Author + information
- Received September 11, 1998
- Revision received February 16, 1999
- Accepted March 19, 1999
- Published online July 1, 1999.
- Hanneke W Wilmink, MD∗,
- Jan D Banga, MD, PhD∗,
- Michel Hijmering, MD†,
- Willem D Erkelens, MD, PhD∗,
- Erik S.G Stroes, MD, PhD† and
- Ton J Rabelink, MD, PhD†,* ()
- ↵*Reprint requests and correspondence: Dr. T. J. Rabelink, Department of Nephrology and Vascular Medicine, F 03.226, University Hospital Utrecht, Heidelberglaan 100, 3584 CX Utrecht, The Netherlands
The purpose of this study was to determine whether endothelial dysfunction as a consequence of direct postprandial lipid response might be favorably influenced by angiotensin-converting enzyme inhibitors or angiotensin AT1 receptor antagonists.
Postprandial triglyceride-rich lipoproteins cause endothelial dysfunction. Angiotensin-converting enzyme inhibitors have been shown to improve vascular reactivity. For angiotensin II type 1 receptor antagonists this effect is as yet uncertain.
A randomized, double-blind, placebo-controlled crossover study in 30 healthy volunteers, aged 18 to 33 years, evaluated the effect of quinapril (40 mg daily for two weeks) and losartan (50 mg daily for two weeks) on basal as well as postprandial endothelial function measured noninvasively as percentage diameter change in the brachial artery after reactive hyperemia. Endothelium-independent dilation was measured after nitroglycerine spray sublingual.
An acute oral fat load impaired endothelial function. Flow-mediated vasodilation (FMD) decreased from a median of 6.2% to 4.2% (p < 0.05). There was no significant difference in preprandial endothelial function after two weeks of treatment with either quinapril or losartan compared with placebo in these healthy volunteers. Both quinapril (FMD 6.4% to 6.3%) and losartan (7.1% to 5.4%) prevented endothelial dysfunction induced by an oral fat load, although the protective effect of quinapril appeared to be more profound. The response to the endothelium-independent vasodilator nitroglycerine was unaltered throughout the study.
Both losartan and quinapril prevent endothelial dysfunction induced by triglyceride-rich lipoproteins in healthy volunteers. However, the protective effect of quinapril is more pronounced.
Endothelial dysfunction is a primary event in atherogenesis and all known cardiovascular risk factors have been associated with endothelial dysfunction before atherosclerotic vascular disease manifests itself clinically (1–3). Pivotal to endothelial dysfunction is a disturbance in the function of endothelium-derived nitric oxide (NO) (4).
It has been generally accepted that low density lipoprotein (LDL) cholesterol is a key factor in atherogenesis. Currently, evidence is accumulating that triglyceride-rich remnant particles may also be involved. In vitro, triglyceride-rich lipoproteins can induce macrophage lipid loading (5), penetrate arterial tissue and accumulate within the subendothelial space, consistent with the site of atherogenesis (6,7). In agreement, triglyceride-rich lipoproteins (endogenous and postprandial) have been shown to relate to the rate of progression of coronary artery disease (8,9)as well as progression of carotid artery intima–media thickness (10).
Recently, abnormal endothelium-dependent vasomotor function has been shown to be associated with triglyceride-rich remnant lipoprotein levels in both the coronary and the brachial artery (11, 12). This may bear direct consequences for disorders in which the clearance of triglyceride-rich lipoproteins is impaired, such as diabetes, familial combined hyperlipidemia, postmenopausal women and apolipoprotein E2/E2 genotype (13). In line with the hypothesis that endothelial function can be used as a surrogate end point for cardiovascular morbidity, therapeutic modulation of postprandial endothelial dysfunction may potentially contribute to prevention of cardiovascular disease in these patients.
