IONIC PERMEABILITY OF, AND DIVALENT-CATION EFFECTS ON, 2 ATP-GATED CATION CHANNELS (P2X RECEPTORS) EXPRESSED IN MAMMALIAN-CELLS

1. Complementary DNAs for the ATP-gated ion channel subunits P2X(1) (f rom human bladder) and P2X(2) (from rat phaeochromocytoma (PC12) cells ) were used to express the receptors in human embryonic kidney cells b y stable transfection, and in Chinese hamster ovary cells by viral inf ection. 2. Membrane currents evoked by ATP were recorded by the whole- cell patch clamp method. The reversal potential of the current was mea sured with various intracellular and extracellular solutions and used to compute the relative permeability of the P2X receptor channels. 3. There was no difference between the two receptors with respect to thei r permeability to monovalent organic cations. The relative permeabilit ies (P-X/P-Na) were 2.3, 1.0, 1.0, 0.95, 0.72, 0.5, 0.29, 0.16, 0.04 a nd 0.03 for guanidinium, potassium, sodium, methylamine, caesium, dime thylamine, 2-methylethanolamine, tris(hydroxymethyl)-aminomethane, tet raethylammonium and N-methyl-D-glucamine, respectively (values for P2X (2) receptor). 4. The calcium permeability of P2X(1) receptors was gre ater than that of P2X(2) receptors. Under biionic conditions (112 mM c alcium outside, 154 mM sodium inside), P-Ca/P-Na values were 3.9 and 2 .2, respectively (corrected for ionic activities). 5. ATP-evoked curre nts in cells expressing the P2X(2) receptor were strongly inhibited wh en the extracellular calcium concentration was increased (0.3-30 mM); the action of ATP could be restored by increasing the ATP concentratio n. ATP-evoked currents in cells expressing the P2X(1) receptor were no t inhibited by such increases in the extracellular calcium concentrati on.