Voltage clamp of rat and human skeletal muscle: measurements with an improved loose-patch technique

Whole-cell patch-clamp recording and parameters

The patch-clamp technique represents an electrophysiology type of method. This is one of several insightful approaches with five major configurations, namely a loose patch, cell-attached (also known as on-cell), whole-cell, inside-out, and outside-out modes. The patch-clamp method is more advanced compared to classical electrophysiology since it elucidates single-channel activation in a tiny portion of the membrane in addition to action potential (AP), junction potential (JP), endplate potential (EP), electrical coupling between two adjacent cells via Gap junction hemi-channels, excitatory/inhibitory postsynaptic potentials, and resting membrane potential (RMP). In fact, a malfunction of only one channel or even one component will alter AP amplitude or duration in vitro. If parameters are inferred appropriately and recordings are performed properly, the patch-clamp trace readouts and results are robust. The main hallmarks of currents via voltage-dependent calcium (Cav), hyperpolarization-activated cyclic nucleotide gated non-selective cation (HCN), inwardly rectifying potassium (Kir), voltage-dependent potassium (Kv), and voltage-dependent sodium (Nav) channels are similar and tractable among cells even when they are derived from evolutionary distinct organs and species. However, the size of the membrane area, where the functional subunits reside, and current magnitudes vary among cells of the same type. Therefore, dividing current magnitudes by cell capacitance- current density enables the estimate of functional and active channels relative to recorded cytoplasmic membrane area. Since the patch-clamp recordings can be performed in both current- and voltage-clamp modes, the action potential or spike durations can be adequately elucidated. Sometimes, optical methods are preferred to patch-clamp electrophysiology, but the obtained signals and traces are not robust. Finally, not only an alternans of AP durations, but also that of 'action potential shape' is observed with electrophysiology.

Ryanodine receptor modulation by caffeine challenge modifies Na+ current properties in intact murine skeletal muscle fibres

We investigated effects of the ryanodine receptor (RyR) modulator caffeine on Na⁺ current (I_Na) activation and inactivation in intact loose-patch clamped murine skeletal muscle fibres subject to a double pulse procedure. I_Na activation was examined using 10-ms depolarising, V₁, steps to varying voltages 0–80 mV positive to resting membrane potential. The dependence of the subsequent, I_Na inactivation on V₁ was examined by superimposed, V₂, steps to a fixed depolarising voltage. Current-voltage activation and inactivation curves indicated that adding 0.5 and 2 mM caffeine prior to establishing the patch seal respectively produced decreased (within 1 min) and increased (after ~2 min) peak I_Na followed by its recovery to pretreatment levels (after ~40 and ~30 min respectively). These changes accompanied negative shifts in the voltage dependence of I_Na inactivation (within 10 min) and subsequent superimposed positive activation and inactivation shifts, following 0.5 mM caffeine challenge. In contrast, 2 mM caffeine elicited delayed negative shifts in both activation and inactivation. These effects were abrogated if caffeine was added after establishing the patch seal or with RyR block by 10 μM dantrolene. These effects precisely paralleled previous reports of persistently (~10 min) increased cytosolic [Ca²⁺] with 0.5 mM, and an early peak rapidly succeeded by persistently reduced [Ca²⁺] likely reflecting gradual RyR inactivation with ≥1.0 mM caffeine. The latter findings suggested inhibitory effects of even resting cytosolic [Ca²⁺] on I_Na. They suggest potentially physiologically significant negative feedback regulation of RyR activity on Na_v1.4 properties through increased or decreased local cytosolic [Ca²⁺], Ca²⁺-calmodulin and FKBP12.

Phase 1 repolarization rate defines Ca2+ dynamics and contractility on intact mouse hearts

Intact heart physiology assessed by local-field optical techniques helps decipher cardiac function at the organ level. Here, we found that the phase 1 repolarization rate of the mouse ventricular action potential defines contractility by regulating the deactivation of an L-type Ca²⁺ current.

In the heart, Ca²⁺ influx through L-type Ca²⁺ channels triggers Ca²⁺ release from the sarcoplasmic reticulum. In most mammals, this influx occurs during the ventricular action potential (AP) plateau phase 2. However, in murine models, the influx through L-type Ca²⁺ channels happens in early repolarizing phase 1. The aim of this work is to assess if changes in the open probability of 4-aminopyridine (4-AP)–sensitive Kv channels defining the outward K⁺ current during phase 1 can modulate Ca²⁺ currents, Ca²⁺ transients, and systolic pressure during the cardiac cycle in intact perfused beating hearts. Pulsed local-field fluorescence microscopy and loose-patch photolysis were used to test the hypothesis that a decrease in a transient K⁺ current (I_to) will enhance Ca²⁺ influx and promote a larger Ca²⁺ transient. Simultaneous recordings of Ca²⁺ transients and APs by pulsed local-field fluorescence microscopy and loose-patch photolysis showed that a reduction in the phase 1 repolarization rate increases the amplitude of Ca²⁺ transients due to an increase in Ca²⁺ influx through L-type Ca²⁺ channels. Moreover, 4-AP induced an increase in the time required for AP to reach 30% repolarization, and the amplitude of Ca²⁺ transients was larger in epicardium than endocardium. On the other hand, the activation of I_to with NS5806 resulted in a reduction of Ca²⁺ current amplitude that led to a reduction of the amplitude of Ca²⁺ transients. Finally, the 4-AP effect on AP phase 1 was significantly smaller when the L-type Ca²⁺ current was partially blocked with nifedipine, indicating that the phase 1 rate of repolarization is defined by the competition between an outward K⁺ current and an inward Ca²⁺ current.

Bundles of Brain Microtubules Generate Electrical Oscillations

Microtubules (MTs) are long cylindrical structures of the cytoskeleton that control cell division, intracellular transport, and the shape of cells. MTs also form bundles, which are particularly prominent in neurons, where they help define axons and dendrites. MTs are bio-electrochemical transistors that form nonlinear electrical transmission lines. However, the electrical properties of most MT structures remain largely unknown. Here we show that bundles of brain MTs spontaneously generate electrical oscillations and bursts of electrical activity similar to action potentials. Under intracellular-like conditions, voltage-clamped MT bundles displayed electrical oscillations with a prominent fundamental frequency at 39 Hz that progressed through various periodic regimes. The electrical oscillations represented, in average, a 258% change in the ionic conductance of the MT structures. Interestingly, voltage-clamped membrane-permeabilized neurites of cultured mouse hippocampal neurons were also capable of both, generating electrical oscillations, and conducting the electrical signals along the length of the structure. Our findings indicate that electrical oscillations are an intrinsic property of brain MT bundles, which may have important implications in the control of various neuronal functions, including the gating and regulation of cytoskeleton-regulated excitable ion channels and electrical activity that may aid and extend to higher brain functions such as memory and consciousness.

