Beneficial versus adverse effects of long-term use of clenbuterol in mdx mice

Breathing in Duchenne muscular dystrophy: Translation to therapy

Duchenne muscular dystrophy (DMD) is an X-linked neuromuscular disease caused by a deficiency in dystrophin - a structural protein which stabilizes muscle during contraction. Dystrophin deficiency adversely affects the respiratory system leading to sleep-disordered breathing, hypoventilation, and weakness of the expiratory and inspiratory musculature, which culminate in severe respiratory dysfunction. Muscle degeneration associated respiratory impairment in neuromuscular disease is a result of disruptions at multiple sites of the respiratory control network, including sensory and motor pathways. As a result of this pathology, respiratory failure is a leading cause of premature death in DMD patients. Currently available treatments for DMD respiratory insufficiency attenuate respiratory symptoms without completely reversing the underlying pathophysiology. This underscores the need to develop curative therapies to improve quality of life and longevity of DMD patients. This review summarises research findings on the pathophysiology of respiratory insufficiencies in DMD disease in humans and animal models, the clinical interventions available to ameliorate symptoms, and gene-based therapeutic strategies uncovered by preclinical animal studies. Abstract figure legend: Summary of the therapeutic strategies for respiratory insufficiency in DMD (Duchenne muscular dystrophy). Treatment options currently in clinical use only attenuate respiratory symptoms without reversing the underlying pathology of DMD-associated respiratory insufficiencies. Ongoing preclinical and clinical research is aimed at developing curative therapies that both improve quality of life and longevity of DMD patients. AAV - adeno-associated virus, PPMO - Peptide-conjugated phosphorodiamidate morpholino oligomer This article is protected by copyright. All rights reserved.

Insight View on the Role of in Ovo Feeding of Clenbuterol on Hatched Chicks: Hatchability, Growth Efficiency, Serum Metabolic Profile, Muscle, and Lipid-Related Markers

Simple Summary

This study examined the effects of ovo injection of clenbuterol on fat deposition and growth performance in chickens, which is prejudicial to poultry consumers and muscle growth-related genes, egg hatchability, and fertility. The achieved result showed a definite effect of clenbuterol on body gain and hatchability. It decreased fat deposition and upregulation of muscle growth-related gene expressions accompanied by modulation of fatty and amino acid composition, reflecting a new insight into the intracellular pathways of clenbuterol supplementation on chicks.

Abstract

The present study aimed to assess the in ovo administration of clenbuterol on chick fertility, growth performance, muscle growth, myogenic gene expression, fatty acid, amino acid profile, intestinal morphology, and hepatic lipid-related gene expressions. In this study, 750 healthy fertile eggs from the local chicken breed Dokki-4 strain were analyzed. Fertile eggs were randomly divided into five experimental groups (150 eggs/3 replicates for each group). On day 14 of incubation, in addition to the control group, four other groups were established where 0.5 mL of worm saline (30 °C) was injected into the second group of eggs. In the third, fourth, and fifth groups, 0.5 mL of worm saline (30 °C), 0.9% of NaCl, and 10, 15, and 20 ppm of clenbuterol were injected into the eggs. Results suggested that clenbuterol increased growth efficiency up to 12 weeks of age, especially at 15 ppm, followed by 10 ppm, decreased abdominal body fat mass, and improved hatchability (p < 0.01). Clenbuterol also modulated saturated fatty acid levels in the breast muscles and improved essential amino acids when administered at 10 and 15 ppm. Additionally, clenbuterol at 15 ppm significantly decreased myostatin gene expression (p < 0.01) and considerably increased IGF1r and IGF-binding protein (IGFBP) expression. Clenbuterol administration led to a significant upregulation of hepatic PPARα, growth hormone receptor, and Lipoprotein lipase (LPL) mRNA expression with a marked decrease in fatty acid synthase (FAS) and sterol regulatory element-binding protein 1 (SREBP-1c) expression. In conclusion, the current study revealed that in ovo injection of clenbuterol showed positive effects on the growth of hatched chicks through reduced abdominal fat deposition, improved intestinal morphology, and modulation of hepatic gene expressions in myogenesis, lipogenesis, and lipolysis.

