Induction of the platelet release reaction by phytohemagglutinin

It was previously shown that the erythroagglutinating phytohemagglutinin (E PHA) from P. vulgaris binds to the surface of intact human platelets and that adenylate cyclase activity in the particulate fraction of E PHA treated platelets is lower than in comparable controls. It is now found that E PHA induces release of [¹⁴C]serotonin from platelets. Release follows binding of E PHA, and a haptenic inhibitor of E PHA binding prevents induction of release. E PHA does not produce platelet lysis and has little effect on [¹⁴C]serotonin uptake. Platelets possess approximately 300,000 receptor sites for E PHA per cell, and it is estimated that about 15% of these sites must be occupied by E PHA to initiate the release reaction. Prior incubation of platelets with prostaglandin E₁, theophylline, or dibutyryl cyclic AMP prevents E PHA induced release, although these agents have little effect on E PHA binding to platelets. Thrombin and E PHA produce different rates and extents of serotonin release. Thrombin (1 U/ml) causes release of 75 to 85% of platelet [¹⁴C]serotonin, with half maximal release occurring less than 0.5 min after thrombin addition. E PHA, however, induces release of only 30 to 60% of platelet serotonin at a 10 fold slower rate. In addition, utilizing electron microscopy, striking differences were observed in the morphologic changes that occur in platelets exposed to E PHA as compared with thrombin. Thus, the platelet release reaction may be triggered in part by binding of E PHA to the cell surface, but this reaction only partially resembles that produced by thrombin.