Contraction intensity and feeding affect collagen and myofibrillar protein synthesis rates differently in human skeletal muscle.

Exercise stimulates muscle protein fractional synthesis rate (FSR), but the importance of contractile intensity and whether it interplays with feeding is not understood. This was investigated following two distinct resistance exercise (RE) contraction intensities using an intrasubject design in the fasted (n = 10) and fed (n = 10) states. RE consisted of 10 sets of knee extensions. One leg worked against light load (LL) at 16% of one-repetition maximum (1RM), the other leg against heavy load (HL) at 70% 1RM, with intensities equalized for total lifted load. Males were infused with [(13)C]leucine, and vastus lateralis biopsies were obtained bilaterally at rest as well as 0.5, 3, and 5.5 h after RE. Western blots were run on muscle lysates and phosphospecific antibodies used to detect phosphorylation status of targets involved in regulation of FSR. The intramuscular collagen FSR was evenly increased following LL- and HL-RE and was not affected by feeding. Myofibrillar FSR was unaffected by LL-RE, whereas HL-RE resulted in a delayed improvement (0.14 +/- 0.02%/h, P < 0.05). Myofibrillar FSR was increased at rest by feeding (P < 0.05) and remained elevated late in the postexercise period compared with the fasting condition. The Rp-s6k-4E-binding protein-1 (BP1) and the mitogen-activated protein kinase (MAPk) pathways were activated by the HL intensity and were suggested to be responsible for regulating myofibrillar FSR in response to adequate contractile activity. Feeding predominantly affected Rp-s6k and eukaryotic elongation factor 2 phosphorylations in correspondence with the observed changes in myofibrillar FSR, whereas 4E-BP1 remained to respond only to the HL contraction intensity. Thus the study design allows us to conclude that the MAPk- and mammalian target of rapamycin-dependent signaling responds to contractile activity, whereas elongation mainly was found to respond to feeding. Furthermore, although functionally linked, the contractile and the supportive matrix structures upregulate their protein synthesis rate quite differently in response to feeding and contractile activity and intensity.