Tachykinin receptors in the circular muscle of the guinea‐pig ileum

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Abstract
1 We have studied the mechanical response of circular strips of the guinea-pig ileum to tachykinins and characterized the receptors involved by means of receptor-selective agonists. 2 The strips responded to both substance P (SP) and neurokinin A (NKA), as well as to [Pro9]-SP sulphone (selective NK1-receptor agonist), [βAla8]-NKA(4–10) (selective NK2-receptor agonist) and [MePhe7]-neurokinin B (selective NK3-receptor agonist). The ED50s of the various peptides (calculated as the concentration of agonist which produced 50% of the response to 10 μm carbachol) were similar, in the range of 40–200 nm, i.e. no clearcut rank order of potency was evident. 3 The response to a submaximal (10 nm) concentration of SP or NKA was unaffected in the presence of peptidase inhibitors (thiorphan, captopril and bestatin, 1 μm each). 4 The response to the NK1-agonist was totally atropine-resistant, but was reduced (about 30% inhibition) by tetrodotoxin. The response to the NK3-receptor agonist was halved by atropine and abolished by tetrodotoxin. The response to the NK2-agonist was unaffected by either atropine or tetrodotoxin. 5 The response to the selective NK2-agonist was unchanged after desensitization of NK1- or NK3-receptors. 6 The response to the NK2-selective agonist was strongly inhibited by [Tyr5, d-Trp6,8,9, Arg10]-NKA(4–10) (MEN 10,207) a selective NK2-receptor antagonist which did not modify the response to the NK1-selective agonist. 7 Our findings indicate that all the three known types of tachykinin receptors mediate the contractile response of the circular muscle of the guinea-pig ileum to peptides of this family. The response to activation of NK3-receptors is totally neurogenic and partially mediated by endogenous acetylcholine, the response to activation of NK1-receptors is partly neurogenic and largely myogenic and the response to activation of NK2-receptors is totally myogenic.