(Circulation. 1997;96:4002-4010.)
© 1997 American Heart Association, Inc.
Articles |
Effect of Angiotensin-Converting Enzyme Inhibition on Myocardial Fibrosis and Function in Hypertrophied and Failing Myocardium From the Spontaneously Hypertensive Rat
From the Department of Veterans Affairs Medical Center, Boston, Mass, and the Department of Medicine, Boston University School of Medicine, Boston.
Correspondence to Wesley W. Brooks, DSc, Research Service(151), Boston VA Medical Center, 150 South Huntington Ave, Boston, Mass 02130. E-mail conrad.chester{at}boston.va.gov
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Background After a period of stable hypertrophy, male spontaneously hypertensive rats (SHR) develop heart failure between 18 to 24 months of age, with depression of active myocardial function and increased passive stiffness. We tested the hypothesis that chronic ACE inhibition by captopril would prevent and possibly reverse impairment of myocardial function.
Methods and Results Male SHR and normotensive Wistar-Kyoto rats (WKY) were assigned to no treatment or captopril treatment (2 g/L in drinking water) begun at ages 12, 18, and 21 months; animals were studied at 24 months of age, or earlier when evidence of heart failure was found in SHR (mean age, 19±2 months). In an additional group, captopril treatment was begun when SHR developed heart failure; surviving animals were studied at 24 months of age. In untreated SHR, relative to WKY, isometric stress development at Lmax, maximum rate of stress development, and shortening velocity were depressed, whereas passive stiffness was increased, in association with the development of myocardial fibrosis. In the SHR treated before cardiac dysfunction, captopril administration attenuated hypertrophy and prevented contractile dysfunction, fibrosis, and increased passive stiffness. Captopril treatment begun after cardiac function was impaired reduced left ventricular hypertrophy but did not restore intrinsic contractile function or reduce fibrosis or passive stiffness.
Conclusions In the male SHR, early treatment with captopril was associated with the most marked attenuation of dysfunction relative to the untreated SHR. Treatment initiated after the onset of heart failure improved clinical signs of heart failure and decreased left ventricular hypertrophy in surviving animals but did not reverse the fibrosis and contractile dysfunction associated with heart failure.
Key Words: angiotensin-converting enzyme inhibition • myocardial fibrosis • myocardial hypertrophy • heart failure
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Introduction |
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The SHR is a laboratory model of naturally developing hypertension that appears to be similar in a number of respects to essential hypertension in humans1–3. In the SHR model, persistent hypertension begins when the rats are
2 months of
age4,5 and is followed by a relatively long
period of stable, compensated hypertrophy. At 18 months
of age in the male SHR, the first indications of left
ventricular pump6,7 and muscle
dysfunction8,9 appear. Evidence of dysfunction is
also observed in the female SHR although events appear to be delayed by
a few months.10 These findings are followed by
pathophysiological manifestations of overt heart
failure (mean age of failure is 20 months in the male SHR, with
evidence of failure in most animals by 24 months of
age).3,9,11 At 18 to 24 months, with the advent
of the heart failure state, the male SHR demonstrates a marked
upregulation of genes encoding extracellular matrix
components12 associated with an increase in
myocardial fibrosis, passive stiffness, and impaired contractile
function relative to age-matched nonfailing SHR and normotensive WKY
rats.3,9 In the clinical setting, patients with heart failure of varying origin are well recognized to benefit from ACE inhibitor therapy.13 Pfeffer et al10 have shown that long-term captopril administration prevents impairment of hemodynamic function in the female SHR. However, the relation between the time treatment is initiated in the course of the disease and the prevention or reversal of intrinsic myocardial dysfunction and hemodynamic impairment has not been elucidated.
The purpose of this study, therefore, was to examine the relation between age of initiation of ACE inhibitor treatment, including its administration before or subsequent to the development of heart failure, and the prevention and/or reversal of fibrosis and intrinsic myocardial dysfunction in the male SHR.
