Distribution of potassium and chloride permeability over the surface and T-tubule membranes of mammalian skeletal muscle

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.

ClC-1 Chloride Channel: Inputs on the Structure-Function Relationship of Myotonia Congenita-Causing Mutations

Myotonia congenita is a hereditary muscle disease mainly characterized by muscle hyperexcitability, which leads to a sustained burst of discharges that correlates with the magnitude and duration of involuntary aftercontractions, muscle stiffness, and hypertrophy. Mutations in the chloride voltage-gated channel 1 (CLCN1) gene that encodes the skeletal muscle chloride channel (ClC-1) are responsible for this disease, which is commonly known as myotonic chloride channelopathy. The biophysical properties of the mutated channel have been explored and analyzed through in vitro approaches, providing important clues to the general function/dysfunction of the wild-type and mutated channels. After an exhaustive search for CLCN1 mutations, we report in this review more than 350 different mutations identified in the literature. We start discussing the physiological role of the ClC-1 channel in skeletal muscle functioning. Then, using the reported functional effects of the naturally occurring mutations, we describe the biophysical and structural characteristics of the ClC-1 channel to update the knowledge of the function of each of the ClC-1 helices, and finally, we attempt to point out some patterns regarding the effects of mutations in the different helices and loops of the protein.

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.

Mechanisms of altered skeletal muscle action potentials in the R6/2 mouse model of Huntington's disease

Huntington's disease (HD) patients suffer from progressive and debilitating motor dysfunction for which only palliative treatment is currently available. Previously, we discovered reduced skeletal muscle Cl^- channel (ClC-1) and inwardly rectifying K⁺ channel (Kir) currents in R6/2 HD transgenic mice. To further investigate the role of ClC-1 and Kir currents in HD skeletal muscle pathology, we measured the effect of reduced ClC-1 and Kir currents on action potential (AP) repetitive firing in R6/2 mice using a two-electrode current clamp. We found that R6/2 APs had a significantly lower peak amplitude, depolarized maximum repolarization, and prolonged decay time compared with wild type (WT). Of these differences, only the maximum repolarization was accounted for by the reduction in ClC-1 and Kir currents, indicating the presence of additional ion channel defects. We found that both K_V1.5 and K_V3.4 mRNA levels were significantly reduced in R6/2 skeletal muscle compared with WT, which explains the prolonged decay time of R6/2 APs. Overall, we found that APs in WT and R6/2 muscle significantly and progressively change during activity to maintain peak amplitude despite buildup of Na⁺ channel inactivation. Even with this resilience, the persistently reduced peak amplitude of R6/2 APs is expected to result in earlier fatigue and may help explain the motor impersistence experienced by HD patients. This work lays the foundation to link electrical changes to force generation defects in R6/2 HD mice and to examine the regulatory events controlling APs in WT muscle.

Defective Gating and Proteostasis of Human ClC-1 Chloride Channel: Molecular Pathophysiology of Myotonia Congenita

The voltage-dependent ClC-1 chloride channel, whose open probability increases with membrane potential depolarization, belongs to the superfamily of CLC channels/transporters. ClC-1 is almost exclusively expressed in skeletal muscles and is essential for stabilizing the excitability of muscle membranes. Elucidation of the molecular structures of human ClC-1 and several CLC homologs provides important insight to the gating and ion permeation mechanisms of this chloride channel. Mutations in the human CLCN1 gene, which encodes the ClC-1 channel, are associated with a hereditary skeletal muscle disease, myotonia congenita. Most disease-causing CLCN1 mutations lead to loss-of-function phenotypes in the ClC-1 channel and thus increase membrane excitability in skeletal muscles, consequently manifesting as delayed relaxations following voluntary muscle contractions in myotonic subjects. The inheritance pattern of myotonia congenita can be autosomal dominant (Thomsen type) or recessive (Becker type). To date over 200 myotonia-associated ClC-1 mutations have been identified, which are scattered throughout the entire protein sequence. The dominant inheritance pattern of some myotonia mutations may be explained by a dominant-negative effect on ClC-1 channel gating. For many other myotonia mutations, however, no clear relationship can be established between the inheritance pattern and the location of the mutation in the ClC-1 protein. Emerging evidence indicates that the effects of some mutations may entail impaired ClC-1 protein homeostasis (proteostasis). Proteostasis of membrane proteins comprises of biogenesis at the endoplasmic reticulum (ER), trafficking to the surface membrane, and protein turn-over at the plasma membrane. Maintenance of proteostasis requires the coordination of a wide variety of different molecular chaperones and protein quality control factors. A number of regulatory molecules have recently been shown to contribute to post-translational modifications of ClC-1 and play critical roles in the ER quality control, membrane trafficking, and peripheral quality control of this chloride channel. Further illumination of the mechanisms of ClC-1 proteostasis network will enhance our understanding of the molecular pathophysiology of myotonia congenita, and may also bring to light novel therapeutic targets for skeletal muscle dysfunction caused by myotonia and other pathological conditions.

A four-electrode method to study dynamics of ion activity and transport in skeletal muscle fibers

Ion movements across biological membranes, driven by electrochemical gradients or active transport mechanisms, control essential cell functions. Membrane ion movements can manifest as electrogenic currents or electroneutral fluxes, and either process can alter the extracellular and/or intracellular concentration of the transported ions. Classic electrophysiological methods allow accurate measurement of membrane ion movements when the transport mechanism produces a net ionic current; however, they cannot directly measure electroneutral fluxes and do not detect any accompanying change in intracellular ion concentrations. Here, we developed a method for simultaneously measuring ion movements and the accompanying dynamic changes in intracellular ion concentrations in intact skeletal muscle fibers under voltage or current clamp in real time. The method combines a two-microelectrode voltage clamp with ion-selective and reference microelectrodes (four-electrode system). We validate the electrical stability of the system and the viability of the preparation for periods of ∼1 h. We demonstrate the power of this method with measurements of intracellular Cl-, H+, and Na+ to show (a) voltage-dependent redistribution of Cl- ions; (b) intracellular pH changes induced by changes in extracellular pCO2; and (c) electroneutral and electrogenic Na+ movements controlled by the Na,K-ATPase. The method is useful for studying a range of transport mechanisms in many cell types, particularly when the transmembrane ion movements are electrically silent and/or when the transport activity measurably changes the intracellular activity of a transported ion.

