Single-channel recording in myelinated nerve fibers reveals one type of Na channel but different K channels

An aptamer-assisted biological nanopore biosensor for ultra-sensitive detection of ochratoxin A with a portable single-molecule measuring instrument

Biological nanopore-based single-molecule detection technology has shown ultrahigh sensitivity to various target analyte. But the detection scope of interesting targets is limited due to the lack of effective signal conversion strategies. In addition, conventional nanopore detection instruments are cumbersome, resulting nanopore detection can only be performed in laboratory. Herein, a customizable nanopore current amplifier is constructed to lower the cost and increase the portability of the nanopore instrument, and then an immobilized aptamer-based signal conversion strategy is proposed for α-hemolysin (α-HL) nanopore to detect small molecules (ochratoxin A, OTA). The presence of OTA in sample would trigger the release of probe single-strand DNA (ssDNA) from magnetic beads, which could subsequently cause current blockage in nanopore. The results show that the signal frequency of probe ssDNA has a linear relationship with the OTA concentration in the range of 2 × 10¹～2 × 10³ pmol/L. Compared to other methods, our sensing system has achieved an ultra-sensitive detection of OTA with the detection limit as low as 1.697 pmol/L. This strategy could broaden the scope of nanopore detection and have the potential for rapid and in-situ detection of other food contaminants in the future.

Function of K2P channels in the mammalian node of Ranvier

In myelinated nerve fibres, action potentials are generated at nodes of Ranvier. These structures are located at interruptions of the myelin sheath, forming narrow gaps with small rings of axolemma freely exposed to the extracellular space. The mammalian node contains a high density of Na⁺ channels and K⁺ -selective leakage channels. Voltage-dependent Kv1 channels are only present in the juxta-paranode. Recently, the leakage channels have been identified as K2P channels (TRAAK, TREK-1). K2P channels are K⁺ -selective 'background' channels, characterized by outward rectification and their ability to be activated, e.g. by temperature, mechanical stretch or arachidonic acid. We are only beginning to elucidate the peculiar functions of nodal K2P channels. I will discuss two functions of the nodal K2P-mediated conductance. First, at body temperature K2P channels have a high open probability, thereby inducing a resting potential of about -85 mV. This negative resting potential reduces steady-state Na⁺ channel inactivation and ensures a large Na⁺ inward current upon a depolarizing stimulus. Second, the K2P conductance is involved in nodal action potential repolarization. The identification of nodal K2P channels is exciting since it shows that the nodal K⁺ conductance is not a fixed value but can be changed: it can be increased or decreased by a broad range of K2P modulators, thereby modulating, for example, the resting potential. The functional importance of nodal K2P channels will be exemplified by describing in more detail the function of the K2P conductance increase by raising the temperature from room temperature to 37°C.

Computer modeling of mild axonal injury: implications for axonal signal transmission

Diffusion imaging and postmortem studies of patients with mild traumatic brain injury (mTBI) of the concussive type are consistent with the observations of diffuse axonal injury to the white matter axons. Mechanical trauma to axons affects the properties of tetrodotoxin-sensitive sodium channels at the nodes of Ranvier, leading to axonal degeneration through intra-axonal accumulation of calcium ions and activation of calcium proteases; however, the immediate implications of axonal trauma regarding axonal functionality and their relevance to transient impairment of function as observed in concussion remain elusive. A biophysically realistic computational model of a myelinated axon was developed to investigate how mTBI could immediately affect axonal function. Traumatized axons showed alterations in signal propagation properties that nonlinearly depended on the level of trauma; subthreshold traumatized axons had decreased spike propagation time, whereas suprathreshold traumatized axons exhibited a slowdown of spike propagation and spike propagation failure. Trauma had consistently reduced axonal spike amplitude. The susceptibility of an axon to trauma could be modulated by the function of an ATP-dependent sodium-potassium pump. The results suggest a mechanism by which concussive mTBI could lead to the immediate impairment of signal propagation through the axon and the emerging dysfunctional neuronal information exchange.

Physiology and pathophysiology of myelinated nerve fibers

The chief role of the axon is that of impulse conduction, which depends on the electrical cable structure and voltage-dependent ion channels of the axonal membrane. Over recent decades, the development of specialized techniques such as patch clamping and site-directed mutagenesis have established the contribution of neuronal ion channel function to the processes of impulse conduction in myelinated nerves. Recently, these insights from in vitro studies have been translated into the clinical realm. In keeping with this progress, clinical axonal excitability techniques have been developed to provide information related to the activity of a variety of ion channels, energy-dependent pumps, and ion exchange processes activated during impulse conduction in peripheral axons. These noninvasive techniques have been extensively applied to the study of the biophysical properties of human peripheral nerves in vivo and have provided important insights into axonal ion channel function in health and neurological disease, particularly in relation to the pathophysiological mechanisms that underlie neuropathy.

Fatigue in multiple sclerosis: mechanisms and management

Multiple sclerosis [MS] is a chronic immune-mediated disorder of the central nervous system [CNS]. Fatigue may be a debilitating symptom in MS patients, adversely impacting on their quality of life. Clinically, fatigue may manifest as exhaustion, lack of energy, increased somnolence, or worsening of MS symptoms. Activity and heat typically serve to exacerbate symptoms of fatigue. There is now strong evidence to suggest that fatigue results from reduced voluntary activation of muscles by means of central mechanisms. Given that axonal demyelination is a pathological hallmark of MS, activity-dependent conduction block [ADCB] has been proposed as a mechanism underlying fatigue in MS. This ADCB results from axonal membrane hyperpolarization, mediated by the Na(+)/K(+) electrogenic pump, with conduction failure precipitated in demyelinated axons with a reduced safety factor of impulse transmission. In addition, Na(+)/K(+) pump dysfunction, as reported in MS, may induce a depolarizing conduction block associated with inactivation of Na(+) channels. These processes may induce secondary effects including axonal degeneration triggered by raised levels of intracellular Ca(2+) through reverse operation of the Na(+)-Ca(2+) exchanger. Restoration of normal conduction in demyelinated axons with selective channel blockers improves fatigue and may yet prove useful as a neuroprotective strategy, in preventing secondary axonal degeneration and consequent functional impairment.

Axonal ion channels from bench to bedside: a translational neuroscience perspective

Over recent decades, the development of specialised techniques such as patch clamping and site-directed mutagenesis have established the contribution of neuronal ion channel dysfunction to the pathophysiology of common neurological conditions including epilepsy, multiple sclerosis, spinal cord injury, peripheral neuropathy, episodic ataxia, amyotrophic lateral sclerosis and neuropathic pain. Recently, these insights from in vitro studies have been translated into the clinical realm. In keeping with this progress, novel clinical axonal excitability techniques have been developed to provide information related to the activity of a variety of ion channels, energy-dependent pumps and ion exchange processes activated during impulse conduction in peripheral axons. These non-invasive techniques have been extensively applied to the study of the biophysical properties of human peripheral nerves in vivo and have provided important insights into axonal ion channel function in health and disease. This review will provide a translational perspective, focusing on an overview of the investigational method, the clinical utility in assessing the biophysical basis of ectopic symptom generation in peripheral nerve disease and a review of the major findings of excitability studies in acquired and inherited neurological disease states.

