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(Circulation. 2001;104:2391.)
© 2001 American Heart Association, Inc.
Brief Rapid Communications |
From the Cardiovascular Research Center and Cardiology Division, Massachusetts General Hospital and Harvard Medical School, Boston, Mass.
Correspondence to Paul L. Huang, MD, PhD, Cardiovascular Research Center, Massachusetts General Hospital East, 149 East 13th Street, Charlestown, MA 02129. E-mail HuangP{at}helix.mgh.harvard.edu
| Abstract |
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Methods and Results Male apoE/eNOS DKO mice were treated with hydralazine in their drinking water (250 mg/L) using a dose that lowers the blood pressure to levels seen in apoE KO mice. The mice were fed a Western-type diet for 16 weeks, and lesion formation was assessed by inspection of the vessel and staining with Sudan IV. Hydralazine-treated, normotensive male apoE/eNOS DKO mice developed increased aortic lesion areas (30.0±2.8%, n=11) compared with male apoE KO mice (14.6±0.8%, n=7). The extent of lesion formation was not significantly different from male apoE/eNOS DKO mice that were not given hydralazine (28.3±3.1%, n=9). Four of 11 hydralazine-treated male apoE/eNOS DKO mice developed abdominal aortic aneurysms.
Conclusions Hypertension is not required for the accelerated atherosclerosis seen in apoE/eNOS DKO animals, and control of hypertension during a 16-week period does not prevent aortic aneurysm formation.
Key Words: arteriosclerosis nitric oxide synthase aneurysm hypertension hydralazine
| Introduction |
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Because eNOS deficiency is associated with hypertension, both by itself3 and in the presence5 of apoE deficiency, it is possible that hypertension contributes to the increased extent of atherosclerotic lesions and the development of aortic aneurysms. To test this hypothesis, we treated male apoE/eNOS DKO mice with hydralazine at a dose that lowers the blood pressure down to the levels seen in apoE KO mice. Hydralazine-treated, normotensive, apoE/eNOS DKO mice still developed increased atherosclerotic lesions compared with apoE KO mice. Hydralazine did not decrease the extent of lesion formation, because lesion areas were comparable between untreated and hydralazine-treated animals. Furthermore, 4 of 11 male mice treated with hydralazine developed abdominal aortic aneurysms, demonstrating that hypertension is not required for the development of aortic aneurysms in the apoE/eNOS DKO mouse model. These findings suggest that the effects of eNOS deficiency extend beyond hypertension and argue that eNOS plays important roles in suppressing atherogenesis separate from blood pressure regulation.
| Methods |
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Blood Pressure Measurement and Hydralazine Treatment
Invasive blood pressure was measured by femoral artery catheterization using a Millar catheter.7 Hydralazine was added to the drinking water at 250 mg/L. Pilot experiments indicated that this dose of hydralazine reduces the blood pressure of eNOS KO mice and apoE/eNOS DKO mice to levels seen in apoE KO mice. The blood pressure of each treated apoE/eNOS DKO mouse was measured immediately before euthanization after 16 weeks of treatment on a Western-type diet.
Lesion Assessment
Animals were euthanized and perfused with PBS (pH 7.4). The aorta was opened longitudinally, and video images were captured using a dissecting microscope. To identify lipid-rich intraluminal lesions, the aortas were stained with Sudan IV. Image analysis was performed using Image Pro Plus (Version 3.0.1; Media Cybernetics). The amount of lesion formation in each animal was expressed as percent lesion area per total area of the aorta. Aortic aneurysms were defined according to the criteria of the Society for Vascular Surgery, as an increase in vessel diameter of >50% over that of the proximal, adjacent, undilated segment.
Statistical Analysis
Statistical analysis was performed using StatView 4.51 (Abacus Concepts, Inc). ANOVA with Scheffes F test for post hoc comparison was used to compare the results from the different groups. P<0.05 was considered significant.
| Results |
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Despite the lowering of blood pressure, the hydralazine-treated apoE/eNOS DKO mice still had increased lesion areas compared with apoE KO mice. In fact, hydralazine treatment did not significantly affect the extent of atherosclerosis in the apoE/eNOS DKO mice, as shown in the Table. The hydralazine-treated apoE/eNOS DKO mice developed lesions with areas of 30.0±2.8% after 16 weeks, whereas the untreated apoE/eNOS DKO mice developed lesion areas of 28.3±3.1% after 16 weeks. Furthermore, the percent lesion area did not increase with mean blood pressure, as shown in Figure 1.
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Four of 11 male mice in this study developed abdominal aortic aneurysms, which were defined as an increase in aortic luminal diameter to >50% of the proximal segment, as seen in Figure 2. In untreated apoE/eNOS DKO mice after 16 weeks of a Western-type diet, 5 of 12 male mice developed aortic aneurysms; 2 additional mice had aortic dissection.5
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| Discussion |
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Treatment with hydralazine lowered blood pressure in the apoE/eNOS DKO mice, but this did not affect the accelerated development of atherosclerosis in the animals. Lesion areas were the same in both hydralazine-treated and untreated groups. Furthermore, 4 of 11 male hydralazine-treated apoE/eNOS DKO mice developed aortic aneurysms compared with 5 of 12 untreated male apoE/eNOS DKO mice. These differences are not statistically significant.
Knowles et al6 also bred apoE/eNOS DKO mice to examine the role of eNOS deficiency in atherogenesis. Despite important differences between their study and our previous one,5 in both cases, apoE/eNOS DKO mice developed significantly greater atherosclerotic lesion areas than control apoE KO mice, confirming a role for eNOS in the suppression of atherogenesis. Knowles et al6 found that enalapril lowered the blood pressure of apoE/eNOS DKO mice and reduced the development of lesions.6 This indicates that the mechanism of enalapril action does not depend on eNOS. However, because of the intrinsic effects of ACE inhibitors on atherogenesis, enalapril treatment does not answer the question of whether the increase in lesion area seen in apoE/eNOS DKO animals is due to their increased blood pressure.
