Does the beta2-agonist clenbuterol help to maintain myocardial potential to recover during mechanical unloading?

Sex-Linked Differences in Cardiac Atrophy After Mechanical Unloading Induced by Heterotopic Heart Transplantation

No information is available about sex-related differences in unloading-induced cardiac atrophy. We aimed to compare the course of unloading-induced cardiac atrophy in intact (without gonadectomy) male and female rats, and in animals after gonadectomy, to obtain insight into the influence of sex hormones on this process. Heterotopic heart transplantation (HT((x)) was used as a model for heart unloading. Cardiac atrophy was assessed as the weight ratio of heterotopically transplanted heart weight (HW) to the native HW on days 7 and 14 after HTx in intact male and female rats. In separate experimental groups, gonadectomy was performed in male and female recipient animals 28 days before HT(x) and the course of cardiac atrophy was again evaluated on days 7 and 14 after HT(x). In intact male rats, HT(x) resulted in significantly greater decreases in whole HW when compared to intact female rats. The dynamics of the left ventricle (LV) and right ventricle (RV) atrophy after HT(x) was quite similar to that of whole hearts. Gonadectomy did not have any significant effect on the decreases in whole HW, LV, and RV weights, with similar results in male and female rats. Our results show that the development of unloading-induced cardiac atrophy is substantially reduced in female rats when compared to male rats. Since gonadectomy did not alter the course of cardiac atrophy after HTx, similarly in both male and female rats, we conclude that sex-linked differences in the development of unloading-induced cardiac atrophy are not caused by the activity of sex hormones.

Clenbuterol Prevents Mechanical Unloading-Induced Myocardial Atrophy via Upregulation of Transient Receptor Potential Channel-3

Left ventricular assist device in combination with clenbuterol has been demonstrated to significantly improve heart function in patients with advanced heart failure. However, the roles of clenbuterol in mechanical unloading and its underlying mechanism are poorly understood. A rat abdominal heart transplantation model has been developed to mimic mechanical unloading of the heart. The recipient rats were randomly segregated into experimental groups for the daily administration of either saline (the "Trans" group; n = 13) or clenbuterol (2 mg/kg, the "Trans + CB" group; n = 12). Another group of 10 rats served as a treatment mimic control/sham animals (the "Sham" group). All interventions were performed via intraperitoneal injections once daily for 4 weeks. The Trans group animals exhibited myocardial atrophy and dysfunction with decreased expression levels of transient receptor potential channel 3 (TRPC3) and phospholipase C-β1 (PLC-β1) at 4 weeks post-transplantation. Administration of clenbuterol improved cardiac function, prevented myocardial atrophy, and restored expression of TRPC3 and PLC-β1 in the unloaded hearts of the "Trans + CB" animals at 4 weeks post-transplantation. Silencing of the TRPC3 gene by siRNA inhibited the pro-hypertrophic effect of clenbuterol in the rat primary cardiomyocytes in vitro. Furthermore, U73122, an inhibitor of the PLC-β1/diacylglycerol (DAG) pathway, significantly attenuated clenbuterol-induced upregulation of TRPC3 in cardiomyocytes. These findings suggest that the anti-atrophic effect of clenbuterol may be dependent on the upregulation of TRPC3 through the activation of the PLC-β1/DAG pathway during mechanical unloading. The results of our study reveal a potential target for the prevention and treatment of mechanical unloading-induced myocardial atrophy.

Effects of carvedilol and metoprolol on the myocardium during mechanical unloading in a rat heterotopic heart transplantation model

BACKGROUND AND OBJECTIVES

Left ventricular assist devices enable recovery from severe heart failure and serve as a bridge to heart transplantation. However, chronic mechanical unloading can impair myocardial recovery. We aimed to assess myocyte size, fibrosis, apoptosis, and β-adrenoreceptor levels after rats with left ventricle unloading induced by heterotopic heart transplantation were administered carvedilol and metoprolol.

METHODS

Thirty rats with heart transplants were divided randomly into control, carvedilol treatment, and metoprolol treatment groups. Follow-up was conducted after 2 and 4 weeks of unloading.

