The aggregation of platelets from a variety of animal species in response to thrombin
receptor-derived activating peptides was evaluated. A series of 14-(SFLLRNPNDKYEPF),
7-(SFLLRNP-NH2), 6-(SFLLRN-HN2) or 5-(SFLLR-NH2) residue peptides, the structures of which were based on the deduced amino acid sequence
of the human thrombin receptor, promoted full aggregation of platelets in plasma from
humans, African Green and Rhesus monkeys, baboons and guinea pigs at 4-50 μM depending
on the peptide used. Platelets in plasma from rabbit, dog, pig, and hamster underwent
a shape change but failed to aggregate in response to these peptides over 3 log units
of peptide up to 800 μM, despite being fully responsive to human thrombin. However,
because the receptor peptides induced shape change in the platelets from these non-aggregating
species, they apparently can activate some of the intracellular signaling system(s)
usually initiated by thrombin in these platelets. In contrast, platelets from rats
did not undergo shape change or aggregate in response to the peptides. A 7-residue
receptor-derived peptide based on the deduced amino acid sequence of the clone of
the hamster thrombin receptor (SFFLRNP-N2) was nearly as efficacious as the corresponding human receptor-derived 7-residue
peptide to promote aggregation of human platelets. However, the hamster peptide could
not promote aggregation of hamster platelets in plasma at up to 800 μM peptide, while
a shape change response was elicited. Platelets from rats, rabbits and pigs also did
not aggregate in response to this peptide derived from the hamster thrombin receptor,
but all species except the rat underwent a shape change. Longer 17-residue peptides
derived from the sequences of the hamster or mouse thrombin receptors elicited aggregation
of human platelets but no aggregation of the hamster platelets. In contrast, the human
14- and 5-residue and the hamster 7-residue thrombin receptor-derived peptides promoted
mitogenesis of CCL39 cells, a hamster fibroblast cell line. Finally, the human 6-residue
thrombin receptor-derived peptide promoted contraction of normal and de-endothelialized
canine coronary artery rings, despite having no pro-aggregatory effect on canine platelets.
Taken together, these results demonstrate that the thrombin receptor-derived peptides
are able to mimic many, but not all, of the activating effect of thrombin in different
tissues from multiple species. The heterogeneity of responsiveness to these peptides
should be taken into account in future experiments designed to elucidate the mechanism
of thrombin stimulation of platelets and other cells.
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