Trained Integrated Postexercise Myofibrillar Protein Synthesis Rates Correlate with Hypertrophy in Young Males and Females

Purpose Resistance training induces skeletal muscle hypertrophy via the summated effects of postexercise elevations in myofibrillar protein synthesis (MyoPS) that persist for up to 48 h, although research in females is currently lacking. MyoPS is regulated by mTOR translocation and colocalization; however, the effects of resistance training on these intracellular processes are unknown. We hypothesized that MyoPS would correlate with hypertrophy only after training in both sexes and would be associated with intracellular redistribution of mTOR. Methods Recreationally active males and females (n = 10 each) underwent 8 wk of whole-body resistance exercise three times a week. Fasted muscle biopsies were obtained immediately before (REST) and 24 and 48 h after acute resistance exercise in the untrained (UT) and trained (T) states to determine integrated MyoPS over 48 h (D2O ingestion) and intracellular mTOR colocalization (immunofluorescence microscopy). Results Training increased (P < 0.01) muscle strength (similar to 20%-126%), muscle thickness (similar to 8%-11%), and average fiber cross-sectional area (similar to 15%-20%). MyoPS increased above REST in UT (P = 0.032) and T (P < 0.01), but to a greater extent in males (similar to 23%; P = 0.023), and was positively (P < 0.01) associated with muscle thickness and fiber cross-sectional area at T only in both males and females. mTOR colocalization with the cell periphery increased (P < 0.01) in T, irrespective of sex or acute exercise. Training increased (P <= 0.043) total mTOR, LAMP2 (lysosomal marker), and their colocalization (P < 0.01), although their colocalization was greater in males at 24 and 48 h independent of training status (P < 0.01). Conclusions MyoPS during prolonged recovery from exercise is greater in males but related to muscle hypertrophy regardless of sex only in the trained state, which may be underpinned by altered mTOR localization.

Automated summary

Resistance training increases muscle strength, thickness, and fiber cross-sectional area. MyoPS increases to a greater extent in males and is associated with muscle hypertrophy, potentially due to altered mTOR localization.