Cardiovascular Research 39 (1998) 224–232
The atherosclerotic Yucatan animal model to study the arterial response
after balloon angioplasty: the natural history of remodeling
B.J.G.L. de Smet a ,b , J. van der Zande a ,b , Y.J.M. van der Helm a ,b , R.E. Kuntz c , C. Borst a ,
c,
M.J. Post *
a
Department of Cardiology, Utrecht University Hospital, Utrecht, Netherlands
Interuniversity Cardiology Institute of the Netherlands, Utrecht, Netherlands
c
Cardiovascular Division, Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, MA, USA
b
Received 8 October 1997; accepted 20 January 1998
Abstract
Objective: Remodeling in de novo atherosclerosis and in restenosis after balloon angioplasty constitutes a change in total arterial
circumference which, together with plaque growth or neointimal formation, determines the lumen of the artery. To better understand the
fundamental biology of neointimal formation, remodeling and their interaction, animal studies are needed. In this study, we described in
detail the methodology used and the natural history of neointimal formation and remodeling after balloon angioplasty in atherosclerotic
Yucatan micropigs. Methods and results: Atherosclerosis was induced in 60 peripheral arteries of sixteen Yucatan micropigs by a
combination of denudation and atherogenic diet. Balloon angioplasty was performed in 38 arteries, with serial intravascular ultrasound
(IVUS) and quantitative angiography before and after intervention and at 2, 4, 7, 14 or 42 days follow-up. Remodeling, expressed as late
media-bounded area (MBA) loss, increased progressively over time. At 42 days, late MBA loss after balloon angioplasty was significantly
different compared to late MBA loss in control arteries, 2.261.0 versus 20.361.1 mm 2 and p50.02. Late lumen loss increased over time
and was highest at 42 days after balloon angioplasty (2.860.7 mm 2 ). The contribution of neointimal formation to late lumen loss
decreased over time and the contribution of late MBA loss to late lumen increased over time and was highest at 42 days (78%). Medial
necrosis was 48% at two days after balloon angioplasty and the repopulation of the media was almost completed at seven days.
Conclusion: Remodeling following balloon angioplasty has an early onset and progresses with neointimal formation to cause restenosis
over the standard 42-day time course for Yucatan micropigs. This correlates to six months renarrowing in humans. In this model,
atherosclerosis and the natural history of restenosis, both with respect to neointimal formation and remodeling, resemble the human
disease quite closely.  1998 Elsevier Science B.V. All rights reserved.
Keywords: Experimental (discipline); Vasculature (object of study); Circulatory physiology (level); Remodeling; Angioplasty; Ultrasound; Arteries;
Atherosclerosis (additional)
1. Introduction
Coronary restenosis after balloon angioplasty, defined as
arterial lumen renarrowing of the treatment site, has long
been considered to be primarily the consequence of
excessive neointimal formation. Recently, however, human
[1,2] and animal studies [3–5] have shown that, in addition
to neointimal formation, changes in the total arterial
circumference (so-called geometric remodeling) also occur
*Corresponding author: Tel.: 617 667 3790; Fax: 617 975 5201;
E-mail: [email protected]
0008-6363 / 98 / $19.00  1998 Elsevier Science B.V. All rights reserved.
PII: S0008-6363( 98 )00085-6
in response to arterial intervention. To better understand
the fundamental biology of neointimal formation, geometric remodeling and their interaction, animal studies are
needed. Preferably, such an animal model should have
arteries with atherosclerotic plaques that bear maximum
similarity to the human disease, and should illustrate the
two paradigms of restenosis after balloon angioplasty,
neointimal formation and remodeling. Small atherosclerotic animal models, especially in the mouse, should be
developed for future transgenic and knockout approaches
Time for primary review 27 days.
B. J.G.L. de Smet et al. / Cardiovascular Research 39 (1998) 224 – 232
to address specific questions in the vascular biology of
restenosis. In addition, a large animal model will remain
necessary to study new recanalization devices that are
usually designed according to human measures, and to
evaluate local drug delivery strategies with specialized
catheters. Sound and fastidious qualitative and quantitative
analysis of atherosclerosis and neointimal formation and
remodeling after balloon angioplasty is a second requisite
of a good animal model. We have discussed previously that
serial intravascular ultrasound is the best way to assess
remodeling after balloon angioplasty [6]. The current size
of intravascular ultrasound catheters also necessitates the
use of large animals.