FKBP8 Enhances Protein Stability of the CLC-1 Chloride Channel at the Plasma Membrane

Mutations in the skeletal muscle-specific CLC-1 chloride channel are associated with the human hereditary disease myotonia congenita. The molecular pathophysiology underlying some of the disease-causing mutations can be ascribed to defective human CLC-1 protein biosynthesis. CLC-1 protein folding is assisted by several molecular chaperones and co-chaperones, including FK506-binding protein 8 (FKBP8). FKBP8 is generally considered an endoplasmic reticulum- and mitochondrion-resident membrane protein, but is not thought to contribute to protein quality control at the cell surface. Herein, we aim to test the hypothesis that FKBP8 may regulate CLC-1 protein at the plasma membrane. Surface biotinylation and subcellular fractionation analyses reveal that a portion of FKBP8 is present at the plasma membrane, and that co-expression with CLC-1 enhances surface localization of FKBP8. Immunoblotting analyses of plasma membrane proteins purified from skeletal muscle further confirm surface localization of FKBP8. Importantly, FKBP8 promotes CLC-1 protein stability at the plasma membrane. Together, our data underscore the importance of FKBP8 in the peripheral quality control of CLC-1 channel.

Measurement of force and calcium release using mechanically skinned fibers from mammalian skeletal muscle

The mechanically skinned (or “peeled”) skeletal muscle fiber technique is a highly versatile procedure that allows controlled examination of each of the steps in the excitation-contraction (EC)-coupling sequence in skeletal muscle fibers, starting with excitation/depolarization of the transverse tubular (T)-system through to Ca2+ release from sarcoplasmic reticulum (SR) and finally force development by the contractile apparatus. It can also show the overall response of the whole EC-coupling sequence together, such as in twitch and tetanic force responses. A major advantage over intact muscle fiber preparations is that it is possible to set and rapidly manipulate the “intracellular” conditions, allowing examination of the effects of key variables (e.g., intracellular pH, ATP levels, redox state, etc.) on each individual step in EC coupling. This Cores of Reproducibility in Physiology (CORP) article describes the rationale, procedures, and experimental details of the various ways in which the mechanically skinned fiber technique is used in our laboratory to examine the physiological mechanisms controlling Ca2+ release and contraction in skeletal muscle fibers and the aberrations and dysfunction occurring with exercise and disease.

Role of physiological ClC-1 Cl- ion channel regulation for the excitability and function of working skeletal muscle

Electrical membrane properties of skeletal muscle fibers have been thoroughly studied over the last five to six decades. This has shown that muscle fibers from a wide range of species, including fish, amphibians, reptiles, birds, and mammals, are all characterized by high resting membrane permeability for Cl⁻ ions. Thus, in resting human muscle, ClC-1 Cl⁻ ion channels account for ∼80% of the membrane conductance, and because active Cl⁻ transport is limited in muscle fibers, the equilibrium potential for Cl⁻ lies close to the resting membrane potential. These conditions—high membrane conductance and passive distribution—enable ClC-1 to conduct membrane current that inhibits muscle excitability. This depressing effect of ClC-1 current on muscle excitability has mostly been associated with skeletal muscle hyperexcitability in myotonia congenita, which arises from loss-of-function mutations in the CLCN1 gene. However, given that ClC-1 must be drastically inhibited (∼80%) before myotonia develops, more recent studies have explored whether acute and more subtle ClC-1 regulation contributes to controlling the excitability of working muscle. Methods were developed to measure ClC-1 function with subsecond temporal resolution in action potential firing muscle fibers. These and other techniques have revealed that ClC-1 function is controlled by multiple cellular signals during muscle activity. Thus, onset of muscle activity triggers ClC-1 inhibition via protein kinase C, intracellular acidosis, and lactate ions. This inhibition is important for preserving excitability of working muscle in the face of activity-induced elevation of extracellular K⁺ and accumulating inactivation of voltage-gated sodium channels. Furthermore, during prolonged activity, a marked ClC-1 activation can develop that compromises muscle excitability. Data from ClC-1 expression systems suggest that this ClC-1 activation may arise from loss of regulation by adenosine nucleotides and/or oxidation. The present review summarizes the current knowledge of the physiological factors that control ClC-1 function in active muscle.

Regulation of CLC-1 chloride channel biosynthesis by FKBP8 and Hsp90β

Mutations in human CLC-1 chloride channel are associated with the skeletal muscle disorder myotonia congenita. The disease-causing mutant A531V manifests enhanced proteasomal degradation of CLC-1. We recently found that CLC-1 degradation is mediated by cullin 4 ubiquitin ligase complex. It is currently unclear how quality control and protein degradation systems coordinate with each other to process the biosynthesis of CLC-1. Herein we aim to ascertain the molecular nature of the protein quality control system for CLC-1. We identified three CLC-1-interacting proteins that are well-known heat shock protein 90 (Hsp90)-associated co-chaperones: FK506-binding protein 8 (FKBP8), activator of Hsp90 ATPase homolog 1 (Aha1), and Hsp70/Hsp90 organizing protein (HOP). These co-chaperones promote both the protein level and the functional expression of CLC-1 wild-type and A531V mutant. CLC-1 biosynthesis is also facilitated by the molecular chaperones Hsc70 and Hsp90β. The protein stability of CLC-1 is notably increased by FKBP8 and the Hsp90β inhibitor 17-allylamino-17-demethoxygeldanamycin (17-AAG) that substantially suppresses cullin 4 expression. We further confirmed that cullin 4 may interact with Hsp90β and FKBP8. Our data are consistent with the idea that FKBP8 and Hsp90β play an essential role in the late phase of CLC-1 quality control by dynamically coordinating protein folding and degradation.

Channelopathies of skeletal muscle excitability

Familial disorders of skeletal muscle excitability were initially described early in the last century and are now known to be caused by mutations of voltage-gated ion channels. The clinical manifestations are often striking, with an inability to relax after voluntary contraction (myotonia) or transient attacks of severe weakness (periodic paralysis). An essential feature of these disorders is fluctuation of symptoms that are strongly impacted by environmental triggers such as exercise, temperature, or serum K(+) levels. These phenomena have intrigued physiologists for decades, and in the past 25 years the molecular lesions underlying these disorders have been identified and mechanistic studies are providing insights for therapeutic strategies of disease modification. These familial disorders of muscle fiber excitability are "channelopathies" caused by mutations of a chloride channel (ClC-1), sodium channel (NaV1.4), calcium channel (CaV1.1), and several potassium channels (Kir2.1, Kir2.6, and Kir3.4). This review provides a synthesis of the mechanistic connections between functional defects of mutant ion channels, their impact on muscle excitability, how these changes cause clinical phenotypes, and approaches toward therapeutics.

