Expert Insights from a Delphi-driven Neurologists' Panel: Real-world Mexiletine use in Patients with Myotonic Disorders in Italy

Background

Myotonic disorders, such as non-dystrophic myotonias (NDMs) and myotonic dystrophies (DMs) are characterized by a delay in muscle relaxation after a contraction stimulus. There is general consensus that protocols to treat myotonia need to be implemented.

Objective

Mexiletine is the only pharmacological agent approved for the symptomatic treatment of myotonia in adult patients with NDM and is considered to be the first-line treatment for DMs; however, its production in Italy was halted in 2022 making its availability to patients problematic.

Methods

A panel of 8 Italian neurologists took part in a two-round Delphi panel between June and October 2022, analyzing the current use of mexiletine in Italian clinical practice.

Results

The panelists assist 1126 patients (69% DM type1, 18% NDM and 13% DM type2). Adult NDM patients receive, on average, 400-600 mg of mexiletine hydrochloride (HCl) while adult DM patients receive 100-600 mg, per day in the long-term. The severity of symptoms is considered the main reason to start mexiletine treatment for both NDM and DM patients. Mexiletine is reckoned to have a clinical impact for both NDM and DM patients, but currently drug access is problematic.

Conclusions

Mexiletine treatment is recognized to have a role in the reduction of the symptomatic burden for NDM and DM patients. Patient management could be improved by facilitating access to therapy and developing new drug formulations.

Respiratory compromise after MgSO4 therapy for preterm labor in a woman with myotonic dystrophy: a case report

BACKGROUND

MgSO4 is widely used for tocolysis. Serious complications are rare as long as dosing is carefully monitored. Adverse effects in muotonic dustrophy have not been previously described.

CASE

A 35-year-old woman, gravida 1, para 0, was hospitalized with suspected mild myotonic dystrophy, polyhydramnios and preterm labor at 33 weeks. MgSO4 infusion rapidly resulted in respiratory compromise. Muscular strength returned to baseline after the infusion was stopped. Mother and infant proved to have myotonic dystrophy.

CONCLUSION

The choice of tocolytic medication in maternal myotonic dystrophy is problematic. Beta-2 sympathomimetics have been reported to precipitate myotonia. This case illustrates the potential for MgSO4 to cause respiratory embarrassment. Indomethacin may be the tocolytic of choice in myotonic dystrophy.

Prolonged myotonia and dystonia after general anaesthesia in a patient taking gabapentin

This is the report of a 55-yr-old female who developed severe myotonia and dystonia after general anaesthesia. Before starting on gabapentin therapy for a neuropathic pain condition, she had undergone numerous uneventful general anaesthetics. Since receiving treatment with gabapentin, she has experienced severe movement disorders on emergence from each subsequent general anaesthetic. The events were unrelated to the choice of anaesthetic or anti-emetic. The most recent event that required a protracted stay in hospital after a day-case surgery is presented in detail, and the possible mechanisms to explain the interaction are discussed.

Clinical effects and plasma concentration determination after 2,4-dichlorophenoxyacetic acid 200 mg/kg administration in the dog

OBJECTIVE

To investigate the clinical effects and to determine the 2,4-dichlorophenoxyacetic acid plasma concentrations after a dose of twice the reported LD50 (100 mg/kg) was administered orally to dogs. Investigation included electromyographic evaluations and biochemical parameter determinations, as well as observable clinical signs.

METHODS

Six beagle dogs were administered 2,4-dichlorophenoxyacetic acid 200 mg/kg orally. Dogs were monitored for the development of clinical signs and were anesthetized at 24 hours for needle electromyography. Blood was collected pre- and 24-hours postadministration. Plasma was analyzed for total and unbound 2,4-dichlorophenoxyacetic acid by high-performance liquid chromatography with fluorescence detection. Serum was submitted for clinical chemistry parameter analysis. Statistical analyses of the chemistry parameters were performed using paired t-tests.

RESULTS

All 6 dogs survived after oral administration of twice the reported LD50. Clinical signs observed were vomiting in 33% and diarrhea in 100% of the dogs. No gait abnormalities were seen in awake dogs. Electromyographic findings revealed predominantly insertional myotonia with 1 dog having spontaneous fibrillations. Decreases from baseline measurements were seen in serum calcium, potassium, and total bilirubin. The mean total and unbound plasma 2,4-dichlorophenoxyacetic acid concentrations were 511 mg/L and 129 mg/L, respectively.

CONCLUSIONS

This study demonstrates that the beagle dog is less sensitive to the acute effects of 2,4-dichlorophenoxyacetic acid than previously reported. The main clinical effects seen after oral administration of twice the reported LD50 were vomiting and diarrhea. Total and unbound plasma 2,4-dichlorophenoxyacetic acid concentrations may be a useful indicator of toxicity.

