Reactive oxygen and nitrogen species in skeletal muscle: acute and long-term effects

Abstract

It is a common belief that oxidative stress imposes a considerable threat to our health and that antioxidants promote health. Along these lines, foodstuffs are considered healthy if they contain antioxidants and advertisements for expensive supplements to increase our defence against oxidants are commonplace. Unfortunately, the term ‘oxidative stress’ is poorly defined and it is uncertain whether relatively minor changes in the dietary intake of antioxidants are important in relation to the internal systems that control the redox balance. Although it is clear that excessive amounts of reactive oxygen species (ROS) and reactive nitrogen species (RNS) can induce pathological processes, numerous recent studies have also shown important effects of ROS/RNS in integral physiological signalling events. The complex effects of ROS/RNS in skeletal muscles were addresses in The Journal of Physiology Symposium Reactive oxygen and nitrogen species in skeletal muscle – acute and long-term effects, which was held in September 2010 in association with the XXXIX European Muscle Conference in Abano Terme, Italy. The goal of this well-attended symposium was to review, and thereby increase our understanding of, how ROS/RNS affect skeletal muscle acutely and in the long term. The symposium started with Hakan Westerblad presenting data relating to acute effects of ROS/RNS on skeletal muscle contractile function and cellular Ca2+ handling. Exposure of single intact muscle fibres to ROS results in biphasic effects with an initial increase in force production followed by a decline during prolonged exposure. Myofibrillar Ca2+ sensitivity is the parameter most sensitive to ROS exposure. Results also show that increases in ROS are difficult to detect during acute fatigue, but are still likely to occur since the recovery after fatigue shows ROS dependency. Graham Lamb then used data obtained from experiments on skinned muscle fibres to elucidate mechanisms underlying the effects seen in intact muscle fibres (Lamb & Westerblad, 2011). His data highlighted the importance of the cellular constituents glutathione and myoglobin in the acute response to ROS/RNS and the difference in response between fast- and slow-twitch fibres, where the former are much more sensitive to ROS/RNS. Moreover, skinned fibre results show that while the sarcoplasmic reticulum SR Ca2+ release channels (the ryanodine receptors) are highly sensitive to ROS/RNS, this has little effect on their physiological activation induced by action potentials, which is in accordance with results obtained in intact fibres. Scott Powers discussed the role of ROS/RNS in cellular signalling pathways involved in muscle adaptations to exercise or the contrary, i.e. periods of prolonged inactivity (Powers et al. 2011). He highlighted the paradox that inactivity leads to increased ROS production, which contributes to disuse muscle atrophy, while exercise also increases ROS production, but in this case they contribute to the beneficial muscle effects induced by training. The mechanism underlying this paradox is not clear, but might involve prolonged vs. acute increases in ROS and/or different locations of ROS production. In this context it is interesting to note that recent data indicate that inactivity leads to increased mitochondral ROS production, whereas the ROS produced during exercise seem to originate mostly from non-mitochondrial sources. Moreover, signalling pathways involving nuclear factor-κB (NF-κB) and peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) were identified as important for training-induced muscle adaptations. Malcolm Jackson presented data related to the effects of increased ROS production induced by contractile activity in young and adult vs. aged muscles (Jackson & McArdle, 2011). The data presented show that in young and adult muscle, contraction-induced increases in ROS production activate redox-sensitive signalling pathways, which results in improved defence against oxidative damage. These pathways involve activation of the ROS-sensitive transcription factors NF-κB, activator protein-1 and heat shock factor 1. On the other hand, recent data indicate that these beneficial responses to contraction-induced ROS are severely attenuated in aged muscle, which then leads to oxidative damage. Paradoxically there are also results showing an attenuated ROS production in response to contractile activity in aged muscle. Tentative mechanisms behind the impaired adaptations to contractile activity in aged muscle were discussed and the exact nature of these remains to be