Long-term effects of clenbuterol on diaphragm morphology and contractile properties in emphysematous hamsters

The conventional isoproterenol-induced heart failure model does not consistently mimic the diaphragmatic dysfunction observed in patients

Heart failure (HF) impairs diaphragm function. Animal models realistically mimicking HF should feature both the cardiac alterations and the diaphragmatic dysfunction characterizing this disease. The isoproterenol-induced HF model is widely used, but whether it presents diaphragmatic dysfunction is unknown. However, indirect data from research in other fields suggest that isoproterenol could increase diaphragm function. The aim of this study was to test the hypothesis that the widespread rodent model of isoproterenol-induced HF results in increased diaphragmatic contractility. Forty C57BL/6J male mice were randomized into 2 groups: HF and healthy controls. After 30 days of isoproterenol infusion to establish HF, in vivo diaphragmatic excursion and ex vivo isolated diaphragm contractibility were measured. As compared with healthy controls, mice with isoproterenol-induced HF showed the expected changes in structural and functional echocardiographic parameters and lung edema. isoproterenol-induced HF increased in vivo diaphragm excursion (by ≈30%, p<0.01) and increased by ≈50% both ex vivo peak specific force (p<0.05) and tetanic force (p<0.05) at almost all 10–100 Hz frequencies (p<0.05), with reduced fatigue resistance (p<0.01) when compared with healthy controls. Expression of myosin genes encoding the main muscle fiber types revealed that Myh4 was higher in isoproterenol-induced HF than in healthy controls (p<0.05), suggesting greater distribution of type IIb fibers. These results show that the conventional isoproterenol-induced HF model increases diaphragm contraction, a finding contrary to what is observed in patients with HF. Therefore, this specific model seems limited for translational an integrative HF research, especially when cardio-respiratory interactions are investigated.

Pulmonary and diaphragmatic pathology in collagen type I α1 mutant mice with osteogenesis imperfecta

BackgroundOsteogenesis imperfecta (OI) is most often caused by mutations in type I collagen genes. Respiratory complications have been largely attributed to spine and ribcage deformities. We hypothesized that direct involvement of the pulmonary parenchyma and/or diaphragm by the disease may occur.MethodsIn Col1a1^Jrt/+ mice, a model of severe dominant OI, mean linear intercept length (Lm) was used to assess the distal airspace size. Cross-sectional area (CSA) and myosin heavy chain (MyHC) phenotype of the diaphragm muscle fibers, as well as contractile properties, were determined. OI mice were also treated with neutralizing antibodies against transforming growth factor-β (TGF-β).ResultsDistal airspace enlargement occurred in OI mice (Lm +27%). Diaphragmatic thickness and fiber number were reduced, with increases in fast-twitch type IIx/IIb MyHC fibers. Ex vivo force generation (normalized for CSA) of the diaphragm was also significantly reduced. The increased Lm values found in OI mice were not prevented by anti-TGF-β antibody treatment.ConclusionsThe Col1a1^Jrt/+ mouse model of OI demonstrates: (1) pulmonary airspace enlargement not driven by TGF-β; and (2) reduced muscle mass and intrinsic contractile weakness of the diaphragm. These results suggest a complex and multifaceted basis for respiratory complications in OI that cannot be solely attributed to bone manifestations.

