Intracardiac detection of angiotensinogen and renin: a localized renin-angiotensin system in neonatal rat heart

Local stress, not systemic factors, regulate gene expression of the cardiac renin-angiotensin system in vivo: a comprehensive study of all its components in the dog

Cardiac hypertrophy is associated with altered expression of the components of the cardiac renin-angiotensin system (RAS). While in vitro data suggest that local mechanical stimuli serve as important regulatory modulators of cardiac RAS activity, no in vivo studies have so far corroborated these observations. The aims of this study were to (i) examine the respective influence of local, mechanical versus systemic, soluble factors on the modulation of cardiac RAS gene expression in vivo; (ii) measure gene expression of all known components of the RAS simultaneously; and (iii) establish sequence information and an assay system for the RAS of the dog, one of the most important model organisms in cardiovascular research. We therefore examined a canine model of right ventricular hypertrophy and failure (RVHF) in which the right ventricle (RV) is hemodynamically loaded, the left ventricle (LV) is hemodynamically unloaded, while both are exposed to the same circulating milieu of soluble factors. Using specific competitive PCR assays, we found that RVHF was associated with significant increases in RV mRNA levels of angiotensin converting enzyme and angiotensin II type 2 receptor, and with significant decreases of RV expression of chymase and the angiotensin II type 1 receptor, while RV angiotensinogen and renin remained unchanged. All components remained unchanged in the LV. We conclude that (i) dissociated regional regulation of RAS components in RV and LV indicates modulation by local, mechanical, not soluble, systemic stimuli; (ii) components of the cardiac RAS are independently and differentially regulated; and (iii) opposite changes in the expression of angiotensin converting enzyme and chymase, and of angiotensin II type I and angiotensin II type 2 receptors, may indicate different physiological roles of these RAS components in RVHF.

Mechanical stretch induces enhanced expression of angiotensin II receptor subtypes in neonatal rat cardiac myocytes

Mechanical stress plays a pivotal role in the development of cardiac hypertrophy during hemodynamic overload, and angiotensin (Ang) II secreted from stretched myocytes plays an important role in mechanical stretch-induced hypertrophy. In the present study, we examined stretch-induced expression of Ang II receptors in an in vitro stretch model using 1-day-old rat myocytes. Both Ang II type 1 receptor (AT1-R) and type 2 receptor (AT2-R) mRNA levels were upregulated by myocyte stretching with similar time courses: significant increases were evident 6 hours after stretching, maximal levels (2.8- and 3.3-fold, respectively) were observed at 12 hours, and these were sustained for up to 18 hours. Ang II receptor expression in fibroblast-rich cultures was not affected by stretching. Conditioned medium in which myocytes were stretched for 12 hours significantly downregulated AT1-R and AT2-R mRNA levels in recipient myocytes, and this effect was almost completely blocked by AT1-R antagonists but not AT2-R antagonists. Stretch-induced expression of AT1-R and AT2-R mRNAs was further increased by 27% and 31%, respectively, after pretreatment with AT1-R antagonists, suggesting that Ang II secreted from stretched myocytes downregulates both AT1-R and AT2-R. Western blot and binding assays showed that the number of AT1-Rs and AT2-Rs increased by 2.4- and 2.6-fold, respectively, without affecting receptor affinities. Inositol phosphate response to 0.5 mumol/L Ang II was enhanced 2.1-fold in stretched myocytes. Nuclear runoff assays and treatment with actinomycin D revealed that stretch-induced upregulation of AT1-R was mainly due to increased transcription, whereas that of AT2-R resulted from a stabilizing effect on AT2-R mRNA metabolism. Stretch-induced changes in levels of Ang II receptors were inhibited by genistein but not by H-7, staurosporin, and protein kinase C depletion or by BAPTA-AM. Exposure to cycloheximide did not affect stretch-induced changes. These findings indicate that nonsecretory pathways activated by myocyte stretching upregulate the expression of Ang II receptor subtypes transcriptionally and posttranscriptionally through mechanisms involving stretch-activated tyrosine kinases independently of de novo protein synthesis and that the AT1-R-mediated action of Ang II is functionally enhanced in stretched cardiac myocytes.

