Heterogeneity of T-tubule geometry in vertebrate skeletal muscle fibres

Exercise and fatigue: integrating the role of K+, Na+ and Cl- in the regulation of sarcolemmal excitability of skeletal muscle

Perturbations in K+ have long been considered a key factor in skeletal muscle fatigue. However, the exercise-induced changes in K+ intra-to-extracellular gradient is by itself insufficiently large to be a major cause for the force decrease during fatigue unless combined to other ion gradient changes such as for Na+. Whilst several studies described K+-induced force depression at high extracellular [K+] ([K+]e), others reported that small increases in [K+]e induced potentiation during submaximal activation frequencies, a finding that has mostly been ignored. There is evidence for decreased Cl- ClC-1 channel activity at muscle activity onset, which may limit K+-induced force depression, and large increases in ClC-1 channel activity during metabolic stress that may enhance K+ induced force depression. The ATP-sensitive K+ channel (KATP channel) is also activated during metabolic stress to lower sarcolemmal excitability. Taking into account all these findings, we propose a revised concept in which K+ has two physiological roles: (1) K+-induced potentiation and (2) K+-induced force depression. During low-moderate intensity muscle contractions, the K+-induced force depression associated with increased [K+]e is prevented by concomitant decreased ClC-1 channel activity, allowing K+-induced potentiation of sub-maximal tetanic contractions to dominate, thereby optimizing muscle performance. When ATP demand exceeds supply, creating metabolic stress, both KATP and ClC-1 channels are activated. KATP channels contribute to force reductions by lowering sarcolemmal generation of action potentials, whilst ClC-1 channel enhances the force-depressing effects of K+, thereby triggering fatigue. The ultimate function of these changes is to preserve the remaining ATP to prevent damaging ATP depletion.

Muscle Ionic Shifts During Exercise: Implications for Fatigue and Exercise Performance

Exercise causes major shifts in multiple ions (e.g., K⁺ , Na⁺ , H⁺ , lactate^- , Ca²⁺ , and Cl^- ) during muscle activity that contributes to development of muscle fatigue. Sarcolemmal processes can be impaired by the trans-sarcolemmal rundown of ion gradients for K⁺ , Na⁺ , and Ca²⁺ during fatiguing exercise, while changes in gradients for Cl^- and Cl^- conductance may exert either protective or detrimental effects on fatigue. Myocellular H⁺ accumulation may also contribute to fatigue development by lowering glycolytic rate and has been shown to act synergistically with inorganic phosphate (Pi) to compromise cross-bridge function. In addition, sarcoplasmic reticulum Ca²⁺ release function is severely affected by fatiguing exercise. Skeletal muscle has a multitude of ion transport systems that counter exercise-related ionic shifts of which the Na⁺ /K⁺ -ATPase is of major importance. Metabolic perturbations occurring during exercise can exacerbate trans-sarcolemmal ionic shifts, in particular for K⁺ and Cl^- , respectively via metabolic regulation of the ATP-sensitive K⁺ channel (K_ATP ) and the chloride channel isoform 1 (ClC-1). Ion transport systems are highly adaptable to exercise training resulting in an enhanced ability to counter ionic disturbances to delay fatigue and improve exercise performance. In this article, we discuss (i) the ionic shifts occurring during exercise, (ii) the role of ion transport systems in skeletal muscle for ionic regulation, (iii) how ionic disturbances affect sarcolemmal processes and muscle fatigue, (iv) how metabolic perturbations exacerbate ionic shifts during exercise, and (v) how pharmacological manipulation and exercise training regulate ion transport systems to influence exercise performance in humans. © 2021 American Physiological Society. Compr Physiol 11:1895-1959, 2021.

Preclinical pharmacological in vitro investigations on low chloride conductance myotonia: effects of potassium regulation

In myotonia, reduced Cl^- conductance of the mutated ClC-1 channels causes hindered muscle relaxation after forceful voluntary contraction due to muscle membrane hyperexcitability. Repetitive contraction temporarily decreases myotonia, a phenomena called "warm up." The underlying mechanism for the reduction of hyperexcitability in warm-up is currently unknown. Since potassium displacement is known to reduce excitability in, for example, muscle fatigue, we characterized the role of potassium in native myotonia congenita (MC) muscle. Muscle specimens of ADR mice (an animal model for low gCl^- conductance myotonia) were exposed to increasing K⁺ concentrations. To characterize functional effects of potassium ion current, the muscle of ADR mice was exposed to agonists and antagonists of the big conductance Ca²⁺-activated K⁺ channel (BK) and the voltage-gated Kv7 channel. Effects were monitored by functional force and membrane potential measurements. By increasing [K⁺]₀ to 5 mM, the warm-up phenomena started earlier and at [K⁺]₀ 7 mM only weak myotonia was detected. The increase of [K⁺]₀ caused a sustained membrane depolarization accompanied with a reduction of myotonic bursts in ADR mice. Retigabine, a Kv7.2-Kv7.5 activator, dose-dependently reduced relaxation deficit of ADR myotonic muscle contraction and promoted the warm-up phenomena. In vitro results of this study suggest that increasing potassium conductivity via activation of voltage-gated potassium channels enhanced the warm-up phenomena, thereby offering a potential therapeutic treatment option for myotonia congenita.

Inhibition of glycogenolysis prolongs action potential repriming period and impairs muscle function in rat skeletal muscle

KEY POINTS

Muscle glycogen content is associated with muscle function, but the physiological link between the two is poorly understood. This study investigated the effects of inhibiting glycogenolysis, while maintaining high overall energy status, on different aspects of muscle function. We demonstrate here that Na⁺ ,K⁺ -ATPase activity depends on glycogenolytically derived ATP regardless of high global ATP, with a decrease in activity leading to reduced force production and accelerated fatigue development. The results support the concept of compartmentalized energy transfer with glycogen metabolism playing a crucial role in intramuscular ATP resynthesis and ion regulation. This study gives specific insights into muscular function and may help towards a better understanding of glycogen storage diseases and muscle fatigue.

ABSTRACT

Skeletal muscle glycogen content is associated with muscle function and fatigability. However, little is known about the physiological link between glycogen content and muscle function. Here we aimed to investigate the importance of glycogenolytically derived ATP per se on muscle force and action potential (AP) repriming period, i.e. the time before a second AP can be produced (indicative of Na⁺ ,K⁺ -ATPase activity). Single fibres from rat extensor digitorum longus muscles were isolated and mechanically skinned in order to investigate force production and the AP repriming period while global ATP and PCr concentrations were kept high. The importance of glycogenolytically derived ATP was studied by inhibition of glycogen phosphorylase (1,4-dideoxy-1,4-imino-d-arabinitol (DAB; 2 mm) or CP-316,819 (CP; 10 µm)) or glycogen removal (amyloglucosidase, 20 U ml^-1 ). Tetanic force decreased by (mean (SD)) 21 (15)% (P < 0.001) and 76 (28)% (DAB) or 94 (6)% (CP, P < 0.001) in well-polarized and partially depolarized fibres, respectively. In depolarized fibres, twitch force decreased by 16 (10)% and 55 (26)% with DAB and CP, respectively, with no effect in well-polarized fibres (84 (10)%, P = 0.14). There was no effect of glycogen phosphorylase inhibition on repriming period in well-polarized fibres (median (25th, 75th percentile): 5 (4, 5) vs. 4 (4, 5) ms, P = 0.26), while the repriming period was prolonged from 6 (5, 7) to 8 (7, 10) ms (P = 0.01) in partially depolarized fibres. In line with this, glycogen removal increased repriming period from 5 (5, 6) to 6 (5, 7) ms (P = 0.003) in depolarized fibres. Together, these data strongly indicate that blocking glycogenolysis attenuates Na⁺ ,K⁺ -ATPase activity, which in turn increases the repriming period and reduces force, demonstrating a functional link between glycogenolytically derived ATP and force production.