Proteasome inhibition improves diaphragm function in an animal model for COPD

Diaphragm muscle weakness in patients with chronic obstructive pulmonary disease (COPD) is associated with increased morbidity and mortality. Recent studies indicate that increased contractile protein degradation by the proteasome contributes to diaphragm weakness in patients with COPD. The aim of the present study was to investigate the effect of proteasome inhibition on diaphragm function and contractile protein concentration in an animal model for COPD. Elastase-induced emphysema in hamsters was used as an animal model for COPD; normal hamsters served as controls. Animals were either treated with the proteasome inhibitor Bortezomib (iv) or its vehicle saline. Nine months after induction of emphysema, specific force-generating capacity of diaphragm bundles was measured. Proteolytic activity of the proteasome was assayed spectrofluorometrically. Protein concentrations of proteasome, myosin, and actin were measured by means of Western blotting. Proteasome activity and concentration were significantly higher in the diaphragm of emphysematous hamsters than in normal hamsters. Bortezomib treatment reduced proteasome activity in the diaphragm of emphysematous and normal hamsters. Specific force-generating capacity and myosin concentration of the diaphragm were reduced by ~25% in emphysematous hamsters compared with normal hamsters. Bortezomib treatment of emphysematous hamsters significantly increased diaphragm-specific force-generating capacity and completely restored myosin concentration. Actin concentration was not affected by emphysema, nor by bortezomib treatment. We conclude that treatment with a proteasome inhibitor improves contractile function of the diaphragm in emphysematous hamsters through restoration of myosin concentration. These findings implicate that the proteasome is a potential target of pharmacological intervention on diaphragm weakness in COPD.

Respiratory muscle fiber remodeling in chronic hyperinflation: dysfunction or adaptation?

The diaphragm and other respiratory muscles undergo extensive remodeling in both animal models of emphysema and in human chronic obstructive pulmonary disease, but the nature of the remodeling is different in many respects. One common feature is a shift toward improved endurance characteristics and increased oxidative capacity. Furthermore, both animals and humans respond to chronic hyperinflation by diaphragm shortening. Although in rodent models this clearly arises by deletion of sarcomeres in series, the mechanism has not been proven conclusively in human chronic obstructive pulmonary disease. Unique characteristics of the adaptation in human diaphragms include shifts to more predominant slow, type I fibers, expressing slower myosin heavy chain isoforms, and type I and type II fiber atrophy. Although some laboratories report reductions in specific force, this may be accounted for by decreases in myosin heavy chain content as the muscles become more oxidative and more efficient. More recent findings have reported reductions in Ca(2+) sensitivity and reduced myofibrillar elastic recoil. In contrast, in rodent models of disease, there is no consistent evidence for loss of specific force, no consistent shift in fiber populations, and atrophy is predominantly seen only in fast, type IIX fibers. This review challenges the hypothesis that the adaptations in human diaphragm represent a form of dysfunction, secondary to systemic disease, and suggest that most findings can as well be attributed to adaptive processes of a complex muscle responding to unique alterations in its working environment.

[Salbutamol improves diaphragm force generation in experimental sepsis]

OBJECTIVE

In a high percentage of cases, severe sepsis is accompanied by acute respiratory failure, in which weakness of the respiratory muscles plays an important role. Weakened respiratory muscles that are subjected to an increased mechanical load may develop muscle fatigue, with exacerbation of the respiratory failure. Because beta2-adrenergic drugs increase muscle contraction force, they may play a role in preventing and managing respiratory failure in septic patients. Our aim was to study the effects of salbutamol on diaphragm function in an animal model of peritoneal sepsis.

MATERIAL AND METHODS

The study included 3 groups of animals: a) a control group (n=7), in which the animals underwent a median laparotomy without visceral manipulation; b) a septic group (n=10), in which peritoneal sepsis was induced by cecal ligation and puncture (CLP); and c) a salbutamol group (n=7), in which peritoneal sepsis (CLP) was treated with salbutamol. Hemodynamic parameters and blood gases were measured in vivo. Diaphragm function was evaluated in vitro.

RESULTS

Salbutamol increased aortic blood flow and heart rate while it reduced mean arterial pressure in the animals with peritoneal sepsis (P< .05). Sepsis produced a significant drop in diaphragmatic force both before and after the application of a muscle-fatigue protocol. Treatment with salbutamol improved muscle contraction force before and after application of the protocol (P< .05).

CONCLUSIONS

The use of beta2-adrenergic drugs such as salbutamol improves diaphragm function in experimental sepsis. The mechanisms that produce this improvement require further study.