Fluorescence Readout of a Patch Clamped Membrane by Laser Scanning Microscopy

In this chapter, we describe how to shield a patch of a cell membrane against extracellularly applied chemoattractant stimuli. Classical patch clamp methodology is applied to allow for controlled shielding of a membrane patch by measuring the seal resistivity. In Dictyostelium cells, a seal resistivity of 50 MΩ proved to be tight enough to exclude molecules from diffusing into the shielded membrane region. This allowed for separating a shielded and a non-shielded region of a cell membrane to study the spatiotemporal dynamics of intracellular chemotactic signaling events at the interface between shielded and non-shielded areas. The spatiotemporal dynamics of signaling events in the membrane was read out by means of appropriate fluorescent markers using laser scanning confocal microscopy.

Intact Heart Loose Patch Photolysis Reveals Ionic Current Kinetics During Ventricular Action Potentials

RATIONALE

Assessing the underlying ionic currents during a triggered action potential (AP) in intact perfused hearts offers the opportunity to link molecular mechanisms with pathophysiological problems in cardiovascular research. The developed loose patch photolysis technique can provide striking new insights into cardiac function at the whole heart level during health and disease.

OBJECTIVE

To measure transmembrane ionic currents during an AP to determine how and when surface Ca(2+) influx that triggers Ca(2+)-induced Ca(2+) release occurs and how Ca(2+)-activated conductances can contribute to the genesis of AP phase 2.

METHODS AND RESULTS

Loose patch photolysis allows the measurement of transmembrane ionic currents in intact hearts. During a triggered AP, a voltage-dependent Ca(2+) conductance was fractionally activated (dis-inhibited) by rapidly photo-degrading nifedipine, the Ca(2+) channel blocker. The ionic currents during a mouse ventricular AP showed a fast early component and a slower late component. Pharmacological studies established that the molecular basis underlying the early component was driven by an influx of Ca(2+) through the L-type channel, CaV 1.2. The late component was identified as an Na(+)-Ca(2+) exchanger current mediated by Ca(2+) released from the sarcoplasmic reticulum.

CONCLUSIONS

The novel loose patch photolysis technique allowed the dissection of transmembrane ionic currents in the intact heart. We were able to determine that during an AP, L-type Ca(2+) current contributes to phase 1, whereas Na(+)-Ca(2+) exchanger contributes to phase 2. In addition, loose patch photolysis revealed that the influx of Ca(2+) through L-type Ca(2+) channels terminates because of voltage-dependent deactivation and not by Ca(2+)-dependent inactivation, as commonly believed.

Physiological and ultrastructural features of human induced pluripotent and embryonic stem cell-derived skeletal myocytes in vitro

Progress has recently been made toward the production of human skeletal muscle cells from induced pluripotent stem (iPS) cells. However, the functional and ultrastructural characterization, which is crucial for disease modeling and drug discovery, remains to be documented. We show, for the first time to our knowledge, that the electrophysiological properties of human iPS-derived skeletal myocytes are strictly similar to those of their embryonic stem (ES) cell counterparts, and both are typical of aneural mammalian skeletal muscle. In both cell types, intracellular calcium signaling that links membrane depolarization to contraction occurs in the absence of extracellular Ca(2+), a unique feature of skeletal muscle. Detailed analysis of the Ca(2+) signal revealed diverse kinetics of the rising phase, and hence various rates in the release of Ca(2+) from the sarcoplasmic reticulum. This was mirrored by ultrastructural evidence of Ca(2+) release units, which varied in location, shape, and size. Thus, the excitation-contraction coupling machinery of both iPS- and ES-derived skeletal myocytes was functional and specific, but did not reach full maturity in culture. This is in contrast with the myofibrillar network, which displayed the same organization as in adult skeletal muscle. Overall, the present study validates the human iPS-based skeletal myocyte model in comparison with the embryonic system, and provides the functional and ultrastructural basis for its application to human skeletal muscle diseases.

Fiber Types in Mammalian Skeletal Muscles

Mammalian skeletal muscle comprises different fiber types, whose identity is first established during embryonic development by intrinsic myogenic control mechanisms and is later modulated by neural and hormonal factors. The relative proportion of the different fiber types varies strikingly between species, and in humans shows significant variability between individuals. Myosin heavy chain isoforms, whose complete inventory and expression pattern are now available, provide a useful marker for fiber types, both for the four major forms present in trunk and limb muscles and the minor forms present in head and neck muscles. However, muscle fiber diversity involves all functional muscle cell compartments, including membrane excitation, excitation-contraction coupling, contractile machinery, cytoskeleton scaffold, and energy supply systems. Variations within each compartment are limited by the need of matching fiber type properties between different compartments. Nerve activity is a major control mechanism of the fiber type profile, and multiple signaling pathways are implicated in activity-dependent changes of muscle fibers. The characterization of these pathways is raising increasing interest in clinical medicine, given the potentially beneficial effects of muscle fiber type switching in the prevention and treatment of metabolic diseases.

Gating behaviour of sodium currents in adult mouse muscle recorded with an improved two-electrode voltage clamp

Muscle contraction is triggered by the spread of an action potential along the fibre. The ionic current to generate the action potential is conducted through voltage-activated sodium channels, and mutations of these channels are known to cause several human muscle disorders. Mouse models have been created by introducing point mutations into the sodium channel gene. This achievement has created the need for a high-fidelity technique to record sodium currents from intact mouse muscle fibres. We have optimized a two-electrode voltage clamp, using sharp microelectrodes to preserve the myoplasmic contents. The voltage-dependent behaviour of sodium channel activation, inactivation and slow-inactivation were characterized. The voltage range for these gating behaviours was remarkably hyperpolarized, in comparison to studies in artificial expression systems. These results provide normative data for sodium channels natively expressed in mouse muscle and illustrate the need to modify model simulations of muscle excitability to account for the hyperpolarized shift.