established. Roberto Bottinelli reconsidered evidence supporting the widely accepted role of ROS in determining disuse muscle atrophy (Pellegrino et al. 2011). The relationship between muscle atrophy and increases in protein and lipid peroxidation and impairment in antioxidant defence systems has been taken as a strong indication of a causal role of oxidative stress. This relationship clearly holds true in the very fast developing and dramatic diaphragm atrophy following mechanical ventilation. Importantly, in this situation, a direct link between oxidative stress and increased proteolysis could be established. However, such a relationship appears less clear in slow-twitch soleus muscles from hindlimb unloaded mice and it appears to be absent in fast-twitch gastrocnemius muscles of the same animals. Moreover, limited evidence exists that oxidative stress could play a determinant role in human limb muscle atrophy following immobilization or bed rest. To explain the variable role of oxidative stress in different muscles, species and models, a tentative unifying hypothesis was presented based on differences in muscle phenotype, rate of oxidative metabolism, and relative decreases in load and neuromuscular activity. The latter variables could affect the extent and time course of ROS production and the degree of activation of different ROS-dependent intracellular signalling pathways and determine whether protein oxidation and rate of proteolysis play a major role in the phenomenon. A pivotal role of altered ROS/RNS is now widely recognized in the pathogenesis of muscle wasting and weakness not only in disuse and ageing, but also in a variety of pathological conditions: muscular dystrophies, chronic heart failure, chronic obstructive pulmonary disease, kidney disease, rheumatoid arthritis, sepsis, cancer, type 2 diabetes, and liver disease. In this context, John Lawler reviewed the role of oxidative stress in the pathology of Duchenne muscular dystrophy (DMD), which is the most common and devastating muscular dystrophy in humans (Lawler, 2011). DMD is caused by X-linked defects in the gene encoding dystrophin, which is an important scaffolding protein in the dystroglycan complex of the sarcolemmal cytoskeleton. Disruption of this complex leads to impaired mechanical integrity of the sarcolemma and altered cellular signalling, including ROS/RNS-dependent signalling. Oxidative stress may contribute to the muscle damage and weakness seen in DMD and recent data indicate that NAD(P)H oxidase plays a key role in this context. NAD(P)H oxidase could then be a part of a cascade through which caveolin-3, stretch activated channels, NF-kB and dislocation of neuronal nitric oxide synthase (nNOS) from the dystroglycan complex all contribute to damage, inflammation and impaired contractile function in DMD. Michael Reid reviewed the role of oxidative stress on skeletal muscle function in subjects with chronic heart failure (CHF) (Reid & Moylan, 2011). He reported that weakness of limb and respiratory muscles in patients afflicted by CHF is not solely due to muscle atrophy, but also to an intrinsic loss of contractile capacity, i.e. the force loss exceeds the loss of muscle mass. The major trigger of the latter contractile dysfunction would be tumour necrosis factor-α (TNF), a cytokine involved in systemic inflammation. Several results support this conclusion; for instance, the serum concentration of TNF is inversely related to muscle strength in patients with CHF. Importantly, the effects of TNF can be mimicked by ROS, and muscle-derived oxidants have been shown to be essential mediators of TNF-induced muscle dysfunction. Although the role of ROS in mediating TNF effects is well supported, the types of oxidants stimulated by TNF, the pathways involved and the intracellular mechanisms causing force depression are still important subjects for future research. In summary, the symposium Reactive oxygen and nitrogen species in skeletal muscle – acute and long-term effects highlighted the central roles of ROS/RNS in numerous processes in skeletal muscle. Their complex effects are striking and increased ROS/RNS production can result in virtually the opposite effects depending on the circumstances. Thus, our knowledge of ROS/RNS effects in muscle has increased substantially during recent years, but there are still major gaps in our understanding. Further progress depends on, for instance, improved methods to accurately detect changes in ROS/RNS and their effects. Also, in many cases there is still a need for wider perspectives where results are interpreted with an open mind and without the preconceived idea that ‘oxidative stress’ must be deleterious.