ABSTRACT LV 83583 (6-anilino-5, 8-quinolinedione) has been reported to lower intracellular cyclic GMP by an unknown mechanism. The objective of the present study was to investigate the effect of LY 83583 on different types of vasorelaxation and to study its mechanism of action. Low concentrations of LV 83583(sO. 1 M) inhibited endothelium-dependent relaxations of rabbit aortic strips induced by acetykhdine or by the caldum knophore A231 87. HIgher concentrations(0.3 M) were required to produce partial inhibition of relaxation to sodium nitroprusside and glyceryl thnitrate. Cydic AMP-mediated relaxations, induced by isoprenaline or forskolin, were not affected by LY 83583(1 0 SM). The site of interference of LV 83583 with endothelium-dependent relaxation was examined with endothelium-denved relaxing fac-tor (EDRF) released from cultured endothelial cells that were grown on microcarner beads and stimulated by superfusion with ATP or thimerosai. EDRF in the superfusate was detected by endothelium-denuded segments of rabbit femoral artery, which responded with dilation and, simultaneously, by purified soluble guanylate cyclase (GC) in test tubes, which was activated by EDAF. When LV 83583 was added to the glutathione-containing GC-assay or to the superfusate from cultured endothelial cells, it did not affect stimulation of soluble GC by EDAF but it slowly reversed the dilator response of the arterial detector segment. Superfusion of cultured endothelial cells with LV 83583(1 M) rapidly and reversibly inhibited EDRF release. If LV 83583 was reduced with DL-dithiothreitol, it inhibited reversibly both, unstim-ulated soluble GC (lCse, 1 zM) and GC stimulated with 100 zM sodium nitroprusside(lCse, 1 0 tM). We conclude that LY 83583 is activated by chemical reduction, either in endothelial cells, where it inhibits release of EDAF(low concentrations), or in smooth muscle cells, where it inhibits soluble GC (higher concentrations). In intact blood vessels both mechanisms can lead to reduced levels of cyclic GMP and thus inhibit relaxation.