© 1997 American Heart Association, Inc.
Articles |
Altered Compliance and Residual Strain Precede Angiographically Detectable Early Atherosclerosis in Low-Density Lipoprotein Receptor Deficiency
From the Department of Internal Medicine (Cardiology Division) of the Washington Hospital Center, Washington, DC; Engineering Research Center, University of District of Columbia, Washington, DC; the Cleveland Clinic, Cleveland, Ohio; Biomedical Engineering Center, Ohio State University, Columbus; and the Molecular Disease Branch and the Laboratory of Animal Medicine and Surgery of the National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Md.
Correspondence to Jeffrey M. Hoeg, MD, Chief, Section of Cell Biology, Molecular Disease Branch, NHBLI, NIH, Bldg 10, Room 7N117, 10 Center Dr MSC 1666, Bethesda, MD 20892-1666.
 |
Abstract |
|---|
 Top Abstract IntroductionMethodsResultsDiscussionReferences |
|---|
Background This study was performed to detect changes in vascular biomechanical properties early in atherogenesis.
Methods and Results Age- and weight-matched LDL-receptor deficient Watanabe hypercholesterolemic male rabbits (Group I: n=11) and normal rabbits (Group II: n=11) were studied. Fasting plasma lipoprotein concentrations, aortic angiography and intravascular ultrasound, in vivo aortic compliance evaluation, ex vivo aortic residual strain measurements, aortic lipid content and histopathology were determined. Plasma cholesterol was increased 9.8 fold and aortic cholesterol content was increased from 20 to 43 fold in Group I compared to Group II, respectively (P<.00005). Angiography revealed no stenoses in either group, whereas intravascular ultrasound and histological studies of Group I showed small circumferential plaques with <10% cross-sectional area involvement. The residual strain in Group I was significantly increased in the ascending thoracic aorta (22.1±6.9% versus 10.4±3.2% in Group II, P<.0001), descending thoracic aorta (15.7±7.2% versus 4.8±1.3% in Group II, P<.0001), and abdominal aorta (18.0±4.8% versus 8.3±6.3% in Group II, P<.005). Changes in residual strain were inversely correlated with the aortic cholesterol content in the ascending thoracic aorta (r=-.72; P=-.001), descending thoracic aorta (r=-.95; P<.001), and abdominal aorta (r=-.51; P=.019).
Conclusions Early atherosclerosis in LDL-receptor deficient rabbits, undetectable by angiography yet observed by intravascular ultrasound imaging and histology, is associated with marked changes in ex vivo residual strain. Alterations in vascular biomechanical properties, associated with changes in cholesterol content, may have physiologic consequences and may be useful in detecting and quantitating early atherosclerosis.
Key Words: cholesterol • compliance • intravascular ultrasound • residual strain • atherosclerosis
|
Introduction |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
Demonstration of atherosclerosis in vivo by documenting luminal compromise, especially following an ischemic episode, is a late event in atherogenesis. Previous studies have shown that acute ischemic syndromes result from plaque rupture of noncritical stenoses.1 2 Recent clinical trials, showing reductions in plasma low density lipoproteins associated with marked reductions in myocardial infarction and death before angiographic changes were evident,3 emphasize the importance of detecting early atherosclerosis and modifying the natural history of disease progression. Thus cineangiography, the accepted gold standard in quantitating the progression or regression of atherosclerosis, has inherent limitations.4 One of the greatest limitations of angiography is its inability to detect and quantitate early or diffuse atherosclerosis.5 Thus, novel methods are needed to both detect and quantitate atherosclerosis in order to assess the impact of new treatment strategies directed toward atherogenesis before the development of clinical manifestations.
One of the means of detecting early lesions is intravascular ultrasound (IVUS). This imaging modality permits detailed cross-sectional pictures of the coronary arteries in vivo and is not subject to the limitations inherent to cineangiography.6 7 8 The ability of intravascular ultrasound to measure coronary arterial cross-sectional dimensions accurately9 and its ability to determine plaque composition have been validated.10
However, we hypothesized that changes in the arterial biomechanical properties in vessels undergoing early atherogenesis may precede even the subtle structural changes that can be observed by IVUS. Biomechanical changes of atherosclerotic arteries, even in the absence of detectable disease by angiography, may be more sensitive in detecting early atherosclerosis. Thus, we conducted a study in spontaneously hypercholesterolemic WHHL rabbits lacking functional LDL receptors. These rabbits develop tissue cholesterol deposition that mimics that of patients homozygous for familial hypercholesterolemia. WHHL rabbits, compared to age-and weight-matched normolipemic rabbits, were hypercholesterolemic but appeared normal by cineangiography. However, these rabbits developed aortic atherosclerosis characterized by the accumulation of increased concentrations of both esterified and unesterified cholesterol. These cholesterol-enriched early lesions were detected by both IVUS and histopathological evaluation. To determine the presence of vascular biomechanical changes that might accompany early lesion development, we used IVUS and intra-aortic pressure measurements to estimate the vascular compliance in vivo and directly determined the residual strain in these aortas ex vivo.
|
Methods |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
The Watanabe heritable hyperlipidemic rabbit has been described previously.11 This animal model for human familial hypercholesterolemia12 arises from a 12 base pair deletion in exon 4 of the LDL receptor gene which disrupts the LDL-binding domain of this receptor protein.13 Due to the defective LDL receptor function, the rabbit has a 10-fold increase in LDL serum concentration and the consequence of markedly elevated serum LDL is pronounced atherosclerosis, particularly the generalized fibrocalcific atherosclerosis that resembles human atherosclerotic lesions.11 Male WHHL rabbits were studied and compared with age- and weight-matched New Zealand White normolipemic male rabbits. This study was approved by the Animal Care and Use Committees of both the Medlantic Research Institute and the National Heart, Lung and Blood Institute of the National Institutes of Health, and conformed to the position of the American Heart Association on research animal use.
