Effects of leucine and citrulline versus non-essential amino acids on muscle protein synthesis in fasted rat: a common activation pathway?

Leucine (LEU) is recognized as a major regulator of muscle protein synthesis (MPS). Citrulline (CIT) is emerging as a potent new regulator. The aim of our study was to compare MPS modulation by CIT and LEU in food-deprived rats and to determine whether their action was driven by similar mechanisms. Rats were either freely fed (F, n = 10) or food deprived for 18 h. Food-deprived rats were randomly assigned to one of four groups and received per os, i.e., gavage, saline (S, n = 10), L: -leucine (1.35 g/kg, LEU, n = 10), L: -citrulline (1.80 g/kg CIT, n = 10) or isonitrogenous non-essential amino acids (NEAA, n = 10). After gavage, the rats were injected with a flooding dose of [(13)C] valine to determine MPS. The rats were killed 50 min after the injection of the flooding dose. Blood was collected for amino acid, glucose and insulin determinations. Tibialis anterior muscles were excised for determination of MPS and for Western blot analyses of the PI3K/Akt, mTORC1, ERK1/2/MAPK pathways and AMP kinase component. MPS was depressed by 61% in starved rats (Saline vs. Fed, P < 0.05). Administration of amino acids (NEAA, LEU or CIT) completely abolished this decrease (NEAA, CIT, LEU vs. Fed, NS). Food deprivation affected the phosphorylation status of the mTORC1 pathway and AMP kinase (Saline vs. Fed, P < 0.05). LEU and CIT administration differently stimulated the mTORC1 pathway (LEU > CIT). LEU but not CIT increased the phosphorylation of rpS6 at serine 235/236. Our findings clearly demonstrated that both CIT and LEU were able to stimulate MPS, but this effect was likely related to the nitrogen load. LEU, CIT and NEAA may have different actions on MPS in this model as they share different mTORC1 regulation capacities.