The specificity of the sodium channel for monovalent cations

A high-throughput HERG potassium channel function assay: an old assay with a new look

In this paper, we describe an assay using radioactive rubidium (86Rb) efflux to screen functional human ether-a go-go-related gene (HERG) K+ channels in a high-throughput screening (HTS) format. This assay offers an alternative way to examine junctional interactions between chemical compounds and HERG K+ channels. Follow-up experiments and discussions were carried out to address a variety of factors that affect potency evaluation within the Rb efflux assay. Factors that can affect the assay results, such as assay time, efflux rate, and compound blocking kinetics, are discussed in detail. Our results provide some explanations for the variances of the assay results and offer some guidelines for using the Rb efflux assay to evaluate compound interactions with HERG K+ channels in the pharmaceutical industry.

Effects of veratridine on sodium currents and fluxes

Veratridine causes Na+ channels to stay open during a sustained membrane depolarization by abolishing inactivation. The consequential Na+ influx, either by itself or by causing a maintained depolarization, leads to many secondary effects such as increasing pump activity, Ca2+ influx, and in turn exocytosis. If the membrane is voltage clamped in the presence of the alkaloid, a lasting depolarizing impulse induces, following the "normal" transient current, another much more slowly developing Na+ current that reaches a constant level after a few seconds. Repolarization then is followed by an inward tail current that slowly subsides. Development of these slow currents is enhanced by additional treatment with agents that inhibit inactivation. Most of these phenomena can be satisfactorily explained by assuming that Na+ channels must open before veratridine binds to them, and that the slow current changes reflect the kinetics of binding and unbinding. It is unclear, however, where the alkaloid stays when it is not bound. Although the effect sets in promptly, once this pool is filled, access to it from outside must be impeded since in most preparations veratridine can only partially be washed out. Cooling acts as if the available concentration is reduced, but this reversible "reduction" takes much longer to develop than the cold-induced changes in kinetics. Several authors assume that the binding site, site 2, is accessed from the lipid phase of the membrane. Considerations of this kind are often based on experiments with batrachotoxin, the widely used site-2 ligand which has a much higher affinity and acts as a full agonist in contrast to the partial agonist veratridine. Batrachotoxin thus lends itself to binding studies using radiolabeled derivatives. Such experiments may eventually lead to the characterization of neurotoxin site 2; the first promising steps have been taken. Modern techniques of molecular biology will almost certainly be successful, and one hopes for point-mutated channels with distinctly different reactions also to veratridine. A considerable amount of research is still required to clarify the structural basis for the numerous allosteric interactions with other sites, the mechanism of the very large potential shift of activation, the reduced single-channel conductance and selectivity, and the chemical nature of the different affinities of the site-2 toxins. Note Added in Proof. A report on point mutations with effects on neurotoxin site 2 (see Sect. 8) has just appeared: Wang S-Y, Wang GK (1988) Point mutations in segment I-S6 render voltage-gated Na+ channels resistant to batrachotoxin. Proc Natl Acad USA 95:2653-2658. In microliter muscle Na+ channels expressed in mammalian cells, mutation Asn434Lys leads to complete, Asn434Ala to partial insensitivity to 5 mM batrachotoxin. (Asn434 corresponds to Asn419 of Trainer et al. 1996). The mutant channel displays almost normal current kinetics and in the presence of veratridine little, if any, slow tail current. However, veratridine inhibits peak Na+ currents in the mutant which may point to a complex structure of site 2.

