Increased activity of calcium leak channels in myotubes of Duchenne human and mdx mouse origin

A patch-clamp study of delayed rectifier currents in skeletal muscle of control and mdx mice

1. Potassium currents were measured in the extensor digitorum longus muscle of normal and mdx mice, which lack the protein dystrophin, using the cell-attached and inside-out patch clamp techniques, in the presence of asymmetrical K+ concentrations (3 mM in the pipette, 160 mM in the bath). 2. In cell-attached patches, the delayed rectifier was the most commonly found potassium channel, with a density of roughly 8 channels microns-2. Outward macroscopic currents were activated in macropatches depolarized to potentials positive to -60 mV. The probability of opening reached half-maximal values around -40 mV for control patches and -31 mV for patches from mdx mice. 3. Tail currents were linear in the range between -60 and +20 mV, reversing close to -100 mV. The single channel current at 0 mV, estimated from non-stationary analysis of variance, was used in conjunction with the slope of the linear part of the tail current to calculate the single channel conductance, yielding a value of 19 +/- 1 pS. 4. At 0 mV, the delayed rectifier inactivated with two time constants, of 70 +/- 20 ms and 600 +/- 200 ms. Prepulses of 500 ms duration to different potentials produced incomplete inactivation with inactivation reaching 50% of its maximum at -50 mV. 5. Single channel activity was recorded using small pipettes. Both single channel conductance and kinetic behaviour were in agreement with the macroscopic current data. 6. In excised patches, the delayed rectifier current ran down, unmasking other K+ channels. A Ca(2+)-dependent K+ channel of 186 pS (BK-like channel) was found frequently in patches bathed in solutions containing appropriate concentrations of calcium, especially at stronger depolarizations. A K+ channel of 63 pS was unmasked in control excised patches bathed in solutions devoid of ATP. This channel was not observed in patches excised from mdx fibers.

Modulation by prednisolone of calcium handling in skeletal muscle cells

1. Increased calcium (Ca2+) influx has been incriminated as a potential pathological mechanism in the chronic skeletal muscle degeneration exhibited by Duchenne muscular dystrophy (DMD) patients. We have studied the influence of the glucocorticoid alpha-methylprednisolone (PDN), the only drug known to have a beneficial effect on the degenerative course of DMD, on Ca2+ handling in the C2 skeletal muscle cell line. 2. PDN, when added 3 days (when myoblasts start to fuse into myotubes) after cell seeding, led to a 2 to 4 fold decrease in cellular Ca2+ uptake. This decrease was independent of the extracellular Ca2+ concentration applied to cells. The effect took at least 24 h in order to become established (PDN of 10(-5) M) and took longer for lower PDN concentrations (EC50 of ca. 10(-6) M at day 5, 10(-6.5) M at day 7 and 10(-7.5) M at day 9 in culture). 3. Cellular calcium accumulation was also decreased in PDN-treated myotubes exposed to 45Ca(2+)-containing medium for 1 to 6 days. 4. No effect of PDN was seen on 45Ca2+ efflux; a decrease in the amount of 45Ca2+ released was observed due to the reduction of cellular 45Ca2+ loading. 5. PDN treatment led to an approximately 2 fold decrease in basal cytosolic Ca2+ concentration. 6. Three antioxidant drugs (lazaroids), previously shown to enhance in vitro skeletal muscle cell differentiation to the same extent as PDN, induced a similar decrease in Ca2+ influx. 7. Our results suggest that long-term incubation of C2 cells with PDN leads to a decrease of the size of the cellular Ca2+ pools and to reduced resting cytosolic Ca2+ levels. Part of the beneficial effect of PDN in DMD patients could be attributed to a reduction of Ca2+ influx and of the size of Ca2+ pools in dystrophic muscle fibres.

Ouabain sensitive Na+/K(+)-ATPase content is elevated in mdx mice: implications for the regulation of ions in dystrophic muscle

Recent evidence indicates that in dystrophin-deficient muscle, intracellular sodium content (Na(i)) may be elevated and sodium regulation may be altered or impaired. If there is an elevation in Na(i), this could be due to decreased active pumping of sodium from the cell or increased passive influx of sodium. The present study has therefore determined the content of plasma membrane-bound Na+/K(+)-ATPase in the skeletal muscle of mdx mice; a genetically homologous model of Duchenne muscular dystrophy. Measurements were made on muscles from 5-6-month-old mdx mice and age-matched controls of the C57B1/10ScSn strain (n = 9 pairs), using the vanadate-facilitated ouabain-binding technique. The Na+/K(+)-ATPase concentration per unit weight increased by 2.3-fold in the longissimus dorsi and 1.4-fold in the gastrocnemius of mdx mice compared with controls. The increase in Na+/K(+)-ATPase content is of similar magnitude to the previously reported increase in ouabain-sensitive Na+/K(+)-ATPase activity in mdx muscle, suggesting that this elevated enzyme activity occurs largely through an increase in its concentration. This compensatory increase in the main regulator of internal sodium is likely to occur in an attempt to maintain homeostasis. Nevertheless, the elevated pump concentration is unable to compensate entirely for the increased Na(i). These results are consistent with a previously proposed hypothesis that sodium regulation is abnormal in dystrophin deficient muscles, and also that cell death in these muscles may be due to abnormal regulation of cell volume.

How does dystrophin deficiency lead to muscle degeneration?--evidence from the mdx mouse

The mdx mouse has a defect in the same gene as boys with Duchenne muscular dystrophy, which results in the absence of the protein product, dystrophin. A large number of recent studies have used the mdx mouse model to examine the potential role of dystrophy in normal muscle and the mechanisms by which dystrophin-deficiency leads to myopathy. This review discusses critically the results of these studies and their relevance to understanding the mechanisms by which dystrophin-deficiency leads to muscle necrosis.

Spontaneous opening of the acetylcholine receptor channel in developing muscle cells from normal and dystrophic mice

Single-channel activity was recorded from cell-attached patches on skeletal muscle cells isolated from wild-type mice and from mice carrying the dy or mdx mutations. Spontaneous openings of the nicotinic acetylcholine receptor channel (nAChR) were detected in virtually all recordings from either dy/dy or dy/+ myotubes, but only infrequently from wild-type or mdx myotubes. Spontaneous openings were also present in most recordings from undifferentiated myoblasts from all of the mouse strains studied. The biophysical properties of the spontaneous activity were similar to those of the embryonic form of the nAChR in the presence of acetylcholine (ACh). Examination of the single-channel currents evoked by low concentrations of ACh showed a reduced sensitivity to the agonist in the dystrophic dy and mdx myotubes, but not in wild-type myotubes. The results suggest that alterations in nAChR function are associated with the pathogenesis of muscular dystrophy in the dy mouse.

