(Hypertension. 2000;35:595.)
© 2000 American Heart Association, Inc.
Scientific Contributions |
From the Departments of Internal Medicine and Pharmacology, Cardiovascular Center, University of Iowa College of Medicine, Iowa City.
Correspondence to Donald D. Heistad, MD, Department of Internal Medicine, University of Iowa College of Medicine, Iowa City, IA 52242. E-mail donald-heistad{at}uiowa.edu
| Abstract |
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Key Words: angiotensin II gene transfer free radicals enzymes acetylcholine
| Introduction |
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In atherosclerosis6 and some forms of hypertension, levels of angiotensin II (Ang II) may be increased. Ang II reportedly impairs vascular relaxation, in part, by increasing the production of superoxide anion via a membrane-bound NADPH oxidase.7 In Ang IIinduced hypertension in rats, the impairment of vasodilator responses to acetylcholine and calcium ionophore is improved by treatment with liposome-encapsulated SOD.8 On the basis of these observations, we hypothesized that the gene transfer of SOD to vessels after treatment with Ang II would improve vascular relaxation in a manner similar to the effects of SOD in normal vessels.9
In previous studies, the generation of superoxide by Ang II in blood vessels was measured using lucigenin-enhanced chemiluminescence.8 10 Because lucigenin primarily detects superoxide from the endothelium and adventitia,11 it has been assumed that these tissues are the source of the superoxide in Ang IItreated animals. In cell culture, however, Ang II also increases the smooth muscle cell generation of superoxide.7 In this study, we used hydroethidine to test the hypothesis that Ang II may also increase superoxide levels in the smooth muscle cells of vessels from Ang IItreated animals.
If superoxide levels are elevated in vessels after Ang II treatment, the gene transfer of eNOS might not improve NO-mediated dilation, because superoxide can react with NO to produce the potent oxidant peroxynitrite.12 However, because NO avidly binds to superoxide,13 the overexpression of NO through the gene transfer of eNOS might augment relaxation by increasing the bioavailability of NO. Therefore, we also examined the effects of the gene transfer of eNOS in Ang IIinduced vasomotor dysfunction to test the hypothesis that increases in eNOS would be sufficient to augment vascular relaxation.
| Methods |
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Adenoviral Vectors
Four different recombinant adenoviruses were used for gene
transfer. (1) a recombinant adenovirus driven by a cytomegalovirus
(cmv) promoter (AdCMVß-gal) carried the reporter gene for
ß-galactosidase and was used as a control virus; (2) AdCMVCuZnSOD (a
gift from Dr John Engelhardt, University of Iowa, Iowa City) contained
cDNA for human copper zinc SOD (CuZnSOD); (3) AdCMVECSOD carried
cDNA for human extracellular SOD (a gift from Dr James D. Crapo,
National Jewish Medical and Research Center, Denver, Colo); and
(4) AdCMVeNOS carried cDNA for bovine eNOS (a gift from Dr David
Harrison, Emory University, Atlanta, Ga). The adenoviral vectors were
constructed by the University of Iowa Gene Transfer Vector Core using a
method described previously.14 PBS containing 3%
sucrose was used as a virus-free control.
Transduction of Arteries
After 7 days of infusion with Ang II or saline, the rabbits were
anesthetized, and blood pressure was measured. The rabbits were
killed by an overdose of pentobarbital, and the thoracic aorta was
removed. Vessels were placed in cold Krebs bicarbonate solution with
the following composition (in mmol/L): NaCl 118, KCl 4.7,
KH2PO4 1.2,
MgSO47H2O 1.2,
D-glucose 11.1, NaHCO3 25.0, and
CaCl2H2O 2.54. Loose
connective tissue was removed, and the vessels were cut into rings
3 mm in length.