After an overnight fast, 10 mL of blood was drawn into sodium EDTA tubes with a final EDTA concentration of 0.1% (weight/volume). The plasma was separated by centrifugation, and the total cholesterol and HDL cholesterol concentrations were determined using an enzymatic assay (Sigma) on a Hitachi 911 Autoanalyzer (Boehringer-Mannheim) as previously described.14 Both the total cholesterol/HDL cholesterol ratios as well as the non-HDL cholesterol concentrations were calculated.
The angiography and IVUS studies were conducted following
anesthesia with intramuscular ketamine (35
mg/kg; Fort Dodge Laboratories) and xylazine (5 mg/kg;
Mobay Corp). The animals were intubated and anesthesia was
maintained by halothane inhalation (1-2%) throughout the procedure.
Via right carotid artery access and 6F sheath insertion, angiography of
the entire abdominal and descending thoracic aortas was performed using
a 6F balloon wedge pressure catheter (Arrow International Inc). Blood
pressure measurements were made with a fluid-filled pressure transducer
(Marquette Electronics, Inc) at the same locations in each animal: (1)
lower abdominal aorta defined as one vertebra above the iliac
bifurcation; (2) mid-abdominal aorta selected at 3 vertebrae above the
iliac bifurcation; (3) upper abdominal aorta chosen at the diaphragm
level; and (4) thoracic aorta defined as two vertebrae above the
diaphragm. Then, over a 0.014-in guidewire, an intravascular ultrasound
(IVUS; Cardiovascular Imaging Systems, Inc) catheter
was exchanged for the balloon wedge pressure catheter and imaging was
performed with the IVUS catheter stationary at the same four locations.
The IVUS images were recorded on s-VHS for off-line quantitative
analysis. Using computerized planimetry, major and minor axes
(in mm), as well as luminal areas (in mm2) in
systole and diastole were measured by an observer unaware
of the animal's lipoprotein status. The compliance of the aorta,
determined as a tube under pressure, was derived from the IVUS
measurements using the following formula: 
[(mean systolic
diameter from IVUS)2 - (mean diastolic diameter
from IVUS)2]/4(pulse pressure) in mm2/kPa
(1 kPa=7.6 mm Hg).15 To overcome inherent
differences in aortic compliance considering gender and age, only male
rabbits that were age- (23.0±8.2 months old) and weight- (3.0±0.6 kg
versus 3.4±0.8 kg) matched, were studied.
After the pressure measurements were obtained, aortography was performed and then each animal was euthanatized with an overdose of sodium pentobarbital. Aortic histopathological studies were performed on segments of aortas from the LDL-receptor deficient and control rabbits. Sections (1 mm) of the descending thoracic aorta from control and LDL-receptor deficient rabbits were taken and further sectioned for the application of hematoxylin and eosin, Gomori Trichrome, and Von Kossa stains. In addition, the extent of lipid deposition within the arterial wall was determined by planimetry in WHHL rabbits. Briefly, aortas from a separate group of WHHL rabbits 6.5 years of age and 19 to 20 months of age were harvested, filleted open, and placed on cardboard. The aortas were stained with Sudan IV, photographed and the images were digitized. The percent of the surface area that stained with Sudan IV was determined by quantitative planimetry.16
Directly after euthanatizing each animal, the aortas were harvested and direct determinations of residual strain were made as previously described.17 18 Briefly, an arterial ring (1 to 2 mm wide) was cut using two parallel sections perpendicular to the axis of the vessel. The ring was placed flat in a saline-containing Petri dish with the ventral side adjacent to a mm scale ruler and photographed. The dorsum of the vessel was oriented to be 180° from the ventral surface. The ring was then cut radially at the dorsum, which caused it to spring open into a horseshoe configuration, and this configuration was also photographed. Using enlarged prints, the inner boundary of an intact ring and its open configuration were measured using a digitizing tablet and a scanning software (Sigma Scan, Jandel Scientific). The residual strain was then computed by dividing the change in curved lengths, before and after the cut by the curved length of the inner edge of the open configuration (at zero-stress state). A total of 20 rings were cut from each aortic segment. Since the aortic residual strain changes from region to region, we divided the entire aorta into three regions: the ascending thoracic aorta, the descending thoracic aorta, and the abdominal aorta. The residual strain values for each ring of each region of the aorta were separately averaged for both control and experimental groups.
The aortic cholesterol content was also determined for each animal. From harvested aorta, 20 to 50 mg (wet weight) of the aortic segments was weighed on an analytical balance and finely minced with a scalpel blade. Lipids were extracted in 5 mL chloroform:methanol (2:1, v/v) in 35 mL glass tubes according to the method by Folch et al.19 Lipids were resolubilized in 1 mL of isopropanol. For subsequent protein determination, tissue remaining in the 35 mL glass tubes was solubilized with 2.5 mL NaOH overnight according to the method by Lowry et al.20 The total aortic lipid was measured gravimetrically on the lipid extracts following solvent evaporation under nitrogen. Total cholesterol was determined using the Cholesterol CII enzymatic colorimetric method (Waco Chemicals, Inc). Unesterified cholesterol was determined using the Free Cholesterol C enzymatic colorimetric method (Waco Chemicals, Inc). Esterified cholesterol was calculated by subtraction of the unesterified from the total cholesterol content. Protein determination was performed following the Enhanced Protocol of the Bicinchoninic Acid method (Pierce).
All data are presented as mean±1 standard deviation. Unpaired t tests were used to compare the values between the WHHL and NZW rabbits. A two-tailed P value <.05 was considered significant. Pearson bivariate correlations were performed using the Windows version of SPSS (release 5.0; SPSS Inc).
|
Results |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
Since WHHL rabbits lack functional LDL receptors, there was an accumulation of cholesteryl ester–enriched lipoprotein particles in the plasma of these animals (Table 1
). The nearly 10 fold
increase in the plasma total cholesterol concentration in
the WHHL rabbits reflected a 40 fold rise in the atherogenic non-HDL
particles that were enriched in triglycerides. In contrast,
the HDL cholesterol concentrations were reduced by 20%.