We developed an atherosclerotic model in the Yucatan
micropig [7] that meets most of these conditions. We found
that the morphology of the atherosclerotic plaque covers
the whole range from hypercellular fibromuscular plaque
to lipid lakes that are covered by a fibrous cap and
complex lesions with calcification, cholesterol clefts and
signs of continuous inflammation and prothrombotic tendency. After balloon angioplasty, 70% of restenosis is
attributed to remodeling and 30% to neointimal formation,
which fits nicely with the reported numbers for human
coronary restenosis [1]. In contrast, neointimal formation is
the sole cause of restenosis in stented arteries.
The time course of remodeling after angioplasty has not
been described yet. Results from the SURE trial with serial
intravascular ultrasound suggested that there was a
biphasic remodeling response of the coronary artery, with
enlargement at one month and shrinkage at six months
after balloon angioplasty [8]. Our aim was to study the
possible differences in the time course of neointimal
formation and remodeling in this animal model. In this
manuscript, we describe in detail the methodology used
and the natural history of neointimal formation and
remodeling after balloon angioplasty in the atherosclerotic
peripheral arteries of Yucatan micropigs.
225
US National Institutes of Health (NIH Publication No.
85-23, revised 1985) and was approved by the ethical
committee on Animal Experiments of the Faculty of
Medicine, Utrecht University.
2.1. Atherogenic diet and anesthesia
In addition to essential nutrients, vitamins and salts,
1.5% cholesterol, 17.5% casein, 14% lard and 6% peanut
oil formed the basic atherogenic components of the diet,
which had a daily nutritional value equivalent to 2400 kcal
(Hope Farms, Netherlands). In pilot experiments, it was
shown that this regimen results in a sustained tenfold
increase in the total cholesterol level to 14.9 (61.3)
mmol / l, and an approximately 3.5-fold increase in the
high-density lipoprotein (HDL) level to 2.2 (60.1) mmol /
l. After discontinuation of the atherogenic diet, the levels
of cholesterol and HDL cholesterol dropped to base levels
(1.560.2 and 0.660.1 mmol / l, respectively). Water intake
was not restricted. The diet during follow-up was a regular,
non-atherogenic chow, with a nutritional value that was
equal to that of the atherogenic diet.
For denudation, intervention and termination, the animals were anesthetized with intravenous metomidate (4
mg / kg) and ventilated (Servo, EM 902) with a mixture of
O 2 –N 2 O (1:2) and halothane, 1–2%. During each procedure, the animals were heparinized, starting with 100
IU / kg thromboliquine (Organon Technika), and then titrating to an activated partial thromboplastin time (APTT)
above 80 s throughout the procedure. Every 15 min, 0.25
mg of atropine was given intravenously.
One day before intervention, acetyl salicylic acid (125
mg) p.o. was given and administration was continued for
two weeks after the intervention. During intervention and
termination, a continuous infusion of nitroglycerin (20
mg / min) was given to prevent arterial spasm.
2.2. Procedures
2. Methods
Sixteen Yucatan micropigs with an average weight of 24
kg at termination were used for this study. All animals
were started on an atherogenic diet at the age of seven–
eight months. Two weeks thereafter, they underwent
Fogarty denudation of the internal iliac, external iliac and
femoral arteries. The atherogenic diet was continued for an
additional four months, after which, selected arteries were
balloon dilated with a standard angioplasty balloon catheter, and the atherogenic diet was replaced by a regular diet.
The immediate and long term results of balloon angioplasty were documented by quantitative angiography and by
serial intravascular ultrasound. After 2, 4, 7, 14 or 42 days,
the animals were killed and the arteries were harvested for
histology. The investigation conforms with the Guide for
the Care and Use of Laboratory Animals published by the
For denudation, intervention and termination, the arterial
tree was accessed through a carotid cutdown and an
arterial 8F sheath was inserted into the descending aorta
under fluoroscopic guidance. An 8F guiding catheter was
advanced to the aortic bifurcation. Through this, contrast
(Telebrix, Laboratoire Guerbet, France) angiography was
performed and the Fogarty balloon catheter, the intravascular ultrasound catheter and the balloon angioplasty catheter
were advanced. After the denudation and the intervention,
the carotid arteries were carefully sutured for future
interventions.