Segments of the thoracic aorta were incubated at 37°C in 150 µL of viral suspension containing 1010 plaque-forming units/mL of virus (either AdCMVß-gal, AdCMVCuZnSOD, AdCMVECSOD, or AdCMVeNOS) or vehicle. After 2 hours, viral suspension or vehicle was removed. Rings were rinsed with PBS and incubated with minimum essential medium containing 100 U/mL penicillin and 100 µg/mL streptomycin at 37°C. The gas in the incubator was 5% CO2 and 95% room air. We did not bubble oxygen through the solution at the time of transduction or subsequent incubation. After 24 hours, rings were removed and used for oxidative fluorescent staining, the detection of superoxide by lucigenin-enhanced chemiluminescence, or the recording of isometric tension in the organ chambers.
Detection of Superoxide
Production of superoxide in aortas was measured by the
response to lucigenin-enhanced chemiluminescence, as described
previously.15 Vascular rings were placed in a
polypropylene tube containing 1 mL PBS and lucigenin (250 µmol/L
in 14 samples and 5 µmol/L in 3 samples). The tube was
placed in a Monolight 2010 luminometer. The luminometer reported
relative light units emitted, which were integrated over 5 minutes.
Dark current readings (photomultiplier background signal) were
automatically subtracted. Background counts, which were determined from
vessel-free preparations, were subtracted from the readings obtained
with vessels. In some rings, superoxide generation was measured after
vessels were preincubated with polyethylene-glycolated (PEG) SOD (250
U/mL) for 30 minutes. The surface area of the lumen was imaged with a
video camera; it was calculated with National Institutes of
Health image software for each vascular segment to normalize superoxide
levels.
Oxidative Fluorescent Microtopography
The oxidative fluorescent dye hydroethidine was used to
evaluate the in situ concentration of superoxide, as described
previously.11 Hydroethidine is freely permeable to cells
and, in the presence of superoxide, is oxidized to ethidium bromide
(EtBr), where it is trapped by intercalation with DNA. EtBr is excited
at 488 nm and has an emission spectrum of 610 nm. In cell-free assays,
hydrogen peroxide, NO, and peroxynitrite do not react with
hydroethidine to increase EtBr fluorescence. Unfixed frozen
rings of aortic segments were cut into 30-µm-thick sections and
placed on a glass slide. Hydroethidine (2x10-6
mol/L) was topically applied to each tissue section and coverslipped.
Slides were incubated in a light-protected, humidified chamber at
37°C for 30 minutes. Images were obtained with a Bio-Rad MRC-1024
laser scanning confocal microscope equipped with a krypton/argon laser.
Laser settings were identical for the acquisition of images from all
samples. Fluorescence was detected with a 585-nm long-pass
filter. Sections were subsequently stained with nuclear fast red.
First, images of aortas from rabbits that were treated with saline were
measured. After adjusting the basal settings of the confocal
microscope, images of aortas from rabbits that were infused with Ang II
were measured.
Vasomotor Function
Aortic rings were suspended in organ chambers (bubbled with 95%
O2 and 5% CO2 at 37°C)
containing Krebs bicarbonate solution to measure isometric tension.
Rings were progressively stretched to 8g of resting
tension, which was determined by repeated stimulation with KCl
(100 mmol/L) to be the optimal tension for the vessels. Vessels
were allowed to equilibrate for 30 minutes; then, they were contracted
twice with 100 mmol/L KCl.
After the vessels were rinsed, their responses to
phenylephrine, the endothelium-dependent
dilator acetylcholine, or the endothelium-independent
dilator sodium nitroprusside were determined. In some rings, responses
to acetylcholine were also determined after a 30-minute incubation in
N
-nitro-L-arginine (L-NNA,
10-4 mol/L), an inhibitor of NO
synthase, or a 60-minute incubation in PEG-SOD (100 U/mL).
Data Analysis
Results are expressed as mean±SE. Relaxations are the percent
change from the precontracted tension. Data were analyzed with
a 1 way ANOVA with post hoc Bonferroni correction.
