Sodium channels near end-plates and nuclei of snake skeletal muscle

Membrane excitability: Ankyrins keep neuromuscular junctions firing

Voltage-gated sodium channels are clustered and immobilized at high densities in electrically excitable cells. A new study shows that ankyrins are essential to tether sodium channels and prevent synaptic fatigue at the neuromuscular junction.

Nature and Action of Antibodies in Myasthenia Gravis

This article discusses antibodies associated with immune-mediated myasthenia gravis and the pathologic action of these antibodies at the neuromuscular junctions of skeletal muscle. To explain how these antibodies act, we consider the physiology of neuromuscular transmission with emphasis on 4 features: the structure of the neuromuscular junction; the roles of postsynaptic acetylcholine receptors and voltage-gated Na⁺ channels and in converting the chemical signal from the nerve terminal into a propagated action potential on the muscle fiber that triggers muscle contraction; the safety factor for neuromuscular transmission; and how the safety factor is reduced in different forms of autoimmune myasthenia gravis.

In vivo microscopy reveals extensive embedding of capillaries within the sarcolemma of skeletal muscle fibers

OBJECTIVE

To provide insight into mitochondrial function in vivo, we evaluated the 3D spatial relationship between capillaries, mitochondria, and muscle fibers in live mice.

METHODS

3D volumes of in vivo murine TA muscles were imaged by MPM. Muscle fiber type, mitochondrial distribution, number of capillaries, and capillary-to-fiber contact were assessed. The role of Mb-facilitated diffusion was examined in Mb KO mice. Distribution of GLUT4 was also evaluated in the context of the capillary and mitochondrial network.

RESULTS

MPM revealed that 43.6 ± 3.3% of oxidative fiber capillaries had ≥50% of their circumference embedded in a groove in the sarcolemma, in vivo. Embedded capillaries were tightly associated with dense mitochondrial populations lateral to capillary grooves and nearly absent below the groove. Mitochondrial distribution, number of embedded capillaries, and capillary-to-fiber contact were proportional to fiber oxidative capacity and unaffected by Mb KO. GLUT4 did not preferentially localize to embedded capillaries.

CONCLUSIONS

Embedding capillaries in the sarcolemma may provide a regulatory mechanism to optimize delivery of oxygen to heterogeneous groups of muscle fibers. We hypothesize that mitochondria locate to PV regions due to myofibril voids created by embedded capillaries, not to enhance the delivery of oxygen to the mitochondria.

A microfluidic device for whole-animal drug screening using electrophysiological measures in the nematode C. elegans

This paper describes the fabrication and use of a microfluidic device for performing whole-animal chemical screens using non-invasive electrophysiological readouts of neuromuscular function in the nematode worm, C. elegans. The device consists of an array of microchannels to which electrodes are attached to form recording modules capable of detecting the electrical activity of the pharynx, a heart-like neuromuscular organ involved in feeding. The array is coupled to a tree-like arrangement of distribution channels that automatically delivers one nematode to each recording module. The same channels are then used to perfuse the recording modules with test solutions while recording the electropharyngeogram (EPG) from each worm with sufficient sensitivity to detect each pharyngeal contraction. The device accurately reported the acute effects of known anthelmintics (anti-nematode drugs) and also correctly distinguished a specific drug-resistant mutant strain of C. elegans from wild type. The approach described here is readily adaptable to parasitic species for the identification of novel anthelmintics. It is also applicable in toxicology and drug discovery programs for human metabolic and degenerative diseases for which C. elegans is used as a model.

Blood vessels and desmin control the positioning of nuclei in skeletal muscle fibers

Skeletal muscle fibers contain hundreds to thousands of nuclei which lie immediately under the plasmalemma and are spaced out along the fiber, except for a small cluster of specialized nuclei at the neuromuscular junction. How the nuclei attain their positions along the fiber is not understood. Here we show that the nuclei are preferentially localized near blood vessels (BV), particularly in slow-twitch, oxidative fibers. Thus, in rat soleus muscle fibers, 81% of the nuclei appear next to BV. Lack of desmin markedly perturbs the distribution of nuclei along the fibers but does not prevent their close association with BV. Consistent with a role for desmin in the spacing of nuclei, we show that denervation affects the organization of desmin filaments as well as the distribution of nuclei. During chronic stimulation of denervated muscles, new BV form, along which muscle nuclei align themselves. We conclude that the positioning of nuclei along muscle fibers is plastic and that BV and desmin intermediate filaments each play a distinct role in the control of this positioning.

