Extracellular magnesium and calcium reduce myotonia in isolated ClC-1 chloride channel-inhibited human muscle

Lower Ca2+ enhances the K+-induced force depression in normal and HyperKPP mouse muscles

Hyperkalemic periodic paralysis (HyperKPP) manifests as stiffness or subclinical myotonic discharges before or during periods of episodic muscle weakness or paralysis. Ingestion of Ca²⁺ alleviates HyperKPP symptoms, but the mechanism is unknown because lowering extracellular [Ca²⁺] ([Ca²⁺]_e) has no effect on force development in normal muscles under normal conditions. Lowering [Ca²⁺]_e, however, is known to increase the inactivation of voltage-gated cation channels, especially when the membrane is depolarized. Two hypotheses were tested: (1) lowering [Ca²⁺]_e depresses force in normal muscles under conditions that depolarize the cell membrane; and (2) HyperKPP muscles have a greater sensitivity to low Ca²⁺-induced force depression because many fibers are depolarized, even at a normal [K⁺]_e. In wild type muscles, lowering [Ca²⁺]_e from 2.4 to 0.3 mM had little effect on tetanic force and membrane excitability at a normal K⁺ concentration of 4.7 mM, whereas it significantly enhanced K⁺-induced depression of force and membrane excitability. In HyperKPP muscles, lowering [Ca²⁺]_e enhanced the K⁺-induced loss of force and membrane excitability not only at elevated [K⁺]_e but also at 4.7 mM K⁺. Lowering [Ca²⁺]_e increased the incidence of generating fast and transient contractures and gave rise to a slower increase in unstimulated force, especially in HyperKPP muscles. Lowering [Ca²⁺]_e reduced the efficacy of salbutamol, a β2 adrenergic receptor agonist and a treatment for HyperKPP, to increase force at elevated [K⁺]_e. Replacing Ca²⁺ by an equivalent concentration of Mg²⁺ neither fully nor consistently reverses the effects of lowering [Ca²⁺]_e. These results suggest that the greater Ca²⁺ sensitivity of HyperKPP muscles primarily relates to (1) a greater effect of Ca²⁺ in depolarized fibers and (2) an increased proportion of depolarized HyperKPP muscle fibers compared with control muscle fibers, even at normal [K⁺]_e.

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.

The mechanism underlying transient weakness in myotonia congenita

In addition to the hallmark muscle stiffness, patients with recessive myotonia congenita (Becker disease) experience debilitating bouts of transient weakness that remain poorly understood despite years of study. We performed intracellular recordings from muscle of both genetic and pharmacologic mouse models of Becker disease to identify the mechanism underlying transient weakness. Our recordings reveal transient depolarizations (plateau potentials) of the membrane potential to -25 to -35 mV in the genetic and pharmacologic models of Becker disease. Both Na+ and Ca2+ currents contribute to plateau potentials. Na+ persistent inward current (NaPIC) through NaV1.4 channels is the key trigger of plateau potentials and current through CaV1.1 Ca2+ channels contributes to the duration of the plateau. Inhibiting NaPIC with ranolazine prevents the development of plateau potentials and eliminates transient weakness in vivo. These data suggest that targeting NaPIC may be an effective treatment to prevent transient weakness in myotonia congenita.

Treatment Updates for Neuromuscular Channelopathies

PURPOSE OF REVIEW

This article aims to review the current and upcoming treatment options of primary muscle channelopathies including the non-dystrophic myotonias and periodic paralyses.

RECENT FINDINGS

The efficacy of mexiletine in the treatment of myotonia is now supported by two randomised placebo-controlled trials, one of which utilised a novel aggregated n-of-1 design. This has resulted in licencing of the drug via orphan drug status. There is also good evidence that mexiletine is well tolerated and safe in this patient group without the need for intensive monitoring. A range of alternative antimyotonic treatment options include lamotrigine, carbamazepine and ranolazine exist with variable evidence base. In vitro studies have shown insight into reasons for treatment failure of some medications with certain genotypes opening the era of mutation-specific therapy such as use of flecainide. In the periodic paralyses, the ability of MRI to distinguish between reversible oedema and irreversible fatty replacement makes it an increasingly useful tool to guide and assess pharmacological treatment. Unfortunately, the striking efficacy of bumetanide in hypokalaemic periodic paralysis animal models was not replicated in a recent pilot study in humans.