RESULTS

Carvedilol and metoprolol treatments did not prevent the decrease in myocyte diameter in unloaded left ventricles. Metoprolol significantly decreased the ratio of the fibrotic area in the unloaded heart, measured using Masson's trichrome staining after 2 weeks. However, carvedilol and metoprolol did not reduce apoptosis, based on measurements of terminal deoxynucleotidyl-transferase-mediated dUTP nick end-labelling positive cells and the expression of caspase-3 in unloaded hearts after 2 and 4 weeks. Metoprolol treatment did not significantly decrease the mRNA expression of myocardial SERCA2a in the unloaded heart after 2 weeks.

CONCLUSIONS

Compared to carvedilol treatment, metoprolol treatment improved myocardial fibrosis and SERCA2a expression to a greater extent; however, neither drug prevented myocardial apoptosis.

Evidence for synergy between sarcomeres and fibroblasts in an in vitro model of myocardial reverse remodeling

We have created a novel in-vitro platform to study reverse remodeling of engineered heart tissue (EHT) after mechanical unloading. EHTs were created by seeding decellularized porcine myocardial sections with a mixture of primary neonatal rat ventricular myocytes and cardiac fibroblasts. Each end of the ribbon-like constructs was fixed to a plastic clip, allowing the tissues to be statically stretched or slackened. Inelastic deformation was introduced by stretching tissues by 20% of their original length. EHTs were subsequently unloaded by returning tissues to their original, shorter length. Mechanical characterization of EHTs immediately after unloading and at subsequent time points confirmed the presence of a reverse-remodeling process, through which stress-free tissue length was increased after chronic stretch but gradually decreased back to its original value within 9 days. When a cardiac myosin inhibitor was applied to tissues after unloading, EHTs failed to completely recover their passive and active mechanical properties, suggesting a role for actomyosin contraction in reverse remodeling. Selectively inhibiting cardiomyocyte contraction or fibroblast activity after mechanical unloading showed that contractile activity of both cell types was required to achieve full remodeling. Similar tests with EHTs formed from human induced pluripotent stem cell-derived cardiomyocytes also showed reverse remodeling that was enhanced when treated with omecamtiv mecarbil, a myosin activator. These experiments suggest essential roles for active sarcomeric contraction and fibroblast activity in reverse remodeling of myocardium after mechanical unloading. Our findings provide a mechanistic rationale for designing potential therapies to encourage reverse remodeling in patient hearts.

Role of β-adrenergic signaling in masseter muscle

In skeletal muscle, the major isoform of β-adrenergic receptor (β-AR) is β2-AR and the minor isoform is β1-AR, which is opposite to the situation in cardiac muscle. Despite extensive studies in cardiac muscle, the physiological roles of the β-AR subtypes in skeletal muscle are not fully understood. Therefore, in this work, we compared the effects of chronic β1- or β2-AR activation with a specific β1-AR agonist, dobutamine (DOB), or a specific β2-AR agonist, clenbuterol (CB), on masseter and cardiac muscles in mice. In cardiac muscle, chronic β1-AR stimulation induced cardiac hypertrophy, fibrosis and myocyte apoptosis, whereas chronic β2-AR stimulation induced cardiac hypertrophy without histological abnormalities. In masseter muscle, however, chronic β1-AR stimulation did not induce muscle hypertrophy, but did induce fibrosis and apoptosis concomitantly with increased levels of p44/42 MAPK (ERK1/2) (Thr-202/Tyr-204), calmodulin kinase II (Thr-286) and mammalian target of rapamycin (mTOR) (Ser-2481) phosphorylation. On the other hand, chronic β2-AR stimulation in masseter muscle induced muscle hypertrophy without histological abnormalities, as in the case of cardiac muscle, concomitantly with phosphorylation of Akt (Ser-473) and mTOR (Ser-2448) and increased expression of microtubule-associated protein light chain 3-II, an autophagosome marker. These results suggest that the β1-AR pathway is deleterious and the β2-AR is protective in masseter muscle. These data should be helpful in developing pharmacological approaches for the treatment of skeletal muscle wasting and weakness.

