Cloning of the gene and cDNA for hamster chymase 2, and expression of chymase 1, chymase 2 and angiotensin-converting enzyme in the terminal stage of cardiomyopathic hearts

Role of chymase in cigarette smoke-induced pulmonary artery remodeling and pulmonary hypertension in hamsters

Background

Cigarette smoking is an important risk factor for pulmonary arterial hypertension (PAH) in chronic obstructive pulmonary disease (COPD). Chymase has been shown to function in the enzymatic production of angiotensin II (AngII) and the activation of transforming growth factor (TGF)-β1 in the cardiovascular system. The aim of this study was to determine the potential role of chymase in cigarette smoke-induced pulmonary artery remodeling and PAH.

Methods

Hamsters were exposed to cigarette smoke; after 4 months, lung morphology and tissue biochemical changes were examined using immunohistochemistry, Western blotting, radioimmunoassay and reverse-transcription polymerase chain reaction.

Results

Our results show that chronic cigarette smoke exposure significantly induced elevation of right ventricular systolic pressures (RVSP) and medial hypertrophy of pulmonary arterioles in hamsters, concurrent with an increase of chymase activity and synthesis in the lung. Elevated Ang II levels and enhanced TGF-β1/Smad signaling activation were also observed in smoke-exposed lungs. Chymase inhibition with chymostatin reduced the cigarette smoke-induced increase in chymase activity and Ang II concentration in the lung, and attenuated the RVSP elevation and the remodeling of pulmonary arterioles. Chymostatin did not affect angiotensin converting enzyme (ACE) activity in hamster lungs.

Conclusions

These results suggest that chronic cigarette smoke exposure can increase chymase activity and expression in hamster lungs. The capability of activated chymase to induce Ang II formation and TGF-β1 signaling may be part of the mechanism for smoking-induced pulmonary vascular remodeling. Thus, our study implies that blockade of chymase might provide benefits to PAH smokers.

Structural basis for elastolytic substrate specificity in rodent alpha-chymases

Divergence of substrate specificity within the context of a common structural framework represents an important mechanism by which new enzyme activity naturally evolves. We present enzymological and x-ray structural data for hamster chymase-2 (HAM2) that provides a detailed explanation for the unusual hydrolytic specificity of this rodent alpha-chymase. In enzymatic characterization, hamster chymase-1 (HAM1) showed typical chymase proteolytic activity. In contrast, HAM2 exhibited atypical substrate specificity, cleaving on the carboxyl side of the P1 substrate residues Ala and Val, characteristic of elastolytic rather than chymotryptic specificity. The 2.5-A resolution crystal structure of HAM2 complexed to the peptidyl inhibitor MeOSuc-Ala-Ala-Pro-Ala-chloromethylketone revealed a narrow and shallow S1 substrate binding pocket that accommodated only a small hydrophobic residue (e.g. Ala or Val). The different substrate specificities of HAM2 and HAM1 are explained by changes in four S1 substrate site residues (positions 189, 190, 216, and 226). Of these, Asn(189), Val(190), and Val(216) form an easily identifiable triplet in all known rodent alpha-chymases that can be used to predict elastolytic specificity for novel chymase-like sequences. Phylogenetic comparison defines guinea pig and rabbit chymases as the closest orthologs to rodent alpha-chymases.

Possible role for mast cell-derived cathepsin G in the adverse remodelling of stenotic aortic valves

AIMS

Aortic stenosis (AS) is characterized by extensive remodelling of the valves, including infiltration of inflammatory cells, extracellular matrix degradation, and fibrosis. The molecular mechanisms behind this adverse remodelling have remained obscure. In this article, we study whether cathepsin G, an angiotensin II (Ang II)-forming elastolytic enzyme, contributes to progression of AS.

