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Aim: Mechanical ventilation (MV) results in diminished diaphragm size and strength, termed ventilator‐induced diaphragm dysfunction (VIDD). VID increases dependence, prolongs weaning, and increases discharge mortality rates. The Janus kinase (JAK)/Signal Transducer and Activator of Transcription (STAT) pathway is implicated in VIDD, upregulated following MV. JAK/STAT inhibition alleviates chronic muscle wasting conditions. This study aimed to explore the therapeutic potential of Ruxolitinib, an FDA approved JAK1/2 inhibitor (JI) for the treatment of VIDD. Methods: Rats were subjected to 5 days controlled MV (CMV) with and without daily Ruxolitinib gavage. Muscle fiber size and function were assessed. RNAseq, mitochondrial morphology, respirometry, and mass spectrometry were determined. Results: CMV significantly reduced diaphragm size and specific force by 45% (p < 0.01), associated with a two‐fold P‐STAT3 upregulation (p < 0.001). CMV disrupted mitochondrial content and reduced the oxygen consumption rate (p < 0.01). Expression of the motor protein myosin was unaffected, however CMV alters myosin function via post‐translational modifications (PTMs). Daily administration of JI increased animal survival (40% vs. 87%; p < 0.05), restricted P‐STAT3 (p < 0.001), and preserved diaphragm size and specific force. JI was associated with preserved mitochondrial content and respiratory function (p < 0.01), and the reversal or augmentation of myosin deamidation PTMs of the rod and head region. Conclusion: JI preserved diaphragm function, leading to increased survival in an experimental model of VIDD. Functional enhancement was associated with maintenance of mitochondrial content and respiration and the reversal of ventilator‐induced PTMs of myosin. These results demonstrate the potential of repurposing Ruxolitinib for treatment of VIDD. [ABSTRACT FROM AUTHOR]

Prolonged mechanical ventilation for critically ill patients with respiratory distress can result in severe muscle wasting with preferential loss of myosin. Systemic inflammation triggered by lung mechanical injury likely contributes to this myopathy, although the exact mechanisms are unknown. In this study, we hypothesized that muscle wasting following mechanical ventilation is accompanied by bone loss. The objective was to determine the rate, nature, and extent of bone loss in the femora of rats ventilated up to 10 days and to relate the bone changes to muscle deterioration. We have developed a rat model of ventilator‐induced muscle wasting and established its feasibility and clinical validity. This model involves pharmacologic paralysis, parenteral nutrition, and continuous mechanical ventilation. We assessed the hindlimb muscle and bone of rats ventilated for 0, 2, 5, 8, and 10 days. Routine histology, microCT, and biomechanical evaluations were performed. Hindlimb muscles developed changes consistent with myopathy, whereas the femurs demonstrated a progressive decline in trabecular bone volume, mineral density, and microarchitecture beginning Day 8 of mechanical ventilation. Biomechanical testing showed a reduction in flexural strength and stiffness on Day 10. The bone changes correlated with the loss of muscle mass and myosin. These results demonstrate that mechanical ventilation leads to progressive trabecular bone loss parallel to muscle deterioration. The results of our study suggest that mechanically ventilated patients may be at risk of compromised bone integrity and muscle weakness, predisposing to post‐ventilator falls and fractures, thereby warranting interventions to prevent progressive bone and muscle decline. [ABSTRACT FROM AUTHOR]

