Effects of sulphydryl modification on skinned rat skeletal muscle fibres using 5,5'-dithiobis(2-nitrobenzoic acid)

Endogenous and Exogenous Antioxidants in Skeletal Muscle Fatigue Development during Exercise

Muscle fatigue is defined as a decrease in maximal force or power generated in response to contractile activity, and it is a risk factor for the development of musculoskeletal injuries. One of the many stressors imposed on skeletal muscle through exercise is the increased production of reactive oxygen species (ROS) and reactive nitrogen species (RNS), which intensifies as a function of exercise intensity and duration. Exposure to ROS/RNS can affect Na⁺/K⁺-ATPase activity, intramyofibrillar calcium turnover and sensitivity, and actin-myosin kinetics to reduce muscle force production. On the other hand, low ROS/RNS concentrations can likely upregulate an array of cellular adaptative responses related to mitochondrial biogenesis, glucose transport and muscle hypertrophy. Consequently, growing evidence suggests that exogenous antioxidant supplementation might hamper exercise-engendering upregulation in the signaling pathways of mitogen-activated protein kinases (MAPKs), peroxisome-proliferator activated co-activator 1α (PGC-1α), or mammalian target of rapamycin (mTOR). Ultimately, both high (exercise-induced) and low (antioxidant intervention) ROS concentrations can trigger beneficial responses as long as they do not override the threshold range for redox balance. The mechanisms underlying the two faces of ROS/RNS in exercise, as well as the role of antioxidants in muscle fatigue, are presented in detail in this review.

Supplementation with Spirulina platensis Prevents Uterine Diseases Related to Muscle Reactivity and Oxidative Stress in Rats Undergoing Strength Training

Strength training increases systemic oxygen consumption, causing the excessive generation of reactive oxygen species, which in turn, provokes oxidative stress reactions and cellular processes that induce uterine contraction. The aim of this study was to evaluate the possible protective effect of Spirulina platensis (SP), an antioxidant blue algae, on the contractile and relaxation reactivity of rat uterus and the balance of oxidative stress/antioxidant defenses. Female Wistar rats were divided into sedentary (CG), trained (TG), and T + supplemented (TG50, TG100) groups. Reactivity was analyzed by AQCAD, oxidative stress was evaluated by the malondialdehyde (MDA) formation, and the antioxidant capacity was measured by the 2,2-diphenyl-1-picrylhydrazyl (DPPH) method. Strength training increased contractile reactivity and decreased the pharmaco-mechanical component of relaxing reactivity in rat uterus. In addition, training decreased oxidation inhibition in the plasma and exercise increased oxidative stress in the uterine tissue; however, supplementation with algae prevented this effect and potentiated the increase in antioxidant capacity. Therefore, this study demonstrated that food supplementation prevents changes in reactivity and oxidative stress induced by strength training in a rat uterus, showing for the first time, that the uterus is a target for this exercise modality and antioxidant supplementation with S. platensis is an alternative means of preventing uterine dysfunction.

Contractile Properties of MHC I and II Fibers From Highly Trained Arm and Leg Muscles of Cross-Country Skiers

Introduction

Little is known about potential differences in contractile properties of muscle fibers of the same type in arms and legs. Accordingly, the present study was designed to compare the force-generating capacity and Ca²⁺ sensitivity of fibers from arm and leg muscles of highly trained cross-country skiers.

Method

Single muscle fibers of m. vastus lateralis and m. triceps brachii of eight highly trained cross-country skiers were analyzed with respect to maximal Ca²⁺-activated force, specific force and Ca²⁺ sensitivity.

Result

The maximal Ca²⁺-activated force was greater for myosin heavy chain (MHC) II than MHC I fibers in both the arm (+62%, P < 0.001) and leg muscle (+77%, P < 0.001), with no differences between limbs for each MHC isoform. In addition, the specific force of MHC II fibers was higher than that of MHC I fibers in both arms (+41%, P = 0.002) and legs (+95%, P < 0.001). The specific force of MHC II fibers was the same in both limbs, whereas MHC I fibers from the m. triceps brachii were, on average, 39% stronger than fibers of the same type from the m. vastus lateralis (P = 0.003). pCa₅₀ was not different between MHC I and II fibers in neither arms nor legs, but the MHC I fibers of m. triceps brachii demonstrated higher Ca²⁺ sensitivity than fibers of the same type from m. vastus lateralis (P = 0.007).

Conclusion

Comparison of muscles in limbs equally well trained revealed that MHC I fibers in the arm muscle exhibited a higher specific force-generating capacity and greater Ca²⁺ sensitivity than the same type of fiber in the leg, with no such difference in the case of MHC II fibers. These distinct differences in the properties of fibers of the same type in equally well-trained muscles open new perspectives in muscle physiology.

Modeling of the mussel catch muscle contractile apparatus in actomyosin suspension

In this paper, we tried to create a contractile model from proteins of the catch muscle of the Gray mussel, similar to the well-described suspension contractile model of vertebrate skeletal muscles. This model makes it possible to characterize the processes in the reconstructed contractile apparatus with the help of monitoring the two characteristics of muscle suspensions - the optical density and the particle size. Contractile model of the catch muscle we constructed was the simplest model consisting of two proteins, actin and myosin. During this work we compared the optical manifestations of the contraction and relaxation states of constructed model with earlier data on the actomyosin suspension of skeletal muscles. It appeared that the approach used in the study of skeletal muscle actomyosin relaxing - the use of an increased amount of ATP - cannot be applied to the contractile model of the molluscan catch muscle. Nevertheless we managed to reach relaxed state of this model with modifying calcium concentration. As a result, we laid the foundation for further reconstruction of the third state of the catch muscle - the catch tone.

