Simulation analyses of the activation phase evoked by a voltage step in the P2X₂ WT channel. (A) Reproduction of the activation phase by simulation. The activation phases evoked by step pulses from −60 to −160 mV in the presence of various [ATP] were simulated. Rate constants used are shown in Table II (A and B). The applied [ATP] relative to Kd is indicated. (B) Comparison of the simulations of the activation phases using various ATP-binding and unbinding rate constants in the presence of an [ATP] that equals the Kd. The k_unbind values used are shown. Rate constants used are shown in Table II (A and C). Red dashed lines indicate lines fitted by a single exponential function for each current trace. (C) Summary of the simulation of the activation kinetics at various voltages and [ATP]. The activation phases evoked by a voltage step from −60 mV to each voltage were simulated using the rate constants in Table II (A and B). The activation phases of the simulated currents could be fitted satisfactorily with a single exponential function, and the time constants of the fittings at various [ATP] relative to Kd were plotted versus membrane potential. (D) Reproduction of [ATP]-dependent changes in the activation kinetics by a simulation assuming that k_bind is voltage dependent and that k_on and k_off are voltage independent. The activation phases evoked by the step pulse from −60 to −160 mV in the presence of high and low [ATP] were simulated. The rate constants used are shown in Table II D. In the case of low [ATP] here, Kd is equal to 100 × [ATP] at −60 mV and 10 × [ATP] at −160 mV due to the voltage-dependent change of k_bind. In the high [ATP] case, Kd is equal to 10 × [ATP] at −60 mV and 1 × [ATP] at −160 mV.