These highly proatherogenic changes in the plasma lipoprotein particle
concentrations were reflected in the total
cholesterol/HDL cholesterol ratio. Based
upon epidemiological studies in man, a total
cholesterol/HDL cholesterol ratio of 5
conferred average cardiovascular disease
risk.21 Thus, the 12.9 fold increase in this ratio in the
WHHL rabbits compared to control rabbits would be expected to be highly
atherogenic.
|
As expected, the aortas of the WHHL rabbits developed
atherosclerosis. Histopathological evaluation
demonstrated that the control rabbits had no intimal cellular
proliferation (Fig 1A), disruption of the elastic tunica (Fig 1B), or
calcification (Fig 1C). In contrast, there was striking cellular
proliferation (Fig 1D), disruption of the fibrillar pattern in the
media (Fig 1E), and diffuse medial calcification (Fig 1F) in WHHL
aorta. This atherogenic
process was present throughout the entire aortic length in WHHL
rabbits. By age 6.5 months, half of the surface area of WHHL rabbits
was covered with sudanophilic atherosclerotic plaque. By 19 to 20
months of age, the age of the WHHL rabbits that were studied, nearly
75% of the aortic surface was covered with atherosclerotic plaque (Fig 2). Thus, WHHL rabbits, with
highly proatherogenic lipoprotein profiles have diffuse fibrocalcific
atherosclerosis detectable by histopathological
analysis and quantitative planimetry.
|
|
Despite the presence of atherosclerosis by sensitive
postmortem studies, angiography did not detect any evidence of
atherosclerosis in vivo in either study group (Fig 3). In both the WHHL rabbits
as well as in the controls, the aortic luminal surface was smooth with
no irregularities or stenoses. Therefore, the proatherogenic
lipoprotein profile as well as the postmortem characterization of
atherosclerosis in the WHHL rabbits did not lead to
changes detectable by aortography.
|
In contrast to angiography, intravascular ultrasound proved more
sensitive in identifying atherosclerosis in WHHL
rabbits. The smooth luminal-vascular interface in the control rabbits
was paralleled by a uniform mural thickness throughout the aortas
(Fig 4A). In contrast, both
epicentric and concentric atherosclerotic plaques were detected in the
aortas of the LDL-receptor deficient rabbits, and the shadows cast by
calcification could be detected in these animals (Fig 4B).
Intravascular ultrasound was more successful in identifying and
characterizing the atherosclerotic plaque in the WHHL rabbits than was
aortography.
|
In addition to the morphologic characterization of aortic
atherosclerosis, intravascular ultrasound was useful to
determine the vascular compliance of the aorta in vivo. The changes in
the aortic luminal diameter in systole and diastole were
quantitated, and using simultaneous blood pressure
determinations, the vascular compliance was determined. Compliance in
the thoracic aorta was reduced by 58% in the WHHL rabbits compared to
control rabbits (P=.018; Table 2). On the other hand, the
compliance in the other aortic segments was not significantly different
between the two groups (Table 2). In general, IVUS showed less changes
in systolic to diastolic cross-sectional dimensions
in the LDL-receptor deficient rabbits.
|
Another measure of vascular biomechanical property, ie, the evaluation
of residual strain, was also applied to the aortas of these rabbits ex
vivo. Residual stress is the stress that remains in the body after the
removal of all the external stresses (or loads). It is the inherent
property of vascular and other biologic tissues.22 23 24 25 The
strain corresponding to the residual stress is called residual strain.
Since stress can not be measured, we have evaluated the corresponding
residual strain. The residual strain in aortic segments was readily
observed after radial sectioning of aortic rings (Fig 5). The residual strain in the
aortas of the WHHL rabbits was 2.1 to 3.3 fold higher than in control
rabbits (Table 2
). The greatest residual strain was detected in the
ascending thoracic aorta in both WHHL and control rabbits. This
significant change in residual strain closely paralleled the
severity of atherosclerosis observed with a proximal to
distal gradient (Figs 1 and 2).
|
The aortic total lipid, total cholesterol, and unesterified
cholesterol contents were determined in both control and
WHHL rabbits (Table 3). No
differences in the total lipid content were detected between control
and WHHL rabbits. In contrast, there were striking changes in the
accumulation of both cholesterol and cholesteryl ester in
the WHHL aortas (Table 3 and Fig 6). In each of the three
segments of the WHHL aortas, the ascending thoracic aorta, the
descending thoracic aorta, and the abdominal aorta, there was from 20
to 40 fold more cholesterol than in control rabbits.
Virtually all of the cholesterol present in the aortas
from control rabbits was unesterified. In contrast, from 38% to 41%
of the cholesterol in the WHHL aortas was esterified.
Moreover, there was a gradient of aortic cholesterol
content in the WHHL rabbits that had the greatest
cholesterol and cholesteryl ester content proximally with
diminishing content detected distally within the aortas (Fig 6). In
contrast, both total cholesterol and esterified
cholesterol in control rabbits remained approximately
constant within the ascending thoracic aorta, the descending thoracic
aorta, and the abdominal aorta.
|
|
Therefore, a gradient of atherosclerosis severity
identified by quantitative planimetry was associated with gradients in
both aortic cholesterol content and in aortic compliance.
Table 4 summarizes the correlations between the lipid content of the
ascending thoracic aorta, descending thoracic aorta, and the abdominal
aorta with the plasma lipid and lipoprotein concentrations as well as
with aortic residual strain. The plasma total and non-HDL
cholesterol concentrations were highly and significantly
correlated with the both total and cholesteryl ester content of the
aortas. The plasma HDL cholesterol concentration was
inversely correlated with the accumulation of aortic
cholesterol content. The ratio of total
cholesterol/HDL cholesterol lead to the
highest correlations with the accumulation of cholesterol
and cholesteryl esters in the vessel wall. There was little, if any,
correlation between these plasma lipoproteins and the total lipid
content of the aortas.
|
These biochemical correlates were paralleled by the altered
biomechanical properties of these vessels (Table 4 and Fig 7).
