Published online before print November 13, 2003, doi: 10.1161/01.RES.0000106135.02935.E1
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© 2003 American Heart Association, Inc.
Integrative Physiology |
Rapid Inhibition of Vasoconstriction in Renal Afferent Arterioles by Aldosterone
From the Department of Physiology and Pharmacology, University of Southern Denmark, Odense, Denmark.
Correspondence to Ole Skøtt, Physiology and Pharmacology, University of Southern Denmark, DK-5000 Odense, Denmark. E-mail oskott{at}health.sdu.dk
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Abstract |
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Aldosterone has been suggested to elicit vessel contraction via a nongenomic mechanism. We tested this proposal in microdissected, perfused rabbit renal afferent arterioles. Aldosterone had no effect on internal diameter in concentrations from 10-10 to 10-5 mol/L, but aldosterone abolished the ability of 100 mmol/L KCl to induce vascular contraction. The inhibitory effect of aldosterone was observed from 1 pmol/L. The inhibitory effect was significant after 5 minutes and maximal after 20 minutes and was fully reversible. Actinomycin D (10-6 mol/L) prolonged the effect of aldosterone. The effect was abolished by the mineralocorticoid receptor antagonist spironolactone (10-7 mol/L) but not by the glucocorticoid receptor antagonist mifepristone (10-6 mol/L). The K+-mediated increase of intracellular calcium concentration in afferent arterioles was not affected by aldosterone. Mineralocorticoid receptor was detected by reverse transcription–polymerase chain reaction and immunohistochemistry in rat renal vasculature and rabbit endothelial cells. Inhibition of phosphatidylinositol (PI)-3 kinase with LY 294002 (3x10-6 mol/L) restored sensitivity to K+ in the presence of aldosterone, and afferent arterioles were immunopositive for PI-3 kinase subunit p110
. Inhibition of NO formation by L-NAME (10-4 mol/L) or inhibition of soluble guanylyl cyclase with 1H-(1,2,4)Oxadiazolo[4,3-a]quinoxaline-1-one restored K+-induced vasoreactivity in the presence of aldosterone. Similar to aldosterone, the NO donor sodium nitroprusside inhibited K+-induced vascular contraction. Geldanamycin (10-6 mol/L), an inhibitor of heat shock protein 90, abolished aldosterone-induced vasorelaxation. We conclude that aldosterone inhibits depolarization-induced vasoconstriction in renal afferent arterioles by a rapid nongenomic mechanism that is initiated by mineralocorticoid receptor activation and involves PI-3 kinase, protein kinase B, and heat shock protein 90–mediated stimulation of NO generation.
Key Words: endothelium • steroid • kidney • smooth muscle
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Introduction |
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Aldosterone mediates its classical effects through binding to a cytoplasmic receptor, the mineralocorticoid receptor (MR), which subsequently leads to activation or repression of target genes, including the early response genes K-Ras and several serum glucocorticoid kinases.1 In addition to the effects on sodium, potassium, and extracellular volume homeostasis, aldosterone stimulates generation of collagen, fibronectin, and laminin and promotes hypertrophy, cardiac remodeling, and fibrosis independent of blood pressure.2,3 The clinical significance of these effects of aldosterone on cardiovascular function was emphasized by the Randomized Aldactone Evaluation Study,4 which demonstrated that inhibition of aldosterone action was associated with a 30% reduction in mortality rates in patients with heart failure after myocardial infarction. In nonepithelial cells (vascular smooth muscle, lymphocytes, and porcine aortic endothelial cells), aldosterone has been shown to activate rapidly Na+-H+ exchange and cause IP3 formation and release of calcium from intracellular stores. These rapid effects are not influenced by antagonists of MR and by inhibitors of transcription and translation.5 Based on the potential ability of aldosterone to acutely increase intracellular calcium concentration, it could be anticipated that aldosterone is a rapid vasoconstrictor. We tested this hypothesis using renal resistance vessels. We examined whether aldosterone had any direct effects on vascular luminal diameter or intracellular calcium concentration in microperfused afferent arterioles. After initial experiments showed that aldosterone did not induce contraction but inhibited depolarization-induced contraction, we pursued this unexpected effect, and we provide evidence that aldosterone inhibits depolarization-induced vasoconstriction in renal afferent arterioles by an NO-dependent pathway.
