Differential expression of sodium channel activities during the development of chick skeletal muscle cells in culture

Expression and electrophysiological characteristics of VGSC during mouse myoblasts differentiation

Voltage-gated sodium channels (VGSC) are essential for triggering and relaying action potentials (AP), which perform critical functions in a variety of physiological processes, such as controlling muscle contractions and facilitating the release of neurotransmitters. In this study, we used a mouse C2C12 cell differentiation model to study the molecular expression and channel dynamics of VGSC and to investigate the exact role of VGSC in the development of muscle regeneration. Immunofluorescence, Real-time quantitative polymerase chain reaction, Western blot, and whole-cell patch clamp were employed for this purpose in mouse myoblasts. The findings revealed an increase in intracellular sodium concentration, NaV1.4 gene expression, and protein expression with the progress of differentiation (days 0, 1, 3, 5 and 7). Furthermore, VGSC dynamics exhibit the following characteristics: ① The increase of sodium current (INa); ② The decrease in the activation threshold and the voltage trigger maximum of INa; ③ A positive shift in the steady-state inactivation curve; ④ The recovery of INa during repolarization is delayed, the activity-dependent decay rate of INa was accelerated, and the proportionate amount of the fraction of activated channels was reduced. Based on these results, it is postulated that the activation threshold of AP could be decreased, and the refractory period could be extended with the extension of differentiation duration, which may contribute to muscle contraction. Taken together, VGSC provides a theoretical and empirical basis for exploring potential targets for neuromuscular diseases and other therapeutic muscle regeneration dysfunctions.

Isolation of two saxitoxin-sensitive sodium channel subtypes from rat brain with distinct biochemical and functional properties

Two different 3H-saxitoxin-binding proteins, with distinct biochemical and functional properties, were isolated from rat brain using a combination of anion exchange and lectin affinity chromatography as well as high resolution size exclusion and anion exchange HPLC. The alpha subunits of the binding proteins had different apparent molecular weights on SDS-PAGE (Type A: 235,000; Type B: 260,000). When reconstituted into planar lipid bilayers, the two saxitoxin-binding proteins formed sodium channels with different apparent single-channel conductances in the presence of batrachotoxin (Type A: 22 pS; Type B: 12 pS) and veratridine (Type A: 9 pS; Type B: 5 pS). The subtypes were further distinguished by scorpion (Leiurus quinquestriatus) venom which had different effects on single-channel conductance and gating of veratridine-activated Type A and Type B channels. Scorpion venom caused a 19% increase in single-channel conductance of Type A channels and a 35-mV hyperpolarizing shift in activation. Scorpion venom doubled the single-channel conductance of Type B channels and shifted activation by at least 85 mV.

Developmental changes in the regulation of glycine-activated Cl- channels of cultured rat medullary neurons

Glycine-activated currents in 1- to 11-day-old rat medullary neurons were studied using patch clamp techniques. Glycine produced neither repeatable whole-cell current responses nor single-channel activity in the cell-attached mode until cells were in culture for a week or more. However, Cl- channels were present at the early stages because glycine-activated channels were seen in excised, inside-out patches. Furthermore, for cells less than a week in culture, 10 patches which did not exhibit glycine-activated Cl- channels in the cell-attached mode did upon excision. Consequently, the activation properties of these Cl- channels undergo a developmental change in that some cellular factor(s) presumably prevents the Cl- channels from opening in the intact cell during the initial stages in culture.

Characterization of resting membrane potential and its electrogenic pump component in cultured chick myotubes

The role of the electrogenic Na+-K+ pump in the determination of the level of the resting membrane potential in cultured chick limb muscle was investigated. Transmembrane resting potential and ouabain-sensitive 86Rb-uptake were measured in myotubes at different ages in culture from 2 to 10 days in vitro. Inhibition of the Na+-K+ pump with ouabain prevented the developmental increase in membrane potential which normally follows fusion of myotubes (day 2-3). In mature myotubes, ouabain caused a dose-related decrease in both membrane potential and 86Rb-uptake, with values for EC50 and maximal effect being nearly the same on both variables. The decrease in membrane potential by ouabain, up to 20 mV maximum, occurred within 2-5 sec and was not accompanied by detectable changes in input resistance. Membrane potential was also reduced by a decrease in temperature of the recording medium and removal of extracellular K+, both of which reduce Na+-K+ pump activity. We also found that the relation between membrane potential and extracellular K+ concentration was completely attenuated by ouabain in the physiological range (2-10 mM). We conclude that the electrogenic Na+-K+ pump plays an important role in the determination of the resting membrane potential of chick myotubes and that regulation of its level is not entirely explained by the diffusion potential hypothesis.

