Tetrodotoxin-sensitive and -insensitive action potentials in myotubes

Combination of agrin and laminin increase acetylcholine receptor clustering and enhance functional neuromuscular junction formation In vitro

Clustering of acetylcholine receptors (AChR) at the postsynaptic membrane is a crucial step in the development of neuromuscular junctions (NMJ). During development and after denervation, aneural AChR clusters form on the sarcolemma. Recent studies suggest that these receptors are critical for guiding and initiating synaptogenesis. The aim of this study is to investigate the effect of agrin and laminin-1; agents with known AChR clustering activity; on NMJ formation and muscle maturation. Primary myoblasts were differentiated in vitro on collagen, laminin or collagen and laminin-coated surfaces in the presence or absence of agrin and laminin. The pretreated cells were then subject to innervation by PC12 cells. The number of neuromuscular junctions was assessed by immunocytochemical co-localization of AChR clusters and the presynaptic marker synaptophysin. Functional neuromuscular junctions were quantitated by analysis of the level of spontaneous as well as neuromuscular blocker responsive contractile activity and muscle maturation was assessed by the degree of myotube striation. Agrin alone did not prime muscle for innervation while a combination of agrin and laminin pretreatment increased the number of neuromuscular junctions formed and enhanced acetylcholine based neurotransmission and myotube striation. This study has direct clinical relevance for treatment of denervation injuries and creating functional neuromuscular constructs for muscle tissue repair.

Intrinsic ionic conductances mediate the spontaneous electrical activity of cultured mouse myotubes

Mouse skeletal myotubes differentiated in vitro exhibited spontaneous contractions associated with electrical activity. The ionic conductances responsible for the origin and modulation of the spontaneous activity were examined using the whole-cell patch-clamp technique and measuring [Ca(2+)](i) transients with the Ca(2+) indicator, fura 2-AM. Regular spontaneous activity was characterized by single TTX-sensitive action potentials, followed by transient increases in [Ca(2+)](i). Since the bath-application of Cd(2+) (300 microM) or Ni(2+) (50 muM) abolished the cell firing, T-type (I(Ca,T)) and L-type (I(Ca,L)) Ca(2+) currents were investigated in spontaneously contracting myotubes. The low activation threshold (around -60 mV) and the high density of I(Ca,T) observed in contracting myotubes suggested that I(Ca,T) initiated action potential firing, by bringing cells to the firing threshold. The results also suggested that the activity of I(Ca,L) could sustain the [Ca(2+)](i) transients associated with the action potential, leading to the activation of apamin-sensitive SK-type Ca(2+)-activated K(+) channels and the afterhyperpolarization (AHP) following single spikes. In conclusion, an interplay between voltage-dependent inward (Na(+) and Ca(2+)) and outward (SK) conductances is proposed to mediate the spontaneous pacemaker activity in cultured muscle myotubes during the process of myogenesis.

Sarcoplasmic-reticulum biogenesis in contraction-inhibited skeletal-muscle cultures

We have previously shown that inhibition of the spontaneous contractile activity of cultured embryonic-chick skeletal-muscle fibres with tetrodotoxin (TTX) leads to decreased sarcoplasmic-reticulum Ca(2+)-transport rates and steady-state concentrations of the high-energy Ca(2+)-ATPase phosphoenzyme intermediate [Charuk & Holland (1983) Exp. Cell Res. 144, 143-157]. In the present study we used a monoclonal antibody to the Ca(2+)-ATPase to show that there is a decreased amount of enzyme accumulated by contraction-inhibited myotubes. Indirect immunofluorescence microscopy using the monoclonal antibody to the Ca(2+)-ATPase also revealed a disordered subcellular organization of the sarcotubular system in contraction-inhibited myotubes. The biogenesis of sarcoplasmic-reticulum proteins in TTX-paralysed myofibres was studied by labelling cells with [35S]methionine before isolation of the active Ca(2+)-pump membrane fraction. Protein turnover was selectively increased in that fraction from TTX-treated muscle cultures. Electrophoretic analysis and quantitative fluorography confirmed that decreased accumulation of the Ca(2+)-ATPase enzyme in contraction-inhibited myotubes was associated with increased turnover of this protein. The present results demonstrate that biogenesis of the sarcoplasmic-reticulum Ca(2+)-ATPase is regulated by the contractile activity of skeletal-muscle fibres.