NHE- and diffusion-dependent proton fluxes across the tubular system membranes of fast-twitch muscle fibers of the rat

The regulation of pH across the t-system membrane of skeletal muscle fibers is poorly understood. Using a sealed tubule preparation, Launikonis et al. reveal Na⁺/H⁺ exchange activity and characterize the properties of the diffusional and NHE proton fluxes across the t-system.

The complex membrane structure of the tubular system (t-system) in skeletal muscle fibers is open to the extracellular environment, which prevents measurements of H⁺ movement across its interface with the cytoplasm by conventional methods. Consequently, little is known about the t-system’s role in the regulation of cytoplasmic pH, which is different from extracellular pH. Here we describe a novel approach to measure H⁺-flux measurements across the t-system of fast-twitch fibers under different conditions. The approach involves loading the t-system of intact rat fast-twitch fibers with a strong pH buffer (20 mM HEPES) and pH-sensitive fluorescent probe (10 mM HPTS) before the t-system is sealed off. The pH changes in the t-system are then tracked by confocal microscopy after rapid changes in cytoplasmic ionic conditions. T-system sealing is achieved by removing the sarcolemma by microdissection (mechanical skinning), which causes the tubules to pinch off and seal tight. After this procedure, the t-system repolarizes to physiological levels and can be electrically stimulated when placed in K⁺-based solutions of cytosolic-like ionic composition. Using this approach, we show that the t-system of fast-twitch skeletal fibers displays amiloride-sensitive Na⁺/H⁺ exchange (NHE), which decreases markedly at alkaline cytosolic pH and has properties similar to that in mammalian cardiac myocytes. We observed mean values for NHE density and proton permeability coefficient of 339 pmol/m² of t-system membrane and 158 µm/s, respectively. We conclude that the cytosolic pH in intact resting muscle can be quantitatively explained with respect to extracellular pH by assuming that these values apply to the t-system membrane and the sarcolemma.

A quantitative description of tubular system Ca(2+) handling in fast- and slow-twitch muscle fibres

KEY POINTS

Current methods do not allow a quantitative description of Ca(2+) movements across the tubular (t-) system membrane without isolating the membranes from their native skeletal muscle fibre. Here we present a fluorescence-based method that allows determination of the t-system [Ca(2+) ] transients and derivation of t-system Ca(2+) fluxes in mechanically skinned skeletal muscle fibres. Differences in t-system Ca(2+) -handling properties between fast- and slow-twitch fibres from rat muscle are resolved for the first time using this new technique. The method can be used to study Ca(2+) handling of the t-system and allows direct comparisons of t-system Ca(2+) transients and Ca(2+) fluxes between groups of fibres and fibres from different strains of animals.

ABSTRACT

The tubular (t-) system of skeletal muscle is an internalization of the plasma membrane that maintains a large Ca(2+) gradient and exchanges Ca(2+) between the extracellular and intracellular environments. Little is known of the Ca(2+) -handling properties of the t-system as the small Ca(2+) fluxes conducted are difficult to resolve with conventional methods. To advance knowledge in this area we calibrated t-system-trapped rhod-5N inside skinned fibres from rat and [Ca(2+) ]t-sys , allowing confocal measurements of Ca(2+) -dependent changes in rhod-5N fluorescence during rapid changes in the intracellular ionic environment to be converted to [Ca(2+) ] transients in the t-system ([Ca(2+) ]t-sys (t)). Furthermore, t-system Ca(2+) -buffering power was determined so that t-system Ca(2+) fluxes could be derived from [Ca(2+) ]t-sys (t). With this new approach, we show that rapid depletion of sarcoplasmic reticulum (SR) Ca(2+) induced a robust store-operated Ca(2+) entry (SOCE) in fast- and slow-twitch fibres, reducing [Ca(2+) ]t-sys to < 0.1 mm. The rapid activation of SOCE upon Ca(2+) release was consistent with the presence of STIM1L in both fibre types. Abruptly introducing internal solutions with 1 mm Mg(2+) and [Ca(2+) ]cyto (28 nm-1.3 μm) to Ca(2+) -depleted fibres generated t-system Ca(2+) uptake rates dependent on [Ca(2+) ]cyto with [Ca(2+) ]t-sys reaching final plateaus in the millimolar range. For the same [Ca(2+) ]cyto , t-system Ca(2+) fluxes of fast-twitch fibres were greater than that in slow-twitch fibres. In addition, simultaneous imaging of t-system and SR Ca(2+) signals indicated that both membrane compartments accumulated Ca(2+) at similar rates and that SOCE was activated early during SR Ca(2+) depletion.

Observation of the molecular organization of calcium release sites in fast- and slow-twitch skeletal muscle with nanoscale imaging

Localization microscopy is a fairly recently introduced super-resolution fluorescence imaging modality capable of achieving nanometre-scale resolution. We have applied the dSTORM variation of this method to image intracellular molecular assemblies in skeletal muscle fibres which are large cells that critically rely on nanoscale signalling domains, the triads. Immunofluorescence staining in fixed adult rat skeletal muscle sections revealed clear differences between fast- and slow-twitch fibres in the molecular organization of ryanodine receptors (RyRs; the primary calcium release channels) within triads. With the improved resolution offered by dSTORM, abutting arrays of RyRs in transverse view of fast fibres were observed in contrast to the fragmented distribution on slow-twitch muscle that were approximately 1.8 times shorter and consisted of approximately 1.6 times fewer receptors. To the best of our knowledge, for the first time, we have quantified the nanometre-scale spatial association between triadic proteins using multi-colour super-resolution, an analysis difficult to conduct with electron microscopy. Our findings confirm that junctophilin-1 (JPH1), which tethers the sarcoplasmic reticulum ((SR) intracellular calcium store) to the tubular (t-) system at triads, was present throughout the RyR array, whereas JPH2 was contained within much smaller nanodomains. Similar imaging of the primary SR calcium buffer, calsequestrin (CSQ), detected less overlap of the triad with CSQ in slow-twitch muscle supporting greater spatial heterogeneity in the luminal Ca2+ buffering when compared with fast twitch muscle. Taken together, these nanoscale differences can explain the fundamentally different physiologies of fast- and slow-twitch muscle.

Three-dimensional reconstruction and analysis of the tubular system of vertebrate skeletal muscle

Skeletal muscle fibres are very large and elongated. In response to excitation there must be a rapid and uniform release of Ca(2+) throughout for contraction. To ensure a uniform spread of excitation throughout the fibre to all the Ca(2+) release sites, the muscle internalizes the plasma membrane, to form the tubular (t-) system. Hence the t-system forms a complex and dense network throughout the fibre that is responsible for excitation-contraction coupling and other signalling mechanisms. However, we currently do not have a very detailed view of this membrane network because of limitations in previously used imaging techniques to visualize it. In this study we serially imaged fluorescent dye trapped in the t-system of fibres from rat and toad muscle using the confocal microscope, and deconvolved and reconstructed these images to produce the first three-dimensional reconstructions of large volumes of the vertebrate t-system. These images showed complex arrangements of tubules that have not been described previously and also allowed the association of the t-system with cellular organelles to be visualized. There was a high density of tubules close to the nuclear envelope because of the close and parallel alignment of the long axes of the myofibrils and the nuclei. Furthermore local fluorescence intensity variations from sub-resolution tubules were converted to tubule diameters. Mean diameters of tubules were 85.9±6.6 and 91.2±8.2 nm, from rat and toad muscle under isotonic conditions, respectively. Under osmotic stress the distribution of tubular diameters shifted significantly in toad muscle only, with change specifically occurring in the transverse but not longitudinal tubules.