Threshold behaviour of human axons explored using subthreshold perturbations to membrane potential

The present study explores the threshold behaviour of human axons and the mechanisms contributing to this behaviour. The changes in excitability of cutaneous afferents in the median nerve at the wrist were recorded to a long-lasting subthreshold conditioning stimulus, with a waveform designed to maximize the contribution of currents active in the just-subthreshold region. The conditioning stimulus produced a decrease in threshold that developed over 3-5 ms following the end of the depolarization and then decayed slowly, in a pattern similar to the recovery of axonal excitability following a discharge. To ensure that the conditioning stimulus did not activate low-threshold axons, similar recordings were then made from single motor axons in the ulnar nerve at the elbow. The findings were comparable, and behaviour with the same pattern and time course could be reproduced by subthreshold stimuli in a model of the human axon. In motor axons, subthreshold depolarizing stimuli, 1 ms long, produced a similar increase in excitability, but the late hyperpolarizing deflection was less prominent. This behaviour was again reproduced by the model axon and could be explained by the passive properties of the nodal membrane and conventional Na+ and K+ currents. The modelling studies emphasized the importance of leak current through the Barrett-Barrett resistance, even in the subthreshold region, and suggested a significant contribution of K+ currents to the threshold behaviour of axons. While the gating of slow K+ channels is slow, the resultant current may not be slow if there are substantial changes in membrane potential. By extrapolation, we suggest that, when human axons discharge, nodal slow K+ currents will be activated sufficiently early to contribute to the early changes in excitability following the action potential.

Two toxins from Conus striatus that individually induce tetanic paralysis

We describe structural properties and biological activities of two related O-glycosylated peptide toxins isolated from injected (milked) venom of Conus striatus, a piscivorous snail that captures prey by injecting a venom that induces a violent, spastic paralysis. One 30 amino acid toxin is identified as kappaA-SIVA (termed s4a here), and another 37 amino acid toxin, s4b, corresponds to a putative peptide encoded by a previously reported cDNA. We confirm the amino acid sequences and carry out structural analyses of both mature toxins using multiple mass spectrometric techniques. These include electrospray ionization ion-trap mass spectrometry and nanoelectrospray techniques for small volume samples, as well as matrix-assisted laser desorption/ionization time of flight mass spectrometric analysis as a complementary method to assist in the determination of posttranslational modifications, including O-linked glycosylation. Physiological experiments indicate that both s4a and s4b induce intense repetitive firing of the frog neuromuscular junction, leading to a tetanic contracture in muscle fiber. These effects apparently involve modification of voltage-gated sodium channels in motor axons. Notably, application of either s4a or s4b alone mimics the biological effects of the whole injected venom on fish prey.

Non-stationary fluctuation analysis of the Na current in myelinated nerve fibers of Xenopus laevis: experiments and stochastic simulations

Na current fluctuations under voltage-clamp conditions during pulse steps in the potential range from -65 to -30 mV were measured in myelinated nerve fibers of Xenopus laevis. The covariance functions for four consecutive 1 ms intervals were calculated. The time courses of the covariance functions were well fitted with monoexponential functions with time constants between 0.5 and 3 ms, larger at the end of the pulse and larger at more positive potentials. To analyze the underlying channel kinetics we simulated current fluctuations at a step to -35 mV of eight published Na channel models and calculated corresponding covariance functions. None of the models did explain the experimental fluctuation results. We therefore developed a new Na channel model that satisfactorily described the results. Features that distinguished this model from the other tested ones were a slower deactivation rate, and an inactivation transition directly from a closed state.

Twenty years of dendrotoxins

Dendrotoxins are small proteins that were isolated 20 years ago from mamba (Dendroaspis) snake venoms (Harvey, A.L., Karlsson, E., 1980. Dendrotoxin from the venom of the green mamba, Dendroaspis angusticeps: a neurotoxin that enhances acetylcholine release at neuromuscular junctions. Naunyn-Schmiedebergs Arch. Pharmacol. 312, 1-6.). Subsequently, a family of related proteins was found in mamba venoms and shown to be homologous to Kunitz-type serine protease inhibitors, such as aprotinin. The dendrotoxins contain 57-60 amino acid residues cross-linked by three disulphide bridges. The dendrotoxins have little or no anti-protease activity, but they were demonstrated to block particular subtypes of voltage-dependent potassium channels in neurons. Studies with cloned K(+) channels indicate that alpha-dendrotoxin from green mamba Dendroaspis angusticeps blocks Kv1.1, Kv1.2 and Kv1.6 channels in the nanomolar range, whereas toxin K from the black mamba Dendroaspis polylepis preferentially blocks Kv1.1 channels. Structural analogues of dendrotoxins have helped to define the molecular recognition properties of different types of K(+) channels, and radiolabelled dendrotoxins have also been useful in helping to discover toxins from other sources that bind to K(+) channels. Because dendrotoxins are useful markers of subtypes of K(+) channels in vivo, dendrotoxins have become widely used as probes for studying the function of K(+) channels in physiology and pathophysiology.

Dynamic control of presynaptic Ca(2+) inflow by fast-inactivating K(+) channels in hippocampal mossy fiber boutons

Analysis of presynaptic determinants of synaptic strength has been difficult at cortical synapses, mainly due to the lack of direct access to presynaptic elements. Here we report patch-clamp recordings from mossy fiber boutons (MFBs) in rat hippocampal slices. The presynaptic action potential is very short during low-frequency stimulation but is prolonged up to 3-fold during high-frequency stimulation. Voltage-gated K(+) channels in MFBs inactivate rapidly but recover from inactivation very slowly, suggesting that cumulative K(+) channel inactivation mediates activity-dependent spike broadening. Prolongation of the presynaptic voltage waveform leads to an increase in the number of Ca(2+) ions entering the terminal per action potential and to a consecutive potentiation of evoked excitatory postsynaptic currents at MFB-CA3 pyramidal cell synapses. Thus, inactivation of presynaptic K(+) channels contributes to the control of efficacy of a glutamatergic synapse in the cortex.

Spatial distribution of NA+ and K+ channels in spinal dorsal horn neurones: role of the soma, axon and dendrites in spike generation

Spinal dorsal horn neurones play an important role in processing sensory information received from primary afferent fibers. The application of the patch-clamp technique to thin slices of rat spinal cord has enabled the study of ionic channels in visually identified dorsal horn neurones. The small soma of these neurones isolated from the slice by means of a novel method of 'entire soma isolation' has become a convenient model for investigating the properties and distributions of ionic channels. The present review summarizes results of recent experiments studying different types of voltage-gated Na+ and K+ channels expressed in dorsal horn neurones. Uneven distribution of the channels between the soma. axon and dendrites appears to play a major role in determining the neuronal excitability. The contribution of the soma, axon and dendrites to generation and propagation of the action potentials in central neurones is discussed.

