Load-induced growth responses in isolated adult rat hearts. Role of the AT1 receptor

Regulation of cardiac melusin gene expression by hypertrophic stimuli in the rat

AIM

Melusin is an integrin β1-interacting protein proposed to act as a biomechanical sensor in the heart. We characterized mechanisms and signalling pathways regulating cardiac melusin expression.

METHODS

Infusion of arginine(8) -vasopressin (AVP) in Sprague-Dawley (SD) rats, spontaneously hypertensive rats (SHR) and double transgenic rats (dTGR) harbouring both human angiotensinogen and renin genes as well as infusion of angiotensin II (Ang II) in SD rats were used. The effect of direct left ventricular (LV) wall stretch was analysed by using isolated perfused rat heart preparation. For the cell culture studies, mouse atrial HL-1 cell line and neonatal rat ventricular myocytes (NRVMs) were used.

RESULTS

Left atrial melusin mRNA levels increased already after 30 min of AVP infusion. Ang II caused significant upregulation of left atrial melusin mRNA (2.1-fold at 6 h, P < 0.05) and protein (1.9-fold at 72 h, P < 0.05) levels. In contrast, LV melusin mRNA levels remained unchanged in response to both infusions, as well as to aortic banding-induced pressure overload. Direct LV wall stress or late-stage hypertensive heart disease did not modify LV melusin gene expression either. Interestingly, in atrial HL-1 cells, cyclic stretching increased melusin mRNA levels. Stretching and treatments with hypertrophic agonists increased melusin mRNA and protein levels in NRVMs, endothelin-1 being the most potent. PD98059, an extracellular signal-regulated protein kinase 1/2 inhibitor, markedly attenuated the endothelin-1-induced upregulation of melusin gene expression in NRVMs.

CONCLUSION

Multiple hypertrophic stimuli regulate melusin expression predominately in the atria, which may represent a necessary initial step in early adaptive remodelling processes.

Rac1 and RhoA differentially regulate angiotensinogen gene expression in stretched cardiac fibroblasts

AIMS

Angiotensin II (Ang II) stimulates cardiac remodelling and fibrosis in the mechanically overloaded myocardium. Although Rho GTPases regulate several cellular processes, including myocardial remodelling, involvement in mediating mechanical stretch-induced regulation of angiotensinogen (Ao), the precursor to Ang II, remains to be determined. We, therefore, examined the role and associated signalling mechanisms of Rho GTPases (Rac1 and RhoA) in regulation of Ao gene expression in a stretch model of neonatal rat cardiac fibroblasts (CFs).

METHODS AND RESULTS

CFs were plated on deformable stretch membranes. Equiaxial mechanical stretch caused significant activation of both Rac1 and RhoA within 2-5 min. Rac1 activity returned to control levels after 4 h, whereas RhoA remained at a high level of activity until the end of the stretch period (24 h). Mechanical stretch initially caused a moderate decrease in Ao gene expression, but was significantly increased at 8-24 h. RhoA had a major role in mediating both the stretch-induced inhibition of Ao at 4 h and the subsequent upregulation of Ao expression at 24 h. β₁ integrin receptor blockade by Tac β₁ expression impaired acute (2 and 15 min) stretch-induced Rac1 activation, but increased RhoA activity. Molecular experiments revealed that Ao gene expression was inhibited by Rac1 through both JNK-dependent and independent mechanisms, and stimulated by RhoA through a p38-dependent mechanism.

CONCLUSION

These results indicate that stretch-induced activation of Rac1 and RhoA differentially regulates Ao gene expression by modulating p38 and JNK activation.

Angiotensin II: a hormone involved in and contributing to pro-hypertrophic cardiac networks and target of anti-hypertrophic cross-talks

