Myotonia induced by chemical agents

Guidelines on clinical presentation and management of nondystrophic myotonias

The nondystrophic myotonias are rare muscle hyperexcitability disorders caused by gain-of-function mutations in the SCN4A gene or loss-of-function mutations in the CLCN1 gene. Clinically, they are characterized by myotonia, defined as delayed muscle relaxation after voluntary contraction, which leads to symptoms of muscle stiffness, pain, fatigue, and weakness. Diagnosis is based on history and examination findings, the presence of electrical myotonia on electromyography, and genetic confirmation. In the absence of genetic confirmation, the diagnosis is supported by detailed electrophysiological testing, exclusion of other related disorders, and analysis of a variant of uncertain significance if present. Symptomatic treatment with a sodium channel blocker, such as mexiletine, is usually the first step in management, as well as educating patients about potential anesthetic complications.

A Comprehensive, Integrated Review and Evaluation of the Scientific Evidence Relating to the Safety of the Herbicide 2,4-D

The safety of 2,4-D to farm and forestry workers, commercial applicators and the general public has been of continuing concern because certain epidemiological studies of groups potentially exposed to 2,4-D have suggested a relationship between 2,4-D use and increased risk of soft tissue sarcoma, Hodgkin's disease or non-Hodgkin's lymphoma. This review on 2,4-D is unique in that the approach taken was to integrate data from worker exposure studies, whole animals, metabolic and other relevant laboratory studies with the epidemiological findings to assess the extent to which there is scientific support for the hypothesis that 2,4-D exposure is associated with any increased risk of human cancer.

The case-control epidemiological studies that have been the source of the cancer risk hypothesis are inconclusive. Problems in assessing exposure based on patients' memories make these studies difficult to interpret. Cohort studies of exposed workers do not generally support the specific hypothesis that 2,4-D causes cancer. Taken together, the epidemiological studies provide, at best, only weak evidence of an association between 2,4-D and the risk of cancer.

Importantly, the cancer hypothesis is not supported by other data. A critical evaluation of the exposure data indicates that exposure to 2,4-D in user groups is intermittent and much lower than the doses tested chronically in long-term animal studies that have not shown evidence of tumor induction. Moreover, the structure of 2,4-D does not suggest it would be a carcinogen. 2,4-D is a simple organic acid, that is largely excreted unchanged, and there is no evidence that it is metabolized to critically reactive metabolites or accumulates in tissues. This evidence is supported by a large body of negative studies on genotoxicity, which taken together with the metabolic studies, clearly indicates that 2,4-D is highly unlikely to be a genotoxic carcinogen. Furthermore, 2,4-D has no known hormonal activity and does not induce proliferative changes in any tissue or organ, indicating that it does not possess any of the characteristics of non-genotoxic animal carcinogens. Thus the available mechanistic studies provide no plausible basis for a hypothesis of carcinogenicity.

In this review, data relating to potential neurotoxicity, immunotoxicity and reproductive toxicity also were evaluated. There is no evidence that 2,4-D adversely affects the immune system and neurotoxic and reproductive effects only have been associated with high toxic doses that would not be encountered by 2,4-D users.

Historical exposures to 2,4-D by user groups, particularly farmers, forestry workers and commercial applicators, would be higher than those sustained under present rigorous standards for application which involve the use of protective clothing and other measures to reduce exposure. Proposed label changes indicate that in the future exposures will be even further reduced. Viewed in this context, the available data indicate that the potential public health impact of 2,4-D, including the risk of human cancer, was negligible in the past and would be expected to be even smaller in the present and future.

Myotonia congenita

Myotonia is a symptom of many different acquired and genetic muscular conditions that impair the relaxation phase of muscular contraction. Myotonia congenita is a specific inherited disorder of muscle membrane hyperexcitability caused by reduced sarcolemmal chloride conductance due to mutations in CLCN1, the gene coding for the main skeletal muscle chloride channel ClC-1. The disorder may be transmitted as either an autosomal-dominant or recessive trait with close to 130 currently known mutations. Although this is a rare disorder, elucidation of the pathophysiology underlying myotonia congenita established the importance of sarcolemmal chloride conductance in the control of muscle excitability and demonstrated the first example of human disease associated with the ClC family of chloride transporting proteins.

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

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

Review of 2,4-dichlorophenoxyacetic acid (2,4-D) epidemiology and toxicology

The scientific evidence in humans and animals relevant to cancer risks, neurologic disease, reproductive risks, and immunotoxicity of 2,4-D was reviewed. Despite several thorough in vitro and in vivo animal studies, no experimental evidence exists supporting the theory that 2,4-D or any of its salts and esters damages DNA under physiologic conditions. Studies in rodents demonstrate a lack of oncogenic or carcinogenic effects following a lifetime dietary administration of 2,4-D. Epidemiologic studies provide scant evidence that exposure to 2,4-D is associated with soft tissue sarcoma, non-Hodgkin's lymphoma, Hodgkin's disease, or any other cancer. Overall, the available evidence from epidemiologic studies is not adequate to conclude that any form of cancer is causally associated with 2,4-D exposure. There is no human evidence of adverse reproductive outcomes related to 2,4-D. The available data from animal studies of acute, subchronic, and chronic exposure to 2,4-D, its salts, and esters show an unequivocal lack of systemic toxicity at doses that do not exceed renal clearance mechanisms. There is no evidence that 2,4-D in any of its forms activates or transforms the immune system in animals at any dose. At high doses, 2,4-D damages the liver and kidney and irritates mucous membranes. Although myotonia and alterations in gait and behavioral indices are observed after overwhelming doses of 2,4-D, alterations in the neurologic system of experimental animals are not observed with the administration of doses in the microgram/kg/day range. It is unlikely that 2,4-D has any neurotoxic potential at doses below those required to induce systemic toxicity.

