Serotonin transport by platelet plasma and granule membranes

Genetic control of serotonin uptake in blood platelets: a twin study

Platelet serotonin (5HT) uptake was studied in 13 pairs of monozygotic (MZ) twins, 13 pairs of dizygotic (DZ) twins, and 14 pairs of unrelated normal volunteers. Significant intraclass correlations (ICC) in the affinity (Km) of 5HT uptake in the blood platelets of MZ and DZ twins and unrelated pairs were found. However, the ICC for maximum velocity (Vmax) was significant only in MZ and DZ twins. The ICC of the Vmax of 5HT uptake of MZ twins was significantly greater than that of DZ twins and unrelated pairs. This suggests that the Vmax of 5HT uptake in blood platelets is, in part, heritable. Thus, low platelet 5HT uptake (Vmax) in major depression and other disorders may be genetically determined.

Vervet monkey (Cercopithecus aethiops sabaeus) whole blood serotonin level is determined by platelet uptake sites

Whole blood serotonin levels in adult male vervet monkeys living in social groups are sensitive to the animals' social environment. The mechanisms that translate different behavioral and environmental cues into altered whole blood serotonin levels are unknown. In this study, we have measured platelet number, size, serotonin content, and serotonin uptake, as well as the serum concentrations of tryptophan, Mg+2 and Ca+2. Results showed that whole blood serotonin levels, platelet serotonin content, and the serotonin uptake parameter Vmax were stable within animals on repeated sampling. The whole blood serotonin level was highly positively associated with platelet serotonin content, and the platelet serotonin content was highly positively associated with Vmax. These findings suggested that whole blood serotonin levels were a function of the number of platelet uptake sites.

Incorporation of a circulating protein into megakaryocyte and platelet granules

To determine whether or not proteins circulating in plasma can be incorporated into megakaryocytes and platelets, horseradish peroxidase (HRP) was injected intravenously into guinea pigs and these cells were examined for its uptake by electron microscopy and cytochemistry. Enriched samples of megakaryocytes enabled ultrastructural analysis of large numbers of these rare cells. In megakaryocytes, 50% of alpha granules contained HRP between 75 min and 7 hr after injection. At 24 hr, 25% of the megakaryocyte granules were peroxidase-positive, less were positive by 48 hr, and there were none at 4 days. Thus, the findings demonstrate that a circulating protein can be endocytosed by megakaryocytes and rapidly packaged into alpha granules. Platelet granules also contain HRP by 7 hr after injection, and they can secrete it in response to thrombin. Unfortunately, our present studies do not allow us to distinguish between direct endocytosis by the platelet and/or shedding of new platelets from recently labeled megakaryocytes. It is concluded that while some alpha granule proteins are synthesized by megakaryocytes, others may be acquired from plasma by endocytosis. In addition to providing evidence that some of the proteins of alpha granules may be of exogenous origin, this study has allowed the definition of a pathway whereby plasma proteins may be temporarily sequestered in megakaryocytes before reentering the circulation in platelets.