Neutral endopeptidase and angiotensin I converting enzyme insertion/deletion gene polymorphism in humans.

Neutral endopeptidase (NEP) hydrolyses angiotensins (Ang) I and II and generates angiotensin-(1-7) [Ang-(1-7)]. In humans, the insertion/deletion (I/D) angiotensin-I converting enzyme (ACE) gene polymorphism determined plasma ACE levels by 40%. In rats, a similar polymorphism determines ACE levels which are inversely associated to NEP activity. The objective of this study is to evaluate the relationship between ACE expression and plasma NEP activity in normotensive subjects and in hypertensive patients. In total, 58 consecutive patients with hypertension, evaluated in our Hypertension Clinic, were compared according to their ACE I/D genotypes with 54 control subjects in terms of both plasma ACE activity and NEP activities. Plasma ACE activity was elevated 51 and 70% in both DD ACE groups (normotensives and hypertensives) compared with their respective ID and II ACE groups (P<0.001). A significant effect of the ACE polymorphism and of the hypertensive status on ACE activity was observed (P<0.001). In normotensive DD ACE subjects, NEP activity was 0.30+/-0.02 U/ml, whereas in the normotensive II ACE and in the normotensive ID ACE subjects NEP activity was increased 65 and 48%, respectively (P<0.001). In the hypertensive DD ACE patients, NEP activity was 0.47+/-0.03 U/mg. An effect of the I/D ACE genotypes on NEP activity (P<0.04) and an interaction effect between the I/D ACE genotype and the hypertensive status were also observed (P<0.001). These results are consistent with a normal and inverse relationship between the ACE polymorphism and NEP activity in normotensive humans (as is also observed in rats). This normal relationship is not observed in hypertensive patients.

[Early prevention of experimental left ventricular hypertrophy in experimental hypertension and angiotensin II levels]. / Prevención precoz de hipertrofia ventricular izquierda en la hipertensión experimental y concentraciones de angiotensina II.

INTRODUCTION: Angiotensin II levels can be partially inhibited during chronic administration of angiotensin converting enzyme (ACE) inhibitors, limiting from a clinical point of view its efficacy in the treatment of hypertension. There are few studies relating ACE activity directly with early prevention of left ventricular hypertrophy (LVH) in systemic hypertension during the administration of an ACE inhibitor (ACEI). AIM: To evaluate the effects of early ACE inhibition with perindopril on the development of hypertension, LVH and levels of angiotensin II (Ang II) in plasma as well as in LV in the rat Goldblatt model (Gb; 2 kidneys-1 clip), 2 weeks after surgery. RESULTS: Systolic blood pressure and relative LV mass increased by 42% and 20% respectively, in the Gb group (p < 0.001). Plasma and LV ACE activities were significantly higher in the Gb rats compared with the control rats. Plasma and LV Ang II levels also increased by 129% and 800%, respectively. Perindorpil prevented hypertension and LVH development by inhibiting plasma ACE (and also LV ACE), and also circulation Ang II in plasma and in the LV. CONCLUSIONS: In this experimental model of hypertensive LVH, there is an early activation of plasma and cardiac ACE. Early administration of an ACE inhibitor prevents the development of hypertension and LVH by inhibiting the increases of plasma and LV Ang II.

Angiotensin I-converting enzyme modulates neutral endopeptidase activity in the rat.

Angiotensin I is a substrate for both ACE and for neutral endopeptidase 24.11 (NEP). We hypothesized that high ACE expression is related to low NEP activity. Accordingly, circulating and tissue NEP and ACE activities were measured by fluorometry in homozygous rats (F(0) and F(2)) for the Lewis microsatellite allele (LL, low ACE) and for the Brown Norway microsatellite allele (BB, high ACE). Plasma, lung, and aortic ACE activities in F(0) and F(2) were higher in BB rats than in LL rats (P<0.01), whereas left ventricular ACE activity was similar in both genotypes. In contrast, NEP activity in the LL group was higher in the serum, aorta, and lungs in F(0) and F(2) homozygous (P<0.05). Plasma ACE activity was inversely correlated with serum (r=-0.6 and -0.598 in F(0) and F(2), respectively; P<0.03) and lung NEP activities (r=-0.77 in F(0) and r=-0.59 in F(2), P<0.01). Aortic ACE and NEP activities were also correlated (r=-0.696 and -0.584 in F(0) and F(2), respectively; P<0.03). In conclusion, genetically determined high ACE expression in rats is inversely related to tissue NEP activity, which could determine lower angiotensin-(1-7) tissue levels.

