(Hypertension. 1995;26:971-978.)
© 1995 American Heart Association, Inc.
Articles |
Biological Determinants of Aldosterone-Induced Cardiac Fibrosis in Rats
From INSERM U127, and Service d'Anatomo-Pathologie (M.W.), Hôpital Lariboisière, IFR "Circulation," Université Denis Diderot, Paris, France.
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Abstract |
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Abstract To determine the events leading to cardiac fibrosis in aldosterone-salt hypertensive rats, we studied protein and mRNA accumulation of procollagens I and III for 60 days. After 3 and 7 days of treatment systolic pressure was normal, and no histological or biochemical changes were seen in rat hearts. At day 15 arterial pressure was raised (+40%) and left ventricular hypertrophy was +15%. Cardiac examination after hemalun-eosin staining and immunolabeling with anticollagen I and III antibodies showed no structural alterations, but an 83% increase in right ventricular type III procollagen mRNA levels was found. At 30 and 60 days we found progressive cardiac fibrosis, with inflammatory cells, myocyte necrosis, and elevation of both types I and III procollagen mRNA levels in both ventricles. To determine whether aldosterone had effects on Na,K-ATPase that might lead to ionic disturbances and induce myocyte necrosis, we studied the major cardiac Na,K-ATPase isoform genes. Although Na,K-ATPase
1- and ß1-subunit mRNA levels were
elevated in kidney at day 1, neither of these cardiac transcripts nor
the specific 2 isoform was altered between 1 and 15
days. These results show that accumulation of procollagen mRNAs occurs
before collagen deposition. Cardiac alterations are late and not
preceded by changes in Na,K-ATPase cardiac gene expression, precluding
a direct modulation of cardiac collagen synthesis and Na,K-ATPase by
aldosterone.
Key Words: aldosterone • collagen • fibrosis • Na+,K+-transporting ATPase • heart
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Introduction |
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Long-term treatment of rats with aldosterone-salt results in arterial hypertension, left ventricular hypertrophy, and fibrosis in both the right and left ventricles.1 2 3 The effect of aldosterone on fibrosis development is blocked by low and high doses of spironolactone, even though the low dose does not reverse the hypertension and hypertrophy.4 These results argue for a predominant hormonal stimulus as inducer of the fibrous tissue response. We have previously shown that in addition to the fibrosis, which is histologically evident at 2 months of aldosterone treatment, the levels of cardiac types I and III collagen mRNAs are greatly increased,3 suggesting that the process of fibrosis is still ongoing at this time. In vitro, aldosterone has been reported to increase hydroxyproline incorporation into collagen of isolated rat cardiac fibroblasts within 24 hours,5 although these results could not be confirmed in another laboratory.6 Therefore, the relationship between aldosterone and cardiac fibrosis remains to be clarified. It is of interest to determine in vivo whether aldosterone is able to rapidly induce the accumulation of collagen mRNAs, which would suggest a direct control of collagen genes by the hormone. If collagen synthesis was not induced directly, ie, rapidly, by aldosterone, it would be important to identify the biological determinants involved in the cardiac fibrosis induced by aldosterone-salt treatment. Aldosterone upregulates the expression of major cardiac Na,K-ATPase isoform genes in isolated cardiomyocytes.7 Ionic disturbances arising from alterations of cardiac Na,K-ATPase activity and/or synthesis might explain myocyte necrosis and subsequent reparative fibrosis, as seen in the aldosterone-salt model.
We quantified the levels of types I and III collagen mRNAs over a
60-day period and followed the progression of cardiac fibrosis by
histological examination and immunolabeling of types I
and III collagens in both ventricles. We also measured the
ventricular content of Na,K-ATPase 1-,
2-, and ß1-subunit mRNAs between 1 and 15
days of treatment. The findings provide clear evidence for a
pretranslational control of collagen accumulation in the
aldosterone-salt model, but the late accumulation of
collagen mRNAs precludes a direct action of aldosterone on
fibroblasts. Unlike kidney, cardiac Na,K-ATPase gene expression was not
modulated by aldosterone-salt treatment, suggesting
that other mechanisms that remain to be defined are involved in the
necrotic process.