Key points: Critical illness myopathy (CIM) is a frequently observed negative consequence of modern critical care.Chronic Janus kinase (JAK)/signal transducer and activator of transcription activation impairs muscle size and function and is prominent following mechanical ventilation.We identify pSTAT‐3 activation in tibialis anterior of CIM patients, before examining the potential benefits of JAK1/2 inhibition in an experimental model of CIM, where muscle mass and function are impaired.CIM activates complement cascade and increased monocyte infiltration in the soleus muscle, which was ameliorated by JAK1/2 inhibition, leading to reduced muscle degeneration and improved muscle force.Here, we demonstrate that JAK1/2 inhibition augments CIM muscle function through regulation of the complement cascade. Critical illness myopathy (CIM) is frequently observed in response to modern critical care with negative consequences for patient quality of life, morbidity, mortality and healthcare costs. Janus kinase (JAK)/signal transducer and activator of transcription (STAT) activation is observed in limb muscles following controlled mechanical ventilation. Chronic JAK/STAT activation promotes loss of muscle mass and function. Thus, we hypothesized that JAK1/2 inhibition would improve muscle outcomes for CIM. Following 12 days of intensive care unit conditions, pSTAT‐3 levels increased in tibialis anterior muscle of CIM patients (P = 0.0489). The potential of JAK1/2 inhibition was assessed in an experimental model of CIM, where soleus muscle size and force are impaired. JAK1/2 inhibition restores soleus force (P < 0.0001). CIM activated muscle complement cascade, which was ameliorated by JAK1/2 inhibition (P < 0.05, respectively). Soleus macrophage number corresponded with complement activity, leading to reduced muscle degeneration and augmented muscle function (P < 0.05). Thus, JAK/STAT inhibition improves soleus function by modulating the complement cascade and muscle monocyte infiltration. Collectively, we demonstrate that JAK/STAT inhibition augments muscle function in CIM. Key points: Critical illness myopathy (CIM) is a frequently observed negative consequence of modern critical care.Chronic Janus kinase (JAK)/signal transducer and activator of transcription activation impairs muscle size and function and is prominent following mechanical ventilation.We identify pSTAT‐3 activation in tibialis anterior of CIM patients, before examining the potential benefits of JAK1/2 inhibition in an experimental model of CIM, where muscle mass and function are impaired.CIM activates complement cascade and increased monocyte infiltration in the soleus muscle, which was ameliorated by JAK1/2 inhibition, leading to reduced muscle degeneration and improved muscle force.Here, we demonstrate that JAK1/2 inhibition augments CIM muscle function through regulation of the complement cascade. [ABSTRACT FROM AUTHOR]

Aim: Critical illness myopathy (CIM) represents a common consequence of modern intensive care, negatively impacting patient health and significantly increasing health care costs; however, there is no treatment available apart from symptomatic and supportive interventions. The chaperone co‐inducer BGP‐15 has previously been shown to have a positive effect on the diaphragm in rats exposed to the intensive care unit (ICU) condition. In this study, we aim to explore the effects of BGP‐15 on a limb muscle (soleus muscle) in response to the ICU condition. Methods: Sprague‐Dawley rats were subjected to the ICU condition for 5, 8 and 10 days and compared with untreated sham‐operated controls. Results: BGP‐15 significantly improved soleus muscle fibre force after 5 days exposure to the ICU condition. This improvement was associated with the protection of myosin from post‐translational myosin modifications, improved mitochondrial structure/biogenesis and reduced the expression of MuRF1 and Fbxo31 E3 ligases. At longer durations (8 and 10 days), BGP‐15 had no protective effect when the hallmark of CIM had become manifest, that is, preferential loss of myosin. Unrelated to the effects on skeletal muscle, BGP‐15 had a strong positive effect on survival compared with untreated animals. Conclusions: BGP‐15 treatment improved soleus muscle fibre and motor protein function after 5 days exposure to the ICU condition, but not at longer durations (8 and 10 days) when the preferential loss of myosin was manifest. Thus, long‐term CIM interventions targeting limb muscle fibre/myosin force generation capacity need to consider both the post‐translational modifications and the loss of myosin. [ABSTRACT FROM AUTHOR]

