Transient Triglyceridemia Decreases Vascular Reactivity in Young, Healthy Men Without Risk Factors for Coronary Heart Disease
Background Hypertriglyceridemia is now accepted as a risk factor for coronary heart disease, although the mechanism behind the increased risk is not well understood. The present study was undertaken to investigate the effects of triglyceridemia on endothelial function, because impaired endothelial function is considered a marker of atherogenesis.
Methods and Results Flow- and nitroglycerin-induced dilatation of the brachial artery was investigated noninvasively by high-resolution ultrasound technique in seven young, healthy men without risk factors for coronary heart disease. Transient triglyceridemia was induced by infusion of a triglyceride emulsion, Intralipid, which raised free fatty acid concentrations twofold and triglyceride levels fourfold. Flow-induced vasodilatation decreased from 7.1±3.0% to 1.6±2.6% (P<.0002), whereas nitroglycerin-induced vasodilatation decreased from 20.5±5.8% to 11.5±3.2% (P<.002) before and after 1 hour of infusion of Intralipid, respectively.
Conclusions Transient triglyceridemia decreases vascular reactivity, presumably by both endothelium-dependent and endothelium-independent mechanisms.
Endothelial dysfunction in the form of impaired endothelium-dependent vasodilatation has been proposed to be a marker of atherogenesis. Patients with coronary heart disease (CHD) have been shown to have impaired endothelium-dependent relaxation in brachial1 and coronary2 arteries, with a close correlation between the two sites, indicating a systemic nature of endothelial dysfunction in atherosclerosis.3 Further support for the role of endothelial dysfunction in the pathogenesis of atherosclerosis includes the fact that patients with risk factors for CHD, such as smoking, hypertension, hypercholesterolemia, and non–insulin-dependent diabetes mellitus, have impaired endothelium-dependent vasodilatation.4–6
Previously, investigators of endothelial dysfunction in hyperlipidemia have focused on hypercholesterolemia. However, there is a lack of studies of hypertriglyceridemia, which also is considered a risk factor for CHD,7 although the mechanism behind the increased risk is less well understood. To find out whether triglyceridemia affects endothelial function, we examined flow- and nitroglycerin (NTG)-induced vasodilatation of the brachial artery noninvasively by high-resolution ultrasound technique in young, healthy men without risk factors for CHD. Infusion of a triglyceride emulsion, Intralipid, was chosen to give a reproducible transient triglyceridemia without the possible interference of carbohydrate-induced hyperinsulinemia.
Seven healthy, nonsmoking men 23±3 years old (mean±SD) participated after giving informed consent. All individuals were normotensive and had no family history of CHD before the age of 65 years. All individuals were shown to have normal fasting plasma triglyceride and cholesterol concentrations and a normal flow-induced dilatation of the brachial artery at a screening visit.
All subjects were studied on two separate occasions at least 4 hours after a light breakfast. First, the response to flow-induced vasodilatation was measured twice before and once after 1 hour of infusion of Intralipid. The experiment was terminated by measurement of vasodilatation in response to 0.4 mg of buccal NTG. Because of the poor response to NTG in the first series of experiments in comparison with the results reported by Celermajer and coworkers,8 a second series of experiments was made in which the response to both flow- and NTG-induced vasodilatation was investigated before and after 1 hour of infusion of Intralipid. Intralipid (Pharmacia-Upjohn) was given as a bolus of 0.15 g/kg body wt followed by continuous infusion of 0.15 g · kg−1 · h−1 until the experiment was finished.
The study was approved by the local Ethics Committee at the Karolinska Hospital.
