Intraplatelet tetrahydrobiopterin plays an important role in regulating canine coronary arterial thrombosis by modulating intraplatelet nitric oxide and superoxide generation

Background-Platelet-derived nitric oxide inhibits platelet aggregation via constitutive NO synthase (NOS). Tetrahydrobiopterin (BH4), a cofactor of NO S, augments NO formation, whereas its deficiency decreases NO bioactivity a nd increases superoxide generation by NOS. The roles of intraplatelet BH4 i n platelet aggregation and thrombus formation, however, are unknown. Accord ingly, we investigated whether intraplatelet BH4 is involved in regulating cyclic flow variations (CFVs) and platelet aggregation in a canine model wi th stenosed and endothelium-injured coronary arteries that mimics acute cor onary syndromes in humans. Methods and Results-After developing CFVs, dogs received saline or BH4 (10 or 30 mg/kg) intravenously. Intraplatelet BH4 and cGMP levels were decrease d and intraplatelet nitrotyrosine production was increased during CFVs. ADP - and U46619-induced ex vivo platelet aggregation and platelet P-selectin e xpression were augmented during CFVs. BH4 administration restored intraplat elet BH4 and cGMP levels and decreased intraplatelet nitrotyrosine producti on, resulting in reduced CFVs and inhibited ex vivo platelet aggregation an d platelet P-sclectin expression. CFVs again developed after N-G-monomethyl -L-arginine, an inhibitor of NOS, in BH4-treated dogs. Ex vivo platelet NOS activity at baseline, during CFVs, and after BH, administration did not di ffer. Conclusions-Intraplatelet BH4 may play an important role in regulating thro mbus formation by modulating platelet-derived nitric oxide and superoxide g eneration by platelet NOS.