Heterotopic Abdominal Rat Heart Transplantation as a Model to Investigate Volume Dependency of Myocardial Remodeling

Heterotopic abdominal rat heart transplantation has been extensively used to investigate ischemic-reperfusion injury, immunological consequences during heart transplantations and also to study remodeling of the myocardium due to volume unloading. We provide a unique review on the latter and present a summary of the experimental studies on rat heart transplantation to illustrate changes that occur to the myocardium due to volume unloading. We divided the literature based on whether normal or failing rat heart models were used. This analysis may provide a basis to understand the physiological effects of mechanical circulatory support therapy.

Suppression of microRNA-16 protects against acute myocardial infarction by reversing beta2-adrenergic receptor down-regulation in rats

microRNA-16 (miR-16) has been shown to be up-regulated in ischemic heart. Beta2-adrenoreceptor (β2-AR) exerts cardioprotective property in ischemic injury. This study aims to determine the effect of miR-16 in cardiac injury in rats and the possible involvement of β2-AR in this process. Acute myocardial infarction (AMI) model in rats was induced by ligation of left coronary artery. Neonatal rat ventricular cells (NRVCs) were cultured in vitro tests. The cardiomyocyte model of oxidative injury was mimicked by hydrogen peroxide. The expression of miR-16 was obviously up-regulated and β2-AR was remarkably down-regulated in both AMI rats and NRVCs under oxidative stress. miR-16 over-expression in NRVCs reduced cell viability and increased apoptosis. Conversely, inhibition of endogenous miR-16 with its specific inhibitor reversed these changes. Over-expression of miR-16 using an miR-16 lentivirus in AMI rats markedly increased cardiac infarct area, lactate dehydrogenase and creatine kinase activity, and exacerbated cardiac dysfunction. Lentivirus-mediated knockdown of miR-16 alleviated acute cardiac injury. Moreover, miR-16 over-expression significantly suppressed β2-AR protein expression in both cultured NRVCs and AMI rats, while inhibition of miR-16 displayed opposite effect on β2-AR protein expression. Luciferase assay confirmed that miR-16 could directly target the 3'untranslated region of β2-AR mRNA. miR-16 is detrimental to the infarct heart and suppression of miR-16 protects rat hearts from ischemic injury via up-regulating of β2-AR by binding to the 3'untranslated region of β2-AR gene. This study indicates that targeting miR-16/β2-AR axis may be a promising strategy for ischemic heart disease.

Stimulation of β-adrenergic receptors plays a protective role via increased expression of RAF-1 and PDX-1 in hyperglycemic rat pancreatic islet (RIN-m5F) cells

INTRODUCTION

It is a widely held view that a progressive reduction of beta-cell mass occurs in the progression of diabetes. RAF-1 kinase and pancreas duodenal homeobox 1 (PDX-1) are major factors that promote survival of cells and maintain normal insulin functions. In this study we investigated the effect of a β-adrenergic receptor agonist and antagonist on RAF-1 and PDX-1, and their respective effects on apoptosis and insulin release in RIN-m5F cells.

MATERIAL AND METHODS

RIN-m5F cells were cultured in normal (5 mM) and high (25 mM) glucose to mimic diabetic conditions, followed by treatment with 5 µM, 10 µM and 20 µM of isoproterenol and isoproterenol + propranolol for 6, 12 and 24 h. Western blotting and reverse transcription analysis were performed to examine the expression of RAF-1 and PDX-1. Annexin-V-FITC and terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) assays were used to investigate apoptosis. ELISA was used to measure insulin levels. Reverse transcription polymerase chain reaction was conducted to investigate the expression of genes.