Crucial role of sodium channel fast inactivation in muscle fibre inexcitability in a rat model of critical illness myopathy

Critical illness myopathy is an acquired disorder in which skeletal muscle becomes electrically inexcitable. We previously demonstrated that inactivation of Na+ channels contributes to inexcitability of affected fibres in an animal model of critical illness myopathy in which denervated rat skeletal muscle is treated with corticosteroids (steroid denervated; SD). Our previous work, however, did not address the relative importance of membrane depolarization versus a shift in the voltage dependence of fast inactivation in causing inexcitability. It also remained unknown whether changes in the voltage dependence of activation or slow inactivation play a role in inexcitability. In the current study we found that a hyperpolarizing shift in the voltage dependence of fast inactivation of Na+ channels is the principal factor underlying inexcitability in SD fibres. Although depolarization tends to decrease excitability, it is insufficient to account for inexcitability in SD fibres since many normal and denervated fibres retain normal excitability when depolarized to the same resting potentials as affected SD fibres. Changes in the voltage dependence of activation and slow inactivation of Na+ channels were also observed in SD fibres; however, the changes appear to increase rather than decrease excitability. These results highlight the importance of the change in fast inactivation in causing inexcitability of SD fibres.

Mechanisms of adaptation in a predator-prey arms race: TTX-resistant sodium channels

Populations of the garter snake Thamnophis sirtalis have evolved geographically variable resistance to tetrodotoxin (TTX) in a coevolutionary arms race with their toxic prey, newts of the genus Taricha. Here, we identify a physiological mechanism, the expression of TTX-resistant sodium channels in skeletal muscle, responsible for adaptive diversification in whole-animal resistance. Both individual and population differences in the ability of skeletal muscle fibers to function in the presence of TTX correlate closely with whole-animal measures of TTX resistance. Demonstration of individual variation in an essential physiological function responsible for the adaptive differences among populations is a step toward linking the selective consequences of coevolutionary interactions to geographic and phylogenetic patterns of diversity.

Sodium channel inactivation in an animal model of acute quadriplegic myopathy

We previously demonstrated that muscle fibers become unable to fire action potentials in both patients and an animal model of acute quadriplegic myopathy (AQM). In the animal model, skeletal muscle is denervated in rats treated with high-dose corticosteroids (steroid-denervated; SD), and muscle fibers become inexcitable despite resting potentials and membrane resistances similar to those of control denervated fibers that remain excitable. We show here that unexcitability of SD fibers is due to increased inactivation of sodium channels at the resting potential of affected fibers. A hyperpolarizing shift in the voltage dependence of inactivation in combination with the depolarization of the resting potential induced by denervation results in inexcitability. Our findings suggest that paralysis in the animal model of AQM is the result of an abnormality in the voltage dependence of sodium channel inactivation.

Sodium channel distribution on uninnervated and innervated embryonic skeletal myotubes

