Adaptations of the diaphragm in emphysema

Muscle-specific deletion of the vitamin D receptor in mice is associated with diaphragm muscle weakness

Diseases or conditions where diaphragm muscle (DIAm) function is impaired, including chronic obstructive pulmonary disease, cachexia, asthma, and aging, are associated with an increased risk of pulmonary symptoms, longer duration of hospitalizations, and increasing requirements for mechanical ventilation. Vitamin D deficiency is associated with proximal muscle weakness that resolves following therapy with vitamin D₃. Skeletal muscle expresses the vitamin D receptor (VDR), which responds to the active form of vitamin D, 1,25-dihydroxyvitamin D₃ by altering gene expression in target cells. In knockout mice without skeletal muscle VDRs, there is marked atrophy of muscle fibers and a change in skeletal muscle biochemistry. We used a tamoxifen-inducible skeletal muscle Cre recombinase in Vdr^fl/fl mice (Vdr^fl/fl actin.iCre+) to assess the role of muscle-specific VDR signaling on DIAm-specific force, fatigability, and fiber type-dependent morphology. Vdr^fl/fl actin.iCre+ mice treated with vehicle and Vdr^fl/fl mice treated with tamoxifen served as controls. Seven days following the final treatment, mice were euthanized, the DIAm was removed, and isometric force and fatigue were assessed in DIAm strips using direct muscle stimulation. The proportion and cross-sectional areas of DIAm fiber types were evaluated by immunolabeling with myosin heavy chain antibodies differentiating type I, IIa and IIx, and/or IIb fibers. We show that in mice with skeletal muscle-specific VDR deletion, maximum specific force and residual force following fatigue are impaired, along with a selective atrophy of type IIx and/or IIb fibers. These results show that the VDR has a significant biological effect on DIAm function independent of systemic effects on mineral metabolism.NEW & NOTEWORTHY Vitamin D deficiency and vitamin D receptor (VDR) polymorphisms are associated with adverse pulmonary and diaphragm muscle (DIAm)-associated respiratory outcomes. We used a skeletal muscle-specific tamoxifen-inducible VDR knockout to investigate DIAm dysfunction following reduced VDR signaling. Marked DIAm weakness and atrophy of type IIx and/or IIb fibers are present in muscle-specific tamoxifen-induced VDR knockout mice compared with controls. These results show that the VDR has a significant biological effect on DIAm function independent of systemic effects on mineral metabolism.

Impact of congenital diaphragmatic hernia on diaphragm muscle function in neonatal rats

Congenital diaphragmatic hernia (CDH) is characterized by incomplete partitioning of the thoracic and abdominal cavities by the diaphragm muscle (DIAm). The resulting in utero invasion of the abdominal viscera into the thoracic cavity leads to impaired fetal breathing movements, severe pulmonary hypoplasia, and pulmonary hypertension. We hypothesized that in a well-established rodent model of Nitrofen-induced CDH, DIAm isometric force generation, and DIAm fiber cross-sectional areas would be reduced compared with nonlesioned littermate and Control pups. In CDH and nonlesioned pups at embryonic day 21 or birth, DIAm isometric force responses to supramaximal field stimulation (200 mA, 0.5 ms duration pulses in 1-s duration trains at rates ranging from 10 to 100 Hz) was measured ex vivo. Further, DIAm fatigue was determined in response to 120 s of repetitive stimulation at 40 Hz in 330-ms duration trains repeated each second. The DIAm was then stretched to L_o, frozen, and fiber cross-sectional areas were measured in 10 μm transverse sections. In CDH pups, there was a marked reduction in DIAm-specific force and force following 120 s of fatiguing contraction. The cross-sectional area of DIAm fibers was also reduced in CDH pups compared with nonlesioned littermates and Control pups. These results show that CDH is associated with a dramatic weakening of the DIAm, which may contribute to poor survival despite various surgical efforts to repair the hernia and improve lung development.NEW & NOTEWORTHY There are notable respiratory deficits related to congenital diaphragmatic hernia (CDH), yet the contribution, if any, of frank diaphragm muscle weakness to CDH is unexplored. Here, we use the well-established Nitrofen teratogen model to induce CDH in rat pups, followed by diaphragm muscle contractility and morphological assessments. Our results show diaphragm muscle weakness in conjunction with reduced muscle fiber density and size, contributing to CDH morbidity.

Therapeutic Treatment with Abdominal Adipose Mesenchymal Cells Does Not Prevent Elastase-Induced Emphysema in Rats

OBJECTIVES

Emphysema and chronic bronchitis have different pathophysiologies but both are significant components of chronic obstructive lung disease (COPD). The levels of Matrix metalloproteinase (MMP)-9 in the bronchoalveloar lavage fluid (BALF) and in serum indicate the presence of emphysema. Intratracheal administration of elastase has been used to create a rat model of emphysema. Adipose tissue-derived mesenchymal stem cells (MSC) have been postulated to prevent or reverse emphysema, however, this has not been examined in the rat model of elastase-induced emphysema.

MATERIALS AND METHODS

In this study, 31 Wistar albino rats aged 6-8 weeks and weighing 250-300 g were assessed. On day 1, the animals were treated intratracheally with 0.5 mL saline (control group, n=10), i.e., 0.5 mL saline solution containing 0.1 IU porcine pancreatic elastase (PPE) (Elastase group, n=12) or PPE plus MSC (Elastase-MSC group, n=9) was adminstered per animal. MSCs suspended in serum were injected via the caudal vein on day 21. At least 10⁶ cells were injected. All animals were sacrificed on day 42 and the emphysema index (EI) was calculated, along with measuring the BALF and serum MMP-9 concentrations.

RESULTS

Porcine pancreatic elastase induced a significant degree of emphysema in the PPE groups as compared to the control group, which was determined by the EI index (p=0.008). This was not reversed by MSC treatment. The EI remained significantly low in comprison with the controls (p=0.001) and measured no different from the Elastase-treated animals. There was no statistically significant difference between the BALF and serum MMP-9 levels between the control and treatment groups.

CONCLUSION

Our findings suggest that therapeutic treatment with adipose tissue-derived MSC in rats has no effect on emphysema or on MMP9 expression, which is a known marker of emphysema.

