(Circulation. 1995;92:562-578.)
© 1995 American Heart Association, Inc.
Articles |
Angiotensin-Converting Enzyme Inhibition and the Progression of Congestive Cardiomyopathy
Effects on Left Ventricular and Myocyte Structure and Function
From the Division of Cardiothoracic Surgery, Medical University of South Carolina, Charleston, and Bristol Myers Squibb Research Institute, Princeton, NJ (H.H.H., J.R.P., W.H.K.).
Correspondence to Francis G. Spinale, MD, PhD, Division of Cardiothoracic Surgery, Room 418 CSB, 171 Ashley Ave, Medical University of South Carolina, Charleston, SC 29425.
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Abstract |
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 Top Abstract IntroductionMethods Results Discussion References |
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Background Clinical trials have demonstrated that angiotensin-converting enzyme inhibition (ACEI) improves survival in patients with long-term left ventricular (LV) dysfunction. However, it remained unclear from these clinical reports whether the beneficial effects of ACEI were due to direct improvements in LV myocardial structure and function. Accordingly, the overall objective of the present study was to examine the direct effects of ACEI on both LV and myocyte structure and function in the setting of cardiomyopathic disease.
Methods and Results LV and isolated myocyte function and structure were examined in control dogs (n=6), in dogs after the development of dilated cardiomyopathy caused by rapid ventricular pacing (RVP, 216 beats per minute, 4 weeks, n=6), and in dogs with RVP and concomitant ACEI (RVP/ACEI, fosinopril 30 mg/kg BID, n=6). LV ejection fraction fell with RVP compared with control values (35±3 versus 73±2%, P<.05) and was higher with RVP/ACEI compared with RVP values (41±4%, P=.048). LV end-diastolic volume increased with RVP compared with control values (78±7 versus 101±7 cm3, P<.05) and was lower with RVP/ACEI (82±3 cm3, P<.05). Isolated myocyte length increased with RVP (182±1 versus 149±1 µm), and the velocity of shortening decreased (36±1 versus 57±1 µm/s) compared with control values (P<.05). With RVP/ACEI, myocyte length was reduced (169±1 µm) and velocity of shortening was increased (45±1 µm/s) compared with RVP values (P<.05). Myocyte velocity of shortening after ß-adrenergic receptor stimulation with 25 nmol/L isoproterenol was reduced with RVP compared with control values (142±5 versus 193±8 µm/s, P<.05) and significantly improved with RVP/ACEI (166±6 µm/s, P<.05). In the RVP group, ß-adrenergic receptor density fell 26%, and cAMP production with ß-adrenergic receptor stimulation was reduced 48% from control values. RVP/ACEI resulted in a normalization of ß-adrenergic receptor density and cAMP production. LV myosin heavy-chain content when normalized to dry weight of myocardium was unchanged with RVP (149±11 mg per gram dry weight of myocardium [gdwt]) and RVP/ACEI (150±4 mg/gdwt) compared with control values (165±4 mg/gdwt). LV collagen content decreased with RVP compared with control values (7.6±0.4 versus 9.6±0.8 mg per gram wet weight of myocardium [gwwt], P<.05) but was increased with RVP/ACEI (14.4±1.3 mg/gwwt, P<.05).
Conclusions Concomitant ACEI with chronic tachycardia reduced LV chamber dilation and improved myocyte contractile function and ß-adrenergic responsiveness. Contributory cellular and extracellular mechanisms for the beneficial effects of ACEI in this model of dilated cardiomyopathy included a normalization of ß-adrenergic receptor function and enhanced myocardial collagen support. The results from this study provide evidence that ACEI during the development of cardiomyopathic disease provided beneficial effects on LV myocyte contractile processes and myocardial structure.
Key Words: cardiomyopathy • contractility • myosin • angiotensin
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Introduction |
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TopAbstractIntroductionMethods Results Discussion References |
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A major focus of cardiovascular research efforts has been to understand the basic mechanisms underlying the development and progression of congestive heart failure. An important etiology of congestive heart failure is dilated cardiomyopathy.1 In patients with progressive left ventricular (LV) dysfunction, significant activation of neurohormonal systems occurs.2 Clinical studies reported that angiotensin-converting enzyme (ACE) inhibition improved indexes of LV function and survival in patients with cardiomyopathic disease.3 4 5 However, it remained unclear from these clinical studies whether the mechanisms of action of ACE inhibitors were due to global hemodynamic and neurohormonal effects (systemic effects) or whether ACE inhibition directly improved myocardial structure and function (myocardial effects).6 Thus, while it has been demonstrated that survival of patients with symptomatic cardiomyopathic disease has improved with concomitant ACE inhibition, the mechanisms of action have not been clearly established. Furthermore, it is unknown whether ACE inhibition may alter the progression of the cardiomyopathic disease process. Accordingly, one objective of the present study was to examine the direct effects of ACE inhibition on both LV and myocyte structure and function in the setting of cardiomyopathic disease.
Past reports from this laboratory and others demonstrated that chronic tachycardia in animals causes a dilated cardiomyopathy with alterations in LV contractile function, myocardial structure, and sarcolemmal transduction systems.7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Specifically, this laboratory demonstrated that chronic pacing-induced tachycardia caused a reduction in myocyte contractile function and alterations in cytoarchitecture.19 20 21 In addition, past studies have reported that the development of tachycardia-induced cardiomyopathy is associated with a reduction in ß-adrenergic receptor density, diminished cAMP production, and blunted responsiveness to ß-adrenergic receptor stimulation.8 18 23 Thus, this model of dilated cardiomyopathy produces abnormalities in LV function and sarcolemmal transduction systems that are similar to those reported in patients with cardiomyopathic disease.2 6 26 Accordingly, this model of dilated cardiomyopathy was used to test the central hypothesis that concomitant ACE inhibition with chronic tachycardia will have direct effects on LV and myocyte structure and function.
