Fig. 2.

MgATP²⁻ activates P2X1 and P2X3, but not P2X2 and P2X4 receptors. Macroscopic currents activated by 10 µM ATP in the absence and presence of Mg²⁺ were recorded at −60 mV from HEK cells expressing P2X1 (A), P2X3 (B), P2X2 (C), or P2X4 (D). (A–D) The solution of 10 μM ATP plus 5 mM MgCl₂ contains ∼0.2 μM ATP⁴⁻ and ∼9.8 μM MgATP²⁻. Between ATP pulses, cells were perfused with divalent-free solution. To estimate rundown, 10 μM ATP was applied twice at 90-s intervals in the divalent-free solution, and all subsequent ATP applications were given at 90-s intervals. Similar experiments were repeated on 3–4 different cells. (Right) Summary of normalized currents in response to each ATP pulse. Numbering indicates the sequence of ATP applications from first (1) to last (4). P2X1–P2X3 subtypes were studied with conventional whole-cell recording, and P2X4 was studied with the perforated-patch configuration. I_R observed in P2X2 and P2X4 are indicated with an asterisk. (E and F) Enlarged views of I_R for P2X2 (E) or P2X4 (F). Scale bars on the left show the amplitude of currents normalized to that activated by 10 μM ATP⁴⁻.