1. No category

Upregulation of Angiotensin-Converting Enzyme 2 After Myocardial Infarction by
Blockade of Angiotensin II Receptors
Yuichiro Ishiyama, Patricia E. Gallagher, David B. Averill, E. Ann Tallant, K. Bridget
Brosnihan and Carlos M. Ferrario
Hypertension. 2004;43:970-976; originally published online March 8, 2004;
doi: 10.1161/01.HYP.0000124667.34652.1a
Hypertension is published by the American Heart Association, 7272 Greenville Avenue, Dallas, TX 75231
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Upregulation of Angiotensin-Converting Enzyme 2
After Myocardial Infarction by Blockade of
Angiotensin II Receptors
Yuichiro Ishiyama, Patricia E. Gallagher, David B. Averill, E. Ann Tallant,
K. Bridget Brosnihan, Carlos M. Ferrario
Abstract—We investigated in Lewis normotensive rats the effect of coronary artery ligation on the expression of cardiac
angiotensin-converting enzymes (ACE and ACE 2) and angiotensin II type-1 receptors (AT1a-R) 28 days after
myocardial infarction. Losartan, olmesartan, or the vehicle (isotonic saline) was administered via osmotic minipumps
for 28 days after coronary artery ligation or sham operation. Coronary artery ligation caused left ventricular dysfunction
and cardiac hypertrophy. These changes were associated with increased plasma concentrations of angiotensin I,
angiotensin II, angiotensin-(1–7), and serum aldosterone, and reduced AT1a-R mRNA. Cardiac ACE and ACE 2 mRNAs
did not change. Both angiotensin II antagonists attenuated cardiac hypertrophy; olmesartan improved ventricular
contractility. Blockade of the AT1a-R was accompanied by a further increase in plasma concentrations of the
angiotensins and reduced serum aldosterone levels. Both losartan and olmesartan completely reversed the reduction in
cardiac AT1a-R mRNA observed after coronary artery ligation while augmenting ACE 2 mRNA by approximately
3-fold. Coadministration of PD123319 did not abate the increase in ACE 2 mRNA induced by losartan. ACE 2 mRNA
correlated significantly with angiotensin II, angiotensin-(1–7), and angiotensin I levels. These results provide evidence
for an effect of angiotensin II blockade on cardiac ACE 2 mRNA that may be due to direct blockade of AT1a receptors
or a modulatory effect of increased angiotensin-(1–7). (Hypertension. 2004;43:970-976.)
Key Words: angiotensin 䡲 receptors, angiotensin 䡲 angiotensin-converting enzyme
䡲 myocardial infarction 䡲 heart failure
T
he renin-angiotensin system (RAS) plays a key role in
structural and functional remodeling after myocardial
infarction (MI), and angiotensin (Ang) II is a major determinant in this process.1 Ang II stimulates cardiac hypertrophy
and fibrosis in ischemic heart failure models,2 whereas Ang II
blockade prevents development of left ventricular (LV) remodeling and hypertrophy after MI.3,4 Recently, a novel
angiotensin-converting enzyme (ACE)-related carboxypeptidase, ACE 2, was identified in the human heart.5,6 ACE 2
degrades Ang I into Ang-(1–9) and Ang II into Ang-(1–7).6,7
Genetic inactivation of ACE 2 in mice resulted in severe
cardiac dysfunction8 while cardiac and renal ACE 2 gene
expression was reduced in three models of hypertension in
which the ACE 2 gene mapped to quantitative trait loci
previously detected by linkage analysis.8
Characterization of the actions of angiotensin-(1–7) [Ang(1–7)] demonstrated that the RAS consists of two biochemical arms: one generates Ang II via the action of ACE on Ang
I, and the second generates Ang-(1–7) from either Ang I or
Ang II via enzymes other than ACE.9,10 The discovery of
ACE 2 and the demonstration that its catalytic efficiency is
approximately 400-fold higher with Ang II as a substrate than
with Ang I11 strengthened our hypothesis that this second arm
of the system acts as a counter-regulator of the first arm. In
further evaluation of this hypothesis, we determined in
normotensive Lewis rats the effect of myocardial ischemia on
the expression of cardiac ACE and ACE 2 both in the absence
and in the presence of systemic blockade of Ang II type-1
receptors (AT1-Rs) with losartan or olmesartan.
