Proteolysis of the platelet surface: dissociation of shape change from aggregation

Stimulation of intact platelets by ADP results in a shape change followed by aggregation in the presence of fibrinogen. ADP was found to induce a shape change in chymotrypsin-treated platelets that was similar in extent and initial velocity to that of intact (untreated) platelets. Scanning-electron microscopy verified an ADP-induced shape change in chymotrypsin-treated platelets. This shape change could be completely blocked by stimulators of platelet adenylate cyclase (forskolin, prostaglandin E1, and prostacyclin). On the other hand, the aggregation of chymotrypsin-treated platelets by fibrinogen was not dependent on the presence of ADP and could not be blocked by forskolin, prostaglandin E1, or prostacyclin, even though the levels of cyclic AMP (cAMP) formed in chymotrypsin-treated platelets were comparable to levels that completely inhibited the ADP-induced aggregation of intact platelets. This lack of inhibition of platelet aggregation was not due to degradation of the adenylate cyclase or prostaglandin receptors, since chymotrypsin-treated platelets were found to have a functional adenylate cyclase system that could be stimulated by forskolin, prostaglandin E1, and prostacyclin and inhibited by ADP and epinephrine, similar to that of intact platelets. These results provide direct evidence that cAMP does not interact with fibrinogen binding sites once they have become permanently exposed on the surface of platelets. Pretreatment of platelets with chymotrypsin therefore appears to be a useful tool that allows for the dissociation of platelet shape change from aggregation, without inhibiting either response.