Heparan sulfate deficiency disrupts developmental angiogenesis and causes congenital diaphragmatic hernia

Congenital diaphragmatic hernia (CDH) is a common birth malformation with a heterogeneous etiology. In this study, we report that ablation of the heparan sulfate biosynthetic enzyme NDST1 in murine endothelium (Ndst1ECKO mice) disrupted vascular development in the diaphragm, which led to hypoxia as well as subsequent diaphragm hypoplasia and CDH. Intriguingly, the phenotypes displayed in Ndst1ECKO mice resembled the developmental defects observed in slit homolog 3 (Slit3) knockout mice. Furthermore, introduction of a heterozygous mutation in roundabout homolog 4 (Robo4), the gene encoding the cognate receptor of SLIT3, aggravated the defect in vascular development in the diaphragm and CDH. NDST1 deficiency diminished SLIT3, but not ROBO4, binding to endothelial heparan sulfate and attenuated EC migration and in vivo neovascularization normally elicited by SLIT3-ROBO4 signaling. Together, these data suggest that heparan sulfate presentation of SLIT3 to ROBO4 facilitates initiation of this signaling cascade. Thus, our results demonstrate that loss of NDST1 causes defective diaphragm vascular development and CDH and that heparan sulfate facilitates angiogenic SLIT3-ROBO4 signaling during vascular development.

Amplification of proinflammatory phenotype, damage, and weakness by oxidative stress in the diaphragm muscle of mdx mice

Duchenne muscular dystrophy (DMD) is a common and devastating type of childhood-onset muscular dystrophy, attributed to an X-linked defect in the gene that encodes dystrophin. Myopathy with DMD is most pronounced in the diaphragm muscle and fast-twitch limb muscles and is dependent upon susceptibility to damage, inflammatory cell infiltration, and proinflammatory signaling (nuclear factor-κB; NF-κB). Although recent papers have reawakened the notion that oxidative stress links inflammatory signaling with pathology in DMD in limb muscle, the importance of redox mechanisms had been clouded by inconsistent results from indirect scavenger approaches, including in the diaphragm muscle. Therefore, we used a novel catalytic mimetic of superoxide dismutase and catalase (EUK-134) as a direct scavenger of oxidative stress in myopathy in the diaphragm of the mdx mouse model. EUK-134 reduced 4-hydroxynonenal and total hydroperoxides, markers of oxidative stress in the mdx diaphragm. EUK-134 also attenuated positive staining of macrophages and T-cells as well as activation of NF-κB and p65 protein abundance. Moreover, EUK-134 ameliorated markers of muscle damage including internalized nuclei, variability of cross-sectional area, and type IIc fibers. Finally, impairment of contractile force was partially rescued by EUK-134 in the diaphragm of mdx mice. We conclude that oxidative stress amplifies DMD pathology in the diaphragm muscle.

[Effects of chronic nicotine exposure on electrogenic activity of the Na+, K(+)-ATPase and contractility in the rat diaphragm muscle]

Rats were chronically treated with nicotine via subcutaneous injections up to a dose 6 mg/kg/day during 2-3 weeks. After this period, resting membrane potential and action potentials of muscle fibres as well as isometric twitch and tetanic (20 s(-1) and 50(-1)) contractions of isolated rat diaphragm were studied. To estimate electrogenic contribution of the alpha2 isoform of the Na+, K(+)-ATPase ouabain in concentration 1 microM was used. Chronic nicotine exposure induced depolarization of resting membrane potential of 2.2 +/- 0.6 mV (p < 0.01). In rats chronically exposed to nicotine, electrogenic contribution of the Na+, K(+)-ATPase alpha2 isoform was twofold lesser than in control animals (3.7 +/- 0.6 mV and 6.4 +/- 0.6 mV, respectively, p < 0.01). Chronic nicotine exposure did not affect force of twitch and tetanic contractions in response to direct or indirect stimulation. A decrease in the twitch contraction time as well as in the rise time of tetanic contractions was observed. Fatigue dynamics was unchanged. The results suggest that chronic nicotine exposure leads to decrease of the Na+, K(+)-ATPase alpha2 isoform electrogenic activity, and as a consequence to damage of the rat diaphragm muscle electogenesis.

[Changes in the diaphragm muscle and its feed artery after chronic respiratory airway obstruction in rats]

A chronic respiratory load was produced in Wistar rats by tracheal binding to produce a twofold increase of pleural pressure oscillation amplitude during respiration. Eight weeks after the surgery, a higher proportion of type-I muscle fibers (MFI) in the costal diaphragm along with a greater MFI cross-section area and a higher succinate dehydrogenase activity in MFII in the crural diaphragm were observed. During recording the mechanical activity of ring preparations of diaphragm arteries under isometric conditions, an increase in endothelium-dependent relaxation was found, whereas endothelium-independent relaxation and arterial reactivity to noradrenaline did not change. Tracheal binding did not produce any changes of MF in the gastrocnemius muscle, but endothelium-dependent relaxation of gastrocnemius feed arteries was reduced. We conclude that chronic respiratory load affects the endothelial function in diaphragm arteries in a manner favorable for blood flow control in the diaphragm. Functional alterations in gastrocnemius arteries may be associated with the reduced locomotor activity of operated rats.

Expression of superoxide dismutase and matrix metalloproteinase type 2 in diaphragm muscles of young rats

Moderate physical activity increases antioxidant defenses, whereas intensive activity is associated with oxidative stress. In this study we investigated the expression of superoxide dismutase (Cu,Zn-SOD), a major antioxidant defense enzyme, and that of the proteolytic enzyme matrix metalloproteinase-2 (MMP-2) in exercising muscle tissue. Treadmill running was used as a model to investigate the mechanism involved in muscle use and over use. Sprague-Dawley female rats (4 months old) were randomly assigned to 3 groups: running group I, trained at a slow speed (18 m/min; approximately 50% VO(2)), running group II, trained at a very fast speed (32 m/min; approximately 75% VO(2)), for 3 weeks, and group III - control, non-running group. Cu,Zn-SOD was measured spectrophotometrically at 320 nm by assessing the inhibition of cytochrome c reduction by xanthine oxidase. MMP-2 levels of protein and mRNA were assessed in the diaphragm by Western blotting and by reverse transcriptase-polymerase chain reaction, respectively. We found that Cu,Zn-SOD level significantly decreased in the crural diaphragm muscle of rats three weeks after fast speed running, whereas it remained unchanged in the sternal diaphragm muscle three weeks after slow speed running. The expression of MMP-2 increased in both fast and slow running groups; however, it was particularly prominent in the fast twitch muscle fibers type IIb. We conclude that the crural diaphragm muscle, which contains significantly more type IIb fibers, was more affected following fast speed running than the sternal/costal diaphragm muscles, which have an equal distribution of slow twitch (type I) and fast twitch (type IIb) muscle fibers.

Habitual exercise induced resistance to oxidative stress

We investigated whether habitual exercise (HE) modulates levels of oxidative DNA damage and responsiveness to oxidative stress induced by renal carcinogen Fe-nitrilotriacetic acid (Fe-NTA). During a ten week protocol, two groups of rats either remained sedentary or underwent swimming for 15--60 min per day, 5 days per week, with or without a weight equivalent to 5% of their body weight. Then we injected Fe-NTA and sacrificed the rats 1 h after the injection. We determined the activity of superoxide dismutase (SOD) in diaphragm and kidney, evaluated levels of 8-hydroxydeoxyguanosine (8OHdG), catalase, and glutathione peroxidase, and assayed OGG1 protein levels in kidney. SOD activity in the diaphragm and kidney was increased in HE rats. By itself, HE had no effect on the level of 8OHdG, but it did significantly suppress induction of 8OHdG by Fe-NTA, and the amount of suppression correlated with intensity of exercise. These results suggest that HE induces resistance to oxidative stress and, at least at the initiation stage, inhibits carcinogenesis.

