Calcium currents in normal and dystrophic human skeletal muscle cells in culture

Regulation of TRPC1 and TRPC4 cation channels requires an alpha1-syntrophin-dependent complex in skeletal mouse myotubes

The dystrophin-associated protein complex (DAPC) is essential for skeletal muscle, and the lack of dystrophin in Duchenne muscular dystrophy results in a reduction of DAPC components such as syntrophins and in fiber necrosis. By anchoring various molecules, the syntrophins may confer a role in cell signaling to the DAPC. Calcium disorders and abnormally elevated cation influx in dystrophic muscle cells have suggested that the DAPC regulates some sarcolemmal cationic channels. We demonstrated previously that mini-dystrophin and alpha1-syntrophin restore normal cation entry in dystrophin-deficient myotubes and that sarcolemmal TRPC1 channels associate with dystrophin and the bound PDZ domain of alpha1-syntrophin. This study shows that small interfering RNA (siRNA) silencing of alpha1-syntrophin dysregulated cation influx in myotubes. Moreover, deletion of the PDZ-containing domain prevented restoration of normal cation entry by alpha1-syntrophin transfection in dystrophin-deficient myotubes. TRPC1 and TRPC4 channels are expressed at the sarcolemma of muscle cells; forced expression or siRNA silencing showed that cation influx regulated by alpha1-syntrophin is supported by TRPC1 and TRPC4. A molecular association was found between TRPC1 and TRPC4 channels and the alpha1-syntrophin-dystrophin complex. TRPC1 and TRPC4 channels may form sarcolemmal channels anchored to the DAPC, and alpha1-syntrophin is necessary to maintain the normal regulation of TRPC-supported cation entry in skeletal muscle. Cation channels with DAPC form a signaling complex that modulates cation entry and may be crucial for normal calcium homeostasis in skeletal muscles.

Calcium current kinetics in young and aged human cultured myotubes

There is evidence that the complex process of sarcopenia in human aged skeletal muscle is linked to the modification of mechanisms controlling Ca(2+) homeostasis. To further clarify this issue, we assessed the changes in the kinetics of activation and inactivation of T- and L-type Ca(2+) currents in in vitro differentiated human myotubes, derived from satellite cells of healthy donors aged 2, 12, 76 and 86 years. The results showed an age-related decrease in the occurrence of T- and L-type currents. Moreover, significant age-dependent alterations were found in L-(but not T) type current density, and activation and inactivation kinetics, although an interesting alteration in the kinetics of T-current inactivation was observed. The T- and L-type Ca(2+) currents play a crucial role in regulating Ca(2+) entry during satellite cells differentiation and fusion into myotubes. Also, the L-type Ca(2+) channels underlie the skeletal muscle excitation-contraction coupling mechanism. Thus, our results support the hypothesis that the aging process could negatively affect the Ca(2+) homeostasis of these cells, by altering Ca(2+) entry through T- and L-type Ca(2+) channels, thereby putting a strain on the ability of human satellite cells to regenerate skeletal muscle in elderly people.

Membrane properties of single muscle cells of the rhabdosphincter of the male urethra

BACKGROUND

The electrophysiological properties of myoblast cultures established from the human and porcine rhabdosphincter (RS) and porcine lower limb muscle (LLSKM) were studied to elucidate their potential for tissue engineering applications in the lower urinary tract.

METHODS

Muscle biopsies were collected from the prostatic part of the RS, the RS of male pigs, and the porcine LLSKM. Ion channels were studied by means of the patch-clamp technique.

RESULTS

Only one subtype each of voltage gated Na+ and Ca2+ channels was observed in porcine RS and LLSKM. Two types of voltage gated Ca2+ channels were identified in human RS cells. The porcine RS and LLSKM myoblasts displayed similar fusion competence.

CONCLUSIONS

Porcine RS and LLSKM myoblasts and human RS and human skeletal muscle cells show a high degree of similarity. Injection of autologous skeletal muscle myoboblasts in the lower urinary tract might, therefore, represent a promising approach to treat stress incontinence after radical prostatectomy.

Normal calcium homeostasis in dystrophin-expressing facioscapulohumeral muscular dystrophy myotubes

The aim of this study was to provide a set of data on mechanisms involved in the calcium homeostasis of facioscapulohumeral muscular dystrophy (FSHD) co-cultured myotubes. In fact, abnormal regulation of calcium have been shown in deficient dystrophin cells like Duchenne muscular dystrophy (DMD) cells, and it seemed interesting to study the calcium regulation in a pathologic cellular model which express dystrophin. T- and L-type calcium currents and contractile responses induced by membrane depolarisations as well as intracellular calcium transients induced by three kinds of stimulus (superfusions of acetylcholine, high K+ or caffeine containing media) were recorded by means of whole-cell patch-clamp and ratiometric cytofluorimetry in co-cultured FSHD myotubes which presented a sarcolemmal localisation of dystrophin. As judged from calcium currents properties, voltage-dependency of contractile responses or amplitude of evoked calcium transients, no clear difference in the calcium handling or calcium signalling was observed between this type of cell and the control cells, at least with the means and the conditions used in the present study. Since FSHD cells, contrary to DMD (Duchenne muscular dystrophy) cells, seemed to display both dystrophin expression and unaltered calcium regulation, the FSHD co-cultured cells appeared as a useful model of dystrophin-expressing pathological muscle cells to further investigate the link between dystrophin expression and intracellular calcium level regulation.

