Potentiation of kinin analogues by ramiprilat is exclusively related to their degradation

The potentiation of kinin actions represents a cardioprotective property of ACE inhibitors. Although a clear contribution to this effect is related to the inhibition of bradykinin (BK) breakdown, the high efficacy of potentiation and the ability of ACE inhibitors to provoke a B₂-receptor-mediated response even after receptor desensitization has also triggered hypotheses concerning additional mechanisms of kinin potentiation. The application of kinin analogues with enhanced metabolic stability for the demonstration of degradation-independent mechanisms of potentiation, however, has yielded inconsistent results. Therefore, the relation between the susceptibility of B₂-agonists to ACE and the potentiation of their actions by ACE inhibitors was investigated with the use of minimally modified kinin derivatives that varied in their degree of ACE resistance. The B₂-agonists BK, D-Arg-[Hyp³]-BK, [Hyp,³ Tyr(Me)⁸]-BK, [ΔPhe⁵]-BK, [D-NMF⁷]-BK, and [Phe⁸ψ(CH₂-NH)Arg⁹]-BK were tested for degradation by purified rabbit ACE and for their potency in contracting the endothelium-denuded rabbit jugular vein in the absence and presence of ramiprilat. Purified ACE degraded D-Arg-[Hyp³]-BK and [Hyp,³ Tyr(Me)⁸]-BK at 81% and 71% of BK degradation activity, respectively, whereas other peptides were highly ([ΔPhe⁵]-BK) or completely ([D-NMF⁷]-BK, [Phe⁸ψ(CH₂-NH)Arg⁹]-BK) resistant. The EC₅₀ of BK-induced venoconstriction (1.15±0.2 nmol/L) was reduced by a factor of 5.7 in the presence of ramiprilat. Likewise, D-Arg-[Hyp³]-BK and [Hyp,³ Tyr(Me)⁸]-BK were both significantly potentiated by a factor of 4.4, whereas the activities of the other agonists were not affected. Ramiprilat exerted no influence on the maximum contraction induced by any of the agonists. It is concluded that the potentiation of kinin analogues during ACE inhibition correlates quantitatively with the susceptibility of each substance to degradation by ACE. As such, no evidence of degradation-independent potentiating actions of ACE inhibitors could be obtained.