METHODS AND RESULTS

Stenotic aortic valves (n = 86) and control valves (n = 17) were analysed for cathepsin G, transforming growth factor-beta1 (TGF-beta1), and collagens I and III with RT-PCR and immunohistochemistry. Valvular collagen/elastin ratio was quantified by histochemistry. In stenotic valves, cathepsin G was present in mast cells and showed increased expression (P < 0.001), which correlated positively (P < 0.001) with the expression levels of TGF-beta1 and collagens I and III. TGF-beta1 was also present in mast cell-rich areas and cathepsin G induced losartan-sensitive TGF-beta1 expression in cultured fibroblasts. Collagen/elastin ratio was increased in stenotic valves (P < 0.001) and correlated positively with smoking (P = 0.02). Nicotine in cigarette smoke activated mast cells and induced TGF-beta1 expression in cultured fibroblasts. Fragmented elastin was observed in stenotic valves containing activated cathepsin G-secreting mast cells and in normal valves treated with cathepsin G.

CONCLUSION

In stenotic aortic valves, mast cell-derived cathepsin G may cause adverse valve remodelling and AS progression.

Effect of mast cell chymase inhibitor on the development of scleroderma in tight-skin mice

1 Although the pathogenesis of scleroderma is not fully understood, activation of connective-tissue-type mast cells (CTMCs) has been implicated in various fibrotic diseases. 2 Our previous study showed that the number of CTMCs was markedly increased during fibrous proliferation in the skin of a scleroderma model, namely tight-skin (Tsk) mice. Because mast cells express numerous bioactive factors, such as cytokines, growth factors, proteases, and others, it is crucial to identify the primary factors that may be involved in the pathogenesis of scleroderma. Our previous study also showed that a CTMC-specific protease, chymase-4, was selectively upregulated in accordance with the development of skin fibrosis in Tsk mice. 3 To further elucidate the role of chymase secreted from CTMCs, we evaluated the therapeutic effects of a synthetic chymase-specific inhibitor, SUN-C8257, on the development of skin fibrosis in Tsk mice. SUN-C8257 (50 mg kg-1 day-1) was administered via intraperitoneal injection in 13-week-old Tsk mice for a period of 2 weeks. 4 Treatment with SUN-C8257 significantly reduced chymase activity by 43% and the chymase-4 mRNA level by 47%, and also decreased the thickness of the subcutaneous fibrous layer of Tsk mice by 42% compared with that of Tsk mice injected with vehicle. 5 Furthermore, immunohistochemical analysis revealed that transforming growth factor (TGF)-beta1 staining in the fibrous layer of Tsk skin was markedly reduced by the treatment with SUN-C8257. This chymase inhibitor may prevent the chymase-dependent pathway that activates the latent TGF-beta1 in fibrous tissue, and may exhibit beneficial effects that inhibit the development of fibrosis. 6 In conclusion, our results strongly support the assumption that CTMC-derived chymase may play a key role in the pathogenesis of scleroderma.

Cardiac chymase: pathophysiological role and therapeutic potential of chymase inhibitors

On release from cardiac mast cells, alpha-chymase converts angiotensin I (Ang I) to Ang II. In addition to Ang II formation, alpha-chymase is capable of activating TGF-beta1 and IL-1beta, forming endothelins consisting of 31 amino acids, degrading endothelin-1, altering lipid metabolism, and degrading the extracellular matrix. Under physiological conditions the role of chymase in the mast cells of the heart is uncertain. In pathological situations, chymase may be secreted and have important effects on the heart. Thus, in animal models of cardiomyopathy, pressure overload, and myocardial infarction, there are increases in both chymase mRNA levels and chymase activity in the heart. In human diseased heart homogenates, alterations in chymase activity have also been reported. These findings have raised the possibility that inhibition of chymase may have a role in the therapy of cardiac disease. The selective chymase inhibitors developed to date include TY-51076, SUN-C8257, BCEAB, NK320, and TEI-E548. These have yet to be tested in humans, but promising results have been obtained in animal models of myocardial infarction, cardiomyopathy, and tachycardia-induced heart failure. It seems likely that orally active inhibitors of chymase could have a place in the treatment of cardiac diseases where injury-induced mast cell degranulation contributes to the pathology.

