Contribution of reactive oxygen species to the endothelial dysfunction. Data show the effect of the blockade of NAD(P)H oxidase activity with apocynin on the dose–response curve to acetylcholine (ACh) in Sham (A), non-treated 2K1C (B) and 2K1C mice treated with sildenafil (C) mice. Bar graphs show the area under the curve (D), maximum response (E) and the sensitivity (pEC50) (E and F, gray color) and the relative changes induced by apocynin (E and F, blue color) in the groups of animals. Values are the means±SEM for 8–10 animals per group. **p < 0.01 (gray bars) and ##p < 0.01 (apocynin effect, green bars), one- (D, E and F) and two-way (A, B and C) ANOVA.