Methods and Materials
Animals
Male Lewis rats (250 to 300 g, Charles River Laboratory, Wilmington, Mass) were housed in individual cages (12-hour light/dark
cycle) with ad libitum access to rat chow and tap water. Procedures
complied with the policies implemented by our Institutional Animal
Care and Use Committee.
Experimental Protocol
Seventy-eight normotensive rats (age range 8 to 10 weeks), randomly divided into 4 groups, were subjected to either shamoperation (Group I, n⫽20) or coronary artery ligation (CAL; n⫽58,
Groups II-IV). Alzet osmotic minipumps (2ML4, Durect Corpora-
Received June 19, 2003; first decision July 10, 2003; revision accepted December 24, 2003.
From the Hypertension and Vascular Disease Center, Wake Forest University Health Science Center, Winston-Salem, NC.
Correspondence to Carlos M. Ferrario, MD, Hypertension and Vascular Disease Center, Wake Forest University School of Medicine, Medical Center
Boulevard, Winston-Salem, NC 27157. E-mail [email protected]
© 2004 American Heart Association, Inc.
Hypertension is available at http://www.hypertensionaha.org
DOI: 10.1161/01.HYP.0000124667.34652.1a
970
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by guest on June 9, 2014
Ishiyama et al
tion, Cupertino, Calif) were implanted subcutaneously under ketamine HCL/xylazine anesthesia (80/12 mg/kg IP, n⫽67) 24 hours
prior to either sham operation or administration of either vehicle or
losartan. Rats randomized to olmesartan had the pump implanted
within 4 hours after CAL. The vehicle (isotonic saline, Groups I and
II), losartan (10 mg/kg per day, Group III), or olmesartan (0.1 mg/kg
per day, Group IV) was infused continuously for 28 days after CAL.
In 11 of the 58 rats undergoing CAL either the vehicle (saline, n⫽4)
or the AT2 antagonist (PD123319, 5 mg/kg every 12 hours, n⫽7)
was given by IP injection for the last 3 days of losartan
administration.
The left anterior descending coronary artery was ligated between the pulmonary outflow tract and the left atrium with a 6-0
silk suture under aseptic conditions in anesthetized rats (ketamine
HCl/xylazine, 80/12 mg/kg IP; 150 mg/kg of ampicillin SC),
mechanically ventilated (70 breaths/min) with room air (SAR830/P, CWE Inc., Ardmore, Pa) through a tube inserted into their
tracheas. Onset of ventricular arrhythmias and the presence of
myocardial blanching distal to the suture confirmed successful
ligation of the artery. After closing the thorax, rats were extubated
after recovery of spontaneous breathing. Sham-operated animals
were intervened in the same manner but a suture was not placed
around the coronary artery.
Hemodynamic measures were obtained 4 weeks after CAL or
sham operation in halothane anesthetized rats (1.5%; Wyeth-Ayerst
Laboratories, Gaithersburg, Md). Aortic pressure was measured with
a catheter (PE-10 connected with PE-50, Clay Adams, Parsipanny,
NJ) inserted via a carotid artery. The catheter was later advanced into
the left ventricle for measurement of left ventricular systolic pressure
(LVSP) and left ventricular end-diastolic pressure (LVEDP), the
maximum rate of isovolumic pressure development (⫹dP/dtmax) and
decay (⫺dP/dtmax), and heart rate (HR). Following collection of
venous blood from the inferior vena cava, deeply anesthetized rats
were euthanized by cardiopulmonary excision. The heart was rinsed
in saline; the cardiac ventricles were separated from the atria,
weighed, and cut transversely from apex to base. A 1 mm slice was
quick-frozen in liquid nitrogen and stored at ⫺80°C. The left and
right ventricles were fixed in formalin (4%) and paraffin embedded.
