The signal transduction pathways of endothelin (ET)-induced vasorelaxation in rat aorta were investigated. An agonist for ETB receptors, IRL 1620, induced transient increases in cytosolic Ca++ (peak at about 10 sec) and cyclic GMP (peak at about 20 sec) accompanied by transient vasorelaxation (peak at about 60 sec) in aortic strips precontracted with 100 nM norepinephrine. The cyclic GMP content was increased 3- to 6-fold from the basal level (1.6 +/- 0.2 fmol/micrograms of protein) with 1 nM to 1 microM IRL 1620. The cyclic GMP elevation was endothelium-dependent, abolished in the presence of 100 microM NG-monomethyl-L-arginine, an inhibitor of nitric oxide synthase, and recovered after the further addition of 1 mM L-arginine. An ETB receptor antagonist, IRL 1038 (3 microM), inhibited completely the cyclic GMP increase induced by 100 nM IRL 1620 (8.1 +/- 0.6 fmol/micrograms of protein) without affecting the basal level. On the other hand, an ETA receptor antagonist, 3 microM BQ-123, enhanced significantly both the basal level (3.7 +/- 0.6 fmol/micrograms of protein) and the IRL IRL 1620-induced production (12.2 +/- 0.8 fmol/micrograms of protein) of cyclic GMP. Specific binding sites for [125I]IRL 1620 were detected in rat aortic membranes with a dissociation constant of 37.0 pM and maximal binding capacity of 36.6 fmol/mg of protein, which disappeared after removing the endothelium. Unlabeled ET-1, ET-3, IRL 1620 and IRL 1038, but not BQ-123, displaced the binding of [125I]IRL 1620 with inhibitory constants of 38.5 pM, 36.6 pM, 97.8 pM and 8.9 nM, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)