Myofibril Damage Research Articles

Changes of Ca2+ release through ryanodine receptor 1 in pork under different postmortem chilling rates

The role of postmortem chilling rate in regulating Ca2+ release through ryanodine receptor 1 (RyR1) in pork microstructure and tenderness were evaluated. The pork was allocated to the low chilling rate (LCR, 4.26°C/h), medium chilling rate (MCR, 8.89°C/h), and high chilling rate (HCR, 13.33°C/h) group. Sarcomere super-contraction and fiber damage were obtained in HCR samples, leading to the lowest shear force (P < 0.05). Under HCR, Ca2+ concentration significantly increased in sarcoplasm and decreased in sarcoplasmic reticulum were found at 2-5 h postmortem compared to other groups (P < 0.05). RyR1 and dihydropyridine receptor (DHPR) levels were higher and 12 kDa FK506 binding protein (FKBP12) level was lower in HCR samples at 6 h postmortem (P < 0.05). Therefore, HCR (13.33°C/h) may enhance Ca2+ release by up-regulating RyR1 level with DHPR activation increasing and FKBP12 inhibition decreasing, causing myofibril damage from sarcomere super-contraction to improve pork tenderness at early postmortem.

Filamin protects myofibrils from contractile damage through changes in its mechanosensory region.

Filamins are mechanosensitive actin crosslinking proteins that organize the actin cytoskeleton in a variety of shapes and tissues. In muscles, filamin crosslinks actin filaments from opposing sarcomeres, the smallest contractile units of muscles. This happens at the Z-disc, the actin-organizing center of sarcomeres. In flies and vertebrates, filamin mutations lead to fragile muscles that appear ruptured, suggesting filamin helps counteract muscle rupturing during muscle contractions by providing elastic support and/or through signaling. An elastic region at the C-terminus of filamin is called the mechanosensitive region and has been proposed to sense and counteract contractile damage. Here we use molecularly defined mutants and microscopy analysis of the Drosophila indirect flight muscles to investigate the molecular details by which filamin provides cohesion to the Z-disc. We made novel filamin mutations affecting the C-terminal region to interrogate the mechanosensitive region and detected three Z-disc phenotypes: dissociation of actin filaments, Z-disc rupture, and Z-disc enlargement. We tested a constitutively closed filamin mutant, which prevents the elastic changes in the mechanosensitive region and results in ruptured Z-discs, and a constitutively open mutant which has the opposite elastic effect on the mechanosensitive region and gives rise to enlarged Z-discs. Finally, we show that muscle contraction is required for Z-disc rupture. We propose that filamin senses myofibril damage by elastic changes in its mechanosensory region, stabilizes the Z-disc, and counteracts contractile damage at the Z-disc.

Fluvastatin-induced myofibrillar damage is associated with elevated ROS, and impaired fatty acid oxidation, and is preceded by mitochondrial morphological changes

Previously, we showed that fluvastatin treatment induces myofibrillar damage and mitochondrial phenotypes in the skeletal muscles of Drosophila. However, the sequential occurrence of mitochondrial phenotypes and myofibril damage remains elusive. To address this, we treated flies with fluvastatin for two and five days and examined their thorax flight muscles using confocal microscopy. In the two-day fluvastatin group, compared to the control, thorax flight muscles exhibited mitochondrial morphological changes, including fragmentation, rounding up and reduced content, while myofibrils remained organized in parallel. In the five-day fluvastatin treatment, not only did mitochondrial morphological changes become more pronounced, but myofibrils became severely disorganized with significantly increased thickness and spacing, along with myofilament abnormalities, suggesting myofibril damage. These findings suggest that fluvastatin-induced mitochondrial changes precede myofibril damage. Moreover, in the five-day fluvastatin group, the mitochondria demonstrated elevated H2O2 and impaired fatty acid oxidation compared to the control group, indicating potential mitochondrial dysfunction. Surprisingly, knocking down Hmgcr (Drosophila homolog of HMGCR) showed normal mitochondrial respiration in all parameters compared to controls or five-day fluvastatin treatment, which suggests that fluvastatin-induced mitochondrial dysfunction might be independent of Hmgcr inhibition. These results provide insights into the sequential occurrence of mitochondria and myofibril damage in statin-induced myopathy for future studies.

