Influence of corticosteroids on myonuclear domain size in the rat diaphragm muscle

Evolution and Functional Differentiation of the Diaphragm Muscle of Mammals

Symmorphosis is a concept of economy of biological design, whereby structural properties are matched to functional demands. According to symmorphosis, biological structures are never over designed to exceed functional demands. Based on this concept, the evolution of the diaphragm muscle (DIAm) in mammals is a tale of two structures, a membrane that separates and partitions the primitive coelomic cavity into separate abdominal and thoracic cavities and a muscle that serves as a pump to generate intra-abdominal (Pab ) and intrathoracic (Pth ) pressures. The DIAm partition evolved in reptiles from folds of the pleural and peritoneal membranes that was driven by the biological advantage of separating organs in the larger coelomic cavity into separate thoracic and abdominal cavities, especially with the evolution of aspiration breathing. The DIAm pump evolved from the advantage afforded by more effective generation of both a negative Pth for ventilation of the lungs and a positive Pab for venous return of blood to the heart and expulsive behaviors such as airway clearance, defecation, micturition, and child birth. © 2019 American Physiological Society. Compr Physiol 9:715-766, 2019.

Neuromuscular electrical stimulation combined with exercise decreases duration of mechanical ventilation in ICU patients: A randomized controlled trial

BACKGROUND

Early mobilization can be employed to minimize the duration of intensive care. However, a protocol combining neuromuscular electrical stimulation (NMES) with early mobilization has not yet been tested in ICU patients. Our aim was to assess the efficacy of NMES, exercise (EX), and combined therapy (NMES + EX) on duration of mechanical ventilation (MV) in critically ill patients.

METHODS

The participants in this randomized double-blind trial were prospectively recruited within 24 hours following admission to the intensive care unit of a tertiary hospital. Eligible patients had 18 years of age or older; MV for less than 72 hours; and no known neuromuscular disease. Computer-generated permuted block randomization was used to assign patients to NMES, EX, NMES + EX, or standard care (control group). The main endpoint was duration of MV. Clinical characteristics were also evaluated and intention to treat analysis was employed.

RESULTS

One hundred forty-four patients were assessed for eligibility to participate in the trial, 51 of whom were enrolled and randomly allocated into four groups: 11 patients in the NMES group, 13 in the EX group, 12 in the NMES + EX group, and 15 in the control group (CG). Duration of MV (days) was significantly shorter in the combined therapy (5.7 ± 1.1) and NMEN (9.0 ± 7.0) groups in comparison to CG (14.8 ± 5.4).

CONCLUSIONS

NMES + EX consisting of NMES and active EXs was well tolerated and resulted in shorter duration of MV in comparison to standard care or isolated therapy (NMES or EX alone).

Exploring the Role of PGC-1α in Defining Nuclear Organisation in Skeletal Muscle Fibres

Muscle fibres are multinucleated cells, with each nucleus controlling the protein synthesis in a finite volume of cytoplasm termed the myonuclear domain (MND). What determines MND size remains unclear. In the present study, we aimed to test the hypothesis that the level of expression of the transcriptional coactivator PGC-1α and subsequent activation of the mitochondrial biogenesis are major contributors. Hence, we used two transgenic mouse models with varying expression of PGC-1α in skeletal muscles. We isolated myofibres from the fast twitch extensor digitorum longus (EDL) and slow twitch diaphragm muscles. We then membrane-permeabilised them and analysed the 3D spatial arrangements of myonuclei. In EDL muscles, when PGC-1α is over-expressed, MND volume decreases; whereas, when PGC-1α is lacking, no change occurs. In the diaphragm, no clear difference was noted. This indicates that PGC-1α and the related mitochondrial biogenesis programme are determinants of MND size. PGC-1α may facilitate the addition of new myonuclei in order to reach MND volumes that can support an increased mitochondrial density. J. Cell. Physiol. 232: 1270-1274, 2017. © 2016 Wiley Periodicals, Inc.