The role of exposed tryptophan residues in the activity of the cardiotonic polypeptide anthopleurin B

Scorpion and sea anemone venoms contain several polypeptides that delay inactivation of voltage-sensitive sodium channels via interaction with a common site. In this report, we target exposed hydrophobic residues at positions 33 and 45 of anthopleurin B (ApB) by polymerase chain reaction mutagenesis to ascertain their contribution to toxin activity. Nonconservative replacements are not permitted at position 33, indicating that Trp-33 may play an important structural role. Strikingly, the relatively conservative substitution of Trp-33 by phenylalanine results in major reductions in binding affinity for both the cardiac and neuronal channel isoforms as measured by ion flux, whereas substitution with tyrosine is tolerated and exhibits near wild-type affinities, suggesting that either the ability to form a hydrogen bond or the amphiphilic nature of the side chain are important at this position. Electrophysiological analysis of W33F indicates that its diminished affinity is primarily due to a decreased association rate. Analysis of a panel of mutants at Trp-45 shows only modest changes in apparent binding affinity for both channel isoforms but significant effects on Vmax. In neuronal channels, the maximal levels of uptake for W45A/S/F are about 50% those seen with ApB. This effect is also observed for W45A and W45S in the cardiac model, wherein W45F is normal. These results suggest that a hydrophobic contact is involved in toxin-induced stabilization of the open conformation of the cardiac sodium channel. We conclude that Trp-33 contributes significantly to apparent affinity, whereas Trp-45 does not appear to affect binding per se. Furthermore, W33F is the first ApB mutant that displays a significantly altered association rate and may prove to be a useful probe of the channel binding site.

3,4 dichlorobenzamil-sensitive, monovalent cation channel induced by palytoxin in cultured aortic myocytes

1. Smooth muscle cells were dispersed from rat aorta and then cultured. The action of palytoxin on rat aortic myocytes was analysed by measurement of 22Na+ uptake and single channel recording techniques. 2. Palytoxin induced an increase in 22Na+ uptake, with a concentration of 50 nM producing half-maximal activation. The action of palytoxin was inhibited by amiloride derivatives and by ouabain. The concentrations of inhibitor producing half-maximal inhibition were 10 microM for 3,4 dichlorobenzamil, 30 microM for benzamil, 100 microM for phenamil and 1 mM for ouabain. 3. In outside-out patches, palytoxin induced single channel currents that reversed near 0 mV with NaCl or KCl in the extracellular solution, but were outward with N-methyl-D-glucamine chloride or CaCl2 (110 mM), indicating that palytoxin induced a cation channel permeable to Na+ and K+ (PK/PNa = 1.2) but not to Ca2+ (PK/PCa > 30) or to N-methyl-D-glucamine (NMDG) (PK/PNMDG > 11) The unit channel conductance was 11-14 pS. 4. A high (> 0.1 mM) extracellular concentration of Ca2+ was necessary to observe channel activation by palytoxin. A high (150 mM) extracellular concentration of K+ partially prevented and reversed channel activation by palytoxin. 5. The channel activity was fully blocked by 3,4 dichlorobenzamil (20 microM) and partially blocked by phenamil (50 microM). It was not reduced by ouabain (200 microM).

Veratridine-induced intoxication: an in vitro model for the characterization of anti-ischemic compounds?

Due to the complexity of ischemia-induced cellular dysfunction many different pharmacological approaches have been tested to improve cellular ischemia resistance. However, despite the importance of [Na+]i for ischemia-induced dysfunction, only very few studies have investigated the contribution of the Na+ channel to ischemia-induced failure of intracellular ion homeostasis. Since an activation of Na+ channels by veratridine also results in a failure of intracellular ion homeostasis, the veratridine- and ischemia-induced alterations of cellular function were compared. Moreover, despite the difference in the electrophysiological changes induced by veratridine and ischemia, the possible involvement of a slowly inactivating, less selective Na+ channel in both veratridine- and ischemia-induced cellular dysfunction is discussed. As a conclusion it is suggested that veratridine intoxication could be a helpful in vitro method for the characterization of putative anti-ischemic compounds.