Human axons contain at least five types of voltage-dependent potassium channel

1. We investigated voltage-gated potassium channels in human peripheral myelinated axons; apart from the I, S and F channels already described in amphibian and rat axons, we identified at least two other channel types. 2. The I channel activated between -70 and -40 mV, and inactivated very slowly (time constant 13.1 s at -40 mV). It had two gating modes: the dominant ('noisy') mode had a conductance of 30 pS (inward current, symmetrical 155 mM K+) and a deactivation time constant (tau) of 25 ms (-80 mV); it accounted for most ( approximately 50-75 %) of the macroscopic K+ current in large patches. The secondary ('flickery') gating mode had a conductance of 22 pS, and showed bi-exponential deactivation (tau = 16 and 102 ms -80 11 mV); it contributed part of the slow macroscopic K+ current. 3. The I channel current was blocked by 1 microM alpha-dendrotoxin (DTX); we also observed two other DTX-sensitive K+ channel types (40 pS and 25 pS). The S and F channels were not blocked by 1 microM DTX. 4. The conductance of the S channel was 7-10 pS, and it activated at slightly more negative potentials than the I channel; its deactivation was slow (tau = 41.7 ms at -100 mV). It contributed a second component of the slow macroscopic K+ current. 5. The F channel had a conductance of 50 pS; it activated at potentials between -40 and +40 V, deactivated very rapidly (tau = 1.4 ms at -100 mV), and inactivated rapidly (tau = 62 ms at +80 mV). It accounted for the fast-deactivating macroscopic K+ current and partly for fast K+ current inactivation. 6. We conclude that human and rat axonal K+ channels are closely similar, but that the correspondence between K+ channel types and the macroscopic currents usually attributed to them is only partial. At least five channel types exist, and their characteristics overlap to a considerable extent.

Functional and molecular differences between voltage-gated K+ channels of fast-spiking interneurons and pyramidal neurons of rat hippocampus

We have examined gating and pharmacological characteristics of somatic K+ channels in fast-spiking interneurons and regularly spiking principal neurons of hippocampal slices. In nucleated patches isolated from basket cells of the dentate gyrus, a fast delayed rectifier K+ current component that was highly sensitive to tetraethylammonium (TEA) and 4-aminopyridine (4-AP) (half-maximal inhibitory concentrations <0.1 mM) predominated, contributing an average of 58% to the total K+ current in these cells. By contrast, in pyramidal neurons of the CA1 region a rapidly inactivating A-type K+ current component that was TEA-resistant prevailed, contributing 61% to the total K+ current. Both types of neurons also showed small amounts of the K+ current component mainly found in the other type of neuron and, in addition, a slow delayed rectifier K+ current component with intermediate properties (slow inactivation, intermediate sensitivity to TEA). Single-cell RT-PCR analysis of mRNA revealed that Kv3 (Kv3.1, Kv3.2) subunit transcripts were expressed in almost all (89%) of the interneurons but only in 17% of the pyramidal neurons. In contrast, Kv4 (Kv4.2, Kv4.3) subunit mRNAs were present in 87% of pyramidal neurons but only in 55% of interneurons. Selective block of fast delayed rectifier K+ channels, presumably assembled from Kv3 subunits, by 4-AP reduced substantially the action potential frequency in interneurons. These results indicate that the differential expression of Kv3 and Kv4 subunits shapes the action potential phenotypes of principal neurons and interneurons in the cortex.

Functional and molecular differences between voltage-gated K+ channels of fast-spiking interneurons and pyramidal neurons of rat hippocampus

We have examined gating and pharmacological characteristics of somatic K+ channels in fast-spiking interneurons and regularly spiking principal neurons of hippocampal slices. In nucleated patches isolated from basket cells of the dentate gyrus, a fast delayed rectifier K+ current component that was highly sensitive to tetraethylammonium (TEA) and 4-aminopyridine (4-AP) (half-maximal inhibitory concentrations <0.1 mM) predominated, contributing an average of 58% to the total K+ current in these cells. By contrast, in pyramidal neurons of the CA1 region a rapidly inactivating A-type K+ current component that was TEA-resistant prevailed, contributing 61% to the total K+ current. Both types of neurons also showed small amounts of the K+ current component mainly found in the other type of neuron and, in addition, a slow delayed rectifier K+ current component with intermediate properties (slow inactivation, intermediate sensitivity to TEA). Single-cell RT-PCR analysis of mRNA revealed that Kv3 (Kv3.1, Kv3.2) subunit transcripts were expressed in almost all (89%) of the interneurons but only in 17% of the pyramidal neurons. In contrast, Kv4 (Kv4.2, Kv4.3) subunit mRNAs were present in 87% of pyramidal neurons but only in 55% of interneurons. Selective block of fast delayed rectifier K+ channels, presumably assembled from Kv3 subunits, by 4-AP reduced substantially the action potential frequency in interneurons. These results indicate that the differential expression of Kv3 and Kv4 subunits shapes the action potential phenotypes of principal neurons and interneurons in the cortex.

Fundamental properties of local anesthetics: half-maximal blocking concentrations for tonic block of Na+ and K+ channels in peripheral nerve

Local anesthetics suppress excitability by interfering with ion channel function. Ensheathment of peripheral nerve fibers, however, impedes diffusion of drugs to the ion channels and may influence the evaluation of local anesthetic potencies. Investigating ion channels in excised membrane patches avoids these diffusion barriers. We investigated the effect of local anesthetics with voltage-dependent Na+ and K+ channels in enzymatically dissociated sciatic nerve fibers of Xenopus laevis using the patch clamp method. The outside-out configuration was chosen to apply drugs to the external face of the membrane. Local anesthetics reversibly blocked the transient Na+ inward current, as well as the steady-state K+ outward current. Half-maximal tonic inhibiting concentrations (IC50), as obtained from concentration-effect curves for Na+ current block were: tetracaine 0.7 microM, etidocaine 18 microM, bupivacaine 27 microM, procaine 60 microM, mepivacaine 149 microM, and lidocaine 204 microM. The values for voltage-dependent K+ current block were: bupivacaine 92 microM, etidocaine 176 microM, tetracaine 946 microM, lidocaine 1118 microM, mepivacaine 2305 microM, and procaine 6302 microM. Correlation of potencies with octanol:buffer partition coefficients (logP0) revealed that ester-bound local anesthetics were more potent in blocking Na+ channels than amide drugs. Within these groups, lipophilicity governed local anesthetic potency. We conclude that local anesthetic action on peripheral nerve ion channels is mediated via lipophilic drug-channel interactions.

IMPLICATIONS

Half-maximal blocking concentrations of commonly used local anesthetics for Na+ and K+ channel block were determined on small membrane patches of peripheral nerve fibers. Because drugs can directly diffuse to the ion channel in this model, these data result from direct interactions of the drugs with ion channels.

Functional differences in Na+ channel gating between fast-spiking interneurones and principal neurones of rat hippocampus

1. GABAergic interneurones differ from glutamatergic principal neurones in their ability to discharge high-frequency trains of action potentials without adaptation. To examine whether Na+ channel gating contributed to these differences, Na+ currents were recorded in nucleated patches from interneurones (dentate gyrus basket cells, BCs) and principal neurones (CA1 pyramidal cells, PCs) of rat hippocampal slices. 2. The voltage dependence of Na+ channel activation in BCs and PCs was similar. The slope factors of the activation curves, fitted with Boltzmann functions raised to the third power, were 11.5 and 11.8 mV, and the mid-point potentials were -25.1 and -23.9 mV, respectively. 3. Whereas the time course of Na+ channel activation (-30 to +40 mV) was similar, the deactivation kinetics (-100 to -40 mV) were faster in BCs than in PCs (tail current decay time constants, 0.13 and 0.20 ms, respectively, at -40 mV). 4. Na+ channels in BCs and PCs differed in the voltage dependence of inactivation. The slope factors of the steady-state inactivation curves fitted with Boltzmann functions were 6.7 and 10.7 mV, and the mid-point potentials were -58.3 and -62.9 mV, respectively. 5. The onset of Na+ channel inactivation at -55 mV was slower in BCs than in PCs; the inactivation time constants were 18.6 and 9.3 ms, respectively. At more positive potentials the differences in inactivation onset were smaller. 6. The time course of recovery of Na+ channels from inactivation induced by a 30 ms pulse was fast and mono-exponential (tau = 2.0 ms at -120 mV) in BCs, whereas it was slower and bi-exponential in PCs (tau 1 = 2.0 ms and tau 2 = 133 ms; amplitude contribution of the slow component, 15%). 7. We conclude that Na+ channels of BCs and PCs differ in gating properties that contribute to the characteristic action potential patterns of the two types of neurones.

