| Growth hormone (GH) is a powerful stimulator of the Janus Kinase 2/Signal Transducers and Activators of Transcription 5 (JAK2/STAT5) pathway in animal and cellular research. The well-documented exercise-induced release of GH provides an ideal model to study the activation of this pathway in the human. The purpose of this study was to determine if the tyrosine phosphorylation of the JAK2/STAT5 pathway in human skeletal muscle occurs in response to an acute bout of aerobic exercise. Twelve young (22.1 +/- 0.7, Mean +/- SE), healthy, aerobically trained males performed 30 minutes of cycling at 70% VO2max. Blood samples were collected at 10 to 15 minute intervals for 175 minutes (85 minute rest, 30 minute exercise, 60 minute recovery). Blood samples were analyzed for human GH, immunofunctional (IF) GH, growth hormone binding protein (GHBP), and insulin-like growth factor-I (IGF-I) using Diagnostic Systems Laboratory ELISA kits (Webster, TX). Muscle biopsies from the vastus lateralis were taken pre-exercise, post-exercise, 30 minutes and 60 minutes into recovery. Muscle samples were analyzed for tyrosine phosphorylation changes in JAK2 and STAT5, as well as, changes in JAK2, STAT5a, and STAT5b protein content.; Results from multivariate ANOVA revealed a significant time effect for all hormones measured (P<0.009). Post hoc comparisons demonstrated that all hormones were significantly elevated immediately after exercise compared to before exercise (P<0.003). Exercise significantly increased the phosphorylation of JAK2 immediately after exercise (1.79 +/- 0.19-fold, P=0.001). In addition, there was a trend in the phosphorylation of STAT5 to increase after exercise (P=0.057). Muscle JAK2, STAT5a and STAT5b protein content did not change during the session. Peak IF GH and absolute change in IF GH (pre-exercise to post-exercise) were positively related to the fold-change in STAT5 phosphorylation (r=0.66, P=0.037 and r=0.72, P=0.02, respectively). These results demonstrate that the JAK2/STAT5 pathway is activated in response to acute aerobic exercise in human skeletal muscle. Furthermore, the data from the present study suggests that the exercise-induced release of GH may play a pivotal role in the phosphorylation and activation of this pathway during exercise. |