Increased expression of bradykinin type-1 receptors in endothelium of intramyocardial coronary vessels in human failing hearts

In experimental animals, bradykinin type-1 receptors (BK-1Rs) are induced during inflammation and ischemia, and, by exerting either cardioprotective or cardiotoxic effects, they may contribute to the pathogenesis of heart failure. Nothing is known about the expression of BK-1Rs in human heart failure. Human heart tissue was obtained from excised hearts of patients undergoing cardiac transplantation (n = 13), due to idiopathic dilated cardiomyopathy (IDC; n = 7) or to coronary heart disease (CHD; n = 6), and from normal hearts (n = 6). The expression of BK-1Rs was analyzed by means of competitive RT-PCR, Western blot analysis, and immunohistochemistry. Expression of BK-1R mRNA was increased in both IDC (2.8-fold) and CHD (2.1-fold) hearts compared with normal hearts. The observed changes were verified at the protein level. Expression of BK-1Rs in failing hearts localized to the endothelium of intramyocardial coronary vessels and correlated with an increased expression of TNF-alpha in the vessel wall. Treatment of human coronary artery endothelial cells with TNF-alpha increases their BK-1R expression. These novel results show that BK-1Rs are induced in the endothelium of intramyocardial coronary vessels in failing human hearts and so may participate in the pathogenesis of heart failure.

Induction of local angiotensin II-producing systems in stenotic aortic valves

OBJECTIVES

The purpose of this study was to investigate the expression of angiotensin II (Ang II)-producing enzyme systems in normal and stenotic aortic valves.

BACKGROUND

Chronic inflammation and fibrosis are involved in the pathogenesis of aortic stenosis (AS), but the detailed molecular mechanisms of this atherosclerosis-like process remain obscure. Angiotensin II, a powerful mediator of inflammation and fibrosis, may participate in AS progression.

METHODS

Stenotic aortic valves (n = 86) were obtained from patients undergoing valve replacement surgery, and control valves (n = 11) were obtained from patients undergoing cardiac transplantation. Angiotensin-converting enzyme (ACE) and mast cell (MC)-derived chymase were quantified by reverse-transcription polymerase chain reaction, autoradiography, and immunostaining. The MCs, macrophages, and T lymphocytes were detected by immunohistochemistry, and angiotensin II type 1 receptor (AT-1R) by autoradiography.

RESULTS

Compared with control valves, stenotic aortic valves showed a significant increase in both messenger ribonucleic acid (mRNA) (p = 0.001) and protein (p < 0.001) expression of ACE, which colocalized with macrophages. Similarly, the expression of AT-1R protein and chymase mRNA and protein was upregulated (p < 0.001), and the number of MCs was six-fold higher in stenotic than in normal valves. The MCs were associated with the calcified areas, and-in contrast to control valves-showed an increased degree of degranulation, a prerequisite for chymase secretion and action.

CONCLUSIONS

Angiotensin-converting enzyme and chymase, two Ang II-forming enzymes, are locally expressed in aortic valves, and owing to infiltration of macrophages and MCs, are further upregulated in stenotic valves. These novel findings, implicating chronic inflammation and an increased expression of local Ang II-forming systems, suggest that therapeutic interventions aiming at inhibiting these processes may slow AS progression.

Insulin activates native and recombinant large conductance Ca(2+)-activated potassium channels via a mitogen-activated protein kinase-dependent process

Evidence is accumulating that, in addition to regulating peripheral energy metabolism, insulin is an important modulator of neuronal function. Indeed, high levels of insulin and insulin receptors are expressed in several brain regions including the hippocampus. We have shown previously that insulin inhibits aberrant synaptic activity in hippocampal neurons via activation of large conductance Ca(2+)-activated K+ (BK) channels. In this study, we have examined further the effects of insulin on native hippocampal and recombinant (hSlo) BK channels expressed in human embryonic kidney (HEK) 293 cells. Pipette or bath application of insulin evoked a rapid increase in hippocampal BK channel activity, an action caused by activation of insulin receptors because insulin-like growth factor 1 (IGF-1) failed to mimic insulin action. In parallel studies, insulin, applied via the pipette or bath, also activated hSlo channels expressed in HEK293 cells. Although phosphoinositide 3-kinase is a key component of insulin and IGF-1 receptor signaling pathways, activation of this lipid kinase does not underlie the effects of insulin because neither 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one (LY294002) nor wortmannin inhibited or reversed insulin action. However, specific inhibitors of mitogen-activated protein kinase (MAPK) activation, 2'-amino-3'-methoxyflavone (PD98059) or 1,4-diamino-2,3-dicyano-1,4-bis(2-aminophenylthio)-butadiene (U0126), attenuated insulin action, indicating that a MAPK-dependent mechanism underlies this process. Furthermore, insulin activation of this pathway enhances BK channel activity by shifting the Ca(2+)-sensitivity such that BK channels are active at more hyperpolarized membrane potentials. Because postsynaptic BK channels are important regulators of neuronal hyperexcitability, insulin-induced activation of BK channels, via stimulation of a MAPK-dependent pathway, may be an important process for regulating hippocampal function under normal and pathological conditions.

