Experimental Models Of Heart Failure Research Articles

Sympathetic Nervous System Neuroplasticity

In human and experimental models of heart failure there is widespread change in the sympathetic nervous system. This involves preferential activation of the sympathetic nervous outflow to the heart, impairment of norepinephrine reuptake by sympathetic nerves, and reduction of the content of norepinephrine in the failing myocardium, attributable in part to sympathetic neuronal rarefaction.1–4 Michael Kreusser and colleagues from the University of Heidelberg, in their article in this issue of Hypertension ,5 report rapid correction of 2 of these abnormalities— reduced norepinephrine reuptake and depleted cardiac norepinephrine stores—in the rat heart failure model they studied, following injection of a single dose of the neurotrophin, nerve growth factor (NGF), into the left stellate ganglion. The neurotrophins are regulators of neural survival, development, and function that have a key role in vertebrate embryonic life but are increasingly documented to have important influences also in maturity.6,7 The article by Kreusser et al5 underscores the extraordinary neuroplasticity of the sympathetic nervous system in adulthood, a structural and functional neuronal ebb and flow that is orchestrated in large part by NGF. Perhaps the most explicit example of human sympathetic neuroplasticity is the regrowth and remodeling of the sympathetic nerves of the heart following cardiac transplantation. With the exception of the sympathetic nerves to the recipient atria, which depending on the surgical technique may be left intact, all cardiac sympathetic nerves are cut at the time of surgical removal of the failing heart. Subsequently, over several years, sympathetic fiber ingrowth leads to substantial reinnervation of …

Expression of Cytoskeletal, Linkage and Extracellular Proteins in Failing Dog Myocardium

In the setting of chronic heart failure (HF), progressive left ventricular (LV) dysfunction and chamber remodeling may be due, in part, to altered expression and disorganization of cytoskeletal, linkage and extracellular proteins. This brief review describes changes in expression of cytoskeletal, linkage and extracellular protein using LV tissue obtained from dogs with progressive HF produced by multiple sequential intracoronary microembolizations. LV tissue samples from 6 untreated HF dogs (LV ejection fraction 20% to 25%) and 3 normal dogs were used. Sections from freshly frozen tissue were prepared, immunostained for specific proteins and studies by confocal microscopy. In failing hearts, confocal microscopy showed disorganization of key cytoskeletal proteins that, when combined with the loss of myofilaments and sarcomeric skeleton, suggest substantial cardiomyocyte remodeling. Cardiomyocytes in areas bordering old infarcts invariably exhibited disorganization of alpha-actinin. The cytoskeleton protein desmin showed increased expression in areas of extensive fibrosis. Staining for pancadherin showed interruptions of intercalated disks in areas of intensive interstitial fibrosis. Observation of increased fibronectin and increased interstitial cellularity based on vimentin labeling is suggestive of ongoing fibrosis. Based on these findings, we conclude that the structural changes observed in failing LV myocardium of dogs with intracoronary microembolizations-induced HF are extensive and typical of those seen and previously described in LV myocardium of explanted failed human hearts. The observed structural changes in this experimental model of HF also support the notion that these cytoskeletal, linkage and extracellular disorganization of structural proteins may be important maladaptations that contribute, albeit in part, to the progression of LV dysfunction and remodeling characteristic of the HF state.

Pharmacological Stimulation of β 2-adrenergic Receptors (β 2AR) Enhances Therapeutic Effectiveness of β 1AR Blockade in Rodent Dilated Ischemic Cardiomyopathy

