Verapamil mitigates chloride and calcium bi-channelopathy in a myotonic dystrophy mouse model

Myotonic dystrophy type 1 (DM1) involves misregulated alternative splicing for specific genes. We used exon or nucleotide deletion to mimic altered splicing of genes central to muscle excitation-contraction coupling in mice. Mice with forced skipping of exon 29 in the CaV1.1 calcium channel combined with loss of ClC-1 chloride channel function displayed markedly reduced lifespan, whereas other combinations of splicing mimics did not affect survival. The Ca2+/Cl- bi-channelopathy mice exhibited myotonia, weakness, and impairment of mobility and respiration. Chronic administration of the calcium channel blocker verapamil rescued survival and improved force generation, myotonia, and respiratory function. These results suggest that Ca2+/Cl- bi-channelopathy contributes to muscle impairment in DM1 and is potentially mitigated by common clinically available calcium channel blockers.

[Myotonia and cardiac conduction defects in myotonic dystrophy and defect in ion channels]

Myotonic dystrophy (DM), the most common hereditary muscle disease in adults, is caused by the unstable genomic expansion of simple sequence repeats. This disease is characterized by myotonia and various multisystemic complications, most commonly those of the cardiac, endocrine, and central nervous systems. The cardiac abnormalities, especially cardiac conduction defects, significantly contribute to morbidity and mortality in DM patients. Therefore, understanding the pathophysiology of cardiac conduction defects in DM is important. The pathomechanism of DM has been thoroughly investigated. The mutant RNA transcripts containing the expanded repeat give rise to a toxic gain-of-function by perturbing splicing factors in the nucleus, leading to the misregulation of alternative pre-mRNA splicing. In particular, several studies, including ours, have shown that myotonia is caused by alternative splicing of the CLCN1 gene coding the voltage-gated chloride channel in skeletal muscle through an "RNA-dominant mechanism". Since the aberrantly spliced isoform does not seem to form a functional channel, the feature of skeletal muscle in DM can be interpreted as a "channelopathy" caused by reduced chloride channel protein. Similarly, we recently identified a misregulation of alternative splicing in an ion channel gene which is known to be responsible for arrhythmic disease showing Mendelian inheritance. Here, we review the cardiac manifestation and RNA-dominant mechanism of DM, and discuss the possible pathophysiology of cardiac conduction defects by referring to hereditary arrhythmic diseases, such as long QT syndrome and Brugada syndrome.

Skeletal-muscle channelopathies: periodic paralysis and nondystrophic myotonias

PURPOSE OF REVIEW

To provide a current review of clinical phenotypes, genetics, molecular pathophysiology, and electro-diagnostic testing strategies of periodic paralysis and nondystrophic myotonias.

RECENT FINDINGS

The number of pathogenic mutations causing periodic paralysis and nondystrophic myotonias continues to increase. Important insight into the molecular pathogenesis of muscle sodium channelopathies has been revealed by the finding of 'leaky' closed sodium channels. Previously, alterations in sodium-channel activation or inactivation have been identified as important disease mechanisms. The recent discovery that substitutions of key arginine residues in the voltage-sensing segment of the channel may lead to a 'pore leak' when the channel is closed suggests a new mechanism. Since similar mutations exist in corresponding positions of other channels, this mechanism may apply to other channel diseases. The recognition of different electrophysiological patterns that are specific to muscle ion-channel genotypes will be useful in diagnosis and in guiding genetic testing. Recent studies demonstrate that magnetic resonance imaging may be used to detect intramuscular accumulation of sodium during episodes of weakness.

SUMMARY

Recent advances have refined our ability to make a precise molecular diagnosis in muscle channelopathies. The description of a pore leak with voltage-sensor mutations may represent a new disease mechanism.

The effect of dehydroepiandrosterone sulfate (DHEAS) on myotonia: intracellular studies

OBJECTIVE

In order to find some appropriate medicine to suppress myotonia without decreasing muscle strength experiments were performed on myotonic (mto) mice whose Cl channel does not develop due to stop codon and serves as an animal model of myotonia. In myotonic dystrophy dehydroepiandrosterone is low in the serum and it has been reported that intravenous injections of DHEAS to human cases improves myotonia and activities of daily living.

MATERIALS AND METHODS

Three pairs of heterozygote mto mice, SWR/J-Clcn1(adr-mto/+) and ten Wistar rats were used. We performed intracellular recordings of myotonia from mto mice and the drug effects on insertion myotonia were recorded from the hemidiaphragm preparations of mto mice with different concentrations of DHEAS. Isometric twitch tension was recorded from rat hemidiaphragm preparations in Tyrode's solution and the effect of DHEAS on the muscle twitch tension was measured at different concentrations of DHEAS from 100 mg/l to 300 mg/l. The effect of mexiletine on ITT was also measured.

RESULTS

In mto mice insertion myotonia was recorded as soon as the microelectrode was inserted in the muscle cells. When DHEAS was added to Tyrode's solution, insertion myotonia was suppressed. DHEAS decreased ITT up to 70% of the original value, though mexiletine decreased ITT to 30% of the original value. Therefore, the decrement of the muscle strength in DHEAS solution is much smaller than that of mexiletine.

CONCLUSION

Since myotonic dystrophy shows progressive muscle weakness in addition to myotonia, medications like DHEAS are more favorable than the typical Na channel blocker.

Myotonia and muscle contractile properties in mice with SIX5 deficiency

Myotonic dystrophy (DM1) is an autosomal-dominant multisystem disease characterized by progressive skeletal muscle weakness, myotonia, cataracts, cardiac arrhythmias, mild mental retardation, and endocrinopathies. Heterozygous loss of SIX5 in mice causes cataracts and cardiac conduction disease, and homozygous loss also leads to sterility and decreased testicular mass, reminiscent of DM1 in humans. The effect of SIX5 deficiency in muscle is unknown. In this study, we found that muscle contractile properties, electromyographic insertional activity, and muscle histology were normal in SIX5 deficient mice. The implications of these findings for the pathogenesis of DM1 are discussed.

