Bagi, Zsolt, Csongor Csekő , Erika Tó th, and Akos Koller. Oxidative stress-induced dysregulation of arteriolar wall shear stress and blood pressure in hyperhomocysteinemia is prevented by chronic vitamin C treatment. Am J Physiol Heart Circ Physiol 285: H2277-H2283, 2003. First published July 17, 2003 10.1152/ajpheart.00448.2003.-We aimed to test the hypothesis that an enhanced level of reactive oxygen species (ROS) is primarily responsible for the impairment of nitric oxide (NO)-mediated regulation of arteriolar wall shear stress (WSS) in hyperhomocysteinemia (HHcy). Thus flow/WSS-induced dilations of pressurized gracilis muscle arterioles (basal diameter: ϳ170 m) isolated from control (serum Hcy: 6 Ϯ 1 M), methionine diet-induced HHcy rats (4 wk, serum Hcy: 30 Ϯ 6 M), and HHcy rats treated with vitamin C, a known antioxidant (4 wk, 150 mg ⅐ kg body wt Ϫ1 ⅐ day Ϫ1 ; serum Hcy: 32 Ϯ 10 M), were investigated. In vessels of HHcy rats, increases in intraluminal flow/WSS-induced dilations were converted to constrictions. Constrictions were unaffected by inhibition of NO synthesis by N -nitro-L-arginine methyl ester (L-NAME). Vitamin C treatment of HHcy rats reversed the WSS-induced arteriolar constrictions to L-NAME-sensitive dilations but did not affect control responses. Similar changes in responses were obtained for the calcium ionophore A-23187. In addition, diastolic and mean arterial blood pressure and serum 8-isoprostane levels (a marker of in vivo oxidative stress) were significantly elevated in rats with HHcy, changes that were normalized by vitamin C treatment. Taken together, our data show that in chronic HHcy long-term vitamin C treatment, by decreasing oxidative stress in vivo, enhanced NO bioavailability, restored the regulation of shear stress in arterioles, and normalized systemic blood pressure. Thus our study provides evidence that oxidative stress is an important in vivo mechanism that is primarily responsible for the development of endothelial dysregulation of WSS in HHcy.homocysteine; nitric oxide; thromboxane A2 power dissipation SEVERAL EPIDEMIOLOGICAL STUDIES have shown that hyperhomocysteinemia (HHcy) increases the risk for cardiovascular diseases, such as ischemic heart diseases; cerebrovascular, peripheral vascular diseases; and hypertension (1,6,7,9,29,32). Homocysteine is formed during the metabolism of the essential amino acid methionine, and its normal plasma concentration is between 5 and 15 M, but it can be increased due to genetic (e.g., cystathione--synthase and methyltetrahydrofolate reductase) and nutritional alterations (deficiency of vitamins, e.g., folic acid, vitamin B 6 , and B 12 ), factors that participate in the metabolism of homocysteine and methionine (8).Although the underlying mechanisms responsible for the elevated risks have not yet been fully elucidated, there is increasing evidence to suggest that endothelial dysfunction of vessels contributes to the development of vascular diseases observed in both humans and animals with HHcy (19,34). Several studies have doc...