Mechanisms of excitation-contraction coupling relevant to skeletal muscle fatigue

Myoelectrical manifestation of fatigue less prominent in patients with cancer related fatigue

PURPOSE

A lack of fatigue-related muscle contractile property changes at time of perceived physical exhaustion and greater central than peripheral fatigue detected by twitch interpolation technique have recently been reported in cancer survivors with fatigue symptoms. Based on these observations, it was hypothesized that compared to healthy people, myoelectrical manifestation of fatigue in the performing muscles would be less significant in these individuals while sustaining a prolonged motor task to self-perceived exhaustion (SPE) since their central fatigue was more prominent. The purpose of this study was to test this hypothesis by examining electromyographic (EMG) signal changes during fatiguing muscle performance.

METHODS

Twelve individuals who had advanced solid cancer and cancer-related fatigue (CRF), and 12 age- and gender-matched healthy controls performed a sustained elbow flexion at 30% maximal voluntary contraction till SPE. Amplitude and mean power frequency (MPF) of EMG signals of the biceps brachii, brachioradialis, and triceps brachii muscles were evaluated when the individuals experienced minimal, moderate, and severe fatigue.

RESULTS

CRF patients perceived physical "exhaustion" significantly sooner than the controls. The myoelectrical manifestation of muscular fatigue assessed by EMG amplitude and MPF was less significant in CRF than controls. The lower MPF even at minimal fatigue stage in CRF may indicate pathophysiologic condition of the muscle.

CONCLUSIONS

CRF patients experience less myoelectrical manifestation of muscle fatigue than healthy individuals near the time of SPE. The data suggest that central nervous system fatigue plays a more important role in limiting endurance-type of motor performance in patients with CRF.

Lack of muscle contractile property changes at the time of perceived physical exhaustion suggests central mechanisms contributing to early motor task failure in patients with cancer-related fatigue

CONTEXT

Fatigue is one of the most common symptoms reported by cancer survivors, and fatigue worsens when patients are engaged in muscle exertion, which results in early motor task failure. Central fatigue plays a significant role, more than muscle (peripheral) fatigue, in contributing to early task failure in cancer-related fatigue (CRF).

OBJECTIVES

The purpose of this study was to determine if muscle contractile property alterations (reflecting muscle fatigue) occurred at the end of a low-intensity muscle contraction to exhaustion and if these properties differed between those with CRF and healthy controls.

METHODS

Ten patients (aged 59.9±10.6 years, seven women) with advanced solid cancer and CRF and 12 age- and gender-matched healthy controls (aged 46.6±12.8 years, nine women) performed a sustained contraction of the right arm elbow flexion at 30% maximal level until exhaustion. Peak twitch force, time to peak twitch force, rate of peak twitch force development, and half relaxation time derived from electrical stimulation-evoked twitches were analyzed pre- and post-sustained contraction.

RESULTS

CRF patients reported significantly greater fatigue as measured by the Brief Fatigue Inventory and failed the motor task earlier, 340±140 vs. 503±155 seconds in controls. All contractile property parameters did not change significantly in CRF but did change significantly in controls.

CONCLUSION

CRF patients perceive physical exhaustion sooner during a motor fatigue task with minimal muscular fatigue. The observation supports that central fatigue is a more significant factor than peripheral fatigue in causing fatigue feelings and limits motor function in cancer survivors with fatigue symptoms.

GLYCOGEN CONTENT AND CONTRACTILE RESPONSIVENESS TO T-SYSTEM DEPOLARIZATION IN SKINNED MUSCLE FIBRES OF THE RAT

1. Glycogen content (determined microfluorometrically), response capacity to transverse tubular (T) system depolarization and the relationship between these two parameters were examined in single, mechanically skinned fibres from rat extensor digitorum longus (EDL) muscle in the presence of high and constant concentrations of ATP and creatine phosphate. 2. The mean total glycogen content (tGlyc) in freshly dissected fibres was 58.1 +/- 4.2 mmol glucosyl units/L fibre (n = 53). 3. A large proportion of tGlyc was retained in the skinned fibres (SFGlyc) after 2 and 30 min exposure to an aqueous relaxing solution (73.1 +/- 2.8 and 64 +/- 12.3%, respectively). 4. When fibres were incubated for 30 min in a high (30 micromol/L)-Ca2+ solution, the proportion of SFGlyc was markedly lower (approximately 28%), which suggests that rat skinned fibres contain a Ca2+-sensitive glycogenolytic system. 5. In rat skinned fibres, T-system depolarization-induced Ca2+ release was not accompanied by a detectable loss of fibre glycogen and there was no correlation between response capacity and initial SFGlyc, indicating that other factors, unrelated to glycogen depletion, ultimately limited the capacity of rat skinned fibres to respond to T-system depolarization. 6. It is concluded that rat mechanically skinned fibre preparations are well suited for studies of glycogenolysis at a cellular level and that, with further refinement of the depolarization protocol, they may be suitable for studies of the non-metabolic role of glycogen in mammalian skeletal muscle contractility.

