Training at high exercise intensity promotes qualitative adaptations of mitochondrial function in human skeletal muscle

This study explored mitochondrial capacities to oxidize carbohydrate and fatty acids and functional optimization of mitochondrial respiratory chain complexes in athletes who regularly train at high exercise intensity (ATH, n = 7) compared with sedentary (SED, n = 7). Peak O(2) uptake (Vo(2max)) was measured, and muscle biopsies of vastus lateralis were collected. Maximal O(2) uptake of saponin-skinned myofibers was evaluated with several metabolic substrates glutamate-malate (V(GM)), pyruvate (V(Pyr)), palmitoyl carnitine (V(PC)), and the activity of the mitochondrial respiratory complexes II and IV were assessed using succinate (V(s)) and N,N,N',N'-tetramethyl-p-phenylenediamine dihydrochloride (V(TMPD)), respectively. Vo(2max) was higher in ATH than in SED (57.8 +/- 2.2 vs. 31.4 +/- 1.3 ml.min(-1).kg(-1), P < 0.001). V(GM) was higher in ATH than in SED (8.6 +/- 0.5 vs. 3.3 +/- 0.3 micromol O(2).min(-1).g dry wt(-1), P < 0.001). V(Pyr) was higher in ATH than in SED (8.7 +/- 1.0 vs. 5.5 +/- 0.2 micromol O(2).min(-1).g dry wt(-1), P < 0.05), whereas V(PC) was not significantly different (5.3 +/- 0.9 vs. 4.4 +/- 0.5 micromol O(2).min(-1).g dry wt(-1)). V(S) was higher in ATH than in SED (11.0 +/- 0.6 vs. 6.0 +/- 0.3 micromol O(2).min(-1).g dry wt(-1), P < 0.001), as well as V(TMPD) (20.1 +/- 1.0 vs. 16.2 +/- 3.4 micromol O(2).min(-1).g dry wt(-1), P < 0.05). The ratios V(S)/V(GM) (1.3 +/- 0.1 vs. 2.0 +/- 0.1, P < 0.001) and V(TMPD)/V(GM) (2.4 +/- 1.0 vs. 5.2 +/- 1.8, P < 0.01) were lower in ATH than in SED. In conclusion, comparison of ATH vs. SED subjects suggests that regular endurance training at high intensity promotes the enhancement of maximal mitochondrial capacities to oxidize carbohydrate rather than fatty acid and induce specific adaptations of the mitochondrial respiratory chain at the level of complex I.