Convergence of pattern generator outputs on a common mechanism of diaphragm motor unit recruitment

Motor units are the final element of neuromotor control. In manner analogous to the organization of neuromotor control in other skeletal muscles, diaphragm motor units comprise phrenic motoneurons located in the cervical spinal cord that innervate the diaphragm muscle, the main inspiratory muscle in mammals. Diaphragm motor units play a primary role in sustaining ventilation but are also active in other nonventilatory behaviors, including coughing, sneezing, vomiting, defecation, and parturition. Diaphragm muscle fibers comprise all fiber types. Thus, diaphragm motor units display substantial differences in contractile and fatigue properties, but importantly, properties of the motoneuron and muscle fibers within a motor unit are matched. As in other skeletal muscles, diaphragm motor units are recruited in order such that motor units that display greater fatigue resistance are recruited earlier and more often than more fatigable motor units. The properties of the motor unit population are critical determinants of the function of a skeletal muscle across the range of possible motor tasks. Accordingly, fatigue-resistant motor units are sufficient to generate the forces necessary for ventilatory behaviors, whereas more fatigable units are only activated during expulsive behaviors important for airway clearance. Neuromotor control of diaphragm motor units may reflect selective inputs from distinct pattern generators distributed according to the motor unit properties necessary to accomplish these different motor tasks. In contrast, widely distributed inputs to phrenic motoneurons from various pattern generators (e.g., for breathing, coughing, or vocalization) would dictate recruitment order based on intrinsic electrophysiological properties.

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.

Mechanisms regulating skeletal muscle growth and atrophy

Skeletal muscle mass increases during postnatal development through a process of hypertrophy, i.e. enlargement of individual muscle fibers, and a similar process may be induced in adult skeletal muscle in response to contractile activity, such as strength exercise, and specific hormones, such as androgens and β-adrenergic agonists. Muscle hypertrophy occurs when the overall rates of protein synthesis exceed the rates of protein degradation. Two major signaling pathways control protein synthesis, the IGF1-Akt-mTOR pathway, acting as a positive regulator, and the myostatin-Smad2/3 pathway, acting as a negative regulator, and additional pathways have recently been identified. Proliferation and fusion of satellite cells, leading to an increase in the number of myonuclei, may also contribute to muscle growth during early but not late stages of postnatal development and in some forms of muscle hypertrophy in the adult. Muscle atrophy occurs when protein degradation rates exceed protein synthesis, and may be induced in adult skeletal muscle in a variety of conditions, including starvation, denervation, cancer cachexia, heart failure and aging. Two major protein degradation pathways, the proteasomal and the autophagic-lysosomal pathways, are activated during muscle atrophy and variably contribute to the loss of muscle mass. These pathways involve a variety of atrophy-related genes or atrogenes, which are controlled by specific transcription factors, such as FoxO3, which is negatively regulated by Akt, and NF-κB, which is activated by inflammatory cytokines.

Non-stationarity and power spectral shifts in EMG activity reflect motor unit recruitment in rat diaphragm muscle

We hypothesized that a shift in diaphragm muscle (DIAm) EMG power spectral density (PSD) to higher frequencies reflects recruitment of more fatigable fast-twitch motor units and motor unit recruitment is reflected by EMG non-stationarity. DIAm EMG was recorded in anesthetized rats during eupnea, hypoxia-hypercapnia (10% O(2)-5% CO(2)), airway occlusion, and sneezing (maximal DIAm force). Although power in all frequency bands increased progressively across motor behaviors, PSD centroid frequency increased only during sneezing (p<0.05). The non-stationary period at the onset of EMG activity ranged from ∼80 ms during airway occlusion to ∼150 ms during eupnea. Within the initial non-stationary period of EMG activity 80-95% of motor units were recruited during different motor behaviors. Motor units augmented their discharge frequencies progressively beyond the non-stationary period; yet, EMG signal became stationary. In conclusion, non-stationarity of DIAm EMG reflects the period of motor unit recruitment, while a shift in the PSD towards higher frequencies reflects recruitment of more fatigable fast-twitch motor units.

