Serotonin uptake in subpopulations of normal human blood platelets

Serotonin uptake was studied in platelet-rich plasma with different proportions of heavy (young) and light (old) platelets. The maximum velocity of serotonin uptake, a measure of the number of uptake sites (Vmax), was greater in the heavy platelets than in the light platelets. However, the uptake sites in the lighter platelets had an increased affinity (Km) for serotonin compared to the heavy platelets, which could compensate for the decreased number of uptake sites. These results indicate differences in Vmax of 5-HT uptake in particular populations could be due to differences in the relative proportions of heavy and light platelets.