Acetylcholine receptor (AChR) and sodium (Na(+)) channel distributions within the membrane of mature vertebrate skeletal muscle fibers maximize the probability of successful neuromuscular transmission and subsequent action potential propagation. AChRs have been studied intensively as a model for understanding the development and regulation of ion channel distribution within the postsynaptic membrane. Na(+) channel distributions have received less attention, although there is evidence that the temporal accumulation of Na(+) channels at developing neuromuscular junctions (NMJs) may differ between species. Even less is known about the development of extrajunctional Na(+) channel distributions. To further our understanding of Na(+) channel distributions within junctional and extrajunctional membranes, we used a novel voltage-clamp method and fluorescent probes to map Na(+) channels on embryonic chick muscle fibers as they developed in vitro and in vivo. Na(+) current densities on uninnervated myotubes were approximately one-tenth the density found within extrajunctional regions of mature fibers, and showed several-fold variations that could not be explained by a random scattering of single channels. Regions of high current density were not correlated with cellular landmarks such as AChR clusters or myonuclei. Under coculture conditions, AChRs rapidly concentrated at developing synapses, while Na(+) channels did not show a significant increase over the 7 day coculture period. In vivo investigations supported a significant temporal separation between Na(+) channel and AChR aggregation at the developing NMJ. These data suggest that extrajunctional Na(+) channels cluster together in a neuronally independent manner and concentrate at the developing avian NMJ much later than AChRs.

Clustering of sodium channels at the neuromuscular junction

Voltage-gated sodium channels (NaChs) are highly concentrated in the postsynaptic region of the neuromuscular junction, especially in the depths of postsynaptic folds and in the perijunctional region. The formation of the high NaCh density occurs during synapse maturation, approximately 2 weeks after initial synaptic contact in the rodent. The concentration of NaChs and their localization in the troughs of the folds increase the safety factor for neuromuscular transmission by reducing the threshold for initiation of the action potential. There is evidence that agrin plays a role in the formation of NaCh aggregation. Molecules such as ankyrin and syntrophin that bind NaChs may be important for maintenance of the high channel density at the endplate.

A novel voltage clamp technique for mapping ionic currents from cultured skeletal myotubes

The biophysical properties and cellular distribution of ion channels largely determine the input/output relationships of electrically excitable cells. A variety of patch pipette voltage clamp techniques are available to characterize ionic currents. However, when used by themselves, such techniques are not well suited to the task of mapping low-density channel distributions. We describe here a new voltage clamp method (the whole cell loose patch (WCLP) method) that combines whole-cell recording through a tight-seal pipette with focal extracellular stimulation through a loose-seal pipette. By moving the stimulation pipette across the cell surface and using a stationary whole-cell pipette to record the evoked patch currents, this method should be suitable for mapping channel distributions, even on large cells possessing low channel densities. When we applied this method to the study of currents in cultured chick myotubes, we found that the cell cable properties and the series resistance of the recording pipette caused significant filtering of the membrane currents, and that the filter characteristics depended in part upon the distance between the stimulating and recording pipettes. We describe here how we determined the filter impulse response for each loose-seal pipette placement and subsequently recovered accurate estimates of patch membrane current through deconvolution.

Mapping fungal ion channel locations

Ion channel mapping techniques are described and the results for two fungal organisms, Saprolegnia ferax and Neurospora crassa, are presented. In these species, two channel types have been characterized, stretch-activated channels exhibiting significant calcium permeability and spontaneous channels having significant potassium permeability. Two distinct analyses of patch clamp data, analysis of channel self-clustering and association between different channel types, and localization along the hyphae, reveal significant differences between the two organisms. S. ferax maintains a tip-high gradient of both channel types which is lost after disruption of the actin cytoskeleton. There is significant self-clustering of the channels, as well as interactions between channel types. N. crassa on the other hand does not maintain tip-high gradients, and clustered distributions are observed only for the stretch-activated channels. In terms of physiological roles, evidence is quite strong that the stretch-activated channels function as a growth sensor in S. ferax, but have an unknown function in N. crassa. In both organisms, the potassium permeable channels presumably function in potassium uptake. The differences between these two organisms may be due, in part, to differences in their normal environment: aquatic versus terrestrial. Copyright 1998 Academic Press.

Effects of length changes on Na+ current amplitude and excitability near and far from the end-plate

Na+ current (INa), membrane capacitance (Cm), action potential (AP) properties, and cable properties were studied on the end-plate (E), the end-plate border (EB), and extrajunctional (EJ) membrane of rat fast twitch muscle fibers. INa normalized to Cm, which is proportional to the density of Na+ channels, was the same on the E and the EB and smallest on EJ membrane. The AP threshold was lower and rate of rise of the AP was larger at the EB compared with EJ membrane. On the E and the EB, Cm and INa did not change in response to changes in fiber length. On EJ membrane, INa, Cm, and membrane cable properties changed in a manner consistent with folding and unfolding of the sarcolemma during length changes. The stiffness of the E membrane may add mechanical stability of the neuromuscular junction so that the electrical properties of the end-plate do not change with fiber length. The higher density of Na+ channels near the end-plate increases the safety factor for neuromuscular transmission by lowering the AP threshold.

