Dense granule secretion stimulated by collagen and Pam₃CSK₄ is partially resistant to PGI₂. Representative (A-D) and average peak (E) extracellular ATP levels monitored by luciferin:luciferase showing the effect of a supramaximal concentration of PGI₂ (60μM; 90 seconds) on dense granule secretion evoked by U46619, thrombin, collagen, and Pam₃CSK₄. Agonist concentrations match those used in Ca²⁺ responses (Figures 1–2), with the exception that no detectable ATP release was observed with the lower concentration of Pam₃CSK₄ (1 μg mL⁻¹). Asterisks indicate the significance level compared with control samples in the absence of PGI₂, and hash symbols indicate the significance level compared with thrombin in the presence of PGI₂.