Summary of the differing cellular mechanisms by which ATP and UTP exert their effects on AMC and bone mineralization. ATP and UTP inhibit both AMC and bone mineralization but there are some differences in the mechanisms mediating these actions. In VSMCs (top), ATP and UTP act via P2 receptors to decrease apoptosis and, as a consequence, calcification. In osteoblasts (OBs, bottom panel), ATP and UTP act via the P2Y2 receptor to inhibit alkaline phosphatase (TNAP) activity. However, for both cell types hydrolysis of ATP and UTP to produce PPi by NPP1 also contributes to the inhibitory effects observed