Impact of aging on diaphragm muscle function in male and female Fischer 344 rats

The diaphragm muscle (DIAm) is the primary inspiratory muscle in mammals and is active during ventilatory behaviors, but it is also involved in higher-force behaviors such as those necessary for clearing the airway. Our laboratory has previously reported DIAm sarcopenia in rats and mice characterized by DIAm atrophy and a reduction in maximum specific force at 24 months of age. In Fischer 344 rats, these studies were limited to male animals, although in other studies, we noted a more rapid increase in body mass from 6 to 24 months of age in females (~140%) compared to males (~110%). This difference in body weight gain suggests a possible sex difference in the manifestation of sarcopenia. In mice, we previously measured transdiaphragmatic pressure (Pdi) to evaluate in vivo DIAm force generation across a range of motor behaviors, but found no evidence of sex-related differences. The purpose of this study in Fischer 344 rats was to evaluate if there are sex-related differences in DIAm sarcopenia, and if such differences translate to a functional impact on Pdi generation across motor behaviors and maximal Pdi (Pdi_max ) elicited by bilateral phrenic nerve stimulation. In both males and females, DIAm sarcopenia was apparent in 24-month-old rats with a ~30% reduction in both maximum specific force and the cross-sectional area of type IIx and/or IIb fibers. Importantly, in both males and females, Pdi generated during ventilatory behaviors was unimpaired by sarcopenia, even during more forceful ventilatory efforts induced via airway occlusion. Although ventilatory behaviors were preserved with aging, there was a ~20% reduction in Pdi_max , which likely impairs the ability of the DIAm to generate higher-force expulsive airway clearance behaviors necessary to maintain airway patency.

Impact of sarcopenia on diaphragm muscle fatigue

NEW FINDINGS

What is the central question of this study? Is the residual force generated by the diaphragm muscle after repeated activation reduced with sarcopenia, and is the residual force generated after fatiguing activation sufficient to sustain ventilatory behaviours of diaphragm muscle in young and old rats? What is the main finding and its importance? After diaphragm muscle fatigue, the residual specific force after 120 s of repeated stimulation was unaffected by ageing and was sufficient to accomplish ventilatory behaviours, but not expulsive manoeuvres (e.g. coughing). The inability to perform expulsive behaviours might underlie the increased susceptibility of older individuals to respiratory tract infections.

ABSTRACT

Type IIx and/or IIb diaphragm muscle (DIAm) fibres make up more fatigable motor units that are more vulnerable to sarcopenia, i.e. age-associated reductions of specific force and cross-sectional area. In contrast, type I and IIa DIAm fibres form fatigue-resistant motor units that are relatively unchanged with age. The fatigue resistance of the DIAm is assessed by normalizing the residual force generated after a period of repeated supramaximal stimulation (e.g. 120 s) to the initial maximal force. Given that sarcopenia primarily affects more fatigable DIAm motor units, apparent fatigue resistance improves with ageing. However, the central question is whether there is an ageing-related difference in the residual force generated by the DIAm after repeated stimulation and whether this force is sufficient to sustain ventilatory behaviours of DIAm. In 6- and 24-month-old Fischer 344 rats, we assessed the loss of ex vivo DIAm force throughout 120 s of repeated supramaximal stimulation at 10, 40 and 75 Hz. We found that relative fatigue resistance improved in older rats at 40 and 75 Hz stimulation. Across all stimulation frequencies, DIAm residual force was unchanged with age (∼5 N cm^-2 ). We conclude that ageing increases the relative contribution of type I and IIa fibres to DIAm force, with decreased contributions of type IIx and/or IIb fibres. The residual force generated by the DIAm after repeated stimulation is sufficient to accomplish ventilatory behaviours, regardless of age.

Obesity modulates diaphragm curvature in subjects with and without COPD

Obesity is a common comorbidity of chronic obstructive pulmonary disease (COPD) and has been associated with worse outcomes. However, it is unknown whether the interaction between obesity and COPD modulates diaphragm shape and consequently its function. The body mass index (BMI) has been used as a correlate of obesity. We tested the hypothesis that the shape of the diaphragm muscle and size of the ring of its insertion in non-COPD and COPD subjects are modulated by BMI. We recruited 48 COPD patients with postbronchiodilator forced expiratory volume in 1 s (FEV1)-to-forced vital capacity (FVC) < 0.7 and 29 age-matched smoker/exsmoker control (non-COPD) subjects, who underwent chest computed tomography (CT) at lung volumes ranging from functional residual capacity (FRC) to total lung capacity (TLC). We then computed maximum principal diaphragm curvature in the midcostal region of the left hemidiaphragm at the end of inspiration during quiet breathing (EI) and at TLC. The radius of maximum curvature of diaphragm muscle increased with BMI in both COPD and non-COPD subjects. The size of diaphragm ring of insertion on the chest wall also increased significantly with increasing BMI. Surprisingly, COPD severity did not appear to cause significant alteration in diaphragm shape except in normal-weight subjects at TLC. Our data uncovered important factors such as BMI, the size of the diaphragm ring of insertion, and disease severity that modulate the structure of the ventilatory pump in non-COPD and COPD subjects.

Aging-related changes in respiratory system mechanics and morphometry in mice

Previous work investigating respiratory system mechanics in mice has reported an aging-related increase in compliance and mean linear intercept (Lm). However, these changes were assessed using only a young (2-mo-old) and old (20- and 26-mo-old) group yet were interpreted to reflect a linear evolution across the life span. Therefore, to investigate respiratory system mechanics and lung morphometry across a more complete spectrum of ages, we utilized 2 (100% survival, n = 6)-, 6 (100% survival, n = 12)-, 18 (90% survival, n = 12)-, 24 (75% survival, n = 12)-, and 30 (25% survival, n = 12)-mo-old C57BL/6 mice. We found a nonlinear aging-related decrease in respiratory system resistance and increase in dynamic compliance and hysteresis between 2- and 24-mo-old mice. However, in 30-mo-old mice, respiratory system resistance increased, and dynamic compliance and hysteresis decreased relative to 24-mo-old mice. Respiratory system impedance spectra were measured between 1-20.5 Hz at positive end-expiratory pressures (PEEP) of 1, 3, 5, and 7 cmH2O. Respiratory system resistance and reactance at each level of PEEP were increased and decreased, respectively, only in 2-mo-old animals. No differences in the respiratory system impedance spectra were observed in 6-, 18-, 24-, and 30-mo-old mice. Additionally, lungs were fixed following tracheal instillation of 4% paraformaldehyde at 25 cmH2O and processed for Lm and airway collagen deposition. There was an aging-related increase in Lm consistent with emphysematous-like changes and no evidence of increased airway collagen deposition. Accordingly, we demonstrate nonlinear aging-related changes in lung mechanics and morphometry in C57BL/6 mice.

