Development of excitability in embryonic chick skeletal muscle cells

Adrenaline diminishes K+ contractures and Ba2+-current in chicken slow skeletal muscle fibres

The effects of adrenaline and the beta-adrenergic agonist isoprenaline on K+-evoked tension (K+-contracture) and Ba2+ current were investigated in chicken slow (anterior latissimus dorsi (ald)) muscle using isometric-tension measurements and current recording. Addition of adrenaline (10(-7) - 10(-5) M) or isoprenaline (10(-6) - 10(-5) M) to the bath reduced the amplitude of the K+-contractures. These effects were blocked by the beta-antagonist propranolol (5 x 10(-6) M). External application of a cAMP analogue (8-bromo cyclic AMP; 1 x 10(-4) M) also decreased the amplitude of the K+-contractures. To analyze the possible relationship between the induced tension reduction and effects on sarcolemmal Ca2+ channels, a slow action potential and a slow inward membrane current were studied in intact ald chicken muscle fibres. When the ald muscle was immersed in a Na+- and Cl--free solution containing Ba2+ and depolarizing pulses were delivered from a -80 mV holding potential, the muscle fibres exhibited a small, slow Ba2+-dependent potential (observed at about -26 mV, peak amplitude, around -10 mV). The response was blocked by the addition of Co2+ (5 mM) or Cd2+ (2 mM). Using the three-microelectrode voltage-clamp technique, a slow inward membrane current underlying the Ba2+ potential could be discerned. The current had a mean threshold of -60 mV, reached maximum at about -5 mV and ranged from ca. 9 to 19 pA/cm2 (depending on the external Ba2+ concentration). It had a mean reversal potential of +45 mV. The Ba2+ inward current was diminished when adrenaline or isoprenaline was added to the bath (1 x 10(-5) M); however, this decrease did not occur when propranolol was present (5 x 10-6 M). These results suggest that the decreases in the tension of K+-contractures induced by adrenaline and isoprenaline may occur through beta-adrenergic effects on sarcolemmal Ca2+ channels in ald chicken slow muscle fibres.

Development of muscle-specific features in cultured frog embryonic skeletal myocytes

To study the development of muscle-specific features during myogenesis, we analysed the ultrastructure and voltage-dependent currents of frog embryonic skeletal myocytes maintained in culture for 10 days. The cells were maintained under culture conditions that prevented cell division, fusion and cell contacts with neuroblasts. The cell surface was estimated morphometrically and from cell capacity and the values obtained were used to calculate ion current densities. It was shown that the expression of all main types of voltage dependent ionic currents occurs during the first 3-5 days. Na+ maximum specific conductance at days 1-2 was low but by day 7 it showed a 20-fold increase. The magnitude of Na+ current densities increased 16-fold from day 1 (3.6 microA/cm) to the day 7 (58.1 microA/cm). The maximum specific K+ conductance increased almost 3-fold during the first 5 days. In contrast to the other types of currents, I(K) undergoes qualitative changes. Sodium action potentials, whose amplitude and time course depend on gNa/gK ratio, appeared from day 4 in culture, when myofibrils and the T-system also developed. The amplitude of DHP-sensitive slow I(Ca) increased in parallel with the development of the T-membrane. I(Ca,S) density per unit of T-membrane area reached an equilibrium of ca., 17 microA/cm2 on the day 4 and then remained stable until the end of the period of observation. These studies demonstrate that muscle-specific characteristics including morphology and excitatory properties begin to develop on the third day and resemble those of adult muscle cells by the sixth day in culture.

Acetylcholine receptor formation in mouse-chick chimera

This study investigated possible interactions between motoneurons and somitic-derived muscle cells in the formation of neuromuscular synapses in the myotome. The peculiarities of the neuromuscular synaptic pattern in chick and mouse embryos provided a model for studying the achievement of synaptogenesis between chick motoneurons and mouse muscle cells. In chick embryo, initial AChR clustering occurs well before innervation of the myotome, whereas in mouse embryo nerve axons invade the myotome extensively before the appearance of AChR clusters. Our approach was to replace somites from a chick host embryo with those derived from mouse donor embryos. We show that muscle cells from mouse myotome can differentiate in the chick embryo environment and form neuromuscular contacts with chick motor axons. Host axons invaded in ovo differentiating mouse myotome at a time when they had not yet reached the host myotome. This particular ingrowth of motor nerves was attributable to the mouse transplant since use of a quail somite did not produce the same effect as the mouse somite, which suggests that developing mouse muscles specifically modify the time course of chick axogenesis. The synaptic areas formed between chick motor axons and mouse myotubes developed according to the mouse pattern. Both the timing of their appearance and their morphology correlated perfectly with events in mouse synaptogenesis. These results indicate the important role played by postsynaptic membrane in controlling the first steps of AChR formation.

