Increased muscle strength in paralyzed patients after spinal cord injury: effect of beta-2 adrenergic agonist

Functional electrical stimulation therapy for upper extremity rehabilitation following spinal cord injury: a pilot study

STUDY DESIGN

Pilot study.

OBJECTIVES

To examine if functional electrical stimulation therapy (FEST) improves neuromuscular factors underlying upper limb function in individuals with SCI.

SETTING

A tertiary spinal cord rehabilitation center specialized in spinal cord injury care in Canada.

METHODS

We examined 29 muscles from 4 individuals living with chronic, cervical, and incomplete SCI. The analysis was focused on the changes in muscle activation, as well as on how the treatment could change the ability to control a given muscle or on how multiple muscles would be coordinated during volitional efforts.

RESULTS

There was evidence of gains in muscle strength, activation, and median frequency after the FEST. Gains in muscle activation indicated the activation of a greater number of motor units and gains in muscle median frequency the involvement of higher threshold, faster motor units. In some individuals, these changes were smaller but accompanied by increased control over muscle contraction, evident in a greater ability to sustain a volitional contraction, reduce the co-contraction of antagonist muscles, and provide cortical drive.

CONCLUSIONS

FEST increases muscle strength and activation. Enhanced control of muscle contraction, reduced co-contraction of antagonist muscles, and a greater presence of cortical drive were some of the findings supporting the effects of FEST at the sensory-motor integration level.

Pharmacologic approaches to prevent skeletal muscle atrophy after spinal cord injury

Skeletal muscle atrophy is a hallmark of severe spinal cord injury (SCI) that is precipitated by the neural insult and paralysis. Additionally, other factors may influence muscle loss, including systemic inflammation, low testosterone, low insulin-like growth factor (IGF)-1, and high-dose glucocorticoid treatment. The signaling cascades that drive SCI-induced muscle loss are common among most forms of disuse atrophy and include ubiquitin-proteasome signaling and others. However, differing magnitudes and patterns of atrophic signals exist after SCI versus other disuse conditions and are accompanied by endogenous inhibition of IGF-1/PI3K/Akt signaling, which combine to produce exceedingly rapid atrophy. Several well-established anabolic agents, including androgens and myostatin inhibitors, display diminished ability to prevent SCI-induced atrophy, while ursolic acid and β2-agonists more effectively attenuate muscle loss. Strategies combining physical rehabilitation regimens to reload the paralyzed limbs with drugs targeting the underlying molecular pathways hold the greatest potential to improve muscle recovery after severe SCI.

Salbutamol modifies the neuromuscular junction in a mouse model of ColQ myasthenic syndrome

The β-adrenergic agonists salbutamol and ephedrine have proven to be effective as therapies for human disorders of the neuromuscular junction, in particular many subsets of congenital myasthenic syndromes. However, the mechanisms underlying this clinical benefit are unknown and improved understanding of the effect of adrenergic signalling on the neuromuscular junction is essential to facilitate the development of more targeted therapies. Here, we investigated the effect of salbutamol treatment on the neuromuscular junction in the ColQ deficient mouse, a model of end-plate acetylcholinesterase deficiency. ColQ-/- mice received 7 weeks of daily salbutamol injection, and the effect on muscle strength and neuromuscular junction morphology was analysed. We show that salbutamol leads to a gradual improvement in muscle strength in ColQ-/- mice. In addition, the neuromuscular junctions of salbutamol treated mice showed significant improvements in several postsynaptic morphological defects, including increased synaptic area, acetylcholine receptor area and density, and extent of postjunctional folds. These changes occurred without alterations in skeletal muscle fibre size or type. These findings suggest that β-adrenergic agonists lead to functional benefit in the ColQ-/- mouse and to long-term structural changes at the neuromuscular junction. These effects are primarily at the postsynaptic membrane and may lead to enhanced neuromuscular transmission.

Beta2 -adrenoceptor agonist salbutamol increases protein turnover rates and alters signalling in skeletal muscle after resistance exercise in young men

KEY POINTS

Animal models have shown that beta₂ -adrenoceptor stimulation increases protein synthesis and attenuates breakdown processes in skeletal muscle. Thus, the beta₂ -adrenoceptor is a potential target in the treatment of disuse-, disease- and age-related muscle atrophy. In the present study, we show that a few days of oral treatment with the commonly prescribed beta₂ -adrenoceptor agonist, salbutamol, increased skeletal muscle protein synthesis and breakdown during the first 5 h after resistance exercise in young men. Salbutamol also counteracted a negative net protein balance in skeletal muscle after resistance exercise. Changes in protein turnover rates induced by salbutamol were associated with protein kinase A-signalling, activation of Akt2 and modulation of mRNA levels of growth-regulating proteins in skeletal muscle. These findings indicate that protein turnover rates can be augmented by beta₂ -adrenoceptor agonist treatment during recovery from resistance exercise in humans.

