Purpose: The objective of this study was to investigate the time course of the endurance training–induced adaptations in two major mitochondrial functions.
Methods: Forty rats were divided into four groups: a control group and three training groups—a 1-d training group, a 5-d training group, and a 10-d training group. The training protocol consisted of 30 min of running on a motorized treadmill (26 m·min−1, 15% grade). Nuclear respiratory factor-1; transcription factor A, mitochondrial; superoxide dismutase-2; glutathione peroxidase-4; and citrate synthase (CS) messenger RNA levels were measured by qPCR. Mitochondrial respiration and H2O2 release were assessed using permeabilized fibers of white gastrocnemius in situ. Calculation of free radical leak was performed in two conditions where substrates were identical in both measurements. CS activity was assessed spectrophotometrically.
Results: An early time-dependent modulation in messenger RNA levels was observed with training: nuclear respiratory factor-1 and superoxide dismutase-2 levels increased after acute exercise, transcription factor A, mitochondrial and CS levels improved after 5 d, and glutathione peroxidase-4 levels increased after 10 d. CS activity improved by 29% ± 8% (P < 0.01) after 5 d together with a 50% ± 7% reduction in the free radical leak (P < 0.05). Finally, 10 d of endurance training did not significantly alter mitochondrial H2O2 release but increased mitochondrial respiration rates in situ (P < 0.05).
Conclusions: Our results demonstrate that mitochondrial adaptations follow a sequential program in which mitochondrial respiration and free radical leak adaptations occur according to a different timing. Collectively, these results suggest early mitochondrial qualitative adaptations in response to endurance training.