ADP evokes multiple inward cationic currents through mechanisms that require P2Y₁ receptors. Simultaneous whole-cell patch clamp and [Ca²⁺]_i recordings at –70 mV under conditions that eliminate K⁺-selective currents. (A) Typical biphasic inward current and [Ca²⁺]_i increase activated by 30 μM ADP. Dashed line shows the background current level. (B) Another cell displaying typical repetitive inward currents observed in many recordings during exposure to ADP. These repetitive events had rapid kinetics, as shown in the expanded section. (C) Inward currents and [Ca²⁺]_i increases were not observed in response to 30 μM in P2Y₁^–/– MKs. (D) Frequency of occurrence (events per minute) of the repetitive transient inward currents for WT and P2Y₁^–/– MKs before (□) and during (▦) exposure to 30 μM ADP. Error bars represent SEM. *Statistical significance of P < .05 compared to WT.