Pyruvate dehydrogenase kinase-4 deficiency lowers blood glucose and improves glucose tolerance in diet-induced obese mice

The effect of pyruvate dehydrogenase kinase-4 (PDK4) deficiency on glucose homeostasis was studied in mice fed a high-fat diet. Expression of PDK4 was greatly increased in skeletal muscle and diaphragm but not liver and kidney of wild-type mice fed the high-fat diet. Wild-type and PDK4(-/-) mice consumed similar amounts of the diet and became equally obese. Insulin resistance developed in both groups. Nevertheless, fasting blood glucose levels were lower, glucose tolerance was slightly improved, and insulin sensitivity was slightly greater in the PDK4(-/-) mice compared with wild-type mice. When the mice were killed in the fed state, the actual activity of the pyruvate dehydrogenase complex (PDC) was higher in the skeletal muscle and diaphragm but not in the liver and kidney of PDK4(-/-) mice compared with wild-type mice. When the mice were killed after overnight fasting, the actual PDC activity was higher only in the kidney of PDK4(-/-) mice compared with wild-type mice. The concentrations of gluconeogenic substrates were lower in the blood of PDK4(-/-) mice compared with wild-type mice, consistent with reduced formation in peripheral tissues. Diaphragms isolated from PDK4(-/-) mice oxidized glucose faster and fatty acids slower than diaphragms from wild-type mice. Fatty acid oxidation inhibited glucose oxidation by diaphragms from wild-type but not PDK4(-/-) mice. NEFA, ketone bodies, and branched-chain amino acids were elevated more in PDK4(-/-) mice, consistent with slower rates of oxidation. These findings show that PDK4 deficiency lowers blood glucose and slightly improves glucose tolerance and insulin sensitivity in mice with diet-induced obesity.

Effects of buthionine sulfoximine treatment on diaphragm contractility and SR Ca2+ pump function in rats

The purpose of this study was to examine the effects of glutathione (GSH) depletion and cellular oxidation on rat diaphragm contractility and sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) function in vitro under basal conditions and following fatiguing stimulation. Buthionine sulfoximine (BSO) treatment (n = 10) for 10 days (20 mM in drinking water) reduced (P < 0.05) diaphragm GSH content (nmol/mg protein) and the ratio of GSH to glutathione disulfide (GSH/GSSG) by 91% and 71%, respectively, compared with controls (CTL) (n = 10). Western blotting showed that Hsp70 expression in diaphragm was not increased (P > 0.05) with BSO treatment. As hypothesized, basal peak twitch force (g/mm(2)) was increased (P < 0.05), and fatigability in response to repetitive stimulation (350-ms trains at 100 Hz once every 1 s for 5 min) was also increased (P < 0.05) in BSO compared with CTL. Both Ca(2+) uptake and maximal SERCA activity (mumol.g protein(-1).min(-1)) measured in diaphragm homogenates that were prepared at rest were increased (P < 0.05) with BSO treatment, an effect that could be partly explained by a twofold increase (P < 0.05) in SERCA2a expression with BSO. In response to the 5-min stimulation protocol, both Ca(2+) uptake and maximal SERCA activity were increased (P < 0.05) in CTL but not (P > 0.05) in BSO diaphragm. We conclude that 1) cellular redox state is more optimal for contractile function and fatigability is increased in rat diaphragm following BSO treatment, 2) SERCA2a expression is modulated by redox signaling, and 3) regulation of SERCA function in working diaphragm is altered following BSO treatment.

Diaphragmatic proteasome function is maintained in the ageing Fisher 344 rat

The diaphragm is the most important inspiratory muscle in mammals and is essential for normal ventilation. Therefore, maintenance of diaphragm function is critical to overall health throughout the lifespan. Evidence indicates that the ubiquitin proteasome pathway (UPP) function is diminished in locomotor skeletal muscle of ageing animals, but the function of the UPP in the senescent diaphragm has not yet been studied. Diaphragms were harvested from 6- and 24- to 26-month-old Fisher 344 rats (n = 8 per group), and a comprehensive assessment of key components of the UPP, proteasome activity and ubiquitin-conjugating enzyme activity was performed. Gene expression and diaphragmatic protein levels of several key proteasome components are not altered in the diaphragm by ageing. Furthermore and most importantly, the senescent diaphragm exhibited no age-related changes in the content of endogenous ubiquitin-protein conjugates or 20S proteasome activity. In conclusion, in contrast to locomotor skeletal muscle, proteasome function and ubiquitin-conjugating enzyme activity are preserved during senescence in diaphragm. A more thorough understanding of the divergent molecular mechanisms and pathways regulating the UPP in different skeletal muscles could lead to the enhancement of therapeutic strategies aimed at improving morbidity and mortality outcomes in different clinical populations.

Role of protein kinase C in the effect of ATP on contractile function of the isolated strip from mouse diaphragm

We studied the effects of adenosine and ATP on contractile function of the isolated strip from mouse diaphragm. ATP significantly increased the strength of muscle contraction induced by carbachol. Adenosine had no effect on carbachol-induced muscle contraction. P2 receptor antagonist suramin abolished the effect of ATP. The positive chronotropic effect of ATP was not observed after treatment with specific protein kinase C inhibitor chelerythrine. Our results indicate that the effect of ATP on contractile function of mouse diaphragm is realized via protein kinase C.

MicroRNA-206 is overexpressed in the diaphragm but not the hindlimb muscle of mdx mouse

MicroRNAs are highly conserved, noncoding RNAs involved in posttranscriptional gene silencing. MicroRNAs have been shown to be involved in a range of biological processes, including myogenesis and muscle regeneration. The objective of this study was to test the hypothesis that microRNA expression is altered in dystrophic muscle, with the greatest change occurring, of the muscles examined, in the diaphragm. The expression of the muscle-enriched microRNAs was determined in the soleus, plantaris, and diaphragm muscles of control and dystrophin-deficient ( mdx) mice by semiquantitative PCR. In the soleus and plantaris, expression of the mature microRNA 133a (miR-133a) and miR-206, respectively, was decreased by ∼25%, whereas in the diaphragm, miR-206 expression increased by 4.5-fold relative to control. The increased expression of miR-206 in the mdx diaphragm was paralleled by a 4.4-fold increase in primary miRNA-206 (pri-miRNA-206) transcript level. Expression of Myod1 was elevated 2.7-fold only in the mdx diaphragm, consistent with an earlier finding demonstrating Myod1 can activate pri-miRNA-206 transcription. Transcript levels of Drosha and Dicer, major components of microRNA biogenesis pathway, were unchanged in mdx muscle, suggesting the pathway is not altered under dystrophic conditions. Previous in vitro analysis found miR-206 was capable of repressing utrophin expression; however, under dystrophic conditions, both utrophin transcript and protein levels were significantly increased by 69% and 3.9-fold, respectively, a finding inconsistent with microRNA regulation. These results are the first to report alterations in expression of muscle-enriched microRNAs in skeletal muscle of the mdx mouse, suggesting microRNAs may have a role in the pathophysiology of muscular dystrophy.

Leupeptin Inhibits Ventilator-induced Diaphragm Dysfunction in Rats

RATIONALE

Controlled mechanical ventilation (CMV) has been shown to result in elevated diaphragmatic proteolysis and atrophy together with diaphragmatic contractile dysfunction.

OBJECTIVES

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.

METHODS

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.

MEASUREMENTS AND MAIN RESULTS

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.

CONCLUSIONS

Administration of the protease inhibitor leupeptin concomitantly with mechanical ventilation completely prevented ventilation-induced diaphragmatic contractile dysfunction and atrophy.

An age-dependent physiologically based pharmacokinetic/pharmacodynamic model for the organophosphorus insecticide chlorpyrifos in the preweanling rat

Juvenile rats are more susceptible than adults to the acute toxicity of organophosphorus insecticides like chlorpyrifos (CPF). Age- and dose-dependent differences in metabolism may be responsible. Of importance are CYP450 activation and detoxification of CPF to chlorpyrifos-oxon (CPF-oxon) and trichloropyridinol (TCP), as well as B-esterase (B-est) and PON-1 (A-esterase) detoxification of CPF-oxon to TCP. In the current study, a physiologically based pharmacokinetic/pharmacodynamic (PBPK/PD) model incorporating age-dependent changes in CYP450, PON-1, and tissue B-est levels for rats was developed. In this model, age was used as a dependent function to estimate body weight which was then used to allometrically scale both metabolism and tissue cholinesterase (ChE) levels. In addition, age-dependent changes in brain, liver, and fat volumes and brain blood flow were obtained from the literature and used in the simulations. Model simulations suggest that preweanling rats are particularly sensitive to CPF toxicity, with levels of CPF-oxon in blood and brain disproportionately increasing, relative to the response in adult rats. This age-dependent nonlinear increase in CPF-oxon concentration may potentially result from both the depletion of nontarget B-est and a lower PON-1 metabolic capacity in younger animals. The PBPK/PD model behaves consistently with the general understanding of CPF toxicity, pharmacokinetics, and tissue ChE inhibition in neonatal and adult rats. Hence, this model represents an important starting point for developing a computational model to assess the neurotoxic potential of environmentally relevant organophosphate exposures in infants and children.

