Intravascular and interstitial degradation of bradykinin in isolated perfused rat heart

Neprilysin Inhibitors and Bradykinin

Bradykinin has important physiological actions related to the regulation of blood vessel tone and renal function, and protection from ischemia reperfusion injury. However, bradykinin also contributes to pathological states such as angioedema and inflammation. Bradykinin is metabolized by many different peptidases that play a major role in the control of bradykinin levels. Peptidase inhibitor therapies such as angiotensin converting enzyme (ACE) and neprilysin inhibitors increase bradykinin levels, and the challenge for such therapies is to achieve the beneficial cardiovascular and renal effects without the adverse consequences such as angioedema that may result from increased bradykinin levels. Neprilysin also metabolizes natriuretic peptides. However, despite the potential therapeutic benefit of increased natriuretic peptide and bradykinin levels, neprilysin inhibitor therapy has only modest efficacy in essential hypertension and heart failure. Initial attempts to combine neprilysin inhibition with inhibition of the renin angiotensin system led to the development of omapatrilat, a drug that combines ACE and neprilysin inhibition. However, omapatrilat produced an unacceptably high incidence of angioedema in patients with hypertension (2.17%) in comparison with the ACE inhibitor enalapril (0.68%), although angioedema incidence was less in patients with heart failure with reduced ejection fraction (HFrEF) treated with omapatrilat (0.8%), and not different from that for enalapril therapy (0.5%). More recently, LCZ696, a drug that combines angiotensin receptor blockade and neprilysin inhibition, was approved for the treatment of HFrEF. The approval of LCZ696 therapy for HFrEF represents the first approval of long-term neprilysin inhibitor administration. While angioedema incidence was acceptably low in HFrEF patients receiving LCZ696 therapy (0.45%), it remains to be seen whether LCZ696 therapy for other conditions such as hypertension is also accompanied by an acceptable incidence of angioedema.

Cardiac myosin binding protein-C as a central target of cardiac sarcomere signaling: a special mini review series

Cardiac myosin binding protein-C (cMyBP-C) is a cardiac-specific thick filament assembly, accessory, and regulatory protein. Physiologically, it is a key regulator of cardiac contractility. With more than 200 mutations in the cMyBP-C gene directly linked to the development of cardiomyopathy and heart failure, cMyBP-C clearly plays a critical role in heart function. At baseline, cMyBP-C is highly phosphorylated, a condition required for normal cardiac function. However, the level of cMyBP-C phosphorylation is significantly decreased during heart failure, indicating that the level of cMyBP-C phosphorylation is directly linked to signaling and cardiac function. Early studies indicated that cMyBP-C interacts with myosin and titin, whereas studies now show that it also interacts with thin filament proteins. However, the exact role(s) of cMyBP-C in the heart remain(s) to be elucidated. As such, we invited experts in the field of cardiac muscle to identify and address key issues related to cMyBP-C by contributing a mini review on such topics as structure, function, regulation, cardiomyopathy, proteolysis, and gene therapy. Starting from this issue, Pflügers Archiv European Journal of Physiology will publish two invited mini review articles each month to discuss the most recent advances in the study of cMyBP-C.

Extracellular and intracellular proteases in cardiac dysfunction due to ischemia-reperfusion injury

Various procedures such as angioplasty, thrombolytic therapy, coronary bypass surgery, and cardiac transplantation are invariably associated with ischemia-reperfusion (I/R) injury. Impaired recovery of cardiac function due to I/R injury is considered to be a consequence of the occurrence of both oxidative stress and intracellular Ca(2+)-overload in the myocardium. These changes in the ischemic myocardium appear to activate both extracellular and intracellular proteases which are responsible for the cleavage of extracellular matrix and subcellular structures involved in the maintenance of cardiac function. It is thus intended to discuss the actions of I/R injury on several proteases, with a focus on calpain, matrix metalloproteinases, and cathepsins as well as their role in inducing alterations both inside and outside the cardiomyocytes. In addition, modifications of subcellular organelles such as myofibrils, sarcoplasmic reticulum and sarcolemma as well as extracellular matrix, and the potential regulatory effects of endogenous inhibitors on protease activities are identified. Both extracellular and intracellular proteolytic activities appear to be imperative in determining the true extent of I/R injury and their inhibition seems to be of critical importance for improving the recovery of cardiac function. Thus, both extracellular and intracellular proteases may serve as potential targets for the development of cardioprotective interventions for reducing damage to the heart and retarding the development of contractile dysfunction caused by I/R injury.

Proteinases as hormones: targets and mechanisms for proteolytic signaling

Proteinases, such as kallikrein-related peptidases, trypsin and thrombin, can play hormone-like 'messenger roles in vivo. They can regulate cell signaling by cleaving and activating a novel family of G-protein-coupled proteinase-activated receptors (PARs 1-4) by unmasking a tethered receptor-triggering ligand. Short synthetic PAR-derived peptide sequences (PAR-APs) can selectively activate PARs 1, 2 and 4, causing physiological responses in vitro and in vivo. Using the PAR-APs to activate the receptors in vivo, it has been found that PARs, like hormone receptors, can affect the vascular, renal, respiratory, gastrointestinal, musculoskeletal and nervous systems (central and peripheral). PARs trigger responses ranging from vasodilatation to intestinal inflammation, increased cytokine production and increased nociception. These PAR-stimulated responses have been implicated in various disease states, including cancer, atherosclerosis, asthma, arthritis, colitis and Alzheimer's disease. In addition to targeting the PARs, proteinases can also cause hormone-like effects by other signaling mechanisms that may be as important as the activation of PARs. Thus, the PARs themselves, their activating serine proteinases and their signaling pathways can be considered as attractive targets for therapeutic drug development. Further, proteinases can be considered as physiologically relevant 'hormone-like' messengers that can convey signals locally or systemically either via PARs or by other mechanisms.

Increased expression of profibrotic neutral endopeptidase and bradykinin type 1 receptors in stenotic aortic valves

AIMS

In aortic stenosis (AS), adverse remodelling of the valves may depend on altered local regulation of pro- and antifibrotic systems. We have recently shown that angiotensin-converting enzyme (ACE), which generates profibrotic angiotensin II and inactivates antifibrotic bradykinin (BK), is upregulated in stenotic aortic valves. Here, we analyse the expression of neutral endopeptidase (NEP), another profibrotic and BK-degrading enzyme, and of BK receptors in aortic valves in AS.

