Abstract-Asymmetric dimethylarginine (ADMA), which inhibits NO synthase, is inactivated by N G ,N G -dimethylarginine dimethylaminohydrolase (DDAH). We tested whether DDAH-1 or -2 regulates serum ADMA (S ADMA ) and/or endothelium-derived relaxing factor (EDRF)/NO. Small inhibitory (si)RNAs targeting DDAH-1 or -2, or an siRNA control were given intravenously to rats. After 72 hours, EDRF/NO was assessed from acetylcholine-induced, NO synthase-dependent relaxation and 4-amino-5-methylamino-2Ј,7Ј-diflouroflourescein diacetate for NO activity in isolated mesenteric resistance vessels (MRVs). Expression of mRNA for DDAH-1 versus -2 was 2-and 7-fold higher in the kidney cortex and liver, respectively, whereas expression of DDAH-2 versus -1 was 5-fold higher in MRVs. The proteins and mRNAs for DDAH-1 or -2 were reduced selectively by 35% to 85% in the kidney cortex, liver, and MRVs 72 hours following the corresponding siRNA. S ADMA was increased only after siDDAH-1 (266Ϯ25 versus; PϽ0.005), whereas EDRF/NO responses and NO activity were not changed consistently by siDDAH-1 but were greatly reduced after siDDAH-2. Mean arterial pressure was not changed significantly by any siRNA. In conclusion, S ADMA is regulated by DDAH-1, which is expressed at sites of ADMA metabolism in the kidney cortex and liver, whereas EDRF/NO is regulated primarily by DDAH-2, which is expressed strongly in blood vessels. This implies specific functions of DDAH isoforms. Key Words: RNA interference Ⅲ hypertension Ⅲ kidney Ⅲ blood vessel Ⅲ endothelium T he endothelium dependent relaxing factor (EDRF) response of resistance vessels is mediated predominantly by NO and an endothelium-dependent hyperpolarizing factor (EDHF). 1,2 Defects in NO occur in blood vessels and the kidneys of hypertensive models, despite often well-preserved expression of constitutive NO synthase (NOS). 3 One candidate to account for this paradox is superoxide (O 2 . ), which can inactivate NO in blood vessels. 4 A second candidate is asymmetric dimethylarginine (ADMA), which inhibits NOS activity, EDRF/NO responses, and L-arginine transport into cells by system y ϩ . 5 Arginine moieties in proteins are methylated by protein arginine methyltransferases. 5 Following protein catabolism, ADMA or its stereoisomer, symmetric dimethylarginine (SDMA), are released within cells and exported into the plasma. SDMA does not inhibit NOS. 6 Many of the patient groups or animal models at risk for cardiovascular disease have endothelial dysfunction and elevated serum levels of ADMA (S ADMA ). 5,7 Although S ADMA is a strong predictor of future cardiovascular events in high-risk patients, 8 it is presently unclear whether these associations are causative.ADMA and L-monomethyl arginine are metabolically inactivated by DDAH, whereas SDMA is not a substrate for this enzyme. 5 DDAH is expressed extensively in the proximal tubules of the rat kidney and the liver. 9,10 However, DDAH is expressed as 2 isoforms in rats and humans. 9 Current studies have shown that a 50% gene deletion for DDAH-1 i...

Tempol corrects hypertension without a compensatory sympathoadrenal activation or salt retention. The response is independent of nitric oxide, endothelin, or catecholamines and occurs despite increased PRA. It is accompanied by a reduction in oxidative stress and is maintained during increased salt intake.