Na+ Modulates Anion Permeation and Block of P2X7 Receptors from Mouse Parotid Glands

We previously reported that mouse parotid acinar cells display anion conductance ( I ATPCl ) when stimulated by external ATP in Na + -free extracellular solutions. It has been suggested that the P2X 7 receptor channel (P2X 7 R) might underlie I ATPCl . In this work we show that I ATPCl can be activated by ATP, ADP, AMP-PNP, ATPγS and CTP. This is consistent with the nucleotide sensitivity of P2X 7 R. Accordingly, acinar cells isolated from P2X 7 R −/− mice lacked I ATPCl . Experiments with P2X 7 R heterologously expressed resulted in ATP-activated currents ( I ATP-P2X7 ) partially carried by anions. In Na + -free solutions, I ATP-P2X7 had an apparent anion permeability sequence of SCN − > I − ≅ NO 3 − > Br − > Cl − > acetate, comparable to that reported for I ATPCl under the same conditions. However, in the presence of physiologically relevant concentrations of external Na + , the Cl − permeability of I ATP-P2X7 was negligible, although permeation of Br − or SCN − was clearly resolved. Relative anion permeabilities were not modified by addition of 1 m m carbenoxolone, a blocker of Pannexin-1. Moreover, cibacron blue 3GA, which blocks the Na + current activated by ATP in acinar cells but not I ATPCl , blocked I ATP-P2X7 in a dose-dependent manner when Na + was present but failed to do so in tetraethylammonium containing solutions. Thus, our data indicate that P2X 7 R is fundamental for I ATPCl generation in acinar cells and that external Na + modulates ion permeability and conductivity, as well as drug affinity, in P2X 7 R.