Abstract
1 The movement of radiolabelled 5-hydroxytryptamine (5-HT) between the extracellular medium, the thrombin-releasable (vesicular) compartment, and a non-thrombin-releasable compartment has been investigated in washed human platelets. 2 Appreciable amounts of extracellular 5-HT can accumulate in a non-releasable compartment. Depending on the incubation conditions and the amount of 5-HT already present inside the cell, non-releasable 5-HT can either remain non-releasable or rapidly migrate into the vesicular compartment. 3 Measurable amounts of vesicular 5-HT can also enter a non-releasable compartment. However, vesicular 5-HT which becomes non-releasable does not appear to mix with 5-HT becoming non-releasable following uptake from the extracellular medium. 4 Extracellular 5-HT can be added to vesicles and to the non-releasable compartment in the presence of appreciable quantities of 5-HT already in one or both compartments. 5 The vesicular and non-releasable compartments appear to accumulate 5-HT independently of one another. Furthermore, most vesicular accumulation of 5-HT occurs from the extracellular medium, rather than by translocation of non-releasable 5-HT into the vesicular compartment.