MHC-based fiber type and E-C coupling characteristics in mechanically skinned muscle fibers of the rat

In this study, we investigated whether the previously established differences between fast- and slow-twitch single skeletal muscle fibers of the rat, in terms of myosin heavy chain (MHC) isoform composition and contractile function, are also detectable in excitation-contraction (E-C) coupling. We compared the contractile responsiveness of electrophoretically typed, mechanically skinned single fibers from the soleus (Sol), the extensor digitorum longus (EDL), and the white region of the sternomastoid (SM) muscle to t-system depolarization-induced activation. The quantitative parameters assessed were the amplitude of the maximum depolarization-induced force response (DIFR(max); normalized to the maximum Ca(2+)-activated force in that fiber) and the number of responses elicited until the force declined by 75% of DIFR(max) (R-D(75%)). The mean DIFR(max) values for type IIB EDL and type IIB SM fibers were not statistically different, and both were greater than the mean DIFR(max) for type I Sol fibers. The mean R-D(75%) for type IIB EDL fibers was greater than that for type I Sol fibers as well as type IIB SM fibers. These data suggest that E-C coupling characteristics of mechanically skinned rat single muscle fibers are related to MHC-based fiber type and the muscle of origin.

Effects of ADP on sarcoplasmic reticulum function in mechanically skinned skeletal muscle fibres of the rat

1. The sarcoplasmic reticulum (SR) Ca(2+) content (expressed in terms of endogenous SR Ca(2+) content under physiologically resting conditions and measured from caffeine-induced force responses) and the effective rates of the SR Ca(2+) pump and SR Ca(2+) leak (measured from the temporal changes in SR Ca(2+) content) were determined in mechanically skinned skeletal muscle fibres of the rat at different [ADP] (< 0.10 microM to 1.04 mM). 2. The estimated SR Ca(2+) pump rate at 200 nM Ca(2+) did not change when [ADP] increased from below 0.10 microM to 10 microM but decreased by about 30 % when [ADP] increased from 10 microM to 1.04 mM. 3. The rate constant of SR Ca(2+) leak increased markedly with rising [ADP] when [Ca(2+)] in solution was 200 nM (apparent dissociation constant Kd(ADP) = 64 +/- 27 microM). Decreasing the [Ca(2+)] in solution from 200 nM to < 10 nM significantly increased the leak rate constant at all [ADP]. The SR Ca(2+) leak rate constant could be significantly reduced by blocking the SR Ca(2+) pump with 2,5-di(tert-butyl)-1,4-hydroquinone (TBQ). 4. The decrease in the SR Ca(2+) pump rate and the increase in the rate constant of SR Ca(2+) leak when the [ADP] increased from < 0.10 microM to 1.04 mM caused a 4.4-fold decrease in SR Ca(2+) loading ability at 200 nM Ca(2+). 5. The results can be fully explained by a mechanism whereby the presence of ADP causes a marked increase in the ADP-sensitive fraction of the phosphorylated pump protein, which can act as a Ca(2+)-Ca(2+) exchanger and demonstrates that ADP is an important modulator of SR function in skeletal muscle.

Glycogen content and excitation-contraction coupling in mechanically skinned muscle fibres of the cane toad

1. Mechanically skinned skeletal muscle fibres from the twitch region of the iliofibularis muscle of cane toads were used to investigate the relationship between fibre glycogen content and fibre capacity to respond to transverse tubular (T-) system depolarization. 2. A large proportion of total fibre glycogen remained in mechanically skinned muscle fibres exposed to aqueous solutions. This glycogen pool (about 80% of total fibre glycogen) was very stable when the preparation was incubated in a rigor solution (pH 7.0) but decreased gradually at a rate of 0.59+/-0.20% min-1 in a relaxing solution (200 nM [Ca2+]). The rate was considerably higher (2.66+/-0.38% min(-1)) when the preparations were exposed to 30 microM [Ca2+]. An even greater rate of glycogen loss was found after T-system depolarization-induced contractions. The Ca2+-dependent loss of fibre glycogen was caused by endogenous glycogenolytic processes. 3. Silver stained SDS gels of components eluted into relaxing solution from single skinned fibres revealed a rapid (2 min) loss of parvalbumin and at least 10 other proteins varying in molecular mass between 10 and 80 kDa but there was essentially no loss of myosin heavy and light chains and actin. Subsequent elution for a further 30 min in either relaxing or maximally Ca2+-activating solution did not result in additional, appreciable detectable loss of fibre protein. 4. Depletion of fibre glycogen was associated with loss of fibre ability to respond to T-system depolarization even though the bathing solutions contained high levels of ATP (8 mM) and creatine phosphate (10 mM). 5. The capacity of mechanically skinned fibres to respond to T-system depolarization was highly positively correlated (P<0.0001) with initial fibre glycogen concentration. 6. In conclusion, the results show that (i) the capacity of skeletal muscle to respond to T-system depolarization is related directly or indirectly to the non-washable glycogen pool in fibres, (ii) this relationship holds for conditions where glycogen is not required as a source of energy and (iii) the mechanically skinned fibre preparation is well suited to study the regulation of endogenous glycogenolytic enzymes.