Chemicals
Acetylcholine chloride, L-phenylephrine
hydrochloride, sodium nitroprusside, L-NNA, PEG-SOD, and lucigenin were
obtained from Sigma and dissolved in PBS. Hydroethidine was obtained
from Molecular Probes, Inc, and suspended in dimethyl sulfoxide at a
concentration of 10-2 mol/L, where it was stored
in aliquots at -80°C until use. Subsequent dilution was performed
in PBS.
| Results |
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Vascular Superoxide Levels After Infusion of Ang II
We used 2 methods to detect superoxide. Levels of superoxide
detected with lucigenin were 2.5 times higher in vessels from rabbits
that received an infusion of Ang II (n=9) than those that received
saline (n=4) (28±11 versus 11±3 relative light units per minute per
area; P<0.05). Addition of exogenous PEG-SOD (250 U/mL)
(n=5) significantly reduced superoxide levels in vessels from rabbits
that received Ang II (Figure 1).
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After 24 hours of incubation in minimum essential medium at 37°C, superoxide levels in vessels from rabbits treated with Ang II (n=4) were 1.7-fold higher than those in control animals (n=4). When measured with a low concentration of lucigenin (5 µmol/L) to avoid the potential autoxidation of lucigenin,16 17 superoxide levels were also increased 1.7-fold in vessels from rabbits treated with Ang II (n=3) compared with controls.
After incubation with hydroethidine, a marked increase in EtBr fluorescence occurred in the aortas from rabbits that received Ang II, which reflected an increase in superoxide (n=3). EtBr fluorescence was most prominent in the endothelium and adventitia, but it was also detected in smooth muscle cells (Figure 2).
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Effects of Gene Transfer of eNOS, CuZnSOD, and Extracellular
SOD on Vascular Responses
Contractile responses to phenylephrine tended to be
enhanced in the aortas from rabbits that received Ang II, but these
differences were not significant when compared with control rabbits
(Figure 3A). The gene transfer of ß-gal
or eNOS did not alter the contraction to phenylephrine
(Figure 3A).
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Maximal relaxation to acetylcholine in aortas from rabbits that received Ang II was impaired compared with vessels from rabbits that received saline (72±5% in Ang IItreated animals versus 87±4% in controls, P<0.01; EC50, 6.77±0.07 log(mol/L) in Ang IItreated animals versus 7.17±0.15 in controls, P<0.05; Figure 3B). Gene transfer of eNOS, but not ß-gal, normalized relaxation to acetylcholine in aortas from Ang IItreated rabbits (82±3% in Ang IItreated rabbits versus 7.25±0.08 in controls for eNOS; 67±4% in Ang IItreated rabbits [P<0.05 versus eNOS] versus 6.82±0.04 in controls [P<0.05 versus eNOS] for ß-gal; Figure 3B).
L-NNA abolished responses to acetylcholine before and after the gene transfer of eNOS (Figure 3B). Relaxation in response to sodium nitroprusside was similar in rabbits that were infused with saline or Ang II and treated with adenovirus containing the gene for ß-gal (Adß-gal), adenovirus containing the gene for eNOS (AdeNOS), and vehicle (Figure 3C).
Gene transfer of CuZnSOD or extracellular SOD (ECSOD) to aortas from rabbits that received Ang II had no effect on responses to acetylcholine (maximal response, 73±6%; EC50, 6.90±0.10 for CuZnSOD [Figure 4A] and maximal response, 66±8%; EC50, 6.80±0.12 for ECSOD [Figure 4B]).
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PEG-SOD augmented the response to acetylcholine in aortas from rabbits that received Ang II (Figure 5). The effects of PEG-SOD treatment were smaller than those of AdeNOS.
