The importance of C‐terminal residues of vertebrate and invertebrate tachykinins for their contractile activities in gut tissues

The C-terminal residues of mammalian tachykinins and urechistachykinins (Uru-TKs), tachykinin-related peptides of echiuroid worm origin, were substituted for each other. Their contractile effects were assayed on the cockroach hindgut and the guinea pig ileum. [Met(10)] substitution of Uru-TKs caused a 1000 times lower activity on the hindgut, but a 1000 times higher activity on the ileum. In contrast, [Arg(11)]substance P (SP) was 100 times more and 400 times less potent than SP on the hindgut and ileum, respectively. A SP antagonist blocked these Uru-TK activities on the hindgut. These results demonstrated that the C-terminal Met-NH(2) is necessary for ileum contraction and the Arg-NH(2) is required for hindgut contraction, which was caused by binding to the cockroach's neurokinin-like receptor.