The residual strain was inversely correlated with the total and
cholesteryl ester content in all the arterial segments that
were analyzed. Again, the changes were specific for
cholesterol since the total vascular lipid content was not
as highly correlated, especially in the ascending and descending
thoracic aortas. The inverse correlations in the residual strain of the
ascending thoracic, descending thoracic, and abdominal aortas with the
aortic total cholesterol content shown in Fig 7 illustrate
that the correlations were better proximally than distally. Therefore,
the gradient of lesion development observed by planimetry and by
histopathology parallels the biochemical and biomechanical changes
present in the WHHL aorta.
|
|
Discussion |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
This study establishes that early atherosclerosis of LDL-receptor deficiency is associated with significantly altered vascular biochemical and biomechanical properties, and that these changes are highly correlated with one another. In addition, these significant alterations occur before disease can be detected by cineangiography. Therefore, early atherosclerosis identified by both intravascular ultrasound and by histopathological evaluation reflects vessels with reduced vascular compliance. These observations have broad physiologic and pathophysiological implications.
Compliance is the ratio of strain to stress in a material under a uniaxial state of stress and is the reciprocal of the modulus of elasticity used by engineers to classify stiffness of materials. Stiffer material has a higher modulus of elasticity (thus, lower compliance) compared to softer material. In hemodynamics, arterial compliance is defined as the ratio of the change in cross-sectional area of the artery divided by the change in pressure. This value reflects not only the softness (or stiffness) of the vessel, but also the geometry of the specimen. Compliance is an inherent mechanical property of the vascular tissue and influences its function in health and disease.
Residual stress has a very old and established history in the investigation of engineering materials and structures. Recently, this concept has been broadly applied to biologic tissues and organs, including arteries, intact heart, trachea, intestine, esophagus, cartilage and bone.17 18 26 It has also been demonstrated that residual strain is a sensitive indicator of biomechanical changes due to a variety of pathologic conditions including hypertension,22 23 24 25 hypoxia,27 diabetes,28 smoking,29 atherosclerosis and calcification,30 31 and growth hormone administration.32 In arteries, residual stress reduces the high stress concentration at the intimal layer and provides more uniform stress distribution across the arterial wall.33 34 35 It has also been hypothesized that residual stress is responsible for growth, development and remodeling of biological tissues and organs.35 36 37 38 39 40 41 42 43 44 45
The current results establish that aortic stiffness, determined by
intravascular ultrasound (Table 2) and by residual strain (Fig 5), are
deranged in WHHL rabbits. The normal rabbits demonstrated a gradient of
compliance with the thoracic aorta>upper abdominal aorta>lower
abdominal aorta. This proximal-to-distal gradient was also evident in
the atherosclerosis that develops in the
hypercholesterolemic WHHL rabbits. The more severely
affected proximal thoracic aorta detected by planimetry (Fig 2) was
also the least compliant region and was 60% lower than in control, NZW
rabbits (Table 2). The marked reductions in compliance were
paralleled by a 2.1 to 3.3 fold increase in residual strain in
these aortas (Table 2) and parallel observations in
cholesterol-fed rabbit aortas.46 Therefore,
the aortic compliance was strikingly reduced in the
hypercholesterolemic rabbits due to loss of functional
LDL receptors while only small, nonocclusive atherosclerotic plaque was
evident by intravascular ultrasound (Fig 4)
This reduced compliance with
hypercholesterolemia was quantitative. The loss
of compliance of the elastic aorta was inversely correlated with the
accumulation of aortic cholesterol (Table 4 and Fig 7). The
initiation of lipoprotein-mediated atherogenesis has been proposed to
begin with direct injury to the vascular
endothelium47 followed by the accumulation
of apolipoprotein B particles in the media of human48 as
well as cholesterol-fed rabbits.49 The WHHL
rabbits, unable to remove the cholesteryl ester-enriched apolipoprotein
B, had plasma total and non-HDL cholesterol concentrations
that were increased 9.7 and 40.2 fold, respectively (Table 1). This
apolipoprotein B enrichment in the intercellular space accounts for the
accumulation of cholesteryl ester that is present in both the core
of the lipoprotein particle as well as in the arterial
wall. However, in addition to esterified cholesterol,
unesterified cholesterol also accumulates in
arterial lesions.49 50 51 52 This unesterified
cholesterol is present in intercellular non-lipoprotein
particle vesicles and has been proposed to have been processed by
monocyte-macrophages.49 53 The accumulation of
unesterified cholesterol has also been demonstrated in the
tendon xanthomata of patients lacking functional LDL
receptors.54 The results of the present study
establish that WHHL rabbits, lacking functional LDL receptors,
accumulate both esterified and unesterified cholesterol in
the aorta, as has been observed in cholesterol-fed rabbit
aorta and in homozygous familial hypercholesterolemic
tendons. In addition, these data suggest that this marked accumulation
of cholesterol correlates with changes in the biomechanical
properties of the aorta.
Cholesterol is present in all mammalian cell membranes
and is important as a structural element.55 Within these
membranes, it serves to reduce membrane fluidity.56 This
effect of cholesterol to stiffen membranes in vitro and in
cell culture may have a parallel in the macro-fluidity and resultant
compliance of the vessel wall. The presence of vesicles rich in
unesterified cholesterol may alter intercellular
proteoglycan function. The observed 20 to 43 fold increase in the
aortic content of cholesterol in all of the regions of the
aorta reflected changes in both the esterified as well as the
unesterified cholesterol content (Table 3). In addition,
disruption of the elastin structure, whether or not there is a causal
relationship, that occurs in the face of mural cholesterol
accumulation may be central to the observed changes in compliance and
residual strain. The highly significant inverse correlation between the
aortic cholesterol content (Table 4 and Fig 7) and residual
strain establishes that this relationship is apparent long before the
presence of angiographically detectable disease (Fig 3).