Angiotensin-converting enzyme (ACE) is strategically positioned on the endothelium to influence the activity of at least three local vasoactive systems: angiotensin II, bradykinin and NO. Angiotensin-converting enzyme inhibition has two major actions: prevention of angiotensin II formation and prevention of bradykinin breakdown. Angiotensin II is a potent vasoconstrictor and a strong stimulator of superoxide production, for example, by vascular nicotinamide adenine dinucleotide, reduced form oxidase (14). Superoxide reacts with NO at a diffusion-limited rate, thus constituting a main degradation pathway for NO (15). Bradykinin is a vasodilator, in part due to direct stimulation of NO synthase. However, it may also cause release of endothelium-dependent hypopolarizing factor (16). In line with these actions, several studies have shown that administration of ACE inhibitors improves endothelial function, for example, in subjects with diabetes (17)and patients with coronary artery disease (18). To what extent angiotensin II type 1 receptor antagonists affect endothelial function is as yet unclear. However, if inhibition of the formation of angiotensin II is important for the improvement of endothelial function observed with ACE inhibition, then a favorable effect of angiotensin II type 1 receptor antagonists could be expected.
The objectives of the present study were to test whether ACE inhibition and angiotensin II type 1 receptor antagonism influence endothelial function, assessed as flow-mediated dilation (FMD), in healthy volunteers and to evaluate whether postprandial endothelial dysfunction can be improved by ACE inhibition or angiotensin II type 1 receptor antagonism.
Thirty healthy volunteers, aged 18 to 33 years, participated. All individuals were normotensive and had no history of cardiovascular disease, or family history of premature vascular disease. All subjects had fasting plasma triglyceride and cholesterol concentrations below 2.0 mmol/liter and 6.5 mmol/liter, respectively. The subjects did not use medication.
The study was approved by the Medical Ethics Committee of University Hospital Utrecht and written informed consent was obtained from all participants.
The effect of two weeks of placebo, quinapril or losartan therapy on FMD before and after oral fat load was studied by use of a randomized, double-blind, placebo-controlled crossover protocol. Subjects were randomized to receive quinapril 40 mg once daily (Acupril, Parke-Davis), losartan 50 mg once daily (Cozaar, Merck Sharpe & Dohme) or a similarly packaged placebo. After two weeks each subject returned for a study of forearm vascular function before and 4 h after an oral fat load. Two weeks of washout followed. After crossover, they were restudied four weeks later. The same routine was performed for the final measurement. The subjects refrained from drinking caffeine-containing beverages, smoking and eating 12 h before each procedure. At each visit blood samples were drawn for laboratory determinations of triglycerides, total cholesterol and high density lipoprotein cholesterol concentrations before and 4 h after an oral fat load, which consisted of 50 g of fat per m2body surface in the form of whipped cream (40% fat).
Forearm vascular test
The ultrasound measurements were performed in the supine position at the elbow of the right arm using a vessel wall-movement system (Wall Track System, Pie Medical, Maastricht, The Netherlands), which consists of an ultrasound imager with a 7.5-MHz linear array transducer connected to a data acquisition system and a personal computer. An optimal two-dimensional B-mode image of the brachial artery was obtained. An M line perpendicular to the vessel was selected. The ultrasound system was switched to M mode, after which storage of data started. The vessel movement detector system repeatedly registered end-diastolic vessel diameter during a period of five to six cardiac cycles. This procedure was performed three times. The measurements were averaged for the baseline diameter.
By inflation of a blood pressure cuff for 4 min at a pressure of 100 mm Hg above the systolic blood pressure, ischemia was applied to the forearm distal to the location of the transducer. Ultrasonography continued for 3 min after cuff release with measurements at 30-s intervals. The widest lumen diameter was taken as a measure for maximal diameter. After 10 min of rest, allowing the artery to return to its baseline diameter, sublingual nitroglycerine spray was administered as an endothelium-independent dilator. Measurements were obtained for another 5 min, at 1-min intervals.
Flow-mediated dilation and nitroglycerine-induced dilation were expressed as a percentage change relative to baseline diameter.
