Diaphragm Specific Force Research Articles

A novel effect of eicosapentaenoic acid: improved diaphragm strength in endotoxemia

Respiratory muscle weakness is commonplace in critically ill patients, impairing the ability of those patients to breath, prolonging the need for ventilatory support, and increasing the likelihood of respiratory failure when that support is removed. Infections and endotoxemia reduce respiratory muscle strength, probably acting through several mechanisms. It is reported that the omega-3 fatty acid eicosapentaenoic acid (EPA) attenuates the loss in diaphragm specific force generation (that is, diaphragm strength) induced by bacterial endotoxin treatment in rats. EPA is found in fish oils. EPA reduces calpain activation, suggesting a specific effect on this proteolytic pathway. It will be important to identify whether this effect occurs in patients receiving EPA.

Calpain activation contributes to endotoxin-induced diaphragmatic dysfunction.

Calpain activation occurs in skeletal muscle in response to infection, but it is unknown if calpain inhibition improves muscle functional capacity. We hypothesized that infection induces diaphragm calpain activation, that calpain activation results in cleavage of important diaphragm cytoskeletal proteins, and that inhibition of calpain attenuates infection-induced diaphragm dysfunction. Mice (n = 4-6/group) were given: (1) saline (intraperitoneal); (2) endotoxin (12 mg/kg intraperitoneal); (3) calpain inhibitor peptide III (12 mg/kg intraperitoneal); and (4) endotoxin (12 mg/kg) plus calpain inhibitor peptide III (12 mg/kg). At 24 hours, diaphragms were removed and the following determined: (1) calpain activity by fluorogenic assay; (2) calpain I and II protein levels; (3) talin protein levels; and (4) the force-frequency relationship. Endotoxin significantly increased diaphragm calpain activity (P < 0.001), active calpain I protein (P < 0.001), active calpain II protein (P < 0.01), levels of a calpain-specific cleavage talin degradation product (P < 0.003), and reduced diaphragm force (P < 0.001). Calpain inhibitor III administration prevented endotoxin-induced increases in calpain activity, reduced talin degradation, and attenuated reductions in diaphragm force. Diaphragm-specific force at 150 Hz stimulation was significantly higher in control, endotoxin plus calpain inhibitor III, and calpain inhibitor III alone groups (23 +/- 1, 20 +/- 1 and 23 +/- 1 N/cm(2), respectively) than in the endotoxin alone group (15 +/- 1 N/cm(2)) (P < 0.01). This model of sepsis results in significant diaphragm calpain activation and calpain-dependent diaphragm cytoskeletal protein cleavage. Moreover, calpain inhibition attenuates endotoxin-induced diaphragm weakness, suggesting that such inhibitors may be a potential treatment to improve respiratory muscle function in infected patients.

EFFECTS OF CHRONIC HYPOXIA ON UPPER AIRWAY AND DIAPHRAGM MUSCLE ENDURANCE IN THE DEVELOPING RAT

Chronic hypoxia (CH) is common in respiratory disease and is known to alter skeletal muscle structure and oxidative capacity. We hypothesized that CH in early development would alter respiratory muscle function. Wistar rats were reared in normobaric normoxia or hypobaric hypoxia (PB=450mmHg) for 7 days beginning at P1, P11, P21 or P31. Isolated muscle function studies were conducted at P19, P29 or P39. Fatigue was assessed in response to repeated tetanic contractions (40Hz, 300msec) every 2 sec for 5 minutes. Chronic hypoxia during early development (beginning P1 and P11) increased sternohyoid specific force but significantly reduced muscle endurance (fatigue index ie ratio of force at 5 min of fatigue to initial force, measured at P19 was 83+/−7% vs. 59+/−7%* and 63+/−4%*; control vs. P1 and P11 hypoxic groups, *P&lt;0.05 ANOVA). CH in older animals did not affect sternohyoid endurance. CH increased diaphragm specific force but had no effect on muscle endurance (eg fatigue index at P19 was 81+/−6% vs. 76+/−4% and 88+/−4%; control vs. P1 and P11 hypoxic groups, P&gt;0.05 ANOVA). We conclude that CH elicits plasticity in respiratory muscles with differential effects in diaphragm and upper airway muscles. Increased fatigue of the sternohyoid muscle may have important consequences for the control of upper airway patency in vivo.

Corticortophin releasing factor 2 receptor agonist treatment significantly slows disease progression in mdx mice

BackgroundDuchenne muscular dystrophy results from mutation of the dystrophin gene, causing skeletal and cardiac muscle loss of function. The mdx mouse model of Duchenne muscular dystrophy is widely utilized to evaluate the potential of therapeutic regimens to modulate the loss of skeletal muscle function associated with dystrophin mutation. Importantly, progressive loss of diaphragm function is the most consistent striated muscle effect observed in the mdx mouse model, which is the same as in patients suffering from Duchenne muscular dystrophy.MethodsUsing the mdx mouse model, we have evaluated the effect that corticotrophin releasing factor 2 receptor (CRF2R) agonist treatment has on diaphragm function, morphology and gene expression.ResultsWe have observed that treatment with the potent CRF2R-selective agonist PG-873637 prevents the progressive loss of diaphragm specific force observed during aging of mdx mice. In addition, the combination of PG-873637 with glucocorticoids not only prevents the loss of diaphragm specific force over time, but also results in recovery of specific force. Pathological analysis of CRF2R agonist-treated diaphragm muscle demonstrates that treatment reduces fibrosis, immune cell infiltration, and muscle architectural disruption. Gene expression analysis of CRF2R-treated diaphragm muscle showed multiple gene expression changes including globally decreased immune cell-related gene expression, decreased extracellular matrix gene expression, increased metabolism-related gene expression, and, surprisingly, modulation of circadian rhythm gene expression.ConclusionTogether, these data demonstrate that CRF2R activation can prevent the progressive degeneration of diaphragm muscle associated with dystrophin gene mutation.

Leupeptin Inhibits Ventilator-induced Diaphragm Dysfunction in Rats

Controlled mechanical ventilation (CMV) has been shown to result in elevated diaphragmatic proteolysis and atrophy together with diaphragmatic contractile dysfunction. To test whether administration of leupeptin, an inhibitor of lysosomal proteases and calpain, concomitantly with 24 hours of CMV, would protect the diaphragm from the deleterious effects of mechanical ventilation. Rats were assigned to either a control group or 24 hours of CMV; animals in the ventilation group received either a single intramuscular injection of saline or 15 mg/kg of the protease inhibitor, leupeptin. Compared with control animals, mechanical ventilation resulted in a significant reduction of the in vitro diaphragm-specific force production at all stimulation frequencies. Leupeptin completely prevented this reduction in force generation. Atrophy of type IIx/b fibers was present after CMV, but not after treatment with leupeptin. Cathepsin B and calpain activities were significantly higher after CMV compared with the other groups; this was abolished by treatment with leupeptin. Significant inverse correlations were found between diaphragmatic force generation and cathepsin B and calpain activity, and illustrate the deleterious role of proteolysis in diminishing diaphragmatic force production after prolonged CMV. Administration of the protease inhibitor leupeptin concomitantly with mechanical ventilation completely prevented ventilation-induced diaphragmatic contractile dysfunction and atrophy.