Calcium currents and transients in co-cultured contracting normal and Duchenne muscular dystrophy human myotubes

1. The goal of the present study was to investigate differences in calcium movements between normal and Duchenne muscular dystrophy (DMD) human contracting myotubes co-cultured with explants of rat spinal cord with attached dorsal root ganglia. Membrane potential, variations of intracellular calcium concentration and T- and L-type calcium currents were recorded. Further, a descriptive and quantitative study by electron microscopy of the ultrastructure of the co-cultures was carried out. 2. The resting membrane potential was slightly less negative in DMD (-61.4 +/- 1.1 mV) than in normal myotubes (-65.5 +/- 0.9 mV). Both types of myotube displayed spontaneous action potentials (mean firing frequency, 0.42 and 0.16 Hz, respectively), which triggered spontaneous calcium transients measured with Indo-1. 3. The time integral under the spontaneous Ca(2+) transients was significantly greater in DMD myotubes (97 +/- 8 nM s) than in normal myotubes (67 +/- 13 nM s). 4. The L- and T-type current densities estimated from patch-clamp recordings were smaller in DMD cells (2.0 +/- 0.5 and 0.90 +/- 0.19 pA pF(-1), respectively) than in normal cells (3.9 +/- 0.7 and 1.39 +/- 0.30 pA pF(-1), respectively). 5. The voltage-dependent inactivation relationships revealed a shift in the conditioning potential at which inactivation is half-maximal (V(h,0.5)) of the T- and L-type currents towards less negative potentials, from -72.1 +/- 0.7 and -53.7 +/- 1.5 mV in normal cells to -61.9 +/- 1.4 and -29.2 +/- 1.4 mV in DMD cells, respectively. 6. Both descriptive and quantitative studies by electron microscopy suggested a more advanced development of DMD myotubes as compared to normal ones. This conclusion was supported by the significantly larger capacitance of the DMD myotubes (408 +/- 45 pF) than of the normal myotubes (299 +/- 34 pF) of the same apparent size. 7. Taken together, these results show that differences in T- and L-type calcium currents between normal and DMD myotubes cannot simply explain all observed alterations in calcium homeostasis in DMD myotubes, thus suggesting that other transmembrane calcium transport mechanisms must also be altered in DMD myotubes compared with normal myotubes.

Cationic channels in normal and dystrophic human myotubes

Human skeletal muscle cells obtained from normal and Duchenne muscular dystrophy patients were cocultured with explants of rat dorsal root ganglions. Single-channel recordings were performed with the cell-attached configuration of the patch-clamp technique and negative pressure was applied via the patch-pipette in order to mechanically stimulate the membrane patch. Inward elementary current activity was recorded under control or negative pressure conditions. Its occurrence and mean open probability were higher in Duchenne muscular dystrophy. Amplitude histograms reveal that these channels have a small unitary conductance of around 10 pS in 110 mM Ca2+ and could be inhibited in a dose-dependent manner by gadolinium. Results show that the membrane stress favoured calcium permeation through these channels. Taken together these data provide arguments for the involvement of such channels in calcium overload previously observed in cocultured dystrophic human (Duchenne muscular dystrophy) muscle cells.

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

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

L-type calcium current activation in cultured human myotubes

The time course of activation of the skeletal muscle L-type calcium channel was studied in voltage-clamped myotubes derived from human satellite cells. The slow L-type current was isolated by inactivating faster calcium current components using appropriate prepulses or by subtracting the currents not blocked by 5 microM nifedipine. The L-type current exhibited a single exponential activation and time constants which showed little voltage dependence in the range +10 to +50mV. Currents blocked by nifedipine could be partially restored by UV-light flash photolysis. When a flash of light was applied during a depolarizing step, the activation time course of the resulting inward current contained a rapid, almost instantaneous component followed by a slower component. The amplitude of the rapid component was different when the flash was applied at different times during the depolarizing step: depolarization first increased and then decreased the fraction of channels which could rapidly be restored from the block by photolysis. Plotted versus time after the onset of the depolarization this fraction closely matched the time course of the L-type current obtained before the block by nifedipine. This indicates that the slow gating recations of the Ca2+ channel remain functional in the nifedipine-blocked state. Large conditioning depolarizations which had been shown to enhance the speed of L-type current activation in frog muscle fibres showed no effect in human myotubes. Numerical simulations using a gating scheme proposed for frog muscle demonstrate that such differences can be caused by changing just a single kinetic parameter.