Angiotensin II (Ang II) plays a major role in the progression of myocardial hypertrophy to heart failure. Inhibiting the angiotensin converting enzyme (ACE) or blockade of the corresponding Ang II receptors is used extensively in clinical practice, but there is scope for refinement of this mode of therapy. This review summarizes the current understanding of the direct effects of Ang II on cardiomyocytes and then focus particularly on interaction of components of the renin-angiotensin system with other hormones and cytokines. New findings described in approximately 400 papers identified in the PubMed database and published during the 2.5 years are discussed in the context of previous relevant literature. The cardiac action of Ang II is influenced by the activity of different isoforms of ACE leading to different amounts of Ang II by comparison with other angiotensinogen-derived peptides. The effect of Ang II is mediated by at least two different AT receptors that are differentially expressed in cardiomyocytes from neonatal, adult and failing hearts. The intracellular effects of Ang II are influenced by nitric oxide (NO)/cGMP-dependent cross talk and are mediated by the release of autocrine factors, such as transforming growth factor (TGF)-beta1 and interleukin (IL)-6. Besides interactions with cytokines, Ang II is involved in systemic networks including aldosterone, parathyroid hormone and adrenomedullin, which have their own effects on cardiomyocytes that modify, amplify or antagonize the primary effect of Ang II. Finally, hyperinsulemia and hyperglycaemia influence Ang II-dependent processes in diabetes and its cardiac sequelae.

Endothelin-1 and angiotensin II contribute to BNP but not c-fos gene expression response to elevated load in isolated mice hearts

The early events in the cardiac hypertrophic process induced by hemodynamic load include activation of B-type natriuretic peptide (BNP) and c-fos gene expression. However, it is unknown whether stretch acts directly or through local paracrine factors to trigger changes in cardiac gene expression. Herein we studied the involvement of endothelin-1 (ET-1) and angiotensin II (Ang II) in load-induced activation of left ventricular BNP and c-fos gene expression using an in vitro stretch model in isolated perfused adult mice hearts. Two-hour stretch induced by increasing coronary flow rate from 2 to 5 ml/min increased the expression of BNP and c-fos genes by 1.9- and 1.5-fold, respectively (P<0.001 and P<0.05). A mixed ET(A/B) receptor antagonist bosentan attenuated the BNP gene expression response to load by 58% (P<0.005). A similar 53% inhibition was observed with the selective ET(A) receptor blocker BQ-123 (P<0.05). Type 1 Ang II receptor antagonist CV-11974 decreased the activation of BNP gene expression by 50% (P<0.05). In contrast, the activation of c-fos gene expression was not inhibited by antagonists of ET(A/B) and AT(1) receptors. Our results show that ET-1 and Ang II play a key role in the induction of BNP, but not c-fos gene expression in response to load in intact adult murine hearts.

AT1receptors are necessary for eccentric training-induced hypertrophy and strength gains in rat skeletal muscle

This study was undertaken to measure the response of skeletal muscle to eccentric contractions (EC) in the presence of the angiotensin type 1 (AT1) receptor blocker, losartan. It was hypothesized that blocking AT1 receptors prior to an initial bout of EC would prevent the muscle from developing the normal adaptation to EC as demonstrated by the repeated bout effect. It was also hypothesized that continuous AT1 receptor blockade during EC training would significantly reduce muscle hypertrophy and strength gains that occur with repeated EC. Rats received losartan in their drinking water at either a low dose (20 mg (kg body weight)-1 day-1) or a high dose (40 mg (kg body weight)-1 day-1). Each bout of EC consisted of a total of 24 contractions. Rats were assigned to four groups: a single acute bout of EC (n=6); two bouts of EC separated by 14 days (n=8); and 4 weeks of training twice a week on the low dose (n=5) or the high dose (n=9). There was no effect of AT1 receptor blockade on the initial loss of function following a single acute bout of EC, or on the repeated bout effect following a second exposure to EC. AT1 receptor blockade did alter the results of EC training, in both the low and high dose groups. Losartan treatments prevented EC training-induced increases in muscle wet and dry weights compared to untreated rats. Finally, the low and high dose losartan treatments also prevented an increase in muscle contractile force following EC training compared to the untreated group. Functional AT1 receptors are therefore not necessary for an acute adaptation to EC as demonstrated by the repeated bout effect, but are necessary for muscle hypertrophy and increased contractile force associated with EC training.