Increased hindrance on the chiral carbon atom of mexiletine enhances the block of rat skeletal muscle Na+ channels in a model of myotonia induced by ATX

1 The antiarrhythmic drug mexiletine (Mex) is also used against myotonia. Searching for a more efficient drug, a new compound (Me5) was synthesized substituting the methyl group on the chiral carbon atom of Mex by an isopropyl group. Effects of Me5 on Na+ channels were compared to those of Mex in rat skeletal muscle fibres using the cell-attached patch clamp method. 2 Me5 (10 microM) reduced the maximal sodium current (INa) by 29.7+/-4.4 % (n=6) at a frequency of stimulation of 0.3 Hz and 65.7+/-4.4 % (n=6) at 1 Hz. At same concentration (10 microM), Mex was incapable of producing any effect (n=3). Me5 also shifted the steady-state inactivation curves by -7. 9+/-0.9 mV (n=6) at 0.3 Hz and -12.2+/-1.0 mV (n=6) at 1 Hz. 3 In the presence of sea anemone toxin II (ATX; 5 microM), INa decayed more slowly and no longer to zero, providing a model of sodium channel myotonia. The effects of Me5 on peak INa were similar whatever ATX was present or not. Interestingly, Me5 did not modify the INa decay time constant nor the steady-state INa to peak INa ratio. 4 Analysis of ATX-induced late Na+ channel activity shows that Me5 did not affect mean open times and single-channel conductance, thus excluding open channel block property. 5 These results indicate that increasing hindrance on the chiral atom of Mex increases drug potency on wild-type and ATX-induced noninactivating INa and that Me5 might improve the prophylaxis of myotonia.

Myotonia associated with sarcoidosis: marked exacerbation with pravastatin

A 37-year-old man with sarcoidosis developed severe electrical and clinical myotonia while taking pravastatin for hypercholesterolemia. Myotonia associated with sarcoidosis is rare. Pravastatin is associated with myotonia in animals. This case suggests that sarcoidosis and pravastatin, two entities not frequently associated with myotonia, may interact in a synergistic manner to produce severe clinical myotonia in humans.

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.

Effect of temperature reduction on myotonia in rat skeletal muscles in vitro

1. The objective of the study was to determine the effect of temperature reduction on the response of rat skeletal muscles to myotonia-inducing agents. 2. A model myotonia was induced in the muscles in vitro, using either the chloride channel blocker anthracene-9-carboxylic acid or chloride-free Krebs solution. This model is similar in its characteristics to the myotonia which occurs in autosomal recessive generalized myotonia congenita in humans. 3. Isometric twitch contractions were recorded in the muscles in Krebs solution before and after the addition of the myotonia-inducing agent. The presence of myotonia was confirmed when the half-relaxation time of the twitch contraction after the addition of the agent was significantly greater than that before its addition. 4. Recordings were made at 37 degrees C, 30 degrees C, 25 degrees C and 15 degrees C. Myotonia developed at 37 degrees C, 30 degrees C and 25 degrees C, but not at 15 degrees C, indicating that at a temperature between 25 degrees C and 15 degrees C, anthracene-9-carboxylic acid-induced myotonia failed to develop. This supports the results obtained in humans suffering from myotonia congenita where myotonic contractions in the adductor pollicis muscle disappeared when the muscle temperature was cooled to 20 degrees C. 5. The myotonia which developed at 37 degrees C could be significantly reduced by exposure to 1 x 10(-4) mol/l ouabain or by elevation of the K+ concentration of the Krebs solution to 7.5 mmol/l. 6. Measurements made using microelectrodes showed that the conditions under which myotonia either did not develop or was significantly reduced, i.e. a temperature of 15 degrees C, exposure to 7.5 mmol/l K+ at 37 degrees C or exposure to 1 x 10(-4) mol/l ouabain at 37 degrees C were each associated with membrane depolarization. The results are discussed in terms of a possible role for depolarization in preventing/reducing the myotonic response.

Suspected herbicide toxicosis in a dog

An 8-year-old 38-kg spayed female Golden Retriever was admitted for vomiting, signs of abdominal pain on palpation, ataxia, anorexia, and generalized weakness of 2 days' duration. Ten hours prior to onset of clinical signs, the dog was found standing in and drinking from large pools of an accidentally spilled herbicide that contained an octanoic acid ester of bromoxynil (3,5-dibromo-4-hydroxybenzonitrile) and an isooctyl ester of (2-methyl-4-chloro) phenoxyacetic acid (MCPA). Appendicular muscles were firm on palpation and persistent muscle contraction (myotonia > 1 minute duration) was found on muscle percussion, using a reflex hammer. Electrical activity indicative of myotonia was identified on electromyographic evaluation. With supportive treatment, the dog eventually recovered from suspected MCPA toxicosis. Although rare, MCPA toxicosis should be considered as a cause of acquired myotonia in dogs.

