Calcium currents and transients of native and heterologously expressed mutant skeletal muscle DHP receptor alpha1 subunits (R528H)

Molecular mechanism of hyperactivation conferred by a truncation of TRPA1

A drastic TRPA1 mutant (R919*) identified in CRAMPT syndrome patients has not been mechanistically characterized. Here, we show that the R919* mutant confers hyperactivity when co-expressed with wild type (WT) TRPA1. Using functional and biochemical assays, we reveal that the R919* mutant co-assembles with WT TRPA1 subunits into heteromeric channels in heterologous cells that are functional at the plasma membrane. The R919* mutant hyperactivates channels by enhancing agonist sensitivity and calcium permeability, which could account for the observed neuronal hypersensitivity-hyperexcitability symptoms. We postulate that R919* TRPA1 subunits contribute to heteromeric channel sensitization by altering pore architecture and lowering energetic barriers to channel activation contributed by the missing regions. Our results expand the physiological impact of nonsense mutations, reveal a genetically tractable mechanism for selective channel sensitization, uncover insights into the process of TRPA1 gating, and provide an impetus for genetic analysis of patients with CRAMPT or other stochastic pain syndromes.

Voltage-Dependent Ca2+ Release Is Impaired in Hypokalemic Periodic Paralysis Caused by CaV1.1-R528H but not by NaV1.4-R669H

Hypokalemic periodic paralysis (HypoPP) is a channelopathy of skeletal muscle caused by missense mutations in the voltage sensor domains (usually at an arginine of the S4 segment) of the Ca_V1.1 calcium channel or of the Na_V1.4 sodium channel. The primary clinical manifestation is recurrent attacks of weakness, resulting from impaired excitability of anomalously depolarized fibers containing leaky mutant channels. While the ictal loss of fiber excitability is sufficient to explain the acute episodes of weakness, a deleterious change in voltage sensor function for Ca_V1.1 mutant channels may also compromise excitation-contraction coupling (EC-coupling). We used the low-affinity Ca²⁺ indicator OGN-5 to assess voltage-dependent Ca²⁺-release as a measure of EC-coupling for our knock-in mutant mouse models of HypoPP. The peak in fibers isolated from Ca_V1.1-R528H mice was about two-thirds of the amplitude observed in WT mice; whereas in HypoPP fibers from Na_V1.4-R669H mice the was indistinguishable from WT. No difference in the voltage dependence of from WT was observed for fibers from either HypoPP mouse model. Because late-onset permanent muscle weakness is more severe for Ca_V1.1-associated HypoPP than for Na_V1.4, we propose the reduced Ca²⁺-release for Ca_V1.1-R528H mutant channels may increase the susceptibility to fixed myopathic weakness. In contrast the episodes of transient weakness are similar for Ca_V1.1- and Na_V1.4-associated HypoPP, consistent with the notion that acute attacks of weakness are primarily caused by leaky channels and are not a consequence of reduced Ca²⁺-release.

Sodium channelopathies of skeletal muscle and brain

Voltage-gated sodium channels initiate action potentials in nerve, skeletal muscle, and other electrically excitable cells. Mutations in them cause a wide range of diseases. These channelopathy mutations affect every aspect of sodium channel function, including voltage sensing, voltage-dependent activation, ion conductance, fast and slow inactivation, and both biosynthesis and assembly. Mutations that cause different forms of periodic paralysis in skeletal muscle were discovered first and have provided a template for understanding structure, function, and pathophysiology at the molecular level. More recent work has revealed multiple sodium channelopathies in the brain. Here we review the well-characterized genetics and pathophysiology of the periodic paralyses of skeletal muscle and then use this information as a foundation for advancing our understanding of mutations in the structurally homologous α-subunits of brain sodium channels that cause epilepsy, migraine, autism, and related comorbidities. We include studies based on molecular and structural biology, cell biology and physiology, pharmacology, and mouse genetics. Our review reveals unexpected connections among these different types of sodium channelopathies.

Morphological Alterations of the Sarcotubular System in Permanent Myopathy of Hereditary Hypokalemic Periodic Paralysis with a Mutation in the CACNA1S Gene

We investigated the immunohistochemical localization of several proteins related to excitation-contraction coupling and ultrastructural alterations of the sarcotubular system in biopsied muscles from a father and a daughter in a family with permanent myopathy with hypokalemic periodic paralysis (PMPP) due to a mutation in calcium channel CACNA1S; p. R1239H hetero. Immunostaining for L-type calcium channels (LCaC) showed linear hyper-stained regions indicating proliferation of longitudinal t-tubules. The margin of vacuoles was positive for ryanodine receptor, LCaC, calsequestrin (CASQ) 1, CASQ 2, SR/ER Ca2+-ATPase (SERCA) 1, SERCA2, dysferlin, dystrophin, α-actinin, LC3, and LAMP 1. Electron microscopy indicated that the vacuoles mainly originated from the sarcoplasmic reticulum (SR). These findings indicate impairment of the muscle contraction system related to Ca2+ dynamics, remodeling of t-tubules and muscle fiber repair. We speculate that PMPP in patients with a CACNA1S mutation might start with abnormal SR function due to impaired LCaC. Subsequent induction of muscular contractile abnormalities and the vacuoles formed by fused SR in the repair process including autophagy might result in permanent myopathy. Our findings may facilitate prediction of the pathomechanisms of PMPP seen on morphological observation.

Skeletal muscle CaV1.1 channelopathies

Ca_V1.1 is specifically expressed in skeletal muscle where it functions as voltage sensor of skeletal muscle excitation-contraction (EC) coupling independently of its functions as L-type calcium channel. Consequently, all known Ca_V1.1-related diseases are muscle diseases and the molecular and cellular disease mechanisms relate to the dual functions of Ca_V1.1 in this tissue. To date, four types of muscle diseases are known that can be linked to mutations in the CACNA1S gene or to splicing defects. These are hypo- and normokalemic periodic paralysis, malignant hyperthermia susceptibility, Ca_V1.1-related myopathies, and myotonic dystrophy type 1. In addition, the Ca_V1.1 function in EC coupling is perturbed in Native American myopathy, arising from mutations in the Ca_V1.1-associated protein STAC3. Here, we first address general considerations concerning the possible roles of Ca_V1.1 in disease and then discuss the state of the art regarding the pathophysiology of the Ca_V1.1-related skeletal muscle diseases with an emphasis on molecular disease mechanisms.