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Methods |
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TopAbstractIntroductionMethods Results Discussion References |
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Experimental Model of Cardiomyopathy
Eighteen adult mongrel dogs of either sex (9 to 16 months of age, 15 to 25 kg; Hazelton) were used in this study. The animals were chronically instrumented to serially measure LV and arterial pressures and to obtain plasma samples. In addition, a pacemaker and stimulating electrode were implanted to produce rapid right ventricular pacing. The animals were anesthetized with thiopental (4 mg/kg, Pentothal, Abbot Labs), intubated, and ventilated with nitrous oxide and oxygen (1:3 ratio). A surgical plane of anesthesia was maintained using 1% to 3% isoflurane (Aurthan, Anaquest), and a left thoracotomy was performed. A shielded stimulating electrode was sutured onto the right ventricular outflow tract, connected to a programmable pacemaker modified for programming heart rates up to 300 beats per minute (bpm) (Spectrax 5985, Medtronic, Inc), and buried in a subcutaneous pocket. A previously calibrated microtipped transducer (model p5-X4, Konigsberg Instruments) was placed into the LV chamber through a small incision at the apex. The connection of the LV transducer was tunneled and externalized in the suprascapular region of each animal. The pericardium was left open, the incision was closed, and the pleural space was evacuated of air. Next, the right carotid artery was exposed, and a vascular access port (model GPV, 9F, Access Technologies) was placed in the artery, advanced to the aortic arch, and sutured in place for subsequent arterial blood pressure measurements and blood sampling. The animals were allowed a 14-day recovery period, at which time a chest roentgenogram was obtained, and proper operation of all implanted instrumentation was confirmed. All animals used in this study were treated and cared for in accordance with the NIH "Guide for the Care and Use of Laboratory Animals" (NIH publication 86-23, 1985).
Experimental Design
After the dogs recovered from the
surgical procedure, baseline
LV pressure and dimensions and arterial pressure were measured, and
plasma samples were obtained for each dog (described below). The
pacemakers were then activated for rapid ventricular pacing (216±2
bpm), and 1:1 capture was confirmed by ECG. The dogs were then randomly
assigned to one of three treatment protocols: ACE inhibition (dogs were
administered the ACE inhibitor fosinopril [30 mg/kg, PO-BID] during
the pacing period, n=6), rapid pacing only (dogs were given gelatin
capsules during the pacing period, n=6), and sham control (these dogs
were instrumented and cared for exactly as the other groups except for
activation of the pacemaker, n=6). Cardiac auscultation and an ECG were
performed frequently during the 28-day pacing protocol to ensure proper
operation of the pacemaker and the presence of 1:1 conduction. At
weekly intervals, the dogs were brought to the laboratory, and the
pacemakers were deactivated. After a 30-minute stabilization period, LV
pressures were recorded, and echocardiographic measurements were
obtained as described in the following section. To determine changes in
neurohormonal status with the progression of tachycardia-induced
cardiomyopathy, plasma samples were obtained
immediately after the LV function measurements were made. After the LV
function studies and plasma collection, the pacemaker was reactivated
(except in the sham controls). At the end of the 28-day pacing
protocol, the dogs were returned to the laboratory for terminal study
as described in the following section.
LV Function Measurements
Indexes of LV systolic and diastolic
function were obtained at
baseline and at weekly intervals during the 28-day pacing period with
simultaneously recorded pressure and echocardiographic measurements
previously described by this laboratory.19 25 All
measurements were performed in a darkened room with the animal resting
quietly in a sling. The arterial access port was punctured with a
22-gauge Huber point needle (Access Technologies) connected to a
fluid-filled catheter. Pressures from the fluid-filled aortic catheter
were obtained with an externally calibrated transducer (Statham P23ID,
Gould). The ECG and pressure waveforms were recorded with a
multichannel recorder (TA4000, Gould) and digitized on computer for
subsequent analysis at a sampling frequency of 250 Hz (PO-NE-MAH).
Two-dimensional (2D) and M-mode echocardiographic studies (ATL
Ultramark 7, 3.5-mHz transducer) were used to image the LV from a right
parasternal approach. LV volumes and ejection fractions were computed
from the 2D and M-mode echocardiographic recordings.27
Peak positive and negative dP/dt and peak systolic wall stress were
computed with methods described previously.19 25
Neurohormonal Measurements
To examine the relation between
changes in neurohormonal status
that accompany changes in LV function with pacing-induced
cardiomyopathy, blood samples were drawn at the
conclusion of each LV function study. With the animal resting quietly,
35 cm3 of blood was drawn from the arterial access port
into tubes containing EDTA (1.5 mg/mL), sodium azide (0.2 mg/mL), and
aprotinin (1.15 TIU/mL). The blood samples were immediately centrifuged
(2000g, 10 minutes, 4°C), and the plasma was decanted into
separate tubes, frozen in a dry ice–methanol bath, and stored at
-80°C until the time of assay. From these plasma samples,
norepinephrine concentration, atrial natriuretic peptide levels, cGMP
content, and plasma renin activity were determined. Plasma
norepinephrine was measured with high-performance liquid chromatography
and normalized to picograms per milliliter of plasma.28
For the atrial natriuretic peptide and cGMP assays, the plasma was
first eluted over a cation exchange column (C-18 Sep-Pak, Waters
Associates). Standardized radioimmunoassay procedures were performed to
determine atrial natriuretic peptide concentrations, cGMP levels, and
plasma renin activity (Peninsula Laboratories). All assays were shown
to have a linear response by use of known standards with <10%
coefficient of variation. All plasma assays were performed in
duplicate.
Terminal Study: Myocardial Sampling and Myocyte Isolation
Four weeks after institution of the protocols described above,
the dogs were brought to the laboratory, and a final series of LV
function measurements and plasma samples was obtained. The animals were
then anesthetized as described, sternotomies were performed, and the
hearts were quickly extirpated and placed in a phosphate-buffered ice
slush. The great vessels, atria, and right ventricle were carefully
trimmed away, and the LV was weighed. The region of the LV free wall
incorporating the circumflex artery (5x5 cm) was excised and prepared
for myocyte isolation. The region of the left ventricular free wall
comprising the left anterior descending artery (3x5 cm) was cannulated
and prepared for perfusion fixation. The posterior region of the LV
free wall (3x3 cm) was snap-frozen in liquid nitrogen for subsequent
biochemical analysis.
Myocytes were isolated from the LV free wall with methods described by this laboratory previously.19 20 21 Briefly, the left circumflex coronary artery was perfused with a collagenase solution (0.5 mg/mL, Worthington type II; 146 U/mg) for 35 minutes. The tissue was then minced into 2-mm sections and gently agitated. After 15 minutes, the supernatant was removed and filtered, and the cells were allowed to settle. The myocyte pellet was then resuspended in Dulbecco's modified Eagle's medium:nutrient mixture F-12 (GIBCO Laboratories). The number of viable myocytes was counted at x100 magnification with a hemocytometer (Reichert-Jung, Cambridge Instruments Inc) and resuspended to a final concentration of 5x104 cells/mL. Viable myocytes were defined as those cells maintaining a rod shape that were quiescent in culture. With this myocyte isolation method, a high yield (70±8%) of viable myocytes was routinely obtained for the myocyte contractile function measurements as described in a following section.
