A comparison of the effects of three substance P antagonists on tachykinin-stimulated [3H]-acetylcholine release in the guinea-pig ileum

Abstract

1 The potencies of three tachykinin antagonists [D-Pro4,D-Trp7,9.10]SP(4-11), [D-Arg1,D-Pro2,D-Trp7,9.Leu11]SP(1-11) and [D-Arg1,D-Trp7,9.Leu11]SP(1-11) (spantide) against eledoisin were examined in the guinea-pig ileum myenteric plexus, where a continuous superfusion system was employed to examine evoked release of [3H]-acetylcholine ([3H]-ACh); effects on mechanical activity of the preparations were also measured. 2 Eldoisin was chosen as the standard tachykinin agonist since the rank order of potency observed in evoking release was eledoisin, kassinin, substance P, physalaemin; on this basis it may be presumed that an ''SP-E'' type receptor was involved in the release process. 3 The two undecapeptide antagonists both significantly reduced the response to eledoisin (10 nM) as assessed by both [3H]-ACh release and mechanical activity which under these conditions was largely dependent on ACh release, and the response levels could be restored by increasing the concentration of eledoisin to 100 nM. The pA2 values for the two antagonists were estimated as 5.3 for [D-Arg1,D-Pro2,D-Trp7,9Leu11]SP(1-11) and 5.2 for [D-Arg1,D-Trp7,9.Leu11]SP(1-11)-[D-Pro4,D-Trp7,9,10]SP(4-11) was markedly less potent with a pA2 value of < 4.8. 4 All three antagonists possessed considerably inherent stimulatory activity as measured both by [3H]-ACh release and mechanical activity, [D-Pro4-D-Trp7,9,10]SP(4-11) being the most active in this respect, a 10 .mu.M concentration producing 50% of the response seen with 10 mM eledoisin. 5 These findings are discussed both in relation to tachykinin receptor classifications and limitations in the use of such antagonists in the study of the role of tachykinins in neurotransmission.