Systems biology of skeletal muscle: fiber type as an organizing principle

Skeletal muscle force generation and contraction are fundamental to countless aspects of human life. The complexity of skeletal muscle physiology is simplified by fiber type classification where differences are observed from neuromuscular transmission to release of intracellular Ca(2+) from the sarcoplasmic reticulum and the resulting recruitment and cycling of cross-bridges. This review uses fiber type classification as an organizing and simplifying principle to explore the complex interactions between the major proteins involved in muscle force generation and contraction.

The satellite cell in male and female, developing and adult mouse muscle: distinct stem cells for growth and regeneration

Satellite cells are myogenic cells found between the basal lamina and the sarcolemma of the muscle fibre. Satellite cells are the source of new myofibres; as such, satellite cell transplantation holds promise as a treatment for muscular dystrophies. We have investigated age and sex differences between mouse satellite cells in vitro and assessed the importance of these factors as mediators of donor cell engraftment in an in vivo model of satellite cell transplantation. We found that satellite cell numbers are increased in growing compared to adult and in male compared to female adult mice. We saw no difference in the expression of the myogenic regulatory factors between male and female mice, but distinct profiles were observed according to developmental stage. We show that, in contrast to adult mice, the majority of satellite cells from two week old mice are proliferating to facilitate myofibre growth; however a small proportion of these cells are quiescent and not contributing to this growth programme. Despite observed changes in satellite cell populations, there is no difference in engraftment efficiency either between satellite cells derived from adult or pre-weaned donor mice, male or female donor cells, or between male and female host muscle environments. We suggest there exist two distinct satellite cell populations: one for muscle growth and maintenance and one for muscle regeneration.

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.

Phrenic motor unit recruitment during ventilatory and non-ventilatory behaviors

Phrenic motoneurons are located in the cervical spinal cord and innervate the diaphragm muscle, the main inspiratory muscle in mammals. Similar to other skeletal muscles, phrenic motoneurons and diaphragm muscle fibers form motor units which are the final element of neuromotor control. In addition to their role in sustaining ventilation, phrenic motor units are active in other non-ventilatory behaviors important for airway clearance such as coughing or sneezing. Diaphragm muscle fibers comprise all fiber types and are commonly classified based on expression of contractile proteins including myosin heavy chain isoforms. Although there are differences in contractile and fatigue properties across motor units, there is a matching of properties for the motor neuron and muscle fibers within a motor unit. Motor units are generally recruited in order such that fatigue-resistant motor units are recruited earlier and more often than more fatigable motor units. Thus, in sustaining ventilation, fatigue-resistant motor units are likely required. Based on a series of studies in cats, hamsters and rats, an orderly model of motor unit recruitment was proposed that takes into consideration the maximum forces generated by single type-identified diaphragm muscle fibers as well as the proportion of the different motor unit types. Using this model, eupnea can be accomplished by activation of only slow-twitch diaphragm motor units and only a subset of fast-twitch, fatigue-resistant units. Activation of fast-twitch fatigable motor units only becomes necessary when accomplishing tasks that require greater force generation by the diaphragm muscle, e.g., sneezing and coughing.

Upregulation of the CaV 1.1-ryanodine receptor complex in a rat model of critical illness myopathy

The processes that trigger severe muscle atrophy and loss of myosin in critical illness myopathy (CIM) are poorly understood. It has been reported that muscle disuse alters Ca(2+) handling by the sarcoplasmic reticulum. Since inactivity is an important contributor to CIM, this finding raises the possibility that elevated levels of the proteins involved in Ca(2+) handling might contribute to development of CIM. CIM was induced in 3- to 5-mo-old rats by sciatic nerve lesion and infusion of dexamethasone for 1 wk. Western blot analysis revealed increased levels of ryanodine receptor (RYR) isoforms-1 and -2 as well as the dihydropyridine receptor/voltage-gated calcium channel type 1.1 (DHPR/Ca(V) 1.1). Immunostaining revealed a subset of fibers with elevation of RYR1 and Ca(V) 1.1 that had severe atrophy and disorganization of sarcomeres. These findings suggest increased Ca(2+) release from the sarcoplasmic reticulum may be an important contributor to development of CIM. To assess the endogenous functional effects of increased intracellular Ca(2+) in CIM, proteolysis of α-fodrin, a well-known target substrate of Ca(2+)-activated proteases, was measured and found to be 50% greater in CIM. There was also selective degradation of myosin heavy chain relative to actin in CIM muscle. Taken together, our findings suggest that increased Ca(2+) release from the sarcoplasmic reticulum may contribute to pathology in CIM.

