Hoe 140 and pseudo-irreversible antagonism in the rat vas deferens in vitro

The effects of bradykinin and the bradykinin B-2 receptor antagonists D-Arg -[Hyp(3),Thi(5.8),D-Phe(7)]-bradykinin (NPC 349) and D-Arg-[Hyp(3),Thi(5),D -TiC7,Oic(8)]-bradykinin (Hoe 140) were examined in the electrically-stimul ated rat vas deferens. Cumulative additions of bradykinin (1-3000 nhl) prod uced two distinct responses: an enhancement in the magnitude of the basal e lectrically-induced twitch response (neurogenic response) and an increase i n the baseline tension (musculotropic response). NPC 349 (10-100 mu M) prod uced concentration-dependent surmountable rightward shifts of both the brad ykinin neurogenic and musculotropic response curves. In contrast, while Hoe 140 (10-100 nM) caused an apparently surmountable antagonism of the bradyk inin neurogenic response, it caused an apparent insurmountable antagonism o f the bradykinin musculotropic response. Interestingly, co-incubation of Ho e 140 (30 nM) with NPC 349 (30 and 100 mu M) resulted in a concentration-re lated upwards displacement of the Hoe 140-suppressed bradykinin musculotrop ic response curve. Thus, Hoe 140 can be described as a pseudo-irreversible antagonist against the bradykinin musculotropic response. No time-dependent changes were observed in the maximum bradykinin musculotropic response att ainable when NPC 349 (100 mu M) additions were made for the final 2 or 18 m in of the Hoe 140 incubation (20 min). These findings indicate that slow re versibility of Hoe 140 from the bradykinin B-2 receptor is unlikely to be t he mechanism responsible for the pseudo-irreversible antagonism of the brad ykinin-induced musculotropic response. Instead, we propose an alternative e xplanation involving a third, unstable and inactive form of the bradykinin B-2 receptor. (C) 2000 Elsevier Science B.V. All rights reserved.