When muscle Ca2+ channels carry monovalent cations through gating pores: insights into the pathophysiology of type 1 hypokalaemic periodic paralysis

Patients suffering from type 1 hypokalaemic periodic paralysis (HypoPP1) experience attacks of muscle paralysis associated with hypokalaemia. The disease arises from missense mutations in the gene encoding the α1 subunit of the dihydropyridine receptor (DHPR), a protein complex anchored in the tubular membrane of skeletal muscle fibres which controls the release of Ca²⁺ from sarcoplasmic reticulum and also functions as a Ca²⁺ channel. The vast majority of mutations consist of the replacement of one of the outer arginines in S4 segments of the α1 subunit by neutral residues. Early studies have shown that muscle fibres from HypoPP1 patients are abnormally depolarized at rest in low K⁺ to the point of inducing muscle inexcitability. The relationship between HypoPP1 mutations and depolarization has long remained unknown. More recent investigations conducted in the closely structurally related voltage-gated Na⁺ and K⁺ channels have shown that comparable S4 arginine substitutions gave rise to elevated inward currents at negative potentials called gating pore currents. Experiments performed in muscle fibres from different models revealed such an inward resting current through HypoPP1 mutated Ca²⁺ channels. In mouse fibres transfected with HypoPP1 mutated channels, the elevated resting current was found to carry H⁺ for the R1239H arginine-to-histidine mutation in a S4 segment and Na⁺ for the V876E HypoPP1 mutation, which has the peculiarity of not being located in S4 segments. Muscle paralysis probably results from the presence of a gating pore current associated with hypokalaemia for both mutations, possibly aggravated by external acidosis for the R1239H mutation.

Stac3 enhances expression of human CaV1.1 in Xenopus oocytes and reveals gating pore currents in HypoPP mutant channels

Hypokalemic periodic paralysis (HypoPP) is thought to be caused by an aberrant inward current through the voltage sensors of mutant Na⁺ or Ca²⁺ channels. Wu et al. use Stac3 to enhance the membrane expression of two HypoPP Ca_V1.1 mutants in oocytes and find that both support gating pore currents.

Mutations of Ca_V1.1, the pore-forming subunit of the L-type Ca²⁺ channel in skeletal muscle, are an established cause of hypokalemic periodic paralysis (HypoPP). However, functional assessment of HypoPP mutant channels has been hampered by difficulties in achieving sufficient plasma membrane expression in cells that are not of muscle origin. In this study, we show that coexpression of Stac3 dramatically increases the expression of human Ca_V1.1 (plus α₂-δ_1b and β_1a subunits) at the plasma membrane of Xenopus laevis oocytes. In voltage-clamp studies with the cut-open oocyte clamp, we observe ionic currents on the order of 1 μA and gating charge displacements of ∼0.5–1 nC. Importantly, this high expression level is sufficient to ascertain whether HypoPP mutant channels are leaky because of missense mutations at arginine residues in S4 segments of the voltage sensor domains. We show that R528H and R528G in S4 of domain II both support gating pore currents, but unlike other R/H HypoPP mutations, R528H does not conduct protons. Stac3-enhanced membrane expression of Ca_V1.1 in oocytes increases the throughput for functional studies of disease-associated mutations and is a new platform for investigating the voltage-dependent properties of Ca_V1.1 without the complexity of the transverse tubule network in skeletal muscle.

Mechanisms Responsible for ω-Pore Currents in Cav Calcium Channel Voltage-Sensing Domains

Mutations of positively charged amino acids in the S4 transmembrane segment of a voltage-gated ion channel form ion-conducting pathways through the voltage-sensing domain, named ω-current. Here, we used structure modeling and MD simulations to predict pathogenic ω-currents in Ca_V1.1 and Ca_V1.3 Ca²⁺ channels bearing several S4 charge mutations. Our modeling predicts that mutations of Ca_V1.1-R1 (R528H/G, R897S) or Ca_V1.1-R2 (R900S, R1239H) linked to hypokalemic periodic paralysis type 1 and of Ca_V1.3-R3 (R990H) identified in aldosterone-producing adenomas conducts ω-currents in resting state, but not during voltage-sensing domain activation. The mechanism responsible for the ω-current and its amplitude depend on the number of charges in S4, the position of the mutated S4 charge and countercharges, and the nature of the replacing amino acid. Functional characterization validates the modeling prediction showing that Ca_V1.3-R990H channels conduct ω-currents at hyperpolarizing potentials, but not upon membrane depolarization compared with wild-type channels.

Na leak with gating pore properties in hypokalemic periodic paralysis V876E mutant muscle Ca channel

Type 1 hypokalemic periodic paralysis (HypoPP1) is a poorly understood genetic neuromuscular disease characterized by episodic attacks of paralysis associated with low blood K⁺ The vast majority of HypoPP1 mutations involve the replacement of an arginine by a neutral residue in one of the S4 segments of the α1 subunit of the skeletal muscle voltage-gated Ca²⁺ channel, which is thought to generate a pathogenic gating pore current. The V876E HypoPP1 mutation has the peculiarity of being located in the S3 segment of domain III, rather than an S4 segment, raising the question of whether such a mutation induces a gating pore current. Here we successfully transfer cDNAs encoding GFP-tagged human wild-type (WT) and V876E HypoPP1 mutant α1 subunits into mouse muscles by electroporation. The expression profile of these WT and V876E channels shows a regular striated pattern, indicative of their localization in the t-tubule membrane. In addition, L-type Ca²⁺ current properties are the same in V876E and WT fibers. However, in the presence of an external solution containing low-Cl^- and lacking Na⁺ and K⁺, V876E fibers display an elevated leak current at negative voltages that is increased by external acidification to a higher extent in V876E fibers, suggesting that the leak current is carried by H⁺ ions. However, in the presence of Tyrode's solution, the rate of change in intracellular pH produced by external acidification was not significantly different in V876E and WT fibers. Simultaneous measurement of intracellular Na⁺ and current in response to Na⁺ readmission in the external solution reveals a rate of Na⁺ influx associated with an inward current, which are both significantly larger in V876E fibers. These data suggest that the V876E mutation generates a gating pore current that carries strong resting Na⁺ inward currents in physiological conditions that are likely responsible for the severe HypoPP1 symptoms associated with this mutation.

Elevated resting H+ current in the R1239H type 1 hypokalaemic periodic paralysis mutated Ca2+ channel

KEY POINTS

Missense mutations in the gene encoding the α1 subunit of the skeletal muscle voltage-gated Ca²⁺ channel induce type 1 hypokalaemic periodic paralysis, a poorly understood neuromuscular disease characterized by episodic attacks of paralysis associated with low serum K⁺ . Acute expression of human wild-type and R1239H HypoPP1 mutant α1 subunits in mature mouse muscles showed that R1239H fibres displayed Ca²⁺ currents of reduced amplitude and larger resting leak inward current increased by external acidification. External acidification also produced intracellular acidification at a higher rate in R1239H fibres and inhibited inward rectifier K⁺ currents. These data suggest that the R1239H mutation induces an elevated leak H⁺ current at rest flowing through a gating pore and could explain why paralytic attacks preferentially occur during the recovery period following muscle exercise.

