Effects of superoxide generating systems on muscle tone, cholinergic and NANC responses in cat airway

To study the possible role of reactive oxygen species in airway hyperreactivity, we examined the effects of the superoxide anion radical (O-2(-)) generating systems, pyrogallol and xanthine with xanthine oxidase, on muscle tone, excitatory and inhibitory neurotransmission in the cat airway. Smooth muscle contraction or non-adrenergic non-cholinergic (NANC) relaxation evoked by electrical field stimulation (EFS) were measured before or after O-2(-) generating systems with or without diethydithiocarbamic acid (DEDTCA), an inhibitor of endogenous superoxide dismutase (SOD). Resting membrane potential or excitatory junction potential (EJP) were also measured in vitro. Both pyrogallol and xanthine/xanthine oxidase produced biphasic changes in basal and elevated (by 5-HT) muscle tone. After SOD pretreatment, both systems consistently produced a prolonged contraction, thereby indicating that O-2(-) was converted to H2O2 by the action of SOD and as a result the actions of O-2(-) were lost but those of H2O2 introduced. The O-2(-) showed no significant effect on smooth muscle contraction or EJP evoked by EFS, however after DEDTCA pretreatment, it evoked initial enhancement followed by suppression of the contraction and EJP. DEDTCA pretreatment ameliorated the inhibitory action of pyrogallol and xanthine/xanthine oxidase on the NANC relaxation, probably because O-2(-) could combine with endogenous NO to form peroxynitrite. These results indicate that the O-2(-) generating systems have multiple actions, presumably due to the presence and simultaneous action of at least two different reactive oxygen species (O-2(-) and H2O2). While H2O2 seems to be responsible for elevation of muscle tone and augmentation of smooth muscle contraction by EFS, O-2(-) inhibits muscle tone, cholinergic and NANC neurotransmission. (C) 2000 Elsevier Science B.V. All rights reserved.