Beeting atrophy: dietary nitrate to protect the powerhouse of the cell?

Disuse of skeletal muscle results in muscle wasting, a debilitating condition characterized by loss of muscle strength, speed and power, as well as increased fatiguability during and decreased recoverability following muscle contractions. These contractile changes are due in part to reduced muscle mass, which results from the activation of proteolytic and/or slowing of protein synthesis pathways within muscle, changes that are also observed with ageing, cancer, AIDS, heart failure as well as neuromuscular diseases (Sartori et al., 2021). Disuse is also accompanied by a decline in muscle mitochondrial content, as a result of an imbalance between mitochondrial protein synthesis and degradation of mitochondria via autophagy (Memme et al., 2021). Lower mitochondrial respiratory capacity has been linked to decreased ATP production rates, increased inflammation, as well as increased reactive oxygen species (ROS) production (Memme et al., 2021). Despite these negative consequences, few viable treatments for muscle wasting are available. In this issue of The Journal of Physiology, Petrick et al. (2023) sought to determine whether dietary nitrate (NO3−) supplementation would attenuate the above effects of disuse-induced muscle atrophy. Dietary NO3−, a source of nitric oxide (NO) via the NO3− → nitrite → NO pathway, was studied because it has been shown to preserve mitochondrial bioenergetics in situations of impaired mitochondrial function (e.g. Zhu et al., 2011), thereby potentially offsetting or reversing some of the effects of disuse. Atrophy was induced in female C57Bl/6N mice by casting one leg for 3 or 7 d, with the drinking water of half the animals supplemented with 1 mm NaNO3. Both 3 and 7 d of single-leg immobilization led to significantly lower myofibrillar protein fractional synthesis rates (FSR) and decreased muscle mass of the soleus and gastrocnemius+plantaris compared to the control limb, with no effect of NO3− supplementation on these changes. Thus, dietary NO3− was unable to prevent disuse-induced muscle atrophy. However, dietary NO3− supplementation did prevent decreases in subsarcolemmal and intermyofibrillar mitochondrial protein FSR in the immobilized limb after 3 d of immobilization. Although this was not true after 7 d, NO3− supplementation preserved mitochondrial bioenergetics (e.g. submaximal and maximal respiratory rates) after both 3 and 7 d of immobilization. Dietary NO3− also prevented the immobilization-induced increase in ROS emission observed in the control animals. Petrick et al. (2023) therefore concluded that dietary NO3− can potentially preserve mitochondrial bioenergetics but does not prevent muscle atrophy due to immobilization. Thus, in this atrophy model dietary NO3− disassociated decreases in mitochondrial function from muscle mass loss. This supports previous findings in a mouse hindlimb nerve injury model, in which the AMP protein kinase activator 5-aminoimidazole-4-carboxamide ribonucleotide elevated peroxisome proliferator-activated receptor γ coactivator 1-α, cytochdrome c, and succinate dehydrogenase protein levels but no influence on the denervation-induced muscle atrophy (Brault et al., 2010). These intriguing findings by Petrick et al. (2023) lead to several questions on potential mechanism of action of NO3− supplementation on atrophy. First, precisely how did increased production of NO maintain submaximal/maximal mitochondrial respiration and prevent the immobilization-induced increase in ROS emission? It is unclear whether the reduction in respiratory capacity is simply an adaptation to reduced ATP demand during immobilization or is mechanistically linked to the decrease in muscle mass. As well, preservation of mitochondrial function has the potential to reduce fatigability during exercise or enhance recoverability following exercise, but this remains to be demonstrated. Second, would the beneficial effects of dietary NO3− be observed in other situations where atrophy occurs, e.g. ageing, cancer, neuromuscular disease, denervation, etc.? Finally, the use of single-limb casting, although representative of common atrophy-inducing clinical conditions (e.g. a broken ankle), may have potentially led to increased use of the contralateral limb, a factor that needs to be kept in mind in interpreting the study's results. Would the effects of NO3− supplementation be the same using a model where whole-body muscle atrophy is induced? Regardless of any outstanding questions, Petrick et al. (2023) have laid the groundwork and provided a strong rationale for future studies of the effects of dietary NO3− administration in various models of atrophy. In particular, it will be important to determine the mechanisms and functional consequences of preserving mitochondrial bioenergetics during disuse-induced atrophy, and whether similar beneficial effects can be demonstrated in other states associated with muscle wasting, such as ageing, denervation or cancer. Given the broad clinical relevance, it is especially important to determine whether dietary NO3− supplementation has similar effects in humans. Petrick et al. (2023) are therefore to be commended not only for their well-conducted study but also for opening new avenues for investigation for combating the deleterious effects of muscle atrophy. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article. No competing interests declared. All authors have read and approved the final version of this manuscript and agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. All persons designated as authors qualify for authorship, and all those who qualify for authorship are listed. This work was funded by a grant from the National Heart, Lung, and Blood Institute (NHLBI) to A.R.C. (HL155858). HL155858 is a multiple PI grant to Linda R. Peterson, Kenneth B. Schechtman.