SUMMARY

The treatment of skeletal muscle channelopathies combines dietary and lifestyle advice together with pharmacological interventions. The rarity of these conditions remains a barrier for clinical studies but the example of the aggregated n-of-1 trial of mexiletine shows that innovative trial design can overcome these hurdles. Further research is required to test efficacy of drugs shown to have promising characteristics in preclinical experiments such as safinamide, riluzule and magnesium for myotonia or bumetanide for hypokalaemic periodic paralysis.

Depletion of ATP Limits Membrane Excitability of Skeletal Muscle by Increasing Both ClC1-Open Probability and Membrane Conductance

Activation of skeletal muscle contractions require that action potentials can be excited and propagated along the muscle fibers. Recent studies have revealed that muscle fiber excitability is regulated during repeated firing of action potentials by cellular signaling systems that control the function of ion channel that determine the resting membrane conductance (G_m). In fast-twitch muscle, prolonged firing of action potentials triggers a marked increase in G_m, reducing muscle fiber excitability and causing action potential failure. Both ClC-1 and K_ATP ion channels contribute to this G_m rise, but the exact molecular regulation underlying their activation remains unclear. Studies in expression systems have revealed that ClC-1 is able to bind adenosine nucleotides, and that low adenosine nucleotide levels result in ClC-1 activation. In three series of experiments, this study aimed to explore whether ClC-1 is also regulated by adenosine nucleotides in native skeletal muscle fibers, and whether the adenosine nucleotide sensitivity of ClC-1 could explain the rise in G_m muscle fibers during prolonged action potential firing. First, whole cell patch clamping of mouse muscle fibers demonstrated that ClC-1 activation shifted in the hyperpolarized direction when clamping pipette solution contained 0 mM ATP compared with 5 mM ATP. Second, three-electrode G_m measurement during muscle fiber stimulation showed that glycolysis inhibition, with 2-deoxy-glucose or iodoacetate, resulted in an accelerated and rapid >400% G_m rise during short periods of repeated action potential firing in both fast-twitch and slow-twitch rat, and in human muscle fibers. Moreover, ClC-1 inhibition with 9-anthracenecarboxylic acid resulted in either an absence or blunted G_m rise during action potential firing in human muscle fibers. Third, G_m measurement during repeated action potential firing in muscle fibers from a murine McArdle disease model suggest that the rise in G_m was accelerated in a subset of fibers. Together, these results are compatible with ClC-1 function being regulated by the level of adenosine nucleotides in native tissue, and that the channel operates as a sensor of skeletal muscle metabolic state, limiting muscle excitability when energy status is low.

Skeletal muscle ClC-1 chloride channels in health and diseases

In 1970, the study of the pathomechanisms underlying myotonia in muscle fibers isolated from myotonic goats highlighted the importance of chloride conductance for skeletal muscle function; 20 years later, the human ClC-1 chloride channel has been cloned; last year, the crystal structure of human protein has been solved. Over the years, the efforts of many researchers led to significant advances in acknowledging the role of ClC-1 in skeletal muscle physiology and the mechanisms through which ClC-1 dysfunctions lead to impaired muscle function. The wide spectrum of pathophysiological conditions associated with modification of ClC-1 activity, either as the primary cause, such as in myotonia congenita, or as a secondary adaptive mechanism in other neuromuscular diseases, supports the idea that ClC-1 is relevant to preserve not only for skeletal muscle excitability, but also for skeletal muscle adaptation to physiological or harmful events. Improving this understanding could open promising avenues toward the development of selective and safe drugs targeting ClC-1, with the aim to restore normal muscle function. This review summarizes the most relevant research on ClC-1 channel physiology, associated diseases, and pharmacology.

Physical activity, dietary calcium to magnesium intake and mortality in the National Health and Examination Survey 1999-2006 cohort