The accumulation of aortic cholesterol in the WHHL rabbits represents not only enhanced deposition of cholesterol into the aortic wall, but it also may reflect impaired egress of cholesterol from the tissue. Reverse cholesterol transport was first proposed to describe the process by which HDL could remove excess cellular cholesterol from nonhepatic tissues and deliver the cholesterol to the liver for secretion into the bile.57 High concentrations of HDL may retard the atherogenic process by this metabolic pathway, and epidemiological studies in man suggest a strong inverse correlation between HDL and the development of cardiovascular disease.58 In contrast to LDL where a 1% increase in concentration leads to a 2% increase in cardiovascular disease incidence, a 1% increase in HDL cholesterol leads to a 3% decrease in the incidence of cardiovascular disease.58 Therefore the 20% reduction in the HDL cholesterol in the WHHL rabbits could account for some of the cholesterol accumulation in the arterial wall. We have recently reported that transgenic rabbits expressing high concentrations of the enzyme lecithin:cholesterol acyl- transferase develop HDL cholesterol concentrations as high as 200 mg/dL (5.14 mmol/L).14 These high concentrations of HDL cholesterol prevent the development of atherosclerosis in cholesterol-fed rabbits.59 These findings indicate that the net arterial accumulation of cholesterol may reflect the competing mechanisms of cholesterol-deposition and removal. In addition, the results of the current study imply that determination of arterial compliance may provide a means of assessing these competing processes in vivo.
Physiologic changes in vascular function have previously focused upon vascular contractility of muscular arteries.60 61 62 These alterations also occur in the absence of significant stenosis detectable by angiography. With intracoronary acetylcholine injection in "angiographically normal coronary arteries," aberrant vasodilatory response can be detected. These alterations have implicated endothelial dysfunction in vessels containing early atherosclerosis. More importantly, subsequent studies showed that such abnormal vasomotor response to vasoactive stimuli can be favorably influenced by either elevated HDL levels63 or by reducing LDL cholesterol levels,64 65 suggesting the reversibility of endothelial injury from the atherosclerotic process and the potential for beneficial effect from treatment. The current study indicates that changes in the function of elastic arteries may also be observed in the absence of arteriographic changes. Taken together, more subtle alterations in vascular function precede the formation of angiographically detectable plaque.
The present results may have several physiologic implications and may help explain results from clinical and epidemiological studies. An intriguing discrepancy between luminal changes during serial angiography and clinical benefit from cholesterol reduction has been observed.66 The overall angiographic changes in the coronary arteries have been minimal in atherosclerosis-regression trials.4 66 However, the reduction in ischemic events was striking and occurred within 6 months of therapy. These findings suggested that factors other than changes in lumen dimension accounted for the clinical benefit. By reducing the plasma cholesterol concentrations, particularly the concentrations of the proatherogenic apolipoprotein B particles, there may have been alteration in vascular compliance that led to reduced plaque rupture and subsequent thrombotic events.
Another implication of these findings is that the aortic compliance may be reduced long before there are any lesions detectable by aortography. During systole, the left ventricle ejects a stroke volume of 60 to 100 mL. Peripheral resistance and elastic extension of the aortic wall are responsible for accommodating 50% of the stroke volume.67 During diastole, the aortic recoil not only maintains forward flow to the periphery, it also is critical for coronary artery perfusion. The current findings that early atherosclerosis reduces vascular compliance suggests that a stiffened aorta would not be as effective in serving as an auxiliary pump. Reduced aortic compliance might then be expected to lead to left ventricular hypertrophy and reduced coronary artery perfusion. Therefore, these data may help to account for the striking predictive power of left ventricular hypertrophy for cardiovascular disease events in the Framingham Heart Study.68 69
The results of this study also suggest that even non-occlusive, "minor" plaque detected by intravascular ultrasound examination may reflect physiologically relevant disease. Patients with detectable lesions may already have substantially altered vascular compliance in a variety of vascular beds. In order to intervene in the atherogenic process long before the disease is clinically manifest, these findings suggest that techniques directed toward quantitation of vascular compliance may become useful. Noninvasive methods to detect biomechanical changes, such as M-mode ultrasonography,70 transesophageal ultrasound,71 intravascular ultrasound,72 and magnetic resonance imaging73 74 75 have all been used to demonstrate altered vascular compliance in patients with cardiovascular disease. These more subtle alterations may be both reversible as well as more sensitive in the assessment of potential therapies to treat and prevent the sequelae of atherosclerosis.
In conclusion, this study establishes that early atherosclerosis, detectable in vivo only by intravascular ultrasound, is associated with marked vascular biomechanical changes. These biomechanical changes precede angiographically detectable atherosclerotic plaque and are highly and inversely correlated with mural cholesterol content in hypercholesterolemic animals. The assessment of vascular compliance may be useful in not only quantitating early atherosclerosis, it may provide a new means to assess the efficacy of therapies directed toward preventing atherosclerosis at an early stage.76
|
Selected Abbreviations and Acronyms |
|---|
|
|
Acknowledgments |
|---|
This research was supported by grants from the Cardiology Research Foundation of the Washington Cardiology Center and Medlantic Research Institute, Washington, DC (M.K.H., G.S.M., M.B.L.); National Institutes of Health, HL-54246-01 (J.F.C., E.E.H.); and National Science Foundation Grant HRD-910452 and USDA Grant 0167398 (J.V.). We would also like to thank Virginia Livingston, Darlene Allen, John DeLeonardis, and Orville Bramwell for their technical assistance and Donna James for her help in preparing the manuscript.
|
Footnotes |
|---|
E mail Jeff @ MDB.NHLBI.NIH.GOV
Received January 27, 1997; accepted April 4, 1997.
|
References |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
1. Ambrose JA, Tannenbaum MA, Alexopoulos D, Hjemdahl-Monsen CE, Leavy J, Weiss M, Borrico S, Gorlin R, Fuster V. Angiographic progression of coronary artery disease and the development of myocardial infarction. J Am Coll Cardiol. 1988;12:56-62.[Abstract]
2.
Little WC, Constantinescu M, Applegate RJ, Kutcher MA,
Burrows MT, Kahl FR, Santamore WP. Can coronary
angiography predict the site of a subsequent myocardial infarction in
patients with mild-to-moderate coronary artery disease?
Circulation. 1988;78:1157-1166.
3. Brown G, Stewart BF, Zhao XQ, Hillger LA, Poulin D, Albers JJ. What benefit can be derived from treating normocholesterolemic patients with coronary artery disease? Am J Cardiol. 1995;76:93C-97C.[Medline] [Order article via Infotrieve]
4.