ABSTRACT

The effect of beta₂ -adrenoceptor stimulation on skeletal muscle protein turnover and intracellular signalling is insufficiently explored in humans, particularly in association with exercise. In a randomized, placebo-controlled, cross-over study investigating 12 trained men, the effects of beta₂ -agonist (6 × 4 mg oral salbutamol) on protein turnover rates, intracellular signalling and mRNA response in skeletal muscle were investigated 0.5-5 h after quadriceps resistance exercise. Each trial was preceded by a 4-day lead-in treatment period. Leg protein turnover rates were assessed by infusion of [¹³ C₆ ]-phenylalanine and sampling of arterial and venous blood, as well as vastus lateralis muscle biopsies 0.5 and 5 h after exercise. Furthermore, myofibrillar fractional synthesis rate, intracellular signalling and mRNA response were measured in muscle biopsies. The mean (95% confidence interval) myofibrillar fractional synthesis rate was higher for salbutamol than placebo [0.079 (95% CI, 0.064 to 0.093) vs. 0.066 (95% CI, 0.056 to 0.075%) × h^-1 ] (P < 0.05). Mean net leg phenylalanine balance 0.5-5 h after exercise was higher for salbutamol than placebo [3.6 (95% CI, 1.0 to 6.2 nmol) × min^-1  × 100 g_{Leg Lean Mass}^-1 ] (P < 0.01). Phosphorylation of Akt2, cAMP response element binding protein and PKA substrate 0.5 and 5 h after exercise, as well as phosphorylation of eEF2 5 h after exercise, was higher (P < 0.05) for salbutamol than placebo. Calpain-1, Forkhead box protein O1, myostatin and Smad3 mRNA content was higher (P < 0.01) for salbutamol than placebo 0.5 h after exercise, as well as Forkhead box protein O1 and myostatin mRNA content 5 h after exercise, whereas ActivinRIIB mRNA content was lower (P < 0.01) for salbutamol 5 h after exercise. These observations suggest that beta₂ -agonist increases protein turnover rates in skeletal muscle after resistance exercise in humans, with concomitant cAMP/PKA and Akt2 signalling, as well as modulation of mRNA response of growth-regulating proteins.

Serum blood metabolite response and evaluation of select organ weight, histology, and cardiac morphology of beef heifers exposed to a dual corticotropin-releasing hormone and vasopressin challenge following supplementation of zilpaterol hydrochloride

The objectives of this study were 1) to determine if supplementation of zilpaterol hydrochloride (ZH) altered select organ weights, histology, and cardiac anatomical features at harvest and 2) to determine if administration of a corticotropin-releasing hormone (CRH) and vasopressin (VP) challenge following 20 d of ZH supplementation altered the blood chemistry profile in cattle. Crossbred heifers ( = 20; 556 ± 7 kg BW) were randomized into 2 treatment groups: 1) control (CON), without ZH, and 2) zilpaterol (ZIL; ZH at 8.33 mg/kg [DM basis] for 20 d). On d 20 of supplementation, heifers were fitted with indwelling jugular catheters. On d 24, starting at 0800 h and continuing until 1600 h, blood samples were collected at 60-min intervals. At 1000 h, heifers received an i.v. bolus of CRH (0.3 µg/kg BW) and VP (1.0 µg/kg BW) to activate the stress axis. Serum was separated and stored at -80°C until analyzed for a large-animal chemistry panel. Following the CRH/VP challenge, heifers were harvested on d 25, 26, and 27 (5, 6, and 7 d after ZH supplementation); BW, HCW, select organ weights, and histology were measured, and a total heart necropsy was performed. A treatment effect ( ≤ 0.02) was observed for Ca, K, creatinine, alkaline phosphatase, and sorbitol dehydrogenase. Zilpaterol-fed heifers had decreased ( ≤ 0.02) concentrations of Ca and K and increased concentrations ( 0.01) of creatinine ( = 0.02) during the CRH/VP challenge when compared to control heifers. Control heifers had greater ( ≤ 0.05) alkaline phosphatase and sorbitol dehydrogenase concentrations when compared with ZIL heifers. A treatment × time interaction ( = 0.02) was observed for P; concentrations were similar between treatments from -2 to 6 h postchallenge, and 7 h postchallenge CON heifers had decreased P. Liver ( = 0.06) and kidney ( = 0.08) weights as a percentage of BW tended ( ≤ 0.08) to be reduced in ZIL heifers. Gross liver weights tended ( = 0.08) to be lower in ZIL heifers. Other organ (heart, lung, adrenals) to BW ratios remained similar ( ≥ 0.41). These data suggest that there are some variations observed between treatments in terms of response to ZH supplementation and the CRH/VP challenge; however, in the environmental conditions of this study, limited variation in blood metabolic responses and organ weights suggests that the supplementation of ZH did not detrimentally alter the physiology of cattle.

