Calcium-dependent regulation of the membrane potential and extrajunctional acetylcholine receptors of rat skeletal muscle

Role for calcium from the sarcoplasmic reticulum in coupling muscle activity to nicotinic acetylcholine receptor gene expression in rat

Neurally evoked muscle electrical activity suppresses nicotinic acetylcholine receptor (nAChR) gene expression in extrajunctional domains of adult muscle fibers. It has been proposed that this regulation is mediated by calcium influx through voltage-dependent L-type calcium channels but bypasses the sarcoplasmic reticulum in chick and mouse C2C12 cells. Here we report that in rat muscle calcium influx through L-type calcium channels preferentially reduced nAChR epsilon-subunit RNA via a post-transcriptional mechanism. In contrast, calcium release from the sarcoplasmic reticulum (SR) suppressed nAChR subunit RNA levels as a result of decreasing nAChR subunit promoter activity. Finally, we show that this decreased promoter activity is mediated through the same DNA sequences that control activity-dependent gene expression. Therefore, we propose that in rat muscle, calcium release from the SR participates in coupling muscle depolarization to nAChR gene expression.

Depolarization-transcription signals in skeletal muscle use calcium flux through L channels, but bypass the sarcoplasmic reticulum

Membrane depolarization inactivates acetylcholine receptor (AChR) genes in skeletal muscle. We have studied this process in C2C12 cells, focusing on the role of calcium. Cytoplasmic calcium was monitored with fluo-3, and the activity of receptor genes was measured with a sensitive transcript elongation assay. Removal of extracellular calcium or blockage of L-type calcium channels disrupts signaling, even when release of calcium from the sarcoplasmic reticulum (SR) is not impeded, whereas L channel agonists induce signaling without membrane depolarization or release of calcium from intracellular stores. Activators of calcium release from the SR do not inhibit AChR genes, either in C2C12 or in chicken skeletal muscle in vivo. It appears that calcium ions do not act as messengers between sarcolemma and nucleus but target a sensor near their port of entry where they initiate a signal that bypasses the SR.

Calcium influx blocks the skeletal muscle acetylcholine receptor alpha-subunit gene in vivo

The transcriptional activity of the acetylcholine receptor alpha-subunit gene was measured in denervated chick skeletal muscle in response to calcium-active drugs, using a ribonuclease protection version of the conventional run-off assay. The L-channel agonist (-)Bay-K6844 and the calcium ionophore A23187 mimicked, and the intracellular chelator BAPTA and the calcium channel blockers D600 and nifedipine blocked, the effect of electrostimulation. These results suggest that influx of extracellular calcium is an integral component of the membrane depolarization-receptor gene inactivation cascade.

Measurement of intracellular Ca2+ in BC3H-1 muscle cells with Fura-2: relationship to acetylcholine receptor synthesis

Synthesis of acetylcholine receptors (AChR) can be affected by calcium, but the role played by this cation is controversial. The effect of changes in extracellular calcium, [Ca2+]o, on AChR synthesis was examined in a cultured mouse muscle cell line, BC3H-1. Reduction of [Ca2+]o for long periods (approximately 22 h) leads to a decrease in total surface AChR levels, a finding that is consistent with inhibition of AChR synthesis. A half-maximal reduction in surface AChR levels is observed when [Ca2+]o is decreased from 1.8 to approximately 5o microM. Under these conditions, however, total protein synthesis is also largely inhibited, suggesting that the effect of [Ca2+]o on AChR synthesis may be relatively non-specific. Increasing [Ca2+]i by adding the Ca2+ ionophore, A23187 (in the presence of 1.8 mM [Ca2+]o) also gives similar and significant reductions of both AChR and protein synthesis. Since the time course of changes in intracellular calcium [( Ca2+]i) produced by these manoeuvres is unknown, we examined the effects of briefer (1-6 h) reductions in [Ca2+]o and achieved a more specific reduction in AChR synthesis. A direct measurement of the changes in [Ca2+]i resulting from changes in [Ca2+]o was made using the fluorescent indicator Fura-2 and video fluorescence microscopy. Our results show that in BC3H-1 muscle cells the resting intracellular calcium decreases reversibly over 20 min when [Ca2+]o is decreased. We suggest that a reduction of [Ca2+]i produced by the lower [Ca2+]o underlies the reduction in AChR synthesis observed in these experiments.

