(Hypertension. 1999;33:1342-1347.)
© 1999 American Heart Association, Inc.
Scientific Contributions |
From the University of Ottawa Heart Institute, Ottawa, Ontario, Canada (T.O., A.J. de B.) and Hoechst Marion Roussel, Frankfurt am Main, Germany (W.L., B.A.S.).
Correspondence to Adolfo J. de Bold, PhD, University of Ottawa Heart Institute at the Ottawa Civic Hospital, 40 Ruskin St, Ottawa, Ontario K1Y 4W7, Canada.
| Abstract |
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Key Words: atrial natriuretic factor renin-angiotensin system kidney deoxycorticosterone hypertension, renal
| Introduction |
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Because the renin-angiotensin system (RAS) and ANF often play counterregulatory roles, we took advantage of the known inhibiting effect of the DOCA-salt treatment on RAS on the one hand and the upregulation of RAS after aortic banding on the other to substantiate the hypothesis that renal ANF gene expression may be influenced by RAS status. Furthermore, the aortic-banded rats were treated with either a low- or a high-dose schedule of the angiotensin-converting enzyme (ACE) inhibitor ramipril. High-dose treatment with ramipril leads to inhibition of both the local and circulating RAS, resulting in normalization of blood pressure, although low-dose treatment inhibits local RAS with persisting hypertension.4 This approach thus allows definition of the differential contributions of high blood pressure and RAS to ANF gene expression. In the present studies we show that renal ANF gene expression is downregulated by DOCA-salt. Conversely, hypertension induced by aortic banding leads to upregulation of renal ANF gene expression. In the latter model, renal ANF mRNA levels were normalized by the ACE inhibitor independently of hypertension. Together, these findings suggest that renal ANF gene expression is RAS dependent but independent of hypertension. They further suggest that the decrease of renal ANF mRNA in DOCA-salt hypertension could play a pathogenic role because it represses a potential sodium excretory mechanism mediated by the local expression of ANF acting on renal receptors. In aortic banding, renal ANF gene expression upregulation suggests a local compensatory function consistent with the consensus role of natriuretic peptides in the modulation of RAS, leading to amelioration of the sodium-retaining effects of renal underperfusion induced by aortic banding.
| Methods |
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Aortic Banding Experiment
The preparation of animals used in these experiments has been
previously described.5 Briefly, adult male Sprague-Dawley
rats weighing 270 to 280 g had the aorta constricted above the
kidneys. Five groups of animals were used: (1) control, (2) sham
operated, (3) aortic banded, (4) aortic banded treated with
high-dose ramipril (1 mg/kg), and (5) aortic banded treated with
low-dose ramipril (10 µg/kg). Ramipril was administered by daily oral
gavage for 6 weeks to rats immediately after the aortic-banding
operation. Ramipril dosage was adjusted weekly according to body
weight. At the end of the treatment period, the animals were
instrumented for measurement of carotid blood pressure because tail
sphygmomanometry is not feasible because of the blood pressure drop
distal to the aortic coarctation. Both methods of blood pressure
measurement give comparable results.6 7 Blood and tissue
samples were obtained as described above for the DOCA-salt
experiment.
Extraction of Plasma and Tissue Samples
Plasma samples were acidified by adding 100 µL/mL of 1 mol/L
HCl and passed through Sep-Pak C18 cartridges
(Millipore) that were prewetted with 5 mL of 80% acetonitrile in 0.1%
trifluoroacetic acid (TFA) and 10 mL of 0.1% TFA. The cartridges with
the absorbed peptides were washed with 20 mL of 0.1% TFA and then
eluted with 3 mL of 60% acetonitrile in 0.1% TFA. Tissue samples were
homogenized in 10 volumes of an extracting mixture
consisting of 0.1N HCl, 1.0 mol/L acetic acid, and 1% NaCl and
centrifuged at 10 000g for 30 minutes at 4°C. The
supernatants were then extracted with the use of Sep-Pak
C18 cartridges as described above for plasma,
except that elution was 80% acetonitrile in 0.1% TFA. The eluates
were freeze-dried and processed for RIA as described below.
Assay Methods
Plasma renin activity (PRA) was measured by determining the
level of angiotensin I (Ang I) generated during 1
hour of incubation at 37°C in the presence of 8-hydroxyquinoline. The
Ang I concentration was measured by RIA kit (Du Pont). PRA values were
expressed as nanograms Ang I synthesized per milliliter per hour.