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Methods |
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Materials and Preparation
Animal Model
One hundred eight male SHR and 12 WKY rats were purchased from Taconic (Germantown, NY). All rats were housed two per container and fed regular rat chow and water. Groups of SHR were treated by addition of captopril to drinking water (2 g/L of drinking water). This concentration has been shown to be effective in preventing hemodynamic impairment associated with chronic hypertrophy in the SHR.10 Groups of animals had treatment initiated at 12, 18, and 21 months of age (SHRRx12, SHRRx18, SHRRx21, respectively). An additional group of SHR was treated with captopril when evidence of impaired cardiac function was detected clinically and documented by echocardiographic studies.14 Fig 1
is a schematic representation
of the times captopril treatment was initiated to the various SHR
groups used in this experimental protocol. Control groups included
untreated SHR and age-matched WKY. Animals were monitored with
measurements of body weight and blood pressure. Animals were also
observed several times a week for the presence of tachypnea and labored
respiration; when these findings were clearly evident, animals were
either killed and studied or treated with captopril. Animals not
demonstrating respiratory difficulties were studied at 24 months of
age.
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Echocardiographic Methods
Two dimensional echocardiographic examination
was performed on conscious, nonfailing SHR (SHR-NF) and failing SHR
(SHR-F) as previously described.14 All studies
were performed with a 7.5-MHz short focus transducer with a 60-Hz frame
acquisition rate (model 77020A, Hewlett-Packard). Images were obtained
in the parasternal long-axis and short-axis and apical two-chamber and
four-chamber views. Gain and contrast settings were adjusted by the
operator (M.T.S.) to achieve a well-balanced gray-scale appearance on
the video display. Real-time images were stored on VHS videotape for
subsequent off-line analysis. Ventricular volume
(V) was calculated assuming the ventricle to be a prolate spheroid,
formed by rotation of an ellipse about its major axis: V=(4/3) ·
· (L/2) · (W/2)2, where L is
the major axis (length) and W the minor axis (width). The relationship
between the long-axis cross-sectional area (A) and the dimensions is
A= · (L/2) · (W/2), or A=/4 · L · W,
so that W can be expressed as W=4 · A/(L), and volume can be
expressed as V=([8/3]/) · A2/L.
End-diastolic volume (EDV) can therefore be
represented as EDV=([8/3]/) ·
LVAd2/LVLd,
where LVAd is end-diastolic area and
LVLd is end-diastolic length;
end-systolic volume (ESV) as ESV=([8/3]/) ·
LVAs2/LVLs,
where LVAs is end-systolic area and
LVLs is end-systolic length. Ejection
fraction ([EDV-ESV]/EDV) was calculated as an index of left
ventricular systolic function. The high-frequency
transducer used in this study allowed us to obtain good-quality
transthoracic images of the beating heart. With a small
sector size and shallow imaging depth, sampling rate for
two-dimensional imaging was 60 Hz. Given heart rates of 200 to 300
bpm, 8 to 16 frames were recorded per cardiac cycle.
Mechanical Studies
Experimental Preparation
After the animal was killed, hearts were quickly removed and
placed in oxygenated Krebs-Henseleit solution at 28°C.
The left ventricular posterior papillary muscle was
dissected free, mounted between two spring clips, and placed vertically
in a 100-mL acrylic chamber containing Krebs-Henseleit solution of the
following composition (mmol/L): NaCl 120, KCl 5.9,
NaHCO3 25,
NaH2PO4 1.2,
MgCl2 1.2, CaCl2 1.0, and
dextrose 11.5. The solution was bubbled with a gas mixture containing
95% O2 and 5% CO2 and
equilibrated to pH 7.4 at 28°C. The muscle preparation was placed
between parallel platinum plate electrodes and field stimulation was
carried out at a rate of 0.2 Hz, using square-wave pulses 5 ms in
duration. The voltage was set 10% above threshold. The spring clip on
the upper end of the muscle was attached to a low-inertia DC pen motor
(G100-PD, General Scanning) and the lower clip to a semiconductor
strain gauge force transducer (DSC-3, Kistler-Morse). A digital
computer with an analog/digital interface allowed control of
either force or length of the preparation. Force and length data were
sampled at a rate of 1 kHz and stored on disk for later
analysis.
After removal of the papillary muscles, the left and right ventricles were carefully separated. Tissues were gently blotted and weighed. Samples of left and right ventricle were dried at 60°C for 24 hours and reweighed to calculate wet/dry ratios. LV/BW and RV/BW were used as indices of ventricular hypertrophy.