Excitation-contraction coupling of cultured human skeletal muscle cells and the relation between basal cytosolic Ca2+ and excitability

Cultured human skeletal muscle cells are frequently used as a model to study muscle pathology, in which Ca2+ homeostasis might be affected. However, their excitation-contraction (E-C) coupling has been poorly investigated. In order to elucidate E-C coupling of cultured muscle cells, we activated the acetylcholine receptors, voltage-dependent Na+ channels, dihydropyridine receptors or ryanodine receptors both in the presence and absence of external Ca2+, as well as after specific inhibition, and measured the effects on the cytosolic Ca2+ concentration ([Ca2+]i) using Fura-2. Furthermore, we examined the excitability of these cells during iterative high (125 mM) K+ stimulation with various repolarisation intervals. The resting [Ca2+]i in muscle cells of controls is about 130 nM. Acetylcholine, veratridine, high K+ and caffeine elicit dose-dependent Ca2+ transients, which are independent of extracellular Ca2+ and can be inhibited by alpha-bungarotoxin, tetrodotoxin, nifedipine or ryanodine. During repetitive K+ stimulation, the excitability of the muscle cells depends on the repolarisation interval between successive stimulations. Upon shortening the repolarisation time the Ca2+ transients become smaller and slower. Thereby, the basal [Ca2+]i rises, the Ca2+ response amplitude declines and both the half-increase and half-decay time increase. However, if the basal [Ca2+]i equals the resting [Ca2+]i the initial Ca2+ response can be recovered. The intracellular pH of 7.23, measured by BCECF, is unaffected by repeated K+ stimulation, whatever the repolarisation interval was. In conclusion, cultured human skeletal muscle cells possess a 'skeletal muscle type' of E-C coupling and their excitability at iterative stimulation is set by their basal [Ca2+]i.

Hypoosmotic shocks induce elevation of resting calcium level in Duchenne muscular dystrophy myotubes contracting in vitro

In Duchenne muscular dystrophy (DMD) muscle cells which lack dystrophin, contraction seems to be a dominant factor contributing to the abnormal elevated intracellular calcium level. Human normal and DMD contracting myotubes cocultured with nervous cells were exposed to a hypotonic medium to mimic contraction-induced mechanical stress on the membrane, and the cytoplasmic calcium activity was simultaneously monitored (Indo-1). Hypotonic shocks induced a reversible [Ca2+]i increase in 81% of the DMD cells vs. 54% of control. In addition, responses were qualitatively different: most of DMD myotubes displayed a fast increase of Ca2+ flowing from the edge of the myotube while the response in normal cells was slow and diffuse. The fact that these responses were not affected by ryanodine, was in favour of an external source of Ca2+ involved in the hypoosmotic shocks. The localized increase of Ca2+ in DMD myotubes, inhibited by Gd3+, could result from sites of high mechanosensitive channel activity or density which could constitute a pathway for Ca2+ entry provided these cells contract.

Properties of calcium currents and contraction in cultured rat diaphragm muscle

The characterization of calcium currents and contraction simultaneously measured in cultured rat diaphragm muscle cells was carried out in the present study. Whole-cell patch-clamp experiments were designed to further elucidate the mechanism of excitation-contraction (E-C) coupling in diaphragm which, though generally considered a skeletal-type muscle, has been reported to exhibit properties indicative of a cardiac-like E-C coupling mechanism. Normalized current/voltage (I/V) curves for two concentrations of external calcium (2.5 and 5 mM) were obtained from diaphragm myoballs. Both curves showed peaks corresponding to the activation of a T-type calcium current and a dihydropyridine-sensitive L-type calcium current. The normalized curve for the voltage dependence of the activation of contraction in diaphragm myoballs followed a typical Boltzmann-type relationship to the peak of contraction. Thereafter, the curve declined in a manner that was more pronounced in diaphragm compared to that measured in additional experiments using cultured rat limb muscle myoballs. This effect could be interpreted in terms of a more pronounced participation of the L-type current in E-C coupling in cultured diaphragm muscle. An increased likelihood of cultured diaphragm muscle to undergo depletion of sarcoplasmic reticular calcium stores during repetitive stimulation, or a heightened propensity for the voltage sensor for E-C coupling in diaphragm to enter the inactive state could also explain this effect. Maximal contractile activity was only slightly affected when the L-type current was blocked by externally applied cadmium (2 mM) or cobalt (3 mM), suggesting that a pronounced calcium-current-dependent component of contraction is unlikely in cultured diaphragm muscle. These results show that T- and L-type calcium channels are expressed in cultured rat diaphragm muscle cells and that, in contrast to cardiac muscle, the entry of calcium ions via L-type voltage-dependent calcium channels is not a prerequisite for contraction. Differences in the voltage sensitivity of contraction, observed at depolarized membrane potentials in cultured rat diaphragm and limb muscle cells, suggest that the voltage sensor for E-C coupling in diaphragm might more readily enter an inactivated configuration - possibly by a mechanism which is dependent on the concentration of external calcium.

