(Circulation. 1995;91:16-19.)
© 1995 American Heart Association, Inc.
Articles |
From the Hypertension Unit, University of Ottawa Heart Institute, 1053 Carling Ave, Ottawa, Ontario, Canada, K1Y 4E9.
Correspondence to Hypertension Unit, University of Ottawa Heart Institute, 1053 Carling Ave, Ottawa, Ontario, Canada K1Y 4E9.
| Abstract |
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Methods and Results In the present study, we assessed the effects of ACE inhibitors with low (enalapril) and high (quinapril) affinity for cardiac tissue ACE on prevention of volume overloadinduced cardiac hypertrophy in relation to their hemodynamic effects. Both blockers were equipotent for circulatory ACE as assessed from the pressure response curve to angiotensin I. Both blockers partially (and similarly) prevented the increase in left ventricular end-diastolic pressure by aortocaval shunt. However, only quinapril prevented or attenuated the development of right ventricular hypertrophy and left ventricular hypertrophy and dilation.
Conclusions The present findings further stress the involvement of the renin-angiotensin system as a trophic stimulus in the development of cardiac hypertrophy in this model. Moreover, the low affinity of enalapril for cardiac ACE appears to lead to continuous angiotensin II generation in the heart and can thus explain the failure of enalapril to attenuate hypertrophic response of the heart induced by shunt despite decreasing cardiac volume overload.
Key Words: angiotensin hypertrophy enzymes volume overload
| Introduction |
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Cardiac hypertrophy induced by volume overload after aortocaval shunt can be prevented by the angiotensin II receptor blocker losartan.10 This effect appears to be only in part related to the decrease in cardiac preload and afterload by losartan, since the ACE inhibitor enalapril showed similar hemodynamic effects but failed to prevent cardiac hypertrophy.10 These data suggested that an activated circulatory and/or cardiac renin-angiotensin system (RAS) (as assessed from increases in plasma and cardiac renin activity10 ) indeed contributes to the remodeling of the heart.10 The failure of enalapril might be explained by its low affinity for cardiac tissue ACE, resulting in continuous generation of angiotensin II in the heart and stimulation of growth of cardiomyocytes.
In the present study, we assessed the effects of doses equipotent for circulatory ACE (as assessed from the pressure-response curve to angiotensin I) of ACE inhibitors with low (enalapril) and high (quinapril) affinity for cardiac tissue ACE on prevention of volume overloadinduced cardiac hypertrophy in relation to their hemodynamic effects.
| Methods |
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Cardiac Hemodynamics
On the day of the study, rats were
anesthetized with
halothanenitrous oxideoxygen, and a PE-50 catheter (Clay Adams)
filled with heparinized saline (100 U/mL) was inserted into the left
ventricle (LV) via the right common carotid artery and into the right
external jugular vein. Catheters were exteriorized on the neck of
animals. After a 4-hour recovery period from anesthesia, LV
end-diastolic pressure (LVEDP) and LV peak systolic
pressure (LVPSP) were assessed in conscious, unrestrained rats after a
30-minute acclimatization period as previously
described.13 Heart rate was calculated from the curves of
LVPSP and LVEDP recorded at a paper speed of 10 mm/s.
Dose-Response Curve to Angiotensin I
The degree of blockade
of plasma and endothelial ACE (in treated
compared with untreated control rats) was assessed from the increase in
LVPSP in response to angiotensin I (0.01, 0.1, 0.5, 1.0, 5.0, and 10.0
ng/kg body wt per minute IV) immediately after resting hemodynamics
were recorded.
Cardiac Anatomy
At the end of the experiment with animals
under pentobarbital
anesthesia, the chest cavity was opened and the heart was arrested in
diastole by intravenous injection of 1 mol/L KCl, rapidly excised, and
placed into ice-cold saline to remain in diastole and to remove the
blood. After removal of the atria and great vessels, the ventricles
were blotted dry, and the right ventricle (RV) was dissected along its
septal insertion from the remainder of the ventricular mass.
Subsequently, LV and RV weights were assessed separately. The mass of
the LV was then divided by two transverse cuts at one third and two
thirds of the length. The middle slice of the LV was used for
assessment of the LV wall thickness and internal diameters as
previously described by Tsoporis et al.14
Statistical Analysis
Results are expressed as
mean±SEM. Differences between groups
at a given treatment period were evaluated by ANOVA and Duncan's
multiple-range test. Differences were considered statistically
significant if P<.05.
| Results |
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Aortocaval shunt significantly increased LVEDP compared with control
rats (Table
). Enalapril and quinapril did not affect LVEDP in
control
rats (Table
). In contrast, enalapril and quinapril decreased
LVEDP in
rats with aortocaval shunt by approximately 50% (Table
). LVEDP
still
remained significantly higher in rats receiving either treatment
compared with control rats (Table
). There were no differences
in heart
rate between the groups.
Dose-Response Curve to Angiotensin I
Blockade of circulatory
ACE by enalapril and quinapril was
assessed from the responses of LVPSP to angiotensin I in control
treated and untreated rats. As clearly shown in Fig 1
,
approximately 10-fold higher doses of angiotensin I were required in
rats treated with enalapril or quinapril compared with untreated
animals. This indicates a similar blockade of the plasma and/or
endothelial ACE by enalapril and quinapril.
|
Cardiac Anatomy
LV weight increased after 1 week of an
aortocaval shunt by
28% compared with control rats (Fig 2
). Enalapril and
quinapril did not significantly affect LV weight in control rats (Fig
2
). In rats that received shunts and enalapril, LV hypertrophy
developed similar to that in untreated rats (Fig 2
). In
contrast,
quinapril significantly attenuated the development of LV hypertrophy
compared with untreated rats that received shunts (Fig 2
).