Tail currents in the myelinated axon of Xenopus laevis suggest a two-open-state Na channel

Na tail currents in the myelinated axon of Xenopus laevis were measured at -70 mV after steps to -10 mV. The tail currents were biexponential, comprising a fast and a slow component. The time constant of the slow tail component, analyzed in the time window 0.35-0.50 ms, was independent of step duration, and had a value of 0.23 ms. The amplitude, extrapolated back to time 0, varied, however, with step duration. It reached a peak after 0.7 ms and inactivated relatively slowly (at 2.1 ms the absolute value was reduced by approximately 30%). The amplitude of the fast component, estimated by subtracting the amplitude of the slow component from the calculated total tail current amplitude, reached a peak (three times larger than that of the slow component) after 0.5 ms and inactivated relatively fast (at 2.1 ms it was reduced by approximately 65%). The results were explained by a novel Na channel model, comprising two open states bifurcating from a common closed state and with separate inactivating pathways. A voltage-regulated use of the two pathways explains a number of findings reported in the literature.

Endogenous firing patterns of murine spiral ganglion neurons

Current-clamp recordings with the use of the whole cell configuration of the patch-clamp technique were made from postnatal mouse spiral ganglion neurons in vitro. Cultures contained neurons that displayed monopolar, bipolar, and pseudomonopolar morphologies. Additionally, a class of neurons having exceptionally large somata was observed. Frequency histograms of the maximum number of action potentials fired from 240-ms step depolarizations showed that neurons could be classified as either slowly adapting or rapidly adapting. Most neurons (85%) were in the rapidly adapting category (58 of 68 recordings). Measurements of elementary properties were used to define the endogenous firing characteristics of both the neuron classes. Action potential number varied with step and holding potential, spike amplitude decayed during prolonged depolarizations, and spike frequency adaptation was observed in both rapidly and slowly adapting neurons. The apparent input resistance, spike amplitude decrement, and instantaneous firing frequency differed significantly between rapidly and slow adapting neurons. Inward rectification was evaluated in response to hyperpolarizing contrast current injections. Present in both electrophysiological classes, its magnitude was graded from neuron to neuron, reflecting differences in number, type, and/or voltage dependence of the underlying channels. These data suggest that spiral ganglion neurons possess intrinsic firing properties that regulate action potential number and timing, features that may be crucial to signal coding in the auditory periphery.

Kinetic parameters of the ionic currents in myelinated axons: characterization of temperature effects in a hibernator and a nonhibernator

Na+ and K+ currents were measured by the patch-clamp method in the paranodal region of single sciatic nerve fibres of rats and of warm-adapted and cold-adapted golden hamsters. Kinetic parameters and temperature dependence of the Na+ currents were determined. The time constant for activation (about 0.2 ms for rats and hamsters) as well as the time constant for inactivation (about 1.6 ms for rats and hamsters) at 15 degrees C and at -35 mV compared well with single fibre voltage-clamp data from the rat. Differences amongst the three groups of animals were not significant. The temperature coefficient, Q10, for the activation and the inactivation time constant as well as for the time-to-peak of the Na+ current ranged between 2.3 and 3.1. No data have previously been published on the temperature dependence of the delayed-rectifier K channels of mammalian nerve fibres. Most of the K+ current was carried by intermediate (KI) and fast (KF) K channels. Dendrotoxin block indicated that "approximate"55% of the K+ current was due to KI channels, with no significant difference amongst the three groups of animals tested. The Arrhenius plot of the time constant of K+ current activation, "tau"n, yielded a mean Q10 of 3.3 at -40 mV (4. 0 at + 60 mV). No significant differences of the channel kinetics between rats, warm-adapted hamsters and cold-adapted hamsters were detected. We observed, however, a significant decrease of the Na channel density in the paranodal region of cold-adapted hamsters.

Characteristics of type I and type II K+ channels in rabbit cultured Schwann cells

1. Voltage-dependent K+ currents were studied in rabbit Schwann cells cultured from neonatal sciatic nerve and from the lumbar or sacral spinal roots of 10-day-old animals. 2. Whole-cell K+ currents, evoked in response to depolarizing voltage-clamp steps, were categorized as type I or type II on the basis of their apparent threshold and activation kinetics. In the presence of a quasi-physiological [K+] gradient, the magnitude of the fully activated type I current varied linearly with membrane potential, whereas type II current always gave rise to a curved and outwardly rectifying current-membrane potential (I-E) relation. 3. Type II whole-cell currents, obtained with long duration voltage-clamp steps (> or = 1 s), have an apparent threshold for activation close to -40 mV. Type II current inactivated slowly, and apparently to completion. The current is more than 90% inactivated over 5 s at 0 mV (time consant of inactivation, tau h, approximately 2 s, 20-22 degrees C). Type I current, which activates at close to -60 mV, inactivated at about half this rate at the same potential, assuming that inactivation also proceeds to completion. 4. Type I whole-cell currents were reversibly blocked by superfused beta-bungarotoxin (beta-BuTX; apparent KD = 46 nM). beta-BuTX did not appear to reduce type II whole-cell currents at concentrations up to 500 nM. 5. In outside-out patches, the type I channel had an almost linear I-E relation over the potential range -60 to +60 mV with a quasi-physiological [K+] gradient. A best linear fit gave a single-channel conductance of 12 pS under these conditions. In symmetrical 170 mM K+, type I channels had a single-channel conductance of 30 pS over the same potential range. 6. More slowly activating type II single-channel currents were also recorded in inside-out patches. With symmetrical 170 mM K+, the major conductance level was close to 9.0 pS. With a quasi-physiological [K+] gradient, type II single channels exhibit outward rectification that is reasonably well described by the Goldman-Hodgkin-Katz current equation. 7. In the presence of 2 nM externally superfused alpha-dendrotoxin (alpha-DTX), or 50 nM superfused beta-BuTX, unitary currents were recorded (outside-out patches, -60 or -50 mV) that were smaller than control type I currents. Virtually all transitions in the presence of 50 nM beta-BuTX were at one-third of the control current level. The currents did not conform to the characteristics of type II. 8. The electrophysiological and pharmacological characteristics of the type I channel strongly suggest that it is a member of the mammalian K+ channel subfamily of Shaker homologues, most similar to the homomultimeric Kv1.1 translation product. The type II channel may be a member of the mammalian Shab subfamily. 9. Possible roles for Na+ channels and type I K+ channels in the Schwann cell are discussed.