A role for cardiac mast cells in the pathogenesis of hypertensive heart disease

OBJECTIVE

Cardiac mast cells participate in myocardial dysfunction, but the mechanisms are presently unknown.

DESIGN

By examining spontaneously hypertensive rats (SHRs) during their entire lifespan, we attempted to define the role of mast cells in the induction of cardiac hypertrophy and transition to heart failure.

METHODS AND RESULTS

By contrast to normotensive littermates, hearts of newborn SHRs already contained mast cells. In the prehypertensive (2-week-old) SHRs, the increased expression of c-kit and soluble stem cell factor correlated with an increased number of cardiac mast cells. The mast cells contained tumour necrosis factor-alpha which, together with nuclear factor kappa-B (NF-kappaB) and interleukin (IL)-6, was significantly induced in the prehypertensive SHRs. Stimulation of cardiac mast cells with compound 48/80 in an ex-vivo Langendorff heart perfusion system resulted in increased expression of nuclear factor Kappa-B (NF-kappaB) (four-fold) and IL-6 (nine-fold) mRNA in the left ventricles of adult rat hearts. In the presence of an inhibitor of mast cell degranulation, disodium cromoglycate, the induced expression of NF-kappaB and IL-6 was inhibited. In the late hypertensive stage, the hearts of SHRs with advanced cardiac hypertrophy (12-month-old) and heart failure (20-month-old) had significantly increased levels of transforming growth factor (TGF)-beta1 and basic fibroblast growth factor (bFGF), and displayed increased myocardial fibrosis. Activated mast cells were a major source of TGF-beta1 and bFGF, and localized to areas of myocardial fibrosis.

CONCLUSIONS

By synthesizing and secreting prohypertrophic cytokines and profibrotic growth factors, cardiac mast cells participate in the induction of cardiac hypertrophy and cardiac fibrosis, which are the key steps in the transition to heart failure.

New developments in the genetics and activation of mast cell proteases

Mast cell chymases and tryptases exhibit an intriguing but potentially confusing variety of forms and functions. Thanks to recent genetic and biochemical advances, a clearer picture of phylogenetic and functional relationships in this large group of mammalian enzymes is emerging. Furthermore, there is increasing appreciation of the diversity of these enzymes among human populations. In humans, there appears to be just one mast cell chymase but multiple expressed tryptases, some of which are allelic variants and others of which are products of separate gene loci. New biological tools, including the dipeptidyl peptidase I (DPPI)-null mouse in which the entire class of mast cell chymases appears to be functionally knocked out, are helping to clarify the importance and specific roles of these most abundant of secreted mast cell proteins.

Down-regulation of cardioprotective bradykinin type-2 receptors in the left ventricle of patients with end-stage heart failure

OBJECTIVES

We sought to study the expression of bradykinin type-2 receptors (BK-2Rs) in patients with heart failure (HF).

BACKGROUND

Recent work in experimental animals has suggested that bradykinin (BK) exerts cardioprotective effects through specific BK-2Rs. However, nothing is known about the regulation of BK-2R expression in the pathogenesis of human HF.

METHODS

Human heart tissue was obtained from excised hearts of patients undergoing cardiac transplantation (n = 13) and from normal hearts (n = 6) unsuitable for donation. The patients had HF due to idiopathic dilated cardiomyopathy (IDC) (n = 7) or coronary heart disease (CHD) (n = 6). Tissue samples from the left ventricles were analyzed by competitive reverse-transcriptase-polymerase chain reaction and Western blotting for the expression of BK-2R messenger ribonucleic acid (mRNA) and protein.