Endogenous cardiac vasoactive factors modulate endothelin production by cardiac fibroblasts in culture

Endothelin, a potent vasoconstrictor, is produced by cardiac fibroblasts in culture and induces hypertrophy in cardiac myoctes. The purpose of this study was to determine whether vasoactive factors endogenous to the heart affect the production of endothelin by cultured cardiac fibroblasts. Vasoactive factors have been shown to play multiple roles in the adaptation of the heart to chronic overload, affecting both vascular tone and cell growth. Both atrial (ANP) and brain (BNP) natriuretic peptides are endogenous cardiac vasodilators and are produced by cultured myocytes in response to stimulation with endothelin. Treatment of cardiac fibroblasts with these peptides decreased endothelin production. Nitroprusside, an activator of guanylyl cyclase, decreased endothelin production indicating the involvement of cGMP in the response. Carbaprostacyclin, a stable derivative of prostacyclin, another endogenous cardiac vasodilator, also decreased endothelin production by fibroblasts. The combination of BNP and carbaprostacyclin was additive in decreasing endothelin production. In contrast, PGF2α and angiotensin II, both endogenous cardiac vasoconstrictors, increased endothelin production and overcame the inhibition induced by BNP and carba-prostacyclin. In summary, endothelin production by cardiac fibroblasts was decreased by the endogenous cardiac vasodilators ANP, BNP, and prostacyclin and increased by the endogenous vasoconstrictors PGF2α and angiotensin II.

Valvular interstitial cells express angiotensinogen and cathepsin D, and generate angiotensin peptides

Cells capable of de novo angiotensin (Ang)II generation in the heart remain unidentified. High-density angiotensin converting enzyme (ACE) binding has been localized to sites of high collagen turnover, such as heart valve leaflets and their valvular interstitial cells (VIC). VIC express ACE mRNA and their membrane-bound ACE utilizes AngI as substrate. Whether VIC also express angiotensinogen (Ao) and an aspartyl protease, and whether they generate AngI and II de novo, is presently unknown. We sought to address these questions in serum-deprived cultured VIC. Ao, renin and cathepsin D (Cat-D) mRNA expression was addressed by RT-PCR. Production of Ao, AngI and AngII peptides were measured in VIC-culture media by radioimmunoassay (RIA). Immunoreactive Cat-D was detected by immunofluorescein labeling and Western blotting. Cat-D and renin activities were determined by spectrofluorometric and autoradiographic methods and AngI generation by RIA. Results showed (a) expression of Ao and Cat-D both at mRNA and protein levels; (b) AngI and AngII peptides in culture media; (c) acceleration of AngII production by exogenous AngI (1 nmol/l), which was blocked by lisinopril (0.1 mumol/l); (d) that dexamethasone (0.1 mumol/l) increased AngII production; (e) a 46 kDa immunoreactive Cat-D protein by Western blotting; (f) aspartyl protease activity, using chromogenic and 125I-labeled Ao as substrates, inhibited by pepstatin-A; and (g) the absence of renin mRNA and activity. It is concluded that at both the mRNA and protein levels, cultured VIC express Ao and Cat-D, and can generate AngI and AngII peptides by the action of a non-renin protease Cat-D and ACE, respectively. VIC therefore appear to represent a constitutive nonendothelial cell found in adult rat heart valve leaflets, which are capable of de novo Ang peptide generation.

Wound healing following myocardial infarction

A wound-healing response that eventuates in fibrous tissue formation appears at the site of myocardial infarction (MI) in the affected ventricle. Fibrosis can likewise appear remote to the MI and cause an extensive structural remodeling of the myocardium of infarcted and noninfarcted ventricles. Substances involved in promoting healing at and remote to MI are of considerable interest and an important clinical issue, given that the healing response is subject to pharmacologic intervention. Angiotensin-converting enzyme (ACE) is expressed by wound-healing fibroblast-like cells; it likely serves to regulate local concentrations of angiotensin II and bradykinin involved in healing and matrix remodeling. Wound healing following MI and its regulation are addressed in this review.

Angiotensin II receptor expression in the conduction system and arterial duct of neonatal and adult rat hearts

Paralleling the classic circulating system, recent evidence has demonstrated the presence of a cardiac renin-angiotensin system, as well as the synthesis of angiotensin II in the heart. Two receptors for angiotensin II have been identified and classified as AT1 and AT2. The proportions of these receptor subtypes vary with the tissues, species and stage of development. From the results of other studies, it might be generalized that the expression of angiotensin II receptors and the proportion of AT2 receptor subtype are much higher in fetal and neonatal tissues than in the same tissues from an adult. The aim of this study was to specifically evaluate the AT1/AT2 ratio in the neonatal and adult conduction systems of rat hearts by means of quantitative autoradiogrphy. In the neonatal hearts, angiotensin II binding sites were highly concentrated in the vasculature, arterial duct, and conduction system, whereas their concentrations were barely detectable in the myocardium. Incubation with selective angiotensin II receptor ligands (losartan and CGP 42112) revealed that AT2 was the major subtype in vasculature (86 +/- 3%) and conduction system (73 +/- 4%). In the adult conduction system, the total expression of angiotensin II receptors was greatly reduced meanwhile the AT1 receptors represented the major proportion of the binding sites (80 +/- 3%). Our results demonstrated that the pattern of angiotensin II receptor expression in the conduction system of the rat heart is developmentally regulated. We suggest, as others have already, that the renin-angiotensin system plays a role during the early stage of cardiac development.