METHODS AND RESULTS

Stenotic aortic valves (n = 86) were obtained at valve replacement surgery and control valves (n = 13) at cardiac transplantation. Expression levels of NEP and BK type 1 and 2 receptors (BK-1R and BK-2R) in aortic valves and in isolated valvular myofibroblasts were analysed by real-time PCR and immunohistochemistry, and NEP activity was quantified by autoradiography. NEP, BK-1R, and BK-2R mRNA levels were higher in stenotic than in non-stenotic valves (P < 0.05 for each) and the respective proteins localized to valvular endothelial cells and myofibroblasts. In stenotic valves, the proteolytic activity of NEP was significantly increased (4.5-fold, P < 0.001), and tumour necrosis factor-alpha induced the expression of NEP in cultured myofibroblasts. Finally, treatment of cultured myofibroblasts with an NEP inhibitor (phosphoramidon) downregulated the expression of profibrotic transforming growth factor-beta1, whereas addition of BK decreased the expression of collagens I and III which was reversed by a BK-2R antagonist.

CONCLUSION

NEP activity is increased in stenotic aortic valves in parallel with increased expression of BK-receptors. The upregulation of NEP and BK-1R have the potential to promote valvular fibrosis and remodelling while the increase in BK-2R may represent a compensatory antifibrotic response. These findings add novel pathogenic insight and raise potential new therapeutic targets in AS.

C-terminal fusion of eGFP to the bradykinin B2 receptor strongly affects down-regulation but not receptor internalization or signaling

A functional comparison was made between the wild-type bradykinin B2 receptor (B2wt) and the chimera B2eGFP (enhanced green-fluorescent protein fused to the C-terminus of B2wt), both stably expressed in HEK 293 cells. There was almost no difference in terms of ligand-inducible receptor phosphorylation and internalization, signal transduction (accumulation of inositol phosphates) or expression and affinity. However, stimulation for up to 8 h with 10 microM bradykinin (BK) resulted in a strong decrease in surface receptors (by 60% within 5 h) in B2wt, but not in B2eGFP. When the expression levels of both constructs where comparably reduced using a weaker promoter, long-term stimulation resulted in a reduction in surface receptors for B2wt(low) to less than 20% within 1 h, whereas the chimera B2eGFP(low) still displayed 50% binding activity after 2 h. A 1-h incubation in the absence of BK resulted in a recovery of 60% of the binding in B2wt(low) after 1-h stimulation with BK, but of only 20% after 7-h stimulation. In contrast, B2eGFP(low) levels were restored to more than 70%, even after 7-h stimulation. These data indicate that although the fusion of eGFP to B2wt does not affect its ligand-induced internalization, it strongly reduces the down-regulation, most likely by promoting receptor recycling over degradation.

Degradation of bradykinin, a cardioprotective substance, during a single passage through isolated rat-heart

Angiotensin converting enzyme (ACE) inhibitors have cardioprotective effects in different species including human. This cardioprotective effect is mainly due to the inhibition of bradykinin (BK) degradation rather than inhibition of the conversion of angiotensin I to angiotensin II. Bradykinin, a nonapeptide, has been considered to be the potential target for various enzymes including ACE, neutral endopeptidase 24.11, carboxypeptidase M, carboxypeptidase N, proline aminopeptidase, endopeptidase 24.15, and meprin. In the present study, the coronary vascular beds of Sprague Dawley rat isolated hearts were perfused (single passage) with Krebs solution alone or with different concentrations of BK i.e. 2.75x10(-10), 10(-7), 10(-6) and 10(-5) M solution. Percent degradation of BK was determined by radioimmunoassay. The degradation products of BK after passing through the isolated rat-hearts were determined using RP-HPLC and mass spectroscopy. All the four doses of BK significantly decreased the perfusion pressure during their passage through the hearts. The percentage degradation of all four doses was decreased as the concentration of drug was increased, implying saturation of a fixed number of active sites involved in BK degradation. Bradykinin during a single passage through the hearts degraded to give [1-7]-BK as the major metabolite, and [1-8]-BK as a minor metabolite, detected on HPLC. Mass spectroscopy not only confirmed the presence of these two metabolites but also detected traces of [1-5]-BK and arginine. These findings showed that primarily ACE is the major cardiac enzyme involved in the degradation of bradykinin during a single passage through the coronary vascular of bed the healthy rat heart, while carboxypeptidase M may have a minor role.

Study of bradykinin metabolism by rat lung tissue membranes and rat kidney brush border membranes by HPLC with inductively coupled plasma-mass spectrometry and orthogonal acceleration time-of-flight mass spectrometry

The coupling of the techniques, high-performance liquid chromatography (HPLC), orthogonal acceleration time-of-flight mass spectrometry (OATOF-MS) and inductively coupled plasma mass spectrometry (ICP-MS) provides a very powerful method for identifying and quantifying the products of bradykinin metabolism. In this study, we were able to identify the major metabolites of bradykinin degradation reported in the literature. In addition, a new bradykinin metabolite corresponding to bradykinin 5,9 fragment (BK-(5,9)-fragment) was identified as a product of neutral endopeptidase (NEP) activity. This finding establishes that NEP cleaves bradykinin simultaneously at the positions 4-5 and 7-8. We also demonstrate the equivalent participation of NEP and angiotensin-converting enzyme (ACE) within the rat lung tissue membranes (RLTM) in bradykinin degradation, suggesting its suitability as a model for the assay of dual ACE/NEP inhibitors. On the contrary, in rat kidney brush border membranes (KBBM), ACE is not significantly involved in bradykinin metabolism, with NEP being the major enzyme.

Rat and mouse membrane aminopeptidase P: structure analysis and tissue distribution

Membrane-bound aminopeptidase P (mAPP) is a highly specific exopeptidase that removes the N-terminal amino acid only from a peptide (three amino acids or longer) that has a prolyl residue in the second position. mAPP can inactivate bradykinin, a potent vasodilating and cardioprotective peptide hormone, by hydrolyzing the Arg(1)-Pro(2) bond. Studies on the rat have shown that the metabolism of bradykinin is an important physiological role of this enzyme. We report here the complete coding sequences for rat and mouse mAPP determined from mRNA isolated from lung tissue. Key structural features that determine post-translational processing and substrate recognition and catalysis were identified based on sequence homologies and the crystal structure of Escherichia coli aminopeptidase P complexed with Pro-Leu. The tissue-specific expression of mAPP was studied using the polymerase chain reaction. The mAPP gene is widely, but variably, expressed in adult tissues of the rat and mouse and in mouse embryos.

