James D. Fluckey

Hindlimb suspension (HS) results in rapid losses of muscle mass, which may in part be explained by attenuated rates of protein synthesis. Mammalian target of rapamycin (mTOR) regulates protein synthesis and has been implicated as a potential mediator of the muscle mass decrement with HS. This study examined the effect of resistance exercise, a muscle hypertrophy stimulant, on rates of protein synthesis after 4 days of HS in mature male Sprague-Dawley rats. Flywheel resistance exercise (2 sets × 25 repetitions) was conducted on days 2 and 4 of HS (HSRE). Sixteen hours after the last exercise bout, soleus muscles were assessed for in vitro rates of protein synthesis, with and without insulin (signaling agonist) and/or rapamycin (mTOR inhibitor). Results demonstrated that soleus mass was reduced ( P < 0.05) with HS, but this loss of mass was not observed ( P > 0.05) with HSRE. Muscle protein synthesis was diminished ( P < 0.05) with HS, with or without insulin. HSRE also had reduced rates of synthesis without insulin; however, insulin administration yielded higher ( P < 0.05) rates in HSRE compared with HS or control. Rapamycin diminished protein synthesis in all groups ( P < 0.05), but insulin rescued synthesis rates in HS and HSRE to levels similar to insulin alone for each group, suggesting that alternate signaling pathways develop to increase protein synthesis with HS. These results demonstrate that the capacity for an augmented anabolic response to resistance exercise is maintained after 4 days of HS and is independent of a rapamycin-sensitive pathway.

A recent report from our group demonstrated that insulin facilitates muscle protein synthesis in obese Zucker rats. The purpose of this study was to determine whether PKC, a probable modulator of insulin signal transduction and/or mRNA translation, has a role in this insulin-mediated anabolic response. In the first portion of the study, gastrocnemius muscles of lean and obese Zucker rats ( n = 5–7 for each phenotype) were bilaterally perfused with or without insulin to assess cytosolic and membrane PKC activity. Limbs perfused with insulin demonstrated greater PKC activity in both lean and obese Zucker rats ( P < 0.05) compared with no insulin, but overall activity was greater in obese animals (by ∼27% compared with lean, P < 0.05). To determine whether PKC plays a role in muscle protein synthesis, hindlimbs ( n = 6–8 for each phenotype) were bilaterally perfused with or without insulin and/or GF-109203X (GF; a PKC inhibitor). The presence of GF did not influence the rates of insulin-mediated protein synthesis in gastrocnemius muscle of lean Zucker rats. However, when obese rats were perfused with GF ( P < 0.05), the effect of insulin on elevating rates of protein synthesis was not observed. We also used phorbol 12-myristate 13-acetate (TPA, a PKC activator; n = 5–7 for each phenotype) with and without insulin to determine the effect of PKC activation on muscle protein synthesis. TPA alone did not elevate muscle protein synthesis in lean or obese rats. However, TPA plus insulin resulted in elevated rates of protein synthesis in both phenotypes that were similar to rates of insulin alone of obese rats. These results suggest that PKC is a modulator and is necessary, but not sufficient, for insulin-mediated protein anabolic responses in skeletal muscle.