Role of Ca(2+) in injury-induced changes in sodium current in rat skeletal muscle

Characteristics of voltage-dependent sodium current recorded from adult rat muscle fibers in loose patch mode were rapidly altered following nearby impalement with a microelectrode. Hyperpolarized shifts in the voltage dependence of activation and fast inactivation occurred within minutes. In addition, the amplitude of the maximal sodium current decreased within 30 min of impalement. Impalement triggered a sustained elevation of intracellular Ca(2+). However, buffering Ca(2+) by loading fibers with AM-BAPTA did not affect the hyperpolarized shifts in activation and inactivation, although it did prevent the reduction in current amplitude. Surprisingly, the rise in intracellular Ca(2+) occurred even in the absence of extracellular Ca(2+). This result indicated that the injury-induced Ca(2+) increase came from an intracellular source, but it was not blocked by an inhibitor of release from the sarcoplasmic reticulum, which suggested involvement of mitochondria. Ca(2+) release from mitochondria triggered by carbonyl cyanide 3-chlorophenylhydrazone was sufficient to cause a reduction in sodium current amplitude but had little effect of the voltage dependence of activation and fast inactivation. Our data suggest the effects of muscle injury can be separated into a Ca(2+)-dependent reduction in amplitude and a largely Ca(2+)-independent shift in activation and fast inactivation. Together, the impalement-induced changes in sodium current reduce the number of sodium channels available to open at the resting potential and may limit further depolarization and thus promote survival of muscle fibers following injury.

Modulation of angiogenesis by dithiolethione-modified NSAIDs and valproic acid

BACKGROUND AND PURPOSE

Angiogenesis involves multiple signaling pathways that must be considered when developing agents to modulate pathological angiogenesis. Because both cyclooxygenase inhibitors and dithioles have demonstrated anti-angiogenic properties, we investigated the activities of a new class of anti-inflammatory drugs containing dithiolethione moieties (S-NSAIDs) and S-valproate.

EXPERIMENTAL APPROACH

Anti-angiogenic activities of S-NSAIDS, S-valproate, and the respective parent compounds were assessed using umbilical vein endothelial cells, muscle and tumor tissue explant angiogenesis assays, and developmental angiogenesis in Fli:EGFP transgenic zebrafish embryos.

KEY RESULTS

Dithiolethione derivatives of diclofenac, valproate, and sulindac inhibited endothelial cell proliferation and induced Ser(78) phosphorylation of hsp27, a known molecular target of anti-angiogenic signaling. The parent drugs lacked this activity, but dithiolethiones were active at comparable concentrations. Although dithiolethiones can potentially release hydrogen sulphide, NaSH did not reproduce some activities of the S-NSAIDs, indicating that the dithioles regulate angiogenesis through mechanisms other than release of H(2)S. In contrast to the parent drugs, S-NSAIDs, S-valproate, NaSH, and dithiolethiones were potent inhibitors of angiogenic responses in muscle and HT29 tumor explants assessed by 3-dimensional collagen matrix assays. Dithiolethiones and valproic acid were also potent inhibitors of developmental angiogenesis in zebrafish embryos, but the S-NSAIDs, remarkably, lacked this activity.

CONCLUSIONS AND IMPLICATION

S-NSAIDs and S-valproate have potent anti-angiogenic activities mediated by their dithiole moieties. The novel properties of S-NSAIDs and S-valproate to inhibit pathological versus developmental angiogenesis suggest that these agents may have a role in cancer treatment.

Modulation of angiogenesis by dithiolethione-modified NSAIDs and valproic acid

BACKGROUND AND PURPOSE

Angiogenesis involves multiple signaling pathways that must be considered when developing agents to modulate pathological angiogenesis. Because both cyclooxygenase inhibitors and dithioles have demonstrated anti-angiogenic properties, we investigated the activities of a new class of anti-inflammatory drugs containing dithiolethione moieties (S-NSAIDs) and S-valproate.

EXPERIMENTAL APPROACH

Anti-angiogenic activities of S-NSAIDS, S-valproate, and the respective parent compounds were assessed using umbilical vein endothelial cells, muscle and tumor tissue explant angiogenesis assays, and developmental angiogenesis in Fli:EGFP transgenic zebrafish embryos.

KEY RESULTS

Dithiolethione derivatives of diclofenac, valproate, and sulindac inhibited endothelial cell proliferation and induced Ser(78) phosphorylation of hsp27, a known molecular target of anti-angiogenic signaling. The parent drugs lacked this activity, but dithiolethiones were active at comparable concentrations. Although dithiolethiones can potentially release hydrogen sulphide, NaSH did not reproduce some activities of the S-NSAIDs, indicating that the dithioles regulate angiogenesis through mechanisms other than release of H(2)S. In contrast to the parent drugs, S-NSAIDs, S-valproate, NaSH, and dithiolethiones were potent inhibitors of angiogenic responses in muscle and HT29 tumor explants assessed by 3-dimensional collagen matrix assays. Dithiolethiones and valproic acid were also potent inhibitors of developmental angiogenesis in zebrafish embryos, but the S-NSAIDs, remarkably, lacked this activity.

CONCLUSIONS AND IMPLICATION

S-NSAIDs and S-valproate have potent anti-angiogenic activities mediated by their dithiole moieties. The novel properties of S-NSAIDs and S-valproate to inhibit pathological versus developmental angiogenesis suggest that these agents may have a role in cancer treatment.

Gating defects of a novel Na+ channel mutant causing hypokalemic periodic paralysis

Hypokalemic periodic paralysis type 2 (hypoPP2) is an inherited skeletal muscle disorder caused by missense mutations in the SCN4A gene encoding the alpha subunit of the skeletal muscle Na+ channel (Nav1.4). All hypoPP2 mutations reported so far target an arginine residue of the voltage sensor S4 of domain II (R672/G/H/S). We identified a novel hypoPP2 mutation that neutralizes an arginine residue in DIII-S4 (R1132Q), and studied its functional consequences in HEK cells transfected with the human SCN4A cDNA. Whole-cell current recordings revealed an enhancement of both fast and slow inactivation, as well as a depolarizing shift of the activation curve. The unitary Na+ conductance remained normal in R1132Q and in R672S mutants, and cannot therefore account for the reduction of Na+ current presumed in hypoPP2. Altogether, our results provide a clear evidence for the role of R1132 in channel activation and inactivation, and confirm loss of function effects of hypoPP2 mutations leading to muscle hypoexcitability.