Abnormal calcium homeostasis in Duchenne muscular dystrophy myotubes contracting in vitro

Resting intracellular calcium activity was recorded in three kinds of human muscle cells in culture: normal (control) and dystrophic (DMD and FSH), by means of a ratiometric fluorescence method using the calcium probe Indo-1 under laser illumination. DMD cells are characterized by a lack of dystrophin whereas FSH cells express normal dystrophin. The aim of this study was to determine whether, in dystrophin-deficient muscle cells (DMD), contraction destabilized internal calcium homeostasis. Muscle cells were cocultured with rat spinal cord explants to improve the maturation of human myotubes up to the stage where contraction appears. The resting intracellular calcium level was significantly higher in contracting DMD cells (107 +/- 8 nM; n = 44) compared to control cells (66 +/- 6 nM; n = 43) or in FSH cells (56 +/- 6 nM; n = 35). DMD myotubes cocultured in the presence of TTX which inhibited contractile activity, did not develop an increase in free cytosolic Ca2+ concentration. The amplitudes of calcium transients elicited by exposure to acetylcholine (ACh) or high K+ medium (100K) were significantly higher in contracting DMD myotubes than in control ones. The extra-responses were not observed in DMD myotubes cocultured with TTX. This study strongly suggest that: (i) contraction is a dominant factor contributing to Ca2+ abnormalities in DMD cells; and (ii) contracting dystrophin-deficient cells have defective calcium handling mechanisms during electrical events which involve sarcolemma.

Dystrophin expression in the hair cells of the cochlea

Dystrophin is normally expressed in a number of tissues including muscle, brain and the outer plexiform layer of the retina. In Duchenne and Becker muscular dystrophy abnormal or deficient dystrophin expression leads to muscle degeneration and has been implicated in mental retardation and a form of night blindness. We have examined the expression of dystrophin immunoreactivity in cochlear tissues of normal guinea-pig and mouse, and whether expression is perturbed in the cochlea of the dystrophic MDX mouse. A single band of approximately 427 kDa, corresponding to a full-length isoform of dystrophin was detected in guinea-pig and normal mouse but was absent from the MDX mouse. Cochleae from guinea-pig, normal and MDX mouse also showed a second dystrophin isoform of 116 kDa molecular weight with the C-terminal specific antibody. Immunostained guinea pig cochlear half turns were examined by laser scanning confocal microscopy. Dystrophin was localized in both inner and outer hair cells with staining patterns which were qualitatively similar with both antibodies. In the outer hair cells labelling of the lateral wall was especially distinctive. The synaptic region of both hair cell types was also strongly labelled.

Extraocular muscles: basic and clinical aspects of structure and function

Although extraocular muscle is perhaps the least understood component of the oculomotor system, these muscles represent the most common site of surgical intervention in the treatment of strabismus and other ocular motility disorders. This review synthesizes information derived from both basic and clinical studies in order to develop a better understanding of how these muscles may respond to surgical or pharmacological interventions and in disease states. In addition, a detailed knowledge of the structural and functional properties of extraocular muscle, that would allow some degree of prediction of the adaptive responses of these muscles, is vital as a basis to guide the development of new treatments for eye movement disorders.

Immunohistochemical study of calpain and its endogenous inhibitor in the skeletal muscle of muscular dystrophy

A calcium-dependent proteinase (calpain) has been suggested to play an important role in muscle degradation in Duchenne muscular dystrophy (DMD). In immunohistochemical studies, calpain and its endogenous inhibitor (calpastatin) were located exclusively in the cytoplasm in normal human muscles. The intensity of the staining was stronger in type 1 than in type 2 fibers. Quantitative immunohistochemical study showed an increase of calpain in biopsied muscles from the patients with DMD and Becker muscular dystrophy. Abnormal increases in calpain and calpastatin were demonstrated mainly in atrophic fibers, whereas necrotic fibers showed moderate or weak immunoreactions for the enzymes. Opaque fibers and hypertrophic fibers were negative. Not all dystrophin-deficient muscle fibers necessarily showed a strong reaction for calpain. We suggest that calpain may play an important role in muscle fiber degradation, especially in the early stage of muscle degradation in muscular dystrophy.

Mechanical function of dystrophin in muscle cells

We have directly measured the contribution of dystrophin to the cortical stiffness of living muscle cells and have demonstrated that lack of dystrophin causes a substantial reduction in stiffness. The inferred molecular structure of dystrophin, its preferential localization underlying the cell surface, and the apparent fragility of muscle cells which lack this protein suggest that dystrophin stabilizes the sarcolemma and protects the myofiber from disruption during contraction. Lacking dystrophin, the muscle cells of persons with Duchenne muscular dystrophy (DMD) are abnormally vulnerable. These facts suggest that muscle cells with dystrophin should be stiffer than similar cells which lack this protein. We have tested this hypothesis by measuring the local stiffness of the membrane skeleton of myotubes cultured from mdx mice and normal controls. Like humans with DMD mdx mice lack dystrophin due to an x-linked mutation and provide a good model for the human disease. Deformability was measured as the resistance to indentation of a small area of the cell surface (to a depth of 1 micron) by a glass probe 1 micron in radius. The stiffness of the membrane skeleton was evaluated as the increment of force (mdyne) per micron of indentation. Normal myotubes with an average stiffness value of 1.23 +/- 0.04 (SE) mdyne/micron were about fourfold stiffer than myotubes cultured from mdx mice (0.34 +/- 0.014 mdyne/micron). We verified by immunofluorescence that both normal and mdx myotubes, which were at a similar developmental stage, expressed sarcomeric myosin, and that dystrophin was detected, diffusely distributed, only in normal, not in mdx myotubes. These results confirm that dystrophin and its associated proteins can reinforce the myotube membrane skeleton by increasing its stiffness and that dystrophin function and, therefore, the efficiency of therapeutic restoration of dystrophin can be assayed through its mechanical effects on muscle cells.

