Effect of potassium and calcium on ATP-induced rapid MMP-9 release. PBMCs (2 × 10⁷/mL) with wild-type P2X₇ receptor were pretreated with or without 50 μM BAPTA-AM for 30 minutes in NaCl medium and washed once. Cells then were incubated with (gray bar) or without (black bar) 1 mM ATP at 37°C in NaCl or KCl medium containing 1 mM CaCl₂ or 50 μM EDTA and 50 μM EGTA. Supernatants were collected after 30 minutes. MMP-9 concentrations were measured by ELISA. Results from individuals were normalized and shown as mean ± SEM (n = 3).