No change in skeletal muscle satellite cells in young and aging rat soleus muscle

Satellite cells are muscle stem cells capable of replenishing or increasing myonuclear number. It is postulated that a reduction in satellite cells may contribute to age-related sarcopenia. Studies investigating an age-related decline in satellite cells have produced equivocal results. This study compared the satellite cell content of young and aging soleus muscle in rat, using four different methods: dystrophin-laminin immunohistochemistry, MyoD immunohistochemistry, electron microscopy, and light microscopy of semi-thin sections. The absolute quantity of satellite cells increase with age, but satellite cell percentages were similar in young and aging soleus muscles. There were no differences in satellite cell quantity among MyoD immunohistochemistry, electron microscopy, and semi-thin sections. All three methods had significantly more satellite cells than with dystrophin-laminin immunohistochemistry. We conclude that satellite cell number does not decrease with age and postulate that satellite cell functionality may be responsible for age-related sarcopenia.

Developmental effects on myonuclear domain size of rat diaphragm fibers

During early postnatal development in rat diaphragm muscle (Diam), significant fiber growth and transitions in myosin heavy chain (MHC) isoform expression occur. Similar to other skeletal muscles, Diam fibers are multinucleated, and each myonucleus regulates the gene products within a finite volume: the myonuclear domain (MND). We hypothesized that postnatal changes in fiber cross-sectional area (CSA) are associated with increased number of myonuclei so that the MND size is maintained. The Diam was removed at postnatal days 14 (P-14) and 28 (P-28). MHC isoform expression was determined by SDS-PAGE. Fiber CSA, myonuclear number, and MND size were measured using confocal microscopy. By P-14, significant coexpression of MHC isoforms was present with no fiber displaying singular expression of MHCNeo. By P-28, singular expression was predominant. MND size was not different across fiber types at P-14. Significant fiber growth was evident by P-28 at all fiber types (fiber CSA increased by 61, 93, and 147% at fibers expressing MHCSlow, MHC2A, and MHC2X, respectively). The number of myonuclei per unit of fiber length was similar across fibers at P-14, but it was greater at fibers expressing MHC2X at P-28. The total number of myonuclei per fiber also increased between P-14 and P-28 at all fiber types. Accordingly, MND size increased significantly by P-28 at all fiber types, and it became larger at fibers expressing MHC2X compared with fibers expressing MHCSlow or MHC2A. These results suggest that MND size is not maintained during the considerable fiber growth associated with postnatal development of the Diam.