RESULTS

In both the IDC and CHD hearts, the level of BK-2R mRNA expression was found to be significantly lower (30% and 38% of control values, respectively) than that in normal hearts. Correspondingly, the BK-2R protein level was significantly reduced in both the IDC and CHD hearts (45% and 62% of control values, respectively) and apparently involved all myocardial cell types. The down-regulation of BK-2R expression in failing hearts did not correlate with decreased cellularity or with the expression pattern of other members of the G-protein-coupled receptor superfamily. However, BK-2R down-regulation in the failing hearts was associated with a decrease in endothelial nitric oxide synthase in both IDC (53% of control value) and CHD (43% of control value) hearts.

CONCLUSIONS

These results are the first to suggest that a loss of BK-2Rs is involved in the pathogenesis of human HF.

Dissecting the role of chymase in angiotensin II formation and heart and blood vessel diseases

Inhibition of angiotensin II action or its formation by angiotensin-converting enzyme has been highly successful in the treatment of cardiovascular diseases. Since the identification of chymase as a major angiotensin II-forming enzyme in the human heart and its vessels more than a decade ago, numerous studies have sought to understand the importance of this enzyme in tissue angiotensin II formation and in the pathogenesis of hypertension, congestive heart failure, and vascular disease. Recent studies show that chymase and angiotensin-converting enzyme regulate angiotensin II production in distinct tissue compartments and that, in the pathogenesis of cardiovascular diseases, chymase-dependent effects extend beyond its ability to regulate tissue angiotensin II levels.

Differential ANG II generation in plasma and tissue of mice with decreased expression of the ACE gene

We utilized mice with homozygous disruption of angiotensin-converting enzyme (ACE) (-/-), mice with heterozygous deletion of ACE (+/-), and wild-type mice (+/+) to test the hypothesis that genetic variation in ACE modulates tissue and plasma angiotensin (ANG) II concentrations. With the use of ANG I as substrate, kidney, heart, and lung ACE activity was reduced 80% in -/- mice compared with +/+ mice. However, ANG II concentrations and ANG II-to-ANG I ratios in the kidney, heart, and lung did not differ among genotypes. In contrast, plasma ANG II concentrations in -/- mice were <2 fmol/ml, whereas plasma ANG I concentrations were extremely high (765 fmol/ml). Chymase activity was increased 14-fold in the kidney (P < 0.05) and 1.5-fold in the heart (P < 0.05) of -/- versus +/+ mice but did not differ among genotypes in the lung. ANG II formation from enzymes other than ACE and chymase contributed <2% of total ANG II formation in all genotypes. These data suggest that ACE is essential to ANG II formation in the vascular space, whereas chymase may provide an important mechanism in maintaining steady-state ANG II levels in tissue.

Evidence for diversity of substrate specificity among members of the chymase family of serine proteases

The term chymase is used to signify a chymotrypsin-like protease stored within the secretory granules of mast cells. Primarily based on amino acid sequence homology, 18 chymases have been identified among different animals. This study, which compares the structure of the primary specificity pocket (S1 subsite), defines a subgroup of four chymases likely to have a substrate specificity with more elastase- than chymotrypsin-like qualities. This difference is due, primarily, to finding a Val instead of a Gly at residue 199, a position corresponding to Gly216 in bovine chymotrypsin and Val216 in neutrophil and porcine elastases. Chymases with Val at 199 are found only in animals expressing multiple chymases, consistent with the premise that their substrate specificity differs from that of chymases with Gly at 199.

Angiotensin converting enzyme is the main contributor to angiotensin I-II conversion in the interstitium of the isolated perfused rat heart

OBJECTIVES

Recent studies in homogenized hearts suggest that chymase rather than angiotensin converting enzyme (ACE) is responsible for cardiac angiotensin I to angiotensin II conversion. We investigated in intact rat hearts whether (i) enzymes other than ACE contribute to angiotensin I to angiotensin II conversion and (ii) the localization (endothelial/extra-endothelial) of converting enzymes.

DESIGN AND METHODS

We used a modified version of the rat Langendorff heart, allowing separate collection of coronary effluent and interstitial fluid. Hearts were perfused with angiotensin I (arterial concentration 5-10 pmol/ml) under control conditions, in the presence of captopril (1 micromol/l) or after endothelium removal with 0.2% triton X-100. Endothelium removal was verified as the absence of a coronary vasodilator response to 10 nmol bradykinin. Angiotensin I and angiotensin II were measured in coronary effluent and interstitial fluid with sensitive radioimmunoassays.

