The Effect of Acute Resistance Exercise On Muscle Protein Synthesis in Atrophied Rat Skeletal Muscle After Unloading
Authors: Ato Satoru , Kido Kohei , Miyake Tatsuki , Yokokawa Takumi , Fujita Satoshi 
Published: November 2023

IntroductionSkeletal muscle is an essential factor that is involved in locomotive ability as well as metabolic capacity. Skeletal muscle mass is mainly determined by net balance of muscle protein turnover (i.e. muscle protein synthesis and breakdown). Skeletal muscle mass is quickly recovered in response to resistance exercise even after prolonged inactivity (also known as muscle memory). Although mechanism of this phenomenon are still unclear, it is assumed that recovery of skeletal muscle mass is caused by changing muscle protein metabolism (i.e. translation capacity) in response to resistance exercise. In eukaryotes, protein synthesis is followed by DNA‐RNA‐protein, and it also known as “Central Dogma”. It is well known from previous that DNA abundance correlate with RNA and protein abundance in yeast. Skeletal muscle fiber (matured skeletal muscle cell) has large number of myonucleus. Interestingly, recent study reported that myonuclear number does not decrease after long inactivity in rodent hind limb muscle. Therefore DNA content is increase relative to the cytosol volume after inactivity, which might contribute to explain a mechanism for the “muscle memory”. However, no study was performed whether atrophied muscle has higher anabolic capacity in response to acute resistance exercise. Therefore, purpose of present study was to investigate the effect of acute resistance exercise on muscle protein synthesis in atrophied rat skeletal muscle after unloading.MethodsMale Sprague‐Dawley rats (age of 10‐week) were randomly assigned to either hindlimb‐unloading (HU) group or control group (CON). Two‐weeks tail suspension was exerted in HU group, while CON group was kept in ground. After the 2 weeks of habituation, acute resistance exercise was exerted on right hind limb by percutaneous electrical stimulation (3sec contraction×10times, total 5sets, 100Hz, ~30V). In order to label nascent polypeptide (i.e. synthetized protein), puromycin was intraperitoneally injected 15min before harvest. Gastrocnemius muscle was obtained as sample at 3h after exercise. Morphometric analysis was performed using isolated single fiber (NaOH macerated). Western blot was use to evaluate amino acid transporter (LAT1, CD98), protein associated with translation initiation (p70S6K, ribosome protein S6) and protein synthesis (puromycin labeled polypeptide).ResultsAfter the 2‐week habituation, gastrocnemius muscle weight and fiber CSA were significantly decreased in HU group as compared with CON group (p<0.05). Myonuclear domain (cytosol volume/nucleus) was significantly decreased after unloading. Phosphorylation of p70S6K was significantly increased at 3h after exercise in both groups (p<0.05). Similarly, phosphorylation of ribosomal protein S6 was significantly increased at 3h after exercise with no difference between groups (p<0.05). LAT1 expression was significantly increased 3h after exercise. Moreover, magnitude of LAT1 expression was higher in HU group as compare with CON group after exercise (p<0.05). CD98 expression did not change in either group. Protein synthesis was significantly increased at 3h after exercise similarly in both groups (p<0.05).ConclusionOur result suggested that translation initiation and protein synthesis in response to acute resistance exercise are retained but not improved in rat skeletal muscle even after unloading‐induced atrophy.