[Selection and evaluation of candidate patients for heart transplant]. / Selección y evaluación de pacientes candidatos a trasplante cardíaco.

Cardiac transplantation is now a well-accepted therapy for patients with advanced heart failure. In appropriately selected recipients, it has shown to significantly improve the survival and quality of life. The shortage of appropriate cardiac donor hearts, the costs of cardiac transplantation and its associated long-term medical follow-up, and the potential morbidity and mortality associated with the procedure and with life after transplantation mandates the judicious application of cardiac transplantation to appropriate recipients. A review of current indications, contraindications and evaluation of patients for cardiac transplantation is presented.

Prevalence of the angiotensin I converting enzyme insertion/deletion polymorphism, plasma angiotensin converting enzyme activity, and left ventricular mass in a normotensive Chilean population.

The aim of this study was to estimate the prevalence of the different alleles of the angiotensin converting enzyme (ACE) gene insertion/deletion (I/D) polymorphism and associated plasma ACE activity, as well as cardiac echocardiographic structure, in a healthy Chilean population. We selected 117 healthy normotensive subjects (aged 45 to 60 years, middle socioeconomic status, nonobese, and nondiabetic) from a population-based study concerning the prevalence of risk factors for chronic diseases (Conjunto de Acciones Para la Reducción Multifactorial de las Enfermedades no Transmisibles [CARMEN]). The frequencies of the I and D alleles were 0.57 and 0.43, respectively. Mean plasma ACE activity was 15.3 +/- 3.9 U/mL. Compared with subjects with the II genotype, plasma ACE activity was significantly higher in subjects with the ID and DD genotypes with no difference between them. No correlation was observed between blood pressure and plasma ACE activity. Among the three different genotypes there was no difference in left ventricular (LV) dimensions or in LV mass. No correlation between plasma ACE activity and LV mass was observed for either gender or different genotypes. Multivariate linear regression analysis using LV mass and LV mass index as dependent variables showed independent effects (P < .05) for gender (higher LV mass in men) and diastolic blood pressure, but not for the DD genotype. In conclusion, in this population, the presence of the D allele on the ACE gene determined higher circulating ACE activity. However, in this normotensive healthy population, male gender and diastolic blood pressure, but not the presence of the D allele, were associated with increased LV mass.

Reproducibility of plasma angiotensin-converting enzyme activity in human subjects determined by fluorimetry with Z-phenylalanine-histidyl-leucine as substrate.

Despite the major physiologic role of angiotensin-converting enzyme (ACE), few studies have evaluated the ideal conditions for measuring human plasma ACE activity, specifically when using Z-phenylalanine-histidyl-leucine as substrate. This study, performed in volunteer patients, assessed the reproducibility of human plasma ACE activity measured by fluorimetry with Z-phenyl-histidyl-leucine as the substrate. After blood centrifugation, plasma was stored under different conditions until processing. The following sources of variability were evaluated: (1) the interval to centrifugation of blood after collection, (2) the temperature and (3) safe time for storing the plasma after cold centrifugation, (4) the effect of fasting. Plasma ACE activity was 20.6+/-7.7 U/mL, 20.9+/-8 U/mL, and 20.5+/-7.9 U/mL (n = 25) when samples were centrifuged immediately, after 1 hour of blood sampling, and after 3 hours of blood sampling, respectively (not significant). In plasma kept at -20 degrees C, ACE activity was not different after 1 week (17.4+/-4.3 U/mL) nor after 1 month (17.9+/-4 U/mL), whereas baseline ACE was 16.7+/-4.3 U/mL (n = 10). In plasma stored at -80 degrees C, ACE activity was 15.5+/-5.7 U/mL after 1 month (baseline 15+/-5.3 U/mL; not significant; n = 12). No evidence for hydrolysis of the reaction product of ACE (his-leu dipeptide) was observed in plasma samples kept for 1 month at -20 degrees C or at -80 degrees C (by high-performance liquid chromatography analysis). In plasma obtained before breakfast, ACE activity was 12.8+/-7.1 U/mL, and it was 12.3+/-7.5 U/mL 2 hours afterwards (not significant; n = 12). Thus, to determine human plasma ACE activity by fluorimetry with reliability, with Z-phenylalanine-histidyl-leucine used as a substrate, there is a safe interval of at least 3 hours before blood centrifugation at -4 degrees C. Plasma may be kept at -20 degrees C or at -80 degrees C for at least 4 weeks before final processing. Fasting does not influence its enzymatic activity.