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Materials and Methods |
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Animal care and use were according to the guidelines of the National Institutes of Health, and permissions according to national Danish guidelines were obtained from the Danish Ministry of Justice. Male Sprague-Dawley rats (n=6) (local animal facility; Biomedical Laboratory, Odense, Denmark) and New Zealand rabbits (n=62) (Harlan, France) were maintained on standard chow and had free access to tap water.
Endothelial Cells
Rabbit thoracic aorta was flushed with PBS and filled with RPMI1640 medium with 0.1% BSA and 0.25 mg/mL collagenase A (Roche) 2x30 minutes at 37°C. At the end of each period, the effluent was collected and stored at 4°C. Cells were harvested and lysed with 400 µL 4 mol/L guanidinium-thiocyanate solution. Total RNA was isolated by phenol-chloroform extraction and subjected to reverse transcription (RT).7 Primary cultures of human umbilical artery endothelial cells (HUAECs) were established from normal human umbilical cords by collagenase treatment. The cords were obtained from the Department of Obstetrics and Gynecology, Odense University Hospital, Denmark. HUAECs were seeded on gelatin-coated tissue culture plastic, and the cultures were maintained in EGM-2 medium (Clonetics) and kept at 37°C, 95% humidity, 5% CO2. Cells were detached for passaging with trypsin-EDTA solution for endothelial cultures (Sigma) and used until passage 7. Cells were incubated with aldosterone (10 nmol/L) for 20 minutes before fixation.
Reverse Transcription–Polymerase Chain Reaction and Ribonuclease Protection Assay
Total RNA from rat organs was isolated by RNeasy midi kit (Qiagen) according to the manufacturer’s instruction. Total RNA from isolated cells and dissected vessels was isolated as above.7 RNA used for polymerase chain reaction (PCR) corresponded to 5 to 10 vascular branching points (preglomerular vessels) or 50 ng total RNA (whole kidney) and 5 of 20 µL from each endothelial cell RT reaction mixture. Primer sequences, RT-PCR protocols, and probe design for MR, glucocorticoid receptor (GR), and 11ßHSD2 were as published.7–9 Probes were hybridized to total RNA from rat organs followed by ribonuclease protection and gel electrophoresis.7 Protected probes were cut out of the gel and quantitated in a ß-counter.
Immunoperoxidase Histochemistry and Cytochemistry for Phosphatidylinositol-3 Kinase and Protein Kinase B/Phospho-Akt
HUAECs were permeabilized with methanol and incubated with 5% goat serum in PBS for 30 minutes. Next, the cells were incubated with rabbit anti-mouse phospho-Akt(ser473) antibody diluted 1:50 in PBS for 18 hour at 5°C (Cell Signaling Technology, No. 9277). Secondary antibody (goat anti-rabbit IgG, HRP-labeled) diluted 1:200 with PBS was applied for 60 minutes. Cryosections (5 µm) from rat kidneys were incubated with 5% goat serum in TRIS-buffered saline (TBS) for 30 minutes followed by rabbit anti-rat phosphatidylinositol (PI)-3 kinase antibody diluted 1:150 in TBS for 18 hours at 5°C (anti-p110, sc-7174, 200 µg/mL, Santa Cruz). Secondary antibody (goat anti-rabbit IgG, HRP-labeled) diluted 1:500 with TBS was applied for 60 minutes. The sections were stained by diaminobenzidine (DAB+ substrate-chromogen system, DAKO) and lightly counterstained by hematoxylin for 30 seconds.