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| Discussion |
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We chose rabbits, rather than rats, in this study because our preliminary experiments showed that the gene transfer of an adenovirus is more efficient in vessels from rabbits than rats. After the infusion of Ang II, the mean blood pressure increased to 107 mm Hg. The magnitude of the elevation of pressure was similar to that from a previous report.18 Compared with the blood pressure of rats that received Ang II (mean, 148 mm Hg),19 however, the magnitude of the blood pressure increase in rabbits was small. Nevertheless, a sustained infusion of Ang II in rabbits produced an impairment in relaxation to acetylcholine.
Lucigenin-enhanced chemiluminescence has been widely used to detect superoxide. Recent studies, however, indicated that the reduced form of lucigenin has the potential for autoxidation, with subsequent generation of superoxide.16 17 The ratio of vascular superoxide production in different groups of rabbits, however, does not depend on the lucigenin concentration that is used.17 Furthermore, we compared the ratio of vascular superoxide production using 5 µmol/L and 250 µmol/L of lucigenin. The ratio of superoxide production in aortas from Ang IItreated rabbits and saline-treated rabbits was similar (1.7) using either concentration of lucigenin. Thus, as suggested previously,17 lucigenin chemiluminescence seems to be useful for comparing relative levels of superoxide in rings from the same vessel that are exposed to different treatments.
We observed, by 2 independent methods (lucigenin and hydroethidine), an increase in the vascular levels of superoxide produced by Ang II. This finding is consistent with previous reports.8 10 Activation of an NADPH oxidase is one of the mechanisms of increasing superoxide levels by Ang II.8 This effect of Ang II may not require hypertension per se because norepinephrine, which produces similar increases in blood pressure, does not affect vascular superoxide production.8 A recent study demonstrated that NADPH oxidase is present in endothelial,20 adventitial,21 and smooth-muscle cells.7 We used hydroethidine to determine the location of superoxide after Ang II. Superoxide was increased in the endothelium and adventitia after the infusion of Ang II, as it was, albeit to a lesser extent, in smooth-muscle cells. Furthermore, the application of exogenous SOD, such as PEG-SOD, significantly decreased superoxide concentrations in the aortas from rabbits that were infused with Ang II. These findings led us to determine whether the gene transfer of SOD might reduce superoxide levels and improve vascular function.
CuZnSOD is an isoform of SOD that is present in the cytoplasm and
is involved in the dismutation of superoxide within
cells.22 ECSOD is the only isoform of SOD that is released
from cells into the extracellular space.23 Although the
concentration of ECSOD is small in most tissues, blood vessels have a
large amount of ECSOD.24 Thus, ECSOD may play an important
role in the regulation of vascular tone. We have reported that, after
the in vitro gene transfer of ß-gal to normal rabbit aortas,
10%
of endothelial cells and 25% of adventitial cells
demonstrate histochemical staining for
ß-galactosidase.25 Similarly, after the
adenoviral-mediated gene transfer of CuZnSOD and ECSOD, overexpression
of transgene is confined to the endothelium and
adventitia.9
Gene transfer of CuZnSOD and ECSOD to the carotid arteries of normal rabbits significantly increases SOD activity (2.8 times and 2 times more than ß-gal, respectively), and the response to acetylcholine in the carotid arteries from normal rabbits is augmented after the gene transfer of CuZnSOD.9 Neither CuZnSOD nor ECSOD, however, improved the vasomotor dysfunction in rabbits that received Ang II in this study. One possibility is that, because the gene transfer to vessels is limited to the endothelium and adventitia, SOD can dismutate superoxide in the endothelium and adventitia of the vessel wall, but not in the media. Findings with hydroethidine indicated that the media, as well as the endothelium and adventitia, of aortas from rabbits that received Ang II generated superoxide. The finding that the gene transfer of SOD does not improve responses to acetylcholine suggests that the superoxide generated by the media, which is not transduced by the gene transfer of SOD, may play a critical role in the impaired responses to acetylcholine. These findings are similar to those in atherosclerotic rabbits in which superoxide levels were increased throughout the aortic wall but in which the gene transfer of CuZnSOD or ECSOD failed to improve relaxation to acetylcholine.11 In contrast, PEG-SOD augmented the response to acetylcholine. It is likely that PEG-SOD is more effective than the gene transfer of CuZnSOD and ECSOD in improving responses to acetylcholine in aortas from Ang IItreated rabbits because PEG-SOD penetrates the entire vascular wall.
