) unstable angina without rest pain, and 4 (19%) unstable angina with rest pain from 24 hours to 1 month before catheterization. Six patients (29%) had diabetes mellitus, 6 (29%) hypertension, and 3 (14%) were current smokers. Medications included ACE inhibitors in 3 patients, statins in 7, and conjugated estrogens in 1. Ten (46%) had 2-vessel disease, 9 (43%) had single-vessel disease, and 2 (10%) had a mild stenosis (30% to 50%) in 1 coronary artery.
Plasma α-tocopherol and vasomotor function were described by mean±SD and related by Pearson correlation coefficient and 95% CI. Linear regression was used to adjust this relationship for other potential sources of oxidant stress (total cholesterol level, anginal class, diabetes, and smoking status).
We studied 21 patients, 15 men and 6 women (age 61±10 years, mean±SD). The mean plasma α-tocopherol concentration was 25.6±6.1 μmol/L (SD), the mean cholesterol 193±27 mg/dL, and the mean triglycerides 115±66 mg/dL.
Responses to the maximal concentration of acetylcholine ranged from −33% to 28% diameter change, mean −1±16% (negative numbers indicate vasoconstriction). Responses to nitroglycerin ranged from 0% to 54% dilation, mean 22±13%. The Figure⇓ depicts the relationships between plasma α-tocopherol concentrations and the coronary vasomotor responses to acetylcholine and nitroglycerin. α-Tocopherol concentrations were positively correlated with the maximal response to acetylcholine (r=0.49, 95% CI=0.07, 0.99; P<0.05). In contrast, the responses to the endothelium-independent vasodilator nitroglycerin were not associated with plasma α-tocopherol concentrations (r=0.13, 95% CI=−0.33, 0.59; P=0.6).
Other potential sources of oxidant stress or endothelial vasodilator dysfunction (including medications) did not diminish the strength of the relationship between plasma α-tocopherol concentrations and the response to acetylcholine. After adjustment for serum cholesterol, diabetes mellitus, smoking, and anginal class, for every 10-μmol/L increase in α-tocopherol there was an 18% increase in the vasodilator response to acetylcholine (95% CI=7%, 30%; P<0.01).
The mean plasma α-tocopherol concentration adjusted for total lipid was 4.2±0.9 μmol · L−1 · (mmol/L)−1. There was no significant relationship between endothelium-dependent vasomotion and lipid-adjusted plasma α-tocopherol levels (r=0.24, 95% CI=−0.23, 0.62; P=0.3, n=20).
In patients with coronary atherosclerosis, we found a positive relationship between plasma concentration of the antioxidant α-tocopherol and endothelium-dependent vasodilator function. Low concentrations of α-tocopherol were associated with a loss of endothelium-dependent vasodilation, whereas high concentrations of α-tocopherol were associated with preserved endothelial function. Thus, the concentration of α-tocopherol may contribute to the variability in this endothelium-dependent response.
Endothelium-dependent vasodilation is impaired in the presence of atherosclerosis or risk factors for atherosclerosis. However, endothelial dysfunction is mutable. For example, administration of lipid-lowering therapy,1 ACE inhibitors,13 estrogen replacement,14 l-arginine,15 16 or antioxidant therapy3 4 5 6 can restore endothelium-dependent vasomotor responses to acetylcholine or to shear stress.
Oxidant stress is an important mechanism of endothelial vasodilator dysfunction by inactivating nitric oxide and reducing its synthesis.1 2 Several pathways have been implicated in generating oxygen-derived free radicals in atherosclerosis, including NADH/NADPH oxidases, xanthine oxidase, lipoxygenases, and nitric oxide synthase.2 17 18 Natural antioxidant defense mechanisms are in place to combat oxidant stress; these include intracellular and extracellular forms of superoxide dismutase, glutathione and glutathione peroxidases, catalase, and the antioxidant vitamins E and C.2 18
α-Tocopherol, an isomer of vitamin E, is the most abundant lipid-soluble antioxidant in humans and concentrates in LDL particles as well as in the arterial wall. Supplementation with vitamin E has been shown to augment the bioavailability of endothelium-derived nitric oxide in experimental atherosclerosis.19 Treatment with vitamin E reversed the impairment of endothelium-dependent vasodilation in the brachial artery of patients with coronary vasospastic angina.5
Low vitamin E concentrations may be a cause or a consequence of endothelial dysfunction and elevated oxidant stress. In the present study, a low concentration of vitamin E may have been a causal factor in endothelial dysfunction because it permitted oxidative damage to occur. The presence of a relationship between endothelial vasomotor function and the absolute concentration of α-tocopherol, and the absence of a relationship between endothelial vasomotor function and α-tocopherol adjusted for total lipid (an index of lipoprotein protection by α-tocopherol) suggest that α-tocopherol acts primarily in the arterial wall.19 α-Tocopherol accounted for ≈25% of the variance in the acetylcholine response (r2=0.24), with the remainder explained by other factors.