Hypertrophic effect of inhaled beta2 -agonist with and without concurrent exercise training: A randomized controlled trial

Due to a high prevalence of asthma and exercise-induced bronchoconstriction in elite athletes, there is a high use of beta₂ -adrenoceptor agonists (beta₂ -agonists) in the athletic population. While anabolic in rodents, no study has been able to detect hypertrophy in humans after chronic beta₂ -agonist inhalation. We investigated whether inhaled beta₂ -agonist, terbutaline, alters body composition and metabolic rate with and without concurrent exercise training in healthy young men. Sixty-seven participants completed a 4-week intervention of daily terbutaline (8 × 0.5 mg) or placebo treatment without concurrent training (habitual; n = 23), with resistance (n = 23) or endurance (n = 21) training 3 times weekly. Before and after the interventions, participant's body composition was determined by dual-energy X-ray absorptiometry and resting metabolic rate and substrate oxidation by indirect calorimetry. Terbutaline increased lean body mass by 1.03 kg (95% CI 0.29-1.76; P < .05) and 1.04 kg (95% CI 0.16-1.93; P < .05) compared to placebo in the habitual and resistance training group, respectively, but had no effect compared to placebo in the endurance training group [-0.56 kg (95% CI -1.74-0.62; P > .05)]. Fat mass, bone mineral content, and resting metabolic rate did not change differently between treatments with the intervention. Daily inhalation of terbutaline in near-therapeutic doses induces skeletal muscle growth. This observation should be a concern for antidoping authorities.

EVALUATION OF MUSCLE STRENGTH IN MEDULLAR INJURY: A LITERATURE REVIEW

ABSTRACT Objective: To identify the tools used to evaluate muscle strength in subjects with spinal cord injury in both clinical practice and scientific research. Methods: Initially, the literature review was carried out to identify the tools used in scientific research. The search was conducted in the following databases: Virtual Health Library (VHL), Pedro, and PubMed. Studies published between 1990 and 2016 were considered and selected, depicting an evaluation of muscle strength as an endpoint or for characterization of the sample. Next, a survey was carried out with physiotherapists to identify the instruments used for evaluation in clinical practice, and the degree of satisfaction of professionals with respect to them. Results: 495 studies were found; 93 were included for qualitative evaluation. In the studies, we verified the use of manual muscle test with different graduation systems, isokinetic dynamometer, hand-held dynamometer, and manual dynamometer. In clinical practice, the manual muscle test using the motor score recommended by the American Spinal Cord Injury Association was the most used method, despite the limitations highlighted by the physiotherapists interviewed. Conclusion: In scientific research, there is great variation in the methods and tools used to evaluate muscle strength in individuals with spinal cord injury, differently from clinical practice. The tools available and currently used have important limitations, which were highlighted by the professionals interviewed. No instrument depicts direct relationship of muscle strength and functionality of the subject. There is no consensus as to the best method for assessing muscle strength in spinal cord injury, and new instruments are needed that are specific for use in this population.

Effect of Salbutamol on Respiratory Muscle Strength in Spinal Muscular Atrophy

BACKGROUND

Oral salbutamol has shown clinical benefits in spinal muscular atrophy (SMA). We studied its effect on the respiratory muscle strength in children with different types of SMA.

METHODS

Lung and respiratory muscle functions were assessed in children receiving daily oral salbutamol for at least one year. The respiratory data of age-matched SMA II historical control subjects were compared with data of SMA II patients receiving salbutamol.

RESULTS

Seven children (6.4 ± 2.0 years old, range four to ten; one SMA I, five SMA II, and one SMA III) treated with salbutamol (duration 23 ± 8 months) were assessed. Maximal static inspiratory pressure, sniff nasal inspiratory pressure, and slow vital capacity were significantly better in the salbutamol-treated SMA II group compared with control subjects (P < 0.05).

CONCLUSIONS

Long-term oral salbutamol showed benefits in respiratory function in children with SMA and appeared to increase the strength of the inspiratory muscles in a small cohort of SMA II patients.

