Effects of chronic administration of clenbuterol on function and metabolism of adult rat cardiac muscle

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.

An analysis of risk factors for a fracture or luxation in recovery from general anaesthesia in horses: a single centre study

Catastrophic fractures or luxations (FoL) sustained during recovery from general anaesthesia are a significant cause of mortality during equine anaesthesia. There is a lack of evidence regarding potential risk factors for a FoL occurring in the immediate anaesthetic recovery period. A single centre, retrospective, case-matched study was performed to identify risk factors for sustaining a catastrophic FoL during recovery from general anaesthesia. Clinical data were obtained for horses which sustained a catastrophic FoL when recovering from general anaesthesia from January 2011- June 2020 in a single centre referral population. Multivariable logistical regression analysis was performed to identify risk factors which were significant in horses where a FoL occurred. Statistically significant risk factors in our population of horses of sustaining a FoL in recovery included intra-operative administration of intra-tracheal salbutamol, intra-operative administration of ketamine and increasing age. Further research in this area, particularly with regards to salbutamol administration, is required.

High Throughput Procedure for Comparative Analysis of In Vivo Cardiac Glucose or Amino Acids Use in Cardiovascular Pathologies and Pharmacological Treatments

The heart is characterized by the prominent flexibility of its energy metabolism and is able to use diverse carbon substrates, including carbohydrates and amino acids. Cardiac substrate preference could have a major impact on the progress of cardiac pathologies. However, the majority of methods to investigate changes in substrates’ use in cardiac metabolism in vivo are complex and not suitable for high throughput testing necessary to understand and reverse these pathologies. Thus, this study aimed to develop a simple method that would allow for the analysis of cardiac metabolic substrate use. The developed methods involved the subcutaneous injection of stable ¹³C isotopomers of glucose, valine, or leucine with mass spectrometric analysis for the investigation of its entry into cardiac metabolic pathways that were deducted from ¹³C alanine and glutamate enrichments in heart extracts. The procedures were validated by confirming the known effects of treatments that modify glucose, free fatty acids, and amino acid metabolism. Furthermore, we studied changes in the energy metabolism of CD73 knock-out mice to demonstrate the potential of our methods in experimental research. The methods created allowed for fast estimation of cardiac glucose and amino acid use in mice and had the potential for high-throughput analysis of changes in pathology and after pharmacological treatments.

Enhanced cardiac hypoxic injury in atherogenic dyslipidaemia results from alterations in the energy metabolism pattern

OBJECTIVE

Dyslipidaemia is a major risk factor for myocardial infarction that is known to correlate with atherosclerosis in the coronary arteries. We sought to clarify whether metabolic alterations induced by dyslipidaemia in cardiomyocytes collectively constitute an alternative pathway that escalates myocardial injury.

METHODS

Dyslipidaemic apolipoprotein E and low-density lipoprotein receptor (ApoE/LDLR) double knockout (ApoE^-/-/LDLR^-/-) and wild-type C57BL/6 (WT) mice aged six months old were studied. Cardiac injury under reduced oxygen supply was evaluated by 5 min exposure to 5% oxygen in the breathing air under electrocardiogram (ECG) recording and with the assessment of troponin I release. To address the mechanisms LC/MS was used to analyse the cardiac proteome pattern or in vivo metabolism of stable isotope-labelled substrates and HPLC was applied to measure concentrations of cardiac high-energy phosphates. Furthermore, the effect of blocking fatty acid use with ranolazine on the substrate preference and cardiac hypoxic damage was studied in ApoE^-/-/LDLR^-/- mice.

RESULTS

Hypoxia induced profound changes in ECG ST-segment and troponin I leakage in ApoE^-/-/LDLR^-/- mice but not in WT mice. The evaluation of the cardiac proteomic pattern revealed that ApoE^-/-/LDLR^-/- as compared with WT mice were characterised by coordinated increased expression of mitochondrial proteins, including enzymes of fatty acids' and branched-chain amino acids' oxidation, accompanied by decreased expression levels of glycolytic enzymes. These findings correlated with in vivo analysis, revealing a reduction in the entry of glucose and enhanced entry of leucine into the cardiac Krebs cycle, with the cardiac high-energy phosphates pool maintained. These changes were accompanied by the activation of molecular targets controlling mitochondrial metabolism. Ranolazine reversed the oxidative metabolic shift in ApoE^-/-/LDLR^-/- mice and reduced cardiac damage induced by hypoxia.

CONCLUSIONS

We suggest a novel mechanism for myocardial injury in dyslipidaemia that is consequent to an increased reliance on oxidative metabolism in the heart. The alterations in the metabolic pattern that we identified constitute an adaptive mechanism that facilitates maintenance of metabolic equilibrium and cardiac function under normoxia. However, this adaptation could account for myocardial injury even in a mild reduction of oxygen supply.

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.

Performance-Enhancing Drugs Abuse Caused Cardiomyopathy and Acute Hepatic Injury in a Young Bodybuilder

A number of performance-enhancing drugs (PEDs) are used illicitly to improve muscle strength by the bodybuilders. The misuse of these drugs is associated with serious adverse effects to different organs. A previously healthy 22-year-old male bodybuilder after taking stanozolol, clenbuterol, and triiodothyronine for 10 days presented to the hospital with symptoms of icteric sclera, progressive dyspnea, intermittent cough, and bloody sputum. He was diagnosed with dilated cardiomyopathy and acute hepatic injury. Rapidly progressive dilated cardiomyopathy and acute hepatic injury among bodybuilders in such a short period of time have not been reported. People using these drugs must monitor liver and cardiac functions regularly, and they should discontinue using PEDs after diagnosis of liver or cardiac abnormalities. Physicians should always consider the possibility of the PED abuse in the context of a young athlete suffering cardiomyopathy or hepatic injury.

