OBJECTIVE: We investigated the role of shear stress in regulating P2 receptors in human umbilical vein. METHODS AND RESULTS: Using a novel, computerized, biomechanical perfusion model, parallel vessel segments were randomized to simultaneous perfusion under high (25 dyn/cm2) or low (<4 dyn/cm2) shear stress at identical mean perfusion pressure (20 mm Hg) for 6 hours. In the endothelium, no significant P2 receptor mRNA differences were found. In smooth muscle cells (SMCs), high shear stress decreased P2X1 receptors, whereas P2Y2 and P2Y6 receptors were upregulated. These findings were consistent at the mRNA level (real-time reverse transcription-polymerase chain reaction), protein level (Western blot), and morphologically (immunohistochemistry). The changes were more pronounced in the subintimal layer of the media. CONCLUSIONS: Our findings suggest that shear stress might regulate gene expression in SMCs more than in the endothelium in intact vessels. Decreased expression of the contractile P2X1 receptor could lead to reduced vascular tonus and increased blood flow. Because P2Y2 and P2Y6 receptors stimulate growth and migration of SMCs, increased expression of these receptors could promote vascular remodeling induced by shear stress. The pattern of upregulation of mitogenic P2Y receptors and downregulation of contractile P2X1 receptor is similar to changes seen in the phenotypic shift from contractile to synthetic SMCs.