We have reported that beta2 adrenoreceptor (beta2AR) stimulation is anti-apoptotic, and has strong beneficial effect on cardiac remodeling in an experimental model of post myocardial infarction chronic heart failure (CHF) in rats. Here we investigate whether the addition of chronic pharmacological beta2AR stimulation enhances the therapeutic effects of beta1AR blockade on cardiac remodeling in the same model. Metoprolol, a beta1AR blocker, given alone (beta1) or in combination with beta2AR agonist, fenoterol (beta1beta2) were administered to rats via drinking water for 6 weeks, beginning 2 weeks following permanent coronary ligation. Progressive left ventricular (LV) remodeling of untreated animals, assessed by repeated echocardiography, occurred during the observation time, i.e., 42% and 25% increases in end-systolic and end-diastolic LV volumes respectively, 27% fall in ejection fraction, and 35% infarct expansion. Pressure-volume loop analyses at 2d and 8th post infarction weeks showed continuous deterioration of systolic and diastolic functions and arterio-ventricular mismatch. Histological evaluation at the end of 8 weeks revealed the MI expansion and hypertrophy of cardiomyocytes. beta1beta2 prevented LV remodeling, MI expansion and cardiomyocytes hypertrophy to a greater degree than beta1, due, in large part, to a vasodilatory effect of beta2AR stimulation and thus improvement of arterio-ventricular mismatch. The abnormal diastolic performance improved only in beta1beta2. beta1beta2 treatment reduced myocardial apoptosis throughout myocardium, but beta1 reduced apoptosis only in the areas remote from MI. The therapeutic effects of chronic beta1AR blockade on cardiac remodeling of heart failure are enhanced and extended when supplemented with beta2AR stimulation.

POLY(ADENOSINE DIPHOSPHATE-RIBOSE) POLYMERASE INHIBITION PRESERVES ERECTILE FUNCTION IN RATS AFTER CAVERNOUS NERVE INJURY

We investigated the involvement of poly(adenosine diphosphate-ribose) (PAR) polymerase (PARP) activation in the development of erectile dysfunction and the therapeutic benefit of the potent PARP inhibitor INO-1001 (Inotek Pharmaceuticals Corp., Beverly, Massachusetts) in a bilateral cavernous nerve crush injury (BCNCI) model in rats. Sprague-Dawley rats were divided into 3 groups, namely sham treated, BCNCI plus vehicle and BCNCI plus the PARP inhibitor INO-1001. One week after surgical intervention all groups underwent in vivo cavernous nerve stimulation. PAR activation, nitrotyrosine and inducible nitric oxide synthase were evaluated by immunohistochemistry and serum levels of INO-1001 were measured by high performance liquid chromatography. Penile tissues were analyzed for levels of malondialdehyde and myeloperoxidase. Data sets were statistically compared in all groups. Neurogenic mediated erectile responses were evaluated. Mean intracavernous pressure (ICP), the ICP-to-blood-pressure ratio and total ICP were significantly decreased in BCNCI plus vehicle rats. These values were not statistically different between the sham and PARP inhibitor treated groups. There was a marked decrease in PAR staining in the treatment group. There was a substantial increase in malondialdehyde tissue levels but not myeloperoxidase in response to BCNCI, which was unchanged with PARP inhibitor treatment. There was a marked increase in tyrosine nitration in the treatment group. Up-regulation of nitric oxide synthase and increased tyrosine nitration were not observed in the penile tissues of the treatment group. These data demonstrate that BCNCI in a rat model causes increased PARP activation, resulting in severe erectile dysfunction. Treatment with the PARP inhibitor INO-1001 decreases the degree of nitrosative stress, prevents PARP activation and provides significant cavernous neuroprotection, which in turn preserves erectile function.

Mechanisms of Increased Vascular Superoxide Production in an Experimental Model of Idiopathic Dilated Cardiomyopathy

In the present study, we sought to identify mechanisms underlying increased oxidative stress in vascular tissue in an experimental animal model of chronic congestive heart failure (CHF). Superoxide and nitric oxide (NO) was measured in vessels from cardiomyopathic hamsters (CHF hamsters) and golden Syrian hamsters. We also determined expression of endothelial nitric oxide synthase (NOSIII), the soluble guanylyl cyclase, the cGMP-dependent kinase, and the NADPH oxidase. To analyze the contribution of the renin-angiotensin system to oxidative stress, CHF hamsters were treated with the angiotensin-converting enzyme inhibitor captopril for 200 days (120 mg . kg(-1) . d(-1)). CHF led to increased superoxide production by NOSIII and the NADPH oxidase. Decreased NO production in CHF was associated with a decrease in the expression of NOSIII and an inhibition of NO downstream signaling in the aorta. NOSIII expression was increased within the left ventricle. Captopril treatment normalized NOSIII expression in vessels and the myocardium, reduced superoxide levels, and prevented NOSIII uncoupling. Accordingly, endothelial function, NO production, and downstream signaling were improved in CHF vessels. Oxidative stress in CHF is mediated by NADPH oxidase and an uncoupled NOSIII secondary to an activation of the renin-angiotensin system leading to impaired NO downstream signaling.