Na channel density in extrajunctional sarcolemma of fast and slow twitch mouse skeletal muscle fibres: functional implications and plasticity after fast motoneuron transplantation on to a slow muscle

Na channel densities were measured in fast and slow twitch mouse skeletal muscle fibres using the loose patch voltage clamp technique. It was found that Na channel density was approximately four times greater in fast twitch fibres than in slow. Computer simulations of action potential propagation in these fibres strongly suggest that the higher channel densities in fast twitch fibres are necessary to maintain action potential amplitude and fidelity of transmission across the neuromuscular junction, especially during the periods of rapid stimulation that these fibres are subjected to by their motoneurons. Transplantation of a foreign nerve containing axons which had previously innervated fast twitch fibres on to a slow twitch muscle resulted in an approximate doubling of the Na channel density in fibres innervated by the foreign nerve. These results suggest that motoneurons may exert considerable control over Na channel density in the muscle fibres they innervate.

Expression and distribution of sodium channels in short- and long-term denervated rodent skeletal muscles

1. Loose-patch voltage-clamp recordings were made from rat and mouse skeletal muscle fibres denervated for up to 6 weeks. Innervated muscles possessed a Na+ current density of 107 +/- 3.3 mA cm-2 in endplate membrane, and 6.3 +/- 0.6 mA cm-2 in extrajunctional membrane. This high concentration of Na+ channels at the endplate was gradually reduced following denervation. After 6 weeks of denervation, the endplate Na+ channel concentration was reduced by 40-50%, and the density of Na+ channels in extrajunctional membrane was increased by about 30%. 2. The tetrodotoxin (TTX)-resistant form of the Na+ channel appeared after 3 days of denervation and comprised approximately 43% of the endplate Na+ channels 5-6 days after denervation. Subsequently, TTX-resistant Na+ channels were reduced in density to approximately 25% of the postjunctional Na+ channels and remained at this level up to 6 weeks after denervation. 3. RNase protection analysis showed that mRNA encoding the TTX-resistant Na+ channel was virtually absent in innervated muscle, rose > 50-fold after 3 days of denervation, then decreased by 95% 6 weeks after denervation. The density of TTX-resistant Na+ channels correlated qualitatively with changes in mRNA levels. 4. These results suggest that the density of Na+ channels at neuromuscular junctions is maintained by two mechanisms, one influenced by the nerve terminal and the other independent of innervation.

Sodium channels aggregate at former synaptic sites in innervated and denervated regenerating muscles

The role of innervation in the establishment and regulation of the synaptic density of voltage-activated Na channels (NaChs) was investigated at regenerating neuromuscular junctions. Rat muscles were induced to degenerate after injection of the Australian tiger snake toxin, notexin. The loose-patch voltage clamp technique was used to measure the density and distribution of NaChs on muscle fibers regenerating with or without innervation. In either case, new myofibers formed within the original basal lamina sheaths, and, NaChs became concentrated at regenerating endplates nearly as soon as they formed. The subsequent increase in synaptic NaCh density followed a time course similar to postnatal muscles. Neuromuscular endplates regenerating after denervation, with no nerve terminals present, had NaCh densities not significantly different from endplates regenerating in the presence of nerve terminals. The results show that the nerve terminal is not required for the development of an enriched NaCh density at regenerating neuromuscular synapses and implicate Schwann cells or basal lamina as the origin of the signal for NaCh aggregation. In contrast, the change in expression from the immature to the mature form of the NaCh isoform that normally accompanies development occurred only partially on muscles regenerating in the absence of innervation. This aspect of NaCh regulation is thus dependent upon innervation.