Myocardial Structural Analysis
The LV section for microscopic
analysis was perfused with a
buffered sodium cacodylate solution containing 2%
paraformaldehyde, 2% glutaraldehyde solution (pH 7.4, 325
milliosmoles per liter) for 20 minutes with a perfusion pressure of 100
mm Hg.19 After perfusion, a 2x2 cm region was finely
minced, placed in additional fixative for 3 hours, and then prepared
for electron microscopy.19 Six tissue blocks in the
circumferential orientation from the LV of each dog were used to obtain
thin sections for electron microscopy. Three grids, each containing
three thin sections, were prepared from each block. Thin sections were
stained with uranyl acetate and lead citrate and examined with a JOEL
100S electron microscope. The central portion of each section was
photographed at a calibrated magnification of x10 000. These electron
micrographs were then coded, and this code was not broken until
completion of the study. From the circumferentially oriented
micrographs, the percent volume of myofibrils within myocytes was
analyzed morphometrically by use of a stereology sampling grid
consisting of 140 sampling points. LV myocardial samples were also
examined by scanning electron microscopy. Perfusion-fixed LV myocardial
samples were flash-frozen in liquid nitrogen and
freeze-fractured.22 The freeze-fractured samples
(0.25x0.25 cm) were then dehydrated, and the samples were
critical-point dried (Ladd Research Inc). The samples were mounted on
10x10-mm stubs with conductive adhesive tape (Scotch commercial tape,
3M Inc) and coated with gold sputter (Hummer II, Technics). The
sections were examined in a JOEL JSM-25S SEM at an accelerating voltage
of 15 kV.
Light microscopic examination was performed on the perfusion-fixed LV myocardium to determine myocyte cross-sectional area, percent area occupied by extracellular space, and connectivity of the extracellular network.22 For examination of the extracellular matrix, LV sections were stained with a silver impregnation method.29 The silver-stained LV sections were then digitized at a final magnification of x320 and analyzed with an image analysis system (Zeiss/Kontron, IBAS).22 The percent area of extracellular staining was computed from 15 random fields within the midmyocardium to exclude large epicardial arteries and veins and any cutting or compression artifact. The integrity or continuity of the collagen network was examined in these same fields by use of a grid pattern of 100-µm horizontal and vertical lines. The percent of collagen profiles intersecting this grid was then computed.22 For determination of myocyte cross-sectional area, LV samples previously embedded in resin were sectioned and stained with toluidine blue. These sections were imaged with an epifluorescent illuminator using a rhodamine filter at a magnification of x1000. Myocytes in a cross-sectional orientation were digitized and analyzed with the previously described image analysis system. Only those myocytes in which the nucleus was centrally located within the cell were digitized and analyzed to ensure that the short axis of the cardiocyte was perpendicular to the microscope objective. With this approach, myocyte cross-sectional area could be determined in situ.
Myocardial Biochemical Analysis
To examine the potential
changes in myocardial composition that
may have occurred with concomitant ACE inhibition with chronic
tachycardia, total myocardial protein, water content, myosin
heavy-chain (MHC) content, and hydroxyproline content were determined.
For total myocardial protein and MHC content, myocardial samples (1 g)
were homogenized in a 62.5 mmol/L Tris-HCl buffer (1:10 wt/vol) with a
tissue grinder (Tissumizer, Tekmar Co). The homogenate was placed in a
vortex and diluted into serial dilutions ranging from 1:100 to 1:1000,
and the protein content was determined with a standardized colorimetric
assay (Bio-Rad Protein Assay, Bio-Rad Laboratories). The MHC content
was determined in these myocardial homogenates with
SDS-PAGE.30 An aliquot of the myocardial homogenate (5
µg total protein) was suspended in a buffer containing 125 mmol/L
Tris-HCl, 4% SDS, 20% glycerol, 10% ß-mercaptoethanol, and 0.1%
bromphenol blue. The samples were boiled for 5 minutes and then
immediately loaded onto a vertical maxigel apparatus (Protean II,
Bio-Rad Laboratories). The samples were initially separated with a 4%
stacking gel and then were resolved with a 10% to 13% gradient with a
constant current and voltage set at 70 mV. The gels were run for 17
hours with a constant temperature of 12°C to 15°C maintained. The
gels were stained for 3 hours in a 0.3% Coomassie blue R-250 solution
and destained for an additional 3 hours. The stained gels were then
scanned and digitized with the previously described image analysis
system and subjected to 2D densitometric analysis. After digital
subtraction of background density, the integrated optical densities
(IODs) of the bands corresponding to MHC were computed. The IOD values
obtained from the 2D quantification were then transformed to microgram
values with purified porcine cardiac MHC standards (1 to 6 µg, Sigma
Chemical Co) that had been simultaneously electrophoresed on each gel.
All experiments were performed in duplicate.
To determine whether myocardial collagen content changed with chronic tachycardia or concomitant ACE inhibition, a biochemical assay for hydroxyproline, an amino acid specific for collagen, was performed on LV samples with methods described previously.31 Briefly, the LV sections were weighed, lyophilized, and reweighed. The sections were then hydrolyzed and measured spectrophotometrically (550 nm) after reaction with Ehrlich's reagent.31 A conversion factor of 7.46 was used to convert the final hydroxyproline values to total collagen values. All measurements were performed in duplicate.
In light of the fact that abnormalities in ß-adrenergic receptor density and transduction have been reported with the development of tachycardia-induced cardiomyopathy,8 18 23 the present study examined ß-receptor density and cAMP production with the development of tachycardia-induced cardiomyopathy and concomitant ACE inhibition during the development of tachycardia-induced cardiomyopathy. ß-Adrenergic binding and function experiments were performed with well-described methods.8 18 23 26 Briefly, myocyte membranes were prepared with ultracentrifugation techniques, and ß-adrenergic receptor antagonist binding studies were performed with 10 concentrations (0.02 to 1 nmol/L) of 25 µL [125I]cyanopindolol (74 Bq/mmol, Amersham Corp). A standard Scatchard linear regression analysis was performed on the amount of bound and free ligand, with r2>.90 as the criterion for acceptability of the data. With this analysis, the maximal number of binding sites, Bmax, expressed as femtomoles per milligram of protein, and the equilibrium dissociation constant, Kd (nanomoles per liter), were computed.23 26 As an index of ß-adrenergic receptor system function, adenylate cyclase activity was determined by timed cAMP production in aliquots of 30 to 50 µg/100 µL of membrane preparation with well-described methods.8 23 In addition to determination of basal adenylate cyclase activity, cAMP production was measured in the presence of 10-3 mol/L (-) isoproterenol and 100 µmol/L forskolin. These concentrations of isoproterenol and forskolin were previously shown to cause maximal adenylate cyclase activity in sarcolemmal preparations.23 Reactions were terminated by placement of the tubes in boiling water and centrifugation at 6500g for 5 minutes. The cAMP content of the supernatant was determined with a competitive radiolabeled assay (Cyclic AMP 125I RIA, Advanced Magnetics Inc). Results were expressed as picomoles of cAMP produced per milligram of sarcolemmal protein per minute. All measurements were performed in duplicate.