Differential sensitivity of blood, peripheral, and central cholinesterases in beagle dogs following dietary exposure to chlorpyrifos

Two studies were performed to find out whether exposure limits that protect brain acetylcholinesterase (AChE) will protect peripheral tissue AChE after exposure to chlorpyrifos (CPF), an organophosphate insecticide. In a methods-development study, male dogs (3/dose) were exposed to 0.0, 0.3, 0.6, or 1.2mg/kg/day CPF in their diets for 4 weeks. Mixed cholinesterase (mChE), AChE, and butyrylcholinesterase (BuChE) activities were measured in plasma, RBC, brain, left atrium and ventricle, diaphragm, quadriceps, and nodose ganglia. Plasma, brain and peripheral tissue BuChE was inhibited at all dose levels. While RBC AChE was inhibited at all doses, brain and peripheral AChE activities were unaffected. In the main study, dogs (4/sex/dose) were exposed to 0.0, 0.5, 1.0, or 2.0mg/kg/day CPF in their diets for six weeks and RBC AChE was significantly inhibited at all doses in both sexes. Diaphragm, quadriceps, and nodose ganglia AChE was unaffected by treatment. Brain AChE was decreased by approximately 6% compared to controls in high-dose groups, and this was considered a threshold effect. Left atrium AChE in high-dose dogs was 25.5% less (males) and 32.1% greater (females) than controls; these differences were attributed to chance. While peripheral tissue and brain AChE were not affected following exposure to 1.0mg/kg/day, RBC AChE was inhibited at all doses. These results show that RBC AChE is more sensitive than brain or peripheral tissue AChE to inhibition by CPF, and that protection of brain AChE would protect peripheral tissue AChE.

Modulation of p38 mitogen-activated protein kinase cascade and metalloproteinase activity in diaphragm muscle in response to free radical scavenger administration in dystrophin-deficient Mdx mice

Duchenne muscular dystrophy muscles undergo increased oxidative stress and altered calcium homeostasis, which contribute to myofiber loss by trigging both necrosis and apoptosis. Here, we asked whether treatment with free radical scavengers could improve the dystrophic pattern of mdx muscles. Five-week-old mdx mice were treated for 2 weeks with alpha-lipoic acid/l-carnitine. This treatment decreased the plasmatic creatine kinase level, the antioxidant enzyme activity, and lipid peroxidation products in mdx diaphragm. Free radical scavengers also modulated the phosphorylation/activity of some component of the mitogen-activated protein kinase (MAPK) cascades: p38 MAPK, the extracellular signal-related kinase, and the Jun kinase. beta-Dystroglycan (beta-DG), a multifunctional adaptor or scaffold capable of interacting with components of the extracellular signal-related kinase-MAP kinase cascade, was also affected after treatment. In the mdx muscles, beta-DG (43 kd) was cleaved by matrix metalloproteinases into a 30-kd form (beta-DG30). We show that the proinflammatory protein nuclear factor-kappaB activator decreased after the treatment, leading to a significant reduction of matrix metalloproteinase activity in the mdx diaphragm. Our data highlight the implication of oxidative stress and cell signaling defects in dystrophin-deficient muscle via the MAP kinase cascade-beta-DG interaction and nuclear factor-kappaB-mediated inflammation process.

Inhibition of blood cholinesterases following intoxication with VX and its derivatives

Nerve agents can be divided into G-agents (sarin, soman, tabun, cyclosarin etc.) and V-agents. The studies dealing with V-agents (O-alkyl S-2-dialkylaminoethyl methyl phosphonothiolates) are limited to one or two representatives only (VX, Russian VX). Anticholinesterase properties of 11 V-agents were studied in rats in vivo. Following intoxication with these agents in doses of 1 x LD(50) (intramuscular administration), activities of cholinesterases in the blood were continuously monitored and half-lives (t(0.5)) of inhibition were determined. These values varied from 3 min (VX and some other agents) to 10-14 min (derivatives substituted on the phosphorus head by O-ethyl- or O-isopropyl-, and by dimethyl-, diethyl- and dibutyl- on the nitrogen). Acetylcholinesterase activities in selected parts of the brain and diaphragm (30 min after the intoxication) were also detected. A correlation between toxicities and rates of inhibition of the blood enzymes was demonstrated. A similar relationship between acetylcholinesterase inhibition in vitro (from literature data) and half-lives of the blood cholinesterases was also observed. Though the chemical similarity of V compounds is evident, marked differences were observed among different derivatives; however, all agents examined had high inhibition potency corresponding to their toxicities.

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

RATIONALE

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

OBJECTIVE

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

METHODS

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

MEASUREMENTS AND MAIN RESULTS

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

CONCLUSIONS

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

A comparison of the efficacy of new asymmetric bispyridinium oximes (K027, K048) with currently available oximes against tabun by in vivo methods

The potency of newly developed asymmetric bispyridinium oximes (K027, K048) in reactivating tabun-inhibited acetylcholinesterase (AChE) and in eliminating tabun-induced acute toxic effects was compared with commonly used oximes (obidoxime, trimedoxime, the oxime HI-6) using in vivo methods. Studies determined the percent of reactivation of tabun-inhibited blood and tissue AChE in poisoned rats and showed that the reactivating efficacy of both newly developed oximes is comparable with obidoxime and trimedoxime, the most efficacious known reactivators of tabun-inhibited AChE. These were also found to be sufficiently efficacious in the elimination of acute lethal toxic effects in tabun-poisoned rats. The oxime HI-6, relatively efficacious against soman, did not seem to be an adequately effective oxime in reactivation of tabun-inhibited AChE and in counteracting acute lethal effects of tabun. In addition, our results confirm that the efficacy of oximes in reactivating tabun-inhibited AChE in blood, diaphragm, and brain correlates with the potency of oximes in protecting rats poisoned with supralethal doses of tabun.

Effects of combined, multiple stressors on pyridostigmine-induced acute toxicity in rats

A number of studies have evaluated the possibility that stress-induced changes in blood-brain barrier permeability enhanced the central effects of the carbamate acetylcholinesterase inhibitor, pyridostigmine. We previously found relatively little evidence of stress-induced changes in the acute toxicity of pyridostigmine in rats using a variety of restraint, forced running and forced swimming stress conditions. In this study, we evaluated the effects of sequential pre-exposure to multiple stressors on the acute toxicity of pyridostigmine. Rats (n = 8 per treatment group) were either un-stressed or stressed by restraint (60 min), forced running (60 min, 15 m/min, 6 degrees incline) and forced swimming (15 min), and then given either vehicle (saline, 1 ml/kg, po) or pyridostigmine (30 mg/kg, po) immediately after the final stressor. Functional signs of cholinergic toxicity (involuntary movements, autonomic dysfunction) were recorded at 0.5, 1 and 2 h after dosing. Body temperature was measured both before stress and 2 h after dosing. Rats were sacrificed immediately after 2-h functional observations to collect tissues (whole blood, diaphragm, frontal cortex, hippocampus and cerebellum) for measurement of cholinesterase activity. Stressed rats treated with pyridostigmine exhibited higher lethality (2/8) compared to unstressed rats given pyridostigmine (0/8). Pyridostigmine elicited classical signs of cholinergic toxicity, but the rats that died did not show increased cholinergic signs and no significant differences in cholinergic signs were noted between treatment groups. Cholinesterase activity was significantly inhibited in blood (47-50%) and diaphragm (80%) following pyridostigmine exposure regardless of stress conditions. Slight but significant inhibition (11-15%) of cerebellar cholinesterase activity was observed following pyridostigmine exposure, but inhibition was not influenced by stress. We conclude that while acute lethality from pyridostigmine may be increased by combined, multiple stressors, increased lethality does not appear due to enhanced cholinergic toxicity or via increased cholinesterase inhibition in either central or peripheral tissues.

Diaphragmatic nitric oxide synthase is not induced during mechanical ventilation

Mechanical ventilation (MV) is associated with diaphragmatic oxidative stress that contributes to both diaphragmatic atrophy and contractile dysfunction. However, the pathways responsible for oxidant production in the diaphragm during MV remain unknown. To address this issue, we tested the hypothesis that diaphragmatic nitric oxide synthase (NOS) activity is elevated during MV, resulting in nitration of diaphragmatic proteins. Rats were mechanically ventilated for 18 h, and time-matched, anesthetized but spontaneously breathing animals served as controls. Protein levels of endothelial NOS, inducible NOS, and neuronal NOS were measured in diaphragms from all animals. 3-Nitrotyrosine levels were also measured as an index of protein nitration, and S-nitrosothiol levels were measured as a marker of nitric oxide reactions with molecules containing sulfhydryl groups. Levels of nitrates and nitrites were measured as markers of stable end products of nitric oxide metabolism. Finally, as a marker of oxidative stress, diaphragmatic levels of reduced GSH were also analyzed. MV did not promote an increase in diaphragmatic protein levels of endothelial NOS or neuronal NOS. Moreover, inducible NOS was not detected in the diaphragms of either experimental group. Consistent with these findings, MV did not elevate diaphragmatic 3-nitrotyrosine levels in any subcellular fraction of the diaphragm, including the cytosolic, mitochondrial, membrane, and insoluble protein fractions. Moreover, prolonged MV did not elevate diaphragmatic levels of S-nitrosothiols, nitrate, or nitrite. Finally, prolonged MV significantly reduced diaphragmatic levels of GSH, which is consistent with diaphragmatic oxidative stress. Collectively, these data reveal that MV-induced oxidative stress in the diaphragm is not due to increases in nitric oxide production by NOS.