Mechanical properties of respiratory muscles

Striated respiratory muscles are necessary for lung ventilation and to maintain the patency of the upper airway. The basic structural and functional properties of respiratory muscles are similar to those of other striated muscles (both skeletal and cardiac). The sarcomere is the fundamental organizational unit of striated muscles and sarcomeric proteins underlie the passive and active mechanical properties of muscle fibers. In this respect, the functional categorization of different fiber types provides a conceptual framework to understand the physiological properties of respiratory muscles. Within the sarcomere, the interaction between the thick and thin filaments at the level of cross-bridges provides the elementary unit of force generation and contraction. Key to an understanding of the unique functional differences across muscle fiber types are differences in cross-bridge recruitment and cycling that relate to the expression of different myosin heavy chain isoforms in the thick filament. The active mechanical properties of muscle fibers are characterized by the relationship between myoplasmic Ca2+ and cross-bridge recruitment, force generation and sarcomere length (also cross-bridge recruitment), external load and shortening velocity (cross-bridge cycling rate), and cross-bridge cycling rate and ATP consumption. Passive mechanical properties are also important reflecting viscoelastic elements within sarcomeres as well as the extracellular matrix. Conditions that affect respiratory muscle performance may have a range of underlying pathophysiological causes, but their manifestations will depend on their impact on these basic elemental structures.

Impact of diaphragm muscle fiber atrophy on neuromotor control

In skeletal muscles, motor units comprise a motoneuron and the group of muscle fibers innervated by it, which are usually classified based on myosin heavy chain isoform expression. Motor units displaying diverse contractile and fatigue properties are important in determining the range of motor behaviors that can be accomplished by a muscle. Muscle fiber atrophy and weakness may disproportionately affect specific fiber types across a variety of diseases or clinical conditions, thus impacting neuromotor control. In this regard, fiber atrophy that affects a specific fiber type will alter the relative contribution of different motor units to overall muscle structure and function. For example, in various diseases there is fairly selective atrophy of type IIx and/or IIb fibers comprising the strongest yet most fatigable motor units. As a result, there is muscle weakness (i.e., reductions in force per cross-sectional area) associated with an apparent improvement in resistance to fatiguing contractions. This review will examine neuromotor control of respiratory muscles such as the diaphragm muscle and the impact of muscle fiber atrophy on motor performance.

Neuromotor control in chronic obstructive pulmonary disease

Neuromotor control of skeletal muscles, including respiratory muscles, is ultimately dependent on the structure and function of the motor units (motoneurons and the muscle fibers they innervate) comprising the muscle. In most muscles, considerable diversity of contractile and fatigue properties exists across motor units, allowing a range of motor behaviors. In diseases such as chronic obstructive pulmonary disease (COPD), there may be disproportional primary (disease related) or secondary effects (related to treatment or other concomitant factors) on the size and contractility of specific muscle fiber types that would influence the relative contribution of different motor units. For example, with COPD there is a disproportionate atrophy of type IIx and/or IIb fibers that comprise more fatigable motor units. Thus fatigue resistance may appear to improve, while overall motor performance (e.g., 6-min walk test) and endurance (e.g., reduced aerobic exercise capacity) are diminished. There are many coexisting factors that might also influence motor performance. For example, in COPD patients, there may be concomitant hypoxia and/or hypercapnia, physical inactivity and unloading of muscles, and corticosteroid treatment, all of which may disproportionately affect specific muscle fiber types, thereby influencing neuromotor control. Future studies should address how plasticity in motor units can be harnessed to mitigate the functional impact of COPD-induced changes.

Respiratory muscle plasticity

Muscle plasticity is defined as the ability of a given muscle to alter its structural and functional properties in accordance with the environmental conditions imposed on it. As such, respiratory muscle is in a constant state of remodeling, and the basis of muscle's plasticity is its ability to change protein expression and resultant protein balance in response to varying environmental conditions. Here, we will describe the changes of respiratory muscle imposed by extrinsic changes in mechanical load, activity, and innervation. Although there is a large body of literature on the structural and functional plasticity of respiratory muscles, we are only beginning to understand the molecular-scale protein changes that contribute to protein balance. We will give an overview of key mechanisms regulating protein synthesis and protein degradation, as well as the complex interactions between them. We suggest future application of a systems biology approach that would develop a mathematical model of protein balance and greatly improve treatments in a variety of clinical settings related to maintaining both muscle mass and optimal contractile function of respiratory muscles.

Elastase-induced pulmonary emphysema: insights from experimental models

Several distinct stimuli can be used to reproduce histological and functional features of human emphysema, a leading cause of disability and death. Since cigarette smoke is the main cause of emphysema in humans, experimental researches have attempted to reproduce this situation. However, this is an expensive and cumbersome method of emphysema induction, and simpler, more efficacious alternatives have been sought. Among these approaches, elastolytic enzymes have been widely used to reproduce some characteristics of human cigarette smoke-induced disease, such as: augmentation of airspaces, inflammatory cell influx into the lungs, and systemic inflammation. Nevertheless, the use of elastase-induced emphysema models is still controversial, since the disease pathways involved in elastase induction may differ from those occurring in smoke-induced emphysema. This indicates that the choice of an emphysema model may impact the results of new therapies or drugs being tested. The aim of this review is to compare the mechanisms of disease induction in smoke and elastase emphysema models, to describe the differences among various elastase models, and to establish the advantages and disadvantages of elastase-induced emphysema models. More studies are required to shed light on the mechanisms of elastase-induced emphysema.

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.

