Amplification of collagen-evoked Ca²⁺ responses by P2X1 receptors is independent of platelet density. Representative (A,C) and average (B,D,E) ATP secretion (A-B) and [Ca²⁺]_i responses (C-E) evoked by collagen (0.5 μg mL⁻¹) at platelet densities of 10% to 100% (where 100% is equivalent to that in plasma). The contribution of P2X1 receptors was assessed by desensitization before addition of external Ca²⁺ using 0.6μM α,βmeATP (desensitized P2X1). (D) Absolute peak Ca²⁺ response under control conditions (dark bars) and after desensitization of P2X1 receptors with 0.6μM α,βmeATP (light bars). (E) Peak Ca²⁺ response as a percentage of control responses after P2X1 receptor desensitization with 0.6μM α,βmeATP; there was no significant difference between responses in panel E across the range of platelet densities.