Myeloperoxidase impairs the contractile function in isolated human cardiomyocytes

We set out to characterize the mechanical effects of myeloperoxidase (MPO) in isolated left-ventricular human cardiomyocytes. Oxidative myofilament protein modifications (sulfhydryl (SH)-group oxidation and carbonylation) induced by the peroxidase and chlorinating activities of MPO were additionally identified. The specificity of the MPO-evoked functional alterations was tested with an MPO inhibitor (MPO-I) and the antioxidant amino acid Met. The combined application of MPO and its substrate, hydrogen peroxide (H2O2), largely reduced the active force (Factive), increased the passive force (Fpassive), and decreased the Ca(2+) sensitivity of force production (pCa50) in permeabilized cardiomyocytes. H2O2 alone had significantly smaller effects on Factive and Fpassive and did not alter pCa50. The MPO-I blocked both the peroxidase and the chlorinating activities, whereas Met selectively inhibited the chlorinating activity of MPO. All of the MPO-induced functional effects could be prevented by the MPO-I and Met. Both H2O2 alone and MPO + H2O2 reduced the SH content of actin and increased the carbonylation of actin and myosin-binding protein C to the same extent. Neither the SH oxidation nor the carbonylation of the giant sarcomeric protein titin was affected by these treatments. MPO activation induces a cardiomyocyte dysfunction by affecting Ca(2+)-regulated active and Ca(2+)-independent passive force production and myofilament Ca(2+) sensitivity, independent of protein SH oxidation and carbonylation. The MPO-induced deleterious functional alterations can be prevented by the MPO-I and Met. Inhibition of MPO may be a promising therapeutic target to limit myocardial contractile dysfunction during inflammation.

Skeletal muscle myofilament adaptations to aging, disease, and disuse and their effects on whole muscle performance in older adult humans

Skeletal muscle contractile function declines with aging, disease, and disuse. In vivo muscle contractile function depends on a variety of factors, but force, contractile velocity and power generating capacity ultimately derive from the summed contribution of single muscle fibers. The contractile performance of these fibers are, in turn, dependent upon the isoform and function of myofilament proteins they express, with myosin protein expression and its mechanical and kinetic characteristics playing a predominant role. Alterations in myofilament protein biology, therefore, may contribute to the development of functional limitations and disability in these conditions. Recent studies suggest that these conditions are associated with altered single fiber performance due to decreased expression of myofilament proteins and/or changes in myosin-actin cross-bridge interactions. Furthermore, cellular and myofilament-level adaptations are related to diminished whole muscle and whole body performance. Notably, the effect of these various conditions on myofilament and single fiber function tends to be larger in older women compared to older men, which may partially contribute to their higher rates of disability. To maintain functionality and provide the most appropriate and effective countermeasures to aging, disease, and disuse in both sexes, a more thorough understanding is needed of the contribution of myofilament adaptations to functional disability in older men and women and their contribution to tissue level function and mobility impairment.

Hydrogen peroxide affects contractile activity and anti-oxidant enzymes in rat uterus

BACKGROUND AND PURPOSE

The effects of hydrogen peroxide (H(2)O(2)) on uterine smooth muscle are not well studied. We have investigated the effect and the mechanism of action of exogenous hydrogen peroxide on rat uteri contractile activity [spontaneous and calcium ion (Ca(2+))-induced] and the effect of such treatment on anti-oxidative enzyme activities.

EXPERIMENTAL APPROACH

Uteri were isolated from virgin Wistar rats and suspended in an organ bath. Uteri were allowed to contract spontaneously or in the presence of Ca(2+) (6 mM) and treated with H(2)O(2) (2 microM-3 mM) over 2 h. Anti-oxidative enzyme activities (manganese superoxide dismutase-MnSOD, copper-zinc superoxide dismutase-CuZnSOD, catalase-CAT, glutathione peroxidase-GSHPx and glutathione reductase-GR) in H(2)O(2)-treated uteri were compared with those in uteri immediately frozen after isolation or undergoing spontaneous or Ca(2+)-induced contractions, without treatment with H(2)O(2). The effect of inhibitors (propranolol, methylene blue, L-NAME, tetraethylamonium, glibenclamide and 4-aminopyridine) on H(2)O(2)-mediated relaxation was explored.

KEY RESULTS

H(2)O(2) caused concentration-dependent relaxation of both spontaneous and Ca(2+)-induced uterine contractions. After H(2)O(2) treatment, GSHPx and MnSOD activities were increased, while CuZnSOD and GR (In Ca(2+)-induced rat uteri) were decreased. N(omega)-nitro-L-arginine methyl ester antagonized the effect of H(2)O(2) on Ca(2+)-induced contractions. H(2)O(2)-induced relaxation was not affected by propranolol, potentiated by methylene blue and antagonized by tetraethylamonium, 4-aminopyridine and glibenclamide, with the last compound being the least effective.

CONCLUSIONS AND IMPLICATIONS

H(2)O(2) induced dose-dependent relaxation of isolated rat uteri mainly via changes in voltage-dependent potassium channels. Decreasing generation of reactive oxygen species by stimulation of anti-oxidative pathways may lead to new approaches to the management of dysfunctional uteri.

Oxidation of myofilament protein sulfhydryl groups reduces the contractile force and its Ca2+ sensitivity in human cardiomyocytes

This study sought to characterize the relation between the oxidation of protein sulfhydryl (SH) groups and Ca2+-activated force production in the human myocardium. Triton-permeabilized left ventricular cardiomyocytes from donor hearts were exposed to an oxidative (2,2'-dithiodipyridine, DTDP) agent in vitro, and the changes in isometric force, its Ca2+ sensitivity, the cross-bridge-sensitive rate constant of force redevelopment at saturating [Ca2+] (k(tr,max)), and protein SH oxidation were monitored. DTDP (0.1-10 mM for 2 min) oxidized the myocardial proteins and diminished the Ca2+-activated force with different concentration dependences (EC(50,SH) = 0.17 +/- 0.02 mM and EC(50,force) = 2.46 +/- 0.22 mM; mean +/- SEM). The application of 2.5 mM DTDP decreased the maximal Ca2+-activated force (to 64%), its Ca2+ sensitivity (DeltapCa(50) = 0.22 +/- 0.02), and the steepness of the Ca2+-force relation (n(Hill), from 2.01 +/- 0.08 to 1.76 +/- 0.08). These changes were paralleled by reductions in the free SH content of the proteins (to 15%) and in k(tr,max) (to 75%). SH-specific labeling identified SH oxidation of myosin light chain 1 and actin at DTDP concentrations at which the changes in the contractile parameters occurred. Our data suggest that SH oxidation in selected myofilament proteins diminishes the Ca2+-activated force and its Ca2+ sensitivity through an impaired Ca2+ regulation of the actin-myosin cycle in the human heart.