Electrical and morphological factors influencing the depolarizing after-potential in rat and lizard myelinated axons

1. Intra-axonal recording and electron microscopy were applied to intramuscular myelinated axons in lizards and rats to investigate factors that influence the amplitude and time course of the depolarizing after-potential. 2. Depolarizing after-potentials in lizard axons had larger peak amplitudes and longer half-decay times than those recorded in rat axons (mean values 10 mV, 35 ms in lizard; 3 mV, 11 ms in rat). These differences were not due to differences in temperature, resting potential or action potential amplitude or duration. 3. For a given axon diameter, the myelin sheath in lizard fibres was thinner and had fewer wraps than in rat fibres. There was no significant difference in myelin periodicity. Calculations suggest that the thinner myelin sheath accounts for < 30% of the difference between depolarizing after-potential amplitudes recorded in lizard and rat axons. 4. Consistent with a passive charging model for the depolarizing after-potential, the half-time of the passive voltage transient following intra-axonal injection of current was shorter in rat than in lizard axons. 5. Aminopyridines prolonged the falling phase of the action potential and increased the amplitude of the depolarizing after-potential in both types of axon. 6. During repetitive stimulation the depolarizing after-potentials following successive action potentials exhibited little or no summation. Axonal input conductance in the interspike interval increased during the train. 7. These findings suggest that the amplitude and time course of the depolarizing after-potential are influenced not only by the passive properties of the axon and myelin sheath, but also by persisting activation of axolemmal K+ channels following action potentials.

Acquired neuromyotonia: evidence for autoantibodies directed against K+ channels of peripheral nerves

Acquired neuromyotonia is characterized by hyperexcitability of motor nerves leading to muscle twitching, cramps, and weakness. The symptoms may improve following plasma exchange, and injection of immunoglobulin G (IgG) from 1 neuromyotonia patient into mice increased the resistance of neuromuscular transmission to d-tubocurarine. Here we examine nerves and muscle in vitro from mice injected with plasma or purified IgG from 6 neuromyotonia patients or pooled control subjects, and cultured dorsal root ganglion cells after treatment with IgG. Three of the patients had antibodies against human voltage-gated potassium channels labeled with 125I-alpha-dendrotoxin. The quantal release of acetylcholine (quantal content) at end-plates in diaphragms from mice treated with neuromyotonia IgG preparations was increased by 21% relative to control values (p = 0.0053). With one IgG preparation, the duration of the superficial peroneal nerve compound action currents was increased by 93%. The dorsal root ganglion cells treated with this IgG showed a marked increase in repetitive firing of action potentials. All effects were similar to those obtained with aminopyridines. We conclude that at least some patients with acquired neuromyotonia have antibodies directed against aminopyridine- or alpha-dendrotoxin-sensitive K+ channels in motor and sensory neurons, and they are likely to be implicated in the disease process.

Modulation of delayed rectifier K+ channel activity by external K+ ions in Xenopus axon

The effect of external K+ ions upon the activation of delayed rectifier K+ channels was studied in demyelinated amphibian nerve fibres by means of the patch-clamp technique. In external 105 mM K+ solution (high-Ko) macroscopic K+ currents activated at more negative potentials (approximately -15 mV) than in external Ringer (2.5 mM K+). Since the rapid substitution of external Ringer with high-Ko solution at holding potentials of -70 mV and -60 mV directly activated K+ concentration from 5 mM to 10,20,50 and 105 mM gradually increased the open probability of the channels. Although Rb+ ions were less permeant through the channels, they were more potent in their interaction with the binding site and shifted K+ channel activation to more negative potentials. In contrast, external Cs+ ions had only a weak effect on the binding site. Thus, external K+ ions at physiological concentrations modulate the activation of delayed rectifier K+ channels at potentials between -90 mV and -60 mV.

Single voltage-activated Na+ and K+ channels in the somata of rat motoneurones

1. Voltage-activated Na+ and K+ channels were investigated in the soma membrane of motoneurones using the patch-clamp technique applied to thin slices of neonatal rat spinal cord. 2. One type of TTX-sensitive Na+ channel, with a conductance of 14.0 pS, was found to underlie the macroscopic Na+ conductance in the somata of motoneurones. These channels activated within a potential range between -60 and -20 mV with a potential of half-maximal activation (E50) of -38.9 mV and steepness factor (k) of 6.1 mV. 3. Kinetics of Na+ channel inactivation could be fitted with a single exponential function at all potentials investigated. The curve of the steady-state inactivation had the following parameters: a half-maximal potential (Eh,50) of -81.6 mV and k of -10.2 mV. 4. Kinetics of recovery of Na+ channels from inactivation at a potential of -80 mV were double exponential with fast and slow components of 16.2 (76%) and 153.7 ms (24%), respectively. It is suggested that the recovery of Na+ channels from inactivation plays a major role in defining the limiting firing frequency of action potentials in motoneurones. 5. Whole-cell K+ currents consisted of transient (A)- and delayed-rectifier (DR)-components. The A-component activated between -60 and +20 mV with an E50 of -33.3 mV and k of 15.7 mV. The curve of steady-state inactivation was best fitted with an Eh,50 of -82.5 mV and k of -10.2 mV. The DR-component of K+ current activated smoothly at more positive potentials. E50 and k for DR-currents were +1.4 and 16.9 mV, respectively. 6. The most frequent single K+ channel found in the somata of motoneurones was the fast inactivating A-channel with a conductance of 19.2 pS in external Ringer solution. In symmetrical high-K+ solutions the conductance was 50.9 and 39.6 pS for inward and outward currents, respectively. The channel activation took place between -60 and +20 mV. The curve of steady-state inactivation of single A-channels had an Eh,50 of -87.1 mV and k of -12.8 mV. In high-Ko+ solution A-channels demonstrated a rapid deactivation at potentials between -110 and -60 mV. The time constant of the channel deactivation depended on the membrane potential and changed from 1.5 ms at -110 mV to 6.3 ms at -60 mV. 7. Delayed-rectifier K+ channels were found in the soma membrane at a moderate density.(ABSTRACT TRUNCATED AT 250 WORDS)

Local anesthetics potently block a potential insensitive potassium channel in myelinated nerve

Effects of some local anesthetics were studied in patch clamp experiments on enzymatically demyelinated peripheral amphibian nerve fibers. Micromolar concentrations of external bupivacaine depolarized the excised membrane considerably. The flicker K+ channel was found to be the most sensitive ion channel to local anesthetics in this preparation. Half-maximum inhibiting concentrations (IC50) for extracellular application of bupivacaine, ropivacaine, etidocaine, mepivacaine, lidocaine, and QX-314 were 0.21, 4.2, 8.6, 56, 220, and > 10,000 microM, respectively. The corresponding concentration-effect curves could be fitted under the assumption of a 1:1 reaction. Application from the axoplasmic side resulted in clearly lower potencies with IC50 values of 2.1, 6.6, 16, 300, 1,200, and 1,250 microM, respectively. The log(IC50)-values of the local anesthetics linearly depended on the logarithm of their octanol:buffer distribution coefficients with two regression lines for the piperidine derivatives and the standard amino-amides indicating an inherently higher potency of the cyclic piperidine series. Amide-linked local anesthetics did not impair the amplitude of the single-channel current but prolonged the time of the channel to be in the closed state derived as time constants tau c from closed-time histograms. With etidocaine and lidocaine tau c was 133 and 7.2 ms, and proved to be independent of concentration. With the most potent bupivacaine time constants of wash in and wash out were 1.8 and 5.2 s for 600 nM bupivacaine. After lowering the extracellular pH from 7.4 to 6.6, externally applied bupivacaine showed a reduced potency, whereas at higher pH of 8.2 the block was slightly enhanced. Intracellular pH of 6.4, 7.2, 8.0 had almost no effect on internal bupivacaine block. It is concluded that local anesthetics block the flicker K+ channel by impeding its gating but not its conductance. The slow blocker bupivacaine and the fast blocker lidocaine compete for the same receptor. Lipophilic interactions are of importance for blockade but besides a hydrophobic pathway, there exists also a hydrophilic pathway to the binding site which could only be reached from the cytoplasmic side of the membrane. Under physiological conditions, blockade of the flicker K+ channel which is more sensitive to bupivacaine than the Na+ channel might lead via membrane depolarization and the resulting sodium channel inactivation to a pronounced block of conduction in thin fibers.