Hong MK, Mintz GS, Popma JJ, Kent KM, Pichard AD,
Satler LF, Leon MB. Limitations of angiography for analyzing
coronary atherosclerosis progression or
regression. Ann Intern Med. 1994;121:348-354.
5.
St.Goar FG, Pinto FJ, Alderman EL, Valantine HA,
Schroeder JS, Gao SZ, Stinson EB, Popp RL. Intracoronary
ultrasound in cardiac transplant recipients: in vivo evidence of
`angiographically silent' intimal thickening.
Circulation. 1992;85:979-987.
6. Coy KM, Maurer G, Siegel RJ. Intravascular ultrasound imaging: a current perspective. J Am Coll Cardiol. 1991;18:1811-1823.[Abstract]
7. Nissen SE, Gurley JC, Booth DC, DeMaria AN. Intravascular ultrasound of the coronary arteries: current applications and future directions. Am J Cardiol. 1992;69:18H-29H.[Medline] [Order article via Infotrieve]
8. Liebson PR, Klein LW. Intravascular ultrasound in coronary atherosclerosis: a new approach to clinical assessment. Am Heart J. 1992;123:1643-1660.[Medline] [Order article via Infotrieve]
9.
Tobis JM, Mallery JA, Gessert J, Griffith J, Mahon D,
Bessen M, Moriuchi M, McLeay L, McRae M, Henry WL. Intravascular
ultrasound cross-sectional arterial imaging before and
after balloon angioplasty in vitro. Circulation. 1989;80:873-882.
10.
Tobis JM, Mallery J, Mahon D, Lahman K, Zalesky P,
Griffith J, Gessert J, Moriuchi M, McRae M, Dwyer M, Greep N, Henry
WL. Intravascular imagining of human coronary arteries
in vivo: analysis of tissue characteristics with comparison to
histologic specimens. Circulation. 1991;83:913-926.
11. Watanabe Y. Serial inbreeding of rabbits with hereditary hyperlipidemia (WHHL-rabbit): incidence and development of atherosclerosis and xanthoma. Atherosclerosis. 1980;36:261-268.[Medline] [Order article via Infotrieve]
12.
Kita T, Brown MS, Watanabe Y, Goldstein JL.
Deficiency of low density lipoprotein receptors in liver and adrenal
gland of the WHHL rabbit, an animal model of familial
hypercholesterolemia. Proc Natl
Acad Sci U S A. 1981;78:2268-2272.
13.
Yamamoto T, Bishop RW, Brown MS, Goldstein JL, Russell
DW. Deletion in cysteine-rich region of LDL receptor impedes
transport to cell surface in WHHL rabbit. Science. 1986;232:1230-1237.
14.
Hoeg JM, Vaisman BL, Demosky SJ Jr, Meyn SM, Talley GD,
Hoyt RF Jr, Feldman S, Berard AM, Sakai N, Wood D, Brousseau ME,
Marcovina S, Brewer HB Jr, Santamarina-Fojo S.
Lecithin-cholesterol acyltransferase overexpression
generates hyperalpha-lipoproteinemia and a nonatherogenic lipoprotein
pattern in transgenic rabbits. J Biol Chem. 1996;271:4396-4402.
15.
Salomaa V, Riley W, Kark JD, Nardo C, Folsom AR.
Non–insulin-dependent diabetes mellitus and fasting glucose and
insulin concentrations are associated with arterial
stiffness indexes: the ARIC study. Circulation. 1995;91:1432-1443.
16. Kolodgie FD, Virmani R, Cornhill JF, Herderick EE, Malcom GT, Mergner WJ. Cocaine: an independent risk factor for aortic sudanophilia: a preliminary report. Atherosclerosis. 1992;97:53-62.[Medline] [Order article via Infotrieve]
17. Vaishnav RN, Vossoughi J. Residual stress and strain in aortic segments. J Biomech. 1987;20:235-239.[Medline] [Order article via Infotrieve]
18. Vaishnav RN, Vossoughi J. Estimation of residual strain in aortic segment. In: Hall CW, ed. Biomedical Engineering II, Recent Developments. Pergamon Press; 1983:330-333.
19.
Folch RJ, Lees M, Sloan-Stanley GH. A simple
method for the isolation and purification of total lipids from animal
tissues. J Biol Chem. 1957;226:497-509.
20.
Lowry OH, Rosebrough NJ, Farr AL, Randall RJ.
Protein measurement with the Folin phenol reagent. J
Biol Chem. 1951;193:265-275.
21. Castelli W, Leaf A. Identification and assessment of cardiac risk: an overview. Cardiol Clin. 1985;3:171-178.
22.
Fung YC, Liu SQ. Change of residual strains in
arteries due to hypertrophy caused by aortic
constriction. Circ Res. 1989;65:1340-1349.
23. Liu SQ, Fung YC. Relationship between hypertension, hypertrophy, and opening angle of zero-stress state of arteries following aortic constriction. J Biomed Eng. 1989;111:325-335.
24. Matsumoto T, Hayashi K. Mechanical response of rat aorta to hypertension. In: Advances in Bioengineering. New York: ASME; 1991:631-634.
25. Matsumoto T, Hayashi K. Stress and strain distribution in hypertensive and normotensive rat aorta considering residual strain. J Biomech Eng. 1996;118:62-73.[Medline] [Order article via Infotrieve]
26. Vossoughi J. Biological residual stress and strain. In: Vossoughi J, ed. Biomedical Engineering/Recent Developments. Washington, DC: Eng Res Center, UDC; 1994:200-206.
27.
Fung YC, Liu SQ. Changes of zero-stress state of
rat pulmonary arteries in hypoxic hypertension.
J Appl Physiol. 1991;70:2455-2470.
