Corticosteroid effects on isotonic contractile properties of rat diaphragm muscle

The effects of corticosteroids (CS) on diaphragm muscle (Diam) fiber morphology and contractile properties were evaluated in three groups of rats: controls (Ctl), surgical sham and weight-matched controls (Sham), and CS-treated (6 mg . kg-1 . day-1 prednisolone at 2.5 ml/h for 3 wk). In the CS-treated Diam, there was a selective atrophy of type IIx and IIb fibers, compared with a generalized atrophy of all fibers in the Sham group. Maximum isometric force was reduced by 20% in the CS group compared with both Ctl and Sham. Maximum shortening velocity in the CS Diam was slowed by approximately 20% compared with Ctl and Sham. Peak power output of the CS Diam was only 60% of Ctl and 70% of Sham. Endurance to repeated isotonic contractions improved in the CS-treated Diam compared with Ctl. We conclude that the atrophy of type IIx and IIb fibers in the Diam can only partially account for the CS-induced changes in isotonic contractile properties. Other factors such as reduced myofibrillar density or altered cross-bridge cycling kinetics are also likely to contribute to the effects of CS treatment.     

Immobilization effects on contractile properties of aging rat skeletal muscle

Recent advances in miniaturization have provided clinicians with hearing aids that can be comfortably inserted as far as the bony portion of the ear canal. It is possible to take advantage of these deeply inserted hearing aids in new ways. For example, the physical vibrations of microphone and receiver components may be used to improve hearing aid gain through bone conduction. Three cases are presented that will introduce this phenomenon for two transcranial CROS fittings and for one unilateral otosclerosis fitting. In each case, functional gain measurements under headphones were obtained with the hearing aid receivers acoustically plugged. Considerable gain was still present. Potential benefits, ramifications, and side effects are discussed.     

The contractile properties of the elderly human diaphragm

It has previously been reported that aging is associated with a substantial decrease in diaphragm strength. To test this hypothesis we studied 15 (10 male, 5 female) subjects with a mean age of 29 (range 21 to 40) and 15 elderly (10 male, 5 female) subjects, mean age 73 (range 67 to 81). We measured transdiaphragmatic pressure (Pdi) during a maximal sniff (Sniff Pdi) and during bilateral cervical magnetic stimulation (CMS) of the phrenic nerve roots (Tw Pdi). Additionally in 17 subjects (9 elderly and 8 young) the Pdi elicited by paired CMS (pTw Pdi) was obtained at interstimulus intervals ranging from 10 to 999 ms (1 to 100 Hz). There was considerable overlap between groups. Mean Sniff Pdi in the elderly was 119 cm H2O compared with 136 cm H2O for the young subjects; this represented a median reduction of 18 cm H2O or 13% (p = 0.05, 95% Cl of difference 0 to 33 cm H2O). Mean Twitch Pdi in the elderly was 26.8 cm H2O compared with 35.2 cm H2O, a median reduction of 8 cm H2O or 23% (p = 0.004, 95% Cl 3 to 13 cm H2O). At 10 Hz the elderly tended to generate a higher fraction of the Pdi obtained at 100 Hz than the young, but this trend did not achieve statistical significance (p = 0.11). We conclude that aging is associated with a reduction in diaphragm strength. However the magnitude of the reduction is small and may be offset by a leftward shift of the force-frequency relationship.     