High-intensity interval training alleviates exhaustive exercise-induced HSP70-assisted selective autophagy in skeletal muscle

This study was designed to probe the effect of chaperone-assisted selective autophagy (CASA) on the maintenance of proteostasis during exhaustive exercise and uncover the alteration of CASA in muscle fibers with pre-high-intensity interval training (HIIT) intervention-induced muscle adaptation in response to exhaustive exercise. Rats were randomly divided into a control group; an exhaustive exercise group; and an HIIT + exhaustive exercise group. Results show myofibril damage and BiP levels were increased after exhaustive exercise, and the levels of the HSP70, BAG3, ubiquitin, autophagy-related proteins, and their interactions were increased. HIIT intervention before exhaustive exercise could decrease myofibril injury and BiP levels, accompanied by down-regulation of HSP70/BAG3 complex and selective autophagy. In conclusion, exhaustive exercise promotes CASA to clear protein aggregation for keeping proteostasis in muscle fibers; pre-HIIT intervention improves myofibril injury and unfold protein response caused by exhaustive exercise, which might contribute to inhibit the augmentation of CASA.

Galangin Attenuates Myocardial Ischemic Reperfusion-Induced Ferroptosis by Targeting Nrf2/Gpx4 Signaling Pathway.

Myocardial ischemic reperfusion injury (MIRI) is a crucial clinical problem globally. The molecular mechanisms of MIRI need to be fully explored to develop new therapeutic methods. Galangin (Gal), which is a natural flavonoid extracted from Alpinia Officinarum Hance and Propolis, possesses a wide range of pharmacological activities, but its effects on MIRI remain unclear. This study aimed to determine the pharmacological effects of Gal on MIRI. C57BL/6 mice underwent reperfusion for 3 h after 45 min of ischemia, and neonatal rat cardiomyocytes (NRCs) subjected to hypoxia and reoxygenation (HR) were cultured as in vivo and in vitro models. Echocardiography and TTC-Evans Blue staining were performed to evaluate the myocardial injury. Transmission electron microscope and JC-1 staining were used to validate the mitochondrial function. Additionally, Western blot detected ferroptosis markers, including Gpx4, FTH, and xCT. Gal treatment alleviated cardiac myofibril damage, reduced infarction size, improved cardiac function, and prevented mitochondrial injury in mice with MIRI. Gal significantly alleviated HR-induced cell death and mitigated mitochondrial membrane potential reduction in NRCs. Furthermore, Gal significantly inhibited ferroptosis by preventing iron overload and lipid peroxidation, as well as regulating Gpx4, FTH, and xCT expression levels. Moreover, Gal up-regulated nuclear transcriptive factor Nrf2 in HR-treated NRCs. Nrf2 inhibition by Brusatol abolished the protective effect of Gal against ferroptosis. This study revealed that Gal alleviates myocardial ischemic reperfusion-induced ferroptosis by targeting Nrf2/Gpx4 signaling pathway.

Statins Induce Locomotion and Muscular Phenotypes in Drosophila melanogaster That Are Reminiscent of Human Myopathy: Evidence for the Role of the Chloride Channel Inhibition in the Muscular Phenotypes.

The underlying mechanisms for statin-induced myopathy (SIM) are still equivocal. In this study, we employ Drosophila melanogaster to dissect possible underlying mechanisms for SIM. We observe that chronic fluvastatin treatment causes reduced general locomotion activity and climbing ability. In addition, transmission microscopy of dissected skeletal muscles of fluvastatin-treated flies reveals strong myofibrillar damage, including increased sarcomere lengths and Z-line streaming, which are reminiscent of myopathy, along with fragmented mitochondria of larger sizes, most of which are round-like shapes. Furthermore, chronic fluvastatin treatment is associated with impaired lipid metabolism and insulin signalling. Mechanistically, knockdown of the statin-target Hmgcr in the skeletal muscles recapitulates fluvastatin-induced mitochondrial phenotypes and lowered general locomotion activity; however, it was not sufficient to alter sarcomere length or elicit myofibrillar damage compared to controls or fluvastatin treatment. Moreover, we found that fluvastatin treatment was associated with reduced expression of the skeletal muscle chloride channel, ClC-a (Drosophila homolog of CLCN1), while selective knockdown of skeletal muscle ClC-a also recapitulated fluvastatin-induced myofibril damage and increased sarcomere lengths. Surprisingly, exercising fluvastatin-treated flies restored ClC-a expression and normalized sarcomere lengths, suggesting that fluvastatin-induced myofibrillar phenotypes could be linked to lowered ClC-a expression. Taken together, these results may indicate the potential role of ClC-a inhibition in statin-associated muscular phenotypes. This study underlines the importance of Drosophila melanogaster as a powerful model system for elucidating the locomotion and muscular phenotypes, promoting a better understanding of the molecular mechanisms underlying SIM.