Treatment with a corticotrophin releasing factor 2 receptor agonist modulates skeletal muscle mass and force production in aged and chronically ill animals

Background

Muscle weakness is associated with a variety of chronic disorders such as emphysema (EMP) and congestive heart failure (CHF) as well as aging. Therapies to treat muscle weakness associated with chronic disease or aging are lacking. Corticotrophin releasing factor 2 receptor (CRF2R) agonists have been shown to maintain skeletal muscle mass and force production in a variety of acute conditions that lead to skeletal muscle wasting.

Hypothesis

We hypothesize that treating animals with a CRF2R agonist will maintain skeletal muscle mass and force production in animals with chronic disease and in aged animals.

Methods

We utilized animal models of aging, CHF and EMP to evaluate the potential of CRF2R agonist treatment to maintain skeletal muscle mass and force production in aged animals and animals with CHF and EMP.

Results

In aged rats, we demonstrate that treatment with a CRF2R agonist for up to 3 months results in greater extensor digitorum longus (EDL) force production, EDL mass, soleus mass and soleus force production compared to age matched untreated animals. In the hamster EMP model, we demonstrate that treatment with a CRF2R agonist for up to 5 months results in greater EDL force production in EMP hamsters when compared to vehicle treated EMP hamsters and greater EDL mass and force in normal hamsters when compared to vehicle treated normal hamsters. In the rat CHF model, we demonstrate that treatment with a CRF2R agonist for up to 3 months results in greater EDL and soleus muscle mass and force production in CHF rats and normal rats when compared to the corresponding vehicle treated animals.

Conclusions

These data demonstrate that the underlying physiological conditions associated with chronic diseases such as CHF and emphysema in addition to aging do not reduce the potential of CRF2R agonists to maintain skeletal muscle mass and force production.

Exercise capacity in hamsters with elastase-induced emphysema compared to normal controls

The purpose of this study was to determine whether hamsters with elastase-induced emphysema (EMP) would demonstrate a reduction in exercise capacity compared to control (CON) hamsters and whether changes in activity levels, muscle function and structure could explain any changes in exercise capacity. Peak oxygen consumption and daily activity levels were measured on two occasions. Inspiratory capacity under deep anesthesia, in vitro measurements of muscle force and fatigability for the diaphragm (DIA) and extensor digitorum longus (EDL) and fiber proportions, muscle cross-sectional area and fiber specific SDH activity from the DIA, EDL and vastus lateralis (VLA) were obtained. Inspiratory capacity was 60% higher in the EMP compared to CON hamsters (p=0.0004). Activity levels and exercise capacity were not significantly different between EMP and CON hamsters. Muscle strength and fatigability, fiber proportions, muscle cross-sectional area and fiber specific SDH activity were similar between EMP and CON hamsters. In conclusion, in hamsters, elastase-induced emphysema did not reduce maximal exercise capacity.

The Effect of Exercise on Peripheral Muscle in Emphysema: A Preliminary Investigation

Emphysema has been associated with loss of aerobic muscle fibers and decreased blood supply. However, when these changes begin and whether exercise can prevent these changes is unknown. The purpose of this study was to examine peripheral muscle at different time points during the development of emphysema and to determine the additional effects of muscle activity. In a series of 3 experiments, emphysema was induced in hamsters. Exercise was simulated through surgical overload (OV) of the plantaris muscle of one leg. Animals were sacrificed at 1, 3, and 5 months following emphysema induction. Fiber type composition and capillary-to-fiber ratio (CFR) were determined. There were no significant changes in fiber type composition in the 1-month group. A significant increase in type IIA fiber composition (mean 72.0 vs. 54.5%) and decrease in type IIB fiber (mean 13.3 vs. 28.1%) was seen in the non-overloaded muscles following 3 months. In the 5-month group, there was a significant decrease in percentage of type I fibers (mean 14.7 vs. 28.0%). There were no significant differences in fiber type composition in the OV limb, regardless of duration. The CFR was significantly lower in the OV limb after 5-months of emphysema (mean 0.92 vs. 1.55 cap/fiber). Muscle overload prevented emphysema-associated changes in fiber type composition, but not in CFR. Peripheral muscle is affected early in the course of emphysema and chronic overload may play an important role in preserving normal muscle composition.

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.

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.

Short-term influences of lung volume reduction surgery on the diaphragm in emphysematous hamsters

With emphysema, diaphragm length adaptation results in shortened fibers. We hypothesize that passive diaphragm stretch occurring acutely after lung volume reduction surgery (LVRS) results in fiber injury. Bilateral LVRS was performed in emphysematous hamsters. Studies were performed 1 (D1) and 4 (D4) days after LVRS, and compared with sham-treated groups. Sarcolemmal rupture was evident in 10.9% of fibers in LVRS-D1 and reduced to 1.6% in LVRS-D4. Ultrastructural analysis revealed focal abnormalities in both LVRS-D1 and LVRS-D4 animals in over one-third of fibers. Myofibrillar disruption was not observed in sham-treated animals. Diaphragm insulin-like growth factor-I (IGF-I) was increased in LVRS-D4 compared with other emphysematous groups. Increased IGF-I immunoreactivity was localized to types IIA and I fibers. The abundance of the splice variant of IGF-I mRNA sensitive to muscle stretch (IGF-IEb) increased 3.2-fold in LVRS D-4 diaphragms, compared with emphysema-sham animals. The main form of IGF-I mRNA was unchanged. Marked force deficit was observed in the LVRS-D1 diaphragm, compared with emphysema-sham and emphysema (no surgery) animals. These data highlight a markedly compromised ventilatory pump acutely after LVRS. Acute fiber stretch predisposes to muscle fiber injury and may also be a necessary mechanotransductive stimulus for fiber remodeling as the diaphragm adapts to reduced lung volume.

Capillary-to-fiber ratio of hind limb muscles in the male Syrian golden hamster

The hamster has been the accepted model of emphysema since the 1970s, demonstrating disease-related effects on respiratory skeletal muscle. However, there is scant information available about the model's ability to replicate the peripheral skeletal muscle changes seen in human disease, such as alterations in capillarity. The present study described the capillary-to-fiber ratio (C/F) of normal hamster plantaris, gastrocnemius, and soleus muscles in eight animals. C/F was 1.72 +/- 0.38 for plantaris, 1.95 +/- 0.40 for gastrocnemius, and 2.22 +/- 0.43 for soleus. C/F of soleus was significantly greater (P < 0.01) than plantaris. The C/F of hamster hindlimb muscles varies from those seen in rat species, and having baseline data on hamsters makes it possible to determine the effects of emphysema on C/F in this model.

