Early effects of mechanical ventilation on isotonic contractile properties and MAF-box gene expression in the diaphragm

Renin-angiotensin system in ventilator-induced diaphragmatic dysfunction: Potential protective role of Angiotensin (1-7)

Ventilator-induced diaphragmatic dysfunction is a feared complication of mechanical ventilation that adversely affects the outcome of intensive care patients. Human and animal studies demonstrate atrophy and ultrastructural alteration of diaphragmatic muscular fibers attributable to increased oxidative stress, depression of the anabolic pathway regulated by Insulin-like growing factor 1 and increased proteolysis. The renin-angiotensin system, through its main peptide Angiotensin II, plays a major role in skeletal muscle diseases, mainly increasing oxidative stress and inducing insulin resistance, atrophy and fibrosis. Conversely, its counter-regulatory peptide Angiotensin (1-7) has a protective role in these processes. Recent data on rodent models show that renin-angiotensin system is activated after mechanical ventilation and that infusion of Angiotensin II induces diaphragmatic skeletal muscle atrophy. Given: (A) common pathways shared by ventilator-induced diaphragmatic dysfunction and skeletal muscle pathology induced by renin-angiotensin system, (B) evidences of an involvement of renin-angiotensin system in diaphragm atrophy and dysfunction, we hypothesize that renin-angiotensin system plays an important role in ventilator-induced diaphragmatic dysfunction, while Angiotensin (1-7) can have a protective effect on this pathological process. The activation of renin-angiotensin system in ventilator-induced diaphragmatic dysfunction can be demonstrated by quantification of its main components in the diaphragm of ventilated humans or animals. The infusion of Angiotensin (1-7) in an established rodent model of ventilator-induced diaphragmatic dysfunction can be used to test its potential protective role, that can be further confirmed with the infusion of Angiotensin (1-7) antagonists like A-779. Verifying this hypothesis can help in understanding the processes involved in ventilator-induced diaphragmatic dysfunction pathophysiology and open new possibilities for its prevention and treatment.

The JAK-STAT pathway is critical in ventilator-induced diaphragm dysfunction

Mechanical ventilation (MV) is one of the lynchpins of modern intensive-care medicine and is life saving in many critically ill patients. Continuous ventilator support, however, results in ventilation-induced diaphragm dysfunction (VIDD) that likely prolongs patients' need for MV and thereby leads to major associated complications and avoidable intensive care unit (ICU) deaths. Oxidative stress is a key pathogenic event in the development of VIDD, but its regulation remains largely undefined. We report here that the JAK-STAT pathway is activated in MV in the human diaphragm, as evidenced by significantly increased phosphorylation of JAK and STAT. Blockage of the JAK-STAT pathway by a JAK inhibitor in a rat MV model prevents diaphragm muscle contractile dysfunction (by ~85%, p < 0.01). We further demonstrate that activated STAT3 compromises mitochondrial function and induces oxidative stress in vivo, and, interestingly, that oxidative stress also activates JAK-STAT. Inhibition of JAK-STAT prevents oxidative stress-induced protein oxidation and polyubiquitination and recovers mitochondrial function in cultured muscle cells. Therefore, in ventilated diaphragm muscle, activation of JAK-STAT is critical in regulating oxidative stress and is thereby central to the downstream pathogenesis of clinical VIDD. These findings establish the molecular basis for the therapeutic promise of JAK-STAT inhibitors in ventilated ICU patients.

Skeletal muscle atrophy and the E3 ubiquitin ligases MuRF1 and MAFbx/atrogin-1

Muscle RING finger 1 (MuRF1) and muscle atrophy F-box (MAFbx)/atrogin-1 were identified more than 10 years ago as two muscle-specific E3 ubiquitin ligases that are increased transcriptionally in skeletal muscle under atrophy-inducing conditions, making them excellent markers of muscle atrophy. In the past 10 years much has been published about MuRF1 and MAFbx with respect to their mRNA expression patterns under atrophy-inducing conditions, their transcriptional regulation, and their putative substrates. However, much remains to be learned about the physiological role of both genes in the regulation of mass and other cellular functions in striated muscle. Although both MuRF1 and MAFbx are enriched in skeletal, cardiac, and smooth muscle, this review will focus on the current understanding of MuRF1 and MAFbx in skeletal muscle, highlighting the critical questions that remain to be answered.

