Pyruvate sparing in avian and mammalian muscle mitochondria

Birds represent unique athletes as they fuel flight, a moderate‐high intensity exercise, with lipid, while mammals primarily rely on carbohydrate running at the same relative intensity. We assessed fuel selection at the level of the matrix in avian and mammalian muscle mitochondria (MITO) presented with multiple fuels. MITO were isolated from sparrow pectoralis and rat hind limb muscles and incubated for ~10 min using the creatine kinase energetic clamp at 3 values of ΔG ATP . Substrates provided were pyruvate (P), malate (M), glutamate (G), and palmitoyl carnitine (PC), as P+M, P+G+M, P+PC+M, or P+G+PC+M. O 2 consumption was measured continuously and P was assayed in acid‐stopped aliquots sampled at the beginning and end of each incubation. In sparrow MITO either PC or G addition to P+M suppressed ~2/3 of P utilization across all values of ΔG ATP examined (65±13% and 66±12%, respectively). Rat MITO were much more intensity dependent: PC or G addition decreased P utilization by 64±7% or 32±15%, respectively, at ΔG ATP = − 14.0 kcal/mol, but by only 32±7% and 10±4% at ΔG ATP = − 13.0. These results, consistent with the pattern of fuel metabolism of the whole organisms, indicate that mechanisms operating within the MITO inner membrane can importantly influence fuel selection during contractile activity.