Respiratory muscle injury, fatigue and serum skeletal troponin I in rat

To evaluate injury to respiratory muscles of rats breathing against an inspiratory resistive load, we measured the release into blood of a myofilament protein, skeletal troponin I (sTnI), and related this release to the time course of changes in arterial blood gases, respiratory drive (phrenic activity), and pressure generation. After approximately 1.5 h of loading, hypercapnic ventilatory failure occurred, coincident with a decrease in the ratio of transdiaphragmatic pressure to integrated phrenic activity (P(di)/ integral Phr) during sighs. This was followed at approximately 1.9 h by a decrease in the P(di)/ integral Phr ratio during normal loaded breaths (diaphragmatic fatigue). Loading was terminated at pump failure (a decline of P(di) to half of steady-state loaded values), approximately 2.4 h after load onset. During 30 s occlusions post loading, rats generated pressure profiles similar to those during occlusions before loading, with comparable blood gases, but at a higher neural drive. In a second series of rats, we tested for sTnI release using Western blot-direct serum analysis of blood samples taken before and during loading to pump failure. We detected only the fast isoform of sTnI, release beginning midway through loading. Differential detection with various monoclonal antibodies indicated the presence of modified forms of fast sTnI. The release of fast sTnI is consistent with load-induced injury of fast glycolytic fibres of inspiratory muscles, probably the diaphragm. Characterization of released fast sTnI may provide insights into the molecular basis of respiratory muscle dysfunction; fast sTnI may also prove useful as a marker of impending respiratory muscle fatigue.

Myosin heavy chain and physiological adaptation of the rat diaphragm in elastase-induced emphysema

BACKGROUND

Several physiological adaptations occur in the respiratory muscles in rodent models of elastase-induced emphysema. Although the contractile properties of the diaphragm are altered in a way that suggests expression of slower isoforms of myosin heavy chain (MHC), it has been difficult to demonstrate a shift in MHCs in an animal model that corresponds to the shift toward slower MHCs seen in human emphysema.

METHODS

We sought to identify MHC and corresponding physiological changes in the diaphragms of rats with elastase-induced emphysema. Nine rats with emphysema and 11 control rats were studied 10 months after instillation with elastase. MHC isoform composition was determined by both reverse transcriptase polymerase chain reaction (RT-PCR) and immunocytochemistry by using specific probes able to identify all known adult isoforms. Physiological adaptation was studied on diaphragm strips stimulated in vitro.

RESULTS

In addition to confirming that emphysematous diaphragm has a decreased fatigability, we identified a significantly longer time-to-peak-tension (63.9 +/- 2.7 ms versus 53.9 +/- 2.4 ms). At both the RNA (RT-PCR) and protein (immunocytochemistry) levels, we found a significant decrease in the fastest, MHC isoform (IIb) in emphysema.

CONCLUSION

This is the first demonstration of MHC shifts and corresponding physiological changes in the diaphragm in an animal model of emphysema. It is established that rodent emphysema, like human emphysema, does result in a physiologically significant shift toward slower diaphragmatic MHC isoforms. In the rat, this occurs at the faster end of the MHC spectrum than in humans.

Lipid peroxidation in the skeletal muscle of hamsters with emphysema

Current evidence suggests that skeletal muscle functional impairments present in emphysema and COPD patients may in part be a consequence of the disease condition per se. The mechanistic basis for these impairments is uncertain. Within the human population, it is difficult to control for confounding effects of concomitantly reduced activity levels. To explore this issue, malondialdehyde (MDA), a marker of lipid peroxidation, and enzymes of the glutathione redox system were measured in selected hindlimb muscles of Syrian Golden hamsters 6 months following intratracheal instillation of either saline (CON, n=7) or elastase (25 U/100 g body weight, EMP, n=5) in an accepted model where physical activity levels between control and EMP groups could be equated. Excised lung volume increased with EMP (CON, 1.3+/-0.2 g; EMP, 3.2+/-0.4 g, P<0.01). MDA was increased in the gastrocnemius (CON, 238+/-87; EMP, 371+/-122 nmol/g protein, P<0.05) of EMP hamsters. Antioxidant concentrations had a disparate response; glutathione (CON, 7.68+/-1.53; and EMP, 10.25+/-0.67 &mgr;mol/g protein, P<0.01) and the activity of glutathione reductase (GR) were increased (CON, 1.87+/-0.17; and EMP, 2.46+/-0.31 U/g protein, P<0.01) in the gastrocnemius, whereas the activity of glutathione peroxidase (GPx) was decreased (CON, 12.7+/-2.65; and EMP, 9.46+/-1.88 U/g protein, P<0.05) in the vastus lateralis of EMP hamsters. CONCLUSION: These data indicate that EMP may induce oxidative stress in peripheral skeletal muscle.

Bioenergetic adaptation of individual human diaphragmatic myofibers to severe COPD

To assess the effect of severe chronic obstructive pulmonary disease (COPD) on the ability of human diaphragmatic myofibers to aerobically generate ATP relative to ATP utilization, we obtained biopsy specimens of the costal diaphragm from seven patients with severe COPD (mean +/- SE; age 56 +/- 1 yr; forced expiratory volume in 1 s 23 +/- 2% predicted; residual volume 267 +/- 30% predicted) and seven age-matched control subjects. We categorized all fibers in these biopsies by using standard techniques, and we carried out the following quantitative histochemical measurements by microdensitometry: 1) succinate dehydrogenase (SDH) activity as an indicator of mitochondrial oxidative capacity and 2) calcium-activated myosin ATPase (mATPase) activity, the ATPase that represents a major portion of ATP consumption by contracting muscle. We noted the following: 1) COPD diaphragms had a larger proportion of type I fibers, a lesser proportion of type IIax fibers, and the same proportion of type IIa fibers as controls. 2) SDH activities of each of the fiber types were higher in COPD than control diaphragms (P < 0.0001); the mean increases (expressed as percent of control values) in types I, IIa, and IIax were 84, 114, and 130%, respectively. 3) COPD elicited no change in mATPase activity of type I and IIa fibers, but mATPase decreased in type IIax fibers (P = 0.02). 4) Mitochondrial oxidative capacity relative to ATP demand (i.e., SDH/mATPase) was higher (P = 0.03) in each of the fiber types in COPD diaphragms than in controls. These results demonstrate that severe COPD elicits an increase in aerobic ATP generating capacity relative to ATP utilization in all diaphragmatic fiber types as well as the previously described fast-to-slow fiber type transformation (Levine S, Kaiser L, Leferovich J, and Tikunov B, N Engl J Med 337: 1799-1806, 1997).

