(Hypertension. 1996;27:392-398.)
© 1996 American Heart Association, Inc.
Articles |
Type 1 Angiotensin II Receptor Subtypes in Kidney of Normal and Salt-Sensitive Hypertensive Rats
From INSERM U90, Hôpital Necker, and INSERM U36, Collège de France (C.L.-C.), Paris, France.
Correspondence to Nadine Bouby, INSERM U90, Hôpital Necker, 161 rue de Sèvres, 75743 Paris Cedex 15, France.
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Abstract |
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 Top Abstract IntroductionMethods Results Discussion References |
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Abstract We studied the localization and regulation of the two type 1 angiotensin II receptor subtypes AT1A and AT1B in different renal zones of the rat kidney by a reverse transcription–polymerase chain reaction amplification method. The yield of the reaction was quantified with an internal standard that was a 63-bp deleted mutant cRNA of the AT1A receptor. In kidneys of male Sprague-Dawley rats (n=4), the levels of AT1A and AT1B receptor mRNAs were highest in the inner stripe of the outer medulla, lowest in the inner medulla, and intermediate in the cortex and outer stripe of the outer medulla. Results (mean±SE) expressed in 105 molecules per microgram total RNA were for cortex, outer stripe, inner stripe, and inner medulla, respectively, 171±15, 152±27, 322±10, and 73±3 for AT1A, and 35±9, 26±1, 71±10, and 53±11 for AT1B. In Sabra rats sensitive (n=6) or resistant (n=6) to salt-induced hypertension and maintained on a normal salt diet, the percentage and level of each receptor subtype mRNA in cortex and outer stripe were similar in the two strains and comparable to those observed in Sprague-Dawley rats. However, AT1A of the inner stripe was significantly decreased in salt-resistant compared with salt-sensitive rats (166±28 and 318±58 105 molecules per microgram total RNA, respectively). These modifications were organ specific because no difference in the level of the receptor mRNAs was observed in the liver of the two Sabra rat strains, whereas a twofold increase in AT1A mRNA level but not in AT1B mRNA level was apparent in adrenal and in one renal zone, the inner stripe of the outer medulla, of hypertension-prone Sabra rats.
Key Words: kidney • RT-PCR • angiotensin II receptors • hypertension, sodium-dependent • rats, Sabra
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Introduction |
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TopAbstractIntroductionMethods Results Discussion References |
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Ang II exerts an essential contribution in controlling several physiological functions of the kidney but in some circumstances also induces pathological actions in this organ. It is normally involved in the regulation of vascular resistance, glomerular filtration rate, and proximal tubular reabsorption.1 The role of the kidney in the pathogenesis of some forms of hypertension has been well established. An alteration in the capacity of the kidney to regulate body fluid and electrolytes under the dependence of Ang II may modify blood pressure.