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Methods |
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Animal Models and Tissues
Male Sprague-Dawley rats weighing 150 to 170 g at the beginning of experiments were used. All sham-operated and treated rats were anesthetized (60 mg/kg pentobarbital) and uninephrectomized. For aldosterone-salt hypertensive rats an osmotic minipump (Alzet, Charles River, Paris, France) was subcutaneously implanted to deliver 0.75 µg/h d-aldosterone (Sigma Chemical Co) and changed every 15 days. Rats received 1% NaCl and 0.3% KCl in the drinking water. Sham-operated rats were also implanted with an osmotic minipump containing only physiological serum and received no salt in the drinking water. Nonoperated rats of the same age were used as controls. Sham-operated rats were not different from controls for any of the parameters studied. All rats were fed ad libitum with M25 pellets (Extralabo). All animal procedures followed were in accordance with institutional guidelines. Systolic pressure was determined by the tail-cuff method. Rats were treated for 3, 7, 15, 30, and 60 days. At the time of euthanasia the heart and kidney were removed, rinsed in ice-cold saline solution, blotted, and weighed. Left ventricles with septum were separated from right ventricles. The cardiac samples were stored at -70°C until use.
Plasma Thyroxine and Aldosterone Levels
Blood samples were collected at the time of euthanasia in tubes
containing 0.5% EDTA. After centrifugation, plasma
samples were collected and stored at -70°C. Plasma
concentrations of aldosterone, free thyroxine (T4), and
free triiodothyronine (T3) were measured by radioimmunoassay.
Total RNA Extraction
Total RNA from ventricles and kidney was prepared according to
Chomczynski and Sacchi.8 RNA concentrations were measured
by 260 nm absorbance, assuming 40 µg/mL for 1 absorbance unit. The
RNAs were resuspended in 0.1% sodium dodecyl sulfate (SDS),
and aliquots were stored at -70°C until use.
Northern Blots and Slot Blots
For Northern blots 20-µg samples of RNA were denatured in 50%
formamide, 2.2 mol/L formaldehyde, and 1x morpholinopropanesulfonic
acid buffer (pH 8. 0) and electrophoresed in a 1% agarose gel. Total
RNA was then transferred to a Hybond-N membrane (Amersham). For dot
blot analysis 1, 2, 4, and 10 µg RNA of each sample were
spotted on the membranes.
Hybridization Conditions
Slots were hybridized with the following oligomers or cDNA
probes: a rat collagen 1(I) cDNA of 1600 bp
complementary to the carboxy-terminal propeptide9 and
a rat collagen 1(III) cDNA containing 1300 bp of the 3'
noncoding and coding regions (kindly provided by Dr
Vuorio).10 Another series of slots were hybridized with
isoform Na,K-ATPase–specific cDNAs (approximately 300 bp) and
a ß1-subunit Na,K-ATPase probe as described previously by
Charlemagne et al.11 All slots were hybridized with a
24-mer oligonucleotide specific for rat 18S rRNA
(Institut Pasteur) for relative quantitation as previously
described.3 Oligonucleotides were 5'-end
labeled with [32P]ATP (NEN) and T4
polynucleotide-kinase (Promega). Hybridizations
with collagen and Na,K-ATPase cDNAs were carried out in 50% formamide,
5x Denhardt's, 200 mg/mL herring sperm DNA, 5x SSC, 0.1% SDS, and
0.1 mol/L phosphate buffer (pH 7. 0) at 42°C. Slots were
prehybridized in this solution for 12 hours and hybridized for 24 hours
with added cDNA radiolabeled by random primer extension with an
Amersham Megaprime DNA labeling system. [32P]dCTP (3000
Ci/mmol) was incorporated to obtain a specific activity of
2x108 to 8x108 counts per milligram. For
collagen hybridization slots were washed in 0.1x SSC and 0.1% SDS at
room temperature and twice more in the same solution at 50°C. For
Na,K-ATPase hybridization slots were washed twice in 2x SSC and 0.1%
SDS at room temperature and in 0.1x SSC and 0.1% SDS at 50°C.
