Methylene Blue Inhibits Vasodilation of Skeletal Muscle Arterioles to Acetyicholine and Nitric Oxide via the Extracellular Generation of Superoxide Anion1

The mechanism of modulation of cyclic GMP-associated vascular responses by methylene blue, an agent employed to inhibit the activation of soluble guanylate cyclase in tissues, was investi-gated in the cremaster muscle microcirculation of pentobarbital-anesthetized rats. The effect of topically applied agents on the diameter of third-order arterioles (1 5-20 m diameter) was de-termined by in vivo television microscopy. Topical application (100 I) of acetylcholine (0.01 zg) or nitric oxide (0.06-6 g) caused vasodilator responses that were inhibited (P <.05, n = 6-8) 64 % and 30 to 1 00%, respectively, by suffusion of the preparation with 5 M methyiene blue. Agents that are thought to produce activation of guanylate cyclase via cellular metabolism to nitric oxide, nitroglycerin (0.5 ng-0.5 g) or nitroprusside (0.5 ng-0.5 tg), also produced vasodilation. However, methylene blue suffusion did not inhibit these responses (n = 6-9). Th