# Arginine Regulates Skeletal Muscle Fiber Type Formation via mTOR Signaling Pathway

## TL;DR

L-arginine promotes slow-twitch muscle fibers and improves endurance by activating the mTOR pathway in mice.

## Contribution

This study reveals that L-arginine modulates muscle fiber type and mitochondrial function via the mTOR signaling pathway.

## Key findings

- Arg supplementation increased slow-twitch fiber genes and mitochondrial markers in mouse muscles.
- Arg activated the mTOR pathway, and its effects were blocked by the mTOR inhibitor rapamycin.
- Endurance performance improved with Arg, along with elevated SDH enzyme activity and gene expression.

## Abstract

The composition of skeletal muscle fiber types affects the quality of livestock meat and human athletic performance and health. L-arginine (Arg), a semi-essential amino acid, has been observed to promote the formation of slow-twitch muscle fibers in animal models. However, the precise molecular mechanisms are still unclear. This study investigates the role of Arg in skeletal muscle fiber composition and mitochondrial function through the mTOR signaling pathway. In vivo, 4-week C56BL/6J male mice were divided into three treatment groups and fed a basal diet supplemented with different concentrations of Arg in their drinking water. The trial lasted 7 weeks. The results show that Arg supplementation significantly improved endurance exercise performance, along with increased SDH enzyme activity and upregulated expression of the MyHC I, MyHC IIA, PGC-1α, and NRF1 genes in the gastrocnemius (GAS) and quadriceps (QUA) muscles compared to the control group. In addition, Arg activated the mTOR signaling pathway in the skeletal muscle of mice. In vitro experiments using cultured C2C12 myotubes demonstrated that Arg elevated the expression of slow-fiber genes (MyHC I and Tnnt1) as well as mitochondrial genes (PGC-1α, TFAM, MEF2C, and NRF1), whereas the effects of Arg were inhibited by the mTOR inhibitor rapamycin. In conclusion, these findings suggest that Arg modulates skeletal muscle fiber type towards slow-twitch fibers and enhances mitochondrial functions by upregulating gene expression through the mTOR signaling pathway.