Mitochondrial function in diaphragm of emphysematous hamsters after treatment with nandrolone

Respiratory failure in patients with COPD may be caused by insufficient force production or insufficient endurance capacity of the respiratory muscles. Anabolic steroids may improve respiratory muscle function in COPD. The effect of anabolic steroids on mitochondrial function in the diaphragm in emphysema is unknown. In an emphysematous male hamster model, we investigated whether administration of the anabolic steroid nandrolone decanoate (ND) altered the activity of mitochondrial respiratory chain complexes in the diaphragm. The bodyweight of hamsters treated with ND was decreased after treatment compared with initial values, and serum testosterone levels were significantly lower in hamsters treated with ND than in control hamsters. No difference in the activity of mitochondrial respiratory chain complexes in the diaphragm between normal and emphysematous hamsters was observed. Treatment with ND did not change the activity of mitochondrial respiratory chain complexes in the diaphragm of both normal and emphysematous hamsters. In emphysematous hamsters, administration of ND decreased the activity of succinate:cytochrome c oxidoreductase compared with ND treatment in normal hamsters. We conclude that anabolic steroids have negative effects on the activity of succinate:cytochrome c oxidoreductase and anabolic status in this emphysematous hamster model.

Isoproterenol hydrochloride augments collagen proliferation and ATPase activity in mice soleus and EDL muscles

Aim of this study is to analyze the effect of chronic administration of beta agonist isoproterenol hydrochloride (60 mg kg(-1) day(-1); 30 days) on soleus (a slow type) and extensor digitorum longus (EDL, a fast type) muscles in young mice. Isoproterenol resulted in significant increase in muscle weight to whole body weight ratio with no increase in hypertrophy index in soleus muscle. A significant increase in noncontractile protein collagen is also observed in both muscles but more prominent in soleus muscle. Collagen proliferation is also analyzed on sodium dodecyle sulphate polyacrylamide gel electrophoresis (SDS-PAGE) of pepsin soluble and Cyanogen Bromide (CN Br) treated pepsin insoluble collagen. Isoproterenol remolded the myofibrillar proteins in both muscles but significant increase in myofibrillar ATPase activity occurred only in soleus muscle. It is concluded that growth stimulatory effect of isoproterenol hydrochloride is more prominent in soleus than FDL muscle. Isoproterenol augmented the proliferation of non-contractile protein collagen in soleus and EDL muscles. The transformation in myofibrillar proteins caused by isoproterenol might lead to an enhancement of contractile performance.

Transdermal treatment with tulobuterol increases isometric contractile properties of diaphragm muscle in mice

Clinically, patients suffering from bronchial asthma are often treated transdermally with tulobuterol patches to dilate the bronchi. Tulobuterol, a synthetic beta(2) agonist, is also thought to act as a diaphragm muscle contractor, like other beta(2) sympathomimetic drugs. However, it has not been clarified that transdermal treatment with tulobuterol influences diaphragm muscle contractility. We therefore examined its effects on contractile properties of such muscles obtained from BALB/c mice. Two systems, a tulobuterol incubation group (in vitro) and a tulobuterol transdermal treatment group (in vivo), were employed. In both groups, the contractile properties of the dissected diaphragm muscles were measured by field stimulation in an organ bath. In the incubation group, the diaphragm muscle of untreated mice was incubated in an organ buffer at 10(-7), 10(-6), or 10(-5) M tulobuterol for 1 hr and then measured for contractility. Tulobuterol significantly increased force-frequency curves at a concentration of 10(-5) M at 1 (p < 0.01), 30, 50, 70, 100, and 120 Hz (p < 0.05, each) compared with the values at 0 M. In the transdermal treatment group, the diaphragm muscle was dissected from animals at 1, 4, 8, 12, or 24 hrs after treatment and measured for contractility, showing that the force-frequency curves were significantly increased and maintained from 4 to 24 hrs (each p < 0.01 as compared with the sham-treated group). We suggest that transdermal tulobuterol treatment in case of bronchial asthma is useful not only for bronchial dilatation, but also for increasing diaphragm muscle contractility.