Veratridine activates a silent sodium channel in rat isolated aorta

To investigate the existence of silent Na+ channels, isolated rat aorta was treated with veratridine (0.1 mM) and the resulting Ca2+ uptake was determined. After 30-min incubation the total tissue uptake of Ca2+ and Ca2+ uptake increased from 2.325 +/- 0.017 to 2.614 +/- 0.080 nmol/mg wet weight (ww) and from 162.6 +/- 9.7 to 218.1 +/- 13.0 pmol/mg ww, respectively. The veratridine-induced Ca2+ uptake was blocked by tetrodotoxin (1 microM; to 17 +/- 5%) but not altered by amiloride (10 microM-1 mM). Activation of Na+/Ca2+ exchange by Na+ removal increased Ca2+ uptake from 74.2 +/- 4.5 to 97.3 +/- 5.3 pmol/mg ww, which was suppressed by amiloride (10 microM-1 mM). Nifedipine (10 nM) and verapamil (0.1 microM) at concentrations at which depolarization-induced Ca2+ uptake was diminished did not attenuate veratridine-induced Ca2+ uptake. Phenytoin at 0.1 mM reduced the Ca2+ uptake induced by veratridine or by depolarization. R 56865 (0.1 microM) and R 59494 (1 microM), novel anti-ischemic compounds inhibiting slowly inactivating Na+ channels, suppressed the veratridine-induced but not the depolarization-induced Ca2+ uptake. Guanidinium uptake was increased by veratridine (0.1 mM) from 371.2 +/- 7.2 to 574.8 +/- 45.9 pmol/mg ww. These results suggest that the rat aorta possesses a Na+ channel which is electrically silent under normal conditions but could be activated by veratridine.

[14C]guanidinium ion influx into Na+ channel preparations from mouse cerebral cortex

[14C]Guanidinium ion influx into Na+ channel preparations from mouse and rat cerebral cortex (purified synaptosomes, and synaptoneurosomes) was characterized and its properties were compared with those for 22Na+ influx. Tetrodotoxin-sensitive influx of [14C]guanidinium ion was stimulated by aconitine, veratridine, and batrachotoxin with a K0.5 of 7, 5 and 0.3 microM, respectively, the maximal influx being the same with all toxins. Scorpion venom shifted the activation curve of veratridine to the left, but did not increase the maximal influx. The potency of the local anesthetic drugs cocaine and tetracaine in inhibiting [14C]guanidinium ion influx depended upon the concentration of veratridine used to activate the Na+ channels. The mechanism of inhibition was of a competitive nature. Other local anesthetic drugs and cocaine congeners inhibited [14C]guanidinium ion influx with potencies very similar to those for inhibition of 22Na+ influx. The results show that [14C]guanidinium ion influx is a valid model for 22Na+ influx through voltage-dependent Na+ channels although there are some differences between the two influx assays. The guanidinium ion assay offers the convenience of the 14C isotope as compared with the strongly radiating 22Na+ isotope.

A model study of the contribution of active Na-K transport to membrane repolarization in cardiac cells

A biochemical model of active Na-K transport in cardiac cells was studied in conjunction with a representation of the passive membrane currents and ion concentration changes. The active transport model is based on the thermodynamic and kinetic properties of a six-step reaction scheme for the Na,K-ATPase. It has a fixed Na:K stoechiometry of 3:2, and its activation is governed by three parameters: membrane potential intracellular Na+ concentration, and interstitial K+ concentration. The Na-K pump current is directly proportional to the density of Na,K-ATPase molecules. The passive membrane currents and ion concentration changes involve only Na+ and K+ ions, and no attempt was made to provide a precise representation of Ca2+ currents or Ca2+ concentration changes. The surface-to-volume ratio of the interstitial compartment is 55 times larger than that of the intracellular compartment. The flux balance conditions are such that the original equilibrium concentration values are re-established at each stimulation cycle. The underlying assumptions of the model were checked against experimental measurements on Na-K pump activity in a variety of preparations. In addition, the qualitative validation of the model was carried out by comparing its behavior following sudden frequency shifts to corresponding experimental observations. The overall behavior of the model is quite satisfactory and it is used to provide the following indications: (1) when the intracellular and interstitial volumes are relatively large, the ion concentration transients are small and the pumping rate depends essentially on average concentration levels. (2) An increase in internal Na+ concentration potentiates the response of the Na-K pump to rapid membrane depolarizations. (3) When the internal Na+ concentration is large enough, the Na-K pump current transient plays an important role in shaping the plateau and repolarization phase of the action potential. (4) A rapid increase in external K+ concentration during voltage clamp in multicellular preparations could saturate the Na-K pump response and lead to a fairly linear dependence of the pump activity on the internal Na+ concentration.