Water vapour measurements from a ground-based Raman lidar and an airborne differential absorption lidar, complemented by high-resolution numerical simulations from two mesoscale models (AROME-WMED and Meso-NH), are considered to investigate three transition events from Mistral/Tramontane to southerly marine flow taking place in the Montpellier region (southern France) in the time frame September-October 2012, during the Hydrological Cycle in the Mediterranean Experiment Special Observation Period 1. Low-level wind reversals associated with these transitions are found to have a strong impact on water vapour transport, leading to a large variability of the water vapour vertical and horizontal distributions. Water vapour mixing ratio within the boundary layer is found to vary from typical values in the range 4-8 g kg−1 during the Mistral/Tramontane flows to values in the range 8-15 g kg−1 during the southerly marine flows. The increase/decrease in water vapour mixing ratio within the boundary layer may be abrupt and marked during these transition periods, with values increasing/decreasing by a factor of 2-4 within 1 h. The high spatial and temporal resolutions of the lidar data allow monitoring the time evolution of the water vapour field during these transitions from predominantly northerly Mistral/Tramontane flow to a predominantly southerly flow, permitting identification of the quite sharp separation between these flows, which is also satisfactorily well captured by the mesoscale models. Water vapour measurements from the ground-based lidar are complemented by particle backscatter measurements from the same system, which reveal the significant variability in the aerosol and cloud fields associated with these transition events. [ABSTRACT FROM AUTHOR]

Initiation and maintenance of atrial fibrillation (AF) is often associated with pharmacologically or pathologically induced bradycardic states. Even drugs specifically developed in order to counteract cardiac arrhythmias often combine their action with bradycardia and, in turn, with development of AF, via still largely unknown mechanisms. This study aims to simulate action potential (AP) conduction between sinoatrial node (SAN) and atrial cells, either arranged in cell pairs or in a one-dimensional strand, where the relative amount of SAN membrane is made varying, in turn, with junctional resistance. The source-sink relationship between the two membrane types is studied in control conditions and under different simulated chronotropic interventions, in order to define a safety factor for pacemaker-to-atrial AP conduction (SASF) for each treatment. Whereas antiarrhythmic-like interventions which involve downregulation of calcium channels or of calcium handling decrease SASF, the simulation of Ivabradine administration does so to a lesser extent. Particularly interesting is the increase of SASF observed when downregulation GKr, which simulates the administration of class III antiarrhythmic agents and is likely sustained by an increase in ICaL. Also, the increase in SASF is accompanied by a decreased conduction delay and a better entrainment of repolarization, which is significant to anti-AF strategies. [ABSTRACT FROM AUTHOR]

Key points Weaning from mechanical ventilation (MV) of long-term intensive care unit (ICU) patients is delayed by impaired respiratory muscle function; however, the mechanisms that cause the impairment are not fully understood., A novel experimental rat ICU model was used for time-resolved analyses (6 h to 2 weeks) of the effects of MV on diaphragm muscle fibre structure and function, and on gene and protein expression., A prompt and progressive decline of diaphragm muscle fibre function, preceding atrophy, occurred with MV, and at the end of 2 weeks residual diaphragm muscle fibre function was <15% of control levels., Cellular and subcellular analyses indicated that oxidative stress-triggered protein modifications had significantly diminished diaphragm muscle fibre function., The novel finding that activation of proteolytic pathways and regulation of contractile protein synthesis were different in diaphragm and limb muscles has direct implications for the design of muscle-specific intervention strategies., Abstract Controlled mechanical ventilation (CMV) plays a key role in triggering the impaired diaphragm muscle function and the concomitant delayed weaning from the respirator in critically ill intensive care unit (ICU) patients. To date, experimental and clinical studies have primarily focused on early effects on the diaphragm by CMV, or at specific time points. To improve our understanding of the mechanisms underlying the impaired diaphragm muscle function in response to mechanical ventilation, we have performed time-resolved analyses between 6 h and 14 days using an experimental rat ICU model allowing detailed studies of the diaphragm in response to long-term CMV. A rapid and early decline in maximum muscle fibre force and preceding muscle fibre atrophy was observed in the diaphragm in response to CMV, resulting in an 85% reduction in residual diaphragm fibre function after 9-14 days of CMV. A modest loss of contractile proteins was observed and linked to an early activation of the ubiquitin proteasome pathway, myosin:actin ratios were not affected and the transcriptional regulation of myosin isoforms did not show any dramatic changes during the observation period. Furthermore, small angle X-ray diffraction analyses demonstrate that myosin can bind to actin in an ATP-dependent manner even after 9-14 days of exposure to CMV. Thus, quantitative changes in muscle fibre size and contractile proteins are not the dominating factors underlying the dramatic decline in diaphragm muscle function in response to CMV, in contrast to earlier observations in limb muscles. The observed early loss of subsarcolemmal neuronal nitric oxide synthase activity, onset of oxidative stress, intracellular lipid accumulation and post-translational protein modifications strongly argue for significant qualitative changes in contractile proteins causing the severely impaired residual function in diaphragm fibres after long-term mechanical ventilation. For the first time, the present study demonstrates novel changes in the diaphragm structure/function and underlying mechanisms at the gene, protein and cellular levels in response to CMV at a high temporal resolution ranging from 6 h to 14 days. [ABSTRACT FROM AUTHOR]