β2-Adrenoceptor agonists as novel, safe and potentially effective therapies for Amyotrophic lateral sclerosis (ALS)

Amyotrophic lateral sclerosis (ALS) is a chronic and progressive neuromuscular disease for which no cure exists and better treatment options are desperately needed. We hypothesize that currently approved β2-adrenoceptor agonists may effectively treat the symptoms and possibly slow the progression of ALS. Although β2-agonists are primarily used to treat asthma, pharmacologic data from animal models of neuromuscular diseases suggest that these agents may have pharmacologic effects of benefit in treating ALS. These include inhibiting protein degradation, stimulating protein synthesis, inducing neurotrophic factor synthesis and release, positively modulating microglial and systemic immune function, maintaining the structural and functional integrity of motor endplates, and improving energy metabolism. Moreover, stimulation of β2-adrenoceptors can activate a range of downstream signaling events in many different cell types that could account for the diverse array of effects of these agents. The evidence supporting the possible therapeutic benefits of β2-agonists is briefly reviewed, followed by a more detailed review of clinical trials testing the efficacy of β-agonists in a variety of human neuromuscular maladies. The weight of evidence of the potential benefits from treating these diseases supports the hypothesis that β2-agonists may be efficacious in ALS. Finally, ways to monitor and manage the side effects that may arise with chronic administration of β2-agonists are evaluated. In sum, effective, safe and orally-active β2-agonists may provide a novel and convenient means to reduce the symptoms of ALS and possibly delay disease progression, affording a unique opportunity to repurpose these approved drugs for treating ALS, and rapidly transforming the management of this serious, unmet medical need.

A center's experience: pulmonary function in spinal cord injury

Traumatic spinal cord injury (SCI) is associated with significant psychological and physical challenges. A multidisciplinary approach to management is essential to ensure recovery during the acute phase, and comprehensive rehabilitative strategies are necessary to foster independence and quality of life throughout the chronic phase of injury. Complications that beset these individuals are often a unique consequence of SCI, and knowledge of the effects of SCI upon organ systems is essential for appropriate management. According to the National SCI Statistical Center (NSCISC), as of 2010 there were an estimated 265,000 persons living with SCI in the United States, with approximately 12,000 incidence cases annually. Although life expectancy for newly injured individuals with SCI is markedly reduced, persons with chronic SCI are expected to live about as long as individuals without SCI; however, longevity varies inversely with level of injury. Since 2005, 56 % of persons with SCI are tetraplegic, and due to paralysis of respiratory muscles, these individuals may be especially prone to pulmonary complications, which remain a major cause of mortality among persons with chronic SCI. We at the VA Rehabilitation Research and Development Center of Excellence for the Medical Consequences of SCI at the James J. Peters VA Medical Center have devoted more than 25 years to the study of secondary medical conditions that complicate SCI. Herein, we review pulmonary research at the Center, both our past and future endeavors, which form an integral part of our multidisciplinary approach toward achieving a greater understanding of and improving care for veterans with SCI.

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.

Therapeutic strategies for the treatment of spinal muscular atrophy

Spinal muscular atrophy (SMA) is an inherited neurodegenerative disease that results in progressive dysfunction of motor neurons of the anterior horn of the spinal cord. SMA is caused by the loss of full-length protein expression from the survival of motor neuron 1 (SMN1) gene. The disease has a unique genetic profile as it is autosomal recessive for the loss of SMN1, but a nearly identical homolog, SMN2, acts as a disease modifier whose expression is inversely correlated to clinical severity. Targeted therapeutic approaches primarily focus on increasing the levels of full-length SMN protein, through either gene replacement or regulation of SMN2 expression. There is currently no US FDA approved treatment for SMA. This is an exciting time as multiple efforts from academic and industrial laboratories are reaching the preclinical and clinical testing stages.

Therapy development for spinal muscular atrophy in SMN independent targets

Spinal muscular atrophy (SMA) is an autosomal recessive neurodegenerative disorder, leading to progressive muscle weakness, atrophy, and sometimes premature death. SMA is caused by mutation or deletion of the survival motor neuron-1 (SMN1) gene. An effective treatment does not presently exist. Since the severity of the SMA phenotype is inversely correlated with expression levels of SMN, the SMN-encoded protein, SMN is the most important therapeutic target for development of an effective treatment for SMA. In recent years, numerous SMN independent targets and therapeutic strategies have been demonstrated to have potential roles in SMA treatment. For example, some neurotrophic, antiapoptotic, and myotrophic factors are able to promote survival of motor neurons or improve muscle strength shown in SMA mouse models or clinical trials. Plastin-3, cpg15, and a Rho-kinase inhibitor regulate axonal dynamics and might reduce the influences of SMN depletion in disarrangement of neuromuscular junction. Stem cell transplantation in SMA model mice resulted in improvement of motor behaviors and extension of survival, likely from trophic support. Although most therapies are still under investigation, these nonclassical treatments might provide an adjunctive method for future SMA therapy.

