Identification of a tetrodotoxin-sensitive Na+ channel in a variety in fibroblast lines

Type II brain sodium channel expression in non-neuronal cells: embryonic rat osteoblasts

Although voltage-sensitive sodium channels play a central role in electrogenesis in neurons, rat brain sodium channels are also present in some glial cells. To determine whether rat brain sodium channel alpha-subunit isotypes are expressed in other cell types, we examined osteoblasts within the embryonic day 17 (E17) vertebral column with in situ hybridization and immunocytochemical methods. For in situ hybridization studies, riboprobes hybridizing to isoform-specific sequences in the 3'-noncoding region of sodium channel mRNAs (NCI, NCII and NCIII) were utilized. Sodium channel mRNA I and III were not detectable in osteoblasts of the vertebra centrum or neural arches in E17 rats. In contrast, sodium channel mRNA II was moderately expressed by osteoblasts in the developing vertebral column of E17 rats. In immunocytochemical experiments, antipeptide antibodies directed against conserved and isotype-specific regions of the sodium channel alpha-subunit were used. Antibody SP20, which recognizes a conserved region of the sodium channel, intensely stains osteoblasts in both the vertebra centrum and neural arches. Antibody SP11-I, which recognizes sodium channel I, exhibited negligible-to-low levels of immunostaining in vertebral column osteoblasts. Osteoblasts reacted with antibody SP11-II, which recognizes sodium channel II, displayed moderate levels of immunostaining. Antibody SP32-III, which recognizes sodium channel III, displayed negligible levels of staining in osteoblasts within vertebra centrum and neural arches. These results demonstrate that osteoblasts in situ within E17 vertebral columns express sodium channel II mRNA and protein. Together with previous electrophysiological observations, the present results suggest that functional sodium channels are expressed in osteoblasts in vivo. These results extend the range of non-neuronal cells known to express rat brain sodium channels.

Characteristics of Na+ current in Schwann cells cultured from frog sciatic nerve

Characteristics of voltage-dependent currents in cultured frog Schwann cells were investigated by the whole-cell clamp technique. An inward current was detectable at a membrane potential level more positive than -50 mV and reached a maximum value at about -10 mV, while no rectifying channel was present. The inward current was carried by Na+ ions, because the extrapolated reversal potential of the current agreed with the calculated ENa, and the current was sensitive to tetrodotoxin. The membrane potential for half-maximal inactivation was -82 mV. The inactivation curve indicated that more than 90% of the Na+ channels were inactivated at the resting membrane potential, suggesting that the cultured frog Schwann cells could not generate an action potential under physiological conditions. The time constant for the inactivation at a maximum current was 5.3 ms (-10 mV, 13 degrees C). The electrophysiological characteristics of the Na+ current in the cultured frog Schwann cells were compared with those in other tissues. This Na+ current was quantitatively different from that observed in the amphibian node of Ranvier but was similar to that in the mammalian Schwann or glial cells, especially in the more hyperpolarized half-maximal inactivation potential and in the slower inactivation time course.

Role of the Na+/K+/Cl- transporter in the positive inotropic effect of ouabain in cardiac myocytes

In this study we have characterized the bumetanide-sensitive K+/Na+/Cl- cotransport in cultured rat cardiac myocytes. 1) It carries about 10% of the total K+ influx. 2) It is sensitive to furosemide (Ki0.5 = 10(-6)M) and bumetanide (Ki0.5 = 10(-7)M). 3) It is strongly dependent on the extracellular concentrations of Na+ and Cl-. 4) It carries out influx of both ions, K+ and Na+. A therapeutic concentration of ouabain (10(-7) M) stimulated the bumetanide-sensitive K+ influx (as measured by 86Rb+), in the cultured myocytes, with no effect on the bumetanide-resistant K+ influx, which was mediated mostly by the Na+/K+ pump. Stimulation of the bumetanide-sensitive Rb+ influx by a low ouabain concentration was strongly dependent on Na+ and Cl- in the extracellular medium. A low concentration of ouabain (10(-7) M) was found to increase the steady-state level of cytosolic Na+ by 15%. This increase was abolished by the addition of bumetanide or furosemide. These findings suggest that ouabain, at a low (10(-7) M) concentration, induced its positive inotropic effect in rat cardiac myocytes by increasing Na+ influx into the cells through the bumetanide-sensitive Na+/K+/Cl- cotransporter. In order to examine this hypothesis, we measured the effect of bumetanide on the increased amplitude of systolic cell motion induced by ouabain. Bumetanide or furosemide, added to cultured cardiac myocytes, inhibited the increased amplitude of systolic cell motion induced by ouabain. Neither bumetanide nor furosemide alone has any significant effect on the basal amplitude of systolic cell motion. We propose that stimulation of bumetanide-sensitive Na+ influx plays an essential role in the positive inotropic effect in rat cardiac myocytes induced by low concentration of ouabain.