An impaired metabolism of nucleotides underpins a novel mechanism of cardiac remodeling leading to Huntington's disease related cardiomyopathy

Huntington's disease (HD) is mainly thought of as a neurological disease, but multiple epidemiological studies have demonstrated a number of cardiovascular events leading to heart failure in HD patients. Our recent studies showed an increased risk of heart contractile dysfunction and dilated cardiomyopathy in HD pre-clinical models. This could potentially involve metabolic remodeling, that is a typical feature of the failing heart, with reduced activities of high energy phosphate generating pathways. In this study, we sought to identify metabolic abnormalities leading to HD-related cardiomyopathy in pre-clinical and clinical settings. We found that HD mouse models developed a profound deterioration in cardiac energy equilibrium, despite AMP-activated protein kinase hyperphosphorylation. This was accompanied by a reduced glucose usage and a significant deregulation of genes involved in de novo purine biosynthesis, in conversion of adenine nucleotides, and in adenosine metabolism. Consequently, we observed increased levels of nucleotide catabolites such as inosine, hypoxanthine, xanthine and uric acid, in murine and human HD serum. These effects may be caused locally by mutant HTT, via gain or loss of function effects, or distally by a lack of trophic signals from central nerve stimulation. Either may lead to energy equilibrium imbalances in cardiac cells, with activation of nucleotide catabolism plus an inhibition of re-synthesis. Our study suggests that future therapies should target cardiac mitochondrial dysfunction to ameliorate energetic dysfunction. Importantly, we describe the first set of biomarkers related to heart and skeletal muscle dysfunction in both pre-clinical and clinical HD settings.

Effects of chronic treatment with the new ultra-long-acting β2 -adrenoceptor agonist indacaterol alone or in combination with the β1 -adrenoceptor blocker metoprolol on cardiac remodelling

BACKGROUND AND PURPOSE

The ability of a chronic treatment with indacaterol, a new ultra-long-acting β2 -adrenoceptor agonist, to reverse cardiac remodelling and its effects in combination with metoprolol, a selective β1 -adrenoceptor antagonist, were investigated on myocardial infarction in a rat model of heart failure (HF).

EXPERIMENTAL APPROACH

We investigated the effects of indacaterol and metoprolol, administered alone or in combination, on myocardial histology, β-adrenoceptor-mediated pathways, markers of remodelling and haemodynamic parameters in a rat model of HF. Five groups of rats were assessed: sham-operated rats; HF rats; HF + indacaterol 0.3 mg·kg(-1) ·day(-1) ; HF + metoprolol 100 mg·kg(-1) ·day(-1) ; HF + metoprolol + indacaterol. All pharmacological treatments continued for 15 weeks.

KEY RESULTS

Treatment with either indacaterol or metoprolol significantly reduced the infarct size in HF rats. However, the combination of indacaterol and metoprolol reduced the infarct size even further, reduced both BP and heart rate, reversed the decrease in ejection fraction, normalized left ventricular systolic and diastolic internal diameters, normalized the decreased β1 adrenoceptor mRNA expression as well as cardiac cAMP levels and reduced cardiac GPCR kinase 2 expression, compared with the untreated HF group.

CONCLUSION AND IMPLICATIONS

The results of our study demonstrated an additive interaction between indacaterol and metoprolol in normalizing and reversing cardiac remodelling in our experimental model of HF. The translation of these findings to clinical practice might be of interest, as this combination of drugs could be safer and more effective in patients suffering from HF and COPD.

HDAC4-myogenin axis as an important marker of HD-related skeletal muscle atrophy

Skeletal muscle remodelling and contractile dysfunction occur through both acute and chronic disease processes. These include the accumulation of insoluble aggregates of misfolded amyloid proteins that is a pathological feature of Huntington’s disease (HD). While HD has been described primarily as a neurological disease, HD patients’ exhibit pronounced skeletal muscle atrophy. Given that huntingtin is a ubiquitously expressed protein, skeletal muscle fibres may be at risk of a cell autonomous HD-related dysfunction. However the mechanism leading to skeletal muscle abnormalities in the clinical and pre-clinical HD settings remains unknown. To unravel this mechanism, we employed the R6/2 transgenic and HdhQ150 knock-in mouse models of HD. We found that symptomatic animals developed a progressive impairment of the contractile characteristics of the hind limb muscles tibialis anterior (TA) and extensor digitorum longus (EDL), accompanied by a significant loss of motor units in the EDL. In symptomatic animals, these pronounced functional changes were accompanied by an aberrant deregulation of contractile protein transcripts and their up-stream transcriptional regulators. In addition, HD mouse models develop a significant reduction in muscle force, possibly as a result of a deterioration in energy metabolism and decreased oxidation that is accompanied by the re-expression of the HDAC4-DACH2-myogenin axis. These results show that muscle dysfunction is a key pathological feature of HD.

Huntington’s disease (HD) is a neurodegenerative disorder in which the mutation results in an extra-long tract of glutamines that causes the huntingtin protein to aggregate. It is characterized by neurological symptoms and brain pathology, which is associated with nuclear and cytoplasmic protein aggregates and with transcriptional deregulation. Despite the fact that HD has been recognized principally as a neurological disease, there are multiple studies indicating that peripheral pathologies including cardiac dysfunction and skeletal muscle atrophy, contribute to the overall progression of HD. To unravel the cause of the skeletal muscle dysfunction, we applied a wide range of molecular and physiological methods to the analysis of two well established genetic mouse models of this disease. We found that symptomatic animals developed muscle dysfunction characterised by a change in the contractile characteristics of fast twitch muscles and a decrease in twitch and tetanic force of hindlimb muscles. In addition, there is a significant decrease in the number of motor units innervating the EDL muscle, and this motor unit loss progresses during the course of the disease. These changes were accompanied by the re-expression of contractile transcripts and markers of muscle denervation such as the HDAC4-Dach2-myogenin axis, as well as the apparent deterioration in energy metabolism and decreased oxidation. Therefore, we conclude, that the HD-related skeletal muscle atrophy is accompanied by progressive loss of functional motor units.

Pharmacology of manipulating lean body mass

Dysfunction and wasting of skeletal muscle as a consequence of illness decreases the length and quality of life. Currently, there are few, if any, effective treatments available to address these conditions. Hence, the existence of this unmet medical need has fuelled large scientific efforts. Fortunately, these efforts have shown many of the underlying mechanisms adversely affecting skeletal muscle health. With increased understanding have come breakthrough disease-specific and broad spectrum interventions, some progressing through clinical development. The present review focuses its attention on the role of the antagonistic process regulating skeletal muscle mass before branching into prospective promising therapeutic targets and interventions. Special attention is given to therapies in development against cancer cachexia and Duchenne muscular dystrophy before closing remarks on design and conceptualization of future therapies are presented to the reader.