Cardiac kinin level in experimental diabetes mellitus: role of kininases

Diabetes mellitus impairs the cardiac kallikrein-kinin system by reducing cardiac kallikrein (KLK) and kininogen levels, a mechanism that may contribute to the deleterious outcome of cardiac ischemia in this disease. We studied left ventricular (LV) function and bradykinin (BK) coronary outflow in buffer-perfused, isolated working hearts (n = 7) of controls and streptozotocin (STZ)-induced diabetic rats before and after global ischemia. With the use of selective kininase inhibitors, the activities of angiotensin I-converting enzyme, aminopeptidase P, and neutral endopeptidase were determined by analyzing the degradation kinetics of exogenously administered BK during sequential coronary passages. Basal LV function and coronary flow were impaired in STZ-induced diabetic rats. Neither basal nor postischemic coronary BK outflow differed between control and diabetic hearts. Reperfusion after 15 min of ischemia induced a peak in coronary BK outflow that was of the same extent and duration in both groups. In diabetic hearts, total cardiac kininase activity was reduced by 41.4% with an unchanged relative kininase contribution compared with controls. In conclusion, despite reduced cardiac KLK synthesis, STZ-induced diabetic hearts are able to maintain kinin liberation under basal and ischemic conditions because of a primary impairment or a secondary downregulation of kinin-degrading enzymes.

Bradykinin, angiotensin-(1-7), and ACE inhibitors: how do they interact?

The beneficial effect of ACE inhibitors in hypertension and heart failure may relate, at least in part, to their capacity to interfere with bradykinin metabolism. In addition, recent studies have provided evidence for bradykinin-potentiating effects of ACE inhibitors that are independent of bradykinin hydrolysis, i.e. ACE-bradykinin type 2 (B(2)) receptor 'cross-talk', resulting in B(2) receptor upregulation and/or more efficient activation of signal transduction pathways, as well as direct activation of bradykinin type 1 receptors by ACE inhibitors. This review critically reviews the current evidence for hydrolysis-independent bradykinin potentiation by ACE inhibitors, evaluating not only the many studies that have been performed with ACE-resistant bradykinin analogues, but also paying attention to angiotensin-(1-7), a metabolite of both angiotensin I and II, that could act as an endogenous ACE inhibitor. The levels of angiotensin-(1-7) are increased during ACE inhibition, and most studies suggest that its hypotensive effects are mediated in a bradykinin-dependent manner.

Inhibition of kinin breakdown prolongs retention and action of bradykinin in a myocardial B2 receptor compartment

1. The high efficacy of ACE inhibitors to potentiate the actions of kinins might be explained by a hypothetical compartment in which B(2)-receptors are colocalized with kinin degrading enzymes. To demonstrate the functional consequence of such a compartment we compared the myocardial uptake and the persistence of action of bradykinin under the influence of kininase inhibitors. 2. Bradykinin-induced vasodilation and uptake of tritiated bradykinin were studied in perfused rat hearts during inhibition of ACE and aminopeptidase P. B(2)-receptors were localized by immuno-gold labelling and electron-microscopy. 3. The EC(50) of bradykinin-induced vasodilation (5.1+/-0.8 nM) was shifted to 14 fold lower concentrations during inhibition of both kininases. The maximum persistence of vasodilation after termination of bradykinin application (half-life 112+/-20 s) was increased by kininase inhibitors to 398+/-130 s. This prolongation was reversed when B(2)-receptors were blocked simultaneously with the termination of bradykinin infusion. 4. Tritiated bradykinin (perfused for 1 min) was partially (1.7+/-0.24%) retained by the myocardium and consecutively released with a half-life of 70+/-9 s. Kinin uptake was increased during kininase inhibition (7.7+/-2.6%), and was normalized by HOE 140 (2.0+/-0.34%), or when a tritiated B(2)-receptor antagonist (NPC 17731) was used as label. 5. B(2)-receptors were localized in plasmalemmal and cytosolic vesicles of capillary endothelium. 6. Bradykinin is locally incorporated and can associate with B(2)-receptors repeatedly when kinin breakdown is inhibited. This is the kinetic and functional consequence of a colocalization of kininases and B(2)-receptors in a compartment constituted by endothelial membrane vesicles.

Investigation of bradykinin metabolism in human and rat plasma in the presence of the dual ACE/NEP inhibitors GW660511X and omapatrilat

Several studies have suggested that the accumulation of bradykinin, or that of one its metabolites BK1-8, is involved in the occurrence of side effects such as AE associated with the use of various ACEi. In this work a novel approach combining HPLC-UV on-line with oaTOF-MS and ICPMS was applied to investigate in human and rat plasma the metabolism of labelled BK (79/81 Br-Phe5) BrBK in the presence of two new dual ACE/NEP inhibitors (GW660511X and omapatrilat) currently under clinical trial. In human plasma the BrBK half-life values in the absence or in the presence of GW660511X (3.8 microM) or omapatrilat (32 nM) were 38.7 +/- 2.4, 51.2 +/- 4.7 and 114.7 +/- 9.3 min, respectively and BrBK was degraded into BrBK1-8, BrBK1-7, BrBK1-5 and Br-Phe. In the presence of inhibitors, however, the levels of these resultant metabolites were different. Unlike GW660511X, omapatrilat abolished the production of BrBK1-5 and BrBK1-7, suggesting a better ACE inhibition effect over GW660511X as no NEP activity was found. In addition the production of BrBK1-8 was enhanced in the presence of these inhibitors with a greater accumulation being observed with omapatrilat. The production of Br-Phe5 was reduced with GW660511X while no significant change was observed with omapatrilat after 4 h of incubation. In rat plasma the BrBK half-life values in the absence or in the presence of GW660511X (530 nM) or omapatrilat (50 nM) were 9.31 +/- 1.7, 22.06 +/- 3.1 and 25.3 +/- 1.7 min, respectively and BrBK was degraded into BrBK1-8, BrBK1-7, BrBK1-5 and Br-Phe5 plus BrBK2-9, BrBK4-8 and BrBK2-8 metabolites not found in human plasma. GW660511X and omapatrilat reduced the production of BrBK1-5 and BrBK1-7 with more effect being observed with omapatrilat. GW660511X and omapatrilat increased the production of both BrBK1-8 and Br-Phe5 but not that of BrBK4-8 and BrBK2-8. This study shows that the potency of GW660511X in comparison with omapatrilat is more than 100-fold lower in human, but less than 10-fold lower in rat plasma, suggesting that rat may not be a suitable in vivo model for the evaluation of ACE/NEP inhibition in relation to effects in humans.