Myotonia in colchicine myoneuropathy

Colchicine may induce a myoneuropathy in patients with renal insufficiency. To date, myotonia has not been described in this disorder. We recently studied 4 patients treated with routine doses of colchicine who, in the setting of renal insufficiency, developed a severe myoneuropathy characterized by prominent myotonic discharges on electromyography. In addition, 1 of the 4 patients had profound clinical myotonia. In the 3 patients in whom biopsies were performed, marked myopathic change with intracytoplasmic vacuolization was identified. All 4 patients improved rapidly with discontinuation of the medication. The patient in whom electrophysiologic studies were repeated had a complete resolution of the myotonic discharges. Colchicine myoneuropathy can present with prominent clinical and electrophysiologic myotonia that resolves completely with discontinuation of the medication.

Electrical myotonia of rabbit skeletal muscles by HMG-CoA reductase inhibitors

HMG-CoA reductase (HCR) inhibitors are effective cholesterol-lowering agents in the treatment of hypercholesterolemia. Using intracellular microelectrodes, we studied the pathomechanism of myotonia experimentally induced in rabbits by HCR inhibitors, simvastatin, and pravastatin. The external intercostal muscle of rabbits showed some electrophysiologic characteristics of myotonia including repetitive firing after administration of simvastatin (50 mg/kg per day, for 4 weeks). The relative chloride conductance, though reduced in both, was more affected in simvastatin-administered muscles. In normal muscles perfused with a solution containing the inhibitors, both simvastatin and pravastatin produced membrane hyperexcitability with repetitive firing similar to that seen in simvastatin-administered rabbits. The minimum concentrations required to cause repetitive firing was 0.3 mg/L for simvastatin and 30 mg/L for pravastatin. These results indicate that HCR inhibitors induce some characteristics of myotonia by blocking the chloride channel in the muscle membrane.

Myotonia induced by potassium repletion in a diabetic patient with secondary hypokalemic paralysis

In a woman suffering from insulin-dependent diabetes mellitus, hypokalemic paralysis developed acutely following an episode of diabetes decompensation. During the treatment of this episode, as soon as serum potassium levels were restored to normal values, a marked increase in muscular excitability with an electromyographic picture of myotonia was observed. The patient showed signs of chronic muscle denervation that accounted for an increased sensitivity to potassium-induced depolarization and contraction and that might have been responsible for the appearance of myotonia during potassium repletion.

Studies on the inhibition by chlorpromazine of myotonia induced by ion channel modulators in mouse skeletal muscle

The myotonic activity of mouse soleus and extensor digitorum longus muscles induced by either a combination of K+ channel blockers (4-aminopyridine) and a Cl- channel blocker (9-anthracene carboxylic acid) or a Cl- channel blocker in low Ca2+ (0.25 mM) Krebs or a Na+ channel activator (veratridine) was characterized in this paper. Myotonic activity was characterized by an increase in both the contraction amplitude and contraction duration accompanied by stimulus-related repeated action potentials. The slow soleus and fast extensor digitorum longus muscles appeared to differ in their responses to these ion channel modifiers. Nevertheless, chlorpromazine at a low concentration of 1 microM significantly inhibited all kinds of myotonic activity; it reduced the prolonged contraction duration and attenuated the stimulus-related repeated action potential firing. This depressant action of chlorpromazine was apparently not correlated with inhibition of either calmodulin or phospholipase A2 activity, since the myotonic depressant action of calmodulin inhibitors, such as dibucaine, flunarizine, chlorpromazine, trifluoperazine and diltiazem, was unrelated to their potency in inhibiting the activity of calmodulin or phospholipase A2. However, phosphatidylcholine was found to inhibit the myotonic depressant action of chlorpromazine. It is therefore, tentatively concluded that chlorpromazine interacted with membrane phospholipids, thereby changing membrane ion channel activity and depressing myotonic activity. These findings indicate that chlorpromazine might be useful in the management of clinical myotonia.