ABSTRACT

Missense mutations in the gene encoding the α1 subunit of the skeletal muscle voltage-gated Ca²⁺ channel induce type 1 hypokalaemic periodic paralysis, a poorly understood neuromuscular disease characterized by episodic attacks of paralysis associated with low serum K⁺ . The present study aimed at identifying the changes in muscle fibre electrical properties induced by acute expression of the R1239H hypokalaemic periodic paralysis human mutant α1 subunit of Ca²⁺ channels in a mature muscle environment to better understand the pathophysiological mechanisms involved in this disorder. We transferred genes encoding wild-type and R1239H mutant human Ca²⁺ channels into hindlimb mouse muscle by electroporation and combined voltage-clamp and intracellular pH measurements on enzymatically dissociated single muscle fibres. As compared to fibres expressing wild-type α1 subunits, R1239H mutant-expressing fibres displayed Ca²⁺ currents of reduced amplitude and a higher resting leak inward current that was increased by external acidification. External acidification also produced intracellular acidification at a higher rate in R1239H fibres and inhibited inward rectifier K⁺ currents. These data indicate that the R1239H mutation induces an elevated leak H⁺ current at rest flowing through a gating pore created by the mutation and that external acidification favours onset of muscle paralysis by potentiating H⁺ depolarizing currents and inhibiting resting inward rectifier K⁺ currents. Our results could thus explain why paralytic attacks preferentially occur during the recovery period following intense muscle exercise.

Transient compartment-like syndrome and normokalaemic periodic paralysis due to a Ca(v)1.1 mutation

We studied a two-generation family presenting with conditions that included progressive permanent weakness, myopathic myopathy, exercise-induced contracture before normokalaemic periodic paralysis or, if localized to the tibial anterior muscle group, transient compartment-like syndrome (painful acute oedema with neuronal compression and drop foot). 23Na and 1H magnetic resonance imaging displayed myoplasmic sodium overload, and oedema. We identified a novel familial Ca(v)1.1 calcium channel mutation, R1242G, localized to the third positive charge of the domain IV voltage sensor. Functional expression of R1242G in the muscular dysgenesis mouse cell line GLT revealed a 28% reduced central pore inward current and a -20 mV shift of the steady-state inactivation curve. Both changes may be at least partially explained by an outward omega (gating pore) current at positive potentials. Moreover, this outward omega current of 27.5 nS/nF may cause the reduction of the overshoot by 13 mV and slowing of the upstroke of action potentials by 36% that are associated with muscle hypoexcitability (permanent weakness and myopathic myopathy). In addition to the outward omega current, we identified an inward omega pore current of 95 nS/nF at negative membrane potentials after long depolarizing pulses that shifts the R1242G residue above the omega pore constriction. A simulation reveals that the inward current might depolarize the fibre sufficiently to trigger calcium release in the absence of an action potential and therefore cause an electrically silent depolarization-induced muscle contracture. Additionally, evidence of the inward current can be found in 23Na magnetic resonance imaging-detected sodium accumulation and 1H magnetic resonance imaging-detected oedema. We hypothesize that the episodes are normokalaemic because of depolarization-induced compensatory outward potassium flux through both delayed rectifiers and omega pore. We conclude that the position of the R1242G residue before elicitation of the omega current is decisive for its conductance: if the residue is located below the gating pore as in the resting state then outward currents are observed; if the residue is above the gating pore because of depolarization, as in the inactivated state, then inward currents are observed. This study shows for the first time that functional characterization of omega pore currents is possible using a cultured cell line expressing mutant Ca(v)1.1 channels. Likewise, it is the first calcium channel mutation for complicated normokalaemic periodic paralysis.

A calcium channel mutant mouse model of hypokalemic periodic paralysis

Hypokalemic periodic paralysis (HypoPP) is a familial skeletal muscle disorder that presents with recurrent episodes of severe weakness lasting hours to days associated with reduced serum potassium (K+). HypoPP is genetically heterogeneous, with missense mutations of a calcium channel (Ca(V)1.1) or a sodium channel (Na(V)1.4) accounting for 60% and 20% of cases, respectively. The mechanistic link between Ca(V)1.1 mutations and the ictal loss of muscle excitability during an attack of weakness in HypoPP is unknown. To address this question, we developed a mouse model for HypoPP with a targeted Ca(V)1.1 R528H mutation. The Ca(V)1.1 R528H mice had a HypoPP phenotype for which low K+ challenge produced a paradoxical depolarization of the resting potential, loss of muscle excitability, and weakness. A vacuolar myopathy with dilated transverse tubules and disruption of the triad junctions impaired Ca2+ release and likely contributed to the mild permanent weakness. Fibers from the Ca(V)1.1 R528H mouse had a small anomalous inward current at the resting potential, similar to our observations in the Na(V)1.4 R669H HypoPP mouse model. This "gating pore current" may be a common mechanism for paradoxical depolarization and susceptibility to HypoPP arising from missense mutations in the S4 voltage sensor of either calcium or sodium channels.

Ca(V)1.1: The atypical prototypical voltage-gated Ca²⁺ channel

Ca(V)1.1 is the prototype for the other nine known Ca(V) channel isoforms, yet it has functional properties that make it truly atypical of this group. Specifically, Ca(V)1.1 is expressed solely in skeletal muscle where it serves multiple purposes; it is the voltage sensor for excitation-contraction coupling and it is an L-type Ca²⁺ channel which contributes to a form of activity-dependent Ca²⁺ entry that has been termed Excitation-coupled Ca²⁺ entry. The ability of Ca(V)1.1 to serve as voltage-sensor for excitation-contraction coupling appears to be unique among Ca(V) channels, whereas the physiological role of its more conventional function as a Ca²⁺ channel has been a matter of uncertainty for nearly 50 years. In this chapter, we discuss how Ca(V)1.1 supports excitation-contraction coupling, the possible relevance of Ca²⁺ entry through Ca(V)1.1 and how alterations of Ca(V)1.1 function can have pathophysiological consequences. This article is part of a Special Issue entitled: Calcium channels.

Pathophysiological role of omega pore current in channelopathies

In voltage-gated cation channels, a recurrent pattern for mutations is the neutralization of positively charged residues in the voltage-sensing S4 transmembrane segments. These mutations cause dominant ion channelopathies affecting many tissues such as brain, heart, and skeletal muscle. Recent studies suggest that the pathogenesis of associated phenotypes is not limited to alterations in the gating of the ion-conducting alpha pore. Instead, aberrant so-called omega currents, facilitated by the movement of mutated S4 segments, also appear to contribute to symptoms. Surprisingly, these omega currents conduct cations with varying ion selectivity and are activated in either a hyperpolarized or depolarized voltage range. This review gives an overview of voltage sensor channelopathies in general and focuses on pathogenesis of skeletal muscle S4 disorders for which current knowledge is most advanced.