Tissues were sectioned (5 ␮m) and stained with picrosirius red (0.1%
solution in saturated aqueous picric acid, Sigma Chemical Co., St.
Louis, Mo). Infarct size, determined by planimetry, was calculated as
the ratio of scar length to circumference from each of 3 slices.
Measurements were expressed as a percentage of the total ventricular
perimeter.
Biochemistry
Plasma concentrations of Ang I, Ang II, and Ang-(1–7) were
determined by radioimmunoassay from blood collected into chilled
tubes containing a mixture of 25 mmol/L ethylene-diaminetetraacetic acid (Sigma Chemical Co., St. Louis, Mo), 0.44 mmol/L
1,20-orthophenanthrolene monohydrate, 1 mmol/L Na⫹ parachloromercuribenzoate, and 3 ␮mol/L of WFML (rat renin inhibitor:
acetyl-His-Pro-Phe-Val-Statine-Leu-Phe). Serum aldosterone concentrations were measured with a commercially available kit (CoatA-Count Aldosterone, Diagnostic Products Corporation, Los
Angeles, Calif).
Quantification of mRNA and Protein
One ␮g of RQ1 DNAase-treated, total RNA, isolated from the
noninfarcted portions of the left ventricle with the Trizol reagent
(GIBCO BRL), was quantified by ultraviolet spectroscopy and
reverse transcriptase-polymerase chain reaction assay was performed
using the primers listed in Table 1.12 Amplification conditions (30
cycles) for measurements of AT1a-R and ACE 2 mRNAs were
performed as follows: denaturation at 94°C for 60 seconds; annealing at 60°C for 60 seconds; and elongation at 72°C for 60 seconds,
with a final elongation step at 72°C for 7 minutes. The ACE
fragment was amplified for 32 cycles (annealing temperature of
62°C). Primers for the control EF1␣ sequence were added after 9
amplification cycles for AT1a-R and ACE 2, and after 6 cycles for
ACE. Amplification products were separated on a 6% polyacryl-
TABLE 1.
ACE 2 in Myocardial Infarction
971
Primer Sets
Gene
Primer Set
Fragment Size (bp)
AT1a R
Forward: 5⬘-GCACACTGGCAATGTAATGC-3’
385
ACE
Forward: 5⬘-CAGCTTCATCATCCAGTTCC-3’
Reverse: 5⬘-GTTGAACAGAACAAGTGACC-3⬘
406
Reverse:5⬘-CTAGGAAGAGCAGCACCCAC-3⬘
ACE 2
Forward: 5⬘-GTGCACAAAGGTGACAATGG-3’
EF1␣
Forward: 5⬘-GGAATGGTGACAACATGCTG-3’
425
Reverse: 5⬘-ATGCGGGGTCACAGTATGTT-3⬘
260
Reverse:5⬘-CTAGGAAGAGCAGCACCCAC-3⬘
amide gel, visualized using a PhosphorImager, and quantified by
computerized densitometry.
Statistical Analysis
All values are expressed as mean⫾SEM. One-way ANOVA followed by the 2-tailed Student t test was used for comparing the
differences at P⬍0.05.
Results
Body weight did not differ among the experimental groups
(Table 2). CAL induced significant increases in heart
weight to body weight ratio accompanied by bradycardia,
reduced mean arterial pressure and LVSP, a 3.1-fold
elevation in LVEDP, and significant changes in both the
maximum rate of isovolumic pressure development and
decay (Table 2).
Chronic administration of either losartan or olmesartan for
28 days after CAL reversed cardiac hypertrophy without
significant changes in infarct size (Table 2). Hemodynamically, blockade of Ang II receptors produced further hypotension, which was more pronounced in rats given olmesartan
(Table 2). Although olmesartan produced a significant decrease in LVSP, the greatest decrease in LVEDP was recorded in rats given losartan. There were no differences in
heart rate, whereas in rats given olmesartan the maximum rate
of isovolumic pressure development or decay increased
toward normal values (Table 2).
