Nitroglycerin (GTN) has reported to induce oxidative stress and impairs endothelial function (EF). Aldehyde dehydrogenase 2 (ALDH2) is an anti-oxidative enzyme, and its activity is lost in the Glu504Lys (*2) genotype. We reported that this genotype impaired EF and increased oxidative stress. We also reported that a HMG-CoA reductase inhibitor (atorvastatin), which has anti-oxidative effect, improved EF after GTN treatment. In this study, we examined whether another anti-oxidative agent, quercetin, improved EF impaired by GTN treatment in ALDH2 *2/*2 carriers. Eighteen volunteers (*1/*1, n=12; *2/*2, n=6) were given GTN (25 mg/day) and quercetin (1000 mg/day) or GTN alone for 7 days using a crossover design. Flow-mediated dilation (FMD) was measured to evaluate EF before and after each treatment. FMD was more reduced in ALDH2*2/*2 carriers than that in *1/*1 carriers after GTN alone treatment. However, FMD was improved in the volunteers co-administered quercetin compared with those given GTN alone in both genotypes. Therefore, co-administration of quercetin might be beneficial when GTN is continuously administered, especially in ALDH2 *2/*2 patients.

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