Atomoxetine prevents dexamethasone-induced skeletal muscle atrophy in mice

Skeletal muscle atrophy remains a clinical problem in numerous pathologic conditions. β2-Adrenergic receptor agonists, such as formoterol, can induce mitochondrial biogenesis (MB) to prevent such atrophy. Additionally, atomoxetine, an FDA-approved norepinephrine reuptake inhibitor, was positive in a cellular assay for MB. We used a mouse model of dexamethasone-induced skeletal muscle atrophy to investigate the potential role of atomoxetine and formoterol to prevent muscle mass loss. Mice were administered dexamethasone once daily in the presence or absence of formoterol (0.3 mg/kg), atomoxetine (0.1 mg/kg), or sterile saline. Animals were euthanized at 8, 16, and 24 hours or 8 days later. Gastrocnemius muscle weights, changes in mRNA and protein expression of peroxisome proliferator-activated receptor-γ coactivator-1 α (PGC-1α) isoforms, ATP synthase β, cytochrome c oxidase subunit I, NADH dehydrogenase (ubiquinone) 1 β subcomplex, 8, ND1, insulin-like growth factor 1 (IGF-1), myostatin, muscle Ring-finger protein-1 (muscle atrophy), phosphorylated forkhead box protein O 3a (p-FoxO3a), Akt, mammalian target of rapamycin (mTOR), and ribosomal protein S6 (rp-S6; muscle hypertrophy) in naive and muscle-atrophied mice were measured. Atomoxetine increased p-mTOR 24 hours after treatment in naïve mice, but did not change any other biomarkers. Formoterol robustly activated the PGC-1α-4-IGF1-Akt-mTOR-rp-S6 pathway and increased p-FoxO3a as early as 8 hours and repressed myostatin at 16 hours. In contrast to what was observed with acute treatment, chronic treatment (7 days) with atomoxetine increased p-Akt and p-FoxO3a, and sustained PGC-1α expression and skeletal muscle mass in dexamethasone-treated mice, in a manner comparable to formoterol. In conclusion, chronic treatment with a low dose of atomoxetine prevented dexamethasone-induced skeletal muscle wasting and supports a potential role in preventing muscle atrophy.

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.

Effects of chronic administration of clenbuterol on contractile properties and calcium homeostasis in rat extensor digitorum longus muscle

Clenbuterol, a β₂-agonist, induces skeletal muscle hypertrophy and a shift from slow-oxidative to fast-glycolytic muscle fiber type profile. However, the cellular mechanisms of the effects of chronic clenbuterol administration on skeletal muscle are not completely understood. As the intracellular Ca²⁺ concentration must be finely regulated in many cellular processes, the aim of this study was to investigate the effects of chronic clenbuterol treatment on force, fatigue, intracellular calcium (Ca²⁺) homeostasis and Ca²⁺-dependent proteolysis in fast-twitch skeletal muscles (the extensor digitorum longus, EDL, muscle), as they are more sensitive to clenbuterol-induced hypertrophy. Male Wistar rats were chronically treated with 4 mg.kg⁻¹ clenbuterol or saline vehicle (controls) for 21 days. Confocal microscopy was used to evaluate sarcoplasmic reticulum Ca²⁺ load, Ca²⁺ -transient amplitude and Ca²⁺ spark properties. EDL muscles from clenbuterol-treated animals displayed hypertrophy, a shift from slow to fast fiber type profile and increased absolute force, while the relative force remained unchanged and resistance to fatigue decreased compared to control muscles from rats treated with saline vehicle. Compared to control animals, clenbuterol treatment decreased Ca²⁺-transient amplitude, Ca²⁺ spark amplitude and frequency and the sarcoplasmic reticulum Ca²⁺ load was markedly reduced. Conversely, calpain activity was increased by clenbuterol chronic treatment. These results indicate that chronic treatment with clenbuterol impairs Ca²⁺ homeostasis and this could contribute to the remodeling and functional impairment of fast-twitch skeletal muscle.

State of the art of mechanical circulatory support

Mechanical circulatory support of the failing heart has become an important means of treating end-stage heart disease. This rapidly growing therapeutic field has produced impressive clinical outcomes and has great potential to help thousands of otherwise terminal patients worldwide. In this review, we examine the state of the art of mechanical circulatory support: current practice, totally implantable systems of the future, evolving biventricular support mechanisms, the potential for myocardial recovery and adjunctive treatment methods, and miniaturized devices with expanded indications for therapy.

Treatment strategies for myocardial recovery in heart failure

OPINION STATEMENT

Heart failure is a progressive disorder characterized by adverse left ventricular remodeling. Until recently, this has been thought to be an irreversible process. Mechanical unloading with a left ventricular assist device (LVAD), particularly if combined with neurohormonal blockade with heart failure medications, can lead to a reversal of the heart failure phenotype, a process called "reverse remodeling." Reverse remodeling refers to the regression of pathologic myocardial hypertrophy and improvement in LV chamber size that can occur in response to treatment. Myocardial recovery is the sustained normalization of structural, molecular, and hemodynamic changes sufficient to allow explant of the LVAD. Despite the fact that reverse remodeling is commonly seen in LVAD patients in clinical practice, myocardial recovery sufficient to allow device explantation is still rare. Previous experience suggests that young patients with short duration of heart failure and less myocardial fibrosis may be more likely to recover. Alternatively, it may just be that clinicians make a greater effort to recover these subgroups. A combined approach of mechanical unloading with LVADs and pharmacological management, together with regular testing of underlying myocardial function with the pump reduced to a speed at which it is not contributing, can increase the frequency of sustained recovery from heart failure. The goal is to achieve optimal unloading of the myocardium, combined with pharmacologic therapy aimed at promoting reverse remodeling. Myocardial recovery must be considered as a therapeutic target. Clinical variables such as pump speed and blood pressure must be optimized to promote maximal unloading, leading to reverse remodeling and myocardial recovery. Frequent echocardiographic and hemodynamic evaluation of underlying myocardial function must be performed. The combination of LVAD therapy with optimal neurohormonal blockade appears promising as an approach to myocardial recovery. In addition, there is a growing body of translational research which, when combined with LVADs, may further promote more durable recovery. Strategies to thicken the myocardium to enhance the durability of recovery prior to explantation, such as clenbuterol (which induces "physiological hypertrophy"), or intermittently reducing the pump speed to increase myocardial load may be beneficial. Emergence of cardiac stem cells and alternative biologic agents, when added to current therapies, may have a complementary role in promoting and maintaining myocardial recovery. This review will summarize both current strategies and emerging therapies.