Voltage-sensor mutations in channelopathies of skeletal muscle

Mutations of voltage-gated ion channels cause several channelopathies of skeletal muscle, which present clinically with myotonia, periodic paralysis, or a combination of both. Expression studies have revealed both loss-of-function and gain-of-function defects for the currents passed by mutant channels. In many cases, these functional changes could be mechanistically linked to the defects of fibre excitability underlying myotonia or periodic paralysis. One remaining enigma was the basis for depolarization-induced weakness in hypokalaemic periodic paralysis (HypoPP) arising from mutations in either sodium or calcium channels. Curiously, 14 of 15 HypoPP mutations are at arginines in S4 voltage sensors, and recent observations show that these substitutions support an alternative pathway for ion conduction, the gating pore, that may be the source of the aberrant depolarization during an attack of paralysis.

Calcium channelopathies: voltage-gated calcium channels

Since the initial identification of native calcium currents, significant progress has been made towards our understanding of the molecular and cellular contributions of voltage-gated calcium channels in multiple physiological processes. Moreover, we are beginning to comprehend their pathophysiological roles through both naturally occurring channelopathies in humans and mice and through targeted gene deletions. The data illustrate that small perturbations in voltage-gated calcium channel function induced by genetic alterations can affect a wide variety of mammalian developmental, physiological and behavioral functions. At least in those instances wherein the channelopathies can be attributed to gain-of-function mechanisms, the data point towards new therapeutic strategies for developing highly selective calcium channel antagonists.

Gating of the HypoPP-1 mutations: I. Mutant-specific effects and cooperativity

Hypokalemic periodic paralysis type 1 (HypoPP-1) is a hereditary muscular disorder caused by point mutations in the gene encoding the voltage-gated Ca(2+) channel alpha subunit (Ca(v)1.1). Despite extensive research, the results on HypoPP-1 mutations are minor and controversial, as it is difficult to analyse Ca(2+) channel activation macroscopically due to an existence of two open states. In this study, we heterologously expressed the wild-type and HypoPP-1 mutations introduced into the rabbit cardiac Ca(2+) channel (R650H, R1362H, R1362G) in HEK-293 cells. To examine the cooperative effects of the mutations on channel gating, we expressed two double mutants (R650H/R1362H, R650H/R1362G). We performed whole-cell patch-clamp and, to obtain more information, applied a global fitting procedure whereby several current traces elicited by different potentials were simultaneously fit to the kinetic model containing four closed, two open and two inactivated states. We found that all HypoPP-1 mutations have "loss-of-function" features: D4/S4 mutations shift the equilibrium to the closed states, which results in reduced open probability, shorter openings and, therefore, in smaller currents, and the D2/S4 mutant slows the activation. In addition, HypoPP-1 histidine mutants favored the second open state O(2) with a possibly lower channel selectivity. Cooperativity between the D2/S4 and D4/S4 HypoPP-1 mutations manifested in dominant effects of the D4/S4 mutations on kinetics of the double mutants, suggesting different roles of D2/S4 and D4/S4 voltage sensors in the gating of voltage-gated calcium channels.

Paroxysmal muscle weakness: the familial periodic paralyses

The familial periodic paralyses (PP) were commonly considered to be benign diseases since frequency and severity of the paralytic attacks decrease in adulthood. However, with increasing age, a third of the patients develop permanent weakness and muscle degeneration with fatty replacement. Another complication, cardiac arrhythmia, can result from the dyskalemia during paralytic attacks. The familial PP are typical dominant ion channelopathies: the function of the mutant muscular channel is compensated in the interictal state but defective under certain conditions which then cause flaccid weakness. A triggering factor is the level of serum potassium, the extracellular ion decisive for membrane excitability. In hyper- and hypokalemic periodic paralysis, the mutations are specifically located in the voltage-gated sodium and calcium channels which are essential for action potential generation or excitation-contraction coupling. The common mechanism for the membrane inexcitability during paralytic attacks is a transient membrane depolarization that inactivates the sodium channels which are then no longer available for action potential generation. For the third PP type, the Andersen syndrome, the responsible gene is also expressed in cardiac muscle, and, independently of paralytic attacks, the hazard of ventricular arrhythmias is inherent. The gene product, an inwardly rectifying potassium channel, is responsible for maintaining the resting membrane potential, and all known mutations cause dominant-negative effects on the tetrameric channel complexes. In this article the clinical consequences of the mutations and the therapeutic strategies for all three types of PP are reported.

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.

PATHOMECHANISMS IN CHANNELOPATHIES OF SKELETAL MUSCLE AND BRAIN

Ion channelopathies are a diverse array of human disorders caused by mutations in ion channel genes. This review focuses on the pathogenic mechanisms of channelopathies affecting skeletal muscle and brain arising from mutations of voltage-gated ion channels and fast ligand-gated ion channels expressed at the surface membrane. Derangements in channel function alter the electrical excitability of the cell and thereby increase susceptibility to transient symptomatic attacks including myasthenia, periodic paralysis, myotonic stiffness, seizures, headache, dyskinesia, or episodic ataxia. Although these disorders are rare, they stand out as exemplary cases for which disease pathogenesis can be traced from a point mutation to altered protein function, to altered cellular activity, and to clinical phenotype. The study of these disorders has provided insights on channel structure-function relations, the physiological roles of ion channels, and rational approaches toward therapeutic intervention for many disorders of cellular excitability.

Muscle channelopathies and critical points in functional and genetic studies

Muscle channelopathies are caused by mutations in ion channel genes, by antibodies directed against ion channel proteins, or by changes of cell homeostasis leading to aberrant splicing of ion channel RNA or to disturbances of modification and localization of channel proteins. As ion channels constitute one of the only protein families that allow functional examination on the molecular level, expression studies of putative mutations have become standard in confirming that the mutations cause disease. Functional changes may not necessarily prove disease causality of a putative mutation but could be brought about by a polymorphism instead. These problems are addressed, and a more critical evaluation of the underlying genetic data is proposed.