Effects of antihypertensive treatment on cardiac IGF-1 during prevention of ventricular hypertrophy in the rat.

There is some evidence that cardiac rather than circulating insulin-like growth factor-1 (IGF-1) levels contribute to the development of renovascular hypertensive left ventricular hypertrophy (LVH), remaining unknown the effects of antihypertensive drugs on IGF-1 levels. We have assessed here the preventive effects of enalapril, losartan, propanolol and alpha-methyldopa on left ventricle (LV) and circulating IGF-1 levels in a rat model of hypertension and LVH (Goldblatt, GB). Our results show that relative LV mass and the LV content of IGF-1 were significantly lower with all antihypertensive drugs in GB rats (p<0.001). Serum concentrations of IGF-1 were lower in GB rats treated with enalapril, alpha-methyldopa and propanolol (p<0.01), but not in those treated with losartan. These results support the hypothesis that local rather than seric IGF-1 contributes to the development of left ventricular hypertrophy induced by pressure overload in the rat.

Selective increase in cardiac IGF-1 in a rat model of ventricular hypertrophy.

There is evidence that insulin-like growth factor-1 (IGF-1) plays a role in the development of left ventricular hypertrophy, but it is uncertain whether cardiac IGF-1 changes before or after hypertension is established, and whether circulating IGF-1 are involved in cardiac hypertrophy. We have investigated changes in circulating and left ventricular IGF-1 and in the expression of the IGF-1 gene in the left ventricles of rats during the development of hypertensive left ventricular hypertrophy (Goldblatt model; 2 kidney-1 clamped). Our results show that the left ventricular contents of IGF-1 and its mRNA were increased at one and four weeks of hypertension and hypertrophy, and that both returned to control values after nine weeks. These changes were unrelated to the seric concentration of IGF-1 in the blood. These results show that local rather than circulating IGF-1 levels contributed to the development of renovascular hypertensive left ventricular hypertrophy.

Regression of left ventricular hypertrophy in experimental renovascular hypertension: diastolic dysfunction depends more on myocardial collagen than it does on myocardial mass.

OBJECTIVE: To evaluate regression of experimental left ventricular hypertrophy (LVH) in terms of its effects both on myocardial collagen levels and on diastolic stiffness. METHODS: Two-kidney, one clip Goldblatt hypertensive rats were left untreated for 4 weeks (HT4W, n = 12) or 12 weeks (HT12W, n = 11) and compared with rats the treatment of which was started after 4 weeks of hypertension with 30 mg/kg per day losartan for 8 weeks (LOS, n = 12), or 50 mg/l enalapril for 8 weeks (ENA, n = 11). A group of sham-operated rats served as controls (SHAM, n = 9). RESULTS: The blood pressure of the rats increased significantly and LVH developed both after 4 and after 12 weeks of hypertension. Treatment with losartan or enalapril significantly decreased blood pressure and induced complete regression of LVH. Myocardial hydroxyproline concentrations increased in groups HT4W and HT12W (530 +/- 153 and 581 +/- 111 micrograms/g, respectively) relative to that in the SHAM group (421 +/- 22 micrograms/g). None of the treatments induced regression of increased myocardial collagen levels. The slopes of the end-diastolic stress-strain relationships in the isolated beating hearts were significantly higher in HT4W, HT12W and in both treated groups compared with those in the SHAM group, indicating increased diastolic myocardial stiffness. CONCLUSION: Losartan and enalapril treatments decreased blood pressure and induced complete regression of LVH in this model of renovascular hypertension. In contrast, none of the treatments induced regression of increased myocardial collagen levels or reduced the abnormal left ventricular diastolic stiffness. These data suggest that diastolic dysfunction depends more on increased myocardial collagen levels than it does on myocardial mass in this model of pathological LVH.

Cyclic AMP-dependent protein kinase and mechanical heart function in ventricular hypertrophy induced by pressure overload or secondary to myocardial infarction.