Studies of Ang II receptors in the kidney with in vitro binding and autoradiographic techniques have shown the presence of receptors in the cortex and inner stripe of the outer medulla2 3 and suggested the existence of different receptor subtypes.4 The synthesis of novel nonpeptide Ang II receptor antagonists revealed the existence of at least two Ang II receptor types, designated AT1 and AT2. Most studies showed that AT1 receptors are predominantly expressed in the rat kidney and are localized in the glomeruli and vasculature, thick ascending limb, and interstitial cells of the inner stripe.5 6 7 8 9 In humans, AT1 receptors were also found in glomeruli10 11 12 and the outer medulla.10 AT2 binding sites were seen in the rabbit, monkey, and human kidney but constituted a minor proportion of Ang II receptors. The known physiological effects of Ang II seem to be mediated by the AT1 receptor. However, the large transient expression of AT2 sites in several organs of the fetus suggests that AT2 receptors play a role during development.13
During the last 4 years, cDNAs for the AT1 receptor have been cloned in several species.14 15 16 17 18 19 20 21 In rat and mouse, the cloning and sequencing of AT1 receptor cDNAs have identified in adrenal and vascular smooth muscle two subtypes, designated AT1A and AT1B.22 23 24 The genes of these two receptor subtypes are localized on different chromosomes (chromosomes 17 and 2 in the rat).25 In the rat, AT1A and AT1B receptor cDNAs exhibit 91% nucleotide sequence homology within the coding region and 58% and 62% within the 5' and 3' untranslated regions, respectively.24 The existence of these two subtypes was also demonstrated in humans.26 27 AT1 receptor subtypes differ in their tissue distribution and regulation, suggesting that these two subtypes could mediate different physiological functions. AT1A receptor mRNA is predominantly expressed in the liver, lung, aorta, and kidney, and AT1B receptor mRNA is predominantly expressed in pituitary. In adrenal, both subtypes are found equally.23 28 29 30
Few studies have quantified AT1 receptor mRNA content within the kidney,31 32 and none has attempted to quantify separately the A and B subtypes. The aim of the present study was to localize and quantify the AT1A and AT1B receptor mRNAs within the four zones of the rat kidney. Each of these zones is characterized by the presence of specific segments of the nephron associated with different vascular patterns and different types of interstitial cells. For this purpose, we used an RT method combined with PCR amplification with an internal standard cRNA. This approach had already been used for the study of the distribution of both subtypes in different rat organs.29
Furthermore, we quantified AT1A and AT1B receptor mRNAs in a salt-sensitive hypertensive rat strain, the Sabra rat. SBH and SBN have been selected for their respective sensitivity or resistance to deoxycorticosterone acetate–salt treatment.33 Submitted to a high sodium diet, SBH develop frank hypertension and SBN do not. The study of SBH and SBN fed a normal sodium diet should reveal whether the sensitivity or resistance to hypertension is associated with a difference in AT1 receptor subtype mRNA expression.
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Methods |
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TopAbstractIntroductionMethods Results Discussion References |
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Animals and Tissue Sampling
All animal procedures were conducted in agreement with our institutional guidelines for the care and use of laboratory animals. Male Sprague-Dawley rats (Iffa Credo) weighing approximately 260 g were used in the first part of the study (n=4). In the second part, male Sabra rats (CNRS) weighing approximately 330 g were studied. Salt-sensitive rats (SBH, n=6) were compared with their controls (SBN, n=6). All rats were fed a normal standard diet (M25 Extralabo) containing 0.32% sodium and were offered water ad libitum. After pentobarbital anesthesia (60 mg/kg body wt), kidneys were removed and cut into coronal slices, and the different renal zones, cortex and inner and outer stripes of the outer medulla and inner medulla, were isolated on ice with microscissors. Since the transition between cortex and outer stripe is irregular, only the superficial cortex and deep outer stripe were taken to avoid tissue zone contamination. Approximately 50 mg of tissue from each zone was used for total RNA extraction. In Sabra rats, adrenal and a piece of liver were also sampled.
RNA Isolation
Total RNA was extracted and purified with the
RNAzol B method
(Biotecx Laboratory) derived from the method of Chomczynski and
Sacchi.34 The RNA pellet was dissolved in 50 to 200 µL
diethylpyrocarbonate-treated water and stored at -80°C
until RT-PCR. An aliquot of RNA preparation was used for determination
of RNA concentration by measurement of optical density at 260 nm. The
quality of isolated RNAs was checked by gel electrophoresis. For this
purpose, approximately 2 µg total RNA was denatured for 5 minutes at
95°C in a solution containing 50% formamide, 6% formaldehyde, 5%
glycerol, 0.25% bromophenol blue, 20 mmol/L
3-(N-morpholino)propanesulfonic acid (MOPS), 5 mmol/L sodium
acetate, and 1 mmol/L EDTA. Denatured RNA to which ethidium bromide was
added (0.05 µg/µL) was fractionated by gel electrophoresis in a 1%
agarose gel containing 2% formaldehyde and was photographed under UV
light. The relative amount of total RNA was quantified by scanning the
18S ribosomal band standardized with a known amount of an RNA ladder
(BRL).