Membranes were exposed to Amersham Hyperfilm at -70°C with
intensifying screens. Different exposures of all
autoradiograms were done to ensure that the signal
stayed within the linear range of densitometry. The relative amounts of
mRNAs were quantified on slot blots by dividing the optical densities
measured for the different probes by the optical density measured for
the 18S probe. Collagen mRNAs were standardized against a synthetic RNA
spotted on every slot. Standard synthetic RNA collagens were prepared
after subcloning a collagen cDNA fragment in a transcription vector
(pJLP 1, 1600-bp fragment in pGEM3 for type I collagen; pJLP 3, 1300-bp
fragment in pGEM3 for type III collagen). After linearization of the
plasmids by appropriate enzymatic digestion, RNA transcripts were
synthesized with SP6 promoter generating 500-bp and 660-bp RNAs for
types I and III collagens, respectively (Promega kit). The
concentration of each collagen mRNA was expressed in picograms per
microgram of total RNA after dividing values by a factor of
3.2 (1600/500=3.2) and 1.97 (1300/660=1.97) for types I and III
collagens, respectively.
Ribonuclease Protection Assay
The level of the mRNA coding for the 3 isoform of
Na,K-ATPase was studied by ribonuclease protection assay. Use of this
sensitive technique was necessary given the extremely low abundance of
this isoform in the adult heart. The antisense RNA complementary to
nucleotides -51 to 331 bp was transcribed by T3 RNA
polymerase in the presence of [32P]UTP (3000 Ci/mmol)
with in vitro transcription kits (Promega). Labeled 3
RNA probe was purified on a polyacrylamide (5%)/urea (8 mol/L)
gel. An amount of 0.1 ng of the probe with a specific radioactivity of
3x108 cpm/µg was hybridized with 25 µg total cardiac
RNA. The assay was carried out as described (Kit Ambion, Cliniscience).
The length of the protected fragment was 382 bp.
Histology and Histochemistry
Cardiac samples were included in mounting solution (Ystosystem)
and frozen at -155°C in isopentane. Equatorial sections (5
µm) were cut in a cryostat at -20°C. One slide for each
specimen was stained with hemalum-eosin for
histological assessment, and the counterpart was used
for double immunofluorescence of isocollagens I and
III. The primary antisera to human skin type I collagen (Chemicon) and
to rat skin type III collagen (Institut Pasteur) were raised in sheep
and rabbit, respectively. Heart sections were first overlaid with 30
µL of type I collagen antiserum (1/100) and incubated overnight at
4°C. Sections were washed in 1x phosphate-buffered saline and
then incubated at 37°C for 30 minutes with 30 µL of type III
collagen antiserum (1/50). All sections were rinsed in
phosphate-buffered saline and incubated for 30 minutes with
biotinylated anti-sheep IgG (1/100). The biotinylated
antibodies were revealed with streptavidin–Texas red complex (1/50).
After washing, sections were incubated with anti-rabbit
immunoglobulin antibodies conjugated to fluorescein
isothiocyanate fluorochrome (Amersham).
Statistical Analysis
Results are expressed as mean±SEM. Comparisons between groups
were performed by the unpaired Student's t test and
Wilcoxon Mann-Whitney test after variance analysis. All
probability values were two-tailed; a value of P<.05
was considered statistically significant.
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Results |
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Blood Pressure and Anatomic Data
Aldosterone-salt treatment induced a marked increase (+40%, P<.001) of systolic pressure at 15 days (Fig 1). Systolic pressure continued to rise for 60 days compared with sham-operated control rats. As a consequence, the ratio of heart weight to body weight was increased by 15% (P<.05) as early as 15 days, reaching 42% at 30 days and 60% at 60 days of treatment (P<.001). In agreement with previous studies using the same model,1 2 3 hypertensive rats developed left ventricular hypertrophy.
