SELECTIVE BLOCKADE OF ENDOTHELIUM-DEPENDENT AND GLYCERYL TRINITRATE-INDUCED RELAXATION BY HEMOGLOBIN AND BY METHYLENE-BLUE IN THE RABBIT AORTA

Abstract

Hb at 1 .mu.M reduced and at 10 .mu.M abolished the endothelium-dependent relaxation induced by acetylcholine or by A23187 in rabbit aortic rings. Similarly, methylene blue at 10 .mu.M reduced and at 50 .mu.M abolished relaxation induced by acetylcholine and by A23187. Hb (1-10 .mu.M) and methylene blue (10-50 .mu.M) each induced a dose-dependent inhibition of the endothelium-independent relaxation produced by glyceryl trinitrate, but neither had any effect on the relaxation produced by isoproterenol. The inhibitory effects of Hb and methylene blue may be due to blockade of guanylate cyclase, as the rises in cGMP content which accompany relaxation induced by acetylcholine, A23187 or glyceryl trinitrate were abolished. Isoproterenol-induced relaxation took place with no change in cGMP content. Hb and methylene blue appear therefore to inhibit selectively vasorelaxation induced by agents which increase cGMP levels. Hb and methylene blue augment tone in aortic rings, particularly when endothelial cells are present, suggesting that the endothelium-derived relaxing factor (EDRF) might be released spontaneously in low concentrations. The possibility that Hb inhibits endothelium-dependent and glyceryl trinitrate-induced relaxation by binding EDRF and NO is discussed with the proposal that methylene blue might produce its effects by oxidizing a component of guanylate cyclase, possibly a ferrous heme group linked to the enzyme molecule. Methylene blue might, in addition, interact directly with EDRF.