Peroxynitrite-induced relaxation in isolated rat aortic rings and mechanisms of action

The present study was designed to evaluate the effects of peroxynitrite (ONOO−), the product of superoxide and nitric oxide, on isolated segments of rat aorta. In the absence of any vasoactive agent, ONOO− (from 10−8 to 10−4 M) failed to alter the basal tension. In phenylephrine (PE; 5 × 10−7 M)-precontracted rat aortic rings (RAR), ONOO− elicited concentration-dependent relaxation at concentrations of from 10−8 to 10−4 M. The effective concentrations producing approximately 50% of maximal relaxation (ED50) to ONOO− were 1.84 × 10−5 M and 1.96 × 10−5 M in intact and denuded RAR, respectively (P > 0.05). No significant differences in the relaxation responses were found between RAR with or without endothelium (P > 0.05). The presence of either 5 μM methylene blue (MB) or 5 μM 1H-[1,2,4]oxadiazolo-[4,3-α]quinoxalin-1-one (ODQ) significantly inhibited the relaxations induced by ONOO−. Sildenafil (10−7 M), on the other hand, significantly potentiated the ONOO−-induced relaxations. Tetraethylammonium chloride (T-2265) significantly decreased the ONOO−-induced relaxations in a concentration-dependent manner. However, ONOO− had no effect on RAR precontracted by high KCL (40 mM, n = 6, P > 0.05). Addition of calyculin A also significantly decreased the ONOO−-induced relaxation in a dose-dependent manner. Furthermore, ONOO− significantly inhibited calcium-induced contractions of K+-depolarized aortic rings in a concentration-related manner. Lastly, a variety of other pharmacological agents and antagonists including l-NMMA, l-arginine, indomethacin, atropine, naloxone, diphenhydramine, cimetine, glibenclamide, haloperidol, superoxide dismutase (SOD), and catalase did not influence the relaxant effects of ONOO− on RAR. Our new results suggest that ONOO−-triggered relaxation on rat aortic rings is mediated by elevation of cGMP levels, membrane hyperpolarization via K+-channel activation, activation of myosin phosphatase activity, and interference with calcium movement and cellular membrane Ca2+ entry.