The role of cyclic AMP-dependent protein kinase (PKA) and systolic function during the development of left ventricular hypertrophy (LVH) still remain uncertain. The aim of this work is to study PkA activity and mechanical heart function in two experimental heart hypertrophy models: specifically, one induced by pressure overload (Goldblatt model: two kidneys, one clamped, Gb); and another secondary to myocardial infarction (MI) generated by ligation of the left coronary artery. Hypertension in the Gb group becomes evident by the third and fourth week after surgery without any significant change in the corresponding sham group. The myocardial infarction group did not show any change in systolic pressure. Different degrees of LVH for the two experimental models were observed. Relative cardiac mass (RCM) and relative ventricular mass (RVM) increased 23 and 16%, respectively, above the sham-operated rats in MI group (P < 0.05). For the pressure overload model, the increase values were 42 and 44%, respectively (P < 0.05). Left ventricular hypertrophy was also evaluated through quantitative changes in cardiac beta-myosin heavy chain which agreed with morphometric studies in Goldblatt rats. Ventricular PKA activity did not show any significant difference with respect to the sham-operated group after induction of pressure overload. For the MI model, ventricular PKA activity changed only at day 7 post-infarction with a 289% increase above the sham-operated group (P < 0.05). The absence of activation of ventricular PKA after constriction of renal artery or myocardial infarction was also corroborated by the patterns of PKA-dependent phosphorylated proteins. While force-generating capacity was increased, there was no change in ventricular PKA activity, indicating that there is no relation between this enzyme and systolic stress-strain regression lines in either pressure overload or myocardial infarction conditions. Cyclic AMP-dependent protein kinase activity had no relation with development of cardiac hypertrophy in the two experimental models of LVH. These findings contribute to the hypothesis for a multifactorial interaction of different intracellular biochemical and molecular mechanisms in the genesis of cardiac hypertrophy.

Amiodarone protection against myocardial injury and fibrosis induced by isoprenaline is abolished by thyroid hormone.

OBJECTIVE: Catecholamines induce myocyte necrosis and myocardial fibrosis. These effects are probably related to beta adrenergic receptor stimulation and to thyroid hormonal status. The aim of the study was to test the hypothesis that amiodarone prevents myocardial damage induced by isoprenaline and that these effects are not observed when thyroid hormone is administered. METHODS: Myocardial injury was assessed in the first experiment. Isoprenaline (1 mg.kg-1 subcutaneously once) was given to two groups of rats which received monoclonal antimyosin. Group 1 (n = 5) was pretreated with amiodarone and group 2 (n = 6) received only isoprenaline. In the second experiment, the effects of amiodarone on isoprenaline induced myocardial fibrosis and of supplementary triiodothyronine (T3) were examined. Group 3 (n = 10) received only isoprenaline for 4 d. Group 4 (n = 10) received amiodarone for 14 d and isoprenaline during 4 d. Group 5 (n = 8) received amiodarone and isoprenaline like group 4, plus T3. Untreated rats served as controls (group 6; n = 10). Collagen volume fraction (CVF) was measured in each heart. RESULTS: No rats pretreated with amiodarone showed antimyosin labelling, while the mean score of rats receiving only isoprenaline was 2.8 (p < 0.05), indicating the presence of significant myocyte injury. In group 3, CVF was significantly higher than in controls, at 7.63(SEM 0.89)% v 1.74(0.07)%, p < 0.001, whereas rats pretreated with amiodarone (group 4) showed significantly less fibrosis [CVF 2.96(0.19)%]. This protective effect was lost when amiodarone and T3 were given together [CVF 7.92(1.8)%, p < 0.005 between groups 4 and 6]. CONCLUSIONS: By preventing isoprenaline induced myocardial injury and fibrosis, amiodarone may have a cardioprotective role. This effect is completely abolished by thyroid hormone.

Changes in cyclic AMP dependent protein kinase and active stiffness in the rat volume overload model of heart hypertrophy.