28. Liu SQ, Fung YC. Influence of STZ-induces diabetes on zero-stress states of rat pulmonary and systemic arteries. Diabetes. 1992;41:136-146.[Abstract]
29. Lin SQ, Fung YC. Changes in the structure and mechanical properties of pulmonary arteries of rats exposed to cigarette smoke. Am Rev Resp Dis. 1993;148:768-777.[Medline] [Order article via Infotrieve]
30. Matsumoto T, Hayashi K, Ide K. Residual strain and local strain distributions in the rabbit atherosclerotic aorta. J Biomech. 1995;28:1207-1217.[Medline] [Order article via Infotrieve]
31. Hong MK, Vossoughi J, Haudenschild CC, Wong SC, Zuckerman BD, Leon MB. Vascular effects of diet-induced hypercalcemia after balloon artery injury in giant Flemish rabbits. Am Heart J. 1995;130:758-764.[Medline] [Order article via Infotrieve]
32. Caperna TJ, Vossoughi J, Hedjazi Z. Growth hormone alters aortic residual strain in growing pigs. In: Vossoughi J, ed. Biomedical Engineering/Recent Developments. Washington, DC: Eng Res Center, UDC; 1994;803-806.
33. Takamizawa K, Hayashi K. Strain energy density function and uniform strain hypothesis for arterial mechanics. J Biomech. 1987;20:7-17.[Medline] [Order article via Infotrieve]
34. Takamizawa K, Hayashi K. Uniform strain hypothesis and thin-walled theory in arterial mechanics. Biorheology. 1988;25:555-565.[Medline] [Order article via Infotrieve]
35. Fung YC. Biomechanics, Mechanical Properties of Living Tissues. New York: Springer-Verlag; 1993.
36. Liu SQ, Fung YC. Changes in rheological properties of blood vessel tissue remodeling in the course of development of diabetes. Biorheology. 1992;29:443-457.[Medline] [Order article via Infotrieve]
37. Fung YC, Liu SQ, Zhou JB. Remodeling of the constitutive equation while a blood vessel remodels itself under stress. J Biomech Eng. 1993;115:453-459.[Medline] [Order article via Infotrieve]
38. Lin I-E, Taber LA. Models for growth and morphogenesis in the developing heart. Ann Biomed Eng. 1993;21:9.[Medline] [Order article via Infotrieve]
39.
Rodriquez EK, Omens JH, Waldman LK, McCulloch
AD. Effect of residual stress on transmural sacromere length
distributions in rat left ventricle. Am J
Physiol. 1993;264:H1048-H1056.
40. Taber LA, Nu N, Pexieder T, Clark EB, Kelly BB. Residual strain in the ventricle of the stage 16-24 chick embryo. Circ Res. 1993;71:455-462.
41. Nevo E, Lanir Y. The effect of residual strain on the diastolic function of the left ventricle as predicted by a structural model. J Biomech. 1994;27:1433-1446.[Medline] [Order article via Infotrieve]
42. Rodriguez EK, Hoger A, McCulloch AD. Stress-dependent finite growth in soft elastic tissue. J Biomech. 1994;27:455-467.[Medline] [Order article via Infotrieve]
43. Humphrey JD. Mechanics of arterial wall: review and directions. Crit Rev Biomed Eng. 1995;23:1-162.[Medline] [Order article via Infotrieve]
44. Taber LA. Biomechanics of growth, remodeling, and morphogenesis. Appl Mech Rev. 1995;48:487-545.
45. Hayashi K, Kamiya A, Ono K. Biomechanics, Functional Adaptation and Remodeling. Tokyo: Springer-Verlag; 1996.
46. Hayashi K, Ide K, Matsumoto T. Aortic walls in atherosclerotic rabbits: mechanical study. J Biomech Eng. 1994;116:284-293.[Medline] [Order article via Infotrieve]
47. Ross R. The pathogenesis of atherosclerosis: a perspective for the 1990s. Nature. 1993;362:801-809.[Medline] [Order article via Infotrieve]
48. Hoff HF, Gaubatz JW. Isolation, purification, and characterization of a lipoprotein containing Apo B from the human aorta. Atherosclerosis. 1982;42:273-297.[Medline] [Order article via Infotrieve]
49. Simionescu N, Vasile E, Lupu F, Popescu G, Simionescu M. Prelesional events in atherogenesis: accumulation of extracellular cholesterol-rich liposomes in the arterial intima and cardiac valves of the hyperlipidemic rabbit. Am J Pathol. 1986;123:109-125.[Abstract]
50. Simionescu N, Mora R, Vasile E, Lupu F, Filip DA, Simionescu M. Prelesional modifications of the vessel wall in hyperlipidemic atherogenesis: Extracellular accumulation of modified and reassembled lipoproteins. Ann N Y Acad Sci. 1990;598:1-16.
51. Mora R, Lupu F, Simionescu N. Cytochemical localization of beta-lipoproteins and their components in successive stages of hyperlipidemic atherogenesis of rabbit aorta. Atherosclerosis. 1989;79:183-195.[Medline] [Order article via Infotrieve]
52. Ghinea N, Leabu M, Hasu M, Muresan V, Colceag J, Simionescu N. Prelesional events in atherogenesis: changes induced by hypercholesterolemia in the cell surface chemistry of arterial endothelium and blood monocytes in rabbit. J Submicrosc Cytol. 1987;19:209-227.[Medline] [Order article via Infotrieve]
53.
Guyton JR, Klemp KF. Development of the
atherosclerotic core region: Chemical and ultrastructural
analysis of microdissected atherosclerotic lesions from human
aorta. Arterioscler Thromb. 1994;14:1305-1314.
54. Kruth HS. Lipid deposition in human tendon xanthomas. Am J Pathol. 1985;121:311.[Abstract]
55. Singer MA, Finegold L. Cholesterol interacts with all of the lipid in bilayer membranes: implications for models. Biophys J. 1990;57:153-156.[Medline] [Order article via Infotrieve]
56. Spector AA, Mathur SN, Kaduce TL, Hyman BT. Lipid nutrition and metabolism of cultured mammalian cells. Prog Lipid Res. 1980;19:155-186.[Medline] [Order article via Infotrieve]
57. Glomset JA. The plasma lecithin:cholesterol acyltransferase reaction. J Lipid Res. 1968;9:155-167.[Abstract]
58. Gordon DJ, Rifkind BM. High-density lipoprotein: the clinical implications of recent studies. N Engl J Med. 1989;321:1311-1316.[Medline] [Order article via Infotrieve]
59.