Influence of acute lung volume change on contractile properties of human diaphragm

The effect of stimulus frequency on the in vivo pressure generating capacity of the human diaphragm is unknown at lung volumes other than functional residual capacity. The transdiaphragmatic pressure (Pdi) produced by a pair of phrenic nerve stimuli may be viewed as the sum of the Pdi elicited by the first (T1 Pdi) and second (T2 Pdi) stimuli. We used bilateral anterior supramaximal magnetic phrenic nerve stimulation and a digital subtraction technique to obtain the T2 Pdi at interstimulus intervals of 999, 100, 50, 33, and 10 ms in eight normal subjects at lung volumes between residual volume and total lung capacity. The reduction in T2 Pdi that we observed as lung volume increased was greatest at long interstimulus intervals, whereas the T2 Pdi obtained with short interstimulus intervals remained relatively stable over the 50% of vital capacity around functional residual capacity. For all interstimulus intervals, the total pressure produced by the pair decreased as a function of increasing lung volume. These data demonstrate that, in the human diaphragm, hyperinflation has a disproportionately severe effect on the summation of pressure responses elicited by low-frequency stimulations; this effect is distinct from and additional to the known length-tension relationship.     

Potassium induced potential changes in rat diaphragm muscle

The effect of different potassium concentrations on the membrane potential and membrane resistance of rat diaphragm muscle fibres was measured by means of a double sucrose gap method and a microelectrode technique. Concentration measurements showed that the muscle fibres gained sodium and lost potassium in the equilibration period. In the absence of external chloride changing the external potassium concentration from 2.8 mM to potassium-free caused a depolarization of the membrane of about 30 mV and a small increase in membrane resistance. This K-dependent potential change (K-response) was induced by ouabain, K-strophanthin, 2,4-dinitrophenol and cyanide, indicating that an energy requiring process is involved. The temperature dependence of the K-response found is consistent with this assumption. Variation in potassium permeability in the absence and presence of external potassium could account for only 13% of the K-response. The K-response amplitude appeared to depend on the external potassium and the internal sodium concentration. Hyperpolarization of the membrane could not only be produced after readmission of potassium but also after addition of thallium, the latter being more potent. Raising the external chloride concentration resulted in a decrease of the K-response and membrane resistance. The current, generating the K-response was shown to be hardly influenced by conditional polarization of the membrane. It is concluded from these results that the K-response is mainly due to the operation of an electrogenic sodium pump.     

Long-term effects of clenbuterol on diaphragm morphology and contractile properties in emphysematous hamsters

The most reliable prognostic factors for patients with primary malignant brain tumors remain histology, age, and functional status. Management of these individuals might be improved by quantifying pertinent molecular markers. We have measured the gene dosage of the epidermal growth factor receptor (EGFR), mouse double minute 2 (MDM2), and cyclin-dependent kinase 4 (CDK4) genes in a series of brain tumor specimens and correlated their amplification status with standard prognostic factors and survival. Individual tumor DNA was successively hybridized with probes for EGFR, MDM2, and CDK4. The signal was quantified by densitometry, and amplification was defined as gene signal > or = 2 times normal. Survival, age, Karnofsky performance status, and histology were correlated with gene amplification. Nineteen astrocytomas, 20 anaplastic astrocytomas, and 70 glioblastomas had complete data available. Median survival with and without any form of gene amplification was 70.7 and 88.6 weeks, respectively (P = 0.0369). For the EGFR gene alone, those with and without amplification had a median survival of 58.9 and 88.6 weeks, respectively (P = 0.0104). By Cox analysis, only tumor histology (P = 0.04) and Karnofsky performance status (P = 0.0157) were significant independent predictors of survival. Gene amplification by itself was not predictive of survival, even for glioblastomas (P = 0.8249). The lack of correlation between gene amplification and survival for patients with primary malignant brain tumors may be because EGFR, MDM2, and CDK4 are only portions of larger signaling systems. Therefore, the lack of a direct correlation between a single gene and outcome is not entirely unexpected.     