R3357 --> 875.7 --> Mechanical ventilation (MV), however brief results in the loss of diaphragm muscle mass and strength, termed ventilator induced diaphragm dysfunction (VIDD). VIDD increases dependence, complicates and prolongs weaning and significantly increases discharge mortality rate and health care costs worldwide. The Janus kinase (JAK)/Signal Transducer and Activator of Transcription (STAT) pathway was recently identified as an important signalling pathway implicated in VIDD, upregulated in the diaphragm following MV and limb muslces during critical care. Regulation of STAT3 is imperritve to skeletal muscle mass and function, as STAT3 is required in proper muscle growth and regeneration, while chronic activation of STAT3 is implicated in muscle dysfunction. As JAK/STAT pathway inhibition can restrict the development of chronic muscle wasting conditons, this study aimed to explore the therapeutic potential of Ruxolitinib, an approved JAK1/2 inhibitor for myelofibrosis, for treatment of CIM. We hypothesised Ruxolitinib would reduce loss of muscle mass and function associated with VIDD. Here, rats were subjected to five days controlled MV (CMV) with and without daily Ruxolitinib gavage. Five‐days CMV significantly reduced diaphragm muscle size and impaired specific force, which was associated with 2‐fold upregulation of P‐STAT3, disrupted mitochondrial structure and respiratory function. Expression of the motor protein myosin was not affected, however CMV may alter myosin function through deamidation post translational modification. Ruxolitinib increases five‐day survival rate, restored P‐STAT3 expression and preserved diaphragm muscle size and specific force. These functional improvements were associated with improved mitochondrial structure, augmented mitochondrial respiratory function and reversal or augmentation of myosin deamidations. These results provide evidence of the preclinical potential of repurposing Ruxolitinib for the treatment of VIDD. [ABSTRACT FROM AUTHOR]