Increase in electrically-stimulated Ca2+ release and suppression of caffeine response in diaphragm muscle of alloxan-diabetic mice compared with the denervation effect

Changes in intracellular Ca2+ release in the diaphragm muscle of alloxan-diabetic mice were compared with changes in normal muscles and non-diabetic denervated muscles. We measured Ca2+ transient aequorin luminescence by direct electrical stimulation of these muscles. External Ca2(+)-free solution readily decreased the Ca2+ transient in normal muscles but had less of an effect in diabetic muscles. Only when the muscles were pre-injected with EGTA (reducing intracellular levels of free Ca2+) did the Ca2+ transients decrease significantly in diabetic muscles, however, there was no effect in denervated muscles. The caffeine-induced increase in Ca2+ transients, however, was delayed in both diabetic muscles and non-diabetic denervated muscles. The caffeine response was observed in normal muscles under the external Ca2(+)-free conditions even after EGTA-pretreatment, whereas it was suppressed, after a brief increase, in both diabetic and non-diabetic denervated muscles. These results demonstrate (1) the insensitivity of intracellular Ca2+ mobilization to external Ca2+ levels and the ready accumulation of intracellular Ca2+ in the cytosol in the diabetic state, (2) increased permeability to Ca2+ in the denervated state and (3) impairment of the Ca2+ pool which responds to caffeine in both diabetes and the non-diabetic denervated state. Diabetic neuromyopathy thus appears to be a state of abnormal Ca2(+)-mobilization caused secondarily by high levels of blood glucose.

Neural regulation of mRNA for the alpha-subunit of acetylcholine receptors: role of neuromuscular transmission

Levels of mRNA for acetylcholine receptor (AChR) subunits are relatively low in innervated skeletal muscles. Following denervation they rise rapidly, leading to increased AChR synthesis. The mechanism by which motor nerves normally regulate these mRNA levels is not yet known. In order to determine the possible role of synaptic transmission in this process, we have compared the effect of blockade of cholinergic ACh transmission with that of surgical denervation. Blockade of quantal ACh transmission was produced by injection of type A botulinum toxin into the soleus muscles of rats. We measured mRNA for the alpha-subunit of the AChR (alpha-AChR mRNA) in RNA extracts of botulinum-treated, denervated, and normal control muscles by hybridization with a highly specific cDNA probe. Our findings show that treatment with botulinum toxin resulted in an increase in alpha-AChR mRNA which was similar to the effect of surgical denervation, although slower in its time course. Since botulinum toxin specifically inhibits quantal ACh release, these results support the concept that cholinergic synaptic transmission plays a key role in mediating the neural control of the alpha-AChR message. The difference between the effects of denervation and botulinum-treatment may be explained by the fact that botulinum toxin does not block the spontaneous non-quantal component of ACh transmission, which has previously been shown to have a partial influence in regulating certain properties of muscles. The present results suggest that synaptic transmission has an important influence in regulating gene expression in the target cell.

Acetylcholine-gated and chloride conductance channel expression in rat muscle membrane