Group values are expressed as mean ± SD. Due to failure of the normality test for FMD data, FMD values are expressed as median (25th to 75th percentile) and statistical tests on ranks were used. Differences in FMD at baseline between the three treatment periods were tested with a one-way repeated measures analysis of variance on ranks. Differences in FMD before and after lipid load within one treatment session were tested with the Wilcoxon signed rank test. Differences between changes in FMD before and after lipid load between the various treatment sessions were tested with one-way repeated measures analysis of variance. If variance ratios reached statistical significance, differences were analyzed with the Student-Newman-Keuls test for p < 0.05.
Lipid changes before and after lipid load and between treatment groups were tested using paired samples ttest with Bonferroni correction. Correlation testing was performed using linear regression. Differences were considered significant at p < 0.05.
The general characteristics of the study group are given in Table 1. Preprandial and postprandial endothelial function values are shown in Table 2. The baseline arterial diameter and FMD before fat loading were not significantly different between placebo, quinapril and losartan treatment. Total cholesterol, triglyceride and high density lipoprotein cholesterol concentrations did not differ between the three groups studied, before as well as after fat loading; values are shown in Table 3. Upon fat loading trygliceride concentrations were significantly increased in all three groups, whereas the other lipid parameters remained unaltered (Table 3).
During placebo therapy an oral fat load induced a significant decrease in FMD (Table 2). There was an inverse relation between the level of postprandial trygliceride concentration and FMD after fat load. The mean change in FMD per mmol increase of triglycerides was −1.71% (95% confidence interval: −3.35 to −0.61, p < 0.05).
During losartan treatment postprandial FMD was no longer significantly different from preprandial FMD (Table 2). Accordingly, the relation between postprandial triglycerides and FMD after fat load was no longer significant. However, there was still a tendency toward a decrease in FMD, which was underscored by the observation that the change in FMD (difference between pre- and postprandial) was not significantly different during placebo compared with losartan treatment; values are shown in Figure 1.
During quinapril treatment postprandial FMD was not significantly different from preprandial FMD (Table 2). Comparable to losartan, there was no relation between postprandial triglycerides and FMD after fat load. The change in postprandial endothelial function compared with preprandial endothelial function was significantly different between quinapril and placebo treatment as well as quinapril and losartan treatment, implying that the ameliorative effect of ACE inhibition on postprandial endothelial dysfunction was more profound than the effect of angiotensin II type 1 receptor antagonism. Values are shown in Figure 1.
Responses to the endothelium-independent vasodilator nitroglycerine were not significantly altered by either an oral fat tolerance test, or quinapril or losartan therapy (Table 4).
In the present study we demonstrate that the impairment of endothelial function upon acute fat load is prevented by two weeks therapy with both the angiotensin II type 1 receptor antagonist losartan and the ACE inhibitor quinapril. The effect of ACE inhibition on postprandial endothelial dysfunction appeared to be more profound.
Postprandial lipemia and endothelial function
Acute fat loading induces a consistent reduction in FMD in healthy volunteers. In the present study we observe a clear correlation between the degree of impairment of FMD and the levels of postprandial triglycerides, which was abolished during angiotensin II type 1 receptor antagonist and ACE inhibition therapy. Whereas most studies have evaluated relations between endothelium-dependent vasodilation and LDL cholesterol (19), two recent studies have emphasized the potential relevance of triglyceride-rich lipoproteins for endothelial function. Remnant lipoprotein levels were independently associated with endothelial dysfunction in epicardial coronary arteries in patients without angiographically proven vascular disease (11). Also, in patients with familial combined hyperlipidemia improvement of endothelial function in the forearm vasculature was significantly correlated to reduction of intermediate density lipoprotein cholesterol after lipid-lowering treatment (20). These findings have been substantiated in vitro, where (oxidized) chylomicron remnants have been shown to impair endothelium-derived NO activity in rat aortic vessel rings (21). Mechanistically, decreased NO bioavailability may be the consequence of decreased NO formation or increased NO degradation by oxygen radicals. Postprandial lipemia was recently shown to be associated with increased free radical generation in vivo in type II diabetic patients (22). Also, pretreatment with the radical scavenger vitamin C could eliminate postprandial endothelial dysfunction (12).
