A K+ channel in Xenopus nerve fibres selectively blocked by bee and snake toxins: binding and voltage-clamp experiments

Snake Venom: A Promising Source of Neurotoxins Targeting Voltage-Gated Potassium Channels

The venom derived from various sources of snakes represents a vast collection of predominantly protein-based toxins that exhibit a wide range of biological actions, including but not limited to inflammation, pain, cytotoxicity, cardiotoxicity, and neurotoxicity. The venom of a particular snake species is composed of several toxins, while the venoms of around 600 venomous snake species collectively encompass a substantial reservoir of pharmacologically intriguing compounds. Despite extensive research efforts, a significant portion of snake venoms remains uncharacterized. Recent findings have demonstrated the potential application of neurotoxins derived from snake venom in selectively targeting voltage-gated potassium channels (Kv). These neurotoxins include BPTI-Kunitz polypeptides, PLA2 neurotoxins, CRISPs, SVSPs, and various others. This study provides a comprehensive analysis of the existing literature on the significance of Kv channels in various tissues, highlighting their crucial role as proteins susceptible to modulation by diverse snake venoms. These toxins have demonstrated potential as valuable pharmacological resources and research tools for investigating the structural and functional characteristics of Kv channels.

Α-Dendrotoxin-sensitive Kv1 channels contribute to conduction failure of polymodal nociceptive C-fibers from rat coccygeal nerve

It is known that some patients with diabetic neuropathy are usually accompanied by abnormal painful sensations. Evidence has accumulated that diabetic neuropathic pain is associated with the hyperexcitability of peripheral nociceptors. Previously, we demonstrated that reduced conduction failure of polymodal nociceptive C-fibers and enhanced voltage-dependent sodium currents of small dorsal root ganglion (DRG) neurons contribute to diabetic hyperalgesia. To further investigate whether and how potassium channels are involved in the conduction failure, α-dendrotoxin (α-DTX), a selective blocker of the low-threshold sustained Kv1 channel, was chosen to examine its functional capability in modulating the conduction properties of polymodal nociceptive C-fibers and the excitability of sensory neurons. We found that α-DTX reduced the conduction failure of C-fibers from coccygeal nerve in vivo accompanied by an increased initial conduction velocity but a decreased activity-dependent slowing of conduction velocity. In addition, the number of APs evoked by step currents was significantly enhanced after the treatment with α-DTX in small-diameter sensory neurons. Further study of the mechanism indicates α-DTX-sensitive K(+) current significantly reduced and the activation of this current in peak and steady state shifted to depolarization for diabetic neurons. Expression of Kv channel subunits Kv1.2 and Kv1.6 was downregulated in both small dorsal root ganglion neurons and peripheral C-fibers. Taken together, these results suggest that α-DTX-sensitive Kv1 channels might play an important role in regulating the conduction properties of polymodal nociceptive C-fibers and firing properties of sensory neurons.

Beyond faithful conduction: short-term dynamics, neuromodulation, and long-term regulation of spike propagation in the axon

Most spiking neurons are divided into functional compartments: a dendritic input region, a soma, a site of action potential initiation, an axon trunk and its collaterals for propagation of action potentials, and distal arborizations and terminals carrying the output synapses. The axon trunk and lower order branches are probably the most neglected and are often assumed to do nothing more than faithfully conducting action potentials. Nevertheless, there are numerous reports of complex membrane properties in non-synaptic axonal regions, owing to the presence of a multitude of different ion channels. Many different types of sodium and potassium channels have been described in axons, as well as calcium transients and hyperpolarization-activated inward currents. The complex time- and voltage-dependence resulting from the properties of ion channels can lead to activity-dependent changes in spike shape and resting potential, affecting the temporal fidelity of spike conduction. Neural coding can be altered by activity-dependent changes in conduction velocity, spike failures, and ectopic spike initiation. This is true under normal physiological conditions, and relevant for a number of neuropathies that lead to abnormal excitability. In addition, a growing number of studies show that the axon trunk can express receptors to glutamate, GABA, acetylcholine or biogenic amines, changing the relative contribution of some channels to axonal excitability and therefore rendering the contribution of this compartment to neural coding conditional on the presence of neuromodulators. Long-term regulatory processes, both during development and in the context of activity-dependent plasticity may also affect axonal properties to an underappreciated extent.

Dendritic D-type potassium currents inhibit the spike afterdepolarization in rat hippocampal CA1 pyramidal neurons

In CA1 pyramidal neurons, burst firing is correlated with hippocampally dependent behaviours and modulation of synaptic strength. One of the mechanisms underlying burst firing in these cells is the afterdepolarization (ADP) that follows each action potential. Previous work has shown that the ADP results from the interaction of several depolarizing and hyperpolarizing conductances located in the soma and the dendrites. By using patch-clamp recordings from acute rat hippocampal slices we show that D-type potassium current modulates the size of the ADP and the bursting of CA1 pyramidal neurons. Sensitivity to alpha-dendrotoxin suggests that Kv1-containing potassium channels mediate this current. Dual somato-dendritic recording, outside-out dendritic recordings, and focal application of dendrotoxin together indicate that the channels mediating this current are located in the apical dendrites. Thus, our data present evidence for a dendritic segregation of Kv1-like channels in CA1 pyramidal neurons and identify a novel action for these channels, showing that they inhibit action potential bursting by restricting the size of the ADP.