Plasma Ang I and plasma angiotensin II (Ang II) levels were
determined with RIA kits (Advanced ChemTech). The Ang I antisera showed
<0.01% cross-reactivity with Ang II peptide, and the Ang II antisera
showed 1.7% cross-reactivity with Ang I peptide. The concentration of
immunoreactive ANF in plasma and tissue samples was determined by RIA
as previously described with the use of anti-rat
ANF99126 serum from Peninsula
Laboratories.8 The ANF antisera showed <0.01%
cross-reactivity with brain natriuretic peptide.
Total RNA Extraction and Northern Blot Analysis
Total RNA extraction and Northern blot analysis were
performed as previously described5 with the following
32P-labeled probes: a 1.4-kb
BamHI/HindIII fragment of the rat renin
cDNA,9 a 1.6-kb EcoRI/HindIII
fragment of the rat angiotensinogen
cDNA,10 a 3.8-kb EcoRI fragment of the
mouse ACE.5 cDNA,11 a 900-bp
EcoRI/HindIII fragment of the rat ANF
cDNA,12 a 2-kb BamHI/BglII
fragment of the mouse phosphoglycerate kinase cDNA,13
and a 5-kb SalI/EcoRI fragment of mouse 28S rRNA
cDNA. Autoradiographs were scanned with the use of an Ultrascan XL
laser densitometer (LKB Produckter) and LKB 2400 Gelscan XL software
package. The scanning values of each mRNA were normalized to 28S rRNA
or phosphoglycerate kinase mRNA as internal controls to correct for
differences in the amount of RNA applied and transfer efficiency.
Quantitative Competitive Reverse Transcription Polymerase
Chain Reaction
A detailed description of the QC-RT-PCR has been previously
published.2 Briefly, RNA samples were reverse transcribed
with Super Script II RNase H- Reverse
Transcriptase and oligo(dT)1218 primer with the
use of a reverse transcription kit (GIBCO BRL). An aliquot of the cDNA
product was used for PCR amplification with ANF primers. A dilution
series of total RNA (5 µg) aliquots was prepared for each sample.
Each dilution was spiked with competitor RNA. After the PCR, aliquots
(5 µL) of the PCR product were electrophoresed on a 2% agarose
gel and visualized by ethidium bromide staining. Photographs were taken
with Polaroid 55 film, and the negatives were scanned with the use of
an Ultrascan XL laser densitometer and Gelscan XL 2000 software
package. The ratio of the density of the competitor RNA to the target
RNA was plotted against the amount of the competitor RNA added to each
reaction.
Statistical Analysis
All data were expressed as mean±SEM, and a level of
P<0.05 was considered significant. ANOVA was performed to
determine statistical differences among multiple groups. When
significance was obtained by ANOVA, Fisher's least squares differences
post hoc analysis was used to determine pairwise
differences.
| Results |
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Plasma RAS
DOCA-salt and salt treatments significantly decreased PRA (Figure 1). Plasma Ang I and Ang II levels
followed a pattern similar to that of PRA. The changes of PRA and
plasma Ang II levels in the aortic-banding experiments have been
previously reported.4 These consisted of a slight,
nonsignificant increase in PRA and a significant increase in Ang II
plasma levels in the banded rats.
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ANF Plasma Levels
In the DOCA-salt experiments, plasma ANF levels of the
DOCA-treated rats (95±7 pg/mL), the salt-treated rats (87±6 pg/mL),
and the DOCA-salttreated rats (117±7 pg/mL) were significantly
higher than those of the control rats (67±5 pg/mL; P<0.01
for all groups versus control). We have previously reported that in the
aortic-banding experiments,5 plasma ANF in the banded
rats was significantly higher than those of the control and
sham-operated rats. In the banded rats treated with high-dose ramipril,
ANF was normalized, but in the banded rats treated with low-dose
ramipril, plasma ANF remained higher than that of the control and
sham-operated rats although lower than that of the banded rats and
untreated animals.
ANF Concentration and mRNA Levels in Cardiac Tissue
Left atrial ANF was partially depleted by DOCA-salt treatment even
though ANF mRNA was significantly higher than those in other groups
(Figure 2). Ventricular ANF
and ANF mRNA levels in the DOCA-salttreated rats were higher than
those of the control groups. We previously reported that in the
aortic-banding experiments,5 atrial ANF and ANF mRNA
levels were similar among the groups. Ventricular ANF and
ANF mRNA levels closely paralleled the changes in plasma ANF
levels. Both hypertensive models are therefore accompanied by
stimulation of cardiac ANF gene expression, although aortic banding is
characterized by displaying a ventricular response
only.