Effects of emphysema on diaphragm microvascular oxygen pressure

Pulmonary emphysema impairs lung and respiratory muscle function leading to restricted physical capacity and accelerated morbidity and mortality consequent to respiratory muscle failure. In the absence of direct evidence, an O2 supply-demand imbalance within the diaphragm and other respiratory muscles in emphysema has been considered the most likely explanation for this failure. To test this hypothesis, we utilized phosphorescence quenching techniques to measure mean microvascular PO2 (PO2m) within the medial costal diaphragm of control (C, n = 10) and emphysematous (E, elastase instilled, n = 7) hamsters. PO2m and mean arterial pressure (MAP) were measured in the spontaneously breathing anesthetized hamster at inspired O2 percentages of 10, 21, and 100, and across a range of mean MAPs from 40 to 115 mm Hg. At each inspired O2, diaphragm PO2m was significantly (p < 0.05) lower in E animals (10%: C, 19 +/- 3; E, 9 +/- 2; 21%: C, 32 +/- 2; E, 21 +/- 2; 100%: C, 60 +/- 8; E, 36 +/- 9 mm Hg). At 21% inspired O2, the PO2m decrease was correlated with reduced MAP in both C (r = 0.968) and E (r = 0.976) animals. We conclude that diaphragmatic PO2m (and therefore microvascular O2 content) is decreased in emphysematous hamsters reflecting a greater diaphragmatic O2 utilization at rest and a lower O2 extraction reserve. According to Fick's law, this lower PO2m will mandate an exaggerated fall in intramyocyte PO2, which is expected to accelerate muscle glycogen depletion and consequently fatigue. This provides empirical evidence in support of one possible mechanism for respiratory muscle failure in emphysema.

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.

Functional, cellular, and biochemical adaptations to elastase-induced emphysema in hamster medial scalene

The scalene has been reported to be an accessory inspiratory muscle in the hamster. We hypothesize that with the chronic loads and/or dynamic hyperinflation associated with emphysema (Emp), the scalene will be actively recruited, resulting in functional, cellular, and biochemical adaptations. Emp was induced in adult hamsters. Inspiratory electromyogram (EMG) activity was recorded from the medial scalene and costal diaphragm. Isometric contractile and fatigue properties were evaluated in vitro. Muscle fibers were classified histochemically and immunohistochemically. Individual fiber cross-sectional areas (CSA) and succinate dehydrogenase (SDH) activities were determined quantitatively. Myosin heavy chain (MHC) isoforms were identified by SDS-PAGE, and their proportions were determined by scanning densitometry. All Emp animals exhibited spontaneous scalene inspiratory EMG activity during quiet breathing, whereas the scalene muscles of controls (Ctl) were silent. There were no differences in contractile and fatigue properties of the scalene between Ctl and Emp. In Emp, the relative amount of MHC(2A) was 15% higher whereas that of MHC(2X) was 14% lower compared with Ctl. Similarly, the proportion of type IIa fibers increased significantly in Emp animals with a concomitant decrease in IIx fibers. CSA of type IIx fibers were significantly smaller in Emp compared with Ctl. SDH activities of all fiber types were significantly increased by 53 to 63% in Emp. We conclude that with Emp the actively recruited scalene exhibits primary-like inspiratory activity in the hamster. Adaptations of the scalene with Emp likely relate both to increased loads and to factors intrinsic to muscle architecture and chest mechanics.

Chronic resistive loading induces diaphragm injury and ventilatory failure in the hamster

The purpose of this study was to examine the effects of tracheal banding for 30 days on arterial blood gases, and diaphragm structure and function. Hamsters were tracheal banded (TB) or underwent a sham procedure (C) (n = 16 and 18, respectively). After 30 days, arterial blood gases from awake TB hamsters showed hypoxemia and a respiratory acidosis. Histochemical analysis of diaphragm cross-sections showed a five-fold greater area fraction of abnormal muscle; a greater variation in fiber size; and a 3% higher proportion of type 1 fibers in TB than C hamsters. In vitro physiologic studies of costal strips from TB hamsters showed lower stress (45-70% over 10-100 Hz) than C values. Maximal esophageal pressure during occlusion was 45% higher and normalized diaphragm mass was 10% higher in TB hamsters than C hamsters. We conclude that the lower stress in vitro was attributable, at least in part, to diaphragm injury. Hypercapnea was present in spite of the higher diaphragm mass and maximal esophageal pressures in banded hamsters.

Relaxation of diaphragm muscle

Relaxation is the process by which, after contraction, the muscle actively returns to its initial conditions of length and load. In rhythmically active muscles such as diaphragm, relaxation is of physiological importance because diaphragm must return to a relatively constant resting position at the end of each contraction-relaxation cycle. Rapid and complete relaxation of the diaphragm is likely to play an important role in adaptation to changes in respiratory load and breathing frequency. Regulation of diaphragm relaxation at the molecular and cellular levels involves Ca(2+) removal from the myofilaments, active Ca(2+) pumping by the sarcoplasmic reticulum (SR), and decrease in the number of working cross bridges. The relative contribution of these mechanisms mainly depends on sarcomere length, muscle tension, and the intrinsic contractile function. Increased capacity of SR to take up Ca(2+) can arise from increased density of active SR pumping sites or in slow-twitch fibers from phosphorylation of phospholamban, whereas impaired coupling between ATP hydrolysis and Ca(2+) transport into the SR or intracellular acidosis reduces SR Ca(2+) pump activity. In experimental conditions of decreased contractile performance, slowed, enhanced, or unchanged relaxation rates have been reported in vitro. In vivo, a slowing in the rate of decline of the respiratory pressure is generally considered an early reliable index of respiratory muscle fatigue. Impaired relaxation rate may, in turn, favor mismatch between blood flow and metabolic demand, especially at high breathing frequencies.