Positive end-expiratory airway pressure does not aggravate ventilator-induced diaphragmatic dysfunction in rabbits

Introduction

Immobilization of hindlimb muscles in a shortened position results in an accelerated rate of inactivity-induced muscle atrophy and contractile dysfunction. Similarly, prolonged controlled mechanical ventilation (CMV) results in diaphragm inactivity and induces diaphragm muscle atrophy and contractile dysfunction. Further, the application of positive end-expiratory airway pressure (PEEP) during mechanical ventilation would result in shortened diaphragm muscle fibers throughout the respiratory cycle. Therefore, we tested the hypothesis that, compared to CMV without PEEP, the combination of PEEP and CMV would accelerate CMV-induced diaphragm muscle atrophy and contractile dysfunction. To test this hypothesis, we combined PEEP with CMV or with assist-control mechanical ventilation (AMV) and determined the effects on diaphragm muscle atrophy and contractile properties.

Methods

The PEEP level (8 cmH₂O) that did not induce lung overdistension or compromise circulation was determined. In vivo segmental length changes of diaphragm muscle fiber were then measured using sonomicrometry. Sedated rabbits were randomized into seven groups: surgical controls and those receiving CMV, AMV or continuous positive airway pressure (CPAP) with or without PEEP for 2 days. We measured in vitro diaphragmatic force, diaphragm muscle morphometry, myosin heavy-chain (MyHC) protein isoforms, caspase 3, insulin-like growth factor 1 (IGF-1), muscle atrophy F-box (MAFbx) and muscle ring finger protein 1 (MuRF1) mRNA.

Results

PEEP shortened end-expiratory diaphragm muscle length by 15%, 14% and 12% with CMV, AMV and CPAP, respectively. Combined PEEP and CMV reduced tidal excursion of segmental diaphragm muscle length; consequently, tidal volume (VT) decreased. VT was maintained with combined PEEP and AMV. CMV alone decreased maximum tetanic force (Po) production by 35% versus control (P < 0.01). Combined PEEP and CMV did not decrease Po further. Po was preserved with AMV, with or without PEEP. Diaphragm muscle atrophy did not occur in any fiber types. Diaphragm MyHC shifted to the fast isoform in the combined PEEP and CMV group. In both the CMV and combined PEEP and CMV groups compared to controls, IGF-1 mRNAs were suppressed, whereas Caspase-3, MAFbx and MuRF1 mRNA expression were elevated.

Conclusions

Two days of diaphragm muscle fiber shortening with PEEP did not exacerbate CMV-induced diaphragm muscle dysfunction.

Ventilator-induced diaphragm dysfunction: cause and effect

Mechanical ventilation (MV) is used clinically to maintain gas exchange in patients that require assistance in maintaining adequate alveolar ventilation. Common indications for MV include respiratory failure, heart failure, drug overdose, and surgery. Although MV can be a life-saving intervention for patients suffering from respiratory failure, prolonged MV can promote diaphragmatic atrophy and contractile dysfunction, which is referred to as ventilator-induced diaphragm dysfunction (VIDD). This is significant because VIDD is thought to contribute to problems in weaning patients from the ventilator. Extended time on the ventilator increases health care costs and greatly increases patient morbidity and mortality. Research reveals that only 18–24 h of MV is sufficient to develop VIDD in both laboratory animals and humans. Studies using animal models reveal that MV-induced diaphragmatic atrophy occurs due to increased diaphragmatic protein breakdown and decreased protein synthesis. Recent investigations have identified calpain, caspase-3, autophagy, and the ubiquitin-proteasome system as key proteases that participate in MV-induced diaphragmatic proteolysis. The challenge for the future is to define the MV-induced signaling pathways that promote the loss of diaphragm protein and depress diaphragm contractility. Indeed, forthcoming studies that delineate the signaling mechanisms responsible for VIDD will provide the knowledge necessary for the development of a pharmacological approach that can prevent VIDD and reduce the incidence of weaning problems.