Circulating RAS Components
Twenty-eight days after CAL, plasma concentrations of
Ang I, Ang II, Ang-(1–7), and serum aldosterone increased
above the values determined in sham-operated controls
(Figure 1). Proportionally, plasma Ang II levels (6.5-fold)
rose more than plasma Ang-(1–7) (1.7-fold), as reflected in
the ratios of Ang II/Ang I and Ang-(1–7)/Ang I in rats
subjected to either sham-operation or MI. The plasma Ang
II/Ang I ratio averaged 0.15⫾0.01 in CAL vehicle-treated
rats and 0.12⫾0.01 (P⬎0.05) in sham-operated controls,
whereas the Ang-(1–7)/Ang I ratio decreased from
0.103⫾0.028 in sham-operated animals to 0.035⫾0.005 in
vehicle-treated CAL rats (P⬍0.01). These data suggest
that MI induced increased formation of Ang II in association with reduced Ang-(1–7) production. This interpretation agreed with the finding of a large decrease in the
Ang-(1–7)/Ang II ratio from 0.76⫾0.13 to 0.30⫾0.05
between sham-operated and CAL-vehicle treated rats,
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May 2004
TABLE 2. Hemodynamic Effects of Either Sham-Operation or Coronary Artery Ligation in Rats
Given Vehicle or Angiotensin II Receptor Blockers
Variables
N
Group I
Sham-Vehicle
Group II
MI-Vehicle
Group III
MI-Losartan
Group IV
MI-Olmesartan
20
20
12
10
Body weight, g
373⫾4
366⫾4
371⫾7
364⫾5
Heart weight/body weight, mg/g
2.95⫾0.03
3.68⫾0.10*
3.15⫾0.11†
3.24⫾0.08†
40⫾1
43⫾1
38⫾1
348⫾6
300⫾9*
323⫾8
319⫾4
Infarct size, %
Heart rate, bpm
Mean arterial pressure, mm Hg
LV systolic pressure, mm Hg
LV end-diastolic pressure, mm Hg
91⫾1
74⫾3*
66⫾2†
62⫾2†
108⫾1
89⫾3*
83⫾2
78⫾2†
7⫾1
22⫾1*
17⫾1†
19⫾1
LV ⫹dP/dtmax, mm Hg/s
4568⫾156
2963⫾132*
2596⫾130
3426⫾181‡
LV ⫺dP/dtmax, mm Hg/s
⫺4425⫾180
⫺2603⫾145*
⫺2397⫾112
⫺2924⫾175‡
Values are mean⫾SEM.
MI indicates myocardial infarction; LV, ventricular; ⫹dP/dtmax, maximum rate of isovolumic pressure development;
⫺dP/dtmax, maximum rate of isovolumic pressure decay.
*P⬍0.05 vs Sham-Vehicle, †P⬍ 0.05 vs MI-Vehicle; ‡P⬍0.05 MI-Losartan vs MI-Olmesartan.
respectively (P⬍0.001). Activation of the RAS post-MI
was also associated with a 3.6-fold rise in serum aldosterone concentration (Figure 1).
Blockade of Ang II receptors during the 28-day post-MI
was associated with higher plasma levels of Ang I, Ang II,
and Ang-(1–7) when compared with sham-operated or
vehicle-treated CAL rats (Figure 1). Figure 1 shows that
plasma Ang I levels in rats given olmesartan increased
significantly above the values obtained in losartan-treated
rats (P⬍0.001). Plasma levels of Ang II were comparable
in rats medicated with either losartan or olmesartan
(P⫽0.306), whereas a tendency for lower values of Ang(1–7) in rats treated with olmesartan was not statistically
significant (P⫽0.06) (Figure 1). Chronic administration of
either Ang II antagonist suppressed the elevations in serum
aldosterone, which for the olmesartan-treated group was
significantly less (P⫽0.03) than for the group of rats given
losartan (Figure 1). Pooled analysis of the relation between
LVSP and plasma concentrations of Ang II and Ang-(1–7)
showed that LVSP correlated inversely with Ang-(1–7)
(r⫽⫺0.53, P⬍0.001) and directly with Ang II (r⫽0.73,
P⬍0.001).
