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Vascular actions of thrombin receptor‐derived polypeptides: structure‐activity profiles for contractile and relaxant effects in rat aorta

1 Using endothelium‐denuded and intact rat aortic rings, we have determined the contractile and relaxant structure‐activity profile for a series of thrombin receptor‐derived polypeptides (TRPs) based on the human and rat receptor sequences: SFLLR (P5), SFLLR‐NH2 (P5‐NH2) SFFLR (Rat P5), SFFLR‐ NH2 (Rat P5‐NH2), SFLLRNP (P7), SFLLRNP‐NH2 (P7‐NH2), SFFLRNP (Rat P7), SFFLRNP‐NH2 (Rat P7‐NH2), and SFLLRNPNDKYEPF (P14). 2 A contractile response to thrombin and the TRPs in the endothelium‐denuded aortic tissue was minimal or absent in preparations obtained from animals weighing less than 180 g (<6 weeks of age), but increased with animal size, plateauing in tissues derived from animals weighing between 320 and 420 g (about 9 to 14 weeks of age). In contrast, the contractile responses to KC1 and noradrenaline did not differ in the tissues and relaxant responses to the TRPs in endothelium‐intact aortic preparations were comparable for tissues obtained from either young (≤ 180 g) or older (≤ 320 g) animals. 3 The contractile response of the endothelium‐denuded preparation to thrombin and the TRPs showed marked cross‐desensitization: the relaxation response of the intact rings did not desensitize to the TRPs. 4 The relative potencies for the TRPs in the aortic contraction assay were comparable to those for the relaxation assay, but were distinct from the relative potencies we measured previously in a rat gastric longitudinal muscle contraction assay. Further, P5 behaved as a partial agonist in the aortic contraction assay, whereas it had been observed to be a full agonist in the gastric contraction assay. 5 The contractile activity of P5‐NH2 in endothelium intact aortic rings was low or absent, but in the presence of the nitric oxide synthase inhibitor, Nω‐nitro‐L‐arginine‐methyl ester (L‐NAME), the contractions in the intact preparation were equivalent to the response of the endothelium‐denuded preparation in the absence of L‐NAME. 6 The contractile response of the endothelium‐denuded aortic preparation to P5‐NH2 was inhibited by nifedipine and the kinase C antagonist, chelerythrine, but was resistant to the action of indomethacin, tetrodotoxin and the tyrosine kinase inhibitor, genistein. 7 We conclude that the receptor system for the TRPs in the aortic smooth muscle elements, responsible for the contractile response, is similar to the aortic endothelial cell receptor responsible for the relaxation response, but is distinct from the receptor that we have previously characterized in gastric longitudinal smooth muscle, results pointing to the presence of receptor subtypes in the vascular and gastric smooth muscle elements.