COMPARISON OF TACHYKININ NK1 AND NK2 RECEPTORS IN THE CIRCULAR MUSCLEOF THE GUINEA-PIG ILEUM AND PROXIMAL COLON

1 The aim of this study was the pharmacological characterization of ta chykinin NK1 and NK2 receptors mediating contraction in the circular m uscle of the guinea-pig ileum and proximal colon. The action of substa nce P (SP), neurokinin A (NKA) and of the synthetic agonists Sar(9)S P sulphone, Glp(6),Pro(9)SP(6-11) (septide) and beta Ala(8)NKA(4-1 0) was investigated. The affinities of various peptide and nonpeptide antagonists for the NK1 and NK2 receptor was estimated by use of recep tor selective agonists. 2 The natural agonists, SP and NKA, produced c oncentration-dependent contraction in both preparations. EC(50) values were 100 pM and 5 nM for SP, 1.2 nM and 19 nM for NKA in the ileum an d colon, respectively. The action of SP and NKA was not significantly modified by peptidase inhibitors (bestatin, captopril and thiorphan, 1 mu M each). 3 Synthetic NK1 and NK2 receptor agonists produced concen tration-dependent contraction of the circular muscle of the ileum and proximal colon. EC(50) values were 83 pM, 36 pM and 10 nM in the ileum , 8 nM, 0.7 nM and 12 nM in the colon for Sar(9)SP sulphone, septide and beta Ala(8)NKA(4-10), respectively. The pseudopeptide derivativ e of NKA(4-10), MDL 28,564 behaved as a full or near-to-full agonist i n both preparations, its EC(50)s being 474 nM and 55 nM in the ileum a nd colon, respectively. 4 Nifedipine (1 mu M) abolished the response t o septide and Sar(9)SP sulphone in the ileum and produced a rightwar d shift and large depression of the response in the colon. The respons e to beta Ala(8)NKA(4-10) was abolished in the ileum and largely una ffected in the colon. 5 The NK1 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 t o beta Ala(8)NKA(4-10). In general, the NK1 receptor antagonists wer e significantly more potent toward septide than Sar(9)SP sulphone in both preparations. 6 The NK2 receptor antagonists, GR 94,800 and SR 4 8,968 selectively antagonized the response to beta Ala(8)NKA(4-10) w ithout affecting that to Sar(9)SP sulphone or septide in the ileum a nd colon. SR 48,968 produced noncompetitive antagonism of the response to the NK2 receptor agonist in the ileum and competitive antagonism i n the colon. 7 MEN 10,376 and the cyclic pseudopeptide MEN 10,573 anta gonized in a competitive manner the response to beta Ala(8)NKA(4-10) in the ileum and colon. While MEN 10,573 was equipotent in both prepa rations, MEN 10,376 was significantly more potent in the colon than in the ileum. MEN 10,376 was also effective against septide in both prep arations, without affecting the response to Sar(9)SP sulphone. MEN 1 0,573 antagonized the response to Sar(9)SP sulphone and septide in b oth preparations, pK(B) values against septide being intermediate, and significantly different from, those measured against beta Ala(8)NKA (4-10) and Sar(9)SP sulphone. 8 These findings show that tachykinin NK1 and NK2 receptors mediate contraction of the circular muscle of th e guinea-pig ileum and colon. In both preparations NK1 receptor antago nists display higher apparent affinity when tested against septide tha n Sar(9)SP sulphone. These findings are compatible with the proposed existence of NK1 receptor subtypes in guinea-pig, although alternativ e explanations (e.g. agonist binding to different epitopes of the same receptor protein) cannot be excluded at present. Furthermore, an intr aspecies heterogeneity of the NK2 receptor in the circular muscle of t he guinea-pig ileum and colon is suggested.