The undecapeptide amide, H-Arg-Pro-Lys-Pro-Gln-Gln-Phe-Phe-Gly-Leu-Met-NH2, corresponding to the entire amino acid sequence of substance P, a bovine hypothalamic peptide, was synthesized by the conventional method. This and C-terminal tetra, penta, hexa, hepta and octapeptide amides were submitted for biological assays. Synthetic substance P caused considerable decrease in blood pressure and heart rate in anesthetized rat, but other shorter chain peptides were less active. When contractility on isolated guinea-pig ileum was examined, the heptapeptide amide exhibited much higher activity than synthetic substance P.
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Yajima et al. (1973) studied this question.