Different flecainide sensitivity of hNav1.4 channels and myotonic mutants explained by state-dependent block

Flecainide, a class IC antiarrhythmic, was shown to improve myotonia caused by sodium channel mutations in situations where the class IB antiarrhythmic drug mexiletine was less efficient. Yet little is known about molecular interactions between flecainide and human skeletal muscle sodium (hNa(v)1.4) channels. Whole-cell sodium currents (I(Na)) were recorded in tsA201 cells expressing wild-type (WT) and mutant hNa(v)1.4 channels (R1448C, paramyotonia congenita; G1306E, potassium-aggravated myotonia). At a holding potential (HP) of -120 mV, flecainide use-dependently blocked WT and G1306E I(Na) equally but was more potent on R1448C channels. For WT, the extent of block depended on a holding voltage more negative than the activation threshold, being greater at -90 mV as compared to -120 and -180 mV. This behaviour was exacerbated by the R1448C mutation since block at -120 mV was greater than that at -180 mV. Thus flecainide can bind to inactivated sodium channels in the absence of channel opening. Nevertheless, all the channels showed the same closed-state affinity constant (K(R) approximately 480 microM) and the same inactivated-state affinity constant (K(I) approximately 18 microM). Simulations according to the modulated receptor hypothesis mimic the voltage-dependent block of WT and mutant channels by flecainide and mexiletine. All the results suggest similar blocking mechanisms for the two drugs. Yet, since flecainide exerts use-dependent block at lower frequency than mexiletine, it may exhibit greater benefit in all myotonic syndromes. Moreover, flecainide blocks hNa(v)1.4 channel mutants with a rightward shift of availability voltage dependence more specifically than mexiletine, owing to a lower K(R)/K(I) ratio. This study offers a pharmacogenetic strategy to better address treatment in individual myotonic patients.

Skeletal muscle channelopathies

Ion channelopathies have common clinical features, recurrent patterns of mutations, and almost predictable mechanisms of pathogenesis. In skeletal muscle, disorders are associated with mutations in voltage-gated Na(+), K(+), Ca(2+), and Cl(-) channels leading to hypoexcitability, causing periodic paralysis and to hyperexcitabilty, resulting in myotonia or susceptibility to malignant hyperthermia.

Slow inactivation in voltage-gated sodium channels: molecular substrates and contributions to channelopathies

Slow inactivation in voltage-gated sodium channels is a biophysical process that governs the availability of sodium channels over extended periods of time. Slow inactivation, therefore, plays an important role in controlling membrane excitability, firing properties, and spike frequency adaptation. Defective slow inactivation is associated with several diseases of cell excitability, such as hyperkalemic periodic paralysis, myotonia, idiopathic ventricular fibrillation and long-QT syndrome. These associations underscore the physiological importance of this phenomenon. Nevertheless, our understanding of the molecular substrates for slow inactivation is still fragmentary. This review covers the current state of knowledge concerning the molecular underpinnings of slow inactivation, and its relationship with other biophysical processes of voltage-gated sodium channels.

Two mutations in the IV/S4-S5 segment of the human skeletal muscle Na+ channel disrupt fast and enhance slow inactivation

Fast and slow inactivation (FI, SI) of the voltage-gated Na+ channel are two kinetically distinct and structurally dissociated processes. The voltage sensor IV/S4 and the intracellular IV/S4-S5 loop have been shown to play an important role in FI mediating the coupling between activation and inactivation. Two mutations in IV/S4-S5 of the human muscle Na+ channel, L1482C/A, disrupt FI by inducing a persistent Na+ current, shifting steady-state inactivation in the depolarizing direction and accelerating its recovery. These effects were more pronounced for L1482A. In contrast, SI of L1482C/A channels was enhanced showing a more complete SI and a 3-fold slowing of its recovery. Effects on SI were more pronounced for L1482C. The results indicate an important role of the IV/S4-S5 loop not only in FI but also in SI of the Na+ channel.

Spectrum of sodium channel disturbances in the nondystrophic myotonias and periodic paralyses

Several heritable forms of myotonia and periodic paralysis are caused by missense mutations in the voltage-gated sodium channel of skeletal muscle. Mutations produce gain-of-function defects, either disrupted inactivation or enhanced activation. Both defects result in too much inward Na current which may either initiate pathologic bursts of action potentials (myotonia) or cause flaccid paralysis by depolarizing fibers to a refractory inexcitable state. Myotonic stiffness and periodic paralysis occur as paroxysmal attacks often triggered by environmental factors such as serum K+, cold, or exercise. Many gaps remain in our understanding of the interactions between genetic predisposition and these environmental influences. Targeted gene manipulation in animals may provide the tools to fill in these gaps.

Maturation and myotonia influence the abundance of cation channels KDR, KIR and CIR differently: a patch-clamp study on mouse interosseus muscle fibres

To detect cation channels, the expression of which is dependent on the physiological state of muscle, single-channel activities of dissociated fibres of the mouse interosseus muscle were recorded using the patch-clamp technique in the cell-attached mode. Fibres were prepared from juvenile and adult wild-type (WT), from chloride channel-deficient myotonic and from denervated adult WT muscles. In all cases delayed-rectifier K+ channels (KDR) with a unitary conductance of 11 pS were recorded in more than 95% of sarcolemmal patches, but with a low, steady-state open probability. Inwards-rectifying K+ channels (KIR) with a conductance of 31 pS in 140 mM [K+]o were active in about 50% of the membrane patches from WT and in more than 90% of those from myotonic fibres. A hitherto undescribed, inwards-rectifying, cation channel, provisionally termed CIR, with fast kinetics and a unitary conductance of 36 pS, was active in nearly every membrane patch from juvenile mice, both WT and myotonic. The abundance of CIR decreased during development, but was not changed 7 days after denervation of adult WT muscle. Ca(2+)-dependent K+ channels were seen sporadically. Channels with the characteristics of adenosine 5'-triphosphate (ATP)-sensitive K+ channels were recorded frequently upon excision of membrane patches, but remained inactive in most cell-attached recordings. In conclusion, of the investigated ion channels, only KIR was responsive to the activity pattern of adult muscle, whereas CIR was down-regulated during muscle maturation.

[Ion-channel related muscular diseases]

INTRODUCTION

Though ion channel-related muscular disorders were described long ago, better understanding of their underlying mechanisms has been more recently achieved. These mechanisms include myotonic syndromes that may be caused by mutations in sodium and chloride channels, as well as periodic paralysis which is due to mutations in sodium and calcium channels.

CURRENT KNOWLEDGE AND KEY POINTS

Knowledge of the involved pathophysiological mechanisms has led to better clinical description of these disorders, as well as more efficacious treatment. In some cases, it is now possible to establish the diagnosis, using genetic tests.

FUTURE PROSPECTS AND PROJECTS

Other neuromuscular disorders might be related to ion channel mutations.