Potassium channel blockers in multiple sclerosis: Neuronal Kv channels and effects of symptomatic treatment

Multiple sclerosis (MS) is an inflammatory disease of the central nervous system (CNS) characterized by demyelination, with a relative sparing of axons. In MS patients, many neurologic signs and symptoms have been attributed to the underlying conduction deficits. The idea that neurologic function might be improved if conduction could be restored in CNS demyelinated axons led to the testing of potassium (K(+)) channel blockers as a symptomatic treatment. To date, only 2 broad-spectrum K(+) channel blockers, 4-aminopyridine (4-AP) and 3,4-diaminopyridine (3,4-DAP), have been tested in MS patients. Although both 4-AP and 3,4-DAP produce clear neurologic benefits, their use has been limited by toxicity. Here we review the current status of basic science and clinical research related to the therapeutic targeting of voltage-gated K(+) channels (K(v)) in MS. By bringing together 3 distinct but interrelated disciplines, we aim to provide perspective on a vast body of work highlighting the lengthy and ongoing process entailed in translating fundamental K(v) channel knowledge into new clinical treatments for patients with MS and other demyelinating diseases. Covered are (1) K(v) channel nomenclature, structure, function, and pharmacology; (2) classic and current experimental morphology and neurophysiology studies of demyelination and conduction deficits; and (3) a comprehensive overview of clinical trials utilizing 4-AP and 3,4-DAP in MS patients.

Platelet-activating factor-induced pulmonary edema is partly mediated by prostaglandin E(2), E-prostanoid 3-receptors, and potassium channels

Platelet-activating factor (PAF) is an important endogenous mediator of pulmonary edema in many models of acute lung injury. PAF triggers edema formation by simultaneous activation of two independent pathways; one is mediated by a cyclooxygenase metabolite, and the other is blocked by quinine. We examined the hypothesis that the cyclooxygenase-dependent part of PAF-induced edema is mediated by prostaglandin E(2) (PGE(2)). In isolated rat lungs, PAF administration stimulated release of PGE(2) into the venous effluate and increased lung weight as a measure of edema formation. Perfusion with a neutralizing PGE(2) antibody attenuated the PAF-induced edema formation. In vivo, E-prostanoid 3-receptor-deficient mice showed less pulmonary Evans blue extravasation in response to PAF injection than did mice deficient in EP1, EP2, or EP4 receptors. Perfusion of rat lungs with PGE(2) caused pulmonary edema, which was largely prevented by inhibition of voltage-gated potassium channels (25 nM beta-dendrotoxin), but not by blocking calcium-dependent potassium currents (100 micro M paxilline). In line with its effects on PGE(2)-induced edema formation, beta-dendrotoxin attenuated PAF-induced edema partly if given alone, and completely in combination with quinine. Our findings suggest that PAF-triggered edema is partly mediated by the release of PGE(2), activation of EP3 receptors, and activation of voltage-gated potassium channels.

A voltage-dependent transient K(+) current in rat dental pulp cells

We characterized a voltage-dependent transient K(+) current in dental pulp fibroblasts on dental pulp slice preparations by using a nystatin perforated-patch recording configuration. The mean resting membrane potential of dental pulp fibroblasts was -53 mV. Depolarizing voltage steps to +60 mV from a holding potential of -80 mV evoked transient outward currents that are activated rapidly and subsequently inactivated during pulses. The activation threshold of the transient outward current was -40 mV. The reversal potential of the current closely followed the K(+) equilibrium potential, indicating that the current was selective for K(+). The steady-state inactivation of the peak outward K(+) currents described by a Boltzmann function with half-inactivation occurred at -47 mV. The K(+) current exhibited rapid activation, and the time to peak amplitude of the current was dependent on the membrane potentials. The inactivation process of the current was well fitted with a single exponential function, and the current exhibited slow inactivating kinetics (the time constants of decay ranged from 353 ms at -20 mV to 217 ms at +60 mV). The K(+) current was sensitive to intracellular Cs(+) and to extracellular 4-aminopyridine in a concentration-dependent manner, but it was not sensitive to tetraethylammonium, mast cell degranulating peptide, and dendrotoxin-I. The blood depressing substance-I failed to block the K(+) current. These results indicated that dental pulp fibroblasts expressed a slow-inactivating transient K(+) current.

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.

What does beta-bungarotoxin do at the neuromuscular junction?

beta-Bungarotoxin from the Taiwan banded krait, Bungarus multicinctus is a basic protein (pI=9.5), with a molecular weight of 21,800 consisting of two different polypeptide subunits. A phospholipase A(2) subunit named the A-chain and a non-phospholipase A(2) subunit named the B-chain, which is homologous to Kunitz protease inhibitors. The A-chain and the B-chain are covalently linked by one disulphide bridge. On mouse hemi-diaphragm nerve-muscle preparations, partially paralysed by lowering the external Ca(2+) concentration, beta-bungarotoxin classically produces triphasic changes in the contraction responses to indirect nerve stimulation. The initial transient inhibition of twitches (phase 1) is followed by a prolonged facilitatory phase (phase 2) and finally a blocking phase (phase 3). These changes in twitch tension are mimicked, to some extent, by similar changes to end plate potential amplitude and miniature end plate potential frequency. The first and second phases are phospholipase-independent and are thought to be due to the B-chain (a dendrotoxin mimetic) binding to or near to voltage-dependent potassium channels. The last phase (phase 3) is phospholipase dependent and is probably due to phospholipase A(2)-mediated destruction of membrane phospholipids in motor nerve terminals.

A role for voltage-gated potassium channels in the outgrowth of retinal axons in the developing visual system

Neural activity is important for establishing proper connectivity in the developing visual system. Tetrodotoxin blockade of sodium (Na(+))-dependent action potentials impairs the refining of synaptic connections made by developing retinal ganglion cells (RGCs), but does not affect their ability to get out to their target. Although this may suggest neural activity is not required for the directed extension of RGC axons, in many species developing RGCs express additional, Na(+)-independent ionic mechanisms. To test whether the ability of RGC axons to extend in a directed fashion is influenced by membrane excitability, we blocked the principal modulators of the neural activity of a neuron, voltage-dependent potassium (Kv) channels. First, we showed that RGCs and their growth cones express Kv channels when they are growing through the brain on the way to their main midbrain target, the optic tectum. Second, a Kv channel blocker, 4-aminopyridine (4-AP), was applied to the developing Xenopus optic projection. Blocking Kv channels inhibited RGC axon extension and caused aberrant routing of many RGC fibers. With the higher doses, <25% of embryos had a normal optic projection. These data suggest that Kv channel activity regulates the guidance of growing axons in the vertebrate brain.

