Hydrogen peroxide decreases Ca²⁺ sensitivity in airway smooth muscle by inhibiting rMLC phosphorylation

The purpose of this study was to determine the mechanism by which hydrogen peroxide (H₂O₂), an important inflammatory mediator, relaxes canine tracheal smooth muscle (CTSM). H₂O₂ caused concentration-dependent relaxations of CTSM strips contracted with ACh or isotonic KCl [EC₅₀ of 0.24 ± 0.04 (SE) and 0.23 ± 0.04 mM, respectively]. Indomethacin (10 μM) decreased the sensitivity of both KCl- and ACh-contracted strips to H₂O₂. H₂O₂ increased intracellular cAMP levels, an increase that was abolished by indomethacin. H₂O₂ did not affect intracellular cGMP levels. In strips treated with indomethacin and contracted with ACh or isotonic KCl, H₂O₂- evoked relaxations were accompanied by increases in intracellular Ca²⁺ concentration and decreases in regulatory myosin light chain phosphorylation. We conclude that H₂O₂ decreases Ca²⁺ sensitivity in CTSM by decreasing regulatory myosin light chain phosphorylation through inhibition of myosin light chain kinase and/or activation of smooth muscle protein phosphatases.