Purinergic Signalling (2006) 2:471–480 P2 receptors in blood cells G-protein-coupled P2Y receptors the activation of which leads to intracellular calcium mobilization have been observed in neutrophils [4, 33, 34], and turkey erythrocytes express both P2Y1 and P2X7 receptors [35, 36]. ATP and UTP act as secretagogues by binding to P2Y2Rs to enhance exocytosis of primary granules in neutrophils [37]. In macrophages, ATP activates a P2X7 receptor that differs from other ligand-gated ion-channel P2X receptors by its ability to generate plasma membrane pores when activated [38]. Human T lymphocytes also have been shown to express P2X7 receptors [39]. Human monocytes and macrophages coexpress P2X1, P2X4, and P2X7 receptors whereas granulocytes only express P2X7 receptors [40]. P2Y1, P2Y2, and P2Y6 receptors also are expressed in monocytes, B lymphocytes, and polymorphonuclear gran- ulocytes [41]. Human platelets express P2Y1, P2Y12, and P2X1 receptors [42–44]. Thus, the diversity of P2-receptor expression in blood cells, as well as endothelial cells, SMC and fibroblasts, suggests that this receptor superfamily plays a significant role in the regulation of cardiovascular functions. P2 receptors regulate nucleotide-induced vascular smooth muscle cell proliferation Proliferation of SMC is a hallmark of vascular diseases such as atherosclerosis and restenosis following angioplas- ty. ATP and other nucleotides are released by aggregating platelets and damaged vascular cells, such as endothelial cells and SMC in pathological or stress conditions [45]. Extracellular nucleotides can act via P2X and P2Y receptors to induce acute responses such as regulation of vascular tone [46]. However, it is generally believed that the ionotropic P2X receptors do not mediate the chronic responses of nucleotides, such as cell proliferation. The mitogenic effect of extracellular nucleotides on vascular SMC (VSMC) has been known for years [47]. However, a potent antiproliferative effect of UTP on VSMC also has been reported in human arterial and venous SMC [48]. In either case, the P2 receptor subtype(s) responsible for these effects on the proliferation of VSMC has not been determined. Earlier studies have shown that ATP or UTP increases DNA and protein synthesis in subcultured rat aortic VSMC [49, 50]. In the same cell-culture model, however, Malam-Souley et al. [51] were unable to detect increases in DNA synthesis after ATP/UTP stimulation although ATP or UTP up-regulated the expression of mRNA to several cell-cycle, progression-related genes. Because P2X receptor agonists were essentially inactive, it was concluded that a P2U-like receptor (now termed 473 P2Y2) was responsible for the mitogenic effects of ATP/ UTP. However, the role of a P2Y4 receptor cannot be excluded because the nucleotide agonist profile between rat P2Y2 and P2Y4 receptors is essentially indistinguishable [52]. Indeed, Harper et al. [53] suggested that the P2Y4 receptor mediated ATP/UTP-induced proliferation of rat aortic VSMC. Recent studies indicated that ATP, UTP, or ITP, three agonists of the cloned porcine P2Y2 receptor, increased DNA and protein synthesis and cell number in coronary artery SMC [54]. Indeed, treatment of pig coronary artery SMC with UTP, ATP, or ITP caused a concentration-dependent increase in DNA and protein synthesis and cell number whereas UDP only caused a small increase in protein synthesis. Intriguingly, ATP was much more potent and efficacious than UTP, ITP, and UDP in increasing DNA synthesis and expression of PCNA, a protein marker of cell proliferation, suggesting that another receptor may contribute to the proliferative response [54]. In vivo experiments have shown that intimal thickening of collared rabbit carotid arteries was greatly enhanced by in situ UTP application and was closely associated with osteopontin expression in medial SMC [6]. Osteopontin is chemotactic for SMC and is associated with arterial SMC proliferation [55]. Moreover, both UTP and ATP increased osteopontin expression in cultured SMC whereas ADP, UDP, and 2-MeSATP were ineffective, which suggests a role for the P2Y2R in which ATP and UTP are equipotent. Direct evidence for involvement of the P2Y2R was provided by inhibition of UTP-induced osteopontin expres- sion in cultured SMC by P2Y2 antisense oligonucleotides [6]. P2Y6 receptors also have been shown to mediate proliferation of SMC in rat aorta [56]. Role of P2 receptors in the migration of vascular SMC Recent studies indicate that the extracellular nucleotides ATP, ADP, UTP, and UDP serve as directional cues for migration of rat aortic SMC [57]. At identical concen- trations, the most powerful migratory response induced by these nucleotides was elicited by UTP. Nucleotide-induced migration of SMC is the consequence of both chemotaxis and chemokinesis and may result either from activation of one particular P2 nucleotide receptor subtype or of several P2 receptor subtypes. The ability of UTP at submicromolar levels to stimulate migration of SMC supports the hypoth- esis that this response could have physiological consequen- ces and is essentially mediated by P2Y2 receptor activation without excluding participation of other P2Y receptor subtypes. The difference in the capacity of UTP and ATP to elicit migration of SMC could be due to inhibition of nucleotide-induced cell migration by adenosine generated from ATP catabolism by cell-surface ectonucleotidases.