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) and sham-operated (
) rats. Data are mean±SEM; n=6
in the treated group and n=7 or 8 in the sham-operated group at
each time point. *P<.01, §P<.05 vs
sham-operated group.
Hormonal Status
Aldosterone administration at a rate of 0.75 µg/h
induced a fourfold to fivefold increase in its plasma concentration
above control (2160±525 versus 394±69 pmol/L measured at 3 days,
P<.001). We and others have previously shown that
aldosterone-salt treatment is also accompanied by a
decrease in plasma concentrations of thyroid hormones.3 12
The Table shows that both T4 and T3 plasma
concentrations decreased as early as 15 days and remained low
thereafter.
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Histological and Immunologic
Examination
Equatorial heart sections including left and right ventricles were
examined after hemalum-eosin staining. No structural alteration was
observed at days 3 to 15 of aldosterone-salt treatment.
Among six rat hearts treated for 15 days, a small focus of
mononucleated inflammatory cells, colocalized with numerous
capillaries, was detected in only one heart.
In contrast, all heart tissue sections of rats treated for 30 days exhibited several lesions (Fig 2). All hearts displayed a marked increase in the number of inflammatory cells, mainly located in thickened fibrous periarterial adventitial layers. These cells were largely lymphocytes, macrophages, and fibroblasts, as identified by their cellular shape (Fig 2F). Some lymphocytes had also infiltrated limited areas of perivascular nonfibrotic myocardium. The endocardium was mildly thickened and contained a few mononucleated inflammatory cells.
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In four of six hearts from rats treated for 30 days, several lesions of segmental or circumferential fibrinoid necrosis of the arterial media were observed in both ventricles. The necrosis was associated with panarterial inflammation and endothelial swelling (Fig 2F).
Small foci of recent or healing myocyte necrosis were also observed, containing myocyte cellular debris or iron-loaded macrophages, respectively. In one rat, a subepicardial focus of replacement fibrosis containing iron-loaded macrophages was present.
At 60 days of treatment additional interstitial fibrosis and numerous large areas of hypervascularized replacement fibrosis were seen in both left and right ventricles (Fig 2D and 2E). At this time fibrinoid necrosis was observed in only one rat among six examined. Moreover, prominent fibromuscular thickening of the wall of intramyocardial arteries was consistently present, with severe narrowing of some arterial lumina.
The immunofluorescence analysis of types I and III collagens showed deposition of both isocollagens, mainly around blood vessels, at day 30; it also extended to the interstitium between days 30 and 60 (Fig 3). The double immunolabeling did not reveal any specific redistribution of an isocollagen relative to the other in fibrotic areas.
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Ventricular Collagen mRNA Levels
Fig 4 presents the time course of changes in
mRNA levels for ventricular 1(I) and
1(III) procollagens after
aldosterone-salt treatment as quantified by dot blot
analysis. It should be noted that in sham-operated rats,
1(I) and 1(III) procollagen mRNA levels
declined significantly between 3 and 60 days, ie, with increasing age,
as previously reported.13 14
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In contrast, 1(I) and 1(III) procollagen
mRNA levels increased as a consequence of
aldosterone-salt treatment after the second week.
At 15 days the increases were not yet significant, except in the right
ventricle, in which the 1(III) mRNA level increased by
83% (P<.05). The levels of both collagen transcripts were
markedly increased in both ventricles at 30 days compared with
age-matched sham-operated rats and were slightly diminished at
60 days. One prominent feature of aldosterone-salt
treatment was the marked accumulation of collagen transcripts in the
right ventricle. For example, procollagen I and III mRNA levels in the
right ventricle were increased by 184% and 230%, respectively, at 30
days, whereas in the left ventricle these transcripts were increased by
only 78% and 162%. This difference was still evident at 60 days.