The mechanism of the angiotensin-converting enzyme inhibitor quinapril is not related to bradykinin level in heart tissue

In order to examine the effect of the angiotensin-converting enzyme inhibitor (ACEi) quinapril, we performed a sensitive and specific radioimmunoassay (RIA) to quantify bradykinin, BK-(1-9), in heart and kidney tissues. The BK-(1-9) level was unaffected in the heart of sham and water-deprived rats treated for 2h with quinapril (10mg/kg), but was significantly higher in the kidneys in the two groups. In these conditions, circulating and tissue angiotensin II (Ang II) levels were significantly decreased by quinapril. Moreover, our results indicated that acute treatment with this dose of quinapril induced kinin-mediated effects which were not related to its action on bradykinin degradation in rat hearts.

Neutral endopeptidase is activated in cardiomyocytes in human aortic valve stenosis and heart failure

BACKGROUND

The regulation of the cardiac neutral endopeptidase (EC 24.10, NEP) that degrades bradykinin and natriuretic peptides has been investigated in human cardiac hypertrophy and heart failure. Methods and Results- NEP mRNA was quantitated by real-time polymerase chain reaction (PCR) in left ventricular biopsies from patients with aortic valve stenosis (AS, n=19) and heart failure due to dilated cardiomyopathy (DCM, n=14), and control subjects with normal systolic function (CON, n=14). Left ventricular NEP mRNA content was increased 3-fold in AS (P<0.005) and 4.1-fold in DCM (P<0.002). The increase in NEP mRNA was related to the increase in end diastolic pressure in AS and DCM. In a second series, myocardial NEP enzymatic activity was determined. It increased 3.6-fold in AS (P<0.02) and 4-fold in DCM (P<0.002). NEP was localized in the myocardium by immunofluorescence microscopy and in situ PCR to myocytes and nonmyocyte areas and cells.

CONCLUSIONS

Elevated cardiac NEP activity in pressure loaded and failing human hearts may increase the local degradation of bradykinin and natriuretic peptides.

Radioiodination of the stable metabolic fragment of bradykinin, RPPGF

Arg-Pro-Pro-Gly-Phe (RPPGF, BK[1-5]), is a stable metabolite of the peptide hormone bradykinin. Considering the short half-life of bradykinin (BK, approximately 15 secs), RPPGF has been used as a marker for BK's endogenous generation. A lack of a radioiodinated RPPGF has precluded the development of a radioimmunoassay for this peptide. The present study describes a two-step reaction that allows for the incorporation of 125I into the aromatic ring of the phenylalanine of RPPGF. This radioiodinated analog is recognized by an antibody to RPPGF, demonstrating its utility for the development of a radioimmunoassay for measurements of RPPGF, a stable metabolic product of bradykinin.

Myocardial bradykinin following acute angiotensin-converting enzyme inhibition, AT1 receptor blockade, or combined inhibition in congestive heart failure

BACKGROUND

The present study examined the effects of acute angiotensin-converting enzyme inhibition (ACEI), AT(1) receptor blockade (AT(1) block), or combined treatment on in vitro and in vivo bradykinin (BK) levels.

METHODS

BK levels were measured in isolated porcine myocyte preparations (n = 13) in the presence of exogenous BK (10(-8) M); with an ACEI (benezaprilat; 0.1 mM) and BK; an AT(1) block (valsartan; 10(-5) M) and BK; and combined treatment and BK. In a second study, myocardial microdialysis was used to measure porcine interstitial BK levels in both normal (n = 14) and pacing-induced congestive heart failure (CHF) (240 beats/min, 3 weeks, n = 16) under the following conditions: baseline, following ACEI (benezaprilat, 0.0625 mg/kg) or AT(1) block (valsartan, 0.1 mg/kg), and a combined treatment (benezaprilat, 0.0625 mg/kg; valsartan, 0.1 mg/kg).

RESULTS

In the left ventricular myocyte study, BK levels increased over 93% with all treatments compared to untreated values (P < 0.05). In the in vivo study, basal interstitial BK values were lower in the CHF group than in controls (2.64 +/- 0.57 vs 5.91 +/- 1.4 nM, respectively, P < 0.05). Following acute infusion of the ACEI, BK levels in the CHF state increased from baseline (57% +/- 22; P < 0.05). Following combined ACEI/AT(1) block, BK levels increased from baseline in both control (42% +/- 11) and CHF groups (60% +/- 22; P < 0.05 for both).

CONCLUSION

These findings suggest that ACEI, or combined ACEI/AT(1) block increased BK at the level of the myocyte and potentiated BK levels in the CHF myocardial interstitium.

Functional expression and characterization of the cytoplasmic aminopeptidase P of Caenorhabditis elegans

Aminopeptidase P (AP-P; X-Pro aminopeptidase; EC 3.4.11.9) cleaves the N-terminal X-Pro bond of peptides and occurs in mammals as both cytosolic and plasma membrane forms, encoded by separate genes. In mammals, the plasma membrane AP-P can function as a kininase, but little is known about the physiological role of the cytosolic enzyme. The C. elegans genome contains a single gene encoding AP-P (W03G9.4), analysis of which predicts regions displaying high levels of amino-acid sequence homology between the predicted gene product and mammalian cytoplasmic AP-P, with the absolute conservation of key catalytic residues. The sequence of an EST (yk91g4), comprising the open reading frame of W03G9.4, confirmed the predicted genomic structure of the gene and the prediction that W03G9.4 codes for a nonsecreted protein with a molecular mass of 68 kDa. Nematodes transformed with a promoter reporter construct, W03G9.4:GFP, showed high levels of fluorescence in the intestine of larvae and adult hermaphrodites, indicating that the intestine is a major site of W03G9.4 expression. yk91g4 tagged with a hexahistidine and DLYDDDDK peptide epitope was expressed in Escherichia coli to yield, after affinity purification, a recombinant protein with a molecular mass of 71 kDa. The recombinant W03G9.4 removed the N-terminal amino acid from bradykinin (RPPGFSPFR), a Caenorhabditis elegans neuropeptide (KPSFVRFamide) and Lem Trp 1 (APSGFLGVRamide), but did not display activity towards angiotensin I (NRVYIHPFHL), des-Arg bradykinin and AF1 (KNEFIRFamide). The activity towards bradykinin was inhibited by EDTA and 1, 10 phenanthroline, as expected for a metalloenzyme, and also by apstatin (IC50, 1 microM), a selective inhibitor of mammalian AP-P. A Km of 45 microM and an optimum pH of 7-8 was observed with bradykinin as the substrate. The activity of the nematode AP-P, like its mammalian counterparts, was strongly influenced by metal ions, with Co2+, Mn2+ and Zn2+ all inhibiting the hydrolysis of bradykinin. We conclude that W03G9.4 codes for a cytoplasmic AP-P with very similar enzymatic properties to those of mammalian AP-P, and we suggest that the enzyme has a physiological role in the intracellular hydrolysis of proline-containing peptides absorbed from the lumen of the intestine.

