Vascular actions of thrombin receptor peptide

We have examined the action of the thrombin receptor-derived polypeptide, S42FLLRNPNDKYEPF55 (TRP42–55), in rat and guinea pig aortic rings and helical arterial strips, and we have compared the actions of the peptide with those of thrombin. In rat preparations, both TRP 42–55 and thrombin caused a concentration-dependent endothelium-dependent relaxation that was blocked by Nω-nitro-L-arginine methyl ester; the relaxation response of the intact rat aortic strip preparation to concentrations of the peptide in the range 30–60 μg/mL (17–34 μM) was equivalent to the response to 0.03–0.1 U/mL of thrombin (about 0.3–0.9 nM), yielding a potency ratio (TRP 42–55:thrombin) of about 38 000: 1. In contrast with the complete desensitization of thrombin-treated rat aortic preparations to a second administration of the enzyme, the rat aortic tissue was not desensitized by repeated exposures to TRP 42–55 and remained responsive to the peptide even after treatment of the tissue by thrombin. In contrast with the rat aortic tissue, in either intact or endothelium-free guinea pig aortic preparations both TRP 42–55 and thrombin caused a concentration-dependent endothelium-independent contraction. The contractile action of 60 μg/mL of receptor peptide (34 μM) in guinea pig aortic strip preparations was equivalent to the contractile action of 0.1–0.3 U/mL thrombin (0.9–3 nM), yielding a potency ratio of about 17 000: 1. In guinea pig aortic preparations with an intact endothelium that were precontracted with noradrenaline, neither thrombin nor TRP 42–55 caused relaxation, whereas substance P did so. As with the rat aortic preparation, thrombin treatment of the guinea pig aorta rendered the tissue refractory to a second exposure to the enzyme, whereas treatment with the receptor peptide did not desensitize the tissue to a subsequent exposure to thrombin. Nonetheless, as opposed to the rat aortic preparation, wherein repeated exposure to the peptide yielded a constant response, repeat doses of TRP 42–55 caused a modest desensitization of its contractile action in the guinea pig aortic strip preparation. None of the effects of TRP 42–55 were affected by the thrombin inhibitor hirudin, which blocked the action of thrombin in both the rat and guinea pig aortic preparations. We conclude that the distinct effects of TRP 42–55 in both rat and guinea pig aortic tissue, which mimic the actions of thrombin in both preparations, are due to the presence of a thrombin receptor in these tissues, akin to the one previously characterized in platelet tissue. Further, the data indicating different TRP 42–55:thrombin potency ratios and different desensitizing properties in the rat and guinea pig aortic tissue support the notion that there may be distinct thrombin receptor signal transduction systems that regulate vascular contractility either directly by acting on smooth muscle elements or indirectly via an action on endothelial cells.Key words: thrombin, thrombin receptor, vasoconstriction, vasorelaxation, aorta, endothelium.