31st g. Heiner sell lectureship: secondary medical consequences of spinal cord injury

Persons with spinal cord injury (SCI) have secondary medical consequences of paralysis and/or the consequences of extreme inactivity. The metabolic changes that result from reduced activity include insulin resistance with carbohydrate disorders and dyslipidemia. A higher prevalence of coronary artery calcification was found in persons with SCI than that in matched able-bodied controls. A depression in anabolic hormones, circulating testosterone and growth hormone, has been described. Adverse soft tissue body composition changes of increased adiposity and reduced skeletal muscle are appreciated. Immobilization is the cause for sublesional disuse osteoporosis with an associated increased risk of fragility fracture. Bowel dysmotility affects all segments of the gastrointestinal tract, with an interest in better defining and addressing gastroesophageal reflux disease and difficulty with evacuation. Developing and testing more effective approaches to cleanse the bowel for elective colonoscopy are being evaluated. The extent of respiratory dysfunction depends on the level and completeness of SCI. Individuals with higher spinal lesions have both restrictive and obstructive airway disease. Pharmacological approaches and expiratory muscle training are being studied as interventions to improve pulmonary function and cough strength with the objective of reducing pulmonary complications. Persons with spinal lesions above the 6th thoracic level lack both cardiac and peripheral vascular mechanisms to maintain blood pressure, and they are frequently hypotensive, with even worse hypotension with upright posture. Persistent and/or orthostatic hypotension may predispose those with SCI to cognitive impairments. The safety and efficacy of anti-hypotensive agents to normalize blood pressure in persons with higher level cord lesions is being investigated.

Formoterol and cancer muscle wasting in rats: Effects on muscle force and total physical activity

Cancer cachexia occurs in the majority of cancer patients before death, and it is responsible for the death of 22% of cancer patients. One of the most relevant characteristics of cachexia is that of asthenia, which reflects significant muscle wasting noted in cachectic cancer patients The aim of the present study was to assess whether the β(2)-adrenergic agonist formoterol is associated with an improvement in physiological parameters such as grip force and total physical activity in cachetic rats. Administration of the β(2)-agonist formoterol (0.3 mg/kg for 7 days) in rats bearing Yoshida AH-130 ascites hepatoma tumors, a model which induces a strong loss of both body and muscle weight, resulted in a significant reversal of the muscle wasting process, as reflected by individual muscle weights. The anti-wasting effects of the drug were also observed in terms of total physical activity and grip force, thus resulting in an improvement in physical performance in cachectic tumor-bearing rats.

beta(2)-Adrenergic agonist administration and strength training

beta(2)-Adrenergic agonists (beta(2)AA) produce myriad effects throughout the human body. Prescribed concurrently with theophylline for the bronchodilatory effects they offer in the treatment of asthma and chronic obstructive pulmonary disease, beta(2)AA actions include many beneficial and adverse changes when administered to animals at supraphysiological doses. Beneficial changes include improved musculoskeletal health and function, which can be maintained because adverse changes are reduced if oral beta(2)AA are given at therapeutic dosages in humans with concurrent resistive exercise administration. Combined oral beta(2)AA-resistive exercise treatments have been shown to produce gains in musculoskeletal health and function in numerous healthy and disuse atrophy human models. The mechanism(s) by which beta(2)AA exert their effects are a function of multiple factors, not the least of which includes the type of model receiving the drug treatment. Combined oral beta(2)AA-resistive exercise treatments in humans showed that adverse effects were greatly reduced when prudent and safer drug administration practices were employed (eg, screening subjects for tolerance before drug treatment). Results from human research trials suggest that administration of the combined treatment improves musculoskeletal function and performance with minimal health risk if proper precautions are followed. A related issue is administration of the combined treatment as an ergogenic aid to athletic performance. Given the results presented in this article, physicians should be wary of potential drug abuse and administer beta(2)AA only under appropriate circumstances when such a treatment is warranted.

Pulmonary function and spinal cord injury

Injury to the cervical and upper thoracic spinal cord disrupts function of inspiratory and expiratory muscles, as reflected by reduction in spirometric and lung volume parameters and static mouth pressures. In association, subjects with tetraplegia have decreased chest wall and lung compliance, increased abdominal wall compliance, and rib cage stiffness with paradoxical chest wall movements, all of which contribute to an increase in the work of breathing. Expiratory muscle function is more compromised than inspiratory muscle function among subjects with tetraplegia and high paraplegia, which can result in ineffective cough and propensity to mucus retention and atelectasis. Subjects with tetraplegia also demonstrate heightened vagal activity with reduction in baseline airway caliber, findings attributed to loss of sympathetic innervation to the lungs. Significant increase in airway caliber following inhalation of ipratropium bromide, an anticholinergic agent, suggests that reduction in airway caliber is not due to acquired airway fibrosis stemming from repeated infections or to abnormal hysteresis secondary to chronic inability of subjects to inhale to predicted total lung capacity. Reduced baseline airway caliber possibly explains why subjects with tetraplegia exhibit airway hyperresponsiveness to methacholine and ultrasonically nebulized distilled water. While it has been well demonstrated that bilateral phrenic nerve pacing or stimulation through intramuscular diaphragmatic electrodes improves inspiratory muscle function, it remains unclear if inspiratory muscle training improves pulmonary function. Recent findings suggest that expiratory muscle training, electrical stimulation of expiratory muscles and administration of a long-acting beta(2)-agonist (salmeterol) improve physiological parameters and cough. It is unknown if baseline bronchoconstriction in tetraplegia contributes to respiratory symptoms, of if the chronic administration of a bronchodilator reduces the work of breathing and/or improves respiratory symptoms. Less is known regarding the benefits of treatment of obstructive sleep apnea, despite evidence indicating that the prevalence of this condition in persons with tetraplegia is far greater than that encountered in able-bodied individuals.