Angiotensin II signalling pathways in cardiac fibroblasts: conventional versus novel mechanisms in mediating cardiac growth and function

Angiotensin II has been demonstrated to be involved in the regulation of cellular growth of several tissues in response to developmental, physiological, and pathophysiological processes. Angiotensin II has been implicated in the developmental growth of the left ventricle in the neonate and remodeling of the heart following chronic hypertension and myocardial infarction. The inhibition of DNA synthesis and collagen deposition in myocardial interstitium following myocardial infarction by angiotensin converting enzyme inhibitor, suggests that angiotensin II mediates interstitial and perivascular fibrobrosis by preventing fibroblast proliferation. In the past, little attention was focused on the identity and functional roles of cardiac fibroblasts. Recent in vitro studies utilizing cultured cardiac fibroblasts demonstrate that angiotensin II, acting via the AT1 receptor, initiates intracellular signalling pathways in common with those of peptide growth factors. Below, we describe growth-related aspects of cardiac fibroblasts with respect to angiotensin II receptors, conventional and novel signal transduction systems, secretion of extracellular matrix proteins and growth factors, and localization of renin-angiotensin system components.

Aging does not affect the activation of the myocyte insulin-like growth factor-1 autocrine system after infarction and ventricular failure in Fischer 344 rats

To determine whether the attenuation in the growth capacity of myocytes in the overloaded aging heart is associated with an impairment in the activation of insulin-like growth factor-1 (IGF-1) and its receptor (IGF-1R) in the stressed cells, large myocardial infarcts were produced in Fischer 344 rats at 4 and 16 months of age, and the animals were killed 6 hours, 3 days, and 7 days later. After the documentation of cardiac failure, the unaffected myocytes were enzymatically dissociated, and the expression of IGF-1 and IGF-1R was measured at these three time points after surgery. The level of expression of IGF-1R mRNA increased at 3 days and remained elevated at 7 days in both age groups. In addition, an increase in IGF-1R protein in these cells was found, with no apparent difference with age. This phenomenon was coupled with an upregulation of IGF-1 mRNA of comparable magnitude in the younger and older animals. In contrast, the increases in the dimensional properties of myocytes were delayed and of smaller magnitude in the older infarcted rats. Moreover, the expression of atrial natriuretic factor, used as a molecular marker of myocyte cellular hypertrophy, was greater at 3 days in 4-month-old rats and at 7 days in 16-month-old rats. Thus, aging may affect the hypertrophic response of myocytes after infarction but has no impact on the ability of the cells to enhance the expression of IGF-1 and IGF-1R, which may sustain only in part the growth reserve mechanisms of the pathological heart.

Effects of angiotensin II and aldosterone on collagen gene expression and protein turnover in cardiac fibroblasts

Earlier studies have demonstrated angiotensin II (AngII) and aldosterone (ALDO) each augment cultured adult rat cardiac fibroblast (CFb) collagen synthesis. Whether this involves type I collagen, the major structural protein of the myocardium, and represents a transcriptional event, is uncertain. Accordingly, the influence of AngII and ALDO on transcription and synthesis of fibrillar collagen and on collagenolytic activity was examined in cultured CFb maintained in serum-deprived media. Using concentrations for AngII (10(-7) M) or ALDO (10(-9) M), shown to influence collagen turnover in these cells, we found: a) total collagen synthesis was significantly (p < 0.05) increased (5.4 +/- 0.41 and 4.8 +/- 0.37 vs. control 3.1 +/- 0.55); b) type I collagen production (6590 +/- 710 and 6150 +/- 410 vs. control 4700 +/- 490 ng/mL) in the medium were significantly (p < 0.01) increased; c) type I collagen mRNA expression was also significantly (p < 0.01) increased by AngII (2.0 fold) and ALDO (1.8 fold) compared with control; d) AngII, but not ALDO, significantly (p < 0.05) decreased collagenolytic activity (0.5 fold) compared with control. Thus, AngII and ALDO each increase CFb type I collagen synthesis at the level of transcription and protein synthesis and AngII, but not ALDO, alters collagenolytic activity. Such hormonally mediated alterations in CFb collagen turnover may contribute to the adverse accumulation of fibrillar collagen found in the myocardium in various disease states, where circulating AngII and/or ALDO are increased.