Examination of the subsarcolemmal tubular system of mammalian skeletal muscle fibers

A subsarcolemmal tubular system network (SSTN) has been detected in skeletal muscle fibers by confocal imaging after the removal of the sarcolemma. Here we confirm the existence and resolve the fine architecture and the localization of the SSTN at an unprecedented level of detail by examining extracellularly applied tubular system markers in skeletal muscle fiber preparations with a combination of three imaging modalities: confocal fluorescence microscopy, direct stochastic optical reconstruction microscopy, and tomographic electron microscopy. Three-dimensional reconstructions showed that the SSTN was a dense two-dimensional network within the subsarcolemmal space around the fiber, running ~500-600 nm underneath and parallel to the sarcolemma. The SSTN is composed of tubules ~95 nm in width with ~60% of the tubules directed transversely and >30% directed longitudinally. The deeper regular transverse tubules located at each A-I boundary of the sarcomeres branched from the SSTN, indicating individual transverse tubules that form triads are continuous with, but do not directly contact the sarcolemma. This suggests that the SSTN plays an important role in affecting the exchange of deeper tubule lumina with the extracellular space.

Relationship between membrane Cl- conductance and contractile endurance in isolated rat muscles

Resting skeletal muscle fibres have a large membrane Cl(-) conductance (G(Cl)) that dampens their excitability. Recently, however, muscle activity was shown to induce PKC-mediated reduction in G(Cl) in rat muscles of 40-90%. To examine the physiological significance of this PKC-mediated G(Cl) reduction for the function of muscles, this study explored effects of G(Cl) reductions on contractile endurance in isolated rat muscles. Contractile endurance was assessed from the ability of muscle to maintain force during prolonged stimulation under conditions when G(Cl) was manipulated by: (i) inhibition of PKC, (ii) reduction of solution Cl(-) or (iii) inhibition of ClC-1 Cl(-) channels using 9-anthracene-carboxylic acid (9-AC). Experiments showed that contractile endurance was optimally preserved by reductions in G(Cl) similar to what occurs in active muscle. Contrastingly, further G(Cl) reductions compromised the endurance. The experiments thus show a biphasic relationship between G(Cl) and contractile endurance in which partial G(Cl) reduction improves endurance while further G(Cl) reduction compromises endurance. Intracellular recordings of trains of action potentials suggest that this biphasic dependency of contractile endurance on G(Cl) reflects that lowering G(Cl) enhances muscle excitability but low G(Cl) also increases the depolarisation of muscle fibres during excitation and reduces their ability to re-accumulate K(+) lost during excitation. If G(Cl) becomes very low, the latter actions dominate causing reduced endurance. It is concluded that the PKC-mediated ClC-1 channel inhibition in active muscle reduces G(Cl) to a level that optimises contractile endurance during intense exercise.

An analysis of the relationships between subthreshold electrical properties and excitability in skeletal muscle

Skeletal muscle activation requires action potential (AP) initiation followed by its sarcolemmal propagation and tubular excitation to trigger Ca²⁺ release and contraction. Recent studies demonstrate that ion channels underlying the resting membrane conductance (G_M) of fast-twitch mammalian muscle fibers are highly regulated during muscle activity. Thus, onset of activity reduces G_M, whereas prolonged activity can markedly elevate G_M. Although these observations implicate G_M regulation in control of muscle excitability, classical theoretical studies in un-myelinated axons predict little influence of G_M on membrane excitability. However, surface membrane morphologies differ markedly between un-myelinated axons and muscle fibers, predominantly because of the tubular (t)-system of muscle fibers. This study develops a linear circuit model of mammalian muscle fiber and uses this to assess the role of subthreshold electrical properties, including G_M changes during muscle activity, for AP initiation, AP propagation, and t-system excitation. Experimental observations of frequency-dependent length constant and membrane-phase properties in fast-twitch rat fibers could only be replicated by models that included t-system luminal resistances. Having quantified these resistances, the resulting models showed enhanced conduction velocity of passive current flow also implicating elevated AP propagation velocity. Furthermore, the resistances filter passive currents such that higher frequency current components would determine sarcolemma AP conduction velocity, whereas lower frequency components excite t-system APs. Because G_M modulation affects only the low-frequency membrane impedance, the G_M changes in active muscle would predominantly affect neuromuscular transmission and low-frequency t-system excitation while exerting little influence on the high-frequency process of sarcolemmal AP propagation. This physiological role of G_M regulation was increased by high Cl⁻ permeability, as in muscle endplate regions, and by increased extracellular [K⁺], as observed in working muscle. Thus, reduced G_M at the onset of exercise would enhance t-system excitation and neuromuscular transmission, whereas elevated G_M after sustained activity would inhibit these processes and thereby accentuate muscle fatigue.

Ultra-rapid activation and deactivation of store-operated Ca(2+) entry in skeletal muscle

Skeletal muscle is highly specialized for the rapid delivery of Ca(2+) to the contractile apparatus during excitation-contraction coupling (EC coupling). Previous studies have shown the presence of a relatively fast-activated store-operated Ca(2+) entry (SOCE) mechanism (<1s) to be present in skeletal muscle, unlike the situation occurring in non-excitable cells. We simultaneously imaged [Ca(2+)] in the t-system and cytoplasm in mechanically skinned fibers during SR Ca(2+) release and observed both cell-wide Ca(2+) release and Ca(2+) waves. SOCE activation followed cell-wide Ca(2+) release from high sarcoplasmic reticulum (SR) [Ca(2+)] ([Ca(2+)](SR)) by seconds, consistent with depletion of [Ca(2+)](SR) to an absolute threshold for SOCE and an unformed SOCE complex at high [Ca(2+)](SR). Ca(2+) waves occurred at low [Ca(2+)](SR), close to the threshold for SOCE, minimizing the time between Ca(2+) release and Ca(2+) influx. Local activation of SOCE during Ca(2+) waves occurred in approximately 27ms following local initiation of SR depletion indicating a steep relationship between [Ca(2+)](SR) and SOCE activation. Most of this delay was due to slow release of Ca(2+) from SR, leaving only milliseconds at most for the activation of Ca(2+) entry following store depletion. SOCE was also observed to deactivate effectively instantly during store refilling at low [Ca(2+)](SR). These rapid kinetics of SOCE persisted as subsequent Ca(2+) waves propagated along the fiber. Thus we show for the first time millisecond activation and deactivation of SOCE during low amplitude [Ca(2+)](SR) oscillations at low [Ca(2+)](SR). To account for the observed Ca(2+) movements we propose the SOCE complex forms during the progressive depletion of [Ca(2+)](SR) prior to reaching the activation threshold of SOCE and this complex remains stable at low [Ca(2+)](SR).

Potassium-transporting proteins in skeletal muscle: cellular location and fibre-type differences

Abstract Potassium (K(+)) displacement in skeletal muscle may be an important factor in the development of muscle fatigue during intense exercise. It has been shown in vitro that an increase in the extracellular K(+) concentration ([K(+)](e)) to values higher than approx. 10 mm significantly reduce force development in unfatigued skeletal muscle. Several in vivo studies have shown that [K(+)](e) increases progressively with increasing work intensity, reaching values higher than 10 mm. This increase in [K(+)](e) is expected to be even higher in the transverse (T)-tubules than the concentration reached in the interstitium. Besides the voltage-sensitive K(+) (K(v)) channels that generate the action potential (AP) it is suggested that the big-conductance Ca(2+)-dependent K(+) (K(Ca)1.1) channel contributes significantly to the K(+) release into the T-tubules. Also the ATP-dependent K(+) (K(ATP)) channel participates, but is suggested primarily to participate in K(+) release to the interstitium. Because there is restricted diffusion of K(+) to the interstitium, K(+) released to the T-tubules during AP propagation will be removed primarily by reuptake mediated by transport proteins located in the T-tubule membrane. The most important protein that mediates K(+) reuptake in the T-tubules is the Na(+),K(+)-ATPase alpha(2) dimers, but a significant contribution of the strong inward rectifier K(+) (Kir2.1) channel is also suggested. The Na(+), K(+), 2Cl(-) 1 (NKCC1) cotransporter also participates in K(+) reuptake but probably mainly from the interstitium. The relative content of the different K(+)-transporting proteins differs in oxidative and glycolytic muscles, and might explain the different [K(+)](e) tolerance observed.

