Potentiation of Bradykinin by Angiotensin-(1-7), Quinaprilat and Captopril Correlates Directly with Inhibition of the ACE C- and N-Terminal Domains.
ACE inhibitors (ACEIs) block bradykinin type 2 (B2) desensitisation, thereby potentiating bradykinin (BK) beyond blocking its hydrolysis (Deddish et al., Hypertension 1998). Angiotensin-(1-7) (A(1-7)) also acts as an ACEI, with preference for the ACE C-domain (Deddish et al.), and causes relaxation in a BK- and AT2 receptor-dependent manner (Gorelik et al., J Pharmacol Exp Ther 1998). Here, we compared the BK-potentiating effects of A(1-7) with those of quinaprilat (Q) and captopril (C). Porcine coronary arteries (PCAs), obtained from 24 pigs, were mounted in organ baths, preconstricted with PGF2α, and exposed to Q, C, BK or A(1-7) in the absence or presence of the AT1 receptor antagonist irbesartan (1 μM) or the AT2 receptor antagonist PD123319 (1 μM). BK induced complete relaxation (EC50 7.9±0.1, mean±SEM), while Q, C and A(1-7) alone, at concentrations up to 10 μM, were without effect. Pretreatment with Q (0.1 pM-10 μM) shifted the BK curve to the left in a biphasic manner: a 5-fold shift at Q concentrations (0.1-1 nM) that are known to be specific for the ACE C-domain, and a 10-fold shift at Q concentrations (> 10 nM) that block both ACE domains. Pretreatment with C (0.1 pM-10 μM) or A(1-7) (1 nM-10 μM) monophasically shifted the BK curve to the left, by a factor of 10 and 5, respectively (both at concentrations of 1 μM and higher). Repeated exposure of PCAs to a BK concentration that causes maximal relaxation (0.1 μM) induced B2 receptor desensitisation. The addition of 10 μM Q, C or A(1-7) to the bath, at a time when BK alone was no longer able to induce relaxation, fully restored the relaxant effects of BK. Irbesartan or PD123319 did not affect any of the observed effects of A(1-7). In conclusion, A(1-7), like Q and C, potentiates BK by acting as an ACE inhibitor and not through AT receptor stimulation. BK potentiation is maximal when both the ACE N- and C-terminal domain are inhibited. At physiological concentrations, the inhibitory effects of A(1-7) are limited to the ACE C-domain, raising the possibility that A(1-7) synergistically increases the blood pressure-lowering effects of N-domain-specific ACE inhibitors.