Splenic release of platelets contributes to increased circulating platelet size and inflammation after myocardial infarction.

Acute myocardial infarction (AMI) is characterized by a rapid increase in circulating platelet size but the mechanism for this is unclear. Large platelets are hyperactive and associated with adverse clinical outcomes. We determined mean platelet volume (MPV) and platelet-monocyte conjugation (PMC) using blood samples from patients, and blood and the spleen from mice with AMI. we further measured changes in platelet size, PMC, cardiac and splenic contents of platelets and leukocyte infiltration into the mouse heart. In AMI patients, circulating MPV and PMC increased at 1~3 h post-MI and MPV returned to reference levels within 24h after admission. In mice with MI, increases in platelet size and PMC became evident within 12h and were sustained up to 72h. Splenic platelets are bigger than circulating platelets in normal or infarct mice. At 24h post-MI, splenic platelet storage was halved whereas cardiac platelets increased by 4-fold. Splenectomy attenuated all changes observed in the blood, reduced leukocyte and platelet accumulation in the infarct myocardium, limited infarct size and alleviated cardiac dilatation and dysfunction. AMI induced elevated circulating levels of adenosine diphosphate and catecholamines in both human and the mouse, which may trigger splenic platelet release. Pharmaological inhibition of angiotensin-converting enzyme, β1-adrenergic receptor or platelet P2Y12 receptor reduced platelet abundance in the murine infarct myocardium albeit having diverse effects on platelet size and PMC. In conclusion, AMI evokes release of splenic platelets, which contributes to the increase in platelet size and PMC and facilitates myocardial accumulation of platelets and leukocytes, thereby promoting post-infarct inflammation.