Isoform-specific functions of synaptopodin-2 variants in cytoskeleton stabilization and autophagy regulation in muscle under mechanical stress

Protein homeostasis (proteostasis) in multicellular organisms depends on the maintenance of force-bearing and force-generating cellular structures. Within myofibrillar Z-discs of striated muscle, isoforms of synaptopodin-2 (SYNPO2/myopodin) act as adapter proteins that are engaged in proteostasis of the actin-crosslinking protein filamin C (FLNc) under mechanical stress. SYNPO2 directly binds F-actin, FLNc and α-actinin and thus contributes to the architectural features of the actin cytoskeleton. By its association with autophagy mediating proteins, i.e. BAG3 and VPS18, SYNPO2 is also engaged in protein quality control and helps to target mechanical unfolded and damaged FLNc for degradation. Here we show that deficiency of all SYNPO2-isoforms in myotubes leads to decreased myofibrillar stability and deregulated autophagy under mechanical stress. In addition, isoform-specific proteostasis functions were revealed. The PDZ-domain containing variant SYNPO2b and the shorter, PDZ-less isoform SYNPO2e both localize to Z-discs. Yet, SYNPO2e is less stably associated with the Z-disc than SYNPO2b, and is dynamically transferred into FLNc-containing myofibrillar lesions under mechanical stress. SYNPO2e also recruits BAG3 into these lesions via interaction with the WW domain of BAG3. Our data provide evidence for a role of myofibrillar lesions as a transient quality control compartment essential to prevent and repair contraction-induced myofibril damage in muscle and indicate an important coordinating activity for SYNPO2 therein.

Comparison of linear versus macrocyclic gadolinium chelates in rat skeletal muscle.

We investigated the effects on skeletal muscle of gadolinium based linear and macrocyclic radiocontrast agents applied at experimental intervals using histopathological methods. Thirty-two male Sprague-Dawley rats were included in the study for histopathological analysis. No procedure was performed on the healthy control group. The sham group received 0.1 ml/kg intravenous (IV) saline solution through the tail vein 4 times weekly for 5 weeks. The gadodiamide group received total 2 mmol/kg IV gadodiamide through the tail vein 4 times weekly for 5 weeks. The gadoteric-acid group received 2 mmol/kg IV gadoteric acid through the tail vein 4 times weekly for 5 weeks. We determined no marked apoptotic myofibrils associated withcaspase-3 expression in these two groups. Furthermore, no calcineurin expression was observed in myofibrils in the two groups. However, quantitative analyses revealed a decrease in muscle-fiber area in the gadodiamide and gadoteric-acid groups compared to the control group (Respectively; p=0.001 and p=0.017). In our experimental study, linear and macrocyclic GBCAs applied at repeated doses played no role in myofibril damage induced by caspase-3 and calcineurin - nuclear factor of activated T-cells in skeletal muscle tissue.

MiR-212/132 Cluster Modulation Prevents Doxorubicin-Mediated Atrophy and Cardiotoxicity