Hoeg JM, Santamarina-Fojo S, Berard AM, Cornhill JF,
Herderick EE, Feldman SJ, Haudenschild CC, Vaisman BL, Hoyt RF Jr,
Demosky SJ Jr, Kauffman RD, Hazel CM, Marcovina SM, Brewer HB Jr.
Overexpression of lecithin:cholesterol acyltransferase in
transgenic rabbits prevents diet-induced
atherosclerosis. Proc Natl Acad Sci U S A. 1996;93:11448-11453.
60. Ludmer PL, Selwyn AP, Shook TL, Wayne RR, Mudge GH, Alexander RW, Ganz P. Paradoxical vasoconstriction induced by acetylcholine in atherosclerotic coronary arteries. N Engl J Med. 1986;315:1046-1051.[Abstract]
61.
Vita JA, Treasure CB, Nabel EG, McLenachan JM, Fish RD,
Yeung AC, Vekshtein VI, Selwyn AP, Ganz P. Coronary
vasomotor response to acetylcholine relates to risk factors for
coronary artery disease. Circulation. 1990;81:491-497.
62.
Seiler C, Hess OM, Buechi M, Suter TM, Krayenbuehl
HP. Influence of serum cholesterol and other
coronary risk factors on vasomotion of angiographically normal
coronary arteries. Circulation. 1993;88:2139-2148.
63.
Zeiher ZM, Schaechinger V, Hohnloser SH, Saurbier B,
Just H. Coronary atherosclerotic wall thickening and
vascular reactivity in humans: elevated high-density lipoprotein levels
ameliorate abnormal vasoconstriction in early
atherosclerosis. Circulation. 1994;89:2525-2532.
64.
Treasure CB, Klein JL, Weintraub WS, Talley JD,
Stillabower ME, Kosinski AS, Zhang J, Boccuzzi SJ, Cedarholm JC,
Alexander RW. Beneficial effects of
cholesterol-lowering therapy on the coronary
endothelium in patients with coronary artery
disease. N Engl J Med. 1995;332:481-487.
65.
Anderson TJ, Meredith AT, Yeung AC, Frei B, Selwyn AP,
Ganz P. The effect of cholesterol-lowering and
antioxidant therapy on endothelium-dependent
coronary vasomotion. N Engl J Med. 1995;332:488-493.
66.
Levine GN, Keaney JF, Vita JA.
Cholesterol reduction in cardiovascular
disease: clinical benefits and possible mechanisms.
N Engl J Med. 1995;332:512-521.
67. Bader H. Importance of the gerontology of elastic arteries in the development of essential hypertension. Clin Physiol Biochem. 1983;1:36-56.[Medline] [Order article via Infotrieve]
68. Bikkina M, Larson MG, Levy D. Asymptomatic ventricular arrhythmias and mortality risk in subjects with left ventricular hypertrophy. J Am Coll Cardiol. 1993;22:1111-1116.[Abstract]
69.
Levy D, Salomon M, D'Agostino RB, Belanger AJ, Kannel
WB. Prognostic implications of baseline electrocardiographic
features and their serial changes in subjects with left
ventricular hypertrophy.
Circulation. 1994;90:1786-1793.
70. Alessandri N, Franzin S, Sulpizii L, Cecchetti F, Rondoni G, Martarelli L, Anaclerio M, Campa PP. Wall elasticity of the ascending aorta in ischemic heart disease. Cardiologia. 1995;40:173-181.
71.
Lang RM, Cholley BP, Korcarz C, Marcus RH, Shroff
SG. Measurement of regional elastic properties of the human
aorta: a new application of transesophageal
echocardiography with automated border detection
and calibrated subclavian pulse tracings.
Circulation. 1994;90:1875-1882.
72. Heintz B, Vom Dahl J, Roeber K, Doettger A, Hanrath P, Sieberth HG. Effects of blood pressure reduction on the elastic profile of the aortic tree in patients with coronary heart disease. Am J Hypertens. 1995;8:584-590.[Medline] [Order article via Infotrieve]
73.
Mohiaddin RH, Firmin DN, Longmore DB.
Age-related changes in human aortic flow wave velocity measured
noninvasively by magnetic resonance imaging. J Appl
Physiol. 1993;74:492-497.
74. Underwood SR, Mohiaddin RH. Magnetic resonance imagining of atherosclerotic vascular disease. Am J Hypertens. 1993;6:335S-339S.[Medline] [Order article via Infotrieve]
75. Forbat SM, Mohiaddin RH, Yang GZ, Firmin DN, Underwood SR. Measurement of regional aortic compliance by MR imaging: a study of reproducibility. J Magn Reson Imaging. 1995;5:635-639.[Medline] [Order article via Infotrieve]
76.
Hoeg JM. Can genes prevent
atherosclerosis? JAMA. 1996;276:989-992.
This article has been cited by other articles:
 |
|
 |
|
  A. L. Pyle, J. B. Atkinson, A. Pozzi, J. Reese, B. Eckes, J. M. Davidson, D. L. Crimmins, and P. P. Young Regulation of the Atheroma-Enriched Protein, SPRR3, in Vascular Smooth Muscle Cells through Cyclic Strain is Dependent on Integrin {alpha}1{beta}1/Collagen Interaction Am. J. Pathol., November 1, 2008; 173(5): 1577 - 1588. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 Y-f Cheung, S J Wong, and M H K Ho Relationship between carotid intima-media thickness and arterial stiffness in children after Kawasaki disease Arch. Dis. Child., January 1, 2007; 92(1): 43 - 47. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 G. G. Zimmermann-Paul, H. H. Quick, P. Vogt, G. K. von Schulthess, D. Kling, and J. F. Debatin High-Resolution Intravascular Magnetic Resonance Imaging : Monitoring of Plaque Formation in Heritable Hyperlipidemic Rabbits Circulation, March 2, 1999; 99(8): 1054 - 1061. [Abstract] [Full Text] [PDF]
|
|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||