Signaling cascades downstream IGF-I receptor in response to resistance exercise relevant to skeletal muscle hypertrophy in rats
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Mechanical loading of a muscle induces stimulation of IGF-I receptor and its signaling cascades within the muscle. This study evaluated the effect of mechanical loading through resistance training utilizing the ladder climbing paradigm of Hornberger and Farrar. Rats were trained for up to 5 weeks and then muscles were evaluated either 12 or 40 hours after their last bout of exercise. The resistance exercise resulted in progressive increases in body mass, FHL muscle mass and maximal load as the exercise was accumulated over 5 weeks. 12 hours following the last bout of the exercise, phosphorylation of ERK1/2 was elevated while that of p70S6K declined. There were no differences in phosphorylation of other representable proteins: Akt, mTOR, and GSK-3β. Furthermore, 40 hours following the last bout of the exercise, there were no significant changes in phosphorylation of selective pathways within either the ERK or PI3Kinase pathways. The early phase of recovery was associated with an increase in phosphorylation of ERK1/2 and a decrease in phosphorylation of p70S6K. While the resistance training induced a significant increase in muscle mass, the relationship between IGF-I signaling and increases in protein accretion need to be evaluated in relationship to the time of evaluation of the last bout of exercise.