Cation selectivity characteristics of the reconstituted voltage-dependent sodium channel purified from rat skeletal muscle sarcolemma

Membrane protein reconstitution for functional and structural studies

Membrane proteins are involved in various critical biological processes, and studying membrane proteins represents a major challenge in protein biochemistry. As shown by both structural and functional studies, the membrane environment plays an essential role for membrane proteins. In vitro studies are reliant on the successful reconstitution of membrane proteins. This review describes the interaction between detergents and lipids that aids the understanding of the reconstitution processes. Then the techniques of detergent removal and a few useful techniques to refine the formed proteoliposomes are reviewed. Finally the applications of reconstitution techniques to study membrane proteins involved in Ca(2+) signaling are summarized.

Voltage-gated sodium and calcium channels in nerve, muscle, and heart

Voltage-gated ion channels are membrane proteins which underlie rapid electrical signals among neurons and the spread of excitation in skeletal muscle and heart. We outline some recent advances in the study of voltage-sensitive sodium and calcium channels. Investigations are providing insight into the changes in molecular conformation associated with open-closed gating of the channels, the mechanisms by which they allow only specific ion species to pass through and carry an electric current, and the pathological consequences of small perturbations in channel structure which result from genetic mutations. Determination of three-dimensional structures, coupled with molecular manipulations by site-directed mutagenesis, and parallel electrophysiological analyses of currents through the ion channels, are providing an understanding of the roles and function of these channels at an unprecedented level of molecular detail. Crucial to these advances are studies of bacterial homologues of ion channels from man and other eukaryotes, and the use of naturally occurring peptide toxins which target different ion channel types with exquisite specificity.

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.

High-level expression and functional reconstitution of Shaker K+ channels

Voltage-gated K+ channels were expressed in COS cells transiently transfected with a plasmid carrying a cDNA for an inactivation-removed Shaker K+ channel driven by an adenovirus promoter. Channel expression was followed by immunological detection, binding of radioactive charybdotoxin (CTX), and functional reconstitution into planar lipid bilayers. About 10(7) channels per transfected cell are expressed on the plasma membrane. The expressed channels are glycosylated and competent to bind CTX with the expected characteristics. Channels observed after insertion into planar lipid bilayers displayed the voltage-dependent gating, conduction, and ion selectivity behavior expected for this channel. Channels were solubilized in several detergents without loss of CTX binding activity. The results make plausible a systematic attack on the purification of milligram-level amounts of functional K+ channels from a heterologous expression system.

Inactivation modifiers of Na+ currents and the gating of rat brain Na+ channels in planar lipid membranes

Rat brain Na+ channels whose inactivation process had been removed either by batrachotoxin (BTX) or veratridine (VT) were reconstituted into planar lipid membranes. The voltage dependence of the open probability (Po) of the channel, of the opening and closing rate constants, and the conductance and relative permeability for Na+ and K+ were studied in voltage-clamp conditions in the presence of agents known to modify the inactivation of Na+ currents. In relation to alkaloids (BTX, VT, and aconitine), it was found that once a Na+ channel was modified by BTX or VT, the addition of another alkaloid did not change further the gating and permeation properties of the channel over a period of about 1 h. Once the inactivation process of the channels is removed by BTX, the addition of a proteolytic enzyme (trypsin) or an halogenated compound (chloramine-T, CT) induced profound and specific modifications on the opening and closing events of Na+ channels: (1) the voltage dependence of the channel Po shifted to more hyperpolarized potentials; (2) this voltage shift can be explained by equal hyperpolarizing voltage shifts of the opening and closing rate constants of the channel; (3) although the gating properties of the channel were modified by these compounds, the permeation properties of the channel, as evaluated by the conductance and the selectivity to Na+ and K+ ions, were unaltered; (4) trypsin and CT were active only in the intracellular side of the channel and were irreversible within the time course of the experiments, suggesting covalent modifications of the channel. Inactivation modifiers also affected the gating of toxin-activated single Na+ channels.(ABSTRACT TRUNCATED AT 250 WORDS)