Caspase activation contributes to endotoxin-induced diaphragm weakness

Infections produce significant respiratory muscle weakness, but the mechanisms by which inflammation reduces muscle force remain incompletely understood. Recent work suggests that caspase 3 releases actin and myosin from the contractile protein lattice, so we postulated that infections may reduce skeletal muscle force by activating caspase 3. The present experiments were designed to test this hypothesis by determining 1) diaphragm caspase 3 activation in the diaphragm after endotoxin and 2) the effect of a broad-spectrum caspase inhibitor, Z-Val-Ala-Asp(OCH3)-fluoromethylketone (zVAD-fmk), and a selective caspase 3 inhibitor, N-acetyl-Asp-Glu-Val-Asp-al (DEVD-CHO), on endotoxin-induced diaphragm weakness. Caspase 3 activation was assessed by measuring caspase protein levels and by measuring cleavage of a fluorogenic substrate. Diaphragm force was measured in response to electrical stimulation (1-150 Hz). Caspase-mediated spectrin degradation was assessed by Western blotting. Parameters were compared in mice given saline, endotoxin (12 mg/kg ip), endotoxin plus zVAD-fmk (3 mg/kg iv), zVAD-fmk alone, or endotoxin plus DEVD-CHO (3 mg/kg iv). Endotoxin increased diaphragm active caspase 3 protein (P<0.003), increased caspase 3 activity (P<0.002), increased diaphragm spectrin degradation (P<0.001), and reduced diaphragm force (P<0.001). Administration of zVAD-fmk or DEVD-CHO prevented endotoxin-induced weakness (e.g., force in response to 150-Hz stimulation was 23.8+/-1.4, 12.1+/-1.3, 23.5+/-0.8, 22.7+/-1.3, and 24.4+/-0.8 N/cm2, respectively, for control, endotoxin, endotoxin plus zVAD-fmk, endotoxin plus DEVD-CHO, and zVAD-fmk alone treated groups, P<0.001). Caspase inhibitors also prevented spectrin degradation. In conclusion, endotoxin administration elicits significant diaphragm caspase 3 activation and caspase-mediated diaphragmatic weakness.

Sustained correction of glycogen storage disease type II using adeno-associated virus serotype 1 vectors

Glycogen storage disease type II (GSDII) is caused by a lack of functional lysosomal acid alpha-glucosidase (GAA). Affected individuals store glycogen in lysosomes beginning during gestation, ultimately resulting in fatal hypertrophic cardiomyopathy and respiratory failure. We have assessed the utility of recombinant adeno-associated virus (rAAV) vectors to restore GAA activity in vivo in a mouse model of GSDII (Gaa(-/-)). A single systemic administration of a rAAV serotype 1 (rAAV1) vector to neonate animals resulted in restored cardiac GAA activity to 6.4 times the normal level (mean=641+/-190% of normal (Gaa(+/+)) levels with concomitant glycogen clearance) at 11 months postinjection. Greater than 20% of normal levels of GAA activity were also observed in the diaphragm and quadriceps muscles. Furthermore, functional correction of the soleus skeletal muscle was also observed compared to age-matched untreated Gaa(-/-) control animals. These results demonstrate that rAAV1 vectors can mediate sustained therapeutic levels of correction of both skeletal and cardiac muscles in a model of fatal cardiomyopathy and muscular dystrophy.

Sprint-interval training-induced alterations of Myosin heavy chain isoforms and enzyme activities in rat diaphragm: effect of normobaric hypoxia

The purpose of this study was twofold: (i) to investigate if sprint-interval training (SIT) alters myosin heavy chain (MyHC) isoform composition and bioenergetic properties within the rat diaphragm, and (ii) to determine if mild normobaric hypoxia would enhance the effects of SIT-induced diaphragmatic adaptation. Male Wistar rats (8 weeks old) were randomly assigned to one of four groups (n = 7/group): (i) normoxic control (NC); (ii) normoxic training (NT); (iii) hypoxic control (HC); or (iv) hypoxic training (HT). The NT and HT groups were engaged in SIT (1 min sprint and 2-5 min rest, 6-10 sets/day, 5-6 days/week) on a treadmill for 9 weeks. Animals in the HC and HT groups were exposed to normobaric hypoxia (14.5% O(2)) during an SIT program from the 4th week of the training period. After completion of the training program, MyHC composition, citrate synthase (CS) activity, and lactate dehydrogenase (LDH) activity in the diaphragm and plantaris muscle were analyzed. An analysis of diaphragmatic MyHC composition demonstrated increased type IIa and decreased type IId/x for both training groups (P < 0.05), with the HT group producing greater changes than the NT group (P < 0.05). The plantaris muscle, however, showed increased Type IIa and IId/x and decreased Type IIb for both the NT and HT groups (P < 0.05). CS activity increased only for the training groups (P < 0.05), and this change was greater for the HT group in the diaphragm and for the NT group in the plantaris muscle (P < 0.05). Further, diaphragmatic LDH activity in HT was significantly lower (P < 0.05) than in HC and NT. These findings demonstrated that SIT could induce alterations in MyHC composition from fast to slow within type II isoforms and also improve the oxidative capacity in the diaphragm and plantaris muscles. It is of importance that our data revealed that SIT-induced diaphragmatic adaptations were enhanced when SIT was performed in normobaric hypoxia.

[Adaptation of myofibrilla, MHC and metabolic enzyme of rabbit diaphragm muscle to different frequency chronic electrical stimulation]

AIM

To detect effect of the different frequency of chronic electrical stimulation (CES) on myofibrillar isoform, myosin heavy chain (MHC) and metabolic enzyme activities.

METHODS

The histochemical method and SDS-polyacrylamide gel electrophoresis were respectively employed.

RESULTS

(1)There were a significant increase in I myo-fibrillar isoform and I MHC isoform and decrease in II B myofibrillar isoform and II B MHC isoforms in the chronic low frequency electrical stimulation (CLFES) 10 Hz and 20 Hz groups, but opposite results were found in the chronic high frequency electrical stimulation (CHFES) 50 Hz and 100 Hz groups. (2) There were a significant increase in the aerobic-oxidative enzyme activities and capacity, and a concomitant significant drop in glycolysis enzyme activities in CLFES groups, but opposite results were found in CHFES 50 Hz and 100 Hz groups.

CONCLUSION

It was suggested that there was a significant dependent relation between chronic electrical stimulation frequency and myofibrilla isoforms, myosin heavy chain (MHC) and metabolic enzyme activities.

Metabolic capacity of the diaphragm in patients with COPD

Chronic obstructive pulmonary disease (COPD) is associated with an increased load on the diaphragm. Chronic loading on skeletal muscles results in metabolic changes and fiber-type shifts. Therefore, we investigated whether the load on the human diaphragm imposed by COPD altered oxidative enzyme activity, glycogenolytic enzyme activity and mitochondrial energy generating capacity and efficiency. Biopsies of the diaphragm from COPD patients and control subjects were obtained and activities of L(+)3-hydroxyacylCoA-dehydrogenase (HADH, marker for beta-oxidation capacity) and phosphorylase (marker for glycogenolytic capacity) were measured spectrophotometrically. Mitochondrial energy generating capacity was measured by spectrophotometrical and radiochemical methods. Fiber-type distribution was determined electrophoretically. We found that HADH activity was increased with increasing severity of COPD (P=0.05). No change in glycogenolytic enzyme activity was observed. The activity of the mitochondrial respiratory chain complexes III and IV and oxidation of pyruvate was increased with increasing airflow obstruction. These results suggest that in COPD the diaphragm adapts to a higher workload by increasing the oxidative capacity and mitochondrial function.