The muscle chloride channel ClC-1 has a double-barreled appearance that is differentially affected in dominant and recessive myotonia

Single-channel recordings of the currents mediated by the muscle Cl- channel, ClC-1, expressed in Xenopus oocytes, provide the first direct evidence that this channel has two equidistant open conductance levels like the Torpedo ClC-0 prototype. As for the case of ClC-0, the probabilities and dwell times of the closed and conducting states are consistent with the presence of two independently gated pathways with approximately 1.2 pS conductance enabled in parallel via a common gate. However, the voltage dependence of the common gate is different and the kinetics are much faster than for ClC-0. Estimates of single-channel parameters from the analysis of macroscopic current fluctuations agree with those from single-channel recordings. Fluctuation analysis was used to characterize changes in the apparent double-gate behavior of the ClC-1 mutations I290M and I556N causing, respectively, a dominant and a recessive form of myotonia. We find that both mutations reduce about equally the open probability of single protopores and that mutation I290M yields a stronger reduction of the common gate open probability than mutation I556N. Our results suggest that the mammalian ClC-homologues have the same structure and mechanism proposed for the Torpedo channel ClC-0. Differential effects on the two gates that appear to modulate the activation of ClC-1 channels may be important determinants for the different patterns of inheritance of dominant and recessive ClC-1 mutations.

Myopathy induced by HMG-CoA reductase inhibitors in rabbits: a pathological, electrophysiological, and biochemical study

A combination of electrophysiological, pathological, and biochemical studies were performed in myopathy induced by 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitors. Simvastatin (a lipophilic inhibitor) or pravastatin (a hydrophilic inhibitor) were administered by gavage to rabbits. In Group I (simvastatin-treated group, 50 mg/kg/day for 4 weeks), four rabbits showed muscle necrosis and high serum creatine kinase (CK) levels, and all six rabbits showed electrical myotonia. In Group II (pravastatin-treated group, 100 mg/kg/day for 4 weeks), no rabbit showed either condition. In Group III (pravastatin-treated group, 200 mg/kg/day for 3 weeks plus 300 mg/kg/day for 3 weeks), one rabbit showed muscle necrosis and high serum CK level and two rabbits showed electrical myotonia. The pathological findings were muscle fiber necrosis and degeneration with increased acid phosphatase activity by light microscopy, autophagic vacuoles and mitochondrial swelling, and disruption and hypercontraction of myofibrils by electron microscopy. Ubiquinone content decreased in skeletal muscle by 22 to 36% in Group I, by 18 to 52% in Group II, and by 49 to 72% in Group III. However, mitochondrial enzyme activities of respiratory chain were normal in all groups. These results indicate that myopathy was not induced by a secondary dysfunction of mitochondrial respiration due to low ubiquinone levels.

Human sodium channel gating defects caused by missense mutations in S6 segments associated with myotonia: S804F and V1293I

1. Missense mutations in the alpha-subunit of the human skeletal muscle sodium channel (hSkM1) have been detected in some heritable forms of myotonia. By recording Na+ currents from cells transfected with cDNA encoding either wild-type or mutant hSkM1, we characterized the functional consequences of two myotonia-associated mutations that lie at the cytoplasmic end of the sixth transmembrane segment in domain II (S804F) or domain III (V1293I). 2. Both mutations caused modest, but unequivocal, alterations in the voltage-dependent gating behaviour of hSkM1. For S804F, the abnormalities were limited to fast inactivation: the persistent Na+ current at the end of a 50 ms depolarization was increased 3-fold, the rate of inactivation from the open state was slowed 2-fold, and the voltage dependence of fast inactivation (h) was shifted by +3 mV. V1293I also disrupted fast inactivation, as evidenced by a 3-fold faster rate of recovery at hyperpolarized potentials (-70 mV). Activation was altered as well for V1293I: the voltage dependence was shifted by -6 mV (hyperpolarized). 3. Slow inactivation was not altered by S804F or V1293I. 4. We conclude that S804F and V1293I are not benign polymorphisms. Either mutation causes detectable alterations in channel gating and, in model simulations, the magnitude of the defects is sufficient to produce runs of myotonic discharges.

Expression of the potassium channel KV3.4 in mouse skeletal muscle parallels fiber type maturation and depends on excitation pattern

We report the detailed expression pattern of the voltage-dependent potassium channel KV3.4 (rat homologue, Raw3) in mouse skeletal muscle. Using semi-quantitative RT-PCR, we show that its expression is detectable at embryonic day 17 and rises to adult levels within 2 weeks after birth. Expression is fiber type-dependent, with mRNA levels being 5-6-fold lower in the mixed slow/fast soleus muscle than in the fast tibialis anterior and extensor digitorum longus muscles. Fast muscles from myotonic mice exhibit low KV3.4 mRNA levels similar to those of wild-type soleus. In denervated extensor digitorum longus, KV3.4 expression declines to perinatal levels. We conclude that KV3.4 expression in mouse skeletal muscle is regulated by the pattern of excitation.

Defective fast inactivation recovery and deactivation account for sodium channel myotonia in the I1160V mutant

The skeletal muscle sodium channel mutant I1160V cosegregates with a disease phenotype producing myotonic discharges (observed as muscle stiffness) that are worsened by elevated K+ levels but unaffected by cooling. The I1160V alpha-subunit was co-expressed with the beta1-subunit in Xenopus oocytes. An electrophysiological characterization was undertaken to examine the underlying biophysical characteristics imposed by this mutation. Two abnormalities were found. 1) The voltage dependence of steady-state fast inactivation was reduced in I1160V, which resulted in faster rates of closed-state fast inactivation onset and recovery in I1160V compared with wild-type channels. 2) The rates of deactivation were slower in I1160V than in wild-type channels. Using a computer-simulated model, the combination of both defects elicited myotonic runs under conditions of elevated K+, consistent with the observed phenotype of the mutant.

Chloride conductance in mouse muscle is subject to post-transcriptional compensation of the functional Cl- channel 1 gene dosage

1. In mature mammalian muscle, the muscular chloride channel ClC-1 contributes about 75% of the sarcolemmal resting conductance (Gm). In mice carrying two defective alleles of the corresponding Clc1 gene, chloride conductance (GCl) is reduced to less than 10% of that of wild-type, and this causes hyperexcitability, the salient feature of the disease myotonia. Potassium conductance (GK) values in myotonic mouse muscle fibres are lowered by about 60% compared with wild-type. 2. The defective Clcadr allele causes loss of the 4.5 kb ClC-1 mRNA. Mice heterozygous for the defective Clc1adr allele contain about 50% functional mRNA in their muscles compared with homozygous wild-type mice. 3. Despite a halved functional gene dosage, heterozygous muscles display an average GCl which is not significantly different from that of homozygous wild-type animals. The GK values in heterozygotes are also indistinguishable from homozygous wild-type animals. 4. These results indicate that a regulatory mechanism acting at the post-transcriptional level limits the density of ClC-1 channels. GK is probably indirectly regulated by muscle activity.