OBJECTIVE: The aim was to clarify the role of cyclic AMP dependent protein kinase (PKA) and changes in mechanical heart function during development of cardiac hypertrophy induced by volume overload. METHODS: Protein and DNA contents, PKA activity, and peak systolic stress-strain relationships in hearts from animals submitted to aortocaval shunt were assessed as a function of time. Sham operated (control) rats were used as controls. RESULTS: Heart weight to body weight ratio and cardiac protein content per heart increased from d 7 (p < 0.005 and p < 0.01, respectively) reaching their highest values by d 56; the same occurred with cardiac DNA content. PKA activity.g-1 tissue in soluble extracts of hearts from rats with aortocaval shunt increased by 2.7-fold on d 2 (p < 0.005), reached a ninefold peak increase by d 7 (p < 0.0001) and declined to fourfold by d 56 with respect to control values. The end peak systolic stress-strain relation slopes were: control, 368(SEM 14) g.cm-2 (n = 16); aortocaval shunt values: 2 d, 514(28) g.cm-2 (n = 6); 7 d, 579(10) g.cm-2 (n = 7); and 56 d, 554(28) g.cm-2 (n = 7). The force generating capacity at 0% strain was also significantly higher in the shunt groups as compared to sham operated controls (p < 0.01). Early activation of PKA was also confirmed through endogenous cardiac protein phosphorylation. SDS-PAGE gel electrophoretogram and autoradiography showed more heavily phosphorylated bands in aortocaval shunt hearts than in the control group. CONCLUSIONS: PKA activity and the slope of systolic stress-strain regression line followed a similar trend throughout the study, with an early increase in both variables by d 2 in the shunt group, reaching a peak at d 7, and decreasing thereafter but remaining higher than in controls. PKA activity appears to be related to increased force generating capacity rather than to hypertrophy or increased cardiac protein content. Thus PKA activation is an early biochemical event after aortocaval shunt, followed later by cardiac hypertrophy. Changes in PKA activity showed a similar trend to mechanical heart function over time. These findings help to explain the changes in the mechanical properties of the heart preceding the development of cardiac hypertrophy in the rat model of volume overload.

[Evaluation of the effect of enalapril, a converting enzyme inhibitor, on lymphocytic beta-adrenergic receptors of patients with chronic cardiac insufficiency]. / Evaluación del efecto de enalapril, inhibidor de la enzima conversiva, en los receptores betaadrenérgicos de linfocitos de pacientes con insuficiencia cardíaca crónica.

We have previously corroborated that lymphocyte beta-adrenergic receptor density is significantly reduced in patients with chronic heart failure. It is well known that angiotensin converting enzyme inhibitors normalize the function of sympathetic nervous system. We have assessed the effect of enalapril on lymphocyte beta-adrenergic receptor system from patients with chronic heart failure (n = 14) using a random, cross and double blind protocol. Our results show that the improvement in clinical score and ventricular function were not related with changes in the number and affinity of beta-adrenergic receptor nor cyclic AMP content in lymphocytes obtained from these patients.

Cardiac myocyte necrosis induced by angiotensin II.

Although the role of angiotensin II (Ang II) in the pathogenesis and progression of the failing heart is uncertain, previous reports have suggested that myocyte injury may be a component in this process. In this study, we investigated this possibility in more detail. Cardiotoxic effects of nonacutely hypertensive doses of Ang II were examined in 90 rats, including those receiving an angiotensin infusion (200 ng/min i.p.) and those with renovascular hypertension, where endogenous stimulation of Ang II occurred. Myocyte injury and wound healing resulting from these treatments were evaluated by 1) immunofluorescence after in vivo monoclonal antibody labeling of myosin to detect abnormal sarcolemmal permeability, 2) [3H]thymidine incorporation into DNA, to detect fibroblast proliferation, and 3) light microscopic evidence of myocytolysis and subsequent scar formation. We found that exogenous Ang II produced multifocal antimyosin labeling of cardiac myocytes and myocytolysis, which were maximal on days 1-2 of the infusion. Subsequently, DNA synthesis rates were increased, with fibroblast proliferation reaching peak levels on day 2 (Ang II-treated rats, 90.0 +/- 18.6 cpm/micrograms DNA; control rats, 11.4 +/- 2.3 cpm/micrograms DNA; p less than 0.05); microscopic scarring was found on day 14 and represented 0.12 +/- 0.02% of the myocardium. Concurrent treatment with both propranolol (30 mg/kg/day s.c.) and phenoxybenzamine (5 mg/kg/day i.m.) did not attenuate Ang II-induced antimyosin labeling. Increased endogenous Ang II, resulting from renal ischemia after abdominal aortic constriction, produced both antimyosin labeling and increased rates of DNA synthesis like that observed with Ang II infusion. Both myocyte injury and fibroplasia were prevented with captopril (65 mg/day p.o.), but this protective effect was not seen with reserpine pretreatment. Infrarenal aortic banding without renal ischemia, on the other hand, produced hypertension without necrosis. We conclude that pathophysiological levels of endogenous as well as low-dose exogenous Ang II were associated with altered sarcolemmal permeability and myocytolysis with subsequent fibroblast proliferation and scar formation. Myocyte injury was unrelated to the hypertensive or enhanced adrenergic effects of Ang II or to hypertension per se. Captopril was effective in preventing myocyte injury in renovascular hypertension. The mechanism(s) responsible for Ang II-induced necrosis will require further study.