R4473 --> 605.26 --> The diaphragm loss of function resulting from Mechanical Ventilation (MV) in Intensive Care Unit (ICU) patients has been termed "Ventilator Induced Diaphragmatic Dysfunction" (VIDD) and is characterized by a rapid and progressive loss of diaphragm muscle mass and function. Hypercapnia is a frequent condition in mechanically ventilated ICU patients and accepted as "permissive hypercapnia" in ARDS. There are numerous negative effects of this gas imbalance on different tissues and their functions are well known, whereas the positive effects are still poorly understood. Acute hypercapnic exposure has shown benefit to in vivo diaphragm function in porcine and rat ICU models, however our understanding of the effects of longer‐term ventilation and the downstream signaling are less known. Here, we hypothesize that normoxic non acidotic hypercapnia would augment diaphragm muscle function during development of VIDD. Adult female Sprague‐Dawley rats were deeply sedated, pharmacologically paralyzed, hydrated, nourished and controlled mechanical ventilated (CMV) for 5 days in normoxic‐normocapnic or normoxic, hypercapnic conditions. Hypercapnic conditions maintained EtCO2 between 55 and 70 mmHg for the duration of the experiment ensuring normal arterial blood pH (7.37 – 7.43), thus they were non‐acidotic hypercapnic conditions. This unique experimental rat model is more representative of the prolonged nature and effects of VIDD and thus provides clinically relevant assessment of hypercapnic treatment efficacy. Diaphragm muscles were dissected and prepared for contractile function assessment and downstream signalling assessed at the gene and protein level. Our results show that: 1) cross sectional area (CSA) was decreased by 40% following normocapnic CMV when compared with CTR (p< 0.001), which was unaltered by hypercapnia treatment; 2) Specific force (SF; force normalised to CSA) was decreased to 58% of CTR following normocapnic CMV (p< 0.001). Non‐acidotic hypercapnia increased SF by 15% when compared with normocapnic rats (p< 0.05); 3) Muscle E3 ligases Murf1 and Atrogin‐1 protein expression is unchanged following 5 days of CMV, irrespective of C02 conditions. LC3B is a known marker for autophagy, with LC3B II:I ratio signifying activation. Following 5 days normocapnic CMV LC3B II:I ratio (p< 0.05) was reduced by 70% in diaphragm muscle. Non‐acidotic hypercapnic conditions failed to alter the activation of LC3B or these muscle degradation pathways; 4) Non‐acidotic hypercapnic conditions however increased TNFα and Interleukin 1β transcript expression in the diaphragm when compared with normocapnic CMV and CTR groups; 5) Non‐acidotic hypercapnic conditions increased stability of respiratory peak pressure, peripheral oxygenation and perfusion, hemodynamic conditions, thus promoting the benefits of non‐acidotic hypercapnic conditions. Our results suggest that non‐acidotic hypercapnic conditions have beneficial effects on single fibre diaphragm function irrespective of significant increase in muscle inflammation in non‐septic animals. This, combined with the positive respiratory and hemodynamic effects, encourage further study of the therapeutic potential of CO2 in ICU contexts. [ABSTRACT FROM AUTHOR]

R3369 --> 605.16 --> Critical illness myopathy (CIM) is a debilitating condition characterized by the preferential loss of the motor protein myosin. CIM is a consequence of critical care, impairs recovery and provides long‐term complications, and mortality. CIM pathophysiology is complex and remains incompletely understood, however loss of mechanical stimuli appears central to critical illness associated muscle atrophy and weakness. Passive mechanical loading (ML) and electrical stimulation (ES) augment muscle mass and function, however the mechanisms underpinning these therapies are less known. Here we aimed to assess the hypothesis that chronic supramaximal ES would ameliorate CIM in a unique experimental rat model of critical care. Rats were subjected to deep sedation, controlled mechanical ventilation, and immobilization with and without direct soleus ES for 8 days. Critical care reduced soleus muscle mass, cross‐sectional area (CSA), Myosin: Actin ratio and single muscle fibre specific force. ES reduced the loss of soleus muscle fibre CSA and Myosin: Actin ratio yet failed to effect specific force. Insulin signalling gene pathway was downregulated following critical care and GLUT4 trafficking was reduced leading to muscle glycogen depletion. ES promoted phosphofructokinase and insulin signalling pathways and maintained GLUT4 translocation and glycogen levels. ES evoked AMPK, but not AKT, signalling pathway, where the downstream target TBC1D4 and AMPK‐specific P‐TBC1D4 levels tended to be increased. Reduced muscle protein degradation promoted soleus CSA, as ES reduced E3 ligases Atrogin‐1 and MuRF1 downstream of AMPK‐FoxO3. These results demonstrate chronic supramaximal ES reduces critical care associated muscle wasting, preserved glucose signalling and reduced muscle protein degradation in CIM. [ABSTRACT FROM AUTHOR]