Myocyte Contractile Function Measurements
Isolated myocytes
were placed in a thermostatically controlled
chamber (37°C) fitted with a coverslip on the bottom for imaging on
an inverted microscope (Axiovert IM35, Zeiss Inc). The 2.5-mL chamber
contained two stimulating platinum electrodes. The myocytes were imaged
with an X20 long-working-distance Hoffmann Modulation Contrast
objective (Modulation Optics Inc). Myocyte contractions were elicited
by field stimulating the tissue chamber at 1 Hz (S11, Grass
Instruments) with 5-millisecond current pulses and voltages 10% above
the contraction threshold. The polarity of the stimulating electrodes
was alternated at every pulse to prevent buildup of electrochemical
by-products. Myocyte contractions were imaged using a charge-coupled
device with a noninterlaced scan rate of 240 Hz (GPCD60, Panasonic).
Myocyte motion signals were captured with the cell parallel to the
video raster lines, and this video signal was input through an edge
detector system (Crescent Electronics). The changes in light intensity
at the myocyte edges were used to track myocyte motion.21
The distances between the left and right myocyte edges were converted
into a voltage signal, digitized, and input into a computer (80286,
ZBV2526, Zenith Data Systems) for subsequent analysis. Stimulated
myocytes were allowed a 5-minute stabilization period, after which
contraction data for each myocyte were recorded from a minimum of 20
consecutive contractions. Parameters computed from the digitized
contraction profiles included percentage shortening (percent), peak
velocity of shortening (micrometers per second), peak velocity of
relengthening (micrometers per second), total contraction duration
(milliseconds), and time to peak contraction (milliseconds). After
collection of baseline indexes of myocyte function, measurements were
performed in the presence of 25 nmol/L (-) isoproterenol. This
concentration of isoproterenol was previously shown to produce a
maximal contractile response in isolated myocyte
preparations.23 32
Data Analysis
Indexes of LV function, myocardial biochemical
composition, and
isolated myocyte function were compared among the three groups by
ANOVA. The morphological data were analyzed by use of the average
measurements obtained for each animal, and the groups were compared by
ANOVA. If the ANOVA revealed significant differences, pairwise tests of
individual group means were compared by use of Bonferroni
probabilities.33 For comparisons of neurohormonal profiles
between groups, the Mann-Whitney rank-sum test was used.33
All statistical procedures were performed using the BMDP
statistical software package (BMDP Statistical Software Inc). Results
are presented as mean±SEM. Values of P<.05 were
considered to be statistically significant.
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Results |
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TopAbstractIntroductionMethods Results Discussion References |
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In the present study, six dogs were successfully studied in each of the following treatment groups: (1) dogs undergoing 28 days of rapid ventricular pacing and concomitant ACE inhibition, (2) dogs undergoing 28 days of rapid pacing with no drugs (gelatin capsule only), and (3) sham-operated controls (no pacing or drug administration). Myocytes were successfully harvested from all animals at the end of the study with no differences in the yield of viable myocytes between groups (P>.75).
LV Function With Chronic Rapid Pacing: Effects of ACE
Inhibition
Fig 1
shows the weekly changes in LV
end-diastolic pressure, volume, and ejection fraction with
long-term rapid pacing. LV end-diastolic pressure and
volume significantly increased after 1 week of pacing compared with
sham controls (P<.05) and increased in a time-dependent
fashion with each week of rapid pacing. In the rapid pacing group, LV
ejection fraction decreased significantly from baseline values and from
sham control values after 1 week of pacing (P<.05) and
continued to decline with each week of rapid ventricular pacing. Thus,
consistent with past reports from this laboratory and others, chronic
pacing-induced tachycardia caused LV dilation and pump
dysfunction.7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
In dogs receiving concomitant ACE
inhibition during chronic tachycardia, there was no significant change
in LV end-diastolic pressure or volume after 1 week of
rapid pacing compared with baseline values or sham controls (Fig
1).
There was no difference in LV end-diastolic pressures
between the group receiving rapid pacing only and the group undergoing
concomitant ACE inhibition. After 2 weeks of rapid pacing, LV
end-diastolic volume increased from baseline values in the
group undergoing concomitant ACE inhibition but remained significantly
lower than values in the group undergoing rapid pacing alone for the
entire pacing protocol (P<.05). After 1 week of pacing, LV
ejection fraction was significantly lower in the group undergoing ACE
inhibition and rapid pacing compared with baseline values or sham
controls (P<.05) and declined with each week of pacing.
After 4 weeks of rapid pacing, LV ejection fraction was higher in the
group undergoing ACE inhibition compared with the value in the group
undergoing rapid pacing alone (P=.048). Thus, concomitant
ACE inhibition significantly reduced the progressive LV dilation
associated with chronic pacing tachycardia and improved LV pump
function.
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), dogs with
long-term rapid pacing (
), and dogs with long-term pacing and
concomitant angiotensin-converting enzyme (ACE) inhibition (
).
Top, Long-term rapid pacing resulted in a significant increase in LVEDP
by week 1 compared with controls (P<.05) and increased with
each week of rapid pacing. In contrast, concomitant ACE inhibition
resulted in lower LVEDP during weeks 1 through 3 compared with
untreated dogs undergoing rapid pacing (P<.05). However, by
week 4, LVEDP was equivalent in both rapid pacing groups. Middle, LV
end-diastolic volume significantly increased with each week
of rapid pacing and appeared to plateau by week 4. LV
end-diastolic volume was significantly lower in the rapid
pacing and concomitant ACE inhibition group at all time points compared
with the group receiving only rapid pacing (P<.05). Bottom,
LV ejection fraction fell in a time-dependent manner with each week of
pacing regardless of drug treatment (P<.05). However, at
weeks 3 and 4, a higher LV ejection fraction was observed in the rapid
pacing and ACE inhibition group compared with the group receiving rapid
pacing alone (P<.05). See Table 1
To more carefully examine the relation between changes in LV
loading
conditions and LV geometry with long-term rapid pacing and with
concomitant ACE inhibition, peak LV wall stress was computed at each
week of the pacing protocol (Fig 2). LV peak wall stress
significantly increased with each week of rapid pacing compared with
the sham control group (P>.05). In the group undergoing ACE
inhibitor and rapid pacing, LV wall stress was not significantly
different from baseline or sham control values after 1 week of pacing
(P=.417). In this group,, LV peak wall stress remained
significantly lower compared with the value in the group undergoing
rapid pacing only for the entire pacing protocol (P<.05).