Hydroxyl radical and glutathione interactions alter calcium sensitivity and maximum force of the contractile apparatus in rat skeletal muscle fibres

Studies on intact muscle fibres indicate that reactive oxygen species (ROS) produced during muscle activity, or applied exogenously, can cause decreased force responses primarily by reducing the Ca(2+) sensitivity of the contractile apparatus. Identification of the molecular basis of this effect is complicated by the fact that studies on skinned muscle fibres in general have not observed reduced contractile Ca(2+) sensitivity when applying ROS, predominantly H(2)O(2). Here, using skinned fibres from rat extensor digitorum longus (EDL) and soleus muscle, it is shown that although H(2)O(2) (> or = 100 microm) has little effect by itself, when added in the presence of myoglobin it causes marked reduction in the Ca(2+) sensitivity of the contractile apparatus, probably due to production of hydroxyl radicals (OH(*)). Maximum force production is also reduced, but only with larger or more prolonged treatments. The effects are not prevented by tempol, a potent superoxide scavenger. Dithiotreitol (DTT) produces little reversal of the sensitivity change if applied afterwards, but it does substantially reverse all the changes if applied before the fibre undergoes an activation sequence. When glutathione (GSH, 5 mM) is present, exposure of EDL fibres to H(2)O(2) and myoglobin causes an increase in Ca(2+) sensitivity, with longer treatments causing a subsequent decrease, whereas in soleus fibres it causes only decreases in sensitivity and maximum force. The increased Ca(2+) sensitivity in EDL fibres is evidently due to the summed actions of (i) a potentiating effect of glutathionylation, which can be reversed by DTT and only occurs in fast-twitch fibres, and (ii) a less reversible reduction in sensitivity. Western blotting showed that reductions in Ca(2+) sensitivity were not due to loss of troponin-C. The present findings help provide a mechanistic basis for diverse findings on the effects of ROS in muscle fibres and implicate OH(*) radicals and glutathione as likely mediators of the effects.

Skeletal Muscle Fatigue: Cellular Mechanisms

Repeated, intense use of muscles leads to a decline in performance known as muscle fatigue. Many muscle properties change during fatigue including the action potential, extracellular and intracellular ions, and many intracellular metabolites. A range of mechanisms have been identified that contribute to the decline of performance. The traditional explanation, accumulation of intracellular lactate and hydrogen ions causing impaired function of the contractile proteins, is probably of limited importance in mammals. Alternative explanations that will be considered are the effects of ionic changes on the action potential, failure of SR Ca2+release by various mechanisms, and the effects of reactive oxygen species. Many different activities lead to fatigue, and an important challenge is to identify the various mechanisms that contribute under different circumstances. Most of the mechanistic studies of fatigue are on isolated animal tissues, and another major challenge is to use the knowledge generated in these studies to identify the mechanisms of fatigue in intact animals and particularly in human diseases.

O2(*-) production at 37 degrees C plays a critical role in depressing tetanic force of isolated rat and mouse skeletal muscle

To find out whether the decrease in muscle performance of isolated mammalian skeletal muscle associated with the increase in temperature toward physiological levels is related to the increase in muscle superoxide (O(2)(*-)) production, O(2)(*-) released extracellularly by intact isolated rat and mouse extensor digitorum longus (EDL) muscles was measured at 22, 32, and 37 degrees C in Krebs-Ringer solution, and tetanic force was measured in both preparations at 22 and 37 degrees C under the same conditions. The rate of O(2)(*-) production increased marginally when the temperature was increased from 22 to 32 degrees C, but increased fivefold when the temperature was increased from 22 to 37 degrees C in both rat and mouse preparations. This increase was accompanied by a marked decrease in tetanic force after 30 min incubation at 37 degrees C in both rat and mouse EDL muscles. Tetanic force remained largely depressed after return to 22 degrees C for up to 120 min. The specific maximum Ca(2+)-activated force measured in mechanically skinned fibers after the temperature treatment was markedly depressed in mouse fibers but was not significantly depressed in rat muscle fibers. The resting membrane and intracellular action potentials were, however, significantly affected by the temperature treatment in the rat fibers. The effects of the temperature treatment on tetanic force, maximum Ca(2+)-activated force, and membrane potential were largely prevented by 1 mM Tempol (4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl), a membrane-permeable superoxide dismutase mimetic, indicating that the increased O(2)(*-) production at physiological temperatures is largely responsible for the observed depression in tetanic force at 37 degrees C by affecting the contractile apparatus and plasma membrane.

In vivo fascicle behavior of synergistic muscles in concentric and eccentric plantar flexions in humans

Ultrasonography was used to directly measure in vivo fascicle behavior of the medial gastrocnemius (MG) and soleus (SOL) muscles while the subjects (n=6 men) performed maximal voluntary concentric and eccentric plantar flexions at 60, 120, 180 and 240 deg/s. Fascicle shortening and lengthening velocities of MG, obtained from fascicle length changes over time, were significantly higher than those of SOL at +/-120, +/-180 and +240 deg/s, possibly reflecting physiological and mechanical differences between these muscles. On the other hand, the effective fascicle shortening and lengthening velocities, defined as the velocities in the longitudinal direction of muscle belly, were not significantly different between MG and SOL. This could be due to difference in fascicle architecture and/or the existence of mechanical linkages between these muscles. Moreover, when the contribution of tendinous tissues to muscle-tendon complex length change was determined from fascicle length, pennation angle, moment arm and joint angle, it accounted for approximately 50% in both concentric and eccentric trials, but showed considerable intra-subject variations. This result quantifiably demonstrates the importance of tendinous tissues in isokinetically controlled joint movements.

