We examined P2X receptor expression and distribution in the mouse collecting duct (CD) and their functional role in Ca2+ signaling. Both P2X(1) and P2X(4) were detected by RT-PCR and Western blot. Immunohistochemistry demonstrated apical P2X(1) and P2X(4) iturminoreactivity in principal cells in the outer medullary CD (OMCD) and inner medullary CD (IMCD). Luminal ATP induced an increase in Ca2+ signaling in native medullary CD (MCD) as measured by fluorescence imaging. ATP also induced an increase in Ca2+ signaling in MCD cells grown in primary culture but not in the presence of P2XR antagonist PPNDS. Short circuit current (I-sc) measurement with mouse IMCD cells showed that P2XR agonist BzATP induced a larger I-sc than did P2YR agonist UTP in the apical membrane. Our data reveal for the first time that P2X, and P2X4 are cell-specific with prominent immunoreactivity in the apical area of MCD cells. The finding that P2XR blockade inhibits ATP-induced Ca2+ signaling suggests that activation of P2XR is a key step in Ca2+-dependent purinergic signaling. The result that activation of P2XR produces large I-sc indicates the necessity of P2XR in renal CD ion transport. Published by Elsevier Inc.