Effects of methylene blue and LY83583 on neuronal nitric oxide synthase and NADPH-diaphorase.

Methylene blue and 6-anilino-5,8-quinolinedione (LY83583) have often been used as 'selective' inhibitors of soluble guanylyl cyclase. We report that in in vitro assays, both these compounds were potent inhibitors of rat cerebellar nitric oxide synthase activity. Methylene blue had an apparent Ki of 2.7 microM, while for LY83583 the Ki was 15.8 microM. Furthermore, methylene blue, but not LY83583, inhibited the NADPH-diaphorase histochemical reaction associated with nitric oxide synthase. Our results indicate that many of the effects of these drugs which have been attributed to inhibition of guanylyl cyclase, may derive from their direct inhibition of nitric oxide synthase activity instead.