The activity-induced reduction of myofibrillar Ca2+ sensitivity in mouse skeletal muscle is reversed by dithiothreitol

The aim of this study was to further characterize the reduction of myofibrillar Ca2+ sensitivity in mouse muscle which has been observed after fatigue at 37 degrees C. Muscle bundles and single fibres were isolated from mouse flexor digitorum brevis muscle and studied at 37 degrees C. The single fibres were injected with the Ca2+ indicator indo-1. Muscle fatigue was produced by 0.4 s tetani repeated at 4 s intervals until force had fallen to less than 50% of initial. Excitation-contraction coupling was assessed by measuring the cytosolic calcium concentration ([Ca2+]i) during tetani, and the maximum Ca2+-activated force and the myofibrillar Ca2+ sensitivity were estimated from a series of tetani at different stimulation frequencies. Two main results were found. (i) The reduction of Ca2+ sensitivity only occurred when the muscle was intensely stimulated leading to fatigue. When the muscle was rested for 10 min at 37 degrees C there was no significant change in Ca2+ sensitivity. (ii) If the membrane-permeant thiol-specific reducing agent dithiothreitol (0.5 mm) was applied to the muscle for 2 min following the fatigue protocol, the reduction in Ca2+ sensitivity was reversed. Dithiothreitol had no effect on Ca2+ sensitivity in unfatigued preparations. There was no effect of fatigue or dithiothreitol on tetanic [Ca2+]i or on the maximum Ca2+-activated force. These results suggest that intense activity of skeletal muscle at 37 degrees C causes the production of reactive oxygen species which oxidize a target protein. We propose that critical sulphydryl groups on the target protein(s) are converted to disulphide bonds and this reaction reduces Ca2+ sensitivity.

N-acetylcysteine fails to modulate the in vitro function of sarcoplasmic reticulum of diaphragm in the final phase of fatigue

AIM

In the present study, we tested the hypothesis whether N-acetylcysteine (NAC), a non-specific antioxidant, might influence fatigue by modulating Ca2+-handling capacity by the sarcoplasmic reticulum (SR).

METHODS

In the presence (10 mm) or absence of NAC, bundles of rat diaphragm were stimulated with tetanic trains (350 ms, 30-40 Hz) at 1 train every 2 s for 300 s. SR functions, as assessed by SR Ca2+-uptake and release rates and SR Ca2+-ATPase activity, were measured in vitro on muscle homogenates.

RESULTS

Following the 300-s stimulation, the force developed by NAC-treated muscles is approximately 1.8-fold higher (P < 0.05) than that of muscles without NAC treatment. Stimulation elicited an 18-30% depression in SR function (P < 0.05). Despite the differing degrees of fatigue between NAC-treated and non-treated muscles, SR functions in these muscles were reduced to similar extents.

CONCLUSIONS

These results suggest that modulation of SR function measured in vitro may not be a major contributor to inhibition of diaphragmic fatigue with antioxidant, at least, in the final phase of fatigue where force output is remarkably reduced.

A novel biosensor for mercuric ions based on motor proteins

We explored the potential of contractile proteins, actin and myosin, as biosensors of solutions containing mercuric ions. We demonstrate that the reaction of HgCl2 with myosin rapidly inhibits actin-activated myosin ATPase activity. Mercuric ions inhibit the in vitro analog of contraction, namely the ATP-initiated superprecipitation of the reconstituted actomyosin complex. Hg reduces both the rate and extent of this reaction. Direct observation of the propulsive movement of actin filaments (10 nm in diameter and 1 microm long) in a motility assay driven by a proteolytic fragment of myosin (heavy meromyosin or HMM) is also inhibited by mercuric ions. Thus, we have demonstrated the biochemical, biophysical and nanotechnological basis of what may prove to be a useful nano-device.

Effects of glycine and proline on the calcium activation properties of skinned muscle fibre segments from crayfish and rat

The effects of the polar amino acid glycine (20 mmol l(-1)) and the non-polar amino acid proline (20 mmol l(-1)) on Ca(2+)-activated contraction have been examined in four types of striated muscle fibres. Single fibres dissected from the claw muscle of a crustacean (long- and short-sarcomere) and the hindlimb muscles of the rat (slow-twitch from soleus and fast-twitch from extensor digitorum longus) were activated in matched solutions that either contained the amino acid ('test') or not ('control'). The steady-state force produced in these solutions was used to determine the relation between force production and pCa (-log10[Ca2+]). The results show that in the concentrations used, glycine and proline had only small effects on the maximum Ca(2+)-activated force, pCa corresponding to 10, 50 and 90% maximum force (pCa10, pCa50, pCa90, respectively) or on the slope of the force-pCa curves in the four different fibre types. The relative lack of effects of glycine and proline on contractile activation would confer a distinct physiological advantage to force production of muscle of Cherax, where the concentrations of glycine and proline vary considerably. Finally, the results show that glycine and proline may be useful to balance control solutions when the effects of other amino acids or zwitterions on contractile activation are examined.

Effects of oxidation and reduction on contractile function in skeletal muscle fibres of the rat

This study investigated the effects of the oxidants hydrogen peroxide (H(2)O(2)) and 2,2'-dithiodipyridine (DTDP), and reductants, glutathione (GSH) and dithiothreitol (DTT), on the properties of the contractile apparatus of rat fast- and slow-twitch skeletal muscle fibres, in order to assess how oxidation affects muscle function. Skinned muscle fibres were activated in heavily-buffered Ca(2+) solutions. The force-[Ca(2+)] relationship before and after various treatments was fitted by a Hill curve described by the maximum Ca(2+)-activated force, pCa(50) (-log(10)[Ca(2+)] giving half-maximum force) and n(H) (the Hill coefficient). Exposing freshly skinned fibres to strong reducing conditions (i.e. 10 mM DTT or 5 mM GSH) had little if any effect on Ca(2+) sensitivity (pCa(50) or n(H)). The effect of oxidants H(2)O(2) and DTDP depended on whether the fibre was relaxed (in pCa > 9) or activated during the exposure. In both fast- and slow-twitch fibres a 5 min exposure to 10 mM H(2)O(2) at pCa > 9 had no effect on pCa(50), causing only a reduction in n(H). In contrast, when fast-twitch fibres were activated in the presence of 10 mM H(2)O(2) (or 100 microM DTDP) there was a substantial increase in pCa(50) (by approximately 0.06 and 0.1, respectively), as well as larger decreases in n(H) than occurred in relaxed fibres, with all effects being reversed by DTT (10 mM, 10 min). In slow-twitch soleus fibres, the activation-dependent effect of DTDP was even greater (pCa(50) increased by ~0.35), and it was found that the rate of reversal in DTT was also increased by activation. A separate important phenomenon was that fast-twitch fibres that had been oxidised with H(2)O(2) or DTDP (while either relaxed or activated) showed a paradoxical increase in Ca(2+) sensitivity (~0.04 and 0.25 increase in pCa(50), respectively) when briefly exposed to the endogenous reductant GSH (5 mM, 2 min). This effect was reversed by DTT or longer (> 20 min) exposure to GSH, did not occur in slow-twitch soleus fibres, and may contribute to post-tetanic potentiation in fast-twitch muscle. Maximum force was not affected by any of the above treatments, whereas exposure to a high concentration of DTDP (1 mM) did greatly reduce force production. These findings reveal a number of novel and probably important effects of oxidation on the contractile apparatus in skeletal muscle fibres.

