Quantity, Assay and Release of Serotonin in Human Platelets

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Abstract
Human platelets contain a vasoconstrictor substance whose activity on the perfused rabbit ear is equivalent to 0.76 [mu]g of serotonin/109 platelets. This is close to the amount of serotonin found by others using chemical methods or different pharmacological indicators. Furthermore, the platelet vasoconstrictor is, like serotonin, blocked by 1-methyl-medmain, a serotonin analogue. Thus, all of the evidence indicates that the vasoconstrictor substance in human platelets is serotonin (5-hydroxytryptamine). The vasoconstrictor activity of an extract of platelets made in plasma is equal to that of an extract made in saline when they are tested on a fresh rabbit ear, but the former is more active on aged vasculature. The vasoconstrictor activity of serum prepared by clotting plasma containing washed platelets is equal to that of a saline or plasma solution or extract of the same number of disrupted platelets. Since it can be assumed that all of the preformed serotonin is released by platelet disruption, it can be concluded that serotonin is completely released from washed human platelets when they are suspended in plasma and the mixture is clotted by recalcification. Only about half of the platelet serotonin is released when whole blood clots in glass. The amount liberated in whole blood is not increased by prolonging the incubation of the blood or by adding thrombin before coagulation occurs. This is presumably due to the fact that platelets in their native plasma are considerably more stable than those which have been subjected to the arduous washing procedures.