R3306 --> 605.15 --> Background: Critical illness myopathy (CIM) is a consequence of modern critical care resulting in general muscle wasting and paralyses of all limb and trunk muscles, and prolonged weaning from the ventilator. CIM is associated with severe morbidity/mortality and significant negative socioeconomic consequences which has become increasingly evident during the current COVID‐19 pandemic, but underlying mechanisms remain elusive. Methods: Ten neuro‐ICU patients exposed to long‐term controlled mechanical ventilation (CMV) were followed with repeated muscle biopsies 3times/week for up to 12 days. Single muscle fiber contractile recordings were conducted on the first and final biopsy. A multiomics approach was taken to analyze gene and protein expression in muscle and plasma at all six timepoints. Findings: i) A progressive preferential myosin loss, the hallmark of CIM, was observed in all neuro‐ICU patients during the observation period. The myosin loss was coupled to a general transcriptional downregulation of myofibrillar proteins and activation of protein degradation pathways, resulting in significant muscle fiber atrophy and loss in force generation capacity which declined >65% during the 12‐day observation period, ii) membrane excitability was not affected as indicated by the maintained compound muscle action potential amplitude upon supramaximal stimulation of upper and lower extremity motor nerves, and iii), analyses of plasma revealed early activation of inflammatory and proinflammatory pathways, as well as a redistribution of zinc ions from plasma in parallel with the activation of zinc transporters. Interpretation: The mechanical ventilation induced lung injury with release of cytokines/chemokines and the complete mechanical silencing uniquely observed in immobilized ICU patients affecting skeletal muscle gene/protein expression are forwarded as the dominant factors triggering CIM. It is therefore suggested that future intervention strategies should target the lung injury and the mechanical silencing in critical ill ICU patients. [ABSTRACT FROM AUTHOR]

Key points Early physical mobilization of mechanically ventilated intensive care unit (ICU) patients can reduce the length of stay in the ICU and hospital and improve muscle strength and functional outcomes., A unique experimental rat ICU model has been used to study the effects and underlying mechanisms of unilateral passive mechanical loading on skeletal muscle size and function at durations varying between 6 h and 2 weeks., Passive mechanical loading attenuated the loss of muscle mass and force-generation capacity associated with the ICU intervention., The maintained muscle mass and function by passive loading is probably due to lower oxidative stress and a reduced loss of the molecular motor protein myosin., The beneficial effects of passive mechanical loading on muscle size and function strongly support the importance of early and intense physical therapy in immobilized ICU patients., Abstract The response to mechanical stimuli, i.e. tensegrity, plays an important role in regulating cell physiological and pathophysiological function, and the mechanical silencing observed in intensive care unit (ICU) patients leads to a severe and specific muscle wasting condition. This study aims to unravel the underlying mechanisms and the effects of passive mechanical loading on skeletal muscle mass and function at the gene, protein and cellular levels. A unique experimental rat ICU model has been used allowing long-term (weeks) time-resolved analyses of the effects of standardized unilateral passive mechanical loading on skeletal muscle size and function and underlying mechanisms. Results show that passive mechanical loading alleviated the muscle wasting and the loss of force-generation associated with the ICU intervention, resulting in a doubling of the functional capacity of the loaded versus the unloaded muscles after a 2-week ICU intervention. We demonstrate that the improved maintenance of muscle mass and function is probably a consequence of a reduced oxidative stress revealed by lower levels of carbonylated proteins, and a reduced loss of the molecular motor protein myosin. A complex temporal gene expression pattern, delineated by microarray analysis, was observed with loading-induced changes in transcript levels of sarcomeric proteins, muscle developmental processes, stress response, extracellular matrix/cell adhesion proteins and metabolism. Thus, the results from this study show that passive mechanical loading alleviates the severe negative consequences on muscle size and function associated with the mechanical silencing in ICU patients, strongly supporting early and intense physical therapy in immobilized ICU patients. [ABSTRACT FROM AUTHOR]

