Abnormal Platelet Serotonin Uptake And Binding Sites In Myeloproliferative Disorders
We previously shown that the platelets of patients suffering from myeloproliferative disorders (MD) present not only a reduced capacity to store serotonin (5 HT) in their dense granules but also a dramatic reduction in the initial velocity (Vi) of 5 HT uptake ; this suggests that the abnormalities are not restricted to the dense granules but involve the transport mechanism accross the platelet membrane.The present study concerns the 5 HT binding sites of MD platelets which present such a reduction of the Vi Max (Li- neweaver-Burke plot) of the 5 HT uptake. Binding assays were performed according to the method of Schik et al. (Biochem. Pharmacol. 1979, 28, 2667). Schatchard plot analysis of the binding data revealed two binding sites both in normals and patients : site A with a high affinity and a low capacity and site B with a low affinity and a high capacity.Thus the abnormal 5 HT transport accross the plasmatic membrane is the consequence of a quantitative reduction of the 5 HT binding sites and not of a qualitative defect of these sites. Nevertheless, in spite of the reduction of the number of binding sites, 5 HT-induced platelet aggregation was found normal in these patients.