Autocrine phosphorylation of p70(S6k) in response to acute stretch in myotubes

Phosphorylation of 70-KDa S6 kinase (p70(S6k)) is correlated with in vivo skeletal muscle hypertrophy. Experiments tested whether mechanical stretch is sufficient to increase p70(S6k) phosphorylation in skeletal myotubes. Immediately following stretch, there was a small increase in p70(S6k) phosphorylation (63.2 +/- 8.5%) with maximal phosphorylation at 3 h (129.5 +/- 22.2%) and it remained elevated through 24 h (46.0 +/- 17.2%). To test whether an autocrine mechanism is involved, unstretched myotubes were incubated with medium from the stretch group for 10 min. Conditioned medium resulted in the phosphorylation of p70(S6k) in unstretched myotubes (92.8 +/- 28.9%) to levels comparable to the 3-h stretch group. These data indicate that p70(S6k) is phosphorylated in stretched myotubes via a mechanism that most likely involves an autocrine signaling pathway.