Immunostaining of Isolated Microvessels
The preglomerular vasculature was microdissected from rat kidneys after HCl maceration and immunostained.10,11 We used goat anti-rat MR antibody (Santa Cruz, sc-6860), diluted 1:25; rabbit anti-rat 11ßHSD2 antibody (Chemicon, AB1296), diluted 1:1000; and rabbit anti-mouse renin antibody (a kind gift from Prof Knud Poulsen, The Royal Veterinary and Agricultural University, Copenhagen, Denmark).
Isolation and Microperfusion of Renal Arterioles
Rabbit afferent arterioles were microdissected and perfused with CO2-HCO3–buffered solutions as described.12 Once the vessels had stabilized for 30 minutes at 37°C, they were challenged with isoosmotic K+ (100 mmol/L) to assure viability. Phentolamine (10-5 mol/L) (Sigma) was added to all solutions to exclude nerve-mediated -adrenergic effects of depolarization.
Series 1
The effect of aldosterone on inner arteriolar diameter was tested in sequentially increasing concentrations from 10-9 to 10-5 mol/L.
Series 2
The ability of aldosterone to interfere with K+-induced vasoconstriction was tested in a step-up protocol with aldosterone at increasing concentrations from 10-16 to 10-8 mol/L.
Series 3
The time dependence of the aldosterone response was assessed by testing the ability of 100 mmol/L K+ to induce vasoconstriction after 5, 20, 50, 70, and 90 minutes of incubation with 10-9 mol/L aldosterone and after 5, 20, and 50 minutes of incubation with 10-8 mol/L aldosterone.
Series 4
The effect of the transcription inhibitor actinomycin D (10-6 mol/L) on the aldosterone-mediated vascular response to K+ was tested. L-NAME (10-4 mol/L) was added for 20 minutes at the end of each experiment.
Series 5
The effects of blockade of the mineralocorticoid and glucocorticoid receptors on the response to aldosterone were tested in this series using spironolactone (10-7 mol/L) and mifepristone (10-6 mol/L), respectively.
Series 6
The effect of the PI-3 kinase inhibitor LY 294002 ]2-(4-morpholinyl)-8-phenyl-1(4H)-benzopyran-4-one hydrochloride; 3x10-6 mol/L; 30 minutes)[ on aldosterone-mediated vascular responses to K+ was assessed.
Series 7
The role of NO for aldosterone-mediated inhibition of vasoreactivity to K+ was tested by incubating the vessels for 10 minutes with the NO synthase inhibitor N
-nitro-L-arginine methyl ester hydrochloride (10-4 mol/L).
Series 8
The ability of the NO donor sodium nitroprusside (SNP; 10-6 to 10-4 mol/L) to interfere with K+-induced vasoconstriction was tested in a step-up protocol.
Series 9
In this series we tested the ability of the NO-sensitive guanylyl cyclase blocker 1H-(1,2,4)Oxadiazolo]4,3-a[quinoxaline-1-one (ODQ; 10-6 mol/L) to interfere with the inhibitory effect of aldosterone on vasoreactivity to K+.
Series 10
The effect of the heat shock protein (hsp) 90 blocker geldanamycin (10-6 mol/L) on the response to aldosterone (10-9 mol/L) was tested in these series.
Measurement of Intracellular Calcium Concentration by Digital Fluorescence-Imaging Microscopy
Intracellular free calcium concentration was measured in microdissected afferent glomerular arterioles.13,14 The -adrenoceptor blocker phentolamine (10-5 mol/L) was present to exclude nerve-mediated effects on the smooth muscle.
Statistics
Data on arteriole contraction were compared after normalizing the responses in the individual vessels to the initial maximal response to high K+. We used Dunnett’s test for multiple comparisons with a control. P<0.05 was considered significant.
