Electrogenesis of embryonic chick skeletal muscle cells differentiated in vitro

Chronic treatment with D600 enhances development of sodium channels in cultured chick skeletal muscle cells

We have studied the long-term effects of D600, a blocker of L-type voltage-dependent Ca channels (VDCC), on the development of voltage-dependent Na channels (VDNC) that are sensitive to tetrodotoxin (TTX), by electrophysiological measurements of the maximum rate of rise of the TTX-sensitive Na spike in cultured chick skeletal muscle cells. Chronic treatment with D600 (2-20 microM) caused a dose-dependent increase in the density of VDNC. The density of VDNC was increased by 150-250% when D600 was added to the cultures at 20 microM from the second day of culture onward. Co-treatment with an inhibitor of the transcription of RNA from DNA, alpha-amanitin, or with cycloheximide, an inhibitor of protein synthesis, prevented the up-regulation by D600. Nifedipine, a different type of blocker of L-type VDCC, was also effective in increasing the density of VDNC, and BAY K 8644, an agonist of L-type VDCC, had the opposite effect. It is suggested that the effect of D600 was mediated via a mechanism specific for L-type VDCC that involves regulation of cytosolic levels of Ca2+ and protein synthesis de novo.

A calcium- and voltage-dependent chloride current in developing chick skeletal muscle

1. Depolarization of embryonic chick myotubes from negative potentials elicits a rapid spike followed by a long-duration after-potential. The ionic basis of the long-duration after-potential was examined by making intracellular recordings from cultured myotubes, and by making whole-cell patch-clamp recordings from myoblasts and myoballs. 2. The peak potential of the long-duration after-potential varied with the chloride gradient, suggesting that a conductance increase to chloride is involved in generating the after-potential. However, a calcium current was also implicated, since lowering the extracellular calcium or replacing extracellular calcium with cobalt abolished the after-potential. 3. When extracellular calcium was replaced with strontium or barium, short-duration spikes similar to calcium spikes were observed, but only strontium was able to support activation of long-duration after-potentials. Intracellular injection of calcium or strontium into myotubes bathed in calcium-free extracellular solutions restored the ability of depolarization to evoke an after-potential. Intracellular injection of magnesium, barium, nickel or cobalt did not restore this ability. These experiments strongly suggested that the long-duration after-potential was due to a calcium- and voltage-activated chloride current. 4. Whole-cell voltage-clamp recordings from myoballs and myoblasts showed that a large chloride conductance could be activated by depolarization when the internal free calcium concentration was buffered at levels greater than 10(-7) M. At 2.5 x 10(-7) M-calcium, the voltage dependence of activation was steepest in the range of -30 to -20 mV and the activation kinetics varied with the membrane potential. The time to half-maximal activation ranged from 0.1 s at positive potentials to greater than 1 s at more negative potentials. The time constant for deactivation was approximately 1 s at -50 mV. No inactivation was observed. 5. The selectivity of the chloride current was measured by substituting other anions for chloride. The following permeability series was found: I- greater than NO3- greater than Br- greater than Cl- greater than acetate greater than F- greater than SO4- = glucuronate. Thus anion permeability decreased as the hydration radius increased. 6. Measurements of the resting potential of developing myoblasts and myotubes under 'physiological' conditions (37 degrees C, bicarbonate buffer) suggest that the after-potential acts to depolarize these cells 10-20 mV above their resting potential (approximately -60 mV) for several seconds. 7. We discuss the possibility that the long-duration after-potential may be involved in triggering myoblast fusion and in the generation of bursts of spontaneous contractions in developing myotubes.

Multiple actions of adenosine 5'-triphosphate on chick skeletal muscle

1. Extracellularly applied adenosine 5'-triphosphate (ATP) is known to have an excitatory action on chick skeletal muscle. By making intracellular recordings from cultured chick myotubes bathed with blockers of several types of voltage-dependent channels, the direct action of ATP could be observed. 2. When muscle cells were studied near their resting potential, ATP usually produced a biphasic response. There was a rapid initial depolarization, followed by a slower repolarization. The repolarization could drive cells negative to their initial resting potential, indicating that it was not due simply to desensitization of the process that produced the depolarization. Thus there are at least two distinct responses to ATP. 3. At room temperature the early response to ATP activated within 20 ms, and the second response activated with a latency of approximately 1 s. In our standard blocking solution, the average reversal potential of the early response was -17 mV, while the late response had a reversal potential that was negative to -70 mV. In a few cells the second response appeared to be absent. 4. The amplitude and time course of the late response were substantially decreased by low temperature (12 degrees C) and increased by high temperature (37 degrees C). In contrast, temperature had much smaller effects on the early response. Both the time course and temperature dependence of the late response suggest that an intracellular second messenger system may be involved in its activation. 5. Ion-substitution experiments were performed to determine the type of conductance changes that evoke each response. These indicated that the early response was due to an increased membrane permeability to sodium, potassium and chloride, but not to large cations or anions, and that the late response was due to an increased permeability to potassium. 6. Measurement of the responses of muscle cells to acetylcholine supported the conclusion that both anions and cations are permeable during the early ATP response. Under conditions in which there was a large negative reversal potential for all cations, and a large positive reversal potential for all anions, the early ATP response reversed approximately 50 mV positive to the acetylcholine response. 8. The possibility that the early ATP response is due to a channel selective for size, but not charge, is discussed.