A role for voltage-gated potassium channels in the outgrowth of retinal axons in the developing visual system

Neural activity is important for establishing proper connectivity in the developing visual system. Tetrodotoxin blockade of sodium (Na(+))-dependent action potentials impairs the refining of synaptic connections made by developing retinal ganglion cells (RGCs), but does not affect their ability to get out to their target. Although this may suggest neural activity is not required for the directed extension of RGC axons, in many species developing RGCs express additional, Na(+)-independent ionic mechanisms. To test whether the ability of RGC axons to extend in a directed fashion is influenced by membrane excitability, we blocked the principal modulators of the neural activity of a neuron, voltage-dependent potassium (Kv) channels. First, we showed that RGCs and their growth cones express Kv channels when they are growing through the brain on the way to their main midbrain target, the optic tectum. Second, a Kv channel blocker, 4-aminopyridine (4-AP), was applied to the developing Xenopus optic projection. Blocking Kv channels inhibited RGC axon extension and caused aberrant routing of many RGC fibers. With the higher doses, <25% of embryos had a normal optic projection. These data suggest that Kv channel activity regulates the guidance of growing axons in the vertebrate brain.

Differential inhibition of glial K(+) currents by 4-AP

Spinal cord astrocytes express four biophysically and pharmacologically distinct voltage-activated potassium (K(+)) channel types. The K(+) channel blocker 4-aminopyridine (4-AP) exhibited differential and concentration-dependent block of all of these currents. Specifically, 100 microM 4-AP selectively inhibited a slowly inactivating outward current (K(SI)) that was insensitive to dendrototoxin (< or = 10 microM) and that activated at -50 mV. At 2 mM, 4-AP inhibited fast-inactivating, low-threshold (-70 mV) A-type currents (K(A)) and sustained, TEA-sensitive noninactivating delayed-rectifier-type currents (K(DR)). At an even higher concentration (8 mM), 4-AP additionally blocked inwardly rectifying, Cs(+)- and Ba(2+)-sensitive K(+) currents (K(IR)). Current injection into current-clamped astrocytes in culture or in acute spinal cord slices induced an overshooting voltage response reminiscent of slow neuronal action potentials. Increasing concentrations of 4-AP selectively modulated different phases in the repolarization of these glial spikes, suggesting that all four K(+) currents serve different roles in stabilization and repolarization of the astrocytic membrane potential. Our data suggest that 4-AP is an useful, dose-dependent inhibitor of all four astrocytic K(+) channels. We show that the slowly inactivating astrocytic K(+) currents, which had not been described as separate current entities in astrocytes, contribute to the resting K(+) conductance and may thus be involved in K(+) homeostatic functions of astrocytes. The high sensitivity of these currents to micromolar 4-AP suggests that application of 4-AP to inhibit neuronal A-currents or to induce epileptiform discharges in brain slices also may influence astrocytic K(+) buffering.

Strength-duration properties of peripheral nerve in acquired neuromyotonia

The strength-duration time constant (SDTC) of a myelinated axon is a property of the nodal membrane and is sensitive to changes in membrane potential. Strength-duration time constants for motor axons and cutaneous afferents of the median nerve were measured in 9 patients with acquired neuromyotonia (NMT), a condition of peripheral nerve hyperexcitability, and 15 control patients. Mean motor axon time constants were significantly prolonged (344 +/- 100 micros) in patients compared to healthy controls (264 +/- 34 micros; P = 0.038), but sensory axon time constants were not significantly different. Motor axon time constants were longer than sensory axon time constants in 4 of the patients with neuromyotonia, suggesting that the nodal membrane was depolarized by an ectopic focus at the site of nerve stimulation at the wrist, ionic conductances were altered at the node, or that the size of the node was increased, possibly as a result of immune-mediated damage. The anti-voltage-gated potassium channel antibodies thought to generate peripheral nerve hyperexcitability in acquired neuromyotonia may be indirectly responsible for changes in motor axon nodal membrane properties.

Potassium currents in freshly dissociated uterine myocytes from nonpregnant and late-pregnant rats

In freshly dissociated uterine myocytes, the outward current is carried by K+ through channels highly selective for K+. Typically, nonpregnant myocytes have rather noisy K+ currents; half of them also have a fast-inactivating transient outward current (ITO). In contrast, the current records are not noisy in late pregnant myocytes, and ITO densities are low. The whole-cell IK of nonpregnant myocytes respond strongly to changes in [Ca2+]o or changes in [Ca2+]i caused by photolysis of caged Ca2+ compounds, nitr 5 or DM-nitrophene, but that of late-pregnant myocytes respond weakly or not at all. The Ca2+ insensitivity of the latter is present before any exposure to dissociating enzymes. By holding at -80, -40, or 0 mV and digital subtractions, the whole-cell IK of each type of myocyte can be separated into one noninactivating and two inactivating components with half-inactivation at approximately -61 and -22 mV. The noninactivating components, which consist mainly of iberiotoxin-susceptible large-conductance Ca2+-activated K+ currents, are half-activated at 39 mV in nonpregnant myocytes, but at 63 mV in late-pregnant myocytes. In detached membrane patches from the latter, identified 139 pS, Ca2+-sensitive K+ channels also have a half-open probability at 68 mV, and are less sensitive to Ca2+ than similar channels in taenia coli myocytes. Ca2+-activated K+ currents, susceptible to tetraethylammonium, charybdotoxin, and iberiotoxin contribute 30-35% of the total IK in nonpregnant myocytes, but <20% in late-pregnant myocytes. Dendrotoxin-susceptible, small-conductance delayed rectifier currents are not seen in nonpregnant myocytes, but contribute approximately 20% of total IK in late-pregnant myocytes. Thus, in late-pregnancy, myometrial excitability is increased by changes in K+ currents that include a suppression of the ITO, a redistribution of IK expression from large-conductance Ca2+-activated channels to smaller-conductance delayed rectifier channels, a lowered Ca2+ sensitivity, and a positive shift of the activation of some large-conductance Ca2+-activated channels.

