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Arterioles on the surface of the mouse brain were observed by in vivo TV microscopy. Four dilators were topically applied to relax the vessels in vivo. Two of the dilators were acetylcholine and bradykinin, whose action in this vascular bed is dependent upon production of endothelium-dependent relaxing factors. The other two dilators were sodium nitroprusside and 8-bromo-cGMP, whose action is not endothelium dependent. The dilations by acetylcholine, bradykinin, and nitroprusside were significantly depressed by 10(-4) M methylene blue applied topically for 7 minutes prior to application of the dilators. The inhibitory effect was reversible, was greatest against acetylcholine, and was least against nitroprusside. These data parallel reports of methylene blue's action against these dilators when applied to large blood vessels in vitro. Our data appear to be the first microvascular data and the first in vivo data showing this effect. The data thus suggest that the mechanisms underlying dilation of cerebral arterioles and large extracerebral vessels are similar. The literature accounts for the effect of methylene blue on the basis of its action as an inhibitor of guanylate cyclase. Our data, including the failure of methylene blue to alter dilation by 8-bromo-cGMP, are in keeping with this hypothesis and with current beliefs that guanylate cyclase and cGMP have a central role in vasodilation. The data do not rule out the possibility that methylene blue has an additional action in the case of acetylcholine and inactivates the endothelium-dependent relaxing factor for that dilator.

In vivo effect of methylene blue on endothelium-dependent and endothelium-independent dilations of brain microvessels in mice.