Potentiation of bradykinin effects and inhibition of kininase activity in isolated smooth muscle.

Prolongation of bradykinin half-life following kininase inhibition has been proposed as the reason for the potentiation of kinin effects. We have reassessed this assumption by using three different isolated smooth muscle preparations and simultaneously studying the inhibition of kininase activity and the potentiation of bradykinin effects by enalaprilat and BPP9a. Rat duodenum displayed higher total kininase activity, metabolizing half of the added bradykinin in 6.5 min, while this time for rat uterus was greater than 60 min. Guinea-pig ileum showed the intermediate value of 14.6 min. Enalaprilat and BPP9a slowed the metabolism of bradykinin by 50-100% in rat duodenum and by 50-180% in guinea-pig ileum, showing that a significant fraction of total kininase activity appears to be due to kininase II. In rat duodenum, an almost complete blockade of kininase activity was achieved when bacitracin and mergetpa were used together with enalaprilat. Enalaprilat and BPP9a potentiated bradykinin effects in guinea-pig ileum and rat uterus. In contrast, bradykinin-induced relaxations and contractions in rat duodenum were not potentiated by enalaprilat, BPP9a, or by the enzyme inhibitor mixture (enalaprilat--bacitracin--mergetpa). The results suggest that inhibition of bradykinin enzymatic metabolism by kininases does not necessarily lead to the potentiation of bradykinin effects.