Negative effect of clenbuterol on physical capacities and neuromuscular control of muscle atrophy in adult rats

INTRODUCTION

Clenbuterol has been used to alleviate chronic obstructive pulmonary disease and elicit an anabolic response in muscles. The aim of this study was to determine the influence of muscle mass variation on physical capacities in rats.

METHODS

The left hindlimbs of Wistar rats were immobilized for 20 days in plantarflexion with a splint and then remobilized for 16 days. The effect of a non-myotoxic dose of clenbuterol during the immobilization period was evaluated. Physical capacities were coordination, free locomotion, grip strength, and bilateral deficit.

RESULTS

Immobilization induced a loss of muscle mass, coordination, and strength without any effect on free locomotion. The positive anabolic effect of clenbuterol did not prevent a loss of physical capacities resulting from immobilization.

CONCLUSIONS

Muscle mass correlated strongly with coordination and isometric strength in untreated rats. Anabolic effect, fiber phenotype modification, and perturbation in neuromuscular communication with clenbuterol improved muscle mass, but it altered physical capacities.

Ventilatory chemosensory drive is blunted in the mdx mouse model of Duchenne Muscular Dystrophy (DMD)

Duchenne Muscular Dystrophy (DMD) is caused by mutations in the DMD gene resulting in an absence of dystrophin in neurons and muscle. Respiratory failure is the most common cause of mortality and previous studies have largely concentrated on diaphragmatic muscle necrosis and respiratory failure component. Here, we investigated the integrity of respiratory control mechanisms in the mdx mouse model of DMD. Whole body plethysmograph in parallel with phrenic nerve activity recordings revealed a lower respiratory rate and minute ventilation during normoxia and a blunting of the hypoxic ventilatory reflex in response to mild levels of hypoxia together with a poor performance on a hypoxic stress test in mdx mice. Arterial blood gas analysis revealed low PaO2 and pH and high PaCO2 in mdx mice. To investigate chemosensory respiratory drive, we analyzed the carotid body by molecular and functional means. Dystrophin mRNA and protein was expressed in normal mice carotid bodies however, they are absent in mdx mice. Functional analysis revealed abnormalities in Dejours test and the early component of the hypercapnic ventilatory reflex in mdx mice. Together, these results demonstrate a malfunction in the peripheral chemosensory drive that would be predicted to contribute to the respiratory failure in mdx mice. These data suggest that investigating and monitoring peripheral chemosensory drive function may be useful for improving the management of DMD patients with respiratory failure.

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.

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.

Low dose formoterol administration improves muscle function in dystrophic mdx mice without increasing fatigue

The beta(2)-adrenoceptor agonist (beta(2)-agonist), formoterol, has been shown to cause muscle hypertrophy in rats even when administered at the micromolar dose of 25 micro g/kg/day. We investigated whether a similar low dose of formoterol could improve muscle function in the dystrophic mdx mouse. Ten-week-old male mdx and wild-type (C57BL/10) mice were administered formoterol (25 micro g/kg/day, i.p.) for 4 weeks. Formoterol treatment increased extensor digitorum longus (EDL) and soleus muscle mass, increased median muscle fibre size in diaphragm, EDL, and soleus muscles, and increased maximum force producing capacity in skeletal muscles of both wild-type and mdx mice. In contrast to other studies where beta(2)-agonists have been administered to mice and rats, generally at higher doses, low dose formoterol treatment did not increase the fatiguability of EDL, soleus or diaphragm muscles. Although others have found formoterol can decrease ubiquitin mRNA and proteasome activity when administered to tumour bearing rats at high doses (2mg/kg/day), in the present study low dose formoterol treatment did not alter ubiquitin or the E1 and E3 ubiquitin ligases in diaphragm muscles of wild-type or mdx mice, but it did reduce the level of ubiquitinated proteins in diaphragm of wild-type mice. The findings indicate that formoterol has considerably more powerful anabolic effects on skeletal muscle than older generation beta(2)-agonists (like clenbuterol and albuterol), and has considerable therapeutic potential for muscular dystrophies and other neuromuscular disorders where muscle wasting is indicated.