Extent of shock-induced membrane leakage in human and mouse myotubes depends on dystrophin

A lack of the cytoskeletal protein dystrophin causes muscle fiber necrosis in Duchenne/Becker muscular dystrophies (DMD/BMD) and in murine X-linked muscular dystrophy (MDX). However, no overt disease symptoms are observed in dystrophin-less cultured myotubes, and the biological function of dystrophin in normal muscle cells is still unknown. In this work, we have extended our studies on a model system, using hypoosmotic shock to determine stress resistance of muscle cells. In frozen sections of control human and mouse myotubes, dystrophin was shown to be localized at the cell periphery as in mature muscle fibers. Dystrophin-less DMD and MDX myotubes were more susceptible to hypoosmotic shock than controls, as monitored by the uptake of external horseradish peroxidase and release of the soluble enzymes creatinine kinase or pyruvate kinase and of radiolabelled proteins. Control experiments indicated that this difference is not due to differences in metabolism or ion fluxes. Treatment with cytochalasin D drastically increased the shock sensitivity of myotubes and abolished the difference between dystrophin-less and control cells. These results lend further support to the suggested stabilizing role of dystrophin in the context of the membrane-cytoskeletal complex.

Mechanosensitive ion channels in skeletal muscle from normal and dystrophic mice

1. We examined the activity of single mechanosensitive ion channels in recordings from cell-attached patches on myoblasts, differentiated myotubes and acutely isolated skeletal muscle fibres from wild-type and mdx and dy mutant mice. The experiments were concerned with the role of these channels in the pathophysiology of muscular dystrophy. 2. The predominant form of channel activity recorded with physiological saline in the patch electrode arose from an approximately 25 pS mechanosensitive ion channel. Channel activity was similar in undifferentiated myoblasts isolated from all three strains of mice. By contrast, channel activity in mdx myotubes was approximately 3-4 times greater than in either wild-type or dy myotubes and arose from a novel mode of mechanosensitive gating. 3. Single mechanosensitive channels in acutely isolated flexor digitorum brevis fibres had properties indistinguishable from those of muscle cells grown in tissue culture. The channel open probability in mdx fibres was approximately 2 times greater than the activity recorded from wild-type fibres. The overall level of activity in fibres, however, was roughly an order of magnitude smaller than in myoblasts or myotubes. 4. Histological examination of the flexor digitorum brevis fibres from mdx mice showed no evidence of myonecrosis or regenerating fibres, suggesting that the elevated channel activity in dystrophin-deficient muscle precedes the onset of fibre degeneration. 5. An early step in the dystrophic process of the mdx mouse, which leads to pathophysiological Ca2+ entry, may be an alteration in the mechanisms that regulate mechanosensitive ion channel activity.

Subcellular distribution of dystrophin in isolated adult and neonatal cardiac myocytes

The subcellular localization of dystrophin was examined in adult rabbit and rat cardiac myocytes with immunofluorescence and at higher resolution with immunogold. The aim was to resolve the conflicting reports on the presence of dystrophin in the transverse tubules (T tubules) of cardiac muscle and to determine its distribution in neonatal myocytes before and during the development of the T tubules. Dystrophin was localized to the peripheral sarcolemma and the T-tubular membrane and was absent from the intercalated disk membranes. In addition, dystrophin localization was followed with immunofluorescence in developing rabbit myocytes at 4 days, 1 wk, and 1 mo after birth. At 4 days of age, T tubules are absent and dystrophin was localized only in the peripheral sarcolemma. Dystrophin was present in the developing T tubules at 1 wk and 1 mo. These results imply that dystrophin is expressed in the T tubules as soon as they develop and confirm the different distribution of dystrophin in the T tubules of cardiac and skeletal muscle.

Myotubes from transgenic mdx mice expressing full-length dystrophin show normal calcium regulation

A lack of dystrophin results in muscle degeneration in Duchenne muscular dystrophy. Dystrophin-deficient human and mouse muscle cells have higher resting levels of intracellular free calcium ([Ca2+]i) and show a related increase in single-channel open probabilities of calcium leak channels. Elevated [Ca2+]i results in high levels of calcium-dependent proteolysis, which in turn increases calcium leak channel activity. This process could initiate muscle degeneration by further increasing [Ca2+]i and proteolysis in a positive feedback loop. Here, we tested the direct effect of restoration of dystrophin on [Ca2+]i and channel activity in primary myotubes from mdx mice made transgenic for full-length dystrophin. Transgenic mdx mice have been previously shown to have normal dystrophin localization and no muscle degeneration. Fura-2 calcium measurements and single-channel patch recordings showed that resting [Ca2+]i levels and open probabilities of calcium leak channels of transgenic mdx myotubes were similar to normal levels and significantly lower than mdx littermate controls (mdx) that lack dystrophin. Thus, restoration of normal calcium regulation in transgenic mdx mice may underlie the resulting absence of degeneration.

Expression and subcellular localization of dystrophin in skeletal, cardiac and smooth muscles during the human development

Dystrophin, the product of the DMD gene, is present in all muscle types in normal individuals. Its function has yet to be elucidated, but its absence or the presence of a truncated version of the protein is responsible for the appearance of Duchenne and Becker muscular dystrophies. Using monoclonal antibodies raised against distinct regions of the dystrophin protein, we have examined its expression and subcellular distribution during the human development in skeletal and smooth muscles. We show that both dystrophin expression and its association to the plasma membrane take place earlier in cardiac and smooth muscles (8 weeks of gestation) than in skeletal muscle. In skeletal muscle, dystrophin is first detected in the cytoplasm, and progressively localizes to the plasma membrane from 10 weeks onwards. Since we have obtained marked differences in staining when using antibodies against either a central region of the protein or the C-terminal part, we suggest that different fetal and adult dystrophin isoforms are expressed, probably differing in their C-terminal domain. These findings are discussed in the context of the pathology of Duchenne muscular dystrophy.

Reduction of the transient outward potassium current in canine X-linked muscular dystrophy

BACKGROUND

The xmd dog develops a cardiomyopathy similar to that seen in Duchenne muscular dystrophy patients. In both the canine and human diseases, ECG abnormalities may precede the development of overt cardiac pathological lesions. The purpose of this study was to determine whether specific cellular electrical abnormalities occur in dystrophic ventricular tissue.

METHODS AND RESULTS

Action potentials were recorded in epicardial tissue strips obtained from normal and xmd dogs. Phase 1 amplitude was increased from 86.8 +/- 2.7 mV in normal dogs to 94.3 +/- 1.8 mV in xmd dogs (mean +/- SEM; P < .05). The 4-aminopyridine-sensitive transient outward potassium current (Ito), as recorded in isolated epicardial myocytes using the whole-cell patch-clamp technique, was reduced in xmd dogs compared with age-matched normal dogs. Cell capacitance also was reduced significantly in xmd compared with normal cells, as was the current density (3.6 +/- 0.3 versus 5.4 +/- 0.8 pA/pF, respectively). No differences were observed in the time constants of current decay or in the kinetics of recovery from inactivation between groups. The slope factor (k) of steady-state inactivation was significantly greater in xmd compared with normal cells (7.2 +/- 0.9 versus 5.4 +/- 0.5, respectively), whereas the V1/2 of inactivation did not differ (-38.2 +/- 2.4 versus -36.8 +/- 1.6 mV, respectively).