Tetrodotoxin-resistant sodium channels

1. Tetrodotoxin (TTX) has been widely used as a chemical tool for blocking Na+ channels. However, reports are accumulating that some Na+ channels are resistant to TTX in various tissues and in different animal species. Studying the sensitivity of Na+ channels to TTX may provide us with an insight into the evolution of Na+ channels. 2. Na+ channels present in TTX-carrying animals such as pufferfish and some types of shellfish, frogs, salamanders, octopuses, etc., are resistant to TTX. 3. Denervation converts TTX-sensitive Na+ channels to TTX-resistant ones in skeletal muscle cells, i.e., reverting-back phenomenon. Also, undifferentiated skeletal muscle cells contain TTX-resistant Na+ channels. Cardiac muscle cells and some types of smooth muscle cells are considerably insensitive to TTX. 4. TTX-resistant Na+ channels have been found in cell bodies of many peripheral nervous system (PNS) neurons in both immature and mature animals. However, TTX-resistant Na+ channels have been reported in only a few types of central nervous system (CNS). Axons of PNS and CNS neurons are sensitive to TTX. However, some glial cells have TTX-resistant Na+ channels. 5. Properties of TTX-sensitive and TTX-resistant Na+ channels are different. Like Ca2+ channels, TTX-resistant Na+ channels can be blocked by inorganic (Co2+, Mn2+, Ni2+, Cd2+, Zn2+, La3+) and organic (D-600) Ca2+ channel blockers. Usually, TTX-resistant Na+ channels show smaller single-channel conductance, slower kinetics, and a more positive current-voltage relation than TTX-sensitive ones. 6. Molecular aspects of the TTX-resistant Na+ channel have been described. The structure of the channel has been revealed, and changing its amino acid(s) alters the sensitivity of the Na+ channel to TTX. 7. TTX-sensitive Na+ channels seem to be used preferentially in differentiated cells and in higher animals instead of TTX-resistant Na+ channels for rapid and effective processing of information. 8. Possible evolution courses for Na+ and Ca2+ channels are discussed with regard to ontogenesis and phylogenesis.

Developmental sequence of expression of voltage-dependent currents in embryonic Xenopus laevis myocytes

Although the development of several of the voltage-dependent currents in embryonic amphibian myocytes has been described, the overall muscle electrical development, particularly the relative times of expression of different voltage-dependent currents, has not been addressed in a single study under one set of conditions. We have found that, in mesoderm isolated and cultured from neurula stage embryos, myocytes are identifiable before they express voltage-gated currents. These ionic currents are absent from all Xenopus mesodermal cells during the late gastrula/early neurula stages of embryonic development. At about the time of first somite segregation an inward rectifier K+ current is expressed in some myocytes, followed within 2 hr by a delayed rectifier K+ current. The density of both currents increases fourfold over the next 24 hr in culture. A Na+ current is not expressed in large numbers of myocytes until late in this culture period, at about the time that a slow Ca2+ current appears. Under our culture conditions the myocytes have a very low chloride conductance. A fast inactivating component to the outward K+ current is expressed in all myocytes by 24 hr in culture. In some experiments we dissociated embryos at later times and made recordings when all previously isolated myocytes expressed currents. In the late dissociations, most myocytes did not express currents, but developed them after a short period in culture. Because we have evidence that in vivo development is more closely approximated by the early dissociations, these results suggest that dissociation causes some degree of dedifferentiation.

Spontaneous calcium influx and its roles in differentiation of spinal neurons in culture

Stimulation of embryonic amphibian spinal neurons has been shown to produce calcium-dependent action potentials of long duration at early stages of development. These impulses become brief and sodium-dependent upon further differentiation. The neurons are now shown to exhibit spontaneous, transient elevations of intracellular calcium in culture during the early developmental period when activity produces greatest calcium influx. Removal of extracellular calcium during this period alone is sufficient to perturb differentiation, and influx through voltage-dependent calcium channels is shown to be required for standard development of neuronal phenotypes. No large changes in steady-state calcium levels occur in the cytoplasm during the maturation of cultured neurons despite a reduction of the calcium-dependent component of the impulse. Transient elevation of intracellular calcium is necessary for standard cytodifferentiation and may provide a link between electrical activity and gene expression.