After muscles were mounted, they were equilibrated by isotonic
contraction at a light load (on the order of 0.4
g/mm2). After this period, muscles were
gradually stretched to the peak of the active stress versus length
relationship (Lmax, defined as the muscle length
resulting in peak active stress). The overall muscle equilibration time
period before study was 90 to 120 minutes. Isometric contraction
parameters from five twitches were averaged; these were
resting stress (
resting,
g/mm2); active stress
(active, g/mm2),
defined as peak isometric stress minus resting stress; peak rate of
isometric stress development (peak+d/dt,
g/mm2/s); electromechanical delay (EMD,
ms), defined as the time from stimulation to the onset of stress
development; time to peak stress (TPS, ms), defined as the time from
the onset of stress development to the time of peak stress; and time
from peak stress to 50% relaxation
(RT1/2, ms). Stress-velocity relations
(shortening velocity versus load) were determined from muscle
shortening velocity measurements after "quick releases" 100 ms
after stimulation to loads ranging from 0.5 g to peak
isometric stress. Values were subsequently normalized for muscle length
and cross-sectional area, as described
previously.3,8
Stress-Strain Analysis
Analysis of passive central segment stiffness was done
as previously described.9,15 Briefly, two silk
markers, 1 to 2 mm apart, were applied to the muscle
preparations. Application of the markers had no significant effect on
active stress development. The preparation is scanned longitudinally by
a laser beam at a rate of 1 kHz, and the resulting decrease in
reflected light is detected by a photodiode. The time between marker
detection events is converted to a central segment length signal
(Lcs), which is sampled by the computer along
with whole muscle length and stress. Central segment dimensions were
measured at three levels in both the frontal and lateral
projections with the use of a cathetometer-telescope combination
(Gaertner Scientific Corp) and cross-sectional area calculated assuming
an elliptical configuration. Passive stress/length relations were
determined by applying length ramps to the whole papillary muscle at a
rate of 1.0 mm/s, corresponding to a normalized rate of length
change on the order of 0.1 muscle lengths/s, with a range of stress on
the order of 0.1 to 1.25 g/mm2. Stress
and central segment length were sampled and central segment
stress-strain relations were derived from these measurements as
previously described.9 It should be noted that
stiffness was determined over a relatively large range of stress, the
upper limit of which was above the usual
physiological range.
Central segment stress (cs) was defined as
tension normalized by instantaneous cross-sectional area, calculated
from the measured cross-sectional area assuming incompressibility:
CSAcs(inst)=CSAcs(ref)[Lcs(ref)/Lcs(inst)],
where CSAcs(inst) is instantaneous central
segment cross-sectional area, CSAcs(ref) is the
cross-sectional area at the reference length,
Lcs(inst) is instantaneous central segment
length, and Lcs(ref) is the reference central
segment length. Natural strain (
) is defined as =ln
(L/Lo), where L is length and
L0 is length at zero stress (or "slack
length"). Because of the exponential nature of the stress-strain
relation, and therefore the shallow slope at low loads, the
determination of true slack length (as used in the traditional
definition of strain) is subject to considerable experimental error.
Therefore, a modified natural strain definition was used in the
present study: cs=ln
(Lcs/L0.1), where
Lcs is instantaneous central segment length, and
L0.1 is central segment length at a load of 0.1
kdyne/mm2. Using this definition, =0 at a
near-slack "reference length" at which =0.1
kdyne/mm2.
Assuming that passive myocardial stress()-strain() relations are
exponential in nature, the relation can be expressed as
=ce(k). Using a log transformation,
log()=log(c)+k. The central segment stiffness constant,
kcs, was derived from the slope of the log()
versus relation.
Histological Analysis
Samples of left ventricular free wall were obtained
at the time the animals were killed, placed in Karnovsky's fixative,
and prepared for microscopy and histological
analysis as previously described.8,9 At
the conclusion of the mechanical studies, the central segment of the
left ventricular papillary muscle was fixed with a preload
equivalent to the resting stress at Lmax.
Histological sections were stained with Masson's
trichrome and with picrosirius red. Picrosirius red sections were
examined under polarized light.16 Areas of
connective tissue network and myocytes were quantified by a
semiautomated computer-based video analysis system with the
ColorImage image analysis software
package.17 Images were acquired with a binocular
microscope (Leitz Ortholux) with a video camera (Javelin model JE3462
RGB). The software has the capability of identifying the area in an
image corresponding to a specified spectral range. For each field of a
given slide, a region of interest was identified and the computer was
used to discriminate all areas within that region meeting the specified
spectral criteria. The relative area occupied by connective tissue and
myocytes (representative of volume fraction, assuming a
random three-dimensional distribution) was quantified in at least 5
fields from each slide, and the results were averaged. Slides from 5 to
13 left ventricles were examined from each group.