CONCLUSIONS

These data indicate that the magnitude of Ito is reduced in dystrophic epicardial myocytes, resulting in an increase in phase 1 amplitude. The reduction of Ito may alter the balance of inward and outward currents in dystrophic myocardium and thereby contribute to the development of cardiac pathology.

Dystrophin deficiency, altered cell signalling and fibre hypertrophy

Dystrophin is a subsarcolemmal protein which is defective in Duchenne and Becker muscular dystrophy (DMD/BMD), and in three animal models. Clinical manifestations of dystrophin deficiency in humans range from a mild calf muscle hypertrophy with cramps to the classical progressive degenerative hypertrophic myopathy of Duchenne. A common feature in the clinical presentation of dystrophin deficiency in humans and in the three documented animal models is the presence of muscle fibre hypertrophy. This paper explores the hypothesis that membrane-bound signalling processes are disrupted in the absence of dystrophin, and suggests that these abnormalities may contribute to both the hypertrophic and degenerative changes of dystrophin deficiency.

Utrophin localization in normal and dystrophin-deficient heart

BACKGROUND

The localization of dystrophin at the sarcolemma of cardiac skeletal fibers and cardiac Purkinje fibers has been described. Dystrophin deficiency produces clinical manifestations of disease in skeletal muscles and hearts of patients with Duchenne and Becker muscular dystrophy. Utrophin (or dystrophin-related protein), a dystrophin homologous protein, was found to be expressed in fetal muscles and reexpressed in dystrophin-deficient skeletal muscle fibers. We therefore examined utrophin expression in normal and in dystrophin-deficient hearts.

METHODS AND RESULTS

The expression and subcellular distribution of utrophin was examined in cardiac muscle by immunoblot and immunofluorescence analysis in normal bovine heart compared with dystrophin. Utrophin expression was also examined in normal and dystrophin-deficient hearts of MDX mice. Three monoclonal antibodies reacting with dystrophin and utrophin solely or reacting with both proteins along with two polyclonal antibodies reacting with either utrophin or dystrophin and utrophin were tested. In normal bovine heart, utrophin was not expressed at the periphery of fibers but was strongly expressed in intercalated disks and in the cytoplasm of cardiac Purkinje fibers. In cardiocytes, utrophin was colocalized along transverse T tubules with dystrophin. Dystrophin was present at the periphery of cardiocytes and cardiac Purkinje fibers as well as in transverse T tubules but was absent or faintly expressed in intercalated disks. The results with monoclonal and polyclonal antibodies were identical. Western blot analysis revealed that the detected molecules corresponded only to a 400-kD protein band and not to possible shorter transcripts of utrophin or dystrophin (apo-utrophin or apo-dystrophin). In dystrophin-deficient hearts of MDX mice, utrophin alone was abundant but not organized in the same networklike distribution.

CONCLUSIONS

This first localization of utrophin in normal heart (in Purkinje fibers, transverse tubules, and intercalated disks) showed a distinct subcellular localization of this protein with dystrophin, suggesting an important function of this protein in intercellular communication. In dystrophin-deficient hearts of MDX mice, utrophin alone is overexpressed as in skeletal muscle sarcolemma, an area normally occupied by dystrophin but not organized in the same networklike distribution.

Dystrophin-dependent efficiency of metabolic pathways in mouse skeletal muscles

Muscles from the mdx mouse (X-linked genetic disorder similar to Duchenne muscular dystrophy) lack dystrophin-associated transsarcolemmal proteins and show reduced maintenance metabolic rates. Here, microcalorimetric comparisons of metabolic stimulation by exogenous substrates in isolated muscles revealed substrate-selective limitation of chemical reaction rates through both glycolytic and TCA-cycle pathways, identical in slow- and fast-twitch mdx muscles. This systemic approach, as opposed to comparisons of single-enzyme activities, sheds new light on the function of dystrophin and associated proteins. The in vivo efficiency of metabolic pathways may depend on stabilization of enzyme complexes by dystrophin-associated elements of the cytoskeleton.

Intracellular Ca2+ concentrations are not elevated in resting cultured muscle from Duchenne (DMD) patients and in MDX mouse muscle fibres

The free intracellular calcium concentration, [Ca2+]i, was studied in single myotubes using the fluorescent Ca2+ indicator fura-2. Myotubes cultured from satellite cells of small muscle specimens from Duchenne muscular dystrophy (DMD) patients were compared with human control myotubes and with myotubes cultured from MDX and control mouse muscle satellite cells. The resting [Ca2+]i levels in DMD and control myotubes were not significantly different, i.e. 104 +/- 26 nM (mean +/- SD, n = 190 cells from eight DMD patients) compared with 97 +/- 25 nM (175/seven controls) and were not significantly lower than the corresponding murine values (154 +/- 33 nM, n = 135 MDX myotubes; 159 +/- 34 nM, n = 135 controls). All myotubes reacted to 10 microM acetylcholine or 40 mM KCl with fast transient increases of [Ca2+]i. After application of a hyposmotic (130 mOsm) solution, [Ca2+]i was increased 1.5- to 3-fold within 2-3 min, the DMD myotubes tending to stronger reactions (significantly higher [Ca2+]i in 2 out of 6 cases). The response was usually transient, [Ca2+]i decreasing to the initial level within 10 min. Gadolinium (50 microM) reduced the response by 50%-70%, indicating that the osmotic shock increased Ca2+ influx. During exposure to high (15 mM) [Ca2+]e, [Ca2+]i of DMD and control cells was 1.5- to 2-fold higher. Adult muscle fibres from MDX mice and controls showed identical Ca2+ resting levels (n = 45 fibres from three mice in each case), but did not respond to decreased external osmolarity with a change in [Ca2+]i. The results indicate that lack of dystrophin in muscle fibres does not necessarily lead to increased [Ca2+]i.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunohistochemical detection of neural cell adhesion molecule and laminin in X-linked dystrophic dogs and mdx mice

Although dystrophin deficiency is known to be the genetic and biochemical defect causing Duchenne muscular dystrophy (DMD), much remains unknown about the underlying factors affecting clinical and pathological expression of the disease. Two animal forms of muscular dystrophy resembling DMD have been described. Neural cell adhesion molecule (NCAM) and laminin expression were examined in the proliferation-competent mdx mouse and non-regenerative "golden retriever muscular dystrophy dog" (GRMD). The results showed that (1) NCAM expression was greater in dystrophic dogs and mice than in age-matched normal animals, (2) myoblast-specific NCAM was greater in mdx mice than in dystrophic dogs, and (3) laminin strongly labelled mdx and GRMD myofibre membranes but was also sometimes found in individual interstitial cells of mdx muscle. Expression of these proteins may partly determine the clinicopathological expression of dystrophin deficiency.