Reversible changes in Ca(2+)-activation properties of rat skeletal muscle exposed to elevated physiological temperatures

Exposure of relaxed rat extensor digitorum longus (EDL; predominantly fast-twitch) muscle to temperatures in the upper physiological range for mammalian skeletal muscle (43-46 degrees C) led to reversible alterations of the contractile activation properties. These properties were studied using the mechanically skinned fibre preparation activated in Ca(2+)-buffered solutions. The maximum Ca(2+)-activated force (maximum force per cross-sectional area) and the steepness of force-pCa (-log(10)[Ca(2+)]) curves as measured by the Hill coefficient (n(H)) reversibly decreased by factors of 8 and 2.5, respectively, when the EDL muscle was treated at 43 degrees C for 30 min and 5 and 2.8, respectively, with treatment at 46 degrees C for 5 min. Treatment at 47 degrees C for 5 min produced an even more marked depression in maximum specific force, which fully recovered after treatment, and in the Hill coefficient, which did not recover after treatment. After all temperature treatments there was no change in the level of [Ca(2+)] at which 50 % maximum force was generated. The temperature-induced depression in force production and steepness of the force-pCa curves were shown to be associated with superoxide (O(2)(-)) production in muscle (apparent rate of O(2)(-) production at room temperature, 0.055 +/- 0.008 nmol min(-1) (g wet weight)(-1); and following treatment to 46 degrees C for 5 min, 1.8 +/- 0.2 nmol min(-1) (g wet weight)(-1)) because 20 mM Tiron, a membrane-permeant O(2)(-) scavenger, was able to markedly suppress the net rate of O(2)(-) production and prevent any temperature-induced depression of contractile parameters. The temperature-induced depression in force production of the contractile apparatus could be reversed either by allowing the intact muscle to recover for 3-4 h at room temperature or by treatment of the skinned fibre preparation with dithiothreitol (a potent reducing agent) in the relaxing solution. These results demonstrate that mammalian skeletal muscle has the ability to uncouple force production reversibly from the activator Ca(2+) as the temperature increases in the upper physiological range through an increase in O(2)(-) production.

Early changes of type 2B fibers after denervation of rat EDL skeletal muscle

Skeletal muscle type 2B fibers normally receive a moderate level of motoneuron discharge. As a consequence, we hypothesize that type 2B fiber properties should be less sensitive to the absence of the nerve. Therefore, we have investigated the response of sarcoplasmic reticulum and myofibrillar proteins of type 2B fibers isolated from rat extensor digitorum longus muscle after denervation (2 and 7 days). Single fibers were identified by SDS-PAGE of myosin heavy chain isoforms. Electrophysiological and isometric contractile properties of the whole muscle were also analyzed. The pCa-tension relationship of type 2B single fibers was shifted to the left at 2 days and to right at 7 days after denervation, with significant differences in the Hill coefficients and pCa threshold values in 2- vs. 7-day-denervated fibers. The sarcoplasmic reticulum Ca2+ uptake capacity and rate significantly decreased after 2 days of denervation, whereas both increased at 7 days. Caffeine sensitivity of sarcoplasmic reticulum Ca2+ release was transitory and markedly increased in 2-day-denervated fibers. Our results indicate that type 2B fiber functional properties are highly sensitive to the interruption of nerve supply. Moreover, most of 2-day-denervated changes were reverted at 7 days.

Hydrogen peroxide increases depolarization-induced contraction of mechanically skinned slow twitch fibres from rat skeletal muscles

The effect of exogenous hydrogen peroxide (H(2)O(2)) on excitation-contraction (E-C) coupling and sarcoplasmic reticulum (SR) function was compared in mechanically skinned slow twitch fibres (prepared from the soleus muscles) and fast twitch fibres (prepared from the extensor digitorum longus; EDL muscles) of adult rats. Equilibration (5 min) with 1 mM H(2)O(2) diminished the ability of the Ca(2+)-depleted SR to reload Ca(2+) in both slow (P < 0.01) and fast twitch fibres (P < 0.05) compared to control. Under conditions when all Ca(2+) uptake was prevented, 1 mM H(2)O(2) increased SR Ca(2+) "leak" in fast twitch fibres by 24 +/- 5 % (P < 0.05), but leak was not altered in slow twitch fibres. Treatment with 1 mM H(2)O(2) also increased the peak force of low [caffeine] contracture by approximately 45% in both fibre types compared to control (P < 0.01), which could be partly reversed following treatment with 10 mM dithiothreitol (DTT). The changes in SR function caused by 1 mM H(2)O(2) were associated with an approximately 65% increase in the peak height of depolarization-induced contractile response (DICR) in slow twitch fibres, compared to control (no H(2)O(2); P < 0.05). In contrast, peak contractile force of fast twitch fibres was not altered by 1 mM H(2)O(2) treatment. Equilibration with 5 mM H(2)O(2) induced a spontaneous force response in both slow and fast twitch fibres, which could be partly reversed by 2 min treatment with 10 mM DTT. Peak DICR was also increased approximately 40% by 5 mM H(2)O(2) in slow twitch fibres compared to control (no H(2)O(2); P < 0.05). Our results indicate that exogenous H(2)O(2) increases depolarization-induced contraction of mechanically skinned slow but not fast twitch fibres. The increase in depolarization-induced contraction in slow twitch fibres might be mediated by an increased SR Ca(2+) release during contraction and/or an increase in Ca(2+) sensitivity.