Apoptosis and Id2 expression in diaphragm and soleus muscle from the emphysematous hamster

During chronic obstructive pulmonary disease (COPD) diaphragm and peripheral muscle weakness occur. Muscle remodeling and wasting may be a result of apoptosis and changes in muscle-specific transcription factors, such as MyoD, altering muscle-specific gene transcription and muscle regenerative capacity. To investigate this, we instilled under ketamine/xylazine anesthesia porcine elastase in the lungs of hamsters to induce emphysema. The emphysematous hamster is an accepted model for COPD. In the diaphragm and peripheral muscles we assessed the occurrence of apoptosis, and in the diaphragm and soleus also the expression of MyoD and inhibitor of differentiation protein 2 (Id2). There was no significant muscle atrophy in emphysematous hamsters. The mRNA levels of TNF-alpha and markers of apoptosis were significantly elevated in the diaphragm and soleus muscles during emphysema. This was accompanied by an increased presence of nucleosomes in the cytosol. Caspase 3 activity and the DNA-binding activity of the p65 subunit of NF-kappaB, however, were unaltered in all muscles. The protein expression of MyoD and Id2 were decreased and increased in the diaphragm and the soleus muscle, respectively. Thus, despite the absence of muscle atrophy in emphysematous hamsters, there was evidence of increased TNF-alpha expression, apoptosis, and altered muscle-specific transcriptional regulation as reflected by decreased MyoD and elevated Id2 levels at least in the soleus and diaphragm muscle. These alterations may impair the regenerative capacity of skeletal muscles and ultimately contribute to muscle wasting.

Respiratory muscle fibres: specialisation and plasticity

Skeletal muscles are composed of fibres of different types, each type being identified by the isoform of myosin heavy chain which is expressed as slow 1, fast 2A, fast 2X, and fast 2B. Slow fibres are resistant to fatigue due to their highly oxidative metabolism whereas 2X and 2B fibres are easily fatiguable and fast 2A fibres exhibit intermediate fatigue resistance. Slow fibres and fast fibres are present in equal proportions in the adult human diaphragm while intercostal muscles contain a higher proportion of fast fibres. A small fibre size, abundance of capillaries, and a high aerobic oxidative enzyme activity are typical features of diaphragm fibres and give them the resistance to fatigue required by their continuous activity. Because of their fibre composition, intercostal muscles are less resistant to fatigue. The structural and functional characteristics of respiratory muscle fibres are not fixed, however, and can be modified in response to several physiological and pathological conditions such as training (adaptation to changes in respiratory load), adaptation to hypoxia, age related changes, and changes associated with respiratory diseases. The properties of respiratory muscle fibres can also be modified by pharmacological agents such as beta2 agonists and corticosteroids used for the treatment of respiratory diseases.

Beta Adrenoceptor Agonists, Clenbuterol, and Isoproterenol Retard Denervation Atrophy in Rat Gastrocnemius Muscle: Use of 3-Methylhistidine as a Marker of Myofibrillar Degeneration

The effects of beta adrenergic agonists, clenbuterol (2 mg/kg body weight/d) and isoproterenol (12 mg/kg body weight/d), in normal innervated and denervated rat gastrocnemius muscle were investigated. The daily administration of beta adrenergic agonists to normal innervated rats for a short period (7 d) resulted in the hypertrophy of gastrocnemius as confirmed from the measurement of total tissue protein contents. The development of denervation atrophy witnessed a stimulation in the expression of acid and alkaline phosphatases, pointing to an enhanced myofibrillar degeneration. An administration of beta adrenergic agonists inhibited the expression of raised levels of these enzymes in denervated muscle. A measurement of 3-methylhistidine in muscle revealed a loss of amino acid with the progress in the development of denervation atrophy. Serum and urine samples from denervated rats showed a progressive accumulation of 3-methylhistidine. Clenbuterol and isoproterenol treatment to these rats resulted in an inhibition of 3-methylhistidine accumulation. When 3-methylhistidine was used as a marker of myofibrillar degeneration, the results seemed to suggest that the degeneration of cyto-contractile apparatus accompanying denervation atrophy is attenuated in the presence of beta adrenergic agonists, implying that these sympathomimetic drugs are capable of reversing denervation atrophy in rat gastrocnemius.

