Serotonin uptake and storage in human platelet density subpopulations

S ummary . Serotonin uptake and storage were studied in human platelet density subpopulations which were isolated after isopycnic centrifugation on a discontinuous iso‐osmolar stractan gradient. Kinetic parameters of the serotonin uptake were calculated (K m, V i max) and the granular storage capacity was determined by comparing the total amount taken up in the presence or absence of reserpine, a specific inhibitor of the uptake at the granular level. Mean platelet volume and the number of mepacrine‐labelled dense bodies were also determined. The results show that the active metabolic process of serotonin uptake is identical whatever the platelet subpopulation but the storage capacity is greater in the densest fraction which contains more dense bodies than the lightest one. Thus active serotonin uptake appears as another well‐defined platelet metabolic process which is independent of platelet density; this argues against the conception that light platelets could be old platelets with reduced functional capacities.