Ventricular Na,K-ATPase Subunit mRNA
Levels
Na,K-ATPase 1-subunit (+28%,
P=.01) and ß1-subunit (+24%,
P=.04) mRNA levels were increased in the kidney as early as
1 day after onset of aldosterone-salt treatment (Fig 5). This increase was transient because
at day 3 both mRNA levels in treated rats were equivalent to those of
sham-operated rats. The renal levels of 1 and
ß1 mRNAs of uninephrectomized rats that drank salt water
(n=3) were not altered, indicating a specific effect of
aldosterone on renal Na,K-ATPase gene expression. To
determine whether cardiac Na,K-ATPase subunit mRNA levels were also
changed by the aldosterone-salt treatment before
collagen mRNA increases, we measured the Na,K-ATPase 1-
and ß1-subunits and tissue-specific
2-subunit mRNA levels at 1, 3, and 15 days of treatment.
As seen in Fig 6 neither 1 and
2 nor ß1 mRNA levels in the left ventricle
were affected by treatment, which was also the case for the right
ventricle (data not shown). The 3-subunit mRNA was not
detected by RNase protection assay in either control or treated groups
(data not shown).
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Discussion |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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This study demonstrates that long-term aldosterone-salt treatment in the rat induced (1) an increase in arterial pressure and left ventricular hypertrophy beginning at 15 days, (2) an increase in cardiac procollagen mRNAs beginning at 15 days, (3) cardiac fibrosis histologically evident at 30 days with the presence of inflammatory cells and myocyte necrosis, and (4) no change in cardiac Na,K-ATPase subunit mRNA levels between 1 and 15 days.
As previously described, cardiac fibrosis induced by a 2-month treatment of rats with aldosterone-salt was associated with an upregulation of both types I and III procollagen mRNA levels. To understand the determinants of aldosterone-induced fibrosis, we investigated the time course of both events. We observed that the increase of collagen mRNAs appeared before collagen deposition. Indeed, the type III procollagen mRNA level was significantly increased in the right ventricle at 15 days. At this time, the levels of type I procollagen in both ventricles and those of type III procollagen in the left ventricle were not significantly elevated. In contrast, histological evidence of cardiac fibrosis was obtained only at 1 month. Therefore, the present study shows that collagen synthesis is regulated at the pretranslational level in aldosterone-salt hypertension. Since there is a dissociation between the hypertrophy restricted to the left ventricle and the fibrosis observed in both the left and right ventricles,1 2 3 we followed the development of fibrosis in both ventricles. Interestingly, the temporal patterns of changes in types I and III procollagen mRNA levels, expressed as a percentage of values in age-matched rats, were very similar in the left ventricle of aldosterone-salt hypertensive rats. In the right ventricle this pattern was slightly different because, as mentioned above, type III procollagen transcripts accumulated before those of type I. However, both types I and III procollagen mRNA levels increased in parallel between 1 and 2 months. These results correlate well with the immunohistochemical study, which did not reveal differences in the spatial deposition of types I and III collagens. Thus, there was no time difference in the stimulation of collagen synthesis (both mRNA and protein) in the ventricles. This eliminates the hypothesis that fibrosis in the right ventricle is a delayed consequence of left ventricular hypertrophy. Since we were able to obtain absolute values for collagen mRNA, we also showed that both the levels of types I and III collagen mRNAs in control rats and the increase of these levels as induced by aldosterone-salt treatment were higher in the right ventricle than the left. This is in agreement with the fact that the concentration of collagen proteins is greater in the right ventricle than in the left15 and may indicate further basal collagen synthesis in the right ventricle.
A major finding of this study is that the upregulation of cardiac collagen mRNAs is delayed in aldosterone-salt–treated rats. Several results from the literature are consistent with a stimulatory effect of aldosterone on cardiac collagen. Cardiac fibrosis is obtained in vivo by long-term treatment with aldosterone-salt but also by aldosterone alone (unpublished results from this laboratory, 1995). In vivo, spironolactone prevents the increase of collagen synthesis by aldosterone-salt.4 However, in vitro studies give conflicting results. Brilla et al5 reported that aldosterone stimulated collagen synthesis in isolated fibroblasts within 24 hours, but Fullerton and Funder6 were not able to confirm this. The reason for this discrepancy is unknown. The hypothesis that aldosterone acts directly on cardiac fibroblasts in vivo needs to be reconsidered in light of the time course of ventricular collagen mRNA changes described in the present work. Indeed, the first signs of increased collagen synthesis occurred around day 15, whereas the plasma concentration of aldosterone was elevated as early as day 1. Thus, our findings clearly suggest an indirect effect of aldosterone on collagen synthesis.