Characterization of outward currents in neurons of the avian nucleus magnocellularis

Characterization of outward currents in neurons of the avian nucleus magnocellularis. J. Neurophysiol. 80: 2824-2835, 1998. Neurons of the nucleus magnocellularis (NM) preserve the timing of auditory signals through the convergence of a variety of voltage- and ligand-gated ion channels. To understand better how these channels interact, we have characterized the kinetics, voltage sensitivity, and pharmacology of outward currents of NM neurons in brain slices. The reversal potential (Erev) of outward currents varied with potassium concentration as expected for currents carried by potassium. However, Erev was consistently more positive than the Nernst potential for potassium (EK). Deviation of Erev from the calculated EK most likely arose from potassium accumulation in extracellular spaces by potassium conductances active at rest and during depolarizing steps. Three outward potassium currents were studied that varied in voltage and pharmacological sensitivity. A tetraethylammonium (TEA)-sensitive, high-threshold current was activated within 1-5 ms of the onset of depolarization, with a half-maximal activation voltage (V1/2) of -19 mV. It was blocked partially by 4-aminopyridine (4-AP) and was the dominant ionic conductance of NM neurons. A dendrotoxin-I (DTX) and 4-AP-sensitive, low-threshold current had a V1/2 of -58 mV, rapid activation kinetics, and only partial inactivation, with decay time constants between 20 and 100 ms. A rapidly inactivating current was observed that was resistant to TEA and DTX and was blocked by intracellular Cs+. The transient current was inactivated almost completely at the resting potential. The onset of inactivation was fastest at potentials negative to those that caused activation. When intracellular K+ was replaced by Cs+, large inward and outward currents were obtained that corresponded respectively to the above-mentioned DTX- and TEA-sensitive currents. Outward, TEA-sensitive current was carried by Cs+, with a PCs/PK of approximately 0.1. In current-clamped neurons, DTX induced repetitive firing and increased membrane time constant near rest but had little effect on action potential duration. These studies indicate that a low-threshold, DTX-sensitive current plays a key role in making NM neurons highly responsive to the onset and offset of synaptic stimuli.

Mechanisms of bupivacaine action on Na+ and K+ channels in myelinated axons of Xenopus laevis

The local anaesthetic bupivacaine has recently been proposed to inhibit Na+ channels indirectly by making the resting potential less negative. To test this hypothesis we analysed the effects of bupivacaine on voltage and current clamped nodes of Ranvier. Contrary to the hypothesis, the leak current and the resting potential were unaffected. The Na+ and K+ channels were, however, affected at relatively low concentrations (33 microM). Steady-state activation curves were decreased without notable shift effects, whereas the Na+ inactivation curve was decreased and shifted in negative direction. The effect on the Na+ current was tentatively explained by a single-site, state-dependent binding model (Kd = 44 microM), while that on the K+ current was explained by two population-specific mechanisms, one open-state dependent (Kd = 550 microM) and one state independent (Kd = 59 microM). The binding stoichiometry was higher than 1:1 for the main sites of action. In conclusion, bupivacaine exerts its main anaesthetic action on myelinated nerve axons by a direct modification of Na+ channels.

Recent studies on dendrotoxins and potassium ion channels

1. Dendrotoxins are small proteins isolated from mamba (Dendroaspis) snake venoms. They block some subtypes of voltage-dependent potassium channels in neurons. 2. Dendrotoxins contain 57-60 amino acid residues crosslinked by three disulfide bridges. They are homologous to Kunitz-type serine protease inhibitors, such as aprotinin, although they have little or no antiprotease activity. 3. Dendrotoxins act mainly on neuronal K+ channels. Studies with cloned K+ channels indicate that alpha-dendrotoxin from green mamba Dendroaspis angusticeps blocks Kv1.1 and Kv1.2 channels in the nanomolar range. In native cells, dendrotoxin appears preferentially to block inactivating forms of K+ current. 4. Dendrotoxins can induce repetitive firing in neurons and facilitate transmitter release. On direct injection to the CNS, dendrotoxins can induce epileptiform activity. 5. Radiolabeled dendrotoxins are useful markers of subtypes of K+ channels in vivo, and structural analogs help to define the molecular recognition properties of different types of K+ channels.

Changes in pharmacological sensitivity of the spinal cord to potassium channel blockers following acute spinal cord injury

In this investigation we studied changes in the pharmacological sensitivity of dorsal column white matter to a variety of K+ channel blockers, including 4-aminopyridine (4-AP), following acute spinal cord injury (SCI) in vitro using a modified aneurysm clip. Compound action potentials (CAPs) were recorded extracellularly with microelectrodes and by the sucrose gap recording technique. With acute trauma, injured axons showed significantly enhanced sensitivity to 4-AP in comparison to uninjured controls as early as 10 min following injury. Microelectrode derived field potential recordings showed a significantly greater increase in a delayed positive component (P2) of the CAP at both 1 and 5 mM 4-AP in injured as compared to noninjured axons. Sucrose gap recordings showed an increase in CAP area and amplitude of injured axons with 1 mM 4-AP at 22 degrees C. The relative improvement in CAP area and amplitude with 4-AP was even more pronounced (P < 0.05) at higher temperatures (37 degrees C). As shown by sucrose gap, 4-AP also caused a delay in repolarization of the CAP and depolarization of the resting membrane potential of acutely injured axons. TEA (0.1 mM and 10 mM), when infused alone and with CsCl (10 mM), produced similar effects on injured and intact axons. In conclusion, the results of this study show an altered sensitivity of the spinal cord to 4-AP following acute SCI. In contrast, TEA and CsCl exhibit no difference in their effects on low frequency axonal conduction between injured and noninjured axons. The data suggest that acute traumatic myelin disruption following SCI causes axonal dysfunction partly due to abnormal activation of 4-AP-sensitive 'fast' K+ channels.

