Question

Performing endurance training 1 hour prior to resistance training (aka concurrent training) may result in the following signaling mechanisms occurring in skeletal muscle:

a. AMPK activating mTOR, resulting in an increase in protein synthesis

b. AMPK inhibiting mTOR, resulting in an increase in protein synthesis

c. mTOR activating AMPK, resulting in a reduction in protein synthesis

d. AMPK inhibiting mTOR, resulting in a reduction in protein synthesis

e. None of the above

Answer 1

In concurrent training, endurance exercise (EE) and resistance exercise (RE) are used in succession. Concurrent training has been used for improvement of cardiovascular functions and muscle strength.

Muscle mass is regulated by mTORC1 by number of stimuli like mechanical stress, growth factors hormones etc. It regulates muscle mass via its effects on initiation of translation and synthesis of muscle proteins. In RE, mTORC1 signaling is increased causing increased phosphorylation of target such as p70S6K, leading to increase protein synthesis in muscles and followed by muscle hypertrophy. In EE, it is AMPK (AMP-activated protein kinase) signaling that is activated. This AMPK signaling is known to inhibit mTORC1 signaling in EE. This leads to decreased protein synthesis and muscle hypertrophy. This inhibition of mTORC1 by AMPK, is via effects on Raptor protein. Raptor is a target for AMPK. It is phosphorylated at Ser792 by AMPK, leading to its inactivation and therefore causes inhibition of mTORC1 signaling. AMPK also phosphorylates Tublero scleorisis complex (TSC2), and activates. This TISC2 is an inhibitor of mTOR. AMPK also prevents mTOR from localizing in to the lysosome.

Normally, 3 hour difference should be maintained between EE and RE. This is to allow AMPK activation to increase and then subside to baseline. This will remove the inhibitory effect of AMPK on mTORC1 signaling. After 3hr, RE can then upregulate mTORC1 for stimulating protein synthesis and muscle hypertrophy. However, when EE is performed just 1hr prior to RE, AMPK is still high enough to inhibit mTORC1 signaling. This will cause reduction in protein synthesis in muscles.

Right choice: d. AMPK inhibiting mTOR, resulting in a reduction in protein synthesis.