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1 GIH, Lidingövägen 2, Box 5626, Stockholm, SE 11486, Sweden; Physiology and Pharmacology, Karolinska Institutet, Sweden; Institute of Sports Science and Clinical Biomechanics, University of Southern Denmark, Odense, Denmark
2 Institute of Sports Science and Clinical Biomechanics, University of Southern Denmark, Odense, Denmark
3 GIH, Lidingövägen 2, Box 5626, Stockholm, SE 11486, Sweden; Physiology and Pharmacology, Karolinska Institutet, Sweden
* To whom correspondence should be addressed. E-mail: kent.sahlin{at}fyfa.ki.se.
The purpose of this study was to investigate fatty acid (FA) oxidation in isolated mitochondrial vesicles (mit) and its relation to training status, fibre type composition and whole body FA oxidation. Trained (VO2peak 60.7±1.6, n=8) and untrained subjects (39.5±2.0 ml min-1 kg-1, n=5) cycled at 40, 80 and 120 W and whole body relative FA oxidation was assessed from respiratory exchange ratio (RER). Mit were isolated from muscle biopsies and maximal ADP stimulated respiration was measured with CHO derived substrate (Pyr: pyruvate+malate) and FA derived substrate (PC: palmitoyl-L-carnitine+malate). Fibre type composition was determined from analysis of myosin heavy chain (MHC) composition. The rate of mit oxidation was lower with PC than with Pyr and the ratio between PC and Pyr oxidation (MFO) varied greatly between subjects (49-93%). MFO was significantly correlated to muscle fibre type distribution i.e. % MHC I (r=0.62, P = 0.03) but was not different between trained (62±5%) and untrained subjects (72±2%). MFO was correlated to RER during submaximal exercise at 80 W (r=-0.62, P=0.02), 120 W (r=-0.71, P=0.007) and interpolated 35% VO2peak (r=-0.74, P=0.004). ADP sensitivity of mit respiration was significantly higher with PC than with Pyr. It is concluded that MFO is influenced by fibre type composition but not by training status. The inverse correlation between RER and MFO implies that intrinsic mit characteristics are of importance for whole body FA oxidation during low intensity exercise. The higher ADP sensitivity with PC than that with Pyr may influence fuel utilization at low rate of respiration.
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