Mechanism of inverted activation of ClC-1 channels caused by a novel myotonia congenita mutation

The voltage-gated chloride channel ClC-1 is the major contributor of membrane conductance in skeletal muscle and has been associated with the inherited muscular disorder myotonia congenita. Here, we report a novel mutation identified in a recessive myotonia congenita family. This mutation, Gly-499 to Arg (G499R) is located in the putative transmembrane domain 10 of the ClC-1 protein. In contrast to normal ClC-1 channels that deactivate upon hyperpolarization, functional expression of G499R ClC-1 yielded a hyperpolarization-activated chloride current when measured in the presence of a high (134 mM) intracellular chloride concentration. Current was abolished when measured with a physiological chloride transmembrane gradient. Electrophysiological analysis of other Gly-499 mutants (G499K, G499Q, and G499E) suggests that the positive charge introduced by the G499R mutation may be responsible for this unique gating behavior. To further explore the function of domain 10, we mutated two charged residues near Gly-499 of ClC-1. Functional analyses of R496Q, R496Q/G499R, R496K, and E500Q mutant channels suggest that the charged residues in domain 10 are important for normal channel function. Study of these mutants may shed further light on the structure and voltage-gating of this channel.

Channelopathies: the nondystrophic myotonias and periodic paralyses

The term channelopathy does not indicate a new group of neuromuscular conditions, but a re-orientation of well- and long-known muscular conditions, the congenital myotonias, and the periodic paralyses. Although, in the past, they have overlapped clinically here and there, both groups were classified differently, as myotonias and as metabolic myopathies, respectively. The discovery of mutations in several ion channels has rewritten nosography of these disorders and procured a new term, the channelopathy-clinical, electrophysiological, and molecular genetic details of which are discussed in this chapter.

Molecular changes in erythrocyte membranes induced by long-term administration of clofibrate

It has been reported that an enantiomer ((S)-(-)-4) of clofibric acid, and the racemate, can block the chloride conductance of skeletal muscle membrane. It has also been reported that several analogs of clofibric acid inhibit the HCO3(-)-Cl- exchange of erythrocytes. Since the two effects are probably similar biophysical membrane phenomena, the possibility of a common molecular mechanism arises. We exposed Sprague-Dawley male rats to long-term administration of clofibrate and 20,25-diazacholesterol (20,25-D) for comparison, at equipotent doses. Clofibrate (but not 20,25-diazacholesterol) produced a significant increase in density of the 220,000 Da band (beta-spectrin) and a decrease, also significant, in density of bands 2.1, 2.2, 2.3, 2.6 (syndeins or ankyrins) and of bands 4.1 and 6. Thus, clofibrate exhibits a manifold effect on the protein profile of the erythrocyte membrane cytoskeleton which, due to the lack of effect of 20-25-D, does not seem to be produced by the hypolipidemic effect per se, and thus deserves further study.

Treatment of myotonia with antiarrhythmic drugs

The effects of disopyramide, phenytoin, mexiletine, and tocainide were compared in 30 patients with myotonic disorders. The severity of myotonia was assessed by clinical and electromyographic criteria at the end of each treatment phase lasting four weeks. Mexiletine (MXT) and tocainide (TCD) were found to be the most potent antimyotonic agents. The antimyotonic efficacy of MXT and TCD is explained by their fast-blocking effect on voltage-dependent sodium channels in the muscle membrane. The benefits of myotonia control with pharmacological agents must be weight against the risk of therapy in the individual patient. Because of the risks of hematologic problems, TCD is not recommended by us for the treatment of myotonia.

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.

Ovine congenital progressive muscular dystrophy: clinical syndrome and distribution of lesions

The distribution and severity of lesions in the skeletal muscles of 37 Merino sheep with congenital progressive muscular dystrophy (CPMD) are described. An explanation for the clinical signs is offered on the basis of functional defects in regional muscle groups. Lesions in the extensors of the hip, stifle and hock joints and flexors of the digits are primarily responsible for the progressive abnormality of hind limb gait that is characteristic of the clinical syndrome. Lesions in extensors of the elbow and flexors of the shoulder, carpus and digits affected fore limb function in advanced cases. The tendency for some affected sheep to develop ruminal tympany is probably caused by lesions in the diaphragmatic crus. Clinically affected sheep had higher resting and post-exercise concentrations of serum creatine phosphokinase and lactic dehydrogenase than unaffected control sheep. The rise in serum creatine phosphokinase after exercise was greater in affected sheep than in controls. Myotonia was not demonstrated in electromyographic studies in one sheep.

Comparison of UV and fluorescence spectrophotometry for the quantification of a potent myotonia inducer: anthracene-9-carboxylic acid, in plasma, urine, and saline perfusion fluids

UV and fluorescence spectrophotometry were used to establish the analytical profile of a potent myotonia inducer, anthracene-9-carboxylic acid (I). UV spectrophotometry is useful for the determination of I when it is dissolved in physiological solutions (Ringer's, Tyrode's, etc). In these fluids there is a linear relationship between UV absorption and I concentration between 500 and 2000 ng/ml (2.25-9.0 X 10(-6)M). However, in biological fluids there are interferences in the UV absorption due to organic substances. On the other hand, fluorescence spectrophotometry is more sensitive than UV for determinations in plasma and urine. Within the range of 200-1000 ng/ml (0.9-4.5 X 10(-6) M) fluorescence intensity increases linearly with concentration. Furthermore, when both emission and excitation spectra are combined there are no interferences due to organic substances normally present in those fluids. An extraction procedure of I from plasma and urine is also described, and the importance of I determinations in relation to the problem of this myotonia-inducing aromatic monocarboxylic acid is discussed.

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.