Changes in Cardiac ACE and ACE 2 mRNA
Cardiac ACE mRNA was not different among the various
experimental groups (Figure 2). Although cardiac ACE 2
mRNA did not change in MI vehicle-treated rats, both
losartan and olmesartan elevated ACE 2 mRNA by an
average of 97% and 42%, respectively. Myocardial AT1a-R
mRNA was reduced 51% (P⬍0.002) in vehicle-treated CAL
rats (Figure 2). Both losartan and olmesartan treatments
reversed the decrease in AT1-R mRNA to values that were not
different from those found in sham-operated rats (Figure 2).
Figure 3 shows that ACE 2 mRNA was statistically correlated
Figure 1. Effect of myocardial infarction
(MI) and the angiotensin II receptor
blockers losartan and olmesartan on circulating levels of angiotensin I (Ang I),
angiotensin II (Ang II), angiotensin-(1–7)
[Ang-(1–7)], and serum aldosterone. Values are mean⫾SEM. *P⬍0.05 vs shamoperated animals (Sham-Veh.); #P⬍0.05
vs rats given either vehicle (MI-Veh) or
the Ang II receptor blockers (MI-Losartan; MI-Olmesartan) after myocardial
infarction (MI); ●P⬍0.05, MI-Losartan vs
MI-Olmesartan.
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Ishiyama et al
Figure 2. Changes in the myocardial mRNA of angiotensinconverting enzyme (ACE), angiotensin-converting enzyme 2
(ACE 2), and Ang II receptor (AT1a-R) in sham-operated (ShamVeh) Lewis rats and those receiving either vehicle (MI-Veh) or
Ang II antagonists (MI-Losartan; MI-Olmesartan) during 28 days
after coronary artery ligation. Expression values for each mRNA
were normalized to EF1␣ mRN〈. Values are mean⫾SEM.
*P⬍0.05 vs sham-operated animals (Sham-Veh.); #P⬍0.05 vs
rats given either vehicle (MI-Veh) or Ang II receptor blockers
(MI-Losartan; MI-Olmesartan) after myocardial infarction (MI);
●P⬍0.05, MI-Losartan versus MI-Olmesartan.
with plasma Ang I, Ang II, and Ang-(1–7) levels. Figure 4
shows that concomitant administration of PD123319 in rats
given losartan post-MI did not abolish the increase in ACE 2
mRNA and had no effect on ACE mRNA.
ACE 2 in Myocardial Infarction
973
Figure 3. Scattergram of the relation among plasma concentrations of angiotensin I (Ang I), angiotensin II (Ang II), and angiotensin-(1–7) [Ang-(1–7)] as a function of ACE 2 mRNA.
Discussion
Left ventricular remodeling 28 days after MI was associated
with cardiac dysfunction, compensatory cardiac hypertrophy,
and stimulation of the RAS. We now report that the chronic
phase of MI-induced cardiac remodeling decreased cardiac
AT1a-R mRNA without changes in cardiac ACE and ACE 2
mRNAs. While MI induced increases in plasma levels of Ang
I, Ang II, and Ang-(1–7), examination of ratios as a function
of Ang I demonstrated a relatively greater concentration of
circulating Ang II compared with Ang-(1–7). Blockade of
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Hypertension
May 2004
Figure 4. Coadministration of the angiotensin II type 2 receptor
antagonist PD123319 for the last 3 of 28 days of losartan treatment post-MI had no effect on the increase of ACE 2 mRNA.
*P⬍0.04 compared with vehicle-treated rats.
AT1 receptors after CAL with 2 separate Ang II antagonists
caused a large increase in ACE 2 mRNA to levels significantly higher than in sham- and vehicle-treated CAL rats.
This increase was associated with restoration of AT1a-R
mRNA. In contrast, coadministration of PD123319 in rats
given losartan had no effect on the increased ACE 2 mRNA.
