There are three mechanisms by which NG-hydroxy-L-arginine (L-HOArg) induces endothelium-dependent relaxations. L-HOArg is a substrate for the constitutive nitric oxide (NO) synthase present in endothelial cells (ECs). It reacts with NO released from EC to form a potent and more stable vasodilator. Moreover, it induces a relatively stable, EC-dependent relaxation that is not blocked by the inhibitors of NO synthesis. Subsequently, we have investigated the effects of hydroxyguanidine (HOG) on the biological activity of endothelium-derived relaxing factor (EDRF). HOG potentiated the relaxant responses of rabbit aortic strips to EDRF released from EC by adenosine diphosphate (ADP) or bradykinin as well as those induced by authentic NO. Importantly, it was not a substrate for NO synthesis and it did not affect the generation of prostacyclin by ECs. Thus, the effects of HOG were due to the chemical reaction of HOG with NO released from ECs and the formation of a more stable vasodilator. Moreover, HOG augmented not only agonist-triggered, but also flow-induced, EC-dependent relaxation and both effects of HOG were abolished by NG-nitro-L-arginine methyl ester (L-NO2-Arg). In contrast, the EC-dependent relaxation induced by L-HOArg was not inhibited by L-NO2Arg. Moreover, it was not affected by the removal of extracellular Ca2+, but was blocked by oxyhemoglobin and potentiated by superoxide dismutase. These results demonstrate the involvement of the hydroxyguanidino moiety of L-HOArg in its reaction with NO. Moreover, they strongly support the notion that nitrix oxide mediates both the agonist-triggered and flow-induced endothelium-dependent relaxation.(ABSTRACT TRUNCATED AT 250 WORDS)