RT-PCR
The method used to quantify the AT1A and
AT1B receptor mRNAs was developed by Llorens-Cortes et
al.29 The primers used were in homologous parts of the
coding region of the rat AT1A and AT1B receptor
genes. Reverse and sense primers corresponded to base pairs 739 to 719
and 295 to 314, respectively, according to Murphy et al.15
Separation of the two amplified PCR products was assessed by
EcoRI digestion because this restriction site is present
only in the AT1A cDNA. The RT reaction and PCR
amplification yield was quantified with an internal standard consisting
of a 63-bp deleted mutant cRNA (nucleotides 502 to 564
according to Murphy et al15 ) of the AT1A
receptor that included the EcoRI restriction site.
Reverse Transcription
cDNAs were synthesized from total
RNA samples, combined with or
without the internal standard, with 200 U Moloney murine leukemia virus
reverse transcriptase (BRL), 40 U RNase inhibitor
(Boehringer), and 0.5 µmol/L reverse primer in an RT-Mix
containing (mmol/L) Tris-HCl buffer 50 (pH 8.3), KCl 75,
MgCl2 3, dNTP 2.5, and dithiothreitol 10, in a final volume
of 20 µL. The RT reaction lasted 90 minutes at 37°C and was stopped
by heating for 10 minutes at 70°C.
PCR Amplification
PCR amplification was performed on one quarter of the RT
reaction sample. The reaction was carried out with 2.5 U Taq
polymerase (Boehringer) and 0.1 µmol/L of each primer with 20
mmol/L Tris-HCl buffer (pH 8.3), 65 mmol/L KCl, 2.1 mmol/L
MgCl2, 0.5 mmol/L dNTP, 3 µCi
[
-3H]dCTP (Amersham), 2 mmol/L dithiothreitol, 0.01%
gelatin, and 10 U RNase inhibitor, in a final volume of 50
µL. The samples were covered with mineral oil. PCR amplification was
conducted for 30 cycles at 94°, 54°, and 72°C for 60, 60, and 90
seconds, respectively, in a DNA thermal cycler (Perkin-Elmer 480).
Possible contamination by genomic DNA was verified by subjecting each
RNA sample to PCR amplification without the RT step. An additional
control without RNA but with reagents was submitted to RT-PCR for each
experiment.
Quantitative Analysis of RT-PCR Products
To distinguish AT1A from AT1B within the
PCR products, we submitted 20 µL of these products to
EcoRI digestion (2000 U/µL) for 90 minutes at 37°C.
Thereafter, the different products, ie, AT1A,
AT1B, and internal standard, were separated by gel
electrophoresis (1.5% low melting point agarose gel in 1x Tris
borate/EDTA buffer) and visualized under UV light. The bands were
excised, solubilized in Tris/HCl/EDTA at 70°C, and counted by liquid
scintillation (Picofluor 15, Packard). Results are expressed as number
of mRNA molecules according to the corresponding number of molecules of
internal standard. Calculations took into account the number of
cytosine residues present in each fragment.
Llorens-Cortes et al29 verified by Southern blot analysis that the amplified fragments correspond to authentic AT1 receptor transcripts. The linearity of the amplification conditions was checked (between 26 and 32 cycles). At 30 cycles, the efficiency of amplification was found to be equal to 90%; the recovery of the RT-PCR reaction was equal for the internal standard and for the AT1 receptor mRNAs. The efficiency of the digestion by EcoRI was checked by observing a complete digestion of an AT1A cDNA amplicon.