RESULTS

Stimulation of β-adrenergic receptors with isoproterenol significantly induced RAF-1 and PDX-1 genes in a concentration-dependent and time-independent manner. Changes were significant both at protein and mRNA levels. Up-regulation of RAF-1 and PDX-1 was accompanied by improved insulin levels and reduced apoptosis. Concentrations of 10 µM and 20 µM for 12 and 24 h were more effective in achieving significant differences in the experimental and control groups. Propranolol reversed the effect of isoproterenol mostly at maximum concentrations and time periods.

CONCLUSIONS

A positive effect of a β-adrenergic agonist on RAF-1 and PDX-1, reduction in β-cell apoptosis and improved insulin contents can help to understand the pathogenesis of diabetes and to develop novel approaches for the β-cell dysfunction in diabetes.

Rat Heterotopic Heart Transplantation Model to Investigate Unloading-Induced Myocardial Remodeling

Unloading of the failing left ventricle in order to achieve myocardial reverse remodeling and improvement of contractile function has been developed as a strategy with the increasing frequency of implantation of left ventricular assist devices in clinical practice. But, reverse remodeling remains an elusive target, with high variability and exact mechanisms still largely unclear. The small animal model of heterotopic heart transplantation (hHTX) in rodents has been widely implemented to study the effects of complete and partial unloading on cardiac failing and non-failing tissue to better understand the structural and molecular changes that underlie myocardial recovery. We herein review the current knowledge on the effects of volume unloading the left ventricle via different methods of hHTX in rats, differentiating between changes that contribute to functional recovery and adverse effects observed in unloaded myocardium. We focus on methodological aspects of heterotopic transplantation, which increase the correlation between the animal model and the setting of the failing unloaded human heart. Last, but not least, we describe the late use of sophisticated techniques to acquire data, such as small animal MRI and catheterization, as well as ways to assess unloaded hearts under "reloaded" conditions. While giving regard to certain limitations, heterotopic rat heart transplantation certainly represents the crucial model to mimic unloading-induced changes in the heart and as such the intricacies and challenges deserve highest consideration. Careful translational research will further improve our knowledge of the reverse remodeling process and how to potentiate its effect in order to achieve recovery of contractile function in more patients.

Impact of combined clenbuterol and metoprolol therapy on reverse remodelling during mechanical unloading

Background

Clenbuterol (Cl), a β₂ agonist, is associated with enhanced myocardial recovery during left ventricular assist device (LVAD) support, and exerts beneficial remodelling effects during mechanical unloading (MU) in rodent heart failure (HF). However, the specific effects of combined Cl+β₁ blockade during MU are unknown.

Methods and Results

We studied the chronic effects (4 weeks) of β₂-adrenoceptor (AR) stimulation via Cl (2 mg/kg/day) alone, and in combination with β₁-AR blockade using metoprolol ((Met), 250 mg/kg/day), on whole heart/cell structure, function and excitation-contraction (EC) coupling in failing (induced by left coronary artery (LCA) ligation), and unloaded (induced by heterotopic abdominal heart transplantation (HATx)) failing rat hearts. Combined Cl+Met therapy displayed favourable effects in HF: Met enhanced Cl's improvement in ejection fraction (EF) whilst preventing Cl-induced hypertrophy and tachycardia. During MU combined therapy was less beneficial than either mono-therapy. Met, not Cl, prevented MU-induced myocardial atrophy, with increased atrophy occurring during combined therapy. MU-induced recovery of Ca²⁺ transient amplitude, speed of Ca²⁺ release and sarcoplasmic reticulum Ca²⁺ content was enhanced equally by Cl or Met mono-therapy, but these benefits, together with Cl's enhancement of sarcomeric contraction speed, and MU-induced recovery of Ca²⁺ spark frequency, disappeared during combined therapy.

Conclusions

Combined Cl+Met therapy shows superior functional effects to mono-therapy in rodent HF, but appears inferior to either mono-therapy in enhancing MU-induced recovery of EC coupling. These results suggest that combined β₂-AR simulation +β₁-AR blockade therapy is likely to be a safe and beneficial therapeutic HF strategy, but is not as effective as mono-therapy in enhancing myocardial recovery during LVAD support.