Molecular changes after left ventricular assist device support for heart failure

Heart failure is associated with remodeling that consists of adverse cellular, structural, and functional changes in the myocardium. Until recently, this was thought to be unidirectional, progressive, and irreversible. However, irreversibility has been shown to be incorrect because complete or partial reversal can occur that can be marked after myocardial unloading with a left ventricular assist device (LVAD). Patients with chronic advanced heart failure can show near-normalization of nearly all structural abnormalities of the myocardium or reverse remodeling after LVAD support. However, reverse remodeling does not always equate with clinical recovery. The molecular changes occurring after LVAD support are reviewed, both those demonstrated with LVAD unloading alone in patients bridged to transplantation and those occurring in the myocardium of patients who have recovered enough myocardial function to have the device removed. Reverse remodeling may be attributable to a reversal of the pathological mechanisms that occur in remodeling or the generation of new pathways. A reduction in cell size occurs after LVAD unloading, which does not necessarily correlate with improved cardiac function. However, some of the changes in both the cardiac myocyte and the matrix after LVAD support are specific to myocardial recovery. In the myocyte, increases in the cytoskeletal proteins and improvements in the Ca²⁺ handling pathway seem to be specifically associated with myocardial recovery. Changes in the matrix are complex, but excessive scarring appears to limit the ability for recovery, and the degree of fibrosis in the myocardium at the time of implantation may predict the ability to recover.

β2-Adrenergic agonists and the treatment of skeletal muscle wasting disorders

β2-Agonists are traditionally used for the treatment of bronchospasm associated with asthma and the treatment of symptomatic patients with COPD. However, β2-agonists are also powerful anabolic agents that trigger skeletal muscle hypertrophy. Investigating the effects of β2-agonists in skeletal muscle over the past 30 years in different animal models has led to the identification of potential therapeutic applications in several muscle wasting disorders, including neuromuscular diseases, cancer cachexia, sepsis or thermal injury. In these conditions, numerous studies indicate that β2-agonists can attenuate and/or reverse the decrease in skeletal muscle mass and associated weakness in animal models of muscle wasting but also in human patients. The purpose of this review is to present the biological and clinical significance of β2-agonists for the treatment of skeletal muscle wasting. After the description of the molecular mechanisms involved in the hypertrophy and anti-atrophy effect of β2-agonists, we will review the anti-atrophy effects of β2-agonist administration in several animal models and human pathologies associated with or leading to skeletal muscle wasting. This article is part of a Directed Issue entitled: Molecular basis of muscle wasting.

Pathophysiological relevance of the cardiac β2-adrenergic receptor and its potential as a therapeutic target to improve cardiac function

β-adrenoceptors are members of the G protein-coupled receptor superfamily which play a key role in the regulation of myocardial function. Their activation increases cardiac performance but can also induce deleterious effects such as cardiac arrhythmias or myocardial apoptosis. In fact, inhibition of β-adrenoceptors exerts a protective effect in patients with sympathetic over-stimulation during heart failure. Although β(2)-adrenoceptor is not the predominant subtype in the heart, it seems to importantly contribute to the cardiac effects of adrenergic stimulation; however, the mechanism by which this occurs is not fully understood. This review summarizes the current knowledge on the role of β(2)-adrenoceptors in the regulation of cardiac contractility, metabolism, cardiomyocyte survival and cardiac arrhythmias. In addition, therapeutic considerations relating to stimulation of the β(2)-adrenoceptor such as an increase in cardiac contractility with low arrythmogenic effect, protection of the myocardium again apoptosis or positive regulation of heart metabolism are discussed.

Myocardial recovery with left ventricular assist devices

OPINION STATEMENT

Advanced heart failure (HF) is a condition that is rarely thought of in terms of cure. Left ventricular assist devices (LVADs), like no therapy before them, provide complete decongestion of the left ventricle, with resulting favorable changes at all levels, from reversal of hypertrophy of cardiomyocytes to recovery of normal geometry and function of the ventricles. Although not a frequent phenomenon at most institutions, LV recovery is achieved in 20-25 % of LVAD recipients in some programs. Patients with good chances for recovery are usually young, with nonischemic cardiomyopathy and short duration of HF symptoms. After LVAD removal, patients with recovered function remain asymptomatic for years. To reach this level of sustainable restoration of cardiac function, several steps need to be taken: 1) myocardial recovery has to be recognized as a therapeutic goal, especially in patients with nonischemic cardiomyopathy; 2) HF medications have to be restarted and aggressively uptitrated after LVAD implantation; 3) regular monitoring for signs of myocardial recovery (eg, echocardiography or hemodynamics) should become a standard practice in LVAD centers; and 4) weaning protocols should be discussed and accepted at each LVAD program. While some protocols involve extensive several-day testing both at rest and with exercise, others are mostly guided by echocardiographic evaluation.

Bridge to recovery: what remains to be discovered?

The use of left ventricular assist devices to induce substantial myocardial recovery with explantation of the device, bridge to recovery (BTR), is an exciting but currently grossly underused application. Recently acquired knowledge relating to BTR and its mechanisms offers unprecedented opportunities to streamline its use and unravel some of the secrets of heart failure with much wider implications. This article reviews the status, challenges, and future of cardiac recovery.

Clenbuterol, a β2-adrenergic agonist, reciprocally alters PGC-1 alpha and RIP140 and reduces fatty acid and pyruvate oxidation in rat skeletal muscle

Clenbuterol, a β2-adrenergic agonist, reduces mitochondrial content and enzyme activities in skeletal muscle, but the mechanism involved has yet to be identified. We examined whether clenbuterol-induced changes in the muscles' metabolic profile and the intrinsic capacity of mitochondria to oxidize substrates are associated with reductions in the nuclear receptor coactivator PGC-1 alpha and/or an increase in the nuclear corepressor RIP140. In rats, clenbuterol was provided in the drinking water (30 mg/l). In 3 wk, this increased body (8%) and muscle weights (12–17%). In red (R) and white (W) muscles, clenbuterol induced reductions in mitochondrial content (citrate synthase: R, 27%; W, 52%; cytochrome- c oxidase: R, 24%; W, 34%), proteins involved in fatty acid transport (fatty acid translocase/CD36: R, 36%; W, 35%) and oxidation [β-hydroxyacyl CoA dehydrogenase (β-HAD): R, 33%; W, 62%], glucose transport (GLUT4: R, 8%; W, 13%), lactate transport monocarboxylate transporter (MCT1: R, 61%; W, 37%), and pyruvate oxidation (PDHE1α, R, 18%; W, 12%). Concurrently, only red muscle lactate dehydrogenase activity (25%) and MCT4 (31%) were increased. Palmitate oxidation was reduced in subsarcolemmal (SS) (R, 30%; W, 52%) and intermyofibrillar (IMF) mitochondria (R, 17%; W, 44%) along with reductions in β-HAD activity (SS: R, 17%; W, 51%; IMF: R, 20%; W, 57%). Pyruvate oxidation was only reduced in SS mitochondria (R, 20%; W, 28%), but this was not attributable solely to PDHE1α, which was reduced in both SS (R, 21%; W, 20%) and IMF mitochondria (R, 15%; W, 43%). These extensive metabolic changes induced by clenbuterol were associated with reductions in PGC-1α (R, 37%; W, 32%) and increases in RIP140 (R, 23%; W, 21%). This is the first evidence that clenbuterol appears to exert its metabolic effects via simultaneous and reciprocal changes in the nuclear receptor coactivator PGC-1α and the nuclear corepressor RIP140.