Statistical Analysis
A two-way ANOVA with replication was used to examine group and
treatment effects. The Newman-Keuls multiple-sample comparison
test18 was used to localize differences where
appropriate. Data are expressed as mean±SD.
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Results |
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Development of Heart Failure and Survival Data
One hundred eight SHR and 12 WKY rats were entered into the study. SHR were studied when failure was suspected, or at 24 months if failure did not occur by that time. Clinical findings suggesting heart failure were tachypnea and labored respiration. Animals were only considered to have heart failure, however, when documented by pathological findings as described previously,3,9,11 which included pleural and/or pericardial effusions, left atrial thrombi, and right ventricular hypertrophy. Forty-five SHR were treated before heart failure (SHRRx12, n=14; SHRRx18, n=16; and SHRRx21, n=15) and 12 SHR were treated after heart failure was documented (SHR-FRx). Fifty-one SHR were never treated (SHRNo Rx). Of the group of 63 initially untreated SHR (51 untreated and 12 treated only after the onset of failure), 54 of 63 (85%) died or developed heart failure by 24 months of age; 17 SHR died of unknown cause (no obvious cardiac pathology) or had to be killed for other reasons (stroke, debilitation, or tumor); 5 SHR died with autopsy evidence of heart failure (the pathological criteria for failure were the same as that used for study animals); and 20 were studied when they developed heart failure. Twelve SHR developed evidence of heart failure and were treated (SHR-FRx group). Overall, 37 of 63 (59%) of untreated SHR (this includes the SHR-FRx group, which developed failure and were then treated) developed heart failure by 24 months of age. None of the SHR treated before heart failure (n=45) developed pathological findings consistent with heart failure.
Physiological Parameters
Peak systolic blood pressure, body weight, cardiac chamber
weights, and chamber weights normalized for body weight are
presented in Table 1. Treatment
of SHR with captopril initiated at 12, 18, and 21 months of age
(SHRRx12, SHRRx18,
SHRRx21) was associated with a significant
decrease in the left ventricular weight/body weight and
right ventricular weight/body weight in all treated SHR
(P<.01) compared with untreated SHR. Both LV/BW and RV/BW,
however, were significantly lower in WKY relative to SHR with treatment
initiated at 12 months of age (SHRRx12 versus
WKY; P<.01). In a subgroup of 12 SHR-F, captopril treatment
was begun at the time of heart failure (SHR-FRx).
In addition to tachypnea and labored respiration, the presence of
depressed function was documented in this subgroup of animals by
echocardiogram (Table 2). Six of 12
SHR-FRx (50%) treated with captopril for 2 to 4
months survived to 24 months of age and were then studied. At the time
of study, 1 of 6 had effusions, 5 of 6 had atrial thrombi, and all had
right ventricular hypertrophy. When SHR-F were
treated after heart failure (SHR-FRx), LV/BW was
significantly reduced (P<.01, Table 2) relative to the
untreated SHR-F group.
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Data from SHR treated at the onset of failure
(SHR-FRx) and SHR-F are summarized in Table 2
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The mean age at study of SHR-F (19±2 months) was slightly but
significantly lower than the other two groups (P<.01)
because the SHR-F were studied when failure developed. In
echocardiographic studies of unanesthetized
rats, SHR-F demonstrated significantly enlarged
end-diastolic and end-systolic left
ventricular volumes relative to SHR-NF. The left
ventricular ejection fraction was 45±7% in the SHR-F and
73±2% in the untreated SHR-NF group (P<.01). Captopril
treatment (SHR-FRx), initiated at the time of
heart failure and continued for 2 to 4 months did not significantly
improve these indices despite a reduction in LV/BW ratio. The RV/BW
ratio was not significantly reduced with captopril in
SHR-FRx relative to SHR-F group.