Calcium and magnesium affect muscle mass and function. Magnesium and calcium are also important for optimal vitamin D status. Vitamin D status modifies the associations between physical activity and risk of incident cardiovascular disease (CVD) and CVD mortality. However, no study examined whether levels of magnesium and calcium and the ratio of dietary calcium to magnesium (Ca:Mg) intake modify the relationship between physical activity and mortality. We included 20,295 National Health and Nutrition Examination Survey participants (1999-2006) aged >20 years with complete dietary, physical activity and mortality data (2,663 deaths). We assessed physical activity based on public health guidelines and sex-specific tertiles of MET-minutes/week. We used Cox proportional hazards models adjusted for potential confounding factors and stratified by the intakes of magnesium, calcium, Ca:Mg ratio. We found higher physical activity was significantly associated with reduced risk of total mortality and cause-specific mortality, regardless of Ca:Mg ratio, magnesium or calcium intake. In contrast, both moderate and high physical activity were significantly associated with substantially reduced risks of mortality due to cancer when magnesium intake was above the RDA level. We also found higher physical activity was significantly associated with a reduced risk of mortality due to cancer only when Ca:Mg ratios were between 1.7 and 2.6, although the interaction was not significant. Overall, dietary magnesium and, potentially, the Ca:Mg ratio modify the relationship between physical activity and cause-specific mortality. Further study is important to understand the modifying effects of the balance between calcium and magnesium intake on physical activity for chronic disease prevention.

Inhibiting persistent inward sodium currents prevents myotonia

OBJECTIVE

Patients with myotonia congenita have muscle hyperexcitability due to loss-of-function mutations in the ClC-1 chloride channel in skeletal muscle, which causes involuntary firing of muscle action potentials (myotonia), producing muscle stiffness. The excitatory events that trigger myotonic action potentials in the absence of stabilizing ClC-1 current are not fully understood. Our goal was to identify currents that trigger spontaneous firing of muscle in the setting of reduced ClC-1 current.

METHODS

In vitro intracellular current clamp and voltage clamp recordings were performed in muscle from a mouse model of myotonia congenita.

RESULTS

Intracellular recordings revealed a slow afterdepolarization (AfD) that triggers myotonic action potentials. The AfD is well explained by a tetrodotoxin-sensitive and voltage-dependent Na⁺ persistent inward current (NaPIC). Notably, this NaPIC undergoes slow inactivation over seconds, suggesting this may contribute to the end of myotonic runs. Highlighting the significance of this mechanism, we found that ranolazine and elevated serum divalent cations eliminate myotonia by inhibiting AfD and NaPIC.

INTERPRETATION

This work significantly changes our understanding of the mechanisms triggering myotonia. Our work suggests that the current focus of treating myotonia, blocking the transient Na⁺ current underlying action potentials, is an inefficient approach. We show that inhibiting NaPIC is paralleled by elimination of myotonia. We suggest the ideal myotonia therapy would selectively block NaPIC and spare the transient Na⁺ current. Ann Neurol 2017;82:385-395.

Sodium channel biophysics, late sodium current and genetic arrhythmic syndromes

Arrhythmias arise from breakdown of orderly action potential (AP) activation, propagation and recovery driven by interactive opening and closing of successive voltage-gated ion channels, in which one or more Na⁺ current components play critical parts. Early peak, Na⁺ currents (I_Na) reflecting channel activation drive the AP upstroke central to cellular activation and its propagation. Sustained late Na⁺ currents (I_Na-L) include contributions from a component with a delayed inactivation timecourse influencing AP duration (APD) and refractoriness, potentially causing pro-arrhythmic phenotypes. The magnitude of I_Na-L can be analysed through overlaps or otherwise in the overall voltage dependences of the steady-state properties and kinetics of activation and inactivation of the Na⁺ conductance. This was useful in analysing repetitive firing associated with paramyotonia congenita in skeletal muscle. Similarly, genetic cardiac Na⁺ channel abnormalities increasing I_Na-L are implicated in triggering phenomena of automaticity, early and delayed afterdepolarisations and arrhythmic substrate. This review illustrates a wide range of situations that may accentuate I_Na-L. These include (1) overlaps between steady-state activation and inactivation increasing window current, (2) kinetic deficiencies in Na⁺ channel inactivation leading to bursting phenomena associated with repetitive channel openings and (3) non-equilibrium gating processes causing channel re-opening due to more rapid recoveries from inactivation. All these biophysical possibilities were identified in a selection of abnormal human SCN5A genotypes. The latter presented as a broad range of clinical arrhythmic phenotypes, for which effective therapeutic intervention would require specific identification and targeting of the diverse electrophysiological abnormalities underlying their increased I_Na-L.

