Native and recombinant ASIC1a receptors conduct negligible Ca2+ entry

Acid Sensing Ion Channels (ASICs) are a family of proton-gated cation channels that play a role in the sensation of noxious stimuli. Of these, ASIC1a is the only family member that is reported to be permeable to Ca2+, although the absolute magnitude of the Ca2+ current is unclear. Here, we used patch-clamp photometry to determine the contribution of Ca2+ to total current through native and recombinant ASIC1a receptors. We found that acidification of the extracellular medium evoked amiloride and psalmotoxin 1-sensitive currents in isolated chick dorsal root ganglion neurons and human embryonic kidney cells, but did not alter fura-2 fluorescence when the bath concentration of Ca2+ was close to that found in normal physiological conditions. Further, activation of recombinant ASIC1a receptors also failed to produce measurable changes in fluorescence despite of the fact that the total cation current through the over-expressed receptor was ten-fold larger than that of the native channels. Finally, we imaged a field of intact DRG neurons loaded with the Ca2+-sensing dye Fluo-4, and found that acidification increased [Ca2+]i in a small population of cells. Thus, although our whole-field imaging data agree with previous studies that activation of ASIC1a receptors can potentially cause elevations in intracellular free Ca2+, our single cell data strongly challenges the view that Ca2+ entry through the ASIC1a receptor itself contributes to this response.