The purpose of this study was to test the hypothesis that H(2)O(2) decreases the amount of force produced by a given intracellular Ca(2+) concentration (i.e., the Ca(2+) sensitivity) in airway smooth muscle (ASM) in part by mechanisms independent of changes in regulatory myosin light chain (rMLC) phosphorylation. A new preparation was developed and validated in which canine ASM strips were first exposed to H(2)O(2) and then permeabilized with 10% Triton X-100 to assess the persistent effects of H(2)O(2) on Ca(2+) sensitivity. Experiments in which H(2)O(2) was administered before permeabilization revealed a novel mechanism that contributed to reduced Ca(2+) sensitivity independently of changes in rMLC phosphorylation, in addition to an rMLC phosphorylation-dependent mechanism. The mechanism depended on factors not available in the permeabilized ASM strip or in the buffer to which the strip was exposed, since there was no effect when H(2)O(2) was added to permeabilized strips. H(2)O(2) treatment of a maximally thiophosphorylated purified myosin subfragment (heavy meromyosin) significantly reduced actomyosin ATPase activity, suggesting one mechanism by which the phosphorylation-independent reduction in Ca(2+) sensitivity may occur.

The effect of H 2 O 2 on smooth muscle heavy meromyosin (HMM) and subfragment 1 (S1) was examined. The number of molecules that retained the ability to bind ATP and the actinactivated rate of P i release were measured by single-turnover kinetics. H 2 O 2 treatment caused a decrease in HMM regulation from 800-to 27-fold. For unphosphorylated and phosphorylated heavy meromyosin and for S1, ϳ50% of the molecules lost the ability to bind to ATP. H 2 O 2 treatment in the presence of EDTA protected against ATPase inactivation and against the loss of total ATP binding. Inactivation of S1 versus time correlated to a loss of reactive thiols. Treatment of H 2 O 2 -inactivated phosphorylated HMM or S1 with dithiothreitol partially reactivated the ATPase but had no effect on total ATP binding. H 2 O 2 -inactivated S1 contained a prominent cross-link between the N-terminal 65-kDa and C-terminal 26-kDa heavy chain regions. Mass spectral studies revealed that at least seven thiols in the heavy chain and the essential light chain were oxidized to cysteic acid. In thiophosphorylated porcine tracheal muscle strips at pCa 9 ؉ 2.1 mM ATP, H 2 O 2 caused a ϳ50% decrease in the amplitude but did not alter the rate of force generation, suggesting that H 2 O 2 directly affects the force generating complex. Dithiothreitol treatment reversed the H 2 O 2 inhibition of the maximal force by ϳ50%. These data, when compared with the in vitro kinetic data, are consistent with a H 2 O 2 -induced loss of functional myosin heads in the muscle.Contraction of smooth muscle is controlled largely by phosphorylation of the 20-kDa myosin regulatory light chain (RLC), 2 resulting in the cyclic attachment and detachment of myosin to actin (cross-bridge cycling) and the hydrolysis of ATP by actin-activated myosin ATPase (1). The level of RLC phosphorylation depends on the balance between the activities of MLCK and SMM phosphatase. MLCK is primarily activated (through Ca 2ϩ -calmodulin) by an increase in [Ca 2ϩ ] i from intracellular stores or influx of extra-cellular Ca 2ϩ , whereas inhibition of myosin phosphatase, which contributes to the amount of force at a given [Ca 2ϩ ] i (Ca 2ϩ sensitivity), occurs through a receptor-G protein-catalyzed signal transduction cascade (2-5).Reactive oxygen species, such as H 2 O 2 , modulate smooth muscle contractility (6) and contribute to the severity of numerous diseases, including acute lung injury, asthma, pulmonary hypertension, ischemia-reperfusion, and arthritis (7). H 2 O 2 reversibly inhibits receptor agonist-induced contraction of both vascular (8 -10) and ASM (11-15).The mechanisms for H 2 O 2 inhibition of smooth muscle contraction are not well understood but have been proposed to involve effects on kinase-mediated signaling cascades, cytoskeletal effects, ionotropic effects, lipid oxidation, direct inhibitory effect on smooth muscle contractile proteins, and potentially confounding effects on epithelial cells (11)(12)(13)(14)(15)(16)(17)(18)(19)(20). In general, healthy smooth muscle is quite resis...