Cardiac and peripheral actions of growth hormone and its releasing peptides: Relevance for the treatment of cardiomyopathies

Ischemic and nonischemic cardiomyopathies are associated with significant morbidity and mortality in industrialized countries. Cardiomyopathies of primary origin, and more specifically the dilated form of the disease, have been associated with a number of gene defects in cytoskeletal, membrane, and sarcomeric proteins. Cardiomyopathies of secondary origin such as ischemic cardiomyopathy remain the leading cause of left ventricular systolic dysfunction and heart failure. Among novel strategies to improve cardiac function in heart failure, treatment with growth hormone, insulin growth factor-1 (IGF-1), and natural and synthetic growth hormone-releasing peptides such as ghrelin and hexarelin have been explored. The present review focuses on the issues involved in the use of exogenous growth hormone and its releasing peptides in experimental animal models of chronic heart failure and in clinical studies on cardiomyopathic patients as potential releasing peptides for the treatment of chronic heart failure developing as a consequence of cardiomyopathy.

Growth hormone therapy in heart failure: a novel therapy worthy of further consideration?

Despite the improvements in survival with angiotensin-converting enzyme inhibitors and β-blockers, the clinical events for patients with heart failure remain elevated. New therapies for heart failure are required to improve the functional capacity, quality of life and prognosis. Growth hormone exerts both direct and indirect effects on cardiac structure and function. Experimental models of heart failure and small studies have demonstrated significant improvements in cardiac function, haemodynamical parameters, functional capacity and quality of life. The results from randomised controlled studies have been mixed with others showing benefit and some that do not. The randomised studies showing benefit consistently used growth hormone every other day. Further studies are needed to assess the potential role of this adjuvant therapy in patients with heart failure.

Improvement of Cardiac Sarcoplasmic Reticulum Calcium Cycling in Dogs with Heart Failure Following Long-Term Therapy with the Acorn Cardiac Support Device

Abnormal Ca(2+)-homeostasis is a hall-marked characteristic of the failing heart. In the normal myocardium, the sarcoplasmic reticulum (SR) is a principal organelle that controls intracellular Ca(2+) concentration during the cardiac cycle. The SR consists of longitudinal and terminal cisternea regions. The former contains the Ca(2+)-ATPase pump or SERCA-2a whose function is to transport cytosolic Ca(2+) into the lumen of the SR during diastole and whose activity is regulated by reversible phosphorylation of the endogenously SR-bound phospholamban (PLB). The SR's terminal cisternea region contains ryanodine-sensitive Ca(2+)-release channels (RR), the activity of which is regulated by direct and indirect reversible phosphorylation. These channels release the SR-stored Ca(2+) during contraction. We have shown that in left ventricular (LV) myocardium from dogs with coronary microembolization-induced heart failure, ability of the SR to sequester and release Ca(2+) during the cardiac cycles is impaired. This abnormality was associated with reduced expression (protein and mRNA) levels of Ca(2+)-ATPase, PLB, and reduced PLB phosphorylation. Long-term therapy with the Acorn Cardiac Support Device (CSD) is associated with restoration of the ability of the SR to sequester Ca(2+). This improvement in SR function following chronic CSD therapy was due primarily to increased affinity of the SERCA-2a for calcium. The later was associated with (1) increased phosphorylation of PLB at serine 16 and threonine 17, (2) unchanged protein expression of PLB and (3) unchanged protein expression of SERCA-2a in LV myocardium of CSD-treated dogs compared to controls. This review summarizes our current understanding of the role of the CSD in modulating SR calcium cycling in an experimental canine model of chronic heart failure produced by multiple sequential intracoronary microembolizations.