L-type Ca2+ channel function is linked to dystrophin expression in mammalian muscle

Background

In dystrophic mdx skeletal muscle, aberrant Ca²⁺ homeostasis and fibre degeneration are found. The absence of dystrophin in models of Duchenne muscular dystrophy (DMD) has been connected to altered ion channel properties e.g. impaired L-type Ca²⁺ currents. In regenerating mdx muscle, ‘revertant’ fibres restore dystrophin expression. Their functionality involving DHPR-Ca²⁺-channels is elusive.

Methods and Results

We developed a novel ‘in-situ’ confocal immuno-fluorescence and imaging technique that allows, for the first time, quantitative subcellular dystrophin-DHPR colocalization in individual, non-fixed, muscle fibres. Tubular DHPR signals alternated with second harmonic generation signals originating from myosin. Dystrophin-DHPR colocalization was substantial in wt fibres, but diminished in most mdx fibres. Mini-dystrophin (MinD) expressing fibres successfully restored colocalization. Interestingly, in some aged mdx fibres, colocalization was similar to wt fibres. Most mdx fibres showed very weak membrane dystrophin staining and were classified ‘mdx-like’. Some mdx fibres, however, had strong ‘wt-like’ dystrophin signals and were identified as ‘revertants’. Split mdx fibres were mostly ‘mdx-like’ and are not generally ‘revertants’. Correlations between membrane dystrophin and DHPR colocalization suggest a restored putative link in ‘revertants’. Using the two-micro-electrode-voltage clamp technique, Ca²⁺-current amplitudes (i_max) showed very similar behaviours: reduced amplitudes in most aged mdx fibres (as seen exclusively in young mdx mice) and a few mdx fibres, most likely ‘revertants’, with amplitudes similar to wt or MinD fibres. Ca²⁺ current activation curves were similar in ‘wt-like’ and ‘mdx-like’ aged mdx fibres and are not the cause for the differences in current amplitudes. i_max amplitudes were fully restored in MinD fibres.

Conclusions

We present evidence for a direct/indirect DHPR-dystrophin interaction present in wt, MinD and ‘revertant’ mdx fibres but absent in remaining mdx fibres. Our imaging technique reliably detects single isolated ‘revertant’ fibres that could be used for subsequent physiological experiments to study mechanisms and therapy concepts in DMD.

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.

Chloride conductance in the transverse tubular system of rat skeletal muscle fibres: importance in excitation-contraction coupling and fatigue

Contraction in skeletal muscle fibres is governed by excitation of the transverse-tubular (t-) system, but the properties of the t-system and their importance in normal excitability are not well defined. Here we investigate the properties of the t-system chloride conductance using rat skinned muscle fibres in which the sarcolemma has been mechanically removed but the normal excitation-contraction coupling mechanism kept functional. When the t-system chloride conductance was eliminated, either by removal of all Cl(-) or by block of the chloride channels with 9-anthracene carboxylic acid (9-AC) or by treating muscles with phorbol 12,13-dibutyrate, there was a marked reduction in the threshold electric field intensity required to elicit a t-system action potential (AP) and twitch response. Calculations of the t-system chloride conductance indicated that it constitutes a large proportion of the total chloride conductance observed in intact fibres. Blocking the chloride conductance increased the size of the twitch response and was indicative that Cl(-) normally carries part of the repolarizing current across the t-system membrane on each AP. Block of the t-system chloride conductance also reduced tetanic force responses at higher frequency stimulation (100 Hz) and greatly reduced twitch responses in the period shortly after a brief tetanus, owing to rapid loss of t-system excitability during the AP train. Blocking activity of the Na(+)-K(+) pump in the t-system membrane caused loss of excitability owing to K(+) build-up in the sealed t-system, and this occurred approximately 3-4 times faster when the chloride conductance was blocked. These findings show that the t-system chloride conductance plays a vital role during normal activity by countering the effects of K(+) accumulation in the t-system and maintaining muscle excitability.

Assembly of transverse tubule architecture in the middle and myotendinous junctional regions in developing rat skeletal muscle fibers

The transverse (t)-tubule is responsible for the rapid inward spread of excitation from the sarcolemma to the inside of the muscle fiber, and the compartments of the t-tubule become highly and regularly organized during development. Although it is known that skeletal muscle fibers lengthen by adding sarcomeres at the myotendinous junction (MTJ) during development, no specific model exists for the assembly of new t-tubule architecture at the MTJ. We performed an electron-microscopic examination of the assembly of t-tubule architecture at the MTJ in developing rat skeletal muscle fibers. Although the longitudinally oriented t-tubule elements represent only a small fraction of the total t-tubule system in adult muscle fibers, they were observed at both A-band and I-band regions of middle and MTJ regions in early developmental stages, and gradually disappeared in the middle regions of muscle fibers during development; however, they remained in the MTJ even in adult muscle fibers. The frequency of pentads and heptads (two or three t-tubule elements with three or four elements of terminal cisternae, closely aligned with terminal cisternae of the sarcoplasmic reticulum) decreased during development, with sudden decrease between 7 and 10 weeks of age in the middle regions. Interestingly, although the frequency of decrease appeared to be higher in the middle region than in the MTJ regions in early (3- to 7-week) development, this pattern reversed, and the frequency of decrease was higher in the MTJ in later development (after 10 weeks of age). The MTJ maintained the features of immature membrane systems involved in e-c coupling much longer than the middle region of the fiber during development. The assembly of t-tubule architecture during postnatal development thus follows different processes in the middle and MTJ regions of skeletal muscle fibers.

Store-operated Ca2+ entry during intracellular Ca2+ release in mammalian skeletal muscle

Store-operated Ca2+ entry (SOCE) is activated following the depletion of internal Ca2+ stores in virtually all eukaryotic cells. Shifted excitation and emission ratioing of fluorescence (SEER) was used to image mag-indo-1 trapped in the tubular (t) system of mechanically skinned rat skeletal muscle fibres to measure SOCE during intracellular Ca2+ release. Cytosolic Ca2+ transients were simultaneously imaged using the fluorescence of rhod-2. Spatially and temporally resolved images of t system [Ca2+] ([Ca2+]t-sys) allowed estimation of Ca2+ entry flux from the rate of decay of [Ca2+]t-sys. Ca2+ release was induced pharmacologically to activate SOCE without voltage-dependent contributions to Ca2+ flux. Inward Ca2+ flux was monotonically dependent on the [Ca2+] gradient, and strongly dependent on the transmembrane potential. The activation of SOCE was controlled locally. It could occur without full Ca2+ store depletion and in less than a second after initiation of store depletion. These results indicate that the molecular agonists of SOCE must be evenly distributed throughout the junctional membranes and can activate rapidly. Termination of SOCE required a net increase in [Ca2+]SR. Activation and termination of SOCE are also demonstrated, for the first time, during a single event of Ca2+ release. At the physiological [Ca2+]t-sys, near 2 mM (relative to t system volume), SOCE flux relative to accessible cytoplasmic volume was at least 18.6 microM s(-1), consistent with times of SR refilling of 1-2 min measured in intact muscle fibres.

Na+-K+ pumps in the transverse tubular system of skeletal muscle fibers preferentially use ATP from glycolysis

The Na(+)-K(+) pumps in the transverse tubular (T) system of a muscle fiber play a vital role keeping K(+) concentration in the T-system sufficiently low during activity to prevent chronic depolarization and consequent loss of excitability. These Na(+)-K(+) pumps are located in the triad junction, the key transduction zone controlling excitation-contraction (EC) coupling, a region rich in glycolytic enzymes and likely having high localized ATP usage and limited substrate diffusion. This study examined whether Na(+)-K(+) pump function is dependent on ATP derived via the glycolytic pathway locally within the triad region. Single fibers from rat fast-twitch muscle were mechanically skinned, sealing off the T-system but retaining normal EC coupling. Intracellular composition was set by the bathing solution and action potentials (APs) triggered in the T-system, eliciting intracellular Ca(2+) release and twitch and tetanic force responses. Conditions were selected such that increased Na(+)-K(+) pump function could be detected from the consequent increase in T-system polarization and resultant faster rate of AP repriming. Na(+)-K(+) pump function was not adequately supported by maintaining cytoplasmic ATP concentration at its normal resting level ( approximately 8 mM), even with 10 or 40 mM creatine phosphate present. Addition of as little as 1 mM phospho(enol)pyruvate resulted in a marked increase in Na(+)-K(+) pump function, supported by endogenous pyruvate kinase bound within the triad. These results demonstrate that the triad junction is a highly restricted microenvironment, where glycolytic resynthesis of ATP is critical to meet the high demand of the Na(+)-K(+) pump and maintain muscle excitability.