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Results |
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Expression of Mineralocorticoid Receptor and 11ß Hydroxysteroid Dehydrogenase type 2 (11ßHSD2) in Vascular Tissue
Ribonuclease protection assays showed significant expression of MR and GR mRNAs in left cardiac ventricle and in aorta of control rats, whereas 11ßHSD-2 was expressed at a low level in heart and aorta (Figure 1A). By RT-PCR, MR and 11ßHSD-2 mRNA were detected in freshly microdissected rat preglomerular microvessels only in the presence of added cDNA (Figure 1B). The ß-actin primer set spans an intron and amplification yielded products with the expected size of cDNA (194 bp) and not genomic DNA (453 bp) excluding a genomic origin of amplified DNA. Next, RNA from freshly isolated rabbit aortic endothelial cells (n=3 separate preparations) was subjected to RT-PCR, which showed expression of MR (Figure 1C). Amplification products were evident only in the presence of cDNA in the PCR and of RT in the reverse-transcription reaction. PCR for ß-actin again yielded products with the expected size of cDNA (194 bp) (Figure 1C).
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X173/HaeIII. C, Expression of MR and ß-actin mRNAs in isolated rabbit aortic endothelial cells. Total RNA was isolated from three separate preparations (E1–3) and subjected to RT-PCR for 35 cycles. cDNA 50 ng from rabbit aorta and brain served as positive controls, and negative controls were water instead of cDNA (-cDNA) and omission of RT in the reverse-transcription reaction (-RT). Molecular weight marker was
Immunolocalization of Mineralocorticoid Receptor and 11ßHSD-2 Protein in Isolated Vessels
Rat preglomerular vessels were microdissected and immunostained with antibodies directed against MR and 11ßHSD-2. Significant labeling was associated with all segments of the preglomerular vasculature (Figure 1D). Negative control vessels for immunolabeling were incubated without primary antibody and with primary antibody that had been preabsorbed with the peptide used for immunization (Figure 1D). Kidney sections labeled with the MR antibody were stained in the collecting ducts from the cortex to the papilla and in the thin limbs of Henle’s loop. Weaker signals were observed in distal convoluted tubules. No or little staining was seen in the vasculature, suggesting that vascular MR is less abundant compared with epithelium (data not shown). As a positive control for preserved physiological localization of proteins in the acid-macerated vessels, we observed that immunostaining for renin resulted in the expected labeling in the distal ends of the afferent arterioles.
Isolated Perfused Arterioles
The effect of aldosterone on basal diameter and K+-evoked contraction was assessed in microperfused rabbit afferent arterioles. The viability of the afferent arteriole was assessed by the response to isosmotic addition of 100 mmol/L K+. Vessels that did not respond to this procedure were discarded. Average basal diameter of 45 perfused vessels was 17.4±0.4 µm (SE), and K+ occluded the lumen totally.
Series 1
Aldosterone was added in increasing concentrations from 10-9 to 10-5 mol/L using a step-up procedure with 5 minutes at each concentration. Aldosterone had no direct effect on the luminal diameter of the microperfused afferent arterioles at any concentration (Figure 2; n=6). At the end of the experiment, viability was tested by addition of K+. Surprisingly, after exposure to aldosterone, 100 mol/L K+ had no effect on internal diameter. In contrast, supraphysiological levels of norepinephrine (10-5 mol/L) occluded the vessels, thus underlining that vessels were viable.
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Series 2
Increasing concentrations of aldosterone were added in a step-up procedure with 20 minutes at each concentration of aldosterone (from 10-16 to 10-8 mol/L). At the end of each 20-minute incubation period, the response to addition of 100 mmol/L K+ was assessed. The ability of aldosterone to inhibit depolarization-mediated vasoconstriction was concentration-dependent and significant at low physiological concentration (10-14 mol/L) (Figure 3A; n=5). In a separate set of experiments, we confirmed that preincubation of the arterioles with 10-9 mol/L aldosterone for 20 minutes (without prior exposure to lower concentrations of aldosterone or depolarization at 5 minutes) blunted depolarization-induced vasoconstriction (n=3). Data from a single experiment are shown in Figures 3B and 3C.