Tissue kallikrein KLK1 is expressed de novo in endothelial cells and mediates relaxation of human umbilical veins

Bradykinin released by the endothelium is thought to play an important local role in cardiovascular regulation. However, the molecular identity of endothelial proteases liberating bradykinin from its precursors remained unclear. Using RT-PCR and Southern blotting techniques we detected mRNA for tissue kallikrein (KLK1) in human umbilical vein endothelial cells and in bovine aortic endothelial cells. Protein expression was confirmed by precipitation of KLK1 from lysates of endothelial cells pre-labeled with [35S]-cysteine/methionine. Partial purification of tissue kallikrein from total endothelial cell extracts resulted in a protein triplet of about 50 kDa in Western blots using specific anti-KLK1 antibodies. The immunodetection of tissue kallikrein antigen in the fractions from ion exchange chromatography correlated with the presence of amidolytic tissue kallikrein activity. Stimulation of endothelial cells with angiotensin II (ANG-II), which recently has been shown to activate the vascular kinin system and to cause vasodilation, resulted in the release of bradykinin and kallidin. ANG-II-dependent relaxation of pre-constricted rings from human umbilical veins was abolished in the presence of a specific tissue kallikrein inhibitor. We conclude that endothelial cells de novo express significant amounts of tissue kallikrein, which likely serves in the local generation of vasoactive kinins.

Effects of hydrosaline treatments on prolyl endopeptidase activity in rat tissues

Enzymatic cleavage of some peptide hormones, neurotransmitters and neuromodulators could be implicated in the regulation of extra- and intracellular fluid volume and osmolality. Prolyl endopeptidase is known to hydrolyze several peptides, which act on hydromineral balance, such as angiotensins, bradykinin, vasopressin, oxytocin, thyrotropin-releasing hormone, neurotensin and opioids. In this work, we analyzed the effects of certain volume and/or osmotic changes in the activity of the soluble and membrane-bound prolyl endopeptidase in several brain areas, heart, lungs, kidney and adrenal and pituitary glands of the rat. Soluble prolyl endopeptidase activity was higher in the renal cortex of the chronic salt-loaded rats than in the control rats. In the water-deprived and polyethylene glycol-treated rats, heart particulate prolyl endopeptidase was lower than in the control rats. Particulate prolyl endopeptidase was also lower in the adrenal gland of the acute salt-loaded rats and in the brain cortex of the water-loaded rats than in the control rats. Data suggest that tissue-dependent peptide hydrolysis evoked by prolyl endopeptidase activity is involved in the water-electrolyte homeostasis.

Apstatin, a selective inhibitor of aminopeptidase P, reduces myocardial infarct size by a kinin-dependent pathway

1. Inhibitors of the angiotensin converting enzyme (ACE) have been shown to exert their cardioprotective actions through a kinin-dependent mechanism. ACE is not the only kinin degrading enzyme in the rat heart. 2. Since aminopeptidase P (APP) has been shown to participate in myocardial kinin metabolism to the same extent as ACE, the aims of the present study were to investigate whether (a) inhibition of APP leads to a reduction of myocardial infarct size in a rat model of acute ischaemia and reperfusion, (b) reduction of infarct size is mediated by bradykinin, and (c) a combination of APP and ACE inhibition leads to a more pronounced effect than APP inhibition alone. 3. Pentobarbital-anaesthetized rats were subjected to 30 min left coronary artery occlusion followed by 3 h reperfusion. The APP inhibitor apstatin, the ACE-inhibitor ramiprilat, or their combination were administered 5 min before ischaemia. Rats receiving HOE140, a specific B(2) receptor antagonist, were pretreated 5 min prior to enzyme inhibitors. Myocardial infarct size (IS) was determined by tetrazolium staining and expressed as percentage of the area at risk (AAR). 4. IS/AAR% was significantly reduced in rats that received apstatin (18+/-2%), ramiprilat (18+/-3%), or apstatin plus ramiprilat (20+/-4%) as compared with those receiving saline (40+/-2%), HOE (43+/-3%) or apstatin plus HOE140 (49+/-4%). 5. Apstatin reduces IS in an in vivo model of acute myocardial ischaemia and reperfusion to the same extent than ramiprilat. Cardioprotection achieved by this selective inhibitor of APP is mediated by bradykinin. Combined inhibition of APP and ACE did not result in a more pronounced reduction of IS than APP-inhibition alone.

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 B(2)-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 B(2)-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 B(2)-agonists BK, D-Arg-[Hyp(3)]-BK, [Hyp,(3) Tyr(Me)(8)]-BK, [DeltaPhe(5)]-BK, [D-NMF(7)]-BK, and [Phe(8)psi(CH(2)-NH)Arg(9)]-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-[Hyp(3)]-BK and [Hyp,(3) Tyr(Me)(8)]-BK at 81% and 71% of BK degradation activity, respectively, whereas other peptides were highly ([DeltaPhe(5)]-BK) or completely ([D-NMF(7)]-BK, [Phe(8)psi(CH(2)-NH)Arg(9)]-BK) resistant. The EC(50) 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-[Hyp(3)]-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.

Bradykinin metabolism in the isolated perfused rabbit heart

BACKGROUND

Bradykinin is a potent cardioprotective hormone, the beneficial role of which in vivo appears to be limited by its rapid metabolism. Inhibitors of peptidases that degrade endogenously formed bradykinin are themselves cardioprotective, presumably by increasing local bradykinin concentrations. As bradykinin-degrading peptidases are potential therapeutic targets, it is important to identify these enzymes in different animal models of cardiac function.

OBJECTIVE

To determine the mechanism of bradykinin degradation in the coronary circulation of the rabbit, using an isolated perfused heart preparation.

DESIGN AND METHODS

[3H]Bradykinin (16 nmol/l) was perfused as a bolus through the isolated rabbit heart in the presence and absence of specific peptidase inhibitors. The effluent was collected and the radiolabeled metabolites of [3H]bradykinin were separated by high performance liquid chromatography, identified, and quantified.

RESULTS

[3H]Bradykinin was metabolized to the extent of 62 +/- 3% in a single passage through the rabbit coronary circulation at a physiological flow rate. The metabolites were identified as [3H]bradykinin(1-5) and [3H]bradykinin(1-7),accounting for 50 +/- 4 and 12 +/- 2% of the radioactivity, respectively. Co-perfusion with the angiotensin converting enzyme inhibitor, ramiprilat, completely blocked formation of these metabolites.

