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      Chronic Mild Hyperhomocysteinemia Induces Aortic Endothelial Dysfunction but Does Not Elevate Arterial Pressure in Rats

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          Abstract

          Mild hyperhomocysteinemia is prevalent in the general population and has been linked to endothelial dysfunction and high arterial pressure (AP) in clinical studies. The present study was designed to determine whether a rise in AP was induced by mild hyperhomocysteinemia and whether the potential rise in AP is secondary or prior to endothelial dysfunction. Experiments were performed in a rat model of mild hyperhomocysteinemia induced by oral administration of homocysteine for 1–4 months. Aortic endothelial dysfunction was observed 2 months after homocysteine treatment while endothelium-independent vasodilation was normal. In parallel, homocysteine treatment increased phenylephrine-induced contraction in aortas with endothelium, but did not modify the contraction in aortas without endothelium, suggesting a decrease of basal NO production. In conscious unrestrained rats, AP was not significantly different 1, 2, 3 and 4 months after homocysteine treatment. In correlation, endothelial function of a resistance vessel (mesenteric artery), mainly non-NO nonprostanoid factor mediated, was preserved, indicating that homocysteine treatment only affected the NO pathway. In conclusion, mild hyperhomocysteinemia alone is not sufficient to elevate arterial blood pressure, at least in the rat model. Aortic endothelial dysfunction produced by mild hyperhomocysteinemia is independent of hemodynamic factors.

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          Most cited references 28

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          Homocysteine and cardiovascular disease.

          An elevated level of total homocysteine (tHcy) in blood, denoted hyperhomocysteinemia, is emerging as a prevalent and strong risk factor for atherosclerotic vascular disease in the coronary, cerebral, and peripheral vessels, and for arterial and venous thromboembolism. The basis for these conclusions is data from about 80 clinical and epidemiological studies including more than 10,000 patients. Elevated tHcy confers a graded risk with no threshold, is independent of but may enhance the effect of the conventional risk factors, and seems to be a particularly strong predictor of cardiovascular mortality. Hyperhomocysteinemia is attributed to commonly occurring genetic and acquired factors including deficiencies of folate and vitamin B12. Supplementation with B-vitamins, in particular with folic acid, is an efficient, safe, and inexpensive means to reduce an elevated tHcy level. Studies are now in progress to establish whether such therapy will reduce cardiovascular risk.
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            Mice deficient in methylenetetrahydrofolate reductase exhibit hyperhomocysteinemia and decreased methylation capacity, with neuropathology and aortic lipid deposition.

            Hyperhomocysteinemia, a risk factor for cardiovascular disease, is caused by nutritional and/or genetic disruptions in homocysteine metabolism. The most common genetic cause of hyperhomocysteinemia is the 677C-->T mutation in the methylenetetrahydrofolate reductase (MTHFR) gene. This variant, with mild enzymatic deficiency, is associated with an increased risk for neural tube defects and pregnancy complications and with a decreased risk for colon cancer and leukemia. Although many studies have reported that this variant is also a risk factor for vascular disease, this area of investigation is still controversial. Severe MTHFR deficiency results in homocystinuria, an inborn error of metabolism with neurological and vascular complications. To investigate the in vivo pathogenetic mechanisms of MTHFR deficiency, we generated mice with a knockout of MTHFR: Plasma total homocysteine levels in heterozygous and homozygous knockout mice are 1.6- and 10-fold higher than those in wild-type littermates, respectively. Both heterozygous and homozygous knockouts have either significantly decreased S-adenosylmethionine levels or significantly increased S-adenosylhomocysteine levels, or both, with global DNA hypomethylation. The heterozygous knockout mice appear normal, whereas the homozygotes are smaller and show developmental retardation with cerebellar pathology. Abnormal lipid deposition in the proximal portion of the aorta was observed in older heterozygotes and homozygotes, alluding to an atherogenic effect of hyperhomocysteinemia in these mice.
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              The importance of the hyperpolarizing mechanism increases as the vessel size decreases in endothelium-dependent relaxations in rat mesenteric circulation.

              Endothelium-dependent relaxations are achieved by a combination of endothelium-derived prostacyclin (PGI2), nitric oxide (NO), and endothelium-derived hyperpolarizing factor (EDHF). However, it remains to be fully clarified whether the relative contribution of these three mechanisms to endothelium-dependent relaxations varies as a function of the vessel size. This study was designed to clarify this point. Acetylcholine (ACh)-induced endothelium-dependent relaxations were examined in isolated blood vessels taken from the aorta and the proximal and distal mesenteric arteries of the rat. The contributions of PGI2, NO, and EDHF were evaluated by the inhibitory effects of indomethacin, N omega-nitro-L-arginine methyl ester (L-NAME) in the presence of indomethacin, and KCl in the presence of indomethacin and L-NAME, respectively. The membrane potentials were recorded with microelectrodes. The expression of endothelial No synthase (eNOS) was examined by both immunostaining and immunoblotting. The contribution of PGI2 was negligible in three different-sized blood vessels. The contribution of NO was most prominent in the aorta, whereas that of EDHF was most prominent in the distal mesenteric arteries. The resting membrane potential was significantly deeper and the ACh-induced hyperpolarization was greater in the distal mesenteric arteries than those in the aorta. The expression of eNOS was the highest in the aorta and the lowest in the distal mesenteric arteries. These results indicate that the importance of EDHF increases as the vessel size decreases in endothelium-dependent relaxations in the rat mesenteric circulation.
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                Author and article information

                Journal
                JVR
                J Vasc Res
                10.1159/issn.1018-1172
                Journal of Vascular Research
                S. Karger AG
                1018-1172
                1423-0135
                2005
                April 2005
                13 April 2005
                : 42
                : 2
                : 148-156
                Affiliations
                aCardiovascular Research Division, SERVIER Research Institute, Suresnes, France; Departments of Pharmacology, bSecond Military Medical University, Shanghai, and cUniversity of Hong Kong, Hong Kong, China
                Article
                83972 J Vasc Res 2005;42:148–156
                10.1159/000083972
                15713986
                © 2005 S. Karger AG, Basel

                Copyright: All rights reserved. No part of this publication may be translated into other languages, reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, microcopying, or by any information storage and retrieval system, without permission in writing from the publisher. Drug Dosage: The authors and the publisher have exerted every effort to ensure that drug selection and dosage set forth in this text are in accord with current recommendations and practice at the time of publication. However, in view of ongoing research, changes in government regulations, and the constant flow of information relating to drug therapy and drug reactions, the reader is urged to check the package insert for each drug for any changes in indications and dosage and for added warnings and precautions. This is particularly important when the recommended agent is a new and/or infrequently employed drug. Disclaimer: The statements, opinions and data contained in this publication are solely those of the individual authors and contributors and not of the publishers and the editor(s). The appearance of advertisements or/and product references in the publication is not a warranty, endorsement, or approval of the products or services advertised or of their effectiveness, quality or safety. The publisher and the editor(s) disclaim responsibility for any injury to persons or property resulting from any ideas, methods, instructions or products referred to in the content or advertisements.

                Page count
                Figures: 5, Tables: 2, References: 43, Pages: 9
                Categories
                Research Paper

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