The Sick and the Weak: Neuropathies/Myopathies in the Critically Ill

Critical illness polyneuropathies (CIP) and myopathies (CIM) are common complications of critical illness. Several weakness syndromes are summarized under the term intensive care unit-acquired weakness (ICUAW). We propose a classification of different ICUAW forms (CIM, CIP, sepsis-induced, steroid-denervation myopathy) and pathophysiological mechanisms from clinical and animal model data. Triggers include sepsis, mechanical ventilation, muscle unloading, steroid treatment, or denervation. Some ICUAW forms require stringent diagnostic features; CIM is marked by membrane hypoexcitability, severe atrophy, preferential myosin loss, ultrastructural alterations, and inadequate autophagy activation while myopathies in pure sepsis do not reproduce marked myosin loss. Reduced membrane excitability results from depolarization and ion channel dysfunction. Mitochondrial dysfunction contributes to energy-dependent processes. Ubiquitin proteasome and calpain activation trigger muscle proteolysis and atrophy while protein synthesis is impaired. Myosin loss is more pronounced than actin loss in CIM. Protein quality control is altered by inadequate autophagy. Ca(2+) dysregulation is present through altered Ca(2+) homeostasis. We highlight clinical hallmarks, trigger factors, and potential mechanisms from human studies and animal models that allow separation of risk factors that may trigger distinct mechanisms contributing to weakness. During critical illness, altered inflammatory (cytokines) and metabolic pathways deteriorate muscle function. ICUAW prevention/treatment is limited, e.g., tight glycemic control, delaying nutrition, and early mobilization. Future challenges include identification of primary/secondary events during the time course of critical illness, the interplay between membrane excitability, bioenergetic failure and differential proteolysis, and finding new therapeutic targets by help of tailored animal models.

The Cullin 4A/B-DDB1-Cereblon E3 Ubiquitin Ligase Complex Mediates the Degradation of CLC-1 Chloride Channels

Voltage-gated CLC-1 chloride channels play a critical role in controlling the membrane excitability of skeletal muscles. Mutations in human CLC-1 channels have been linked to the hereditary muscle disorder myotonia congenita. We have previously demonstrated that disease-associated CLC-1 A531V mutant protein may fail to pass the endoplasmic reticulum quality control system and display enhanced protein degradation as well as defective membrane trafficking. Currently the molecular basis of protein degradation for CLC-1 channels is virtually unknown. Here we aim to identify the E3 ubiquitin ligase of CLC-1 channels. The protein abundance of CLC-1 was notably enhanced in the presence of MLN4924, a specific inhibitor of cullin-RING E3 ligases. Subsequent investigation with dominant-negative constructs against specific subtypes of cullin-RING E3 ligases suggested that CLC-1 seemed to serve as the substrate for cullin 4A (CUL4A) and 4B (CUL4B). Biochemical examinations further indicated that CUL4A/B, damage-specific DNA binding protein 1 (DDB1), and cereblon (CRBN) appeared to co-exist in the same protein complex with CLC-1. Moreover, suppression of CUL4A/B E3 ligase activity significantly enhanced the functional expression of the A531V mutant. Our data are consistent with the idea that the CUL4A/B-DDB1-CRBN complex catalyses the polyubiquitination and thus controls the degradation of CLC-1 channels.

The determinants of transverse tubular volume in resting skeletal muscle

The transverse tubular (t)-system of skeletal muscle couples sarcolemmal electrical excitation with contraction deep within the fibre. Exercise, pathology and the composition of the extracellular fluid (ECF) can alter t-system volume (t-volume). T-volume changes are thought to contribute to fatigue, rhabdomyolysis and disruption of excitation–contraction coupling. However, mechanisms that underlie t-volume changes are poorly understood. A multicompartment, history-independent computer model of rat skeletal muscle was developed to define the minimum conditions for t-volume stability. It was found that the t-system tends to swell due to net ionic fluxes from the ECF across the access resistance. However, a stable t-volume is possible when this is offset by a net efflux from the t-system to the cell and thence to the ECF, forming a net ion cycle ECF→t-system→sarcoplasm→ECF that ultimately depends on Na⁺/K⁺-ATPase activity. Membrane properties that maximize this circuit flux decrease t-volume, including P_Na(t) > P_Na(s), P_K(t) < P_K(s) and N_(t) < N_(s) [P, permeability; N, Na⁺/K⁺-ATPase density; (t), t-system membrane; (s), sarcolemma]. Hydrostatic pressures, fixed charges and/or osmoles in the t-system can influence the magnitude of t-volume changes that result from alterations in this circuit flux. Using a parameter set derived from literature values where possible, this novel theory of t-volume was tested against data from previous experiments where t-volume was measured during manipulations of ECF composition. Predicted t-volume changes correlated satisfactorily. The present work provides a robust, unifying theoretical framework for understanding the determinants of t-volume.

Electrical coupling between the human serotonin transporter and voltage-gated Ca(2+) channels

Monoamine transporters have been implicated in dopamine or serotonin release in response to abused drugs such as methamphetamine or ecstasy (MDMA). In addition, monoamine transporters show substrate-induced inward currents that may modulate excitability and Ca(2+) mobilization, which could also contribute to neurotransmitter release. How monoamine transporters modulate Ca(2+) permeability is currently unknown. We investigate the functional interaction between the human serotonin transporter (hSERT) and voltage-gated Ca(2+) channels (CaV). We introduce an excitable expression system consisting of cultured muscle cells genetically engineered to express hSERT. Both 5HT and S(+)MDMA depolarize these cells and activate the excitation-contraction (EC)-coupling mechanism. However, hSERT substrates fail to activate EC-coupling in CaV1.1-null muscle cells, thus implicating Ca(2+) channels. CaV1.3 and CaV2.2 channels are natively expressed in neurons. When these channels are co-expressed with hSERT in HEK293T cells, only cells expressing the lower-threshold L-type CaV1.3 channel show Ca(2+) transients evoked by 5HT or S(+)MDMA. In addition, the electrical coupling between hSERT and CaV1.3 takes place at physiological 5HT concentrations. The electrical coupling between monoamine neurotransmitter transporters and Ca(2+) channels such as CaV1.3 is a novel mechanism by which endogenous substrates (neurotransmitters) or exogenous substrates (like ecstasy) could modulate Ca(2+)-driven signals in excitable cells.

Electrophysiological characteristics of six mutations in hClC-1 of Korean patients with myotonia congenita

ClC-1 is a member of a large family of voltage-gated chloride channels, abundantly expressed in human skeletal muscle. Mutations in ClC-1 are associated with myotonia congenita (MC) and result in loss of regulation of membrane excitability in skeletal muscle. We studied the electrophysiological characteristics of six mutants found among Korean MC patients, using patch clamp methods in HEK293 cells. Here, we found that the autosomal dominant mutants S189C and P480S displayed reduced chloride conductances compared to WT. Autosomal recessive mutant M128I did not show a typical rapid deactivation of Cl(-) currents. While sporadic mutant G523D displayed sustained activation of Cl(-) currents in the whole cell traces, the other sporadic mutants, M373L and M609K, demonstrated rapid deactivations. V1/2 of these mutants was shifted to more depolarizing potentials. In order to identify potential effects on gating processes, slow and fast gating was analyzed for each mutant. We show that slow gating of the mutants tends to be shifted toward more positive potentials in comparison to WT. Collectively, these six mutants found among Korean patients demonstrated modifications of channel gating behaviors and reduced chloride conductances that likely contribute to the physiologic changes of MC.