Functional interaction of CaV channel isoforms with ryanodine receptors studied in dysgenic myotubes

The L-type Ca(2+) channels Ca(V)1.1 (alpha(1S)) and Ca(V)1.2 (alpha(1C)) share properties of targeting but differ by their mode of coupling to ryanodine receptors in muscle cells. The brain isoform Ca(V)2.1 (alpha(1A)) lacks ryanodine receptor targeting. We studied these three isoforms in myotubes of the alpha(1S)-deficient skeletal muscle cell line GLT under voltage-clamp conditions and estimated the flux of Ca(2+) (Ca(2+) input flux) resulting from Ca(2+) entry and release. Surprisingly, amplitude and kinetics of the input flux were similar for alpha(1C) and alpha(1A) despite a previously reported strong difference in responsiveness to extracellular stimulation. The kinetic flux characteristics of alpha(1C) and alpha(1A) resembled those in alpha(1S)-expressing cells but the contribution of Ca(2+) entry was much larger. alpha(1C) but not alpha(1A)-expressing cells revealed a distinct transient flux component sensitive to sarcoplasmic reticulum depletion by 30 microM cyclopiazonic acid and 10 mM caffeine. This component likely results from synchronized Ca(2+)-induced Ca(2+) release that is absent in alpha(1A)-expressing myotubes. In cells expressing an alpha(1A)-derivative (alpha(1)Aas(1592-clip)) containing the putative targeting sequence of alpha(1S), a similar transient component was noticeable. Yet, it was considerably smaller than in alpha(1C), indicating that the local Ca(2+) entry produced by the chimera is less effective in triggering Ca(2+) release despite similar global Ca(2+) inward current density.

Electrophysiology and molecular pharmacology of muscle channelopathies

As voltage-gated ion channels are essential for membrane excitation, it is not surprising that mutations in the respective channel genes cause diseases characterised by altered cell excitability. Skeletal muscle was the first tIssue in which such diseases, namely the myotonias and periodic paralyses, were recognised as ion channelopathies. The detection of the functional defect that is brought about by the disease-causing mutation is essential for the understanding of the pathology. Much progress on the road to this aim was achieved by the combination of molecular biology and electrophysiological patch clamp techniques. The functional expression of the mutations in expression systems allows to study the functional alterations of mutant channels and to develop new strategies for the therapy of ion channelopathies, e.g. by designing drugs that specifically suppress the effects of malfunctioning channels.

Cardiac-type EC-coupling in dysgenic myotubes restored with Ca2+ channel subunit isoforms alpha1C and alpha1D does not correlate with current density

Ca(2+)-induced Ca(2+)-release (CICR)-the mechanism of cardiac excitation-contraction (EC) coupling-also contributes to skeletal muscle contraction; however, its properties are still poorly understood. CICR in skeletal muscle can be induced independently of direct, calcium-independent activation of sarcoplasmic reticulum Ca(2+) release, by reconstituting dysgenic myotubes with the cardiac Ca(2+) channel alpha(1C) (Ca(V)1.2) subunit. Ca(2+) influx through alpha(1C) provides the trigger for opening the sarcoplasmic reticulum Ca(2+) release channels. Here we show that also the Ca(2+) channel alpha(1D) isoform (Ca(V)1.3) can restore cardiac-type EC-coupling. GFP-alpha(1D) expressed in dysgenic myotubes is correctly targeted into the triad junctions and generates action potential-induced Ca(2+) transients with the same efficiency as GFP-alpha(1C) despite threefold smaller Ca(2+) currents. In contrast, GFP-alpha(1A), which generates large currents but is not targeted into triads, rarely restores action potential-induced Ca(2+) transients. Thus, cardiac-type EC-coupling in skeletal myotubes depends primarily on the correct targeting of the voltage-gated Ca(2+) channels and less on their current size. Combined patch-clamp/fluo-4 Ca(2+) recordings revealed that the induction of Ca(2+) transients and their maximal amplitudes are independent of the different current densities of GFP-alpha(1C) and GFP-alpha(1D). These properties of cardiac-type EC-coupling in dysgenic myotubes are consistent with a CICR mechanism under the control of local Ca(2+) gradients in the triad junctions.

Voltage-activated calcium signals in myotubes loaded with high concentrations of EGTA

In the present study we describe the analysis of optically recorded whole cell Ca(2+) transients elicited by depolarization in cultured skeletal myotubes. Myotubes were obtained from the mouse muscle-derived cell line C2C12 and from mouse satellite cells. The cells were voltage-clamped and perfused with an artificial intracellular solution containing 15 mM EGTA to ensure that the bulk of the Ca(2+) mobilized by depolarization is bound to this extrinsic buffer. The apparent on- and off-rate constants of EGTA and the dissociation rate constant of fura-2 in the cell were estimated by investigating the Ca(2+)-dependence of kinetic components of the fluorescence decay after repolarization. These parameters were used to calculate the time course of the total voltage-controlled flux of Ca(2+) to the myoplasmic space (Ca(2+) input flux). The validity of the procedure was confirmed by model simulations using artificial Ca(2+) input fluxes. Both C2C12 and primary-cultured myotubes showed a very similar phasic-tonic time course of the Ca(2+) input flux. In most measurements, the input flux was considerably larger and showed a different time course than the estimated Ca(2+) flux carried by the L-type Ca(2+) channels, indicating that it consists mainly of voltage-controlled Ca(2+) release from the sarcoplasmic reticulum. In cells with extremely small fluorescence transients, the calculated input fluxes matched the kinetic characteristics of the Ca(2+) inward current, indicating that Ca(2+) release was absent. These measurements served as a control for the fidelity of the fluorimetric flux analysis. The procedures promise a deeper insight into alterations of Ca(2+) release gating in studies employing myotube expression systems for mutant or chimeric protein components of excitation-contraction coupling.

Genetic disorders of neuromuscular ion channels

Ion channels are complex proteins that span the lipid bilayer of the cell membrane, where they orchestrate the electrical signals necessary for normal function of the central nervous system, peripheral nerve, and both skeletal and cardiac muscle. The role of ion channel defects in the pathogenesis of numerous disorders, many of them neuromuscular, has become increasingly apparent over the last decade. Progress in molecular biology has allowed cloning and expression of genes that encode channel proteins, while comparable advances in biophysics, including patch-clamp electrophysiology and related techniques, have made the study of expressed proteins at the level of single channel molecules possible. Understanding the molecular basis of ion channel function and dysfunction will facilitate both the accurate classification of these disorders and the rational development of specific therapeutic interventions. This review encompasses clinical, genetic, and pathophysiological aspects of ion channels disorders, focusing mainly on those with neuromuscular manifestations.

An expanding view for the molecular basis of familial periodic paralysis

The periodic paralyses are rare disorders of skeletal muscle characterized by episodic attacks of weakness due to intermittent failure of electrical excitability. Familial forms of periodic paralysis are all caused by mutations in genes coding for voltage-gated ion channels. New discoveries in the past 2 years have broadened our views on the diversity of phenotypes produced by mutations of a single channel gene and have led to the identification of potassium channel mutations, in addition to those previously found in sodium and calcium channels. This review focuses on the clinical features, molecular genetic defects, and pathophysiologic mechanisms that underlie familial periodic paralysis.