Improved therapy of cancer has significantly increased the lifespan of patients. However, cancer survivors face an increased risk of cardiovascular complications due to adverse effects of cancer therapies. The chemotherapy drug doxorubicin is well known to induce myofibril damage and cardiac atrophy. Our aim was to test potential counteracting effects of the pro-hypertrophic miR-212/132 family in doxorubicin-induced cardiotoxicity. Invitro, overexpression of the pro-hypertrophic miR-212/132 cluster in primary rodent and human iPSC-derived cardiomyocytes inhibited doxorubicin-induced toxicity. Next, a disease model of doxorubicin-induced cardiotoxicity was established in male C57BL/6N mice. Mice were administered either adeno-associated virus (AAV)9-control or AAV9-miR-212/132 to achieve myocardial overexpression of the miR-212/132 cluster. AAV9-mediated overexpression limited cardiac atrophy by increasing left ventricular mass and wall thickness, decreased doxorubicin-mediated apoptosis, and prevented myofibril damage. Based on a transcriptomic profiling we identified fat storage-inducing transmembrane protein 2 (Fitm2) as a novel target and downstream effector molecule responsible, at least in part, for the observed miR-212/132 anti-cardiotoxic effects. Overexpression of Fitm2 partially reversed the effects of miR-212/132. Overexpression of the miR-212/132 family reduces development of doxorubicin-induced cardiotoxicity and thus could be a therapeutic entry point to limit doxorubicin-mediated adverse cardiac effects.

Sacubitril and valsartan protect from experimental myocardial infarction by ameliorating oxidative damage in Wistar rats

ABSTRACTBackground: Sacubitril (SAC), a neprilysin inhibitor prevent degradation of neprilysin and activate cGMP signaling pathways leading to rise in blood volume concurrent to blood pressure by means of vasoactive peptides, adrenomedullin, and bradykinin.Objective: The aim of this study was to evaluate the anti-ischemic effects of SAC through inhibiting neprilysin in isoproterenol (ISO) induced myocardial infarction (MI) in Wistar albino rats. ISO (85 mg/kg) was injected subcutaneously at the end of 14 days pre-treatment with SAC and valsartan (VAL).Result: Biochemical investigation revealed that SAC along with VAL significantly prevented the antioxidant enzymes (SOD, Catalase, GR, GPx, GST, and GSH) degradation and malondialdehyde (MDA) induced by ISO intoxication in Wistar rats. Along with this, cardiac biomarkers (LDH, CK-MB, ALT, AST, and ALP) were also significantly ameliorated by SACand VAL in ISO-treated rats. Concurrently, decreased infarction area (IA)and marked reduction in myofibril damage by SACand VAL further supported its protective benefits in MI.Conclusion: Taken together, the results suggest that inhibition of enzyme neprilysin alleviated the ISO induces myocardial damage mediated by its strong antioxidant potential.

Levosimendan suppresses oxidative injury, apoptotic signaling and mitochondrial degeneration in streptozotocin-induced diabetic cardiomyopathy

Diabetic cardiomyopathy plays a major role in morbidity and mortality among cardiovascular disorder-related complications. This study was designed to explore long-term benefits of Levosimendan (LEVO) along with Ramipril and Insulin. Diabetic cardiomyopathy was induced using streptozotocin (STZ) at the dose of 25 mg/kg/body weight/day for three consecutive days in Wistar rats. Rats were randomly divided into 10 groups and treatments were started after 2 weeks of STZ administration. A gradual but severe hyperglycemia (§§§p < 0.001) was observed in all STZ-treated groups except those received insulin (2 U/day). LEVO alone and in combination with Ramipril and Insulin normalized (**p < 0.01) mean arterial pressure and heart rate, restored catalase, superoxide dismutase, malondialdehyde, glutathione level and also attenuated (***p < 0.001) the raised serum levels of creatine kinase-heart type, lactate dehydrogenase, tumor necrosis factor-alpha, C-reactive protein, and caspase-3 level in heart tissue altered after STZ treatment. Myofibril degeneration, mitochondrial fibrosis and vacuolization occurred after STZ treatment, were also reversed by LEVO in combination with Ramipril and Insulin. The combination of LEVO with Ramipril and Insulin improved hemodynamic functions, maintained cardiac enzymes and ameliorated myofibril damage in diabetic cardiomyopathy.