Cardiac effects of AII. AT1A receptor signaling, desensitization, and internalization

Angiotensin II receptors present in cardiomyocytes, nonmyocytes (predominantly fibroblasts), nerve terminals, and the heart vasculature mediate the multiple actions of angiotensin II (AII) in the heart, including modulation of normal and pathophysiological cardiac growth. Although the cellular processes that couple AII receptors (principally the AT1 subtype) to effector responses are not completely understood, recent studies have identified an array of signal transduction pathways activated by AII in cardiac cells. These include: the stimulation of phospholipase C which results in the activation of protein kinase C and the release of calcium from intracellular stores; an enhancement of phosphaditic acid formation; the coupling to soluble tyrosine kinase phosphorylation events; the initiation of the mitogen activated protein kinase (MAPK) cascade; and the induction of the STAT (Signal Transducers and Activators of Transcription) signaling pathway. It is tempting to speculate that these latter responses, which have been previously associated with growth factor signaling pathways, are involved in AII-induced cardiac growth. Interestingly, some of these novel pathways are apparently not under the same strict control imposed upon the more classical signaling pathways. Thus, while AII-induced calcium transients are rapidly (within minutes) desensitized following exposure to AII, the MAP kinase pathway is not, and activation of the STAT pathway requires hours of agonist exposure for maximal induction. These observations support an emerging picture in which the downstream signal transduction pathways of AII receptors are initiated and terminated with a distinct temporal arrangement. This organization allows appropriate rapid responses (e.g. vascular contraction) to transient AII exposure, some of which are rapidly terminated, perhaps for protective reasons, and others not. In contrast, additional responses (e.g. growth) probably require prolonged exposure to agonist.

Upregulation of cardiac angiotensin II AT1 receptors in congenital cardiomyopathic hamsters

Angiotensin II (Ang II) is a growth factor that stimulates protein synthesis and induces cellular hypertrophy in cardiac myocytes. To gain insight into the role of Ang II in cardiac hypertrophy, we examined the expression and subtype distribution of Ang II receptors in the ventricles of embryonic and of 25- to 350-day-old inbred control and cardiomyopathic (CHF 146) hamsters. Studies were also performed with heterozygous (cardiomyopathic x control) animals. Compared with the control hamsters, cardiomyopathic hamsters presented decreased body weights and increased ratios of ventricular weight to body weight in every adult group studied. Typical histological lesions appeared in the left ventricle of cardiomyopathic animals around 70 to 75 days, and their severity increased with time. Radioligand binding studies with cardiac ventricular membranes indicated that iodinated [Sar1,Ile8]Ang II (sarile) binds to a homogeneous population of sites in membranes derived from adult normal and cardiomyopathic animals. Competition curves using specific receptor subtype antagonists revealed that 125I-sarile binding sites were exclusively of the AT1 subtype in both groups of animals. Importantly, the density of AT1 receptors was found to be significantly increased (90% augmentation at 70 to 75 days) in the ventricles of cardiomyopathic hamsters. This augmented expression was observed in all adult groups and was already present at 25 days, when no histological lesions were visible. The affinity of the receptor for losartan did not vary significantly between adult normal and cardiomyopathic animals (mean Kd, 19.6 and 16.7 nmol/L, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of intracellular renin on heart cell communication

The influence of intracellular renin and angiotensinogen on the control of cell-to-cell communication in heart muscle was investigated in cell pairs isolated from adult rat ventricle. Junctional conductance was measured with two separated voltage-clamp circuits. Intracellular dialysis of renin (0.2 pmol/L) caused a decrease in junctional conductance of 29 +/- 3.8% (+/- SEM, P < .05) in 7 minutes. The effect of renin on junctional conductance seems to be mainly due to the synthesis of Ang II because enalaprilat (10(-9) mol/L) dialyzed into the cell caused an appreciable reduction in the effect of renin. The intracellular administration of renin (0.2 pmol/L) plus angiotensinogen (0.4 pmol/L) produced a faster and stronger fall in junctional conductance (84.3 +/- 1.35%, P < .05), and the effect was greatly reduced by enalaprilat. The effects of both renin and angiotensinogen on junctional conductance were not related to a fall in surface cell membrane resistance or a change in series resistance. The effect of renin on junctional conductance was blocked by intracellular administration of a renin inhibitor (S 2864). Moreover, renin dialyzed into just one cell of the pair induced rectification of the junctional membrane, which was prevented by enalaprilat. The results support the view that an intracrine renin-angiotensin system in the heart regulates intercellular communication.

The distribution of angiotensin II type 1 receptors, and the tissue renin-angiotensin systems

Since its discovery, the functions of the renin-angiotensin system (RAS) have attracted a great deal of attention, and the roles it plays under normal conditions, and in disease, acquire a deepening significance with every year. In general, the RAS has been considered largely in terms of its roles in sodium and potassium homeostasis and the regulation of blood pressure. The continued acquisition of information on the distribution of angiotensin receptors, however, emphasizes that our interpretation needs to be widened, and it is now clear that angiotensin II has an array of functions in the tissues, which are unrelated to its systemic roles. This paracrine function is brought about by the existence of complete, localized tissue RASs, which respond to physiological demand independently from the systemic system.