Respiratory work in elastase treated hamsters

Biomechanical adaptations of the diaphragm in the hamster model of emphysema are similar to those observed in skeletal muscle with exercise training. The aim of this study was to evaluate whether the dynamic pressure-volume (PV) work of breathing in hamsters with elastase-induced emphysema may contribute to these adaptations. PV work in elastase treated animals was compared to healthy controls. The studies were performed in adult hamsters 14-16 months following intratracheal administration of elastase (elastase treated group, ET) or saline (control group, CTL). Airway and esophageal pressures and air flows were measured during spontaneous breathing in anesthetized, supine animals. Pulmonary work (WL) was computed from transpulmonary pressures and airflows. Functional residual capacity (FRC) and total lung capacity (TLC; defined as volume at 25 cmH2O) in ET were increased 2 and 1.8 times, respectively, compared with CTL. Averaged tidal volume (VT) and inspiratory flows were comparable between groups. Total work of breathing (WT) normalized per ml VT was not significantly affected with elastase treatment but the pulmonary elastance work (WE) was significantly less in ET animals than controls (0.88 +/- 0.61 g cm(-2) vs. 1.63 +/- 0.32). Pulmonary resistive work was not significantly different between ET and CTL animals. These results suggest that biomechanical adaptations of the diaphragm observed in ET hamsters are caused by mechanisms other than the changes in dynamic mechanical properties of the lung following elastase treatment.

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

The aim of the present study was to investigate the effect of chronic long-term clenbuterol treatment (1 mg/kg subcutaneously twice a day for 12 wk) on diaphragm morphology and function in emphysematous (EH) and normal hamsters (NH). Clenbuterol increased body weight, diaphragm weight, and skeletal muscle weight in both EH and NH to a similar extent. In the diaphragm, clenbuterol significantly increased myosin heavy chain type I, IIa, and IIx muscle fiber cross-sectional areas by approximately 35-55% in both EH and NH. This response to clenbuterol treatment was not significantly different between EH and NH diaphragm. In EH, twitch force (Pt), maximal tetanic force, and force-frequency curve were significantly reduced compared with NH. In EH, clenbuterol increased Pt by approximately 10%, restoring Pt to NH level. A similar improvement was observed in the force-frequency characteristics. Clenbuterol did not alter contractile properties in NH. In conclusion, long-term clenbuterol treatment resulted in an increased size of all diaphragm muscle fiber types in both NH and EH. Clenbuterol completely abolished the reduced force generation induced by emphysema.

Pulmonary emphysema decreases hamster skeletal muscle oxidative enzyme capacity

Skeletal muscle oxidative enzyme capacity is impaired in patients suffering from emphysema and chronic obstructive pulmonary disease. This effect may result as a consequence of the physiological derangements because of the emphysema condition or, alternatively, as a consequence of the reduced physical activity level in these patients. To explore this issue, citrate synthase (CS) activity was measured in selected hindlimb muscles and the diaphragm of Syrian Golden hamsters 6 mo after intratracheal instillation of either saline (Con, n = 7) or elastase [emphysema (Emp); 25 units/100 g body weight, n = 8]. Activity level was monitored, and no difference between groups was found. Excised lung volume increased with emphysema (Con, 1.5 +/- 0.3 g; Emp, 3.0 +/- 0.3 g, P < 0.002). Emphysema significantly reduced CS activity in the gastrocnemius (Con, 45.1 +/- 2.0; Emp, 39.2 +/- 0.8 micromol . min-1 . g wet wt-1, P < 0.05) and vastus lateralis (Con, 48.5 +/- 1.5; Emp, 44.9 +/- 0.8 micromol . min-1 . g wet wt-1, P < 0.05) but not in the plantaris (Con, 47.4 +/- 3.9; Emp, 48.0 +/- 2.1 micromol . min-1 . g wet wt-1, P < 0.05) muscle. In contrast, CS activity increased in the costal (Con, 61.1 +/- 1.8; Emp, 65.1 +/- 1.5 micromol . min-1 . g wet wt-1, P < 0.05) and crural (Con, 58.5 +/- 2.0; Emp, 65.7 +/- 2.2 micromol . min-1 . g wet wt-1, P < 0.05) regions of the diaphragm. These data indicate that emphysema per se can induce decrements in the oxidative capacity of certain nonventilatory skeletal muscles that may contribute to exercise limitations in the emphysematous patient.

Effects of emphysema on diaphragm blood flow during exercise

Chronic hyperinflation of the lung in emphysema displaces the diaphragm caudally, thereby placing it in a mechanically disadvantageous position and contributing to the increased work of breathing. We tested the hypothesis that total and regional diaphragm blood flows are increased in emphysema, presumably reflecting an increased diaphragm energetic demand. Male Syrian Golden hamsters were randomly divided into emphysema (E; intratracheal elastase 25 units/100 g body wt) and control (C; saline) groups, and experiments were performed 16-20 wk later. The regional distribution of blood flow within the diaphragm was determined by using radiolabeled microspheres in hamsters at rest and during treadmill exercise (walking at 20 feet/min, 20% grade). Consistent with pronounced emphysema, lung volume per unit body weight was greater in E hamsters (C, 59.3 +/- 1.8; E, 84.5 +/- 5.0 ml/kg; P < 0.001) and arterial PO2 was lower both at rest (C, 74 +/- 3; E, 59 +/- 2 Torr; P < 0.001) and during exercise (C, 93 +/- 3; E, 69 +/- 4 Torr; P < 0.001). At rest, total diaphragm blood flow was not different between C and E hamsters (C, 47 +/- 4; E, 38 +/- 4 ml . min-1 . 100 g-1; P = 0.18). In both C and E hamsters, blood flow at rest was lower in the ventral costal region of the diaphragm than in the dorsal and medial costal regions and the crural diaphragm. During exercise in both C and E hamsters, blood flows increased more in the dorsal and medial costal regions and in the crural diaphragm than in the ventral costal region. Total diaphragm blood flow was greater in E hamsters during exercise (C, 58 +/- 7; E, 90 +/- 14 ml . min-1 . 100 g-1; P = 0.03), as a consequence of significantly higher blood flows in the medial and ventral costal regions and crural diaphragm. In addition, exercise-induced increases in intercostal (P < 0.005) and abdominal (P < 0.05) muscle blood flows were greater in E hamsters. The finding that diaphragm blood flow was greater in E hamsters during exercise supports the contention that emphysema increases the energetic requirements of the diaphragm.

