Transient repetitive currents in the presence and absence of exogenous ADP are mainly caused by P2X₁ receptors. (A) Scatter plot of activation and inactivation kinetics for the repetitively activated inward currents observed during application of 30 μM ADP. Each point shows the time to peak current and the inactivation time constant for a single transient event when this could be fitted by a single exponential decay. Data from 15 cells. (B) Frequency of occurrence (events per minute) of all transient currents during exposure to 30 μM ADP for control, P2X₁^–/– MKs, after preexposure to 10 μM α,β-meATP and in the presence of 1 μM NF449. (C) Example of the slower transient events and concurrent [Ca²⁺]_i transient events evoked by ADP in the absence of P2X₁ receptors. (D) Example of the spontaneous [Ca²⁺]_i increases and transient inward currents displaying a mixture of rapid and slow current events. (E) Frequency of all spontaneous current transient events in control cells, P2X₁^–/– MKs (n = 11), after exposure to 10 μM α,β-meATP (n = 7), and in the presence of 1 μM NF449 (n = 7). Error bars represent SEM. Statistical significance compared with control (*P < .05; **P < .01; ***P < .001).