Figure 4. Mitochondrial respiration declines in the tibialis anterior, but it is maintained in the soleus upon aging. Respiration in mitochondrial fractions isolated from the tibialis anterior (A, C, E, G) and soleus (B, D, F, H) muscles of young (3 mo) and old (28-29 mo) mice was assessed using pyruvate (Pyr), succinate (Succ), and palmitoylcarnitine (PCar) as substrates (see also Figures S4 and S5). Basal and ADP-dependent mitochondrial respiration is shown. Data were normalized for either the protein content of the mitochondrial fraction (A-D; see Figure S4B) or citrate synthase activity/µg of protein (E-H; see Figure S4C), and the means plus standard deviations of 6-11 (Pyr) or 5-6 (Succ, PCar) mice/group are shown. Congenic CD45.2 and CD45.1 old mice were used (young mice were all CD45.2): white dots = CD45.2, black dots = CD45.1. *p ≤ 0.05, **p ≤ 0.01.