Connective tissue and repair in the heart. Potential regulatory mechanisms

The heart is composed of highly differentiated cardiac myocytes, which constitute parenchyma, and stroma or connective tissue. Fibrillar collagen turnover in the heart and its valve leaflets, in particular, is dynamic and essential to tissue repair. Emerging evidence further suggests connective tissue is a metabolically active entity, where peptide hormones are generated and degraded and, in turn, these peptides regulate collagen turnover. This concept arose from quantitative in vitro autoradiography using an iodinated derivative of lisinopril (125I-351A) as ligand to localize angiotensin converting enzyme (ACE) binding density within the heart. A heterogeneous distribution was found: low-density ACE binding within atria and ventricles; high ACE binding density at sites of high collagen turnover, such as valve leaflets, adventitia, and fibrous tissue of diverse etiologic origins. ACE-producing cells at these latter sites were identified by monoclonal ACE antibody. They included valvular interstitial cells (VIC) and fibroblast-like cells each of which also contained alpha-smooth muscle actin and the transcript for type I collagen (in situ hybridization). Substrate utilization in cultured VIC was found to include angiotensin I and bradykinin. Angiotensin II and bradykinin receptor-ligand binding was observed in VIC and at fibrous tissue sites. Connective tissue ACE is independent of circulating angiotensin II. In vivo, fibrous tissue formation is attenuated by ACE inhibition or antagonism of AT1 receptor. Angiotensin II and bradykinin are stimulatory and inhibitory, respectively, to cultured adult cardiac fibroblast collagen synthesis suggesting a paradigm of reciprocal regulation to fibroblast collagen turnover. Stroma and its cellular constituents represent a dynamic metabolic entity that regulates its own peptide hormone composition and turnover of fibrillar collagen. These findings may provide insights that could be used to advantage to either promote or forestall fibrous tissue formation depending on the nature of cardiovascular disease.

Function of human renin proximal promoter DNA

An understanding of the mechanisms involved in the control of the human renin promoter have been hampered and confounded in work to date because of deficiencies in material available and experimental design. The promoter appears to be weak and a good cell model is lacking. Chorio-decidual cultures have been used since these have high renin synthesis, are readily available and grow well in culture. They suffer, however, from phenotypic variability and do not transfect well in transient expression analyses. Recent evidence suggests that 2.6 kb of proximal 5'-flanking DNA is unable to induce native promoter activity under basal conditions. Experiments in which an exogenous enhancer was introduced have raised the possibility that an endogenous enhancer residing outside of the 2.6 kb 5'-flanking region could be required. Cell-type specific factors also appear to be needed. The proximal flanking DNA does, however, appear to be capable of conferring activity on the promoter in chorio-decidual cells under stimulated conditions, suggesting that factors so activated may have considerable importance. Evidence suggests that forskolin-responsive signal transduction pathways may lead cyclic AMP responsive element (CRE) binding protein (CREB) to act on a CRE at -222 in the proximal REN promoter DNA. Activation of the mouse promoter by cAMP appears to involve a different element, however. Furthermore, overall control of renin synthesis is likely to involve post-transcriptional mechanisms as well. Thus, despite being the first cardiovascular gene to be cloned, much more work is required before the control of the human renin gene is fully understood.

Proximal 2.6 kb of 5'-flanking DNA is insufficient for human renin promoter activity in renin-synthesizing chorio-decidual cells

In order to determine the influence of proximal 5'-flanking DNA of the human renin gene (REN) in cells that express human renin, transient expression analyses were carried out in chorio-decidual cells. Constructs containing different lengths of REN promoter DNA, extending as far as 2595 bp upstream of the transcription start site, were unable to drive transcription of a chloramphenicol acetyl transferase reporter gene in chorio-decidual cells, nor in noncognate 293 or JEG-3 cells. The tk promoter was similarly inactive in constructs containing -2595 to -453 fragments of REN 5'-flanking DNA. In each cell type, the -2595 to -1300 DNA exerted a negative influence. Additional promoter- and cell type-dependent negative influences were noted for other regions of REN 5'-flanking DNA and the -453 to -145 DNA increased tk promoter activity 2.5-fold in chorio-decidual cells. By introducing the SV40 enhancer into constructs, a weak stimulation of the REN promoter was observed in chorio-decidual cells, but not in noncognate, JEG-3 cells, although the -2595 to -1300 DNA retained its negative influence in the cognate cell type. These results show that the proximal 2.6 kb of REN 5'-flanking DNA is unable to drive reporter gene activity in renin-synthesizing, chorio-decidual cells under basal conditions and suggest that trans-acting factors unique to at least this cell type, together with enhancer(s) located outside of the proximal 2.6 kb of REN promoter DNA tested, could be required for human renin promoter activity.