Identification of an inducible nitric oxide synthase in diaphragm mitochondria from septic mice: its relation with mitochondrial dysfunction and prevention by melatonin

Sepsis provokes an induction of inducible nitric oxide synthase (iNOS) and melatonin down-regulates its expression and activity. Looking for an inducible mtNOS isoform, we induced sepsis by cecal ligation and puncture in both normal and iNOS knockout mice and studied the changes in mtNOS activity. We also studied the effects of mtNOS induction in mitochondrial function, and the role of melatonin against induced mtNOS and mitochondrial dysfunction. The activity of mtNOS and nitrite levels significantly increased after sepsis in iNOS+/+ mice. These animals showed a significant inhibition of the respiratory chain activity and an increase in mitochondrial oxidative stress, reflected in the disulfide/glutathione ratio, glutathione redox cycling enzymes activity and lipid peroxidation levels. Interestingly, mtNOS activity remained unchanged in iNOS-/- septic mice, and mitochondria of these animals were unaffected by sepsis. Melatonin administration to iNOS+/+ mice counteracted mtNOS induction and respiratory chain failure, restoring the redox status. The results support the existence of an inducible mtNOS that is likely coded by the same gene as iNOS. The results also suggest that sepsis-induced mtNOS is responsible for the increase of mitochondrial impairment due to oxidative stress in sepsis, perhaps due to the high production of NO. Melatonin treatment prevents mitochondrial failure at the same extend as the lack of iNOS gene.

Effect of prolonged mechanical ventilation on diaphragm muscle mitochondria in piglets

BACKGROUND

Respiratory muscle weakness is a common problem in the intensive care unit and could be involved in difficulties in weaning from the ventilator after prolonged mechanical ventilation. Animal models have shown that mechanical ventilation itself impairs diaphragm muscle function. In this study we investigated whether diaphragm contractile impairment caused by mechanical ventilation and immobilization in piglets is associated with a derangement in diaphragm mitochondria.

METHODS

Seven piglets received controlled mechanical ventilation during 5 days. A control group of eight piglets were anaesthetized and surgically manipulated in the same way, but were mechanically ventilated for 4-6 h. After mechanical ventilation, diaphragm muscle biopsies were taken for measurements of mitochondria content, mitochondrial respiratory enzymes and markers of oxidative stress.

RESULTS

Diaphragm mitochondrial content, as assessed by citrate synthase activities and volume density, was not different between the control and ventilated piglets. Activity of complex IV of the mitochondrial respiratory chain decreased by 21% (P=0.02) when expressed per muscle weight and by 11% (P=0.03) when expressed per citrate synthase activity. There were no changes in the markers of oxidative stress between the two groups.

CONCLUSION

Five days of mechanical ventilation and immobilization decreased the activity of complex IV of the mitochondrial respiratory chain in the diaphragm muscle of the piglets.

Effects of oximes on muscle force and acetylcholinesterase activity in isolated mouse hemidiaphragms exposed to paraoxon

Toxicity of organophosphates (OP) is caused by inhibition of acetylcholinesterase (AChE), resulting in accumulation of acetylcholine. While cholinolytics such as atropine are able to counteract muscarinic symptoms, they are unable to restore the impaired neuromuscular transmission (NMT). Here, oximes as potential reactivators of inhibited AChE may be effective. Until now, no unequivocal relation between oxime-induced increase in muscle force and reactivation has been demonstrated. To address this issue the isolated circumfused mouse hemidiaphragm was used as an experimental model. The muscle force generation upon tetanic stimuli was recorded during AChE inhibition by 1 microM paraoxon and after a wash-out period in the presence of obidoxime, pralidoxime and the experimental oximes HI 6, and HLö 7, 10 microM each. At the end of the experiments AChE activity was determined in the diaphragm homogenates by a radiometric assay. At 50-Hz stimulation, recovery was complete with obidoxime, nearly complete with HLö 7 but incomplete with HI 6 and pralidoxime. Only with obidoxime a significant increase in AChE activity was found. An increase of AChE to 10% of normal was sufficient to allow normal muscle force generation. When paraoxon was still present, obidoxime and HLö 7 were effective at 0.1 microM paraoxon, but failed so at paraoxon >1 microM. The data show different effectiveness of the oximes investigated in reactivation of muscle AChE and recovery of NMT after inhibition by paraoxon. Although an increase in muscle force by the oximes was accompanied by a measurable increase in AChE activity only in the case of obidoxime, the plot of muscle force against AChE activity as well as lacking evidence for a direct effect and adaptive processes indicate that reactivation of the enzyme is the main mechanism of NMT recovery. In agreement, in presence of AChE inhibitory concentrations of paraoxon during reactivation a reduced effectiveness of oximes was found.

Effect of chronic obstructive pulmonary disease on calcium pump ATPase expression in human diaphragm

We have previously demonstrated that human diaphragm remodeling elicited by severe chronic obstructive pulmonary disease (COPD) is characterized by a fast-to-slow myosin heavy chain isoform transformation. To test the hypothesis that COPD-induced diaphragm remodeling also elicits a fast-to-slow isoform shift in the sarcoendoplasmic reticulum Ca(2+) ATPase (SERCA), the other major ATPase in skeletal muscle, we obtained intraoperative biopsies of the costal diaphragm from 10 severe COPD patients and 10 control subjects. We then used isoform-specific monoclonal antibodies to characterize diaphragm fibers with respect to the expression of SERCA isoforms. Compared with control diaphragms, COPD diaphragms exhibited a 63% decrease in fibers expressing only fast SERCA (i.e., SERCA1; P < 0.001), a 190% increase in fibers containing both fast and slow SERCA isoforms (P < 0.01), and a 19% increase (P < 0.05) in fibers expressing only the slow SERCA isoform (i.e., SERCA2). Additionally, immunoblot experiments carried out on diaphragm homogenates indicated that COPD diaphragms expressed only one-third the SERCA1 content noted in control diaphragms; in contrast, COPD and control diaphragms did not differ with respect to SERCA2 content. The combination of these histological and immunoblot results is consistent with the hypothesis that diaphragm remodeling elicited by severe COPD is characterized by a fast-to-slow SERCA isoform transformation. Moreover, the combination of these SERCA data and our previously reported myosin heavy chain isoform data (Levine S, Nguyen T, Kaiser LR, Rubinstein NA, Maislin G, Gregory C, Rome LC, Dudley GA, Sieck GC, and Shrager JB. Am J Respir Crit Care Med 168: 706-713, 2003) suggests that diaphragm remodeling elicited by severe COPD should decrease ATP utilization by the diaphragm.

Adaptation of the diaphragm and the vastus lateralis in mild-to-moderate COPD

The chronology of diaphragm and vastus lateralis adaptation in chronic obstructive pulmonary disease (COPD) has not been studied. The hypothesis of this study was that muscle changes would occur earlier in the diaphragm than in the vastus lateralis in COPD, a finding that would suggest that local factors would be more important than systemic factors in determining the muscle phenotypic expression, at least in mild-to-moderate disease. The adaptation of the vastus lateralis and diaphragm muscles was evaluated in patients with mild-to-moderate COPD and in subjects with normal pulmonary function. In both groups, the oxidative potential and the number of lipofuscin inclusions were higher in the diaphragm than in the vastus lateralis. Compared to control, the diaphragm in COPD had a higher oxidative capacity and a higher proportion of type I fibres, with a reciprocal decrease in type IIA fibres, while there was no difference in diaphragmatic cross sectional areas, capillarisation and lipofuscin inclusions. No significant differences were found in the vastus lateralis from both groups. In conclusion, these data indicate that the diaphragm in controls and in chronic obstructive pulmonary disease has a higher oxidative potential than the vastus lateralis, and that muscle adaptation occurs earlier in the diaphragm than in the vastus lateralis in mild-to-moderate chronic obstructive pulmonary disease.

Reduced acetylcholine receptor density, morphological remodeling, and butyrylcholinesterase activity can sustain muscle function in acetylcholinesterase knockout mice

Nerve-evoked contractions were studied in vitro in phrenic nerve-hemidiaphragm preparations from strain 129X1 acetylcholinesterase knockout (AChE-/-) mice and their wild-type littermates (AChE+/+). The AChE-/- mice fail to express AChE but have normal levels of butyrylcholinesterase (BChE) and can survive into adulthood. Twitch tensions elicited in diaphragms of AChE-/- mice by single supramaximal stimuli had larger amplitudes and slower rise and decay times than did those in wild-type animals. In AChE-/- preparations, repetitive stimulation at frequencies of 20 and 50 Hz and at 200 and 400 Hz produced decremental muscle tensions; however, stimulation at 70 and 100 Hz resulted in little or no loss of tension during trains. Muscles from AChE+/+ mice maintained tension at all frequencies examined but exhibited tetanic fade after exposure to the selective AChE inhibitor 1,5-bis(4-allyldimethyl-ammoniumphenyl)pentane-3-one (BW 284C51). The ability of diaphragm muscles from AChE-/- mice to maintain tension at 70 and 100 Hz suggests a partial compensation for impairment of acetylcholine (ACh) hydrolysis. Three mechanisms--including a reliance on BChE activity for termination of ACh action, downregulation of nicotinic acetylcholine receptors (nAChRs), and morphological remodeling of the endplate region--were identified. Studies of neuromuscular transmission in this model system provide an excellent opportunity to evaluate the role of AChE without complications arising from use of inhibitors.