Reverse remodeling in heart failure--mechanisms and therapeutic opportunities

Heart failure (HF) involves changes in cardiac structure, myocardial composition, myocyte deformation, and multiple biochemical and molecular alterations that impact heart function and reserve capacity. Collectively, these changes have been referred to as 'cardiac remodeling'. Understanding the components of this process with the goal of stopping or reversing its progression has become a major objective. This concept is often termed 'reverse remodeling', and is successfully achieved by inhibitors of the renin-angiotensin-aldosterone system, β-blockers, and device therapies such as cardiac resynchronization or ventricular assist devices. Not every method of reverse remodeling has long-lasting clinical efficacy. However, thus far, every successful clinical treatment with long-term benefits on the morbidity and mortality of patients with HF reverses remodeling. Reverse remodeling is defined by lower chamber volumes (particularly end-systolic volume) and is often accompanied by improved β-adrenergic and heart-rate responsiveness. At the cellular level, reverse remodeling impacts on myocyte size, function, excitation-contraction coupling, bioenergetics, and a host of molecular pathways that regulate contraction, cell survival, mitochondrial function, oxidative stress, and other features. Here, we review the current evidence for reverse remodeling by existing therapies, and discuss novel approaches that are rapidly moving from preclinical to clinical trials.

Adult human heart slices are a multicellular system suitable for electrophysiological and pharmacological studies

Electrophysiological and pharmacological data from the human heart are limited due to the absence of simple but representative experimental model systems of human myocardium. The aim of this study was to establish and characterise adult human myocardial slices from small patients' heart biopsies as a simple, reproducible and relevant preparation suitable for the study of human cardiac tissue at the multicellular level. Vibratome-cut myocardial slices were prepared from left ventricular biopsies obtained from end-stage heart failure patients undergoing heart transplant or ventricular assist device implantation, and from hearts of normal dogs. Multiple slices were prepared from each biopsy. Regular contractility was observed at a range of stimulation frequencies (0.1-2 Hz), and stable electrical activity, monitored using multi-electrode arrays (MEA), was maintained for at least 8 h from slice preparation. ATP/ADP and phosphocreatine/creatine ratios were comparable to intact organ values, and morphology and gap junction distribution were representative of native myocardium. MEA recordings showed that field potential duration (FPD) and conduction velocity (CV) in human and dog slices were similar to the values previously reported for papillary muscles, ventricular wedges and whole hearts. Longitudinal CV was significantly faster than transversal CV, with an anisotropic ratio of 3:1 for human and 2.3:1 for dog slices. Importantly, slices responded to the application of E-4031, chromanol and 4-aminopyridine, three potassium channel blockers known to affect action potential duration, with an increase in FPD. We conclude that viable myocardial slices with preserved structural, biochemical and electrophysiological properties can be prepared from adult human and canine heart biopsies and offer a novel preparation suitable for the study of heart failure and drug screening.

Regulation of contractility and metabolic signaling by the β2-adrenergic receptor in rat ventricular muscle

AIMS

Cardiac function is modulated by the sympathetic nervous system through β-adrenergic receptor (β-AR) activity and this represents the main regulatory mechanism for cardiac performance. To date, however, the metabolic and molecular responses to β(2)-agonists are not well characterized. Therefore, we studied the inotropic effect and signaling response to selective β(2)-AR activation by tulobuterol.

MAIN METHODS

Strips of rat right ventricle were electrically stimulated (1Hz) in standard Tyrode solution (95% O(2), 5% CO(2)) in the presence of the β(1)-antagonist CGP-20712A (1μM). A cumulative dose-response curve for tulobuterol (0.1-10μM), in the presence or absence of the phosphodiesterase (PDE) inhibitor IBMX (30μM), or 10min incubation (1μM) with the β(2)-agonist tulobuterol was performed.

KEY FINDINGS

β(2)-AR stimulation induced a positive inotropic effect (maximal effect=33±3.3%) and a decrease in the time required for half relaxation (from 45±0.6 to 31±1.8ms, -30%, p<0.001) after the inhibition of PDEs. After 10min of β(2)-AR stimulation, p-AMPKα(T172) (54%), p-PKB(T308) (38%), p-AS160(T642) (46%) and p-CREB(S133) (63%) increased, without any change in p-PKA(T197).

SIGNIFICANCE

These results suggest that the regulation of ventricular contractility is not the primary function of the β(2)-AR. Rather, β(2)-AR could function to activate PKB and AMPK signaling, thereby modulating muscle mass and energetic metabolism of rat ventricular muscle.

Cardiovascular effects of chronic treatment with a β2-adrenoceptor agonist relieving neuropathic pain in mice

Neuropathic pain is often a chronic condition, disabling and difficult to treat. Using a murine model of neuropathic pain induced by placing a polyethylene cuff around the main branch of the sciatic nerve, we have shown that chronic treatment with β-AR agonists is effective against neuropathic allodynia. β-mimetics are widely used against asthma and chronic obstructive pulmonary disease and may offer an interesting option for neuropathic pain management. The most prominent adverse effects of chronic treatment with β-mimetics are cardiovascular. In this study, we compared the action of low doses of the selective β(2)-AR agonist terbutaline and of a high dose of the mixed β(1)/β(2)-AR agonist isoproterenol on cardiovascular parameters in a neuropathic pain context. Isoproterenol was used as a positive control for some heart-related changes. Cardiac functions were studied by echocardiography, hemodynamic measurements, histological analysis of fibrosis and cardiac hypertrophy, and by quantitative real time PCR analysis of atrial natriuretic peptide (Nppa), periostin (Postn), connective tissue growth factor (Ctgf) and β-myosin heavy chain (Myh7). Our data show that a chronic treatment with the β(2)-AR agonist terbutaline at low antiallodynic dose does not affect cardiovascular parameters, whereas the mixed β(1)/β(2)-AR agonist isoproterenol induces cardiac hypertrophy. These data suggest that low doses of β(2)-AR agonists may provide a suitable treatment with rare side effects in neuropathic pain management. This study conducted in an animal model requires clinical confirmation in humans.