Paylean alters myosin heavy chain isoform content in pig muscle

Feeding beta-adrenergic agonists promotes muscle growth. Early histological techniques failed to show precisely how feeding ractopamine-HCl (Paylean) alters muscle growth in pigs. To understand these effects, an indirect enzyme-linked immunosorbent assay (ELISA) was used to determine the abundance of each adult skeletal muscle myosin heavy chain isoform, one means of assigning muscle fiber type, in fast and slow muscles of pigs fed Paylean. Sixty growing pigs (-85 kg) were randomly assigned to three Paylean doses (0, 20, or 60 ppm). At 3, 7, 14, 28, and 42 d of treatment, four pigs per dose were harvested and white (WST) and red (RST) semitendinosus and longissimus (LM) muscles were removed and processed, and myosin heavy chain was quantified by ELISA. Feeding Paylean enhanced (P < 0.05) pigs' average daily gain. Muscle myosin heavy chain (slow, 2A, 2AX, and 2B) composition differed (P < 0.05) across muscles. Compared with LM, RST contained approximately five times more (P < 0.0001) slow and type 2A myosin heavy chain and three times more 2AX myosin heavy chain but nearly undetectable amounts of 2B myosin heavy chain. Myosin heavy chain composition of the WST closely resembled that of the LM (i.e., greater 2AX and 2B and less slow and 2A). After 42d of 60 ppm Paylean, the amount of slow, 2A, and 2AX myosin heavy chain decreased (P < 0.05) across the three muscles whereas the amount of 2B myosin heavy chain increased (P < 0.05). In contrast, relative amounts of 2A and 2AX myosin heavy chain increased (P < 0.05) in muscle of control pigs at 42d. Changes associated with the 20-ppm dose were intermediate to and different from (P < 0.05) control and 60 ppm treatments. Correlations (P < 0.05) among various myosin heavy chain within muscles suggest that slow, type 2A, and 2X decrease with increases in 2B myosin heavy chain. These data show that administration of Paylean affects myosin heavy chain isoform composition in a time- and dose-dependent manner and provides a mechanism of action for Paylean altering animal growth.

Alteration of contractile force and mass in the senescent diaphragm with beta(2)-agonist treatment

Aging is associated with a decrease in diaphragmatic maximal tetanic force production (P(o)) in senescent rats. Treatment with the beta(2)-agonist clenbuterol (CB) has been shown to increase skeletal muscle mass and P(o) in weak locomotor skeletal muscles from dystrophic rodents. It is unknown whether CB can increase diaphragmatic mass and P(o) in senescent rats. Therefore, we tested the hypothesis that CB treatment will increase specific P(o) (i.e., force per cross-sectional area) and mass in the diaphragm of old rats. Young (5 mo) and old (23 mo) male Fischer 344 rats were randomly assigned to one of the following groups (n = 10/group): 1) young CB treated; 2) young control; 3) old CB treated; and 4) old control. Animals were injected daily with either CB (2 mg/kg) or saline for 28 days. CB increased (P < 0.05) the mass of the costal diaphragm in both young and old animals. CB treatment increased diaphragmatic-specific P(o) in old animals (approximately 15%; P < 0.05) but did not alter (P > 0.05) diaphragmatic-specific P(o) in young animals. Biochemical analysis indicated that the improved maximal specific P(o) in the diaphragm of CB-treated old animals was not due to increased myofibrillar protein concentration. Analysis of the myosin heavy chain (MHC) content of the costal diaphragm revealed a CB-induced increase (P < 0.05) in type IIb MHC and a decrease in type I, IIa, and IIx MHC in both young and old animals. These data support the hypothesis that CB treatment can restore the age-associated decline in both diaphragmatic-specific P(o) and muscle mass.