Mitogen-activated Protein Kinases p38 and ERK 1/2 Mediate the Wall Stress-induced Activation of GATA-4 Binding in Adult Heart

The zinc finger transcription factor GATA-4 has been implicated as a critical regulator of inducible cardiac gene expression and as a potential mediator of the hypertrophic program. However, the precise intracellular mechanisms that regulate the DNA-binding activity of GATA-4 are not fully understood. The aim of the present study was to examine the role of mitogen-activated protein kinases (p38 kinase, extracellular signal-regulated protein kinase, and c-Jun N-terminal protein kinase) in the left ventricular wall stress-induced activation of GATA-4 DNA binding in adult heart. Isolated perfused rat hearts were subjected to increased left ventricular wall stress by inflating a balloon in the ventricle. Gel mobility shift assays were used to analyze the transacting factors that interact with the GATA motifs of the B-type natriuretic peptide promoter. The left ventricular wall stress rapidly activated GATA-4 DNA binding and significantly increased the levels of phosphorylated p38 kinase, extracellular signal-regulated protein kinase, and c-Jun N-terminal protein kinase. The wall stress-induced increase in the DNA-binding activity of GATA-4 was abolished both in the presence of the p38 inhibitor SB239063 and MEK1/2 inhibitor U0126. In contrast, the inhibition of c-Jun N-terminal protein kinase by CEP11004 had no effect on the baseline or stretch-induced GATA-4 DNA binding. Moreover, GATA-4 DNA binding was up-regulated by mechanical stretch in the isolated rat atria via p38 and extracellular signal-regulated protein kinase. In conclusion, the present study demonstrates that both p38 and extracellular signal-regulated protein kinase are required for the stretch-induced GATA-4 binding in intact heart.

Angiotensin II stimulates hyperplasia but not hypertrophy in immature ovine cardiomyocytes

Rat and sheep cardiac myocytes become binucleate as they complete the 'terminal differentiation' process soon after birth and are not able to divide thereafter. Angiotensin II (Ang II) is known to stimulate hypertrophic changes in rodent cardiomyocytes under both in vivo and in vitro conditions via the AT1 receptor and intracellular extracellular regulated kinase (ERK) signalling cascade. We sought to develop culture methods for immature sheep cardiomyocytes in order to test the hypothesis that Ang II is a hypertrophic agent in the immature myocardium of the sheep. We isolated fetal sheep cardiomyocytes and cultured them for 96 h, added Ang II and phenylephrine (PE) for 48 h, and measured footprint area and proliferation (5-bromo-2'-deoxyuridine (BrdU) uptake) separately in mono- vs. binucleate myocytes. We found that neither Ang II nor PE changed the footprint area of mononucleated cells. PE stimulated an increase in footprint area of binucleate cells but Ang II did not. Ang II increased myocyte BrdU uptake compared to serum free conditions, but PE did not affect BrdU uptake. The MAP kinase kinase (MEK) inhibitor UO126 prevented BrdU uptake in Ang II-stimulated cells and prevented cell hypertrophy in PE-stimulated cells. This paper establishes culture methods for immature sheep cardiomyocytes and reports that: (1) Ang II is not a hypertrophic agent; (2) Ang II stimulates hyperplastic growth among mononucleate myocytes; (3) PE is a hypertrophic agent in binucleate myocytes; and (4) the ERK cascade is required for the proliferation effect of Ang II and the hypertrophic effect of PE.

Release of preformed Ang II from myocytes mediates angiotensinogen and ET-1 gene overexpression in vivo via AT1 receptor

The role of angiotensin II in pressure overload is still debated because notwithstanding its effects on myocyte contractility angiotensin II is not an obligatory factor for the development of hypertrophy. To define the role of angiotensin II in acute pressure overload we studied the effects of AT1 blockade (valsartan 80mg per day) on myocardial contractility, cardiac growth factor gene expression, and myocardial hypertrophy in aortic banded (60mmHg) pigs. Acute pressure overload caused an abrupt reduction of myocardial contractility, measured by the end-systolic stiffness constant, and a sharp increase in end-systolic stress which rapidly normalized (within 12h) in the placebo group. In AT1-blocked animals end-systolic stiffness constant remained significantly depressed up to 24h and end-systolic stress was still elevated up to 48h (both P<0.05 vs placebo). In both groups confocal microscopy revealed that granular staining of angiotensin II in cardiomyocyte cytoplasm disappeared after 30min of pressure overload. AT1 blockade abolished following cardiac overexpression of angiotensinogen and endothelin-1 genes as shown in RT-PCR studies and the consequent angiotensin II and endothelin-1 release in the coronary circulation. Conversely, insulin-like growth factor-I and ACE mRNA overexpression, as well as the onset of left ventricular mass increase, were not significantly affected by AT1 blockade.

IN CONCLUSION

(1) mechanical stress releases preformed angiotensin II from myocyte in vivo; (2) the AT1 blockade abolishes cardiac angiotensin II and endothelin-1 production with delayed recovery of myocardial contractility; whereas (3) the overexpression of insulin-like growth factor-I gene and the development of myocardial hypertrophy are not angiotensin II-mediated effects.

