A selective agonist of endothelin type B receptor, IRL 1620, stimulates cyclic GMP increase via nitric oxide formation in rat aorta
Fujitani, Y.; Ueda, H.; Okada, T.; Urade, Y.; Karaki, H.
Journal of Pharmacology and Experimental Therapeutics 267(2): 683-689
1993
ISSN/ISBN: 0022-3565 PMID: 7504100 Document Number: 417320
The signal transduction pathways of endothelin (ET)-induced vasorelaxation in rat aorta were investigated. An agonist for ET-B 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/mu-g of protein) with 1 nM to 1 mu-M IRL 1620. The cyclic GMP elevation was endothelium-dependent, abolished in the presence of 100 mu-M N-G-monomethyl-L-arginine, an inhibitor of nitric oxide synthase, and recovered after the further addition of 1 mM L-arginine. An ET-B receptor antagonist, IRL 1038 (3 mu-M), inhibited completely the cyclic GMP increase induced by 100 nM IRL 1620 (8.1 +- 0.6 fmol/mu-g of protein) without affecting the basal level. On the other hand, an ET-A receptor antagonist, 3 mu-M BQ-123, enhanced significantly both the basal level (3.7 +- 0.6 fmol/mu-g of protein) and the IRL 1620-induced production (12.2 +- 0.8 fmol/mu-g 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. These results show that stimulation of ET-B receptors leads to increases in cytosolic Ca++ and production of nitric oxide in the endothelium. Nitric oxide, in turn, stimulates guanylate cyclase to increase production of cyclic GMP leading to relaxation of the aorta.