Objective:To observe the L-Arg(L-arginine) transport,the nitric oxide (NO) production,and NO synthase (NOS) activity in platelets,investigate the significance of the L-Arg-NO system in the pathogenesis of microvascular angina(MVA),and to study the reversing effects of intravenous L-Arg infusion on L-Arg transport.Methods:The 3H-L-Arg transport,NO production,and NOS activity in platelets were examined in 15 patients with MVA and 15 healthy controls.The 15 patients were given intravenous L-Arg infusion (20 g/d) after basic physical examination and were examined again 10 days later.Results:The L-Arg transport in platelets of MVA patients was obviously lower than that in the normal group; the maximum transport velocity (Vmax) decreased by 34.4% compared with the normal group (P<0.01); and the Michaelis constant (Km) increased by 21.4% (P<0.05).The production of NO-2 and the activity of NOS in platelets were decreased by 47.1% (P<0.05) and 25.4% (P<0.05) compared with the normal group,respectively.Intravenous L-Arg infusion reversed the above changes in MVA patients; it increased the Vmax by 11.9% (P<0.01) and decreased Km by 18% (P<0.05); it also increased production of NO-2 by 1.33 folds (P<0.05) and NOS activity by 1.2 folds (P<0.05).Especially,the attack of angina and patient ECG were greatly improved after intravenous L-Arg infusion.Conclusion:L-Arg-NO pathway is impaired in MVA patients,which might be responsible for the endothelium-dependent vascular relaxation in MVA patients.Intravenous L-Arg infusion may benefit the impaired function of L-Arg-NO transport in patients with MVA.