Tubular system excitability: an essential component of excitation–contraction coupling in fast-twitch fibres of vertebrate skeletal muscle

The tubular (t-) system is the main interface between the myoplasm and the extracellular environment and is responsible for the rapid inward spread of excitation from the sarcolemma to the inner parts of the skeletal muscle fibre as well as for signal transfer to the sarcoplasmic reticulum to release Ca2+ that, in turn, activates the contractile apparatus. In this review, I explore the insights provided by the mechanically skinned muscle fibre preparation to the better understanding of the importance of the t-system excitability in determining the force response under physiologically relevant conditions. In the mechanically skinned muscle fibre, the t-system seals off after is physically separated from the sarcolemma and its excitability can be investigated by electrical stimulation under controlled conditions. Parameters that can be assessed include the threshold for action potential generation, specific electrical resistance and time constant of the tubular wall, quantity of charge transferred during an action potential, refractory period, length constant and velocity of excitation propagation. Results obtained with mechanically skinned fibres from fast-twitch muscles show that decreased t-system excitability does not necessarily translate into reduced force output, but for any particular set of physiologically relevant conditions there is a level below which a further decrease in t-system excitability markedly decreases the force output. There are several built-in mechanisms linked to the metabolic/energetic state of the muscle fibre which prevent complete action potential failure in the t-system, thus allowing the muscle to respond to nerve stimulation, even if the response becomes markedly attenuated.

Osmotic properties of the sealed tubular system of toad and rat skeletal muscle

A method was developed that allows conversion of changes in maximum Ca(2+)-dependent fluorescence of a fixed amount of fluo-3 into volume changes of the fluo-3-containing solution. This method was then applied to investigate by confocal microscopy the osmotic properties of the sealed tubular (t-) system of toad and rat mechanically skinned fibers in which a certain amount of fluo-3 was trapped. When the osmolality of the myoplasmic environment was altered by simple dilution or addition of sucrose within the range 190-638 mosmol kg(-1), the sealed t-system of toad fibers behaved almost like an ideal osmometer, changing its volume inverse proportionally to osmolality. However, increasing the osmolality above 638 to 2,550 mosmol kg(-1) caused hardly any change in t-system volume. In myoplasmic solutions made hypotonic to 128 mosmol kg(-1), a loss of Ca(2+) from the sealed t-system of toad fibers occurred, presumably through either stretch-activated cationic channels or store-operated Ca(2+) channels. In contrast to the behavior of the t-system in toad fibers, the volume of the sealed t-system of rat fibers changed little (by <20%) when the osmolality of the myoplasmic environment changed between 210 and 2,800 mosmol kg(-1). Results were also validated with calcein. Clear differences between rat and toad fibers were also found with respect to the t-system permeability for glycerol. Thus, glycerol equilibrated across the rat t-system within seconds to minutes, but was not equilibrated across the t-system of toad fibers even after 20 min. These results have broad implications for understanding osmotic properties of the t-system and reversible vacuolation in muscle fibers. Furthermore, we observed for the first time in mammalian fibers an orderly lateral shift of the t-system networks whereby t-tubule networks to the left of the Z-line crossover to become t-tubule networks to the right of the Z-line in the adjacent sarcomere (and vice versa). This orderly rearrangement can provide a pathway for longitudinal continuity of the t-system along the fiber axis.

Tubular system volume changes in twitch fibres from toad and rat skeletal muscle assessed by confocal microscopy

The volume of the extracellular compartment (tubular system) within intact muscle fibres from cane toad and rat was measured under various conditions using confocal microscopy. Under physiological conditions at rest, the fractional volume of the tubular system (t-sys(Vol)) was 1.38 +/- 0.09 % (n = 17), 1.41 +/- 0.09 % (n = 12) and 0.83 +/- 0.07 % (n = 12) of the total fibre volume in the twitch fibres from toad iliofibularis muscle, rat extensor digitorum longus muscle and rat soleus muscle, respectively. In toad muscle fibres, the t-sys(Vol) decreased by 30 % when the tubular system was fully depolarized and decreased by 15 % when membrane cholesterol was depleted from the tubular system with methyl-beta-cyclodextrin but did not change as the sarcomere length was changed from 1.93 to 3.30 microm. There was also an increase by 30 % and a decrease by 25 % in t-sys(Vol) when toad fibres were equilibrated in solutions that were 2.5-fold hypertonic and 50 % hypotonic, respectively. When the changes in total fibre volume were taken into consideration, the t-sys(Vol) expressed as a percentage of the isotonic fibre volume did actually decrease as tonicity increased, revealing that the tubular system in intact fibres cannot be compressed below 0.9 % of the isotonic fibre volume. The results can be explained in terms of forces acting at the level of the tubular wall. These observations have important physiological implications showing that the tubular system is a dynamic membrane structure capable of changing its volume in response to the membrane potential, cholesterol depletion and osmotic stress but not when the sarcomere length is changed in resting muscle.

Small-conductance calcium-activated potassium currents in mouse hyperexcitable denervated skeletal muscle

1. Hyperexcitability in denervated skeletal muscle is associated with the expression of SK3, a small-conductance Ca2+-activated K+ channel (SK channel). SK currents were examined in dissociated fibres from flexor digitorum brevis (FDB) muscle using the whole-cell patch clamp configuration. 2. Depolarization activated a K+-selective, apamin-sensitive and iberiotoxin-insensitive current, detected as a tail current upon repolarization, in fibres from denervated but not innervated muscle. Dialysis of the fibres with 20 mM EGTA in the patch pipette solution eliminated the tail current, consistent with this current reflecting Ca2+-activated SK channels expressed only in denervated muscle. 3. Activation of SK tail currents depended on the duration of the depolarizing pulse, consistent with a rise in intracellular Ca2+ due to release from the sarcoplasmic reticulum (SR) and influx through voltage-gated Ca2+ channels. 4. The envelope of SK tail currents was diminished by 10 microM ryanodine for all pulse durations, whereas 2 mM cobalt reduced the SK tail current for pulses greater than 80 ms, demonstrating that Ca2+ release from the SR during short pulses primarily activated SK channels. 5. In current clamp mode with the resting membrane potential set at -70 mV, denervation decreased the action potential threshold by approximately 8 mV. Application of apamin increased the action potential threshold in denervated fibres to that measured in innervated fibres, suggesting that SK channel activity modulates the apparent action potential threshold. 6. These results are consistent with a model in which SK channel activity in the T-tubules of denervated skeletal muscle causes a local increase in K+ concentration that results in hyperexcitability.

Reversible vacuolation of T-tubules in skeletal muscle: mechanisms and implications for cell biology

The majority of investigations of the transverse tubules (T-system) of skeletal muscle have been devoted to their role in excitation-contraction coupling, with particular reference to contact with the sarcoplasmic reticulum and the mechanism of Ca2- release. By contrast, this review is concerned with structural and functional aspects of the vacuolation of T-tubules. It covers experimental procedures used in reversible vacuolation induced by the efflux-influx of glycerol and other small nonelectrolytes, sugars, and ions. The characteristics of the phenomenon, associated alterations in muscle function, and the swelling of analogous structures in nonmuscle cells are considered. Possible functions of reversible vacuolation in water balance, transport, membrane repair, muscle pathology, and fatigue are considered, and the potential application of reversible vacuolation in the transfection of skeletal muscle is discussed. In relation to the possible mechanisms involved in reversible vacuolation, particular attention is given to the dynamic and structural aspects of the opening and closing of T-tubules, the origin of vacuolar membranes, and the localized character of tubular swelling.

