THE ROLE OF CHELATION AND OF HUMAN PLASMA IN THE UPTAKE OF SEROTONIN BY HUMAN PLATELETS*

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Abstract
The problem of uptake of serotonin by human platelets was investigated by (1) a screening method which measured dye (2,6-dichloroindophenol) reduction by intact platelets, and (2) measurable uptake of serotonin by intact platelets, all in the presence of human plasma. The effect of various agents on these in vitro systems was observed. Enhancement of dye reduction by platelets in the presence of serotonin and a plasma factor or factors ("5HT redox effect") correlated with increased uptake of serotonin by intact platelets. Serotonin was shown to solubilize calcium phosphate in vitro. Prior mixing of serotonin with Mg++ followed by chelation with disodium EDTA markedly diminished both 5HT redox effect and uptake of 5HT by intact human platelets in the presence of plasma. Disodium calcium EDTA was without effect, as was Mg++ alone. The possibility is entertained that in the presence of a factor present in human plasma, serotonin is taken up by human platelets as a complex with a divalent cation bound to a heat-stable, nondialyzable plasma factor.