Isometric Contraction Parameters and Shortening
Velocity
Papillary muscle data for the untreated SHR versus
captopril-treated SHR groups are presented in Table 3. Papillary muscle cross-sectional area was not
significantly different among SHR groups. Active isometric stress
(active, normalized for muscle cross-sectional
area) and maximum rate of stress development (+d/dt) were increased
with captopril treatment (SHRRx12,
SHRRx18, SHRRx21) compared
with untreated SHR (SHRNo Rx) (treatment effect
P<.001 by ANOVA; see Table 3 for subgroup comparisons).
There was no significant difference in active stress or maximum rate of
stress development in SHR treated with captopril for 12 months versus
untreated WKY (WKY versus SHRRx12; Table 3).
Captopril treatment with the onset of failure resulted in no
significant improvement in either of these contractile indices
(SHR-FRx compared with SHR-F; Table 3).
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In animals treated with captopril at 12, 18, and 21 months of age to 24
months (SHRRx12, SHRRx18,
SHRRx21 groups), electromechanical delay time
(EMD) was significantly decreased versus untreated SHR (SHRNo
Rx; Table 3). Time to peak stress (TPS) of SHR treated at
12 and 18 months was significantly abbreviated compared with untreated
SHR (P<.01). The relaxation time index
(RT1/2) was significantly abbreviated in
untreated SHR relative to SHR treated at 12, 18, and 21 months of age
(P<.01). There was no significant effect of treatment of
failing SHR (SHR-FRx) versus untreated SHR-F
(SHR-F) on any of the measured physiological
parameters (Table 3).
Quick-release stress-velocity relationships are presented in
Fig 2. At all loads examined, velocity of
shortening was less in papillary muscles from SHR-F than WKY and SHR-NF
(left panel), although to a lesser degree at higher loads. Captopril
treatment (SHRRx12,
SHRRx18, SHRRx21 groups;
right panel) before heart failure was associated with increased
V0.5 in all groups relative to untreated SHR
(SHRRx12, SHRRx18,
SHRRx21 groups combined versus all untreated SHR;
treatment effect P<.01, Table 3). However, captopril
treatment of the failing SHR did not significantly improve
V0.5 (SHR-FRx versus
untreated SHR-F (SHR-F; Table 3) or stress-velocity relationships (Fig 2).
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Myocardial Stiffness
Fig 3 demonstrates the effects
of captopril treatment on myocardial stiffness. Captopril treatment
before failure was associated with significantly reduced myocardial
stiffness, kcs, in SHR captopril-treated groups
(SHRRx12, SHRRx18,
SHRRx21 groups) versus untreated SHR
(SHRNo Rx) (Table 3). In contrast, in the SHR-F
group, treatment did not significantly reduce myocardial stiffness
(SHR-F versus SHR-FRx; Table 3).
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The relationship between myocardial stiffness and shortening velocity
is presented in Fig 4. There is
an overall negative correlation between shortening velocity and
stiffness of the left ventricular papillary muscle
(r=-.96).
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Histology
Representative histological
sections of left ventricular free wall from the WKY, SHR-NF
and SHR-F and 12-month captopril-treated SHR are presented in
Fig 5, and a quantitative
histological analysis of left
ventricular papillary muscles in Table 4. Fig 6 is
a cross-section of the left ventricular free wall from
normotensive WKY and failing SHR stained with picrosirius red, viewed
under polarized light. Myocardial fibrosis was increased and myocyte
area was decreased in failing SHR (SHR-F) relative to WKY
(P<.05). Fibrosis and myocyte area in captopril-treated SHR
(SHRRx12, SHRRx18) was
comparable to that in WKY (Table 4). Fibrosis was not significantly
reduced by captopril when treatment was begun at 21 months of age
(SHRRx21). Furthermore, fibrosis and reduced
myocyte area were not altered when captopril administration was
initiated at the time of failure (SHR-FRx versus
WKY; Table 4).