1. During the differentiation of skeletal muscle, there is a synchronized expression of a number of muscle-specific proteins including the acetylcholine-gated ion channel (AChR). Another muscle-specific ion channel, responsible for chloride conductance, was shown to be expressed in an anticoordinate fashion to AChR. An organ culture system for rat lumbrical muscles was developed to manipulate the expression of these two ion channels. 2. Denervation induced a change in expression of both channels that was mimicked in culture and reversed by direct electrical stimulation. 3. The time course of the disappearance of both channels was similar and started immediately after denervation (chloride conductance) or stimulation (AChR). The time course of the appearance of AChR was delayed several days after denervation and culture but chloride conductance increased immediately upon stimulation. 4. The loss of chloride conductance in muscle cultured in cycloheximide exhibited first-order kinetics, providing an estimate of the half-life (2.3 days) for the chloride conductance channel. This resembled the disappearance of chloride conductance in normal medium, suggesting that synthesis of this channel ceases following denervation. The decrease in chloride conductance characteristic of denervated muscle was not halted by cycloheximide. 5. Changes in chloride conductance presumably alter the intracellular concentration of chloride. The possibility that chloride might regulate the expression of AChRs in skeletal muscle was tested by altering the intracellular concentration of chloride in muscles maintained in organ culture. 6. Denervated muscles, whose intracellular concentration of chloride is elevated, were cultured in medium containing 9 mM-chloride (low-Cl- medium). AChR expression was reduced by either low-Cl- medium or electrical stimulation. Together, low-Cl- medium and electrical stimulation reduced expression more than either treatment alone. 7. The loss of AChRs in low-Cl- medium was blocked when muscle fibrillation was halted by TTX. 8. When chloride conductance was blocked by 9AC (9-anthracene carboxylic acid) intracellular chloride was elevated to the levels seen in denervated muscle. The elevated levels of chloride did not prevent the reduction in AChR expression induced by electrical stimulation. 9. The uncoupling of AChR expression and the intracellular concentration of chloride showed that they were not rigidly linked. Chloride affects the expression of AChR indirectly, by altering the activity of muscle cells.

Activity-dependent regulation of gene expression in muscle and neuronal cells

In both the central and the peripheral nervous systems, impulse activity regulates the expression of a vast number of genes that code for synaptic proteins, including neuropeptides, enzymes involved in neurotransmitter biosynthesis and degradation, and membrane receptors. In recent years, the mechanisms involved in these regulations became amenable to investigation by the methods of recombinant DNA technology. The first part of this review focuses on the activity-dependent control of nicotinic acetylcholine receptor biosynthesis in vertebrate muscle, a model case for the regulation of synaptic protein biosynthesis at the postsynaptic level. The second part summarizes some examples of neuronal proteins whose biosynthesis is under the control of transsynaptic impulse activity. The first, second, and third intracellular messengers involved in membrane-to-gene signaling are discussed, as are possible posttranscriptional control mechanisms. Finally, models are proposed for a role of neuronal activity in the genesis and stabilization of the synapse.

Effects of prolonged depolarization on the nicotinic acetylcholine receptors of PC12 cells

To determine whether prolonged depolarization and/or changes in intracellular Ca2+ concentrations stimulate adaptive responses of neuronal nicotinic acetylcholine receptors, PC12 pheochromocytoma cells were grown in medium containing various concentrations of K+. Nicotinic receptor function was determined as carbachol-stimulated uptake of 86Rb+. Cells were exposed to 50 mM K+ for up to 4 days and then allowed to repolarize for 60 min. Under these conditions, no changes in basal or carbachol-stimulated uptake of 86Rb+ were observed. Furthermore, neither the time course of carbachol-stimulated uptake or the carbachol concentration dependence of 86Rb+ uptake was altered. Finally, concurrent depolarization did not affect the functional down-regulation produced by chronic exposure of the cells to carbachol. Thus, neuronal nicotinic acetylcholine receptors on PC12 cells do not appear to be regulated by depolarization or prolonged elevation of the intracellular Ca2+ level.

Mechanism of action of lithium on acetylcholine receptor metabolism in skeletal muscle