Thus, the significant reduction in LV chamber size with concomitant ACE
inhibition was associated with significantly reduced LV peak wall
stress.
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Table 1 summarizes the LV function and
hemodynamics
obtained in sham controls and after 28 days of rapid pacing and 28 days
of rapid pacing with concomitant ACE inhibition. Resting heart rate was
increased and mean arterial pressure was reduced in the group receiving
rapid pacing only compared with sham controls. Similarly, concomitant
ACE inhibition caused a significant reduction in mean arterial pressure
compared with sham controls but was not significantly different from
the value in the group undergoing rapid pacing only
(P=.261). After 4 weeks of rapid pacing, LV peak systolic
pressure and peak +dP/dt were significantly lower with rapid pacing
compared with the control group, regardless of ACE inhibition. As
outlined in Fig 1
and presented in Table 1, the
significant LV
dilation that occurred with long-term rapid pacing was ameliorated with
concomitant ACE inhibition.
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Neurohormonal Changes With Chronic Rapid Pacing and Concomitant ACE
Inhibition
Fig 3 shows a summary of weekly changes in
plasma
norepinephrine, atrial natriuretic factor (ANF), and cGMP. Plasma
norepinephrine significantly increased from baseline values after 1
week in the groups undergoing rapid pacing only and undergoing rapid
pacing with concomitant ACE inhibition. However, these 1-week plasma
norepinephrine values were lower in the group undergoing ACE inhibitor
compared with the values in the group undergoing rapid pacing only
(P<.05). With longer durations of pacing, plasma
norepinephrine values appeared to plateau but remained significantly
elevated from baseline values. In the group receiving ACE inhibition
with longer durations of pacing, plasma norepinephrine remained higher
than baseline values but was consistently lower than the values in the
group undergoing rapid pacing only (P<.05). After 1 week of
rapid pacing, plasma ANF and cGMP concentrations significantly
increased from baseline values and remained elevated throughout the
pacing protocol. With rapid pacing and concomitant ACE inhibition,
plasma ANF was increased from control values after 1 and 2 weeks of
pacing but remained significantly lower than values in the group
undergoing rapid pacing only (P<.05). With rapid pacing and
concomitant ACE inhibition, cGMP was not increased from baseline
values. In the group undergoing rapid pacing only, plasma renin
activity remained unchanged from sham control values after 28 days of
rapid pacing (3.2±0.9 versus 3.1±0.9
ng · mL-1 · h-1,
respectively). With rapid pacing and concomitant ACE inhibition, plasma
renin activity was significantly higher than in the sham control group
and the group undergoing rapid pacing only (6.9±1.6
ng · mL-1 · h-1,
P<.05). The significant increase in plasma renin activity
with chronic ACE inhibition is an expected pharmacological effect of
interruption of the renin-angiotensin system.12 34
Thus,
concomitant ACE inhibition caused a significant reduction in the degree
of neurohormonal activation associated with chronic tachycardia.
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LV Structure With Rapid Pacing: Effects of ACE Inhibition
Table 2 summarizes the LV mass obtained at autopsy
and myocardial composition for the three groups of dogs. There was no
significant change in LV mass in the group undergoing long-term rapid
pacing compared with the sham control group. Absolute LV mass was lower
in the group with rapid pacing and concomitant ACE inhibition compared
with that of the sham control group (P=.025). Body weight
was lower in the dogs with chronic ACE inhibition, but this difference
did not reach statistical significance (P>.10). The ratios
of LV mass to body weight obtained in the present study for the
control group and the groups undergoing rapid pacing are all within
normal limits for dogs of this size and were not significantly
different between groups.35 When LV mass was normalized to
tibial length, the ratio of LV mass to tibial length was lower in the
group undergoing rapid pacing and ACE inhibitor compared with the sham
control group (P=.041). Thus, as previously reported by this
laboratory and
others,7 9 18 19 20 21 22 23 25
the development of
tachycardia-induced cardiomyopathy was not
associated with LV hypertrophy. Concomitant ACE inhibition caused a
small but statistically significant reduction in LV mass.
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LV myocardial
structure and composition were examined by morphometric
analysis of perfusion-fixed myocardial sections. Myocyte
cross-sectional area was computed from a minimum of 350 myocyte
profiles from each group. Table 2 gives the summary values for
this
analysis; Fig 4 shows the frequency distribution for
this parameter. Myocyte cross-sectional area decreased from sham
control values in the group undergoing rapid pacing. In the group
undergoing rapid pacing and concomitant ACE inhibitor, myocyte
cross-sectional area was significantly reduced from that of the sham
control group and the group undergoing rapid pacing only.
Ultrastructural quantification was performed on perfusion-fixed
myocardial sections taken from each group with coded electron
micrographs. These electron micrographs were examined in a blinded
fashion, and the code was not broken until the end of the study. Table
2 summarizes this analysis, and Fig 5 shows
representative electron micrographs of myocardial sections
taken in cross-sectional orientation and longitudinal orientations.
Absolute myofibril and mitochondrial volumes did not
significantly change from control values in either of the groups
undergoing rapid pacing. However, changes in mitochondrial structure
and cristae formation were apparent in the groups undergoing rapid
pacing compared with the control group (Fig 5). In longitudinal
orientation, sarcomeric units appeared out of register in adjoining
myofibrils in the group receiving rapid pacing compared with the
control group. This myofibril misalignment was not as apparent in
longitudinal sections taken from the group receiving ACE inhibition and
rapid pacing.
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To more carefully examine potential changes in myocardial
composition
with rapid pacing and with concomitant ACE inhibition, biochemical
quantification of total myocardial protein and MHC was performed. Table
2 summarizes these biochemical studies. When expressed in terms
of wet
weight of myocardium, total myocardial protein was significantly
reduced in the group undergoing rapid pacing and the group receiving
concomitant ACE inhibition. However, when this value was expressed in
terms of dry weight, there were no significant differences in total
myocardial protein. Fig 6 shows a representative
SDS-PAGE gel for MHC. A strong positive band for MHC was obtained from
all the myocardial samples used in this study, and Table 2
gives the
results from 2D quantification of these gels. When expressed in terms
of wet weight of myocardium, MHC content was reduced in the group
receiving ACE inhibition and rapid pacing compared with the control
group. However, when MHC was expressed in terms of dry weight, there
was no significant change in this value in either of the groups
undergoing rapid pacing compared with the control group. Thus, while
MHC was consistently reduced with rapid pacing, this difference did not
achieve statistical significance.