RESULTS

In control hearts, 45% of arterial angiotensin I was metabolized during coronary passage, partly through conversion to angiotensin II. At steady-state, the angiotensin I concentration in interstitial fluid was three to four-fold lower than in coronary effluent, while the angiotensin II concentrations in both fluids were similar. Captopril and endothelium removal did not affect coronary angiotensin I extraction, but increased the interstitial fluid levels of angiotensin I two- and three-fold, respectively, thereby demonstrating that metabolism (by ACE) as well as the physical presence of the endothelium normally prevent arterial angiotensin I from reaching similar levels in coronary effluent and interstitial fluid. Captopril, but not endothelium removal, greatly reduced the angiotensin II levels in coronary effluent and interstitial fluid. With the ACE inhibitor, the angiotensin II/I ratios in coronary effluent and interstitial fluid were 83 and 93% lower, while after endothelium removal, the ratios were 33 and 71% lower.

CONCLUSIONS

In the intact rat heart, ACE is the main contributor to angiotensin I to angiotensin II conversion, both in the coronary vascular bed and the interstitium. Cardiac ACE is not limited to the coronary vascular endothelium.

Isoform-selective upregulation of mast cell chymase in the development of skin fibrosis in scleroderma model mice

The involvement of connective-tissue-type mast cells and chymase, a protease unique to their secretory granules, has been implicated in fibrotic diseases. To elucidate the role of chymase in fibroproliferative inflammation, in this study we examined the enzymatic activity and mRNA expression of chymase in the sclerotic skin of tight-skin mice; syngeneic Pallid mice served as the control. Dorsal skin specimens from mice aged 5, 10, and 20 wk were evaluated by morphometric and biochemical analyses. At ages 10 and 20 wk, the hydroxyproline concentration in tight-skin dermis was higher than that in Pallid. At any age, the subcutaneous fibrous layer was thicker in tight-skin than in Pallid. In accordance with these fibrous changes, both connective-tissue-type mast cell counts and chymase activity were higher in tight-skin skin than in Pallid skin up to 20 wk of age. Age-matched (10-wk-old) tight-skin and Pallid were quantified for their mRNA of connective-tissue-type mast-cell-specific chymase, mouse mast cell protease-4, by the competitive reverse transcriptase polymerase chain reaction technique, which revealed its higher level in tight-skin than Pallid. In contrast, the mRNA level of mouse mast cell protease-5, the chymase isoform of undifferentiated mast cells, in tight-skin skin was only a tenth that of mouse mast cell protease-4 and no different from the mouse mast cell protease-5 mRNA level of Pallid mice. An in situ hybridization study confirmed the higher expression of mouse mast cell protease-4 by connective-tissue-type mast cells in tight-skin skin than Pallid skin. These results strongly support the contention that the connective-tissue-type mast cell chymase plays a crucial role in fibroproliferative remodeling of the skin.

Angiotensin II generation by mast cell alpha- and beta-chymases

Mast cells secrete alpha- and beta-chymases. Primate alpha-chymases generate angiotensin (AT) II by selectively hydrolyzing AT I's Phe(8)-His(9) bond. This is distinct from the AT converting enzyme (ACE) pathway. In humans, alpha-chymase is the major non-ACE AT II-generator. In rats, beta-chymases destroy AT II by cleaving at Tyr(4)-Ile(5). Past studies predicted that AT II production versus destruction discriminates alpha- from beta-chymases and that Lys(40) in the substrate-binding pocket determines alpha-chymase Phe(8) specificity. This study examines these hypotheses by comparing AT II generation by human alpha-chymase (containing Lys(40)), dog alpha-chymase (lacking Lys(40)), and mouse mMCP-4 (a beta-chymase lacking Lys(40); orthologous to AT II-destroying rat chymase rMCP-1). The results suggest that human and dog alpha-chymase generate AT II exclusively and with comparable efficiency, although dog chymase contains Ala(40) rather than Lys(40). Furthermore, AT II is the major product generated by degranulation supernatants from cultured dog mast cells, which release tryptases and dipeptidylpeptidase as well as alpha-chymase. In contrast to rMCP-1, mMCP-4 beta-chymase readily generates AT II. Although there is competing AT I hydrolysis at Tyr(4), mMCP-4 does not destroy AT II quickly once it is formed. We conclude (1) that chymases are the dominant AT I-hydrolyzing mast cell peptidases, (2) that residues other than Lys(40) are key determinants of alpha-chymase AT I Phe(8) specificity, (3) that beta-chymases can generate AT II, and (4) that alpha- and beta-chymases are not strictly dichotomous regarding AT I cleavage specificity.

