Mitochondrial respiratory chain (RC) enzymatic activity and citrate synthase (CS) activity were measured spectrophotometrically in muscle homogenates. Muscle homogenates were prepared as described [9]. Briefly, about 50 mg of SM was cut into small fragments and homogenized (glass-glass grinder) in 20 volumes of ice-cold homogenization buffer (250 mM sucrose, 20 mM Tris, 40 mM KCl, 2 mM EGTA, protease inhibitor cocktail, pH 7.4). The homogenates were then centrifuged for 1 min at 600g at 4°C and the supernatants were immediately used for analysis. Protein concentration was measured in aliquots using the bicinchoninic acid (BCA) assay (Sigma). Activity of complex I-IV and CS was determined accordingly to previously published protocols [10-11], that were modified for measurement in a microplate reader (Infinite M200 PRO, Tecan). All measurements were made in tetraplicates. Activities of RC and CS were expressed as nmol/min/mg of total proteins, except for complex IV, which was expressed as Δ log (A550)/min/mg of total proteins. The enzymatic activities of the RC complexes were also normalized to the activity of CS, which is used as a marker of the abundance of mitochondria within a tissue.