Intrinsic apoptosis in mechanically ventilated human diaphragm: linkage to a novel Fos/FoxO1/Stat3-Bim axis

Mechanical ventilation (MV) is a life-saving measure in many critically ill patients. However, prolonged MV results in diaphragm dysfunction that contributes to the frequent difficulty in weaning patients from the ventilator. The molecular mechanisms underlying ventilator-induced diaphragm dysfunction (VIDD) remain poorly understood. We report here that MV induces myonuclear DNA fragmentation (3-fold increase; P<0.01) and selective activation of caspase 9 (P<0.05) and Bcl2-interacting mediator of cell death (Bim; 2- to 7-fold increase; P<0.05) in human diaphragm. MV also statistically significantly down-regulates mitochondrial gene expression and induces oxidative stress. In cultured muscle cells, we show that oxidative stress activates each of the catabolic pathways thought to underlie VIDD: apoptotic (P<0.05), proteasomal (P<0.05), and autophagic (P<0.01). Further, silencing Bim expression blocks (P<0.05) oxidative stress-induced apoptosis. Overlapping the gene expression profiles of MV human diaphragm and H₂O₂-treated muscle cells, we identify Fos, FoxO1, and Stat3 as regulators of Bim expression as well as of expression of the catabolic markers atrogin and LC3. We thus identify a novel Fos/FoxO1/Stat3-Bim intrinsic apoptotic pathway and establish the centrality of oxidative stress in the development of VIDD. This information may help in the design of specific drugs to prevent this condition.

Clinical review: ventilator-induced diaphragmatic dysfunction--human studies confirm animal model findings!

Diaphragmatic function is a major determinant of the ability to successfully wean patients from mechanical ventilation. However, the use of controlled mechanical ventilation in animal models results in a major reduction of diaphragmatic force-generating capacity together with structural injury and atrophy of diaphragm muscle fibers, a condition termed ventilator-induced diaphragmatic dysfunction (VIDD). Increased oxidative stress and exaggerated proteolysis in the diaphragm have been linked to the development of VIDD in animal models, but much less is known about the extent to which these phenomena occur in humans undergoing mechanical ventilation in the ICU. In the present review, we first briefly summarize the large body of evidence demonstrating the existence of VIDD in animal models, and outline the major cellular mechanisms that have been implicated in this process. We then relate these findings to very recently published data in critically ill patients, which have thus far been found to exhibit a remarkable degree of similarity with the animal model data. Hence, the human studies to date have indicated that mechanical ventilation is associated with increased oxidative stress, atrophy, and injury of diaphragmatic muscle fibers along with a rapid loss of diaphragmatic force production. These changes are, to a large extent, directly proportional to the duration of mechanical ventilation. In the context of these human data, we also review the methods that can be used in the clinical setting to diagnose and/or monitor the development of VIDD in critically ill patients. Finally, we discuss the potential for using different mechanical ventilation strategies and pharmacological approaches to prevent and/or to treat VIDD and suggest promising avenues for future research in this area.

Mechanical ventilation-induced diaphragm disuse in humans triggers autophagy

RATIONALE

Controlled mechanical ventilation (CMV) results in atrophy of the human diaphragm. The autophagy-lysosome pathway (ALP) contributes to skeletal muscle proteolysis, but its contribution to diaphragmatic protein degradation in mechanically ventilated patients is unknown.

OBJECTIVES

To evaluate the autophagy pathway responses to CMV in the diaphragm and limb muscles of humans and to identify the roles of FOXO transcription factors in these responses.