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Renal RAS
Figures 3 and 4 show Northern blot analysis of
renal renin, angiotensinogen, and ACE. DOCA, salt, and
DOCA-salt significantly lowered renal renin mRNA. In aortic-banded
rats, renal renin mRNA levels were significantly lower than those of
the control and sham-operated rats. High-dose ramipril treatment
increased renin mRNA levels. Animals treated with low-dose ramipril had
renal renin mRNA levels comparable to those of untreated, banded
animals. Angiotensinogen mRNA levels and ACE mRNA levels
were similar among all aortic-banded groups. ACE mRNA levels in rats
treated with high-dose or low-dose ramipril were normalized with
respect to control and sham-operated rats.
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Renal ANF Concentration
Renal ANF levels were similar among all groups of the DOCA-salt
experiment and aortic-banding experiment (Figure 5).
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Renal ANF mRNA Levels
DOCA-salt treatment significantly depressed renal ANF mRNA levels
compared with all other groups. Aortic banding, on the other hand,
significantly increased renal ANF mRNA levels over those of the control
and sham-operated groups (Figures 5 and 6). High-dose or low-dose ramipril
treatment significantly lowered renal ANF mRNA levels in the banded
rats to levels similar to those in the control and sham-operated
rats.
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| Discussion |
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The differences in renal ANF gene expression occur even though the animals in the 2 hypertension models had similar systolic blood pressure differences between treated and control groups, resulting in a similar increase in the left ventricular weight/body weight and kidney weight/body weight ratios.
Renal ANF levels were not significantly increased in aortic-banded rats banded for 6 weeks, during which time ANF mRNA levels were also significantly increased. The significance of renal ANF changes is obscured by the fact that high-performance liquid chromatography analysis of ANF extracted from the renal tissue shows that most of the ANF is processed ANF99126 and not ANF1126, which is the expected storage form of ANF.2 Therefore, it is likely that the former represents mostly blood-borne or receptor-bound ANF.
The decreased values for PRA, plasma Ang I, and Ang II values found in the present study in the DOCA-salt experiments confirm the characteristic downregulation of circulating RAS caused by DOCA-salt treatment. Renal renin mRNA levels in the animals treated with DOCA alone, salt alone, or DOCA-salt were significantly decreased compared with the control rats. This finding is consistent with the expected effect of volume expansion and with the known role of sodium in the regulation of renal renin mRNA.14
In aortic-banded rats, we have previously reported a slight increase in PRA and a significant increase in plasma Ang II levels.4 5 The latter4 may be expected to exert a negative feedback inhibition on renin release15 16 and a reduction of renal renin mRNA.15 16 Although we did not perform a complete evaluation of renal RAS both in terms of message and product activity or concentration, the significant decrease in plasma Ang II levels in the DOCA-salt rats and its increase previously reported in the banded rats4 may explain the opposite regulation of the renal ANF mRNA levels between these models given that Ang II stimulates ANF gene expression either in culture17 or in vivo independently of hypertrophy.18
An insight into the relative influence of plasma and local RAS on the changes of renal ANF transcript levels found in the present study is provided by the fact that high-dose ramipril normalized renal ANF synthesis together with renal ACE mRNA levels (this study) and plasma ACE and decreased plasma Ang II.6 Low-dose ramipril also decreased renal ANF gene expression and normalized renal ACE mRNA levels, but, from previous studies, it is known not to decrease plasma ACE activity or plasma Ang II,6 suggesting that inhibition of local renal RAS is sufficient to prevent upregulation of renal ANF synthesis. It remains to be determined whether the effects of ACE inhibition described here are the direct result of interference with Ang II generation or some other process, including kinin formation.
The upregulation of renal ANF gene expression observed after aortic banding suggests a local compensatory role consistent with the consensus role of natriuretic peptides in the modulation of RAS. Thus, it may be hypothesized that intrinsic renal ANF ameliorates the sodium-retaining effects brought about by renal underperfusion. This function of ANF could also partly explain the development of sodium-sensitive hypertension observed in ANF knockout mice.19 The decrease in renal ANF mRNA levels in the DOCA-salttreated rats may contribute to the development of volume expansion and hypertension despite the increased cardiac and plasma ANF levels. DOCA-salt treatment increases proximal tubule neutral endopeptidase activity.20 21 This results in increased degradation of filtered ANF, as demonstrated by the fact that administration of neutral endopeptidase inhibitors increases the renal actions of ANF in volume-expanded states, including the DOCA-salt model.21 22 23 24 25 The significant decrease of renal ANF mRNA in DOCA-salt hypertension found in the present investigation suggests that this decrease, together with increased degradation of blood-borne ANF, could play a pathogenic role in the development of mineralocorticoid hypertension. Supporting this view is our previous finding that blockade of the natriuretic peptide receptor impairs the ability of the kidneys to escape the salt-retaining effects of mineralocorticoid administration.26
| Acknowledgments |
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Received January 29, 1999; first decision February 10, 1999; accepted February 12, 1999.
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