Na+ currents near and away from endplates on human fast and slow twitch muscle fibers

Fast and slow twitch muscle fibers have distinct contractile properties. Here we determined that membrane excitability also varies with fiber type. Na+ currents (INa) were studied with the loose-patch voltage clamp technique on 29 histochemically classified human intercostal skeletal muscle fibers at the endplate border and > 200 microns from the endplate (extrajunctional). Fast and slow twitch fibers showed slow inactivation of endplate border and extrajunctional INa and had increased INa at the endplate border compared to extrajunctional membrane. The voltage dependencies of INa were similar on the endplate border and extrajunctional membrane, which suggests that both regions have physiologically similar channels. Fast twitch fibers had larger INa on the endplate border and extrajunctional membrane and manifest fast and slow inactivation of INa at more negative potentials than slow twitch fibers. For normal muscle, the differences between INa on fast and slow twitch fibers might: (1) enable fast twitch fibers to operate at high firing frequencies for brief periods; and (2) enable slow twitch fibers to operate at low firing frequencies for prolonged times. Disorders of skeletal membrane excitability, such as the periodic paralyses and myotonias, may impact fast and slow twitch fibers differently due to the distinctive Na+ channel properties of each fiber type.

Na+ current densities and voltage dependence in human intercostal muscle fibres

1. Voltage-clamp Na+ currents (INa) were studied in human intercostal muscle fibres using the loose-patch-clamp technique. 2. The fibres could be divided into two groups based upon the properties of INa. The two groups of fibres were called type 1 and type 2. 3. Both type 1 and type 2 fibres demonstrated fast and slow inactivation of INa. 4. Type 1 fibres had lower INa on the endplate border and extrajunctional membrane than type 2 fibres and required larger membrane depolarizations to inactivate Na+ channels by fast or slow inactivation of INa. 5. Type 2 fibres had a higher ratio of INa at the endplate border compared to extrajunctional membrane than Type 1 fibres. 6. Measurement of membrane capacitance suggested that the increase in INa at the endplate border was due to increased Na+ channel density. 7. Histochemical staining of some fibres suggested that type 1 fibres were slow twitch and type 2 fibres were fast twitch. 8. Differences in the properties of Na+ channels between fast- and slow-twitch fibres may contribute to the ability of fast-twitch fibres to operate at high firing frequencies and slow-twitch fibres to be tonically active.

Fast and slow twitch skeletal muscle fibres differ in their distribution of Na channels near the endplate

Sodium channel distributions were measured in fast and slow twitch rodent skeletal muscle fibres using the loose patch voltage clamp technique. Large differences were found between these fibre types with respect to Na channel density in the perijunctional region. Fast twitch fibres exhibited a large increase in Na channel density near the endplate, while slow twitch fibres did not.

Na current density at and away from end plates on rat fast- and slow-twitch skeletal muscle fibers

Na current density and membrane capacitance were studied with the loose patch voltage clamp technique on rat fast- and slow-twitch skeletal muscle fibers at three different regions on the fibers: 1) the end plate border, 2) greater than 200 microns from the end plate (extrajunctional), and 3) on the end plate postsynaptic membrane. Fibers were treated with collagenase to improve visualization of the end plate and to enzymatically remove the nerve terminal. The capacitance of membrane patches was similar on fast- and slow-twitch fibers and patches of membrane on the end plate had twice the capacitance of patches elsewhere. For fast- and slow-twitch fibers, the sizes of the Na current normalized to the area of the patch were as follows: end plate greater than end plate border greater than extrajunctional. For both types of fibers, the amplitudes of the Na current normalized to the capacitance of the membrane patch were as follows: end plate approximately end plate border greater than extrajunctional. At each of the three regions, the Na current densities were larger on fast-twitch fibers and fast-twitch fibers had a larger increase in Na current density at the end plate border compared with extrajunctional membrane.

