Thrombin-Induced Surface Changes of Human Platelets in Plasma: Their Relation to Serotonin Release

Membrane proteins of both control and thrombin-treated platelets were labeled by NaB³H₄, reduction of Schiff bases formed between pyridoxal 5′-phosphate and protein amino groups. Examination of the labeled polypeptides by SDS-polyacrylamide gel electrophoresis and fluorography disclosed a different labeling pattern for thrombin-treated platelets. The distributions of Coomassie blue-stained protein from treated and untreated cells were, by contrast, almost identical. Fluorographs of control platelets showed a single intensely labeled protein band (mol wt 90,000) whereas with cells exposed to thrombin (30-60 milliunits) about 10 protein bands with mol wts ranging from 43,000 to 200,000 were typically present. Among these were: thrombin-sensitive protein (mol wt 188,000), glycoprotein I (mol wt 150,000) and actin (mol wt 43,000). When serotonin release was prevented, either by reversing platelet aggregation with low amounts of ADP (0.1-0.3 μM) or by preincubating with 3',5'-ADP (20 μM), an inhibitor of both ADP- and thrombin-induced platelet function, the labeling patterns on fluorographs were similar to the control. These results indicate that blood platelets can undergo reversible aggregation without major changes in their surface topography, whereas thrombin-induced serotonin release appears related to structural alterations in platelet membrane proteins.