Hydroxyl Radical—a Mediator of Acetylcholine-induced Vascular Relaxation

It is well known that acetylcholine (Ach)-induced vasodilation is mediated through the release of the endothelium-derived relaxing factor (EDRF), nitric oxide (NO•). It has been suggested that NO interacts with superoxide anion (O−2) to generate peroxynitrite which at physiological pH gives rise to peroxynitrous acid that rapidly decomposes to hydroxyl radical (•OH) and nitrogen dioxide. •OH relaxes isolated aorta. It was hypothesized that Ach-induced vascular relaxation is mediated by •OH derived from interaction of NO and O−2. To test this hypothesis we investigated the effect of Ach on norepinephrine (NE)-precontracted isolated rabbit aortic preparations in the absence or presence of scavengers of O−2[superoxide dismutase (SOD)] and of •OH [dimethylthiourea (DMTU) or mannitol]. •OH and Ach produced relaxation of NE-precontracted preparations in a concentration-dependent manner. Relaxation produced by •OH generating system was prevented by mannitol, a •OH scavenger suggesting that relaxation is due to •OH. SOD, DMTU, mannitol or combination of SOD and DMTU markedly reduced the Ach-induced relaxation. Glyburide (an ATP-sensitive K+channel blocker) was ineffective in blocking the Ach-induced remaining relaxation in the presence of SOD and DMTU. These results suggest that •OH formed from interaction of O−2and NO• is the mediator of Ach-induced vascular relaxation. Thus while EDRF is NO•, its mechanism of action involves •OH.