l-Leucine Increases Skeletal Muscle IGF-1 but Does Not Differentially Increase Akt/mTORC1 Signaling and Serum IGF-1 Compared to Ursolic Acid in Response to Resistance Exercise in Resistance-Trained Men

Abstract

Objective: Ursolic acid administration following resistance exercise increases mammalian target of rapamycin complex 1 (mTORC1) activity and skeletal muscle IGF-1 concentration in murines in a manner similar to l-leucine yet remains unexamined in humans. This study examined serum and skeletal muscle insulin-like growth factor-1 (IGF-1) and Akt/mTORC1 signaling activity following ingestion of either ursolic acid or l-leucine immediately after resistance exercise.

Methods: Nine resistance-trained men performed 3 lower-body resistance exercise sessions involving 4 sets of 8–10 repetitions at 75%–80% one repetition maximum (1-RM) on the angled leg press and knee extension exercises. Immediately following each session, participants orally ingested 3 g cellulose placebo (PLC), l-leucine (LEU), or ursolic acid (UA). Blood samples were obtained pre-exercise and at 0.5, 2, and 6 hours postexercise. Muscle biopsies were obtained pre-exercise and at 2 and 6 hours postexercise.

Results: Plasma leucine increased in LEU at 2 hours postexercise compared to PLC (p = 0.04). Plasma ursolic acid increased in UA at 2 h and 6 hours postexercise compared to PLC and LEU (p < 0.003). No significant differences were observed for serum insulin (p = 0.98) and IGF-1 (p = 0.99) or skeletal muscle IGF-1 receptor (IGF-1R; p = 0.84), Akt (p = 0.55), mTOR (p = 0.09), and p70S6K (p = 0.98). Skeletal muscle IGF-1 was significantly increased in LEU at 2 hours postexercise (p = 0.03) and 6 hours postexercise (p = 0.04) compared to PLC and UA.

Conclusion: Three grams of l-leucine and ursolic acid had no effect on Akt/mTORC1 signaling or serum insulin or IGF-1; however, l-leucine increased skeletal muscle IGF-1 concentration in resistance-trained men.

Key words:

• cell signaling
• muscle protein synthesis
• branched-chain amino acid
• resistance exercise

ACKNOWLEDGMENTS

This study was supported by a Department of Health, Human Performance, and Recreation graduate study research award and the Exercise and Biochemical Nutrition Laboratory at Baylor University.

Author Contributions

D.D.C. served as the study coordinator and was involved in participant recruitment, testing, data collection, and laboratory analyses and assisted in article preparation. N.A.S., M.B.S., S.K.M.B., and T.L.A. were involved in data collection and testing. A.J.R. was involved in conducting the liquid chromatography–mass spectrometry for plasma ursolic acid. D.S.W. was the principal investigator and was responsible for developing the experimental design. He was also involved in training and mentoring graduate students for laboratory analyses, testing, data collection, laboratory analyses and provided primary oversight during the course of the study as well as supervised article preparation.