472 addition, P2 receptors mediate both vasorelaxation and vasoconstriction of arteries that may be involved in the vascular remodeling accompanying atherosclerosis and postangioplasty restenosis [5]. A better understanding of the causative agents and mechanisms of proliferation and migration of vascular SMC as well as recruitment of blood-derived cells by the endothelium could lead to prevention, attenuation, or even reversal of intimal thick- ening, which may dramatically reduce morbidity and mortality from vascular diseases such as atherosclerosis and restenosis after angioplasty. In this respect, a better understanding of the physiological role of P2 receptors in both normal and pathological blood vessels could poten- tially lead to a breakthrough in the fight against vascular disease. P2 receptors in the cardiovascular system Extracellular nucleotides bind to cell-surface receptors known as P2 receptors, which are present in many tissues. To date, these receptors have been classified into two main families: the P2X receptors that are ligand-gated ion channels comprised of homo- or hetero-oligomers [12], and P2Y receptors that are seven-membrane-spanning receptors coupled via G proteins (Gq/11 or Gi/o) to phos- pholipase C (PLC) and/or adenylate cyclase [12–14]. In turn, PLC activation generates inositol 1,4,5-triphosphate (IP3), a mediator of Ca2+ release from intracellular stores, and diacylglycerol, an activator of protein kinase C (PKC) whereas adenylate cyclase generates cyclic AMP, an activator of protein kinase A (PKA). The cloning of seven P2X (P2X1, P2X2, P2X3, P2X4, P2X5, P2X6, P2X7) and eight P2Y (P2Y1, P2Y2, P2Y4, P2Y6, P2Y11, P2Y12, P2Y13, P2Y14) receptor subtypes has made it possible to use molecular and pharmacological approaches to study the distribution and functional properties of specific P2 receptor subtypes at the tissue and cellular level. P2 receptors in vascular cells The normal arterial wall consists of three layers: intima, media, and adventitia. The single layer of endothelial cells facing the vessel lumen is a very important component of the vascular wall in terms of releasing both vasodilators such as nitric oxide (NO) and prostacyclin (PGI2), and vasoconstrictors such as thromboxane A2 and endothelin. The principal P2Y receptor subtypes that have been functionally characterized in endothelial cells are P2Y1 and P2Y2, but mRNAs for P2Y4 and P2Y6 receptors have also been detected [4]. Endothelium-dependent vasorelax- ation has been attributed to the release of NO and PGI2 Purinergic Signalling (2006) 2:471–480 after binding of nucleotides to P2Y1 and P2Y2 receptors in endothelial cells [15] whereas vasoconstrictor effects in SMC result from the action of nucleotides on P2Y2 and P2X receptors [16, 17]. In most blood vessels, P2Y1 re- ceptors for ADP are present on the endothelium and regulate vasodilatation by Ca2+-dependent (PLC-mediated) activation of NO synthase (NOS) and generation of endothelial-dependent relaxing factor (EDRF) [4]. Endo- thelial prostacyclin production is also stimulated by P2Y1 and P2Y2 receptors, but this seems to play a minimal role in vasodilatation, at least under physiological con- ditions [18]. Recent studies have indicated that in the aorta of P2Y2-null mice, the endothelium-dependent relaxation by ATP and ATPγS was inhibited, demonstrating the role of the P2Y2 receptors, but that a relaxation by UTP and UDP was maintained, suggesting the additional involve- ment of P2Y6 receptors [19]. The majority of cells in intact blood vessels are SMC, which occupy most of the media and are involved in vessel vasoconstriction and vaso- relaxation. P2Y2Rs in SMC mediate the induction of immediate-early and delayed-early cell-cycle-dependent genes, consistent with a role for P2Y2Rs in vascular proliferation of SMC [20, 21]. A recent study demonstrated that P2Y2 is the predominant functional receptor that responds to ATP and UTP in rat aortic SMC [22]. In human cerebral arteries, P2Y6 seems to be the predominant subtype and induces vasoconstriction when activated by UDP/UTP [23, 24]. This is consistent with findings from rat pulmonary and mesenteric arteries [25, 26]. In addition, a recent study on P2X1 knock-out mice further supports the prominent contractile effect of the P2Y6 subtype in mesenteric arterial trees [27]. Taken together, it seems that the principal receptor mediating UTP/UDP-induced con- tractile responses in blood vessels might be the P2Y6 subtype. Other studies have reported the presence of both P2Y4 and P2Y6 receptors in rat aortic SMC [21, 28, 29]. P2Y1 receptors are expressed in SMC of a number of blood vessel types and, like their endothelial cell counterparts, mediate vasodilatation most likely through the activation of K+ channels [18]. The presence of several P2X receptor subtypes also has been reported in human saphenous vein SMC, including P2X1, P2X2, P2X4, and P2X7 receptors [30]. The outermost layer of the blood vessel consists of connective tissue and fibroblasts, which have not been appreciated in the regulation of vascular tone. A recent study showed that fibroblasts can migrate into the neointima, suggesting their possible involvement in the development of vascular diseases such as atherosclerosis and restenosis after angioplasty [31]. Since human and rat fibroblasts are known to express P2Y1, P2Y2, P2Y4, P2Y6, P2X3, P2X4, and P2X7 receptors [32], further investigation is needed to establish the role of fibroblast P2 receptors in either physiological or pathophysiological conditions.