A kinematic analysis of hindlimb motility in 9- and 10-day-old chick embryos

Although chick embryonic leg movements appear jerky and disorganized, the underlying motor patterns are coordinated. This apparent conflict in results was investigated using kinematic analyses to provide detailed quantitative descriptions of leg movements during spontaneous motility in 9- and 10-day-old chick embryos. In many respects, hip, knee, and ankle movements were highly variable. There was variation in movement durations and in the number of leg joints that participated in a given movement. Motion could begin with either flexion or extension. Furthermore, the limb could return to its rest position between movements or move continuously to produce sequences of variable lengths. Patterns of interlimb coordination included alternating, synchronous and independent movements of the legs. We propose that these variable features account for the uncoordinated appearance of embryonic leg movements. In addition to variable features, however, some consistent characteristics were identified. For example, when more than one joint was active, activity typically began and ended synchronously. Furthermore, all active joints generally extended or flexed together and movements were symmetrical. These data are consistent with previous EMG results. Therefore, despite the variability in some parameters that results in the perception that embryonic leg movements are random and uncoordinated, our kinematic analyses show that an organized pattern of interjoint coordination is a prominent feature. This basic pattern shows some similarities to, but is less complex than, kinematic patterns found during a variety of posthatching behaviors.

The effect of Ba ions on human muscle cultured in monolayer and innervated with fetal rat spinal cord

In human muscle culture, 7% of aneurally (AMs) and 5.9% of innervated contracting muscle cells (ICMs) showed slow repolarization components (SRCs: duration, less than 10 ms) of action potentials. After an application of 10 mM Ba ion, prolonged SRCs, which were blocked by nifedipine, appeared in 96% of the AMs and 70% of the ICMs. The duration of SRCs under the application of 10 mM Ba solution were not significantly different between AM and ICM (2.8 and 2.0 s, respectively). The resting membrane potentials (RMPs) of AMs and ICMs decreased to 78% and 74% in the medium with 10 mM Ba solution, respectively, and the input resistance (Rin) of AM increased to 161% in the medium with 10 mM Ba. Slow hyperpolarizing afterpotentials were observed both in AMs and ICMs, and completely disappeared during the application of Ba.

Chloride action potentials and currents in embryonic skeletal muscle of the chick

Chloride-dependent action potentials were elicited from embryonic skeletal muscle fibers of the chick during the last week of in ovo development. The duration of the action potentials was extremely long (greater than 8 sec). The action potentials were reversibly blocked by the stilbene derivative, SITS, a specific blocker of chloride permeability. Using patch clamp pipettes, in which the intracellular chloride concentration was controlled and with other types of ion channels blocked, the membrane potential at the peak of the action potential closely coincided with the chloride equilibrium potential calculated from the Nernst equation. These data indicate that activation of a chloride-selective conductance underlies the long duration action potential. The occurrence of the chloride-dependent action potential was found to increase during embryonic development. The percentage of fibers that displayed the action potential increased from approximately 20% at embryonic day 13 to approximately 70% at hatching. Chloride-dependent action potentials were not found in adult fibers. The voltage and time-dependent currents underlying the action potential were recorded under voltage clamp using the whole-cell version of the patch pipette technique. The reversal potential of the currents was found to shift with the chloride concentration gradient in a manner predicted by the Nernst equation, and the currents were blocked by SITS. These data indicate that chloride ions were the charge carriers. The conductance was activated by depolarization and exhibited very slow activation and deactivation kinetics.

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.

Development alters the expression of calcium currents in chick limb motoneurons

Calcium current types expressed in vertebrate spinal motoneurons have not been previously resolved. We have resolved three types in chick limb motoneurons identified by retrograde labeling and report that dramatic changes in their expression take place during development in vivo. T-, N-, and L-type calcium currents were distinguished on the basis of kinetics, voltage dependence, and unique pharmacological sensitivities. Developmental changes were characterized by studying motoneurons isolated from embryos at three stages spanning neuromuscular system development. T currents were dominant at the earliest stage. Motoneurons from embryos 2 days older showed much reduced T currents and much increased N and L currents. We suggest that mature motoneurons will be dominated by N- and L-type calcium currents and that T current may serve developmental roles.

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.

Elimination of distributed synaptic acetylcholine receptor clusters on developing avian fast-twitch muscle fibres accompanies loss of polyneuronal innervation