Selective β2-adrenoreceptor stimulation attenuates myocardial cell death and preserves cardiac function after ischemia-reperfusion injury

OBJECTIVE

β(2)-adrenoreceptor activation has been shown to protect cardiac myocytes from cell death. We hypothesized that acute β(2)-adrenoreceptor stimulation, using arformoterol (ARF), would attenuate myocardial ischemia/reperfusion (R) injury via NO synthase activation and cause a subsequent increase in NO bioavailability.

METHODS AND RESULTS

Male C57BL/6J and endothelial NO synthase (eNOS) knockout mice were subjected to 45 minutes of myocardial ischemia and 24 hours of R. ARF or vehicle was administered 5 minutes before R. Serum troponin-I was measured, and infarct size per area-at-risk was evaluated at 24 hours of R. Echocardiography was performed at baseline and 2 weeks after R. Myocardial cAMP, protein kinase A, eNOS/Akt phosphorylation status, and NO metabolite levels were assayed. ARF (1 µg/kg) reduced infarct size per area-at-risk by 53.1% (P<0.001 versus vehicle) and significantly reduced troponin-I levels (P<0.001 versus vehicle). Ejection fraction was significantly preserved in ARF-treated hearts compared with vehicle hearts at 2 weeks of R. Serum cAMP and nuclear protein kinase A C-α increased 5 and 15 minutes after ARF injection, respectively (P<0.01). ARF increased Akt phosphorylation at Thr(308) (P<0.001) and Ser(473) (P<0.01), and eNOS phosphorylation at Ser(1177) (P<0.01). ARF treatment increased heart nitrosothiol levels (P<0.001) at 15 min after injection. ARF failed to reduce infarct size in eNOS(-/-) mice.

CONCLUSIONS

Our results indicate that β(2)-adrenoreceptor stimulation activates cAMP, protein kinase A, Akt, and eNOS and augments NO bioavailability. Activation of this prosurvival signaling pathway attenuates myocardial cell death and preserves cardiac function after ischemia/reperfusion.

Clinical, molecular, and genomic changes in response to a left ventricular assist device

The use of left ventricular assist devices in treating patients with end-stage heart failure has increased significantly in recent years, both as a bridge to transplantation and as destination therapy in those who are ineligible for cardiac transplantation. This increase is based largely on the results of several recently completed clinical trials with the new second-generation continuous-flow devices that showed significant improvements in survival, functional capacity, and quality of life. Additional information on the use of the first- and second-generation left ventricular assist devices has come from a recently released report spanning the years 2006 to 2009, from the Interagency Registry for Mechanically Assisted Circulatory Support, a National Heart, Lung, and Blood Institute-sponsored collaboration between the U.S. Food and Drug Administration, the Centers for Medicare and Medicaid Services, and the scientific community. The authors review the latest clinical trials and data from the registry, with tight integration of the landmark molecular, cellular, and genomic research that accompanies the reverse remodeling of the human heart in response to a left ventricular assist device and functional recovery that has been reported in a subset of these patients.

Thyroid hormone and chronically unloaded hearts

The heart is subjected to chronic mechanical unloading during prolonged spaceflight and microgravity. The heart in patients with end-stage heart failure is also unloaded in prolonged duration after left ventricular assist devices (LVAD) are implanted. Heterotopic heart transplantation in rats is an established model of chronic cardiac unloading, and has been used to investigate the effects of chronic cardiac unloading on the heart. Observations that have been found using this experimental model are as follow. Chronic cardiac unloading induces time-dependent depressions of Ca2+ handling and myocyte contractility, which are associated with the shift of myosin heavy chain (MHC) isozymes and altered expressions of Ca2+ cycling-related proteins. Treatment with the physiological treatment dose of thyroid hormone restores the expression levels of Ca2+ cycling-related proteins, Ca2+ handling, and contractile function of cardiac myocytes in chronically unloaded hearts. Although future studies are required to determine precise mechanisms of the beneficial effects of thyroid hormone on chronically unloaded hearts, these observations may have clinical implications in the future for chronic cardiac unloading in the space industry as well as in the treatment of patients with end-stage heart failure supported by LVAD.