Differential expression of sodium channel mRNAs in rat peripheral nervous system and innervated tissues

RNA blot hybridization analyses using probes specific for sodium channels I, II and III revealed high levels of sodium channel I mRNA and low levels of sodium channel II and III mRNAs in peripheral nervous system (PNS) tissues. The developmental expression patterns of these mRNAs were generally similar to those described for the central nervous system. The small amounts of sodium channel I and III mRNAs present in tongue muscle were greatly reduced after partial denervation. Expression of the three sodium channels thus appears to be restricted to the nervous system. Putative novel additional mRNAs, specifically expressed in the PNS, were detected with a probe that recognizes nucleotide sequences common to sodium channels I, II and III.

Veratridine stimulation of sodium influx in carotid body cells from newborn rabbits in primary culture

It was examined whether or not 22Na+ influx into cultured cells of the carotid body (CB) from newborn rabbits might be stimulated by veratridine (VRT), using superior cervical ganglion (SCG) cells as a standard, showing the VRT-stimulating effects on 22Na+ influx. In a CB glomus cell-rich culture, VRT induced a 22Na+ influx increase, as seen in a SCG neuronal cell-rich culture, which was entirely inhibited by tetrodotoxin (TTX). In contrast, in a CB non-glomus cell culture as well as in a SCG non-neuronal cell culture, the VRT-stimulating effect was not seen. This indicates that the VRT-stimulating effect found in the CB glomus cell-rich culture was evoked from only glomus cells. It is concluded that glomus cells have TTX-sensitive voltage-dependent sodium channels, which might be indirectly involved in the chemotransduction mechanism in the CB.

The sodium channels of the neuroblastoma x glioma 108 CC 15 hybrid cell change their sensitivity for volatile and local anesthetics upon continuous passage

We have studied the ion flux through the sodium channels of low passage number (less than 50 p.) and high passage number (greater than 150 p.) neuroblastoma x glioma hybrid cells using [14C] guanidinium and specific neurotoxins to induce channel opening and closing. The sodium channels of low passage number hybrid cells could be opened by veratridine alone, suggesting the presence of voltage dependent channels in agreement with electrophysiological studies reported in the literature. The sodium channels of the high passage number hybrid cells, however, needed the synergistic action of veratridine and scorpion toxin for activation suggesting that these channels are "silent". The [14C] guanidinium ion flux through the sodium channels of the high passage number hybrid cells was inhibited by significantly lower concentrations of the volatile anesthetics (halothane, isoflurane and enflurane) and the local anesthetics (tetracaine and bupivacaine) than the comparable flux through the sodium channels of the low passage number hybrid cells.

