Chronic dietary supplementation with L-arginine inhibits platelet aggregation and thromboxane A2 synthesis in hypercholesterolaemic rabbits in vivo

Objectives: L-arginine exerts anti-atherosclerotic effects in hypercholesterolaemic rabbits via modulating endogenous NO production. We investigated whether L-arginine inhibits thromboxane formation in vivo and platelet aggregation ex vivo in this animal model. Methods: The urinary excretion rates of 2,3-dinor-6-keto-PGF(1 alpha). (major urinary metabolite of PGI(2)) and 2,3-dinor-TXB2, (major urinary metabolite of thromboxane A(2)) were used as indicators of platelet-endothelial cell interactions in vivo. Rabbits were fed 1% cholesterol (Cholesterol group, N = 8), 1% cholesterol plus 2,25% L-arginine (Cholesterol + L-arginine, N = 8), or normal rabbit chow(Control, N = 4) for 12 weeks. Urine samples were collected in weekly intervals. At the end of the study period platelet aggregation ex vivo and endothelium-dependent and -independent vascular function of isolated aortic rings in vitro was assessed. Results: Urinary 2,3-dinor-TXB2 excretion significantly increased in the cholesterol group (p < 0.05), and endogenous NO formation (measured as urinary nitrate excretion) decreased (p < 0.05). Both parameters were significantly correlated with each other (R = 0.48, p < 0.01). L-arginine partly restored urinary nitrate excretion and significantly reduced TXA(2), production to values even below those in the control group (p < 0.001). Urinary 2,3-dinor-6-keto-PGF(1 alpha) excretion increased in early hypercholesterolaemia and returned to control values in the second half of the study period. The early increase in urinary 2,3-dinor-6-keto-PGF(1 alpha) excretion was attenuated by L-arginine. Platelet aggregation was significantly enhanced in cholesterol-fed rabbits and attenuated by dietary L-arginine. L-arginine also improved the impaired endothelium-dependent relaxations to ADP, and normalized the vasoconstrictor effects of 5-HT in isolated aortic rings. Conclusions: Cholesterol-feeding enhances platelet aggregation and TXA(2) formation, and stimulates platelet-endothelial cell interaction in rabbits. These effects are probably due to impaired NO elaboration. as indicated by decreased urinary nitrate excretion. Chronic dietary supplementation with L-arginine elevates systemic NO elaboration and significantly increases the PGI(2)/TXA(2) ratio. It thus beneficially influences the homeostasis between vasodilator and vasoconstrictor prostanoids in vivo. (C) 1998 Elsevier Science B.V.