Studies on mercury-induced myotonia in the mouse diaphragm

Muscle contracture and myotonia of mouse diaphragm, induced by HgCl2, were studied. HgCl2 induced myotonia of mouse diaphragm only on condition that the Hg2+ contracture was inhibited by 0.005-0.01 mM NaCN. A higher concentration of 0.05 mM NaCN abolished both the Hg2+ contracture and Hg2+ myotonia. This finding suggests that the Hg2+ contracture masked the Hg2+ myotonia which was less sensitive to the inhibitory action of NaCN. This differential inhibitory action of NaCN on Hg2+ contracture from Hg2+ myotonia implies a possibility that NaCN antagonized the actions of HgCl2 not only through a simple chemical interaction. Hg2+ myotonia was characterized by an increase in contractile amplitude and a prolongation of contractile duration which were associated with stimulus-bound repetitive action potentials and an increase in membrane input resistance. A low Cl- medium as well as a Cl- channel blocker (9-anthracene carboxylic acid) not only by themselves induced myotonia, but also antagonized Hg2+ myotonia. Thus, Hg2+ appeared to mimic the Cl- channel blocker in inducing myotonia through a blockade of the Cl- channel. K+ channel blockers (4-aminopyridine, uranyl nitrate and tetraethylammonium chloride), as well as low (0.25 mM) Ca2+ Krebs, augmented Hg2+ myotonia while ATP-sensitive K+ channel blockers (tolbutamide and glibenclamide) antagonized Hg2+ myotonia (in the presence of NaCN). Since glibenclamide did not affect myotonia induced by a Cl- channel blocker, it was suggested that glibenclamide inhibited the Hg2+ myotonia through an interaction either directly or indirectly with NaCN on the sarcolemma. All of these findings suggest that K+ channels (delayed rectifier and Ca(2+)-activated K+ channel) functionally cooperated with the Cl- channel of the sarcolemma in the regulation of the skeletal muscle contraction. In this study, K+ channel blockers synergistically cooperated with Cl- channel blockers in inducing myotonia of the mouse diaphragm, while an ATP-sensitive K+ channel blocker exerted only an opposite effect on NaCN. Ca2+ appeared to play an important role in regulating the ionic channel activities, especially the Cl- channel, since low Ca2+ markedly potentiated not only Hg2+ but also low Cl- in inducing myotonia.

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)

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.

The effects of taurine on pharmacologically induced myotonia

Taurine reduces the excitability of striated muscle fibers by increasing the membrane conductance to chloride ions (GCl). This action was tested on rats made myotonic by drugs that block GCl by different mechanisms. Experiments were made "in vivo" using electromyographic (EMG) recordings and "in vitro" with intracellular microelectrode recordings from extensor digitorum longus muscle fibers. Taurine did not antagonize the myotonic discharges produced in vivo by anthracene-9-carboxylic acid, nor did it restore GCl lowered in vitro by this agent. However, when myotonia was chronically induced by 20,25 diazacholesterol, taurine given chronically in vivo or acutely in vitro antagonized the EMG myotonia as well as the reduced GCl and increased excitability of single fibers. We conclude that taurine acts directly on chloride channels to modify their kinetics. Our findings suggest that further clinical studies on the use of taurine in muscle disease involving abnormal excitability or chloride channel function will be useful.

Anthracene-9-carboxylic acid and haematological and biochemical variables in the rat

Chronic administration of anthracene-9-carboxylic acid in Long-Evans rats had no significant effect on the following haematological and biochemical variables: haemoglobin, haematocrit, serum sodium, potassium, glucose, cholesterol and proteins. In agreement with previous reports, significant age-related differences of glucose and cholesterol concentration were observed. A decrease of body weight was the main finding. The similarities between the myotonia induced by anthracene-9-carboxylic acid and the characteristic myotonic, haematological and biochemical alterations in patients with myotonia congenita, suggest that, despite untoward effects such as a decrease of body weight, chronic anthracene-9-carboxylic acid administration appears to be a good model for the study of mytonia congenita.

Studies on cadmium-induced myotonia in the mouse diaphragm

The purpose of this investigation was to study the possible mechanism of the potentiation of the contractile response and myotonia caused by Cd2+ in the mouse diaphragm. Cd2+ increased both amplitude and duration of the contractile response to direct stimulation in either 0.25 mM Ca2+ Krebs or 2.5 mM Ca2+ Krebs containing the K+-channel blockers, 4-aminopyridine, uranyl nitrate or tetraethylammonium ion. High K+ and tetrodotoxin inhibited these effects of Cd2+. Electrophysiological studies revealed that only one or two action potentials were triggered by passing a short depolarizing current across the muscle fibre membrane in 0.25 mM Ca2+ Krebs, but in the presence of Cd2+, a train of action potentials (153 +/- 21 Hz) which lasted for 0.7 +/- 0.2 s was induced. Furthermore, Cd2+ triggered a train of action potentials evoked by a single extracellular direct stimulation on the muscle fibre in 2.5 mM Ca2+ Krebs solution containing either 4-aminopyridine or uranyl nitrate. The membrane depolarized during the repetitive firing and then repolarized immediately after the cessation of repetitive firing. Cd2+ (0.1 mM) increased the input resistance of the muscle fibre by 53 +/- 7% and this effect was inhibited in low [Cl-]o. These findings suggest that the contractile potentiation and myotonia induced by Cd2+ in the mouse diaphragm are mediated by lowering the Cl- conductance of the membrane.

Effect of diltiazem on 2,4-D-induced myotonia in rats

Myotonia is characterized by prolonged contraction (delay in onset of relaxation) of skeletal muscle fibers with characteristic electromyographic findings. Calcium channel blocking drugs may be expected to reduce myotonia, should they promote the onset of relaxation in a contracted skeletal muscle. This study was aimed at evaluating the effect of diltiazem, a calcium channel blocking agent, on myotonia induced by 2,4-dichlorophenoxyacetic acid (2,4-D). In rat diaphragm, exposed to 2.5 mM 2,4-D in a tissue bath, myotonia was quantified by documenting the contraction time in response to direct stimulation with supramaximal electric stimuli. At the peak of myotonia, different concentrations of diltiazem were added to the tissue bath and the effect on evoked contraction studied over a period of 6 minutes. A concentration of 5 x 10(-5) M was found to be the most effective, causing a decrease in contraction time of more than 90% in 3 minutes in 100% of specimens (n = 7). The above findings raise the possibility of using diltiazem as an antimyotonic agent.