Threshold fluctuations in an N sodium channel model of the node of Ranvier

Computer simulations of stochastic single-channel open-close kinetics are applied to an N sodium channel model of a node of Ranvier. Up to 32,000 voltage-gated sodium channels have been simulated with modified amphibian sodium channel kinetics. Poststimulus time histograms are obtained with 1000 monophasic pulse stimuli, and measurements are made of changes in the relative spread of threshold (RS) with changes in the model parameters. RS is found to be invariant with pulse durations from 100 microseconds to 3 ms. RS is approximately of inverse proportion to square-root of N. It decreases with increasing temperature and is dependent on passive electrical properties of the membrane as well as the single-channel conductance. The simulated RS and its independence of pulse duration is consistent with experimental results from the literature. Thus, the microscopic fluctuations of single, voltage-sensitive sodium channels in the amphibian peripheral node of Ranvier are sufficient to account for the macroscopic fluctuation if threshold to electrical stimulation.

Na(+)-activated K+ channels localized in the nodal region of myelinated axons of Xenopus

1. A potassium channel activated by internal Na+ ions (K+Na channel) was identified in peripheral myelinated axons of Xenopus laevis using the cell-attached and excised configurations of the patch clamp technique. 2. The single-channel conductance for the main open state was 88 pS with [K+]o = 105 mM and pS with [K+]o = 2.5 mM ([K+]i = 105 mM). The channel was selectively permeable to K+ over Na+ ions. A characteristic feature of the K+Na channel was the frequent occurrence of subconductance states. 3. The open probability of the channel was strongly dependent on the concentration of Na+ ions at the inner side of the membrane. The half-maximal activating Na+ concentration and the Hill coefficient were 33 mM and 2.9, respectively. The open probability of the channel showed only weak potential dependence. 4. The K+Na channel was relatively insensitive to external tetraethylammonium (TEA+) in comparison with voltage-dependent axonal K+ channels; the half-maximal inhibitory concentration (IC50) was 21.3 mM (at -90 mV). In contrast, the channel was blocked by low concentrations of external Ba2+ and Cs+ ions, with IC50 values of 0.7 and 1.1 mM, respectively (at -90 mV). The block by Ba2+ and Cs+ was more pronounced at negative than at positive membrane potentials. 5. A comparison of the number of K+Na channels in nodal and paranodal patches from the same axon revealed that the channel density was about 10-fold higher at the node of Ranvier than at the paranode. Moreover, a correlation between the number of K+Na channels and voltage-dependent Na+ channels in the same patches was found, suggesting co-localization of both channel types. 6. As weakly potential-dependent ('leakage') channels, axonal K+Na channels may be involved in setting the resting potential of vertebrate axons. Simulations of Na+ ion diffusion suggest two possible mechanisms of activation of K+Na channels: the local increase of Na+ concentration in a cluster of Na+ channels during a single action potential or the accumulation in the intracellular axonal compartment during a train of action potentials.

Resolving three types of chloride channels in demyelinated Xenopus axons

Axons from Xenopus sciatic nerve were demyelinated by intraneural injection of lysolecithin rendering the entire internodal axolema accessible to a patch electrode. In this region, three types of anion selective pores were found and characterized at the single-channel level. These included outwardly rectifying, inwardly rectifying, and maxi Cl- channels. The outwardly rectifying Cl- channels (24 pS) are activated by depolarization with a weak voltage dependence of 42 mV per e-fold change in open probability. The inwardly rectifying Cl- channels (27 pS) are insensitive to voltage, but can be blocked by internal application of 100 microM SITS or DIDS. The I-V curves of rectifying channels are S-shaped and can be fitted by a kinetic model with a single free energy barrier. The rectification may be related to the location of this barrier. The maxi Cl- channel (335 pS) is often open at the resting potential, but is inactivated by a large depolarization. The rectification, voltage dependence, and inactivation of these channels may contribute to the regulation of axonal Cl- balance and resting potential.

Selective block by alpha-dendrotoxin of the K+ inward rectifier at the Vicia guard cell plasma membrane

The efficacy and mechanism of alpha-dendrotoxin (DTX) block of K+ channel currents in Vicia stomatal guard cells was examined. Currents carried by inward- and outward-rectifying K+ channels were determined under voltage clamp in intact guard cells, and block was characterized as a function of DTX and external K+ (K+o) concentrations. Added to the bath, 0.1-30 nM DTX blocked the inward-rectifying K+ current (IK,in), but was ineffective in blocking current through the outward-rectifying K+ channels (IK,out) even at concentrations of 30 nM. DTX block was independent of clamp voltage and had no significant effect on the voltage-dependent kinetics for IK,in, neither altering its activation at voltages negative of -120 mV nor its deactivation at more positive voltages. No evidence was found for a use dependence to DTX action. Block of IK,in followed a simple titration function with an apparent K1/2 for block of 2.2 nM in 3 mM K+o. However, DTX block was dependent on the external K+ concentration. Raising K+o from 3 to 30 mM slowed block and resulted in a 60-70% reduction in its efficacy (apparent Ki = 10 mM in 10 nM DTX). The effect of K+ in protecting IK,in was competitive with DTX and specific for permeant cations. A joint analysis of IK,in block with DTX and K+ concentration was consistent with a single class of binding sites with a Kd for DTX of 240 pM. A Kd of 410 microM for extracellular K+ was also indicated. These results complement previous studies implicating a binding site requiring extracellular K+ (K1/2 approximately 1 mM) for IK,in activation; they parallel features of K+ channel block by DTX and related peptide toxins in many animal cells, demonstrating the sensitivity of plant plasma membrane K+ channels to nanomolar toxin concentrations under physiological conditions; the data also highlight one main difference: in the guard cells, DTX action appears specific to the K+ inward rectifier.