CONCLUSIONS

Angiotensin-converting enzyme fully accounts for the metabolism of [3H]bradykinin in the rabbit coronary circulation. This result contrasts with data obtained using rat heart, which demonstrated a prominent role for aminopeptidase P in bradykinin metabolism in this species.

Acute effect of the dual angiotensin-converting enzyme and neutral endopeptidase 24-11 inhibitor mixanpril on insulin sensitivity in obese Zucker rat

The aim of this study was to determine whether acute dual angiotensin-converting enzyme (ACE)/neutral endopeptidase 24-11 (NEP) inhibition could improve whole body insulin-mediated glucose disposal (IMGD) more than ACE inhibition alone and whether this effect was mediated by the kinin-nitric oxide (NO) pathway activation. We therefore compared in anaesthetized obese (fa/fa) Zucker rats (ZOs) the effects of captopril (2 mg kg(-1), i.v.+2 mg kg(-1) h(-1)), retrothiorphan (25 mg kg(-1), i.v. +25 mg kg(-1) h(-1)), a selective NEP inhibitor, and mixanpril (25 mg kg(-1), i.v. +25 mg kg(-1) h(-1)), a dual ACE/NEP inhibitor, on IMGD using hyperinsulinaemic euglycaemic clamp technique. The role of the kinin-NO pathway in the effects of mixanpril was tested using a bradykinin B2 receptor antagonist (Hoe-140, 300 microg kg(-1)) and a NO-synthase inhibitor (N(omega)-nitro-L-arginine methyl ester, L-NAME, 10 mg kg(-1) i.v. +10 mg kg(-1) h(-1)) as pretreatments. Insulin sensitivity index (ISI) was lower in ZO controls than in lean littermates. Increases in ISI were observed in captopril- and retrothiorphan-treated ZOs. In mixanpril-treated ZOs, ISI was further increased, compared to captopril- and retrothiorphan-treated ZOs. In ZOs, Hoe-140 and L-NAME alone did not significantly alter and slightly reduced the ISI respectively. Hoe-140 and L-NAME markedly inhibited the ISI improvement induced by mixanpril. These results show that in obese insulin-resistant Zucker rats, under acute conditions, NEP or ACE inhibition can improve IMGD and that dual ACE/NEP inhibition improves IMGD more effectively than does either single inhibition. This effect is linked to an increased activation of the kinin-NO pathway.

Pathways of bradykinin degradation in blood and plasma of normotensive and hypertensive rats

Kinins are vasoactive peptide hormones that can confer protection against the development of hypertension. Because their efficacy is greatly influenced by the rate of enzymatic degradation, the activities of various kininases in plasma and blood of spontaneously hypertensive rats (SHR) were compared with those in normotensive Wistar-Kyoto rats (WKY) to identify pathogenic alterations. Either plasma or whole blood was incubated with bradykinin (10 microM). Bradykinin and kinin metabolites were measured by high-performance liquid chromatography. Kininase activities were determined by cumulative inhibition of angiotensin I-converting enzyme (ACE), carboxypeptidase N (CPN), and aminopeptidase P (APP), using selective inhibitors. Plasma of WKY rats degraded bradykinin at a rate of 13.3 +/- 0.94 micromol x min(-1) x l(-1). The enzymes ACE, APP, and CPN represented 92% of this kininase activity, with relative contributions of 52, 25, and 16%, respectively. Inclusion of blood cells at physiological concentrations did not extend the activities of these plasma kininases further. No differences of kinin degradation were found between WKY and SHR. The identical conditions of kinin degradation in WKY and SHR suggest no pathogenic role of kininases in the SHR model of genetic hypertension.

The kallikrein-kininogen-kinin system: lessons from the quantification of endogenous kinins

The purpose of the present review is to describe the place of endogenous kinins, mainly bradykinin (BK) and des-Arg(9)-BK in the kallikrein-kininogen-kinin system, to review and compare the different analytical methods reported for the assessment of endogenous kinins, to explain the difficulties and the pitfalls for their quantifications in biologic samples and finally to see how the results obtained by these methods could complement and extend the pharmacological evidence of their pathophysiological role.

Vascular catabolism of bradykinin in the isolated perfused rat kidney

Kinins in the circulation are rapidly metabolized by several different peptidases. The purpose of this study was to evaluate the contribution of membrane-bound peptidases to kinin metabolism in the renal circulation. Experiments were performed in vitro, in isolated rat kidneys perfused at a constant flow rate (8 ml/min) with Tyrode's solution. The effects of peptidase inhibitors were evaluated on the functional vasodilator response caused by bradykinin (30 nM) or [Tyr(Me)(8)]bradykinin (10 nM) via activation of bradykinin B2 receptors in kidneys precontracted with prostaglandin F2alpha. Angiotensin converting enzyme inhibitors, enalaprilat (3 microM), ramiprilat (1 microM) or lisinopril (1 microM), increased the bradykinin-induced renal vasodilation by 40% or more. Inhibitors of neutral endopeptidase (thiorphan or phosphoramidon, 10 microM), basic carboxypeptidase (DL-2-mercaptomethyl-3-guanidino-ethylthiopropanoic acid or MGTPA, 10 microM) and aminopeptidase P (apstatin, 20 microM) however did not enhance the renal vasodilator response elicited by kinins, whatever tested alone or in the presence of lisinopril. These findings indicate that angiotensin converting enzyme is the major peptidase whose inhibition potentiates the renal bradykinin B2 receptor mediated vasodilator response of kinins. The relative contribution in this potentiation of inhibition of kinin inactivation and of cross-talk of angiotensin converting enzyme with bradykinin B2 receptor remains however to be clarified.

Increased kallikrein expression protects against cardiac ischemia

Multiple indirect lines of evidence point at a cardioprotective role for enhanced bradykinin formation. In particular, the inhibition of angiotensin-converting enzyme, also known as kininase II, can protect against cardiac ischemia, putatively via accumulation of bradykinin. To address whether an increase in kinin formation is sufficient to protect against cardiac ischemia, we studied transgenic rats harboring the human tissue kallikrein gene TGR(hKLK1) under the control of the metallothionein promoter, which drives expression of the transgene in various organs including the heart. We subjected the isolated hearts from transgenic rats and their transgene negative littermates to ex vivo regional cardiac ischemia and reperfusion. During the experiment, the hearts were treated with either vehicle or the specific bradykinin type 2 receptor antagonist HOE 140 (10-9 M). In the transgenic rats, overflow of nucleotide breakdown products upon reperfusion was significantly less (455 +-54 nmol/min/g in transgene negative rats vs. 270+-57 nmol/min/g in the transgenic rats, P.