METHODS

Muscle biopsies were obtained from nine control subjects and nine brain-dead organ donors. Subjects were mechanically ventilated for 2 to 4 hours and 15 to 276 hours, respectively. Activation of the ubiquitin-proteasome system was detected by measuring mRNA expressions of Atrogin-1, MURF1, and protein expressions of UBC2, UBC4, and the α subunits of the 20S proteasome (MCP231). Activation of the ALP was detected by electron microscopy and by measuring the expressions of several autophagy-related genes. Total carbonyl content and HNE-protein adduct formation were measured to assess oxidative stress. Total AKT, phosphorylated and total FOXO1, and FOXO3A protein levels were also measured.

MEASUREMENTS AND MAIN RESULTS

Prolonged CMV triggered activation of the ALP as measured by the appearance of autophagosomes in the diaphragm and increased expressions of autophagy-related genes, as compared with controls. Induction of autophagy was associated with increased protein oxidation and enhanced expression of the FOXO1 gene, but not the FOXO3A gene. CMV also triggered the inhibition of both AKT expression and FOXO1 phosphorylation.

CONCLUSIONS

We propose that prolonged CMV causes diaphragm disuse, which, in turn, leads to activation of the ALP through oxidative stress and the induction of the FOXO1 transcription factor.

Prolonged mechanical ventilation alters diaphragmatic structure and function

OBJECTIVE

To review current knowledge about the impact of prolonged mechanical ventilation on diaphragmatic function and biology.

MEASUREMENTS

Systematic literature review.

CONCLUSIONS

Prolonged mechanical ventilation can promote diaphragmatic atrophy and contractile dysfunction. As few as 18 hrs of mechanical ventilation results in diaphragmatic atrophy in both laboratory animals and humans. Prolonged mechanical ventilation is also associated with diaphragmatic contractile dysfunction. Studies using animal models revealed that mechanical ventilation-induced diaphragmatic atrophy is due to increased diaphragmatic protein breakdown and decreased protein synthesis. Recent investigations have identified calpain, caspase-3, and the ubiquitin-proteasome system as key proteases that contribute to mechanical ventilation-induced diaphragmatic proteolysis. The scientific challenge for the future is to delineate the mechanical ventilation-induced signaling pathways that activate these proteases and depress protein synthesis in the diaphragm. Future investigations that define the signaling mechanisms responsible for mechanical ventilation-induced diaphragmatic weakness will provide the knowledge required for the development of new medicines that can maintain diaphragmatic mass and function during prolonged mechanical ventilation.

Bench-to-bedside review: Diaphragm muscle function in disuse and acute high-dose corticosteroid treatment

Critically ill patients may require mechanical ventilatory support and short-term high-dose corticosteroid to treat some specific underlying disease processes. Diaphragm muscle inactivity induced by controlled mechanical ventilation produces dramatic alterations in diaphragm muscle structure and significant losses in function. Although the exact mechanisms responsible for losses in diaphragm muscle function are still unknown, recent studies have highlighted the importance of proteolysis and oxidative stress. In experimental animals, short-term strategies that maintain partial diaphragm muscle neuromechanical activation mitigate diaphragmatic force loss. In animal models, studies on the influence of combined controlled mechanical ventilation and short-term high-dose methylprednisolone have given inconsistent results in regard to the effects on diaphragm muscle function. In the critically ill patient, further research is needed to establish the prevalence and mechanisms of ventilator-induced diaphragm muscle dysfunction, and the possible interaction between mechanical ventilation and the administration of high-dose corticosteroid. Until then, in caring for these patients, it is imperative to allow partial activation of the diaphragm, and to administer the lowest dose of corticosteroid for the shortest duration possible.

