APPS November 2002 Meeting Abstract 125

Reactive oxygen species are generated in skeletal muscle¹, but there is considerable uncertainty about what effects oxidation has on the various steps in excitation-contraction (E-C) coupling and what role if any it plays in muscle fatigue. Here, we used mechanically-skinned fibres from fast-twitch (extensor digitorum longus, EDL) and slow-twitch (soleus) muscles of the rat. Electrical stimulation was used to trigger action potential-induced twitch responses². The Ca²⁺-sensitivity and maximum force production of the contractile apparatus was examined by direct activation with heavily-buffered Ca²⁺ solutions. Strong reducing conditions had little if any effect on the properties of the contractile apparatus or on twitch responses, showing that it is not necessary to have some level of fibre oxidation for normal muscle function. In EDL fibres oxidised by H₂O₂ or dithiodipyridine, exposure to the endogenous reductant, glutathione (GSH), caused a paradoxical increase in the Ca²⁺-sensitivity of the contractile apparatus that was reversed by exposure to dithiothreitol or longer exposure to GSH. This effect, which did not occur in slow-twitch fibres, could contribute to the well-known phenomenon of post-tetanic potentiation. Oxidants also increased Ca²⁺-sensitivity by another mechanism which depended on fibre activation. Prolonged exposure to H₂O₂ however reduced both sensitivity and maximum force. The amount of Ca²⁺ released from the sarcoplasmic reticulum in a twitch response was not altered by H₂O₂ treatment, even though such treatment greatly potentiated the sensitivity to Ca²⁺-induced Ca²⁺ release³. Strong oxidative treatment was also found to reduce or abolish twitch responses by interfering in some way with activation of the voltage sensors. These findings explain how H₂O₂ treatment initially potentiates, then reduces, twitch and sub-maximal tetanic force responses in intact fibres⁴ and show how oxidation can have both beneficial and deleterious effects in skeletal muscle.