Functional effects of RBC-derived NO. (A) Flow rate of RBCs through a microfilter after incubation of whole blood with l-arginine, l-NNA, and, in addition, oxyhemoglobin (oxyHb) was taken as a measure of RBC membrane deformability (n = 7). l-arginine (l-arg) increased deformability of RBCs, whereas inhibition of NOS drastically decreased deformability. Removal of bioactive NO through the addition of oxyhemoglobin further reduced deformability. (B) Phosphorylation of platelet VASP was induced by a functional RBC-NOS. Western blot of VASP phosphorylation in the presence of the phosphodiesterase-5 inhibitor sildenafil was observed when whole blood was incubated with l-arginine (P < .05). (C) Changes in ADP-induced platelet aggregation were measured after incubation of whole blood with either l-arginine, l-NNA, or buffer (PBS) and subsequent centrifugation of platelet-rich plasma shown as original registration (inset, summarized data). RBC-derived NO induced by l-arginine significantly decreased platelet aggregation (n = 3), which was prevented by NOS inhibition with l-NNA. l-arginine inhibited aggregation of platelets only in the presence of RBCs. (A, C) ^*Significant difference from control. #Significant difference from l-NNA.