Smooth-muscle effects of C-terminal tripeptide of substance P.

The effects of the C-terminal tripeptide fragment of substance P(SP-C-TP) and its interactions with substance P(SP) were studied on isolated smooth-muscle preparations (rat musculus anococcygeus and vas deferens and rabbit ear central artery). Like SP, SP-C-TP (1 x 10(-8) M-1 x 10(-4)M) potentiated contractions of vas deferens and musculus anococcygeus elicited by low frequency electrical stimulation (LFES), the effect of the fragment being much weaker than that of the neuropeptide. The potentiating effect of SP was strongly reduced by SP-C-TP. The fragment slightly decreased the noradrenaline (NA)-induced contractions of vas deferens only. When the pharmacological agents were applied extralumenally only high SP concentrations (1 x 10(-7)M and 1 x 10(-6)M) significantly potentiated the LFES-evoked smooth-muscle contractions. SP-induced potentiation was abolished by SP-C-TP. Intralumenal administration of the neuropeptide and its fragment always led to a marked inhibition of the LFES contractile effects. The observed effects suggest that SP-C-TP acts as a competitive dualist of SP. The data is interpreted in terms of the hypothesis that the neuropeptide fragments might perform as a natural antagonist of the peptides and may control their effects.