Effect of agrin on the distribution of acetylcholine receptors and sodium channels on adult skeletal muscle fibers in culture

We used the loose patch voltage clamp technique and rhodamine-conjugated alpha-bungarotoxin to study the regulation of Na channel (NaCh) and acetylcholine receptor (AChR) distribution on dissociated adult skeletal muscle fibers in culture. The aggregate of AChRs and NaChs normally found in the postsynaptic membrane of these cells gradually fragmented and dispersed from the synaptic region after several days in culture. This dispersal was the result of the collagenase treatment used to dissociate the cells, suggesting that a factor associated with the extracellular matrix was responsible for maintaining the high concentration of AchRs and NaChs at the neuromuscular junction. We tested whether the basal lamina protein agrin, which has been shown to induce the aggregation of AChRs on embryonic myotubes, could similarly influence the distribution of NaChs. By following identified fibers, we found that agrin accelerated both the fragmentation of the endplate AChR cluster into smaller patches as well as the appearance of new AChR clusters away from the endplate. AChR patches which were fragments of the original endplate retained a high density of NaChs, but no new NaCh hotspots were found elsewhere on the fiber, including sites of newly formed AChR clusters. The results are consistent with the hypothesis that extracellular signals regulate the distribution of AChRs and NaChs on skeletal muscle fibers. While agrin probably serves this function for the AChR, it does not appear to play a role in the regulation of the NaCh distribution.

How do patch clamp seals form? A lipid bleb model

Individual ion channels are electrically isolated and studied in living cells with the tight patch voltage clamp method. Channels are identified, categorized, and sometimes named on the basis of the biophysical properties obtained with this method. Although it is usually presumed that these recordings are from native, undisturbed membrane, the physical basis of this technique is not well established. Observations that lipid blebs readily form when suction is applied to patch clamp electrodes suggest that many single channel recordings are from ion channels in these blebs.

Localization of voltage-sensitive sodium channels on the extrasynaptic membrane surface of mouse skeletal muscle by autoradiography of scorpion toxin binding sites

Voltage-dependent sodium channels (Na+ channels) were localized by autoradiography on mouse skeletal muscle using both light and electron microscopy. 125I-scorpion toxins (ScTx) of both the alpha and beta type were used as probes. The specificity of labelling was verified by competitive inhibition with unlabelled toxin and by inhibition of alpha ScTx labelling in depolarizing conditions. Under light microscopy, the labelling of the myocyte surface appeared randomly distributed with both the alpha and beta toxins. No difference in the labelling density obtained with beta ScTx was observed between a 2 mm central segment of the fibre containing the endplate and an adjacent segment not containing the endplate. At the endplate, however, the beta ScTx binding site density was about seven fold higher at the edge of the synaptic primary clefts. This density decreased with distance from the synaptic cleft reaching the extrasynaptic value at 30-40 microns. An analysis of myocyte labelling using electron microscopy provided evidence for a specific, but very low labelling of the myocyte interior which can be attributed to the T-tubules. These results confirm a relatively high density of Na+ channels in a perijunctional zone about 50 microns in width, which could ensure the initial spread of the surface depolarization with a high safety factor, and a homogeneous distribution over the remaining surface with a low density evaluated at 5-10 per microns2. However, the very low labelling of T-tubules could be attributed mainly to a low density of tubular Na+ channels.

Factors released by ciliary neurons and spinal cord explants induce acetylcholine receptor mRNA expression in cultured muscle cells

The nuclei of cultured noninnervated muscle cells are heterogeneous with respect to production of mRNA for the nicotinic acetylcholine receptor (AChR). Some nuclei actively express AChR mRNA while others have a low level of activity or are inactive. To determine if innervation, or a factor released by neurons, influences nuclear expression of AChR mRNA, we examined mRNA at a single cell level via in situ hybridization and autoradiography with an alpha-subunit AChR genomic probe. Four days after plating, we co-cultured chicken primary muscle cells with spinal cord explants, ciliary neurons, or dorsal root ganglia (DRG) cells. In situ hybridization of the spinal-cord and muscle-cell co-cultures with the AChR alpha-subunit probe revealed a high density of silver grains on muscle cells, which were within two explant diameters of the spinal cord explant, and a graded decrease in silver grain density as the distance from the explant increased, as well as the appearance of a strikingly nonhomogenous distribution of active and inactive muscle cell nuclei. When ciliary neurons were uniformly distributed over the muscle cells, a high level of AChR mRNA was induced, but no gradients appeared. Neither an increased mRNA level nor a gradient was observed when DRG cells were co-cultured with muscle cells. When ciliary neurons are cultured within Costar permeable inserts, which prevent any contact between the neurons and the underlying muscle cells, AChR messenger RNA is still induced, showing that diffusible factors are responsible. Our results indicate that molecules released by cholinergic neurons regulate the expression of AChR mRNA in the myotubes and raise the possibility that AChR expression depends on both neuronal signals and on intracellular information from the muscle cell.