During denudation, 3–4 cm segments (measured by a
radiopaque ruler) in the iliac and femoral arteries were
denuded by triple withdrawal of a 4F Fogarty catheter that
was manually inflated with a 50:50 (v / v) water–contrast
mixture.
Four months after the denudation, selected sites of
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B. J.G.L. de Smet et al. / Cardiovascular Research 39 (1998) 224 – 232
angiographic arterial narrowing were balloon dilated.
Before and after each intervention, angiography and intravascular ultrasound were performed under fluoroscopic
guidance. For balloon dilation, a standard peripheral
(length, 2–4 cm; diameter, 4–7 mm) or coronary (length, 2
cm; diameter, 2–4 mm) balloon catheter was advanced
over a 0.03599 or a 0.01499 guidewire. The balloon was
inflated three times for 1 min at a pressure of 10 atm. Of
all of the arteries available, 21 arteries were used for
another study that did not interfere with this study regarding drug therapy or repeated catheter handling.
After a follow-up of 2, 4, 7, 14 or 42 days, the pigs were
anesthetized and angiograms and intravascular ultrasound
measurements were made. The animals were killed after
angiography and intravascular ultrasound was performed.
2.3. Angiography and intravascular ultrasound
Angiography was performed before and after each
intervention and at follow-up. Contrast (Telebrix) was
injected selectively into the artery under study through an
8F guiding catheter. The fluoroscopy was recorded at a
cine rate of 12 images / s using a digital C-arm (Philips,
Eindhoven, Netherlands). The image with the highest
contrast was selected and stored on DAT tape for later
analysis.
The angiographic diameters of the arteries were measured using a semi-automated program [7]. In each artery,
lumen diameters were measured at intervals of 0.5 cm,
including the treated segment and a proximal and distal
reference segment (RLD). Reference segments were
chosen in an angiographically normal adjacent segment at
a distance of at least 1 cm proximal and distal from the
balloon-dilated site. To locate the treated segment repeatedly at different time points (pre- and post-procedure
and at follow-up), its position was documented relative to
an anatomic landmark with the support of a radiopaque
ruler placed along the spine in a reproducible manner. The
ruler was also used for calibration of the angiography [7].
All quantitative analyses were performed off-line. The
minimum lumen diameter (MLD) of the lesion was
determined. Angiographic acute gain was defined as the
difference between post- and pre-procedure lumen diameters, and angiographic late lumen loss was defined as the
difference between post-procedure and follow-up lumen
diameters. Angiographic percentage of stenosis was calculated as (12MLD/ RLD)3100. Dilation ratio was defined
as the diameter of the inflated balloon on fluoroscopy
divided by the angiographic diameter of the reference
segment.
Intravascular ultrasound recordings were made before
and after intervention and at follow-up, using a 30-MHz
ultrasound transducer (Du-MED, Rotterdam, Netherlands),
which rotated up to 16 times per second within a 4.1
French catheter. The axial resolution of the system was 0.1
mm. The images were displayed on a monitor and recorded
on VHS video-tape (Fig. 1). Intravascular ultrasound
Fig. 1. Representative serial IVUS images of an atherosclerotic lesion (A)
before balloon angioplasty, (B) immediately after balloon angioplasty and
(C) at 42 days follow-up. M5media-bounded area; the distance between
indicators is 1 mm.
B. J.G.L. de Smet et al. / Cardiovascular Research 39 (1998) 224 – 232
images were analyzed with a digital video analyzer, as
described previously [9]. Fluoroscopy was performed
during intravascular ultrasound (IVUS) so that the IVUS
images were documented relative to an anatomic landmark, to match the IVUS images at different time points
(pre- and post-procedure and at follow-up). In the IVUS
images, the area circumscribed by the interface between
the echodense intimal layer and the echolucent media was
manually traced and taken as the media-bounded area
(MBA). In addition, the lumen area (LA) was traced and
the minimum lumen area was determined (MLA). Echographic acute gain was defined as the difference between the
post- and pre-intervention lumen area, and echographic
late lumen loss as the difference between post-intervention
and follow-up lumen area. In addition to late lumen loss,
late MBA loss, being a measure of remodeling after
angioplasty, was introduced. Late MBA loss is defined as
the difference between the post-intervention MBA and the
follow-up MBA at either the site of the initial stenosis or
the reference sites. Intimal area was taken as the difference
between MBA and the lumen area. At follow-up, the
intima is the sum of plaque and neointimal formation.