Intramuscular pH and endurance are abnormal in skeletal muscles from rats with experimental myotonia

The relation between resistance to fatigue and intramuscular pH was studied in fast muscles (anterior tibialis and extensor digitorum longus) from rats treated with 2,4-dichlorophenoxyacetic acid (2,4-D) to induce myotonia. Fatiguability was studied in muscles indirectly stimulated at 30 Hz (330 ms/s; 1 train/s) for 2 min. The resistance to fatigue decreased significantly 1 h after drug treatment and remained low 24 h later. The intramuscular pH was lower than normal in resting muscles from 2,4-D-treated rats. After 2 min of stimulation the pH decreased in both control and drug-treated muscles. However, this decrement was reduced in the experimental muscles. The pH of control and of 2,4-D-treated muscles were similar after the stimulation period, but only the drug-treated muscles were fatigued. Therefore, a decrease in intramuscular pH would not be the cause of the observed decrease in muscle resistance to fatigue after 2,4-D treatment. The reduced endurance of drug-treated muscles could not be attributed to impaired neuromuscular transmission.

Drug-induced myotonia in human intercostal muscle

The myotonia-inducing effects of furosemide and clofibrate, two widely used pharmaceutical agents, were investigated in excised human external intercostal muscle. The effects of anthracene-9-carboxylic acid (9-AC), a well-known myotonia-producing chemical, were also tested for comparison. In the presence of these drugs the electrical threshold was lowered, and a constant current pulse produced multiple spiking. Short trains of direct stimuli were often followed by after-activity, and this caused a myotonia-like prolongation of muscle contraction. Voltage-clamp experiments showed that 0.05 mM anthracene-9-carboxylic acid, 1 mM furosemide, and 1 mM clofibrate decreased the chloride conductance of the muscle fiber membrane to 14, 18, and 40%, respectively, of the normal value, and the myotonia-inducing potency of the 3 drugs was correlated with the decreased chloride conductance. The potassium currents were not affected by these compounds.

Activators of protein kinase C induce myotonia by lowering chloride conductance in muscle

Certain phorbol esters and bryostatin 1, known activators of protein kinase C, were found to induce, in genetically normal muscle, an electrical membrane instability leading to unscheduled series of action potentials. Like in hereditary myotonias of man, goat and mouse, these symptoms were caused by a drastically lowered sarcolemmal chloride conductance. Our results indicate that the chloride channels of mammalian muscle may be subject to modulation by the protein kinase C-diacylglycerol system.

The influence of skeletal muscle reinnervation on experimentally induced myotonia

Earlier studies have shown that prior denervation of muscle prevents myotonia induced by 2,4-dichlorophenoxy acetic acid (2,4-D) both in vivo and in vitro. This work studied the effect of reinnervation on 2,4-D myotonia. Twenty Sprague-Dawley rats were injected with 2,4-D at specific intervals following unilateral sciatic nerve crushing; the gastrocnemius muscle on both sides was studied electromyographically to assess myotonia and to document denervation and reinnervation. All the rats gradually became amyotonic following denervation; myotonia reappeared during reinnervation. Myotonic discharges were no longer detectable 1 week after nerve crushing, but returned completely within 3 weeks. Blocking axoplasmic transport with colchicine had essentially the same effect on myotonia. A reciprocal temporal relationship was noted between the occurrence of fibrillations and myotonic discharges. These findings substantiate the view that innervation is essential to maintain the muscle membrane in a state that will support myotonic discharges.