[3H]phenamil binding protein of the renal epithelium Na+ channel. Purification, affinity labeling, and functional reconstitution

This paper describes a large-scale purification procedure of the amiloride binding component of the epithelium Na+ channel. [3H]Phenamil was used as a labeled ligand to follow the purification. The first two steps are identical with those previously described [Barbry, P., Chassande, O., Vigne, P., Frelin, C., Ellory, C., Cragoe, E. J., Jr., & Lazdunski, M. (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 4836-4840]. A third step was a hydroxyapatite column. The purified material consisted of a homodimer of two 88-kDa proteins that migrated anomalously in SDS-PAGE to give an apparent Mr of 105,000. Deglycosylation by treatment with neuraminidase and endoglycosidase F or with neuraminidase and glycopeptidase F indicated that less than 5% of the mass of the native receptor was carbohydrate. Sedimentation analysis of the purified Na+ channel in H2O and D2O sucrose gradients and gel filtration experiments led to an estimated molecular weight of the [3H]phenamil receptor protein-detergent-phospholipid complex of 288,000 and of the native [3H]phenamil receptor protein of 158,000. [3H]Br-benzamil is another labeled derivative of amiloride that recognized binding sites that had the same pharmacological properties as [3H]phenamil binding sites and that copurified with them. Upon irradiation of kidney membranes, [3H]Br-benzamil incorporated specifically into a 185-kDa polypeptide chain under nonreducing electrophoretic conditions and a 105-kDa protein under reducing conditions. The same labeling pattern was observed at the different steps of the purification. Reconstitution of the purified phenamil receptor into large unilamellar vesicles was carried out. A low but significant phenamil- and amiloride-sensitive electrogenic Na+ transport was observed.

The inactivation of sodium channels in the node of Ranvier and its chemical modification

The many experimental studies reported demonstrate the complexity of what is termed inactivation, the decrease of current flow through sodium channels at maintained depolarization. Even at the normal resting potential of, say, -70 mV for a frog node of Ranvier, ca. 20% of the channels are closed and inactivated, i.e., incapable of passing current on a sudden depolarization, in contrast to the remaining 80% of closed but resting channels. The term inactivation has thus evolved from bulk current ("macroscopic") phenomena and is applied to channels although its single-channel ("microscopic") basis is not entirely clear and may even vary among preparations. It is conceivable that the macroscopic phenomenon may have more than a single microscopic cause; this point will probably not be settled until a physical description of the conformational states of the channel macromolecule becomes available. At any rate, channel transition into an inactivated closed state can be easily affected by numerous reagents of highly diverse chemical nature and, most likely, different primary sites of action as already suggested by the sidedness of effective application, e.g., iodate and endopeptidases to the inside, polypeptide toxins to the outside. But also the search for a common denominator, a secondary target of all these treatments, has not been very successful as demonstrated by the experiments with group-specific reagents. Since modification of inactivation is often accompanied by shifts in the voltage dependence of gating parameters, a target could be the "voltage sensor" of the channel, charged and/or dipolar components of the channel macromolecule that, by being moved in the electric field, somehow induce gating and whose movement is measured as gating current (e.g, Hille, 1984). The fraction of open channels as a function of membrane potential, F(E), may serve as an indicator. It may be simply shifted (to more negative potentials) as by veratridine (Leibowitz et al., 1987) or flattened (reduction of gating charge?) and shifted (in the positive direction) as by Anemonia sulcata toxin II (Ulbricht and Schmidtmayer, 1981) or chloramine-T (Drews, 1987). On the other hand, the steady-state inactivation curve is shifted to more negative potentials by the toxin (Ulbricht and Schmidtmayer, 1981), but to more positive potentials by chloramine-T (Wang, 1984a; Schmidtmayer, 1985). Obviously, modifiers may affect activation and inactivation quite differently, a result that touches on the question as to what extent inactivation derives its potential dependence from activation.(ABSTRACT TRUNCATED AT 400 WORDS)