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Series 3
In this series, the time dependence of the inhibitory effect of aldosterone was assessed. In two separate series, aldosterone (10-9 and 10-8 mol/L) was permanently added to the bath. Aldosterone 10-9 mol/L significantly inhibited K+-induced vasoconstriction after only 5 minutes. The response was maximal after 20 minutes and waned after 90 minutes (n=5). Similar results were obtained with 10-8 mol/L aldosterone (n=6) (Figures 4A and 4B). In a separate series, the effect of corticosterone (10-8 mol/L, n=5), was tested after 20 minutes of incubation. Similar to aldosterone, corticosterone inhibited K+-mediated contraction (Emax, 59.5±9.1%; data not shown).
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Series 4
Preincubation with actinomycin D (10-6 mol/L; n=4) had no effect by itself on K+-induced vasoconstriction and did not abolish the inhibitory effect of aldosterone (10-9 mol/L) on K+-induced vasoreactivity. In contrast, preincubation with actinomycin D converted the transient response into a sustained effect of aldosterone. The effect of aldosterone was reversed by L-NAME (10-4 mol/L) in the presence of actinomycin D (Figure 4C).
Series 5
Preincubation with the MR antagonist spironolactone (10-7 mol/L, 30 minutes) had no effect by itself on K+-induced vasoconstriction, whereas it abolished the inhibitory effect of aldosterone (10-8 mol/L) on K+-induced vasoreactivity (Figure 5A, n=6). Preincubation with the inhibitor of the glucocorticoid receptor, mifepristone (10-6 mol/L), also had no effect by itself on K+-induced vasoconstriction and did not alter the effect of aldosterone (10-9 mol/L) on K+-induced vasoconstriction (Figure 5B, n=5).
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Series 6
PI-3 kinase is a downstream target for activated MR. The PI-3 kinase inhibitor LY 294002 (3x10-6 mol/L) had no effect on the basal inner diameter of the arterioles and no effect on K+-induced vasoconstriction, whereas it completely blocked the effect of aldosterone (10-9 mol/L) on K+-induced vasoconstriction (Figure 6E, n=5), suggesting that PI-3 kinase is critically involved in the signal transduction. At a higher concentration (3x10-5 mol/L), LY 294002 blocked the ability of the afferent arteriole to contract (data not shown).
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Localization of PI-3 Kinase and Protein Kinase B/Phospho-Akt(ser473) by Immunohistochemical and Cytochemical Analyses
In rat kidney, immunoperoxidase microscopy for the PI-3 kinase p110-subunit using cryosections showed a distinct labeling in renal vessels. Immunopositive labeling was associated with smooth muscle and endothelium. Also, the renal afferent arterioles were immunopositive for the p110-subunit (Figures 6A through 6D). Control sections that were incubated in the absence of primary antibody were negative.
After exposure to aldosterone, cultured HUAECs showed distinct labeling for the phosphorylated protein kinase B (PKB) isoform [phospho-Akt(ser473), Figures 6E and 6F)]. Controls incubated in the absence of primary antibody were negative.
Series 7, 8, and 9
Endothelial nitric oxide synthase (eNOS) is a downstream target for PI-3 kinase. The involvement of NO in the response to aldosterone was tested by addition of L-NAME, SNP, and ODQ. Addition of 10-4 mol/L L-NAME did not alter basal diameter of the vessels, but it completely reversed the inhibitory effect of aldosterone (10-9 mol/L, Figure 7A, n=5) and 10-8 (n=2, not shown). The NO donor, SNP (10-6 to 10-4 mol/L), caused a concentration-dependent and reversible reduction of K+-induced vasoconstriction (Figure 7B, n=5). Blockade of the soluble guanylyl cyclase with ODQ (10-6 mol/L) had no effect on basal diameter, whereas it abolished the inhibitory effect of aldosterone (10-9 mol/L) on K+-induced vasoreactivity (Figure 7C).
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Series 10
Incubation of the arterioles with the hsp90 blocker geldanamycin (10-6 mol/L) did not affect basal diameter but abolished the inhibitory effect of 10-9 mol/L aldosterone on K+-induced contraction (Figure 8A).