Benidipine prevents oxidative stress, inflammatory changes and apoptosis related myofibril damage in isoproterenol-induced myocardial infarction in rats

Objective: The effects of benidipine on oxidative stress and myocardial apoptosis were assessed in isoproterenol (ISO)-induced myocardial infarction (MI) in wistar rats.Materials and method: Animals were pretreated with benidipine (1, 3, 10 μg/kg/day Body weight) intravenously for a period of 28 days. After pretreatment, ISO (85 mg/kg Body weight, subcutaneous) was injected in rats at an interval of 24 h to induce MI. Myocardial oxidative stress, cardiac biomarkers, apoptosis, inflammatory mediators, and ultrastructural architecture of the cardiac tissue were assessed in ISO-induced MI in rats.Result: Significant variation in the level of TBA, antioxidant enzymes (GSH, CAT, SOD, GPx, GRx, GST) in myocardium, cardiac biomarkers (CK-MB, LDH) in serum, Caspase-3, C-reactive protein (CRP), and alteration in ultrastructural architecture of cardiac tissue confirmed the cardiotoxicity induced by ISO. Pretreatment with benidipine preserved the lipid peroxide and antioxidant enzymes, and furthermore showed maintained levels of myocardial biomarker, CRP and caspase-3. Ultrastructure architecture of cardiac tissue was also found to be well preserved.Conclusion: The present study suggested cardioprotective effect of benidipine which may possibly be due to its antioxidant activity and antiapoptotic nature.

MDMA induces cardiac contractile dysfunction through autophagy upregulation and lysosome destabilization in rats

The underlying mechanisms of cardiotoxicity of 3,4-methylenedioxymethylamphetamine (MDMA, “ecstasy”) abuse are unclear. Autophagy exerts either adaptive or maladaptive effects on cardiac function in various pathological settings, but nothing is known on the role of autophagy in the MDMA cardiotoxicity. Here, we investigated the mechanism through which autophagy may be involved in MDMA-induced cardiac contractile dysfunction. Rats were injected intraperitoneally with MDMA (20mg/kg) or saline. Left ventricular (LV) echocardiography and LV pressure measurement demonstrated reduction of LV systolic contractility 24h after MDMA administration. Western blot analysis showed a time-dependent increase in the levels of microtubule-associated protein light chain 3-II (LC3-II) and cathepsin-D after MDMA administration. Electron microscopy showed the presence of autophagic vacuoles in cardiomyocytes. MDMA upregulated phosphorylation of adenosine monophosphate-activated protein kinase (AMPK) at Thr172, mammalian target of rapamycin (mTOR) at Thr2446, Raptor at Ser792, and Unc51-like kinase (ULK1) at Ser555, suggesting activation of autophagy through the AMPK-mTOR pathway. The effects of autophagic inhibitors 3-methyladenine (3-MA) and chloroquine (CQ) on LC3-II levels indicated that MDMA enhanced autophagosome formation, but attenuated autophagosome clearance. MDMA also induced release of cathepsins into cytosol, and western blotting and electron microscopy showed cardiac troponin I (cTnI) degradation and myofibril damage, respectively. 3-MA, CQ, and a lysosomal inhibitor, E64c, inhibited cTnI proteolysis and improved contractile dysfunction after MDMA administration. In conclusion, MDMA causes lysosome destabilization following activation of the autophagy-lysosomal pathway, through which released lysosomal proteases damage myofibrils and induce LV systolic dysfunction in rat heart.

The silenced diaphragm: The good and the bad

Summary With controlled mechanical ventilation (CMV) the diaphragm is inactive. The application of short-term CMV at the onset of cardiogenic shock or sepsis exerts protective effects on the diaphragm muscle. On the other hand, CMV can produce detrimental effects on healthy diaphragm muscle. The effects are rapid and progressive, and are associated with either muscle atrophy or myofibril damage; the mechanisms of both involve decreased protein synthesis and increased contractile protein degradation. Protein degradation is mediated via interactive molecular signaling, including oxidative stress, apoptosis, and proteasome-proteolysis. Auspiciously, strategies to maintain partial diaphragmatic contractions can mitigate ventilator-induced diaphragmatic dysfunction.

Effects of Thalidomide on Isoprenaline‐Induced Acute Myocardial Injury: A Haemodynamic, Histopathological and Ultrastructural Study