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To identify a potential structural
basis for the changes in LV geometry
with rapid pacing and with concomitant ACE inhibition, extracellular
matrix structure and composition were examined (Table 2).
Morphometric
analysis of silver-stained LV myocardial sections revealed a
significant reduction in positive staining in the group receiving rapid
pacing compared with the control group. Furthermore, the confluent
nature of the extracellular space was reduced in the group receiving
rapid pacing compared with the controls. Fig 7 shows
representative scanning electron micrographs of control
myocardium and myocardial samples taken after 28 days of rapid pacing.
In control myocardium, a fine weave of collagen was observed within the
interstitial space. With chronic rapid pacing, this collagen weave
appeared significantly disrupted, and the fine fibrillar nature of the
collagen weave could not be readily appreciated. In the group
undergoing ACE inhibition and rapid pacing, the percent area of
positive silver staining significantly increased from the group
receiving rapid pacing only and the control group. This increased
silver staining with concomitant ACE inhibition was accompanied by
significantly increased confluence of the extracellular staining
pattern. With rapid pacing and concomitant ACE inhibition, scanning
electron micrographs revealed a well-developed collagen weave
surrounding individual myocytes that was distributed in a meshlike
pattern throughout the interstitial space. To more carefully quantify
changes in LV myocardial collagen, biochemical analysis of
hydroxyproline was performed and converted to collagen values (Table
2). A small but significant reduction in total myocardial
collagen was
observed in the group receiving rapid pacing only compared with the
control group (P=.035). In the group receiving concomitant
ACE inhibition, myocardial collagen content was significantly increased
from the values in the group undergoing rapid pacing only and from
control values.
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Sarcolemmal Receptor Systems With Rapid Pacing: Effects of ACE
Inhibition
Past reports demonstrated that chronic tachycardia caused
alterations in the ß-adrenergic receptor
systems.8 18 23
Accordingly, ß-adrenergic receptor density and affinity and cAMP
production were examined to determine whether concomitant ACE
inhibition during long-term rapid pacing influenced this sarcolemmal
transduction system. Table 3 gives the results from this
portion of the study. Consistent with past
reports,8 18 23
ß-adrenergic receptor density fell significantly with chronic rapid
pacing with no change in affinity. In marked contrast, there was no
change in ß-adrenergic receptor density or affinity with concomitant
ACE inhibition and long-term rapid pacing. In control sarcolemmal
preparations, cAMP production significantly increased from basal levels
after ß-adrenergic receptor stimulation with isoproterenol and after
direct adenylate cyclase activation with forskolin. Long-term rapid
pacing caused a reduction in basal cAMP levels compared with controls.
Furthermore, cAMP production with ß-adrenergic receptor stimulation
or by direct adenylate cyclase activation was reduced in the group
receiving rapid pacing compared with the control group. Concomitant ACE
inhibition and rapid pacing resulted in a normalization of cAMP
production both after ß-adrenergic receptor stimulation and by direct
activation of adenylate cyclase. Thus, concomitant ACE inhibition
with long-term rapid pacing maintained ß-receptor density and
normalized cAMP production. The direct effects of these changes in
ß-receptor density and transduction with chronic rapid pacing and
with concomitant ACE inhibition on myocyte ß-adrenergic
responsiveness were examined next.
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Isolated Myocyte Contractile Function
Table 4
summarizes the isolated myocyte resting
length and contractile function at baseline and after ß-adrenergic
stimulation with isoproterenol. Fig 8 shows
representative photomicrographs of isolated myocytes taken from
sham controls, from dogs undergoing long-term rapid pacing, and from
dogs receiving long-term rapid pacing and concomitant ACE inhibition.
Isolated myocyte resting length significantly increased from control
values in the group receiving rapid pacing and the group receiving
concomitant ACE inhibition. Isolated myocyte length was shorter in the
group receiving concomitant ACE inhibition and rapid pacing compared
with values in the group undergoing rapid pacing alone. Myocyte percent
and velocity of shortening fell significantly from control values in
the group undergoing rapid pacing and the group receiving concomitant
ACE inhibition. However, in the group receiving rapid pacing and
concomitant ACE inhibition, myocyte percent and velocity of shortening
were higher than in the group receiving rapid pacing only. The velocity
of myocyte lengthening was lower in both groups with long-term rapid
pacing. However, in the group receiving concomitant ACE inhibition, the
velocity of myocyte lengthening was significantly higher than in the
group receiving rapid pacing only. The time to peak myocyte contraction
and total duration of contraction were significantly prolonged in the
group undergoing rapid pacing without drug treatment. In the group
receiving concomitant ACE inhibition, time to peak myocyte contraction
returned to control values. Thus, consistent with past reports,
long-term pacing-induced tachycardia caused a significant reduction in
isolated myocyte contractile function.21 23 Results
from
the present study demonstrated that chronic pacing-induced
tachycardia with concomitant ACE inhibition improved indexes of myocyte
contractile function.
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In light of the fact that rapid pacing with concomitant ACE inhibition improved ß-adrenergic receptor density and cAMP production compared with rapid pacing alone, myocyte contractile function was examined in the presence of the ß-adrenergic agonist isoproterenol. ß-Adrenergic receptor stimulation caused a consistent and significant increase in contractile function from baseline values in the control myocytes, in myocytes after 28 days of rapid pacing, and in myocytes with concomitant ACE inhibition. Myocyte contractile function in the presence of isoproterenol was significantly lower than control values in the group undergoing rapid pacing only and the group receiving ACE inhibition. However, ß-adrenergic responsiveness was significantly greater in the group receiving concomitant ACE inhibition compared with the values from the group undergoing rapid pacing only. Thus, the normalization of ß-adrenergic receptor density and transduction with long-term rapid pacing and concomitant ACE inhibition caused a direct improvement in isolated myocyte ß-adrenergic responsiveness.
With
long-term rapid pacing, myocyte lengthening velocity significantly
fell from control values and increased with concomitant ACE inhibition.