The T-tubular network and its triads in the sole plate sarcoplasm of the motor end-plate of mammals

The transition between the subsynaptic folds and the T-tubules demonstrated in a former paper was further investigated in the sole plate area by using the extracellular marker Lanthanum. A tubular network of the T-system of the sole plate area which is connected to the subsynaptic folds and to the t-tubular elements between the myofibrils is described for the first time. T-tubules of this network criss-cross through the sarcoplasm of the sole plate and lie in close proximity to sole plate nuclei and mitochondria. Cisterns of sarcoplasmic reticulum of the sole plate area make contact with these t-tubules forming triads. The possible physiological role of this sole plate network and its triads will be discussed with regard to a transport of substances in tubules with the dimension of nanotubes and Ca2+ activated processes in the sole plate area.

Shape, size, and distribution of Ca(2+) release units and couplons in skeletal and cardiac muscles

Excitation contraction (e-c) coupling in skeletal and cardiac muscles involves an interaction between specialized junctional domains of the sarcoplasmic reticulum (SR) and of exterior membranes (either surface membrane or transverse (T) tubules). This interaction occurs at special structures named calcium release units (CRUs). CRUs contain two proteins essential to e-c coupling: dihydropyridine receptors (DHPRs), L-type Ca(2+) channels of exterior membranes; and ryanodine receptors (RyRs), the Ca(2+) release channels of the SR. Special CRUs in cardiac muscle are constituted by SR domains bearing RyRs that are not associated with exterior membranes (the corbular and extended junctional SR or EjSR). Functional groupings of RyRs and DHPRs within calcium release units have been named couplons, and the term is also loosely applied to the EjSR of cardiac muscle. Knowledge of the structure, geometry, and disposition of couplons is essential to understand the mechanism of Ca(2+) release during muscle activation. This paper presents a compilation of quantitative data on couplons in a variety of skeletal and cardiac muscles, which is useful in modeling calcium release events, both macroscopic and microscopic ("sparks").

Calcium currents during contraction and shortening in enzymatically isolated murine skeletal muscle fibres

1. Calcium currents (ICa) were monitored in enzymatically isolated murine toe muscle fibres using the two-microelectrode voltage-clamp technique. ICa was recorded (i) in hypertonic solution to suppress contraction, and (ii) in actively shortening fibres in isotonic solution. 2. In hypertonic solution the threshold potential for ICa was about -30 mV for both 2 and 10 mM external Ca2+ solution. Maximum peak currents measured -12.6 +/- 2.3 nA (mean +/- s.d.; n = 4) in 2 mM Ca2+ and -65 +/- 15 nA (n = 7) in 10 mM Ca2+. The time to peak (TTP) ICa was 96 +/- 22 ms (n = 4) in 2 mM Ca2+ and 132 +/- 13 ms (n = 7) in 10 mM Ca2+. The exponential decay of ICa was similar in 2 and 10 mM Ca2+ with rate constants (tau-1(V)) of 3.7 s-1 (2 mM) and 3.8 s-1 (10 mM) at +10 mV. 3. ICa in isotonic 10 mM Ca2+ solution was recorded by inserting the micropipettes exactly opposite to each other close to the centre of mass of the fibre where negligible contraction-induced movement occurs. 4. In isotonic 10 mM Ca2+ solution ICa had a smaller peak amplitude (-45 +/- 5 nA; n = 7) and faster TTP (82.8 +/- 22.1 ms; n = 7) than in hypertonic solution. The exponential decay of ICa showed a significantly larger tau-1(V) of 6.4 s-1 at +10 mV (P < 0.03). 5. To test for calcium depletion, extracellular Ca2+ was buffered by malic acid in isotonic solution to 9 mM. The decay of ICa had a time constant of 348 +/- 175 ms (n = 14) vs. 107 +/- 24 ms (n = 12; P < 0.001) at 0 mV in unbuffered 10 mM Ca2+ solution. 6. We conclude that calcium depletion from the transverse tubular system contributes significantly to the decay of calcium currents in murine toe muscle fibres under hypertonic as well as isotonic conditions. In the latter, depletion is even more prominent.

Effect of transverse-tubular chloride conductance on excitability in skinned skeletal muscle fibres of rat and toad

1. The influence of the transverse-tubular (T-) system Cl- conductance on membrane excitability in skeletal muscle fibres of toad and rat was examined because of conflicting conclusions of previous studies on Cl- conductance. A mechanically skinned fibre preparation was used that permitted investigation of Ca2+ release via the normal T-system voltage-sensor mechanism after complete removal of the surface membrane, which thereby allowed estimation of the T-system potential from force measurements. 2. When a skinned fibre was bathed in a high-[K+] solution, the sealed T-system became polarized and could be rapidly depolarized by replacing the K+ with Na+, thereby eliciting Ca2+ release from the sarcoplasmic reticulum. In rat skinned fibres, addition of 20 mM Cl- to the 'myoplasm' (i.e. bathing solution) partially depolarized the T-system, inducing Ca2+ release and subsequent voltage-sensor inactivation. These effects were completely abolished with 100 microM of the Cl- channel blocker 9-anthracene carboxylic acid (9-AC). Voltage-sensor inactivation increased in a graded manner over the range 3-20 mM myoplasmic Cl-. 3. In toad fibres, voltage-sensor inactivation was only detectable at > 10 mM myoplasmic Cl-, and 20 mM Cl- was only able to depolarize the T-system sufficiently to trigger Ca2+ release if the myoplasmic [K+] was reduced by 50 %. In toad fibres, 100 microM 9-AC caused little if any block of the T-system Cl- conductance. 4. It was also found that when skinned fibres were obtained from muscles that had been bathed in a zero Cl- extracellular solution, the initial Na+ substitutions were more effective at depolarizing the T-system. This is consistent with Cl- trapped in the sealed T-system exerting a polarizing effect on T-system potential. 5. These results unequivocally demonstrate that there is a large 9-AC-sensitive Cl- conductance in the T-system of rat fibres, and a smaller, though still appreciable, Cl- conductance in the T-system of toad fibres, which is relatively insensitive to 9-AC. The results are important for understanding the basis of the Cl- channel aberration in myotonia.

Membrane domain localization of pH-regulating transporters in frog skeletal muscle membrane vesicles

The distribution of pH-regulating transporters in surface and transverse (T) tubular membrane (TTM) domains of frog skeletal muscle was studied. 2',7'-Bis(carboxyethyl)-5(6)- carboxyfluorescein-loaded giant sarcolemmal vesicles, containing surface membrane, exhibited reversible Na+/H+ exchange. A microsomal vesicle fraction was shown to be enriched in TTM on the basis of high Na(+)-K(+)-ATPase and Mg(2+)-ATPase activity, high ouabain and nitrendipine binding, and low Ca(2+)-ATPase activity. TTM vesicles were well sealed and oriented inside out. Vesicles were loaded with the pH-sensitive dye pyranine. In response to an inwardly directed Na+ gradient, vesicles displayed virtually no alkalinization unless monensin was present. No pH response to an imposed Na+ gradient was seen regardless of the direction of the pH gradient across the vesicles, after phosphorylation of the vesicles with protein kinase C, or when exposed to guanosine 5'-O-(3-thiotriphosphate). In the presence of CO2, addition of Na+ or Cl- had no effect on vesicle pH. These data indicate that the TTM lacks functional pH-regulating transporters [Na+/H+ and (Na+ + HCO3-)/Cl- exchangers], suggesting that pH-regulating transporters are localized only to the surface membrane domain in frog muscle.

