ASK1 activation is inhibited by sildenafil. (A) Quantitative measurements of optical density (O.D.) of p-ASK1 (Ser83). [F(3,21) = 9.505, P < 0.001]. (B) Quantitative measurements of Western blot analysis of the phosphorylation status of the p38-activating kinases MKK3 and MKK6 and the JNK-activating kinase MKK7. Note that values from sham and sildenafil groups were so weak that were not taken into account for quantitative measurement. Statistical analysis yielded the following results: for p-MKK3/6 [t(12) = 6.778, P < 0.001] and p-MKK7 [t(10) = 2.829, P < 0.05]. Results are mean ± SEM, n = 5–8. Different from the corresponding sham group: **P < 0.01. Different from malonate-only animals: §P < 0.05; §§§P < 0.001. (C) Representative blots showing that sildenafil reverted malonate-induced alterations on the phosphorylation status of ASK-1, MKK3/6 and MKK7.