Development of ionic currents of zebrafish slow and fast skeletal muscle fibers

Voltage-gated Na+ and K+ channels play key roles in the excitability of skeletal muscle fibers. In this study we investigated the steady-state and kinetic properties of voltage-gated Na+ and K+ currents of slow and fast skeletal muscle fibers in zebrafish ranging in age from 1 day postfertilization (dpf) to 4-6 dpf. The inner white (fast) fibers possess an A-type inactivating K+ current that increases in peak current density and accelerates its rise and decay times during development. As the muscle matured, the V50s of activation and inactivation of the A-type current became more depolarized, and then hyperpolarized again in older animals. The activation kinetics of the delayed outward K+ current in red (slow) fibers accelerated within the first week of development. The tail currents of the outward K+ currents were too small to allow an accurate determination of the V50s of activation. Red fibers did not show any evidence of inward Na+ currents; however, white fibers expressed Na+ currents that increased their peak current density, accelerated their inactivation kinetics, and hyperpolarized their V50 of inactivation during development. The action potentials of white fibers exhibited significant changes in the threshold voltage and the half width. These findings indicate that there are significant differences in the ionic current profiles between the red and white fibers and that a number of changes occur in the steady-state and kinetic properties of Na+ and K+ currents of developing zebrafish skeletal muscle fibers, with the most dramatic changes occurring around the end of the first day following egg fertilization.

Sodium and potassium currents of larval zebrafish muscle fibres

The steady-state and kinetic properties of Na(+) and K(+) currents of inner (white) and outer (red) muscles of zebrafish larvae 4-6 days post-fertilization (d.p.f.) are described. In inner muscle, the outward currents were half-activated at -1.0 mV and half-inactivated at -30.4 mV, and completely inactivated within 100 ms of depolarization. The inward currents of inner fibres were half-activated at -7.3 mV and half-inactivated at -74.5 mV and completely inactivated within 5 ms of depolarization. Inner muscle fibres were found to support action potentials, while no action potentials could be evoked in outer muscles. In inner muscle fibres, all tested levels of depolarizing current above a threshold value evoked only one action potential. However, spiking at frequencies of up to 200 cycles s(-1) was evoked by the injection of depolarizing pulses separated by short hyperpolarizing currents. We suggest that the properties of the inward sodium and outward potassium currents permit high frequency firing in response to a pulsatile depolarizing input of the kind expected in fast swimming, whilst safeguarding against tetany during a strong depolarization.

Functional characterization and cold sensitivity of T1313A, a new mutation of the skeletal muscle sodium channel causing paramyotonia congenita in humans

Paramyotonia congenita (PC) is a dominantly inherited skeletal muscle disorder caused by missense mutations in the SCN4A gene encoding the pore-forming alpha subunit (hSkM1) of the skeletal muscle Na+ channel. Muscle stiffness is the predominant clinical symptom. It is usually induced by exposure to cold and is aggravated by exercise. The most prevalent PC mutations occur at T1313 on DIII-DIV linker, and at R1448 on DIV-S4 of the alpha subunit. Only one substitution has been described at T1313 (T1313M), whereas four distinct amino-acid substitutions were found at R1448 (R1448C/H/P/S). We report herein a novel mutation at position 1313 (T1313A) associated with a typical phenotype of PC. We stably expressed T1313A or wild-type (hSkM1) channels in HEK293 cells, and performed a detailed study on mutant channel gating defects using the whole-cell configuration of the patch-clamp technique. T1313A mutation impaired Na+ channel fast inactivation: it slowed and reduced the voltage sensitivity of the kinetics, accelerated the recovery, and decreased the voltage-dependence of the steady state. Slow inactivation was slightly enhanced by the T1313A mutation: the voltage dependence was shifted toward hyperpolarization and its steepness was reduced compared to wild-type. Deactivation from the open state assessed by the tail current decay was only slowed at positive potentials. This may be an indirect consequence of disrupted fast inactivation. Deactivation from the inactivation state was hastened. The T1313A mutation did not modify the temperature sensitivity of the Na+ channel per se. However, gating kinetics of the mutant channels were further slowed with cooling, and reached levels that may represent the threshold for myotonia. In conclusion, our results confirm the role of T1313 residue in Na+ channel fast inactivation, and unveil subtle changes in other gating processes that may influence the clinical phenotype.

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.

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.

Human skeletal muscle fibres: molecular and functional diversity

Contractile and energetic properties of human skeletal muscle have been studied for many years in vivo in the body. It has been, however, difficult to identify the specific role of muscle fibres in modulating muscle performance. Recently it has become possible to dissect short segments of single human muscle fibres from biopsy samples and make them work in nearly physiologic conditions in vitro. At the same time, the development of molecular biology has provided a wealth of information on muscle proteins and their genes and new techniques have allowed analysis of the protein isoform composition of the same fibre segments used for functional studies. In this way the histological identification of three main human muscle fibre types (I, IIA and IIX, previously called IIB) has been followed by a precise description of molecular composition and functional and biochemical properties. It has become apparent that the expression of different protein isoforms and therefore the existence of distinct muscle fibre phenotypes is one of the main determinants of the muscle performance in vivo. The present review will first describe the mechanisms through which molecular diversity is generated and how fibre types can be identified on the basis of structural and functional characteristics. Then the molecular and functional diversity will be examined with regard to (1) the myofibrillar apparatus; (2) the sarcolemma and the sarcoplasmic reticulum; and (3) the metabolic systems devoted to producing ATP. The last section of the review will discuss the advantage that fibre diversity can offer in optimizing muscle contractile performance.

Effects of temperature on slow and fast inactivation of rat skeletal muscle Na(+) channels

Patch-clamp studies of mammalian skeletal muscle Na(+) channels are commonly done at subphysiological temperatures, usually room temperature. However, at subphysiological temperatures, most Na(+) channels are inactivated at the cell resting potential. This study examined the effects of temperature on fast and slow inactivation of Na(+) channels to determine if temperature changed the fraction of Na(+) channels that were excitable at resting potential. The loose patch voltage clamp recorded Na(+) currents (I(Na)) in vitro at 19, 25, 31, and 37 degrees C from the sarcolemma of rat type IIb fast-twitch omohyoid skeletal muscle fibers. Temperature affected the fraction of Na(+) channels that were excitable at the resting potential. At 19 degrees C, only 30% of channels were excitable at the resting potential. In contrast, at 37 degrees C, 93% of Na(+) channels were excitable at the resting potential. Temperature did not alter the resting potential or the voltage dependencies of activation or fast inactivation. I(Na) available at the resting potential increased with temperature because the steady-state voltage dependence of slow inactivation shifted in a depolarizing direction with increasing temperature. The membrane potential at which half of the Na(+) channels were in the slow inactivated state was shifted by +16 mV at 37 degrees C compared with 19 degrees C. Consequently, the low availability of excitable Na(+) channels at subphysiological temperatures resulted from channels being in the slow, inactivated state at the resting potential.