Neointimal formation is therefore calculated as the difference between the intima at follow-up and the intima before
intervention. Echographic percentage of stenosis is calculated as (12MLA / RLA)3100. The loss index was defined
as the coefficient of the regression with echographic late
lumen loss as the dependent variable and echographic
acute gain as the independent variable.
227
buffer (pH 7.4) for 2 h, followed by overnight storage in
sterile 15% sucrose–phosphate-buffered saline (PBS) solution at 48C. After fixation, the arteries were divided into
segments of 3–4 mm and embedded in optimal cutting
temperature compound (OCT) and frozen in liquid nitrogen. Serial cross-sections were either stained with hematoxylin and eosin or with elastin van Gieson stain. Three
cross-sections per artery were studied to evaluate medial
necrosis and the occurrence of thrombosis. In hematoxylin
and eosin stained cross-sections, medial necrosis was
quantitatively analyzed in the following fashion. Four
fields per cross-section were chosen in clockwise order (at
3, 6, 9 and 12 h). In each field, the total number of cell
nuclei in the media was counted. Cell counting was
performed at 3200 magnification using a video microscope that projected the image onto a visual display unit.
Nuclei were counted on the screen using a computer
¨
program, analySIS (Soft-Imaging Software, Munster,
Germany), which allowed tagging of counted nuclei. The
percentage medial necrosis was calculated as: (12number
of cell nuclei in dilated artery / average number of cell
nuclei of all control arteries)3100, assuming no medial
necrosis in control arteries. Cell density was defined as the
number of cell nuclei in the media per mm 2 . Thrombosis
was evaluated at two days after balloon angioplasty in both
hematoxylin and eosin stained and elastin van Gieson
stained cross-sections. For this purpose, three cross-sections per artery, at 5 mm intervals, were evaluated.
2.5. Statistical analysis
2.4. Histology
The arteries were removed ‘en block’ and fixed by
immersion in 4% paraformaldehyde in 0.1 mol / l NaPO 4
All data in text and table are presented as
mean6standard error of the mean. SPSS 6.1 was used for
all statistical calculations. To identify differences between
Fig. 2. Representative serial angiographic images of an iliac artery (A) before balloon angioplasty, (B) the balloon, (C) immediately after balloon
angioplasty and (D) at 42 days follow-up. Bar indicates 1 cm.
228
B. J.G.L. de Smet et al. / Cardiovascular Research 39 (1998) 224 – 232
Fig. 3. Representative histological cross-sections of control iliac arteries (elastin van Gieson stain) (A), an eccentric lesion with calcifications (C) and a
fibrocellular intima (F), (B) a concentric lesion with lipid accumulation (L) and a thin fibrous layer (F). I5internal elastic lamina, E5external elastic
lamina. Bar indicates 1 mm. Note that the arteries are not perfusion fixed and are frozen sections, paraformaldehyde-fixed.
B. J.G.L. de Smet et al. / Cardiovascular Research 39 (1998) 224 – 232
subgroups, we used one-way ANOVA and Duncan’s range
test. Pearson correlation coefficients were calculated when
indicated.
3. Results
Balloon angioplasty was performed in 38 arteries in 16
Yucatan micropigs. Twenty-two arteries were not balloon
dilated and served as control arteries. Complete IVUS and
angiographic imaging were performed in all arteries. An
example of the serial IVUS images and serial angiography
is shown in Figs. 1 and 2. The analyses in this study were
based on 34 / 38 balloon-dilated arteries where a positive
echographic acute gain was achieved. The negative gain in
the remaining arteries resulted from spasm after balloon
angioplasty, which potentially influences the relation between acute gain and late loss, and introduces a group of
post-angioplasty arteries that are enlarged, but not necessarily remodeled. The four excluded arteries were comparable before balloon angioplasty to the arteries that were
successfully dilated. There were no statistically significant
differences between the excluded and included arteries in
parameters such as angiographic and echographic reference
diameter, angiographic and echographic percentage
stenosis and dilation ratio.