Our study enrolled patients who derived vitamin E from their diet only and excluded patients consuming vitamin supplements. Further studies that assess the impact of vitamin E supplementation on coronary endothelial function are necessary to assess the role of vitamin E as potentially useful therapy. The present study supports the hypothesis that plasma α-tocopherol concentration favorably affects coronary endothelial function in patients with coronary atherosclerosis, thus lowering the risk of clinical events.9
This study was supported by NIH grants PO1-HL-48743 and 1P50-HL-56985.
- Received April 7, 1999.
- Revision received May 21, 1999.
- Accepted May 26, 1999.
- Copyright © 1999 by American Heart Association
Motoyama T, Kawano H, Kugiyama K, Hirashima O, Ohgushi M, Tsunoda R, Moriyama Y, Miyao Y, Yoshimura M, Ogawa H, Yasue H. Vitamin E administration improves impairment of endothelium-dependent vasodilation in patients with coronary spastic angina. J Am Coll Cardiol. 1998;32:1672–1679.
Levine GN, Frei B, Koulouris SN, Gerhard MD, Keaney JF Jr, Vita JA. Ascorbic acid reverses endothelial vasomotor dysfunction in patients with coronary artery disease. Circulation. 1996;93:1107–1113.
Gordon JB, Ganz P, Nabel EG, Fish RD, Zebede J, Mudge GH, Alexander RW, Selwyn AP. Atherosclerosis influences the vasomotor response of epicardial coronary arteries to exercise. J Clin Invest. 1989;83:1946–1952.
Stocker R, Bowry VW, Frei B. Ubiquinol-10 protects human low density lipoprotein more efficiently against lipid peroxidation than does alpha-tocopherol. Proc Natl Acad Sci U S A. 1991;88:1646–1650.
Thurnham DI, Davies JA, Crump BJ, Situnayake RD, Davis M. The use of different lipids to express serum tocopherol: lipid ratios for the measurement of vitamin E status. Ann Clin Biochem. 1986;23:514–520.
Braunwald E. Unstable angina: a classification. Circulation. 1989;80:410–414.
Mancini GBJ, Henry GC, Macaya C, O’Neill BJ, Pucillo AL, Carere RG, Wargovich TJ, Mudra H, Lüscher TF, Klibaner MI, Haber HE, Uprichard ACG, Pepine CJ, Pitt B. Angiotensin-converting enzyme inhibition with quinapril improves endothelial vasomotor dysfunction in patients with coronary artery disease: the TREND (Trial on Reversing ENdothelial Dysfunction) study. Circulation. 1996;94:258–265.
Kinlay S, Gerhard M, Selwyn AP, Creager MA, Ganz P. Estrogen’s effects on human coronary and peripheral vasoreactivity. In: Rubani GM, Kauffman R, eds. Estrogen and the Vessel Wall. Amsterdam, Netherlands: Harwood Academic Publishers; 1998:251–260.
Lerman A, Burnett JC Jr, Higano ST, McKinley LJ, Holmes DR Jr. Long-term l-arginine supplementation improves small-vessel coronary endothelial function in humans. Circulation. 1998;97:2123–2128.
Harrison DG. Endothelial function and oxidant stress. Clin Cardiol. 1997;20:II-11–17.
Wever RM, Luscher TF, Cosentino F, Rabelink TJ. Atherosclerosis and the two faces of endothelial nitric oxide synthase. Circulation. 1998;97:108–112.