The hemodynamic effects obtained after CAL agreed with
those reported elsewhere.2,13 Loss of myocardial mass was
associated with progression to heart failure characterized by
increased LVEDP and decreased cardiac contractility. Several lines of evidence suggest that Ang II plays a critical role
in mediating myocardial hypertrophy through direct effects
on contractility, induction of growth-promoting genes, increased protein synthesis, and cell growth.1,3 In the mature
heart, Ang II causes cardiac hypertrophy independent of its
effect on blood pressure, whereas blockade of the RAS
attenuates or reverses the cellular adaptations to pressureoverload.13,14 In contrast, Ang-(1–7) attenuated development
of heart failure post-MI as well as acting as an antiarrhythmogenic factor during myocardial ischemia-reperfusion.15–17
Recently, we showed that cardiac myocytes, but not cardiac
fibroblasts, contain a high density of Ang-(1–7) positive
staining. 18,19 Importantly, the myocyte content of Ang-(1–7)
was significantly augmented in the functional myocardium of
rats after CAL.18,19
Administration of either losartan or olmesartan caused
partial reversal of cardiac hypertrophy and left ventricular
dysfunction while further augmenting plasma angiotensin
levels. This was associated with recovery of cardiac AT1a-R
mRNA, increased cardiac ACE 2 mRNA, and no changes in
cardiac ACE mRNA. Although 2 other studies20,21 found
increased AT1-R mRNA levels between 24 hours and 7 days
after MI, our data now suggest that this may be a transient
phenomena due to the acute inflammatory response to ischemia,22 because at day 28 post-MI AT1a-R mRNA was reduced
by almost 50% in the noninfarcted portion of the heart.
Consistent with this interpretation, cardiac AT1-R mRNA
concentration and AT1-R density decreased in a canine model
of pressure-overloaded hypertrophy and in human hearts
post-MI.23,24 Reduction in AT1a-R mRNA post-MI may be a
compensatory mechanism in response to the increase in
circulating Ang II and reduced plasma Ang-(1–7) levels. This
interpretation agrees with the finding that AT1-R blockade
reversed this process. Thus, different mechanisms may regulate AT1a-R mRNA during the acute and chronic stages of
ventricular remodeling post-MI.
ACE 2 is a carboxypeptidase insensitive to known ACE
inhibitors.5– 8 ACE 2 exhibits a high catalytic efficiency for
the generation of Ang-(1–7) from Ang II since only dynorphin A and apelin 13 were hydrolyzed by ACE 2 with kinetics
comparable to those of Ang II to Ang-(1–7) hydrolysis.11
Ablation of ACE 2 in mice caused severe cardiac dysfunction, a finding that suggests an important function of ACE 2
as a regulator of heart function.8 We now show for the first
time that ACE 2 is unchanged during the process of ventricular remodeling post-MI but that sustained blockade of
AT1-R with two different Ang II antagonists increases ACE 2
mRNA. While the data obtained with two different Ang II
antagonists implicates the AT1a-R in the regulation of ACE 2
post-MI, the negative effect of coadministration of PD123319
on ACE 2 mRNA excluded the possibility that the underlying
mechanism may involve a counter-regulatory effect of Ang II
on AT2 receptors.13,25–27 The possibility that Ang II has a
regulatory role of ACE 2 mRNA expression agrees with the
demonstration that Ang II downregulates ACE 2 mRNA in
cerebellar astrocytes in culture.28
A potential role of Ang-(1–7) in stimulating ACE 2 mRNA
cannot be totally excluded since the increases in urinary
Ang-(1–7) produced by dual inhibition of ACE and neprilysin
in spontaneously hypertensive rats are accompanied by a
marked rise in kidney ACE 2 mRNA.29 In our experiments,
plasma Ang II and Ang-(1–7) levels correlated significantly
with cardiac ACE 2 mRNA whether the data were pooled for
all groups studied or selected from animals given losartan or
olmesartan. Although we did not investigate the molecular
stimuli accounting for the upregulation of ACE 2 after
blockade of the AT1-R, the consistent and highly significant
increases in ACE 2 mRNA after AT1 blockade suggests that
ACE 2 may contribute to the beneficial effects of Ang II
blockade after MI. Since ACE mRNA was unaffected by
blockade of AT1 receptors, these data further demonstrate that
ACE and ACE 2 are regulated differentially following AT1-R
blockade.