In contrast to the effects of the gene transfer of CuZnSOD and ECSOD, the gene transfer of eNOS greatly improved responses to acetylcholine in aortas from rabbits that were treated with Ang II. The augmented response is mediated by NO; L-NNA completely blocks the relaxation. Several reports indicate that the gene transfer of eNOS to normal vessels augments the response to acetylcholine.14 26 Regarding the gene transfer of eNOS, we previously confirmed the efficacy of the staining for eNOS using immunocytochemistry in carotid artery from normal rabbits that were transduced with eNOS.14 Gene transfer of eNOS to the adventitia of carotid arteries from rabbits is sufficient to augment the response to calcium ionophore.14 It is of interest that eNOS improved responses to acetylcholine, although SOD failed to improve responses. We speculate that the gene transfer of eNOS may be more effective than that of SOD because the reaction of NO with superoxide is more rapid than the response of SOD with superoxide13 and because the NO generated by the gene transfer of eNOS may diffuse more efficiently than SOD through the wall of the aorta to decrease superoxide concentrations.
Recent studies indicate that Ang II stimulates the synthesis of eNOS and induces NO production in the vascular endothelium.27 This effect of Ang II on endothelial cells may attenuate its contractile effect on smooth muscle cells.28 Attenuation of responses to acetylcholine after the infusion of Ang II in this and other studies,8 despite the increased synthesis of eNOS,27 emphasizes the effectiveness of the superoxide that is generated by Ang II in attenuating responses to acetylcholine.
Another possibility for the improvement of Ang II-induced vasomotor dysfunction by gene transfer of eNOS relates to the finding that Ang II increases the production of endothelin.29 Endothelin generally produces vasoconstriction by activating endothelin A receptors. Endothelin also stimulates the generation of NO through the activation of endothelin B receptors30 ; NO then modulates vasoconstriction by an autocrine feedback mechanism.31 Thus, we speculate that excess generation of NO by the gene transfer of eNOS may reduce Ang II-induced endothelial dysfunction, both by augmenting NO levels and attenuating the endothelin-mediated constrictor effect.
It seems that the primary mechanism by which Ang II produces endothelial dysfunction is by generating superoxide, perhaps via NADPH oxidase,8 rather than by elevating arterial pressure.10 Nevertheless, we cannot exclude the possibility that increases in arterial pressure produced by Ang II may contribute to endothelial dysfunction. For example, it is possible that the elevation of arterial pressure by Ang II may impair eNOS, and the gene transfer of eNOS may then improve endothelial function.
In conclusion, Ang II increased levels of superoxide throughout the wall of the aorta. Gene transfer of CuZnSOD and ECSOD did not improve vasomotor dysfunction in aortas from rabbits that received Ang II. Gene transfer of eNOS, however, restored the response to acetylcholine. We speculate that the NO produced by the gene transfer of eNOS may decrease superoxide in aortas and improve the vasomotor dysfunction produced by angiotensin II.
| Acknowledgments |
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Received June 20, 1999; first decision August 19, 1999; accepted September 29, 1999.
