title: Mechanism underlying H2O2-induced inhibition of acetylcholine-induced contraction in rabbit tracheal smooth muscle.
abstract: The mechanism underlying the inhibition by H2O2 of acetylcholine-induced contraction was investigated in epithelium-denuded strips of rabbit trachea. Acetylcholine (10 microM) generated a phasic, followed by a tonic increase in both the intracellular Ca2+ concentration ([Ca2+]i) and force. Although the acetylcholine-induced tonic contraction was around 9 times the high K+ (80 mM)-induced one, the two stimulants induced similar [Ca2+]i increases (around 0.2 microM), indicating that acetylcholine generates tonic contraction via increases in both [Ca2+]i and myofilament Ca2+-sensitivity. H2O2 (30 microM) (a) enhanced the acetylcholine-induced tonic (not phasic) increase in [Ca2+]i but attenuated both phases of the acetylcholine-induced contraction and (b) enhanced the high K+-induced increase in [Ca2+]i but did not modify the high K+-induced contraction. In beta-escin-skinned strips, application of acetylcholine in the presence of GTP enhanced the contraction induced by 0.3 microM Ca2+ so that its amplitude became similar to that induced by 1 microM Ca2+. H2O2 (30 microM) attenuated the contraction induced by 0.3 microM Ca2+ (alone or in the presence of acetylcholine) but not those induced by higher concentrations of Ca2+ alone (0.5 microM and 1 microM). These results indicate that H2O2 acts directly on contractile proteins in rabbit tracheal smooth muscle to inhibit the contraction induced by low concentrations of Ca2+ (<0.5 microM). An action of H2O2 that increases [Ca2+]i (and thereby masks this reactive-oxygen-induced inhibition of myofilament Ca2+-sensitivity) is apparent in the presence of high K+ but not of acetylcholine. Thus, in rabbit tracheal smooth muscle H2O2 downregulates myofilament Ca2+-sensitivity more potently during acetylcholine-induced contraction than during high-K+-induced contraction, leading to an effective inhibition of the former contraction.
authors.affiliation: Department of Pharmacology, Graduate School of Medical Sciences, Nagoya City University, Nagoya 467-8601, Japan.
authors.forename: Michihiro
authors.initials: M
authors.lastname: Saito
authors.forename: Yoshimasa
authors.initials: Y
authors.lastname: Watanabe
authors.forename: Takeo
authors.initials: T
authors.lastname: Itoh
mesh_terms: D000109:Acetylcholine / Q000037:antagonists & inhibitors* / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000032:analysis / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004594:Electrophysiology; D004928:Escin / Q000494:pharmacology; D006861:Hydrogen Peroxide / Q000494:pharmacology*; D007700:Kinetics; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects*; D009130:Muscle, Smooth / Q000187:drug effects*; D011817:Rabbits; D014132:Trachea / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism
chemical_list: D004928:Escin; D006861:Hydrogen Peroxide; D000109:Acetylcholine; D002118:Calcium