Myotonia and the muscle chloride channel: dominant mutations show variable penetrance and founder effect

The delayed relaxation or sustained contraction of skeletal muscle-myotonia-is frequently seen in myotonic dystrophy and sodium channelopathies (hyperkalemic periodic paralysis, paramyotonia congenita). Many cases of congenital myotonia without other clinical symptoms have been associated with mutations in the muscle chloride channel gene. Most cases reported to date show a recessive inheritance pattern, with loss of function of the corresponding protein. Six families have been reported with dominantly inherited myotonia and mutations of the chloride channel gene. Here we report clinical and molecular data on 38 family members from four new families with dominantly inherited myotonia congenita. Three families show a previously characterized G230E mutation, and we show that these three share a common affected ancestor despite living in different regions of the United States (linkage disequilibrium). One Italian family is shown to have a novel dominant mutation-I290M. This is the sixth mutation identified in Thomsen's myotonia. Genotype/phenotype correlations in these four families showed that both of the dominant mutations resulted in a mild clinical picture in 90% of the patients, and no symptoms in 10% of mutation-positive patients. The EMG was the clinical feature that most closely correlated with mutation data; however, 3 of 16 (19%) mutation-positive patients tested negative by electromyography at least once, and 1 (6%) tested negative despite multiple tests. Only about half (55%) of the mutation-positive patients tested positive for percussion myotonia. Most of the clinically symptomatic individuals stated that cold temperatures and stress substantially worsened their myotonia. Our data show that dominantly inherited Thomsen's myotonia is most often a very mild disorder that shows considerable clinical heterogeneity.

Inactivation defects caused by myotonia-associated mutations in the sodium channel III-IV linker

Missense mutations in the skeletal muscle Na+ channel alpha subunit occur in several heritable forms of myotonia and periodic paralysis. Distinct phenotypes arise from mutations at two sites within the III-IV cytoplasmic loop: myotonia without weakness due to substitutions at glycine 1306, and myotonia plus weakness caused by a mutation at threonine 1313. Heterologous expression in HEK cells showed that substitutions at either site disrupted inactivation, as reflected by slower inactivation rates, shifts in steady-state inactivation, and larger persistent Na+ currents. For T1313M, however, the changes were an order of magnitude larger than any of three substitutions at G1306, and recovery from inactivation was hastened as well. Model simulations demonstrate that these functional difference have distinct phenotypic consequences. In particular, a large persistent Na+ current predisposes to paralysis due to depolarization-induced block of action potential generation.

Different effects on gating of three myotonia-causing mutations in the inactivation gate of the human muscle sodium channel

1. Three mutations at the same site in the inactivation gate of the alpha-subunit of the human muscle Na+ channel, G1306E, G1306V and G1306A, cause three phenotypes of K(+)-aggravated myotonia: G1306E as the most severe and G1306A as the most benign form. 2. Recombinant wildtype (WT) and mutant (G1306E, G1306V and G1306A) human Na+ channels were expressed in human embryonic kidney cells (HEK293). G1306E and G1306V channels showed a distinct increase in the time constants of inactivation (tau h1 and tau h2) and in the ratios of steady-state to peak currents (Iss/Ipeak) (e.g. at 0 mV, G1306E vs. WT; tau h1, 1.29 +/- 0.10 vs. 0.52 +/- 0.01 ms; Iss/Ipeak, 2.90 +/- 0.40 vs. 0.93 +/- 0.19%). G1306A channels showed only an increase in tau h1 (0.74 +/- 0.07 ms). For G1306E and G1306V channels, the steady-state inactivation curves, as well as the voltage dependence of the rate of recovery from inactivation, were shifted by +15 mV. For G1306A the h infinity curve was shifted by only +5 mV. 3. G1306E and G1306V channels showed prolonged current rise times and later first openings suggesting slowing of activation. For G1306E channels only, the steady-state activation curve was shifted by -7 mV. For all mutants the deactivation time constants were increased. 4. We conclude that (i) the combination of alterations in inactivation and activation produces the slowing of the current decay, (ii) the slowed inactivation is most responsible for myotonia, and (iii) the shift of the steady-state activation curve, seen only with G1306E channels, may explain the severity of this phenotype. 5. The results suggest that two of the mutations in the Na+ channel inactivation gate also alter channel activation and deactivation.

Myotonia levior is a chloride channel disorder

The group of dominant non-dystrophic myotonias, comprising disorders characterized by clinically similar forms of myogenic muscle stiffness, is genetically inhomogeneous. Dominant myotonia congenita (Thomsen's disease) is linked to CLCN1, the gene encoding the major muscle chloride channel, localized on chromosome 7q35. In contrast, dominant myotonias sensitive to potassium are caused by point mutations in SCN4A on chromosome 17q, the gene for the alpha subunit of the adult skeletal muscle sodium channel. No linkage or molecular genetic data are as yet available on 'myotonia levior' characterized by milder symptoms and later onset of myotonia than in Thomsen's disease, and absence of muscle hypertrophy. We report a CLCN1 Gln-552-Arg substitution for a family with dominant inheritance previously diagnosed to have myotonia levior. Thus, this disorder appears as a variant of Thomsen's disease due to mutations leading to low clinical expressivity. In addition, we report a novel Ile-290-Met CLCN1 mutation for a typical Thomsen pedigree. In another family previously diagnosed as having Thomsen's disease, we unexpectedly found a CLCN1 14 bp deletion known to cause recessive myotonia, and a rare Trp-118-Gly polymorphism.

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.