Rescue of enzyme deficiency in embryonic diaphragm in a mouse model of metabolic myopathy: Pompe disease

Several human genetic diseases that affect striated muscle have been modeled by creating knockout mouse strains. However, many of these are perinatal lethal mutations that result in death from respiratory distress within hours after birth. As the diaphragm muscle does not contract until birth, the sudden increase in diaphragm activity creates permanent injury to the muscle causing it to fail to meet respiratory demands. Therefore, the impact of these mutations remains hidden throughout embryonic development and early death prevents investigators from performing detailed studies of other striated muscle groups past the neonatal stage. Glycogen storage disease type II (GSDII), caused by a deficiency in acid alpha-glucosidase (GAA), leads to lysosomal accumulation of glycogen in all cell types and abnormal myofibrillogenesis in striated muscle. Contractile function of the diaphragm muscle is severely affected in both infantile-onset and late-onset individuals, with death often resulting from respiratory failure. The knockout mouse model of GSDII survives well into adulthood despite the gradual weakening of all striated muscle groups. Using this model, we investigated the delivery of recombinant adeno-associated virus (rAAV) vectors encoding the human GAA cDNA to the developing embryo. Results indicate specific high-level transduction of diaphragm tissue, leading to activity levels up to 10-fold higher than normal and restoration of normal contractile function. Up to an estimated 50 vector copies per diploid genome were quantified in treated diaphragms. Histological glycogen staining of treated diaphragms revealed prevention of lysosomal glycogen accumulation in almost all fibers when compared with untreated controls. This method could be employed with disease models where specific rescue of the diaphragm would allow for increased survival and thus further investigation into the impact of the gene deletion on other striated muscle groups.

Altered activity of signaling pathways in diaphragm and tibialis anterior muscle of dystrophic mice

Duchenne muscular dystrophy is a musculoskeletal disease caused by mutations in the dystrophin gene. The purpose of this study was to use the mouse model of muscular dystrophy (mdx) to determine if the progression of the dystrophic phenotype in the diaphragm (costal) versus limb skeletal muscle (tibialis anterior) is associated with specific changes in extracellular regulated kinase (ERK1/2), p70 S6 kinase (p70(S6k)), or p38 signaling pathways. The studies detected that consistent with an earlier dystrophic phenotype, phosphorylation of p70(S6k) is elevated by 40% in the diaphragm with no change in limb muscle. In addition, phosphorylation of p38 kinase was decreased by 33% in the mdx diaphragm muscle. Levels of ERK1/2 as well as phosphorylation states were elevated in the diaphragm and limb muscle of mdx mice compared with age-matched control muscles. These results indicate that distinct signaling pathways are differentially activated in skeletal muscle of mdx mice. The specificity of these responses, particularly in the diaphragm, provides insight for potential targets for blunting the progression of the muscular dystrophy phenotype.

Interactive toxicity of the organophosphorus insecticides chlorpyrifos and methyl parathion in adult rats

The acute interactive toxicity following exposure to two common organophosphorus (OP) insecticides, chlorpyrifos (CPF) and methyl parathion (MPS), was investigated in adult male rats. Oral LD1 values were estimated by dose-response studies (CPF = 80 mg/kg; MPS = 4 mg/kg, in peanut oil, 1 ml/kg). Rats were treated with both toxicants (0.5 or 1 x LD1) either concurrently or sequentially, with 4-h intervals between dosing. Functional signs of toxicity (1-96 h) and cumulative lethality (96 h) were recorded. Rats treated with CPF (1 x LD1) did not show any signs of toxicity although MPS (1 x LD1) elicited slight to moderate signs (involuntary movements) within 1-2 h. Concurrent exposure (LD1 dosages of both CPF and MPS) caused slight signs of toxicity only apparent between 24 and 48 h after dosing. When rats were treated sequentially with MPS first followed by CPF 4 h later, slight signs of toxicity were noted between 6 and 24 h, whereas reversing the sequence resulted in 100% lethality within 1 h of the second dosage. Following exposure to lower dosages (0.5 x LD1), the CPF first group showed higher signs of cholinergic toxicity compared with MPS first or concurrent groups. Cholinesterase inhibition in plasma, diaphragm, and frontal cortex was generally higher in rats treated sequentially with CPF first than in those treated initially with MPS from 4 to 24 h after dosing. Plasma and liver carboxylesterase inhibition at 4 h was also significantly higher in the CPF first (62-90%) compared with MPS first (22-43%) group, while at 8 and 24 h, there was no significant difference between any of the treatment groups. ChE inhibition assays to evaluate in vitro hepatic detoxification of oxons indicated that carboxylesterase (CE)- and A-esterase-mediated pathways are markedly less important for methyl paraoxon (MPO) than chlorpyrifos oxon (CPO) detoxification. CPF pretreatment blocked hepatic detoxification of methyl paraoxon while MPS pretreatment had minimal effect on hepatic CPO detoxification ex vivo. These findings suggest that the sequence of exposure to two insecticides that elicit toxicity through a common mechanism can markedly influence the cumulative action at the target site (acetylcholinesterase, AChE) and consequent functional toxicity.

Biochemical effects of low level exposure to soman vapour

The aim of this study was to demonstrate changes in acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) activities, tyrosine aminotransferase activity (TAT) and plasma corticosterone level, neuroexcitability and behavior following 24 hours and 4 weeks of soman sublethal inhalation exposure at low level. AChE activity in erythrocytes and BuChE activity in plasma was decreased (dependent on the concentration of soman) 24 h and 4 weeks after the exposure. Similar decrease in AChE activity in different brain parts was observed. One of stressogenic parameters (TAT) was changed after 24 h exposure only. 4 weeks after the exposure, these parameters (corticosterone and TAT) were in the range of normal values. Behaviour of experimental animals was changed 24 h after the exposure persisting 4 weeks after the exposure as well as neuroexcitability.

Effects of dietary calcium restriction and acute exercise on the antioxidant enzyme system and oxidative stress in rat diaphragm

Calcium deficiency is considered to increase intracellular calcium level; thus the aim of the current study was to elucidate whether dietary calcium restriction enhanced exercise-induced oxidative stress in rat diaphragm. Twenty male Wistar rats were randomly assigned to either a control group or a group subjected to 1 mo of calcium restriction. In addition, each group was subsequently subdivided into rested or acutely exercised group. Dietary calcium restriction significantly (P < 0.05) upregulated the activities of manganese-superoxide dismutase (Mn-SOD), copper-zinc-superoxide dismutase (Cu-Zn-SOD), and glutathione peroxidase (Gpx) but not catalase. Acute exercise, in addition to calcium restriction, decreased both SOD isoenzymes in the diaphragm of calcium-restricted rats (P < 0.05). On the other hand, calcium restriction resulted in increased Gpx mRNA expression (P < 0.05). In control rats, acute exercise significantly (P < 0.05) increased the expressions of both SOD mRNAs, whereas in the calcium-restricted rats, it increased that of Mn-SOD mRNA (P < 0.05) but decreased that of Gpx mRNA (P < 0.05). Furthermore, reactive carbonyl derivative, a marker of protein oxidation, was significantly greater in the calcium-restricted rats than in the control rats after acute exercise (P < 0.05). The results suggest that antioxidant enzymes in rat diaphragm were upregulated in response to an increased oxidative stress by dietary calcium restriction but that upregulation is not enough to cope with exercise-induced further increase of oxidative stress.

[Porcine kidney extract contains a specific inhibitor of the ouabain-sensitive alpha2-isoform of Na, K-ATPase present in rat diaphragm fibres]

In experiments on isolated rat diaphragm muscle, acetylcholine (100 nmol/l) hyperpolarized muscle fibres due to activation of the alpha 2 isoform of Na,K-ATPase. This hyperpolarization was blocked in a dose-dependent manner by ouabain (K0.5 = 8 +/- 4 nmol/l) as well as by a solution of porcine kidney extract (10 kDa cut-off filtration), with the K0.5 approximately equal to a 1:20,000-fold dilution. The inhibitory activity of the developed slowly over a period of 3 hours and, in contrast to ouabain, was still present after 1 hour of washing. Ouabain, but not the extract, inhibits Rb+ uptake in human erythrocytes that only express the alpha = 1 isoform of Na, K-ATPase. Our data suggest that in rat skeletal muscle the alpha 1 isoform of Na,K-ATPase is primarily responsible for ionic homeostasis, while the alpha 2 isoform provides a "regulatable" function and may be controlled by cholinergic stimulation and/or endogenous digitalis-like factors (EDLFs). Porcine kidney extract contains a factor (M. W. < 10 kDa) that selectively inhibits the rat alpha 2 isoform and differs from ouabain. Our experimental protocol can be used as a highly sensitive physiological assay for factors that selectively inhibit the alpha 2 isoform of Na,K-ATPase.