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.

Daily salbutamol in young patients with SMA type II

The aim of this open pilot study was to establish the profile of tolerability and clinical response of salbutamol (albuterol) in a cohort of young children affected by type II spinal muscular atrophy (SMA). Twenty-three children between 30 months and 6 years of age were treated with salbutamol (2 mg three times a day) for 1 year. All children were longitudinally assessed using the Hammersmith motor functional scale 6 months before treatment started (T0), at baseline (T1) and 6 and 12 months later. There was no significant change in function between T0 and T1 assessments, but the functional scores recorded after 6 and 12 months of treatment were significantly higher than those recorded at baseline (p=0.006). Our results suggest that salbutamol may be beneficial to SMA patients without producing any major side effect. Larger prospective randomized, double-blind, placebo controlled trials are needed to confirm these preliminary findings.

Skeletal muscle changes after hemiparetic stroke and potential beneficial effects of exercise intervention strategies

Stroke is the leading cause of disability in the United States. New evidence reveals significant structural and metabolic changes in skeletal muscle after stroke. Muscle alterations include gross atrophy and shift to fast myosin heavy chain in the hemiparetic (contralateral) leg muscle; both are related to gait deficit severity. The underlying molecular mechanisms of this atrophy and muscle phenotype shift are not known. Inflammatory markers are also present in contralateral leg muscle after stroke. Individuals with stroke have a high prevalence of insulin resistance and diabetes. Skeletal muscle is a major site for insulin-glucose metabolism. Increasing evidence suggests that inflammatory pathway activation and oxidative injury could lead to wasting, altered function, and impaired insulin action in skeletal muscle. The health benefits of exercise in disabled populations have now been recognized. Aerobic exercise improves fitness, strength, and ambulatory performance in subjects with chronic stroke. Therapeutic exercise may modify or reverse skeletal muscle abnormalities.

Nutrient supplementation post ambulation in persons with incomplete spinal cord injuries: a randomized, double-blinded, placebo-controlled case series

OBJECTIVE

To examine effects of protein-carbohydrate intake on ambulation performance in persons with incomplete spinal cord injury (SCI).

DESIGN

Double-blinded treatment with washout and placebo crossover.

SETTING

Academic medical center.

PARTICIPANTS

Three subjects aged 34 to 43 years with incomplete SCI at C5-T4.

INTERVENTIONS

Subjects walked to fatigue on 5 consecutive days. On fatigue, participants consumed 48g of vanilla-flavored whey and 1g/kg of body weight of carbohydrate (CH(2)O). Weekend rest followed, and the process was repeated. A 2-week washout was interposed and the process repeated using 48g of vanilla-flavored soy.

MAIN OUTCOME MEASURES

Oxygen consumed (Vo(2); in L/min), carbon dioxide evolved (Vco(2)), respiratory exchange ratio (RER: Vco(2)/Vo(2)), time (in minutes), and distance walked (in meters) were recorded. Caloric expenditure was computed as Vo(2) by time by 21kJ/L (5kcal/L) of oxygen consumed. Data were averaged across the final 2 ambulation sessions for each testing condition.

RESULTS

Despite slow ambulation velocities (range, .11-.34m/s), RERs near or above unity reflected reliance on CH(2)O fuel substrates. Average ambulation time to fatigue was 17.8% longer; distance walked 37.9% longer, and energy expenditure 12.2% greater with the whey and CH(2)O supplement than with the soy drink.

CONCLUSIONS

Whey and CH(2)O ingestion after fatiguing ambulation enhanced ensuing ambulation by increasing ambulation distance, time, and caloric expenditure in persons with incomplete SCI.

