Discrimination between Citrulline and Arginine Transport in Activated Murine Macrophages: Recycling of Citrulline to Arginine Does Not Sustain Maximal No Synthesis

1. The kinetics, specificity, pH-and Na +-dependency of L-citrulline transport were examined-2-in unstimulated and lipopolysaccharide (LPS) activated murine macrophage J774 cells. The dependency of nitric oxide production on extracellular arginine or citrulline was investigated in cells activated with LPS (1 μg ml-1) for 24 h. 2. In unstimulated J774 cells transport of citrulline was saturable (K t = 0.16 mM and V max = 32 pmol μg protein-1 min-1), pH-insensitive and partially Na +-dependent. In contrast to arginine (Baydoun et al., 1993), transport of citrulline was unchanged in LPS-activated (1 μg ml-1 , 24 h) cells. 3. Kinetic inhibition experiments revealed that arginine was a relatively poor inhibitor of citrulline transport, whilst citrulline was a more potent inhibitor (K i = 3.4 mM) of arginine transport but only in the presence of extracellular Na +. Neutral amino acids inhibited citrulline transport (K i = 0.2-0.3 mM), but were poor inhibitors of arginine transport. 4. Activated J774 cells did not release nitrite in the absence of exogenous arginine.