Effects of chronic low frequency stimulation on contractile and elastic properties of hindlimb suspended rat soleus muscle

Chronic low frequency stimulation (10 Hz, 8 h x day[-1]) was used in this study to prevent the changes in the contractile and elastic properties of rat soleus muscles induced by 3 weeks of hindlimb suspension (HS). Results showed that electrostimulation was able to counteract in part the decrease in soleus muscle mass and tension output induced by unweighting. On the other hand, the increases in maximal shortening velocity and twitch speed following HS were not prevented by stimulation. Unweighting was responsible for an increase in series elastic compliance of soleus muscle. Chronic stimulation successfully counteracted this increase in series compliance probably by changing the properties of the tendon. The partial recovery of muscle mass and tension output as a result of stimulation enhanced the role of contractile activity in preventing muscle atrophy. Moreover, the inefficiency of the tonic activity imposed by stimulation in preventing the increase in twitch speed of soleus muscle during HS demonstrated the primacy of neuronal activity. Discrepant results concerning changes in contraction kinetics deduced from the twitch could have been due to the fact that such myograms also depend on the series compliance.     

Sprint-training effects on some contractile properties of single skinned human muscle fibres

The effects of sprint training on the contractile properties of human muscle fibres obtained by needle biopsy were investigated. Individual fibres were mechanically skinned and activated by Ca(2+)- and Sr(2+)-buffered solutions at pH 7.1, and allocated to distinct populations on the basis of their contractile characteristics. The majority of fibres sampled pre-training could be separated into the three major fibre groups: Populations I (24/70, 34%), II (25/70, 36%) and III (18/70, 26%), which exhibited characteristics similar to those of histochemically classified type I, IIA and IIB fibres, respectively. The remainder (3/70, 4%) represented another fibre group, with intermediate characteristics. The muscle fibres were also activated by Ca2+ at a reduced pH of 6.6, to mimic the intracellular acidification that occurs during intense exercise. Lowering pH increased the threshold for contraction by Ca2+, reduced Ca2+ sensitivity, and increased the steepness of the force-pCa relationship, in all fibres sampled from the three major fibre groups. Maximum force was not significantly reduced in any fibre population. In the post-training sample, the three major fibre types were present in different proportions: Populations I (10/52, 19%), II (20/52, 38.5%) and III (11/52, 21%). Three other fibre groups sampled in low numbers exhibited contractile characteristics intermediate between Population I and Population II. Following sprint training all of the three main fibre populations exhibited higher thresholds for contraction by, and lower sensitivities to, Sr2+ but not Ca2+, compared with the fibres sampled pre-training. Maximum force was significantly lower in Population II fibres after sprint training. At pH 6.6, post-trained Population III fibres exhibited even lower Ca2+ sensitivity, with concomitant increases in the threshold for contraction and force-pCa curve steepness.     

Combined myofibrillar and mitochondrial creatine kinase deficiency impairs mouse diaphragm isotonic function

Creatine kinase (CK) is an enzyme central to cellular high-energy phosphate metabolism in muscle. To characterize the physiological role of CK in respiratory muscle during dynamic contractions, we compared the force-velocity relationships, power, and work output characteristics of the diaphragm (Dia) from mice with combined myofibrillar and sarcomeric mitochondrial CK deficiency (CK[-/-]) with CK-sufficient controls (Ctl). Maximum velocity of shortening was significantly lower in CK[-/-] Dia (14.1 +/- 0.9 Lo/s, where Lo is optimal fiber length) compared with Ctl Dia (17.5 +/- 1.1 Lo/s) (P < 0.01). Maximum power was obtained at 0.4-0.5 tetanic force in both groups; absolute maximum power (2,293 +/- 138 W/m2) and work (201 +/- 9 J/m2) were lower in CK[-/-] Dia compared with Ctl Dia (2,744 +/- 146 W/m2 and 284 +/- 26 J/m2, respectively) (P < 0.05). The ability of CK[-/-] Dia to sustain shortening during repetitive isotonic activation (75 Hz, 330-ms duration repeated each second at 0.4 tetanic force load) was markedly impaired, with CK[-/-] Dia power and work declining to zero by 37 +/- 4 s, compared with 61 +/- 5 s in Ctl Dia. We conclude that combined myofibrillar and sarcomeric mitochondrial CK deficiency profoundly impairs Dia power and work output, underscoring the functional importance of CK during dynamic contractions in skeletal muscle.     