Changes in the distribution of large acetylcholine receptor clusters (AChR-Cs) on developing fast-twitch fibres of the chicken posterior latissimus dorsi (PLD) muscle have been studied during the period of loss of polyneuronal innervation using fluorescein-conjugated alpha-bungarotoxin. Embryonic muscles were ultrasonically dissociated into single fibre fragments and presumptive fast-twitch fibres were distinguished from the minority of slow-type fibres in the PLD by immunofluorescence using an antibody against slow-type myosin. Whereas mature PLD muscle fibres are focally innervated, at embryonic day 11 (E11) many fibre fragments from the PLD displayed two or more large (longer than 2 micron) AChR-Cs. Double labelling with anti-neurofilament antibody suggested that most of these AChR-Cs (82 +/- 2%) were associated with neuromuscular contacts. There was a progressive decline in the number of large (synaptic) AChR-Cs per 1000 micron of fibre, from 3.2 +/- 0.5 at E11 to 0.4 +/- 0.1 at E18. No further decline occurred between E18 and one week post-hatch. Primary generation muscle cells identified at E11 and E16 by tritiated thymidine labelling showed a decline in the number of large AChR-Cs per 1000 micron proportional to that seen in the fibre population as a whole, suggesting that distributed synaptic AChR-Cs are eliminated from individual fibres as they mature. When embryos were treated with d-tubocurarine starting at E6 the loss of distributed AChR-Cs from fast-type PLD fibres between E11 and E14 did not occur, suggesting that neuromuscular activity may play an important role in establishing the focal synaptic site AChR-C.

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.

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.

Contractile activation in myotomes from developing larvae of Xenopus laevis

Contractile activation in response to application of different perfusing solutions was observed in developing myotomes from larvae of Xenopus laevis. Contractures resulted from treatment with isotonic solutions containing high K+ concentrations in myotomes from embryos from stage 24 onwards. These developed over 5-10 s and inactivated over 20-30 s. Contractures persisted in curarized preparations. The applied K+ concentrations at the estimated mechanical threshold in embryos at stages 26 and 34 were about 30 and 20 mM respectively. Maximum activity was achieved at K+ concentrations of 78 and 48 mM respectively. The lyotropic ion thiocyanate (10 mM) potentiated K+ contractures, and shifted the threshold K+ concentration to lower values. Contractures persisted in the presence of 3 mM-Mn2+ and in Ca2+-free solutions, with unaltered mechanical thresholds. K+ contractures were reversibly abolished by 2 mM-tetracaine. Comparisons of resting potentials in 3 and 20 mM-K+ confirmed that the resting potential remained sensitive to external K+ concentration, whether in control, tetracaine-containing, or low-Ca2+ bathing solutions. Caffeine (10-20 mM) caused sustained contractures from stage 24 onwards. Caffeine sensitivity was greatest at stages 26-28, then appeared to decline at stages 33-36. These observations suggest that mechanisms for excitation-contraction coupling develop in Xenopus embryos at the same time as do completed sarcomeres, close to stage 24. Activation of contraction then assumes the adult pattern, involving voltage-dependent release of the intracellular store of activator, independent of entry of extracellular Ca2+.

Proceedings of a conference on steroid hormones as trophic agents in neural development. UCLA, October 9 & 10, 1984

The electric communication system includes both special muscle-derived cells or electrocytes that produce species-typical electric signals, or electric organ discharges (EODs), and specialized sensory receptors, or electroreceptors, that encode the electric fields set up by EODs. Steroid hormones can influence the characteristic properties of both EODs and electroreceptors. Steroids appear to directly effect the anatomy and physiology of the electrocytes that generate an EOD. In contrast, the steroid effect on electroreceptors may be predominantly via an indirect mechanism whereby changes in the spectral characteristics of the EOD appear to induce changes in the spectral sensitivity of electroreceptors. Continued studies of electrosensory and electromotor systems will offer insights into the cellular bases for the development and evolution of steroid-sensitive pathways in the vertebrate nervous system.

Early differentiation of vertebrate spinal neurons in the absence of voltage-dependent Ca2+ and Na+ influx

The development of the action potential and responses to neurotransmitters have been described for a population of embryonic spinal neurons developing in vivo. A comparable pattern is seen for spinal neurons developing in dissociated cell culture. The impulse appears very early in this developmental sequence, and the action potential involves a large inward Ca2+ current. Since Ca2+ is a ubiquitous intracellular regulator, we questioned whether a large influx of Ca2+ is necessary for the subsequent differentiation of membrane properties. Embryonic Xenopus neurons grown in normal culture medium do not make Ca2+- or Na+-dependent action potentials in their cell bodies in a Ca2+-free saline containing tetrodotoxin (TTX). To achieve a chronic blockade of impulse activity, neurons were grown in a medium in which Ca2+ was replaced by Mg2+, and to which 1 mM EGTA was added. In some instances TTX was present. Neurons grown in these experimental culture media extend neurites more rapidly than controls. Action potentials cannot be elicited from neurons when examined in experimental medium. However, examination in saline reveals that the change in the ionic dependence of the impulse is indistinguishable from that observed in neurons grown in normal medium. Furthermore, the time of onset of responses to GABA is unaffected by this experimental treatment. Thus the expression of Ca2+- and Na+-dependent action potentials seems not to play a part in the early differentiation of these membrane properties. However, the later development of GABA sensitivity is reduced.