We also showed for the first time that activation of the
RAS after MI is associated with augmented plasma Ang(1–7) concentrations that correlated inversely with LVSP and
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Ishiyama et al
mean arterial pressure. Loot et al17 reported that in rats
systemic infusion of Ang-(1–7) preserved cardiac function,
coronary perfusion, and aortic endothelial function following
induction of MI while in the isolated perfused rat heart
Ang-(1–7) restored contractility and reduced the occurrence
of ventricular fibrillation after coronary artery occlusion.15,16
That these effects of Ang-(1–7) may reflect a paracrine or
autocrine action of the peptide is suggested by our demonstration of Ang-(1–7) in myocytes of Lewis rats18,19 and the
finding of intracardiac formation of Ang-(1–7) from Ang I or
Ang II in the interstitium of the left ventricle of a dog.30 The
demonstration that attenuation of ventricular dysfunction and
remodeling was associated with further increases in circulating Ang-(1–7) that correlated with perfusion pressure provide
further evidence that Ang-(1–7) may function to oppose the
mechanisms stimulating cardiac remodeling post-MI.
While increased afterload sets into motion a structural and
hemodynamic response of the left ventricle, other studies
showed that ACE inhibitors and Ang II antagonists reversed
ventricular remodeling by mechanisms independent of
changes in blood pressure.3,20 In agreement with these findings, we showed that ACE 2 mRNA did not correlate with
changes in either arterial or left ventricular pressures. These
data suggest that the consequences of increased ACE 2
expression following blockade of Ang II receptors may affect
signaling mechanisms involved in cardiac remodeling rather
than ventricular hemodynamics. In support of this hypothesis,
Ang-(1–7) inhibits vascular neointima proliferation by a
mechanism independent of blood pressure.31
In assessing the effects of AT1-R blockade with 2 different
Ang II antagonists, we excluded the possibility that the ACE
2 response was drug-specific because uricosuric32 and inhibition of platelet aggregation33 effects of losartan have not
been reported with other Ang II antagonists. Although olmesartan reduced mean arterial pressure and LVSP more than
losartan, the difference was not statistically significant. However, olmesartan treatment improved left ventricular contractility, whereas losartan had no effect. Olmesartan produced
significantly higher levels of plasma Ang I but comparable
increases in plasma Ang II or Ang-(1–7) levels in MI-treated
rats when compared with CAL rats given vehicle. In contrast,
serum aldosterone inhibition was significantly higher in rats
given losartan. Quantitative rather than qualitative differences
in the response between the two agents may be related to the
doses employed, although in these experiments both drugs
were given at concentrations shown to inhibit the effects of
Ang II.34
Perspectives
Suppression of Ang II plays an important role in preventing
ventricular hypertrophy and cardiac dysfunction after MI.
Blockade of Ang II receptors after CAL reversed cardiac
hypertrophy and augmented cardiac ACE 2 and AT1a-R
mRNA independent of its effects on blood pressure and
infarct size. These data suggest that the beneficial effects of
Ang II blockade on cardiac remodeling are accompanied by
upregulation of AT1a-R and increased expression of ACE 2.
The data obtained here support the hypothesis that this second
arm of the RAS acts as counter-regulator of the first arm
ACE 2 in Myocardial Infarction
975
wherein ACE catalyzes the formation of Ang II.35,36 The data
further suggest that upregulation of ACE 2, through increased
conversion of Ang II into Ang-(1–7) may counterbalance the
vasopressor effects of ACE that are mediated by Ang II
formation.
Acknowledgments
This work was supported in part by a grant from the National
Institutes of Health (HL-51952 and HL-68258) and an unrestricted
grant from Sankyo Pharmaceuticals (Parsipanny, NY). We thank Drs
Jun Agata and Julie Chao (Medical College of South Carolina,
Charleston) for their technical advice and Robert Lanning for
technical assistance.
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