Pharmacokinetics of anthracene-9-carboxylic acid, a potent myotonia-inducer

The pharmacokinetics of anthracene-9-carboxylic acid (9-AC), a potent myotonia-inducer, was determined in 9 New Zealand White male rabbits after the intravenous injection of 36 microM X kg-1. Myotonic contractions appeared 30 to 80 sec after the beginning of injection. One minute after injection, 25% of the administered dose remains in the blood (9-AC in plasma + 9-AC in cells). Of the 9-AC present in whole blood, 78% is in plasma and can be totally extracted with chloroform whereas the remaining 22% is in the blood cells and can not be extracted. The disappearance of 9-AC from plasma can be explained numerically by assuming an open two-compartment model. By means of a non-linear regression analysis the equation Cp = 20.65e-0.3216t + 28.35e-0.0147t was obtained. This equation gives the best fir for the mean values of all the experiments. The equilibrium between the two compartments is reached in 9.0 min and the Vdss X kg-1 at this time is equal to 273 +/- 18.6 ml X kg-1 which is only marginally larger (10%) than the estimated extracellular volume (246.0 ml X kg-1), suggesting that 9-AC barely penetrates into the cells. The plasma concentration values at t1/2e1 and 3t1/2e1 are 1.07 X 10(-4) M and 2.69 X 10(-5) M, respectively. Similar concentration values were found by other authors as capable of reducing the chloride conductance of skeletal muscle membrane and producing myotonia in vivo and in vitro. Thus, there is a good correspondence between the concentrations active in vitro and the concentrations of plasma in vivo found in this study. Other pharmacokinetic parameters are: t1/2 alpha = 2.8 +/- 0.3 min; t1/2 beta = 45.7 +/- 2.2 min; Area under the curve (AUC) = 1920 +/- 125.0 micrograms X min X ml-1; K12 = 0.085 +/- 0.01 min-1; K21 = 0.162 +/- 0.02 min-1; t1/2e1 = 28.84 +/- 2.2 min., and V1 X kg-1 = 172.1 +/- 9.7 ml. All our findings plus the rapid appearance of myotonic signs and the correspondence with the in vitro studies support the previous assertion that 9-AC acts on the cellular membrane of the skeletal muscles.

Fast to slow transition induced by experimental myotonia in rat EDL muscle

Experimental myotonia was induced by feeding rats with 20,25-diazacholesterol for up to 8 months. Histochemical analysis of myotonic extensor digitorum longus (EDL) muscle showed a progressive decrease of type IIB fibres and a concomitant increase of type IIA and type I fibres. A transient hypertrophy of type IIA fibres was observed 6 months after beginning the treatment. Analysis of the pattern of myosin light chains of single fibres from EDL showed that myotonia caused a progressive decrease of fibres showing a pure fast myosin light chain pattern and an increase of fibres showing coexistence of fast and slow myosin light chains (intermediate fibres). Only a small percentage of intermediate fibres showed coexistence of fast and slow myosin heavy chains. Myotonic fibres presented an increased sensitivity to caffeine which approached that of normal soleus fibres. Furthermore, sarcoplasmic reticulum (SR) vesicles isolated from hind limb fast muscles of myotonic rats demonstrated a decrease of Ca2+-dependent ATPase and Ca2+-transport activities as well as a decrease of immunoreactivity with anti-rabbit SR fast Ca2+-ATPase antibody. These results suggest that the increased electrical activity brought about by 20,25-diazacholesterol-induced myotonia, caused a fast to slow transition in the phenotypic expression of myosin and sarcoplasmic reticulum proteins.

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.

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.

Membrane fluidity and myotonia: effects of cholesterol and desmosterol on erythrocyte membrane fluidity in rats with 20,25-diazacholesterol-induced myotonia and on phospholipid liposomes

Previous spin-label and electromyographic experiments with rats fed 20,25-diazacholesterol, an inhibitor of the biosynthetic conversion of desmosterol to cholesterol, demonstrated an increased erythrocyte membrane fluidity and myotonia, a prolonged muscle contraction upon stimulation. The current studies with rats showed normal erythrocyte fluidity in animals fed 20,25-diazacholesterol but maintained on a high-cholesterol diet and no myotonia. Studies of model membrane systems composed of phospholipid vesicles containing desmosterol, cholesterol, or both demonstrated that desmosterol increased membrane lipid fluidity relative to cholesterol, suggesting that in 20,25-diazacholesterol-induced myotonia, in which desmosterol accounts for 85% of the plasma sterol, the increased membrane fluidity previously observed in erythrocytes and sarcolemma in this animal model of human congenital myotonia may be due to desmosterol.

Clofibrate-induced myotonia in rats

Determination of enzymatic activity, protein profile and phospholipid composition of muscle plasma membranes and sarcoplasmic reticulum in rats were carried out after clofibrate injections in a dose of 0.4 g/kg body weight. In the plasma membranes, the activity of Na+ + K+, Mg2+ ATPase was insignificantly decreased, and that of 5'-nucleotidase significantly diminished. A non-significant change was observed in the total amount of phopholipids. The amount of phosphoethanolamine appeared to be lower. Changes in the protein profile were seen. In the sarcoplasmic reticulum, the major abberation was the decrease of Mg2+ ATPase activity. No evident changes were observed in the phospholipid behaviour. Abnormalities in the protein profile appeared. In the myofibrillar proteins, increases of alpha-actinin and troponin at the expense of myosin were observed. In the clofibrate model of myotonia in rats, the changes in the biochemical parameters were less pronounced as compared to the previously tested 20,25-diazacholesterol model.