Molecular aspects of human brain sodium channel

The sodium channel content of human brain was measured by tritiated tetrodotoxin specific binding. After solubilization, the sodium channel was submitted to chromatography on diethylaminoethyl(cellulose) Sephadex, hydroxylapatite and wheat germ agglutinin sepharose. An increase of tritiated tetrodotoxin binding specific activity was subsequently observed. Eluted sodium channels from wheat germ agglutinin sepharose were overlaid on a sucrose gradient. Electrophoretical analysis of the material obtained after the sedimentation step revealed two co-purified peptides, alpha (Mr = 275,000 mol. wt) and beta (Mr = 30,000-36,000 mol. wt.). Alpha showed an exceptionally high free electrophoretic mobility, which is a common feature for all sodium channels previously described. However, the high denaturation rate of the solubilized tetrodotoxin receptor site 1 did not allow tetrodotoxin receptor quantification by the tritiated toxin binding in sucrose fractions. Sodium channel effective reconstitution in liposomes was demonstrated: (1) 22Na+ influx in proteoliposomes was sensitive to sodium channel-specific neurotoxins: (2) reconstituted proteins showed a cation selectivity similar to that previously described for animal sodium channels. The sodium channel preparation obtained after four chromatographic steps shows two peptides on the electrophoretic analysis. Reconstituted sodium channels displayed some physiological properties found in intact conducting membranes.

Solubilization and reconstitution of voltage-dependent calcium channel from bovine cardiac muscle. Ca2+ influx assay using the fluorescent dye Quin2

Highly purified sarcolemmal membranes, prepared from fresh bovine heart left ventricle, were solubilized by n-octyl beta-D-glucopyranoside and reconstituted into proteoliposomes with soybean phospholipids by the detergent-dialysis method. Ca2+ flux into the proteoliposomes was determined using the fluorescent probe Quin2. A membrane potential (negative in the proteoliposome interior) that was created by K+ diffusion mediated by valinomycin accelerated the Ca2+ influx. The voltage-dependent Ca2+ influx was dependent on pretreatment of the sarcolemmal membranes with Bay K 8644 and was inhibited by various calcium antagonists including nicardipine (K0.5 = 4.5.10(-7) M), verapamil (K0.5 = 9.2.10(-9) M), diltiazem (K0.5 = 26.10(-8) M) and omega-conotoxin (K0.5 = 9.5.10(-9) M).

Structure and function of voltage-sensitive ion channels

Voltage-sensitive ion channels mediate action potentials in electrically excitable cells and play important roles in signal transduction in other cell types. In the past several years, their protein components have been identified, isolated, and restored to functional form in the purified state. Na+ and Ca2+ channels consist of a principal transmembrane subunit, which forms the ion-conducting pore and is expressed with a variable number of associated subunits in different cell types. The principal subunits of voltage-sensitive Na+, Ca2+, and K+ channels are homologous members of a gene family. Models relating the primary structures of these principal subunits to their functional properties have been proposed, and experimental results have begun to define a functional map of these proteins. Coordinated application of biochemical, biophysical, and molecular genetic methods should lead to a clear understanding of the molecular basis of electrical excitability.

Anemone toxin II receptor site of the lobster nerve sodium channel. Studies in membrane vesicles and in proteoliposomes

The receptor-site for the sea anemone toxin II from Anemonia sulcata (ATX) and its functional relationship with the Na+ channel were studied in plasma membrane preparations from lobster walking leg nerves. The modification of the 22Na influx by ATX was determined in membrane vesicles and in proteoliposomes prepared by reconstitution of detergent-extracted, unfractionated membrane particles into soybean liposomes. The effects of two other toxins, veratridine (VER) and tetrodotoxin (TTX), which bind to Na+ channel receptor-sites other than that for polypeptide toxins, were also studied, ATX and VER stimulated 22Na flux into membrane vesicles with K0.5 values in the order of 10(-7) and 10(-5) M, respectively. Positive cooperativity among these toxins was also seen; ATX displaces the K0.5 for VER towards lower VER concentrations. TTX abolishes the 22Na influx increment caused by ATX and/or VER with a K0.5 in the order of 10(-8) M. In proteoliposomes, in contrast, ATX modified the 22Na influx only at high concentrations (greater than 1 microM) and in the presence of VER. VER stimulation and TTX inhibition of the VER and the VER plus ATX modified fluxes, had the same characteristics as in the vesicle preparations. Measurable ATX and VER toxin effects were only seen in the presence of an outwardly directed K+ gradient for both vesicles and proteoliposomes. Detergent treatment and the reconstitution procedure seem to affect the functional properties of the ATX receptor site whereas the VER and the TTX sites remain unaltered.

