Hydrogen peroxide modulates Ca2+-activation of single permeabilized fibresfrom fast- and slow-twitch skeletal muscles of rats

We examined the effects of redox modulation on single membrane-permeabilize d fibre segments from the fast-twitch extensor digitorum longus (EDL) and s low-twitch soleus muscles of adult rats to determine whether the contractil e apparatus was the redox target responsible for the increased contractilit y of muscles exposed to low concentrations of H2O2. The effects of H2O2 on maximum Ca2+-activated force were dose-dependent with 30 min exposure to 5 mM H2O2 causing a progressive decrease by 22 +/- 3 and 13 +/- 2% in soleus and EDL permeabilized muscle fibres, respectively. Lower concentrations of exogenous H2O2 (100 muM and 1 mM) had no effect on maximum Ca2+-activated f orce. Subsequent exposure to the reductant dithiothreitol (DTT, 10 mM, 10 m in) fully reversed the H2O2-induced depression of force in EDL, but not in soleus muscle fibres. Incubation with DTT alone for 10 min did not alter Ca 2+-activated force in either soleus or EDL muscle fibres. The sensitivity o f the contractile filaments to Ca2+ (pCa(50)) was not altered by exposure t o any concentration of exogenous H2O2. However, all concentrations of H2O2 diminished the Hill coefficient in permeabilized fibres from the EDL muscle , indicating that the cooperativity of Ca2+ binding to troponin is altered. H2O2 (5 mM) did not affect rigor force, which indicates that the number of crossbridges participating in contraction was not reduced. In conclusion, H2O2 may reduce the maximum Ca2+ activated force production in skinned musc le fibres by decreasing the force per crossbridge.