Particular
Settings for the Kidney Zones
For quantification of the RT-PCR
reaction products, a
constant quantity of standard cRNA has to undergo RT-PCR with
variable unknown amounts of target mRNA. In this approach, it is
crucial that the optimal amount of internal standard to use be
determined so that a sufficient signal is obtained without induction of
a significant competition between synthetic and target RNAs. An
additional difficulty in the kidney is that the proportion of
AT1B was found to be far lower than that of
AT1A in three of the four renal zones. This fact further
limits the range of target and synthetic RNAs that can be used.
We
carried out preliminary experiments to determine the optimal
conditions of quantification of PCR products in each renal zone.
Fig 1
illustrates the results obtained for the cortex.
It shows the amount of PCR products obtained with increasing
quantities of target RNA in the presence or absence of synthetic cRNA.
Results obtained for each subtype with and without the internal
standard were superimposable. Consequently, addition of the synthetic
cRNA did not induce a significant competition and did not change the
efficiency of the RT-PCR amplification. A linear relationship was
observed between the amount of PCR product and the amount of
starting material in the range of 15 to 125 ng total RNA. Within this
range, when the amount of target total RNA was doubled, the PCR
products of AT1A and AT1B were also
doubled, indicating that the yield of the RT-PCR reaction was identical
for both receptor subtypes. With a higher amount of target total RNA
(250 ng), the curve was no longer linear for AT1A. The
pattern observed for outer and inner stripes was similar to that for
cortex. For inner medulla, a linear relationship was observed for both
subtypes between 30 and 500 ng RNA, at least. In subsequent
experiments, data shown for cortex and outer and inner stripes are
averages of results obtained with 50 and 75 ng total RNA combined with
4.2x105 molecules of standard cRNA, and data for inner
medulla are averages of results obtained with 180 and 300 ng RNA and
2.1x105 molecules standard cRNA.
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Calculations
Results are expressed as mean±SE. The
statistical
significance of the differences observed between the two receptor
subtype mRNA levels in each zone and between SBN and SBH for each
subtype was analyzed by the Mann-Whitney rank sum test.
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Results |
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TopAbstractIntroductionMethods Results Discussion References |
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Results obtained in Sprague-Dawley rats are shown in Table 1
. AT1 receptor mRNA was more abundant
in the inner stripe than in the cortex and outer stripe. In these three
zones, the AT1A subtype was predominant and
represented more than 80% of total AT1
receptor mRNA. In the inner medulla, AT1 receptor mRNA was
poorly expressed and both subtypes were equally
represented. Noteworthy is the fact that the quantity of
total RNA extracted per milligram of tissue was different for each
zone. More than 3 µg total RNA per milligram tissue was extracted
from cortex, 2.5 to 3 µg/mg from outer and inner stripes, and only
approximately 1 µg/mg from inner medulla. When this different content
of tissue total RNA is taken into account, AT1 receptor
mRNA content expressed per milligram of tissue was highest in the inner
stripe followed by cortex, outer stripe, and inner medulla (960, 690,
490, and 140x105 molecules per milligram tissue,
respectively).
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Results obtained in Sabra rats are shown in Table 2 and
Figs 2 and 3. In these rats, as in
Sprague-Dawley rats, AT1A represents the
major part (>80%) of total AT1 receptor mRNA in the
cortex and outer and inner stripes. The percentage of each subtype in
these three zones was not significantly different between SBH and SBN
(Table 2). However, the absolute amount of AT1
receptor
mRNA of inner medulla was higher in SBH than SBN mainly because of the
higher level of AT1A (318±58 versus 166±28
105 molecules per microgram total RNA) (Figs 2 and
3). In
inner medulla the total amount of AT1A plus
AT1B was similar in SBN and SBH. Still, in this zone,
AT1B in SBN was slightly and significantly higher than
AT1A, and in SBH, the reverse was observed.
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174 RF DNA Hae III
Digest.
