Reactive oxygen and nitrogen species as intracellular signals in skeletal muscle.

It is well established that contracting skeletal muscles produce free radicals. Given that radicals are known to play a prominent role in the pathogenesis of several diseases, the 1980s-90s dogma was that contraction-induced radical production was detrimental to muscle because of oxidative damage to macromolecules within the fibre. In contrast to this early outlook, it is now clear that both reactive oxygen species (ROS) and reactive nitrogen species (RNS) play important roles in cell signalling pathways involved in muscle adaptation to exercise and the remodelling that occurs in skeletal muscle during periods of prolonged inactivity. This review will highlight two important redox sensitive signalling pathways that contribute to ROS and RNS-induced skeletal muscle adaptation to endurance exercise. We begin with a historical overview of radical production in skeletal muscles followed by a discussion of the intracellular sites for ROS and RNS production in muscle fibres. We will then provide a synopsis of the redox-sensitive NF-B and PGC-1α signalling pathways that contribute to skeletal muscle adaptation in response to exercise training. We will conclude with a discussion of unanswered questions in redox signalling in skeletal muscle in the hope of promoting additional research interest in this field.