Dystrophin predominantly localizes to the transverse tubule/Z-line regions of single ventricular myocytes and exhibits distinct associations with the membrane

Dystrophin is a high molecular weight protein present at low abundance in skeletal, cardiac and smooth muscle and in trace amounts in brain. In skeletal muscle, dystrophin is uniformly distributed along the inner surface of the plasma membrane. Biochemical fractionation studies have shown that all detectable skeletal muscle dystrophin is tightly associated with a complex of wheat germ agglutinin (WGA)-binding and concanavalin A (Con A) binding sarcolemmal glycoproteins. Absence of dystrophin is the primary biochemical defect in patients with Duchenne muscular dystrophy and leads to segmental necrosis of their skeletal myofibers. Although present in similar amounts in normal cardiac and skeletal muscle, the absence of dystrophin from cardiac muscle has less severe effects on the survival of cardiac cells. We have therefore examined whether there are differences in the properties of cardiac and skeletal dystrophin. We report that in contrast to skeletal muscle, cardiac dystrophin is distributed between distinct pools: a soluble cytoplasmic pool, a membrane-bound pool not associated with WGA-binding glycoproteins and a membrane-bound pool associated with WGA-binding glycoproteins. Cardiac dystrophin was not associated with any Con A binding glycoproteins. Immunohistochemical localization studies in isolated ventricular myocytes reveal a distinct punctate staining pattern for dystrophin, approximating to the level of the transverse tubule/Z-line and contrasting with the uniform sarcolemmal staining reported for skeletal muscle fibers. The distinct properties of cardiac dystrophin suggest unique roles for this protein in cardiac versus skeletal muscle function.

Further impairment of muscle phosphate kinetics by lengthening exercise in DMD/BMD carriers. An in vivo 31P-NMR spectroscopy study

We used phosphorus magnetic resonance spectroscopy (31P-MRS) to study the effect of exercise-induced muscle injury in the calf muscle of 7 DMD/BMD carriers and 6 non-carrier females. All subjects performed 50-80 lengthening contractions with the right calf muscles. 48 h after lengthening exercise non-carriers showed increased sensitivity to pressure in their gastrocnemius accompanied by increased T2 relaxation times and by elevated Pi/PCr ratios at rest. DMD/BMD carriers did not show any effect of lengthening exercise on these measurements. In-magnet exercise revealed in all carriers a reduced initial rate of Pi recovery and an increased time to fully recovery the resting value of intracellular pH. Lengthening exercise further decreased the initial rate of Pi recovery. Non-carriers did not show any variation attributable to lengthening exercise either during in-magnet work or during recovery from exercise. We found that lengthening exercise contractions causes: (1) less muscle injury in carriers compared to non-carriers, (2) even slower rate of Pi recovery, but (3) no effect on Pi recovery in non-carriers. The use of lengthening exercise and measurements of Pi recovery may be a useful method to evaluate the disease process in DMD/BMD.

Neurohormone D increases the intracellular Ca2+ level in cockroach neurones through a Cd(2+)-sensitive Ca2+ influx

The modulating effect of the octapeptide neurohormone D (NHD) on the intracellular calcium level [Ca2+]i of neurones from the dorsal midline of the cockroach terminal ganglion was investigated with fluorescence measurements. [Ca2+]i of cells loaded with Fura 2 was determined by photon counting and imaging at wavelengths of 340 and 380 nm. After application of NHD, [Ca2+]i increased within 3 min from a value of 93 +/- 36 nM to 153 +/- 51 nM, corresponding to an enhancement to 164 +/- 35%. In Ca(2+)-free solution, [Ca2+]i was lowered (52 +/- 6 nM) and NHD no longer affected the intracellular calcium level. The presence of 0.1 mM Cd2+, in normal saline, prevented the NHD-induced increase of [Ca2+]i. The results were explained by postulating a Ca2+ resting current in these cells which is augmented by NHD.

Membrane potential, resting calcium and calcium transients in isolated muscle fibres from normal and dystrophic mice

1. Single skeletal muscle fibres were enzymatically isolated from the flexor digitorum brevis muscles (FDB) of dystrophic mdx and control C57BL/10 mice aged 3-9 weeks. In this age range the majority (> 95%) of the mdx fibres were morphologically normal. 2. There was no significant difference between the resting membrane potential (RMP) of mdx and control mice, -71.2 +/- 1.21 (n = 26) and -70.6 +/- 1.15 mV (n = 42), respectively. 3. At RMP more negative than -60 mV the resting calcium (recorded with fura-2, free acid ionophoresed into cell) in the dystrophic mdx cells was not significantly different from the normal animals, 45.7 +/- 4.1 (n = 10) and 46.2 +/- 3.9 nM (n = 9), respectively. 4. The resting cytosolic calcium concentration was measured simultaneously with the RMP. At RMP between -60 to -17 mV there was an increase in the resting calcium concentration in both mdx and control ranging from 79.3 to 252 nM. This increase was most probably due to the activation of the slow calcium current. 5. Fura-2 calcium transients were produced via single action potential stimulation using an intracellular microelectrode both to stimulate the cell and record potential changes. There was no significant difference between the rise time (Tp) or half-decay time (T1/2) at 22 degrees C of the calcium transient in response to a single action potential in mdx compared to normal animals, 5.9 +/- 0.34 (n = 8) and 5.4 +/- 0.36 ms (n = 7); 39.5 +/- 2.9 (n = 8) and 40.75 +/- 3.7 ms (n = 7), respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Heterokaryon myotubes with normal mouse and Duchenne nuclei exhibit sarcolemmal dystrophin staining and efficient intracellular free calcium control

Duchenne and mdx muscle tissues lack dystrophin where it normally interacts with glycoproteins in the sarcolemma. Intracellular free calcium ([Ca2+]i) is elevated in Duchenne and mdx myotubes and is correlated with abnormally active calcium-specific leak channels in dystrophic myotubes. We fused Duchenne human and normal mouse myoblasts and identified heterokaryon myotubes by Hoechst 33342 staining to measure the degree to which dystrophin introduced by normal nuclei could incorporate throughout the myotube at the sarcolemma and restore normal calcium homeostasis. Dystrophin expression in myotubes was determined by immunofluorescence and confocal laser scanning microscopy. Dystrophin was expressed at the sarcolemma in normal mouse and heterokaryon myotubes, but not in Duchenne myotubes. In heterokaryons, extensive dystrophin localization occurred at the sarcolemma even where only Duchenne nuclei were present, indicating that dystrophin does not exhibit nuclear domains. Heterokaryon, normal mouse and Duchenne myotube [Ca2+]i was measured using fura-2 and fluorescence ratio imaging. Heterokaryon and normal mouse myotubes were found to maintain similar levels of [Ca2+]i. In contrast, Duchenne myotubes had significantly higher [Ca2+]i (p < 0.001). Furthermore, the ability of heterokaryons to maintain normal [Ca2+]i did not depend on greater numbers of normal nuclei than Duchenne being present in the myotube. These results support the view that dystrophin expression in heterokaryons allows for efficient control of [Ca2+]i.

