Citrulline directly modulates muscle protein synthesis: evidence from in vitro study

: The ability of citrulline (CIT) to stimulate muscle protein synthesis has been recently reported. The question of how CIT can stimulate muscle protein synthesis thus arises. The aim of this work is to evaluate the direct effect of CIT on muscle protein synthesis rate using a model of incubated skeletal muscle. The study was performed in healthy and malnourished rats. Material and methods : 3 month‐old rats were submitted to dietary restriction (50 % of the spontaneous food intake with a 5% protein diet) during 6 weeks (R, n=10) or fed ad libitum (AL, n=10). Epitrochlearis muscles were removed and were placed in a tube containing 3mL of appropriate medium (Krebs‐Ringer). Muscles were first incubated for 30 min at 37°C with 95%O 2 :5%CO 2 . Muscles were then incubated for 2 hours in medium containing insulin (0.01 U/ml) and [ring‐ 13 C 6 ] L‐phenylalanine (1mM) with (left) or without (right) CIT (2.5 mM) under the conditions described above. At the end of the incubation period, muscles and media were frozen and stored at −80°C. The absolute synthesis rate (ASR) was determined using the [ring‐ 13 C 6 ]‐phe enrichment of intracellular free amino acids and proteins by mass spectrometry. Results: Conclusions: this study demonstrates for the first time that CIT has a direct effect on muscle protein synthesis and that action is not related to the generation of other amino acids. This work was supported by funds from the Ministry of Research and Technology