Coronary vascular remodeling and myocardial fibrosis in the rat with renovascular hypertension. Response to captopril.

Progressive myocardial fibrosis, including the accumulation of collagen within the adventitia of intramyocardial coronary arteries, is seen in the hypertrophied rat myocardium secondary to renovascular hypertension (RHT) and has been held responsible for alterations in myocardial diastolic stiffness. This study was undertaken to test the hypothesis that this presumptive angiotensin-aldosterone mediated fibrosis and its functional consequences could be favorably altered by an antihypertensive oral dose (50 mg/kg/day) of the angiotensin converting enzyme (ACE) inhibitor captopril. Three groups were studied: control; untreated RHT for 8 weeks; treated RHT, with captopril started 48 h before banding and continued for 8 weeks. Interstitial collagen volume fraction and perivascular collagen area (morphometry), the fibrillar nature of collagen (picrosirius polarization), and the end diastolic stress-strain relation of the intact left ventricle were examined in each group. In comparison to untreated animals with RHT, we found that captopril, begun prior to banding, attenuated interstitial and perivascular fibrosis and prevented hypertrophy and the rise in diastolic stiffness 8 weeks later. Thus, an adverse accumulation of collagen in the interstitium and around intramyocardial coronary arteries, and its functional consequences in the rat with RHT, can be prevented by captopril. Other ACE inhibitors may have similar salutary effects, but remain to be evaluated. The pathogenetic origin of myocardial fibrosis in RHT requires further investigation, but appears to be related to the angiotensin-aldosterone system.

[Post-infarct myocardial remodelling: from experimental data to the prevention of heart failure]. / Remodelamiento miocárdico post infarto del miocardio: desde los datos experimentales a la prevención de insuficiencia cardiaca.

Structural and biochemical modifications of the myocardium (remodeling) occur after acute myocardial infarction. An important part of this process of myocardial remodeling takes place in the interstitial compartment which is composed mainly of fibrillar collagen. These remodeling changes are associated with modifications in left ventricular geometry and function that could be deleterious and have significant clinical manifestations. Some salutary effects of the treatment are related to modifications of the process of interstitial remodeling. Clinical studies with calcium channel antagonists, nitrates and, specially converting enzyme inhibitors have shown significant improvement in the degree of ventricular dilation, hemodynamics and exercise tolerance as a compared to placebo treated patients. Ongoing clinical studies will provide us with more definite information on the effects of converting enzyme inhibitors on long term prognosis as well as on myocardial remodeling after acute myocardial infarction.

Reactive and reparative fibrillar collagen remodelling in the hypertrophied rat left ventricle: two experimental models of myocardial fibrosis.

STUDY OBJECTIVE: The aim was to compare the temporal sequence and structural relationship between perivascular and interstitial fibrosis and microscopic scarring seen in the left ventricle in response to either a transient or sustained stimulus to fibrosis. DESIGN: In 72 male Wistar rats (250-350 g) the transient stimulus model was based on the administration of isoprenaline (500 micrograms.kg-1) while the sustained stimulus model was produced by abdominal aortic banding with right renal artery constriction. Serial sections of myocardium were examined and compared at 4 and 12 weeks in each model and to corresponding controls. EXPERIMENTAL MATERIAL: The collagen specific stain, Sirius Red F3BA, was used to determine collagen volume fraction and the fibrillar nature of the fibrous tissue response seen by light microscopy. MEASUREMENTS AND MAIN RESULTS: Following isoprenaline a stable reparative fibrosis of the endomyocardium and increase in collagen volume fraction was seen without an interstitial or perivascular fibrosis of the non-involved myocardium. In unilateral renal ischemia, on the other hand, a progressive perivascular fibrosis was evident throughout the myocardium and from which fibrillar collagen extended into the extracellular space between muscle bundles creating an interstitial fibrosis; microscopic scarring of the endomyocardium became evident at 12 weeks. CONCLUSIONS: The reactive perivascular fibrosis of intramyocardial coronary arteries seen in renovascular hypertension is a progressive process that leads to an interstitial fibrosis and eventual microscopic scarring. In contrast, the endomyocardial scarring that follows isoprenaline induced myocyte necrosis is stable and intramural vessels in remote regions are not involved.

