Effect of homocysteine on the L-arginine/nitric oxide synthase/nitric oxide pathway in human platelets

Recent evidence indicates that chronic hyperhomocysteinemia is an independent risk factor for the development of atherosclerosis, thrombosis, and other cardiovascular diseases. This may be secondary to impaired fibrinolysis, or increased platelet reactivity. L-Arginine/ nitric oxide synthase/nitric oxide (NO) plays an important role in the regulation of platelet function. The present study was undertaken to determine the effect of homocysteine (HCY) on the L-arginine/NO pathway of human platelets. Washed human platelets were incubated in the presence or absence of HCY for 2h at 37degreesC followed by a measurement of indices of the L-arginine/NO pathway. HCY caused a concentration-dependent reduction in the platelet uptake Of L-[H-3]arginine. HCY also caused a concentration-dependent decrease in nitrite production concurrent with a decrease in cyclic guanosine monophosphate, whereas NO synthase activity of the platelets, measured as conversion of L- [H-3]arginine to L- [H-3]citrulline, remained unchanged on incubation with HCY. These observations indicate that the L-arginine/NO pathway is involved in the mechanism responsible for the effects of HCY on platelets by diminishing NO production through decreased uptake Of L-arginine.