Captopril–furosemide survival study in experimental heart failure

The strategy of diuretic drug administration remains to be defined in heart failure as experimental and clinical evidence have clearly established the benefit of neurohormonal blockade. The impact of addition of diuretic treatment to angiotensin-converting enzyme inhibition on survival remains to be demonstrated. The objectives of the study were to evaluate cardiovascular and renal effects of addition of diuretic treatment to captopril (2 g/L, in drinking water), on survival, on the experimental model of heart failure. Rats were followed for 10 months after coronary ligation. Echocardiographic, hemodynamic, morphometry and renal function investigations were then performed on surviving rats. Survival (from 34 to 61%), diuresis and natriuresis were significantly improved compared to control group only in animals treated with a combination of captopril + furosemide. In treated group: cardiac dimensions were reduced with left ventricular fractional shortening significantly increased in combination group (from 22.4 to 28%); captopril + furosemide animals had highest heart rate and lowest systolic and diastolic blood pressures; body and heart weight were reduced, but kidney weight was significantly increased with furosemide (1.7 g in control vs. 2 g in capto + furo); plasma renin activity and angiotensin 1 were greatly increased, and moderately stimulated in control. In conclusion, this combination of drugs significantly improved cardiac remodeling and survival of animals, increased diuresis and natriuresis despite stimulation of plasma renin activity and kidney hypertrophy.

Functional profiling of human atrial and ventricular gene expression

The purpose of our investigation was to identify the transcriptional basis for ultrastructural and functional specialization of human atria and ventricles. Using exploratory microarray analysis (Affymetrix U133A+B), we detected 11,740 transcripts expressed in human heart, representing the most comprehensive report of the human myocardial transcriptome to date. Variation in gene expression between atria and ventricles accounted for the largest differences in this data set, as 3.300 and 2.974 transcripts showed higher expression in atria and ventricles, respectively. Functional classification based on Gene Ontology identified chamber-specific patterns of gene expression and provided molecular insights into the regional specialization of cardiomyocytes, correlating important functional pathways to transcriptional activity: Ventricular myocytes preferentially express genes satisfying contractile and energetic requirements, while atrial myocytes exhibit specific transcriptional activities related to neurohumoral function. In addition, several pro-fibrotic and apoptotic pathways were concentrated in atrial myocardium, substantiating the higher susceptibility of atria to programmed cell death and extracellular matrix remodelling observed in human and experimental animal models of heart failure. Differences in transcriptional profiles of atrial and ventricular myocardium thus provide molecular insights into myocardial cell diversity and distinct region-specific adaptations to physiological and pathophysiological conditions. Moreover, as major functional classes of atrial- and ventricular-specific transcripts were common to human and murine myocardium, an evolutionarily conserved chamber-specific expression pattern in mammalian myocardium is suggested.

G-protein-coupled receptor kinase activity in human heart failure: Effects of β-adrenoceptor blockade

In human end-stage heart failure as well as in experimental animal models of heart failure, G-protein-coupled receptor kinase activity (GRK) is increased while beta-adrenoceptor responsiveness is diminished. In animal studies, beta-adrenoceptor blockers reverse the GRK-mediated desensitization and down-regulation of myocardial beta-adrenoceptors. The aim of this study was to investigate whether alterations in GRK activity are an early or late accompaniment of human heart failure and whether also in humans beta-adrenoceptor blocker treatment is able to influence myocardial GRK activity. We assessed in right atria, obtained from patients at different stages of heart failure, treated with or not treated with beta-adrenoceptor blockers, and in the four chambers of explanted hearts, obtained from patients with end-stage heart failure, beta-adrenoceptor density (by (-)-[(125)I]-iodocyanopindolol binding) and GRK activity (by an in vitro rhodopsin phosphorylation assay). With increasing severity of heart failure, plasma noradrenaline levels increased while myocardial beta-adrenoceptor density decreased with a maximum in GRK activity in end-stage heart failure. However, in relation to the progression of heart failure, we found that GRK activity transiently increased at an early stage of heart failure (NYHA I and II) but decreased back to control values in patients at NYHA III and IV. beta-Adrenoceptor blockers were able to reduce the early increase in GRK activity at NYHA I and II to control levels, whereas in those patients who did not have increased GRK activity (NYHA III and IV), they had only a marginal effect. According to our results, an increase in GRK activity is an early and transient event in the course of heart failure that can be prevented by beta-adrenoceptor blocker treatment.