Duchenne muscular dystrophy: focus on pharmaceutical and nutritional interventions

Duchenne muscular dystrophy is a lethal X-linked muscle disease resulting from a defect in the muscle membrane protein dystrophin. The absence of dystrophin leads to muscle membrane fragility, muscle death (necrosis) and eventual replacement of skeletal muscle by fat and fibrous connective tissue. Extensive muscle wasting and respiratory failure results in premature death often by the early 20s. This short review evaluates drug and nutritional interventions designed to reduce the severity of muscular dystrophy, while awaiting the outcome of research into therapies to correct the fundamental gene defect. Combinations of dietary supplementation with amino-acids such as creatine, specific anti-inflammatory drugs and perhaps drugs that target ion channels might have immediate realistic clinical benefits although rigorous research is required to determine optimal combinations of such interventions.

Clenbuterol antagonizes glucocorticoid-induced atrophy and fibre type transformation in mice

Beta-agonists and glucocorticoids are frequently coprescribed for chronic asthma treatment. In this study the effects of 4 week treatment with beta-agonist clenbuterol (CL) and glucocorticoid dexamethasone (DEX) on respiratory (diaphragm and parasternal) and limb (soleus and tibialis) muscles of the mouse were studied. Myosin heavy chain (MHC) distribution, fibres cross sectional area (CSA), glycolytic (phosphofructokinase, PFK; lactate dehydrogenase, LDH) and oxidative enzyme (citrate synthase, CS; cytochrome oxidase, COX) activities were determined. Muscle samples were obtained from four groups of adult C57/B16 mice: (1) Control (2) Mice receiving CL (CL, 1.5 mg kg(-1) day(-1) in drinking water) (3) Mice receiving DEX (DEX, 5.7 mg kg(-1) day(-1) s.c.) (4) Mice receiving both treatments (DEX + CL). As a general rule, CL and DEX showed opposite effects on CSA, MHC distribution, glycolytic and mitochondrial enzyme activities: CL alone stimulated a slow-to-fast transition of MHCs, an increase of PFK and LDH and an increase of muscle weight and fibre CSA; DEX produced an opposite (fast-to-slow transition) change of MHC distribution, a decrease of muscle weight and fibre CSA and in some case an increase of CS. The response varied from muscle to muscle with mixed muscles, as soleus and diaphragm, being more responsive than fast muscles, as tibialis and parasternal. In combined treatments (DEX + CL), the changes induced by DEX or CL alone were generally minimized: in soleus, however, the effects of CL predominated over those of DEX, whereas in diaphragm DEX prevailed over CL. Taken together the results suggest that CL might counteract the unwanted effects on skeletal muscles of chronic treatment with glucocorticoids.

Altered activity of signaling pathways in diaphragm and tibialis anterior muscle of dystrophic mice

Duchenne muscular dystrophy is a musculoskeletal disease caused by mutations in the dystrophin gene. The purpose of this study was to use the mouse model of muscular dystrophy (mdx) to determine if the progression of the dystrophic phenotype in the diaphragm (costal) versus limb skeletal muscle (tibialis anterior) is associated with specific changes in extracellular regulated kinase (ERK1/2), p70 S6 kinase (p70(S6k)), or p38 signaling pathways. The studies detected that consistent with an earlier dystrophic phenotype, phosphorylation of p70(S6k) is elevated by 40% in the diaphragm with no change in limb muscle. In addition, phosphorylation of p38 kinase was decreased by 33% in the mdx diaphragm muscle. Levels of ERK1/2 as well as phosphorylation states were elevated in the diaphragm and limb muscle of mdx mice compared with age-matched control muscles. These results indicate that distinct signaling pathways are differentially activated in skeletal muscle of mdx mice. The specificity of these responses, particularly in the diaphragm, provides insight for potential targets for blunting the progression of the muscular dystrophy phenotype.

Enhancement of hybrid-fiber types in rat soleus muscle after clenbuterol administration during hindlimb unloading

Our objective was to determine the effects of a clenbuterol (CB) treatment orally administered (2 mg per kg) to rats submitted to 14 days of hindlimb unloading (HU). The morphological and the contractile properties as well as the myosin heavy chain isoforms contained in each fiber type were determined in whole soleus muscles. As classically described after HU, a decrease in muscle wet weight and in body mass associated with a loss of muscular force, an evolution of the contractile parameters towards those of a fast muscle type, and the emergence of fast myosin heavy chain isoforms were observed. The CB treatment in the HU rats helped reduce the decrease in 1) muscle and body weights, 2) force and 3) the proportion of slow fibers, without preventing the emergence of fast myosin isoforms. Clenbuterol induced a complex remodelling of the muscle typing promoting the combination of both slow and fast myosin isoforms within one fiber. To conclude, our data demonstrate that CB administration partially counteracts the effects produced by HU, and they allow us to anticipate advances in the treatment of muscular atrophy.Key words: β2 agonist, clenbuterol, soleus, contractile parameters, myosin, immunohistochemistry, simulated microgravity, countermeasure.