In the present study, we investigated the cardioprotective effects of thalidomide in a rat model of acute myocardial injury, induced by subcutaneous injection of isoprenaline hemisulphate (85 mg/kg per day for 2 days). Thalidomide (75/150/300 mg/kg) or vehicle (dimethylsulphoxide) or saline (0.9% NaCl) was administered orally for 14 days and isoprenaline injection on the 12th and 13th days. Cardiovascular responses (arterial and left ventricular haemodynamic parameters and heart rate) were obtained in anaesthetized rats on the 14th day. Histopathological and electronmicroscopical analysis of myocardial injury was done. The results showed that thalidomide 300 mg/kg per day orally caused significant improvement in isoprenaline-induced reduction of cardiac function with increases in maximum rate of pressure development (+LVdP/dt, P < 0.001) and maximum rate of pressure decline (-LVdP/dt, P < 0.001) and decreases in left ventricular end-diastolic pressure (P < 0.01), systolic arterial pressure (P < 0.001), diastolic arterial pressure (P < 0.001), mean arterial pressure (P < 0.001) and heart rate (P < 0.001). The myocardial injury caused by isoprenaline was significantly reduced by thalidomide treatment as judged by the reduction of myocardial necrosis, ultrastructural changes such as mitochondria and myofibril damage, 300 mg/kg being the most effective dose. In conclusion, oral administration of thalidomide is able to ameliorate isoprenaline-induced myocardial injury and impaired myocardial function in spite of decreases in systolic arterial pressure, diastolic arterial pressure and mean arterial pressure, which may be due to its depressant effect on the sino-atrial node and sedative action.

Elevated Levels of Both Cardiomyocyte Membrane and Myofibril Damage Markers Predict Adverse Outcomes in Patients With Chronic Heart Failure

Recent studies have shown the presence of ongoing myocardial damage in patients with chronic heart failure (CHF) detected by myofibril and membrane damage markers, cardiac troponin T (TnT) and heart-type fatty acid-binding protein (H-FABP), which identifies patients at increased risk of a future cardiac event (CE: death or rehospitalization because of worsening CHF). There is a difference between TnT and H-FABP in their release kinetics following myocardial damage. TnT and H-FABP were measured in 103 patients with CHF and in 31 controls. Patients were classified into 4 groups based on detectable (>or=0.01 ng/ml) or undetectable TnT (TnT+ or TnT-) and H-FABP >or= or <4.5 ng/ml (mean + 2 standard deviations in controls) (high-H-FABP or low-H-FABP). Kaplan-Meier analysis showed that the CE-free rate (n=43) was significantly lower in patients with TnT+ and high-H-FABP than in patients in the other 3 groups (patients with TnT+ and low-H-FABP, TnT- and high-H-FABP, and TnT- and low-H-FABP; p=0.02, p=0.001 and p=0.0002, respectively). In stepwise multivariate Cox proportional hazard analysis, TnT+ (p=0.01) and high-H-FABP (p=0.04) were independent predictors of future CE. Elevated levels of both TnT and H-FABP predict adverse outcomes in CHF patients.

Evidence for myofibril remodeling as opposed to myofibril damage in human muscles with DOMS: an ultrastructural and immunoelectron microscopic study

The myofibrillar and cytoskeletal alterations observed in delayed onset muscle soreness (DOMS) caused by eccentric exercise are generally considered to represent damage. By contrast our recent immunohistochemical studies suggested that the alterations reflect myofibrillar remodeling (Yu and Thornell 2002; Yu et al. 2003). In the present study the same human muscle biopsies were further analyzed with transmission electron microscopy and immunoelectron microscopy. We show that the ultrastructural hallmarks of DOMS, Z-disc streaming, Z-disc smearing, and Z-disc disruption were present in the biopsies and were significantly more frequent in biopsies taken 2-3 days and 7-8 days after exercise than in those from controls and 1 h after exercise. Four main types of changes were observed: amorphous widened Z-discs, amorphous sarcomeres, double Z-discs, and supernumerary sarcomeres. We confirm by immunoelectron microscopy that the main Z-disc protein alpha-actinin is not present in Z-disc alterations or in the links of electron-dense material between Z-discs in longitudinal register. These alterations were related to an increase of F-actin and desmin, where F-actin was present within the strands of amorphous material. Desmin, on the other hand, was seen in less dense regions of the alterations. Our results strongly support that the myofibrillar and cytoskeletal alterations, considered to be the hallmarks of DOMS, reflect an adaptive remodeling of the myofibrils.

Effects of mechanical ventilation on diaphragm function and biology.