Effect of denervation on ATP consumption rate of diaphragm muscle fibers

Denervation (DNV) of rat diaphragm muscle (DIAm) decreases myosin heavy chain (MHC) content in fibers expressing MHC(2X) isoform but not in fibers expressing MHC(slow) and MHC(2A). Since MHC is the site of ATP hydrolysis during muscle contraction, we hypothesized that ATP consumption rate during maximum isometric activation (ATP(iso)) is reduced following unilateral DIAm DNV and that this effect is most pronounced in fibers expressing MHC(2X). In single-type-identified, permeabilized DIAm fibers, ATP(iso) was measured using NADH-linked fluorometry. The maximum velocity of the actomyosin ATPase reaction (V(max) ATPase) was determined using quantitative histochemistry. The effect of DNV on maximum unloaded shortening velocity (V(o)) and cross-bridge cycling rate [estimated from the rate constant for force redevelopment (k(TR)) following quick release and restretch] was also examined. Two weeks after DNV, ATP(iso) was significantly reduced in fibers expressing MHC(2X), but unaffected in fibers expressing MHC(slow) and MHC(2A). This effect of DNV on fibers expressing MHC(2X) persisted even after normalization for DNV-induced reduction in MHC content. With DNV, V(o) and k(TR) were slowed in fibers expressing MHC(2X), consistent with the effect on ATP(iso). The difference between V(max) ATPase and ATP(iso) reflects reserve capacity for ATP consumption, which was reduced across all fibers following DNV; however, this effect was most pronounced in fibers expressing MHC(2X). DNV-induced reductions in ATP(iso) and V(max) ATPase of fibers expressing MHC(2X) reflect the underlying decrease in MHC content, while reduction in ATP(iso) also reflects a slowing of cross-bridge cycling rate.

Oxidative stress and disuse muscle atrophy

Skeletal muscle inactivity is associated with a loss of muscle protein and reduced force-generating capacity. This disuse-induced muscle atrophy results from both increased proteolysis and decreased protein synthesis. Investigations of the cell signaling pathways that regulate disuse muscle atrophy have increased our understanding of this complex process. Emerging evidence implicates oxidative stress as a key regulator of cell signaling pathways, leading to increased proteolysis and muscle atrophy during periods of prolonged disuse. This review will discuss the role of reactive oxygen species in the regulation of inactivity-induced skeletal muscle atrophy. The specific objectives of this article are to provide an overview of muscle proteases, outline intracellular sources of reactive oxygen species, and summarize the evidence that connects oxidative stress to signaling pathways contributing to disuse muscle atrophy. Moreover, this review will also discuss the specific role that oxidative stress plays in signaling pathways responsible for muscle proteolysis and myonuclear apoptosis and highlight gaps in our knowledge of disuse muscle atrophy. By presenting unresolved issues and suggesting topics for future research, it is hoped that this review will serve as a stimulus for the expansion of knowledge in this exciting field.

Caspase-3 regulation of diaphragm myonuclear domain during mechanical ventilation-induced atrophy

RATIONALE

Unloading the diaphragm via mechanical ventilation (MV) results in rapid diaphragmatic fiber atrophy. It is unknown whether the myonuclear domain (cytoplasmic myofiber volume/myonucleus) of diaphragm myofibers is altered during MV.

OBJECTIVE

We tested the hypothesis that MV-induced diaphragmatic atrophy is associated with a loss of myonuclei via a caspase-3-mediated, apoptotic-like mechanism resulting in a constant myonuclear domain.

METHODS

To test this postulate, Sprague-Dawley rats were randomly assigned to a control group or to experimental groups exposed to 6 or 12 h of MV with or without administration of a caspase-3 inhibitor.

MEASUREMENTS AND MAIN RESULTS

After 12 h of MV, type I and type IIa diaphragm myofiber areas were decreased by 17 and 23%, respectively, and caspase-3 inhibition attenuated this decrease. Diaphragmatic myonuclear content decreased after 12 h of MV and resulted in the maintenance of a constant myonuclear domain in all fiber types. Both 6 and 12 h of MV resulted in caspase-3-dependent increases in apoptotic markers in the diaphragm (e.g., number of terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling positive nuclei and DNA fragmentation). Caspase-3-dependent increases in apoptotic markers occurred after 6 h of MV, before the onset of myofiber atrophy.

CONCLUSIONS

Collectively, these data support the hypothesis that the myonuclear domain of diaphragm myofibers is maintained during prolonged MV and that caspase-3-mediated myonuclear apoptosis contributes to this process.

