The primordial intercellular signaling molecule ATP acts through two families of cell-surface P2 receptors – the P2Y family of G-protein-coupled receptors and the P2X family of ligand-gated cation channels. Multiple P2 receptors are expressed in a variety of cell types. However, the significance of these networks of receptors in any biological system remains unknown. Using osteoblasts as a model system, we found that a low concentration of ATP (10 µM, ATP_low) induced transient elevation of cytosolic Ca²⁺, whereas a high concentration of ATP (1 mM, ATP_high) elicited more sustained elevation. Moreover, graded increases in the Ca²⁺ signal were achieved over a remarkable million-fold range of ATP concentrations (1 nM to 1 mM). Next, we demonstrated that ATP_low caused transient nuclear localization of the Ca²⁺-regulated transcription factor NFATc1; whereas, ATP_high elicited more sustained localization. When stimulated with ATP_high, osteoblasts from P2X7 loss-of-function mice showed only transient Ca²⁺-NFATc1 signaling; in contrast, sustained signaling was observed in wild-type cells. Additional experiments revealed a role for P2Y receptors in mediating transient signaling induced by low ATP concentrations. Thus, distinct P2 receptors with varying affinities for ATP account for this wide range of sensitivity to extracellular nucleotides. Finally, ATP_high, but not ATP_low, was shown to elicit robust expression of the NFAT target gene Ptgs2 (encoding COX-2), consistent with a crucial role for the duration of Ca²⁺-NFAT signaling in regulating target gene expression. Taken together, ensembles of P2 receptors provide a mechanism by which cells sense ATP over a wide concentration range and transduce this input into distinct cellular signals.