The skeletal muscle sodium and chloride channel diseases

The cause of several familial muscular diseases have recently been linked to mutations within skeletal muscle sodium and chloride channel genes. Thomsen's and Becker's diseases are autosomal dominant and recessive, respectively, and are caused by at least seven different mutations in the CLCN1 (ClC-1) skeletal muscle chloride channel gene on chromosome 7q35. Hyperkalaemic periodic paralysis, paramyotonia congenita and a small heterogeneous group of related 'pure' myotonias are autosomal dominant disorders and are due to at least 16 different mutations in the SCN4A (SkM1) adult skeletal muscle sodium channel gene on chromosome 17q23-25. There is generally little correlation between the position of a mutation in the channel and the phenotype. Indeed, identical sodium channel mutations in unrelated subjects and sometimes in different members of the same family can have different clinical expressions. It seems, however, that mutations of the inactivation gate (ID3-4 loop) of the sodium channel tend to produce paramyotonia or pure, sometimes severe, myotonia and respond most favourably to the same medications (tocainide and mexiletine). The structure and polarity of substituted amino acids at a mutation site, especially in highly evolutionally conserved regions of the gene, are undoubtedly important to the expression of a channel disease and may partly explain phenotypic variability. In addition, genetic polymorphisms elsewhere, either in the gene or other channel-related loci, and the net effect of other types of muscle ion channels on the electrical potential of the plasma membrane probably contribute to disease expression.

K(+)-aggravated myotonia: destabilization of the inactivated state of the human muscle Na+ channel by the V1589M mutation

1. Wild type (WT) and V1589M channels were expressed in human embryonic kidney (HEK293) cells for the study of the pathophysiology of the V1589M muscle Na+ channel mutation leading to K(+)-aggravated myotonia. 2. In comparison to WT, whole-cell recordings with V1589M channels showed an increased Na+ steady-state to peak current ratio (Iss/Ipeak) (3.15 +/- 0.70 vs. 0.87 +/- 0.10%, at -15 mV) and a significantly faster recovery from inactivation. The recovery time constants, tau r1 and tau r2, were decreased from 1.28 +/- 0.12 to 0.92 +/- 0.08 ms and from 4.74 +/- 0.94 to 2.66 +/- 0.51 ms for the WT and mutant channels, respectively. 3. Single-channel recordings with mutant channels showed higher probability of short isolated late openings (0.40 +/- 0.09 vs. 0.06 +/- 0.02, at -30 mV) and bursts of late openings (0.011 +/- 0.003 vs. 0.003 +/- 0.001, at -30 mV) compared to WT. 4. These results suggest that the mutation increases the probabilities for channel transitions from the inactivated to the closed and the opened states. 5. Increased extracellular concentrations of K+ had no effects on either V1589M or WT currents in HEK293 cells. The aggravation of myotonia seen in patients during increased serum K+ may arise from the associated membrane depolarization which favours the occurrence of late openings in the mutant channel.

Ca2+ homeostasis in Brody's disease. A study in skeletal muscle and cultured muscle cells and the effects of dantrolene an verapamil

Brody's disease, i.e., sarcoplasmic reticulum (SR) Ca(2+)-dependent Mg(2+)-ATPase (Ca(2+)-ATPase) deficiency, is a rare inherited disorder of skeletal muscle function. Pseudo-myotonia is the most important clinical feature. SR Ca(2+)-ATPase and Ca2+ homeostasis are examined in m. quadriceps and/or cultured muscle cells of controls and 10 patients suffering from Brody's disease. In both m. quadriceps and cultured muscle cells of patients, the SR Ca(2+)-ATPase activity is decreased by approximately 50%. However, the concentration of SR Ca(2+)-ATPase and SERCA1 are normal. SERCA1 accounts for 83 and 100% of total SR Ca(2+)-ATPase in m. quadriceps and cultured muscle cells, respectively. This implies a reduction of the molecular activity of SERCA1 in Brody's disease. The cytosolic Ca2+ concentration ([Ca2+]i) at rest and the increase of [Ca2+]i after addition of acetylcholine are the same in cultured muscle cells of controls and patients. The half-life of the maximal response, however, is raised three times in the pathological muscle cells. Addition of dantrolene or verapamil after the maximal response accelerates the restoration of the [Ca2+]i in these muscle cells. The differences in Ca2+ handling disappear by administration of dantrolene or verapamil concomitantly with acetylcholine. The reduced Ca2+ re-uptake from the cytosol presumably due to structural modification(s) of SERCA1 may explain the pseudo-myotonia in Brody's disease. Single cell measurements suggest a beneficial effect of dantrolene or verapamil in treating patients suffering from Brody's disease.

Subtractive cDNA cloning as a tool to analyse secondary effects of a muscle disease. Characterization of affected genes in the myotonic ADR mouse

In myotonic ADR mice that are homozygous for a defect in the muscular chloride channel gene adr/Clc-1, the hyperexcitability of fast muscles is associated with secondary changes in gene expression and fibre type composition. cDNA clones derived from a set of genes down regulated in fast muscles of the myotonic ADR mouse were isolated by a subtractive cloning procedure. A total of 1200 clones were analysed for high expression in fast muscle of wild type and low expression in mutant mouse. Differential transcript levels were verified by northern blot hybridizations. The identities of the corresponding transcripts were determined by sequencing as myosin heavy chain IIB, alpha-tropomyosin, troponin C, a Ca2+ ATPase and parvalbumin mRNAs. Of these, mRNAs for parvalbumin and myosin heavy chain IIB were drastically downregulated in myotonic muscle (to < 10% of control). A full length cDNA clone for skeletal muscle alpha-tropomyosin was homologous to the mouse fibroblast tropomyosin isoform 2, except for the portion encoding the alpha-tropomyosin specific amino acids 258-284. A cDNA derived from the 1100 nucleotide parvalbumin transcript was cloned and the sequence for the as yet unknown 3' extended trailer, generated by alternative polyadenylation, was determined.

Genotype-phenotype correlations in human skeletal muscle sodium channel diseases

BACKGROUND

Over the past 3 years, the genetics of the myotonic diseases have been substantially elaborated. Three genetically different groups of myotonic disease can be discerned: (1) the chloride channel myotonias, (2) the adynamia-paramyotonia complex, and (3) myotonic dystrophy.

METHODS AND RESULTS

Electrophysiology has suggested and molecular biology has proven that the diseases belonging to the adynamia-paramyotonia complex, ie, paramyotonia congenita, hyperkalemic and normokalemic periodic paralysis, and some rare forms of myotonic disease, are caused by point mutations in the gene encoding the alpha subunit of the sodium channel in adult human skeletal muscle, located on chromosome 17q23. Thirteen different mutations have been described by various groups in the United States and Germany. The various mutations causing a particular form of the complex are not located in the gene in a predictable or easily understandable regular manner.

CONCLUSIONS

Further study of the genotype-phenotype correlations should not only increase our understanding of the variability of signs in this group of diseases, it could also provide us with a deeper insight in the function of the various regions of the sodium channel protein.

