Intraplatelet serotonin and plasma 5-hydroxyindoles in health and disease

Using a fluorometric test sensitive to serotonin (5-HT) and to other 5-hydroxyindoles (5-HIs) it was shown that platelets take up 5-HT and that the added 5-HT and the inherent test-positive material in platelet lysate can be directly measured. However, platelets do not take up 5-hydroxyindole acetic acid or the 5-HIs from the plasma. Thus, 5-HT and the other 5-HIs can be distinguished. Various methods of liberating intraplatelets 5-HT were investigated. Several anticoagulants, temperatures, and speeds of centrifugation were employed, with no effect on the results. It was found that in healthy donors, with increasing age, there was a decrease in intraplatelet 5-HT and an increase in plasma 5-HIs. The quantities of these substances were inversely related. In acute myocardial infarction, the 5-HT was normal taking age into account, but the plasma 5-HIs were increased. In postoperative patients, the platelet 5-HT was low and the plasma 5-HIs were normal. In patients with chest pain but no myocardial infarction, both platelet 5-HT and plasma 5-HIs were normal. The relevance of these findings to 5-HT metabolism and the evidence for in vivo activation of platelets is discussed.