Temperature and synaptic efficacy in frog skeletal muscle

1. Intracellular recording and voltage-clamp techniques were used to measure synaptic efficacy and the safety factor for neuromuscular transmission in frog skeletal muscle. All measurements were made in normal Ringer solution, in the absence of presynaptic or postsynaptic blocking agents. 2. Over a broad temperature range (10-30 degrees C), a small percentage of sartorius fibres (about 6%) could be found which produced only subthreshold end-plate potentials and no action potential in response to single, supramaximal nerve shock. At lower temperatures the proportion of such fibres increased; 42% of the fibres had subthreshold transmission at 5 degrees C, and 59% were subthreshold at 2.5 degrees C. 3. Threshold current, measured by intracellularly injecting short pulses of depolarizing current at end-plate regions, was independent of temperature between 2.5 and 20 degrees C. Thus, the reduced synaptic efficacy observed at low temperatures was not due to decreased electrical excitability of the postsynaptic membrane. 4. The amplitude of evoked end-plate currents (EPCs) decreased with cooling. At temperatures below 10 degrees C, the evoked EPCs at many end-plates were too small to initiate action potentials. The decline in EPC amplitude was due to three factors: a decrease in the amplitude of single quantum currents (MEPCs), an increase in the temporal dispersion of transmitter release, and (below 5 degrees C) a decrease in quantal content. 5. The safety factor for neuromuscular transmission decreased dramatically as temperature was lowered. At 30 degrees C average safety factor was large and positive (540 nA), but at 2.5 degrees C it was negative (-78 nA). 6. The quantal content of evoked transmitter release was independent of temperature change between 5 and 30 degrees C, the average value over this range being 180. However, at temperatures below 5 degrees C, quantal content fell off sharply (average value = 37). 7. The thermal independence of transmitter release may be an important mechanism in allowing poikilothermic animals to maintain physiological function over a wide range of body temperatures.

Sodium channel distribution in normal and denervated rodent and snake skeletal muscle

1. Sodium channel current density was measured using the loose-patch voltage clamp technique. Innervated rat, mouse and snake muscle had the highest density of Na+ channels in the end-plate region. These high Na+ channel densities were maintained in denervated muscle. 2. Perijunctional membrane had a Na+ current density 5- to 10-fold greater than the density several hundred micrometres from the end-plate. In all muscles this concentration of channels near the end-plate persisted following denervation. 3. At the tendon Na+ current density fell to low values (approximately 1 mA/cm2). The decrease in density began about 300-500 microns from the tendon. This pattern was found in all snake twitch fibres and fast-twitch (EDL) rat and mouse muscle fibres. This reduction in channel density near the tendon was not affected by denervation. 4. Sodium channels in all regions of innervated rat and snake muscle fibres were highly sensitive to tetrodotoxin (TTX). Sodium channels in snake muscle remained sensitive to TTX after denervation. Sodium channels that are relatively resistant to TTX appeared in rat muscle after denervation. TTX-resistant channels were even more concentrated near the end-plate than were TTX-sensitive channels in innervated muscle. At the tendon TTX-resistant Na+ channel density decreased. 5. We conclude that although the nerve presumably directs the localization of Na+ channels during development, the ability to maintain this distribution and to control the distribution of newly appearing channels persists long after the nerve has been removed.