Spontaneous opening at zero membrane potential of sodium channels from eel electroplax reconstituted into lipid vesicles

The voltage-dependent sodium channel from the eel electroplax was purified and reconstituted into vesicles of varying lipid composition. Isotopic sodium uptake experiments were conducted with vesicles at zero membrane potential, using veratridine to activate channels and tetrodotoxin to block them. Under these conditions, channel-dependent uptake of isotopic sodium by the vesicles was observed, demonstrating that a certain fraction of the reconstituted protein was capable of mediating ion fluxes. In addition, vesicles untreated with veratridine showed significant background uptake of sodium; a considerable proportion of this flux was blocked by tetrodotoxin. Thus these measurements showed that a significant subpopulation of channels was present that could mediate ionic fluxes in the absence of activating toxins. The proportion of channels exhibiting this behavior was dependent on the lipid composition of the vesicles and the temperature at which the uptake was measured; furthermore, the effect of temperature was reversible. However, the phenomenon was not affected by the degree of purification of the protein used for reconstitution, and channels in resealed electroplax membrane fragments or reconstituted solely into native eel lipids did not show this behavior. The kinetics of vesicular uptake through these spontaneously-opening channels was slow, and we attribute this behavior to a modification of sodium channel inactivation.

Neurotoxin-modulated uptake of sodium by highly purified preparations of the electroplax tetrodotoxin-binding glycopeptide reconstituted into lipid vesicles

Using the dialysable detergent CHAPS (3-[(3-cholamidopropyl)-dimethylammonio]-1-propane sulfonate), the tetrodotoxin-binding protein from the electroplax of the electric eel has been purified to a high degree of both chemical homogeneity and toxin-binding activity. On sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the best preparations showed only a single microheterogeneous band at Mr approximately 260,000, despite attempts to visualize smaller bands by sample overloading. Upon dialysis, this material became incorporated into the membranes of small unilamellar vesicles, and in this form the purified protein exhibited tetrodotoxin-binding properties similar to the component in the original electroplax membrane. Furthermore, in the presence of activator neurotoxins the vesicles were able to accumulate isotopic sodium in a manner similar to that previously described for less active or less pure preparations of vesicles containing either mammalian or eel electroplax toxin-binding proteins. Quantitative consideration of the isotopic transport activity of this pure material, along with the high degree of purity of the protein, strongly suggests that the 260-kDa glycopeptide from electroplax is necessary and sufficient to account for the sodium channel function seen in these studies, and eliminates the possible involvement of smaller peptides in the channel phenomena observed.

Effects of ethanol on the functional properties of sodium channels in brain synaptosomes

Voltage-sensitive sodium channels in excitable cell membranes are responsible for the rapid increase in permeability to sodium ions that occurs during depolarization. Neurotoxins that bind with high affinity and specificity to voltage-sensitive sodium channels have been widely used to identify and characterize the structure and function of sodium channels in nerve and skeletal muscle. This chapter describes the actions of ethanol on the functional properties of voltage-sensitive sodium channels in mammalian brain nerve endings. The effects of acute and chronic ethanol administration are also reviewed. Alterations in the function of neuronal membrane sodium channels may be involved in the depressant effect of ethanol.

Biochemistry of sodium channels from mammalian muscle

Sodium channels from rat and rabbit skeletal muscle have been isolated, characterized, and functionally reconstituted. These channel proteins closely resemble those purified from the eel electroplax and from rat brain. Work now in progress on the determination of the primary sequence of the 260 kDa subunit through the cloning and sequencing of the muscle message should shed further light on the relationship between these unique voltage-sensitive sodium channels.