Adaptations in myosin heavy chain expression and contractile function in dystrophic mouse diaphragm

The X chromosome-linked muscular dystrophic (mdx) mouse lacks the subsarcolemmal protein dystrophin and thus represents a genetic homologue of human Duchenne muscular dystrophy. The present study examined alterations in diaphragm contractile properties and myosin heavy chain (MHC) expression in young (3-4 mo) and old (22-24 mo) control and mdx mice. In young mdx mice, maximum isometric tension (Po) was reduced to 50% of control values. An increase in fibers coexpressing types I (slow) and IIa MHC as well as regenerating fibers expressing embryonic MHC occurred, whereas IIx/b fibers were decreased. In the old mdx group, Po underwent a further reduction to 25% of control, and there was a slowing of twitch kinetics along with markedly increased diaphragm endurance. These changes were associated with an approximate sevenfold increase in type I MHC fibers and virtual elimination of the IIx/b fiber population; there was no detectable embryonic MHC expression. We conclude that the mdx diaphragm responds to progressive muscle degeneration with transition to a slower phenotype associated with reduced power output and augmented muscle endurance. In the setting of progressive muscle fiber destruction, these changes may help preserve contractile function and promote greater survival of remaining muscle fibers by decreasing cellular energy requirements.

Changes in cytosolic resting ionized calcium level and in calcium transients during in vitro development of normal and Duchenne muscular dystrophy cultured skeletal muscle measured by laser cytofluorimetry using indo-1

Intracellular calcium activity was recorded during in vitro myogenesis of human normal and DMD muscle, using the calcium probe Indo-1 under laser illumination, at rest and during different kinds of stimulation (acetylcholine, high K+, caffeine). In myoblasts, the resting intracellular calcium level was significantly larger in DMD cells (89 +/- 9 nM; n = 40 vs 37 +/- 5 nM; n = 22) but there was no significant difference in myotubes, after fusion (44 +/- 4 nM; n = 34 vs 36 +/- 4 nM; n = 52). A similar evolution was observed in cells cultured from FSH biopsies. The amplitude of ACh- and high K(+)-induced calcium transients was significantly halved in DMD myotubes as compared to control ones and non-significantly decreased for caffeine responses. Some alterations in the kinetics of responses were observed in DMD muscle: the rising phases of ACh- and high K(+)-elicited transients and the decaying phase of the ACh-responses were significantly slowed down. It is concluded that: (i) in aneurally cultured human muscle, an increase in the basal level of internal calcium can occur at early stages of myogenesis before the expression of the dystrophin gene; and (ii) the changes in calcium transients induced by depolarization or direct stimulation of sarcoplasmic reticulum are not susceptible of inducing a calcium overload in DMD cells.

Prednisolone enhances myogenesis and dystrophin-related protein in skeletal muscle cell cultures from mdx mouse

The differentiation of skeletal muscle cells from mdx mice which lack dystrophin expression was examined after glucocorticoid treatment, namely alpha-methylprednisolone (PDN). Primary skeletal muscle cell cultures were established from newborn mdx, congenic C57BL/10, and allogenic BALB/C mice. We show that PDN promotes the myogenesis of both mdx- and control mice-derived cultures as determined by 1) the number of myotubes, 2) acetylcholine receptors, and 3) dystrophin and dystrophin-related protein levels. These results support the hypothesis that PDN could enhance the myogenesis of satellite cells and increase dystrophin-related protein expression in DMD treated patients.

Critical evaluation of cytosolic calcium determination in resting muscle fibres from normal and dystrophic (mdx) mice

The fluorescent probe Fura-2/AM was used to determine cytosolic free calcium concentration in soleus muscle and in isolated flexor digitorum brevis fibres. This required a precise calibration; therefore, each calibration parameter was studied in situ. The influence of the dye concentration on calcium measurements was also examined. This precise calibration technique was used to compare absolute free calcium concentration in resting preparations from dystrophic (mdx) and control (C57) mice. We showed that the behavior of the dye was not similar in C57 and in mdx muscles. For this reason, we did not confirm the previous results that cytosolic free calcium concentration is increased in mdx muscles.

Dystrophin and dystrophin-related protein expression in Torpedo marmorata electric organ

The presence of different dystrophin-related protein forms was investigated in electric organ as compared to cardiac, white or red skeletal muscles from Torpedo marmorata. Two strategies were followed. First, we used specific C-terminal dystrophin and dystrophin-related protein monoclonal antibodies which we characterized in the present study. 400 kDa protein bands were detected in the tissues mentioned above with both specific types of antibodies. Second, we produced monoclonal antibodies raised against a dystrophin-enriched preparation from T. marmorata electric organ. Western blot and immunofluorescence analyses showed the tissue specificity of T. marmorata antibodies and allowed us to classify them as types I, II and III. Vessel walls and neuromuscular junctions were labeled with T. marmorata type II and III antibodies in human muscles (skeletal and smooth). Both approaches demonstrated that the T. marmorata electric organ contained different proteins related with dystrophin: a dystrophin form, a dystrophin-related protein form and a dystrophin-related protein isoform, homologous to the dystrophin-related protein present in muscle vessel walls and at the neuromuscular junctions of human tissues. The presence of dystrophin and dystrophin-related protein is finally discussed relative to their functions and organ specificities.

New mdx mutation disrupts expression of muscle and nonmuscle isoforms of dystrophin

The dystrophin gene encodes several tissue-specific protein isoforms that are generated by alternative splicing and by transcription from at least three separate promoters. We have characterized the mutation in a new strain of mdx mice that results in aberrant splicing of both the 14 and 4.8 kilobase dystrophin mRNAs and disrupts expression of the muscle and brain 427K and nonmuscle 70K isoforms of dystrophin. In contrast, we have determined that expression of the 70K isoform is normal in the original mdx mutant. We have cloned the unique 5' exon of the murine 4.8 kb mRNA and have analysed the tissue distribution and aberrant splicing of this transcript in the mdx3Cv mutant. This new mdx mutant will provide an improved model system for functional studies of the dystrophin C-terminus in muscle and nonmuscle tissues.

