Comparative study on the mechanism of bradykinin potentiation induced by bradykinin-potentiating peptide 9a, enalaprilat and kinin-potentiating peptide.

The action of a kinin-potentiating peptide (KPP) obtained from tryptic digestion of human serum proteins was compared with that of bradykinin-potentiating peptide 9a (BPP9a; obtained from snake venom) and enalaprilat (a synthetic inhibitor of angiotensin-converting enzyme; ACE) as a means of understanding the mechanism of action of KPP on smooth muscle. KPP potentiated bradykinin-induced contractile effects in guinea-pig ileum and rat uterus, but not the bradykinin-induced relaxation of pre-contracted ileum, whereas BPP9a and enalaprilat potentiated both bradykinin effects. The receptor mediating both the contraction and the relaxation elicited by bradykinin in the ileum was found to be of the B2 type. KPP retained its potentiating effect in the presence of enalaprilat in the guinea-pig ileum and rat uterus, whereas the potentiation evoked by BPP9a was abolished. Enalaprilat inhibited the activity of purified ACE, whereas KPP was completely devoid of such an effect. The potentiating effect of KPP, but not that of BPP9a or enalaprilat, was blocked by compounds that inhibit phospholipase A2 and lipoxygenase activity but not by inhibitors of cyclo-oxygenase or phosphodiesterases. The results suggest that the potentiating effect of KPP (i) does not involve inhibition of ACE; (ii) is not due to an increased affinity of the receptor for bradykinin, and (iii) probably involves post-receptor events linked to phospholipase A2 and to the lipoxygenase pathway.