ACE inhibition in aortic stenosis: dangerous medicine or golden opportunity?

Conventionally angiotensin-converting enzyme (ACE) inhibitors are contraindicated in patients with aortic stenosis. Abundant evidence is now available showing that angiotensin II has a central role in the development of left ventricular hypertrophy (LVH), myocardial contractile failure and diastolic dysfunction in response to pressure overload. In animal models, ACE inhibitors have been shown to attenuate these pathological responses. In humans there is no such evidence available, however uncontrolled studies have shown that these agents are not only tolerated but are associated with acute improvements in haemodynamics and diastolic function. Further studies are merited to assess the possible role of ACE inhibitors in aortic stenosis both before and after valve replacement. Potential benefits may include prevention of LVH, improved diastolic function, reduction of arrhythmias and preservation of left ventricular function.

Signaling pathway and chronotropic action of parathyroid hormone in isolated perfused rat heart

Parathyroid hormone (PTH) activates both adenylyl cyclase and phospholipase C via the PTH-1 receptor. We previously reported that PTH increased heart rate and that this effect was mediated via the pacemaker current (I f ). However, it has been reported that PTH exerts its chronotropic effect via an interaction with adrenergic receptors or via L-type calcium channels. Thus, the objective of the study was to elucidate the exact mechanism of the chronotropic effect of PTH. We tested whether its chronotropic effects could be abolished by inhibitors of the following systems in isolated perfused rat hearts: alpha-adrenergic (prazosin); beta-adrenergic (propranolol); angiotensin II (CV11974); endothelin-1 (TAK044); calcium channel (verapamil); adenylyl cyclase (miconazole); phospholipase C (U73122) or I f (CsCl). In addition, we measured the cyclic adenosine monophosphate level of the heart after PTH administration. Whereas prazosin, propranolol, CV11974, TAK044, verapamil, and U73122 did not inhibit the chronotropic effect of PTH, CsCl or miconazole suppressed it significantly. PTH increased the cyclic adenosine monophosphate level of the atrium but not the left ventricle. These results indicate that the chronotropic actions of PTH are mediated via selective activation of adenylyl cyclase to increase the I f current.

Alterations of load-induced p38 MAP kinase activation in failing rat hearts

Hemodynamic load-induced cardiac p38 mitogen-activated protein kinase (MAPK) activation was studied in normotensive control Dahl rats (n = 10) and hypertensive Dahl rats with heart failure (n = 16). The isolated heart from each animal was stretched on a Langendorff apparatus at an equivalent diastolic wall stress, and the p38-MAPK activity of the left ventricular (LV) myocardium was analyzed by immunoprecipitation-kinase assay. Compared to the control hearts, the stretch-induced p38-MAPK activities were significantly decreased, and inversely correlated with the LV diameter (r = -0.73, P < 0.01). Chronic treatment with an angiotensin II AT1-receptor antagonist, valsartan (10 mg/kg/day), ameliorated cardiac function and remodeling process in the failing hearts, which was associated with an improvement of the p38-MAPK activities. Thus, the mechano-signal transduction of p38-MAPK pathway is downregulated in the failing hearts, along with progressive ventricular remodeling. The data also suggest that the beneficial effects of the AT1-receptor antagonists are potentially mediated by the restoration of cardiac growth-related signal transduction.

Renin angiotensin system-dependent hypertrophy as a contributor to heart failure in hypertensive rats: different characteristics from renin angiotensin system-independent hypertrophy

OBJECTIVES

This study aimed to characterize the difference between renin angiotensin system (RAS)-dependent and RAS-independent hypertrophy and their differential contribution to the transition to heart failure.

BACKGROUND

Hypertensive left ventricular (LV) hypertrophy develops with RAS activation in the heart; however, LV hypertrophy develops even without RAS activation.

METHODS

Left ventricular geometry and function were assessed in Dahl salt-sensitive rats placed on an 8% NaCl diet from seven weeks old (hypertensive rats) and in those placed on an 0.3% NaCl diet (control rats, n = 8). The hypertensive rats were randomized to no treatment (n = 8) or treatment with the angiotensin type 1 receptor (AT1R) antagonist candesartan (1 mg/kg per day, n = 10) after the baseline echocardiography study.

