Purpose: The aim of this study was to characterize skeletal muscle protein breakdown and mitochondrial dynamics markers at different points of endurance exercise.
Methods: Mice run at 10 m.min-1 during 1h and running speed was increased by 0.5 m.min-1 every minute during 40 min and then by 1 m.min-1 until exhaustion. Animals were killed by cervical dislocation at 30, 60, 90, 120 min, time to exhaustion (Te), and 3 and 24h during recovery. The soleus and the deep red regions of the quadriceps muscles were pooled.
Results: AMPK phosphorylation (Thr172) increased from 30 min to Te, and FoxO3a phosphorylation (Thr32 and Ser253) decreased from 120 min to 3h post-exercise. FoxO3a-dependent E3 ligases Mul1 and MuRF1 proteins increased from 30 min to Te and at Te and 3h post-exercise respectively, whereas MAFbx/atrogin-1 protein expression did not change significantly. The autophagic markers LC3B-II increased at 120 min and Te, and p62 significantly decreased at Te. The AMPK-dependent phosphorylation of Ulk1 at Ser317 and Ser555 increased from 60 min to Te, and at 30 min and 60 min respectively. Akt (Ser473), MTOR (Ser2448) and 4E-BP1 (Thr37/46) phosphorylation decreased from 90 min to Te, and the MTOR-dependent phosphorylation of Ulk1 (Ser757) decreased from 120 min to Te. Ser616 phosphorylation of the mitochondrial fission marker DRP1 increased from 60 min to Te, but protein expression of the fusion markers mitofusin-2, a substrate of Mul1, and OPA1 did not significantly change.
Conclusion: These results fit with a regulation of protein breakdown triggered by FoxO3a and Ulk1 pathways after AMPK activation and Akt/MTOR inhibition. Furthermore, our data suggest that mitochondrial fission is quickly increased and mitochondrial fusion is unchanged during exercise.
(C) 2014 American College of Sports Medicine