Loss of electrical excitability in an animal model of acute quadriplegic myopathy

In rats treated with high-dose corticosteroids, skeletal muscle that is denervated in vivo (steroid-denervated [S-D]) develops electrical inexcitability similar to that seen in patients with acute quadriplegic myopathy. In studies of affected muscles in vitro, the majority of S-D fibers failed to generate action potentials in response to intracellular stimulation although the average resting potential of these fibers was no different from that of control denervated muscle. The downregulation of membrane chloride conductance (G[Cl]) seen in normal muscle after denervation did not occur in S-D muscle. Although block of chloride channels in S-D muscle produced high specific membrane resistance, comparable to similarly treated control denervated muscle, and partially restored excitability in many fibers, action potential amplitude was still reduced in S-D fibers, suggesting a concomitant reduction in sodium current. 3H-saxitoxin binding measurements revealed a reduction in the density of the adult muscle sodium channel isoform in S-D muscle, suggesting that a decrease in the number of sodium channels present may play a role in the reduction of sodium current, although altered properties of channels may also contribute. The weakness seen in S-D muscle may involve the interaction of a number of factors that modify membrane excitability, including membrane depolarization, persistence of G(Cl), and reduced voltage-gated sodium currents.

Voltage-dependent sodium currents recorded from dissociated rat taste cells

Voltage-dependent sodium currents were analyzed in detail from dissociated mammalian taste receptor cells using the whole-cell patch clamp technique. Approximately 50-75% of all taste receptor cells expressed sodium currents. These currents activated close to -50 mV (holding potential = -80 mV) with maximal currents most often occurring at -10 mV. The distribution of maximal inward currents across all cells appeared to display two peaks, at -254 pA and -477 pA, possibly due to differences in sodium channel density. Inward currents were eliminated by replacing 90% of external sodium with N-methyl-D-l-glucamine. The current-voltage relationship of the activated current, as measured by a tail current analysis, was linear, suggesting an ohmic nature of the open channel conductance. The relationship between the time to the peak activated current and the step potential was well fit by a double exponential curve (tau1 = 6.18, tau2 = 37.8 msec). Development of inactivation of the sodium current was dependent upon both voltage- and temporal-parameters. The voltage dependence of the time constant (tau) obtained from removal of inactivation, development of inactivation, and decay of the sodium current displayed a bell-shaped curve with a maximum of 55 msec at -70 mV. In addition to fast inactivation (half maximal at -50 mV), these currents also displayed a slow inactivation (half maximal at -65 mV). Voltage-dependent sodium currents were reversibly inhibited by nanomolar concentrations of tetrodotoxin (Kd = 10(-8) M). There was no evidence of a TTX-insensitive sodium current. This description broadens our understanding of gustatory transduction mechanisms with a particular relevance to the physiological role of receptor cell action potentials.

Ionic currents during action potentials in mammalian skeletal muscle fibers analyzed with loose patch clamp

The loose patch-clamp technique was applied to analyze transmembrane currents during propagating action potentials in superficial fibers of musculi extensor digitorum longus of the mouse in vitro. Experimentally three components were identified in the transmembrane current: 1) a capacitive, 2) an inward sodium, and 3) an outward potassium current. Other components were negligible. The capacitive current was similar in shape to the first derivative of the intracellularly measured action potential. Tetrodotoxin, tetraethylammonium, and 4-aminopyridine, applied in the pipette, were used to identify the contribution in the current by sodium and potassium ions. With extracellularly applied depolarization steps only a sodium current was observed, not a potassium current. Occasionally found outward currents were artifactual. The behaviour of delayed rectifier potassium channels in muscle fiber membranes is discussed in the light of these unexpected findings. We conclude that potassium channel activity contributing to and measured during action potential generation is in some way inaccessible to loose patch extracellular voltage-clamp stimulation and that loose patch action current recording is a useful noninvasive method to analyze membrane conductances involved in action potential generation.

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.

Comparison of Na+ currents from type IIa and IIb human intercostal muscle fibers

The voltage dependence and amplitude of Na+ currents (INa) were studied with the loose-patch voltage-clamp technique on 19 fast-twitch human intercostal skeletal muscle fibers at the endplate border and > 200 microns from the endplate (extrajunctional). The fibers were histochemically classified as fast-twitch oxidative-glycolytic (type IIa, n = 9) or fast-twitch glycolytic (type IIb, n = 10). The voltage dependence of activation and fast and slow inactivation of INa were similar for membrane patches recorded on the endplate border and on extrajunctional membrane for both fiber types. INa was about fivefold larger on the endplate border compared with extrajunctional membrane for both fiber types. Type IIb fibers had larger values of INa and manifest fast inactivation of INa at more negative potentials than type IIa fibers. The difference between type IIa and IIb fibers may enable IIb fibers to operate at higher firing frequencies for brief periods.

Theoretical reconstruction of myotonia and paralysis caused by incomplete inactivation of sodium channels

Muscle fibers from individuals with hyperkalemic periodic paralysis generate repetitive trains of action potentials (myotonia) or large depolarizations and block of spike production (paralysis) when the extracellular K+ is elevated. These pathologic features are thought to arise from mutations of the sodium channel alpha subunit which cause a partial loss of inactivation (steady-state Popen approximately 0.02, compared to < 0.001 in normal channels). We present a model that provides a possible mechanism for how this small persistent sodium current leads to repetitive firing, why the integrity of the T-tubule system is required to produce myotonia, and why paralysis will occur when a slightly larger proportion of channels fails to inactivate. The model consists of a two-compartment system to simulate the surface and T-tubule membranes. When the steady-state sodium channel open probability exceeds 0.0075, trains of repetitive discharges occur in response to constant current injection. At the end of the current injection, the membrane potential may either return to the normal resting value, continue to discharge repetitive spikes, or settle to a new depolarized equilibrium potential. This after-response depends on both the proportion of noninactivating sodium channels and the magnitude of the activity-driven K+ accumulation in the T-tubular space. A reduced form of model is presented in which a two-dimensional phase-plane analysis shows graphically how this diversity of after-responses arises as extracellular [K+] and the proportion of noninactivating sodium channels are varied.

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.