Proteolysis results in altered leak channel kinetics and elevated free calcium in mdx muscle

Resting free calcium levels ([Ca2+]i) are elevated in Duchenne human myotubes and mdx mouse muscle and myotubes which lack the gene product dystrophin at the sarcolemma. Increased net muscle protein degradation has been directly related to this elevated [Ca2+]i. The [Ca2+]i rise may result from increased calcium influx via leak channels, which have increased opening probabilities (Po) in dystrophic cells. Dystrophin, therefore, might directly regulate leak channel activity. In intact mdx soleus muscles, protein degradation was reduced to normal levels by leupeptin, a thiol protease inhibitor. In muscle homogenates, leupeptin also abolished calcium-induced increases in protein degradation. When mouse myotubes were cultured in the continuous presence of leupeptin (50 microM), the elevation in mdx resting [Ca2+]i was prevented. Leak channel Po increased with age in mdx myotubes, whereas leupeptin-treated mdx leak channel opening probabilities were always lower or equal to the Po for untreated normal myotubes. These results indicate that increased leak channel activity in dystrophic muscle results in elevated [Ca2+]i levels, but also suggest that dystrophin does not directly regulate channel activity. Instead the results suggest that proteolysis may be responsible for the altered gating of calcium leak channels. The resultant increased channel Po in turn elevates [Ca2+]i, which further increases proteolytic activity in a positive feedback loop, leading to the eventual necrosis of the muscle fibers.

Dystrophin protects the sarcolemma from stresses developed during muscle contraction

The protein dystrophin, normally found on the cytoplasmic surface of skeletal muscle cell membranes, is absent in patients with Duchenne muscular dystrophy as well as mdx (X-linked muscular dystrophy) mice. Although its primary structure has been determined, the precise functional role of dystrophin remains the subject of speculation. In the present study, we demonstrate that dystrophin-deficient muscle fibers of the mdx mouse exhibit an increased susceptibility to contraction-induced sarcolemmal rupture. The level of sarcolemmal damage is directly correlated with the magnitude of mechanical stress placed upon the membrane during contraction rather than the number of activations of the muscle. These findings strongly support the proposition that the primary function of dystrophin is to provide mechanical reinforcement to the sarcolemma and thereby protect it from the membrane stresses developed during muscle contraction. Furthermore, the methodology used in this study should prove useful in assessing the efficacy of dystrophin gene therapy in the mdx mouse.

The structural and functional diversity of dystrophin

Duchenne and Becker muscular dystrophies are caused by defects of the dystrophin gene. Expression of this large X-linked gene is under elaborate transcriptional and splicing control. At least five independent promoters specify the transcription of their respective alternative first exons in a cell-specific and developmentally controlled manner. Three promoters express full-length dystrophin, while two promoters near the C terminus express the last domains in a mutually exclusive manner. Six exons of the C terminus are alternatively spliced, giving rise to several alternative forms. Genetic, biochemical and anatomical studies of dystrophin suggest that a number of distinct functions are subserved by its great structural diversity. Extensive studies of dystrophin may lead to an understanding of the cause and perhaps a rational treatment for muscular dystrophy.

Intracellular calcium transients induced by different kinds of stimulus during myogenesis of rat skeletal muscle cells studied by laser cytofluorimetry with Indo-1

Resting intracellular calcium levels and intracellular calcium transients induced by three types of stimulus (acetylcholine, high potassium and caffeine) were recorded, during in vitro myogenesis, by means of a ratiometric fluorescence method using the calcium probe Indo-1 under laser illumination. Resting levels seemed to decrease with the age of cultured cells and the depolarization-induced transients, through 100 mM K+ or Ach application, were progressively faster and larger as the muscle cells developed. An additive mechanism, likely due to calcium entry into the cell through nicotinic acetylcholine receptors, could explain the differences observed in Ach-induced responses as compared with the 100 mM K(+)-induced ones. In myoballs (the older cells) the calcium transients exhibited progressively a biphasic shape. From data obtained in different conditions (tetrodotoxin, nifedipine, strontium and free Ca EGTA) and those indicating the appearance of caffeine-releasable intracellular calcium stores only at 2-3 days stage, and from the previously reported developmental appearance of calcium currents and contraction, it was proposed that, in young myotubes, the calcium transients were more dependent on extracellular calcium than in older cells. These developmental data are discussed in the light of a known model of the in situ biogenesis of the structures involved in excitation-contraction coupling (ECC) like transverse tubules and triads.

Complementary distributions of vinculin and dystrophin define two distinct sarcolemma domains in smooth muscle

The sarcolemma of the smooth muscle cell displays two alternating structural domains in the electron microscope: densely-staining plaques that correspond to the adherens junctions and intervening uncoated regions which are rich in membrane invaginations, or caveolae. The adherens junctions serve as membrane anchorage sites for the actin cytoskeleton and are typically marked by antibodies to vinculin. We show here by immunofluorescence and immunoelectron microscopy that dystrophin is specifically localized in the caveolae-rich domains of the smooth muscle sarcolemma, together with the caveolae-associated molecule caveolin. Additional labeling experiments revealed that beta 1 integrin and fibronectin are confined to the adherens junctions, as indicated by their codistribution with vinculin and tensin. Laminin, on the other hand, is distributed around the entire cell perimeter. The sarcolemma of the smooth muscle cell is thus divided into two distinct domains, featuring different and mutually exclusive components. This simple bipartite domain organization contrasts with the more complex organization of the skeletal muscle sarcolemma: smooth muscle thus offers itself as a useful system for localizing, among other components, potential interacting partners of dystrophin.