Acute effects of high-dose methylprednisolone on diaphragm muscle function

The time- and dose-dependent effects of acute high-dose corticosteroids on the diaphragm muscle are poorly defined. This study aimed to examine in rabbits the temporal relationships and dose-response effects of acute high-dose methylprednisolone succinate on diaphragmatic contractile and structural properties. Animals were assigned to groups receiving: (1) 80 mg/kg/day methylprednisolone (MP80) intramuscularly for 1, 2, and 3 days; (2) 10 mg/kg/day methylprednisolone (MP10, pulse-dose) for 3 days; or (3) saline (placebo) for 3 days; and (4) a control group. Diaphragmatic in vitro force-frequency and force-velocity relationships, myosin heavy chain (MyHC) isoform protein and mRNA, insulin-like growth factor-1 (IGF-1), muscle atrophy F-box (MAF-box) mRNA, and volume density of abnormal myofibrils were measured at each time-point. MP80 did not affect animal nutritional state or fiber cross-sectional area as assessed in separate pair-fed groups receiving methylprednisolone or saline for 3 days. Compared with control values, MP80 decreased diaphragmatic maximum tetanic tension (Po) by 19%, 24%, and 34% after 1, 2, and 3 days (P < 0.05), respectively, whereas MP10 decreased Po modestly (12%; P > 0.05). Vmax and MyHC protein proportions were unchanged in both the MP80 and MP10 groups. Maximum power output decreased after 2 and 3 days of MP80. Suppression of IGF-1 and overexpression of MAF-box mRNA occurred in both MP groups. Significant myofibrillar disarray was also observed in both MP groups. The decline in Po was significantly associated with the increased volume density of abnormal myofibrils. Thus, very high-dose methylprednisolone (MP80) can produce rapid reductions in diaphragmatic function, whereas pulse-dose methylprednisolone (MP10) produces only modest functional loss.

Effects of acute administration of corticosteroids during mechanical ventilation on rat diaphragm

RATIONALE

Mechanical ventilation is known to induce ventilator-induced diaphragm dysfunction. Patients submitted to mechanical ventilation often receive massive doses of corticosteroids that may cause further deterioration of diaphragm function.

OBJECTIVES

To examine whether the combination of 24 hours of controlled mechanical ventilation with corticosteroid administration would exacerbate ventilator-induced diaphragm dysfunction.

METHODS

Rats were randomly assigned to a group submitted to 24 hours of controlled mechanical ventilation receiving an intramuscular injection of saline or 80 mg/kg methylprednisolone, a group submitted to 24 hours of spontaneous breathing receiving saline, or methylprednisolone and a control group.

MEASUREMENTS AND MAIN RESULTS

The diaphragm force-frequency curve was shifted downward in the mechanical ventilation group, but this deleterious effect was prevented when corticosteroids were administered. Diaphragm cross-sectional area of type I fibers was similarly decreased in both mechanical ventilation groups while atrophy of type IIx/b fibers was attenuated after corticosteroid administration. The mechanical ventilation-induced reduction in diaphragm MyoD and myogenin protein expression was attenuated after corticosteroids. Plasma cytokine levels were unchanged while diaphragm lipid hydroperoxides were similarly increased in both mechanical ventilation groups. Diaphragmatic calpain activity was significantly increased in the mechanical ventilation group, but calpain activation was abated with corticosteroid administration. Inverse correlations were found between calpain activity and diaphragm force.

CONCLUSIONS

A single high dose of methylprednisolone combined with controlled mechanical ventilation protected diaphragm function from the deleterious effects of controlled mechanical ventilation. Inhibition of the calpain system is most likely the mechanism by which corticosteroids induce this protective effect.

Rocuronium exacerbates mechanical ventilation-induced diaphragm dysfunction in rats

OBJECTIVE

Nondepolarizing neuromuscular blocking agents are commonly used in the intensive care setting, but they have occasionally been associated with development of myopathy. In addition, diaphragmatic atrophy and a reduction in diaphragmatic force were reported after short-term controlled mechanical ventilation in animal models. We hypothesized that infusion of rocuronium, an aminosteroidal neuromuscular blocking agent, during 24 hrs of controlled mechanical ventilation would further alter diaphragm function and would enhance activation of the ubiquitin- proteasome pathway.