MESENTERIC VASCULAR DYSFUNCTION AFTER CARDIOPULMONARY BYPASS WITH CARDIAC ARREST IS AGGRAVATED BY COEXISTENT HEART FAILURE

Although patients suffering from heart failure (HF) have an increased incidence of nonocclusive mesenteric ischemia after opened heart surgery, the impact of cardiopulmonary bypass with cardiac arrest (CPB) on mesenteric vascular circulation in such situation remains unexplored. Therefore, the present study investigates the effects of CPB on mesenteric vascular reactivity, regional metabolism, and oxidative stress in an experimental model of HF. Volume-overload HF was induced in six dogs by bilateral femoral arteriovenous fistula. Six sham-operated dogs were used as controls. Eight weeks later, the short-term effects of 90 min of CPB were assessed in vivo during acute experiments. The significant increase in left ventricular end-diastolic volume in HF animals did not influence the vasodilator response of the superior mesenteric artery to acetylcholine (ACH) and nitroprusside (SNP) under baseline conditions. However, reduced mesenteric oxygen delivery, increased oxygen extraction, and lactate release were found during CPB in the HF group. In addition, an increased free radical production was assessed in the HF group during (89 +/- 23 x 10 relative light units [RLU]) and after CPB (93 +/- 15 x 10 RLU) compared with controls (45 +/- 15 and 49 +/- 7 x 10 RLU, respectively). Finally, 90 min of CPB led to a more pronounced decrease of ACH- (-22% +/- 5% vs. -42% +/- 9%, P < 0.05) and SNP- (-14% +/- 4% vs. -50% +/- 7%, P < 0.002) induced mesenteric vasodilations in the HF group compared with controls. We conclude that coexistent HF significantly enhances the pathological effects of CPB on the mesenteric vascular circulation by additionally altering endothelial and smooth muscle vascular function consequent to augmented oxidative stress.

Probucol attenuates oxidative stress and energy decline in isoproterenol-induced heart failure in rat

In the present study, we examined whether the powerful antioxidant probucol (a clinically used lipid-lowering drug) would attenuate the oxidative stress and energy starvation in experimental model of heart failure (HF) using isoproterenol. Rats were injected subcutaneously with isoproterenol (2.4 mg kg −1) daily for 1 week, and then treated with probucol (61 mg/kg) daily for 2 weeks. Oxidative stress was assessed by measuring myocardial lipid peroxides level and antioxidant enzymes activities, glutathione peroxidase (GPx) and superoxide dismutase. In addition, cardiac metabolic damage was estimated by measuring myocardial ATP, ADP and AMP levels as well as ATP/ADP ratio. It was found that isoproterenol induced a significant increase in heart rate by approximately 30% as compared with the pre-value. These changes were significantly attenuated by post-treatment of rats with probucol. Also, isoproterenol induced several pathological changes including lymphocyte infiltration, myofibrillar hemorrhage and degeneration, and these changes were attenuated by probucol. In addition, animals treated with isoproterenol showed a significant increase in myocardial lipid peroxides level up to 163% and a significant decrease in myocardial GPx activity by 35% as compared with the control group. Probucol not only counteracted significantly the pronounced oxidative stress effect of isoproterenol but also it induced a significant increase in myocardial GPx as compared with the control group. The major new finding of the present study is that treatment with probucol induced a significant increase in myocardial ATP level (the source of energy) and ATP/ADP ratio. Moreover, there is a significant correlation between ATP/ADP ratio and myocardial probucol level. In conclusion, the cardioprotective effect of probucol in treatment of HF is a result of not only its antioxidant properties and an enhancement of endogenous antioxidant reserve (mainly GPx) but also an enhancement of myocardial energy state.

