Expression of inducible nitric oxide synthase inhibits platelet adhesion and restores blood flow in the injured artery.

NO generated by endothelial cells is vasoprotective by antagonizing platelet adhesion and smooth muscle contraction. Since vascular smooth muscle cells (VSMCs) produce NO in response to cytokine stimulation and after arterial injury, we speculated that NO produced by VSMCs could compensate for the loss of endothelium. Using balloon catheter denudation of the rat carotid artery as a model for arterial injury and restenosis, we have evaluated the time course of expression of inducible NO synthase (iNOS) by in situ hybridization and immunohistochemistry and studied the effect of iNOS on platelet adhesion and blood flow of the injured vessel. iNOS mRNA and protein were expressed in the innermost layer of the media from day 1 and were subsequently detected in the neointima, whereas no expression was detectable in the uninjured artery. Systemic administration of N omega-nitro-L-arginine methyl ester (L-NAME) resulted in a twofold to threefold increase in the adhesion of 111In-labeled platelets to the injured vessel wall. Platelet adhesion was also enhanced threefold by local delivery of L-NAME from a gel surrounding the injured vessel, whereas the stereoisomer, D-NAME, had no effect. Finally, inhibition of NO synthase led to a 24% reduction of the blood flow in the injured carotid artery. These results demonstrate that arterial injury triggers the expression of iNOS in the lesion and that NO produced by iNOS inhibits platelet adhesion and restores blood flow. This could explain the disappearance of platelet thrombi from deendothelialized arterial surfaces within a few days after injury and indicates the importance of NO generated by iNOS for the maintenance of vascular tone. Thus, expression of iNOS in lesions may represent a protective mechanism that compensates for the loss of endothelium.