Effect of methylene blue on vasoreactivity in dog lung.

We examined the effect of methylene blue (MB), a putative inhibitor of guanylate cyclase (GC) activation by endothelium-derived relaxing factor (EDRF) and nitrovasodilator compounds, on vascular tone and reactivity to vasoactive substances in the isolated, blood-perfused canine lower left lung lobe. Lobar vascular resistance was partitioned into arterial and venous segments by venous outflow occlusion. Because MB did not alter vasoconstriction to either serotonin or acetylcholine (P greater than 0.05) except after cyclooxygenase inhibition (COI), we determined the effectiveness of MB as an inhibitor of GC activation by nitrovasodilators. Lobes were given graded bolus doses of nitroglycerin (GTN), sodium nitroprusside (SNP), and bradykinin (BK) at baseline vascular tone, after COI, and after vascular tone was raised by either U-46619, a thromboxane analogue, or MB infusion. GTN and BK but not SNP induced dose-dependent vasodilation when vascular tone was raised by U-46619. However, when vascular tone was increased to a similar level by 30 mg MB and 0.5 mg/min infusion, vasodilation to GTN, SNP, and BK was enhanced from U-46619 infusion. In contrast to MB, NG-nitro-L-arginine, a putative inhibitor of EDRF synthesis, diminished vasodilation to BK in cyclooxygenase-inhibited lobes with elevated vascular tone. Because MB potentiated vasodilation to GTN, SNP, and BK, it is questionable whether MB is an effective inhibitor of vasodilation to nitrovasodilators or BK in the isolated, blood-perfused canine lung.