Human sodium channel myotonia: slowed channel inactivation due to substitutions for a glycine within the III-IV linker

1. Three families with a form of myotonia (muscle stiffness due to membrane hyperexcitability) clinically distinct from previously classified myotonias were examined. The severity of the disease greatly differed among the families. 2. Three dominant point mutations were discovered at the same nucleotide position of the SCN4A gene encoding the adult skeletal muscle Na+ channel alpha-subunit. They predict the substitution of either glutamic acid, valine or alanine for glycine1306, a highly conserved residue within the supposed inactivation gate. Additional SCN4A mutations were excluded. 3. Electrophysiological studies were performed on biopsied muscle specimens obtained for each mutation. Patch clamp recordings on sarcolemmal blebs revealed an increase in the time constant of fast Na+ channel inactivation, tau h, and in late channel openings as compared to normal controls. tau h was increased from 1.2 to 1.6-2.1 ms and the average late currents from 0.4 to 1-6% of the peak early current. 4. Intracellular recordings on resealed fibre segments revealed an abnormal tetrodotoxin-sensitive steady-state inward current, and repetitive action potentials. Since K+ and Cl- conductances were normal, only the increase in the number of non-inactivating Na+ channels has to be responsible for the membrane hyperexcitability. 5. Length, ramification and charge of the side-chains of the substitutions correlated well with the Na+ channel dysfunction and the severity of myotonia, with alanine as the most benign and glutamic acid as the substitution with a major steric effect. 6. Our electrophysiological and molecular genetic studies strongly suggest that these Na+ channel mutations cause myotonia. The naturally occurring mutants allowed us to gain further insight into the mechanism of Na+ channel inactivation.

Theoretical reconstruction of myotonia and paralysis caused by incomplete inactivation of sodium channels

Muscle fibers from individuals with hyperkalemic periodic paralysis generate repetitive trains of action potentials (myotonia) or large depolarizations and block of spike production (paralysis) when the extracellular K+ is elevated. These pathologic features are thought to arise from mutations of the sodium channel alpha subunit which cause a partial loss of inactivation (steady-state Popen approximately 0.02, compared to < 0.001 in normal channels). We present a model that provides a possible mechanism for how this small persistent sodium current leads to repetitive firing, why the integrity of the T-tubule system is required to produce myotonia, and why paralysis will occur when a slightly larger proportion of channels fails to inactivate. The model consists of a two-compartment system to simulate the surface and T-tubule membranes. When the steady-state sodium channel open probability exceeds 0.0075, trains of repetitive discharges occur in response to constant current injection. At the end of the current injection, the membrane potential may either return to the normal resting value, continue to discharge repetitive spikes, or settle to a new depolarized equilibrium potential. This after-response depends on both the proportion of noninactivating sodium channels and the magnitude of the activity-driven K+ accumulation in the T-tubular space. A reduced form of model is presented in which a two-dimensional phase-plane analysis shows graphically how this diversity of after-responses arises as extracellular [K+] and the proportion of noninactivating sodium channels are varied.

Calcium, calmodulin and 3',5'-cyclic nucleotide phosphodiesterase activity in human muscular disorders

3',5'-Cyclic nucleotide phosphodiesterase (PDE) is known to play an important role in the regulation of cyclic nucleotide levels in various tissues including the muscle. Previous studies have estimated the level of this enzyme in several neuromuscular disorders but the results have been variable. Moreover, there was no attempt made to correlate the enzyme levels with the levels of calcium and calmodulin, both of which regulate diverse biological processes including muscle contraction. In the present study we have estimated phosphodiesterase in the muscle of normal controls as well as patients with myotonic (MyD) and Duchenne muscular dystrophy (DMD) and amyotrophic lateral sclerosis (ALS). PDE was found to be increased significantly in all of the diseased muscles as compared to controls (P less than 0.01). But the increase could be coupled with an increase in calcium and calmodulin only in Duchenne dystrophic muscle.

Effect of myotonia induced by anthracene-9-carboxylic acid on mitochondrial calcium, plasma creatinine-phosphokinase and aldolase activity in the rat

The frequent association of myotonia with dystrophy and the knowledge that calcium is increased in injured skeletal muscle cells suggest a possible relationship between cell calcium and myotonic alterations. This investigation has been performed to study the role of calcium in experimental myotonia induced by anthracene-9-carboxylic acid (9-AC) in rats treated with several regimens of food and exercise. Thirty-two rats were divided into 4 groups of 8 rats each, one control and 3 experimental groups. The treatments included caffeine plus exercise (group 2), and a calcium-rich diet (group 3); these procedures were designed to increase intracellular calcium; another group was treated with 9-AC as a myotonia-inducer (group 4). The treatment for all groups lasted 60 days. No significant differences in plasma sodium, potassium, chloride and calcium between control and experimental groups were observed. Whole muscle calcium in wet tissue samples did no change with any treatment. On the contrary, mitochondrial calcium showed a significantly higher concentration in group 3 and 4. CPK and aldolase activities in groups 1, 2 and 3 were similar; but in group 4 these enzyme activities were significantly higher (p less than 0.05). The electrical and mechanical responses were not altered in any rat with any experimental treatment. Our data suggest that myotonia is a predisposing factor for an altered mitochondrial calcium homeostasis in this model; in addition, the enzyme activities of CPK and aldolase were increased in the rats of group 4 implicating that myotonia is a crucial factor in the development of enzymatic abnormalities.(ABSTRACT TRUNCATED AT 250 WORDS)

Chloride channel regulation in the skeletal muscle of normal and myotonic goats

External intercostal muscle biopsies from normal and congenitally myotonic goats were studied in vitro at 30 degrees C using a two-microelectrode square-pulse cable analysis assisted by computer. The resting chloride conductance (Gcl) was estimated from the difference between the mean membrane conductance in chloride-containing and chloride-free bathing media. The protein kinase C (PKC) activator, 4-beta-phorbol-12,13-dibutyrate. (0.1-2.0 microM) blocks a maximum of 76% of Gcl in normal goat fibers and induces myotonic hyperexcitability similar to that of congenitally myotonic goat fibers. The Gcl block was partially antagonized by pretreatment with the PKC inhibitor, staurosporine (10 microM). The "inactive" 4-alpha-phorbol-12,13-didecanoate had no effect at 50 microM, whereas the "active" 4-beta isomer blocked 41% Gcl at 1 microM. The nearly absent Gcl of congenitally myotonic goat fibers was not restored by treatment with high concentrations of the PKC inhibitors staurosporine, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H7), or tetrahydropapaveralone (THP). Also, forskolin and cholera toxin, which may increase cyclic adenosine monophosphate (cAMP) levels, or the R(+) clofibric acid enantiomers and taurine, which increase Gcl in normal fibers, were also unable to restore Gcl in myotonic goat fibers. The data suggest that PKC may be a chloride channel regulator in normal goat skeletal muscle fibers, however the molecular defect of congenitally myotonic fiber does not appear to be due to excessive activity of PKC.