The voltage-dependent Na+ channel of insect nervous system identified by receptor sites for tetrodotoxin, and scorpion and sea anemone toxins

Receptor sites for some of the most important toxins known to be specific for voltage-sensitive Na+ channel in the mammalian nervous system have been identified in a purified membrane preparation of house fly brain. Very high affinities have been found for the association of tetrodotoxin or tetrodotoxin derivatives with the insect Na+ channel (Kd = 0.03 - 0.08 nM). The gamma toxin from the Brazilian scorpion Tityus serrulatus forms a complex with the Na+ channel having a Kd of 6.1 pM. The Kd value for toxin II from the sea anemone Anemonia sulcata is 0.12 microM. These results show a high degree of conservation of the pharmacological properties of the brain Na+ channels between insects and mammals.

A structural and dynamic molecular model for the sodium channel of Electrophorus electricus

Chemical logic and single group rotation (SGR) theory are applied to the primary structure determined by Noda et al. [(1984) Nature 312, 121-127] to construct a molecular model of the sodium channel of Electrophorus electricus. Both structural and dynamic aspects of the channel are accounted for, including gating current, sensitivity to changes in membrane potential, channel opening, a binding site for sodium, selectivity for sodium over potassium, capacity for rapid sodium flow, sensitivity to batrachotoxin (or other toxins) and inactivation.

The molecular basis of neuronal excitability

Neurons process and transmit information in the form of electrical signals. Their electrical excitability is due to the presence of voltage-sensitive ion channels in the neuronal plasma membrane. In recent years, the voltage-sensitive sodium channel of mammalian brain has become the first of these important neuronal components to be studied at the molecular level. This article describes the distribution of sodium channels among the functional compartments of the neuron and reviews work leading to the identification, purification, and characterization of this membrane glycoprotein.

The coexistence in rat muscle cells of two distinct classes of Ca2+-dependent K+ channels with different pharmacological properties and different physiological functions

Ca2+-dependent K+ channels responsible for the long-lasting after-hyperpolarization in rat muscle cells in culture are not those extensively studied by the patch-clamp technique. The first ones are blocked by apamin, a bee venom polypeptide, and they are unaffected by tetraethylammonium (TEA) whereas the second ones are blocked by TEA and unaffected by apamin. These two Ca2+-dependent K+ channels coexist in rat muscle cells in culture but also probably in many other cellular types.

Single sodium channels from rat brain incorporated into planar lipid bilayer membranes

A voltage- and time-dependent conductance for sodium ions is responsible for the generation of impulses in most nerve and muscle cells. Changes in the sodium conductance are produced by the opening and closing of many discrete transmembrane channels. We present here the first report of electrical recordings from voltage-dependent sodium channels incorporated into planar lipid bilayers. In bilayers with many channels, batrachotoxin (BTX) induced a steady-state sodium current that was blocked by saxitoxin (STX) at nanomolar concentrations. All channels appeared in the bilayer with their STX blocking sites facing the side of vesicle addition, allowing us to define that as the extracellular side. Current fluctuations due to the opening and closing of single BTX-activated sodium channels were voltage-dependent (unit conductance, 30 pS in 0.5 M NaCl): the channels closed at large hyperpolarizing potentials. Slower fluctuations of the same amplitude, due to the blocking and unblocking of individual channels, were seen after addition of STX. Block of the sodium channels by STX was voltage-dependent, with hyperpolarizing potentials favouring block. The voltage-dependent gating, ionic selectivity and neurotoxin sensitivity suggest that these are the channels that normally underlie the sodium conductance change during the nerve impulse.