The delayed induction of collagen synthesis we observed is in contrast with other models of hypertension in which an early and transient stimulation of collagen gene expression has been reported. The work of Chapman et al,16 confirmed by that of Villarreal and Dillmann,17 indicates a transient increase of the ventricular content of types I and III procollagen mRNAs on the first day after abdominal stenosis in the rat. In hypertension obtained by continuous infusion of angiotensin II, the levels of cardiac fibronectin and type I collagen mRNA increase rapidly and return to control levels within 1 week.18 These differences in the onset of collagen rise may thus be related to the particular stimulating agent. Because the rise of arterial pressure parallels collagen mRNA accumulation, it might be argued that hypertension is the trigger for collagen synthesis. In all models of hypertension cardiac fibrosis appears concomitant with the elevation of blood pressure and the subsequent left ventricular hypertrophy and originates in the perivascular compartment to spread out thereafter into neighboring interstitial space. Our observation that fibrosis is perivascular at 30 days and perivascular and interstitial at 60 days is in agreement with this mechanogenic hypothesis.
However, several observations indicate that aldosterone induces fibrosis independently of its effects on arterial pressure. First, cardiac fibrosis is elicited in both ventricles of rats chronically treated by aldosterone,13 whereas atrial natriuretic peptide gene expression is stimulated only in the hypertrophied left ventricle and not in the nonhypertrophied right ventricle.3 Second, even at sub-antihypertensive doses, spironolactone prevents the appearance of cardiac fibrosis by aldosterone.4 Third, the intracerebroventricular injection of the mineraloreceptor antagonist RU 28318 in aldosterone-treated rats suppresses the increase in arterial pressure but not the interstitial cardiac fibrosis.19
Thus, there are indications that aldosterone by itself is responsible for at least part of the fibrosis. It also should be mentioned that other biochemical changes resulting in fibrosis arise before the increase in collagen synthesis.17 20 This suggests that the process of aldosterone-induced fibrosis begins before the increase in blood pressure.
Another potentially important factor for the development of cardiac fibrosis in aldosterone-salt–induced hypertension is cellular necrosis. Although the cardiac histological structure was normal at 15 days, it was profoundly altered at 30 days. At the latter time point we observed fibrinoid necrosis; direct and indirect signs of myocyte necrosis, such as the presence of damaged myocytes or cellular debris in the fibroinflammatory areas; and intramacrophagic iron deposition, which is known to originate from myoglobin released by the phagocytosis of damaged myocytes. Injured myocytes and activated macrophages21 are known to produce growth factors such as transforming growth factor–ß, which increases collagen synthesis. In addition, in spontaneously hypertensive rats fibroblasts expressing collagen mRNA are colocalized with lymphocytes and macrophages in fibrotic and necrotic areas.22 The presence of lymphocytes and macrophages in the fibrotic areas of hearts of aldosterone-treated rats suggests the possibility that transforming growth factor–ß synthesis might contribute to the fibrous tissue response we observed. It may be worthy to note that aldosterone-salt treatment was accompanied by a decreased plasma concentration of thyroid hormones. This decrease was statistically significant at 15 days of treatment and was maintained thereafter. These results need to be considered for interpreting collagen mRNA increase because previous studies have demonstrated that thyroid hormone partly inhibits collagen gene expression.23 The sustained decrease of thyroid hormone level in the plasma may thus suppress the partial inhibition of collagen synthesis by T4 and participate in synergy with other fibrotic factors in the genesis of cardiac fibrosis.