Whether these changes in myocyte lengthening rate indicated a change in
intrinsic myocyte relaxation properties was not clear because
lengthening rate can be changed by an alteration in shortening extent
alone.36 To differentiate shortening-dependent changes in
lengthening rates from changes in lengthening rates caused by intrinsic
alterations in relaxation properties, an additional set of experiments
was performed. In these experiments, the relation between the extent of
myocyte shortening and the velocity of relengthening was developed as
previously described.36 Isolated myocytes were stimulated
after a period of quiescence, and the beat-to-beat changes in
shortening extent and lengthening velocity for the first 20 beats were
computed.36 Sequential contraction profiles were obtained
at baseline and in the presence of 25 nmol/L isoproterenol. Table
5 summarizes the results of the regression analysis
for this relation. Fig 9 shows the regression lines
obtained for the relation between the extent of shortening and the
velocity of lengthening. The slope of this relation fell significantly
from control values in both groups receiving rapid pacing and remained
depressed with ß-adrenergic stimulation. In the group undergoing
rapid pacing and ACE inhibition, the slope of the relation between the
extent of shortening and the velocity of lengthening was significantly
higher than that for the group undergoing rapid pacing alone. Thus,
results from this analysis suggest that one mechanism for the
improved myocyte contractile function observed with ACE inhibition in
this model of chronic tachycardia is enhanced myocyte active relaxation
properties.
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Discussion |
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TopAbstractIntroductionMethods Results Discussion References |
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Cardiomyopathic disease is accompanied by LV dilation and dysfunction with activation of neurohormonal systems.1 2 5 6 26 Recent clinical reports demonstrated improved survival in patients with LV dysfunction treated with ACE inhibitors.3 4 However, the direct effects of ACE inhibition on LV myocardial structure and myocyte contractile processes during the progression of cardiomyopathic disease remain unclear. To address this issue, the present study examined the direct effects of concomitant ACE inhibition during the development of tachycardia-induced cardiomyopathy on LV function and structure and myocyte contractile function and ß-adrenergic responsiveness. The significant and unique findings of this study were fourfold. First, concomitant ACE inhibition reduced the LV dilation that accompanies the development of tachycardia-induced cardiomyopathy. Second, the improved LV geometry with ACE inhibition was associated with a preservation of the LV myocardial collagen matrix. Third, ACE inhibition with chronic tachycardia improved myocyte contractile function and ß-adrenergic responsiveness compared with the group undergoing rapid pacing only. Fourth, the fundamental mechanism for the improved myocyte ß-adrenergic responsiveness with concomitant ACE inhibition during chronic tachycardia was a normalization of ß-adrenergic receptor density and transduction. Thus, results from the present study demonstrated for the first time that ACE inhibition during the development of a cardiomyopathic disease process has direct and beneficial effects on LV myocardial structure and myocyte contractile processes.
It is well established that in animals, 3 to 5 weeks of chronic tachycardia causes a dilated cardiomyopathy.7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Specifically, serial measurements of LV function with chronic tachycardia revealed a progressive increase in LV diastolic volume and an incremental fall in LV ejection fraction with chronic pacing tachycardia.7 11 18 24 In the present study, LV function was serially measured with the progression of tachycardia-induced cardiomyopathy and with concomitant ACE inhibition. Concomitant ACE inhibition with long-term rapid pacing resulted in a significant reduction in LV end-diastolic volume and a small improvement in LV ejection fraction compared with values in dogs undergoing long-term pacing only. The reduced LV end-diastolic volume and increased LV ejection fraction observed after 4 weeks of rapid pacing and concomitant ACE inhibition are probably due to both cellular and extracellular factors. First, long-term ACE inhibition with chronic tachycardia resulted in improved myocyte contractile function. Thus, a contributory mechanism for the increased LV pump function with ACE inhibition and chronic tachycardia is improved myocyte contractile function. Second, ACE inhibition with chronic rapid pacing increased myocardial collagen content. It has been postulated that the fibrillar collagen network of the heart ensures structural integrity of adjoining myocytes, provides the means by which myocyte shortening is translated into overall ventricular pump function, and contributes significantly to the maintenance of LV geometry.37 38 39 In the present study, abnormalities in LV myocardial collagen structure and content were observed with the development of tachycardia-induced cardiomyopathy, and these findings are similar to past reports.22 40 41 Thus, the reduced LV dilation observed with concomitant ACE inhibition during rapid pacing may have been due, at least in part, to increased myocardial collagen content and improved extracellular support. While the present study demonstrated that the reduced LV dilation with rapid pacing and concomitant ACE inhibition was associated with a preservation of collagen content and structure, the effects of these changes on myocardial stiffness properties were not examined. Furthermore, the present study quantified the absolute amount of myocardial collagen content by hydroxyproline assays and did not address whether possible changes in collagen phenotypes occurred with long-term rapid pacing or concomitant ACE inhibition. It was demonstrated that alterations in myocardial collagen content and composition can influence myocardial stiffness properties.42 In addition, long-term ACE inhibition has been demonstrated to directly influence collagen phenotypes.43 In light of the findings of the present study that ACE inhibition influenced collagen content and structure with long-term rapid pacing, future studies of the direct effects of ACE inhibition on myocardial stiffness properties and collagen phenotypes in this model of cardiomyopathic disease would be appropriate.
Concomitant ACE inhibition significantly reduced peak LV myocardial wall stress after 4 weeks of rapid pacing compared with untreated dogs with chronic tachycardia. In light of the fact that there were similar changes in LV peak systolic pressure and wall thickness between the groups receiving rapid pacing alone and undergoing rapid pacing with ACE inhibition, a major contributory factor for the reduction in peak LV wall stress with long-term ACE inhibition was the reduction in LV chamber size. Because LV ejection fraction is dependent on afterload, an additional contributory mechanism for the improved LV ejection fraction with ACE inhibition may have been a reduction in afterload. However, the present study demonstrated that isolated myocyte contractile function, which is independent of systemic loading conditions, was significantly improved with concomitant ACE inhibition with chronic tachycardia. Thus, the beneficial effects of ACE inhibition on LV function in this model of cardiomyopathic disease were twofold. First, ACE inhibition ameliorated the LV dilation that invariably develops with chronic pacing tachycardia7 9 11 18 19 20 21 22 23 25 and thereby reduced LV myocardial wall stress. Second, concomitant ACE inhibition with long-term rapid pacing improved myocyte function and potentially enhanced LV contractile performance.