Ultrasound Technique and Lipid Determinations
Endothelium-dependent flow-induced and endothelium-independent NTG-induced dilatation of the left brachial artery were measured noninvasively by high-resolution ultrasound Doppler technique.8 All ultrasound scans were made with Acuson 128 XP/10 equipped with a 7-MHz ART linear array transducer. The subjects were examined after 30 minutes at rest in a quiet, dark room with a temperature of 22°C to 23°C. The brachial artery was scanned longitudinally 2 to 10 cm above the elbow, and the transducer was held in the same position during the experiment by a special device. The baseline measurements of blood flow and the ID of the brachial artery were taken at rest and 1 minute after reactive hyperemia (RH) provoked by distal forearm artery occlusion with a 12.5-cm-wide blood pressure cuff placed on the forearm and inflated up to 300 mm Hg for 4.5 minutes, followed by release. The subjects then rested for 10 minutes. New ultrasound measurements were performed after a new RH or 4 minutes after administration of 0.4 mg buccal NTG. All measurements of the ID of the brachial artery were performed with the investigator blinded to the sequence of the recordings. The variation in differences between flow-induced vasodilatation determined twice during the same day and between three determinations on separate days were 0.88±0.82% and 3.34±2.68%, respectively.
Blood samples were drawn into vacuum tubes containing 1.4 mg Na2-EDTA/mL before and during the infusion of Intralipid. Baseline samples were drawn before the first ultrasound measurement and then repeated after 30, 60, and 90 minutes during the infusion. Plasma was recovered after low-speed centrifugation at 3000 rpm for 15 minutes and immediately frozen at −20°C. Triglyceride, cholesterol, and free fatty acid concentrations were determined after extraction9,10 by chemical methods11–13 on thawed samples.
Values are expressed as mean±SD. Statistical testing of differences between flow- and NTG-induced vasodilatation before and after 1 hour of infusion of Intralipid was made by Student’s paired t test and ANOVA.
Fasting plasma concentrations of triglyceride and cholesterol, sampled on two different occasions before infusion of Intralipid, were 0.75±0.10 and 3.34±0.33 mmol/L, respectively. Triglyceride and cholesterol concentrations in plasma increased in response to infusion of Intralipid to 2.85±0.82 and 3.67±0.35 mmol/L, respectively, taking into account all determinations during the two infusions. Free fatty acid concentrations in plasma increased from a fasting level of 572±112 to 1230±206 μmol/L during infusion of Intralipid (Fig 1⇓).
In the first series of experiments, RH induced by occlusion of the distal forearm resulted in an increase in the ID of the brachial artery by 7.1±3.0%, from 3.91±0.52 to 4.18±0.53 mm. After 1 hour of infusion of Intralipid, flow-induced vasodilatation was only 1.6±2.6%, increasing from 3.96±0.58 to 4.02±0.56 mm (P<.0002, Student’s paired t test), compared with vasodilatation before infusion (Fig 2A⇓). NTG-induced vasodilatation after infusion of Intralipid was 13.8±3.8%. In the second series of experiments, administration of 0.4 mg of buccal NTG resulted in an increase in the ID of the brachial artery by 20.5±5.8%, from 3.77±0.49 to 4.55±0.67 mm. After 1 hour of infusion of Intralipid, NTG-induced vasodilatation was only 11.5±3.2%, increasing from 4.01±0.54 to 4.47±0.68 mm (P<.002, ANOVA), compared with vasodilatation before infusion (Fig 2B⇓). If the increase in ID of the brachial artery after NTG was related to the baseline diameter before the start of the infusion of Intralipid, the increase was 18.5±0.7% (P=NS) compared with vasodilatation before infusion. Flow-induced dilatation of the brachial artery was investigated also in the second series of experiments with results similar to those in the first series, resulting in a decrease in dilatation of the brachial artery from 7.4±2.0% to 2.3±2.1% before and after 1 hour of infusion of Intralipid, respectively (P<.0005, ANOVA).
The results of the present study showed that transient triglyceridemia induced by the triglyceride emulsion Intralipid results in decreased vasoreactivity of the brachial artery as measured by noninvasive high-resolution ultrasound technique.