Susceptibility to cartap-induced lethal effect and diaphragmatic injury via ocular exposure in rabbits

Cartap is extensively used to control agricultural pests. Pertinent literatures have indicated that it causes no eye irritation [D.E. Ray, Insecticides derived from plants and other organisms, in: W.J. Hayes, E.R. Laws (Eds.), Handbook of Insecticide Toxicology, Classes of Insecticides, vol. 2, Academic Press, New York, 1991, p. 611; C. Tomlin, Cartap, in: C. Tomlin (Ed.), The Insecticide Manual, 12th ed., British Crop Protection Council, Surrey, UK, 2000, p. 144]; however, the instillation of a little cartap through the eye has caused death in rabbits. The aim of this study was to determine the ocular toxicity of cartap in New Zealand White rabbits. Cartap was directly instilled into the low conjunctival sac of eyes, at doses of 0, 5, 7.5, 10 and 12.5 mg/kg body weight. The changes in the enzymes and isoenzymes of creatine kinase (CK), lactate dehydrogenase (LD), as well as pathological changes in the muscles of the heart, thigh and diaphragm were determined in the cartap-treated rabbits. Moreover, the neuromuscular effect of cartap was examined using the isolated rabbit phrenic-nerve diaphragm model. The results indicated that rabbits developed severe signs and they died within 20 min of ocular instillation. The ocular LD50 of cartap was 8.1 mg/kg body weight. Treatment with cartap increased the activities of CK and LD enzymes and their isoenzymes, CK-1, CK-2, and CK-3 in serum, and CK-3 and LD-5 in the diaphragm. Microscopically, hypercontraction bands and the rupture of myofibers of the diaphragm were observed in dead rabbits. Cartap did not affect nerve-evoked twitch but induced irreversible contracture and twitch depression on the isolated rabbit's diaphragm. These results indicate that the rabbit is susceptible to cartap toxicity; the effect of cartap caused contracture and damage to the diaphragm might play a pivotal role in respiratory paralysis and death of rabbits during intoxication.

Combined forced running stress and subclinical paraoxon exposure have little effect on pyridostigmine-induced acute toxicity in rats

Pyridostigmine is a short-acting inhibitor of cholinesterase (ChE) used as a pretreatment against potential nerve agent exposure during the Persian Gulf War. As pyridostigmine contains a quaternary ammonium group, it is generally believed to elicit changes in the peripheral nervous system function only. It has been hypothesized, however, that the neurotoxicity of pyridostigmine may be altered by either stress or combined exposures to other toxicants. We evaluated the effects of forced running stress, exposure to the organophosphate anticholinesterase paraoxon, or a combination of both on the acute neurotoxicity of pyridostigmine. ChE (blood, diaphragm, and selected brain regions) and carboxylesterase (CE; liver, plasma) inhibition was also evaluated. Young adult male Sprague-Dawley rats were either given vehicle or paraoxon (0.1 mg/kg, i.m.) and subsets placed in their home cage or forced to run on a treadmill for 60 min. Pyridostigmine (0, 10 or 30 mg/kg, p.o.) was given 60 min after paraoxon dosing and rats were evaluated for cholinergic toxicity just prior to sacrifice 60 min later. No signs of toxicity were noted following paraoxon exposure while both dosages of pyridostigmine (10 and 30 mg/kg, p.o.) elicited signs of functional toxicity. Toxicity was not different with combined paraoxon-pyridostigmine exposures and forced running did not influence toxicity under any conditions. Paraoxon (0.1 mg/kg, i.m.) caused moderate (23-46%) ChE inhibition in blood, diaphragm and brain 2 h after exposure. Pyridostigmine (10 or 30 mg/kg, p.o.) caused extensive inhibition of blood (88-94%) and diaphragm (75-85%) ChE activity but no significant effect on brain regional ChE activity. Forced running stress did not influence the degree of tissue ChE inhibition following either paraoxon, pyridostigmine or paraoxon-pyridostigmine combined exposures. CE activities were inhibited (26-43%) in plasma and liver by paraoxon but inhibition was not influenced by either stress or combined paraoxon-pyridostigmine exposures. These results suggest that subclinical paraoxon exposure and forced running stress, by themselves or in combination, have little effect on acute pyridostigmine toxicity in rats.

Effects of daily stress or repeated paraoxon exposures on subacute pyridostigmine toxicity in rats

Pyridostigmine (PYR) is a carbamate cholinesterase (ChE) inhibitor used during the Persian Gulf War as a pretreatment against possible chemical nerve agent attack. Because of its quaternary structure, PYR entry into the central nervous system is limited by the blood-brain barrier (BBB). Following reports of unexplained illnesses among Gulf War veterans, however, central nervous system effects of PYR have been postulated through either stress-induced alteration of BBB permeability or via interactions with other neurotoxic agents. We evaluated the effects of daily physical (treadmill running) stress or daily exposure to a subclinical dosage of the organophosphate ChE inhibitor paraoxon (PO) on ChE inhibition in blood, diaphragm and selected brain regions in young adult male Sprague-Dawley rats following subacute PYR exposures. In physical stress studies, rats were placed on a treadmill for 90 min each day for 14 days just prior to PYR (0, 3, or 10 mg/kg per day) administration. In PO-PYR interaction studies, rats were treated with PO (0, 0.05, or 0.1 mg/kg per day) 1 h prior to daily PYR (0 or 3 mg/kg per day) administration for 14 consecutive days. Rats were evaluated daily for signs of cholinergic toxicity and were killed 1 h after the final PYR treatment. Forced running increased plasma corticosterone levels throughout the experiment (on days 1, 3, 7 and 14) when measured immediately after termination of stress. PYR-treated rats in the high dosage (10 mg/kg per day) group exhibited slight signs of toxicity (involuntary movements) for the first 6 days, after which tolerance developed. Interestingly, signs of cholinergic toxicity following PYR were slightly but significantly increased in rats forced to run on the treadmill prior to dosing. ChE activities in whole blood and diaphragm were significantly reduced 1 h after the final PYR challenge, and ChE inhibition in diaphragm was significantly greater in stressed rats than in non-stressed controls following high dose PYR (10 mg/kg per day). No significant effects of treadmill running on PYR-induced ChE inhibition in brain regions were noted, however. Repeated subclinical PO exposure had no apparent effect on functional signs of PYR toxicity. As with repeated treadmill running, whole blood and diaphragm ChE activities were significantly reduced 1 h after the final PYR administration, and ChE inhibition was significantly greater with combined PO and PYR exposures. Brain regional ChE activity was significantly inhibited after daily PO exposure, but no increased inhibition was noted following combined PO and PYR dosing. We conclude that, while some stressors may under some conditions affect functional signs of toxicity following repeated pyridostigmine exposures, these changes are likely to occur via alteration of peripheral cholinergic mechanisms and not through enhanced entry of pyridostigmine into the brain.

Stability of diacylglycerol acyltransferase in dehydrated bovine muscle tissue

Meaningful estimates of diacylglycerol acyltransferase (EC 2.3.1.20) activity in different tissue samples require effective, unbiased methods of sample storage. Samples of the pars costalis diaphragmatis muscle (skirt muscle of the diaphragm) were obtained from 18- to 20-month-old cattle and assayed for microsomal protein content and diacylglycerol acyltransferase activity after having been stored under various conditions as dissected tissue or microsomes prepared from dissected tissue. There was relative enrichment of diacylglycerol acyltransferase specific activity (p<0.05) when samples prepared from the pars costalis diaphragmatis muscle were dehydrated and stored for 2 weeks, as compared to the control condition (in which the microsome fraction was prepared from fresh pars costalis diaphragmatis muscle and assayed immediately). The results suggested that dehydration was an effective method of storage for bovine muscle samples destined for estimation of the microsomal diacylglycerol acyltransferase activity. The dehydration approach for preparing samples for analysis of diacylglycerol acyltransferase activity might also prove useful to investigators who are interested in obtaining reliable estimates of the activity of other enzymes in tissue samples.