A combined study of sodium current and T-type calcium current in isolated cardiac cells

Sodium currents (INa) and T-type calcium currents (ICa,T) of isolated guinea-pig ventricular myocytes were recorded using the whole-cell voltage-clamp technique. Separation of the two currents was obtained by using the difference current method in the presence and absence of 2 mM extracellular Na (Nao). Time to peak and the time constant of inactivation of. INa were about 5 times faster than that of ICa,T (test potential -30 mV), and ICa,T had an activation range positive to -50 mV, were inactivated at -50 mV, and their current/voltage relationships peaked at -22.3 +/- 1.8 mV (n = 18) and -29.3 +/- 0.5 mV (n = 18) respectively, with a reversal potential of +40.3 +/- 4 mV (n = 18) and +30 +/- 10 mV (n = 18), respectively [2 mM Nao; 5.4 mM extracellular Ca (Cao)]. INa was blocked by 30 microM tetrodotoxin (TTX), 500 microM lidocaine, partly inhibited by 1 mM amiloride, but not affected by 100 microM nickel (Ni). ICa,T was neither affected by 30 microM TTX nor 500 microM lidocaine, but blocked by 100 microM Ni, 1 mM amiloride, 10 microM R 56865 and use-dependently reduced by 5 microM flunarizine. Adenosine (500 microM) affected neither INa nor ICa,T, whereas 1 microM isoprenaline did not affect ICa,T, but slightly increased INa. Our results demonstrate that the characteristics of ICa,T are not affected by the concomitant activation of INa, and vice versa. We conclude that ICa,T are not Ca currents through Na channels.

Voltage dependent sodium currents in cultured rat cerebellar granule cells

Sodium currents were studied in granule cells dissociated from rat cerebellum. Macroscopic currents were recorded using the patch-clamp technique. Sodium currents, which are TTX sensitive, reached a maximum peak value of 0.42 +/- 0.08 pA/microns2 at 18.4 +/- 2.2 mV (n = 6). Activation and inactivation kinetics and steady-state properties were described in terms of Hodgkin and Huxley parameters. The properties of sodium channels in cultured rat cerebellar granule cells are very similar to those reported for various neural preparations.

A tetrodotoxin- and Mn2(+)-insensitive Na+ current in Duchenne muscular dystrophy

Muscle myotube cultures were obtained from normal and Duchenne muscular dystrophy (DMD) biopsies by using an explant technique. The current-voltage (I/V) curve of the whole sodium (Na+) current (INa) in normal myotubes was similar to that obtained from DMD myotubes. However, the inactivation curve of the whole INa was different in normal myotubes when compared to that obtained from DMD myotubes. Addition of 10(-4) M tetrodotoxin (TTX, a fast INa blocker) decreased the whole INa in both preparations. The inorganic calcium (Ca2+) blocker manganese (Mn2+) completely blocked the remaining TTX-resistant INa of normal myotubes and decreased this current in DMD myotubes leaving behind a TTX- and Mn2(+)-insensitive INa that was insensitive to the Ca2+ blocker desmetoxyverapamil ((-)D888). The slow inward barium current (IBa) of both normal and DMD myotubes was blocked by Mn2+ and (-)D888. However the kinetics of the slow channel in normal myotubes was different from that of DMD myotubes. This study demonstrates the presence of a TTX- and Mn2(+)-insensitive INa in DMD myotubes. This channel may contribute to the increase of intracellular Na+ [( Na]i) in DMD and allow Ca2+ to enter the cells through the Na(+)-Ca2+ exchanger, thus contributing to calcium loading.

Open-channel block of Na+ channels by intracellular Mg2+

1. Macroscopic and single-channel currents through several types of cloned rat brain Na+ channels, expressed in Xenopus oocytes, were measured using the patch-clamp technique. 2. For all cloned channel types and for endogenous Na+ channels in chromaffin cells, intracellular Mg2+ blocks outward currents in a voltage-dependent manner similar to that in rat brain type II Na+ channel (Pusch et al. 1989). 3. A sodium-channel mutant ('cZ-2') with long single-channel open times was used to examine the voltage-dependent reduction of single-channel outward current amplitudes by intracellular Mg2+. This reduction could be described by a simple blocking mechanism with half-maximal blockage at 0 mV in 1.8 mM intracellular Mg2+ and a voltage-dependence of e-fold per 39 mV (in approximately 125 mM [Na]i); this corresponds to a binding-site at an electrical distance of 0.32 from the inside of the membrane. 4. At low Mg2+ concentrations and high voltages, the open-channel current variance is significantly elevated with respect to zero [Mg]i. This indicates that Mg2+ acts as a fast blocker rather than gradually decreasing current, e.g. by screening of surface charges. Analysis of the open-channel variance yielded estimates of the block and unblock rate constants, which are of the order of 2.10(8) M-1 S-1 and 3.6.10(5) S-1 at 0 mV for the mutant cZ-2. 5. A quantitative analysis of tail-currents of wild-type II channels showed that the apparent affinity for intracellular Mg2+ strongly depends on [Na]i. This effect could be explained in terms of a multi-ion pore model. 6. Simulated action potentials, calculated on the basis of the Hodgkin-Huxley theory, are significantly reduced in their amplitude and delayed in their onset by postulating Mg2+ block at physiological levels of [Mg]i.

Sodium currents in Schwann cells from myelinated and non-myelinated nerves of neonatal and adult rabbits

1. Patch-clamp methods were used to study sodium channels in Schwann cells obtained from four different tissue sources. Primary cultures of Schwann cells were prepared from the sciatic nerve and from the vagus nerve of neonatal and of adult rabbits. In the adult, the sciatic is predominantly myelinated whereas the vagus is predominantly non-myelinated. Whole-cell currents, and single-channel currents in outside-out membrane patches, were analysed. 2. No substantial differences were noted in the passive electrical properties (input resistance, cell capacitance, resting membrane potential) of the four groups of cells. Similarly, no substantial differences were found in the average properties of sodium currents (maximum current, maximum conductance, time-to-peak current, current-voltage relation, h infinity relation) recorded from each type of cell in cultures less than 8 days old. At 10-17 days a fall in the size of the sodium currents recorded from cells in the vagal cultures was found. 3. Exposure of the cells to proteolytic enzymes or collagenase, under conditions similar to those used when the cells were put in culture initially, substantially reduced the size of the peak sodium currents recorded from the cells 24 h later. 4. The results of experiments on Schwann cells with retracted processes indicated that sodium channels are present in the processes extending from each pole of the cell soma and that the plasmalemmal density of these channels in the processes is about the same as it is at the soma. 5. Recordings from outside-out patches revealed no apparent differences in the properties of single-channel sodium currents in patches from cells obtained from the four different sources. The single-channel conductance was about 20 pS for each of the four groups. Ensemble currents from single-channel records were similar in time course to those of whole-cell currents. 6. Saxitoxin reduced the maximum sodium conductance in Schwann cells and bound to the cells with equally high affinity. The equilibrium dissociation constant was about 2 nM at 20-22 degrees C. 7. It is argued that the expression of sodium channels in myelinating Schwann cells does not differ substantially from that of non-myelinating Schwann cells.