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.

Brain extract induces the tetrodotoxin-sensitive action potentials in a rat skeletal muscle cell line (L6)

Tetrodotoxin (TTX) usually blocks the action potential in adult vertebrate innervated skeletal muscle, i.e. the action potential is operated by a TTX-sensitive sodium channel. The L6 cultured myotube, however, has TTX-resistant action potential that is usually found in skeletal muscle following denervation or during fetal and neonatal stages in rat. The TTX-resistivity of the L6 myotubes might be due to a lack of a neurotrophic substance. The present study revealed that brain extract had a trophic substance that is capable of inducing the TTX-sensitive sodium channels in uninnervated cultured L6 myotubes. Seventeen to 18 days old cultures were used and the maximum rate of rise of action potential was measured as an index of the sodium channel density. In control cultures, the maximum rate of rise was about 120 V/s and did not change by TTX application. However, in cultures with the brain extract the value was 142 V/s and reduced to 86 V/s by TTX. The reduced maximum rate of rise by TTX in the latter cultures means that exposure of L6 myotubes to brain extract resulted in an increase in TTX-sensitive sodium channels and a decrease in TTX-resistant sodium channels. Therefore, exposure to a factor in brain extract can elicit development of TTX-sensitive sodium channels in isolated L6 muscle cells that have been grown as a cell line in the absence of neurons for over 10 years.

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.

Neurotrophic substance develops tetrodotoxin-sensitive action potential and increases curare-sensitivity of acetylcholine response in cultured rat myotubes

We examined the trophic effects of a partially purified trophic substance from mouse spinal cord extract on the tetrodotoxin (TTX)-sensitivity of action potentials and on acetylcholine-sensitivity of rat skeletal myotubes in 7- and 8-day-old cultures. Many myotubes grown in control medium generate action potentials in the presence of TTX (10(-6) M). The addition of fraction E (Fr.E) from a Biogel P2 column, which exhibited trophic activity on adult denervated muscle in organ culture, decreased TTX-resistivity of action potentials of myotubes in cell culture. The trophic substance was also effective when further purified by paper chromatography and electrophoresis. The response to iontophoretically applied acetylcholine of Fr.E-treated myotubes was much more reduced by D-tubocurarine (10(-7) g/ml) than those of control cultured myotubes. No difference in morphological differentiation, protein synthesis, creatine phosphokinase activity or specific binding of [125I]alpha-bungarotoxin was observed between control and Fr.E-treated cultures. These results suggest that the trophic substance in Fr.E may be involved in the normal development of TTX-sensitive sodium channels and of acetylcholine receptor properties.

Acetylcholine receptor distribution on myotubes in culture correlated to acetylcholine sensitivity

1. A linear relation, with a slope of 0-9 +/- 0-2 on a log-log plot, was obtained between acetylcholine (ACh) sensitivity and alpha-bungarotoxin (alpha-BTX) binding site density in developing L6 and rat primary myotubes. ACh sensitivity was defined as g/Qn where g is conductance, Q is ACh charge and n is the Hill coefficient. Experimentally we found n approximately 1-7 for our myotubes, which is similar in value to that reported for adult systems. 2. The linear relationship is compatible with an organization whereby each ion channel is always complexed with a fixed number of ACh receptors such that the dose-response characteristics of each such complex are independent of average ACh receptor density. 3. Light microscope autoradiography showed that the alpha-bungarotoxin binding sites on L6 myotubes are uniformly distributed over the surface, while primary rat myotubes exhibit gradients and hot spots. Electron microscope autoradiography indicated that about 70% of the [125I]alpha-bungarotoxin label was on the surface of the myotubes. The alpha-bungarotoxin site density, after subtracting myoblast background, varied from 5 to 400 sites/micrometer2 on different L6 myotubes, and from 54 to 900 sites/micrometer2 on primary rat myotubes, with occasional hot spots of 3000-4000 sites/micrometer2. The conductance sensitivities varied from 10(-4) to 2 X 10(-2) Momega-1/nC1-7.