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Determination of Intracellular Calcium Concentration in Response to Aldosterone
The intracellular calcium concentration was estimated in FURA-2–loaded nonperfused arterioles. The effect of K+-induced depolarization on intracellular calcium concentration was determined before and after addition of aldosterone (Figure 8B). The resting calcium concentration was 126±5 nmol/L, and K+ elicited a peak average increase in calcium to 262±38 nmol/L (n=7; Figure 8C). Quantitatively similar responses were repeatedly induced in the individual specimen, which excludes tachyphylaxis to the response. Aldosterone (10-9 mol/L) had no effect on resting calcium concentration in the course of 20 minutes of incubation and did not affect the response to K+-induced depolarization (resting concentration, 118±28 nmol/L; stimulation, 275±16 nmol/L), showing that the ability of aldosterone to blunt vasoconstriction after depolarization is not dependent on inhibition of calcium influx or calcium mobilization.
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Discussion |
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In the present study we tested the hypothesis that aldosterone is an acute vasoconstrictor in isolated microperfused afferent arterioles.15,16 However, aldosterone did not change vascular diameter but blunted the ability of a depolarizing concentration of potassium to elicit vasoconstriction. This effect of aldosterone was rapid, nongenomic, and mediated through the MR and involved PI-3 kinase–mediated activation of NOS.
Aldosterone mediates its genomic effects by regulation of several target genes, eg, K-ras2 and serum glucocorticoid kinase,1 and, in addition, has several rapid actions. In rat and rabbit vascular smooth muscle, aldosterone activates Na+-H+ exchange, enhances IP3 formation, and increases intracellular calcium concentration within minutes.15,17,18 Based on the rapid effect and the lack of effect of transcriptional inhibitors, it has been argued that these responses are nongenomic. It is not entirely clear whether the nongenomic responses of aldosterone in smooth muscle involve MR.17,19 The present effect of aldosterone on vasoreactivity was nongenomic, because it was significant already after 5 minutes and was completely resistant to actinomycin D. Actinomycin D converted the transient response to aldosterone into a persistent, NO-dependent, vasorelaxant response. Thus, in microvessels, aldosterone initiates gene transcription that counteracts the putative vasorelaxant effect. The inhibitory effect of spironolactone and the lack of effect of the GR antagonist mifepristone are compatible with an MR-mediated effect of aldosterone in the afferent arteriole. In accordance with this notion, MR was detected at the mRNA and protein level in preglomerular vessels and in isolated aortic endothelial cells. MR has previously been shown in heart and vessels20 but not in the microvasculature. The enzyme 11ßHSD-2 confers specificity to aldosterone-sensitive tissues by metabolizing glucocorticoids to inactive derivatives, and thereby it prevents illicit MR activation. 11ßHSD-2 mRNA and protein was associated with renal resistance vessels and thereby provided the molecular basis for a selective in vivo action of aldosterone on microvessels, as seen in human arteries.19 On the other hand, 11ßHSD-2 activity in cardiovascular tissue is several orders of magnitude lower than in the kidney-collecting duct.19,20 A low level of 11ßHSD-2 activity may become overwhelmed by glucocorticoids and may explain our observation that also corticosterone affected vasoreactivity. The present data demonstrate the existence of a functional MR pathway in afferent arterioles, but the relative contribution of aldosterone or glucocorticoid to MR activation in resistance vessels needs to be determined.
