Hydrogen peroxide Modulates Ca2+-Activation of Single permeabilized Fibres from Fast- and Slow-twitch Skeletal Muscles of Rats

Abstract

We examined the effects of redox modulation on single membrane-permeabilized fibre segments from the fast-twitch extensor digitorum longus (EDL) and slow-twitch soleus muscles of adult rats to determine whether the contractile apparatus was the redox target responsible for the increased contractility 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 μM and 1 mM) had no effect on maximum Ca²⁺-activated force. Subsequent exposure to the reductant dithiothreitol (DTT, 10 mM, 10 min) 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 Ca²⁺-activated force in either soleus or EDL muscle fibres. The sensitivity of the contractile filaments to Ca²⁺ (pCa₅₀) was not altered by exposure to 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 muscle fibres by decreasing the force per crossbridge.