Myotonia congenita mutation enhances the degradation of human CLC-1 chloride channels

Myotonia congenita is a hereditary muscle disorder caused by mutations in the human voltage-gated chloride (Cl⁻) channel CLC-1. Myotonia congenita can be inherited in an autosomal recessive (Becker type) or dominant (Thomsen type) fashion. One hypothesis for myotonia congenita is that the inheritance pattern of the disease is determined by the functional consequence of the mutation on the gating of CLC-1 channels. Several disease-related mutations, however, have been shown to yield functional CLC-1 channels with no detectable gating defects. In this study, we have functionally and biochemically characterized a myotonia mutant: A531V. Despite a gating property similar to that of wild-type (WT) channels, the mutant CLC-1 channel displayed a diminished whole-cell current density and a reduction in the total protein expression level. Our biochemical analyses further demonstrated that the reduced expression of A531V can be largely attributed to an enhanced proteasomal degradation as well as a defect in protein trafficking to surface membranes. Moreover, the A531V mutant protein also appeared to be associated with excessive endosomal-lysosomal degradation. Neither the reduced protein expression nor the diminished current density was rescued by incubating A531V-expressing cells at 27°C. These results demonstrate that the molecular pathophysiology of A531V does not involve anomalous channel gating, but rather a disruption of the balance between the synthesis and degradation of the CLC-1 channel protein.

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.

Longitudinal and transversal propagation of excitation along the tubular system of rat fast-twitch muscle fibres studied by high speed confocal microscopy

Mammalian skeletal muscle fibres possess a tubular (t-) system that consists of regularly spaced transverse elements which are also connected in the longitudinal direction. This tubular network provides a pathway for the propagation of action potentials (APs) both radially and longitudinally within the fibre, but little is known about the actual radial and longitudinal AP conduction velocities along the tubular network in mammalian skeletal muscle fibres. The aim of this study was to track AP propagation within the t-system network of fast-twitch rat muscle fibres with high spatio-temporal resolution when the t-system was isolated from the surface membrane. For this we used high speed confocal imaging of AP-induced Ca(2+) release in contraction-suppressed mechanically skinned fast-twitch fibres where the t-system can be electrically excited in the absence of the surface membrane. Supramaximal field pulses normally elicited a synchronous AP-induced release of Ca(2+) along one side of the fibre axis which propagated uniformly across the fibre. In some cases up to 80 or more adjacent transverse tubules failed to be excited by the field pulse, while adjacent areas responded with normal Ca(2+) release. In these cases a continuous front of Ca(2+) release with an angle to the scanning line was observed due to APs propagating longitudinally. From these observations the radial/transversal and longitudinal AP conduction velocities along the tubular network deeper in the fibre under our conditions (19 ± 1°C) ranged between 8 and 11 μm ms(-1) and 5 to 9 μm ms(-1), respectively, using different methods of estimation. The longitudinal propagation of APs appeared to be markedly faster closer to the edge of the fibre, in agreement with the presence of dense longitudinal connections immediately below the surface of the fibre and more sparse connections at deeper planes within the fibre. During long trains of closely spaced field pulses the AP-elicited Ca(2+) releases became non-synchronous along the fibre axis. This is most likely caused by local tubular K(+) accumulation that produces local depolarization and local slowing of AP propagation. Longitudinally propagating APs may reduce such inhomogeneities by exciting areas of delayed AP onset. Clearly, the longitudinal tubular pathways within the fibre for excitation are used as a safety mechanism in situations where a local depolarization obstructs immediate excitation from the sarcolemma. Results obtained from this study also provide an explanation for the pattern of contractures observed in rippling muscle disease.

Physiology and pathophysiology of CLC-1: mechanisms of a chloride channel disease, myotonia

The CLC-1 chloride channel, a member of the CLC-channel/transporter family, plays important roles for the physiological functions of skeletal muscles. The opening of this chloride channel is voltage dependent and is also regulated by protons and chloride ions. Mutations of the gene encoding CLC-1 result in a genetic disease, myotonia congenita, which can be inherited as an autosmal dominant (Thomsen type) or an autosomal recessive (Becker type) pattern. These mutations are scattered throughout the entire protein sequence, and no clear relationship exists between the inheritance pattern of the mutation and the location of the mutation in the channel protein. The inheritance pattern of some but not all myotonia mutants can be explained by a working hypothesis that these mutations may exert a “dominant negative” effect on the gating function of the channel. However, other mutations may be due to different pathophysiological mechanisms, such as the defect of protein trafficking to membranes. Thus, the underlying mechanisms of myotonia are likely to be quite diverse, and elucidating the pathophysiology of myotonia mutations will require the understanding of multiple molecular/cellular mechanisms of CLC-1 channels in skeletal muscles, including molecular operation, protein synthesis, and membrane trafficking mechanisms.

Relationships between resting conductances, excitability, and t-system ionic homeostasis in skeletal muscle

Activation of skeletal muscle fibers requires rapid sarcolemmal action potential (AP) conduction to ensure uniform excitation along the fiber length, as well as successful tubular excitation to initiate excitation–contraction coupling. In our companion paper in this issue, Pedersen et al. (2011. J. Gen. Physiol. doi:10.1085/jgp.201010510) quantify, for subthreshold stimuli, the influence upon both surface conduction velocity and tubular (t)-system excitation of the large changes in resting membrane conductance (G_M) that occur during repetitive AP firing. The present work extends the analysis by developing a multi-compartment modification of the charge–difference model of Fraser and Huang to provide a quantitative description of the conduction velocity of actively propagated APs; the influence of voltage-gated ion channels within the t-system; the influence of t-system APs on ionic homeostasis within the t-system; the influence of t-system ion concentration changes on membrane potentials; and the influence of Phase I and Phase II G_M changes on these relationships. Passive conduction properties of the novel model agreed with established linear circuit analysis and previous experimental results, while key simulations of AP firing were tested against focused experimental microelectrode measurements of membrane potential. This study thereby first quantified the effects of the t-system luminal resistance and voltage-gated Na⁺ channel density on surface AP propagation and the resultant electrical response of the t-system. Second, it demonstrated the influence of G_M changes during repetitive AP firing upon surface and t-system excitability. Third, it showed that significant K⁺ accumulation occurs within the t-system during repetitive AP firing and produces a baseline depolarization of the surface membrane potential. Finally, it indicated that G_M changes during repetitive AP firing significantly influence both t-system K⁺ accumulation and its influence on the resting membrane potential. Thus, the present study emerges with a quantitative description of the changes in membrane potential, excitability, and t-system ionic homeostasis that occur during repetitive AP firing in skeletal muscle.