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Discussion |
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The present study demonstrates that captopril administered prior to the onset of heart failure prevents impairment of myocardial function in the SHR. Treatment decreases hypertrophy and prevents the development of structural changes (fibrosis and cell loss) and mechanical dysfunction associated with the development of heart failure. The degree to which captopril prevents contractile dysfunction appears to be related to the age at which captopril treatment was initiated and the duration of captopril administration (ie, a greater effect with earlier and more prolonged treatment). In contrast, captopril treatment after the onset of failure results in a reduction of left ventricular hypertrophy, but contractile dysfunction and abnormal structure are not reversed. It is likely that there are irreversible events with heart failure (eg, myocyte loss) and other changes that may be irreversible or only slowly reversible (eg, fibrosis). From these studies, one cannot exclude the possibility that improvement in myocardial function in these animals with heart failure might be demonstrable if captopril treatment were to be continued for a longer period of time. On the other hand, there may be little or no improvement in intrinsic myocardial function, and the functional improvement with treatment seen in animal models and in the clinical setting may reflect stabilization of disease progression, with improved cardiac function secondary to alterations in loading conditions. It should be noted that the observed reduction in systolic function in the SHR (as measured in vivo by ejection fraction, Table 2
)
may be due in part to differences in loading conditions (ie, different
levels of end-systolic stress). Previous studies in the SHR19 have demonstrated that in addition to left ventricular hypertrophy there is also an increase in interstitial collagen. Structural remodeling of the extracellular matrix has been implicated in alterations in myocardial stiffness,16,19 which may contribute to both systolic and diastolic dysfunction in hypertrophied hearts. In previous studies, we have observed a marked increase in myocardial fibrosis and stiffness in the male SHR during the transition to failure at 18 to 24 months of age,9 a time that corresponds to the first appearance of hemodynamic and functional impairment.3,6–12,14 Consistent with these findings, histological analysis of the myocardium in the present study demonstrates increased fibrosis and decreased fractional myocyte area in the SHR with heart failure.
In studies of the myocardial infarction model, hypertrophied
noninfarcted papillary muscles from rats with large infarctions were
found to have a marked increase in hydroxyproline content and increased
myocardial stiffness 6 weeks after coronary artery
ligation20; treatment with captopril for 3 weeks
starting 3 weeks after infarction did not prevent the increase in
myocardial hydroxyproline content and passive stiffness. Smits et
al,21 on the other hand, demonstrated that
captopril treatment early after myocardial infarction not only reduced
left ventricular hypertrophy but also
completely inhibited increased DNA synthesis and collagen deposition.
Thus the effects of early treatment of animals with myocardial
infarction may be analogous to those in the SHR treated before the
onset of contractile dysfunction, fibrosis, and heart failure. In the
present study the increased myocardial stiffness in the aging SHR
was associated with decreased shortening velocity (Fig 4). A similar
relationship between stiffness and shortening velocity with myocardial
hypertrophy has been noted by
others.22 Treatment with captopril ameliorated
the depression of shortening velocity seen in untreated animals. It is
not possible, however, from the present studies, to determine if
there is a direct relationship between stiffness and shortening or
whether both result from other structural and functional changes.
The protection afforded by captopril may be attributed in part to a
reduction in blood pressure and reduced left ventricular
loading caused by inhibition of the renin-angiotensin
system.13 In the present study, peak
systolic pressure was reduced on average from 180 to 145
mm Hg over a 3- to 12-month period of treatment. However, the
relation between blood pressure reduction and protection by
angiotensin converting enzyme inhibition is not entirely
straightforward. Laboratory data have demonstrated regression of left
ventricular hypertrophy by ACE
inhibitors at doses that do not lower blood
pressure23,24 as well as in the presence of a
fixed pressure overload as the result of aortic
constriction.25–27 In the male SHR-F, blood
pressure was reduced to 140 mm Hg in comparison to
nonfailing SHR, and captopril treatment of SHR-F did not further
decrease peak systolic pressure although
hypertrophy was reduced. Captopril has also been shown to
significantly reduce LV/BW ratio of normotensive female rats yet did
not lower the blood pressure.10 Thus although the
increase in myocardial hypertrophy and fibrosis seems to be
partially related to elevated blood pressure in male and female
animals, other factors may also be involved.
The transition from hypertrophy to failure has been
observed to be associated with elevations of transforming growth
factor-ß1, fibronectin, and pro-
1(I) and
pro-1(III) collagen mRNA.12 Transforming
growth factor (TGF-ß1), which increases before
increases in collagen mRNA in the heart, has been shown to induce
expression of mRNAs coding for extracellular matrix
proteins28 including collagen type I and
III29 and to inhibit collagenase
activity.30 Angiotensin II has been
shown to increase fibroblast proliferation31,32
and augments, up to threefold, the expression and secretion of latent
TGF-ß1 in neonatal cardiac
fibroblasts.33 While there may be differences
between neonatal and adult fibroblasts, angiotensin
II–induced activation of TGF-ß1 gene
expression may be an important mechanism by which collagen genes are
stimulated in fibroblasts, resulting in increased extracellular matrix
production.
