Biological activity of neurotensin and its C-terminal partial sequences

A hypothalamic tridecapeptide, neurotensin, and its C-terminal partial sequences down to the dipeptide were synthesized. These peptides were assayed for smooth muscle contracting and blood pressure lowering properties using preparations of isolated stomach fundus, uterus and duodenum of the rat, isolated guinea pig ileum and rabbit carotid artery. Sequences of 6 or more terminal amino acids produced a strong stomach fundus contracting effect, the potencies of the fragments being approximately equivalent to or slightly more than that of the parent tridecapeptide. These fragments also elicited the ileum contracting activity, but the potencies were only one fifth to one tenth that of neurotensin. The tetra- or dipeptide hardly stimulated either the fundus or the ileum. The stimulating effect on the uterus or relaxing effect on the duodenum of neurotensin was not consistent. Rabbit blood pressure was lowered markedly by neurotensin and weakly by its dodecapeptide. From these results, the arginine or arginine-arginine residue down to the C-terminal leucine appears to be essential for the smooth muscle contracting activity of neurotensin. The full length sequence may be needed for the hypotensive effect.