EFFECTS OF HYDROGEN-PEROXIDE ON ISOLATED TRACHEALIS MUSCLE OF HORSES

During acute bouts of recurrent airway obstruction (heaves) in horses, neutrophils that are capable of increased production of reactive oxyg en species accumulate in the airways. In the study reported here, the effect of hydrogen peroxide (H2O2; 1 mu M to 0.1M), one of these react ive oxygen species products, on the responses of isolated trachealis m uscle of horses was determined. Before and after incubation with H2O2, contractile responses to acetylcholine, electrical field stimulation (EFS), 127 mM KCl, and relaxation responses to isoproterenol and activ ation of the nonadrenergic noncholinergic inhibitory response (iNANC) were evaluated. Beginning at 1 mM, H2O2 contracted trachealis muscle i n a concentration-dependent manner. This contraction was unaffected by atropine (1 mu M), tetrodotoxin (1 mu M), or 1 mu M meclofenamate. Co ntraction of trachealis muscle in response to H2O2 is, therefore, not attributable to release of prostaglandins, acetylcholine, or other neu rotransmitters. Above a concentration of 0.1 mM, H2O2 depressed the re sponses to EFS, acetylcholine, and KCl in a concentration-dependent ma nner. At 0.1M, H2O2 decreased the maximal responses to EPS, acetylchol ine, and KCl by 62.7 +/- 7.2, 60.58 +/- 6.12, and 37.8 +/- 9.54%, resp ectively. In the presence of meclofenamate (1 mu M), partial but signi ficant protection against 1 to 100 mM H2O2 was observed. In tracheal s trips contracted with 0.3 mu M methacholine, H2O2 had no effect on the isoproterenol concentration-response curve. Up to a concentration of 100 mM, H2O2 had no effect on iNANC response. However, in the presence of 100 mM H2O2, this response was abolished in 2 of 4 horses. We conc lude that high concentrations of H2O2 affected the responses of airway smooth muscle by actions on neurotransmission, muscarinic receptors, and downstream from receptors; some of the H2O2 effects were in part m ediated by cyclooxygenase products; and H2O2 had no effect on beta-adr energic- or iNANC-induced relaxation.