Calcium currents in embryonic and neonatal mammalian skeletal muscle

The whole-cell patch-clamp technique was used to study the properties of inward ionic currents found in primary cultures of rat and mouse skeletal myotubes and in freshly dissociated fibers of the flexor digitorum brevis muscle of rats. In each of these cell types, test depolarizations from the holding potential (-80 or -90 mV) elicited three distinct inward currents: a sodium current (INa) and two calcium currents. INa was the dominant inward current: under physiological conditions, the maximum inward INa was estimated to be at least 30-fold larger than either of the calcium currents. The two calcium currents have been termed Ifast and Islow, corresponding to their relative rates of activation. Ifast was activated by test depolarizations to around -40 mV and above, peaked in 10-20 ms, and decayed to baseline in 50-100 ms. Islow was activated by depolarizations to approximately 0 mV and above, peaked in 50-150 ms, and decayed little during a 200-ms test pulse. Ifast was inactivated by brief, moderate depolarizations; for a 1-s change in holding potential, half-inactivation occurred at -55 to -45 mV and complete inactivation occurred at -40 to -30 mV. Similar changes in holding potential had no effect on Islow. Islow was, however, inactivated by brief, strong depolarizations (e.g., 0 mV for 2 s) or maintained, moderate depolarizations (e.g., -40 mV for 60 s). Substitution of barium for calcium had little effect on the magnitude or time course of either Ifast or Islow. The same substitution shifted the activation curve for Islow approximately 10 mV in the hyperpolarizing direction without affecting the activation of Ifast. At low concentrations (50 microM), cadmium preferentially blocked Islow compared with Ifast, while at high concentrations (1 mM), it blocked both Ifast and Islow completely. The dihydropyridine calcium channel antagonist (+)-PN 200-110 (1 microM) caused a nearly complete block of Islow without affecting Ifast. At a holding potential of -80 mV, the half-maximal blocking concentration (K0.5) for the block of Islow by (+)-PN 200-110 was 182 nM. At depolarized holding potentials that inactivated Islow by 35-65%, K0.5 decreased to 5.5 nM.

Species specificity of transferrin binding, endocytosis and iron internalization by cultured chick myogenic cells

The ability of unlabelled heterologous transferrin to interact with transferrin receptors on developing chick myogenic cells was investigated by measuring their capacity to inhibit the surface-binding and internalization of 125I- and 59Fe-labelled ovotransferrin. Transferrins from rat, rabbit, human, and a species of kangaroo (Macropus fuliginosus) were unable to inhibit either surface-binding or internalization of labelled ovotransferrin even at concentrations ten times the molar concentration of the ovotransferrin. Transferrins isolated from the serum of a toad (Bufo marinus) and a lizard (Teliqua rugosa), when added at high concentrations, were found to reduce surface-binding of 125I-Tf by 20-25% but did not inhibit internalization of either 125I-Tf or 59Fe. This suggests that the effects of toad and lizard transferrins are due to non-specific binding to the myogenic cells. In contrast, inhibition of both surface-binding and internalization of labelled ovotransferrin was found when myogenic cells were incubated in the presence of the homologous transferrin (ovotransferrin). The species-specificity of transferrin binding, endocytosis and iron internalization did not vary with the state of proliferation or differentiation of the myogenic cells. However, the intracellular iron utilization was found to differ between differentiating presumptive and terminally differentiated myotubes. Internalized 59Fe was fractioned by gel filtration. In dividing and non-dividing presumptive myoblasts 59Fe was found to elute in three peaks, two with elution volumes corresponding to ferritin and transferrin and one at greater elution volume than that of myoglobin.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium channel components of action potential in chick skeletal muscle cells developing in culture