The mechanism by which aldosterone lowers vasoreactivity after depolarization did not include a change in the K+-mediated global rise of intracellular calcium concentration in the smooth muscle cells of the afferent arterioles and thereby suggests a calcium-independent pathway for the effect. PI-3 kinase is a downstream target for activated MR, which subsequently activates protein kinase Akt (PKB). PKB phosphorylates and activates eNOS, thereby stimulating NO production21 and blood flow.22,23 High-dose corticosteroids lead to GR-mediated activation of eNOS and vasorelaxation through this nongenomic pathway,24,23 and the same pathway is responsible for estrogen-induced vasodilation in rat aortic rings.25 In cultured endothelial cells, high doses of dexamethasone did not activate PI-3 kinase through MR,23 but in sodium-transporting A6 cells, aldosterone did activate PI-3 kinase.26 The present data suggest a similar pathway in afferent arterioles. Thus, PI-3 kinase p110 catalytic subunit and phosphorylated PKB were observed in endothelial cells, a PI-3 kinase inhibitor abolished the effect of aldosterone on vasoreactivity, and inhibition of NO synthesis or NO targets abolished aldosterone-mediated effects on vasoreactivity, whereas the NO donor sodium nitroprusside mimicked the effect of aldosterone on K+-mediated vasoconstriction. In its nonliganded state, MR binds two hsp90 molecules.27 The dissociation of hsp90 from MR on binding of aldosterone may assist in stimulation of eNOS, as reported for estrogens,25 because hsp90 enhances the activation of eNOS.28 Inhibition of the vasorelaxant effect of aldosterone by a hsp90 blocker is consistent involvement of this pathway in the response.
In conclusion, our data indicate that aldosterone inhibits vasoconstriction in renal afferent arterioles by a nongenomic effect, which is initiated by MR activation and is calcium independent. The data are compatible with an inhibitory pathway that involves activation of endothelial PI-3 kinase, Akt, and NOS, leading to NO formation. In vascular smooth muscle, aldosterone phosphorylates extracellular signal–regulated kinase (ERK) 1 and ERK2.29 However, studies on aortic endothelial cells do not support a contribution of the ERK1/ERK2 pathway to eNOS phosphorylation.30
We are not aware of studies on the interaction between aldosterone and NO in the acute regulation of renal blood flow at the integrated level, but chronic aldosterone excess is associated with increases in blood pressure, renal blood flow, and glomerular filtration rate. In conscious dogs, infusion of aldosterone caused an NO-dependent increase in renal blood flow of 15%, of glomerular filtration rate by 20%, and of urinary nitrate/nitrite excretion by 60%, whereas the ability of aldosterone to induce sodium retention and increase blood pressure was unchanged.31 These data are consistent with our in vitro results. Only little is known about the effect of aldosterone on whole-body control of vascular resistance in humans. Wehling and colleagues16,32 reported an increase in systemic vascular resistance within 5 minutes after acute infusion of aldosterone in human volunteers, which was followed by vasodilation. Aldosterone-induced release of NO could provide some of the explanation for this dissipation of the vasoconstrictor response. In healthy individuals, ingestion of a low-sodium diet leads to an increase in the circulating concentration of angiotensin II and aldosterone, which is not associated with detrimental effects on the cardiovascular system. On the other hand, in several animal models with endothelial dysfunction, aldosterone plays a major role for pathological vascular responses.33–35 The low concentrations of aldosterone necessary to initiate vasorelaxation suggest a tonic effect of aldosterone on vascular reactivity. We propose that in healthy individuals with a functioning NO system, the detrimental effects of aldosterone on cardiovascular function are balanced by activation of the potentially beneficial effect of NO. On the other hand, in situations with endothelial dysfunction, such as congestive heart failure and hypertension, the negative effects of aldosterone are unopposed and inhibition of aldosterone is warranted.
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Acknowledgments |
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This work was supported by grants from the Danish Medical Research Council (9601829, 9902742, and 9903058), the Novo Nordisk Foundation, the Danish Heart Foundation (02-1-2-28A-22983, 01-2-1-61A-22939, 99223622743, 01123022896, and 02-1-2-33A-22982), the Danish Medical Association Research Fund, and Kønig Petersen’s Foundation. The technical assistance of Mette Fredenslund, Karin Kejling, and Inge Andersen is gratefully acknowledged. We thank Anthony M. Carter for language revision and Peter Ottosen for help with photographs.
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Footnotes |
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Original received September 3, 2002; first resubmission received April 22, 2003; second resubmission received September 2, 2003; revised resubmission received October 6, 2003; accepted October 30, 2003.
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