Enhanced inactivation and pH sensitivity of Na(+) channel mutations causing hypokalaemic periodic paralysis type II

Hypokalaemic periodic paralysis (hypoPP) is a dominantly inherited muscle disorder characterized by episodes of flaccid weakness. Previous genetic studies revealed mutations in the voltage-gated calcium channel alpha1-subunit (CACNA1S gene) in families with hypoPP (type I). Electrophysiological studies on these mutants in different expression systems could not explain the pathophysiology of the disease. In addition, several mutations (Arg669His, Arg672His, Arg672Gly and Arg672Ser) in the voltage sensor of the skeletal muscle sodium channel alpha-subunit (SCN4A gene) have been found in families with hypoPP (type II). For Arg672Gly/His a fast inactivation defect was described, and for Arg669His an impairment of slow inactivation was reported. Except for the substitution for serine, we have now expressed all mutants in a human cell-line and studied them electrophysiologically. Patch-clamp recordings show an enhanced fast inactivation for all three mutations, whereas two of them reveal enhanced slow inactivation. This may reduce the number of functional sodium channels at resting membrane potential and contribute to the long-lasting periods of paralysis experienced by hypoPP patients. The gating of both histidine mutants (Arg669His, Arg672His) can be modulated by changes of extra- or intracellular pH. The inactivation defects of Arg669His and Arg672His can be alleviated by low pH to a significant degree, suggesting that the decrease of pH in muscle cells (e.g. during muscle work) might lead to an auto-compensation of functional defects. This may explain a delay or prevention of paralytic attacks in patients by slight physical activity. Moreover, the histidine residues may be the target for a potential therapeutic action by acetazolamide.

Mutations linked to familial hypokalaemic periodic paralysis in the calcium channel alpha1 subunit gene (Cav1.1) are not associated with thyrotoxic hypokalaemic periodic paralysis

OBJECTIVE

To investigate whether patients with thyrotoxic hypokalaemic periodic paralysis (THPP) have the same molecular defect in the calcium channel gene described in familial hypokalaemic periodic paralysis (FHPP), as the symptoms of both diseases are comparable, we analysed, in patients with THPP, the presence of mutations R528H, R1239H and R1239G on the S4 voltage-sensing transmembrane segment of the alpha1 subunit of the calcium channel gene (Cav1.1).

DESIGN AND PATIENTS

Genomic DNA was extracted from peripheral blood from 14 patients with THPP, 13 sporadic cases and one with a family history. An FHPP family was selected as a positive control. The exons bearing the described mutations were amplified by PCR, screened by single-strand conformation polymorphism (SSCP), and further sequenced.

MEASUREMENTS

THPP was diagnosed both clinically and through laboratory tests, all patients having elevated levels of thyroid hormones (T4, T3 or free T4), suppressed TSH and plasma potassium below 3 small middle dot5 mmol/l.

RESULTS

No evidence of the described mutations was found in patients with THPP. Furthermore, we did not detect any mutations in any of the four full S4 voltage-sensing transmembrane segments of Cav1 small middle dot1 (DIS4, DIIS4, DIIIS4 and DIVS4) by direct sequencing. However, close to the R528H mutation, we identified two single nucleotide polymorphisms at nucleotides 1551 and 1564 in both familial and sporadic cases with THPP. In addition, we were able to detect the R528H mutation in the DIIS4 transmembrane segment in all members of the FHPP family.

CONCLUSION

Mutations linked to familial hypokalaemic periodic paralysis in the calcium channel alpha1 subunit gene (Cav1.1) are not associated with thyrotoxic hypokalaemic periodic paralysis. However, polymorphisms in nucleotides 1551 and 1564 in the exon 11 were found in patients with familial hypokalaemic periodic paralysis and thyrotoxic hypokalaemic periodic paralysis in higher frequency than in controls. The polymorphisms identified within the Cav1.1 gene are associated with thyrotoxic hypokalaemic periodic paralysis and represent a novel finding.

Force assessment in periodic paralysis after electrical muscle stimulation

OBJECTIVE

To obtain an objective measure of muscle force in periodic paralysis, we studied ankle dorsiflexion torque during induced paralytic attacks in hyperkalemic and hypokalemic patients. SUBJECTS, PATIENTS, AND METHODS: Dorsiflexor torque after peroneal nerve stimulation was recorded during provocative tests on 5 patients with hypokalemic or hyperkalemic disorders and on 2 control subjects (1995-2001). Manual strength assessment was simultaneously performed in a blinded fashion. Standardized provocation procedures were used.

RESULTS

The loss of torque in hyperkalemic patients roughly paralleled the loss of clinically detectable strength, whereas in the hypokalemic patients, pronounced torque loss occurred well before observed clinical effects. No dramatic changes occurred in the control subjects. Torque amplitude decreased more than 70% in all patients during the provocation tests; such decreases were associated with alterations induced in serum potassium concentrations.

CONCLUSIONS

Stimulated torque measurement offers several advantages in characterizing muscle dysfunction in periodic paralysis: (1) it is independent of patient effort; (2) it can show a definitely abnormal response early during provocative maneuvers; and (3) characteristics of muscle contraction can be measured that are unobservable during voluntary contraction. Stimulated torque measurements can characterize phenotypic muscle function in neuromuscular diseases.

Periodic paralysis: understanding channelopathies

Familial periodic paralyses are typical channelopathies (i.e., caused by functional disturbances of ion channel proteins). The episodes of flaccid muscle weakness observed in these disorders are due to underexcitability of sarcolemma leading to a silent electromyogram and the lack of action potentials even upon electrical stimulation. Interictally, ion channel malfunction is well compensated, so that special exogenous or endogenous triggers are required to produce symptoms in the patients. An especially obvious trigger is the level of serum potassium (K+), the ion responsible for resting membrane potential and degree of excitability. The clinical symptoms can be caused by mutations in genes coding for ion channels that mediate different functions for maintaining the resting potential or propagating the action potential, the basis of excitability. The phenotype is determined by the type of functional defect brought about by the mutations, rather than the channel effected, because the contrary phenotypes hyperkalemic periodic paralysis (HyperPP) and hypokalemic periodic paralysis (HypoPP) may be caused by point mutations in the same gene. Still, the common mechanism for inexcitability in all known episodic-weakness phenotypes is a long-lasting depolarization that inactivates sodium ion (Na+) channels, initiating the action potential.