Hydrogen peroxide modulates Ca2+-activation of single permeabilized fibres from fast- and slow-twitch skeletal muscles of rats

We examined the effects of redox modulation on single membrane-permeabilized fibre segments from the fast-twitch extensor digitorum longus (EDL) and slow-twitch soleus muscles of adult rats to determine whether the contractile apparatus was the redox target responsible for the increased contractility of muscles exposed to low concentrations of H2O2. The effects of H2O2 on maximum Ca2+-activated force were dose-dependent with 30 min exposure to 5 mM H2O2 causing a progressive decrease by 22+/-3 and 13+/-2% in soleus and EDL permeabilized muscle fibres, respectively. Lower concentrations of exogenous H2O2 (100 microM and 1 mM) had no effect on maximum Ca2+-activated force. Subsequent exposure to the reductant dithiothreitol (DTT, 10 mM, 10 min) fully reversed the H2O2-induced depression of force in EDL, but not in soleus muscle fibres. Incubation with DTT alone for 10 min did not alter Ca2+-activated force in either soleus or EDL muscle fibres. The sensitivity of the contractile filaments to Ca2+ (pCa50) was not altered by exposure to any concentration of exogenous H2O2. However, all concentrations of H2O2 diminished the Hill coefficient in permeabilized fibres from the EDL muscle, indicating that the cooperativity of Ca2+ binding to troponin is altered. H2O2 (5 mM) did not affect rigor force, which indicates that the number of crossbridges participating in contraction was not reduced. In conclusion, H2O2 may reduce the maximum Ca2+ activated force production in skinned muscle fibres by decreasing the force per crossbridge.

Rigor force responses of permeabilized fibres from fast and slow skeletal muscles of aged rats

1. Ageing is generally associated with a decline in skeletal muscle mass and strength and a slowing of muscle contraction, factors that impact upon the quality of life for the elderly. The mechanisms underlying this age-related muscle weakness have not been fully resolved. The purpose of the present study was to determine whether the decrease in muscle force as a consequence of age could be attributed partly to a decrease in the number of cross-bridges participating during contraction. 2. Given that the rigor force is proportional to the approximate total number of interacting sites between the actin and myosin filaments, we tested the null hypothesis that the rigor force of permeabilized muscle fibres from young and old rats would not be different. 3. Permeabilized fibres from the extensor digitorum longus (fast-twitch; EDL) and soleus (predominantly slow-twitch) muscles of young (6 months of age) and old (27 months of age) male F344 rats were activated in Ca2+-buffered solutions to determine force-pCa characteristics (where pCa = -log(10)[Ca2+]) and then in solutions lacking ATP and Ca2+ to determine rigor force levels. 4. The rigor forces for EDL and soleus muscle fibres were not different between young and old rats, indicating that the approximate total number of cross-bridges that can be formed between filaments did not decline with age. We conclude that the age-related decrease in force output is more likely attributed to a decrease in the force per cross-bridge and/or decreases in the efficiency of excitation-contraction coupling.

Contractile response of skeletal muscle to low peroxide concentrations: myofibrillar calcium sensitivity as a likely target for redox-modulation

Endogenous peroxides and related reactive oxygen species may influence various steps in the contractile process. Single mouse skeletal muscle fibers were used to study the effects of hydrogen peroxide (H2O2) and t-butyl hydroperoxide (t-BOOH) on force and myoplasmic Ca2+ concentration ([Ca2+]i). Both peroxides (1010 to 105 M) decreased tetanic [Ca2+]i and increased force during submaximal tetani. Catalase (1 kU/ml) blocked the effect of H2O2, but not of t-BOOH. The decrease in tetanic [Ca2+]i was constant, while the effect on force was biphasic: A transitory increase was followed by a steady decline to the initial level. Myofibrillar Ca2+ sensitivity remained increased during incubation with either peroxide. Only the highest peroxide concentration (10 mM) increased resting [Ca2+]i and slowed the return of [Ca2+]i to its resting level after a contraction, evidence of impaired sarcoplasmic reticulum Ca2+ re-uptake. The peroxides increased maximal force production and the rate of force redevelopment, and decreased maximum shortening velocity. N-ethylmaleimide (25 mM, thiol-alkylating agent) prevented the response to 1 mM H2O2. These results show that myofibrillar Ca2+ sensitivity and cross-bridge kinetics are influenced by H2O2 and t-BOOH concentrations that approach those found physiologically, and these findings indicate a role for endogenous oxidants in the regulation of skeletal muscle function.

Superoxide, hydroxyl radical, and hydrogen peroxide effects on single-diaphragm fiber contractile apparatus

Reactive oxygen species contribute to diaphragm dysfunction in certain pathophysiological conditions (i.e., sepsis and fatigue). However, the precise alterations induced by reactive oxygen species or the specific species that are responsible for the derangements in skeletal muscle function are incompletely understood. In this study, we evaluated the effect of the superoxide anion radical (O(2)(-).), hydroxyl radical (.OH), and hydrogen peroxide (H(2)O(2)) on maximum calcium-activated force (F(max)) and calcium sensitivity of the contractile apparatus in chemically skinned (Triton X-100) single rat diaphragm fibers. O(2)(-). was generated using the xanthine/xanthine oxidase system;.OH was generated using 1 mM FeCl(2), 1 mM ascorbate, and 1 mM H(2)O(2); and H(2)O(2) was added directly to the bathing medium. Exposure to O(2)(-). or.OH significantly decreased F(max) by 14.5% (P < 0.05) and 43.9% (P < 0. 005), respectively.OH had no effect on Ca(2+) sensitivity. Neither 10 nor 1,000 microM H(2)O(2) significantly altered F(max) or Ca(2+) sensitivity. We conclude that the diaphragm is susceptible to alterations induced by a direct effect of.OH and O(2)(-)., but not H(2)O(2), on the contractile proteins, which could, in part, be responsible for prolonged depression in contractility associated with respiratory muscle dysfunction in certain pathophysiological conditions.

