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Altered skeletal muscle redox status, structural integrity, and ryanodine receptor are associated with contractile dysfunction following systemic iron loading in mice

We previously reported that a moderate degree of systemic iron loading caused muscle functional impairment in mice. Such deterioration was not attributed to oxidative stress since it did not incur protein or lipid oxidative damage. On the other hand, muscle contractile dysfunction was associated with increased thioredoxin protein levels, suggestive of altered redox signaling. Thus, the purpose of this study was to further examine skeletal muscle redox status and potential meditators of contractile dysfunction following a physiologically‐relevant degree of iron loading. Twelve‐week‐old male CD‐1 mice received subcutaneous injections of iron (4 mg iron dextran/200 μL) or vehicle 5 days/week for 2 weeks (n=10/group). One day following the last injection the gastrocnemius muscles were harvested for measurement of biochemical parameters. Systemic iron loading elevated the muscle non‐heme iron (NHI) concentration by approximately 4‐fold (P < 0.001). Although reduced glutathione GSH (P = 0.39) and glutathione peroxidase (GPX) activity (P = 0.82) measures were similar between the two groups, iron‐loaded mice displayed greater glutathione disulfide (GSSG; P = 0.03) and GSSG/GSH ratio (P = 0.03), which are sensitive markers of oxidative stress and redox status. In addition, redox regulator glutaredoxin‐2 (GRX2) protein expression was lower in the muscle of iron‐loaded mice compared with controls (P = 0.04). Muscle structural integrity marker αII‐spectrin at 240 kDa was lower in the iron‐loaded muscles compared with controls (P = 0.02). No differences were observed in its cleavage products at 150 kDa (P = 0.41) or 120 kDa (P = 0.11). Similar to αII‐spectrin, protein levels of focal adhesion α‐actinin were also lower in the iron‐loaded gastrocnemius (P = 0.04). Levels of calstabin‐1 associated with ryanodine receptor (RyR1) were lower in iron‐loaded muscles compared with controls (P = 0.04), while total RyR1 (P = 0.99) was similar between groups. In summary, contractile dysfunction that results from moderate iron loading may be mediated by a disturbance in the muscle redox balance and from changes arising from an increased proteolytic response and aberrant sarcoplasmic reticulum Ca2+ release.