Since our results support an indirect mechanism of induction of cardiac
fibrosis by aldosterone, one can conclude that binding of
aldosterone to mineralocorticoid receptors in the
heart24 25 induces intermediary biochemical steps that
remain to be defined. One possibility is that of intracellular ionic
disturbances. Mineralocorticoids have a known early stimulatory
action on renal Na,K-ATPase activity and gene
expression.26 27 Aldosterone has been reported
to also increase Na,K-ATPase 1- and
ß1-subunit mRNA levels in isolated cardiac myocytes at 6
hours.7 Thus, it is conceivable that
aldosterone binding in heart could induce changes in
myocyte Na,K-ATPase gene expression, with possible subsequent ionic
disturbances causing myocyte necrosis and reparative fibrosis.
The hypothesis of ionic imbalance is reinforced by the observation that
the potassium-sparing agent amiloride prevents microscopic scarring
associated with aldosterone-salt treatment but appears
ineffective on reactive fibrosis.28
To investigate early cardiac effects of aldosterone, we
have studied Na,K-ATPase gene expression between 1 and 15 days, ie,
before stimulation of collagen gene expression. We observed an increase
of Na,K-ATPase 1- and ß1-subunits mRNA
levels in the kidney 1 day after onset of aldosterone
perfusion, indicating that the aldosterone plasma level
reached in our experiments was adequate to stimulate Na,K-ATPase gene
expression in vivo. In contrast, neither 1 and
2 nor ß1 mRNA levels were affected by the
aldosterone treatment in heart between 1 and 15 days, nor
was the 3 isoform, which is expressed in the neonatal
rat heart,29 reinduced. Similar results have already been
reported in deoxycorticosterone acetate–salt hypertensive
rats.30 In this model as in other models of
hypertrophy,11 only a decrease of the relative
level of the 2 mRNA was detected and related to
increased blood pressure. Thus, our results indicate that cardiac
Na,K-ATPase is not a target for aldosterone and demonstrate
a tissue-specific response to this hormone. The present results
also emphasize the fact that experiments using isolated cells may give
results that are different from those of experiments in intact
animals.
However, even though the cardiac Na,K-ATPase remained unaltered by aldosterone-salt treatment, it is possible that other cardiac ionic pumps known to be regulated by aldosterone, such as the Na-H exchanger,26 might be modified. To avoid myocyte damage by secondary effects of aldosterone-salt treatment such as hypokalemia31 32 we supplemented drinking water with KCl at appropriate doses.3
Another possibility is that myocyte necrosis is ischemic in nature and related to fibrinoid necrosis or fibromuscular narrowing of coronary arteries. It has been proposed that angiotensin II plays a major role in these lesions. Indeed, blockade of the angiotensin II action with losartan is effective in preventing fibrinoid necrosis in the heart of stroke-prone spontaneously hypertensive rats despite no decrease in blood pressure.33 Furthermore, in the heart of aldosterone-salt–treated rats, Sun and Weber34 found an increase in cardiac angiotensin II binding between 2 and 8 weeks of treatment. Thus, it is possible that the action of aldosterone is to potentiate the vasoconstrictor and/or fibrogenic effects of angiotensin II.
In conclusion, we have obtained evidence that cardiac collagen gene expression is increased before that of mature proteins, suggesting transcriptional regulation of collagen in aldosterone-salt hypertensive rats. However, the time course of cardiac fibrosis development in our experiments suggests that rather than a direct induction of collagen in cardiac cells, aldosterone has an unknown cardiac action that can induce or facilitate fibrosis. Alternatively, possible secondary effects of the aldosterone treatment, which remain to be defined, may participate in the observed fibrogenic response.
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Acknowledgments |
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This study was supported by grants from Institut National de la Santé et de la Recherche Medicale, Centre National de la Recherche Scientifique, and Laboratoires Searle France (No 92036-2). The authors thank Drs J. Perennec and J.L. Samuel and N. Van Thiem for helpful discussions, C. Mouas and T. Dakhli for animal handling, and G. Ondola and M. De Villedon for secretarial support.
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Footnotes |
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Reprint requests to Claude Delcayre, INSERM U127, Hôpital Lariboisière, 41 Bd de la Chapelle, 75010 Paris, France.
Received August 7, 1995; first decision September 7, 1995; accepted September 18, 1995.
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