Depolarization accelerates the decay of K+ contractures in rat skeletal muscle fibers

The experiments examine the effects of membrane potential on the time course of K+ contractures in small bundles of rat soleus and extensor digitorum longus (EDL) muscle fibers. Control K+ contractures were induced by exposure to a 200 mmol/L K+ solution in polarized fibers with a resting membrane potential of -83 mV (3.5 mmol/L K+), while test contractures were evoked with 200 mmol/L K+ from -46 mV, after 5, 10, and 30 min in a 30 mmol/L K+ conditioning solution. The decay times of the test K+ contracture in depolarized fibers were faster than those of the control K+ contracture in both soleus and EDL. A maximum reduction of 60% in the time for the contracture to decay from 90% to 10% was seen in soleus fibers after depolarizations lasting 10 min, while a reduction of 45% was seen in the decay time of EDL fibers after a 5-min depolarization. The amplitudes of the test contractures were 30% less than control after 5-min and 10-min depolarization and 50% less than control after 30 min. Analysis of the results suggests that the kinetics of excitation-contraction coupling may be altered in damaged muscle fibers.

Raised intracellular [Ca2+] abolishes excitation-contraction coupling in skeletal muscle fibres of rat and toad

1. Raising the intracellular [Ca2+] for 10 s at 23 degrees C abolished depolarization-induced force responses in mechanically skinned muscle fibres of toad and rat (half-maximal effect at 10 and 23 microM, respectively), without affecting the ability of caffeine or low [Mg2+] to open the ryanodine receptor (RyR)/Ca2+ release channels. Thus, excitation-contraction coupling was lost, even though the Ca2+ release channels were still functional. Coupling could not be restored in the duration of an experiment (up to 1 h). 2. The Ca(2+)-dependent uncoupling had a Q10 > 3.5, and was three times slower at pH 5.8 than at pH 7.1. Sr2+ caused similar uncoupling at twenty times higher concentration, but Mg2+, even at 10 mM, was ineffective. Uncoupling was not noticeably affected by removal of ATP or application of protein kinase or phosphatase inhibitors. 3. Confocal laser scanning microscopy showed that the transverse tubular system was sealed in its entirety in mechanically skinned fibres and that its integrity was maintained in uncoupled fibres. Electron microscopy revealed distorted or severed triad junctions and Z-line aberrations in uncoupled fibres. 4. Only when uncoupling was induced at a relatively slow rate (e.g. over 60 s with 2.5 microM Ca2+) could it be prevented by the protease inhibitor leupeptin (1 mM). Immunostaining of Western blots showed no evidence of proteolysis of the RyR, the alpha 1-subunit of dihydropyridine receptor (DHPR) or triadin in uncoupled fibres. 5. Fibres which, whilst intact, were stimulated repeatedly by potassium depolarization with simultaneous application of 30 mM caffeine showed reduced responsiveness after skinning to depolarization but not to caffeine. Rapid release of endogenous Ca2+, or raised [Ca2+] under conditions which minimized the loss of endogenous diffusible myoplasmic molecules from the skinned fibre, caused complete uncoupling. Taken together, these results suggest that Ca(2+)-dependent uncoupling can also occur in intact fibres. 6. This Ca(2+)-dependent loss of depolarization-induced Ca2+ release may play an important feedback role in muscle by stopping Ca2+ release in localized areas where it is excessive and may be responsible for long-lasting muscle fatigue after severe exercise, as well as contributing to muscle weakness in various dystrophies.

High-frequency fatigue in rat skeletal muscle: role of extracellular ion concentrations

High-frequency fatigue (HFF), the decline of force during continuous tetanic stimulation (lasting 4-40 s), was studied in isolated bundles of rat skeletal muscle fibers. HFF was slower in slow-twitch soleus fibers than in fast-twitch red or white sternomastoid fibers; denervation accelerated fatigue in soleus. Maximal 200-mmol/L potassium contractures of normal amplitude were induced in fatigued fibers, suggesting that crossbridge cycling and the voltage activation of excitation-contraction coupling could still occur maximally, but that activation by action potentials was impaired. An increase in [Na+]o slowed HFF, while a small increase in [K+]o or reduction in [Cl(-)]o accelerated HFF. Increasing the tetanic stimulation frequency exacerbated fatigue. Recovery from HFF proceeded rapidly since force increased markedly within a few seconds when stimulation ceased. These results support the hypothesis that a redistribution of Na+, K+, and Cl- across the transverse tubular membranes during repeated action potential activity induces fatigue by reducing the amplitude and conduction of action potentials.

Reversible vacuolation of the transverse tubules of frog skeletal muscle: a confocal fluorescence microscopy study

A confocal microscope was used to investigate the reversible vacuolation of frog skeletal muscle fibres produced by the efflux and entry of glycerol (80-100 mM). The formation, development and disappearance of vacuoles was observed in the fibres by staining simultaneously with two fluorescent membrane probes, RH414 and DiOC6(3). The styryl dye, RH414, stains only the plasmalemma and the membranes of the transverse tubules. In normal and glycerol-loaded fibres, RH414 revealed regular, narrow dotted bands located at the position of the Z-lines. Glycerol removal produced, within 2-10 min, many empty round vacuoles (0.4-1.5 microns in diameter) that were apparently anchored to the stained bands. Later on, individual vacuoles tended to enlarge and align into longitudinal chains of vacuoles. Neighbouring vacuoles that contacted each other fused to form large vacuoles up to several sarcomeres in length. Neither the T-tubules, nor the vacuoles, were stained by DiOC6(3). However, glycerol efflux was also accompanied by a redistribution of sarcoplasmic reticulum membranes and by changes in mitochondria that were revealed on staining the same fibres with the carbocyanine dye, DiOC6(3). The alterations in staining patterns revealed by RH414 and DiOC6(3) were completely reversible. Within 5-10 min after a second application of glycerol, the pattern of staining returned to normal with the exception of very bright, spots stained with RH414, which appeared in place of many but not all of the vacuoles, and probably correspond to the irregular nets of T-tubules observed under the electron microscope in such fibres. They are considered to be defects in regeneration of the T-system after vacuolation. The vacuolation/devacuolation cycle could be repeated several times following glycerol efflux and entry. The development and disappearance of vacuoles then mainly involved conversion of bright spots to large vacuoles and vice versa. Some possible mechanisms of vacuole formation and disappearance are discussed, and it is suggested that vacuolation of the T-system may be important in relation to regulating the volume of skeletal muscle cells.

Ultrastructure of sarcoballs on the surface of skinned amphibian skeletal muscle fibres

The formation of sarcoballs on the surface of skinned fibres from semitendinosus muscles of Xenopus laevis, and the sarcoplasmic reticulum content of the structures, have been studied using conventional electron microscopic techniques and immunoelectron microscopy. Examination of the fibres showed many membrane-bound blebs projecting from the surface in areas where vesicles of internal membranes (including sarcoplasmic reticulum, T-tubules and mitochondria) were clustered in interfilament spaces. The blebs varied in size from 1 micron to 150 microns and those with diameters > 10 microns are referred to as sarcoballs. Small blebs were often seen in close association with each other and might have fused during sarcoball formation. The interior of the sarcoball was filled with foam-like material made up of vesicles with diameters of 100 nm to 1.0 microns. The sarcoplasmic reticulum membrane content of the sarcoballs was evaluated using two monoclonal antibodies, one to the Ca2+ ATPase of the sarcoplasmic reticulum and the second to ryanodine receptor calcium release channels in the junctional-face membrane. The antibodies bound to some components of the surface and interior of the sarcoball, but not to mitochondrial-like structures and tubular vesicles. The results show that a large component of the sarcoball and its surface is derived from sarcoplasmic reticulum and suggest that mitochondria and T-tubules might also contribute membranes to the structures. Our hypothesis is that (a) blebs bud out from the surface of the skinned fibre following fusion of internal vesicles that are extruded along interfilament channels during unrestrained contractures, (b) blebs grow into sarcoballs by additional fusion of internal membrane vesicles and fusion of adjacent blebs, and (c) the sarcoball is a foam-like structure composed of bathing medium and membrane lipid (containing membrane proteins).