Chloride currents from the transverse tubular system in adult mammalian skeletal muscle fibers

Chloride fluxes are the main contributors to the resting conductance of mammalian skeletal muscle fibers. ClC-1, the most abundant chloride channel isoform in this preparation, is believed to be responsible for this conductance. However, the actual distribution of ClC-1 channels between the surface and transverse tubular system (TTS) membranes has not been assessed in intact muscle fibers. To investigate this issue, we voltageclamped enzymatically dissociated short fibers using a two-microelectrode configuration and simultaneously recorded chloride currents (I_Cl) and di-8-ANEPPS fluorescence signals to assess membrane potential changes in the TTS. Experiments were conducted in conditions that blocked all but the chloride conductance. Fibers were equilibrated with 40 or 70 mM intracellular chloride to enhance the magnitude of inward I_Cl, and the specific ClC-1 blocker 9-ACA was used to eliminate these currents whenever necessary. Voltage-dependent di-8-ANEPPS signals and I_Cl acquired before (control) and after the addition of 9-ACA were comparatively assessed. Early after the onset of stimulus pulses, di-8-ANEPPS signals under control conditions were smaller than those recorded in the presence of 9-ACA. We defined as attenuation the normalized time-dependent difference between these signals. Attenuation was discovered to be I_Cl dependent since its magnitude varied in close correlation with the amplitude and time course of I_Cl. While the properties of I_Cl, and those of the attenuation seen in optical records, could be simultaneously predicted by model simulations when the chloride permeability (P_Cl) at the surface and TTS membranes were approximately equal, the model failed to explain the optical data if P_Cl was precluded from the TTS membranes. Since the ratio between the areas of TTS membranes and the sarcolemma is large in mammalian muscle fibers, our results demonstrate that a significant fraction of the experimentally recorded I_Cl arises from TTS contributions.

Sarcolemmal-restricted localization of functional ClC-1 channels in mouse skeletal muscle

Skeletal muscle fibers exhibit a high resting chloride conductance primarily determined by ClC-1 chloride channels that stabilize the resting membrane potential during repetitive stimulation. Although the importance of ClC-1 channel activity in maintaining normal muscle excitability is well appreciated, the subcellular location of this conductance remains highly controversial. Using a three-pronged multidisciplinary approach, we determined the location of functional ClC-1 channels in adult mouse skeletal muscle. First, formamide-induced detubulation of single flexor digitorum brevis (FDB) muscle fibers from 15-16-day-old mice did not significantly alter macroscopic ClC-1 current magnitude (at -140 mV; -39.0 +/- 4.5 and -42.3 +/- 5.0 nA, respectively), deactivation kinetics, or voltage dependence of channel activation (V(1/2) was -61.0 +/- 1.7 and -64.5 +/- 2.8 mV; k was 20.5 ± 0.8 and 22.8 +/- 1.2 mV, respectively), despite a 33% reduction in cell capacitance (from 465 +/- 36 to 312 +/- 23 pF). In paired whole cell voltage clamp experiments, where ClC-1 activity was measured before and after detubulation in the same fiber, no reduction in ClC-1 activity was observed, despite an approximately 40 and 60% reduction in membrane capacitance in FDB fibers from 15-16-day-old and adult mice, respectively. Second, using immunofluorescence and confocal microscopy, native ClC-1 channels in adult mouse FDB fibers were localized within the sarcolemma, 90 degrees out of phase with double rows of dihydropyridine receptor immunostaining of the T-tubule system. Third, adenoviral-mediated expression of green fluorescent protein-tagged ClC-1 channels in adult skeletal muscle of a mouse model of myotonic dystrophy type 1 resulted in a significant reduction in myotonia and localization of channels to the sarcolemma. Collectively, these results demonstrate that the majority of functional ClC-1 channels localize to the sarcolemma and provide essential insight into the basis of myofiber excitability in normal and diseased skeletal muscle.

Extracellular ATP inhibits chloride channels in mature mammalian skeletal muscle by activating P2Y1 receptors

ATP is released from skeletal muscle during exercise, a discovery dating back to 1969. Surprisingly, few studies have examined the effects of extracellular ATP on mature mammalian skeletal muscle. This electrophysiological study examined the effects of extracellular ATP on fully innervated rat levator auris longus using two intracellular microelectrodes. The effects of ATP were determined by measuring the relative changes of miniature endplate potentials (mEPPs) and voltage responses to step current pulses in individual muscle fibres. Exposure to ATP (20 microm) prolonged the mEPP falling phase by 31 +/- 7.5% (values +/- s.d., n = 3 fibres). Concurrently, the input resistance increased by 31 +/- 2.0% and the time course of the voltage responses increased by 59 +/- 3.0%. Analogous effects were observed using 2 and 5 microm ATP, and on regions distal from the neuromuscular junction, indicating that physiologically relevant levels of ATP enhanced electrical signalling over the entire muscle fibre. The effects of extracellular ATP were blocked by 200 microm anthracene-9-carboxylic acid, a chloride channel inhibitor, and reduced concentrations of extracellular chloride, indicating that ATP inhibited chloride channels. A high affinity agonist for P2Y receptors, 2-methylthioadenosine-5-O-diphosphate (2MeSADP), induced similar effects to ATP with an EC(50) of 160 +/- 30 nm. The effects of 250 nm2MeSADP were blocked by 500 nmMRS2179, a specific P2Y(1) receptor inhibitor, suggesting that ATP acts on P2Y(1) receptors to inhibit chloride channels. The inhibition of chloride channels by extracellular ATP has implications for muscle excitability and fatigue, and the pathophysiology of myotonias.

DHPR activation underlies SR Ca2+ release induced by osmotic stress in isolated rat skeletal muscle fibers