Acetylcholine receptors and sodium channels in denervated and botulinum-toxin-treated adult rat muscle

1. The number of acetylcholine (ACh) receptors and Na channels was measured in adult rat hind-limb muscles after denervation or injection of botulinum toxin type A (BoTX), using specific binding of radiolabelled neurotoxins. 2. Denervation by sciatic nerve section increased the number of [125I]iodo-alpha-bungarotoxin ([125I]BTX) binding sites from low, unmeasurable levels to 39 +/- 3 fmol of toxin bound per milligram muscle protein at 21 days. 3. Subcutaneous injection of BoTX produced complete neuromuscular blockade for 11-14 days over which time the number of [125I]BTX binding sites increased with the same time course and to the same extent as following denervation. 4. Neither denervation nor BoTX treatment significantly altered the number of tritiated saxitoxin ([3H]STX) binding sites from normal values of 7.8 fmol/mg muscle weight or 57 +/- 3 fmol/mg homogenate protein. This may, however, correspond to a lower density of [3H]STX sites in the muscle membrane. 5. It was concluded that neuromuscular blockade with BoTX is equivalent to denervation in its effects on synthesis of ACh receptors. Numbers of Na channels are more stable than ACh receptors but may also be modulated by neuromuscular activity.

Sodium channels in vertebrate hearts. Three types of saxitoxin binding sites in heart

The affinity of saxitoxin binding to cardiac sarcolemmal and cytosolic fractions was examined across species. In amphibia (frog) the plasma membrane site demonstrated a high affinity (Kd approx. 5 X 10(-9) M) but the majority of the total sites in the homogenate appeared to be high affinity soluble sites (Kd approx. 2 X 10(-9) M). Chicken and turtle cardiac plasma membrane fractions bound [3H]saxitoxin with 500-fold less affinity (Kd values of approx. 2 X 10(-6) M). No binding was seen in the cytosol. The affinity of cardiac sarcolemmal binding in amphibians correlates quantitatively with the K0.5 for the inhibition of sodium currents. Physiological correlation of the low affinity saxitoxin sites in chicken and turtle with toxin concentrations necessary to inhibit the sodium current remains unclear. The hypothesis that frog cytosolic saxitoxin binding sites originated from sarcolemma during homogenization is examined. The presence of three types of saxitoxin binding sites in cardiac preparations supports the existence of sodium channel subtypes.

Components of the plasma membrane of growing axons. III. Saxitoxin binding to sodium channels

The density of sodium channels was measured in growing and mature axons of the olfactory nerve of the bullfrog, using as a probe the drug saxitoxin (STX). The toxin binds to control nerves from adult animals in a saturable manner with a dissociation constant of approximately 23 nM at 4 degrees C and a capacity of 72 fmol/mg wet weight, equivalent to about five sites per square micrometer of axolemma. In growing nerves, obtained from adult frogs 4-5 wk following removal of the original nerve, the STX-binding capacity per wet weight of tissue is markedly reduced, to approximately 25% of control values, and appears to decrease in the proximodistal direction. STX-binding data, expressed as STX/mg wet weight, was converted to STX/micron 2 of axolemma using stereologically derived values of membrane area per milligram wet weight of nerve. The axolemmal content (area/mg wet weight) of all regions of growing nerve is substantially decreased compared to controls, but increases in the proximodistal direction by 60%. These changes in axolemmal area result in calculated STX receptor densities (per unit axolemmal area) which, in distal regions, are approximately at the level of the mature nerve and, in proximal regions, are actually increased above controls by 50 to 70%. Upon comparing the axolemmal density of intramembrane particles, reported in the companion paper, with the calculated density of STX receptors in both mature and growing nerves, we find a correlation between STX receptors and intramembrane particles with diameters of 11.5-14.0 nm. The growing axon's gradient of sodium channels and the shift from this gradient to a uniform distribution in the mature axon suggest (a) that sodium channels are inserted into the perikaryal plasmalemma and diffuse from there into the growing axolemma, and (b) that the axolemma undergoes functional maturation during growth.