Sodium channel inactivation kinetics of rat sensory and motor nerve fibres and their modulation by glutathione

Na+ channel currents of rat motor and sensory nerve fibres were studied with the patch-clamp technique on enzymatically demyelinated axons. Differences between motor and sensory fibres in multi-channel inactivation kinetics and the gating of late single-channel currents were investigated. In the axon-attached mode, inactivation of multi-channel Na+ currents in sensory axons was best fitted with a single time constant while for motor axons two time constants were needed. Late single-channel currents in sensory axons were characterized by short openings whereas motor axons exhibited additional long single-channel openings. In contrast, in excised, inside-out membrane patches, no differences between motor and sensory fibres were found; in both types of fibre inactivation of multi-channel Na+ currents proceeded with two time constants and late single-channel currents showed short and long openings. After application of the reducing agent glutathione to the cytoplasmic side of excised inside-out patches, inactivation of Na+ currents in both motor and sensory fibres proceeded with a single, fast exponential time constant and late currents appeared with short openings only. These data indicate that the axonal metabolism may contribute to the different inactivation kinetics of Na+ current in motor and sensory nerve fibres.

Hyperglycaemic hypoxia alters after-potential and fast K+ conductance of rat axons by cytoplasmic acidification

1. The effects of hyperglycaemic hypoxia (a condition possibly involved in the pathogenesis of diabetic neuropathy) on the depolarizing after-potential and the potassium conductance of myelinated rat spinal root axons were investigated using electrophysiological recordings from intact spinal roots and from excised, inside-out axonal membrane patches. 2. Isolated spinal roots were exposed to hypoxia in solutions containing normal or high glucose concentrations. The depolarizing after-potential of compound action potentials was only enhanced in spinal roots exposed to hyperglycaemic (25 mM D-glucose) hypoxia. A maximal effect was seen in bathing solutions with low buffering power. 3. The depolarizing after-potential was also enhanced by cytoplasmic acidification after replacement of 10-30 mM chloride in the bathing solution by propionate. 4. Multi-channel current recordings from excised, inside-out axonal membrane patches were used to study the effects of cytoplasmic acidification on voltage-dependent K+ conductances with fast (F channels) and intermediate (I channels) kinetics of deactivation. 5. F channels were blocked by small changes in cytoplasmic pH (50% inhibition at pH 6.9). I channels were much less sensitive to intra-axonal acidification. 6. In conclusion, our data show that hyperglycaemic hypoxia enhances the depolarizing after-potential in peripheral rat axons. The underlying mechanism seems to be an inhibition of a fast, voltage-dependent axonal K+ conductance by cytoplasmic acidification. This alteration in membrane conductance may contribute to positive symptoms in diabetic neuropathy.

A method for rapid exchange of solutions at membrane patches using a 10-microliters microcapsule

A rapid exchange (less than 2 ms) of the bath solution facing a membrane patch is accomplished by driving the tip of a pipette from the bath through a 100-microns oil layer into a small capsule filled with 10 microliters test solution. The microcapsule method can be applied to both excised patch configurations, inside-out and outside-out patches. On and off reactions of Ca(2+)-activated K+ channel activity have been recorded after changing the intracellular Ca2+ concentration using an inside-out patch. A blockade of these K+ channels by external tetraethylammonium ions is demonstrated with an outside-out patch. The blocking kinetics of delayed-rectifier K+ channels by a purified peptide toxin from snake venom, dendrotoxin, could be measured with our microcapsule method. Using tiny volumes of test solutions this method can be helpful in experiments involving scarce or expensive solutions.

Single voltage-dependent potassium channels in rat peripheral nerve membrane

1. Voltage-dependent potassium channels were investigated in rat axonal membrane by means of the patch-clamp recording technique. Three different types of channels (F, I and S) have been characterized on the basis of their single-channel conductance, activation, deactivation and inactivation properties. 2. The fast (F) channels were activated smoothly at potentials (E) between -50 and 50 mV (E50 = 4.6 mV). They had a conductance of 55 pS for inward current and 30 pS for outward current in solutions containing 155 mM K+ (high K+) on both sides of the membrane at 21-23 degrees C. The F-channels demonstrated the fastest deactivation, within 1-2 ms, and inactivated in a few hundreds of milliseconds. The time constant of inactivation was 143 ms at E = +40 mV. 3. The intermediate (I) channels activated steeply between E = -70 and -50 mV (E50 = -64.2 mv) and had a single-channel conductance of 33 pS for inward and 18 ps for outward currents. The I-channels deactivated with intermediate kinetics with the time constants of 20.4 ms and 10.1 ms at E = -80 mV and E = -100 mV, respectively. Complete inactivation of the channels developed over tens of seconds. The time constant of inactivation was 7.4 s at E = +40 mV. 4. The slow (S) channels were active at potentials positive to -90 mV. Their conductance was 10 pS for inward currents. The time constant of activation of the S-channels was strongly potential dependent. At a holding potential of -100 mV the channels deactivated during a long time interval between 30 ms and 1 s, producing long-lasting tail currents. The mean time constant of deactivation for S-channels was 129 ms. 5. The conductances of F- and I-channels measured under normal physiological conditions (Ringer solution in bath) were 17 and 10 pS, respectively. 6. Tetraethylammonium (TEA), the classic blocker of potassium channels, suppressed F-, I- and S-channels. It gradually reduced the apparent amplitude of unitary currents in a dose-dependent manner with IC50 equal to 1.2 mM for F-channels, 0.6 mM for I-channels and 1.4 mM for S-channels. Dendrotoxin (DTX), a toxin from the green mamba snake, considerably inhibited the I tail currents at nanomolar concentrations (IC50 = 2.8 nM) while the amplitudes of single I-channel currents were not affected. 7. The K+ channels of F, I and S types form the basis of the potassium conductivity in mammalian peripheral myelinated axon.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of laser-induced hyperthermia treatment on ionic permeability of myelinated nerve

The effect of laser-induced hyperthermia on the ionic permeability of nerve membranes was studied using the nodes of Ranvier in amphibian myelinated nerve as a model. To effect a photothermal modification of nerve membrane functions, controlled laser irradiation consisting of a 5-sec thermal pulse was applied to the nodal membrane, increasing the temperature to a maximum of 48-58 degrees C at the node. Major electrophysiological changes observed in the nodal membrane following laser-induced hyperthermia were a differential reduction of the sodium and potassium permeability, an increase in the leakage current, and a negative shift on the potential axis of the steady-state Na inactivation. There was no significant change in the kinetics of ion channel activation and inactivation for treatments below 56 degrees C. The results suggest that a primary photothermal damage mechanism at temperatures below 56 degrees C could be a reduction in the number of active Na channels in the node, rather than a change in individual channel kinetics, or in the properties of the lipid bilayer of intervening nerve membrane. A differential heat sensitivity between the noninactivated and the inactivated Na channels is also suggested. For the treatments of 56 degrees C and above, a significant increase of membrane leakage current suggests an irreversible thermal damage to the lipid bilayer.

A TEA-insensitive flickering potassium channel active around the resting potential in myelinated nerve

A novel potassium-selective channel which is active at membrane potentials between -100 mV and +40 mV has been identified in peripheral myelinated axons of Xenopus laevis using the patch-clamp technique. At negative potentials with 105 mM-K on both sides of the membrane, the channel at 1 kHz resolution showed a series of brief openings and closings interrupted by longer closings, resulting in a flickery bursting activity. Measurements with resolution up to 10 kHz revealed a single-channel conductance of 49 pS with 105 mM-K and 17 pS with 2.5 mM-K on the outer side of the membrane. The channel was selective for K ions over Na ions (PNa/PK = 0.033). The probability of being within a burst in outside-out patches varied from patch to patch (> 0.2, but often > 0.9), and was independent of membrane potential. Open-time histograms were satisfactorily described with a single exponential (tau o = 0.09 msec), closed times with the sum of three exponentials (tau c = 0.13, 5.9, and 36.6 msec). Sensitivity to external tetraethylammonium was comparatively low (IC50 = 19.0 mM). External Cs ions reduced the apparent unitary conductance for inward currents at Em = -90 mV (IC50 = 1.1 mM). Ba and, more potently, Zn ions lowered not only the apparent single-channel conductance but also open probability. The local anesthetic bupivacaine with high potency reduced probability of being within a burst (IC50 = 165 nM). The flickering K channel is clearly different from the other five types of K channels identified so far in the same preparation. We suggest that this channel may form the molecular basis of the resting potential in vertebrate myelinated axons.