Refilling the inositol 1,4,5-trisphosphate-sensitive Ca2+ store in neuroblastoma x glioma hybrid NG108-15 cells

Bradykinin-induced increases in the intracellular free Ca2+ concentration ([Ca2+]i) were recorded in single NG108-15 cells with indo-1-based dual-emission microfluorimetry (50% effective concentration, 16 nM). A 1-min exposure to 30 nM bradykinin completely depleted the inositol 1,4,5-trisphosphate (IP3)-sensitive Ca2+ store; refilling the store required extracellular Ca2+ (half time, 2 min). Refilling the IP3-sensitive store was completely blocked by 1 microM La3+ and 10 microM nitrendipine, but not 10 microM verapamil, 10 microM flunarizine, 1 microM nitrendipine, or 0.1 microM La3+. Thapsigargin irreversibly depleted the Ca2+ store and prevented its refilling (half-maximal inhibitory concentration, 3 nM). Influx of Ca2+ across the plasma membrane did not increase after depletion of the IP3-sensitive store by exposure to bradykinin, although maintained presence of the agonist produced significant Ca2+ influx. Similarly, Mn2+ and Ba2+ influx, as measured by indo-1 quenching and spectral shifts, did not increase following depletion of IP3-sensitive store. In contrast to depletion of the IP3-sensitive Ca2+ store by bradykinin, thapsigargin (10 nM) treatment produced Ca2+ and Ba2+ influx. We conclude that after Ca2+ mobilization, the IP3-sensitive Ca2+ store in NG108-15 cells is refilled with cytoplasmic Ca2+ via a thapsigargin-sensitive Ca(2+)-Mg(2+)-ATPase. Cytoplasmic Ca2+ is replenished by a persistent leak of Ca2+ across the plasma membrane. This leak is not modulated by the status of the intracellular Ca2+ store. In NG108-15 cells, agonist and thapsigargin-evoked Ca2+ entry are mediated by activation of plasmalemmal Ca2+ channels independent of the status of the IP3-sensitive intracellular Ca2+ store.

Lack of dystrophin but normal calcium homeostasis in smooth muscle from dystrophic mdx mice

The free cytosolic Ca2+ concentration ([Ca2+]i) in the dystrophin-lacking smooth muscle from mdx mice was studied to gain new insights into the relation between dystrophin and cytoplasmic Ca2+ homeostasis, which was reported to be impaired in the mdx skeletal muscle. We observed that [Ca2+]i, as measured with the fluorescent Ca2+ indicator fura-2, was not elevated in resting smooth muscle of the vas deferens from mdx mice, in comparison with control C57 mice. Changes of the external Ca2+ concentration evoked similar changes of [Ca2+]i in mdx and control vas deferens. During contraction, cytosolic Ca2+ transients were identical, both in amplitude and in kinetics, whether or not dystrophin was present. Stretches evoked similar Ca2+ increases in muscles from both strains. Intracellular Ca2+ homeostasis appears to be unimpaired in mdx smooth muscle. Thus, the lack of dystrophin per se does not automatically induce a perturbation of Ca metabolism in muscle cells.

Sodium is elevated in mdx muscles: ionic interactions in dystrophic cells

Recent evidence suggests that cellular sodium regulation may be abnormal in muscular dystrophy. We have measured intracellular sodium concentration (Nai) in muscles of mdx mice (a model of Duchenne muscular dystrophy) using two techniques. Nai in isolated diaphragm (measured using a microelectrode) was 13.0 +/- 0.3 mM and 23.5 +/- 0.7 mM (mean +/- SE) in the control and mdx mice respectively. Nai in gastrocnemius muscle (calculated from extra- and intracellular volumes using serum and whole-muscle sodium concentrations) was 13 +/- 3 mM and 24 +/- 2 mM (mean +/- SE) in control and mdx, respectively. We argue that this abnormality in mdx tissues could reflect a reduced flux through the Na/K ATPase, although a contribution from increased Na leak cannot be ruled out. We also discuss possible consequences of an increased Nai: for example, raised Nai may lead to defective cell volume control in Duchenne dystrophy and the mdx mouse.

Ca2+ levels in myotubes grown from the skeletal muscle of dystrophic (mdx) and normal mice

1. Myotubes were grown in culture from normal (C57BL/ScSn) and mdx mice and the cytosolic [Ca2+] was monitored through development (5-21 days in culture) using fura-2 loaded via ionophoresis. Simultaneous measurements of the membrane potential and cytosolic [Ca2+] were made in normal and mdx myotubes before, during and after stimulation by action potentials elicited following anode break excitation. All experiments were undertaken at 22 degrees C. All data are expressed as means +/- S.E.M. 2. A new method was developed which enabled accurate determination of the fluorescence characteristics of fura-2 in murine skeletal muscle fibres. In the under in vitro conditions by 14.60 +/- 0.05, 9.40 +/- 0.15 and 6.90 +/- 0.43% respectively. 3. The resting cytosolic [Ca2+] in the mdx myotubes was consistently higher than in the normal myotubes throughout the developmental period measured. Overall, the resting cytosolic [Ca2+] in mdx myotubes (134 +/- 9 nM, n = 22) was twofold higher than in normal myotubes (66 +/- 6 nM, n = 26). After stimulation (one to three action potentials) the cytosolic [Ca2+] of both mdx and normal myotubes remained elevated. The mdx myotubes (236 +/- 55 nM, n = 5) again had approximately double the cytosolic [Ca2+] of normal myotubes (109 +/- 19 nM, n = 9). 4. The time course and amplitude of the Ca2+ responses measured in the mdx and normal myotubes after action potential stimulation were variable. Two categories of Ca2+ response were observed in mdx and normal myotubes, the first consisted of a small, slow rise in [Ca2+] that remained elevated and the second consisted of a rapid (time to peak 7.4 +/- 1.5 ms) (n = 8) rise in [Ca2+] with amplitudes in the range 61-773 nM and a [Ca2+] decay rate constant of 4.35 +/- 1.57 s-1 (n = 8) (range 0.96-15 s-1). 5. In conclusion, the elevated cytosolic [Ca2+] reported here through development of cultured mdx myotubes suggests that this genetic disorder results in a defect which compromises the ability of the myotubes to strictly regulate cytosolic [Ca2+]. The results are consistent with the presence of functionally abnormal Ca2+ channels recently reported in mdx myotubes.

A rapid preparation of primary cultures of mouse skeletal muscle cells

We describe a rapid and reproducible technique for establishing primary cultures of skeletal muscle cells from mouse origin. This method was aimed at avoiding extensive enzymatic proteolysis which is commonly used for preparation of primary skeletal muscle cultures. It relies on a Stomacher blender that allows a rapid and regular mechanical dissociation of muscle samples by repeated shocks. Cultures have been compared to those obtained by a modification of the method of Yaffé (1993) based on tryptic dissociation of rat muscle thighs. The time of preparation was reduced to 1 h and 15 min as compared to 4 h with the technique of Yaffé. Both cultures displayed similar morphologies and exhibited comparable myogenesis processes. Cellular yield, rate of myotube formation and myotube numbers were similar. The expression of myogenesis markers were identical as assessed by determination of acetylcholine receptor number, creatine kinase activity and level of myosin light chain.