Gender differences in isometric contractile properties and fatigability in elderly human muscle

This study had two purposes: to compare the isometric contractile characteristics and fatigability in the elbow flexors (EF) and ankle dorsiflexors (DF) in older males and females (60-80 years), and to determine the effects of almost 2 years of twice weekly dynamic weight-lifting training on these properties One hundred nine male and female subjects completed the 22-month intervention, 54 in the exercise group and 55 in the control group. Isometric contractile properties and fatigue characteristics were assessed at baseline, 10 months, and 22 months. At baseline, males were significantly stronger (p < .05) than females in evoked twitch torque (Pt) and maximal voluntary strength (MVC) in both EF and DF. Females exhibited significantly less fatigue (expressed either absolutely or relatively) than males in both Pt and MVC during a 3-min protocol of intermittent MVCs (5 s contraction; 2 s rest). There were no significant effects of the 22 months of dynamic training on either the isometric contractile properties or the fatigue characteristics, suggesting a very strong and persistent specificity of training effect.     

Isometric and isotonic contractions in airway smooth muscle

Canine tracheal smooth muscle was used as an in vitro model of smooth muscle in intrapulmonary airways to determine whether active tension curves derived from isometric and isotonic muscles are similar, and thus resemble striated muscle in this respect. Isometric, isotonic after-loaded, and isotonic free-loaded contractions elicited at different lengths and loads, were analysed. The data demonstrate that length-tension (L-T) diagrams were different in these various types of contractions for electrically and carbachol driven tracheal smooth muscles strips. In general, at any given length active tension is less in isotonic and free-loaded modes of contraction as compared with isometric. We conclude that the ability to actively develop tension at a given length in airway smooth muscle depends on the mode of contraction.     

Myosin heavy chain isoforms and dynamic contractile properties: skeletal versus smooth muscle

Torsades de pointes is a potentially life-threatening form of polymorphic ventricular tachyarrhythmia typically seen in the presence of repolarization-prolonging agents. We investigated this particular form of tachyarrhythmia in the isolated, perfused rabbit heart. The experimental model was designed to reproduce conditions that are clinically known to be associated with an increased propensity to the development of torsades de pointes. The class III agent clofilium (1 microM) and d,l-sotalol (10 microM), as well as the antibiotic erythromycin (30-150 microM) were infused in the presence of either normal (5.88 mM) or low (1.5 mM) potassium concentration in sinus-driven or atrioventricular (AV)-blocked hearts. Ventricular tachyarrhythmias spontaneously emerged in the clofilium-, d,l-sotalol-, and erythromycin-treated AV-blocked hearts. The episodes showed typical features of torsades de pointes found in humans. They developed within 4-12 min after the onset of infusion, were normally nonsustained, and only rarely degenerated into ventricular fibrillation. Electrical stimulation at cycle lengths <600 ms and perfusion with MgSO4 suppressed arrhythmic activity. In the d,l-sotalol- and erythromycin-treated hearts, torsades de pointes occurred only in the presence of hypokalemia and bradycardia, whereas, in the presence of clofilium, bradycardia alone caused torsades de pointes. Monophasic action-potential recordings demonstrated early afterdepolarizations in endocardial and epicardial recordings. Thus the isolated AV-blocked rabbit heart represents a model for studying drug-related torsades de pointes and its mechanism.     

N-acetylcysteine depresses contractile function and inhibits fatigue of diaphragm in vitro