RESULTS

From 7 to 13 weeks, AT1R blockade at a subdepressor dose did not restrain the development of LV hypertrophy but prevented narrowing of LV diastolic dimension, leading to the normalization of abnormally decreased end-systolic wall stress in the untreated rats. Progressive development of LV hypertrophy in spite of lower than normal end-systolic wall stress (excessive hypertrophy) after 13 weeks was suppressed by the AT1R blockade. Elevation of LV end-diastolic pressure and prolongation of Tau were associated with histological evidence of myocyte hypertrophy and massive interstitial fibrosis in the untreated rats, and none of these was evident in the treated rats.

CONCLUSIONS

Renin-angiotensin system activation and AT1R signaling may be dispensable for the development of early adaptive LV hypertrophy and closely linked to the transition to heart failure.

Effect of pressure overload on angiotensin receptor expression in the rat heart during early postnatal life

The development of cardiac hypertrophy during neonatal life and in adults implies different processes. The angiotensin II (Ang II) system is involved in the development of cardiac hypertrophy in adults, but its role in neonates remains unclear. The aim of this study was to estimate the influence of increased hemodynamic load on the developmental pattern of the AT1/AT2 receptor expression in the heart. Two-day-old rats submitted to abdominal aortic constriction (AC) or sham operation were sacrificed 2 h, and 1, 3, and 8 days after surgery. Ang II was evaluated in sera and immunohistology was performed to define the cardiac hypertrophy process. The Ang II receptor subtypes 1 and 2 were quantified at the receptor and mRNA levels by(125)I-Ang II binding and RT-PCR, respectively. Ang II content in sera increased transiently 2 h after surgery in the AC group. In sham-operated, AT1 and AT2 decreased throughout the period studied at both mRNA and receptor levels. However, the AT1 mRNA level decrease was more pronounced than that of AT2 (by 57% and 27%, respectively). AC not only prevented the postnatal decrease in AT mRNA level but resulted in an increase in AT1 mRNA 8 days after surgery (P<0.05). Besides in the AC groups, AT2 mRNA levels but not those of AT1 mRNA were linearly correlated with the left ventricular mass. At the receptor level, a significant transient (1 day after surgery) increase in both AT1 and AT2 was observed. In conclusion, our data demonstrated that imposition of pressure overload soon after birth altered the pattern of AT receptor expression.

What can knockout mice contribute to an understanding of hypertension?

The generation of knockout mice using homologous recombination in embryonic stem cells is a powerful tool for physiologic investigations. This experimental approach has provided unique insights into the study of hypertension. Studies using knockout mice have shed new light on blood pressure regulatory mechanisms, molecular mechanisms of end-organ injury, and genetic mechanisms for hypertension. With the development of more accessible approaches for carrying out sophisticated manipulation of the mouse genome, there will be continuing utility of this technique for future studies of hypertension.

Induction of tenascin-C in cardiac myocytes by mechanical deformation. Role of reactive oxygen species

Mechanical overload may change cardiac structure through angiotensin II-dependent and angiotensin II-independent mechanisms. We investigated the effects of mechanical strain on the gene expression of tenascin-C, a prominent extracellular molecule in actively remodeling tissues, in neonatal rat cardiac myocytes. Mechanical strain induced tenascin-C mRNA (3.9 +/- 0.5-fold, p < 0.01, n = 13) and tenascin-C protein in an amplitude-dependent manner but did not induce secreted protein acidic and rich in cysteine nor fibronectin. RNase protection assay demonstrated that mechanical strain induced all three alternatively spliced isoforms of tenascin-C. An angiotensin II receptor type 1 antagonist inhibited mechanical induction of brain natriuretic peptide but not tenascin-C. Antioxidants such as N-acetyl-L-cysteine, catalase, and 1, 2-dihydroxy-benzene-3,5-disulfonate significantly inhibited induction of tenascin-C. Truncated tenascin-C promoter-reporter assays using dominant negative mutants of IkappaBalpha and IkappaB kinase beta and electrophoretic mobility shift assays indicated that mechanical strain increases tenascin-C gene transcription by activating nuclear factor-kappaB through reactive oxygen species. Our findings demonstrate that mechanical strain induces tenascin-C in cardiac myocytes through a nuclear factor-kappaB-dependent and angiotensin II-independent mechanism. These data also suggest that reactive oxygen species may participate in mechanically induced left ventricular remodeling.