Rapamycin inhibits the growth and muscle-sparing effects of clenbuterol

Clenbuterol and other β2-adrenergic agonists are effective at inducing muscle growth and attenuating muscle atrophy through unknown mechanisms. This study tested the hypothesis that clenbuterol-induced growth and muscle sparing is mediated through the activation of Akt and mammalian target of rapamycin (mTOR) signaling pathways. Clenbuterol was administered to normal weight-bearing adult rats to examine the growth-inducing effects and to adult rats undergoing muscle atrophy as the result of hindlimb suspension or denervation to examine the muscle-sparing effects. The pharmacological inhibitor rapamycin was administered in combination with clenbuterol in vivo to determine whether activation of mTOR was involved in mediating the effects of clenbuterol. Clenbuterol administration increased the phosphorylation status of PKB/Akt, S6 kinase 1/p70s6k, and eukaryotic initiation factor 4E binding protein 1/PHAS-1. Clenbuterol treatment induced growth by 27–41% in normal rats and attenuated muscle loss during hindlimb suspension by 10–20%. Rapamycin treatment resulted in a 37–97% suppression of clenbuterol-induced growth and a 100% reduction of the muscle-sparing effect. In contrast, rapamycin was unable to block the muscle-sparing effects of clenbuterol after denervation. Clenbuterol was also shown to suppress the expression of the MuRF1 and MAFbx transcripts in muscles from normal, denervated, and hindlimb-suspended rats. These results demonstrate that the effects of clenbuterol are mediated, in part, through the activation of Akt and mTOR signaling pathways.

Salmeterol improves pulmonary function in persons with tetraplegia

beta(2)-Adrenergic agonists are known to improve muscle strength because of anabolic properties. The purpose of this study was to determine if long-term administration of a long-acting beta(2)-adrenergic agonist to subjects with tetraplegia is associated with improvement in pulmonary function parameters and maximal static inspiratory and expiratory mouth pressures (MIP and MEP, respectively), measures of respiratory muscle strength. The study was a randomized, prospective, double-blind, placebo-controlled, crossover trial and conducted at the James J. Peters Veterans Affairs Medical Center. Thirteen subjects who had complete or incomplete tetraplegia for more than one year participated in the study. Eleven subjects completed the study. All were clinically stable outpatients without any history of asthma or use of inhaled bronchodilators. Following baseline measurements, patients were randomized to receive salmeterol or placebo from identically marked Diskus containers for 4 weeks. Following a 4-week washout period, the subjects were randomized to receive the alternate preparation for 4 weeks. Pulmonary function parameters and static mouth pressure were measured during baseline and during the fourth week of the two study periods. During the 4-week period of salmeterol administration, forced vital capacity, forced expiratory volume in 1 s, peak expiratory flow, MIP, and MEP improved significantly compared with placebo and baseline. Expiratory reserve volume increased significantly compared to baseline. Increases in MIP and MEP during salmeterol administration suggest improvement in respiratory muscle strength. However, this cannot be stated with certainty because MIP and MEP are dependent on volume parameters at which they are measured. Regardless of the mechanism, improvement in static mouth pressures indicates that salmeterol should benefit these individuals by improving cough effectiveness.

Skeletal muscle hypertrophy and anti-atrophy effects of clenbuterol are mediated by the beta2-adrenergic receptor

Analyses were performed to evaluate the roles of the beta1- and beta2-adrenergic receptors in the skeletal muscle hypertrophy and anti-atrophy response to the beta-adrenergic agonist, clenbuterol. Treatment of wild-type mice with clenbuterol resulted in statistically significant hypertrophy of the innervated tibialis anterior and medial gastrocnemius muscles and inhibition of denervation-induced atrophy of these muscles. Treatment of beta1-adrenergic receptor knockout mice with clenbuterol also resulted in statistically significant hypertrophy of the innervated tibialis anterior and medial gastrocnemius muscles and inhibition of denervation-induced atrophy of these muscles. In contrast, in beta2-adrenergic receptor knockout mice and in mice lacking both the beta1- and beta2-adrenergic receptors, clenbuterol treatment did not result in hypertrophy of the innervated tibialis anterior and medial gastrocnemius muscles, nor did it inhibit denervation-induced atrophy in these muscles. Together these data demonstrate that the beta2-adrenergic receptor is responsible for both the skeletal muscle hypertrophy and anti-atrophy effects of the beta-adrenergic agonist clenbuterol.

Salbutamol effect in spinal cord injured individuals undergoing functional electrical stimulation training

OBJECTIVE

Preliminary study to investigate possible changes in skeletal muscle morphology and function, as well as hormonal and metabolic effects, after treatment with a selective beta2-adrenergic receptor agonist.

DESIGN

Double-blind, placebo-controlled trial.

PARTICIPANTS

Three individuals with spinal cord injury (SCI).

INTERVENTION

Two-week treatment with salbutamol (2mg) or placebo (ascorbic acid, 50mg) twice a day. Program of functional electronic stimulation (FES) cycling for 30 minutes twice a week.

MAIN OUTCOME MEASURES

Body weight, three measures of leg circumference (gluteal furrow, one third of subischial height up from tibial-femoral joint space, and minimum circumference above the knee), muscle fiber area, and total work output per session.