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To determine whether the modifications seen in the kidney of SBN and
SBH were organ specific, we also quantified AT1 receptor
subtype mRNAs in liver and adrenal. Results are shown in Table
3. In the liver, AT1A receptor mRNA was
expressed in high amounts, and AT1B was not detectable. No
significant difference was observed between SBN and SBH. In adrenal of
SBN, AT1A and AT1B mRNAs were expressed
approximately equally (45% AT1A and 55%
AT1B). In SBH adrenal, AT1B receptor mRNA
abundance was similar to that observed in SBN adrenal, whereas
AT1A was twofold higher than in SBN; this value did not
reach significance (P=.07) because of the large scatter of
values.
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Discussion |
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TopAbstractIntroductionMethods Results Discussion References |
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The present study localized and quantified AT1A and AT1B receptor mRNAs in the different zones of the kidney in normal rats and Sabra rats prone or resistant to salt-induced hypertension. In the kidney of Sprague-Dawley rats, AT1A is the predominant subtype expressed in the cortex and outer and inner stripes, whereas both subtypes are equally represented in the inner medulla. The level of total AT1 (AT1A plus AT1B) is highest in the inner stripe, lowest in the inner medulla, and intermediate in the cortex and outer stripe. In the kidney of Sabra rats, the percentage of AT1A and AT1B in each of the four zones is roughly the same as that in Sprague-Dawley rats. However, salt-sensitive rats (SBH) differ in two respects from their salt-resistant counterparts (SBN). First, a marked increase in the expression level of AT1A is observed in the inner stripe, and second, the percentage of AT1A and AT1B mRNAs is reversed in the inner medulla.
Until now, no precise information was available on the quantitative distribution of AT1A and AT1B receptor mRNAs in the different zones of the kidney. It was shown that the predominant subtype in the whole kidney was AT1A,28 29 as was also the case in the cortical zone.35 Very recently, by in situ hybridization with specific riboprobes, Gasc et al30 have studied the qualitative distribution of both subtype mRNAs in the kidney. Concerning the quantification, Llorens-Cortes et al,29 using the same method, found a level of AT1 receptor mRNAs in the whole kidney of Wistar rats lower than that detected in Sprague-Dawley rats in the present work. Such differences among rat strains were also observed by Paxton et al,9 who reported variations in the intensity of the immunoreactive AT1 protein within the glomerular mesangial cells of four different rat strains.
To better understand the respective roles of the two receptor subtypes, the study of their localization within different organs and its relation to Ang II actions on these organs is necessary, as well as the determination of quantitative modifications in their expression levels in response to different pathophysiological situations. The localization of both subtypes within the kidney and the possible role of Ang II on the different renal structures, as well as the modulation of AT1 receptor expression in Sabra rats, are discussed below.
Intrarenal Localization
Our study permits the precise
quantification of AT1A
and AT1B mRNAs in the four renal zones but not the cellular
localization of receptors within each zone. Taking into account total
RNA content of cortex per unit tissue weight and the volume occupied by
the cortex in the kidney, it could be assumed that the cortex is the
zone with the highest AT1A mRNA expression. This fit with
the fact that in previous studies Ang II receptors are found along all
segments of the nephron with, however, markedly higher levels in the
proximal tubule,32 36 where Ang II acts on
Na+
and CO3H- transport. In the cortex,
AT1 receptor proteins or mRNAs are also found in the
glomerulus and preglomerular vessels30 37
and in mesangial11 30 38 and
extramesangial31 cells. The significant
level of AT1 mRNA observed in the outer stripe in the
present study fit with the presence of Ang II binding sites on pars
recta found by Mujais et al.36 Noteworthy is the fact that
Ang II receptor binding was not observed in this zone in
autoradiographic studies on kidney
sections.2 3 This discrepancy could be due to the low
sensitivity of the binding techniques when the sites are dispersed or
to posttranscriptional regulatory mechanisms that alter the mRNA
translation rate or affect the turnover rate of the mRNA or protein.