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).

Trifluoperazine stimulates acetylcholine receptor synthesis in cultured chick myotubes

Acetylcholine receptor appearance rate in the presence of the phenothiazines trifluoperazine and chlorpromazine was measured in cultured embryonic chick myotubes by means of 125I-alpha-bungarotoxin. At drug concentrations of 5 to 10 X 10(-6) M, receptor appearance rate was significantly enhanced while receptor half-life, cellular protein, net protein synthesis rate, and acetylcholinesterase levels were not similarly affected. The sulfoxide derivatives were without effect. At concentrations of 3 X 10(-5) M and above, both trifluoperazine and chlorpromazine caused myotube contracture and cell loss. Drug combination experiments revealed that receptor stimulation caused by phenothiazines is overcome by low concentrations of veratridine and ryanodine, but not by membrane depolarization with 20 mM KCl. These results lend support to the role of calcium as an intracellular messenger in acetylcholine receptor synthesis regulation, but are difficult to reconcile with the notion that cytosolic calmodulin serves as the calcium receptor in this signaling pathway. Since the trifluoperazine effect resembles that caused by the calcium antagonist D-600, phenothiazines may stimulate receptor synthesis by blocking a voltage-gated calcium channel.

Influence of thyroid hormone on some electrophysiological properties of developing rat skeletal muscle cells in culture

Effects of thyroxine (T4) were examined on some electrophysiological properties of developing rat myotubes in culture. Thyroxine caused an increase in transmembrane resting potential (Em) of skeletal myotubes. Ouabain decreases Em in both control and T4-treated cells within 5-15 min of addition to the culture. Moreover, the effect of T4 is nearly eliminated by short-term ouabain treatment and by reduced extracellular K+ concentration. The change in Em in response to a 10-fold increase in extracellular K+ concentration is not altered by T4-treatment despite the higher Em of T4-treated cells. Studies on developmental effects of T4 showed that Em of T4-treated cells is higher than controls by as early as day 5. By this time, ouabain decreases Em of T4-treated cells but not that of untreated controls. Treatment with T4 also causes an increase in frequency of spontaneously-occurring action potentials generated by cultured myotubes. Chronic treatment with tetrodotoxin blocks this effect without affecting the T4-induced increase in Em. The findings demonstrate that activity of electrogenic Na,K-ATPase is increased by T4-treatment and that this effect is primarily responsible for the elevated Em in T4-treated cells. In addition, it appears that T4 causes the activity of this enzyme to be manifest earlier in development than might otherwise occur in cultured muscle. Finally, stimulation of electrogenic pump activity is not secondary to increased influx of Na+ ions associated with increased electrical activity and probably results from synthesis of new enzyme units in both developing and mature myotubes.

Activation patterns of embryonic chick hind limb muscles recorded in ovo and in an isolated spinal cord preparation

Muscle activation patterns of embryonic chick hind limb muscles were determined from electromyographic (e.m.g.) recordings in an isolated spinal cord/hind limb preparation of stage 34-36 embryos, and were compared with in ovo e.m.g. activity from similarly staged embryos. Muscle activity in ovo consisted of periodically recurring sequences of bursts during which antagonistic muscles often alternated and synergistic muscles were co-active, as compatible with their mature function. However, more variable behaviour was also observed. Burst sequences in ovo were often initiated by a short-duration, high-amplitude discharge that occurred synchronously in all muscles studied, and which was followed by a period of electrical silence that was longest in the flexor muscles. This type of activity has not been described previously in mature animals. In ovo movement sequences were generally initiated by extensor activity which progressively declined in duration and intensity throughout the sequence, while flexor activity progressively intensified. The onset of activity in extensor muscles was accompanied by an abrupt decrease in flexor activity, whereas the converse was not observed. Spontaneous movement sequences also occurred when the spinal cord and hind limb were isolated and maintained in oxygenated Tyrode solution for several hours. Deafferentation experiments indicated that the motor pattern in this preparation was generated centrally by circuits within the spinal cord. Activity from the isolated cord was less variable than that occurring in ovo, consisting of sequences of highly regular recurring bursts. Each burst began with a brief high-amplitude discharge that occurred synchronously in all muscles and which was similar to that observed in ovo. This was followed by a silent period, which was longest in the flexors, and then by a more prolonged burst. Although its behaviour differs from that in ovo in some respects, it is concluded that the isolated cord maintained in vitro produces a spontaneous and patterned motor output.

