Background— Extracellular uridine 5′-triphosphate (UTP) induces mitogenic activation of smooth muscle cells (SMCs) through binding to P2Y ₂ nucleotide receptors. P2Y ₂ receptor mRNA is upregulated in intimal lesions of rat aorta, but it is unclear how this G-protein–coupled receptor contributes to development of intimal hyperplasia. Methods and Results— This study used a silicone collar placed around rabbit carotid arteries to induce vascular injury and intimal thickening. Collar placement caused rapid upregulation of P2Y ₂ receptor mRNA in medial SMCs before appearance of neointima. Fura-2 digital imaging of single SMCs was used to measure changes in myoplasmic calcium concentration (Ca _m ) in response to P2Y receptor agonists. In contrast to UDP, activation by UTP or adenosine 5′-triphosphate (ATP) greatly increased Ca _m , which indicates upregulation of functional P2Y ₂ receptors at which UTP and ATP are equipotent agonists. The number of responsive cells was significantly greater for freshly dispersed SMCs from collared arteries than for controls. Perivascular infusion of UTP (100 μmol/L) within the collar significantly enhanced neointimal development. Intimas that resulted from UTP exposure were infiltrated by macrophages. Moreover, increased expression of osteopontin occurred in response to in situ application of UTP. ATP or UTP also stimulated osteopontin expression in cultured SMCs in a dose-dependent manner. Furthermore, P2Y ₂ antisense oligonucleotide inhibited osteopontin expression induced by UTP. Conclusions— These findings indicate for the first time a role for the UTP/ATP receptor, P2Y ₂ , in development of intimal hyperplasia associated with atherosclerosis and restenosis.

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