Summary
Based on studies with thrombin, it has been proposed that human platelets exposed
to strong release-inducing agents undergo irreversible aggregation and cannot be deaggregated
without the use of proteolytic enzymes. We tested the hypothesis that irreversible
human platelet aggregation occurs as a result of thrombin-specific platelet alterations
rather than induction of the release reaction per se. Washed human platelets were
exposed to either thrombin (THR) or the aminophospholipid N-(7-Nitro- 2,1,3-benzoxydiazol-4-yl)
phosphatidylserine (NBD-PS) for 20 seconds. Both agents caused similarly extensive
release of platelet dense- and α-granule contents. After neutralization of thrombin
and NBD-PS, and addition of PGE1 and apyrase, the platelets were sedimented, resuspended and incubated at 37° C with
gentle agitation. Single, disc-shaped, degranulated platelets which were recovered
in both systems were capable of aggregation in response to a second exposure to aggregating
and release- inducing stimuli. Deaggregation was more rapid, more extensive, and more
reproducible with NBD-PS- than with THR-degranulated platelets. Platelets exposed
to thrombin for longer than 20 seconds showed a progressive loss of deaggregability
which was not observed after prolonged incubation with NBD-PS. These findings do not
support the concept that extensive secretion per se causes irreversible aggregation
of human platelets. Instead it appears that formation of irreversible linkages between
platelets involves the specific, time-dependent interaction of THR with platelets,
released fibrinogen and possibly one or more other substances secreted from platelets.
Keywords
Irreversible platelet aggregation - Thrombin - NBD-phosphatidylserine
