Rapid Procoagulant Phosphatidylserine Exposure Relies on High Cytosolic Calcium Rather Than on Mitochondrial Depolarization

Objective-Relationships between intracellular Ca2+ concentration ([Ca2+](cyt)) and apoptotic events, such as mitochondrial depolarization (Delta Psi m loss) and Bcl-2 and Bad phosphorylation, were analyzed in platelets and Jurkat cells in relation to rapid procoagulant phosphatidylserine (PS) exposure. Methods and Results-Platelets were stimulated with A23187, thapsigargin (TG) and thrombin plus convulxin (Thr/Cvx), and Jurkat cells with ionomycin, in the presence or absence of cyclosporin A (CsA), a mitochondrial permeability transition pore inhibitor. Delta Psi m loss occurred when platelets were stimulated in Ca2+ medium in conditions exposing PS, but also in EGTA medium. CsA inhibited PS exposure, [Ca2+](cyt) increase, and Delta Psi m loss in platelets stimulated with TG and Thr/Cvx, but had no inhibitory effect on A23187 stimulation. CsA reduced TG-induced Ca2+ release from the endoplasmic reticulum and, consequently, external Ca2+ influx. In ionomycin-stimulated Jurkat cells, rapid PS exposure was evidenced but not Delta Psi m loss, and CsA did not inhibit the process. The status of phosphorylated Bad and Bcl-2 in both cell types remained unchanged on stimulation. Conclusions-Whether Delta Psi m loss occurs or not, PS exposure is triggered by a high [Ca2+](cyt) increase. Data further demonstrate that CsA prevents membrane scrambling by inhibiting the high [Ca2+](cyt) increase, independently of its effect on mitochondrial permeability transition pore. (Arterioscler Thromb Vasc Biol. 2009; 29: 1883-1889.)