The effect of peroxynitrite on sphincter of Oddi motility

Background. Nitric oxide (NO*) is an inhibitory neurotransmitter that induces sphincter of Oddi relaxation. Superoxide (O₂/(·-)-scavenging enzymes are present in enteric plexuses of the sphincter of Oddi and O₂/(·- ) alters sphincter of Oddi motor function. O₂/(·-) rapidly oxidizes nitric oxide (NO*) to form peroxynitrite (ONOO^-), thus terminating the biological activity of NO*. The aim of our study was to determine the effects of ONOO^- on sphincter of Oddi motility in vitro. Materials and methods. Adult opossums were sacrificed and the sphincter of Oddi was removed and placed in a tissue bath containing oxygenated Krebs solution at 37°C. In the first series of experiments, force transducers recorded tension in a transverse orientation at two sites along the spontaneously contracting sphincter of Oddi. In a second series of experiments, circular muscle strips were precontracted with carbachol and stimulated by an electrical field. Results. ONOO^-, superoxide dismutase (SOD), Nω-nitro-L-arginine (L-NNA), or oxyhemoglobin were added to the tissue baths. ONOO^- decreased the frequency of contractions in the spontaneously contracting sphincter of Oddi. Adding hemoglobin increased the frequency of contractions. ONOO^- also increased the stimulation-induced relaxation compared to controls. The increase in relaxation induced by ONOO^- was inhibited by oxyhemoglobin and L-NNA but not SOD. Pretreatment with oxyhemoglobin prevented the increase in the stimulation-induced relaxation caused by ONOO^-. Conclusion. These results suggest that hemoglobin binds ONOO^- or that ONOO^- generates NO.