Mechanisms of relaxation by pyrogallol and methylthioninium chloride in perfused rat mesenteric artery.

In perfused rat mesenteric arteries without endothelium, pyrogallol (Pyr) or methylthioninium chloride (methylene blue, Met) produced a concentration-dependent relaxation. Superoxide dismutase abolished inhibition by Pyr, but not Met, of vasoconstrictor responses to transmural nerve stimulation (TNS). Neither catalase nor deferoxamine had any effect on vasodilator responses to Pyr or Met. Vasodilator responses to Pyr were unaltered by N omega-nitro-L-arginine methyl ester (L-NAME), indomethacin, or capsaicin. Similarly, the relaxing effect of Met was unaffected by indomethacin or capsaicin. These findings suggest that vasodilator responses to Pyr may be due to endothelial-independent generation of superoxide anion. In contrast the relaxation produced by Met appears to be due to a direct action on vascular smooth muscle independent of superoxide anion generation.