Reversal of Severe Heart Failure With a Continuous-Flow Left Ventricular Assist Device and Pharmacological Therapy

Background—

We have previously shown that a specific combination of drug therapy and left ventricular assist device unloading results in significant myocardial recovery, sufficient to allow pump removal, in two thirds of patients with dilated cardiomyopathy receiving a Heartmate I pulsatile device. However, this protocol has not been used with nonpulsatile devices.

Methods and Results—

We report the results of a prospective study of 20 patients who received a combination of angiotensin-converting enzymes, β-blockers, angiotensin II inhibitors, and aldosterone antagonists followed by the β 2 -agonist clenbuterol and were regularly tested (echocardiograms, exercise tests, catheterizations) with the pump at low speed. Before left ventricular assist device insertion, patient age was 35.2±12.6 years (16 male patients), patients were on 2.0±0.9 inotropes, 7 (35) had an intra-aortic balloon pump, 2 were hemofiltered, 2 were ventilated, 3 had a prior Levitronix device, and 1 had extracorporeal membrane oxygenation. Cardiac index was 1.39±0.43 L · min −1 · m −2 , pulmonary capillary wedge pressure was 31.5±5.7 mm Hg, and heart failure history was 3.4±3.5 years. One patient was lost to follow-up and died after 240 days of support. Of the remaining 19 patients, 12 (63.2) were explanted after 286±97 days. Eight had symptomatic heart failure for ≤6 months and 4 for >6 months (48 to 132 months). Before explantation, at low flow for 15 minutes, ejection fraction was 70±7, left ventricular end-diastolic diameter was 48.6±5.7 mm, left ventricular end-systolic diameter was 32.3±5.7 mm, mV̇ o 2 was 21.6±4 mL · kg −1 · min −1 , pulmonary capillary wedge pressure was 5.9±4.6 mm Hg, and cardiac index was 3.6±0.6 L · min −1 · m −2 . Estimated survival without heart failure recurrence was 83.3 at 1 and 3 years. After a 430.7±337.1-day follow-up, surviving explants had an ejection fraction of 58.1±13.8, left ventricular end-diastolic diameter of 59.0±9.3 mm, left ventricular end-systolic diameter of 42.0±10.7 mm, and mV̇ o 2 of 22.6±5.3 mL · kg −1 · min −1 .

Conclusions—

Reversal of end-stage heart failure secondary to nonischemic cardiomyopathy can be achieved in a substantial proportion of patients with nonpulsatile flow through the use of a combination of mechanical and pharmacological therapy.

Contemporary use of ventricular assist devices

The introduction of the heart lung machine more than 50 years ago proved in principle that heart function can be replaced, albeit for short periods. This was followed by attempts to produce total or partial artificial hearts that could function for prolonged periods of time. Progress in this field has been intermittent but has accelerated considerably in the past 10 years, with ventricular assist devices (VADs) reaching an impressive degree of sophistication and complexity owing to the contributions from clinicians, engineers, scientists, industrialists, and others. This review describes the currently available types of VADs, their current clinical use, the patient selection process, the trend toward use of VADs in patients with less severe heart failure, and the use of VADs for myocardial recovery in combination with novel pharmacological strategies, gene therapy, and cell therapy.

Effects of clenbuterol, a β₂-adrenergic agonist, on sizes of masseter, temporalis, digastric, and tongue muscles

We compared the hypertrophic effects of clenbuterol, a β₂-adrenergic agonist, on the masseter, digastric, and temporalis with those on the tongue, tibialis anterior, soleus, diaphragm, and heart. The weights of masseter, digastric and temporalis in the clenbuterol group were 36 ~ 56% greater than those in the control group, whereas those of the tibialis anterior, diaphragm, and heart weights in the clenbuterol group were 9 ~ 33% greater than those in the control group. No significant difference in the weights of the soleus and tongue was found between the control and clenbuterol groups. Taken together with our present and previously reported results, it is suggested that the hypertrophic effects of clenbuterol on the masseter, digastric, and temporalis are greater than those on the limb, trunk, and heart.

Chronic treatment with clenbuterol modulates endothelial progenitor cells and circulating factors in a murine model of cardiomyopathy

The purpose of this study was to determine the effects of chronic treatment with the beta 2 adrenergic receptor agonist clenbuterol on endothelial progenitor cells (EPC) in a well-characterized model of heart failure, the muscle LIM protein knockout (MLP(-/-)) mouse. MLP(-/-) mice were treated daily with clenbuterol (2 mg/kg) or saline subcutaneously for 6 weeks. Clenbuterol led to a 30% increase in CD31(+) cells in the bone marrow of MLP(-/-) heart failure mice (p < 0.004). Clenbuterol did not improve ejection fraction. Clenbuterol treatment in MLP(-/-) mice was associated with significant changes in the following circulating factors: tissue inhibitor of metalloproteinase-type 1, leukemia inhibitory factor 1, C-reactive protein, apolipoprotein A1, fibroblast growth factor 2, serum glutamic oxaloacetic transaminase, macrophage-derived chemokine, and monocyte chemoattractant protein-3. Clenbuterol treatment in the MLP(-/-) model of heart failure did not rescue heart function, yet did increase CD31(+) cells in the bone marrow. This is the first evidence that a beta 2 agonist increases EPC proliferation in the bone marrow in a preclinical model of heart failure.

