Summary
Synthetic peptides (TRAP or Thrombin Receptor Activating Peptide) corresponding to
at least the first five aminoacids of the new N-terminal tail generated after thrombin
proteolysis of its receptor are effective to mimic thrombin. We have studied two different
TRAPs (SFLLR, and SFLLRN) in their effectiveness to induce the different platelet
responses in comparison with thrombin. Using Indo-1/AM- labelled platelets, the maximum
rise in cytoplasmic ionized calcium was lower with TRAPs than with thrombin. At threshold
concentrations allowing maximal aggregation (50 μM SFLLR, 5 μM SFLLRN and 1 nM thrombin)
the TRAPs-induced release reaction was about the same level as with thrombin, except
when external calcium was removed by addition of 1 mM EDTA. In these conditions, the
dense granule release induced by TRAPs was reduced by over 60%, that of lysosome release
by 75%, compared to only 15% of reduction in the presence of thrombin. Thus calcium
influx was more important for TRAPs-induced release than for thrombin-induced release.
At strong concentrations giving maximal aggregation and release in the absence of
secondary mediators (by pretreatment with ADP scavengers plus aspirin), SFLLRN mobilized
less calcium, with a fast return towards the basal level and induced smaller lysosome
release than did thrombin. The results further demonstrate the essential role of external
calcium in triggering sustained and full platelet responses, and emphasize the major
difference between TRAP and thrombin in mobilizing [Ca2+]j. Thus, apart from the proteolysis of the seven transmembrane receptor, another thrombin
binding site or thrombin receptor interaction is required to obtain full and complete
responses.