Characteristics of Na+ channels and Cl- conductance in resealed muscle fibre segments from patients with myotonic dystrophy

1. Electrical and contractile properties of resealed fibre segments were investigated by a variety of in vitro techniques. The preparations were removed from skeletal muscles of normal subjects and of eight patients with myotonic dystrophy. 2. Several hours after removal, fibre segments from normal subjects and those patients in whom myotonia was the primary symptom had resting membrane potentials of approximately -80 mV. In contrast, fibre segments obtained from patients in whom muscle dystrophy was more expressed were depolarized (-60 to -70 mV). 3. Contractions induced in fibre segments of myotonic muscle which had normal potentials were characterized by slowed relaxation which was due to electrical after-activity. 4. After single stimuli, long-lasting (3-100) runs of action potentials were recorded intracellularly from the myotonic muscle. In some of these fibre segments complex repetitive discharges were observed: multiple sites of locally gated currents were identified. 5. The three-electrode voltage clamp was used to determine the total membrane conductance, gm, and the ion component conductances. All fibres of a particular patient had similar conductances. However, the Cl- conductance varied from patient to patient from normal (74% of gm) to low values (30% of gm). The K+ conductance was normal in all fibres of all patients. 6. The patch-clamp technique was used to record currents through single Na+ channels of the sarcolemma. After treatment of the fibre segments with collagenase gigaohm seals were routinely obtained. The rate of success was greater when using the cell-attached mode than the inside-out mode. 7. Sodium channel currents were elicited by depolarizing voltage steps which produced an initial burst of Na+ channel openings. Up to ten channels were activated simultaneously when the patch was depolarized to potentials more positive than -30 mV. The Na+ channels re-opened very rarely in controls. The macroscopic sodium current, INa, was reconstructed by averaging depolarizing pulses. The time constant of rapid decay of INa reflecting macroscopic inactivation, the onset of INa and the amplitude of INa were voltage dependent. The mean amplitude of the current produced by re-openings was on average only 0.11 +/- 0.04% of the amplitude of the peak current. 8. Late openings of the Na+ channels were frequent in patches on the myotonic fibre segments. The amplitude of the current produced by re-openings was as high as about 0.75 +/- 0.11% of the amplitude of the peak current.(ABSTRACT TRUNCATED AT 400 WORDS)

Voltage-gated sodium channels expressed in the human cerebellar medulloblastoma cell line TE671

A characterization of the properties of voltage-gated sodium channels expressed in the human cerebellar medulloblastoma cell line TE671 is presented. Membrane currents were recorded under voltage clamp conditions using the patch clamp technique in both the whole-cell and the excised-patch configurations. Macroscopic sodium currents display a typical transient time course with a sigmoidal rise to a peak followed by an exponential decay. The rates of early activation and subsequent inactivation accelerate and approach a maximum in response to test potentials, V, of greater depolarization. The magnitude of peak sodium current increased from negligible values below V = -50 mV and reached a maximum at V = -3.6 mV +/- 2.7 mV (mean +/- S.E.M., n = 12). Sodium currents reversed at V = + 70 mV, near the predicted Nernst equilibrium potential for a Na+ selective channel. The peak sodium conductance, gpeak increased with depolarizing voltages to a maximum at V = approximately 0 mV, exhibiting half-activation voltage at V approximately equal to -36.8 mV and an e-fold change in gpeak/9.5 mV. The Hodgkin-Huxley inactivation parameter h infinity indicates that at V = -73.6 mV half of the sodium currents were inactivated. Single channel current recordings demonstrated the occurrence of discrete events: the latency for first opening was shorter as the depolarizing pulse became more positive. The single-channel current amplitude was ohmic with a slope conductance, gamma = 17.13 pS +/- 0.66 pS. Sodium channel currents were reversibly blocked by tetrodotoxin (TTX).(ABSTRACT TRUNCATED AT 250 WORDS)

Characteristics of single Na+ channels of adult human skeletal muscle

The patch-clamp technique was used to study Na+ channels of human skeletal muscle. Preparations were from biopsies of quadriceps muscle from adults who were not suffering from neuromuscular diseases. Activity of Na+ channels was recorded from inside-out patches when the membrane potential was stepped from a holding potential of -110 mV to potential above a threshold of about -65 mV. Single channel activity increased within minutes after hyperpolarizing the patch due to recovery from ultra-slow inactivation. Up to ten Na+ channels were active in individual patches. Macroscopic currents were reconstructed by averaging single channel currents. The time-to-peak current declined from 1.6 ms at -60 mV to 0.5 ms at + 10 mV. The currents decayed mono-exponentially with time constants between 12.1 ms at -60 mV and 0.4 ms at + 10 mV (21 C). The conductance of single Na+ channels was 1.65 pS and the mean open time was voltage-dependent. At -50 mV, the mean open time was 0.4 ms, while positive to -10 mV it increased to values above 1 ms. In the threshold potential range, the number of openings per depolarizing pulse was larger than the number of channels under the patch-clamp pipette, indicating reopening of Na+ channels at this potential. Openings could be observed only rarely 10 ms after onset of depolarization and the macroscopic current produced by late openings was less than 0.1% of the peak current. Human skeletal muscle is thus suitable for investigation with the patch-clamp technique and the determination of properties of Na+ channels with this technique could be the basis for an assessment of possible defects of these channels in diseased muscle.

Intracellular magnesium blocks sodium outward currents in a voltage- and dose-dependent manner

Tail currents through Na+ channels have been measured in inside-out patches from Xenopus laevis oocytes injected with cDNA-derived mRNA coding for the rat brain type II Na+ channel. It is shown that intracellular Mg2+ blocks outward currents in a voltage- and dose-dependent manner with a half blocking concentration between 3 and 4 mM at 0 mV and a voltage dependence of e-fold per 49 mV.