Load responsiveness of protein synthesis in adult mammalian myocardium: role of cardiac deformation linked to sodium influx

Exposure of adult mammalian myocardium to increased hemodynamic loads augments cardiac protein synthesis, ultimately leading to hypertrophy of the affected chamber. This established relationship between loading conditions and protein synthesis was examined in terms of two questions. First, is there a basic difference between the anabolic effect of a passive load imposed on diastolic myocardium and that of an active load generated by systolic myocardium? This issue was addressed by measuring [3H]phenylalanine incorporation into muscle protein in either quiescent or contracting ferret papillary muscles, set at known isometric lengths. Myocardial protein synthesis increased in proportion to total muscle tension in each case, with an equivalent relation describing both quiescent and contracting muscles. Synthesis of two contractile proteins, actin and myosin heavy chain, were enhanced by muscle loading. Thus, a quantitative rather than qualitative difference between the anabolic effects of diastolic and systolic loading was demonstrated. Second, since increased sodium influx is an initial cellular response requisite to the growth-inducing activity of many substances, and since sodium entry through stretch-activated ion channels is stimulated by deformation of the sarcolemma, does cardiac deformation during increased loading promote sodium influx as a signal to increase anabolic activity? In either quiescent or contracting papillary muscles, the rate of 24Na+ uptake was found to increase with load. Streptomycin, a cationic blocker of the mechanotransducer ion channels, was without effect on protein synthesis in stimulated but slack muscles; however, it inhibited, in a dose-related manner, the augmented protein synthesis otherwise observed in contracting muscles developing tension. At 500 microM, streptomycin did not reduce active tension, but it did reduce the synthesis of both actin and myosin heavy chain. In a second pharmacologic approach, inotropic agents were chosen which uniformly increased muscle tension development but which had contrasting effects on sodium influx. Protein synthesis increased in the presence of Na+ influx enhancers, monensin or veratridine; however, protein synthesis decreased in the presence of amiloride, a sodium influx inhibitor. Thus, myocardial protein synthesis varied directly with sodium influx despite the positive inotropic effect observed with each of these agents. In addition, inhibition of protein synthesis by ouabain demonstrated that activation of the Na+ pump is required for the anabolic effect of load.(ABSTRACT TRUNCATED AT 400 WORDS)

Lithium transport in human fibroblasts: relationship to RBC lithium transport and psychiatric diagnoses

Cultured fibroblasts were prepared from six normal controls, five DSM-III manic patients, and six DSM-III schizophrenic patients. Lithium (Li+) uptake, 24-hour Li+ ratios, and steady-state membrane potential were measured in these cell lines. The uptake of 10 mM Li+ reached maximum at 2 hours, with an intracellular concentration of approximately 15 mM. No significant difference in uptake was found among subject groups. Twenty-four hour Li+ (ratio of intracellular/extracellular Li+) ratios were determined by incubating the cell lines for 24 hours in the presence of 2 mM Li+. No significant difference was observed among groups; nor was there any significant correlation between the fibroblast 24-hour ratios and 24-hour in vitro ratios determined in donor red cells. The relationship between membrane potential and the 24 hour Li+ ratio in fibroblasts was determined. The average potential in these cell lines was -56 mV and was not affected by Li+ treatment. No correlation between the Li+ ratio and membrane potential was found.

[3H]-tetrodotoxin binding in neuronal and non-neuronal spinal cord cultures

The binding of [3H]-tetrodotoxin [TTX], a sodium channel blocker, was studied in dissociated spinal cord cultures derived from fetal mice. A comparison was made between cultures that consisted of a mixture of neurons and non-neuronal background cells (N + BG) with those that were comprised of only background cells (BG). Specific binding of 1 nM [3H]-TTX was studied in 28-day-old cultures. The IC50 for TTX displacement of [3H]-TTX binding was 10 nM for (N + BG) cultures and 15 nM for (BG). The binding of [3H]-TTX to (N + BG) cultures was approximately 9-fold greater than that observed for the (BG) cultures. During development from day 6 to day 28, the binding of [3H]-TTX in (N + BG) cultures increased about 10-fold per dish or about 30% as expressed as fmol bound per mg protein. Nitrendipine did not displace [3H]-TTX in day 6 (N + BG) cultures, although previous studies indicated that TTX displaced [3H]-nitrendipine binding in developing spinal cord cultures.

