Bronchoconstrictor effect of thrombin and thrombin receptor activating peptide in guinea-pigs in vivo

1 Several thrombin cellular effects are dependent upon stimulation of prote inase activated receptor-1 (PAR-1) localized over the cellular surface. Fol lowing activation by thrombin, a new N-terminus peptide is unmasked on PAR- 1 receptor, which functions as a tethered ligand for the receptor itself. S ynthetic peptides called thrombin receptor activating peptides (TRAPs), cor responding to the N-terminus residue unmasked, reproduce several thrombin c ellular effects, but are devoid of catalytic activity. We have evaluated th e bronchial response to intravenous administration of human alpha-thrombin or a thrombin receptor activating peptide (TRAP-9) in anaesthetized, artifi cially ventilated guinea-pigs. 2 Intravenous injection of thrombin (100 u kg(-1)) caused bronchoconstricti on that was recapitulated by injection of TRAP-9 (1 mg kg(-1)). animal pret reatment with the thrombin inhibitor Hirulog(TM) (10 mg kg(-1) i.v.) preven ted thrombin-induced bronchoconstriction, but did not affect bronchoconstri ction induced by TRAP-9. Both agents did not induce bronchoconstriction whe n injected intravenously to rats. 3 The bronchoconstrictor effect of thrombin and TRAP-9 was subjected to tol erance: however, in animals desensitized to thrombin effect, TRAP-9 was sti ll capable of inducing bronchoconstriction, but not vice versa. 4 Depleting animals of circulating platelets prevented bronchoconstriction induced by both thrombin and TRAP-9. 5 Bronchoconstriction was paralleled by a biphasic change in arterial blood pressure, characterized by a hypotensive phase followed by a hypertensive phase. Thrombin-induced hypotension was not subject to tolerance and was in hibited by Hirulog(TM); conversely, hypertension was subject to tolerance a nd was not inhibited by Hirulog(TM). Hypotension and hypertension induced b y TRAP-9 were neither subject to tolerance nor inhibited by Hirulog(TM) 6 Our results indicate that thrombin causes bronchoconstriction in guinea-p igs through a mechanism that requires proteolytic activation of its recepto r and the exposure of the tethered ligand peptide. Platelet activation migh t be triggered by the thrombin effect.