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 P2X7 receptor channel (P2X7R) 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 P2X7R. Accordingly, acinar cells isolated from P2X 7 R −/− mice lacked I ATPCl. Experiments with P2X7R 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 mm 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 P2X7R is fundamental for I ATPCl generation in acinar cells and that external Na+ modulates ion permeability and conductivity, as well as drug affinity, in P2X7R.
The Journal of Membrane Biology – Springer Journals
Published: Jul 1, 2008
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