The average echographic stenosis was 2862%. The
echographic acute gain was 5.760.8 mm 2 . Acute gain and
the dilation ratio are considered to reflect the severity of
injury imparted on the arterial wall. The mean pre-existing
229
plaque area was 1.860.2 mm 2 . Representative histologic
cross-sections show the heterogeneous nature of the
atherosclerotic plaques, varying from a concentric lesion
with accumulations of lipids covered by a fibrocellular
layer (Fig. 3A) to an eccentric lesion with calcifications
(Fig. 3B). The echographic and angiographic percentage of
stenosis, echographic and angiographic acute gain and the
dilation ratio were not statistically different at the different
time points (Tables 1 and 2). Lumen areas, MBAs and
lumen diameters before balloon angioplasty are given in
Table 1 for both the site of stenosis and the reference site.
3.1. Remodeling and neointimal formation after balloon
angioplasty over time
We used late MBA loss as a measure of remodeling
after balloon angioplasty. Late MBA loss increased progressively over time (Fig. 4A). Even by seven days, we
observed an increase in late MBA loss. At 42 days, late
MBA loss after balloon angioplasty was significantly
different compared to late MBA loss in control arteries,
2.261.0 versus 20.361.1 mm 2 and p50.02.
Echographic and angiographic late lumen losses increased over time and were highest at 42 days after balloon
angioplasty (2.860.7 mm 2 and 0.360.2 mm, respectively).
The contribution of late MBA loss and neointimal formation to late lumen loss is shown in Fig. 4A. At 42 days,
echographic late lumen loss correlated with late MBA loss
(r50.86, p50.03) but not with neointimal formation (r5
20.17, p50.75). The relative contribution of late MBA
Table 1
Angiographic parameters
Number of days after balloon angioplasty
Number of arteries
MLD pre (mm)
LD reference pre (mm)
Stenosis angio (%)
Acute gain angio (mm)
Dilation ratio
2
4
7
14
42
Control
8
2.460.2
3.560.3
30.766.8
0.860.2
1.260.1
5
2.560.3
3.060.1
15.7610.9
1.060.4
1.460.4
7
2.460.2
2.960.2
18.764.0
0.960.3
1.460.3
8
2.760.2
3.460.1
21.965.9
0.860.2
1.260.1
6
2.360.3
3.260.2
29.067.7
0.860.2
1.260.0
22
2.660.1
3.260.1
15.162.4
Data are presented as the mean6SEM, pre5before balloon dilation, MLD5minimum lumen diameter and LD5lumen diameter.
Table 2
Echographic parameters and loss index
Number of days after balloon angioplasty
Number of arteries
MLA pre (mm 2 )
MBA pre (mm 2 )
LA reference pre (mm 2 )
MBA reference pre (mm 2 )
Stenosis IVUS (%)
Acute gain IVUS (mm 2 )
Loss index
2
4
7
14
42
Control
8
7.660.5
9.460.6
12.562.2
13.762.1
29.969.6
5.161.8
20.160.06
5
6.160.3
7.560.8
11.062.6
11.462.6
33.9612.1
5.661.6
0.1460.08
7
6.060.9
7.461.2
8.562.0
8.761.9
26.767.9
7.162.1
0.2260.1
8
9.861.0
12.361.5
13.361.4
13.561.6
28.065.7
4.661.5
0.2660.22
6
7.061.2
8.961.4
11.262.0
12.062.0
34.768.1
6.361.5
0.4260.08
22
7.460.7
8.260.7
9.861.0
10.561.0
24.562.5
Data are presented as the mean6SEM, pre5before balloon dilation, MLA5minimum lumen area, LA5lumen area and MBA5media-bounded area.
230
B. J.G.L. de Smet et al. / Cardiovascular Research 39 (1998) 224 – 232
Fig. 5. Cell density after balloon angioplasty over time. Note that zero
days are the control arteries. *5p,0.05 (ANOVA) and compared with 0,
7, 14 and 42 days.
