Abstract
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 1 h, 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, and 120 min; at time to exhaustion (T-e); and at 3 and 24 h during recovery. The soleus and the deep red regions of the quadriceps muscles were pooled. Results: AMPK phosphorylation (Thr172) increased from 30 min to T-e, and FoxO3a phosphorylation (Thr32 and Ser253) decreased from 120 min to 3 h after exercise. FoxO3a-dependent E3 ligases Mul1 and MuRF1 proteins increased from 30 min to T-e and at T-e and 3 h after exercise, respectively, whereas MAFbx/atrogin-1 protein expression did not change significantly. The autophagic markers LC3B-II increased at 120 min and T-e, and p62 significantly decreased at T-e. The AMPK-dependent phosphorylation of Ulk1 at Ser317 and Ser555 increased from 60 min to T-e and at 30 and 60 min, respectively. Akt (Ser473), MTOR (Ser2448), and 4E-BP1 (Thr37/46) phosphorylation decreased from 90 min to T-e, and the MTOR-dependent phosphorylation of Ulk1 (Ser757) decreased from 120 min to T-e. Ser616 phosphorylation of the mitochondrial fission marker DRP1 increased from 60 min to T-e, but protein expression of the fusion markers mitofusin-2, a substrate of Mul1, and OPA1 did not significantly change. Conclusions: 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.