The high level of AT1 mRNA found in the inner stripe
probably results from the presence of receptors in the thick ascending
limb and medullary collecting duct32 36 as well as in
the
vasa recta and interstitial cells.39 Ang II
was reported to increase cytosolic calcium concentration in the
medullary thick ascending limb.40 Intrarenal Ang II was
responsible for vasoconstriction of the vasa recta and the consequent
reduction in medullary blood flow.41 Zhuo et
al39 proposed paracrine and/or autocrine influences of
endogenous Ang II on the medullary interstitial
cells. The use of the quantitative RT-PCR method permits the detection
of very small amounts of AT1 mRNA in the inner medulla that
could be localized in collecting ducts,32 thin limbs,
vascular structures, and/or interstitial cells. Further
studies are necessary to determine whether the presence of the mRNA
corresponds with that of a receptor and to elucidate the role of the
hormone at the inner medulla.
The role of each AT1 receptor subtype has not yet been elucidated; however, some hypotheses can be proposed. The major physiological effects of Ang II on its different target tissues are vasoconstriction, reabsorption of sodium, and stimulation of hormone release. In most tissues studied, AT1A seems to be predominant (kidney, aorta, lung, liver), but AT1B is more abundant in the anterior pituitary gland and adrenal zona glomerulosa.23 28 29 30 In these latter tissues, Ang II induces the secretion of prolactin and corticotropin in the anterior pituitary gland and aldosterone in the adrenal zona glomerulosa. Consequently, AT1B could mediate the direct action of Ang II on secretory cells. In the kidney, three main types of Ang II action can be distinguished: modulation of transport activity, contraction of mesangial and vascular smooth muscle cells, and regulation of renin secretion by the juxtaglomerular apparatus. Our data show that AT1B receptor mRNAs represent approximately 15% of total kidney AT1 receptor mRNA and are present in each zone. However, in situ hybridization with specific AT1A and AT1B receptor riboprobes suggests that the AT1B receptor mRNA is predominantly expressed in mesangial cells and to a lower level in juxtaglomerular cells.30 The more precise localization of receptor subtypes along the nephron on the one hand and the determination of the effects of the intrarenally synthesized Ang II1 on the other is necessary for elucidating the respective roles of the AT1 receptor subtypes in the kidney.
Strain Differences in AT1 Receptor
Expression
Regulation of AT1 receptor expression is known
to be
different according to tissue. Concerning the kidney, it seems that
AT1 receptor expression is upregulated when Ang II levels
decrease. In cultured mesangial cells, Makita et
al42 observed a decrease in AT1A receptor mRNA
in the presence of Ang II. AT1A mRNA content of the whole
kidney was found to be increased in rats fed a low sodium diet compared
with rats fed a normal sodium diet.43 A low protein diet,
which induces a decrease in plasma renin activity, results in an
increase in the number of Ang II binding sites in
glomeruli44 45 and enhanced AT1 receptor
mRNA
levels in cortex and medulla.45
We chose to study Sabra
rats because they develop hypertension when fed
a high sodium diet, a situation in which plasma renin activity is
diminished. During the present experiment, we maintained the two
Sabra rat strains (SBH and SBN) on a regular salt diet to determine
whether their susceptibility to the development of hypertension was
associated with a constitutive difference in Ang II receptor mRNA
expression. During a regular salt diet, blood pressure of SBH was only
slightly higher than that of SBN.46 It has been shown that
the SBH kidney differs from SBN kidney even before the induction of
hypertension. In basal conditions, SBH exhibit a reduced urine flow
rate and impaired capacity to excrete a sodium load compared with
SBN.47 48 An increased mineralocorticoid activity in
SBH
was suggested by a greater number of mineralocorticoid receptors in SBH
than SBN kidney,49 despite a slightly lower plasma
aldosterone level in SBH.46 Similarly, we
observed an increase in AT1 receptor mRNA in SBH despite
the reported similar plasma renin activity in the two
strains.46 This increase in AT1 receptor mRNA
level is localized in the inner stripe and essentially concerns the
AT1A subtype. In fact, comparison of the two Sabra strains
with Sprague-Dawley rats (see Table 1