Fig. 4. Panel A: The contribution of remodeling (REM) and neointimal
formation (NI) to late lumen loss (LL) over time after balloon angioplasty, as assessed by IVUS. Panel B: The contribution of remodeling (REM)
and neointimal formation (NI) to late lumen loss (LL) in control arteries
on different days of follow-up, as assessed by IVUS.
loss to late lumen loss increased over time and was 79% at
42 days. The relative contribution of neointimal formation
to late lumen loss decreased over time and was 21% at 42
days. The loss index increased from 20.1060.06 at two
days follow-up to 0.4260.08 at 42 days follow-up. Two
days after balloon angioplasty there was a tendency
towards a negative late lumen loss. However, this was not
significant compared to control arteries. For control arteries, the relative contributions of late MBA loss and
neointimal formation are shown in Fig. 4B. Late lumen
loss, late MBA loss or neointimal formation in the control
arteries were not significantly different from zero at the
different time points.
3.2. Thrombus formation and medial necrosis
Two days after balloon dilation, thrombi were seen in
six out of eight arteries. Only in one artery was a medium
size thrombus present in all cross-sections, covering a
large part of the luminal surface without compromising the
lumen area. In five arteries, the thrombi were minor and
present in only one of three cross-sections. In those five
arteries, the thrombi were only present at medial tears,
filling up the crevices in the tissue. The thrombi did not
protrude into the lumen.
Medial necrosis at two days was 48% and at four days,
it was 45%. The cell densities at two and four days were
significantly different (ANOVA, p,0.05) from the cell
density at 7, 14 and 42 days and from the cell density in
the control arteries (Fig. 5). The cell densities at 7, 14 and
42 days were not significantly different from each other
and also were not significantly different from the cell
density in control arteries. However, the cell density at 7
and 42 days was approximately 79% of the cell density in
the control arteries (29346174 and 28896134, respectively, versus 36696188). At 14 days, the cell density was
92% of the cell density in the control arteries (33646168
versus 36696188). Medial necrosis was patchy and
asymmetrically distributed over the circumference of the
cross-section.
4. Discussion
Remodeling after balloon angioplasty has been recognized as a major determinant of restenosis. After balloon
angioplasty, shrinkage seems to be the predominant,
although certainly not the only, mode of remodeling.
Post-dilation remodeling, therefore, usually leads to a
reduction in lumen size [1–5] that is additional to the
lumen encroachment by neointimal formation. Previously,
we reported that remodeling after balloon angioplasty
importantly determines restenosis in this model [10], to an
extent that is equivalent to that reported for human
B. J.G.L. de Smet et al. / Cardiovascular Research 39 (1998) 224 – 232
coronary arteries [1]. Thus, this animal model seems to be
highly suited for the study of remodeling after angioplasty.
In this combined angiographic, serial ultrasound study in
an atherosclerosis model in the Yucatan micropig, we
studied the natural history of neointimal formation and
remodeling after balloon angioplasty. Our principal finding
is that remodeling following balloon angioplasty has an
early onset and progresses with neointimal formation over
a 42-day time course. The histology of the atherosclerotic
plaque is heterogeneous and is very similar to that of
advanced human atherosclerosis. However, spontaneous
plaque rupture, an important source for thrombus accumulation and peripheral embolization [11], was not observed
in these atherosclerotic animals. In this model, the relationship between acute gain and late loss after balloon
angioplasty is almost identical to that seen in various
human angioplasty trials, and we observed previously that
this relationship is based on a proportional remodeling
response [10].
The follow-up of 42 days was chosen because, by this
time, the neointimal formation has reached a steady state
[12,13]. Steele et al. [12] showed in a pig model that
neointima thickness was maximal at 14 days after balloon
angioplasty and did not change between 14 and 60 days
after balloon angioplasty. A similar time course was
observed by Groves et al. [13] in a study where neointimal
area was already maximal at seven days after balloon
angioplasty, without rupture of the internal elastic lamina,
and was maximal at 21 days after balloon angioplasty,
which had caused rupture of the internal elastic lamina.
Furthermore, serial angiographic studies in humans have
shown that the vast majority of restenosis occurs in the
first three months after balloon angioplasty [14,15], a time
period that corresponds with the 42 days in pigs. However,
we cannot exclude the possibility that late lumen loss and
late MBA loss will further increase after 42 days.
