Chelation of the extracellular Zn²⁺ is responsible for potentiation of the ASIC current. A, B, Representative current traces and summary data showing potentiation of the ASIC current by the membrane-impermeable Zn²⁺ cheating agents EDTA, DTPA, and ADA. All three agents induced more than a twofold increase in the amplitude of ASIC current; n = 8. C, D, Chelation of intracellular free Zn²⁺ by inclusion of 100 μm EDTA in the pipette solution does not affect the potentiation of ASIC current by bath application of TPEN (10 μm); n = 4. E, F, Representative current traces and summary data showing potentiation of ASIC current by Ca²⁺-EDTA but not by Zn²⁺-EDTA; n = 5. G, H, TPEN potentiation of the ASIC current is reversed by the presence of 30 μm Zn²⁺, which by itself has no effect on the current in the majority of cortical neurons; n = 5; ^**p < 0.01.