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Aortocaval shunt increased RV weight by 44% compared with control rats
(Fig 2
). Neither blocker affected RV weight in control rats
(Fig 2
).
Enalapril did not prevent the development of RV hypertrophy in rats
that received shunts, and an increase in RV weight by 42% was
present in rats with aortocaval shunt receiving enalapril compared
with control rats receiving enalapril (Fig 2
). In contrast,
quinapril
nearly completely prevented RV hypertrophy in rats that received shunts
compared with (treated and untreated) controls (Fig 2
).
LV internal diameter increased by 14% after 1 week of volume overload
by an aortocaval shunt (Table
). Enalapril and quinapril did not
affect
LV internal diameter in control rats (Table
). In rats that
received
shunts, enalapril failed to prevent the increase in LV internal
diameter, whereas quinapril attenuated LV dilation by approximately
50%.
Aortocaval shunt caused only a small increase in LV wall thickness
(Table
). Enalapril and quinapril did not affect LV wall
thickness in
control rats (Table
). Quinapril significantly attenuated the
increase
in LV wall thickness in rats that received shunts compared with
enalapril (Table
).
Body weights did not differ significantly between the groups at the end of the experiment (control, 292±8 g; control plus enalapril, 295±7 g; control plus quinapril, 289±6 g; shunt, 288±7 g; shunt plus enalapril, 291±5 g; and shunt plus quinapril, 295±7 g).
| Discussion |
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Effects of an Aortocaval Shunt on Central Hemodynamics and Cardiac
Anatomy
As shown previously, the opening of an aortocaval shunt
results in
cardiac volume overload as reflected by the increase in
LVEDP.10 15 16 LVPSP significantly
decreases,10 16 likely as the result of a major
decrease
in total peripheral resistance.10 15 16
RV and LV
hypertrophy and dilation develop over a period of approximately 4
weeks, but most of the response is present at 1 week after the
shunt.10 15
Effects of Enalapril and Quinapril on Changes in Central
Hemodynamics Induced by an Aortocaval Shunt
The doses of enalapril and
quinapril used in this study have been
shown to nearly completely inhibit plasma ACE
activity8 9
and to normalize blood pressure in spontaneously hypertensive or
two-kidney, one clip hypertensive rats.6 11 In the
present study, dose-response curves to angiotensin I clearly
indicate a similar degree of blockade of the plasma and/or endothelial
ACE in enalapril- versus quinapril-treated rats. In addition, there are
no major differences regarding the affinity of the two blockers for
renal ACE.8 9 Enalapril and quinapril blunted the
increase
in LVEDP by shunt to similar extents. In addition to
venodilation17 and improved ventricular relaxation by the
two blockers,18 natriuresis and the reduction of the
volume expansion caused by a shunt19 could contribute to
the attenuation of the increase in LVEDP. Because enalapril has only
minimal effects on stroke volume, heart rate, and blood pressure in
rats that received shunts,10 improvement in LV diastolic
function by enalapril likely contributes to the attenuation of the
increase in LVEDP. Whether quinapril causes a similar improvement
cannot be answered from our data. The absence of a decrease in LVPSP by
the two ACE inhibitors at 1 week after the aortocaval shunt in the
present study is consistent with previous studies showing that
angiotensin II only plays a role in the maintenance of blood pressure
and total peripheral resistance shortly after induction of an
aortocaval shunt.20 Overall, there were no differences in
hemodynamic effects between the two ACE inhibitors.
Effects of Enalapril and Quinapril on Changes in Cardiac Anatomy
Induced by an Aortocaval Shunt in Relation to Their Hemodynamic
Effects
In agreement with our previous results,10 in the
present study enalapril failed to prevent or attenuate the RV and
LV hypertrophy and dilation despite decreasing LVEDP. The ACE inhibitor
quinapril, however, significantly attenuated the LV hypertrophy and
dilation and nearly completely prevented RV hypertrophy while
decreasing LVEDP similarly to enalapril.
In rats that received shunts, the degree of volume overload determines the extent of the cardiac hypertrophic response.10 21 Consistent with the findings of Sadoshima et al1 2 showing that angiotensin II mediates the hypertrophic response of the cardiomyocyte to stretch in vitro, our previous results suggested that RAS acts as a direct trophic stimulus during the development of cardiac hypertrophy in this model.10 First, the plasma and cardiac RAS is activated (as indicated by increases in plasma and RV and LV renin activity) during the development of cardiac hypertrophy after aortocaval shunt.10 Second, a similar attenuation of the increase in LVEDP by enalapril and losartan resulted in blunted hypertrophic response in losartan-treated rats only.10 Third, expanding our previous findings, the results of the present study point to the low affinity for cardiac ACE of enalapril as the explanation for its failure to prevent cardiac remodeling in response to volume overload by an aortocaval shunt. At the same time, these results make the presence in the volume overloaded rat heart of an angiotensin IIforming enzyme resistant to ACE inhibitors unlikely. However, assessment of the effects of enalapril versus quinapril on cardiac angiotensin II and/or ACE activity in this model is necessary for definite conclusions.
This study clearly shows that similar blockade of the plasma and/or endothelial ACE by different ACE inhibitors is relevant for their hemodynamic effects. However, differences in their affinity for cardiac tissue ACE may determine their effect on prevention of cardiac hypertrophy, when the cardiac RAS acts as a trophic factor.
| Acknowledgments |
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Received August 30, 1994; accepted October 30, 1994.
| References |
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