Early onset of the maximum protein anabolic effect induced by isoproterenol in chick skeletal and cardiac muscle

Prolonged (120 days) oral administration of a beta adrenoceptor agonist, isoproterenol hydrochloride (dose = 1.5 mg/kg body weight) resulted in an increase in the live weight of growing chicks (Callus domesticus). Measurement of dry muscle mass and total proteins in muscle homogenates from M. pectoralis major. M. petoralis minor suggested a muscle hypertrophy largely responsible for this live weight increase. Further, an increase in organ weight and total tissue proteins supported cardiac hypertrophy in chicks as a result of isoproterenol administration. Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) revealed alterations in actin myosin profiles implying a drug induced change in phenotypic expression of myofibrillar component of both skeletal and cardiac muscle. The results suggest that prolonged treatment of chicks produced changes that were not much different from those recorded immediately within a fortnight.

Running endurance abnormality in mdx mice

The mdx mouse lacks dystrophin and has histological features of Duchenne muscular dystrophy but little weakness in the first year of life. We report here an early deficit in voluntary wheel running, as assayed with a computerized wheel. All mdx mice showed an intermittent running pattern, in contrast to the continuous running seen in controls. The average continuous running time differed significantly between mdx and control mice at all ages tested (5-21 weeks). This assay is noninvasive, has the advantage of unbiased automatic data collection, and should be useful for quantifying the mdx deficit in therapeutic studies.

Novel approaches to treat muscular dystrophies

Muscular dystrophies (MD) are a clinically and genetically heterogeneous group of skeletal muscle-wasting diseases. Mutations in the dystrophin gene result in dystrophin deficiency, which constitutes the pathogenic basis of Duchenne and Becker MD (DMD and BMD). Several MD are caused by mutations in other recently identified genes coding for proteins linked to the sarcolemma, the nuclear envelope or the contractile apparatus. In addition, several MD have been mapped to different chromosomal loci and for most of them, the identification of the molecular defect is underway. The immediate result is an ongoing reclassification of the MD into disorders defined not by clinical characteristics but specific genetic mutations. At present, therapy of MD is based on symptomatic treatment and supportive care. Convincing evidence for clinical efficacy is only available for corticosteroids that also suffer from frequent and severe side effects. Up to now, curative therapy is not available, although promising new molecular therapies are under investigation in animal models of MD. Current treatment strategies are discussed and a perspective for effective molecular therapy is given.

Force and power output of diaphragm muscle strips from mdx and control mice after clenbuterol treatment

Based on its anabolic properties, treatment with the beta(2)-adrenoceptor agonist, clenbuterol, has been proposed as a strategy for ameliorating the symptoms of muscular dystrophy. In the dystrophic mdx mouse, only the diaphragm muscle exhibits progressive and severe degeneration in muscle structure and function similar to that observed in Duchenne muscular dystrophy. We tested the hypothesis that 20 weeks of clenbuterol treatment ( approximately 1.5-2 mg kg(-1)day(-1)) would increase the force and power output of diaphragm muscle strips of 6-month-old mdx and control mice. At this age, the diaphragm muscles of mdx mice show extensive degeneration and impaired contractility compared with control mice. Clenbuterol treatment did not increase the normalized force or power output of diaphragm strips from either mdx or control mice. The degeneration and necrosis within the diaphragm muscle of mdx mice was also not ameliorated by clenbuterol treatment. The results indicate that clenbuterol treatment does not improve the structure or function of diaphragm muscles from mdx mice.