DESIGN

Randomized, controlled experiment.

SETTING

Basic animal science laboratory.

SUBJECTS

Male Wistar rats, 14 wks old.

INTERVENTIONS

Rats were divided into four groups: a control group, a group of anesthetized rats breathing spontaneously for 24 hrs, and two groups submitted to mechanical ventilation for 24 hrs, receiving a continuous infusion of either 0.9% NaCl or rocuronium.

MEASUREMENTS AND MAIN RESULTS

In vitro diaphragm force was decreased more significantly after 24 hrs of mechanical ventilation combined with rocuronium infusion than after mechanical ventilation alone (e.g., tetanic force, -27%; p < .001 vs. mechanical ventilation). Similarly, the decrease in diaphragm type IIx/b fiber dimensions was more pronounced after mechanical ventilation with rocuronium treatment than with saline treatment (-38% and -29%, respectively; p < .001 vs. control). Diaphragm hydroperoxide levels increased similarly in both mechanically ventilated groups. Diaphragm muscle RING-finger protein-1 (MURF-1) messenger RNA expression, an E3 ligase of the ubiquitin-proteasome pathway, increased after mechanical ventilation (+212%, p < .001 vs. control) and increased further with combination of rocuronium (+320%, p < .001 vs. control). Significant correlations were found between expression of MURF-1 messenger RNA, diaphragm force, and type IIx/b fiber dimensions.

CONCLUSIONS

Infusion of rocuronium during controlled mechanical ventilation leads to further deterioration of diaphragm function, additional atrophy of type IIx/b fibers, and an increase in MURF-1 messenger RNA in the diaphragm, which suggests an activation of the ubiquitin-proteasome pathway. These findings could be important with regard to weaning failure in patients receiving this drug for prolonged periods in the intensive care unit setting.

Impact of post-synaptic block of neuromuscular transmission, muscle unloading and mechanical ventilation on skeletal muscle protein and mRNA expression

To analyse mechanisms of muscle wasting in intensive care unit patients, we developed an experimental model where rats were pharmacologically paralysed by post-synaptic block of neuromuscular transmission (NMB) and mechanically ventilated for 9+/-2 days. Specific interest was focused on the effects on protein and mRNA expression of sarcomeric proteins, i.e., myosin heavy chain (MyHC), actin, myosin-binding protein C (MyBP-C) and myosin-binding protein H (MyBP-H) in fast- and slow-twitch limb, respiratory and masticatory muscles. Muscle-specific differences were observed in response to NMB at both the protein and mRNA levels. At the protein level, a decreased MyHC-to-actin ratio was observed in all muscles excluding the diaphragm, whereas at the mRNA level a decreased expression of the dominating MyHC isoform(s) was observed in the hind limb and intercostal muscles, but not in the diaphragm and masseter muscles. MyBP-C mRNA expression was decreased in the limb muscles, but it otherwise remained unaffected. MyBP-H conversely increased in all muscles. Furthermore, we found myofibrillar protein and mRNA expression to be affected differently when comparing NMB animals with peripherally denervated (DEN) ambulatory rats. We report that NMB has both a larger and different impact on muscle, at the protein and mRNA levels, than DEN has.

