NONSPECIFIC BLOCKADE OF VASCULAR FREE-RADICAL SIGNALS BY METHYLATED ARGININE ANALOGS

Methylated arginine analogues are often used as probes of the effect o f nitric oxide; however, their specificity is unclear and seems to be frequently overestimated. This study analyzed the effects of N-G-methy l-L-arginine (L-NMMA) on the endothelium-dependent release of vascular superoxide radicals triggered by increased flow. Plasma ascorbyl radi cal signals measured by direct electron paramagnetic resonance spectro scopy in 25 rabbits increased by 3.8 +/- 0.7 nmol/l vs baseline (28.7 +/- 1.4 nmol/l, P<0.001) in response to papaverine-induced flow increa ses of 121 +/- 12%. In contrast, after similar papaverine-induced flow increases simultaneously with L-NMMA infusions, ascorbyl levels were not significantly changed compared to baseline. Similar results were o btained in isolated rabbit aortas perfused ex vivo with the spin trap alpha-phenyl-N-tert-butylnitrone (N = 22). However, in both preparatio ns, this complete blockade was not reversed by co-infusion of excess L -arginine and was also obtained by N-methyl-D-arginine, thus indicatin g that it is not related to nitric oxide synthase. L-arginine alone wa s ineffective, as previously demonstrated for N-G-methyl-L-arginine ea ter (L-NAME),ln vitro, neither L-arginine nor its analogues scavenged superoxide radicals. This nonspecific activity of methylated arginine analogues underscores the need for careful controls in order to assess nitric oxide effects, particularly those related to interactions with active oxygen species.