Investigation of the vasorelaxant effects of moxonidine and its relaxation mechanism on the human radial artery when used as a coronary bypass graft

In both low- and high-risk patients undergoing coronary artery bypass grafting, the internal mammary artery is the first choice of arterial graft, and the second choice is the radial artery (RA). Unfortunately, RA spasms are a significant problem for a surgical team to overcome in the perioperative and postoperative period. In current surgical practice, the use of vasodilator agents perioperatively in the pending graft preparation is generally accepted and these may be implemented topically, endoluminally or both ways. Moxonidine is the latest second-generation, centrally acting antihypertensive agent, and the intention in this paper is to investigate its direct vasorelaxant effects and relaxation mechanisms on the human radial artery in vitro. RA rings were mounted in an organ bath and tested for changes in isometric tension in its relaxation response to moxonidine in the presence and absence of NG-nitro-L-arginine methyl ester (L-NAME, non-specific inhibitor of nitric oxide synthase), idazoxan (non-selective I1 and alpha 2-antagonist) and yohimbine (selective alpha 2-antagonist). Moxonidine induced concentration-dependent relaxations on the RA rings precontracted with phenylephrine (P < 0.05). L-NAME and idazoxan significantly reduced the relaxation caused by moxonidine (P < 0.05), while yohimbine significantly increased the relaxation by moxonidine (P < 0.05). In the presence of L-NAME + idazoxan, the relaxation by moxonidine was eliminated completely (P < 0.05). We speculate that the relaxant effect of moxonidine may be attributed partly to the synthesis and/or release of nitric oxide, and partly to the stimulation of imidazoline I1 receptors. We suggest that moxonidine may help to prevent RA spasms during the preparation period in operation when used topically or/and endoluminally.