Cardiac fibrosis in mice lacking brain natriuretic peptide

Cardiac fibrosis, defined as a proliferation of interstitial fibroblasts and biosynthesis of extracellular matrix components in the ventricles of the heart, is a consequence of remodeling processes initiated by pathologic events associated with a variety of cardiovascular disorders, which leads to abnormal myocardial stiffness and, ultimately, ventricular dysfunction. Brain natriuretic peptide (BNP) is a cardiac hormone produced primarily by ventricular myocytes, and its plasma concentrations are markedly elevated in patients with congestive heart failure and acute myocardial infarction. However, its precise functional significance has been undefined. In this paper, we report the generation of mice with targeted disruption of BNP (Nppb(-/-) mice). We observed multifocal fibrotic lesions in the ventricles from Nppb(-/-) mice. No signs of systemic hypertension and ventricular hypertrophy are noted in Nppb(-/-) mice. In response to ventricular pressure overload, focal fibrotic lesions are increased in size and number in Nppb(-/-) mice, whereas no focal fibrotic changes are found in wild-type littermates (Nppb(+/+) mice). This study establishes BNP as a cardiomyocyte-derived antifibrotic factor in vivo and provides evidence for its role as a local regulator of ventricular remodeling.

Cell coupling and impulse propagation in the failing heart

Cell coupling and impulse propagation were investigated in the ventricle of cardiomyopathic hamsters at an advanced stage of heart failure. An appreciable decline in junctional conductance was found, a phenomenon in part related to activation of the plasma and cardiac renin-angiotensin systems. Decreased expression of connexin43 or an alteration of junctional proteins also might be implicated in the decreased cell coupling. Morphologic abnormalities such as fibrosis, necrosis, and rupture of cell contacts contribute to the decline of conduction velocity or to the blockade of impulse propagation in some areas of the ventricle, creating the conditions for anisotropic conduction and cardiac arrhythmias. The decrease in membrane potential found in myopathic cells is related in part to depression of Na-KATPase activity, and the lack of action of beta-adrenergic agonists on junctional conductance is explained by down-regulation of beta receptors and an abnormality of adenyl cyclase.

Differential suppression of pressure-overload cardiac and aortic hypertrophy in rats by angiotensin-converting enzyme inhibitors

Role of tissue angiotensin-converting enzyme (ACE) in the development of pressure-overload cardiovascular hypertrophy was examined in rats by comparing the inhibitory effect of trandolapril (high efficiency on tissue ACE) with that of enalapril (low efficiency) at equally antihypertensive doses. Rats with abdominal aorta banded or sham-operated were orally treated with trandolapril (0.5 mg/kg per day), enalapril (20 mg/kg per day) or vehicle for 8 weeks after the surgical maneuvers. In vehicle-treated rats, the banding raised the intra-aortic systolic pressure by 58%, diastolic pressure by 31%, maximum velocity of pressure rise by 65%, left ventricular (LV) weight by 41%, LV hydroxyproline concentration by 56%, aortic mass by 46%, LV ACE activity by 45%, and aortic ACE activity by 265%. Although both drugs equally reduced the aortic systolic pressure to approx. 70% and diastolic pressure to approx. 80% that of banded rats receiving vehicle, trandolapril partially prevented the LV hypertrophy, whereas enalapril yielded nonsignificant suppression. Trandolapril completely prevented the LV increments in hydroxyproline and ACE activity, whereas enalapril partially inhibited the LV hydroxyproline increase with little inhibition of LV ACE activity. In contrast, both inhibitors almost completely prevented the aortic hypertrophy, with the ACE activity of the aorta being potently inhibited. These results suggest that tissue ACE is the principal factor for pressure-induced aortic hypertrophy and an important yet non-essential factor for LV hypertrophy.