Molecular properties of voltage-gated K+ channels

Subfamilies of voltage-activated K+ channels (Kv1-4) contribute to controlling neuron excitability and the underlying functional parameters. Genes encoding the multiple alpha subunits from each of these protein groups have been cloned, expressed and the resultant distinct K+ currents characterized. The predicted amino acid sequences showed that each alpha subunit contains six putative membrane-spanning alpha-helical segments (S1-6), with one (S4) being deemed responsible for the channels' voltage sensing. Additionally, there is an H5 region, of incompletely defined structure, that traverses the membrane and forms the ion pore; residues therein responsible for K+ selectively have been identified. Susceptibility of certain K+ currents produced by the Shaker-related subfamily (Kv1) to inhibition by alpha-dendrotoxin has allowed purification of authentic K+ channels from mammalian brain. These are large (M(r) approximately 400 kD), octomeric sialoglycoproteins composed of alpha and beta subunits in a stoichiometry of (alpha)4(beta)4, with subtypes being created by combinations of subunit isoforms. Subsequent cloning of the genes for beta 1, beta 2 and beta 3 subunits revealed novel sequences for these hydrophilic proteins that are postulated to be associated with the alpha subunits on the inner side of the membrane. Coexpression of beta 1 and Kv1.4 subunits demonstrated that this auxiliary beta protein accelerates the inactivation of the K+ current, a striking effect mediate by an N-terminal moiety. Models are presented that indicate the functional domains pinpointed in the channel proteins.

Single voltage-gated K+ channels and their functions in small dorsal root ganglion neurones of rat

1. Single voltage-activated K+ channels were investigated by means of the patch-clamp technique in small dorsal root ganglion (DRG) neurones in 150 microns thin slices of new-born rat DRG. It was found that K+ conductance in small DRG neurones is formed by one type of fast inactivating A-channel and four types of delayed rectifier K+ channels, which could be separated on the basis of their single-channel conductance, kinetics and sensitivity to external tetraethylammonium (TEA). 2. Potassium A-channels were observed at relatively moderate density. They were weakly sensitive to TEA and activated between -70 and +20 mV. The conductance of A-channels was about 40 pS for inward currents in symmetrical high-K+ solutions with external 5 mM TEA added to suppress other types of K+ channels. The time constant of channel inactivation (tau in) was 18.8 ms at -70 mV and 6 ms at potentials positive to -20 mV. 3. A fast delayed rectifier (DRF) channel with a conductance of 55 pS in symmetrical high-K+ solutions was the most frequent type of K+ channel. The channel activated in a broad potential range between -50 and +60 mV and demonstrated a fast deactivation within 1-3 ms after potential return to -80 mV in high-Ko+ solution. The tau in value was 90-150 ms at positive membrane potentials. The single-channel current amplitudes were blocked to 55% by 1 mM TEA. 4. Three further types of delayed rectifier K+ channels were called DR1-, DR2- and DR3- channels. Their single-channel conductances for inward currents in symmetrical high-K+ solutions were distributed between 30 and 44 pS. The channels activated in almost the same voltage range between -60 and -10 mV. Deactivation of the channels at -80 mV lasted tens of milliseconds. The channels were separated on the basis of their sensitivities to TEA. DR1-channel currents were reduced to 50% in the presence of 1 mM TEA, DR2-channel currents were reduced to about 50% by 5 mM TEA, whereas the amplitudes of currents through DR3-channels were almost unaffected by 5 mM TEA. 5. Addition of external 1 and 5 mM TEA to whole cells under current-clamp condition depolarized the cell membrane, lowered the threshold for action potential firing, prolonged action potential duration and reduced the amplitude of after-hyperpolarization. 6. It is concluded that potassium A-, DRF-, DR1-, DR2- and DR3-channels play multiple roles in the excitability of DRG neurones. Possible influences of these channels on the shape of the action potential, its firing threshold and the resting membrane potential of small DRG neurones are discussed.

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.

Ion channels and transporters in the electroreceptive ampullary epithelium from skates

Two ampullary epithelial properties necessary for electroreception were used to identify the types of ion channels and transporters found in apical and basal membranes of ampullary receptor cells of skates and to assess their individual role under voltage-clamp conditions. The two essential properties are (1) a steady-state negative conductance generated in apical membranes and (2) a small, spontaneous current oscillation originating in basal membranes (Lu and Fishman, 1995). The effects of pharmacological agents and ion substitutions on these properties were evaluated from transorgan or transepithelial complex admittance determinations in the frequency range 0.125 to 50 Hz measured in individual, isolated ampullary organs. In apical membranes, L-type Ca channels were found to be responsible for generation of the steady-state negative conductance. In basal membranes, K and Ca-dependent Cl (Cl(Ca)) channels were demonstrated to contribute to a net positive membrane conductance. L-type Ca channels were also evident in basal membranes and are thought to function in synaptic transmission from the electroreceptive epithelium to the primary afferent nerve. In addition to ion channels in basal membranes, two transporters (Na+/K+ pump and Na(+)-Ca+ exchanger) were apparent. Rapid (minutes) cessation of the current oscillation after blockage of any of the basal ion channels (Ca, Cl(Ca), K) suggests critical involvement of each of these channel types in the generation of the oscillation. Suppression of either Na+/K+ transport or Na(+)-Ca2+ exchange also eliminated the oscillation but at a slower rate, indicating an indirect effect.

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.