Glutathione accelerates sodium channel inactivation in excised rat axonal membrane patches

The effects of glutathione were studied on the gating behaviour of sodium channels in membrane patches of rat axons. Depolarizing pulses from -120 to -40 mV elicited sodium currents of up to 500 pA, indicating the simultaneous activation of up to 250 sodium channels. Inactivation of these channels in the excised, inside-out configuration was fitted by two time constants (tau h1 = 0.81 ms; tau h2 = 5.03 ms) and open time histograms at 0 mV revealed a biexponential distribution of channel openings (tau short = 0.28 ms; tau long = 3.68 ms). Both, the slow time constant of inactivation and the long lasting single channel openings disappeared after addition of the reducing agent glutathione (2-5 mM) to the bathing solution. Sodium channels of excised patches with glutathione present on the cytoplasmatic face of the membrane had inactivation kinetics similar to channels recorded in the cell-attached configuration. These observations indicate that redox processes may contribute to the gating of axonal sodium channels.

The effect of phloretin on single potassium channels in myelinated nerve

The effect of phloretin, a dipolar organic compound, on single potassium channel currents of myelinated nerve fibres of Xenopus laevis has been investigated, using inside-out patches prepared by the method of Jonas et al. (1989). The I channel, a potential dependent K channel with intermediate deactivation kinetics, was reversibly blocked by 20 microM phloretin applied on the inside; the block was strongest at negative membrane potentials and less pronounced at positive potentials. Phloretin shifted the curve relating open probability to membrane potential towards more positive potentials and reduced its slope and maximum. This confirms previous findings on the effect of phloretin on the voltage dependence of the fast macroscopic K conductance. Single channel conductance and deactivation kinetics were not altered by phloretin.

Evidence that action potentials activate an internodal potassium conductance in lizard myelinated axons

1. We have studied action potentials and after-potentials evoked in the internodal region of visualized lizard intramuscular nerve fibres by stimulation of the proximal nerve trunk. Voltage recordings were obtained using microelectrodes inserted into the axon (intra-axonal) or into the layers of myelin (peri-internodal), with the goal of studying conditions required to activate internodal K+ currents. 2. Peri-internodal recordings made using K2SO4-, KCl- or NaCl-filled electrodes exhibited a negligible resting potential (less than 2 mV), but showed action potentials with peak amplitudes of up to 78 mV and a duration less than or equal to that of the intra-axonally recorded action potential. 3. Following ionophoretic application of potassium from a peri-internodal microelectrode, the peri-internodal action potential was followed by a prolonged (hundreds of milliseconds) negative plateau. This plateau was not seen following peri-internodal ionophoresis of sodium. The prolonged negative potential (PNP) was confined to the K(+)-injected internode: it could be recorded by a second peri-internodal microelectrode inserted into the same internode, but not into an adjacent internode. 4. The peri-internodally recorded PNP was accompanied by an equally prolonged intra-axonal depolarizing after-potential, and by an increase in the conductance of the internodal axolemma. However, the K+ ionophoresis that produced the PNP had little or no detectable effect on the intra-axonally or peri-internodally recorded resting potential or action potential. These findings suggest that the PNP is generated by an inward current across the axolemma of the K(+)-injected internode, through channels opened following the action potential. 5. Following peri-internodal K+ ionophoresis a PNP could also be evoked by passage of depolarizing current pulses through an intra-axonal electrode or by passage of negative current pulses through an electrode in the K(+)-filled peri-internodal region. The threshold for evoking a PNP was less than the threshold for evoking an action potential, and the PNP persisted in 10 microM-tetrodotoxin. Thus the PNP is evoked by depolarization of the axolemma rather than by Na+ influx. 6. The PNP was reversibly blocked by tetraethylammonium (TEA, 2-10 mM), but was not blocked by 100 microM-3,4-diaminopyridine or 5 mM-4-aminopyridine.(ABSTRACT TRUNCATED AT 400 WORDS)

Phloretin affects the fast potassium channels in frog nerve fibres

The effect of phloretin (20-100 microM), a dipolar organic compound, on the voltage clamp currents of the frog node of Ranvier has been investigated. The Na currents are simply reduced in size but not otherwise affected. Phloretin has no effect on the slow 4-aminopyridine-resistant K channels. However, the voltage dependence and time course of the fast K conductance (gK) is markedly altered. The gK (E) curve, determined by measuring fast tail currents at different pulse potentials, normally exhibits a bend at -50 mV, indicating the existence of two types of fast K channels. Phloretin shifts the gK (E) curve to more positive potentials, reduces its slope and its maximum and abolishes the distinction between the two types of fast K channels. The effect becomes more pronounced with time. Phloretin also markedly slows the opening of the fast K channels, but has much less effect on the closing. Opening can be accelerated again by a long depolarizing prepulse which presumably removes part of the phloretin block. It is concluded that phloretin selectively affects the fast K channels of the nodal membrane. The results are compared with similar observations on the squid giant axon.

Selectivity and gating of the type L potassium channel in mouse lymphocytes

Type l voltage-gated K+ channels in murine lymphocytes were studied under voltage clamp in cell-attached patches and in the whole-cell configuration. The kinetics of activation of whole-cell currents during depolarizing pulses could be fit by a single exponential after an initial delay. Deactivation upon repolarization of both macroscopic and microscopic currents was mono-exponential, except in Rb-Ringer or Cs-Ringer solution in which tail currents often displayed "hooks," wherein the current first increased or remained constant before decaying. In some cells type l currents were contaminated by a small component due to type n K+ channels, which deactivate approximately 10 times slower than type l channels. Both macroscopic and single channel currents could be dissected either kinetically or pharmacologically into these two K+ channel types. The ionic selectivity and conductance of type l channels were studied by varying the internal and external permeant ion. With 160 mM K+ in the cell, the relative permeability calculated from the reversal potential with the Goldman-Hodgkin-Katz equation was K+ (identical to 1.0) greater than Rb+ (0.76) greater than NH4+ = Cs+ (0.12) much greater than Na+ (less than 0.004). Measured 30 mV negative to the reversal potential, the relative conductance sequence was quite different: NH4+ (1.5) greater than K+ (identical to 1.0) greater than Rb+ (0.5) greater than Cs+ (0.06) much greater than Na+, Li+, TMA+ (unmeasurable). Single channel current rectification resembled that of the whole-cell instantaneous I-V relation. Anomalous mole-fraction dependence of the relative permeability PNH4/PK was observed in NH4(+)-K+ mixtures, indicating that the type l K+ channel is a multi-ion pore. Compared with other K+ channels, lymphocyte type l K+ channels are most similar to "g12" channels in myelinated nerve.