The anti-convulsants lacosamide, lamotrigine, and rufinamide reduce myotonia in isolated human and rat skeletal muscle

INTRODUCTION

In myotonia congenita, loss of ClC-1 Cl^- channel function results in skeletal muscle hyperexcitability and myotonia. Anti-myotonic treatment has typically targeted the voltage-gated sodium channel in skeletal muscle (Nav1.4). In this study we explored whether 3 sodium channel-modulating anti-epileptics can reduce myotonia in isolated rat and human muscle.

METHODS

Dissected muscles were rendered myotonic by ClC-1 channel inhibition. The ability of the drugs to suppress myotonia was then assessed from subclinical to maximal clinical concentrations. Drug synergy was determined using isobole plots.

RESULTS

All drugs were capable of abolishing myotonia in both rat and human muscles. Lamotrigine and rufinamide completely suppressed myotonia at submaximal clinical concentrations, whereas lacosamide had to be raised above the maximal clinical concentration to suppress myotonia completely. A synergistic effect of lamotrigine and rufinamide was observed.

CONCLUSION

These findings suggest that lamotrigine and rufinamide could be considered for anti-myotonic treatment in myotonia congenita. Muscle Nerve 56: 136-142, 2017.

Protein kinase C-dependent regulation of ClC-1 channels in active human muscle and its effect on fast and slow gating

KEY POINTS

Regulation of ion channel function during repeated firing of action potentials is commonly observed in excitable cells. Recently it was shown that muscle activity is associated with rapid, protein kinase C (PKC)-dependent ClC-1 Cl(-) channel inhibition in rodent muscle. While this PKC-dependent ClC-1 inhibition during muscle activity was shown to be important for the maintenance of contractile endurance in rat muscle it is unknown whether a similar regulation exists in human muscle. Also, the molecular mechanisms underlying the observed PKC-dependent ClC-1 inhibition are unclear. Here we present the first demonstration of ClC-1 inhibition in active human muscle fibres, and we determine the changes in ClC-1 gating that underlie the PKC-dependent ClC-1 inhibition in active muscle using human ClC-1 expressed in Xenopus oocytes. This activity-induced ClC-1 inhibition is suggested to represent a mechanism by which human muscle fibres maintain their excitability during sustained activity.

ABSTRACT

Repeated firing of action potentials (APs) is known to trigger rapid, protein kinase C (PKC)-dependent inhibition of ClC-1 Cl(-) ion channels in rodent muscle and this inhibition is important for contractile endurance. It is currently unknown whether similar regulation exists in human muscle, and the molecular mechanisms underlying PKC-dependent ClC-1 inhibition are unclear. This study first determined whether PKC-dependent ClC-1 inhibition exists in active human muscle, and second, it clarified how PKC alters the gating of human ClC-1 expressed in Xenopus oocytes. In human abdominal and intercostal muscles, repeated AP firing was associated with 30-60% reduction of ClC-1 function, which could be completely prevented by PKC inhibition (1 μm GF109203X). The role of the PKC-dependent ClC-1 inhibition was evaluated from rheobase currents before and after firing 1000 APs: while rheobase current was well maintained after activity under control conditions it rose dramatically if PKC-dependent ClC-1 inhibition had been prevented with the inhibitor. This demonstrates that the ClC-1 inhibition is important for maintenance of excitability in active human muscle fibres. Oocyte experiments showed that PKC activation lowered the overall open probability of ClC-1 in the voltage range relevant for AP initiation in muscle fibres. More detailed analysis of this reduction showed that PKC mostly affected the slow gate of ClC-1. Indeed, there was no effect of PKC activation in C277S mutated ClC-1 in which the slow gate is effectively locked open. It is concluded that regulation of excitability of active human muscle fibres relies on PKC-dependent ClC-1 inhibition via a gating mechanism.

Satellite cell response to erythropoietin treatment and endurance training in healthy young men