An angiotensin II receptor antagonist reduces myocardial damage in an animal model of myocarditis

BACKGROUND

Recently, an angiotensin-converting enzyme inhibitor was shown to have a beneficial effect on virus-induced myocardial injury. We investigated the effect of a new angiotensin II type 1 receptor antagonist, (+-)-1-(cyclohexyloxycarbonyloxy)ethyl 2-ethoxy-1-([2'-(1H-tetrazol-5-yl)biphenyl-4-yl]methyl)-1H-benz imi dazole-7- carboxylate (TCV-116), in an animal model of viral myocarditis induced by encephalomyocarditis virus.

METHODS AND RESULTS

Four-week-old DBA/2 mice were inoculated with the encephalomyocarditis virus. TCV-116 (in 5% gum arabic) was given 1 day before (1 or 10 mg/kg) or 2 days after virus inoculation (0.3 or 3 mg/kg). Control mice received the vehicle only. All drugs were administered orally on a daily basis, and the animals were killed on day 14. When treatment was started 1 day before inoculation, the survival of mice receiving 10 mg/kg of TCV-116 improved (17 of 20 [85%] versus 14 of 22 [64%] control mice), but the difference was not significant. Heart weight (106 +/- 24 mg versus 133 +/- 33 mg, P < .05), histological scores for myocardial necrosis (1.1 +/- 0.3 versus 2.3 +/- 1.2, P < .01), cellular infiltration (1.4 +/- 0.7 versus 2.6 +/- 1.3, P < .05), and calcification (1.1 +/- 0.3 versus 2.1 +/- 1.1, P < .01) were significantly decreased in mice given TCV-116 at 3 mg/kg compared with the vehicle control mice. The plasma angiotensin II level was significantly higher in infected mice than in noninfected mice (71.8 +/- 30.2 versus 31.8 +/- 22.5 pg/mL, P < .05). TCV-116 did not inhibit viral replication in the heart.

CONCLUSIONS

This study suggests that angiotensin II plays an important pathophysiological role in viral myocarditis. Treatment with TCV-116, an angiotensin II receptor antagonist, had a cardioprotective effect.

Cardiac renin and angiotensins. Uptake from plasma versus in situ synthesis

The existence of a cardiac renin-angiotensin system, independent of the circulating renin-angiotensin system, is still controversial. We compared the tissue levels of renin-angiotensin system components in the heart with the levels in blood plasma in healthy pigs and 30 hours after nephrectomy. Angiotensin I (Ang I)-generating activity of cardiac tissue was identified as renin by its inhibition with a specific active site-directed renin inhibitor. We took precautions to prevent the ex vivo generation and breakdown of cardiac angiotensins and made appropriate corrections for any losses of intact Ang I and II during extraction and assay. Tissue levels of renin (n = 11) and Ang I (n = 7) and II (n = 7) in the left and right atria were higher than in the corresponding ventricles (P < .05). Cardiac renin and Ang I levels (expressed per gram wet weight) were similar to the plasma levels, and Ang II in cardiac tissue was higher than in plasma (P < .05). The presence of these renin-angiotensin system components in cardiac tissue therefore cannot be accounted for by trapped plasma or simple diffusion from plasma into the interstitial fluid. Angiotensinogen levels (n = 11) in cardiac tissue were 10% to 25% of the levels in plasma, which is compatible with its diffusion from plasma into the interstitium. Like angiotensin-converting enzyme, renin was enriched in a purified cardiac membrane fraction prepared from left ventricular tissue, as compared with crude homogenate, and 12 +/- 3% (mean +/- SD, n = 6) of renin in crude homogenate was found in the cardiac membrane fraction and could be solubilized with 1% Triton X-100. Tissue levels of renin and Ang I and II in the atria and ventricles were directly correlated with plasma levels (P < .05), and in both tissue and plasma the levels were undetectably low after nephrectomy. We conclude that most if not all renin in cardiac tissue originates from the kidney. Results support the contentions that in the healthy heart, angiotensin production depends on plasma-derived renin and that plasma-derived angiotensinogen in the interstitial fluid is a potential source of cardiac angiotensins. Binding of renin to cardiac membranes may be part of a mechanism by which renin is taken up from plasma.