Perturbation of Na+ and K+ gradients in human fibroblasts incubated in unsupplemented saline solutions

Changes in the intracellular concentrations of Na+ and K+ of fetal human fibroblasts have been followed after replacement of serum-containing growth media with unsupplemented and serum-supplemented saline solution (Earle's balanced salt solution). Incubation in unsupplemented salt solution was followed by a progressive increase of the internal Na+ counterbalanced by a decrease of internal K+, without major alterations of the internal osmolarity. After 3 h incubation the intracellular Na+ and K+ concentrations were 120 mM and 50 mM, respectively. These intracellular ion derangements were not associated with a failure of the (Na+ + K+)-ATPase pump, whose activity actually increased with enhanced intracellular Na+ concentration. Ion changes did not take place when serum (in excess of 0.5%, final concentration) was present in the saline solution and a complete restoration to normal of the Na+ and K+ gradients occurred upon addition of serum to cells previously incubated in plain saline solution. The effects of serum were mimicked by furosemide, thus suggesting that channels sensitive to this diuretic are involved in the movement of Na+ and K+ following fibroblast incubation in unsupplemented saline solution.

The presence of Na+ channels in myometrial smooth muscle cells is revealed by specific neurotoxins

This paper shows the presence, in rat myometrial smooth muscles, of low affinity binding sites for tetrodotoxin with a K0.5 value of 2 microM. Electrophysiological experiments using both intact strips and single isolated myometrial cells in culture have shown that veratridine and sea anemone toxins reveal functional Na+ channels. The activity of these channels was blocked by tetrodotoxin (10 microM) or by removal of Na+ ions. Results presented here are the first direct demonstration of the existence in rat myometrium of Na+ channels of the tetrodotoxin-resistant type.

Tetrodotoxin-sensitive sodium channels in a continuously cultured cell line derived from the adult rat cerebellum

A method to establish continuously cultured cell lines from adult cerebellar cortex is reported. Clones (prepared by this procedure) were isolated from cerebellar established cultures at the 25th passage and after 15 months in vitro. One clone (UCHCC1) was maintained in culture and studied while the others were frozen. The cerebellar cell line UCHCC1 retained a neuronal-like morphology; the addition of dimethylsulfoxide (DMSO) to the culture medium elicited a reproducible morphological 'differentiation' event, characterized mainly by process extension. In 'differentiated' cells, veratridine significantly increased the uptake of 22Na. Such enhanced uptake was blocked by tetrodotoxin (TTX) with a half-maximal inhibitory concentration of 0.9 nM. Binding of an [3H]ethylenediamine derivative of TTX ([3H]en-TTX) to the microsomal fraction prepared from same DMSO-treated cells, showed a single class of receptors with a maximal binding (Bmax) of 173 fmol/mg protein and a Kd of 1.1 nM. Thyroid UCHT1 cells and 'undifferentiated' (cultured without DMSO) cerebellar cells, did not show significant effects of veratridine on 22Na-uptake, or [3H]en-TTX binding. The 'differentiated' nerve-like properties, induced by appropriate environmental manipulation, demonstrate the usefulness of cerebellar UCHCC1 cells as a model system for the developing central neuron. On the other hand, the novel transforming procedure opens new possibilities for obtaining permanent cell lines from other regions of the adult CNS.

The presence of voltage-gated sodium, potassium and chloride channels in rat cultured astrocytes

Patch-clamp recording from the plasmalemma of rat cultured astrocytes reveals the presence of both voltage-dependent sodium and voltage-dependent potassium conductances. These conductances are similar but not identical to the corresponding conductances in the axolemma. Whereas the h infinity relation of the sodium channels has the same voltage dependence as in the nodal axolemma, the peak current-voltage relation is shifted by about 30 mV along the voltage axis in the depolarizing direction. It is speculated that the glial cells synthesize sodium and potassium channels for later insertion into the axolemma of neighbouring axons. The astrocytes also express a plasmalemmal voltage-dependent anion conductance that is turned on at about -40 mV (that is, near the resting potential of the cultured astrocytes). The channels involved are large enough to be just permeable to glutamate but not to ascorbate. It is suggested that the conductance of this channel for chloride plays a physiological role in the spatial buffering of potassium by glial cells.