The skeletal muscle channelopathies: basic science, clinical genetics and treatment

The human neurological channelopathies are a rapidly expanding group of mainly genetic conditions that are characterized by dysfunction of membrane-bound glycoproteins (ion channels). The skeletal muscle channelopathies were the first to be characterized in this group. In recent years significant progress has been made in our understanding of the molecular genetic and cellular electrophysiological bases of these disorders. DNA-based diagnosis is now a reality for many of the channelopathies. The advances made have implications for both genetic counselling and for tailoring treatment to specific channelopathies.

A K(ATP) channel deficiency affects resting tension, not contractile force, during fatigue in skeletal muscle

The objective of this study was to determine how an ATP-sensitive K(+) (K(ATP)) channel deficiency affects the contractile and fatigue characteristics of extensor digitorum longus (EDL) and soleus muscle of 2- to 3-mo-old and 1-yr-old mice. K(ATP) channel-deficient mice were obtained by disrupting the Kir6.2 gene that encodes for the protein forming the pore of the channel. At 2-3 mo of age, the force-frequency curve, the twitch, and the tetanic force of EDL and soleus muscle of K(ATP) channel-deficient mice were not significantly different from those in wild-type mice. However, the tetanic force and maximum rate of force development decreased with aging to a greater extent in EDL and soleus muscle of K(ATP) channel-deficient mice (24-40%) than in muscle of wild-type mice (7-17%). During fatigue, the K(ATP) channel deficiency had no effect on the decrease in tetanic force in EDL and soleus muscle, whereas it caused a significantly greater increase in resting tension when compared with muscle of wild-type mice. The recovery of tetanic force after fatigue was not affected by the deficiency in 2- to 3-mo-old mice, whereas in 1-yr-old mice, force recovery was significantly less in muscle of K(ATP) channel-deficient than wild-type mice. It is suggested that the major function of the K(ATP) channel during fatigue is to reduce the development of a resting tension and not to contribute to the decrease in force. It is also suggested that the K(ATP) channel plays an important role in protecting muscle function in older mice.

Voltage-sensor sodium channel mutations cause hypokalemic periodic paralysis type 2 by enhanced inactivation and reduced current

The pathomechanism of familial hypokalemic periodic paralysis (HypoPP) is a mystery, despite knowledge of the underlying dominant point mutations in the dihydropyridine receptor (DHPR) voltage sensor. In five HypoPP families without DHPR gene defects, we identified two mutations, Arg-672-->His and -->Gly, in the voltage sensor of domain 2 of a different protein: the skeletal muscle sodium channel alpha subunit, known to be responsible for hereditary muscle diseases associated with myotonia. Excised skeletal muscle fibers from a patient heterozygous for Arg-672-->Gly displayed depolarization and weakness in low-potassium extracellular solution. Slowing and smaller size of action potentials were suggestive of excitability of the wild-type channel population only. Heterologous expression of the two sodium channel mutations revealed a 10-mV left shift of the steady-state fast inactivation curve enhancing inactivation and a sodium current density that was reduced even at potentials at which inactivation was removed. Decreased current and small action potentials suggested a low channel protein density. The alterations are decisive for the pathogenesis of episodic muscle weakness by reducing the number of excitable sodium channels particularly at sustained membrane depolarization. The results prove that SCN4A, the gene encoding the sodium channel alpha subunit of skeletal muscle is responsible for HypoPP-2 which does not differ clinically from DHPR-HypoPP. HypoPP-2 represents a disease caused by enhanced channel inactivation and current reduction showing no myotonia.

Calcium channelopathies

Calcium is an important intracellular signaling molecule, and altered calcium channel function can cause widespread cellular changes. Genetic mutations in calcium channels that cause what appear to be trivial alterations of calcium currents in vitro can result in serious diseases in muscles and the nervous system. This article reviews calcium channelopathies in humans and mice.

Channelopathies

In patients with mutations in the genes that encode the chloride, sodium, and calcium channels in skeletal muscle, there is abnormal function of the muscle membrane, which can cause myotonia or attacks of weakness. Mutations in the chloride and sodium channels can lead to myotonia, which typically begins in early childhood. Mexiletine is usually effective in controlling myotonia in these patients. Mexiletine is also effective in preventing attacks of cold-provoked muscle paralysis in patients with paramyotonia congenita, a sodium channel disorder. Certain mutations in the sodium channel cause attacks of hyperkalemic periodic paralysis; these attacks are often controlled with thiazide diuretics. Mutations in the skeletal muscle calcium channel cause periodic attacks of weakness, but hypokalemia (not hyperkalemia) occurs during these episodes. The carbonic anhydrase inhibitors acetazolamide and dichlorphenamide prevent attacks of hypokalemic periodic paralysis, although the mechanism by which they produce this protective effect remains a mystery. Interestingly, the hypokalemic attacks with periodic weakness that occur in some thyrotoxic patients are made worse by acetazolamide. This undesirable response to treatment emphasizes that not all disorders associated with hypokalemic periodic paralysis will benefit from carbonic anhydrase inhibitor therapy. DNA analysis to search for a mutation in the genes that encode for chloride, sodium, or calcium channels in skeletal muscle is helpful to establish the diagnosis. Some patients may eventually require provocative testing, however, to evaluate the attack of weakness and to reach a final diagnosis. Fortunately, there are effective treatments for the channelopathies that affect the skeletal muscle membrane.

Neurological channelopathies: diagnosis and therapy in the new millennium

Rapid progress in the complementary fields of molecular genetics and cellular electrophysiology has led to a better understanding of many disorders which are caused by ion channel dysfunction. These channelopathies may manifest in a multitude of ways depending on the tissue specificity of the channel that is affected. Several important general medical conditions are now known to be channelopathies but the neurological members of this family are amongst the best characterized. Over recent years, ion channel dysfunction in skeletal muscle in particular has emerged as a paradigm for understanding neurological ion channel disorders. This review concentrates mainly on the diseases caused by dysfunction of the voltage-gated ion channels. We initially focus on the skeletal muscle channelopathies (the periodic paralyses, malignant hyperthermia, paramyotonia congenita and myotonia congenita). The central nervous system channelopathies are then explored, with particular reference to the advances which have implications for understanding the mechanisms of common neurological disorders such as epilepsy and migraine. Looking towards the new millennium, DNA-based diagnosis will become a realistic proposition for most neurological channelopathies. Furthermore, it seems likely that new therapies will be designed based on genotype and mode of ion channel dysfunction.