Effects of fatigue on sarcoplasmic reticulum and myofibrillar properties of rat single muscle fibers

Force decline during fatigue in skeletal muscle is attributed mainly to progressive alterations of the intracellular milieu. Metabolite changes and the decline in free myoplasmic calcium influence the activation and contractile processes. This study was aimed at evaluating whether fatigue also causes persistent modifications of key myofibrillar and sarcoplasmic reticulum (SR) proteins that contribute to tension reduction. The presence of such modifications was investigated in chemically skinned fibers, a procedure that replaces the fatigued cytoplasm from the muscle fiber with a normal medium. Myofibrillar Ca(2+) sensitivity was reduced in slow-twitch muscle (for example, the pCa value corresponding to 50% of maximum tension was 6.23 +/- 0.03 vs. 5.99 + 0.05, P < 0.01, in rested and fatigued fibers) and not modified in fast-twitch muscle. Phosphorylation of the regulatory myosin light chain isoform increased in fast-twitch muscle. The rate of SR Ca(2+) uptake was increased in slow-twitch muscle fibers (14.2 +/- 1.0 vs. 19.6 +/- 2. 5 nmol. min(-1). mg fiber protein(-1), P < 0.05) and not altered in fast-twitch fibers. No persistent modifications of SR Ca(2+) release properties were found. These results indicate that persistent modifications of myofibrillar and SR properties contribute to fatigue-induced muscle force decline only in slow fibers. These alterations may be either enhanced or counteracted, in vivo, by the metabolic changes that normally occur during fatigue development.

Effect of hydrogen peroxide and dithiothreitol on contractile function of single skeletal muscle fibres from the mouse

1. We used intact single fibres from a mouse foot muscle to study the role of oxidation-reduction in the modulation of contractile function. 2. The oxidant hydrogen peroxide (H2O2, 100-300 microM) for brief periods did not change myoplasmic Ca2+ concentrations ([Ca2+]i) during submaximal tetani. However, force increased by 27 % during the same contractions. 3. The effects of H2O2 were time dependent. Prolonged exposures resulted in increased resting and tetanic [Ca2+]i, while force was significantly diminished. The force decline was mainly due to reduced myofibrillar Ca2+ sensitivity. There was also evidence of altered sarcoplasmic reticulum (SR) function: passive Ca2+ leak was increased and Ca2+ uptake was decreased. 4. The reductant dithiothreitol (DTT, 0.5-1 mM) did not change tetanic [Ca2+]i, but decreased force by over 40 %. This was completely reversed by subsequent incubations with H2O2. The force decline induced by prolonged exposure to H2O2 was reversed by subsequent exposure to DTT. 5. These results show that the elements of the contractile machinery are differentially responsive to changes in the oxidation-reduction balance of the muscle fibres. Myofibrillar Ca2+ sensitivity appears to be especially susceptible, while the SR functions (Ca2+ leak and uptake) are less so.

Dithiothreitol improves recovery from in vitro diaphragm fatigue

There is increasing evidence that reactive oxygen species are produced during strenuous skeletal muscle work and that they contribute to the development of muscle fatigue. Although the precise cellular mechanisms underlying such a phenomenon remain obscure, it has been hypothesized that endogenously produced reactive oxygen species may down-regulate force production during fatigue by oxidizing critical sulfhydryl groups on important contractile proteins. To test this hypothesis, we fatigued rat diaphragm strips in vitro for 4 min at 20 Hz stimulation and a duty cycle of 0.33. Following fatigue, the tissue baths were drained and randomly replaced with either physiologic saline or physiologic saline containing the disulfide reducing agent, dithiothreitol (DTT) at varying doses (0.1-5.0 mM). Force-frequency characteristics were then measured over a 90-min recovery period. At the 0.5 and 1.0 mM doses, DTT treatment was associated with significantly greater force production in the recovery period. DTT's effects were observed at most frequencies tested, but appeared more prominent at the higher frequencies. The beneficial effects of DTT were not evident at the 0.1 or 5.0 mM doses and appeared to be specific for fatigued muscle. These recovery-enhancing effects of a potent disulfide reducing agent suggest that important contractile proteins may be oxidized during fatigue; such changes may be readily reversible.

Events of the excitation-contraction-relaxation (E-C-R) cycle in fast- and slow-twitch mammalian muscle fibres relevant to muscle fatigue

The excitation-contraction-relaxation cycle (E-C-R) in the mammalian twitch muscle comprises the following major events: (1) initiation and propagation of an action potential along the sarcolemma and transverse (T)-tubular system; (2) detection of the T-system depolarization signal and signal transmission from the T-tubule to the sarcoplasmic reticulum (SR) membrane; (3) Ca2+ release from the SR; (4) transient rise of myoplasmic [Ca2+]; (5) transient activation of the Ca2+-regulatory system and of the contractile apparatus; (6) Ca2+ reuptake by the SR Ca2+ pump and Ca2+ binding to myoplasmic sites. There are many steps in the E-C-R cycle which can be seen as potential sites for muscle fatigue and this review explores how structural and functional differences between the fast- and slow-twitch fibres with respect to the E-C-R cycle events can explain to a great extent differences in their fatiguability profiles.

Skeletal muscle force and actomyosin ATPase activity reduced by nitric oxide donor

Nitric oxide (NO) may exert direct effects on actin-myosin cross-bridge cycling by modulating critical thiols on the myosin head. In the present study, the effects of the NO donor sodium nitroprusside (SNP; 100 microM to 10 mM) on mechanical properties and actomyosin adenosinetriphosphatase (ATPase) activity of single permeabilized muscle fibers from the rabbit psoas muscle were determined. The effects of N-ethylmaleimide (NEM; 5-250 microM), a thiol-specific alkylating reagent, on mechanical properties of single fibers were also evaluated. Both NEM (>/=25 microM) and SNP (>/=1 mM) significantly inhibited isometric force and actomyosin ATPase activity. The unloaded shortening velocity of SNP-treated single fibers was decreased, but to a lesser extent, suggesting that SNP effects on isometric force and actomyosin ATPase were largely due to decreased cross-bridge recruitment. The calcium sensitivity of SNP-treated single fibers was also decreased. The effects of SNP, but not NEM, on force and actomyosin ATPase activity were reversed by treatment with 10 mM DL-dithiothreitol, a thiol-reducing agent. We conclude that the NO donor SNP inhibits contractile function caused by reversible oxidation of contractile protein thiols.