Organization of ion channels in the myelinated nerve fiber

The functional organization of the mammalian myelinated nerve fiber is complex and elegant. In contrast to nonmyelinated axons, whose membranes have a relatively uniform structure, the mammalian myelinated axon exhibits a high degree of regional specialization that extends to the location of voltage-dependent ion channels within the axon membrane. Sodium and potassium channels are segregated into complementary membrane domains, with a distribution reflecting that of the overlying Schwann or glial cells. This complexity of organization has important implications for physiology and pathophysiology, particularly with respect to the development of myelinated fibers.

Coupling of a loop diuretic-sensitive Na+ influx with the net loop diuretic-sensitive K+ efflux in mouse NIH 3T3 cells

Mouse 3T3 fibroblasts have a loop diuretic sensitive Na+ transport system, responsible for more than 50% of the total Na+ influx. This transport system is dependent on the simultaneous presence of all three ions; Na+, K+, (Rb+) and Cl- in the extracellular medium. The same requirement for these three ions was also found for the loop diuretic-sensitive K+ efflux. In addition, the sensitivities of Na+ influx and Rb+ efflux for the two loop diuretics, furosemide and bumetanide were found to be similar. The similar ionic requirement and sensitivity towards loop diuretics of the two fluxes, support the hypothesis, that this loop diuretic-sensitive Na+ influx in mouse 3T3 cells, is accompanied by the net loop diuretic-sensitive K+ efflux.

Clonal sublines of rat neurotumor RT4 and cell differentiation. V. Comparison of Na+ influx, Rb+ efflux, and action potential among stem-cell, neuronal, and glial cell types

A multipotential stem-cell-type cell line (RT4-AC) isolated from a rat peripheral neurotumor differentiates in culture into two neuronal-type cells (RT4-B and RT4-E) or into a glial-type cell (RT4-D). The neuronal classification of RT4-B and RT4-E cells is based on their positive response to veratridine in the tetrodotoxin-sensitive Na+-influx and Rb+-efflux assays and on the action potential observed upon hyperpolarized stimulation. In addition, these neuronal cell types do not synthesize two glial proteins, S100 protein (S100P) and glial fibrillary acidic protein (GFAP). The glial classification of RT4-D is based on the syntheses of S100P and GFAP. Additionally, RT4-D does not display veratridine-activated Na+ influx and Rb+ efflux nor action potential. The stem cell type, RT4-AC, expresses both neuronal and glial properties to a lesser degree. In the neuronal-type cell lines of the RT4 family (RT4-B and RT4-E), the large veratridine-activated Na+ influx can further be stimulated by scorpion toxin. The Na+ influx of the stem cell (RT4-AC), however, is only slightly stimulated by veratridine alone, but greatly stimulated by the addition of veratridine and scorpion toxin. These observations suggest that a progressive differentiation of voltage-dependent Na+ channels may have occurred by the cell-type conversion from the stem cell type to the neuronal cell types. The exact nature of the change in Na+ channels is currently not known.

Halothane inhibits the neurotoxin stimulated [14C]guanidinium influx through 'silent' sodium channels in rat glioma C6 cells

We have investigated the effect of pharmacological agents on [14C]guanidinium ion influx through sodium channels in C6 rat glioma and N18 mouse neuroblastoma cells. The sodium channels of the N18 cells can be activated by aconitine alone, indicating that they are voltage-dependent channels. In contrast, sodium channels in the C6 cells require the synergistic action of aconitine and scorpion toxin for activation and are therefore characterized as so-called silent channels. The general anesthetic halothane used at clinical concentrations, specifically inhibited the ion flux through the silent sodium channel of C6 rat glioma cells. The voltage-dependent channels of the N18 cells were insensitive to halothane at the concentrations tested.