Expression of functional sodium channels from cloned cDNA

The voltage-gated sodium channel is a transmembrane protein essential for the generation of action potentials in excitable cells. It has been reported that sodium channels purified from the electric organ of the electric eel, Electrophorus electricus, and from chick cardiac muscle consist of a single polypeptide of relative molecular mass (Mr) approximately 260,000, whereas those purified from rat brain and from rat and rabbit skeletal muscle contain, in addition to the large polypeptide, one or two smaller polypeptides of Mr 33,000-43,000. The primary structures of the Electrophorus sodium channel and two distinct sodium channel large polypeptides (designated as sodium channels I and II) from rat brain have been elucidated by cloning and sequencing the complementary DNAs. The purified sodium channel preparations from Electrophorus electroplax and from mammalian muscle and brain, when reconstituted into lipid vesicles or planar lipid bilayers, exhibit some functional activities. The successful reconstitution with the Electrophorus preparation would imply that the large polypeptide alone is sufficient to form functional sodium channels. However, studies with the rat brain preparation suggest that the smaller polypeptide of Mr 36,000 is also required for the integrity of the saxitoxin (STX) or tetrodotoxin (TTX) binding site of the sodium channel. Here we report that the messenger RNAs generated by transcription of the cloned cDNAs encoding the rat brain sodium channel large polypeptides, when injected into Xenopus oocytes, can direct the formation of functional sodium channels.

Ionizing radiation alters the properties of sodium channels in rat brain synaptosomes

The effect of ionizing radiation on neuronal membrane function was assessed by measurement of neurotoxin-stimulated 22Na+ uptake by rat brain synaptosomes. High-energy electrons and gamma photons were equally effective in reducing the maximal uptake of 22Na+ with no significant change in the affinity of veratridine for its binding site in the channel. Ionizing radiation reduced the veratridine-stimulated uptake at the earliest times measured (3 and 5 s), when the rate of uptake was greatest. Batrachotoxin-stimulated 22Na+ uptake was less sensitive to inhibition by radiation. The binding of [3H]saxitoxin to its receptor in the sodium channel was unaffected by exposure to ionizing radiation. The effect of ionizing radiation on the lipid order of rat brain synaptic plasma membranes was measured by the fluorescence polarization of the molecular probes 1,6-diphenyl-1,3,5-hexatriene and 1-[4-(trimethylammonium)phenyl]-6-phenyl-1,3,5-hexatriene. A dose of radiation that reduced the veratridine-stimulated uptake of 22Na+ had no effect on the fluorescence polarization of either probe. These results demonstrate an inhibitory effect of ionizing radiation on the voltage-sensitive sodium channels in rat brain synaptosomes. This effect of radiation is not dependent on changes in the order of membrane lipids.

Components of purified sarcolemma from porcine skeletal muscle

Sarcolemmal membranes were isolated from porcine skeletal muscle by modifications of a LiBr-extraction technique. Latency determinations of acetylcholinesterase, ouabain-sensitive p-nitrophenylphosphatase, [3H]ouabain binding, and (Na+ + K+)-ATPase activities indicated that 65-76% of the membranes were sealed inside-out vesicles. The preparations were enriched in cholesterol and phospholipid, and demonstrated adenylate cyclase activity and both cAMP and cGMP phosphodiesterase activities. An indication of the purity of this fraction was that the Ca2+-ATPase activity (0.13 mumol Pi mg-1 min-1 at 37 degrees C) was 3.8% of that of porcine skeletal muscle sarcoplasmic reticulum preparations. Pertussis toxin specifically catalyzed the ADP-ribosylation of a Mr 41,000 sarcolemmal protein, indicating the presence of the inhibitory guanine nucleotide regulatory protein of adenylate cyclase, Ni. An endogenous ADP-ribosyltransferase activity, with several membrane protein substrates, was also demonstrated. The addition of exogenous cAMP-dependent protein kinase or calmodulin promoted the phosphorylation of a number of sarcolemmal proteins. The calmodulin-dependent phosphorylation exhibited an approximate K 1/2 for Ca2+ of 0.5 microM, and an approximate K 1/2 for calmodulin of 0.1 microM. 125I-Calmodulin affinity labeling of the sarcolemma, using dithiobis(succinimidyl propionate), demonstrated the presence of Mr 160,000 and 280,000 calmodulin-binding components in these membranes. These results demonstrate that this porcine preparation will be valuable in the study of skeletal muscle sarcolemmal ion transport, protein and hormonal receptors, and protein kinase-catalyzed phosphorylation.