E-C coupling failure in mouse EDL muscle after in vivo eccentric contractions

The objectives of this research were to determine the contribution of excitation-contraction (E-C) coupling failure to the decrement in maximal isometric tetanic force (Po) in mouse extensor digitorum longus (EDL) muscles after eccentric contractions and to elucidate possible mechanisms. The left anterior crural muscles of female ICR mice (n = 164) were injured in vivo with 150 eccentric contractions. Po, caffeine-, 4-chloro-m-cresol-, and K+-induced contracture forces, sarcoplasmic reticulum (SR) Ca2+ release and uptake rates, and intracellular Ca2+ concentration ([Ca2+]i) were then measured in vitro in injured and contralateral control EDL muscles at various times after injury up to 14 days. On the basis of the disproportional reduction in Po (approximately 51%) compared with caffeine-induced force (approximately 11-21%), we estimate that E-C coupling failure can explain 57-75% of the Po decrement from 0 to 5 days postinjury. Comparable reductions in Po and K+-induced force (51%), and minor reductions (0-6%) in the maximal SR Ca2+ release rate, suggest that the E-C coupling defect site is located at the t tubule-SR interface immediately after injury. Confocal laser scanning microscopy indicated that resting [Ca2+]i was elevated and peak tetanic [Ca2+]i was reduced, whereas peak 4-chloro-m-cresol-induced [Ca2+]i was unchanged immediately after injury. By 3 days postinjury, 4-chloro-m-cresol-induced [Ca2+]i became depressed, probably because of decreased SR Ca2+ release and uptake rates (17-31%). These data indicate that the decrease in Po during the first several days after injury primarily stems from a failure in the E-C coupling process.

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 fatigue in normal subjects and heart failure patients. Is there a common mechanism?

Skeletal muscle fatigue develops gradually during all forms of exercise, and develops more rapidly in heart failure patients. The fatigue mechanism is still not known, but is most likely localized to the muscle cells themselves. During high intensity exercise the perturbations of the Na+ and K+ balance in the exercising muscle favour depolarization, smaller action potentials and inexcitability. The Na+, K+ pump becomes strongly activated and limits, but does not prevent the rise in extracellular Na+, K+ pump concentration and intracellular Na+ concentration. However, by virtue of its electrogenic property the pump may contribute in maintaining excitability and contractility by keeping the cells more polarized than the ion gradients predict. With prolonged exercise perturbations of Na+ and K+ are smaller and fatigue may be associated with altered cellular handling of Ca2+ and Mg2+. Release of Ca2+ from the sarcoplasmic reticulum (SR) is reduced in the absence of changes of the cellular content of Ca2+ and Mg2+. In heart failure several clinical reports indicate severe electrolyte perturbations in skeletal muscle. However, in well controlled studies small or insignificant changes are found. We conclude that with high intensity exercise perturbations of Na+ and K+ in muscle cells may contribute to fatigue, whereas with endurance type of exercise and in heart failure patients the skeletal muscle fatigue is more likely to reside in the intracellular control of Ca2+ release and reuptake.

Lactate inhibits Ca(2+) -activated Ca(2+)-channel activity from skeletal muscle sarcoplasmic reticulum

Sarcoplasmic reticulum (SR) Ca(2+)-release channel function is modified by ligands that are generated during about of exercise. We have examined the effects of lactate on Ca(2+)- and caffeine-stimulated Ca2+ release, [3H]ryanodine binding, and single Ca(2+)-release channel activity of SR isolated from rabbit white skeletal muscle. Lactate, at concentrations from 10 to 30 mM, inhibited Ca(2+)- and caffeine-stimulated nodine binding to and inhibited Ca(2+)- and caffeine-stimulated [3H]ryanodine binding to and inhibited Ca(2+)- and caffeine-stimulated Ca2+ release from SR vesicles. Lactate also inhibited caffeine activation of single-channel activity in bilayer reconstitution experiments. These findings suggest that intense muscle activity, which generates high concentrations of lactate, will disrupt excitation-contraction coupling. This may lead to decreases in Ca2+ transients promoting a decline in tension development and contribute to muscle fatigue.

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.