Recent studies on cloning of the thrombin receptor, which belongs to the family of G-protein-coupled receptors, suggest that thrombin cleaves a peptide from the extracellular N-terminus. A synthetic peptide of 14 amino acids corresponding to the sequence of the newly generated N-terminus was found to possess thrombin-like activity in several cells endowed with thrombin receptors. The relaxant and contractile effects of this thrombin receptor activating peptide (TRAP, Ser-Phe-Leu-Leu-Arg-Asn-Pro-Asn-Asp-Lys-Tyr-Glu-Pro-Phe) were investigated in porcine pulmonary arteries and compared with the action of thrombin. In PGF2 alpha-precontracted vessels with intact endothelium, TRAP (0.3-10 mumol/l) caused reversible transient and concentration-dependent relaxation which was absent after mechanical removal of the endothelium. Preincubation of the vessels with NG-nitro-L-arginine (200 mumol/l) markedly reduced the relaxation. The TRAP-induced relaxation was associated with an increase in cGMP in the arteries. In comparison to thrombin, TRAP (EC50: 0.8 mumol/l) was less potent by more than three orders of magnitude. In endothelium-denuded pulmonary arteries TRAP (1-20 mumol/l) caused a concentration-dependent contraction which was reversible after washout. The TRAP-induced contractile response was preceded by an increase in generation of inositol 1,4,5-triphosphate (IP3); the peak of IP3 accumulation was reached after 30 s. Compared with the contractile effect of thrombin, that of TRAP was weaker by three of magnitude. The vascular effect of TRAP was not inhibited by the thrombin inhibitors hirudin or heparin while the protein kinase C inhibitor staurosporine (0.1 mumol/l) preferentially inhibited the tonic phase of contraction.(ABSTRACT TRUNCATED AT 250 WORDS)