Consecutive-Day Intake of Whey Protein Upregulates mTOR mRNA and Protein Expression in Resting Skeletal Muscle of Mice

Abstract

Background: Although the effect of whey protein intake on protein metabolism in exercise-loaded skeletal muscle has been well documented, little has been reported on its effect on resting muscle. The effects of whey protein intake on protein metabolism in resting mouse skeletal muscle were investigated. Methods: Mice were fed AIN-93G composed of either casein or whey protein as the protein source for 3 or 7 consecutive days. The gastrocnemius muscle was excised, and the expression levels of the regulatory factor, mTOR, and its subunits, Raptor and Rictor, were measured by real-time PCR. The protein expression levels of mTOR and its phosphorylated form were measured by immunofluorescent western blotting. The effects of whey protein were compared to those of the case in control. Results: mTOR expression increased in the gastrocnemius muscle of mice fed whey protein for 7 consecutive days. The expression of Raptor significantly increased, whereas that of Rictor did not change, suggesting a dominant formation of mTORC1 relating to the upregulation of protein synthesis. The protein levels of mTOR and its phosphorylated form significantly increased in mice fed whey protein, indicating enhanced protein synthesis. Increased mTOR expression was not seen in the gastrocnemius muscle of mice fed whey protein for 3 consecutive days. Conclusions: These results indicate that the intake of whey protein for 7 consecutive days, but not 3 days, upregulates the mRNA and protein expression of mTOR in the resting gastrocnemius muscle of mice, suggesting its ability to enhance protein synthesis. Consecutive-day intake of whey protein may induce constitutive alteration of the skeletal muscle, including continuous upregulation of muscle protein synthesis.