Correlation between the relaxation of the rabbit aortic smooth muscle induced by nitroglycerin(NG), sodium nitroprusside(NP) and atriopeptin (AP) and the increase in cGMP levels

Using the isolated smooth muscle preparations of the rabbit aorta, the relaxant effects of NG, NP and AP were studied in relation to the increases in tissue cGMP level. Endothelium free aortic rings were prepared from reserpinized male Japanese white rabbits and mounted in an organ bath. Radioimmunoassay of cGMP was conducted with Yamasa cGMP assay kit. The relaxations produced by NG(10-6 M),NP(10-6 M) and AP(10-8 M) in phenylephrine (10-7 M)-contracted aortic rings were of a similar magnitude with a duration longer than 20 min. Methylene blue (MB; 10-5 M) attenuated the magnitude of relaxation induced by NG without affecting the initial rate of relaxation, while the effect of AP was reduced by MB during the entire course of its development. The relaxant effect of NP was potentiated. NG, NP and AP produced increases in cGMP. The time course of increases in tissue CGMP induced by NP and AP paralleled that of the relaxation, while the increase produced by NG was transient. MB inhibited the increase in cCMP produced by NG, but not those produced by NP and AP. These data suggested a close correlation between the relaxation of the smooth muscle of the rabbit aorta produced by NG, NP and AP and the increase in tissue cGMP level.