Abstract

Hemoglobin at 1 microM reduced and at 10 microM abolished the endothelium-dependent relaxation induced by acetylcholine or by A23187 in rabbit aortic rings. Similarly, methylene blue at 10 microM reduced and at 50 microM abolished relaxation induced by acetylcholine and by A23187. Furthermore, hemoglobin (1-10 microM) and methylene blue (10-50 microM) 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 hemoglobin and methylene blue may be due to blockade of guanylate cyclase, as the rises in cyclic GMP content which accompany relaxation induced by acetylcholine, A23187 or glyceryl trinitrate were abolished. Isoproterenol-induced relaxation took place with no change in cyclic GMP content. Hemoglobin and methylene blue appear therefore to inhibit selectively vaso-relaxation induced by agents which increase cyclic GMP levels. Hemoglobin 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 hemoglobin inhibits endothelium-dependent and glyceryl trinitrate-induced relaxation by binding EDRF and nitric oxide, respectively, is discussed together 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.