Effects of the beta(2)-agonist clenbuterol on respiratory and limb muscles of weaning rats

The aim of this study was to analyze the effects of chronic administration of the beta(2)-agonist clenbuterol (1.5 mg x kg(-1) x day(-1) for 4 wk in the drinking water) on respiratory (diaphragm and parasternal intercostal) and hindlimb (tibialis and soleus) muscles in young rats during postnatal development (21 to 49 postnatal days). The treatment resulted in very little stimulation of muscle growth. Significant slow-to-fast transitions in the expression of myosin heavy chain isoforms and significant increases in the myofibrillar ATPase activity were found in the diaphragm and soleus, whereas tibialis anterior and intercostal muscles did not show any significant fiber-type alteration. Decrease of oxidative enzyme activities and increase of glycolytic enzyme activities were also observed. It is concluded that whereas the growth stimulation is age dependent and only detectable in adult rats, the fiber-type transformation is also present in weaning rats and particularly evident in the soleus and diaphragm. The fiber-type transformation caused by clenbuterol might lead to an enhancement of contractile performance and also to a reduced resistance to fatigue.

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.

Lung volume reduction surgery does not improve diaphragmatic contractile properties or atrophy in hamsters with elastase-induced emphysema

It is claimed that lung volume reduction surgery (LVRS) improves inspiratory muscle function. As diaphragm structure and function are not directly appraisable in patients, we studied the effects of LVRS on the diaphragm in vitro contractile properties and morphology in hamsters with elastase-induced emphysema. Four months after intratracheal instillation of elastase (40 U/100 g), hamsters underwent either bilateral LVRS (LVRS, n = 11) or a sham operation (SHAM, n = 8). Four animals died during the perioperative period in LVRS (n = 7). Hamsters instilled with saline served as control (CTL, n = 8). Animals were studied at the age of 9 mo. LVRS was associated with a significant 25% decrease in functional residual capacity compared to SHAM (p < 0.05). Compared with CTL, LVRS and SHAM showed a significant 18% and 14% reduction in diaphragm mass, respectively (p = 0.02). LVRS had a significantly decreased twitch tension compared to CTL and SHAM (p < 0.01). Both LVRS and SHAM showed increased resistance to muscle fatigue compared with CTL. The histochemical analysis revealed a significant shift from type IIx/b toward type IIa fibers in LVRS and SHAM compared with CTL. In conclusion, emphysema is associated with functional adaptations but LVRS does not appear to beneficially alter the diaphragm contractile and morphological characteristics in hamsters with elastase-induced emphysema.

Effects of emphysema and training on glutathione oxidation in the hamster diaphragm

Loading of skeletal muscles is associated with increased generation of oxidants, which in turn may impair muscle contractility. We investigated whether the load on the hamster diaphragm imposed by pulmonary emphysema induces oxidative stress, as indicated by glutathione oxidation, and whether the degree of glutathione oxidation is correlated with contractility of the diaphragm. In addition, the effect of 12 wk of treadmill exercise training on contractility and glutathione content in the normal (NH) and emphysematous hamster (EH) diaphragm was investigated. Training started 6 mo after elastase instillation. After the training period, glutathione content and in vitro contractility of the diaphragm were determined. Twitch force and maximal tetanic force were significantly reduced (by approximately 30 and approximately 15%, respectively) in EH compared with NH. In sedentary hamsters, the GSSG-to-GSH ratio was significantly elevated in the EH compared with the NH diaphragm. A significant inverse correlation was found between GSSG-to-GSH ratio and twitch force in the diaphragm (P < 0. 01). Training improved maximal tetanic force and reduced fatigability of the EH diaphragm but did not alter its glutathione content. In conclusion, 1) emphysema induces oxidative stress in the diaphragm, 2) training improves the contractile properties of the EH diaphragm, and 3) this improvement is not accompanied by changes in glutathione redox status.