Characterization of angiotensin II receptors in cultured adult rat cardiac fibroblasts. Coupling to signaling systems and gene expression

Cardiac hypertrophy is largely due to cardiac fibroblast growth and increased synthesis of extracellular matrix. This study has investigated the contribution of the vasoactive hormone, angiotensin II, toward this hypertrophic process. We have demonstrated that cultures of adult rat cardiac fibroblasts express AT1 but not AT2 receptors for angiotensin II. The ability of angiotensin II to stimulate phosphoinositide catabolism and to elevate intracellular calcium concentrations in these cells was blocked by losartan, a specific AT1 receptor antagonist, but not by the AT2 receptor antagonist CGP 42112. Exposure of adult cardiac fibroblasts to angiotensin II resulted in the induction of several growth-related metabolic events including c-fos protooncogene expression and increased synthesis of DNA, RNA, and protein. Angiotensin II was also found to induce collagen type I, alpha 1 chain transcript expression in cardiac fibroblasts as well as the synthesis and secretion of collagen by these cells. The data demonstrate that angiotensin II, via AT1 receptors, can stimulate cardiac fibroblast growth and increase collagen synthesis in cardiac tissue. Thus, angiotensin II may contribute toward the development of cardiac hypertrophy in conditions of hypertension that are associated with elevated concentrations of angiotensin II.

Angiotensin II induces fibronectin expression associated with cardiac fibrosis in the rat

Fibronectin expression was studied in the heart of rats given a continuous infusion of angiotensin II (Ang II). Northern blot analysis showed that left ventricular fibronectin steady-state mRNA increased fivefold to eightfold in response to pressor doses of Ang II after 24 hours. Accumulation of immunodetectable fibronectin in the ventricles occurred after the mRNA levels increased. The changes in fibronectin expression were reversible when Ang II treatment was withdrawn. The Ang II-induced increase in fibronectin mRNA accompanied similar increases for collagen type I, collagen type IV, and atrial natriuretic factor steady-state mRNA. Interstitial and perivascular fibrosis was identified in both ventricles of angiotensin-treated rats within 3 days. In situ hybridization identified cells associated with areas of fibrosis as the principal site of fibronectin mRNA accumulation in treated animals. By comparison, normal hearts showed fibronectin expression primarily within ventricular vascular tissue and the atrial endocardium. A dose-dependent reduction of fibronectin expression followed treatment with losartan, indicating an Ang II type 1 receptor-mediated effect. Normalization of blood pressure during Ang II infusion by either hydralazine or prazosin had different effects on the level of fibronectin steady-state mRNA, indicating that blood pressure elevation was not the principal factor responsible for fibronectin induction. Concurrent administration of angiotensin-converting enzyme inhibitors with Ang II attenuated the increased fibronectin expression. Our data indicate that Ang II induces an acute fibrotic response within the heart and suggests that Ang II stimulates fibronectin expression within nonmyocytic cardiac cells by a direct action.

Systemic hypertension and the renin-angiotensin system in diabetic vascular complications

Antihypertensive treatment in the diabetic patient is a critical issue because hypertension has an impact on all of the vascular complications of diabetes, including nephropathy, retinopathy, atherosclerosis, and left ventricular hypertrophy. These complications are a consequence of altered endothelial-vascular smooth muscle interrelations that ultimately enhance vasoconstriction and alter the remodeling processes in the vascular wall. Several observations suggest that the renin-angiotensin system (RAS) may be an important contributor to these processes in diabetes mellitus. In both animal and human studies, angiotensin-converting enzyme (ACE) inhibitors have been demonstrated to slow the progression of glomerulosclerosis, prevent abnormal remodeling processes in the heart following injury, and slow the progression of atherosclerosis. In particular, ACE inhibitors appear to protect the kidney more than would be expected from simply the lowering of blood pressure and decreasing of intraglomerular pressure, possibly because angiotensin II has both hemodynamic and direct effects on the glomerulus. Paradoxically, however, the activity of the circulating RAS is low in diabetic patients. Part of these seemingly inconsistent observations may be due to (1) potential activity of tissue RASs, (2) increased sensitivity to angiotensin II in diabetes, or (3) an effect of ACE inhibition on other systems in addition to the RAS. Investigation of these mechanisms will be important in determining the therapeutic role of inhibition of the RAS in diabetes mellitus.

Autocrine release of angiotensin II mediates stretch-induced hypertrophy of cardiac myocytes in vitro

Hypertrophy is a fundamental adaptive process employed by postmitotic cardiac and skeletal muscle in response to mechanical load. How muscle cells convert mechanical stimuli into growth signals has been a long-standing question. Using an in vitro model of load (stretch)-induced cardiac hypertrophy, we demonstrate that mechanical stretch causes release of angiotensin II (Ang II) from cardiac myocytes and that Ang II acts as an initial mediator of the stretch-induced hypertrophic response. The results not only provide direct evidence for the autocrine mechanism in load-induced growth of cardiac muscle cells, but also define the pathophysiological role of the local (cardiac) renin-angiotensin system.

Identification of functional angiotensin II receptors on rat cardiac fibroblasts

BACKGROUND

Cardiac hypertrophy results in an increased deposition of the extracellular matrix (ECM) proteins fibronectin and collagen. Recent evidence indicates that angiotensin II (Ang II) might have an important role in the development of myocardial fibrosis accompanying cardiac hypertrophy. We sought to determine whether fibroblasts of cardiac origin (isolated from neonatal and adult animals) express receptors for Ang II and to examine the ability of this peptide to regulate fibronectin and collagen gene expression in a cultured adult cardiac fibroblast cell preparation.