Captopril has been shown to prevent the increase in DNA and collagen synthesis in noninfarcted cardiac muscle after coronary artery ligation.34 Enalapril treatment has also been shown to prevent the increase in the fraction of myocardium occupied by fibrosis and the decrease in myocellular cross-sectional area in male SHR when treatment was begun at 3 months of age and continued up to 14 months of age.35 It has been shown, however, that cardiac angiotensin II formation is not blocked by ACE inhibition (suggesting that other proteases are involved36). Thus the effect of captopril may result, at least in part, from effects other than ACE inhibition. In particular, it is possible that the captopril may act by inhibition of degradation of the vasodepressor peptide bradykinin.37 In a study of the noninfarcted region of the rat myocardial infarction model, however, Smits et al21 have shown that the angiotensin II receptor blocker losartan prevents fibrosis at sites remote from the infarction, suggesting that angiotensin II mediated events may be more important than the bradykinin pathway with respect to the development of myocardial fibrosis.
Gene expression of components of the renin-angiotensin system have been demonstrated in myocardium.38–40 Increased ACE activity in right and left myocardium after myocardial infarction has been observed while plasma renin and serum ACE remain at normal levels.41 The colocalization of ACE and angiotensin II receptors further supports the concept that angiotensin II is produced locally,42 particularly in the SHR, in which the tissue renin-angiotensin system has been reported to be abnormally activated.43 In chronic pressure overload, both ACE mRNA and ACE activity have been found to be increased in rat heart as was the fractional conversion of angiotensin I to II.44 In the aging SHR, left ventricular ACE mRNA expression is also upregulated in the SHR-NF and to a greater extent in the SHR-F (unpublished observations). Cardiac ACE45 and AT1 receptor binding density46 have been studied in the myocardial infarction model in the rat. AT1 receptor binding increased by day 3 after infarction and was followed by increased ACE binding at 1 week. ACE binding density subsequently increased progressively from 2 to 8 weeks in fibroblasts and fibroblast-like cells and was associated with collagen I gene expression and fibrillar collagen accumulation.46 These data demonstrate a sequence of local events in stressed myocardium resulting in a connective tissue response.
In the present study, treatment with captopril after the onset of heart failure did not reverse structural or functional abnormalities. Captopril administered to 12 animals with heart failure (6 of which survived until study) for 2 to 4 months was effective in reducing left ventricular hypertrophy but not fibrosis. In a study of SHR with established hypertension and cardiac and vascular hypertrophy, 16 weeks of high-dose ACE inhibitors reduced blood pressure and cardiac hypertrophy but did not reverse vascular fibrosis.47 Although it is possible that with more prolonged treatment, improvement in active and passive mechanical properties might be demonstrated in myocardium from male SHR-F, these findings appear to be generally consistent with the concept that captopril prevents fibroblast proliferation and collagen synthesis, but is ineffective in reversing these effects once fibrosis is well established.
In conclusion, the present study demonstrates that long-term captopril administration can prevent impaired contractile function and heart failure in the aging male SHR. A point of clinical relevance suggested by the data is that the earlier captopril is initiated, the greater the degree of prevention of pathological changes to the myocardium and myocardial dysfunction. Treatment initiated at the time of failure (SHR-FRx) reduces left ventricular hypertrophy but does not significantly ameliorate fibrosis, increased passive myocardial stiffness, or contractile dysfunction.
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Selected Abbreviations and Acronyms |
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Acknowledgments |
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This study was supported by Medical Research Funds from the Department of Veterans Affairs. This work was done during the tenure of a Clinician-Scientist Award from the American Heart Association (C.H.C.).
Received May 23, 1997; revision received August 4, 1997; accepted August 22, 1997.
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References |
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1. Trippodo NC, Frohlich ED. Similarities of genetic (spontaneous) hypertension: man and rat. Circ Res. 1981;48:309–319.
2. Frohlich ED, Trippodo NC. Response to `The rat with spontaneous genetic hypertension is a suitable model of human essential hypertension.' Circ Res. 1981;48:464. Letter.
3.
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