Schwartz-Jampel syndrome: II. Na+ channel defect causes myotonia

Skeletal muscle fibers from a patient with Schwartz-Jampel syndrome were studied in vitro. The fibers had normal resting membrane potentials, but their resting [Ca2+]i was elevated. The resting potentials were unstable and spontaneous depolarizations caused twitching in all fibers. Stimulated contractions were characterized by markedly slowed relaxation which was due to electrical after-activity. Neither curare (0.7 microM), tocainide (50 microM), nor phenytoin (80 microM) had an effect on the myotonic activity. In contrast, procainamide (200 microM) suppressed the hyperexcitability without affecting the twitch amplitude. The steady-state current-voltage relation was normal in 5 fibers, but altered in 3 others. These latter fibers had an increased specific membrane resistance owing to a decreased Cl- conductance. The Na+ channels were investigated in the cell-attached patch clamp mode. In all patches on either type of fiber, depolarizing pulses elicited delayed, synchronized openings of Na+ channels. These abnormal openings occurred even after the surface membrane repolarized. We hypothesize that these altered membrane conductances are responsible for the hyperexcitability and the associated slowed relaxation.

Energetic metabolism and fatigability in experimental myotonia

Experimental myotonia was induced in rats by 2,4-dichloro-phenoxyacetic acid (2,4-D). After 4 to 24 h of treatment, the anterior tibialis muscles exhibited increased fatigue at low frequency (30 Hz) nerve stimulation, but they developed normal tension at high-frequency (100 Hz) stimulation. Glycogen content and the activities of glycogen phosphorylase, lactate dehydrogenase and malate dehydrogenase remained normal. The absence of correlation between fatigability and energetic metabolism in this experimental model of myotonia suggests a dysfunction in excitation-contraction coupling.

Potassium uptake in muscle during paramyotonic weakness

Previous studies have suggested that an abnormal release of potassium from muscle may accompany attacks of paramyotonic weakness. We investigated 3 patients with paramyotonia congenita before and after the induction of forearm muscle weakness by exercise in cold water. Two of these patients had paralysis periodica paramyotonica and the 3rd had paramyotonia congenita. At the time of paramyotonic weakness there was a marked increase in the arterialized-venous concentration difference of potassium across forearm muscle. This indicated a significant uptake of potassium by forearm muscle in all 3 patients. Normal controls showed a slight release of potassium both at rest and after exercise in cold water. These results suggest that (1) the sodium-potassium pump of the muscle fiber is operating efficiently during paramyotonic weakness; and (2) there is a different mechanism responsible for the generalized weakness that occurs in hyperkalemic periodic paralysis.

Opposite regulation of the mRNAs for parvalbumin and p19/6.8 in myotonic mouse muscle

The gene mutation in the mouse, 'arrested development of righting response', adr, causes a defect of chloride conductance of the muscle fibre membrane leading to the symptoms of myotonia [Mehrke, G., Brinkmeier, H. and Jockusch, H. (1988) Muscle & Nerve 11, 440-446]. In fast muscle, the myotonic phenotype is accompanied by a drastic reduction of the Ca2+-binding protein, parvalbumin. Messenger RNA levels in organs of myotonic (ADR) mice were analysed. In fast muscles of the mutant, in-vitro-translatable parvalbumin mRNA was strongly reduced, whereas the mRNA for the slow-muscle-specific protein, p19/6.8, was increased. In contrast, the parvalbumin mRNA in the cerebellum was not affected by the adr mutation. A reduction of the two parvalbumin mRNA species (700 and 1100 nucleotides) in ADR fast muscle and unaltered parvalbumin mRNA levels in mutant cerebella were demonstrated by cDNA/mRNA hybridisation, using a rat parvalbumin cDNA as a probe. The mRNA level for another Ca2+-binding protein, calmodulin, was low in muscle and high in the central nervous system but was unaffected by the mutation. When adr/adr mice were fed a diet containing the membrane-stabilising drug, tocainide, the levels in muscle of the mRNAs for parvalbumin and p19/6.8 were partially normalised.

A novel protein, p19/6.8, specific for cardiac and slow skeletal muscle

Upon in vitro translation of mRNAs from slow (soleus) muscles of the mouse a hitherto undescribed translation product has been detected that was absent from fast skeletal muscles and was termed p19/6.8 according to its position in 2-dimensional gels. mRNA for p19/6.8 was also found in the ventricle of the heart. p19/6.8 was not detectable by Coomassie blue staining but could be characterised by fractionation of in vivo labelled muscle tissue. It was found to sediment with the particulate fraction at 14,000 x g. The expression of p19/6.8 mRNA appears to be down-regulated in muscles with phasic activity.

Reduction of myosin-light-chain phosphorylation and of parvalbumin content in myotonic mouse muscle and its reversal by tocainide

In muscle of the myotonic mouse mutant, 'arrested development of righting response', ADR, a reduced level of fast-myosin-light-chain-2 (LC2f) phosphorylation was observed in addition to a lowered parvalbumin content. In fast muscles, average phosphorylation levels of LC2f (LC2-P/LC2 total) were 0.76 mol/mol for wild type and 0.59 mol/mol for the myotonic mutant. The difference was not due to short-term activity prior to freezing because it was also found in curare-paralyzed muscles. Long-term treatment of genetically myotonic animals with the membrane-stabilizing drug, tocainide, led to an increase of parvalbumin content and LC2-P level. In wild-type mice, tocainide had a similar effect, leading to supranormal parvalbumin concentrations. It is concluded that both the basal level of LC2-P and parvalbumin concentration are regulated by a common factor, related to long-term muscle activity.

Sarcoplasmic reticulum ATPase in tibialis anterior muscle from rats with experimental myotonia

The sarcoplasmic reticulum ATPase was studied (after 3 h to 14 days) in rats treated with 2,4-dichlorophenoxyacetate to induce myotonia. It was found that ATPase decreased in the treated rats after the establishment of myotonia.