KEY POINT

Erythropoietin (Epo) treatment may induce myogenic differentiation factor (MyoD) expression and prevent apoptosis in satellite cells (SCs) in murine and in vitro models. Endurance training stimulates SC proliferation in vivo in murine and human skeletal muscle. In the present study, we show, in human skeletal muscle, that treatment with an Epo-stimulating agent (darbepoetin-α) in vivo increases the content of MyoD(+) SCs in healthy young men. Moreover, we report that Epo receptor mRNA is expressed in adult human SCs, suggesting that Epo may directly target SCs through ligand-receptor interaction. Moreover, endurance training, but not Epo treatment, increases the SC content in type II myofibres, as well as the content of MyoD(+) SCs. Collectively, our results suggest that Epo treatment can regulate human SCs in vivo, supported by Epo receptor mRNA expression in human SCs. In effect, long-term Epo treatment during disease conditions involving anaemia may impact SCs and warrants further investigation. Satellite cell (SC) proliferation is observed following erythropoitin treatment in vitro in murine myoblasts and endurance training in vivo in human skeletal muscle. The present study aimed to investigate the effects of prolonged erythropoiesis-stimulating agent (ESA; darbepoetin-α) treatment and endurance training, separately and combined, on SC quantity and commitment in human skeletal muscle. Thirty-five healthy, untrained men were randomized into four groups: sedentary-placebo (SP, n = 9), sedentary-ESA (SE, n = 9), training-placebo (TP, n = 9) or training-ESA (TE, n = 8). ESA/placebo was injected once weekly and training consisted of ergometer cycling three times a week for 10 weeks. Prior to and following the intervention period, blood samples and muscle biopsies were obtained and maximal oxygen uptake (V̇O2, max) was measured. Immunohistochemical analyses were used to quantify fibre type specific SCs (Pax7(+)), myonuclei and active SCs (Pax7(+)/MyoD(+)). ESA treatment led to elevated haematocrit, whereas endurance training increased V̇O2, max. Endurance training led to an increase in SCs associated with type II fibres (P < 0.05), whereas type I fibres showed no changes. Both ESA treatment and endurance training increased Pax7(+)/MyoD(+) cells, whereas only ESA treatment increased the total content of MyoD(+) cells. Epo-R mRNA presence in adult SC was tested with real-time RT-PCR using fluorescence-activated cell sorting (CD56(+)/CD45(-)/CD31(-)) to isolate cells from a human rectus abdominis muscle and was found to be considerably higher than in whole muscle. In conclusion, endurance training and ESA treatment may separately stimulate SC commitment to the myogenic program. Furthermore, ESA-treatment may alter SC activity by direct interaction with the Epo-R expressed on SCs.

Divalent cation-responsive myotonia and muscle paralysis in skeletal muscle sodium channelopathy

We report a patient with paramyotonia congenita/hyperkalemic periodic paralysis due to Nav1.4 I693T mutation who had worsening of myotonia and muscle weakness in the setting of hypomagnesemia and hypocalcemia with marked recovery after magnesium administration. Computer simulations of the effects of the I693T mutation were introduced in the muscle fiber model by both hyperpolarizing shifts in the Nav1.4 channel activation and a faster recovery from slow channel inactivation. A further shift in the Nav1.4 channel activation in the hyperpolarizing direction as expected with low divalent cations resulted in myotonia that progressed to membrane inexcitability. Shifting the channel activation in the depolarizing direction as would be anticipated from magnesium supplementation abolished the myotonia. These observations provide clinical and biophysical evidence that the muscle symptoms in sodium channelopathy are sensitive to divalent cations. Exploration of the role of magnesium administration in therapy or prophylaxis is warranted with a randomized clinical trial.

ClC-1 chloride channels: state-of-the-art research and future challenges

The voltage-dependent ClC-1 chloride channel belongs to the CLC channel/transporter family. It is a homodimer comprising two individual pores which can operate independently or simultaneously according to two gating modes, the fast and the slow gate of the channel. ClC-1 is preferentially expressed in the skeletal muscle fibers where the presence of an efficient Cl(-) homeostasis is crucial for the correct membrane repolarization and propagation of action potential. As a consequence, mutations in the CLCN1 gene cause dominant and recessive forms of myotonia congenita (MC), a rare skeletal muscle channelopathy caused by abnormal membrane excitation, and clinically characterized by muscle stiffness and various degrees of transitory weakness. Elucidation of the mechanistic link between the genetic defects and the disease pathogenesis is still incomplete and, at this time, there is no specific treatment for MC. Still controversial is the subcellular localization pattern of ClC-1 channels in skeletal muscle as well as its modulation by some intracellular factors. The expression of ClC-1 in other tissues such as in brain and heart and the possible assembly of ClC-1/ClC-2 heterodimers further expand the physiological properties of ClC-1 and its involvement in diseases. A recent de novo CLCN1 truncation mutation in a patient with generalized epilepsy indeed postulates an unexpected role of this channel in the control of neuronal network excitability. This review summarizes the most relevant and state-of-the-art research on ClC-1 chloride channels physiology and associated diseases.