Clenbuterol reduces soleus muscle fatigue during disuse in aged rats

High levels of clenbuterol have been shown to preserve muscle mass and function during disuse. In this study we report that a low dose of clenbuterol (10 microg/kg per day) lessened the loss of in situ soleus muscle isometric force normalized to wet muscle weight (P(o)/g wet weight) by 8% and reduced isometric fatigue by approximately 30% in senescent rats after 21 days of hindlimb suspension (HS). Clenbuterol did not reduce the loss of relative force in the soleus of adult rats or the plantaris of old or adult rats. Furthermore, clenbuterol failed to improve muscle force or isometric fatigue in the soleus of adult rats or in the plantaris of either age group after HS. We conclude that low levels of clenbuterol reduce muscle fatigue in slow muscles during disuse and this beta-agonist may also have therapeutic value for reducing fatigue in slow muscles (e.g., postural muscles) in the elderly during disuse.

A physiological level of clenbuterol does not prevent atrophy or loss of force in skeletal muscle of old rats

Supraphysiological levels of clenbuterol (CL) reduce muscle degradation in both young and old animals; however, these pharmacological levels induce side effects that are unacceptable in the elderly. In this study, we tested the hypothesis that a "physiological" dose of CL (10 microg. kg(-1). day(-1)) would attenuate the loss of in situ isometric force and mass in muscles of senescent rats during hindlimb suspension (HS). Adult (3 mo) and senescent (38 mo) Fischer 344 x Brown Norway rats received CL or a placebo during 21 days of normal-weight-bearing or HS conditions (8 rats/age group). HS reduced soleus muscle weight-to-body weight ratio by 31%, muscle cross-sectional area by 37%, and maximal isometric tetanic force (P(o)) by 76% in senescent rats. CL attenuated the loss of P(o) and muscle weight by 17 and 8%, respectively, in the soleus of senescent rats relative to HS+placebo conditions, but it did not improve muscle weight normalized for body weight. CL did not reduce the decrease in soleus P(o) or mass after HS in adult rats. CL failed to reduce the loss of plantaris weight (-20%) and P(o) (-46%) in senescent rats after HS. Our data support the conclusion that physiological levels of CL do not improve fast muscle atrophy and only modestly reduce slow muscle atrophy, and, therefore, it is largely an ineffective countermeasure for preventing muscle wasting from HS in senescent rats.

Clenbuterol reduces degeneration of exercised or aged dystrophic (mdx) muscle

Evidence of dystrophic muscle degeneration in the hind limb muscles of young (20-week-old) treadmill-exercised or aged (87-week-old) sedentary mdx mice was greatly reduced by treatment with clenbuterol, a beta(2)-adrenoceptor agonist. Daily treadmill exercise for 10 weeks increased the size of regions within the mdx plantaris but not the soleus or gastrocnemius muscles, in which necrotic muscle fibers or the absence of fibers was observed. Clenbuterol reduced the size of these abnormal regions from 21% of total muscle cross-sectional area to levels (4%) found in sedentary mdx mice. In addition, the muscles obtained from aged clenbuterol-treated mdx or wild-type mice did not display the extensive fibrosis or fiber loss observed in untreated mdx mice. These observations are consistent with a mechanism of dystrophic muscle degeneration caused by work load-induced injury that is cumulative with aging and is opposed by beta(2)-adrenoceptor activation. Optimization of beta(2)-agonist treatment of muscular dystrophy in mdx mice may lead to a useful therapeutic modality for human forms of the disease.

Year-long clenbuterol treatment of mice increases mass, but not specific force or normalized power, of skeletal muscles

1. Clenbuterol has been proposed for the treatment of muscle wasting disorders, but its long-term effects on skeletal muscle function have not been tested rigorously. We tested the hypothesis that year-long treatment of young (6 months) mice with clenbuterol would increase skeletal muscle mass and in vitro measurements of specific force (Po) and power output. 2. Male mice (C57BL/10ScSn) were divided into treated (n = 6) or untreated (n = 8) groups. Treated mice received clenbuterol (1.5-2 mg/kg per day) in their drinking water for 52 weeks, following a staggered 3 day on/3 day off schedule to attenuate the response to clenbuterol. 3. Clenbuterol treatment increased the absolute mass of each muscle tested: the heart by 28%, extensor digitorum longus (EDL) by 16%, soleus by 22% and tibialis anterior by 17%. For treated compared with untreated mice, absolute Po (mN) was greater in soleus muscles but not different in EDL muscles. Absolute power output (mW) of the EDL and soleus muscles was not different and no differences were observed for the specific Po (kN/m2) or normalized power output (W/kg) of EDL muscles, soleus muscles or diaphragm muscle strips. 4. We conclude that, following year-long treatment of mice with clenbuterol, the mass of the heart and both fast and slow skeletal muscles is increased, but the lack of any change in normalized Po or power output indicates that clenbuterol has little therapeutic effect on the functional properties of skeletal muscle.