ANF is a peptide hormone of cardiac origin with multiple biological effects, and cGMP is generated after stimulation of ANF receptors. Thus, circulating levels of cGMP can be used as an index of ANF receptor activation. In the present study, pacing-induced tachycardia caused a significant increase in plasma ANF and cGMP. This is consistent with a number of past reports that demonstrated that the institution of rapid ventricular pacing results in increased atrial pressures with concomitant elevations in plasma ANF.7 12 14 15 16 24 For example, Moe and colleagues14 serially measured neurohormonal profiles in dogs during the development of pacing-induced heart failure and reported a progressive increase in plasma ANF. In the present study, concomitant ACE inhibition with chronic tachycardia resulted in a significant reduction in ANF and cGMP levels compared with those in untreated dogs with chronic tachycardia. This reduction in ANF and cGMP was associated with reduced LV dilation and preservation of sarcolemmal ß-adrenergic receptor density and transduction. Thus, potential mechanisms for this reduction in ANF and cGMP levels with long-term ACE inhibition include reduced activation of atrial stretch receptors, diminished local ANF production resulting from modulation of local neuroendocrine function, and enhanced ANF degradation. However, the specific mechanisms responsible for the significant reduction in plasma ANF and cGMP with rapid pacing and concomitant ACE inhibition remain speculative and were beyond the scope of the present study. Nevertheless, the present study demonstrated that ACE inhibition with rapid pacing significantly attenuated the degree of neurohormonal activation, which has been well established in this model of cardiomyopathic disease.7 8 9 10 12 13 14 15 16 17 18 23 24
Consistent with past reports,8 18 23 44 long-term rapid pacing increased plasma norepinephrine concentrations and was associated with abnormalities in ß-adrenergic receptor density and cAMP production. Similar changes in the ß-adrenergic receptor system have also been reported in patients with cardiomyopathic disease.26 45 In the present study, these abnormalities in the ß-adrenergic receptor system with the development of tachycardia-induced cardiomyopathy were accompanied by reduced myocyte ß-adrenergic responsiveness. More importantly, the present study demonstrated that concomitant ACE inhibition with chronic tachycardia prevented the downregulation of ß-adrenergic receptor density and normalized cAMP production. One proposed mechanism for the changes in the ß-adrenergic receptor system with cardiomyopathic disease is that the elevated circulating catecholamine levels result in long-term ß-receptor activation with subsequent receptor downregulation and alterations in the transduction system.26 While elevated from baseline values, chronic tachycardia with concomitant ACE inhibition reduced plasma catecholamines from values in dogs undergoing rapid pacing only. These results suggest that one mechanism for the preservation of ß-receptor density and the increase in cAMP production with ACE inhibition was a reduction in circulating catecholamines and diminished ß-receptor activation. Bristow et al46 demonstrated that abnormalities in the ß-receptor transduction system can occur as a result of local adrenergic activity within the LV myocardium. In addition, Maisel et al47 demonstrated that long-term ACE inhibition can directly modulate ß-receptor density. Thus, a second potential mechanism for the beneficial effects of ACE inhibition on the ß-adrenergic receptor system with chronic tachycardia includes direct modulation of local cardiac neuroendocrine function. In the present study, the normalization of ß-receptor density and cAMP production with ACE inhibition and chronic tachycardia was associated with increased myocyte ß-adrenergic responsiveness. However, concomitant ACE inhibition with chronic tachycardia did not return myocyte ß-adrenergic responsiveness to control levels. This laboratory and others previously demonstrated that the abnormalities in ß-adrenergic receptor transduction and responsiveness with the development of tachycardia-induced cardiomyopathy were associated with alterations in the guanine nucleotide–binding regulatory protein complex (G-protein complex).18 44 Furthermore, the development of cardiomyopathic disease was demonstrated to cause alterations in ß-adrenergic receptor subtype expression.26 Thus, potential contributory mechanisms for the failure of concomitant ACE inhibition with chronic tachycardia to normalize myocyte ß-adrenergic responsiveness are persistent abnormalities in ß-adrenergic receptor subtype expression or in the G-protein complex. However, future studies that more carefully examine specific cellular and molecular changes in the ß-adrenergic system with concomitant ACE inhibition and the progression of tachycardia-induced cardiomyopathy are necessary to directly address this issue.
While myocyte contractile function was improved with concomitant ACE inhibition during long-term rapid pacing, significant abnormalities in myocyte function were observed compared with control myocytes. First, in the present study, myofibrillar disarray was observed within the myocytes after chronic tachycardia, and ACE inhibition did not appear to improve this defect in cytoarchitecture. Moreover, when expressed in terms of wet weight of LV myocardium, ACE inhibition with chronic tachycardia caused a reduction in MHC content. Next, it was demonstrated that chronic tachycardia is associated with abnormalities in sarcolemmal receptor systems such as the Na+,K+-ATPase system.10 20 Finally, downregulation of Ca2+ transport systems within the sarcoplasmic reticulum, alterations in Ca2+ homeostasis, and diminished myocyte responsiveness to Ca2+ all have been reported to occur with tachycardia-induced cardiomyopathy.17 21 Thus, potential mechanisms for the persistent abnormalities in myocyte function associated with ACE inhibition and chronic tachycardia include abnormalities in myocyte composition, changes in sarcolemmal function, and alterations in Ca2+ homeostasis.
The present study also provides evidence to suggest that the mechanism for the protective effects of ACE inhibition in this model of dilated cardiomyopathy was not due to prevention of the activation of the endocrine-humoral renin-angiotensin system or systemic hemodynamic effects. First, chronic tachycardia was not associated with increased plasma renin activity. This was probably due to stable hemodynamics in this model of cardiomyopathy. This is consistent with past experimental reports in which a significant rise in plasma renin activity was not observed with chronic tachycardia until severe hemodynamic compromise or end-organ perfusion occurred.7 16 24 Thus, it is unlikely that the protective effects of ACE inhibition in this model of tachycardia-induced cardiomyopathy were due to a reduction in circulating plasma angiotensin II concentrations. However, it must be recognized that the present study did not directly measure plasma angiotensin II levels or ACE activity with long-term rapid pacing. Second, mean arterial pressure, while reduced with concomitant ACE inhibition, was similar compared with that in dogs with tachycardia-induced cardiomyopathy. Thus, it is unlikely that prevention of LV dilation and improved indexes of myocyte contractile function with ACE inhibition were due simply to differences in systemic hemodynamic effects.
The potential contributory mechanisms for the improved LV and myocyte function with ACE inhibition during chronic tachycardia include changes in the intrinsic myocardial renin-angiotensin system and modulation of alternative enzymatic pathways. In vitro binding studies, steady-state mRNA measurements, and in situ hybridization studies provided evidence for a local renin-angiotensin system within the myocardium.48 49 Furthermore, increased myocardial ACE mRNA levels and activity were reported with tachycardia-induced heart failure in rats.9 In the present study, ACE inhibition during chronic tachycardia may have prevented the deleterious effects of endogenous production of angiotensin II within the myocardium. ACE inhibition was achieved through the use of the nonsulfhydryl compound fosinopril. This particular ACE inhibitor is converted to the biologically active fosinoprilic acid after oral administration.50 It was demonstrated previously that fosinoprilic acid penetrates the myocardium and suppresses myocardial ACE activity to a greater degree than other ACE inhib