Differential release of calcium, magnesium and serotonin by rabbit and human platelets

Calcium and magnesium are extracellularly released by platelets during the release reaction. The characteristics of the release of these components by rabbit and human platelets were compared with the release of serotonin. When rabbit platelets were activated by thrombin which induces maximum response, approximately 80% of the platelet serotonin and calcium, but only 50% of platelet magnesium were released. It was suggested that only about one-half of the amount of platelet magnesium was localized in the releasable compartments in the cells. In response to a lower concentration of thrombin, calcium and magnesium were released to a greater extent than was serotonin. When phorbol myristate acetate was added to the rabbit and human platelet suspension serotonin was slowly but calcium and magnesium were rapidly released into extracellular fluid. Platelet factor-4 was also more rapidly released than serotonin by human platelets. The release of calcium and magnesium was analogous to the release of platelet factor-4. It is conceivable that possible pools containing calcium and magnesium but little or no serotonin exist in the platelets and that their contents are more susceptible to mild stimuli than granular serotonin.