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.

Ca2+ channel activities in the limb bud of early embryonic chick

Ca2+ channel activities were recorded in the limb bud of embryonic day 4 chick with Ca2+ sensitive fluorescence (Fura-2) measurements and patch clamp techniques. Rises in intracellular Ca2+ concentrations were evoked by depolarization with the application of 100 mM K+ and this Ca2+ response was abolished by removing extracellular Ca2+. The Ca2+ response was blocked by 10 microM nifedipine and enhanced by 5 microM Bay K 8644. Long-lasting inward currents were revealed by whole-cell patch clamp recordings from dissociated cells of the limb bud. The inward current was also blocked by 10 microM nifedipine. Our study suggested the presence of L-type Ca2+ channels in the limb bud cells.

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.

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)

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.

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.

Appearance and evolution of calcium currents and contraction during the early post-fusional stages of rat skeletal muscle cells developing in primary culture

Primary cultures from enzymatically dissociated satellite cells of newborn rat skeletal muscles enabled developmental in vitro studies of mechanical and electrical properties during the first steps of myogenesis. The present work focused on the appearance, evolution and roles of two types of calcium currents (ICa,T and ICa,L) and of depolarization-induced contractile activity during the early stages of muscle cell development in primary culture. Prefusional mononucleated cells (myoblasts), young myotubes of 1 day (with less than 10 nuclei) or 2–3 days (more than 9 nuclei) and myoballs from 4–6, 7–9, 10–12 and 13–16 days cultures were patch-clamped (whole-cell configuration), and calcium currents and contraction simultaneously recorded. Sodium but not calcium currents could be recorded at the myoblast stage. In young myotubes (1 day), ICa,L was present with high incidence as compared to ICa,T, which was poorly expressed. Contractile responses appeared at the next stage (2-3 days) while the occurrence of ICa,T progressively increased. This developmental evolution of the calcium currents and contraction expression was accompanied by some changes in their characteristics: the ICa,T/ICa,L amplitudes ratio progressively increased and the time-to-peak of contraction progressively decreased with the age of myoballs. Physiological functions for calcium currents in developing muscle are suggested and discussed: ICa,T, which is transiently expressed, could be involved in the pacemaker-like activity while ICa,L could serve as an early contraction triggering mechanism and/or initially to fill and then to maintain the intracellular calcium stores.

31P-NMR spectroscopy of skeletal muscle in Becker dystrophy and DMD/BMD carriers. Altered rate of phosphate transport

Muscle energy metabolism was studied by phosphorus nuclear magnetic resonance spectroscopy (31P-NMR) in 6 patients with Becker dystrophy, and in 24 female DMD/BMD carriers (n = 18) and non-carriers (n = 6). At rest all patients showed a high Pi/PCr ratio due to low PCr and high Pi contents, and a high intracellular IpH. 31P-NMR of carriers and non-carriers did not differ from controls. In patients and carriers in-magnet exercise revealed a reduced ability to perform work and Pi/PCr ratios higher than controls for comparable relative levels of steady-state work. Post-exercise Pi recovery was found abnormal in patients and in carriers. The 31P-NMR abnormalities found in the working muscle of both BMD patients and female DMD/BMD carriers indicate a defect of phosphate metabolism that, be it primary or secondary, reflects a deficit of energy metabolism.

A third type of calcium current in cultured human skeletal muscle cells

A third type of calcium current could be recorded on a non-negligible number of human skeletal muscle cells (normal and Duchenne dystrophic (DMD)) in primary culture. This transient current exhibited a maximum at 0 mV, a time-to-peak around 30 ms, an inactivation time constant around 70 ms and was insensitive to nifedipine. On these basis, it differentiates from T- and L-type previously described and looks like the neuronal N-type. However, this third type of current was not sensitive to omega-Cgtx, a specific N-type blocker. The occurrence and the possible role of this current are briefly discussed.

Recent advances in dystrophin research

Evidence suggesting that dystrophin is a component of the membrane cytoskeleton of excitable cells continues to accumulate. Whereas the specific mechanisms leading to muscle pathology in Duchenne muscular dystrophy are still being debated it is apparent that the progressive weakness that occurs in this disease is the result of a chronic process that is initiated by dystrophin deficiency.