New studies of the excitatory sodium currents in heart muscle

After decades of frustration with inadequate methods, cardiac electrophysiologists have developed new techniques for superior control of membrane potential by use of single cells, and they have begun careful study of cardiac Na+ currents. Direct recordings of the behavior of single Na+ channels have been made by the newly developed patch clamp technique. Biochemists have made excellent progress purifying and characterizing the Na+ channel proteins, and there has been some initial success in reconstituting these partially purified channels into lipid bilayers, where their function can be studied. Even at this early stage of development of these new techniques, several conclusions are warranted: The cardiac Na+ currents are not accurately described by the original Hodgkin-Huxley mathematical formulation, making undesirable the further use of this model for study of cardiac excitation and conduction. We need to keep an open mind as to the kinetic behavior of Na+ channels, until the newer experimental techniques provide a more complete picture. Although the cardiac Na+ channel strongly resembles Na+ channels in other excitable tissues, important differences remain, reinforcing the idea that the detailed molecular structure of the cardiac Na+ channel will be different from its close relatives in other excitable cells. The density of Na+ channels in heart cell membranes is much less than in nerve and fast twitch skeletal muscle. The Na+ channels are the focus of action of many drugs and pathological processes. The tools are at hand for a complete description of the Na+ channel, including its gating and its molecular structure. We can expect considerable progress in this decade.

Sodium channels in vertebrate hearts. Three types of saxitoxin binding sites in heart

The affinity of saxitoxin binding to cardiac sarcolemmal and cytosolic fractions was examined across species. In amphibia (frog) the plasma membrane site demonstrated a high affinity (Kd approx. 5 X 10(-9) M) but the majority of the total sites in the homogenate appeared to be high affinity soluble sites (Kd approx. 2 X 10(-9) M). Chicken and turtle cardiac plasma membrane fractions bound [3H]saxitoxin with 500-fold less affinity (Kd values of approx. 2 X 10(-6) M). No binding was seen in the cytosol. The affinity of cardiac sarcolemmal binding in amphibians correlates quantitatively with the K0.5 for the inhibition of sodium currents. Physiological correlation of the low affinity saxitoxin sites in chicken and turtle with toxin concentrations necessary to inhibit the sodium current remains unclear. The hypothesis that frog cytosolic saxitoxin binding sites originated from sarcolemma during homogenization is examined. The presence of three types of saxitoxin binding sites in cardiac preparations supports the existence of sodium channel subtypes.

An apparatus for rapid kinetic analysis of isotopic efflux from membrane vesicles and of ligand dissociation from membrane proteins

A new method for the measurement of rapid isotopic release from a membrane compartment is described. Membrane vesicles loaded with isotope, or broken membranes with bound radioactive ligand, are filtered onto the surface of a cellulose ester filter; the rate of the loss of isotope from the membrane compartment is followed continuously by collecting fluid which is passed through the filters under high pressure. A change in release rate is initiated by changing the solution or by delivering a flash of light to a photosensitive sample. The approach has been used to study rapid 22Na efflux from membrane vesicles rich in the ouabain-sensitive Na pump, and to examine dissociation of 32P and 86Rb from membrane-bound Na,K-ATPase. Since the rate of efflux is measured, and not the total counts remaining on the filter, the technique has high sensitivity. A complete time course is obtained using only a few micrograms of membrane protein. The apparatus described is simple, inexpensive, and easily constructed; with the present device, time resolution is approximately 10 ms.

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.