Mitochondrial Calcium and Reactive Oxygen Species Regulate Agonist-Initiated Platelet Phosphatidylserine Exposure

Abstract

Objective—: To study the interactions of cytoplasmic calcium elevation, mitochondrial permeability transition pore (mPTP) formation, and reactive oxygen species formation in the regulation of phosphatidylserine (PS) exposure in platelets. Methods and Results—: mPTP formation, but not the degree of cytoplasmic calcium elevation, was associated with PS exposure in wild-type, cyclophilin D–null, ionomycin-treated, and reactive oxygen species–treated platelets. In the absence of the mPTP regulator cyclophilin D, agonist-initiated mPTP formation and high-level PS exposure were markedly blunted, but cytoplasmic calcium transients were unchanged. Mitochondrial calcium (Ca ²⁺ _mit ) transients and reactive oxygen species, key regulators of mPTP formation, were examined in strongly stimulated platelets. Increased reactive oxygen species production occurred in strongly stimulated platelets and was dependent on extracellular calcium entry, but not the presence of cyclophilin D. Ca ²⁺ _mit increased significantly in strongly stimulated platelets. Abrogation of Ca ²⁺ _mit entry, either by inhibition of the Ca ²⁺ _mit uniporter or mitochondrial depolarization, prevented mPTP formation and exposure but not platelet aggregation or granule release. Conclusion—: Sustained cytoplasmic calcium levels are necessary, but not sufficient, for high-level PS exposure in response to agonists. Increased Ca ²⁺ _mit levels are a key signal initiating mPTP formation and PS exposure. Blockade of Ca ²⁺ _mit entry allows the specific inhibition of platelet procoagulant activity.