l-Carnitine attenuates oxidative stress in hypertensive rats

Abstract

The present study aimed to investigate whether l-carnitine (LC) protects the vascular endothelium and tissues against oxidative damage in hypertension. Antioxidant enzyme activities, glutathione and lipid peroxidation were measured in the liver and heart of spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats. Nitrite and nitrate levels and total antioxidant status (TAS) were evaluated in plasma, and the expression of endothelial nitric oxide synthase (eNOS) and p22phox subunit of NAD(P)H oxidase was determined in aorta. Glutathione peroxidase activity was lower in SHR than in WKY rats, and LC increased this activity in SHR up to values close to those observed in normotensive animals. Glutathione reductase and catalase activities, which were higher in SHR, tended to increase after LC treatment. No differences were found in the activity of superoxide dismutase among any animal group. The ratio between reduced and oxidized glutathione and the levels of lipid peroxidation were respectively decreased and increased in hypertensive rats, and both parameters were normalized after the treatment. Similarly, LC was able to reverse the reduced plasma nitrite and nitrate levels and TAS observed in SHR. We found no alterations in the expression of aortic eNOS among any group; however, p22phox mRNA levels showed an increase in SHR that was reversed by LC. In conclusion, chronic administration of LC leads to an increase in hepatic and cardiac antioxidant defense and a reduction in the systemic oxidative process in SHR. Therefore, LC might increase NO availability in SHR aorta by a reduction in superoxide anion production.

Introduction

There is increasing evidence indicating that oxidative stress plays an important role in the pathogenesis of hypertension [1]. Superoxide anions and other reactive oxygen species (ROS) contribute to the generation and/or maintenance of hypertension through several mechanisms, such as inactivation of endothelium-derived nitric oxide (NO) to produce peroxynitrite, a potent constrictor and lipid-oxidizing radical [2], [3], [4].

Many studies support a role for altered redox status in hypertension [5]. At the cellular level, an increased production of superoxide anions and activation of NAD(P)H oxidase, the major superoxide anion-generating enzyme in vascular cells, have been reported as an important mechanism involved in endothelial dysfunction observed in arterial hypertension [6], [7]. In addition, hypertension is associated with changes in hepatic and heart redox system [8], [9], [10]. In view of these considerations, an ROS-decreasing treatment has been suggested in order to ameliorate the adverse effects of oxygen-derived free radicals [11], [12], and dietary antioxidants such as vitamin C and E have reduced arterial blood pressure in experimental hypertension models such as spontaneously hypertensive or Dahl salt-sensitive rats [13].

l-Carnitine (β-hydroxy-γ-N-trimethylammonium-butyrate; LC) is a vital component in lipid metabolism for the production of ATP through the β-oxidation of long-chain fatty acids. An antioxidant promoting action has been suggested for this compound. Thus, LC acts as a free-radical scavenger [14], [15] and protects cells from ROS [16]. Previous studies have described antioxidant properties of carnitine in aging [17] and atherosclerotic rats [18] and hypercholesterolaemic rabbits [19]. In this sense, recent studies in our laboratory have demonstrated an improvement in the aortic endothelial dysfunction of spontaneously hypertensive rats (SHR) after LC chronic treatment, and results obtained in the presence of superoxide dismutase (SOD) plus catalase (CAT) indicate that this improvement is related to the antioxidant properties of LC [20].

The present study therefore aimed to test the hypothesis that LC might somehow protect tissues against hypertension-induced oxidative damage. To this purpose, the specific activities of the antioxidant enzymes, glutathione peroxidase (GSH-Px), glutathione reductase (GSH-Red), SOD and CAT, as well as the ratio between reduced and oxidized glutathione (GSH/GSSG) and the levels of thiobarbituric acid-reactive substances (TBARS), were measured in liver and heart homogenates from SHR and normotensive Wistar-Kyoto (WKY) rats treated or untreated with LC. Plasma nitrite and nitrate levels (NO_x) and total antioxidant status (TAS) were also evaluated in all experimental groups. In addition, protein and mRNA expression of endothelial NO synthase (eNOS) and mRNA expression of the p22phox subunit of NAD(P)H oxidase were also studied in aorta from the same rats, in order to study the effect of LC on vascular endothelium.