Comparison of tachykinin NK 1 and NK 2 receptors in the circular muscle of the guinea‐pig ileum and proximal colon

1 The aim of this study was the pharmacological characterization of tachykinin NK 1 and NK 2 receptors mediating contraction in the circular muscle of the guinea‐pig ileum and proximal colon. The action of substance P (SP), neurokinin A (NKA) and of the synthetic agonists [Sar 9 ]SP sulphone, [Glp 6 ,Pro 9 ]SP(6–11) (septide) and [βAla 8 ]NKA(4–10) was investigated. The affinities of various peptide and nonpeptide antagonists for the NK 1 and NK 2 receptor was estimated by use of receptor selective agonists. 2 The natural agonists, SP and NKA, produced concentration‐dependent contraction in both preparations. EC 50 values were 100 p m and 5 n m for SP, 1.2 n m and 19 n m for NKA in the ileum and colon, respectively. The action of SP and NKA was not significantly modified by peptidase inhibitors (bestatin, captopril and thiorphan, 1 μ m each). 3 Synthetic NK 1 and NK 2 receptor agonists produced concentration‐dependent contraction of the circular muscle of the ileum and proximal colon. EC 50 values were 83 p m , 36 p m and 10 n m in the ileum, 8 n m , 0.7 n m and 12 n m in the colon for [Sar 9 ]SP sulphone, septide and [βAla 8 ]NKA(4–10), respectively. The pseudopeptide derivative of NKA(4–10), MDL 28,564 behaved as a full or near‐to‐full agonist in both preparations, its EC 50 s being 474 n m and 55 n m in the ileum and colon, respectively. 4 Nifedipine (1 μ m ) abolished the response to septide and [Sar 9 ]SP sulphone in the ileum and produced a rightward shift and large depression of the response in the colon. The response to [βAla 8 ]NKA(4–10) was abolished in the ileum and largely unaffected in the colon. 5 The NK 1 receptor antagonists, (±)‐CP 96,34, FK 888 and GR 82,334 competitively antagonized the response to septide and [Sar 9 ]SP sulphone in both preparations without affecting that to [βAla 8 ]NKA(4–10). In general, the NK 1 receptor antagonists were significantly more potent toward septide than [Sar 9 ]SP sulphone in both preparations. 6 The NK 2 receptor antagonists, GR 94,800 and SR 48,968 selectively antagonized the response to [βAla 8 ]NKA(4–10) without affecting that to [Sar 9 ]SP sulphone or septide in the ileum and colon. SR 48,968 produced noncompetitive antagonism of the response to the NK 2 receptor agonist in the ileum and competitive antagonism in the colon. 7 MEN 10,376 and the cyclic pseudopeptide MEN 10,573 antagonized in a competitive manner the response to [βAla 8 ]NKA(4–10) in the ileum and colon. While MEN 10,573 was equipotent in both preparations, MEN 10,376 was significantly more potent in the colon than in the ileum. MEN 10,376 was also effective against septide in both preparations, without affecting the response to [Sar 9 ]SP sulphone. MEN 10,573 antagonized the response to [Sar 9 ]SP sulphone and septide in both preparations, p K B values against septide being intermediate, and significantly different from, those measured against [βAla 8 ]NKA(4–10) and [Sar 9 ]SP sulphone. 8 These findings show that tachykinin NK 1 and NK 2 receptors mediate contraction of the circular muscle of the guinea‐pig ileum and colon. In both preparations NK 1 receptor antagonists display higher apparent affinity when tested against septide than [Sar 9 ]SP sulphone. These findings are compatible with the proposed existence of NK 1 receptor subtypes in guinea‐pig, although alternative explanations (e.g. agonist binding to different epitopes of the same receptor protein) cannot be excluded at present. Furthermore, an intraspecies heterogeneity of the NK 2 receptor in the circular muscle of the guinea‐pig ileum and colon is suggested.