Changes in the levels of cations within skeletal muscle are thought to mediate the neural regulation of turnover of extrajunctional acetylcholine receptors (AChRs). We have used lithium as a probe of these cation influences because of its resemblance to calcium and other ions. In the present experiments we studied the mechanism of action of lithium on AChR metabolism in cultured mammalian skeletal muscle. We measured the effects of lithium on AChR turnover (using [125I]alpha-bungarotoxin binding), and evaluated the resemblance of lithium and calcium in producing their effects on AChR metabolism. Our results provide insight into the mechanisms of action of lithium and the cellular processes controlling AChR metabolism in muscle. Lithium reduces the number of AChRs in skeletal muscle in vitro to a degree similar to that which we previously reported in vivo. Lithium appears to enter cells via both sodium and calcium channels. It then produces its effect on levels of AChRs primarily by selectively reducing AChR synthesis and insertion into the surface membrane. Lithium induces this change in AChR metabolism in a manner resembling neural and calcium-mediated effects on AChRs. Phosphoinositide pathways may be involved in the lithium-induced effects. Further analysis of the effects of lithium on AChR turnover should provide new information about the mechanisms underlying the cellular control of receptor metabolism.

Increases in muscle Ca2+ mediate changes in acetylcholinesterase and acetylcholine receptors caused by muscle contraction

The synthesis of acetylcholinesterase (AcChoE; acetylcholine acetylhydrolase, EC 3.1.1.7) and of acetylcholine receptors (AcChoR) by cultured rat muscle fibers is influenced strongly by the level of muscle contractile activity. If fibers are grown in the presence of tetrodotoxin (TTX) to block spontaneous contraction, the total amount of AcChoE decreases markedly, as does the percentage of AcChoE assembled as the collagen-tailed presumed synaptic form of the enzyme. Under these conditions, however, the number of AcChoR increases. We demonstrate here that each effect of TTX can be prevented by treating the muscle cells with the calcium ionophore A23187. Thus, cells treated with A23187 and TTX have 30- to 40-fold higher levels of collagen-tailed AcChoE and lower levels of AcChoR by a factor of 4-5 than do cells grown in TTX alone. These results suggest that an increase in muscle cytoplasmic Ca2+ mediates the known effects of muscle contraction on these cholinergic macromolecules.

Acetylcholine receptor: an allosteric protein

The nicotine receptor for the neurotransmitter acetylcholine is an allosteric protein composed of four different subunits assembled in a transmembrane pentamer alpha 2 beta gamma delta. The protein carries two acetylcholine sites at the level of the alpha subunits and contains the ion channel. The complete sequence of the four subunits is known. The membrane-bound protein undergoes conformational transitions that regulate the opening of the ion channel and are affected by various categories of pharmacologically active ligands.

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.

Glucocorticoid regulation of synaptic development

The effect of glucocorticoid hormones on the developmental step in which a presynaptic neuron acquires the ability to transmit excitatory information across a synapse was explored using a retina muscle cell culture system. Cholinergic neurons dissociated from the perinatal rat retina form functional synapses in culture with rat striated muscle cells. Early in the functional maturation of these retina muscle synapses, there is a period in which release of acetylcholine occurs spontaneously, but cannot be evoked. This stage is followed by the emergence of neurotransmitter release that is stimulus-evoked and dependent on extracellular calcium. Here, it is reported that glucocorticoid hormones accelerate this developmental sequence. Experimental findings indicate that this hormonal effect occurs at physiological concentrations, involves glucocorticoid receptors, acts at the transcriptional level and requires protein synthesis. A hypothesis is that glucocorticoids regulate the development of mechanisms which couple neuronal depolarization with release of neurotransmitter. The acceleration of the functional maturation of cholinergic retinal neurons also can occur in utero if pregnant rats are injected with a synthetic glucocorticoid or stressed by cold exposure. Thus, alterations in the time-course of synaptic maturation are not restricted to manipulation of culture conditions. The results presented here indicate that glucocorticoid hormones can regulate the timing of the developmental step in which cholinergic neurons of the rat retina become capable of releasing acetylcholine in response to excitatory stimulation.

Levels of enzymes of energy metabolism are controlled by activity of cultured rat myotubes

We have investigated the effects of inhibiting the spontaneous activity of cultured rat myotubes on several representative enzymes of glycolytic and oxidative metabolism. The results presented demonstrate that contractile activity in the absence of nerves can regulate the amounts of these enzymes and indicate that muscle activity may partially control development of the metabolic types of muscle fibers. Control muscle cells have relatively high levels of glycolytic enzymes and low oxidative enzymes and metabolically most closely resemble fast glycolytic fibers. The divalent cation ionophore A23187 caused enzyme levels of the cultured cells to change towards those found in tonically contracting skeletal muscle fibers in vivo. The evidence presented suggests that calcium may mediate certain of the effects associated with muscle contraction on enzymes of energy metabolism.