Increase of muscle peroxisomal enzymes and myotonia induced by nafenopin, a hypolipidemic drug

The chronic administration of nafenopin, a hypolipidemic drug, induced an increase in catalase and acyl-CoA oxidase activities in various skeletal muscles, including the gracilis, diaphragm, soleus, and extensor digitorum longus. The magnitude of the increase was around 100% for both enzymes in each of the muscles studied in spite of the different basal level. These changes seem to be specific of the peroxisomal enzymes because acetylcholinesterase, which is not peroxisomal, did not follow the same pattern in all the muscles. Concomitant with the increase in muscle peroxisomal enzymes, the skeletal muscles presented an altered electromyogram with prolonged insertional activity, repetitive firing of action potentials, and myotonic runs characteristic of myotonia. Our results suggest a role for peroxisomes in the myotonic disorder.

Adverse effects of drugs on muscle

A variety of drugs used in clinical practice may cause myopathy or interfere with neuromuscular transmission. The precise incidence of such disorders is not known, but it is almost certainly higher than is generally suspected. An important aspect of drug-induced muscular disorders is their reversibility if the offending agent is withdrawn, whereas failure to do so may lead to unnecessary morbidity. The study of drug effects on muscle provides a means of investigating the pathological reactions of muscle, and of producing experimental models of naturally occurring myopathies. Drug-induced myopathies may result from a direct toxic effect, which may be local when the drug is injected into a muscle or more diffuse when the drug is taken systemically, or may be secondary to electrolyte disturbances, muscle compression, ischaemia, neural activation or to the development of an immunological reaction directed against muscle. Repeated injections of antibiotics or drugs of addiction may lead to severe muscle fibrosis and contractures. A variety of drugs may cause an acute or subacute painful necrotising myopathy which may be associated with myoglobinuria, at times leading to acute renal failure. Clofibrate and epsilon aminocaproic acid are the drugs most frequently implicated, but a similar syndrome may occur in alcoholics and heroin addicts. Certain hypocholesterolaemic agents may induce myotonia by altering the sterol composition of the muscle cell membrane, while certain drugs including beta-adrenergic blockers and agonists, succinylcholine and diuretics may exacerbate or unmask pre-existing myotonia. In the syndrome of malignant hyperpyrexia, halothane, succinylcholine and various other agents may induce a potentially fatal state of muscular rigidity and hypermetabolism in susceptible individuals as a result of a defect in the calcium transport function of the sarcoplasmic reticulum and possibly of other cellular membranes. In corticosteroid myopathy, which is the most common form of drug-induced myopathy, there is selective atrophy of type 2 muscle fibres and the primary metabolic effect is an inhibition of RNA and protein synthesis, although protein degradation is also increased. Chloroquine and a number of related drugs with amphiphilic cationic properties may induce lysosomal storage myopathy, which may be associated with cardiomyopathy and with a more widespread form of lipidosis.

Myotonia in the rat diaphragm preparation caused by the sulfhydryl inhibiting para-substituted mercuribenzoates

The sulfhydryl (SH) inhibiting para-substituted mercuribenzoates like pOHMB caused a myotonia which appeared as a smooth myotonic profile during recording of the response to twitch stimulation (0.1/s) of isolated rat diaphragm preparations. The maximum myotonic tension varied from about 10% to 200% of the twitch tension at pOHMB addition, and the myotonic repetitive action potential activity varied from a few to more than one hundred action potentials in different cells of the same preparation. The myotonia did not appear after pretreatment with the SH-reducing agent dithiothreitol, and both dithiothreitol and N-ethyl-maleimide inhibited the motonia. The myotonia increased with temperature, appearing at about 32 degrees C. Increased twitch frequency and tetanic stimulation decreased the myotonia. No change of threshold was observed in myotonic preparations. The myotonia was depressed in K+-free solution, and it was blocked by K+ concentrations exceeding 1.5 X normal. The myotonia was reduced when the NaCl was replaced by sucrose or choline chloride. In Ca2+-free solution the time to maximal myotonic tension and the variability of the maximal myotonia were reduced. CA2+ concentrations above normal inhibited the myotonia. No myotonia was observed in the slow twitch soleus muscle. pOHMB also caused a twitch depression during indirect and direct stimulation. The depression was observed in soleus muscle and in the diaphragm below 30 degrees C. The depression was thus independent of the myotonia.

Sarcolemmal desmosterol accumulation and membrane physical properties in 20,25-diazacholesterol myotonia

In rats treated biweekly with 20,25-diazacholesterol (200 mg/kg orally), the desmosterol level in skeletal muscle sarcolemma increased progressively to about 80% of membrane sterol while total sterol levels remained constant. Following a single oral dose of 20,25-D, the kinetics of desmosterol accumulation and subsequent loss in sarcolemma were more rapid than in whole muscle homogenates. The anisotropy of diphenylhexatriene fluorescence and the calculated microviscosity of the probe's microenvironment decreased significantly with increasing desmosterol levels in a temperature-dependent manner, although fluorescent lifetimes were not altered. Fluorescent probes which localize in more superficial regions of the membrane detected no change. Studies with erythrocyte ghost membranes yielded results comparable to sarcolemma. Replacement of membrane yielded results comparable to sarcolemma. Replacement of membrane cholesterol with desmosterol reduced the local microviscosity of membrane cholesterol with desmosterol reduced the local microviscosity of the membrane hydrophobic region associated with phospholipid acyl chains and sterol side chains, but had little apparent effect on more superficial, polar regions. These observations can be correlated with the membrane location of the unsaturated side chain in desmosterol.