Changes in skeletal muscle volume induce localized sarcoplasmic reticulum (SR) Ca(2+) release (LCR) events, which are sustained for many minutes, suggesting a possible signaling role in plasticity or pathology. However, the mechanism by which cell volume influences SR Ca(2+) release is uncertain. In the present study, rat flexor digitorum brevis fibers were superfused with isoosmotic Tyrode's solution before exposure to either hyperosmotic (404 mOsm) or hypoosmotic (254 mOsm) solutions, and the effects on cell volume, membrane potential (E(m)), and intracellular Ca(2+) ([Ca(2+)](i)) were determined. To allow comparison with previous studies, solutions were made hyperosmotic by the addition of sugars or divalent cations, or they were made hypoosmotic by reducing [NaCl](o). All hyperosmotic solutions induced a sustained decrease in cell volume, which was accompanied by membrane depolarization (by 14-18 mV; n = 40) and SR Ca(2+) release. However, sugar solutions caused a global increase in [Ca(2+)](i), whereas solutions made hyperosmotic by the addition of divalent cations only induced LCR. Decreasing osmolarity induced an increase in cell volume and a negative shift in E(m) (by 15.04 +/- 1.85 mV; n = 8), whereas [Ca(2+)](i) was unaffected. However, on return to the isoosmotic solution, restoration of cell volume and E(m) was associated with LCR. Both global and localized SR Ca(2+) release were abolished by the dihydropyridine receptor inhibitor nifedipine by sustained depolarization of the sarcolemmal or by the addition of the ryanodine receptor 1 inhibitor tetracaine. Inhibitors of the Na-K-2Cl (NKCC) cotransporter markedly inhibited the depolarization associated with hyperosmotic shrinkage and the associated SR Ca(2+) release. These findings suggest (1) that the depolarization that accompanies a decrease in cell volume is the primary event leading to SR Ca(2+) release, and (2) that volume-dependent regulation of the NKCC cotransporter contributes to the observed changes in E(m). The differing effects of the osmotic agents can be explained by the screening of fixed charges by divalent ions.

Do multiple ionic interactions contribute to skeletal muscle fatigue?

During intense exercise or electrical stimulation of skeletal muscle the concentrations of several ions change simultaneously in interstitial, transverse tubular and intracellular compartments. Consequently the functional effects of multiple ionic changes need to be considered together. A diminished transsarcolemmal K(+) gradient per se can reduce maximal force in non-fatigued muscle suggesting that K(+) causes fatigue. However, this effect requires extremely large, although physiological, K(+) shifts. In contrast, moderate elevations of extracellular [K(+)] ([K(+)](o)) potentiate submaximal contractions, enhance local blood flow and influence afferent feedback to assist exercise performance. Changed transsarcolemmal Na(+), Ca(2+), Cl(-) and H(+) gradients are insufficient by themselves to cause much fatigue but each ion can interact with K(+) effects. Lowered Na(+), Ca(2+) and Cl(-) gradients further impair force by modulating the peak tetanic force-[K(+)](o) and peak tetanic force-resting membrane potential relationships. In contrast, raised [Ca(2+)](o), acidosis and reduced Cl(-) conductance during late fatigue provide resistance against K(+)-induced force depression. The detrimental effects of K(+) are exacerbated by metabolic changes such as lowered [ATP](i), depleted carbohydrate, and possibly reactive oxygen species. We hypothesize that during high-intensity exercise a rundown of the transsarcolemmal K(+) gradient is the dominant cellular process around which interactions with other ions and metabolites occur, thereby contributing to fatigue.

Impaired calcium release during fatigue

Impaired calcium release from the sarcoplasmic reticulum (SR) has been identified as a contributor to fatigue in isolated skeletal muscle fibers. The functional importance of this phenomenon can be quantified by the use of agents, such as caffeine, which can increase SR Ca2+release during fatigue. A number of possible mechanisms for impaired calcium release have been proposed. These include reduction in the amplitude of the action potential, potentially caused by extracellular K+accumulation, which may reduce voltage sensor activation but is counteracted by a number of mechanisms in intact animals. Reduced effectiveness of SR Ca2+channel opening is caused by the fall in intracellular ATP and the rise in Mg2+concentrations that occur during fatigue. Reduced Ca2+available for release within the SR can occur if inorganic phosphate enters the SR and precipitates with Ca2+. Further progress requires the development of methods that can identify impaired SR Ca2+release in intact, blood-perfused muscles and that can distinguish between the various mechanisms proposed.

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.

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.

Transverse tubular system depolarization reduces tetanic force in rat skeletal muscle fibers by impairing action potential repriming

When muscle fibers are repeatedly stimulated, they may become depolarized and force output decline. Excitation of the transverse tubular system (T-system) is critical for activation, but its role in muscle fatigue is poorly understood. Here, mechanically skinned fibers from rat fast-twitch muscle were used, because the sarcolemma is absent but the T-system retains normal excitability and its properties can be studied in isolation. The T-system membrane was fully polarized by bathing the skinned fiber in an internal solution with 126 mM K+ (control solution) or set at partially depolarized levels (approximately −63 and −58 mV) in solutions with 66 or 55 mM K+, respectively, and action potentials (APs) were triggered in the sealed T-system by field stimulation. Prolonged depolarization of the T-system reduced tetanic force proportionately more than twitch force, with greater effect at higher stimulation frequency (responses at 20 and 100 Hz reduced to 71 and 62% in 66 mM K+ and to 54 and 35% in 55 mM K+, respectively). Double-pulse stimulation showed that depolarization increased the repriming period (estimated minimum time before a second AP can be produced) from ∼4 ms to ∼7.5 and 15 ms in the 66 and 55 mM K+ solutions, respectively. These results demonstrate that T-system depolarization reduces tetanic force by impairing AP repriming, rather than by preventing AP generation per se or by inactivating the T-system voltage sensors. The findings also explain why it is advantageous to reduce the rate of motoneuron stimulation to muscles during repeated or prolonged periods of activity.

Excitation-contraction coupling from the 1950s into the new millennium

1. Excitation-contraction coupling is broadly defined as the process linking the action potential to contraction in striated muscle or, more narrowly, as the process coupling surface membrane depolarization to Ca(2+) release from the sarcoplasmic reticulum. 2. We now know that excitation-contraction coupling depends on a macromolecular protein complex or 'calcium release unit'. The complex extends the extracellular space within the transverse tubule invaginations of the surface membrane, across the transverse tubule membrane into the cytoplasm and then across the sarcoplasmic reticulum membrane and into the lumen of the sarcoplasmic reticulum. 3. The central element of the macromolecular complex is the ryanodine receptor calcium release channel in the sarcoplasmic reticulum membrane. The ryanodine receptor has recruited a surface membrane L-type calcium channel as a 'voltage sensor' to detect the action potential and the calcium-binding protein calsequestrin to detect in the environment within the sarcoplasmic reticulum. Consequently, the calcium release channel is able to respond to surface depolarization in a manner that depends on the Ca(2+) load within the calcium store. 4. The molecular components of the 'calcium release unit' are the same in skeletal and cardiac muscle. However, the mechanism of excitation-contraction coupling is different. The signal from the voltage sensor to ryanodine receptor is chemical in the heart, depending on an influx of external Ca(2+) through the surface calcium channel. In contrast, conformational coupling links the voltage sensor and the ryanodine receptor in skeletal muscle. 5. Our current understanding of this amazingly efficient molecular signal transduction machine has evolved over the past 50 years. None of the proteins had been identified in the 1950s; indeed, there was debate about whether the molecules involved were, in fact, protein. Nevertheless, a multitude of questions about the molecular interactions and structures of the proteins and their interaction sites remain to be answered and provide a challenge for the next 50 years.

