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 c ytochrome P450 reductase system in mediating the biotransformation of glyceryl trinitrate (GTN) to nitric oxide (or some closely related spe cies), resulting in increased cyclic GMP accumulation and vasodilation . In this study we tested the effect of the flavoprotein inhibitor, di phenyleneiodonium sulfate (DPI) on GTN action in isolated rat aorta. E xposure of phenylephrine-contracted tissues to DPI (10 nM-0.3 mu M) re sulted in 3- to 10-fold increases in the EC(50) 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 accumulat ion induced by 0.3 or 2 mu M GTN was inhibited by 0.3 mu M DPI in both endothelium-intact and endothelium-denuded aortic strips, whereas cyc lic GMP accumulation induced by 0.1 mu M sodium nitroprusside was unaf fected. The regioselective formation of glyceryl-1,2-dinitrate observe d during the aortic biotransformation of GTN was inhibited markedly by DPI. In tissues incubated with 0.3 or 10 mu M DPI for 30 min followed by washout for 60 min, the EC(50) values for GTN-induced relaxation w ere increased 2-fold, and both GTN-induced cyclic GMP accumulation and vascular GTN biotransformation were decreased. This suggests an irrev ersible component to the inhibitory action of DPI. Together, these dat a 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.