and Fig 3)
shows that
similar levels of AT1 receptor mRNA are present in
Sprague-Dawley rats and SBH and a similar pattern of
distribution of both subtypes is observed in the different zones, with
the highest level of AT1A and AT1B in the inner
stripe. In contrast, SBN exhibit an "abnormal" pattern, with a
lower level of AT1A and AT1B in the inner
stripe that was not significantly different from that seen in the
cortex. Although the percentage of each subtype in the inner medulla
was similar for Sprague-Dawley rats and SBH
(AT1A>AT1B), AT1A levels were
lower than AT1B levels in the inner medulla of SBN. As
noted by Ben-Ishay et al,50 "the unusual element of the
Sabra rats is the SBN rat with its ability to maintain a normal blood
pressure under experimental condition[s] that would
[ordinarily]
induce hypertension." This ability may be facilitated by a lower
level of AT1 receptors in a region of the kidney in which
the level of this receptor is usually greatest. Nonetheless, it has not
yet been ascertained whether this "basal downregulation" of the
receptors plays a major role in the control of sodium homeostasis and
blood pressure in these rats and whether it is causal or secondary.
As discussed previously, the AT1 receptor mRNA detected in the inner stripe could be localized in several different cell types. However, the density of the mRNA and the well-known effect of Ang II on vasculature tonicity suggest that the higher amount of AT1A mRNA observed in SBH than in SBN most likely concerns the vasa recta. This localization would agree with the recent finding of Lu et al,41 who showed that intrarenal Ang II contributes to the hemodynamic resetting of the pressure-natriuretic relationship and the development of hypertension through modification of the medullary blood flow.
No significant difference was observed in AT1 receptor mRNA content of the cortex between SBH and SBN, although Ang II is known to influence sodium transport in the proximal tubules, the major nephron segment present in this zone. Nonetheless, this result is consistent with the observation that there is no difference in basal Na,K-ATPase activity in the cortical and medullary thick ascending limbs of SBN and SBH.51
The difference in proportion but not in quantity of AT1A and AT1B in inner medulla between SBN and SBH is intriguing. It could result from differences within a given cell type (an alteration in the regulation of transcription or stability of each mRNA subtype) or from recruitment of another cell type.
In liver, only AT1A receptor mRNA was detected, as has been already reported,29 30 and no difference was observed in the level of AT1A receptor expression between SBH and SBN, attesting to a tissue-specific regulation. In the adrenal, SBH showed a higher level of AT1A receptor mRNA than SBN, without any difference in the level of AT1B mRNA expression. Surprisingly, in pathophysiological situations such as sodium depletion or renovascular hypertension, it was the AT1B receptor mRNA level that was primarily modified in this gland.29 Ang II, among other effects, stimulates the release of catecholamines from adrenal medulla.52 The lower level of AT1A receptor mRNA in SBN thus could be related to the decreased norepinephrine turnover observed in different tissues of SBN compared with SBH.50
In conclusion, our results describe for the first time the specific and quantitative distribution of AT1A and AT1B receptor mRNAs in the different zones of the kidney and will permit the study of their regulation under various physiopathologic conditions. A modification of AT1 receptor mRNA level (zone-specific and subtype-specific) was found in a genetic rat model susceptible to salt-dependent hypertension.
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Selected Abbreviations and Acronyms |
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Acknowledgments |
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This work was partly supported by a grant from Merck Sharp & Dohme. We gratefully acknowledge Pr P. Corvol for critical reading of the manuscript.
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Footnotes |
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Preliminary results of this work were previously published in abstract form (J Am Soc Nephrol. 1993;4:482 and Néphrologie. 1994;15:406).
Received July 6, 1995; first decision August 14, 1995; accepted November 17, 1995.
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