Iron supplementation improves skeletal muscle contractile properties in mice with chronic kidney disease

Adult chronic kidney disease (CKD) patients commonly display decreased physical function, leading to increased mortality, which is exacerbated with iron deficiency and anemia. Interventions to mitigate CKD myopathy are scarce, potentially because the underlying contractile deficits have not been fully investigated. We examined single fiber force production, myosin-actin interactions and myofilament properties in soleus and extensor digitorum longus muscles of 16-week-old CKD mice without (CKD, N=12) and with iron supplementation (CKDFE, N=6, weekly injections of 0.2 mg/kg iron dextran) and controls (N=10). Late-stage CKD was induced by 0.2% adenine diet for 8 weeks. CKD with iron supplementation (CKDFE) improved anemia status compared to CKD, but not back to control levels. Maximally activated (pCa 4.5, 25°C) specific tension (maximal isometric force divided by cross-sectional area) was reduced 36-51% with CKD due, in a large part, to a reduction in strongly-bound myosin-actin cross-bridges in all myosin heavy chain (MHC) I, IIA, and IIB fibers. Iron supplementation improved specific tension and number of strongly-bound cross-bridges in MHC IIA and IIB fibers. Myosin-actin cross-bridge kinetics were slower with CKD in MHC I and IIA fibers, with iron supplementation restoring kinetics closer to control values in MHC IIA fibers. Higher hemoglobin levels were correlated to improvements in specific tension and strongly-bound cross-bridges in MHC IIA and IIB fibers, and with faster cross-bridge kinetics in MHC IIA fibers. However, CKD induced decreases in myofilament lattice stiffness for MHC IIA and IIB fibers were not improved with iron supplementation. Our findings show that iron supplementation improved various aspects of the cellular and molecular CKD-induced myopathy in fast-contracting fiber types. Thus, correction of iron imbalance and/or anemia may be an effective countermeasure to reduce contractile deficits and improve quality of life in late-stage CKD patients.