Diaphragm structure and function in health and disease

The diaphragm is the primary muscle of inspiration, and as such uncompromised function is essential to support the ventilatory and gas exchange demands associated with physical activity. The normal healthy diaphragm may fatigue during intense exercise, and diaphragm function is compromised with aging and obesity. However, more insidiously, respiratory diseases such as emphysema mechanically disadvantage the diaphragm, sometimes leading to muscle failure and death. Based on metabolic considerations, recent evidence suggests that specific regions of the diaphragm may be or may become more susceptible to failure than others. This paper reviews the regional differences in mechanical and metabolic activity within the diaphragm and how such heterogeneities might influence diaphragm function in health and disease. Our objective is to address five principal areas: 1) Regional diaphragm structure and mechanics (GAF). 2) Regional differences in blood flow within the diaphragm (WLS). 3) Structural and functional interrelationships within the diaphragm microcirculation (DCP). 4) Nitric oxide and its vasoactive and contractile influences within the diaphragm (MBR). 5) Metabolic and contractile protein plasticity in the diaphragm (SKP). These topics have been incorporated into three discrete sections: Functional Anatomy and Morphology, Physiology, and Plasticity in Health and Disease. Where pertinent, limitations in our understanding of diaphragm function are addressed along with potential avenues for future research.

Effect of pulmonary emphysema on diaphragm capillary geometry

In emphysema, the diaphragm shortens by losing sarcomeres. We hypothesized that unless capillaries undergo a similar shortening, capillary geometry must be altered. Without quantifying this geometry, capillary length and surface area per fiber volume, which are critical measurements of the structural potential for blood-tissue exchange, cannot be resolved. Five months after intratracheal elastase (E) or saline (control; C) instillation, diaphragms from male Syrian golden hamsters were glutaraldehyde perfusion fixed in situ at reference lung positions (residual volume, functional residual capacity, total lung capacity) to provide diaphragms fixed over a range of sarcomere lengths. Subsequently, diaphragms were processed for electron microscopy and analyzed morphometrically. Emphysema increased lung volume changes from -20 to 25 cmH2O airway pressure (i.e., passive vital capacity) and excised lung volume (both P < 0.001). In each region of the costal diaphragm (i.e., ventral, medial, dorsal), sarcomere number was reduced (all P < 0.05). Capillary-to-fiber ratio increased (C = 2.2 +/- 0.1, E = 2.8 +/- 0.1; P < 0.01) and fibers hypertrophied (C = 815 +/- 35, E = 987 +/- 67 microns2; P < 0.05; both values at 2.5 microns sarcomere length). Capillary geometry was markedly altered by the loss of sarcomeres in series. Specifically, the additional capillary length derived from capillary tortuosity and branching was increased by 183% at 2.5 microns sarcomere length compared with C values (C, 359 +/- 43; E, 1,020 +/- 158 mm-2, P < 0.01). This significantly increased total capillary length (C, 3,115 +/- 173; E, 3,851 +/- 219mm-2 at 2.5 microns, P < 0.05) and surface area (C, 456 +/- 13; E, 519 +/- 24 cm-1, P < 0.05) per fiber volume. Thus emphysema substantially alters diaphragm capillary geometry and augments the capillary length and surface area available for blood-tissue exchange.

Functional role and structure of the scalene: an accessory inspiratory muscle in hamster

Although the scalene muscle (Sca) is a primary inspiratory muscle in humans, its respiratory function in other species is less clear. The electromyographic (EMG) activity of the Sca was studied during resting ventilation (eupnea) in both the awake and anesthetized hamster and after a variety of respiratory challenges in the anesthetized animal. The EMG activities of the medial Sca and the costal diaphragm were compared. The medial Sca, the major component of the Sca, originates from cervical transverse processes 2 to 5 and inserts primarily onto rib 4, with a small segment onto rib 3. In both the anesthetized and awake animal, the Sca was always silent during quiet breathing. With CO2-stimulated hyperpnea, the Sca was always recruited during inspiration in phase with the diaphragm. Active recruitment of the Sca was also observed after resistive loading and total airway occlusion. After ipsilateral phrenicotomy, the Sca was persistently recruited during eupnea. The specificity of the EMG signals was tested both by excluding cross contamination from other rib cage muscles and by selective denervation studies. Muscle spindles were identified in the medial Sca histochemically, suggesting that the respiratory activity of the Sca can also be modulated by changes in muscle length and/or load. These results indicate that the Sca functions as an accessory inspiratory muscle in the hamster and may play an important role in conditions of chronic load.

Diaphragm mechanics in dogs with unilateral emphysema

We studied dogs with unilateral papain-induced emphysema to answer two questions: (1) Do emphysema lung-apposed hemidiaphragm (DiE) and normal lung-apposed hemidiaphragm (DiN) have equal capacities for lowering lung surface pressure? and (2) Are side-to-side differences in intrathoracic pressure the result of unequal force outputs by DiE and DiN or are they caused by differences in their mechanical efficiency as pressure generators? After the airways of the emphysematous and normal lungs were intubated with a dual lumen endotracheal tube, both phrenic nerves were maximally stimulated at rates between 1 and 50 Hz and the changes in airway occlusion pressure (delta PaoE,N) and diaphragm length (sonomicrometry) were recorded. In all animals, delta PaoN exceeded delta PaoE. Differences in pressure ranged from 1.2 +/- 0.6 cm H2O during a twitch to 6.0 +/- 2.9 cm H2O during a 50-Hz tetanus. Midcostal bundles of DiE shortened less than corresponding bundles of DiN, but both reached the same active length relative to their optimal lengths, which were measured in vitro. There was no significant difference in fiber type distribution, fiber cross-sectional area, or maximal isometric tetanic tensions among midcostal regions of DiE and DiN. We conclude that unilateral hyperinflation impairs the mechanical efficiency of the apposing hemidiaphragm as a pressure generator.