Degradation of bradykinin by peritoneal and alveolar macrophages of the guinea pig

Peptidase inhibitors and identification of the peptide fragments were used for the characterization of the bradykinin metabolism by alveolar and peritoneal macrophages. Both cell types show differences in the rate of inactivation and in the quantity of the metabolites generated. BK(1-5), BK(1-8), and BK(1-7) are the predominant direct metabolites. Metalloendopeptidase 24.15, carboxypeptidase M, and an unidentified peptidase are responsible for their formation. Angiotensin-converting enzyme and neutral endopeptidase 24.11 do not play a crucial role in the degradation of bradykinin by macrophages. In the bronchoalveolar space, other cells than the macrophages are more important to the breakdown of this peptide.

Inactivation of bradykinin by angiotensin-converting enzyme and by carboxypeptidase N in human plasma

Because bradykinin (BK) appears to have cardioprotective effects ranging from improved hemodynamics to antiproliferative effects, inhibition of BK-degrading enzymes should potentiate such actions. The purpose of this study was to find out which enzymes are responsible for the degradation of BK in human plasma. Human plasma from healthy donors (n = 10) was incubated with BK in the presence or absence of specific enzyme inhibitors. At high (micromolar) concentrations, BK was mostly (>90%) degraded by carboxypeptidase N (CPN)-like activity. In contrast, at low (nanomolar) substrate concentrations, at which the velocity of the catalytic reaction is equivalent to that under physiological conditions, BK was mostly (>90%) converted into an inactive metabolite, BK-(1-7), by angiotensin-converting enzyme (ACE). BK-(1-7) was further converted by ACE into BK-(1-5), with accumulation of this active peptide. A minor fraction (<10%) of the BK was converted into another active metabolite, BK-(1-8), by CPN-like activity. The present study shows that the most critical step in plasma kinin metabolism, i.e., inactivation of BK, is mediated by ACE. Thus inhibition of plasma ACE activity would be cardioprotective by elevating the concentration of BK in the circulation.

The angiotensin-converting enzyme gene polymorphism and responses to angiotensins and bradykinin in the human forearm

The deletion (D) allele of the angiotensin-converting enzyme (ACE) is associated with high ACE levels. Subjects homozygous for the D allele should therefore exhibit enhanced angiotensin I-induced vasoconstrictor responses and diminished bradykinin-induced vasodilator responses as compared with subjects homozygous for the insertion (I) allele. In eight II and eight DD normotensive male subjects, angiotensin I, bradykinin, and angiotensin II were infused in the forearm. Changes in forearm blood flow were registered with venous occlusion plethysmography. Blood was sampled to quantify angiotensin I to II conversion. Plasma ACE levels were 60% higher, and DD subjects showed an enhanced response to angiotensin I infusion (p < 0.05). No differences in angiotensin I to II conversion, angiotensin H vasoconstriction, and bradykinin vasorelaxation were found. The ACE-inhibitor enalaprilate inhibited angiotensin I-induced vasoconstriction, but did not significantly affect bradykinin-induced vasodilation. The AT1-receptor antagonist losartan (3,000 ng/kg/min) inhibited angiotensin II-induced vasoconstriction. In conclusion, subjects with the DD genotype display an enhanced vasoconstrictor response to angiotensin I, which cannot be explained on the basis of a similarly enhanced angiotensin I to II conversion rate or a difference in vascular reactivity. Possibly therefore, differences in angiotensin I to II conversion occur within the vascular wall only, at a site that does not readily equilibrate with blood plasma.

Potentiation of the vascular response to kinins by inhibition of myocardial kininases

Inhibitors of angiotensin I-converting enzyme (ACE) are very efficacious in the potentiation of the actions of bradykinin (BK) and are able to provoke a B(2) receptor-mediated vasodilation even after desensitization of this receptor. Because this activity cannot be easily explained only by an inhibition of kinin degradation, direct interactions of ACE inhibitors with the B(2) receptor or its signal transduction have been hypothesized. To clarify the significance of degradation-independent potentiation, we studied the vasodilatory effects of BK and 2 degradation-resistant B(2) receptor agonists in the isolated rat heart, a model in which ACE and aminopeptidase P (APP) contribute equally to the degradation of BK. Coronary vasodilation to BK and to a peptidic (B6014) and a nonpeptidic (FR190997) degradation-resistant B(2) agonist was assessed in the presence or absence of the ACE inhibitor ramiprilat, the APP inhibitor mercaptoethanol, or both. Ramiprilat or mercaptoethanol induced leftward shifts in the BK dose-response curve (EC(50)=3.4 nmol/L) by a factor of 4.6 or 4.9, respectively. Combined inhibition of ACE and APP reduced the EC(50) of BK to 0.18 nmol/L (ie, by a factor of 19) but potentiated the activity of B6014 (EC(50)=1.9 nmol/L) only weakly without altering that of FR190997 (EC(50)=0.34 nmol/L). Desensitization of B(2) receptors was induced by the administration of BK (0.2 micromol/L) or FR190997 (0.1 micromol/L) for 30 minutes; the vascular reactivity to ramiprilat or increasing doses of BK was tested thereafter. After desensitization with BK, but not FR190997, an additional application of ramiprilat provoked a B(2) receptor-mediated vasodilation. High BK concentrations were still effective at the desensitized receptor. The process of desensitization was not altered by ramiprilat. These results show that in this model, all potentiating actions of ACE inhibitors on kinin-induced vasodilation are exclusively related to the reduction in BK breakdown and are equivalently provoked by APP inhibition. The desensitization of B(2) receptors is overcome by increasing BK concentrations, either directly or through the inhibition of ACE. These observations do not suggest any direct interactions of ACE inhibitors with the B(2) receptor or its signal transduction but point to a very high activity of BK degradation in the vicinity of the B(2) receptor in combination with a stimulation-dependent reduction in receptor affinity.