Factors contributing to pressure overload-induced immediate early gene expression in adult rat hearts in vivo

We determined the contributions of angiotensin II type 1 receptor (AT(1)) stimulation, adrenergic stimulation, and autonomic activation to pressure overload-induced c-fos expression in the adult rat heart in vivo. c-fos expression was increased in pressure-overloaded hearts created by aortic banding compared with sham-operated rats (458 +/- 100% vs. sham, P < 0.05). GR-138950, a selective AT(1) antagonist, did not blunt this expression (banding vs. banding + GR-138950: 458 +/- 100% vs. 500 +/- 125%, not significant). Atropine and hexamethonium partially decreased c-fos expression (banding vs. banding + atropine/hexamethonium: 700 +/- 67% vs. 400 +/- 67%, P < 0.05). Phentolamine had no significant effect on c-fos expression; however, propranolol inhibited the expression (banding vs. banding + propranolol: 492 +/- 108% vs. 154 +/- 15%, P < 0.05). The inhibition by propranolol was independent of the decreases in heart rate. Thus factors contributing to pressure overload-induced c-fos expression in adult rat hearts in vivo are different from those in neonatal myocytes in vitro undergoing stretch.

Role of angiotensin AT1, and AT2 receptors in cardiac hypertrophy and disease

Angiotensin II modulates beat-to-beat cardiac performance as a potent vasocontrictor, inotrope, and regulator of water and electrolyte balance. It is also a growth factor that can stimulate the early molecular growth responses of proto-oncogene activation and new protein synthesis, and the later event of cardiocyte hypertrophy independent from load. Its effects are mediated through the angiotensin II type 1 (AT1) receptor, which exists as the AT1a and AT1b isoforms, and the angiotensin II type 2 (AT2) receptor. There is still controversy regarding the role of activation of the AT1 receptor subtype(s) as a mandatory signal versus modulatory regulator of the transduction of mechanical load in pressure-overload hypertrophy due to hypertension or aortic stenosis. The role of the AT2 receptor subtype in the heart is even less well understood, although this receptor appears to serve as an antigrowth signal in proliferating cells. Here we review current data on these controversies, including new data that support the notion that angiotensin II activation of the cardiac AT2 receptor subtype inhibits the effects of angiotensin II on the immediate growth response in the adult heart.

Hypertrophic response to hemodynamic overload: role of load vs. renin-angiotensin system activation

Myocardial hypertrophy is one of the basic mechanisms by which the heart compensates for hemodynamic overload. The mechanisms by which hemodynamic overload is transduced by the cardiac muscle cell and translated into cardiac hypertrophy are not completely understood. Candidates include activation of the renin-angiotensin system (RAS) and angiotensin II receptor (AT1) stimulation. In this study, we tested the hypothesis that load, independent of the RAS, is sufficient to stimulate cardiac growth. Four groups of cats were studied: 14 normal controls, 20 pulmonary artery-banded (PAB) cats, 7 PAB cats in whom the AT1 was concomitantly and continuously blocked with losartan, and 8 PAB cats in whom the angiotensin-converting enzyme (ACE) was concomitantly and continuously blocked with captopril. Losartan cats had at least a one-log order increase in the ED50 of the blood pressure response to angiotensin II infusion. Right ventricular (RV) hypertrophy was assessed using the RV mass-to-body weight ratio and ventricular cardiocyte size. RV hemodynamic overload was assessed by measuring RV systolic and diastolic pressures. Neither the extent of RV pressure overload nor RV hypertrophy that resulted from PAB was affected by AT1 blockade with losartan or ACE inhibition with captopril. RV systolic pressure was increased from 21 +/- 3 mmHg in normals to 68 +/- 4 mmHg in PAB, 65 +/- 5 mmHg in PAB plus losartan and 62 +/- 3 mmHg in PAB plus captopril. RV-to-body weight ratio increased from 0.52 +/- 0.04 g/kg in normals to 1.11 +/- 0.06 g/kg in PAB, 1.06 +/- 0.06 g/kg in PAB plus losartan and 1.06 +/- 0.06 g/kg in PAB plus captopril. Thus 1) pharmacological modulation of the RAS with losartan and captopril did not change the extent of the hemodynamic overload or the hypertrophic response induced by PAB; 2) neither RAS activation nor angiotensin II receptor stimulation is an obligatory and necessary component of the signaling pathway that acts as an intermediary coupling load to the hypertrophic response; and 3) load, independent of the RAS, is capable of stimulating cardiac growth.