Large- and small-conductance Ca(2+)-activated K+ channels: their role in the nicotinic receptor-mediated catecholamine secretion in bovine adrenal medulla

In cultured bovine adrenal chromaffin cells, charybdotoxin and iberiotoxin (inhibitors of the large-conductance Ca(2+)-activated K+ channel) as well as apamin (an inhibitor of the small-conductance Ca(2+)-activated K+ channel), at 1-100 nM, suppressed carbachol-induced 86RB+ efflux, augmented carbachol-induced 45Ca2+ influx via voltage-dependent Ca2+ channels and catecholamine secretion and had no effect on carbachol-induced 22Na+ influx via nicotinic receptors, a prerequisite for Ca2+ channel activation by carbachol. 45Ca2+ influx caused by high K+ (a direct activation of voltage-dependent Ca2+ channels) was also enhanced by these K+ channel inhibitors, with the concentration-response curves being similar to those for carbachol-induced 45Ca2+ influx. Dendrotoxin and mast cell degranulating peptide (inhibitors of voltage-dependent K+ channels), on the other hand, did not alter carbachol-induced 86Rb+ efflux or 45Ca2+ influx. These results suggest that the stimulation of nicotinic receptors eventually opens large- and small-conductance Ca(2+)-activated K+ channels, and that the blockade of these Ca(2+)-activated K+ channels results in gating of voltage-dependent Ca2+ channels and thereby augments catecholamine secretion from bovine adrenal chromaffin cells.

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.

Blockade by dendrotoxin homologues of voltage-dependent K+ currents in cultured sensory neurones from neonatal rats

1. Homologues of dendrotoxin (Dtx) were isolated from the crude venom of Green and Black Mamba snakes and examined for K+ channel blocking activity in neonatal rat dorsal root ganglion cells (DRGs) by whole-cell patch clamp recording. 2. Outward potassium current activated by depolarization was composed of two major components: a slowly inactivating current (SIC, tau decay approximately 50 ms, 200 ms and 2s), and a non-inactivating current (NIC, tau decay > 2 min). Tail current analysis revealed two time constants of deactivation of total outward current, 3-12 ms and 50-150 ms (at -80 mV) which corresponded to SIC and NIC, respectively. 3. All the homologues (alpha-, beta-, gamma- and delta-Dtx and toxins I and K) blocked outward current activated by depolarization in a dose-dependent manner. The most potent in blocking total outward current was delta-Dtx (EC50 of 0.5 +/- 0.2 nM), although there were no statistically significant differences in potency between any of the homologues. 4. Qualitative differences in the nature of the block were noted between homologues. In particular, the block by delta-Dtx was time-dependent, whereas that by alpha-Dtx was not. 5. alpha-Dtx was a much better blocker of SIC (EC50 = 1.0 +/- 0.4 nM) than was delta-Dtx (EC50 = 17.6 +/- 5.8 nM). Furthermore, delta-Dtx was selective for NIC (EC50 +/- 0.24 +/- 0.03 nM) over SIC and reduced the slow component of tail currents (NIC), preferentially. On the other hand, a-Dtx did not significantly distinguish between SIC and NIC although tail current analysis showed that a-Dtxpreferentially reduced the fast component of tail currents (SIC).6. The results confirm, using direct electrophysiological methods, that homologues of dendrotoxins from Mamba snake venom block K+ channels in rat sensory neurones. Furthermore, a-Dtx and 6-Dtx distinguish between sub-types of K+ channels in these cells and may thus be useful pharmacological tools in other neuronal K+ channel studies.

Axons regulate the expression of Shaker-like potassium channel genes in Schwann cells in peripheral nerve

We examined potassium channel gene expression of two members of the Shaker subfamily, MK1 and MK2, in sciatic nerves from rats and mice. In Northern blot analysis, MK1 and MK2 probes detected single transcripts of approximately 8 kb and approximately 9.5 kb, respectively, in sciatic nerve and brain from both species. Polymerase chain reaction amplification of a cDNA library of cultured rat Schwann cells using MK1- and MK2- specific primers produced DNA fragments that were highly homologous to MK1 and MK2. To determine whether these channel genes were axonally regulated, we performed Northern blot analysis of developing, permanently transected, and crushed rat sciatic nerves. The mRNA levels for both MK1 and MK2 increased from P1 to P15 and then declined modestly. Permanent nerve transection in adult animals resulted in a dramatic and permanent reduction in the mRNA levels for both MK1 and MK2, whereas normal levels of MK1 and MK2 were restored when regeneration was allowed to occur following crush injury. In all cases, MK1 and MK2 mRNA levels paralleled that of the myelin gene P0. Elevating the cAMP in cultured Schwann cells by forskolin, which mimics axonal contact but not myelination, did not induce detectable levels of MK1 and MK2 mRNA by Northern blot analysis. Further, the level of MK1 mRNA in the vagus nerve, which contains relatively fewer myelinating Schwann cells and relatively more non-myelinating Schwann cells than the sciatic nerve, is reduced relative to the sciatic nerve. In conclusion, we have identified two Shaker-like potassium channel genes in sciatic nerves whose expressions are regulated by axons. We suggest that MK1 and MK2 mRNA are expressed in high levels only in myelinating Schwann cells and that these Shaker-like potassium channel genes have specialized roles in these cells.

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.

Delayed rectifier potassium channels in ventricle and sinoatrial node of the guinea pig: molecular and regulatory properties

We focus on the regulatory properties of delayed rectifier K+ (IK) channels in guinea-pig sinoatrial node (SAN) and compare SAN IK to the better characterized ventricular IK. Despite demonstrated similarities in the properties of IK in guinea-pig ventricle and SAN, the possibility remains that expression of IK channels can vary regionally within the same heart. Like ventricular IK, SAN IK can be enhanced by beta-adrenergic stimulation and exposure to phorbol ester. However, in contrast to ventricular IK, regulation of SAN IK by protein kinases A and C is not temperature dependent. Basal SAN IK can be diminished by muscarinic agonists, while beta-adrenergic stimulation is a precondition for reduction of ventricular IK by cholinergic agonists. Nonstationary state fluctuation analysis predicts a small single-channel current (1 pA) and a large number of functional channels (308) associated with whole-cell SAN IK. The corresponding single-channel conductance of 6 pS is somewhat larger than that estimated for ventricular IK. Overall comparisons of guinea-pig ventricular and SAN IK to the current associated with the minK channel clone suggest that the native guinea-pig cardiac IK channels may be related not only to each other but lso to the minK channel protein.