RESULTS

There were increases in body weight (2.30 +/- .70kg), leg circumferences (gluteal furrow 1.70 +/- .27cm, one third subischial height 1.53 +/- 1.65cm, minimum circumference above the knee .43 +/- .04cm), and muscle (vastus lateralis) cross-sectional area (1,374 +/- 493 to 2,446 +/- 1,177microm2) after salbutamol treatment, whereas quadriceps muscle contractile function was not modified. Total work output during FES cycling sessions was increased more during salbutamol treatment (64%) compared with training alone (27%). Salbutamol treatment was associated with a large decrease in skeletal muscle beta-adrenergic receptor density.

CONCLUSION

Although some side effects were noted, these results suggest that a short treatment with the beta2-adrenergic receptor agonist salbutamol during a training program with FES cycling could be beneficial in patients with SCI.

[Adrenergic beta-2 agonists and their impact on physical performance]

That asthmatics benefit not only from physical exercise but that they can even reach performance levels comparable to that of non asthmatics is becoming increasingly clear. To reach such a level, asthmatics need to use drugs to prevent effort-related asthma attacks in addition to taking appropriate therapeutic measures. beta 2 agonists are drugs that potentially produce a certain amount of "anabolic" effect, depending on the dose and permanence in tissues, in laboratory and farm animals as well as in humans. We must conclude that the dose needed to obtain this effect is higher than that used for therapeutic purposes in asthma or respiratory diseases. Bearing in mind that oral treatments are considerably less effective than inhaled drugs in exercise-related asthma attacks, that the doses of the latter are lower and that the ergogenic effects of the substances studied are nil or below detection levels, it seems logical to allow individuals with asthma who use such substances to engage in physical exercise. Moreover, the asthmatic whose disease is so severe as to require the use of orally administered beta 2 agonists will in all likelihood be too ill to participate in a sport while that status persists.

Effects of oral albuterol on serum lipids and carbohydrate metabolism in healthy men

beta(2)-Selective adrenergic agonists are used in the management of bronchial asthma and preterm labor. Due to their ability to increase muscle strength and size in animal models, new applications for these agents are also being explored for neuromuscular disorders and in rehabilitation. However, the effects of long-term beta(2)-agonist administration on lipoprotein and carbohydrate metabolism are incompletely understood. This investigation evaluated the effects of a beta(2)-agonist, albuterol, on serum lipids and carbohydrate homeostasis in eight healthy nonsmoking men aged 24 to 61 years. Collection of fasting blood samples was completed in duplicate on separate days at baseline, during 14 days of oral albuterol administration (Proventil Repetabs, 8 mg twice daily; Schering Pharmaceuticals, Kenilworth, NJ) and during a 7-day washout period. Carbohydrate homeostasis was evaluated using the minimal model technique at the end of the baseline and albuterol periods. Fasting glucose and insulin, intravenous glucose tolerance, acute insulin response to intravenous glucose (AIRg), insulin sensitivity (Si), and glucose effectiveness (Sg) were not significantly changed during albuterol administration. Significant alterations (P < or = .02) were observed in total cholesterol ([TC] -9.1% +/- 2.5%), low-density lipoprotein cholesterol ([LDL-C] -15.0% +/- 2.9%), and high-density lipoprotein cholesterol ([HDL-C] +10.4% +/- 3.2%) concentrations, as well as the TC/HDL-C (-17.4% +/- 2.6%) and LDL-C/HDL-C (-22.9% +/- 2.4%) ratios. During washout, TC and LDL-C returned to baseline levels, whereas HDL-C remained elevated by 5.8% +/- 2.4% (P < .05). Thus, albuterol administration was associated with favorable changes in the serum lipid profile without marked impairment of glucose tolerance or its physiologic determinants.

Clenbuterol has a greater influence on untrained than on previously trained skeletal muscle in rats

The effects of clenbuterol, a selective beta 2-adrenergic agonist, and of exercise training on the properties of skeletal muscle were studied in the hindlimb of sedentary and trained rats. A 2-week training programme, consisting of climbing on a grid with a load attached to the tail, did not increase the muscle mass of the soleus, the plantaris and the gastrocnemius muscles or modify the isometric in situ contractile properties of the medial gastrocnemius muscle. The only change observed in a 12-week training regimen was a significant increase in contractile forces (expressed in grams per gram of muscle) of the medial gastrocnemius muscle at sub-tetanic stimulating frequencies (twitch 42%, 25Hz 45% and 50Hz 47%). Both training programmes significantly increased fatigue resistance of the medial gastrocnemius muscle. A 2-week oral treatment with clenbuterol significantly increased the muscle mass of the soleus (19.8%), plantaris (16.9%) and gastrocnemius (15.3%) muscles in all animals treated with the agonist. However, clenbuterol had different effects in animals beginning their training programme than in animals that had been trained for the previous 10 weeks. Specifically, clenbuterol caused a significant increase in gastrocnemius muscle mass in the former group but not in the latter. These results suggest that the responses to the combination of clenbuterol and training in previously trained skeletal muscles are not as marked as those observed in untrained muscles.