Diaphragm muscle development in bottlenose dolphins (Tursiops truncatus)

Being born directly into the aquatic environment creates unique challenges for the breathing muscles of neonatal cetaceans. Not only must these muscles be active at the instant of birth to ventilate the lungs, but their activities must also be coordinated with those of the locomotor muscles such that breathing takes place only at the water's surface. At least one major locomotory muscle of bottlenose dolphins (Tursiops truncatus) has been demonstrated to be well developed and, therefore, able to power the neonatal dolphin's early movements (Dearolf et al. [2000] J Morphol 244:203-215). Thus, because of the demands for coordinated behavior with the locomotor muscles, it is hypothesized that the breathing muscles of bottlenose dolphins, represented in this study by the diaphragm, will also demonstrate adult morphology at birth. However, histochemical and biochemical analyses demonstrate that neonatal dolphins have immature diaphragms, with only 52% of the adult slow fiber-type profile (neonates: 34% slow-twitch fibers; adults: 66% slow-twitch fibers). The developmental state of the dolphin diaphragm is compared to those of other neonatal mammals, using a muscle development index (% slow-twitch fibers in neonatal muscle / % slow-twitch fibers in adult muscle). Fiber-type profiles reported in the literature are used to calculate index values for the diaphragms of altricial rats, rabbits, and cats, intermediate baboons and humans, and precocial sheep and horses. The dolphin is not unique in having an immature diaphragm at birth; however, there is a positive relationship between the developmental state of the diaphragm and the overall developmental state of the neonate. The presence of type IIc ("undifferentiated") fibers in the diaphragms of altricial developers (e.g., rats, rabbits, and cats) is correlated with the slow contraction speeds recorded from their diaphragms. The diaphragms of neonatal horses and dolphins express little to no type IIc fibers and, thus, may have the ability to contract at the speeds required for their increased ventilation rates. These results lead to the modification of the criterion for evaluating the developmental state of a muscle at birth. Thus, the developmental state of a neonatal muscle should be based on both its value of Dearolf et al.'s (2000) developmental index, as well as the percentage of type IIc fibers found in that muscle.

Nitric oxide is not involved in the endotoxemia-induced alterations in Ca2+ and ryanodine responses in mouse diaphragms

Lipopolysaccharide (LPS, endotoxin)-induced diaphragmatic contractile dysfunction and sarcolemmal injury in animals has been identified. However, the precise nature of sepsis-related alterations in diaphragm myofiber function and the activity of Ca(2+) release from sarcoplasmic reticulum of skeletal muscle remain unclear. The present study investigated the in vivo effects of LPS on the Ca(2+)-dependent mechanical activity and ryanodine response in mouse diaphragm and Ca(2+) release from isolated sarcoplasmic reticulum membrane vesicles, and aimed to examine the role of nitric oxide (NO) in these responses. When diaphragms were bathed in a solution that was Cl(-)-free, Na(+)-free, but contained high K(+), a Ca(2+)-induced contracture was elicited. Increases in external Ca(2+) concentration produced increases in peak tension of Ca(2+)-induced contracture in control diaphragm, while a decrease was seen in endotoxemic diaphragm. Ryanodine induced a marked contracture in control diaphragms, which was diminished after endotoxemia. This finding is correlated with the decrease of ryanodine-induced Ca(2+) release and the suppression of [(3)H]ryanodine binding on the isolated SR of the skeletal muscle from LPS-treated rats. In mice treated with LPS significantly increased levels of plasma nitrite and serum TNF-alpha were observed, changes inhibited by aminoguanidine [an inhibitor of inducible NO synthase (iNOS)] and pentoxifylline (an inhibitor of tumor necrosis factor-alpha formation), respectively. Moreover, LPS treatment resulted in a significant expression of mRNA for iNOS in mouse diaphragms. The inhibitory effects on Ca(2+)- and ryanodine responses by LPS could be prevented by treatment with polymyxin B (LPS neutralizer) and pentoxifylline, but not by treatment with dexamethasone, N(G)-nitro- L-arginine or aminoguanidine (NOS inhibitors). These results imply that the NO-related pathway may not be involved in the dysfunction of the Ca(2+) release mechanism in the sarcoplasmic reticulum of mouse diaphragm during endotoxemia.

Structural and oxidative enzyme characteristics of the diaphragm

During exercise, the horse can achieve oxygen uptakes and ventilations in excess of 200 ml/kg/min and 1800 l/min, respectively. Whether the diaphragm has the capacity to contribute substantially to inspiratory effort in the exercising horse is not known. To investigate the potential for the horse diaphragm to generate tension, lung displacement and sustain ventilatory function, we measured diaphragm thickness, muscle length and oxidative enzyme activity (citrate synthase) within the ventral, medial and dorsal costal and crural diaphragm. In the diaphragms of 6 mature horses (5 Thoroughbreds, one Quarter Horse; body mass (mean +/- s.e.) 475 +/- 14 kg, age 4 +/- 1 years), the mass of the freshly-excised diaphragm was 4.54 +/- 0.19 kg of which 79% was the costal diaphragm, 17% the crural diaphragm and 4% the central tendon. The medial costal region (2.1 +/- 0.1 cm) was significantly thicker (P<0.05) than either the ventral (1.4 +/- 0.1 cm) or dorsal (1.2 +/- 0.2 cm) costal regions and the crural diaphragm was significantly thicker (>3.2 +/- 0.3 cm, P<0.05) than any costal diaphragm region. With respect to the costal diaphragm, excised muscle length was greatest (P<0.05) in the medial costal (17.2 +/- 1.0 cm) than either the ventral costal (<12.6 +/- 1.5 cm) or dorsal costal (<13.9 +/- 1.8 cm) regions and therefore the medial region would be expected to exhibit the greatest absolute length change on inspiration. Citrate synthase activity was high throughout the diaphragm (40.8 +/- 113 to 55.3 +/- 9.7 micromol/g/min), but was not significantly different among regions. These structural characteristics and the oxidative potential of the horse diaphragm are consistent with the diaphragm providing a significant and substantial contribution to the inspiratory effort during exercise in the horse. Consequently, clinical and physiological investigations of exercise performance should not ignore the potentially crucial importance of the diaphragm.

Control of Ser2448 phosphorylation in the mammalian target of rapamycin by insulin and skeletal muscle load

We have investigated the effects of insulin, amino acids, and the degree of muscle loading on the phosphorylation of Ser(2448), a site in the mammalian target of rapamycin (mTOR) phosphorylated by protein kinase B (PKB) in vitro. Phosphorylation was assessed by immunoblotting with a phosphospecific antibody (anti-Ser(P)(2448)) and with mTAb1, an activating antibody whose binding is inhibited by phosphorylation in the region of mTOR that contains Ser(2448). Incubating rat diaphragm muscles with insulin increased Ser(2448) phosphorylation but did not change the total amount of mTOR. Insulin, but not amino acids, activated PKB, as evidenced by increased phosphorylation of both Ser(308) and Thr(473) in the kinase. Ser(2448) phosphorylation was also modulated by muscle-loading. Overloading the rat plantaris muscle by synergist muscle ablation, which promotes hypertrophy of the plantaris muscle, increased Ser(2448) phosphorylation. In contrast, unloading the gastrocnemius muscle by hindlimb suspension, which promotes atrophy of the muscle, decreased Ser(2448) phosphorylation, an effect that was fully reversible. Neither overloading nor hindlimb suspension significantly changed the total amount of mTOR. In summary, our results demonstrate that atrophy and hypertrophy of skeletal muscle are associated with decreases and increases in Ser(2448) phosphorylation, suggesting that modulation of this site may have an important role in the control of protein synthesis.

Alteration of contractile force and mass in the senescent diaphragm with beta(2)-agonist treatment

Aging is associated with a decrease in diaphragmatic maximal tetanic force production (P(o)) in senescent rats. Treatment with the beta(2)-agonist clenbuterol (CB) has been shown to increase skeletal muscle mass and P(o) in weak locomotor skeletal muscles from dystrophic rodents. It is unknown whether CB can increase diaphragmatic mass and P(o) in senescent rats. Therefore, we tested the hypothesis that CB treatment will increase specific P(o) (i.e., force per cross-sectional area) and mass in the diaphragm of old rats. Young (5 mo) and old (23 mo) male Fischer 344 rats were randomly assigned to one of the following groups (n = 10/group): 1) young CB treated; 2) young control; 3) old CB treated; and 4) old control. Animals were injected daily with either CB (2 mg/kg) or saline for 28 days. CB increased (P < 0.05) the mass of the costal diaphragm in both young and old animals. CB treatment increased diaphragmatic-specific P(o) in old animals (approximately 15%; P < 0.05) but did not alter (P > 0.05) diaphragmatic-specific P(o) in young animals. Biochemical analysis indicated that the improved maximal specific P(o) in the diaphragm of CB-treated old animals was not due to increased myofibrillar protein concentration. Analysis of the myosin heavy chain (MHC) content of the costal diaphragm revealed a CB-induced increase (P < 0.05) in type IIb MHC and a decrease in type I, IIa, and IIx MHC in both young and old animals. These data support the hypothesis that CB treatment can restore the age-associated decline in both diaphragmatic-specific P(o) and muscle mass.