Action of alpha-dendrotoxin on K+ currents in nerve terminal regions of axons in rat olfactory cortex

1. In the rat olfactory cortex, unmyelinated axons give rise to synapses en passant. This tissue was used to study the pharmacology of axonal K(+)-currents. Responses were measured from a group of these axons as unclamped field currents, with a polarizable suction electrode. 2. A single stimulus to the axons elicited a tetrodotoxin-sensitive Na(+)-dependent transient K(+)-currents were revealed by positive polarization of the suction electrode and were manifest as a negative current following the Na(+)-component. 3. In the presence of tetraethylammonium (TEA, 5 mM) and Cd2+ (100 microM), the K(+)-component was depressed by 3,4-diaminopyridine (3,4-DAP; 1 to 20 microM; IC50 2.0 +/- 0.4 microM). alpha-Dendrotoxin (DTX; 15-1500 nM) also attenuated the aminopyridine-sensitive component (IC50 93 +/- 4 nM). At the highest DTX concentration, depression of the K(+)-current was incomplete, the residual K+ current being reduced by 3,4-DAP (0.1 to 5 microM). 4. These results indicate the presence of two aminopyridine-sensitive K+ currents in this preparation distinguished by their susceptibility to DTX.

Two types of 4-aminopyridine-sensitive potassium current in rabbit Schwann cells

1. Delayed rectifier K+ currents were studied in Schwann cells cultured from neonatal rabbit sciatic nerves with the whole-cell patch-clamp technique. 2. Depolarizing voltage steps (40 ms duration) activated two types of K+ current: type I, whose apparent activation threshold was about -60 mV (half-maximal conductance at -40 +/- 1 mV, n = 10); and type II, whose apparent activation threshold was about -25 mV (half-maximal conductance at + 11 +/- 1 mV, n = 9). 3. Type I current was blocked by alpha-dendrotoxin (alpha-DTX) with an apparent equilibrium dissociation constant (KD) of 1.3 nM, whereas the type II current was unaffected by exposure to 500 nM toxin. The action of alpha-DTX on the type I current was reversible. 4. Most cells exhibited both types of current, but occasionally some cells displayed just type I or just type II. 5. Type I current activated rapidly and then showed a much slower fade, which became more noticeable with larger depolarizations. Activation of type II current was slower than that of type I and depended less steeply on voltage. The time constants of activation for type I and type II currents were derived with a Hodgkin-Huxley formalism (based on second-power activation and deactivation kinetics). The longest activation time constant for type II gating was more than twice the corresponding time constant for type I; however, the time constants determined from tail current decays at potentials more negative than -60 mV were shorter for the type II currents than for the type I currents. 6. Both type I and type II currents were sensitive to micromolar concentrations of 4-aminopyridine (4-AP). The KD for 4-AP blockade of type II current was 630 microM (pH 7.2, membrane potential (Em) = -10 mV), which is about 6 times higher than the corresponding value for 4-AP blockade of type I current at negative membrane potentials. The differential sensitivity of the type I and type II currents to 4-AP may account for the apparent voltage dependence of 4-AP block of delayed rectifier K+ currents. 7. In addition to types I and II, a third type of outward K+ current (type III) was generated in most cells at positive membrane potentials. This latter current was insensitive to millimolar concentrations of 4-AP. 8. Similarities between Schwann cell and neuronal potassium channels are discussed.

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.

Fast K channels are more sensitive to riluzole than slow K channels in myelinated nerve fibre

The effects of 1-500 microM riluzole, a novel psychotropic agent, were studied on the nodal K current of isolated nerve fibres of the frog. When added to the external solution, the substance rapidly and reversibly inhibited slow, fast 1 and fast 2 K components of the tail K current. The concentrations of riluzole inducing half maximum reduction of slow, fast 1 and fast 2 K conductances were 413 microM, 24 microM and 21 microM respectively. It is concluded that the substance is about 20 times more effective in blocking fast than slow K channels.

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)

Inhibition of phosphorylation of synapsin I and other synaptosomal proteins by beta-bungarotoxin, a phospholipase A2 neurotoxin

Some snake venom neurotoxins, such as beta-bungarotoxin (beta-BuTX), which possess relatively low phospholipase A2 (PLA2) activity, act presynaptically to alter acetylcholine (ACh) release both in the periphery and in the CNS. In investigating the mechanism of this action, we found that beta-BuTX (5 and 15 nM) inhibited phosphorylation, in both resting and depolarized synaptosomes, of a wide range of proteins, including synapsin I. Naja naja atra PLA2, which has higher PLA2 activity, also inhibited phosphorylation but was less potent than beta-BuTX. At 1 nM, beta-BuTX and N. n. atra PLA2 inhibited phosphorylation of synapsin I only in depolarized synaptosomes. Synaptosomal ATP levels were not affected by 5 or 15 nM beta-BuTX or by 5 nM N. n. atra PLA2. Limited proteolysis, using Staphylococcus aureus V-8 protease, indicated that beta-BuTX inhibited phosphorylation of synapsin I in both the head and the tail regions. The inhibition of phosphorylation was not antagonized by nordihydroguaiaretic acid or indomethacin, suggesting that arachidonic acid derivatives do not mediate this inhibition. Furthermore, inhibition of phosphorylation by beta-BuTX and N. n. atra PLA2 was not altered in the presence of the phosphatase inhibitor okadaic acid, suggesting that stimulation of phosphatase activity is not responsible for this inhibition. Inhibition of protein phosphorylation by PLA2 neurotoxins and enzymes may be associated with an inhibition of ACh release.