A gene expression profile of the myocardial response to clenbuterol

Clenbuterol is currently being used as part of a clinical trial into a novel therapeutic approach for the treatment of end-stage heart failure. The purpose of this study was to determine the global pattern of myocardial gene expression in response to clenbuterol and to identify novel targets and pathways involved. Rats were treated with clenbuterol (n = 6) or saline (n = 6) for periods of 1, 3, 9, or 28 days. Rats treated for 28 days were also subject to continuous electrocardiogram analysis using implantable telemetry. RNA was extracted from rats at days 1 and 28 and used from microarray analysis, and further samples from rats at days 1, 3, 9, and 28 were used for analysis by real-time polymerase chain reaction. Clenbuterol treatment induced rapid development of cardiac hypertrophy with increased muscle mass at day 1 and elevated heart rate and QT interval throughout the 28-day period. Microarray analysis revealed a marked but largely transitory change in gene expression with 1,423 genes up-regulated and 964 genes down-regulated at day 1. Up-regulated genes revealed an unexpected association with angiogenesis and integrin-mediated cell adhesion and signaling. Moreover, direct treatment of endothelial cells cultured in vitro resulted in increased cell proliferation and tube formation. Our data show that clenbuterol treatment is associated with rapid cardiac hypertrophy and identify angiogenesis and integrin signaling as novel pathways of clenbuterol action. The data have implications both for our understanding of the physiologic hypertrophy induced by clenbuterol and for treatment of heart failure.

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 of beta-adrenoceptor signaling in skeletal muscle: implications for muscle wasting and disease

The importance of beta-adrenergic signaling in the heart has been well documented, but it is only more recently that we have begun to understand the importance of this signaling pathway in skeletal muscle. There is considerable evidence regarding the stimulation of the beta-adrenergic system with beta-adrenoceptor agonists (beta-agonists). Although traditionally used for treating bronchospasm, it became apparent that some beta-agonists could increase skeletal muscle mass and decrease body fat. These so-called "repartitioning effects" proved desirable for the livestock industry trying to improve feed efficiency and meat quality. Studying beta-agonist effects on skeletal muscle has identified potential therapeutic applications for muscle wasting conditions such as sarcopenia, cancer cachexia, denervation, and neuromuscular diseases, aiming to attenuate (or potentially reverse) the muscle wasting and associated muscle weakness, and to enhance muscle growth and repair after injury. Some undesirable cardiovascular side effects of beta-agonists have so far limited their therapeutic potential. This review describes the physiological significance of beta-adrenergic signaling in skeletal muscle and examines the effects of beta-agonists on skeletal muscle structure and function. In addition, we examine the proposed beneficial effects of beta-agonist administration on skeletal muscle along with some of the less desirable cardiovascular effects. Understanding beta-adrenergic signaling in skeletal muscle is important for identifying new therapeutic targets and identifying novel approaches to attenuate the muscle wasting concomitant with many diseases.

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.

The role of the cardiac Na+/Ca2+ exchanger in reverse remodeling: relevance for LVAD-recovery

Different strategies can, at least in certain conditions, prevent or reverse myocardial remodeling due to heart failure and induce myocardial functional improvement. Na+/Ca2+ exchanger (NCX) is considered a major player in the pathophysiology of heart failure but its role in reverse remodeling is unknown. A combination of mechanical unloading by left ventricular assist devices (LVADs) and pharmacological therapy has been shown to induce clinical recovery in a limited number of patients with end-stage heart failure. In myocytes isolated from these patients we found that, after LVAD treatment, NCX1/SERCA2a mRNA was 38% higher than at device implant. We studied the ability of NCX to extrude Ca2+ during caffeine-induced SR Ca2+ release in isolated ventricular myocytes from these patients. The time constant of decline was slower in heart failure. In myocytes from patients with clinical recovery following mechanical and pharmacological treatment, NCX1-mediated Ca2+ extrusion was faster compared with myocytes from patient who, despite identical treatment, did not recover. We propose that increased NCX function may be associated with reverse remodeling in patients and that factors that regulate NCX function (i.e., phosphorylation or intracellular [Na+]) other than NCX expression levels alone, may have detrimental consequences on cardiac function.

Left ventricular assist device and drug therapy for the reversal of heart failure

BACKGROUND

In patients with severe heart failure, prolonged unloading of the myocardium with the use of a left ventricular assist device has been reported to lead to myocardial recovery in small numbers of patients for varying periods of time. Increasing the frequency and durability of myocardial recovery could reduce or postpone the need for subsequent heart transplantation.

METHODS

We enrolled 15 patients with severe heart failure due to nonischemic cardiomyopathy and with no histologic evidence of active myocarditis. All had markedly reduced cardiac output and were receiving inotropes. The patients underwent implantation of left ventricular assist devices and were treated with lisinopril, carvedilol, spironolactone, and losartan to enhance reverse remodeling. Once regression of left ventricular enlargement had been achieved, the beta2-adrenergic-receptor agonist clenbuterol was administered to prevent myocardial atrophy.

RESULTS

Eleven of the 15 patients had sufficient myocardial recovery to undergo explantation of the left ventricular assist device a mean (+/-SD) of 320+/-186 days after implantation of the device. One patient died of intractable arrhythmias 24 hours after explantation; another died of carcinoma of the lung 27 months after explantation. The cumulative rate of freedom from recurrent heart failure among the surviving patients was 100% and 88.9% 1 and 4 years after explantation, respectively. The quality of life as assessed by the Minnesota Living with Heart Failure Questionnaire score at 3 years was nearly normal. Fifty-nine months after explantation, the mean left ventricular ejection fraction was 64+/-12%, the mean left ventricular end-diastolic diameter was 59.4+/-12.1 mm, the mean left ventricular end-systolic diameter was 42.5+/-13.2 mm, and the mean maximal oxygen uptake with exercise was 26.3+/-6.0 ml per kilogram of body weight per minute.

CONCLUSIONS

In this single-center study, we found that sustained reversal of severe heart failure secondary to nonischemic cardiomyopathy could be achieved in selected patients with the use of a left ventricular assist device and a specific pharmacologic regimen.

Relative myotoxic and haemodynamic effects of the beta-agonists fenoterol and clenbuterol measured in conscious unrestrained rats