ClC-1 CHLORIDE CHANNEL: MATCHING ITS PROPERTIES TO A ROLE IN SKELETAL MUSCLE

1. ClC-1 is a Cl- channel in mammalian skeletal muscle that plays an important role in membrane repolarization following muscular contraction. Reduction of ClC-1 conductance results in myotonia, a state characterized by muscle hyperexcitability. 2. As is the case for other members of the ClC family, ClC-1 exists as a dimer that forms a double-barrelled channel. Each barrel, or pore, of ClC-1 is gated by its own gate ('fast' or 'single pore' gate), whereas both pores are gated simultaneously by another mechanism ('slow' or 'common' gate). 3. Comparison of the biophysical and pharmacological properties of heterologously expressed ClC-1 with the properties of the Cl- conductance measured in skeletal muscle strongly suggests that ClC-1 is the major Cl- channel responsible for muscle repolarization. However, not all results obtained in experiments on whole muscle or muscle fibres support this notion. 4. In the present review we attempt to bring together the current knowledge of ClC-1 with the physiology of skeletal muscle.

Ion channels and ion transporters of the transverse tubular system of skeletal muscle

This review focuses on the electrical properties of the transverse (T) tubular membrane of skeletal muscle, with reference to the contribution of the T-tubular system (TTS) to the surface action potential, the radial spread of excitation and its role in excitation-contraction coupling. Particularly, the most important ion channels and ion transporters that enable proper depolarization and repolarization of the T-tubular membrane are described. Since propagation of excitation along the TTS into the depth of the fibers is a delicate balance between excitatory and inhibitory currents, the composition of channels and transporters is specific to the TTS and different from the surface membrane. The TTS normally enables the radial spread of excitation and the signal transfer to the sarcoplasmic reticulum to release calcium that activates the contractile apparatus. However, due to its structure, even slight shifts of ions may alter its volume, Nernstian potentials, ion permeabilities, and consequently T-tubular membrane potential and excitability.

Membrane proteins involved in potassium shifts during muscle activity and fatigue

Muscle activity is associated with potassium displacements, which may cause fatigue. It was reported previously that the density of the large-conductance Ca2+-dependent K+ (BK(Ca)) channel is higher in the T tubule membrane than in the sarcolemmal membrane and that the opposite is the case for the ATP-sensitive K+ (K(ATP)) channel. In the present experiments, we investigated the subcellular localizations of the strong inward rectifier 2.1 K+ (Kir2.1) channel and the Na+-K+-2Cl- (NKCC)1 cotransporter with Western blot analysis of different muscle fractions. Furthermore, muscle function was studied while trying to manipulate the opening probability or transport capacity of these proteins during electrical stimulation of isolated soleus muscles. All experiments were made with excised muscle from male Wistar rats. Kir2.1 channels were almost undetectable in the sarcolemmal membrane but present in the T tubule membrane, whereas NKCC1 cotransporters were present in the sarcolemmal membrane. For muscles incubated in a buffer containing pinacidil, NS1619, Ba2+, or bumetanide, there was a faster reduction in peak force (P < 0.05). Furthermore, bumetanide incubation reduced the peak force at the onset of electrical stimulation (P < 0.05). Thus the effects on muscle force indicate that these drugs can affect K+-transporting proteins and thereby influence K+ accumulation, especially in the T tubules, suggesting that K(ATP) and BK(Ca) channels are responsible for K+ release and decrease in force during repeated muscle contractions, whereas Kir2.1 and NKCC1 may have a role in K+ reuptake.

Reducing chloride conductance prevents hyperkalaemia-induced loss of twitch force in rat slow-twitch muscle

Exercise-induced loss of skeletal muscle K(+) can seriously impede muscle performance through membrane depolarization. Thus far, it has been assumed that the negative equilibrium potential and large membrane conductance of Cl(-) attenuate the loss of force during hyperkalaemia. We questioned this idea because there is some evidence that Cl(-) itself can exert a depolarizing influence on membrane potential (V(m)). With this study we tried to identify the possible roles played by Cl(-) during hyperkalaemia. Isolated rat soleus muscles were kept at 25 degrees C and twitch contractions were evoked by current pulses. Reducing [Cl(-)](o) to 5 mM, prior to introducing 12.5 mM K(o), prevented the otherwise occurring loss of force. Reversing the order of introducing these two solutions revealed an additional effect, i.e. the ongoing hyperkalaemia-related loss of force was sped up tenfold after reducing [Cl(-)](o). However, hereafter twitch force recovered completely. The recovery of force was absent at [K(+)](o) exceeding 14 mM. In addition, reducing [Cl(-)](o) increased membrane excitability by 24%, as shown by a shift in the relationship between force and current level. Measurements of V(m) indicated that the antagonistic effect of reducing [Cl(-)](o) on hyperkalaemia-induced loss of force was due to low-Cl(-)-induced membrane hyperpolarization. The involvement of specific Cl(-) conductance was established with 9-anthracene carboxylic acid (9-AC). At 100 microm, 9-AC reduced the loss of force due to hyperkalaemia, while at 200 microm, 9-AC completely prevented loss of force. To study the role of the Na(+)-K(+)-2Cl(-) cotransporter (NKCC1) in this matter, we added 400 microm of the NKCC inhibitor bumetanide to the incubation medium. This did not affect the hyperkalaemia-induced loss of force. We conclude that Cl(-) exerts a permanent depolarizing influence on V(m). This influence of Cl(-) on V(m), in combination with a large membrane conductance, can apparently have two different effects on hyperkalaemia-induced loss of force. It might exert a stabilizing influence on force production during short periods of hyperkalaemia, but it can add to the loss of force during prolonged periods of hyperkalaemia.

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.

Intracellular Acidosis Enhances the Excitability of Working Muscle

Intracellular acidification of skeletal muscles is commonly thought to contribute to muscle fatigue. However, intracellular acidosis also acts to preserve muscle excitability when muscles become depolarized, which occurs with working muscles. Here, we show that this process may be mediated by decreased chloride permeability, which enables action potentials to still be propagated along the internal network of tubules in a muscle fiber (the T system) despite muscle depolarization. These results implicate chloride ion channels in muscle function and emphasize that intracellular acidosis of muscle has protective effects during muscle fatigue.