Effect of clenbuterol on sarcoplasmic reticulum function in single skinned mammalian skeletal muscle fibers

We examined the effect of the beta2-agonist clenbuterol (50 microM) on depolarization-induced force responses and sarcoplasmic reticulum (SR) function in muscle fibers of the rat (Rattus norvegicus; killed by halothane overdose) that had been mechanically skinned, rendering the beta2-agonist pathway inoperable. Clenbuterol decreased the peak of depolarization-induced force responses in the extensor digitorum longus (EDL) and soleus fibers to 77.2 +/- 9.0 and 55.6 +/- 5.4%, respectively, of controls. The soleus fibers did not recover. Clenbuterol significantly and reversibly reduced SR Ca2+ loading in EDL and soleus fibers to 81.5 +/- 2.8 and 78.7 +/- 4.0%, respectively, of controls. Clenbuterol also produced an approximately 25% increase in passive leak of Ca2+ from the SR of the EDL and soleus fibers. These results indicate that clenbuterol has direct effects on fast- and slow-twitch skeletal muscle, in the absence of the beta2-agonist pathway. The increased Ca2+ leak in the triad region may lead to excitation-contraction coupling damage in the soleus fibers and could also contribute to the anabolic effect of clenbuterol in vivo.

Respiratory muscle injury in animal models and humans

Respiratory muscle injury may result from excessive loading due to a decrease in respiratory muscle strength, an increase in the work of breathing, or an increase in the rate of ventilation. Other conditions such as hypoxemia, hypercapnia, aging, decreased nutrition, and immobilization may potentiate respiratory muscle injury. Respiratory muscle injury has been shown in animal models using direct muscle or phrenic nerve stimulation, acute inspiratory resistive loading, tracheal banding, corticosteroids, phrenic nerve section, and the mdx mouse. Although numerous examples of diaphragm injury have been shown in animal models, evidence in humans is sparse. Potential mechanisms which may contribute to respiratory muscle injury include high levels of intracellular calcium-activated degradative enzymes, non-uniformity of stresses and strains, plasma membrane disruptions, and activation of the inflammatory process.

Pharmacological modulation of pressure-overload cardiac hypertrophy: changes in ventricular function, extracellular matrix, and gene expression

BACKGROUND

Appropriate cardiac hypertrophy (CH) is necessary in several clinical settings, such as pulmonary artery banding in the two-stage arterial switch operation for transposition of the great arteries. Pressure-overload CH, however, produces ventricular dysfunction due to structural and molecular changes. The beta2-adrenergic receptor agonist clenbuterol has been shown to induce CH without such adverse effects to the rat heart. This study was performed to determine its effects on left ventricular (LV) function, structure, and gene expression in pressure-overload CH.

METHODS AND RESULTS

Sprague-Dawley rats were assigned to one of four groups: 1, sham-operated (n=15); 2, banding of ascending aorta (n=22); 3, banding+clenbuterol (n=18); and 4, banding+thyroxine (n= 17). At the end of 3 weeks, groups 2, 3, and 4 showed an increase in LV mass index of 49.7+/-5.1%, 66.1+/-3.8%, and 47.6+/-4.6%, respectively, relative to group 1. A subgroup with severe CH (>50%) in group 2 was found to have significantly impaired developed pressure and diastolic relaxation and an increase in passive stiffness, with significantly reduced LV expression of sarcoplasmic reticulum Ca2+-ATPase2a (SERCA2a) mRNA and increased LV collagen concentration. In comparison, similarly hypertrophied animals in groups 3 and 4 demonstrated improved developed pressure, normal relaxation and diastolic stiffness with normal collagen concentration, and a greater abundance of SERCA2a mRNA.

CONCLUSIONS

Clenbuterol administration in conjunction with pressure overload produces a specific type of CH with preserved LV function. In addition, an increase in LV mass was associated with less fibrosis and greater expression of SERCA2a mRNA than banding alone.