Absorption of Serotonin (5-Hydroxytryptamine) by Canine and Human Platelets

Serotonin absorption was studied by incubating platelets for 5 or 10 minutes at room temperature with synthetic serotonin in a concentration of 0.2–4 µg/ml. Serotonin disappeared from the supernatant in two experiments with canine platelet-rich plasma, seven experiments with human platelet-rich plasma and three out of four experiments with native whole human blood. It did not disappear in four out of nine experiments using washed dog platelets suspended in saline and in none of five experiments with washed human platelets. Assay of the platelets in two experiments with human platelet-rich plasma demonstrated that much or all of the serotonin could be recovered in the platelets. The ability of the platelets to absorb serotonin could be correlated with their morphological appearance. Normal disc-shaped platelets absorbed serotonin, whereas spiny spherical platelets often seen in shed blood did not. The quantity of serotonin absorbed by washed dog platelets or by human platelets in platelet-rich plasma from a solution of 0.2 µg/ml was about 0.38 µg/109 platelets, and virtually all of the serotonin was taken up. When the initial concentration was 2 µg/ml, human platelets absorbed up to 2.8 µg/109 platelets, and appreciable quantities of serotonin were left in the supernatant.