Skeletal muscle IL-15/IL-15Rα and myofibrillar protein synthesis after resistance exercise

In vitro and in vivo studies described the myokine IL-15 and its receptor IL-15Rα as anabolic/anti-atrophy agents, however the protein expression of IL-15Rα has not been measured in human skeletal muscle and data regarding IL-15 expression remain inconclusive. The purpose of the study was to determine serum and skeletal muscle IL-15 and IL-15Rα responses to resistance exercise session and to analyse their association with myofibrillar protein synthesis (MPS). Fourteen participants performed a bilateral leg resistance exercise composed of 4 sets of leg press and 4 sets of knee extension at 75% 1RM to task failure. Muscle biopsies were obtained at rest, 0, 4 and 24h post-exercise and blood samples at rest, mid-exercise, 0, 0.3, 1, 2, 4 and 24h post-exercise. Serum IL-15 was increased by ~5.3-fold immediately post-exercise, while serum IL-15Rα decreased ~75% over 1h post-exercise (P<0.001). Skeletal muscle IL-15Rα mRNA and protein expression were increased at 4h post-exercise by ~2-fold (P<0.001) and ~1.3-fold above rest (P=0.020), respectively. At 24h post-exercise IL-15 (P=0.003) and IL-15Rα mRNAs increased by ~2-fold (P=0.002). Myofibrillar fractional synthetic rate between 0-4h was associated with IL-15Rα mRNA at rest (r=0.662, P=0.019), 4h (r=0.612, P=0.029) and 24h post-exercise (r=0.627, P=0.029). Finally, the muscle IL-15Rα protein up-regulation was related to Leg press 1RM (r=0.688, P=0.003) and total weight lifted (r=0.628, P=0.009). In conclusion, IL-15/IL-15Rα signalling pathway is activated in skeletal muscle in response to a session of resistance exercise. The increase of IL-15Rα after resistance exercise is associated with an elevated MPS 0-4h post-exercise.