Transmembrane calcium movement in 20,25-diazacholesterol myotonia

An abnormality in myoplasmic Ca2+ regulation has frequently been proposed in 20,25-diazacholesterol (20,25-D) myotonia. We report here the results of several studies of transmembrane Ca2+ movement in this animal model. (i) Physiologic Ca2+ release by intact sarcoplasmic reticulum (SR) was examined in chemically skinned single muscle fibers preloaded in EGTA-buffered Ca2+ solutions (pCa2+7.0 to 6.4). Isometric tension development and Ca2+ release thresholds in response to Cl- or caffeine showed no differences between control and 20,25-D fibers at any pCa2+. (ii) The kinetics of energy-dependent Ca2+ accumulation in purified SR vesicles were followed spectrophotometrically using Ca2+-sensitive dyes. The apparent rate for ATP-dependent Ca2+ uptake and Ca2+ sequestering capacity were unchanged in SR from 20,25-D animals vs. controls. (iii) Surface membrane Ca2+ATPase activity was measured in red blood cell ghosts and sarcolemma. Enzyme Vmax was decreased by 25 to 50% in both membranes in the 20,25-D-treated animals with a compensatory increase in the number of Ca2+ATPase molecules. In general, the SR handling of Ca2+ appears normal in 20,25-D myotonia, although the activity of Ca2+ATPase in membranes with high sterol content may be altered in response to changes in the lipid environment in this model.

Reduced adrenal androgens in patients with myotonic dystrophy

Endocrine disturbances associated with myotonic dystrophy (MD) include testicular atrophy, hyperinsulinemic glucose intolerance, thyroid abnormalities, and low or low normal urinary 17-ketosteroid (17-KS) excretion. Since the major circulating precursors of urinary 17-KS are dehydroepiandrosterone sulfate (DHAS) and dehydroepiandrosterone (DHA), a decrease in adrenal androgen production has been suggested. This possibility was studied in 19 MD patients and 19 age- and sex-matched normal subjects. Each patient had a 24-h urine collection for 17-KS and cortisol determinations, a 4-h iv infusion of 25 micrograms tetracosactrin with serial measurements of serum DHAS, DHA, and cortisol, and an insulin-induced hypoglycemia test. Sixteen patients had 0800 and 2400 h serum collections for cortisol estimations. Serum DHAS [1.0 +/- 0.5 (+/- SD) vs. 3.9 +/- 1.9 mumol/liter; P less than 0.0005] and DHA (5.9 +/- 2.7 vs. 11.0 +/- 7.1 nmol/liter; P less than 0.005) levels were significantly lower in MD patients than in normal subjects; cortisol levels were higher (540 +/- 222 vs. 394 +/- 128 nmol/liter; P less than 0.01), almost certainly a reflection of stress. A normal diurnal cortisol rhythm was found in all 16 subjects. Cortisol responses to insulin-induced hypoglycemia were normal, increasing from 345 +/- 243 nmol/liter to a maximum of 831 +/- 282 nmol/liter. Urinary 17-KS excretion was low or low normal, while urinary cortisol levels were normal in 18 and mildly elevated in 1 patient. There was a significant correlation between 17-KS and DHAS levels (r = 0.46; P less than 0.05). DHAS, DHA, and cortisol responses to tetracosactrin infusion were similar in patients and normal subjects. It is concluded that 1) in MD patients, serum DHAS and DHA concentrations are significantly lower than those in normal subjects, explaining the frequent reports of low or low normal 17-KS excretion; 2) the reduced DHAS and DHA concentrations are most likely due to decreased production rather than increased clearance; and 3) glucocorticoid production is normal.

Myotonia induced by low chloride solution: intracellular studies by Cl liquid ion exchanger microelectrode

By exposing the rat hemidiaphragm preparations to various low chloride solutions, it was demonstrated that myotonia can be induced when the extracellular chloride concentration was reduced below 82 mEq/L. Myotonia can be induced simply by reducing the extracellular chloride concentration without any significant reduction of RMP. The intracellular and extracellular chloride activity was measured by the liquid ion exchanger microelectrode. The control intracellular chloride activity was 10.8 mEq/L and that of myotonic specimen in a low chloride solution of 47 mEq/L was 4.4 mEq/L. Chloride conductance was closely related to the extracellular chloride concentration and myotonia was induced when gc1 was 38.3% of the control.

Erythrocyte calcium transport in myotonic and facioscapulohumeral muscular dystrophy

We studied calcium transport in inside-out erythrocyte vesicles from patients with myotonic or facioscapulohumeral dystrophy and age- and sex-matched controls. No significant difference was noted in the affinity of the transporter for calcium or the maximum reaction velocity. Under identical conditions, we previously found that Duchenne dystrophy membranes differed from controls in affinity for calcium and maximum velocity. The results reported here imply that the abnormality in Duchenne dystrophy is specific and not an abnormality found in all forms of dystrophy.

Chronic phenytoin administration alters the metabolic profile of superficial gastrocnemius muscle fibers in dystrophic mice

Phenytoin is known to reduce neural overactivity (pseudomyotonia) affecting the hind limb musculature in C57B1/6J dystrophic (dy2J/dy2J) mice. This study reports a change in the metabolic profile of superficial gastrocnemius muscle fibers from dy2J/dy2J animals after chronic phenytoin treatment. The superficial gastrocnemius muscle region from normal mice is composed of 98% fast-twitch glycolytic muscle fibers. In dystrophic mice these fibers (FG) show increased oxidative capacity without evidence of morphologic degeneration during the first few months ex utero. Many of these fibers also store abnormally large amounts of glycogen as determined by periodic acid-Schiff histochemistry. After 104 days of phenytoin treatment, the dy2J/dy2J FG muscle fibers showed a reduction in abnormally high oxidative capacity as monitored by succinic dehydrogenase activity; there was also a reduction of glycogen storage in a number of dy2J/dy2J fibers. One hypothesis suggests that the increase in oxidative capacity of the dy2J/dy2J superficial gastrocnemius muscle fibers is the expected result of overstimulation by the pseudomyotonia. Our experiments indicated that the abnormal metabolic profile observed in those fibers can be altered simply by a reduction in pseudomyotonia. These results mimic those seen after short-term denervation of the same dy2J/dy2J muscle. After phenytoin treatment the mean dy2J/dy2J superficial gastrocnemius muscle fiber cross-sectional area was significantly increased compared with untreated animals. Cursory examination of the degenerated deep region of this same muscle suggested that similar changes did not occur after drug treatment. This suggests that the pseudomyotonia was partially different from the factor(s) causing early degeneration of the oxidative muscle fibers in the dy2J/dy2J animals.