Hypersensitivity of platelets to thrombin: formation of stable thrombin-receptor complexes and the role of shape change.

The rate of rat platelet shape change increases sigmoidally with respect to thrombin concentration under conditions where free Ca++ and ADP are limited to prevent platelet aggregation. In addition, the rate of shape change due to thrombin is considerably enhanced when the platelets are first treated with concanavalin A (Con A), an agent which itself produces shape change. In the presence of both agents the rate is considerably greater than the sum of the rates due to Con A and thrombin separately. This suggests that shape change itself may trigger increased platelet sensitivity to thrombin. One possible mechanism through which this might occur is that shape change promotes binding of thrombin to its surface receptors. If so, then Con A-induced shape change might facilitate binding of 125I-thrombin. Initial binding studies using previously described methods showed that nonspecific trapping of 125I-thrombin, which is bound specifically to platelets during shape change, prevents accurate measurements of thrombin binding. We found, however, that a portion of 125I-thrombin which is bound specifically to platelets forms a stable complex with a 40,000 dalton platelet protein. This complex cannot be disrupted by boiling in SDS buffer containing 2-mercaptoethanol. Linkage of 125I-thrombin to this protein is specific for thrombin, since it can be competed for by an excess of unlabeled thrombin and because a similar complex does not form using 125I-trypsin. Concentrations of Con A that induce platelet shape change also markedly increase the amount of complex produced by a given thrombin concentration. In addition, colchicine, an inhibitor of Con A-induced platelet function, markedly inhibits formation of the 125I-thrombin-receptor complex. We suggest that the sigmoidal response to thrombin might be related to appearance of new thrombin receptors on the platelet surface.