GK(Ca)-dependent cyclic potential changes in the sensory nerve terminal of frog muscle spindle

Spontaneous cyclic hyperpolarizations along the sensory nerve terminal of frog muscle spindles were observed during the application of 1-9 nA depolarizing currents across an air-gap on which the axon was bridged. An increase in the current intensity increased the amplitude and duration of the cyclic changes. Upon subthreshold depolarization, single or repetitive hyperpolarizations could be elicited after a brief electric pulse or during stretch of the receptors, respectively. The threshold was decreased in higher Ca2+, Sr2+ or Ba2+ solutions. The cyclic changes were reversibly blocked by K+- or Ca2+-blockers and quinine. These results suggest that the changes are due to GK(Ca). The site of origin of the changes was at the branching node in the capsule, as confirmed by the following results: (1) the cyclic changes were abolished upon inactivating the node by UV-irradiation; (2) in normal Ringer's solution, the rate of afferent impulses, which reflects the membrane potential at the encoding site along the non-myelinated filaments, was unmodified by the cyclic changes and was independent of the intensity of the polarizing currents within a certain range; however, it was sensitively dependent on this intensity after treatment with K+-blockers; (3) the amplitude of the impulses reaching the branching node was attenuated during the cyclic changes, but not after GK-blockade.

Evidence for the role of non-quantal acetylcholine in the maintenance of the membrane potential of rat skeletal muscle

1. Resting membrane potentials of rat diaphragm muscles cultured in Trowell T8 medium were measured in vitro. After 3 hr in culture the resting membrane potential of muscle fibres within 2.5 mm of nerve section (;near') was -68.3 +/- 0.4 mV (nineteen preparations). This was significantly lower (P < 0.001) than the resting potential (-74.0 +/- 0.4 mV) measured in muscle fibres 8-10 mm from the site of nerve section (;far') in the same preparations. A difference between the ;near' and the ;far' fibres was maintained in muscles cultured for 6 and 12 hr. Miniature end-plate potentials were present in both ;near' and ;far' fibres cultured for 3 and 6 hr and ceased after 12-15 hr.2. The presence of carbamylcholine (10(-7) or 10(-8) M) maintained the resting membrane potential of ;near' fibres close to that of ;far' fibres at 3, 6 and 12 hr. For example, at 3 hr in the presence of 10(-8) M-carbamylcholine the mean resting potential was 75.6 +/- 0.5 mV in ;near' fibres and 76.1 +/- 0.4 mV in ;far' fibres (four preparations). A similar effect was produced in preparations exposed to anticholinesterases: diisopropylphosphorofluoridate (DFP) (10(-7) M), neostigmine (10(-7) M) or physostigmine (10(-5) M).3. Agents that blocked acetylcholine receptors had the reverse effect. In the presence of alpha-bungarotoxin (1 mug/ml.) or d-tubocurarine (10(-5) M) the resting membrane potential of ;far' fibres was reduced to the level of ;near' fibres over the 24 hr period of observation. For example, at 3 hr in the presence of alpha-bungarotoxin the mean resting potential was 67.2 +/- 0.5 mV in ;near' fibres and 68.5 +/- 0.6 mV in ;far' fibres (six preparations). The effect of d-tubocurarine was reversible.4. When muscles were cultured in Ca(2+)-free medium containing 1 mM-EGTA and 10 mM-Mg(2+), there was no difference in membrane potential between ;near' and ;far' fibres and physostigmine (10(-5) M) was ineffective in raising the membrane potential of ;near' fibres.5. It is suggested that non-quantal acetylcholine released from nerve terminals maintains the membrane potential of muscle fibres through a Ca(2+)-dependent mechanism.