The beta(2)-adrenoceptor (beta(2)-AR) agonists clenbuterol and fenoterol have similar beneficial effects in animal models of heart failure. However, large doses of clenbuterol can induce cardiomyocyte death, and it is not known which of these agents has the most favourable therapeutic profile. We have investigated the cardiotoxicity of clenbuterol and fenoterol alongside that of isoprenaline, and compared their haemodynamic effects. Wistar rats (n = 6 per group) were subcutaneously injected with each beta-agonist (0.003-3 mmol kg(-1)) or saline, and cardiomyocyte apoptosis was detected by caspase 3 immunohistochemistry. In a separate experiment, rats (n = 4) were given equivalent doses to those used in the myotoxicity studies, in a randomized cross-over design, and their blood pressure recorded via radiotelemetry. Injection of 0.3 mmol kg(-1) fenoterol or isoprenaline, but not clenbuterol, induced significant cardiomyocyte apoptosis (0.4 +/- 0.05%; P < 0.05). At 3 mmol kg(-1), all agonists induced apoptosis (fenoterol, 1.1 +/- 0.1%; isoprenaline, 0.9 +/- 0.8%; and clenbuterol, 0.4 +/- 0.07%; P < 0.05). beta(1)-Adrenoceptor antagonism (10 mg kg(-1) bisoprolol) prevented 92% (P < 0.05) of apoptosis induced by all three agonists, but clenbuterol-induced apoptosis could also be prevented by 96% (P < 0.05) by beta(2)-AR antagonism (10 mg kg(-1) ICI 118 551). Clenbuterol decreased diastolic (1.3- to 1.6-fold; P < 0.05) and systolic blood pressure (1.3-fold; P < 0.05), and doses > 0.3 mmol kg(-1) increased heart rate (1.4-fold; P < 0.05). Fenoterol increased heart rate (1.2- to 1.4-fold; P < 0.05), and doses > 0.3 mmol kg(-1) decreased diastolic blood pressure (1.3-fold; P < 0.05). In conclusion, the cardiotoxicity of fenoterol was similar to isoprenaline and greater than clenbuterol, and fenoterol had less desirable haemodynamic effects.

Transforming growth factor betas are upregulated in the rat masseter muscle hypertrophied by clenbuterol, a beta2 adrenergic agonist

1. The regulatory mechanism for the hypertrophy of skeletal muscles induced by clenbuterol is unclear. The purpose of the present study was to determine the extent to which transforming growth factor betas (TGFbetas), fibroblast growth factors (FGFs), hepatocyte growth factor (HGF), and platelet-derived growth factors (PDGFs) are involved in the hypertrophy of rat masseter muscle induced by clenbuterol. 2. We measured the mRNA expression levels for TGFbetas, FGFs, HGF, and PDGFs in rat masseter muscle hypertrophied by oral administration of clenbuterol for 3 weeks and determined correlations between the weight of masseter muscle and mRNA expression levels by regression analysis. We determined immunolocalizations of TGFbetas and their receptors (TGFbetaRs). 3. The mRNA expression levels for TGFbeta1, 2, and 3, and for PDGF-B demonstrated clenbuterol-induced elevations and positive correlations with the weight of masseter muscle. In particular, TGFbeta1, 2, and 3 showed strong positive correlations (correlation coefficients >0.6). The mRNA expression levels for PDGF-A, FGF-1 and 2, and HGF showed no significant differences between the control and clenbuterol groups, and no significant correlations. TGFbeta1, 2, and 3 were principally localized in the connective tissues interspaced among myofibers, and TGFbetaRI and II were localized in the periphery and sarcoplasm of the myofibers. 4. These results suggest that paracrine actions of TGFbeta1, 2, and 3 via TGFbetaRI and II could be involved in the hypertrophy of rat masseter muscle induced by clenbuterol. This is the first study to document the involvement of TGFbetas in the hypertrophy of skeletal muscles induced by clenbuterol.

LVAD-Induced Reverse Remodeling: Basic and Clinical Implications for Myocardial Recovery

BACKGROUND

With improved technology, increasing clinical experience, and expanding indications for use, left ventricular assist devices (LVADs) are assuming a greater role in the care of patients with end-stage heart failure. Early in the course of LVAD use as a bridge to transplant, it became evident that some patients exhibit substantial recovery of ventricular function, which led to the concept of reverse remodeling.

METHODS AND RESULTS

Herein we summarize and integrate insights derived from a multitude of studies that have investigated how LVAD support influences ventricular structural, cellular, extracellular matrix, molecular, biochemical, and metabolic characteristics of the end-stage failing heart. The focus includes a review of the extent and sustainability of reverse remodeling, the important advances in understanding of the pathophysiology of heart failure derived from these studies and the implications of these findings for development of new therapeutic strategies.

CONCLUSION

In brief, studies of LVAD-heart interactions have led to the understanding that although we once considered the end-stage failing heart of patients near death to be irreversibly diseased, when given sufficient mechanical unloading and restoration of more normal neurohormonal milieu, a relatively large degree of myocardial recovery is possible. Comparison of effects on right and left ventricles have provided mechanistic insights by implicating hemodynamic unloading as primarily regulating certain aspects of reverse remodeling, neurohormonal factors as regulating other aspects, and joint regulation of still other aspects. As such these observations have driven a shift of thinking of chronic heart failure as a progressive irreversible disease process to a potentially treatable entity.

Homeostatic responses to caloric restriction: influence of background metabolic rate

The biological responses to caloric restriction (CR) are generally examined in rats with elevated metabolic rates due to being housed at ambient temperatures (Ta) below the zone of thermoneutrality. We determined the physiological and behavioral responses to 2 wk of 30–40% CR in male FBNF1 rats housed in cool (Ta = 12°C) or thermoneutral (TMN; Ta = 30°C) conditions. Rats were instrumented with telemetry devices and housed continuously in home-cage calorimeters for the entire experiment. At baseline, rats housed in cool Ta had reduced rate of weight gain; thus a mild CR (5%) group at thermoneutrality for weight maintenance was also studied. Rats housed in cool Ta exhibited elevated caloric intake (cool = 77 ± 1; TMN = 54 ± 2 kcal), oxygen consumption (V̇o2; cool = 9.9 ± 0.1; TMN = 5.5 ± 0.1 ml/min), mean arterial pressure (cool = 103 ± 1; TMN = 80 ± 2 mmHg), and heart rate (cool = 374 ± 3; TMN = 275 ± 4 beats/min). Cool-CR rats exhibited greater CR-induced weight loss (cool = −62 ± 3; TMN = −42 ± 3 g) and reductions in V̇o2 (cool = −2.6 ± 0.1; TMN = −1.5 ± 0.1 ml/min) but similar CR-induced reductions in heart rate (cool = −59 ± 1; TMN= −51 ± 7 beats/min). CR had no effect on arterial blood pressure or locomotor activity in either group. Unexpectedly, weight maintenance produced significant reductions in V̇o2 and heart rate. At thermoneutrality, a single day of refeeding effectively abolished CR-induced reductions in V̇o2 and heart rate. The results reveal that rats with low or high baseline metabolic rate exhibit comparable compensatory reductions in V̇o2 and heart rate and suggest that Ta can be used to modulate the metabolic background on which the more prolonged effects of CR can be studied.