Role of the vascular renin–angiotensin system and endothelial function
In the present study neither ACE inhibition nor angiotensin II type 1 receptor antagonism affected endothelial function before lipid load. In a previous study endothelial function was improved during ACE inhibition in healthy subjects (23). However, in the latter study the ACE inhibitor was infused acutely, which may (partly) account for the differences in observation. Postprandial endothelial dysfunction could be blocked by both losartan as well as quinapril, whereas endothelium-independent responses were unaltered during the various treatments. This finding further corroborates data suggesting a protective effect of ACE inhibition against lipid-associated endothelial dysfunction.
The toxic effect of ox-LDL on endothelial function in aortic rings could be abolished by ACE inhibition (24), whereas ACE inhibition also slowed progression of atherosclerosis in cholesterol-fed rabbits (25). Accordingly, the ameliorative effect of ACE inhibition on endothelial function was larger with higher cholesterol levels (26). Angiotensin-converting enzyme inhibition also retarded progression of coronary atherosclerosis in subjects with LDL cholesterol levels above 130 mg/dl (27). This enhanced effectivity of ACE inhibition during higher cholesterol levels, combined with the observations of the present study, suggest that the vascular renin–angiotensin system may be somehow involved in the mechanism of lipid-induced endothelial dysfunction.
Theoretically, two mechanisms can be involved. Triglyceride-rich lipoproteins may, in analogy to LDL, induce increased oxygen radical production by a renin–angiotensin modifiable pathway. Membrane oxidases, which have been implicated as important superoxide-producing enzymes in the vasculature, are (in part) regulated by angiotensin II (14). Hence, decreased angiotensin II formation may result in down-regulation of the expression of membrane oxidases, resulting in decreased lipid-induced superoxide production. The ensuing increase in NO bioavailability can arise either due to decreased destruction of NO by decreased superoxide itself or due to decreased formation of, for example, oxidatively modified chylomicrons, which may interfere with NO production and/or NO bioavailability. In addition, bradykinin-mediated stimulation of endothelial function may also be involved, which is supported by the significant difference in effect between quinapril and losartan in preventing the change in postprandial endothelial function. In agreement, the protective effect of ACE inhibition against the toxicity of ox-LDL in rat aortic rings was also shown to be bradykinin dependent (23). Interestingly, preprandial endothelial dysfunction was unaffected by ACE inhibition, which may imply that the bradykinin pathway is of particular importance during dyslipidemia. In this respect, bradykinin-stimulated endothelial function has been shown to be preserved in hypercholesterolemic patients, whereas other agonist-stimulated endothelial function, that is, acetylcholine, serotonin and alpha2receptor stimulation, is impaired (28). Alternatively, differences in pharmacokinetics, such as binding and tissue penetration, may have contributed to the observed differences between quinapril and losartan. It is less likely that the difference in effect is the result of the dosage of losartan used in this study, since losartan reaches a plateau in the dose–response curve after 50 mg (29).
The exact nature of the interaction between lipid-induced endothelial dysfunction and the renin–angiotensin system deserves further research. Nevertheless, the observation that postprandial endothelial dysfunction can be improved by blockade of the renin–angiotensin system provides a rationale for ACE inhibition and/or angiotensin type 1 receptor antagonist therapy in patients with impaired remnant particle clearance such as diabetes and familial combined hyperlipidemia.
☆ There was no financial support for this study.
- angiotensin-converting enzyme
- flow-mediated dilation
- low density lipoprotein
- nitric oxide
- Nitroglycerin induced dilation
- Received September 11, 1998.
- Revision received February 16, 1999.
- Accepted March 19, 1999.
- American College of Cardiology
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