Adaptation of action potential duration (APD) to pacing cycle length (CL) has been previously characterized in isolated cardiomyocytes for sudden changes in constant CL and for pre-/postmature stimuli following constant pacing trains. However, random fluctuations characterize both physiological sinus rhythm (up to 10% of mean CL) and intrinsic beat-to-beat APD at constant pacing rate. We analysed the beat-to-beat sensitivity of each APD to the preceding CL during constant–sudden, random or linearly changing pacing trains in single patch clamped rat left ventricular myocytes, in the absence of the autonomic and electrotonic effects that modulate rate dependency in the intact heart. Beat-to-beat variability of APD at −60 mV (APD−60 mV), quantified ass.d. over 10-beat sequences, increased with corresponding mean APD. When measured as coefficient of variability (CV), APD−60 mV variability was inversely proportional to pacing frequency (from 1.2% at 5 Hz to 3.2% at 0.2 Hz). It was increased, at a basic CL (BCL) of 250 ms, by 55% by the L-type calcium current ( ICaL) blocker nifedipine, and decreased by 23% by the transient-outward potassium current ( Ito) blocker 4-aminopyridine. Variability of APD at BCL of 250 ms prevented the detection of random changes of CL smaller than ∼5%. Ten per cent random changes in CL were detected as a 40% increase in CV of APD and tended to correlate with it ( r= 0.43). Block of ICaL depressed this correlation ( r= 0.23), whereas block of Ito significantly increased it ( r= 0.67); this was similar with linearly changing CL ramps (ranging ±10% and ±20% of 250 ms). We conclude that beat-to-beat APD variability, a major determinant of the propensity for development of arrhythmia in the heart, is present in isolated myocytes, where it is dependent on mean APD and pacing rate. Action potential duration shows a beat-to-beat positive correlation with preceding randomly/linearly changing CL, which can be pharmacologically modulated. [ABSTRACT FROM AUTHOR]

The aim of this study was to test the hypothesis that swimming training might impact differentially myostatin expression in skeletal muscles, depending on fibre type composition, and in cardiac muscle of rats. Myostatin expression was analysed by real time reverse transcriptase-polymerase chain reaction, Western blot and immunohistochemistry of the red deep portion (mainly composed of slow and type II A fibres) and in the superficial, white portion (composed of fast type II X and II B fibres) of the gastrocnemius muscle in adult male Wistar rats: (i) subjected to two consecutive swimming bouts for 3 h; (ii) subjected to intensive swimming training for 4 weeks; and (iii) sedentary control rats. Myostatin mRNA content was in all cases higher in white than in red muscles. Two bouts of swimming did not alter myostatin expression, whereas swimming training for 4 weeks resulted in a significant reduction of myostatin mRNA contents, significant both in white and red muscles but more pronounced in white muscles. Western blot did not detect any change in the amount of myostatin protein. Immunohistochemistry showed that, in control rats, myostatin was localized in presumptive satellite cells of a few muscle fibres. After training, the number of myostatin-positive spots decreased significantly. Myostatin mRNA content in cardiac muscle was lower than in skeletal muscle and was significantly increased by swimming training. In conclusion, the results obtained showed that intense training caused a decreased expression of myostatin mRNA in white and red skeletal muscles but an increase in cardiac muscle. [ABSTRACT FROM AUTHOR]