Structural requirements for B2-agonists with improved degradation stability

Studies on bradykinin (BK) have been impeded by the fact that this peptide is rapidly degraded by various kininases. Modifications enacted to stabilize the BK sequence have usually resulted in a loss of agonistic activity. In this study, new structural modifications were investigated with the aim to identify degradation-resistant agonists on the bradykinin B2-receptor. The efficacy and degradation stability of several potentially agonistic derivatives were examined using a B2-receptor model (FURA-stained rat fibroblasts) and rat serum kininases. Modifications of the investigated BK analogues included amino-terminal (D-Arg) or carboxy-terminal (Ile-Tyr) prolongation, various substitutions at positions 2, 5, 7, 8 (tetrahydroisoquinoline-3-carboxylic acid, octahydroindole-2-carboxylic acid, hydroxy-proline, beta-2-thienylalanine, 2,3-dehydro-phenylalanine, erythro-beta-phenylserine, erythro-alpha-amino-beta-phenyl-butyric acid, N-methyl-phenylalanine), or intramolecular cyclization via lactam bridges. Kinin inactivation was investigated in rat serum, where the activities of angiotensin I-converting enzyme (ACE), carboxypeptidase N (CPN), aminopeptidase P (APP) and aminopeptidase M (APM) could be differentiated by selective inhibitors. Analogues derived from phyllokinin (BK-Ile-Tyr-SO4) and cyclic peptides had no receptor affinity. Useful modifications compatible with agonistic activity included D-Arg0 (protects against APP), D-N-methyl-Phe7 and dehydro-Phe5 (protect against ACE), and erythro-phenylserine or erythro-amino-phenyl-butyric acid at position 8 (protect against ACE and CPN). Finally, the kinin derivatives D-Arg0-[Hyp3, Thi5, epsilonSer(betaPh)8]-BK and D-Arg0-[Hyp3, Thi5, epsilonAbu(betaPh)8]-BK proved to be potent B2-agonists with extensive stability against rat serum kininases.

Cardioprotective effects of the aminopeptidase P inhibitor apstatin: studies on ischemia/reperfusion injury in the isolated rat heart

Aminopeptidase P and angiotensin-converting enzyme (ACE) are responsible for the metabolism of exogenously administered bradykinin in the coronary circulation of the rat. It has been shown that ACE inhibitors decrease cytosolic enzyme release from the ischemic rat heart and reduce reperfusion-induced ventricular arrhythmias by increasing endogenous levels of bradykinin. It was hypothesized that the aminopeptidase P inhibitor apstatin could do the same. In an isolated perfused rat heart preparation subjected to global ischemia and reperfusion, both apstatin and ramiprilat (an ACE inhibitor) significantly decreased creatine kinase (CK) and lactate dehydrogenase (LDH) release. The difference between the postischemia and preischemia levels of released CK was reduced 68% by apstatin and 68% by ramiprilat compared with control. The corresponding reductions in LDH release were 74% for apstatin and 81% for ramiprilat. A combination of the inhibitors was not significantly better than either one alone. Apstatin and ramiprilat also significantly reduced the duration of reperfusion-induced ventricular fibrillation by 69 and 61%, respectively. The antiarrhythmic effect of apstatin was reversed by HOE140, a bradykinin B2-receptor antagonist, suggesting that apstatin is acting by potentiating endogenously formed bradykinin. The results demonstrate that the aminopeptidase P inhibitor apstatin is cardioprotective in this model of cardiac ischemia/ reperfusion injury.

Effects of aminopeptidase P inhibition on kinin-mediated vasodepressor responses

We studied in anesthetized rats whether aminopeptidase P (AMP) may be involved in bradykinin (BK) metabolism and responses. For this we inhibited AMP with the specific inhibitor apstatin (Aps). Studies were done with Aps alone or together with the angiotensin-converting enzyme inhibitor lisinopril (Lis). Aps increased the vasodepressor response to an intravenous bolus of BK (400 ng/kg): vehicle, -3.0 +/- 0.7 mmHg; Aps, -7.8 +/- 0.7 mmHg (P < 0.01 vs. vehicle); Lis, -23.8 +/- 1.8 mmHg; Aps + Lis, -37.5 +/- 1.9 mmHg (P < 0.01 vs. Lis). Aps did not affect the vasodepressor response to BK given into the descending aorta. Plasma BK increased only in Aps + Lis-treated rats (in pg/ml): control, 48.0 +/- 1.4; Lis, 57.5 +/- 7.6; Aps + Lis, 121. 8 +/- 30.6 (P < 0.05 vs. control or Lis), whereas in rats infused with BK (400 ng. kg-1. min-1 for 5 min), Aps increased plasma BK (in pg/ml): control, 51.9 +/- 2.5; Aps, 83.5 +/- 20.5; Lis, 725 +/- 225; Aps + Lis, 1,668 +/- 318 (P < 0.05, Aps vs. control and Lis vs. Aps + Lis). In rats with aortic coarctation hypertension, the acute antihypertensive effects of Aps plus Lis were greater than Lis alone (P < 0.01). Hoe-140, a BK B2-receptor antagonist, abolished the difference. We concluded that in the rat AMP contributes to regulation of BK metabolism and responses.

Pharmacology and cardiovascular implications of the kinin-kallikrein system

Kinins are peptide hormones that can exert a significant influence on the regulation of blood pressure and vascular tone due to their vasodilatatory, natriuretic and growth modulating activity. Their cardiovascular involvement in physiological and pathophysiological situations has been studied intensively since inhibitors for angiotensin I-converting enzyme and selective receptor antagonists have become available for pharmacologically potentiating or inhibiting kinin-mediated reactions. Molecular biological analysis and the establishment of genetically modified animal models have also allowed newer information to be acquired on this subject. In this review, the components and cardiovascularly relevant mechanisms of the kinin-kallikrein system shall be described. Organ-specific effects concerning the kidneys, the vascular system, the heart and nervous tissue shall also be illustrated. On this issue, the physiological functions and pathophysiological implications of the kinin-kallikrein system should be clearly distinguished from the many, mostly endothelium-mediated protective effects which occur during ACE inhibition due to the potentiation of kinin effects. Finally, a view shall also be cast upon newly discovered targets of action, which could be exploited for therapeutically altering the kinin-kallikrein system.

Identification of kallidin degrading enzymes in the isolated perfused rat heart

Kallidin (KD) is an important vasoactive kinin whose physiological effects are strongly dependent on its degradation through local kininases. In the present study, we examined the spectrum of these enzymes and their contribution to KD degradation in isolated perfused rat hearts. By inhibiting angiotensin-converting enzyme (ACE), aminopeptidase M (APM) and neutral endopeptidase (NEP) with ramiprilat (0.25 microM), amastatin (40 microM) and phosphoramidon (1 microM), respectively, relative kininase activities were obtained. APM (44%) and ACE (35%) are the main KD degrading enzymes in rat heart; NEP (7%) plays a minor role. A participation of carboxypeptidase N (CPN) could not be found.