Membrane electrical properties of developing fast-twitch and slow-tonic muscle fibres of the chick

Isolated single fibres from the anterior (a.l.d.) and the posterior (p.l.d.) lattissimus dorsi muscles of embryonic and young chicks were used to study in vivo development of membrane electrical properties. Isolated fibres were obtained by an enzymatic dissociation procedure. Intracellular micro-electrode recordings from isolated fibres and from fibres in intact muscles showed that the dissociation procedure did not significantly alter resting membrane potentials, input resistances or membrane time constants (tau m). The 14 day embryonic fibres of a.l.d. and p.l.d. did not have a measurable resting conductance to Cl-. At hatching, about 70% of the resting conductance in p.l.d. fibres was due to Cl-. Membrane electrical properties were estimated from the analysis of voltage responses to intracellular injection of rectangular pulses of current. At 14 days in ovo, membrane resistance (Rm) was approximately 20 k omega cm2 and membrane capacitance (Cm) was 1-2 microF/cm2 for both a.l.d. and p.l.d. The mean membrane length constants (lambda) were 1.7 mm for a.l.d. and 1.5 mm for p.l.d. For p.l.d., the values of Rm, tau m and lambda decreased as development proceeded. For a.l.d., there was no change in these values by the time of hatching (21 days). The decreases in the electrical constants for p.l.d. fibres were partly explained by the appearance of a resting Cl- conductance during the last week of embryonic development.

The regulation of extrajunctional acetylcholine receptors in the denervated rat diaphragm muscle in culture

The regulation of the number of extrajunctional acetylcholine (ACh) receptors was assayed by 125I-labelled alpha-bungarotoxin binding sites in denervated rat diaphragm muscle in culture. Sustained K depolarization does not eliminate extrajunctional ACh receptors. In fact, muscle cultured in high-K medium (normal Cl) for 3 days exhibits a greater binding capacity than controls. Under conditions in which the intracellular Cl concentration is unaltered (high-K-low-Cl medium) this effect of high-K medium on the number of extra-junctional ACh receptors is blocked. The number of extrajunctional receptors increases 24-48 h after exposure to high-K-normal Cl medium, similar to the time course of the initial appearance of extrajunctional receptors in the denervated diaphragm muscle in vivo or in organ culture in normal media. High-K-normal Cl medium did not alter the rate of receptor degradation. Electrical stimulation of denervated muscle strips cultured in low-Ca medium containing D-600 eliminated extrajunctional receptors as efficiently as stimulation of muscles in control medium. Electrical stimulation did not reduce the extrajunctional ACh receptor population in glycerol-treated uncoupled muscles to the same extent as in untreated muscles. The extrajunctional ACh receptor content of denervated muscle cultured for 3 days in 2 and 5 mM-caffeine was reduced by about half respectively. Denervated muscle cultured in 0.3 mM-caffeine did not differ from control denervated muscle. Other agents which may alter intracellular cyclic nucleotide levels: dibutyryl cyclic GMP, dibutyryl cyclic AMP, papaverine, and sodium nitroprusside, did not mimic the effect of caffeine or electrical stimulation in lowering the levels of extrajunctional ACh receptors. We conclude that intracellular Ca release from the sarcoplasmic reticulum is necessary for the elimination of extrajunctional ACh receptors in denervated muscle. The levels of intracellular Cl also influence the population of extrajunctional receptors. Conditions which lead to higher levels of intracellular Cl result in greater rates of synthesis of ACh receptors.

Optical evidence for calcium-action potentials in early embryonic precontractile chick heart using a potential-sensitive dye

Using an optical method for monitoring membrane potential, spontaneous action potentials in the 7- to 9-somite embryonic precontractile chick hearts were measured. The optical action potential in the 7- to 9-somite embryonic heart was lacking 'phase 0' and 'phase 1' attributable to the fast Na+ current. The embryonic precontractile heart continued to generate spontaneous action potentials in a Na+-free solution or in the presence of tetrodotoxin. Such an action potential was blocked by adding Co2+, Mn2+, Ni2+, La3+, D-600 or GEDTA, and the frequency, the amplitude, and the rate of rise of the spontaneous action potentials depended closely upon the external Ca2+ concentration; reducing the external Ca2+ concentration resulted in suppression of the spontaneous excitability. From the above results, we concluded that the spontaneous action potential in the early phases of cardiogenesis is characterized as a Ca2+-dependent action potential.