Myocardial collagen remodeling in pressure overload hypertrophy. A case for interstitial heart disease.

The accumulation of collagen within the myocardium is termed fibrosis. In left ventricular pressure overload a reactive interstitial fibrosis, having distinctive biochemical and structural features, is seen. This reactive fibrosis occurs in the absence of myocyte necrosis, is progressive in nature, and initially is an adaptive response that preserves the force generating capacity, or active (systolic) stiffness, of the hypertrophied myocardium. Later in hypertrophy a reparative (or replacement) fibrosis occurs in response to cell loss, the pathogenesis of which is not clear. Nevertheless, independently of cell loss, interstitial fibrosis can have a detrimental influence on the diastolic and systolic stiffness of the myocardium and can result in pathologic hypertrophy with heart failure. In established hypertrophy with disproportionate collagen matrix remodeling (ie, interstitial heart disease), it would be desirable to retard the continued formation of collagen and, if necessary, degrade collagen fibers that are responsible for impeding the stretching and shortening of muscle fibers. Prevention of interstitial fibrosis in pressure overload hypertrophy with pharmacologic agents with both antihypertensive and antifibrotic properties must also be considered. Future research should address these issues with a view toward developing corrective and preventative forms of therapy. Such advances will require a better understanding of cardiac fibroblast growth, collagen synthesis and the regulation of collagen gene expression in the heart.

Patterns of myocardial fibrosis.

The interstitium of the myocardium is composed of predominantly type I collagen; type III collagen is present to a lesser extent. The fibrillar collagens serve as tethers between muscle cells, muscle fibers, and blood vessels while also providing a scaffolding that supports the muscular and vascular compartments. In pressure overload hypertrophy, a continuous structural remodeling of the fibrillar collagen matrix is seen. What is initially an adaptive process that enhances tensile strength can eventuate in pathologic hypertrophy with muscle fiber entrapment, cell loss, and abnormal diastolic and systolic stiffness of the myocardium. Morphologically distinct patterns of myocardial collagen accumulation, or fibrosis, have been identified based on the alignment of thick and thin collagen fibers to one another and to cardiac muscle. Each pattern, representing either a reactive (without necrosis) or reparative process, can alter stiffness in a unique manner. The manner in which the interstitium regulates the nature and proportion of fibrillar collagen formation is unknown and deserving of further study. Such information may lead to the development of antifibrotic agents that counteract, prevent or modify disproportionate collagen remodeling in pressure overload hypertrophy. These agents may thereby ultimately represent corrective forms of therapy for the management of heart failure.

Isoproterenol-induced myocardial fibrosis in relation to myocyte necrosis.

Treatment of rats with the beta-adrenergic agonist isoproterenol results in cardiac hypertrophy, myocyte necrosis, and interstitial cell fibrosis. Our objectives in this study have been to examine whether hypertrophy and fibrosis occur in a compensatory and reparative response to myocyte loss or whether either process may be occurring independently of myocyte loss and thus be a reactive response to adrenergic hormone stimulation. We have examined this question by evaluating each of these responses in rats treated with different doses and forms of isoproterenol administration. Myocyte necrosis was evaluated using in vivo labeling with monoclonal antimyosin for identification of myocytes with permeable sarcolemma, which was indicative of irreversible injury. Myocardial fibrosis was evaluated by morphometric point counting of Gomori-stained tissue sections and by assessment of the stimulation of fibroblast proliferation by determination of increased levels of DNA synthesis. Stimulation of fibroblast DNA synthesis was determined from DNA specific radioactivities and radioautography after pulse labeling with [3H]thymidine. The evidence provided by this study suggests that the degree and timing of myocardial hypertrophy does not follow the course of myocyte loss and, thus, appears to be either a response to altered cardiac loading or a reactive response to beta-adrenergic hormone stimulation rather than a compensation for myocyte loss. Myocardial fibrosis, on the other hand, appears to be more closely related to myocyte necrosis with respect to collagen accumulation in the same areas of the heart, its dose-response relation to the amount of isoproterenol administered, and the timing of increased DNA synthesis, or fibroblast proliferation, after myocyte loss.