Our hypothesis was that plasma triglycerides or their catabolites free fatty acids formed during lipolysis at the endothelial surface would impair endothelium-dependent vasodilatation. To our surprise, however, the results showed that infusion of Intralipid decreased both flow- and NTG-induced dilatation of the brachial artery. Unfortunately, there was a drift in baseline ID of the brachial artery in the second series of experiments that might have influenced the results of NTG-induced vasodilatation. Therefore, it cannot be ascertained from the present results whether the impairment of vascular reactivity is endothelium-dependent or endothelium-independent. Presumably, the effect of transient triglyceridemia on brachial artery dilatation is both endothelium-dependent and endothelium-independent.
The cause of the decreased vasoreactivity after infusion of Intralipid is not clear but certainly includes effects of triglycerides or free fatty acids, which both increase in plasma during the infusion. It can be speculated that the increased concentrations of lipids inside and subintimal to the endothelium might decrease the diffusion of NO, causing spontaneous degradation before NO reaches the smooth vasculature. Furthermore, the increased concentrations of free fatty acid might inactivate NO by oxidative mechanisms. Other possible mechanisms include interference of free fatty acids with the synthesis of prostaglandins, resulting in an imbalance between vasoconstrictive and vasodilating substances.
The results of the present study might have several clinical implications. First, the increase in triglyceride and free fatty acid concentrations acquired during infusion of Intralipid is of a magnitude similar to the changes after a fatty meal. A decrease in the ability to vasodilate coronary arteries after a fatty meal on stimuli such as exercise could explain the phenomenon of postalimentary angina pectoris in patients with known coronary artery disease.14 Second, intensive care patients receiving parenteral nutrition with Intralipid would clearly not benefit from the inability to vasodilate on endogenous as well as exogenous stimuli.
The use of infusion of an artificial lipid emulsion such as Intralipid to increase the concentrations of triglyceride and free fatty acid is a possible limitation to the study. Another approach would have been to examine vasoreactivity of the brachial artery after the ingestion of a fatty meal. The latter was adopted by Vogel and coworkers,15 who showed that flow-induced dilatation of the brachial artery was impaired after intake of a fatty meal and that the increase in triglycerides correlated with the decrease in vasodilatation. However, they did not exclude an effect on endothelium-independent mechanisms, with the exception of a subgroup consisting of four individuals examined only once after intake of the fatty meal. The advantage of our approach is that the vascular effects of triglycerides and free fatty acids can be investigated without disturbances of vasoactive hormones, such as insulin, which increase after a fatty meal. In addition, the rise in triglyceride and free fatty acid concentrations after infusion of Intralipid is more reproducible than after a fatty meal.
Further support for the results of the present study comes from investigations of patients with non–insulin-dependent diabetes mellitus,5,6 which showed an impairment in both endothelium-dependent and endothelium-independent forearm blood flow compared with healthy control subjects. In fact, in the study by Watts and coworkers,6 this impairment was associated with increased triglyceride concentrations. In contrast, in a study by Chowienczyk and coworkers,16 patients with severe hypertriglyc-eridemia due to lack of lipoprotein lipase showed no impairment in forearm blood flow compared with normolipidemic control subjects. The discrepancy between the results of the present study and the study by Chowienczyk might indeed be explained by the lack of lipoprotein lipase at the endothelial surface, resulting in decreased generation and concentration of free fatty acid at this site, indicating a role for free fatty acids in the decreased vascular reactivity found in our study.
In summary, the results of the present study showed that transient triglyceridemia decreased vascular reactivity in the brachial artery in young, healthy men without risk factors for CHD. Further studies are needed to examine the role of triglycerides versus free fatty acids and to study the mechanisms behind the impairment in vascular reactivity. There is also a need to study whether this occurs in coronary arteries and in chronic hypertriglyceridemia.
This study was supported by the Swedish Heart and Lung Foundation and the Swedish Medical Research Council.
- Received August 27, 1997.
- Revision received September 18, 1997.
- Accepted September 23, 1997.
- Copyright © 1997 by American Heart Association
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