Denervation effects on myonuclear domain size of rat diaphragm fibers

Denervation (DNV) of rat diaphragm muscle (DIAm) leads to selective atrophy of type IIx and IIb fibers, whereas the cross-sectional area of type I and IIa fibers remains unchanged or slightly hypertrophied. DIAm DNV also increases satellite cell mitotic activity and myonuclear apoptosis. Similar to other skeletal muscles, DIAm fibers are multinucleated, and each myonucleus regulates the gene products in a finite fiber volume, i.e., myonuclear domain (MND). MND size varies across DIAm fiber types in rank order, I < IIa < IIx < IIb [fiber type based on myosin heavy chain isoform expression]. We hypothesized that, after DNV, the total number of myonuclei per fiber does not change and, accordingly, that MND changes proportionately to the change in fiber size regardless of fiber type. Adult rats underwent unilateral (right side) DIAm DNV, and after 2 wk single fibers were dissected. Fiber cross-sectional area, myonuclear number, and MND were measured by confocal microscopy, and these values in DNV DIAm were compared with those obtained in controls. After DNV, type I fibers hypertrophied, type IIa fiber size was unchanged, and type IIx and IIb fibers atrophied compared with control. The total number of myonuclei per fiber was not affected by DNV. Accordingly, after DNV, type I fiber MND increased by 25%, whereas it decreased in type IIx and IIb fibers by 50 and 70%, respectively. These results suggest that MND is not maintained after DNV-induced DIAm fiber hypertrophy or atrophy. These results are interpreted with respect to consequent effects of DNV on myonuclear transcriptional activity and protein turnover.

Activity-dependent influences are greater for fibers in rat medial gastrocnemius than tibialis anterior muscle

Skeletal muscles are highly adaptive to changes in loading or activation. A model of neuromuscular inactivity (spinal cord isolation, SI) was used to determine the role of activity-independent and -dependent neural influences on the size and myonuclei number in type-identified fibers of a fast extensor (medial gastrocnemius, MG) and flexor (tibialis anterior, TA) rat muscle. Fibers were categorized based on myosin heavy chain isoform composition. Four days after SI, all fiber types tended to atrophy and lose myonuclei, with the percent loss of myonuclei being disproportionately less than the decrease in fiber size. At 60 days after SI, all fiber types in MG and the fastest fibers in TA were significantly smaller and had fewer myonuclei than control. The disproportionate amount of atrophy resulted in a smaller myonuclear domain. These effects were greater in MG than TA, indicating that activity-dependent influences were greater in the extensor than flexor. The smaller myonuclear domains after a period of chronic inactivity suggest the presence of intrinsic mechanisms operating to maintain the genetic material necessary to recover from atrophic conditions.

Effects of inactivity on fiber size and myonuclear number in rat soleus muscle

The effects of short-term (4 days) and long-term (60 days) neuromuscular inactivity on myonuclear number, size, and myosin heavy chain (MHC) composition of isolated rat soleus fibers were determined using confocal microscopy and gel electrophoresis. Inactivity was produced via spinal cord isolation (SI), i.e., complete spinal cord transections at a midthoracic and a high sacral level and bilateral deafferentation between the transection sites. Compared with control, there was an increase in the percentage of fibers containing the faster MHC isoforms after 60, but not 4, days of SI. The mean sizes of type I and type I+IIa fibers were 41 and 27% and 66 and 56% smaller after 4 and 60 days of SI, respectively. Thus atrophy occurred earlier than the shift in myosin heavy chain (MHC) profile. The number of myonuclei was approximately 30% higher in type I than type I+IIa fibers in control soleus, but after 60 days of SI these values were similar. The number of myonuclei per millimeter in type I fibers was significantly lower than control after 60 days of SI, whereas there was no change in type I+IIa fibers. Thus myonuclei were eliminated from fibers containing only type I MHC. Because the magnitude of the loss of myonuclei was less than the level of atrophy, the myonuclear domains of both type I and type I+IIa fibers were significantly lower than control. Thus chronic (60 days) inactivity results in smaller, faster fibers that contain a higher than normal amount of DNA per unit of cytoplasm. The absence of activation of muscle fibers that are normally the most active (pure type I fibers) resulted in most, but not all, fibers expressing some fast MHC isoforms. The results also indicate that a loss of myonuclei is not a prerequisite for sustained muscle fiber atrophy.