Extra-junctional ryanodine receptors in the terminal cisternae of mammalian skeletal muscle fibres

The distribution of ryanodine receptor calcium-release channels over the terminal cisternae (TC) membrane of skeletal muscle fibres was examined by using immunogold electron microscopy. Two monoclonal antibodies (5C3 and 8E2) that bound to monomers of the ryanodine receptor protein on Western blots of SDS-polyacrylamide gels were used to locate calcium-release channels in longitudinal sections of rat sternomastoid and diaphragm fibres. Up to 21% of 5C3 binding on TC membranes was extra-junctional, compared with 46% for 8E2. Binding of 8E2 to the fibres was less than half that of 5C3, possibly because of steric shielding of the 8E2 antigenic site at the junction. The distances between neighbouring particles in clusters was 20-40 nm, i.e. the distance between subunits of the ryanodine receptor or between neighbouring foot structures. We suggest that, during activation, extra-junctional ryanodine receptors may release Ca2+ directly into the myoplasm, rather than into the restricted space of the triad junction.

Biochemical properties of isolated transverse tubular membranes

This review addresses the major biochemical and structural characteristics of isolated transverse tubule (T-tubule) membranes, including methods of isolation and morphology of purified membranes, evaluation of attendant membrane activities, including ion pumps and channels, and structural and compositional analyses of functionally relevant components. Particular emphasis is placed on the Mg2+-ATPase, its localization in the T-system, its unusual kinetic properties, its possible functions, and its potential regulation by diacylglycerol and other biologically-relevant lipids. Conclusions are drawn with respect to the biochemical markers characteristic of T-tubule membranes and the criteria to be applied in the assessment of isolated T-tubule membrane purity.

Quantitation of 'junctional feet' content in two types of muscle fiber from hind limb muscles of the rat

1. Transverse tubules in fibers from rat soleus and extensor digitorum longus (EDL) muscles of the rat were infiltrated with silver dichromate (black reaction of Golgi). This provides a faithful, high-contrast outline of the tubules, which allows distinction between segments involved in junction formation with the sarcoplasmic reticulum and segments that are free. 2. Electron micrographs of semithin transverse sections were used to quantitate T tubule parameters and to measure cross-sectional area and perimeter of individual fibers. Thin sections and data from the literature were used to obtain the contribution of caveolae to external surface area and the frequency of junctional feet along the junctional T tubule membrane. 3. From the above data we calculate the ratio of number of feet to total external surface area for a given fiber segment. The ratio is compared with data in the literature on the total amount of 'charge movement' (in nC/uF of total external surface area). 4. The average feet/surface area ratio is twice as large in EDL than in soleus fibers, while the charge movement is up to five-fold larger. Probably some of the total charge movement is not directly associated with events related to the turning on of the SR permeability to calcium.

Rods in the terminal cisternae of skeletal muscle

The purpose of the study was to investigate rod-like structures (rods) in the terminal cisternae membrane of freeze fracture replicas of fast- and slow-twitch mammalian muscle. The 9 X 50 nm rods crossed the junctional gap perpendicular to the T-tubule membrane and terminated near indentations. Rods are likely to have a structural basis because (1) grooves were seen in the complimentary membrane leaflet, (2) rods were seen in tissue fixed in 0.5, 5, or 6% glutaraldehyde or 5% acrolein, (3) rods were seen in tissue fractured over a range of temperatures from -40 to -196 degrees C, (4) the number of rods was correlated with the contractile properties of fibers, and (5) the density of rods increased when fibers were depolarized before fixation. The rods are in a unique location that would allow them to participate in excitation-contraction coupling, perhaps by transmitting an electrical signal from the T-tubule membrane to calcium release sites in the terminal cisternae.

Golgi stain identifies three types of fibres in fish muscle

Using Golgi infiltration we have studied the structure and disposition of transverse tubules in muscle fibres from the sand dab fin musculature. Three types of fibres differ significantly from each other in the extent and disposition of junctions between transverse tubules and the sarcoplasmic reticulum. These correlate with the three groups of fibres having different relaxation times shown in the accompanying paper (Gilly & Aladjem, 1987). Fibres with very slow relaxation (tonic fibres) correspond to those which have an unusual disposition of T tubules and very rare T-SR junctions. In the fast twitch fibres the peripheral T tubules segments converge into tangentially arranged tubules before joining the plasmalemma.

Differential effects of thyroid hormone on T-tubules and terminal cisternae in rat muscles: an electrophysiological and morphometric analysis

Isometric twitches, passive electrical properties and the amounts of transverse (T) tubule system and terminal cisternae in extensor digitorum longus (EDL) and soleus muscle fibres were measured in normal rats and rats given daily injections of triiodothyronine (T3, 150 micrograms kg-1) for 15-25 days. Isometric twitches in both muscles were more rapid after the T3-treatment, particularly in soleus. Cable properties were measured using a three-microelectrode, end-of-fibre, voltage clamp technique. In order to increase the space constant of the T-tubule system, extracellular solutions were used that reduced ionic, particularly chloride, conductance. Fibre diameter was less than normal in the hyperthyroid rats. Membrane capacity, per cm2 of fibre surface, increased in both EDL and soleus muscles and there was a decrease in membrane resistance. The volume and surface area of the T-system and terminal cisternae were measured using standard morphometric techniques. Following T3-treatment the amount of T-tubule system per 100 micron3 of fibre volume, in both EDL and soleus fibres, was twofold higher than in normal fibres. The larger area of T-tubule membrane per unit volume was sufficient to account for the increase in membrane capacity. In contrast, the amount of terminal cisternae per 100 micron3 of fibre was unchanged in EDL following T3-treatment and there was only a small increase in soleus. As a consequence, the normal relationship between the T-tubules and terminal cisternae was changed in both muscles. There was an increase in the numbers of 'bare' T-tubules and an increased occurrence of diadic, pentadic and heptadic junctions between the membranes of the T-tubules and terminal cisternae. The results suggest that thyroid hormone has a differential effect on the synthesis of T-tubule and terminal cisternae membrane, resulting in a disproportionately large amount of T-tubule membrane.

Indentations in the terminal cisternae of denervated rat EDL and soleus muscle fibers

The effects of denervation on the structure of the triad and longitudinal sarcoplasmic reticulum have been investigated in freeze-fracture replicas of extensor digitorum longus (EDL) and soleus fibers that had been denervated for 2 to 70 days. In EDL fibers the density of indentations along 1 micron of terminal cisternae fell during the first 2 weeks after nerve section, from a normal value of 7.3 +/- 0.2 (mean +/- 1 SEM) to an average value of 2.0 +/- 0.5 in fibers denervated for 16 to 17 days. Denervation did not change the numbers of indentations in soleus fibers. There was a significant change in the orientation of the triads and organization of the longitudinal sarcoplasmic reticulum in denervated EDL and soleus fibers. The effect of denervation on the density of indentations was best correlated with the effect on asymmetric charge movement, when indentation density was compared with twitch contraction time, the voltage sensitivity of tension, and charge movement. The close relationship between indentation density and charge movement provides compelling evidence for a functional link between the two during excitation-contraction coupling.

The membrane capacity of mammalian skeletal muscle fibres

Membrane capacity was measured as a function of fibre diameter in mammalian skeletal muscle fibres under normal conditions and under conditions designed to reduce the membrane chloride conductance, i.e. in solutions in which chloride ions were replaced by sulphate or methylsulphate ions, or in normal Krebs solutions containing 2,4-dichlorophenoxyacetic acid (2.5 mM). The experiments were done on rat sternomastoid, extensor digitorum longus and soleus muscle fibres. The average membrane capacity of fibres in each muscle was greater than normal when chloride conductance was reduced and the slope of the relationship between membrane capacity and fibre diameter increased. The results were consistent with the hypothesis that the space constant of the transverse tubule system in mammalian fibres is normally short because the transverse tubule membrane has a high chloride conductance. The experimental results imply that the space constant of the transverse tubule system was less than 40 microns for fibres in normal Krebs solution and greater than 100 microns for fibres with low membrane chloride conductance. The space constant was calculated using measured geometrical parameters of the transverse tubule, and measured membrane conductance, and the values were close to 20 microns for fibres in normal Krebs solution and between 50 and 120 microns for fibres with low chloride conductance.