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.

Genetic and physiological evidence concerning the development of chemically sensitive voltage-dependent inophores in L6 cells

The electrophysiological properties of a tissue culture muscle line, L6, and a K+ resistant mutant (MK1) derived from L6 were determined to elucidate certain aspects of membrane differentiation and function. MK1 was selected as a clone of myoblasts resistant to the toxic effects of 55 mM K+. The resting potentials of L6 and MK1 myoblasts and myotubes were K+ dependent and equal. The amplitudes of the action potentials were equal in normal medium, but 27.7 mM K+ interfered with or eliminated the ability of L6 myotubes to produce action potentials. MK1 myotubes produced nearly normal action potentials under these conditions. Thus, the K+ resistant myoblasts differentiate into myotubes which have an action potential generating mechanism much less sensitive to K+ than the normal mechanism. Also, both d-tubocurarine and alpha-bungarotoxin enhance the amplitude of the action potentials produced by L6 myotubes in the presence of 27.7 mM K+; these compounds do not enhance the amplitude of the action potentials produced by MK1 myotubes under the same conditions. It is proposed that as a consequence of differentiation a type of ionophore present in myoblasts becomes a voltage-dependent inophore in myotubes. Furthermore, these voltage-dependent ionophores can be chemically sensitive.

Pharmacologic characterization of the Na+ ionophores in L6 myotubes

We present a pharmacologic characterization of the Na+ ionophores present in L6 myotubes in vitro. Action potentials are abolished by replacement of the external Na+ by Tris. The amplitude of the action potential is generally resistant to high concentrations of tetrodotoxin (10(-5) M) and saxitoxin (10(-6 M), but the effect of these agents is highly variable. Veratridine (10(-4 M) consistently induces, as a short-term effect, a marked prolongation of the falling phase of the action potential. As a long-term effect, veratridine consistently induces a Na+-dependent reduction in the resting potential of the cell. The effects of veratridine on the action potential are not antagonized by tetrodotoxin or saxitoxin. However, the effects of veratridine on the resting potential are strongly antagonized by tetrodotoxin (10(-5) M) and fully inhibited by saxitoxin (10(-6) M). Significantly, under conditions where saxitoxin has fully inhibited the effects of veratridine on the resting potential, the myotubes are capable of generating overshooting action potentials. In contrast to their sensitivity to veratridine, L6 myotubes are insensitive to 10(-5) M alpha-dihydro-grayanotoxin-II. These results are discussed in the contexts of developmental significance and current views about Na+ ionophores.

Electrophysiological properties of the membrane and acetylcholine receptor in developing rat and chick myotubes

Membrane properties of rat and chick myotubes in various stages of development were studied. Resting membrane potentials (Em) increased from -8 to -55 mV in both rat and chick as the myotubes developed from myoblasts to large multinucleated fibers. In the rat myotubes, this increase was not accompanied by significant changes in specific membrane resistivity or changes in Na+ and K+ ion distribution. Nor have we observed a significant electrogenic component to the resting Em of mature rat myotubues under normal circumstances. A progressive increase in the passive permeability of the membrane to K+ relative to Na+ ions has been observed which can account for the changes in Em with development. In contrast to the changes in the ionic selectivity of the membrane, we have found that the ionic selectivity of the ACh receptor of rat and chick myotubes remains constant during the same period of myotube development.