Effects of reducing agents and oxidants on excitation-contraction coupling in skeletal muscle fibres of rat and toad

1. The mechanically skinned fibre technique was used to examine the role of oxidation-reduction in the control of Ca2+ release and contraction in rat and toad skeletal muscle fibres under physiological conditions of myoplasmic [Mg2+] and [ATP] and sarcoplasmic reticulum (SR) Ca2+ load. 2. None of the reducing agents, dithiothreitol (DTT, 10 mM), glutathione (GSH, 10 mM) or cysteine (1 and 5 mM), had any detectable effect on the peak force, duration or the total number of depolarization-induced responses that could be elicited in skinned fibres from either toad or rat muscle, except for a slight alteration in one case (GSH on the duration of the response in rat fibres) caused by an effect of the agent of the Ca2+ sensitivity of the contractile apparatus. 3. Application of the reactive disulphide, 2,2'-dithiodipyridine (DTDP, 100 microM), a potent oxidizing agent, never induced any measurable force response or noticeable depletion of SR Ca2+ in any fibre under the conditions used. When all Ca2+ uptake was prevented, DTDP treatment of rat fibres was found to cause a 2- to 3-fold increase in the low rate of Ca2+ "leak' from the SR. DTDP treatment also increased the responsiveness of toad muscle fibres to 1 or 2 mM caffeine. These effects could be largely reversed by treatment with DTT. These results indicate that oxidation of the Ca2+ release channel does not cause substantial channel opening under physiological conditions. 4. Depolarization-induced force responses in both rat and toad fibres were rapidly abolished in the presence of DTDP (10 or 100 microM), in a manner favoured by inactivation of the voltage sensors. The relatively impermeant oxidant, 5,5'-dithionitrobenzoic acid (DTNB, 100 microM), had an effect very similar to DTDP if applied intracellularly, but unlike DTDP, had little or no effect if applied extracellularly (at 5 mM) before skinning. Depolarization-induced responses could be restored by treatment with DTT (10 mM). Intracellular application of the sulfhydryl-alkylating agent, N-ethylmaleimide (NEM, 100 microM), had effects very similar to DTDP and DTNB. 5. These results are not consistent with the proposal that excitation-contraction coupling in skeletal muscle primarily involves the oxidative linkage of the voltage sensors to the Ca2+ release channels, but do show that oxidation of an intracellularly accessible site can interfere with the coupling, in a process made more sensitive by voltage sensor inactivation.

Molecular cogs in machina carnis

1. This review explores the complexity of skeletal muscle function mainly from the perspective of work performed by the author over the past two decades.

Effects of the reactive oxygen species hypochlorous acid and hydrogen peroxide on force production and calcium sensitivity of rat cardiac myofilaments

Neutrophil activation occurs after myocardial infarction/ischaemia. They produce the reactive oxygen species (ROS) hypochlorous acid (HOCl) and hydrogen peroxide (H2O2) which could contribute to contractile dysfunction upon reperfusion. The myofilaments of 'skinned' rat cardiac muscle were exposed to ROS in various states of activation. Isometric force was measured at controlled degrees of activation. A single application of 10 microM HOCl for 1 min increased log [Ca2+] for half-maximal activation (log K1/2) from 5.23 to 5.32, initial maximum Ca-activated force (FCa, max) was reduced by 18.8 +/- 5.8% and resting tension increased to 15.4 +/- 8.0% of FCa, max. At 50 microM, a 1-min exposure to HOCl produced a greater increase in Ca-sensitivity (log K1/2 increased from 5.23 to 5.47), a greater reduction in FCa, max (falling by 42.3 +/- 23.2%) and a greater increase in resting tension (to 25 +/- 10.7% of FCa, max). The nature of the resting tension rise was examined by reducing pH before and during exposure to HOCl; the results are consistent with 'rigor-like' cross-bridges being involved. H2O2 was without effect on the myofilaments at physiologically relevant (< 10 microM) concentrations. These results suggest that ROS associated with inflammation could contribute to post-ischaemic myocardial dysfunction.

Effects of sulfhydryl inhibitors on depolarizations-contraction coupling in frog skeletal muscle fibers

We have studied the effects of the sulfhydryl reagents on contractile responses, using either electrically stimulated single muscle fibers or short muscle fibers that were voltage-clamped with a two-microelectrode voltage-clamp technique that allows the fiber tension in response to membrane depolarization to be recorded. The sulfhydryl inhibitors para-chloromercuribenzoic acid (PCMB) and parahydroximercuriphenyl sulfonic acid (PHMPS), at concentrations from 0.5 to 2 mM, cause loss of the contractile ability; however, before this effect is completed, they change the fiber contractile behavior in a complex way. After relatively short exposure to the compounds, < 20 min, before the fibers lose their contractile capacity, secondary tension responses may appear after electrically elicited twitches or tetani. After losing their ability to contract in response to electrical stimulation, the fibers maintain their capacity to develop caffeine contractures, even after prolonged periods (120 min) of exposure to PHMPS. In fibers under voltage-clamp conditions, contractility is also lost; however, before this happens, long-lasting (i.e., minutes) episodes of spontaneous contractile activity may occur with the membrane polarized at -100 mV. After more prolonged exposure (> 30 min), the responses to membrane depolarization are reduced and eventually disappear. The agent DTT at a concentration of 2 mM appears to protect the fibers from the effects of PCMB and PHMPS. Furthermore, after loss of the contractile responses by the action of PCMB or PHMPS, addition of 2 mM DTT causes recovery of tension development capacity.