Effects of reactive oxygen species on aspects of excitation-contraction coupling in chemically skinned rabbit diaphragm muscle fibres

Oxidants have been suggested to enhance contractile function in unfatigued muscle. In this study we aimed to determine the effect of oxidants on 'chemically skinned' diaphragm muscle fibre bundles. The sarcoplasmic reticulum and contractile proteins were exposed to superoxide anions (O-2(-)) and hydrogen peroxide (H2O2) under controlled conditions. Application of O-2(-) initially increased maximum Ca2+-activated force but subsequently reduced maximum Ca2+-activated force without altering myofilament Ca2+ sensitivity. Unlike myocardium, caffeine-induced Ca2+ release from the sarcoplasmic reticulum was also inhibited by O-2(-) exposure in diaphragm fibre bundles. Application of H2O2 also increased maximum Ca2+-activated force but had additional effects on resting tension (which increased to 25 % of the control maximum Ca(2+)activated force). H2O2 was without effect on myofilament Ca2+ sensitivitS or caffeine-induced Ca2+ release from the sarcoplasmic reticulum, These data demonstrate that oxidants can potentiate contractile force in the diaphragm through a direct action on the contractile proteins. The potentiation of force is not sustained, however, and under these conditions the detrimental effects of O-2(-) on Ca2+ release from the sarcoplasmic reticulum combined with the effects of oxidants on the contractile proteins mill ultimately compromise excitation-contraction coupling in the diaphragm.