The action potential was recorded from cultured chick skeletal muscle cells in Na-, Ca-, Cl-free saline containing Ba and tetraethylammonium ions (Ba saline). The action potential consisted of two components: a low-threshold, fast inactivating component and a high-threshold, long-lasting component. Both components of the action potential were dependent on external Ba ions and eliminated by Co ions. It is concluded that both components are generated by inward currents carried by Ba ions through Ca channels. The two Ca channel components of the action potential differed with regard to activation and inactivation potential, presence or absence of fast inactivation, sensitivity to an organic Ca channel blocker, and developmental profile. In addition, the failure of occurrence of one or the other components was observed in some cells. These results could be explained by assuming that two components of the action potential in Ba saline were mediated by the two different Ca channels. Furthermore, there was a tendency for younger cells to have more prominent Ca channel components. This may suggest that Ca channels have some function in the early stages of myogenesis.

Iron supports myogenic cell differentiation to the same degree as does iron-bound transferrin

T. Hasegawa, K. Saito, I. Kimura, and E. Ozawa (1981, Proc. Jopan Acad. B 57, 206-210) have shown that Fe ion can promote myogenic cell growth as Fe-bound transferrin. In the present work, the effects of these substances in supporting myogenic cell differentiation were examined. The hallmarks of differentiation adopted were appearance of structural and regulatory proteins, myofibrils, sarcoplasmic reticulum, and Ca-activated activities of myosin B and phosphorylase kinase; isoform transition of creatine kinase; and acquisition of cell membrane excitability and contractility following electrical stimulation of myotubes. The degree of differentiation of myotubes cultured in the presence of Fe ion was almost the same as that of myotubes cultured in the presence of Fe-bound transferrin. These facts suggest that transferrin protein molecules do not play a primary role in differentiation. Further, it has also been shown that myotubes acquire excitation-contraction and metabolism coupling qualitatively similar to that of adult muscle fiber.

Caffeine contracture in the cultured chick myotube

A possible function of Ca store site in cultured chick myotubes was examined by recording contraction of the myotube with special reference to the effect of caffeine. Caffeine at low concentrations (below 1 mM), applied focally on the myotube through a micropipette with a pressure pulse, elicited focal contraction without membrane potential changes. Procaine inhibited the caffeine contracture. Deuterium oxide also inhibited the caffeine contracture at low concentrations, but enhanced the maximal contracture. These observations are similar to those in the mature frog muscle fiber in which the sarcoplasmic reticulum (SR) is a main site of caffeine action. On the basis of these similarities, it was considered that caffeine acts on SR to elicit contracture in the myotube. The ability of SR to accumulate and release Ca ion seemed to be low, because caffeine contracture decreased or disappeared in a Ca-free solution in many myotubes.

Excitability changes of presumptive ectoderm following mesodermal induction

Dissociated ectodermal cells of the early newt gastrula which have been treated with CMF (Ca-Mg-free saline) for 5 hr differentiate into muscle cells when cultured in HFCS (heated fetal calf serum) for up to 9-12 days. Similarly dissociated cells placed into FCS (fetal calf serum) culture differentiate into epidermis. Differences in cell-cluster formation have been found between HFCS and FCS in early cell cultures (6 hr), and membrane excitability phenomena associated with the differentiation of these clusters into the muscle cells or epidermal cells have been investigated, respectively. The HFCS cultures consist of cell clusters which have few of microvilli at their surfaces and which form loose contacts by means of lamellipodia. FCS cultures consist of cell clusters which have numerous microvilli at their surfaces and which make tight contacts between cells by means of ridge-structure precursors. The different reaggregation pattern of dissociated ectoderm cells in HFCS reflects changes in the cell membrane surface induced by HFCS. The sequential genesis of action potentials in cells destined to form muscle cells in HFCS is very similar to those produced by somitic muscle cells in vivo and their ionic dependence for generating action potentials is related to epidermal action potentials in vitro (FCS).

Tunicamycin inhibits the expression of surface Na+ channels in cultured muscle cells

We have investigated the effect of tunicamycin (TM), an inhibitor of protein glycosylation, on surface Na+ channels in cultured chick skeletal muscle cells. The expression of Na+ channels, estimated by the measurement of batrachotoxin (BTX)-activated 22Na+ uptake, was found to be significantly diminished after exposure of muscle cells to TM. This effect is partially reversed by the protease inhibitor leupeptin and is associated with a markedly enhanced rate of disappearance of Na+ channels from the surface of TM-treated cells. Our findings suggest that protein glycosylation contributes to the metabolic stability of voltage-sensitive Na+ channels.

