Identification of system y<sup>+</sup>L as the high-affinity transporter for L- arginine in human platelets: Up-regulation of L-arginine influx in uraemia
Kinetic studies of L-arginine transport in human platelets have identified a high-affinity, low-capacity transport system [Michaelis-Menten constant (K(m)) about 10 μM] for cationic amino acids that also transports neutral amino acids with high affinity in the presence of Na+ but not K+. These characteristics, together with our kinetic cis-inhibition studies, indicate that saturable L-arginine transport in human platelets is mediated via the system y+L and not the classic cationic transporter system y+. We present here the first evidence that L-arginine transport via system y+L is increased twofold in platelets from patients with chronic renal failure. System y+L has been described in human erythrocytes, peripheral blood mononuclear cells and placenta, and up-regulation of system y+L activity in human platelets could explain the paradox of increased nitric oxide (NO) production by uraemic platelets under conditions of decreased plasma L- arginine and elevated N(G)-monomethyl-L-arginine (L-NMMA) concentrations.