We have previously shown that antioxidant enzymes (superoxide dismutase and catalase) depress contractility of unfatigued diaphragm fiber bundles and inhibit development of acute fatigue. In the present study, we tested for similar effects of N-acetyl-cysteine (NAC), a nonspecific antioxidant approved for clinical use. Diaphragms were excised from deeply anesthetized rats. Fiber bundles were removed, mounted isometrically at 37 degrees C, and stimulated directly using supramaximal current intensity. Studies of unfatigued muscle showed that 10 mM NAC reduced peak twitch stress (P < 0.0001), shortened time to peak twitch stress (P < 0.002), and shifted the stress-frequency curve down and to the right (P < 0.05). Fiber bundles incubated in 0.1-10 mM NAC exhibited a dose-dependent decrease in relative stresses developed during 30-Hz contraction (P < 0.0001) with no change in maximal tetanic (200 Hz) stress. NAC (10 mM) also inhibited acute fatigue. Throughout 10 min of intermittent contraction at 30-40 Hz, treated bundles developed higher stresses than time-matched control bundles (P < 0.0001). NAC concentrations > or = 30 mM were toxic, causing a prompt irreversible decrease in maximal tetanic stress (P < 0.0001). Because NAC effects mimic the effects of other antioxidant agents with different mechanisms of action, we conclude that exogenous antioxidants exert stereotypical effects on contractile function that differ between unfatigued and fatiguing muscle. Unlike antioxidant enzymes, however, NAC has been approved for clinical use and may be used in future studies of human muscle fatigue.     

Structural changes in smooth muscle cells during isotonic contraction

Smooth muscle cells of the guinea-pig taenia coli were studied in light and electron microscopy, in condition of mild stretch or of isotonic contraction. During contraction the cells increase in transverse sectional area and their packing density passes from 94,000-mm-2 to 18,000-mm-2. The percentage increase in transverse sectional area of the taenia is approximately the same as the percentage decrease in length. Measurements of cell transverse sectional area suggest that the individual cells shorten and fatten more than the taenia as a whole. Whereas stretched muscle cells run parallel to each other and show a fairly smooth surface, isotonically contracted cells are twisted and entwine around each other. Their surfaces are covered with myriad processes and folds. Longitudinal, transverse or oblique stripes are seen in light microscopy in the contracted muscle cells and it is suggested that they are related to the characteristics of the cell surface. In electron microscopy a complex pattern of interdigitating finger-like and laminar processes is observed. Caveolae are mainly found on the evaginated parts of the cell surface, dense patches are mainly (but not always) found on the invaginated parts. Desmosome-like attachments between contracted cells are frequent. The collagen fibrils run approximately parallel to the stretched muscle cells; on the other hand, they run obliquely and transversely around the isotonically contracted cells.     

Electron microscopical characterization of muscle fiber types of the rat diaphragm using a cluster analysis

Muscle fibers of one rat diaphragm were selected for sterological investigation of some ultrastructural parameters. Histograms and scattergrams did not exhibit consistently the existence of several subgroups of muscle fibers. A cluster analysis was performed to reveal groupings. 2 populations were defined using a stepwise discriminant analysis. The most important parameters for the discrimination of the 2 groups proved to be the volume density of mitochondria and the width of the Z line, all other parameters determined were of minor importance. It was tried to apply numerical taxonomic procedures to the problem of the ultrastructural characterization of skeletal muscle fibers. Thus difficulties arising from subjective determinations of several types of muscle fibers could be avoided.     

Sarcomere relaxation in hamster diaphragm muscle

We characterized instantaneous sarcomere relaxation over the load continuum in isolated hamster diaphragm muscles by means of laser diffraction. In afterloaded twitches, sarcomere relaxation displayed two consecutive phases. The bulk of sarcomere lengthening occurred during the first phase and corresponded in time to muscle lengthening. The second phase of sarcomere relaxation was slower and corresponded in time to tension decay. At initial muscle length, the peak velocity of sarcomere lengthening (SVL) was linearly related to both the maximum extent of sarcomere shortening (delta SL) and sarcomere length at peak shortening (SLmin; each P < 0.01). Varying preload modified the SVL vs. SLmin relationship but not the SVL vs. delta SL relationship. At a given preload, muscle tension decay began at a similar sarcomere length, regardless of the afterload level. In conclusion, our results support the role played by sarcomere length in regulating the diaphragm muscle-lengthening rate but not the rate of tension decline.