Increased K+ inhibits spontaneous contractions reduces myosin accumulation in cultured chick myotubes

Increasing the K+ from 5.4 mM to 12 mM in the culture medium of developing chick myotubes causes an immediate cessation of spontaneous contractions and leads to an inhibition of myosin accumulation. The synthesis of myosin continues at the same rate in 12 mM K+ as in 5.4 mM K+ as measured by [3H]leucine incorporation into myosin corrected for differences in pool specific activity. Total protein synthesis and total protein accumulation are unaffected by growth in 12 mM K+. In addition, growth in 12 mM K+ did not alter the type of myosin heavy-chain isoform expression nor did it alter the pattern of myosin light-chain synthesis. However, the rate of myosin turnover increased threefold in cultures grown in 12 mM K+ compared to cultures grown in 5.4 mM K+, while total protein turnover was only marginally increased. We conclude that suppressed electrical or contractile activity of myotubes leads to an increased rate of myofibrillar protein turnover and that spontaneous mechanical and or electrical activity is required for continued myotube maturation in culture.

Neurotrophic effect of nerve extract on development of tetrodotoxin-sensitive spike potential in skeletal muscle cells in culture

The effect of the presence of nerve extracts on the development of tetrodotoxin (TTX)-sensitive sodium channels in cultures of dissociated embryonic chick skeletal muscle cells was examined by measuring the maximum rate of rise of TTX-sensitive spike potential. The addition of the nerve extract prepared from brain or spinal cord of chick embryos to the culture medium caused an increase in the channel density. Extracts of non-neural tissues, i.e., lung, kidney, and muscle, were ineffective. Liver extract, however, produced an effect similar to the nerve extracts. These results suggest that the TTX-sensitive sodium channels in the muscle cell membrane are regulated by a diffusible chemical substance independently of innervation, and that this substance resides in neural tissues, and perhaps also in liver.

Maturation of human skeletal muscle fibres in explant tissue culture

Human foetal skeletal muscle was grown in explant cultures and the development of myotubes was monitored morphologically and by the development of membrane potential and acetycholine sensitivity. Migration of uninucleate cells from the explants occurred in the first day of culture and formation of multinucleated myotubes took place between 7 to 10 days. Early myotubes were variable in appearance, being either flat and nonrefractile or narrow and cylindrical. Some cross-striated cells were observed. Resting membrane potentials were around -25mV and there was great variation in myotube sensitivity to acetylcholine. After about 6 weeks in culture most myotubes were of the refractile type. Many had hypolemmal nuclei and were cross-striated; some contracted spontaneously. All myotubes had developed high sensitivity to acetylcholine although there had been no increase in resting membrane potential.

Development of spike potentials in skeletal muscle cells differentiated in vitro from chick embryo

The development of spike potential mechanisms during cell differentiation was studied in chick myotubes formed in vitro from trypsin-dissociated myoblasts. The spike potential and its rate of rise were measured in myotubes from 4-14 day old cultures. A depolarizing current pulse was delivered to evoke the spike potential after the steady membrane potential had been adjusted to a standard level of -80 mV in all cases. This gives the greatest maximum rate of rise of the spike potential and eliminates variation due to differences in the resting membrane potential of the myotubes. The size and maximum rate of rise of the spike potential increased significantly during the period examined. The spike potential was blocked by tetrodotoxin in almost all myotubes. These results suggest that during differentiation myotubes develop the ability to generate a spike potential due to an inward current carried by sodium ions.

Development of excitability in embryonic chick skeletal muscle cells

During embryonic and early postnatal development, the chick leg muscle cells undergo a series of changes in their electrical responses in the following sequence: passive response, plateau response, plateau plus spike response and spike response. This suggests that the electrogenetic mechanism of muscles matures during development; a mechanism producing the plateau may first be induced, and then that producing the spike. The plateau is sensitive to manganese or cobalt ions, while the spike to tetrodotoxin. This suggests that the plateau is related to the increase in permeability to calcium ions, while the spike to sodium ions.

Calcium and sodium contributions to regenerative responses in the embryonic excitable cell membrane

Ionic dependence of regenerative responses of the embryonic cell mnembrane has been studied successively at each stage of development from the unfertilized egg to the differentiated striated muscle in the tadpole larva of the tunicate. The unfertilized egg cell itself showed a type of regenerative response dependent on both sodium and calcium ions, while the spike potentials exclusively dependent on calcium ions were elicited in the differentiated muscle cell.