Protective role of extracellular chloride in fatigue of isolated mammalian skeletal muscle

A possible role of extracellular Cl(-) concentration ([Cl(-)](o)) in fatigue was investigated in isolated skeletal muscles of the mouse. When [Cl(-)](o) was lowered from 128 to 10 mM, peak tetanic force was unchanged, fade was exacerbated (wire stimulation electrodes), and a hump appeared during tetanic relaxation in both nonfatigued slow-twitch soleus and fast-twitch extensor digitorum longus (EDL) muscles. Low [Cl(-)](o) increased the rate of fatigue 1) with prolonged, continuous tetanic stimulation in soleus, 2) with repeated intermittent tetanic stimulation in soleus or EDL, and 3) to a greater extent with repeated tetanic stimulation when wire stimulation electrodes were used rather than plate stimulation electrodes in soleus. In nonfatigued soleus muscles, application of 9 mM K(+) with low [Cl(-)](o) caused more rapid and greater tetanic force depression, along with greater depolarization, than was evident at normal [Cl(-)](o). These effects of raised [K(+)](o) and low [Cl(-)](o) were synergistic. From these data, we suggest that normal [Cl(-)](o) provides protection against fatigue involving high-intensity contractions in both fast- and slow-twitch mammalian muscle. This phenomenon possibly involves attenuation of the depolarization caused by stimulation- or exercise-induced run-down of the transsarcolemmal K(+) gradient.

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.

Chloride channels in toad skeletal muscle fibers

Chloride currents were measured in short lumbricalis fibers of toads (Bufo arenarum) with voltage and patch clamp techniques. For the availability of chloride currents we applied a double-pulse technique in voltage-clamped fibers. When the test pulse was preceded by a positive prepulse, the initial current was larger than with a negative prepulse and exhibited a different rate of decline to its steady-state value. At the single-channel level we found that in most of the experiments with symmetrical 110 mM NaCl solutions, two levels of conductance, 20 ("small channel") and 360 pS ("maxi channel"), occurred with the highest probabilities. The openings of the maxi channels were more frequent at potentials close to 0 mV, whereas for the small channels the openings were at negative potentials. In contrast with the results with the macroscopic currents, a change of 2 orders of magnitude in the pH, from 7.3 to 5, had only minor effects on the channels' conductance. As with some other anion channels, the selectivity of the channels described here is low, the p(Cl)/p(Na) ratio being 1.9 and 3.7 for the small and maxi Cl(-) channels, respectively. The behavior of these Cl(-) channels with a relative high Na(+) permeability could contribute to the relatively low resting membrane potential of the lumbricalis fibers measured in the standard 110 mM NaCl solution.

Dynamics and consequences of potassium shifts in skeletal muscle and heart during exercise

Since it became clear that K(+) shifts with exercise are extensive and can cause more than a doubling of the extracellular [K(+)] ([K(+)](s)) as reviewed here, it has been suggested that these shifts may cause fatigue through the effect on muscle excitability and action potentials (AP). The cause of the K(+) shifts is a transient or long-lasting mismatch between outward repolarizing K(+) currents and K(+) influx carried by the Na(+)-K(+) pump. Several factors modify the effect of raised [K(+)](s) during exercise on membrane potential (E(m)) and force production. 1) Membrane conductance to K(+) is variable and controlled by various K(+) channels. Low relative K(+) conductance will reduce the contribution of [K(+)](s) to the E(m). In addition, high Cl(-) conductance may stabilize the E(m) during brief periods of large K(+) shifts. 2) The Na(+)-K(+) pump contributes with a hyperpolarizing current. 3) Cell swelling accompanies muscle contractions especially in fast-twitch muscle, although little in the heart. This will contribute considerably to the lowering of intracellular [K(+)] ([K(+)](c)) and will attenuate the exercise-induced rise of intracellular [Na(+)] ([Na(+)](c)). 4) The rise of [Na(+)](c) is sufficient to activate the Na(+)-K(+) pump to completely compensate increased K(+) release in the heart, yet not in skeletal muscle. In skeletal muscle there is strong evidence for control of pump activity not only through hormones, but through a hitherto unidentified mechanism. 5) Ionic shifts within the skeletal muscle t tubules and in the heart in extracellular clefts may markedly affect excitation-contraction coupling. 6) Age and state of training together with nutritional state modify muscle K(+) content and the abundance of Na(+)-K(+) pumps. We conclude that despite modifying factors coming into play during muscle activity, the K(+) shifts with high-intensity exercise may contribute substantially to fatigue in skeletal muscle, whereas in the heart, except during ischemia, the K(+) balance is controlled much more effectively.

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.

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.

Characteristics of two types of chloride channel in sarcoplasmic reticulum vesicles from rabbit skeletal muscle

A comparison is made of two types of chloride-selective channel in skeletal muscle sarcoplasmic reticulum (SR) vesicles incorporated into lipid bilayers. The I/V relationships of both channels, in 250/50 mM Cl- (cis/trans), were linear between -20 and +60 mV (cis potential,) reversed near Ecl and had slope conductances of approximately 250 pS for the big chloride (BCl) channel and approximately 70 pS for the novel, small chloride (SCl) channel. The protein composition of vesicles indicated that both channels originated from longitudinal SR and terminal cisternae. BCl and SCl channels responded differently to cis SO4(2-) (30-70 mM), 4,4'-diisothiocyanatostilbene 2,2'-disulfonic acid (8-80 microM) and to bilayer potential. The BCl channel open probability was high at all potentials, whereas SCl channels exhibited time-dependent activation and inactivation at negative potentials and deactivation at positive potentials. The duration and frequency of SCl channel openings were minimal at positive potentials and maximal at -40 mV, and were stationary during periods of activity. A substate analysis was performed using the Hidden Markov Model (S. H. Chung, J. B. Moore, L. Xia, L. S. Premkumar, and P. W. Gage, 1990, Phil. Trans. R. Soc. Lond. B., 329:265-285) and the algorithm EVPROC (evaluated here). SCl channels exhibited transitions between 5 and 7 conductance levels. BCl channels had 7-13 predominant levels plus many more short-lived substates. SCl channels have not been described in previous reports of Cl- channels in skeletal muscle SR.

Absence of the skeletal muscle sarcolemma chloride channel ClC-1 in myotonic mice

The voltage-dependent chloride channel ClC-1 stabilizes resting membrane potential in skeletal muscle. Mutations in the ClC-1 gene are responsible for both human autosomal recessive generalized myotonia and autosomal dominant myotonia congenita. To understand the tissue distribution and subcellular localization of ClC-1 and to evaluate its role in an animal model of myotonia, antibodies were raised against the carboxyl terminus of this protein. Expression of the 130-kDa ClC-1 protein is unique to skeletal muscle, consistent with its mRNA tissue distribution. Immunolocalization shows prominent ClC-1 antigen in the sarcolemma of both type I and II muscle fibers. Sarcolemma localization is confirmed by Western analysis of skeletal muscle subcellular fractions. The ADR myotonic mouse (phenotype ADR, genotype adr/adr), in which defective ClC-1 mRNA has been identified, is shown here to be absent in ClC-1 protein expression, whereas other skeletal muscle sarcolemma protein expression appears normal. Immunohistochemistry of skeletal muscle from ADR and other mouse models of human muscle disease demonstrate that the absence of ClC-1 chloride channel is a defect specific to ADR mice.