Poly(ADP-Ribose) Polymerase Promotes Cardiac Remodeling, Contractile Failure, and Translocation of Apoptosis-Inducing Factor in a Murine Experimental Model of Aortic Banding and Heart Failure

Oxidant stress-induced activation of poly(ADP-ribose) polymerase (PARP) plays a role in the pathogenesis of various cardiovascular diseases. We have now investigated the role of PARP in the process of cardiac remodeling and heart failure in a mouse model of heart failure induced by transverse aortic constriction (banding). The catalytic activity of PARP was inhibited by the potent isoindolinone-based PARP inhibitor INO-1001 or by PARP-1 genetic deficiency. PARP inhibition prevented the pressure overload-induced decrease in cardiac contractile function, despite the pressure gradient between both carotid arteries being comparable in the two experimental groups. The development of hypertrophy, the formation of collagen in the hearts, and the mitochondrial-to-nuclear translocation of the cell death factor apoptosis-inducing factor (AIF) were attenuated by PARP inhibition. The ability of the inhibitor to block the catalytic activity of PARP was confirmed by immunohistochemical detection of poly(ADP-ribose), the product of the enzyme in the heart. Plasma levels of INO-1001, as measured at the end of the experiments, were in the concentration range sufficient to block the oxidant-mediated activation of PARP in murine cardiac myocytes in vitro. Myocardial hypertrophy and AIF translocation was also reduced in PARP-1-deficient mice undergoing aortic banding, compared with their wild-type counterparts. Overall, the current results demonstrate the importance of poly(ADP-ribos)ylation in the pathogenesis of banding-induced heart failure.

Furosemide and the progression of left ventricular dysfunction in experimental heart failure

We tested the hypothesis that furosemide accelerates the progression of left ventricular systolic dysfunction in a tachycardia-induced porcine model of heart failure. Furosemide activates the renin-angiotensin-aldosterone system in patients with congestive heart failure (CHF). Such activation may contribute to CHF progression, but prospective data are lacking. Thirty-two Yorkshire pigs were randomized to furosemide (1 mg/kg intramuscularly daily, mean 16.1 +/- 0.9 mg) or placebo. Thereafter, a pacing model of heart failure was utilized to produce systolic dysfunction in both sets of animals (fractional shortening <0.16 by echocardiogram). The goal was to determine if furosemide would accelerate the progression of left ventricular dysfunction in the "treated" group. After sacrifice, sodium-calcium exchanger currents and their responsiveness to isoproterenol were measured during voltage clamp. All investigators were blinded to treatment assignment. Furosemide shortened the time to left ventricular dysfunction (35.1 +/- 5.1 days in placebo versus 21.4 +/- 3.2 days for furosemide animals; p = 0.038, log-rank test). By day 14, aldosterone levels were significantly higher in furosemide animals (43.0 +/- 11.8 ng/dl vs. 17.6 +/- 4.5 ng/dl; p < 0.05). Serum sodium was reduced (133.0 +/- 0.9 mmol/l furosemide vs. 135.7 +/- 0.8 mmol/l placebo; p < 0.05), but no difference in norepinephrine, potassium, magnesium, creatinine, or urea nitrogen was present. Basal sodium-calcium exchanger currents were significantly increased and isoproterenol responsiveness depressed by furosemide. Tachycardic pigs given furosemide had significant acceleration of both contractile and metabolic features of CHF, including left ventricular systolic dysfunction, elevated serum aldosterone levels, and altered calcium handling in a controlled experimental model of heart failure.

Early administration of either spironolactone or eprosartan reduces cardiac hypertrophy and fibrosis in rats with heart failure: possible involvement of ace and ace-2