[The influence of several membrane stabilizing drugs on the induced hereditary myotonia--a methodic way to test the effectiveness of drugs on myotonia (author's transl)]

The therapeutic effectiveness of several membrane stabilizing drugs was investigated on experimentally induced myotonia by 2,4-Dichlorophenoxyacetate (2,4-D) and in 20 patients with myotonia congenita. Procainamide and quinine showed a better antimyotonic effect in-vitro- as well as in-vivo-experiments on the cold blood muscle and the rat than Sparteine sulfate and Tachmaline (Ajmalin). The two last-mentioned drugs had nearly the same effect. Because of these experimental results only procainamide and sparteine sulfate were clinically used. Quinine was not used because of the well known side-effects. Mention is made of the fact, that the therapeutic effect depends on the dose or the concentration. The results support not only the theoretic considerations on the pathogenesis of myotonia but also recommend to carry out further pharmacological investigations with this method.

Effects of vanadium on different adenosinetriphosphatases and binding of 3H-labeled ouabain and calcium-45 to rat brain synaptosomes

The effect of vanadium chloride on rat brain synaptosomal adenosinetriphosphatase (ATPase) activities was determined in vitro and in rats treated at 1 mg/kg.d ip and 10 mg/kg.d po for 10 d. Additional experiments were conducted to determine the effect of vanadium chloride on binding of [3H] ouabain and 45Ca to rat brain synaptosomes. Na+ + K+ - and Ca2+-ATPase activities were inhibited significantly in a concentration-dependent manner by V in vitro. Mg2+ -ATPase inhibition was neither dose-dependent nor significant except at 10(-5) M. Na+ + K+ -ATPase inhibition by V was more pronounced than that of other ATPases studied. Vanadium inhibited [3H] ouabain binding to synaptosomes by 90% at 10(-3) M; the inhibition was concentration-dependent. Binding of 45Ca was inhibited 50% at 10(-4) M; but concentration-dependent inhibition was not evident. Rats treated with vanadium chloride neither became myotonic nor showed any changes in ATPase activities or binding of [3H] ouabain and 45Ca to brain synaptosomes. Lineweaver-Burke plots of the in vitro inhibition of Na+ + K+ -ATPase and [3H] ouabain binding revealed that (1) Na+ + K+ -ATPase activation by ATP was inhibited by V with an increase in Km and a decrease in Vmax; (2) Na+ activation was inhibited noncompetitively by V, as evidenced by a decrease in Vmax and no change in Km; (3)K+ activation was inhibited by V with a decrease in both Vmax and Km; (4) noncompetitive inhibition of Mg2+ -ATPase by V was observed; and (5) the kinetic behavior of [3H] ouabain binding inhibition by V with respect to ATP and Na+ activation was mixed and noncompetitive, respectively. These results suggest that V is a potent inhibitor of Na+ + K+ -ATPase activity in rat brain synaptosomes.

20,25-diazacholesterol myotonia: an electrophysiological study

The electrophysiological characteristics of the myotonic syndrome produced in mammalian skeletal muscle by administration of 20,25-diazacholesterol (20,25-D) were studied in detail. In vivo electromyographic recordings confirmed widespread repetitive electrical activity, but delayed relaxation was evanescent and required isotonic rather than isometric conditions, with long recovery periods between stimuli, for clear demonstration. Subsequent administration of a potent inhibitor of membrane chloride conductance (GCl) induced profound delays in relaxation different from that after chronic 20,25-D alone. Intracellular passive cable analysis revealed only a small decrease in membrane GCl and none in potassium conductance. Potassium current-voltage relationships did not differ in control and treated animals. Intracellular microelectrode recordings consistently showed multiple driven action potentials during long depolarizations but no spontaneous myotonic discharges after cessation of the stimulus. Variations in temperature, buffer, and external ionic concentrations also failed to produce spontaneous activity. Anode break excitation under mild depolarizing conditions, however, did elicit repetitive membrane electrical activity. The myotonia induced by 20,25-D is not due to low membrane GCl. The relationship between delayed mechanical relaxation and membrane repetitive electrical activity remains to be clearly established in this myotonic syndrome.

Ionic conductance and experimentally induced myotonia

Experiments were conducted to study the effect of altering external potassium on in vitro induction of myotonia in rat diaphragm using 2,4-dichlorophenoxy acetate (2,4-D). An increase in external potassium inhibited, while a decrease enhanced, the myotonic response. However, in preparations which have undergone prior denervation, there was no myotonic response to 2,4-D, even when the external potassium was lowered. The experiments also support the existence of neural factors influencing the resting ionic conductance of the muscle membrane.