The effect of concanavalin A on human platelets and their response to thrombin

Concanavalin A (con A), a phytohemagglutinin, weakly aggregated washed platelets but inhibited their aggregation by thrombin. Concanavalin A and thrombin both released serotonin, nucleotides, and α-mannosidase from platelets. At suboptimal concentrations of con A and thrombin, a cumulative effect upon release of these substances was often observed. Glycosidases other than α-mannosidase (β-glucuronidase, N-acetyl-galactosaminidase, N-acetyl-glucosaminidase, and β-galactosidase) were minimally released from platelets by con A, and their release by thrombin was inhibited. Serotonin released initially by con A was re-accumulated by platelets within 30 minutes; however, thereafter these cells did not release serotonin after exposure to thrombin. The results suggest competition between con A and thrombin for receptor sites on the platelet membrane and suggest the possibility of more than one kind of receptor for thrombin.