Extracellular potassium and the regulation of acetylcholine receptor synthesis in embryonic chick muscle cells

The effect of elevated extracellular potassium on acetylcholine receptor synthesis was studied in chick embryonic muscle cultures. At physiological ionic strength, potassium chloride, in the 3.3 to 50 mM range, gave rise to a complex dose-response curve whose prominent features are a considerable reduction of receptor appearance rate at 20 mM and a more than 2-fold increase at higher concentrations. The effect of potassium chloride on receptor synthesis appears to be fairly specific: neither was there a duplication of its effect by other electrolytes or solutes, nor did it alter total protein synthesis or receptor stability by more than 30% at any concentration tested; cellular acetylcholinesterase levels actually declined with increasing KCl concentrations. In order to explore the mechanism of the potassium effect, tetrodotoxin (10(-6) M), veratridine (3 X 10(-6) M), D-600 (1.6 X 10(-5) M), and ryanodine (3 X 10(-7) M) were tested in the presence of various concentrations of potassium. Sodium channel toxins as well as calcium effectors modified the potassium response. Based on these findings we propose that the effects of potassium are due to: (a) cessation of spontaneous muscle activity upon raising KCl from 3 to 10 mM; (b) depolarization of the muscle membrane and persistent activation of a calcium channel as concentration is raised from 10 to 20 mM; (c) finally, inactivation or desensitization of the calcium channel, or some other signaling element proximal to the sarcoplasmic reticulum, upon further depolarization.

Development of sodium, calcium and potassium channels in the cleavage-arrested embryo of an ascidian

1. The cleavage of the embryos of an ascidian, Halocynthia roretzi, was arrested with cytochalasin B (1 microgram/ml.) at the 16- or 64-cell stage. These cleavage-arrested embryos were still able to develop membrane excitability. 2. In the cleavage-arrested 64-cell embryos at the time when control embryos became hatched larvae, Ca spikes were evoked in the presumptive muscle blastomeres, and Na- and Ca-dependent action potentials were induced in some ectodermal blastomeres. 3. Membrane currents of the cleavage-arrested 16-cell embryos were recorded with the voltage-clamp technique and analysed as a function of developmental time at 15 degrees C. For this purpose, intact eggs, 4- and 8-cell embryos were also used. The cleavage-arrested embryos behaved electrically like single cells, due to tight electrical coupling between blastomeres. After the 25-hr stage decoupling occurred. 4. Both Na and Ca currents decreased during the initial 10 hr. Na current became less than one third and Ca current almost disappeared. At 17 hr both Na and Ca currents increased again. 5. The potential-dependence of the Na and Ca currents after 17 hr was similar to that in the egg, although substantial parallel shifts in the current-voltage relations were observed: a 5 mV positive shift for the Na current and a 15-20 mV negative shift for the Ca current. 6. Delayed (outward) K current developed gradually until 20 hr and then increased abruptly. The activation level for the delayed rectification was markedly negative (around -10 mV) in comparison with that of the egg (around + 100 mV). Anomalous (inward) K current, on the other hand, increased gradually without changes in the potential-dependence throughout development. 7. The results suggest that the differentiation of excitable membranes in the ascidian embryo does not involve changes in the properties of the individual channels, but rather changes in the numbers of various kinds of ion channels.

Characterization of the methylxanthine-induced propagated wave phenomenon in striated muscle

Escalation is a propagated wave phenomenon readily observable in chick skeletal muscle fibers growing in culture. It occurs in fibers bathed in methylxanthines, halothane, or quinine, at concentrations associated with twitch potentiation. From the results of a variety of experiments, an explanation of the phenomenon is proposed. Cation requirements suggest that a wave may be initiated in conjunction with a spontaneous calcium influx or calcium spike; and drug concentration, temperature, and extracellular potassium effects support the theory that wave propagation occurs by calcium-induced calcium release from the sarcoplasmic reticulum. The optical effect may then reflect osmotic changes associated with the transient calcium release. Escalation is not restricted to cultured muscle, nor to chick. At appropriate temperatures and in the presence of methylxanthines, escalation has been observed in chick, frog, and rat skeletal muscle. This suggests that it is a subthreshold event, related to contraction, capable of providing further insight into excitation-contraction coupling. The superior visibility conditions and accessibility to experimental manipulation make cultured chick skeletal muscle fibers suitable subjects for such study.

Tetrodotoxin sensitivity and Ca component of action potentials of mouse dorsal root ganglion cells cultured in vitro

In the mouse dorsal root ganglia cultured in vitro, neurons were classified into 3 groups according to the responses of their action potentials to tetrodotoxin (TTX) and removal of Na ions from bathing medium: (1) the neurons whose action potentials were not affected by TTX by TTX (10(-6) - 10 (-5)g/ml) and which generated Ca-dependent regenerative responses under Na-free condition, (2) the neurons whose spike potentials were resistant to TTX but failed to survive in Na-free saline and (3) the neurons whose action potentials were suppressed by TTX(10(-8)g/ml) as well as Na removal. The mean duration of spike and after-hyperpolarization was longest in the first group of the neurons and shortest in the third, probably reflecting the difference in the contribution of Ca currents to action potentials. The unresponsiveness of the neurons to TTX was shown to be due to the insensitivity of Na as well as Ca components of action potentials to the toxin. It was discussed that the occurrence of TTX-resistant action potentials to the toxin. It was discussed that the occurrence of TTX-resistant action might be related to the neuronal development.