Effects of clenbuterol on contractility and Ca2+ homeostasis of isolated rat ventricular myocytes

Clenbuterol, a compound classified as a beta2-adrenoceptor (AR) agonist, has been employed in combination with left ventricular assist devices (LVADs) to treat patients with severe heart failure. Previous studies have shown that chronic administration of clenbuterol affects cardiac excitation-contraction coupling. However, the acute effects of clenbuterol and the signaling pathway involved remain undefined. We investigated the acute effects of clenbuterol on isolated ventricular myocyte sarcomere shortening, Ca2+ transients, and L-type Ca2+ current and compared these effects to two other clinically used beta2-AR agonists: fenoterol and salbutamol. Clenbuterol (30 microM) produced a negative inotropic response, whereas fenoterol showed a positive inotropic response. Salbutamol had no significant effects. Clenbuterol reduced Ca2+ transient amplitude and L-type Ca2+ current. Selective beta1-AR blockade did not affect the action of clenbuterol on sarcomere shortening but significantly reduced contractility in the presence of fenoterol and salbutamol (P < 0.05). Incubation with 2 microg/ml pertussis toxin significantly reduced the negative inotropic effects of 30 microM clenbuterol. In addition, overexpression of inhibitory G protein (Gi) by adenoviral transfection induced a stronger clenbuterol-mediated negative inotropic effect, suggesting the involvement of the Gi protein. We conclude that clenbuterol does not increase and, at high concentrations, significantly depresses contractility of isolated ventricular myocytes, an effect not seen with fenoterol or salbutamol. In its negative inotropism, clenbuterol predominantly acts through Gi, and the consequent downstream signaling pathways activation may explain the beneficial effects observed during chronic administration of clenbuterol in patients treated with LVADs.

Effect of clenbuterol administration on the healthy murine heart

Clenbuterol has recently been shown to reverse pathologic cardiac remodeling during left ventricular assist device (LVAD) support, leading to restored ventricular function and explantation of LVAD devices. However, others have not been able to support these observations. Our hypothesis is that the β2-adrenergic activity of clenbuterol induces cardiac extracellular matrix (ECM) remodeling, resulting in increased interstitial fibrillar collagen content and altered diastolic function that may account for these conflicting reports. The intent of this study is to characterize the effect of clenbuterol on healthy murine hearts with transthoracic echo and histology. C57BL/6 female mice were administered 2.4 µg/kg/day of clenbuterol in the drinking water for 7 days and analysis conducted on day 8–24 hours after the last dose of clenbuterol. Histological analysis demonstrated an increase in left ventricular ECM collagen content in a control group compared with the clenbuterol group (density 0.32 ± 0.16 compared to 2.01 ± 0.30 RD/mm2). The ventricular fibrosis was supported by altered diastolic function measured by transthoracic echo where there was a significant increase in isovolumic relaxation time, and left atrial dimension and a decrease in left ventricular free wall tissue Doppler ratios. Our study showed no significant differences in left ventricular ejection fraction, cardiac output, or heart rate between the clenbuterol and control groups. These data suggest that the β-2 adrenergic activity of clenbuterol increases ECM fibrillar collagen concentrations in normal hearts, resulting in altered diastolic function.

Role and possible mechanisms of clenbuterol in enhancing reverse remodelling during mechanical unloading in murine heart failure

Aims

Combined left ventricular assist device (LVAD) and pharmacological therapy has been proposed to favour myocardial recovery in patients with end-stage heart failure (HF). Clenbuterol (Clen), a β₂-adrenoceptor (β₂-AR) agonist, has been used as a part of this strategy. In this study, we investigated the direct effects of clenbuterol on unloaded myocardium in HF.