We have analysed alterations of α-skeletal actin expression and volume fraction of fibrosis in the ventricular myocardium and their functional counterpart in terms of arrhythmogenesis and haemodynamic variables, in rats with different degrees of compensated cardiac hypertrophy induced by infra-renal abdominal aortic coarctation. The following coarctation calibres were used: 1.3 (AC1.3 group), 0.7 (AC0.7) and 0.4 mm (AC0.4); age-matched rats were used as controls (C group). One month after surgery, spontaneous and sympathetic-induced ventricular arrhythmias were telemetrically recorded from conscious freely moving animals, and invasive haemodynamic measurements were performed in anaesthetized animals. After killing, subgroups of AC and C rats were used to evaluate in the left ventricle the expression and spatial distribution of α-skeletal actin and the amount of perivascular and interstitial fibrosis. As compared with C, all AC groups exhibited higher values of systolic pressure, ventricular weight and ventricular wall thickness. AC0.7 and AC0.4 rats also showed a larger amount of fibrosis and upregulation of α-skeletal actin expression associated with a higher vulnerability to ventricular arrhythmias (AC0.7 and AC0.4) and enhanced myocardial contractility (AC0.4). Our results illustrate the progressive changes in the extracellular matrix features accompanying early ventricular remodelling in response to different degrees of pressure overload that may be involved in the development of cardiac electrical instability. We also demonstrate for the first time a linear correlation between an increase in α-skeletal actin expression and the degree of compensated cardiac hypertrophy, possibly acting as an early compensatory mechanism to maintain normal mechanical performance. [ABSTRACT FROM AUTHOR]

In normal rats, we analysed the arrhythmogenic role of intrinsic action potential duration (APD) heterogeneity. In each animal, ventricular arrhythmic events (VAEs) occurring spontaneously and during the exposure to an acute social challenge were telemetrically recorded. Action potentials were recorded from isolated left ventricular myocytes, at a pacing rate of 5 Hz (patch clamp: current-clamp mode). APDs were measured at -20 mV, -30 mV, -40 mV, -50 mV and -60 mV. The difference between the shortest and the longest APD was also computed, as an index of individual APD heterogeneity. Animals predisposed to stress-induced arrhythmias showed higher values of APD and APD heterogeneity as compared with the remaining rats. We concluded that, in the normal heart, a large intrinsic APD heterogeneity resulting from specific electrophysiological properties of ventricular myocytes is not in itself arrhythmogenic, but can predispose towards arrhythmia development under certain conditions, such as autonomic activation. [ABSTRACT FROM AUTHOR]

Aim: Critical illness myopathy (CIM) is a consequence of modern critical care, leading to skeletal muscle atrophy/paralysis with negative consequences for mortality/morbidity and health care costs. Glucocorticoids (GCs) have been proposed to trigger CIM. Here, we compare outcomes of two GCs, the commonly used prednisolone and the newly developed dissociative vamorolone in response to the intensive care unit (ICU) condition for 5 days, ie, sedation, immobilization, and mechanical ventilation. Methods: Rats were divided into a 0‐day sham‐operated control group, and three groups exposed to 5 days ICU condition during treatment with prednisolone (PRED) or vamorolone (VAM) or none of these GCs (ICU‐group). Survival, body and muscle weights, cytokine concentrations, regulation of muscle contraction in single fast‐ and slow‐twitch muscle fibres, myofibrillar protein expression and protein degradation pathways were studied. Results: Critical illness myopathy geno‐ and pheno‐types were confirmed in the ICU group. However, VAM and PRED groups showed reduced atrophy/weakness than the ICU group, and muscle specific differences with more severe negative effects on fast‐twitch muscle fibres in the PRED than the other groups. Conclusion: These results show that vamorolone provides a GC intervention superior to typical GCs in improving CIM outcomes. Further, the findings do not support the notion that moderate‐dose GC treatment represents a factor triggering CIM. [ABSTRACT FROM AUTHOR]

A letter to the editor is presented from Lars Larsson, Nicola Cacciani, and Barry Dworkin, authors of articles on the effects of mechanical ventilation on diaphragm muscle structure and function which appeared in "The Journal Physiology," in response to the comments of Schellekens and co-workers.