Charybdotoxin, dendrotoxin and mast cell degranulating peptide block the voltage-activated K+ current of fibroblast cells stably transfected with NGK1 (Kv1.2) K+ channel complementary DNA

The blocking actions of the K+ channel toxins charybdotoxin, dendrotoxin and mast cell degranulating peptide were studied in B82 mouse fibroblast cells transformed to express NGK1 (Kv1.2) K+ channels. All three toxins were potent blockers of the K+ current in these cells, with KD values of 1.7, 2.8 and 185 nM, respectively. The toxin block exhibited a weak voltage-dependence with the degree of inhibition decreasing at positive membrane potentials. For charybdotoxin and dendrotoxin, reducing [K+]i did not increase the fractional block, demonstrating that the relief of block at positive membrane potentials is not due to displacement of the toxin molecules by outward flow of K+ ions. A voltage-jump protocol was used to determine the rates of binding and unbinding of dendrotoxin and mast cell degranulating peptide; binding of charybdotoxin was too rapid to be quantitatively evaluated in this manner. The binding rates (dendrotoxin, approximately 5 x 10(7)/M per s; mast cell degranulating peptide, approximately 0.8 x 10(7)/M per s) were largely voltage-independent, suggesting that association of the toxin molecules with the channel is diffusion limited. The rates of unbinding (dendrotoxin, approximately 0.3/s; mast cell degranulating peptide, approximately 3/s at +60 mV) of both toxins increased e-fold per approximately 40 mV change in membrane potential, thus accounting for the voltage-dependence of the equilibrium block. Internal perfusion with the three toxins failed to affect the K+ current (in contrast to internal tetraethylammonium which strongly blocked the current), indicating that the toxins exert their blocking action by binding to extracellular sites.

Protection against dendrotoxin-induced clonic seizures in mice by anticonvulsant drugs

Various anticonvulsant drugs were evaluated for their ability to protect against clonic seizures induced in mice by intraventricular injection of the K+ channel blocking peptide dendrotoxin (DTX). Phenytoin, the phenytoin-like anticonvulsant carbamazepine and the broad spectrum drug valproate were effective in this model, whereas the GABA-enhancers diazepam and tiagabine, the NMDA antagonists (+/-)-CPP and (+)-MK-801, the AMPA antagonist NBQX, the antiabsence drug ethosuximide and the Ca2+ channel antagonist nimodipine were inactive. In contrast to the lack of activity of other NMDA antagonists, phencyclidine and ADCI [(+/-)-aminocarbonyl-10,11-dihydro-5H-dibenzo [a,d]cyclohepten-5,10-imine] were potent antagonists of DTX-induced seizures.

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.

MCD peptide and dendrotoxin I activate c-fos and c-jun expression by acting on two different types of K+ channels. A discrimination using the K+ channel opener lemakalim

Mast cell degranulating (MCD) peptide and Dendrotoxin I (DTXI), two potent hyperexcitability-inducing toxins acting on voltage-dependent potassium channels, induce the expression of both c-fos and c-jun mRNA in i.c.v. treated rats. The distribution of c-fos and c-jun expression has been analyzed by in situ hybridization. The expression is particularly high in the cerebral cortex and hippocampus for both toxins. However differences of expression between MCD and DTXI-treated animals have been observed in hypothalamus and thalamic and amygdaloid nuclei. Moreover, brain areas such as cerebellum which have high amounts of binding sites for both MCD and DTXI do not show any induction of c-fos and c-jun. Lemakalim, a K+ channel opener, prevents the MCD-induced activation of both 'immediate-early genes' in all brain areas but is unable to inhibit the induction of c-fos and c-jun induced by DTXI. These two toxins which are generally believed--from molecular approaches--to act on the same voltage-dependent K+ channel, clearly act in vivo on two distinct classes of channels.

Two types of fast K+ channels in rat myelinated nerve fibres and their sensitivity to dendrotoxin

The effect of dendrotoxin (DTX), a component of the venom of the Eastern green mamba snake, Dendroaspis angusticeps, on K+ currents in rat myelinated nerve fibres was studied in voltage clamp experiments, immunocytochemistry and binding experiments. The analysis of K+ tail currents in 160 mM KCl solution revealed that K+ channels with slow gating kinetics predominate in the intact node of Ranvier. These slow K+ channels were not blocked by DTX. Intact nerve fibres additionally showed fast K+ tail currents of small amplitude which could be blocked by DTX. After enzymatic demyelination with pronase, fast K+ currents of large amplitude appeared. Analysis of the non-monotonous voltage dependence of the fast K+ conductance and the partial pharmacological block by DTX suggest the presence of two subtypes of fast K+ channels in rat nerve fibres similar to the Kf1 and Kf2 channels previously described in the frog and toad node of Ranvier. The DTX concentration required for 50% inhibition (IC50) for the Kf1 component was 8 nM. The IC50 of the blocked Kf2 component was the same as that for Kf1, but the Kf2 component was only partially blocked (about 50%). In contrast to frog nerve, these two fast K+ channel subtypes are located predominantly in the paranodal region. Immunocytochemical staining experiments with DTX using the peroxidase-antiperoxidase technique confirmed the electrophysiological data. In intact nodes, either no staining or only slight staining in some fibres was found. After demyelination, extensive staining of paranodal and internodal regions occurred.(ABSTRACT TRUNCATED AT 250 WORDS)

ATP-sensitive and Ca-activated K channels in vertebrate axons: novel links between metabolism and excitability

Two types of metabolically regulated K channels have been identified for the first time in enzymatically demyelinated fibres of amphibian sciatic nerve using the patch-clamp technique. A maxi K channel with a single-channel conductance of 132 pS (105 mM K on both sides of the membrane, 15 degrees C) is activated both by micromolar concentrations of internal Ca and by depolarization. A second type of K channel with a conductance of 44 pS is inhibited by intracellular adenosine 5'-triphosphate (ATP) with a half-maximal inhibitory concentration (IC50) of 35 microM. It is blocked by submicromolar concentrations of external glibenclamide. Both channels are sensitive to external tetraethylammonium chloride (IC50 = 0.2 mM for the maxi K channel and 4.2 mM for the ATP-sensitive channel). They may be part of a complex feedback system regulating axonal excitability under various metabolic conditions.