4.1. Relationship between neointimal formation and
remodeling
Although in the present study both neointimal formation
and remodeling progress over time and contribute to late
lumen loss, only remodeling was positively correlated with
late lumen loss. The extent of neointimal formation did not
correlate with remodeling [10]. This suggests that the
pathogeneses of both are dissimilar and that changes in the
size and probably the composition of the intima do not
affect total arterial size. Lafont et al. [5] found adventitial
changes to correlate with remodeling and, indeed, by its
high collagen content, the adventitia is most likely involved in the remodeling process. In a previous study in
this animal model [16], we showed that expression of
mRNA of collagen types I and III was markedly increased
seven and fourteen days after balloon angioplasty and that
this expression was highest in the adventitia. This is in
agreement with the results of Strauss et al. [17,18], who
231
showed an increase of total collagen synthesis one week
after balloon angioplasty in a rabbit model.
The repopulation of the media is almost finished at
seven days. The finishing of the repopulation of the media
seems to coincide with both the start of the remodeling
process and the increase in collagen synthesis, which is
maximal at seven days [16]. This suggests that a repopulated media is needed for the start of the remodeling
process, in which the increase in collagen synthesis is
likely to play an important role.
4.2. Drawbacks of the Yucatan atherosclerosis model
The drawbacks of this Yucatan atherosclerosis model are
shared with other large animal atherosclerosis models. The
model is expensive since it requires long term housing and
care of large animals that are being fed an expensive,
custom-made atherogenic diet. The incubation period of
four months precludes quick screening of a new device or
a new drug. Furthermore, the time pattern of the development of restenosis seems to be faster in pigs than in
humans. In this model, the relative contributions at 42 days
of late MBA loss and neointimal formation to late lumen
loss at 42 days were 79 and 21%, respectively. This is
similar to the results in human coronary arteries at six
months [1,2]. Also, the neointima formation appears to be
more rapid in the pig than in the human. Neointima
formation after balloon dilation stabilizes between two and
four weeks after balloon dilation in the pig, but in humans,
it increases for up to six months after balloon dilation [8].
Another possible drawback is that the average age of these
pigs is approximately one year, which does not reflect the
average age in the clinical situation.
The micropigs are expensive to purchase, but over four
months, they end up being cheaper than domestic Yorkshire pigs because of the smaller amount of feed required.
Given these limitations, the atherosclerotic Yucatan model
is extremely useful for fundamental research on vascular
wall biology after balloon angioplasty. It can also be
effectively used in the final, preclinical, stages of deviceor drug testing.
4.3. Limitations of the study
This study was performed in peripheral arteries in an
animal model of advanced atherosclerosis and it remains to
be determined if results obtained in peripheral arteries in
animal atherosclerosis models can be extrapolated to
atherosclerotic human coronary arteries. Separate serial
intravascular ultrasound studies by Mintz et al. [1] and by
Di Mario et al. [2], however, showed similar results with
respect to the relative contribution of remodeling to late
lumen loss [3–5].
Although the arteries contained considerable pre-existing plaques, the echographic stenoses were moderate, with
an average of 2862% and a lumen reduction of 4.060.8
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B. J.G.L. de Smet et al. / Cardiovascular Research 39 (1998) 224 – 232
mm 2 . The average acute gain was 5.760.8 mm 2 and, thus,
30% of the acute gain was achieved through over dilation
of the arteries. However, the dilation ratio we employed in
this study was 1.2, which is similar to the dilation ratios in
coronary arteries of humans. Moreover, we found that the
observed loss index of 0.4260.08 at 42 days follow-up
was also similar to values reported for clinical studies [19].
5. Conclusion
From this serial intravascular ultrasound study in an
atherosclerotic Yucatan micropig model, we conclude that
remodeling following balloon angioplasty has an early
onset and progresses with neointimal formation to cause
restenosis over the standard 42-day time course for
Yucatan micropigs. This correlates to six months renarrowing in humans. In this model, atherosclerosis and the
natural history of restenosis, both with respect to neointimal formation and remodeling, resemble that of the human
disease quite closely.
Acknowledgements
This study was supported by the Netherlands Heart
Foundation (grant NHS 92.365) and in part by the
Interuniversity Cardiology Institute of the Netherlands.
[5]
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[7]
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[10]
[11]
[12]
[13]
[14]
[15]
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