Competition Between Platelets and Lung for 5-Hydroxytryptamine Uptake

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Abstract
The pulmonary endothelial cell removes 5-hydroxytryptamine (5-HT) from the circulation by a process of uptake and metabolism. The platelet, the major source of 5-HT in the circulation, represents another potential site of uptake for this amine. Platelet uptake of 5-HT infused into the pulmonary circulation was evaluated using isolated, ventilated rat lungs. Lungs were perfused with buffer, platelet-poor, platelet-rich or thrombin-treated platelet-rich plasma followed by an infusion of (14C) 5-HT. Platelet counts, aggregation response to ADP and in vitro uptake of 5-HT were unchanged after circulation through the lung. In the presence of buffer or platelet-poor plasma, the lung removed 65.5 (.+-. 1.4% and 67.8 .+-. 0.8%, respectively, of the infused (14C) 5-HT. In the presence of platelet-rich plasma, the lung removed 51.3 .+-. 3.0 and platelet fraction 30.1 .+-. 1.4% of the infused (14C) 5-HT. Recently aggregated platelets removed 24.4 .+-. 1.8%; platelets exposed to low O2 tensions removed 14.7 .+-. 0.6% of the (14C) 5-HT. Apparently, 5-HT released into the pulmonary circulation is taken up by the endothelium and metabolized or taken back up by the platelet and stored.