Figure 2.

Potentiation of P2Y₁ receptor-evoked Ca²⁺ increase by depolarization is greatest at near-threshold concentrations of ADP. (a, b, d) Effect of 80 mV depolarizations (−75 to +5 mV) at a near-threshold (a, d) and maximal (b) concentration of ADP. At 0.03 μM ADP, cells either showed a small Ca²⁺ increase (a) or failed to respond (d) to the agonist alone, whereas all cells displayed a subsequent, marked depolarization-evoked Ca²⁺ increase. (c) Average peak initial [Ca²⁺]_i increases evoked by agonist and an 80-mV depolarization at different ADP concentrations. Only cells that displayed a Ca²⁺ response to 0.03 μM ADP are included in the average response.