Mechanical strain suppresses inducible nitric-oxide synthase in cardiac myocytes

We investigated the effects of precisely controlled mechanical strain on nitric-oxide synthase activity in cultured neonatal rat cardiac myocytes. Incubation of cardiac myocytes for 24 h with 4 ng/ml interleukin-1beta and 100 units/ml interferon-gamma stimulated an increase in nitric oxide production, inducible nitric-oxide synthase (iNOS) mRNA, and iNOS protein. Mechanical strain suppressed nitric oxide production, iNOS mRNA, and iNOS protein stimulated by cytokines in an amplitude-dependent manner. Losartan (1 microM), an angiotensin II type 1 receptor antagonist, weakly inhibited the effect of strain, suggesting that paracrine angiotensin II is not the mediator of the strain effect. In addition, cycloheximide (10 microM), a protein synthesis inhibitor, inhibited the effect of strain by 46%. Transforming growth factor-beta (1 ng/ml) suppressed iNOS mRNA expression, but anti-transforming growth factor-beta antibody (30 microg/ml) did not block the effect of strain. In contrast, staurosporine (100 nM; a nonselective protein kinase inhibitor), calphostin C (1 microM; a selective protein kinase C inhibitor), and pretreatment with phorbol 12-myristate 13-acetate abolished the effect of strain. Genistein (100 microM), a tyrosine kinase inhibitor, partially inhibited the effect of strain. Thus, cyclic mechanical deformation suppresses cytokine-induced iNOS expression in cardiac myocytes, and this effect is mediated at least partially via activation of protein kinase C.

Myosin heavy chain gene expression changes in the diaphragm of patients with chronic lung hyperinflation

In striated muscle, chronic increases in workload result in changes in myosin phenotype. The aim of this study was to determine whether such changes occur in the diaphragm of patients with severe chronic obstructive pulmonary disease, a situation characterized by a chronic increase in respiratory load and lung volume. Diaphragm biopsies were obtained from 22 patients who underwent thoracic surgery. Myosin was characterized with electrophoresis in nondenaturing conditions, SDS-glycerol PAGE, and Western blotting with monoclonal antibodies specific for slow and fast myosin heavy chain (MHC) isoforms. Flow volume curves, total lung capacity, and functional residual capacity were measured before surgery in 20 patients. We found that the human diaphragm is composed of at least four myosin isoforms, one slow and three fast, resulting from the combination of three MHC species. Chronic overload was associated with an increase in the slow beta-MHC species at the expense of the fast species (beta-MHC, 78.2 +/- 4.6 and 50.0 +/- 6.5% in emphysematous and control patients, respectively; P < 0.005). Linear correlations were found between beta-MHC percentage and forced expiratory volume in 1 s (r = -0.52; P < 0.02), total lung capacity (r = 0.44; P < 0.05), and functional residual capacity (r = 0.65; P < 0.003). The human adult diaphragm is composed of a balanced proportion of slow and fast myosin isoforms. In patients with chronic obstructive pulmonary disease, the proportion of fast myosins decreases, whereas that of slow myosin increases. This increase appears to be closely related to lung hyperinflation and may reflect an adaptation of the diaphragm to the new functional requirements.

Pressure-overload hypertrophy is unabated in mice devoid of AT1A receptors

Mechanisms controlling cardiac growth are under intense investigation. Among these, the renin-angiotensin system has received great interest. In the current study, we tested the hypothesis that the renin-angiotensin system was not an obligate factor in cardiac hypertrophy. We examined the left ventricular hypertrophic response to a pressure overload in mice devoid of the AT1A receptor, the putative major effector of the growth response of the renin-angiotensin system. Aortic banding produced similar transband gradients in wild-type and AT1A knockout mice. The left ventricular mass-to-body weight ratio increased from 3.44 +/- 0.08 to 5.62 +/- 0.25 in wild-type ascending aortic-banded mice. The response in the knockout mice was not different (from 2.97 +/- 0.13 to 5.24 +/- 0.37). We conclude that the magnitude of cardiac hypertrophy is not affected by the absence of the AT1A receptor and its signaling pathway and that this component of the renin-angiotensin system is not necessary in cardiac hypertrophy.