Effects of mutations causing hypokalaemic periodic paralysis on the skeletal muscle L-type Ca2+ channel expressed in Xenopus laevis oocytes

1. A truncated form of the rabbit alpha1S Ca2+ channel subunit (alpha1SDeltaC) was expressed with the beta1b, alpha2delta and gamma auxiliary subunits in Xenopus laevis oocytes. After 5-7 days, skeletal muscle L-type currents were measured (469 +/- 48 nA in 10 mM Ba2+). All three of the auxiliary subunits were necessary to record significant L-type current. A rapidly inactivating, dihydropyridine-insensitive endogenous Ba2+ current was observed in oocytes expressing the auxiliary subunits without an exogenous alpha subunit. Expression of full-length alpha1S gave 10-fold smaller currents than the truncated form. 2. Three missense mutations causing hypokalaemic periodic paralysis (R528H in domain II S4 of the alpha1S subunit; R1239H and R1239G in domain IV S4) were introduced into alpha1SDeltaC and expressed in oocytes. L-type current was separated from the endogenous current by nimodipine subtraction. All three of the mutations reduced L-type current amplitude ( approximately 40 % for R528H, approximately 60-70 % for R1239H and R1239G). 3. The disease mutations altered the activation properties of L-type current. R528H shifted the G(V) curve approximately 5 mV to the left and modestly reduced the voltage dependence of the activation time constant, tauact. R1239H and R1239G shifted the G(V) curve approximately 5-10 mV to the right and dramatically slowed tauact at depolarized test potentials. 4. The voltage dependence of steady-state inactivation was not significantly altered by any of the disease mutations. 5. Wild-type and mutant L-type currents were also measured in the presence of (-)-Bay K8644, which boosted the amplitude approximately 5- to 7-fold. The effects of the mutations on the position of the G(V) curve and the voltage dependence of tauact were essentially the same as in the absence of agonist. Bay K-enhanced tail currents were slowed by R528H and accelerated by R1239H and R1239G. 6. We conclude that the domain IV mutations R1239H and R1239G have similar effects on the gating properties of the skeletal muscle L-type Ca2+ channel expressed in Xenopus oocytes, while the domain II mutation R528H has distinct effects. This result implies that the location of the substitutions is more important than their degree of conservation in determining their biophysical consequences.

Voltage-gated ion channels and hereditary disease

By the introduction of technological advancement in methods of structural analysis, electronics, and recombinant DNA techniques, research in physiology has become molecular. Additionally, focus of interest has been moving away from classical physiology to become increasingly centered on mechanisms of disease. A wonderful example for this development, as evident by this review, is the field of ion channel research which would not be nearly as advanced had it not been for human diseases to clarify. It is for this reason that structure-function relationships and ion channel electrophysiology cannot be separated from the genetic and clinical description of ion channelopathies. Unique among reviews of this topic is that all known human hereditary diseases of voltage-gated ion channels are described covering various fields of medicine such as neurology (nocturnal frontal lobe epilepsy, benign neonatal convulsions, episodic ataxia, hemiplegic migraine, deafness, stationary night blindness), nephrology (X-linked recessive nephrolithiasis, Bartter), myology (hypokalemic and hyperkalemic periodic paralysis, myotonia congenita, paramyotonia, malignant hyperthermia), cardiology (LQT syndrome), and interesting parallels in mechanisms of disease emphasized. Likewise, all types of voltage-gated ion channels for cations (sodium, calcium, and potassium channels) and anions (chloride channels) are described together with all knowledge about pharmacology, structure, expression, isoforms, and encoding genes.

Kinetics of inactivation and restoration from inactivation of the L-type calcium current in human myotubes

1. Inactivation and recovery kinetics of L-type calcium currents were measured in myotubes derived from satellite cells of human skeletal muscle using the whole cell patch clamp technique. 2. The time course of inactivation at potentials above the activation threshold was obtained from the decay of the current during 15 s depolarizing pulses. At subthreshold potentials, prepulses of different durations, followed by +20 mV test pulses, were used. The time course could be well described by single exponential functions of time. The time constant decreased from 17.8 +/- 7.5 s at -30 mV to 1.78 +/- 0.15 s at +50 mV. 3. Restoration from inactivation caused by 15 s depolarization to +20 mV was slowed by depolarization in the restoration interval. The time constant increased from 1.11 +/- 0.17 s at -90 mV to 7.57 +/- 2.54 s at -10 mV. 4. Restoration showed different kinetics depending on the duration of the conditioning depolarization. While the time constant was similar at restoration potentials of -90 and -50 mV after a 1 s conditioning prepulse, it increased with increasing prepulse duration at -50 mV and decreased at -90 mV. 5. The experiments showed that the rates of inactivation and restoration of the L-type calcium current in human myotubes were not identical when observed at the same potential. The results indicate the presence of more than one inactivated state and point to different voltage-dependent pathways for inactivation and restoration.

Impairment of skeletal muscle adenosine triphosphate-sensitive K+ channels in patients with hypokalemic periodic paralysis

The adenosine triphosphate (ATP)-sensitive K+ (KATP) channel is the most abundant K+ channel active in the skeletal muscle fibers of humans and animals. In the present work, we demonstrate the involvement of the muscular KATP channel in a skeletal muscle disorder known as hypokalemic periodic paralysis (HOPP), which is caused by mutations of the dihydropyridine receptor of the Ca2+ channel. Muscle biopsies excised from three patients with HOPP carrying the R528H mutation of the dihydropyridine receptor showed a reduced sarcolemma KATP current that was not stimulated by magnesium adenosine diphosphate (MgADP; 50-100 microM) and was partially restored by cromakalim. In contrast, large KATP currents stimulated by MgADP were recorded in the healthy subjects. At channel level, an abnormal KATP channel showing several subconductance states was detected in the patients with HOPP. None of these were surveyed in the healthy subjects. Transitions of the KATP channel between subconductance states were also observed after in vitro incubation of the rat muscle with low-K+ solution. The lack of the sarcolemma KATP current observed in these patients explains the symptoms of the disease, i.e., hypokalemia, depolarization of the fibers, and possibly the paralysis following insulin administration.

Gating of the L-type Ca channel in human skeletal myotubes: an activation defect caused by the hypokalemic periodic paralysis mutation R528H

The skeletal muscle L-type Ca channel serves a dual role as a calcium-conducting pore and as the voltage sensor coupling t-tubule depolarization to calcium release from the sarcoplasmic reticulum. Mutations in this channel cause hypokalemic periodic paralysis (HypoPP), a human autosomal dominant disorder characterized by episodic failure of muscle excitability that occurs in association with a decrease in serum potassium. The voltage-dependent gating of L-type Ca channels was characterized by recording whole-cell Ca currents in myotubes cultured from three normal individuals and from a patient carrying the HypoPP mutation R528H. We found two effects of the R528H mutation on the L-type Ca current in HypoPP myotubes: (1) a mild reduction in current density and (2) a significant slowing of the rate of activation. We also measured the voltage dependence of steady-state L-type Ca current inactivation and characterized, for the first time in a mammalian preparation, the kinetics of both entry into and recovery from inactivation over a wide range of voltages. The R528H mutation had no effect on the kinetics or voltage dependence of inactivation.