Platelet Function in Acute Ischemic Stroke: Relevance of Granule Secretion

We studied platelet aggregation and secretion in 48 patients with ischemic stroke, within 24 h of onset of symptoms, and 17 healthy controls. Aggregation was studied in platelet-rich plasma with the impedance method. As markers of dense and α-granule secretion we used adenosine triphosphate (ATP) and platelet factor 4 (PF4), respectively. Furthermore, we determined the basal platelet serotonin levels as an indicator of previous in vivo platelet activation. Platelet aggregation and secretion of ATP and PF4 were significantly increased in the patients in response to 1.0 and 2.0 µg/ml collagen. Similarly, the patients exhibited significantly increased platelet aggregation and PF4 secretion induced by 1.0 and 10 µmol/l adenosine diphosphate. Platelet aggregation in response to 1.0 µmol/l of platelet-activating factor was similar in patients and controls, but the parallel ATP secretion was significantly higher in patients. Platelets in patients thus exhibited an enhanced responsiveness to all three agonists, suggesting a general lower threshold for activation. The basal platelet levels of serotonin were significantly decreased in patients, indicating a previous in vivo platelet activation. Our data suggest that platelets are hyperaggregable with an increased secretion in the acute phase of cerebral infarct. Apart from the established importance of platelet hyperaggregatability in clot formation, platelet hypersecretion of serotonin and α-granule proteins may be of significance in the pathophysiology of ischemic stroke.