Collagen-evoked ATP release requires PLC activation but not P2X1 receptors and involves costimulation of Ca²⁺ mobilization and PKC. Sample (A,C,D) and average (B,E) ATP secretion responses to collagen (0.5 μg mL⁻¹), monitored by luciferin:luciferase. (A-B) Effect of desensitization of P2X1 receptors using 0.6μM α,βmeATP before addition of external Ca²⁺. (C-E) Effect of the PLC inhibitor U73122 (10μM; 10 minutes), cytosolic loading of the Ca²⁺ chelator BAPTA (15μM BAPTA-AM), the PKC inhibitor GF109203X (20μM; 10 minutes; GF), or combined BAPTA loading and GF109203X.