Aside from its potent neuroendocrine action, angiotensin II also has paracrine and autocrine vascular effects, including smooth muscle cell growth and migration, macrophage activation, platelet aggregation, and increased oxidative stress.9,10 Thus, the difference in outcome after treatment with hydralazine and enalapril may reflect the effects of enalapril itself on atherogenesis. Similar effects of ACE inhibitors that are independent of blood pressure lowering have also been noted in other studies.11 Furthermore, a direct comparison of hydralazine and lisinopril at doses that lower blood pressure equally also shows additional effects of ACE inhibitors unrelated to blood pressure effects.12
In another study, Kauser et al13 reported that apoE KO mice treated with L-N-arginine methyl ester (L-NAME) at doses that do not raise blood pressure still have increased lesion areas. These results agree with ours in that the predominant effect of NOS inhibition on atherogenesis seems independent of blood pressure.
Aneurysm formation is thought to occur by a weakening of the media by tissue remodeling and local changes in the vessel wall. Hypertension may play an additional role in terms of mechanical forces and shear stress. However, the presence of aneurysms in hydralazine-treated male apoE/eNOS DKO mice suggest that the inherent changes in the vessel wall that predispose to aneurysm formation can occur independent of hypertension.
Our results indicate that the effects of eNOS gene deficiency on accelerating atherogenesis are not solely due to hypertension, in that lowering the blood pressure to normal in the apoE/eNOS DKO mice did not reduce the lesion area. Furthermore, control of blood pressure in these animals does not prevent aneurysm formation. Overall, these results suggest the existence of blood pressureindependent mechanisms by which eNOS deficiency accelerates atherogenesis. These may include well-established effects of NO on vascular smooth muscle proliferation, leukocyte-endothelial interactions, and modulation of platelet aggregation.14,15
| Acknowledgments |
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| Footnotes |
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Received August 7, 2001; revision received September 20, 2001; accepted September 21, 2001.
| References |
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2.
Nakashima Y, Plump AS, Raines EW, et al. ApoE-deficient mice develop lesions of all phases of atherosclerosis throughout the arterial tree. Arterioscler Thromb. 1994; 14: 133140.
3. Huang PL, Huang Z, Mashimo H, et al. Hypertension in mice lacking the gene for endothelial nitric oxide synthase. Nature. 1995; 377: 239242.[Medline] [Order article via Infotrieve]
4.
Shesely EG, Maeda N, Kim HS, et al. Elevated blood pressures in mice lacking endothelial nitric oxide synthase. Proc Natl Acad Sci U S A. 1996; 93: 1317613181.
5.
Kuhlencordt PJ, Gyurko R, Han F, et al. Accelerated atherosclerosis, aortic aneurysm formation, and ischemic heart disease in apolipoprotein E/endothelial nitric oxide synthase double-knockout mice. Circulation. 2001; 104: 448454.
6. Knowles JW, Reddick RL, Jennette JC, et al. Enhanced atherosclerosis and kidney dysfunction in eNOS(-/-)Apoe(-/-) mice are ameliorated by enalapril treatment. J Clin Invest. 2000; 105: 451458.[Medline] [Order article via Infotrieve]
7. Gyurko R, Kuhlencordt P, Fishman MC, et al. Modulation of mouse cardiac function in vivo by eNOS and ANP. Am J Physiol. 2000; 278: H971H981.
8.
Yang R, Powell-Braxton L, Ogaoawara AK, et al. Hypertension and endothelial dysfunction in apolipoprotein E knockout mice. Arterioscler Thromb Vasc Biol. 1999; 19: 27622768.
9.
OKeefe JH, Wetzel M, Moe RR, et al. Should an angiotensin-converting enzyme inhibitor be standard therapy for patients with atherosclerotic disease? J Am Coll Cardiol. 2001; 37: 18.
10. Weir MR, Dzau VJ. The renin-angiotensin-aldosterone system: a specific target for hypertension management. Am J Hypertens. 1999; 12: 205S213S.[Medline] [Order article via Infotrieve]
11.
Hayek T, Attias J, Coleman R, et al. The angiotensin-converting enzyme inhibitor, fosinopril, and the angiotensin II receptor antagonist, losartan, inhibit LDL oxidation and attenuate atherosclerosis independent of lowering blood pressure in apolipoprotein E deficient mice. Cardiovasc Res. 1999; 44: 579587.
12. Kai T, Ishikawa K. Lisinopril reduces left ventricular hypertrophy and cardiac polyamine concentrations without a reduction in left ventricular wall stress in transgenic Tsukuba hypertensive mice. Hypertens Res. 2000; 23: 625631.[Medline] [Order article via Infotrieve]
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Kauser K, da Cunha V, Fitch R, et al. Role of endogenous nitric oxide in progression of atherosclerosis in apolipoprotein E-deficient mice. Am J Physiol. 2000; 278: H1679H1685.
14. Mooradian DL, Hutsell TC, Keefer LK. Nitric oxide (NO) donor molecules: effect of NO release rate on vascular smooth muscle cell proliferation in vitro. J Cardiovasc Pharmacol. 1995; 25: 674678.[Medline] [Order article via Infotrieve]
15. Lefer AM, Ma XL, Weyrich A, et al. Endothelial dysfunction and neutrophil adherence as critical events in the development of reperfusion injury. Agents Actions Suppl. 1993; 41: 127135.[Medline] [Order article via Infotrieve]
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