Inhibition of the biotransformation and pharmacological actions of glyceryl trinitrate by the flavoprotein inhibitor, diphenyleneiodonium sulfate.

Recent studies suggest a role for the vascular cytochrome P450-NADPH cytochrome P450 reductase system in mediating the biotransformation of glyceryl trinitrate (GTN) to nitric oxide (or some closely related species), resulting in increased cyclic GMP accumulation and vasodilation. In this study we tested the effect of the flavoprotein inhibitor, diphenyleneiodonium sulfate (DPI) on GTN action in isolated rat aorta. Exposure of phenylephrine-contracted tissues to DPI (10 nM-0.3 microM) resulted in 3- to 10-fold increases in the EC50 values for GTN-induced relaxation in both endothelium-intact tissues and endothelium-denuded tissues, whereas the vasodilator response to sodium nitroprusside was unaffected. Consistent with the relaxation data, cyclic GMP accumulation induced by 0.3 or 2 microM GTN was inhibited by 0.3 microM DPI in both endothelium-intact and endothelium-denuded aortic strips, whereas cyclic GMP accumulation induced by 0.1 microM sodium nitroprusside was unaffected. The regioselective formation of glyceryl-1,2-dinitrate observed during the aortic biotransformation of GTN was inhibited markedly by DPI. In tissues incubated with 0.3 or 10 microM DPI for 30 min followed by washout for 60 min, the EC50 values for GTN-induced relaxation were increased 2-fold, and both GTN-induced cyclic GMP accumulation and vascular GTN biotransformation were decreased. This suggests an irreversible component to the inhibitory action of DPI. Together, these data provide evidence for the involvement of a flavoprotein (e.g., NADPH cytochrome P450 reductase) in the metabolic activation of GTN required for expression of its vasodilator activity.