Action potentials of embryonic dorsal root ganglion neurones in Xenopus tadpoles

1. Several classes of action potentials can be distinguished in dorsal root ganglion cells, studied by intracellular recording techniques in Xenopus laevis tadpoles 4.5--51 days old. The ionic basis of the action potential was investigated by changing the ionic environment of the cells and applying various blocking agents. 2. The Ca2+-dependent action potential is a plateau of relatively long duration (mean 8.7 msec). It is unaffected by removal of Na+ but blocked by mM quantities of Co2+. It is present only in small cells. 3. Ca2+/Na+-dependent action potentials. Type I is a spike followed by a plateau or hump of different durations (mean 8.1 msec). The spike is selectively blocked by removal of Na+, leaving the plateau which is in turn blocked by Co2+. It is present in cells of small and intermediate size. Type II is a spike of short duration (mean 2.0 msec) with only an inflection on the falling phase. The spike is blocked by removal of Na+ and no other components can be elicited. The inflection is blocked by Co2+. It is present in cells of all sizes. Type III is similar to type I but is seen only in solutions in which the outward current is blocked. It was observed only very infrequently. 4. Na+-dependent action potentials. Type I a is a short duration spike (mean 1.1 msec). It is abolished by removal of Na+ or addition of tetrodotoxin (TTX), but largely unaffected by Co2+ or La3+. It is present in cells of all sizes. When the outward current channels are blocked and cells exposed to Na+-free solutions, all cells are capable of producing an action potential in which the inward current is carried by divalent cations. Type I b is a spike with a smooth, more slowly falling phase. It has the same pharmacological properties as type I a action potential and is present in cells of small size. 5. Na+-dependent action potentials. Type II is a spike with an inflection on the falling phase (mean duration 3.4 msec). It is prolonged by Co2+ and La3+. Removal of Na+ abolishes the spike but TTX does not block it. It is present in cells of all sizes. The mean resting potential is less than that of cells with Na+-dependent type I action potentials, while the mean input resistance is greater. 6. Tetraethylammonium chloride (TEA) prolongs the different kinds of action potentials. The amount of prolongation varies among cells with a given type of action potential, so that no distinction could be made of the different actionpotential types based on the effect of TEA. 7. The percent of cells with each kind of action potential varies with the developmental age of the animal. The number of cells with Ca2+ and Ca2+/Na+ action potentials decreases with age, while the number of cells with a Na+ type I action potentials increases. The Na+ type II action potential appears only at later stages. 8...

Developmental changes in the inward current of the action potential of Rohon-Beard neurones

1. Rohon-Beard cells in the spinal cord of Xenopus tadpoles have been studied in animals from early neural tube to free-swimming larval stages. The onset and further development of electrical excitability of these neurones has been investigated in different ionic environments, to determine the ionic species carrying the inward current of the action potential.2. The cells appear inexcitable at early stages (Nieuwkoop & Faber stages 18-20) and do not give action potentials to depolarizing current pulses.3. The action potential is first recorded at stage 20. (A) The inward current is carried by Ca(2+) at stages 20-25, since it is blocked by mm quantitites of La(3+), Co(2+) or Mn(2+) and is unaffected by removal of Na(+) or the addition of tetrodotoxin (TTX). (B) The action potential is an elevated plateau of long duration (mean 190 msec at stages 20-22). The duration decreases exponentially with repetitive stimulation. (C) The specific Ca(2+) conductance (g(Ca)) at the onset of the plateau of the action potential is 2.6 x 10(-4) mho/cm(2). Calculations show that a single action potential raises [Ca(2+)](1) by more than 100-fold.4. At later times (stages 25-40), the inward current of the action potential is carried by both Na(+) and Ca(2+): the action potential has two components, an initial spike which is blocked by removal of Na(+) or addition of TTX, followed by a plateau which is blocked by La(3+), Co(2+) or Mn(2+).5. Finally (stages 40-51), the inward current is primarily carried by Na(+), since the action potential is blocked only by removal of Na(+) or addition of TTX, and the overshoot agrees with the prediction of the Nernst equation for a Na-selective membrane. When the outward current channel is blocked and cells exposed to Na-free solutions, 67% of cells at the latest stages studied were incapable of producing action potentials in which the inward current is carried by divalent cations.6. The duration of the action potential decreases from a maximum of about 1000 msec to about 1 msec during development. The maximum input resistance (R(in)) decreases from ca. 1000 to 100 MOmega.7. The calcium action potential may play a role in the development of excitability and the growth of the neurones.