Mechanical ventilation promotes redox status alterations in the diaphragm

Oxidative stress is an important mediator of diaphragm muscle atrophy and contractile dysfunction during prolonged periods of controlled mechanical ventilation (MV). To date, specific details related to the impact of MV on diaphragmatic redox status remain unknown. To fill this void, we tested the hypothesis that MV-induced diaphragmatic oxidative stress is the consequence of both an elevation in intracellular oxidant production in conjunction with a decrease in the antioxidant buffering capacity. Adult rats were assigned to one of two experimental groups: 1) control or 2) 12 h of MV. Compared with controls, diaphragms from MV animals demonstrated increased oxidant production, diminished total antioxidant capacity, and decreased glutathione levels. Heme oxygenase-1 (HO-1) mRNA and protein levels increased (23.0- and 5.1-fold, respectively) following MV. Thioredoxin reductase-1 and manganese superoxide dismutase mRNA levels were also increased in the diaphragm following MV (2.4- and 1.6-fold, respectively), although no change was detected in the levels of either protein. Furthermore, copper-zinc superoxide dismutase and glutathione peroxidase mRNA were not altered following MV, although protein content decreased -1.3- and -1.7-fold, respectively. We conclude that MV promotes increased oxidant production and impairment of key antioxidant defenses in the diaphragm; collectively, these changes contribute to the MV-induced oxidative stress in this key inspiratory muscle.

Bench-to-bedside review: weaning failure--should we rest the respiratory muscles with controlled mechanical ventilation?

The use of controlled mechanical ventilation (CMV) in patients who experience weaning failure after a spontaneous breathing trial or after extubation is a strategy based on the premise that respiratory muscle fatigue (requiring rest to recover) is the cause of weaning failure. Recent evidence, however, does not support the existence of low frequency fatigue (the type of fatigue that is long-lasting) in patients who fail to wean despite the excessive respiratory muscle load. This is because physicians have adopted criteria for the definition of spontaneous breathing trial failure and thus termination of unassisted breathing, which lead them to put patients back on the ventilator before the development of low frequency respiratory muscle fatigue. Thus, no reason exists to completely unload the respiratory muscles with CMV for low frequency fatigue reversal if weaning is terminated based on widely accepted predefined criteria. This is important, since experimental evidence suggests that CMV can induce dysfunction of the diaphragm, resulting in decreased diaphragmatic force generating capacity, which has been called ventilator-induced diaphragmatic dysfunction (VIDD). The mechanisms of VIDD are not fully elucidated, but include muscle atrophy, oxidative stress and structural injury. Partial modes of ventilatory support should be used whenever possible, since these modes attenuate the deleterious effects of mechanical ventilation on respiratory muscles. When CMV is used, concurrent administration of antioxidants (which decrease oxidative stress and thus attenuate VIDD) seems justified, since antioxidants may be beneficial (and are certainly not harmful) in critical care patients.

Diaphragm unloading via controlled mechanical ventilation alters the gene expression profile

RATIONALE

Prolonged controlled mechanical ventilation results in diaphragmatic inactivity and promotes oxidative injury, atrophy, and contractile dysfunction in this important inspiratory muscle. However, the impact of controlled mechanical ventilation on global mRNA alterations in the diaphragm remains unknown.

OBJECTIVES

In these experiments, we used an Affymetrix oligonucleotide array to identify the temporal changes in diaphragmatic gene expression during controlled mechanical ventilation in the rat.

METHODS

Adult Sprague-Dawley rats were assigned to either control or mechanical ventilation groups (n = 5/group). Mechanically ventilated animals were anesthetized, tracheostomized, and ventilated with room air for 6 or 18 h. Animals in the control group were acutely anesthetized but not exposed to mechanical ventilation.

MEASUREMENTS AND MAIN RESULTS

Compared with control diaphragms, microarray analysis identified 354 differentially expressed, unique gene products after 6 and 18 h of mechanical ventilation. In general, genes in the cell growth/cell maintenance, stress response, and nucleic acid metabolism categories showed predominant upregulation, whereas genes in the structural protein and energy metabolism categories were predominantly downregulated.

CONCLUSIONS

We conclude that mechanical ventilation results in rapid changes in diaphragmatic gene expression, and subsequently, many of these changes may contribute to atrophy and muscle fiber remodeling associated with unloading this primary inspiratory muscle. Importantly, this study also provides new insights into why the diaphragm, after the onset of contractile inactivity, atrophies more rapidly than locomotor skeletal muscles and also highlights unique differences that exist between these muscles in the mRNA response to inactivity.