Action of some natural polypeptides on the longitudinal muscle of the guinea pig ileum.

The structure--activity relationship of some natural tachykinins was investigated in the longitudinal muscle of the guinea pig ileum. The order of potency was eledoisin greater than uperolein greater than substance P greater than phyllomedusin greater than physalaemin. This ratio of activity is different from that observed in other experimental conditions, and in different in vivo or in vitro preparations; it is suggested that the N-terminal part of the molecule of the tachykinins plays a role in determining the degree of potency of these compounds.