The pathophysiological mechanisms of weaning from mechanical ventilation are not fully known, but there is accumulating evidence that mechanical ventilation induces inspiratory muscle dysfunction. Recently, several animal models have provided potential mechanisms for mechanical ventilation-induced effects on muscle function. In patients, weaning difficulties are associated with inspiratory muscle weakness and reduced endurance capacity. Animal studies demonstrated that diaphragm force was already decreased after 12 h of controlled mechanical ventilation and this worsened with time spent on the ventilator. Diaphragmatic myofibril damage observed after 3-days controlled mechanical ventilation was inversely correlated with maximal diaphragmatic force. Downregulation of the diaphragm insulin-like growth factor-I and MyoD/myogenin messenger ribonucleic acid occurred after 24 h and diaphragmatic oxidative stress and increased protease activity after 18 h. In keeping with these findings, diaphragm fibre atrophy was shown after 12 h and reduced diaphragm mass was reported after 48 h of controlled mechanical ventilation. These animal studies show that early alterations in diaphragm function develop after short-term mechanical ventilation. These alterations may contribute to the difficulties in weaning from mechanical ventilation seen in patients. Strategies to preserve respiratory muscle mass and function during mechanical ventilation should be developed. These may include: adaptation of medication, training of the diaphragm, stabilisation of the catabolic state and pharmacotherapy.

Altered diaphragm contractile properties with controlled mechanical ventilation

This study shows that, over time, diaphragm inactivity with controlled mechanical ventilation (CMV) decreases diaphragm force and produces myofibril damage contributing to the reduced force. We measured in vivo and in vitro diaphragm contractile and morphological properties in 30 sedated rabbits grouped (n = 6) as follows: 1 or 3 days of CMV, 1 or 3 days of 0 cmH(2)O continuous positive airway pressure, and control. The CMV rate was set sufficient to suppress diaphragm electrical activity. Compared with the control group, phrenic-stimulated maximum transdiaphragmatic pressure did not decrease with continuous positive airway pressure but decreased to 63% after 1 day of CMV and to 49% after 3 days of CMV. The in vitro tetanic force decreased to 86% after 1 day of CMV and to 44% after 3 days of CMV. After 3 days of CMV, significant myofibril damage occurred in the diaphragm but not in the soleus. The decrease in tetanic force correlated with the volume density of abnormal myofibrils. We conclude that CMV had a detrimental effect on diaphragm contractile properties.

EFFECTS OF ESTRADIOL ON CREATINE KINASE RELEASE AND SUBSEQUENT MUSCLE DAMAGE IN THE RAT 594

The mechanisms associated with delayed onset muscle soreness (DOMS), including an increase in serum creatine kinase (CK) activity, and subsequent damage to the contractile filaments of the myofibrils remain unknown. Females have been shown to exhibit a lower serum CK activity post-exercise compared to males. This gender difference has been linked to the female sex hormone estradiol (E2). The effects of E2 on CK release and muscle damage was assessed in female rats following eccentric treadmill exercise. Sprague Dawley female rats were ovariectomized prior to sexual maturity and treated with an E2 (n=5) or placebo (n=4) pellet insert for 21 days prior to the exercise bout. Exercise consisted of 60 minutes of -10° treadmill running at a speed of 19 m·min-1. Serum samples for CK activity (U/L @ 30°C) were obtained pre-exercise, and at times 0, 2, 6, 12, 24, and 48 hours post-exercise. Following the 48 hour post-exercise blood sample, rats were sacrificed and the soleus muscle was disected and prepared for light and electron microscopy. Prior to exercise, E2 levels were significantly higher in the E2 treated animals compared to placebo treated animals. Compared to pre-exercise, CK release in the placebo rats was significantly (p<0.05) elevated at times 0, 2, 6, and 24 hours post-exercise, with the peak response occurring at immediately post-exercise (time 0). Compared to pre-exercise, CK release in the E2 treated rats was significantly (p<0.05) elevated at times 0, and 2 hours post-exercise, with the peak response occuring 2 hours post-exercise. Placebo rats exhibited significantly greater CK activities compared to E2 treated rats at time points 0, 6, and 24 hours post-exercise. Evidence of microscopic damage (A-band disruption and Z-line streaming) was limited to two placebo treated animals only. Therefore, it is difficult to speculate on the precise protective mechanism that E2 may exhert on the muscle. The effects of E2 appear limited to CK release and not directly related to that of myofibril damage.