P-140 Key Words: Aldosterone, Angiotensin II, Cardiac Hypertrophy

Role of at2 receptor in the protective effect of telmisartan in cardiac tissue

P-339 Key Words: Hypertension, Angiotensin-Receptor Blockers, Kinins

SUPPORTING EFFECT OF ARTIFICIAL MYOCARDIUM ON CONGESTIVE HEART FAILURE

Recently, cardiovascular events including ischemic heart diseases have been on the rise in Japan. What kind of assistance can be provided to patients with congestive heart failure? This assistance should focus on the contraction of myocardium. To achieve this objective, we have initiated the ARTIFICIAL MYOCARDIUM development project. In the first step, the satisfactory cardiac supporting effect of the pneumatic artificial myocardium was confirmed by the animal experiment. Next, the development of an implantable type device commenced. The artificial myocardium development using a ball screw motor was completed. We also planned the development of an electrohydraulic driving system. The drive system was equipped between ribs. Energy was supplied by a transcutaneous energy transmission system from the outside of the body. In this study, the supporting effect of an electrohydraulic artificial myocardium (EHAM) was evaluated in an experimental heart failure model. Ischemic heart failure was experimentally induced by the coronary ligation based on the Korean group. Firstly, we ligated the left anterior descending artery, and subsequently the 1st diagonal was ligated. EHAM was started for providing assistance after the induction of the ischemic heart failure. EHAM was attached to the left ventricle and fixed with banding. The results showed that a satisfactory supporting effect was observed during the EHAM assistance. Hemodynamic parameters returned to their normal range during the EHAM assistance. The results suggested the clinical feasibility of our devices.

Alterations of adenylyl cyclase and G proteins in aortocaval shunt-induced heart failure.

Unlike most other experimental models of congestive heart failure, the volume overload model induced by aortocaval shunt (AVS) in rats was found to exhibit enhanced beta-adrenoceptor (beta-AR) signaling. To study whether the adenylyl cyclase (AC)-G protein system is involved in such a change, we examined cardiac AC activity and protein content as well as G(s)alpha and G(i)alpha activities, protein contents, and mRNA levels in both left (LV) and right (RV) ventricles at the failing stage (16 wk after surgery). Basal and forskolin-stimulated AC activities were significantly increased in both LV and RV from the failing hearts; this change was associated with an upregulation of type V/VI AC protein. In contrast to 5'-guanylyl imidodiphosphate and NaF, the stimulatory effect of isoproterenol on AC was increased in the failing heart. Although G(s)alpha and G(i)alpha protein contents in the failing hearts were not altered, the mRNA level for G(s)alpha was decreased by 20% and that for G(i)alpha was increased by 20%. In addition, the activity of G(s)alpha, but not G(i)alpha, as assessed by toxin-catalyzed ADP ribosylation, was significantly decreased in the failing heart. Losartan and imidapril treatments improved cardiac function and attenuated alterations in mRNA levels for G(s)alpha and G(i)alpha proteins, as well as G(s)alpha activity, without affecting changes in AC protein content or activities in heart failure due to volume overload. These data suggest that increased AC activity may contribute to the enhanced beta-AR signaling in the AVS model of heart failure, whereas alterations in gene expression for G proteins may be of an adaptive nature at this stage of heart failure.

Endothelin Receptor Antagonists in Heart Failure

Experimental evidence suggests that endothelin substantially contributes to left ventricular remodelling and progression of heart failure. Plasma endothelin (ET)-1 levels are increased in patients with heart failure, independent of the aetiology, and correlate with the severity of the disease. Furthermore, tissue endothelin levels and endothelin receptors are upregulated in myocardium from animals and humans with heart failure. In several experimental models of left ventricular remodelling and/or heart failure, treatment with nonselective ET-A and -B as well as selective ET-A antagonists exerted beneficial cardiovascular effects. In patients with heart failure, short-term studies of treatment with endothelin antagonists demonstrated an improvement of haemodynamic parameters; however, long-term treatment with these drugs did not significantly improve combined morbidity/mortality endpoints. Furthermore, in the recently completed Endothelin-A Receptor Antagonist Trial in Heart Failure (EARTH) trial in patients with chronic heart failure, the selective ET-A receptor antagonist darusentan did not significantly affect left ventricular remodelling as assessed by cardiac magnetic resonance imaging. Potential reasons for the lack of beneficial effects of long-term treatment with ET antagonists in patients with heart failure include the following. Firstly, adverse effects on left ventricular healing have been observed when endothelin antagonist therapy was introduced early after myocardial infarction in rats. Secondly, the role of the ET-B receptor in the pathophysiology of heart failure and remodelling processes has not been clearly defined. Finally, for the detection of improvement in left ventricular remodelling, a study needs to be conducted in patients with recent myocardial infarction and signs of heart failure.