Methods and results

Left coronary artery ligation or sham operation was performed in male Lewis rats. After 4–6 weeks, heterotopic abdominal transplantation of the failing hearts into normal recipients was performed to induce LV unloading (UN). Recipient rats were treated with saline (Sal) or clenbuterol (2 mg/kg/day) via osmotic minipumps (HF + UN + Sal or HF + UN + Clen) for 7 days. Non-transplanted HF animals were treated with Sal (Sham + Sal, HF + Sal) or clenbuterol (HF + Clen). LV myocytes were isolated and studied using optical, fluorescence, and electrophysiological techniques. Clenbuterol treatment improved in vivo LV function measured with echocardiography (LVEF (%): HF 35.9 ± 2 [16], HF + Clen 52.1 ± 1.4 [16]; P < 0.001; mean ± SEM [n]). In combination with unloading, clenbuterol increased sarcomere shortening (amplitude (µm): HF + UN + Clen 0.1 ± 0.01 [50], HF + UN + Sal 0.07 ± 0.01 [38]; P < 0.001) by normalizing the depressed myofilament sensitivity to Ca²⁺ (slope of the linear relationship between Ca²⁺ transient and sarcomere shortening hysteresis loop during relaxation (μm/ratio unit): HF + UN + Clen 2.13 ± 0.2 [52], HF + UN + Sal 1.42 ± 0.13 [38]; P < 0.05).

Conclusion

Clenbuterol treatment of failing rat hearts, alone or in combination with mechanical unloading, improves LV function at the whole-heart and cellular levels by affecting cell morphology, excitation–contraction coupling, and myofilament sensitivity to calcium. This study supports the use of this drug in the strategy to enhance recovery in HF patients treated with LVADs and also begins to elucidate some of the possible cellular mechanisms responsible for the improvement in LV function.

Reverse remodeling during long-term mechanical unloading of the left ventricle

A significant proportion of patients placed on long-term mechanical circulatory support for end-stage heart failure can be weaned from mechanical assistance after functional recovery of their native heart ("bridge to recovery"). The pathophysiological mechanisms implicated in reverse remodeling that cause a sustained functional myocardial recovery have recently become the subject of intensive research, expected to provide information with a view to accurately identify reliable prognostic indicators of recovery. In addition, this kind of information will enable changes in the strategy of myocardial recovery by modifying the duration and scale of the unloading regimen or by combining it with other treatments that promote reverse remodeling.

Depressed contractile reserve and impaired calcium handling of cardiac myocytes from chronically unloaded hearts are ameliorated with the administration of physiological treatment dose of T3 in rats

AIM

Chronic cardiac unloading causes a time-dependent upregulation of phospholamban (PLB) and depression of myocyte contractility in normal rat hearts. As thyroid hormone is known to decrease PLB expression, we examined whether thyroid hormone restores the depressed contractile performance of myocytes from chronically unloaded hearts.

METHODS

Cardiac unloading was induced by heterotopic heart transplantation in isogenic rats for 5 weeks. Animals were treated with either vehicle or physiological treatment dose of 3,5,3'-triiodo-L-thyronine (T3) that does not cause hyperthyroidism for the last 3 weeks (n=20 each).

RESULTS

In vehicle-treated animals, myocyte relaxation and [Ca2+]i decay were slower in unloaded hearts than in recipient hearts. Myocyte shortening in response to high [Ca2+]o was also depressed with impaired augmentation of peak-systolic [Ca2+]i in unloaded hearts compared with recipient hearts. In vehicle-treated rats, protein levels of PLB were increased by 136% and the phosphorylation level of PLB at Ser16 were decreased by 32% in unloaded hearts compared with recipient hearts. By contrast, in the T3-treated animals, the slower relaxation, delayed [Ca2+]i decay, and depressed contractile reserve in myocytes from unloaded hearts were all returned to normal levels. Furthermore, in the T3-treated animals, there was no difference either in the PLB protein level or in its Ser16-phosphorylation level between unloaded and recipient hearts.

CONCLUSION

These results suggest that the treatment with physiological treatment dose of thyroid hormone rescues the impaired myocyte relaxation and depressed contractile reserve at least partially through the restoration of PLB protein levels and its phosphorylation state in chronically unloaded hearts.