Alamethicin and related membrane channel forming polypeptides

Alamethicin and several related microbial polypeptides, which contain a high proportion of alpha-aminoisobutyric acid (Aib) residues, possess the ability to modify the permeability properties of phospholipid bilayer membranes. Alamethicin induces excitability phenomena in model membranes and has served as an excellent model for the study of voltage sensitive transmembrane channels. This review summarizes various aspects of the structural chemistry and membrane modifying properties of alamethicin and related Aib containing peptides. The presence of Aib residues in these sequences, constrains the polypeptides to 3(10) or alpha-helical conformations. Functional membrane channels are formed by aggregation of cylindrical peptide helices, which span the lipid bilayer, forming a scaffolding for an aqueous column across the membrane. After consideration of the available data on the conductance characteristics of alamethicin channels, a working hypothesis for a channel model is outlined. Channel aggregates in the lipid phase may be stabilized by intermolecular hydrogen bonding, involving a central glutamine residue and also by interactions between the macro-dipoles of proximate peptide helices. Fluctuations between different conductance states are rationalized by transitions between states of different aggregation and hence altered dimensions of the aqueous core or by changes in net dipole moment of the aggregate. Ion fluxes through the channel may also be affected by the electric field within the aqueous core.

Na+ channels with binding sites of high and low affinity for tetrodotoxin in different excitable and non-excitable cells

The properties of interaction of tetrodotoxin with its receptor site on the voltage-sensitive Na+ channel were analysed in two ways: (a) by titrating Na+ channels with a tetrodotoxin derivative, [3H]ethylenediamine-tetrodotoxin; (b) by studying the physiological properties of interaction of the toxin with its receptor from 22Na flux measurements. Using a variety of cell types in culture, three different kinds of situations were observed. 1. Cells like N1E 115 neuroblastoma, CCl 39 fibroblasts, embryonic chick cardiomyocytes and chick skeletal myotubes only have one family of Na+ channels with high-affinity binding sites (in the nanomolar range) for tetrodotoxin. These Na+ channels are the same ones as those that are activated by the alkaloid and polypeptide toxins that accelerate 22Na+ influx. 2. C9 cells have Na+ channels with low-affinity binding sites for tetrodotoxin. These Na+ channels are also the ones that are activated by alkaloid and polypeptide toxins (the median inhibitory concentration for tetrodotoxin inhibition of 22Na+ influx through these Na+ channels in 300 nM). 3. Rat myotubes that have differentiated in culture in the absence of neuronal influence have both high-affinity binding sites (in the nanomolar range) detected with the tritiated tetrodotoxin derivative and low-affinity binding sites (on the micromolar range) detected by 22Na+ flux experiments. Only low-affinity binding sites correspond to Na+ channels that can be activated with alkaloid and polypeptide toxins.

Apamin as a selective blocker of the calcium-dependent potassium channel in neuroblastoma cells: voltage-clamp and biochemical characterization of the toxin receptor

This paper describes the interaction of apamin, a bee venom neurotoxin, with the mouse neuroblastoma cell membrane. Voltage-clamp analyses have shown that apamin at low concentrations specifically blocks the Ca2+-dependent K+ channel in differentiated neuroblastoma cells. Binding experiments with highly radiolabeled toxin indicate that the dissociation constant of the apamin-receptor complex in differentiated neuroblastoma cells is 15-22 pM and the maximal binding capacity is 12 fmol/mg of protein. The receptor is destroyed by proteases, suggesting that it is a protein. The binding capacity of neuroblastoma cells for radiolabeled apamin dramatically increases during the transition from the nondifferentiated to the differentiated state. The number of Ca2+-dependent K+ channels appears to be at most 1/5th the number of fast Na+ channels in differentiated neuroblastoma. The binding of radiolabeled apamin to its receptor is antagonized by monovalent and divalent cations. Na+ inhibition of the binding of 125I-labeled apamin is of the competitive type (Kd(Na+) = 44 mM). Guanidinium and guanidinated compounds such as amiloride or neurotensin prevent binding of 125I-labeled apamin, the best antagonist being neurotensin.