Measurement of nonuniform current densities and current kinetics in Aplysia neurons using a large patch method

A large patch electrode was used to measure local currents from the cell bodies of Aplysia neurons that were voltage-clamped by a two-microelectrode method. Patch currents recorded at the soma cap, antipodal to the origin of the axon, and whole-cell currents were recorded simultaneously and normalized to membrane capacitance. The patch electrode could be reused and moved to different locations which allowed currents from adjacent patches on a single cell to be compared. The results show that the current density at the soma cap is smaller than the average current density in the cell body for three components of membrane current: the inward Na current (INa), the delayed outward current (Iout), and the transient outward current (IA). Of these three classes of ionic currents, IA is found to reach the highest relative density at the soma cap. Current density varies between adjacent patches on the same cell, suggesting that ion channels occur in clusters. The kinetics of Iout, and on rare occasions IA, were also found to vary between patches. Possible sources of error inherent to this combination of voltage clamp techniques were identified and the maximum amplitudes of the errors estimated. Procedures necessary to reduce errors to acceptable levels are described in an appendix.

Ca-dependent slow action potentials in human skeletal muscle

Slow Ca-action potentials (CaAP) were studied in normal human skeletal muscle fibers obtained during surgery (fibers with both ends cut). Control studies also were carried out with intact as well as cut rat skeletal muscle fibers. Experiments were performed in hypertonic Cl-free saline with 10 or 84 mM Ca and K-channel blockers; muscles were preincubated in a saline containing Cs and tetraethylammonium. A current-clamp technique with two intracellular microelectrodes was used. In human muscle, 14.5% of the fibers showed fully developed CaAPs, 21% displayed nonregenerative Ca responses, and 64.5% showed only passive responses; CaAPs were never observed in 10 mM Ca. In rat muscle, nearly 90% of the fibers showed CaAPs, which were not affected by the cut-end condition. Human and rat muscle fibers had similar membrane potential and conductance in the resting state. In human muscle (22-32 degrees C, 84 mM Ca), the threshold and peak potential during a CaAP were +26 +/- 6 mV and +70 +/- 3 mV, respectively, and the duration measured at threshold level was 1.7 +/- 0.5 sec. In rat muscle, the duration was four times longer. During a CaAP, membrane conductance was assumed to be a leak conductance in parallel with a Ca and a K conductance. In human muscle (22-32 degrees C, 84 mM Ca, 40 micron fiber diameter), values were 0.4 +/- 0.1 microS, 1.1 +/- 0.7 microS, and 0.9 +/- 0.4 microS, respectively. Rat muscle (22-24 degrees C, 84 mM Ca) showed leak and K conductances similar to those found in human fibers. Ca-conductance in rat muscle was double the values obtained in human muscle fibers.

Hodgkin-Huxley parameters of the sodium channels in human myoballs

Myoballs were cultured from biopsies of adult human skeletal muscle without the use of antimitotic drugs. The sodium currents flowing during stepwise depolarization of the myoball membrane were investigated with the wholecell recording technique. The temperature range covered 10-37 degrees C. Two types of sodium channel were distinguished by their different sensitivity to tetrodotoxin (TTX). The channel with normal TTX sensitivity seemed identical with the sodium channel in adult muscle, the channel with less TTX sensitivity seemed identical with the juvenile channel found in developing and in denervated muscle. The activation and inactivation parameters of both channel types were quantitatively determined. The activation parameters of the two channel types were identical, but in comparison to the h infinity-curve of the adult sodium channels the h infinity-curve of the juvenile channels was positioned at more negative potentials, had a less steep slope, and when the temperature was decreased, its point of inflection shifted more in negative direction.

Membrane currents of rat satellite cells attached to intact skeletal muscle fibers

Muscle satellite cells play an important role in the postnatal growth of skeletal muscle and in the regeneration of damaged muscle during adult life. Little is known about the physiological properties of satellite cells in their dormant state as they lie adjacent to the intact muscle fibers, underneath the basement membrane. Our recent experiments, using patch clamp techniques, indicate that no tight electrical coupling is present between satellite cells and the muscle fiber dissociated from rat flexor digitorum brevis. Satellite cells possess sodium channels with low sensitivity to tetrodotoxin and at a much lower density than muscle. In addition, satellite cells are insensitive to acetylcholine (ACh) for at least 24 hr after having been removed from the animal, even when detached from their muscle fiber. However, we could measure ACh-evoked currents from satellite cells 48-72 hr in culture, indicating that ACh sensitivity develops with time.

Slow sodium channel inactivation in mammalian muscle: a possible role in regulating excitability

Sodium currents were recorded in rat fast and slow twitch muscle fibers. Changes in the membrane potential around the resting potential produced slow changes in the sodium current amplitude due to alterations of the slow inactivation process that was increased by steady depolarization and removed by prolonged hyperpolarization. In contrast, classical fast inactivation was not operative around the resting potential, and depolarizations of greater than 20 mV were required to close half of the channels by fast inactivation. Because slow inactivation is operative around the resting potential of mammalian muscle fibers, it may partially explain why small depolarizations, such as those that occur in some patients with periodic paralysis, can reduce excitability.

Currents through ionic channels in multicellular cardiac tissue and single heart cells

Ionic channels are elementary excitable elements in the cell membranes of heart and other tissues. They produce and transduce electrical signals. After decades of trouble with quantitative interpretation of voltage-clamp data from multicellular heart tissue, due to its morphological complexness and methodological limitations, cardiac electrophysiologists have developed new techniques for better control of membrane potential and of the ionic and metabolic environment on both sides of the plasma membrane, by the use of single heart cells. Direct recordings of the behavior of single ionic channels have become possible by using the patch-clamp technique, which was developed simultaneously. Biochemists have made excellent progress in purifying and characterizing ionic channel proteins, and there has been initial success in reconstituting some partially purified channels into lipid bilayers, where their function can be studied.

Cultivation, morphology, and electrophysiology of contractile rat myoballs

Myoballs were cultured from neonatal rat skeletal muscle without the use of antimitotic drugs. Electron microscopic investigation showed that 7-day-old myoballs are multinucleated syncytia in a state of differentiation where filaments are abundant and already in hexagonal arrays. The resting potential of 142 myoballs kept at 20 degrees C was not correlated with the cell size. Its mean value was -64 mV. Cells with a high resting potential were capable of generating action potentials with a threshold of -51 mV, an overshoot of +31 mV, and a rate of rise of 100 V/s. The steady-state current-voltage relation showed inward rectification on hyperpolarization and outward rectification on depolarization. The dynamic sodium and potassium currents were investigated at 37 degrees C with the whole-cell-recording technique. The sodium current had its maximum at -20 mV. The potassium current showed delayed activation and a very slow and incomplete inactivation. The electrophysiological results from these cultured cells are very similar to those obtained from adult cells.