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D. M. Tham, B. Martin-McNulty, Y.-X. Wang, V. Da Cunha, D. W. Wilson, C. N. Athanassious, A. F. Powers, M. E. Sullivan, and J. C. Rutledge Angiotensin II injures the arterial wall causing increased aortic stiffening in apolipoprotein E-deficient mice Am J Physiol Regulatory Integrative Comp Physiol, December 1, 2002; 283(6): R1442 - R1449. [Abstract] [Full Text] [PDF] |
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S. P. Didion, M. J. Ryan, L. A. Didion, P. E. Fegan, C. D. Sigmund, and F. M. Faraci Increased Superoxide and Vascular Dysfunction in CuZnSOD-Deficient Mice Circ. Res., November 15, 2002; 91(10): 938 - 944. [Abstract] [Full Text] [PDF] |
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S. P. Didion, M. J. Ryan, G. L. Baumbach, C. D. Sigmund, and F. M. Faraci Superoxide contributes to vascular dysfunction in mice that express human renin and angiotensinogen Am J Physiol Heart Circ Physiol, October 1, 2002; 283(4): H1569 - H1576. [Abstract] [Full Text] [PDF] |
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L Van Heerebeek, C Meischl, W Stooker, C J L M Meijer, H W M Niessen, and D Roos NADPH oxidase(s): new source(s) of reactive oxygen species in the vascular system? J. Clin. Pathol., August 1, 2002; 55(8): 561 - 568. [Abstract] [Full Text] [PDF] |
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T. J. Guzik, N. E.J. West, R. Pillai, D. P. Taggart, and K. M. Channon Nitric Oxide Modulates Superoxide Release and Peroxynitrite Formation in Human Blood Vessels Hypertension, June 1, 2002; 39(6): 1088 - 1094. [Abstract] [Full Text] [PDF] |
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L. Li, E. Crockett, D. H. Wang, J. J. Galligan, G. D. Fink, and A. F. Chen Gene Transfer of Endothelial NO Synthase and Manganese Superoxide Dismutase on Arterial Vascular Cell Adhesion Molecule-1 Expression and Superoxide Production in Deoxycorticosterone Acetate-Salt Hypertension Arterioscler Thromb Vasc Biol, February 1, 2002; 22(2): 249 - 255. [Abstract] [Full Text] [PDF] |
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S. P. Didion, C. A. Hathaway, and F. M. Faraci Superoxide levels and function of cerebral blood vessels after inhibition of CuZn-SOD Am J Physiol Heart Circ Physiol, October 1, 2001; 281(4): H1697 - H1703. [Abstract] [Full Text] [PDF] |
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N. E.J. West, H. Qian, T. J. Guzik, E. Black, S. Cai, S. E. George, and K. M. Channon Nitric Oxide Synthase (nNOS) Gene Transfer Modifies Venous Bypass Graft Remodeling: Effects on Vascular Smooth Muscle Cell Differentiation and Superoxide Production Circulation, September 25, 2001; 104(13): 1526 - 1532. [Abstract] [Full Text] [PDF] |
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C. A. Gunnett, D. D. Lund, Y. Chu, R. M. Brooks II, F. M. Faraci, and D. D. Heistad NO-Dependent Vasorelaxation Is Impaired After Gene Transfer of Inducible NO-Synthase Arterioscler Thromb Vasc Biol, August 1, 2001; 21(8): 1281 - 1287. [Abstract] [Full Text] [PDF] |
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M.-S. Zhou, A. Adam, and L. Raij Review: Interaction among angiotensin II, nitric oxide and oxidative stress Journal of Renin-Angiotensin-Aldosterone System, March 1, 2001; 2(1_suppl): S59 - S63. [PDF] |
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K. M. Channon, H. Qian, and S. E. George Nitric Oxide Synthase in Atherosclerosis and Vascular Injury : Insights From Experimental Gene Therapy Arterioscler Thromb Vasc Biol, August 1, 2000; 20(8): 1873 - 1881. [Abstract] [Full Text] [PDF] |
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S. P. Didion and F. M. Faraci Effects of NADH and NADPH on superoxide levels and cerebral vascular tone Am J Physiol Heart Circ Physiol, February 1, 2002; 282(2): H688 - H695. [Abstract] [Full Text] [PDF] |
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