Differential responses to endurance training in subsarcolemmal and intermyofibrillar mitochondria

To examinethe effect of endurance training (6 wk of treadmill running) onregional mitochondrial adaptations within skeletal muscle,subsarcolemmal (SS) and intermyofibrillar (IMF) mitochondria wereisolated from trained and control rat hindlimb muscles.Mitochondrial oxygen consumption(O₂) was measuredpolarographically by using the following substrates: 1 mM pyruvate + 1 mM malate (P+M), 10 mM 2-oxoglutarate, 45 µMpalmitoyl-DL-carnitine + 1 mMmalate, and 10 mM glutamate. Spectrophotometric assays ofcytochrome-c reductase andNAD-specific isocitrate dehydrogenase (IDH) activity were alsoperformed. Maximal (state III) and resting (state IV) O₂ were lower in SS than inIMF mitochondria in both trained and control groups. In SSmitochondria, training elicited significant 36 and 20% increases instate III O₂ with P+M andglutamate, respectively. In IMF mitochondria, training resulted in asmaller (20%), yet significant, increase in state IIIO₂ with P+M as a substrate,whereas state IIIO₂ increased 33 and 27% with 2-oxoglutarate andpalmitoyl-DL-carnitine + malate,respectively. Within groups,cytochrome-c reductase and IDHactivities were lower in SS when compared with IMF mitochondria.Training increased succinate-cytochrome-c reductase inboth SS (30%) and IMF mitochondria (28%). IDH activity increased 32%in the trained IMF but remained unchanged in SS mitochondria. Weconclude that endurance training promotes substantial changes inprotein stoichiometry and composition of both SS and IMF mitochondria.