METHODS AND RESULTS

Binding of 125I-Ang II to both neonatal and adult cardiac fibroblasts in culture was specific, reversible, and saturable, with the receptor evenly distributed over the cell population. Competition binding experiments with receptor-specific antagonists indicate that Ang II receptors found on both fibroblast types were of the AT1 subtype. Analysis of mRNA levels for the AT1 receptor indicates that adult cardiac fibroblasts express higher levels of the message than neonatal fibroblasts or cardiac myocytes. Addition of 10(-9) mol/L Ang II to adult cardiac fibroblasts resulted in an induction of ECM proteins above control levels, as determined through Northern blots and total collagen assays.

CONCLUSIONS

Results from this study indicate that neonatal and adult rat cardiac fibroblasts in culture express AT1 receptors for Ang II. Ang II stimulation of AT1 receptors results in an increased gene expression for ECM proteins. These data suggest that Ang II might have important regulatory roles over cardiac fibroblast function under normal and pathological conditions.

Rat angiotensin II (type 1A) receptor mRNA regulation and subtype expression in myocardial growth and hypertrophy

Two subtypes of angiotensin II (Ang II) receptors (AT1 and AT2) are distinguished by using the respective specific antagonists. In the present study, we report the regulation of cardiac AT1 type A (AT1A) receptor mRNA levels and the expression pattern of AT1 and AT2 receptors in the growth of the heart and the development and regression of cardiac hypertrophy. The ventricular AT1A mRNA level and the density of Ang II receptors at the neonatal period were significantly increased (3.5-fold and 2.5-fold, respectively) and then downregulated with maturation. The cardiac hypertrophy established in spontaneously hypertensive rats or two-kidney one-clip renovascular hypertensive rats resulted in substantial increases in ventricular AT1A mRNA levels (threefold) and Ang II receptor densities (twofold) as compared with those in respective control rats, whereas the receptor affinity was similar. The proportion of AT1 and AT2 subtypes in the specific Ang II binding in ventricular membranes prepared from normal adult rats was nearly equal. This proportion did not change significantly in the development of myocardial hypertrophy. The regression of cardiac hypertrophy by the normalization of elevated blood pressure completely reversed the increased levels of AT1A mRNA and the receptor density to the control level. Thus, AT1 and AT2 receptors are present in rat ventricular myocardium, and their expression is developmentally regulated and upregulated in response to hypertrophic change. Ang II action exerted through the increased number of Ang II receptors may contribute to the growth of the heart and thus to the maintenance of established hypertrophy as one of the hormones involved in hypertrophy development.

Angiotensin II is mitogenic in neonatal rat cardiac fibroblasts

Angiotensin II has been reported to be a hormonal stimulus of cardiac growth, a response that may involve myocyte hypertrophy as well as growth of nonmyocytes. This study was designed to determine whether neonatal rat cardiac fibroblasts have an angiotensin II receptor that is coupled with hypertrophic and/or proliferative growth. Competitive radioligand binding studies showed that cardiac fibroblasts have a single class of high-affinity (IC50, 1.0 nM) angiotensin II binding sites (Bmax, 778 fmol/mg protein) that are sensitive to the competitive nonpeptide AT1 receptor antagonist losartan (IC50, 13 nM). Other angiotensin peptides competed for [125I]angiotensin II binding in the following rank order: angiotensin II > angiotensin III > angiotensin I > > [des-Asp1-des-Arg2]angiotensin II. A nonhydrolyzable analogue of guanosine triphosphate increased the dissociation rate of bound [125I]angiotensin II and decreased hormone binding to the receptor at equilibrium. The angiotensin II receptor was coupled with increases in intracellular calcium. Incorporation of precursors into protein, DNA, and RNA in response to angiotensin II was determined. In serum-deprived cultures, a 24-hour exposure to 1 microM [Sar1]angiotensin II increased rates of phenylalanine, thymidine, and uridine incorporation by 58%, 103%, and 118%, respectively. These increases were blocked by the noncompetitive AT1 receptor antagonist EXP3174. After 48 hours, [Sar1]angiotensin II increased total protein and DNA of cardiac fibroblasts by 23% and 15%, respectively, with no change in the protein/DNA ratio. [Sar1]Angiotensin II increased cell number by 138% after a 24-hour exposure, without affecting cell area. In summary, cardiac fibroblasts have G protein-linked AT1 receptors that are coupled with proliferative growth. These results suggest that angiotensin II-induced cardiac hypertrophy is, in part, secondary to stimulated increases in nonmyocyte cellular growth.