Voltage-dependent sodium and potassium channels in mammalian cultured Schwann cells

Cultured Schwann cells from sciatic nerves of newborn rabbits and rats have been examined with patch-clamp techniques. In rabbit cells, single sodium and potassium channels have been detected with single channel conductances of 20 pS and 19 pS, respectively. Single sodium channels have a reversal potential within 15 mV of ENa, are blocked by tetrodotoxin, and have rapid and voltage-independent inactivation kinetics. Single potassium channels show current reversal close to EK and are blocked by 4-aminopyridine. From these results, and from comparisons of single-channel and whole-cell data, we show that these Schwann cells contain voltage-dependent sodium and potassium channels that are similar in most respects to the corresponding channels in mammalian axonal membranes. Cultured rat Schwann cells also have sodium channels, but at a density about 1/10th that of rabbit cells, a result in agreement with saxitoxin binding experiments on axon-free sectioned nerves. Saxitoxin binding to cultured cells suggests that there are up to 25,000 sodium channels in a single rabbit Schwann cell. We speculate that in vivo Schwann cells in myelinated axons might act as a local source for sodium channels at the nodal axolemma.

Different functional states of tetrodotoxin sensitive and tetrodotoxin resistant Na+ channels occur during the in vitro development of rat skeletal muscle

There are three stages of differentiation of voltage dependent Na+ channels during the in vitro development of rat skeletal muscle. Myoblasts which are less than 60 h old in culture have Na+ channels which normally do not give rise to action potentials but do so after treatment of the cells with very low concentrations of sea anemone toxin. These Na+ channels revealed by sea anemone toxin are resistant to TTX. Myoblasts prior to fusion are electrically excitable (Vmax = 10 V/s). Electrically activated Na+ channels are only blocked by high concentrations of TTX. Titration of TTX resistant Na+ channels with a tritiated derivative of TTX indicates a dissociation constant of the TTX-Na+ channel complex of 50 nM. Myotubes have both high and low affinity binding sites for TTX (Frelin et al. 1983). Action potentials (Vmax = 100-200 V/s) are only inhibited at high concentrations of TTX. Experiments with rat myoballs indicate that only Na+ channels with a low affinity binding site for TTX are functional in voltage-clamp studies. The K0.5 value for TTX inhibition of the peak Na+ current is observed at 70 nM. Spontaneous contractions of myotubes are blocked by TTX with a K0.5 value of 100 nM, suggesting that TTX resistant Na+ channels are also the ones responsible for the spontaneous contractions in rat myotubes in culture. 22Na+ flux studies after activation of the Na+ channel with neurotoxins have been carried out at the different stages of differentiation. Toxin activated Na+ channels have the same high affinity for sea anemone toxins at all stages of development; likewise, the sensitivity for TTX is the same.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuronal-type Na+ and K+ channels in rabbit cultured Schwann cells

Nerve axons in the central and peripheral nervous system are normally surrounded by satellite cells. These cells, known as Schwann cells in the peripheral nervous system, interact with axons to form a myelin sheath, so allowing nerve impulses to proceed at high speed. Schwann cells are thought to differ from neurones in their membrane properties in one important aspect: they lack excitability. Using the patch-clamp technique we have now measured directly the ionic currents across the membrane of single Schwann cells cultured from newborn rabbits. Surprisingly, we found that these Schwann cells possess voltage-gated sodium and potassium channels that are similar to those present in neuronal membranes.

A specific mutation abolishing Na+/H+ antiport activity in hamster fibroblasts precludes growth at neutral and acidic pH

A H+-suicide technique based on the reversibility of Na+/H+ antiport was developed for the selection of mutants deficient in this membrane-bound activity. The strategy was to use the Na+/H+ antiporter as a H+-vector killing device. Chinese hamster lung fibroblasts (CCL39) were loaded with LiCl and incubated in Na+-, Li+-free choline Cl saline solution (pH 5.5). Under these conditions, intracellular pH dropped in 5 min from 7.1 to 4.8, leading to a rapid loss of cell viability (less than 0.1% survival after 30 min). Cytoplasmic acidification and cell death were prevented by treatment with 5-N,N-dimethylamiloride, a potent inhibitor of Na+/H+ antiport. Of the H+-suicide resistant clones that survived two cycles of selection, 90% were found deficient in Na+/H+ antiport activity. One class of mutants (PS10, PS12) fully resistant to the H+-suicide test, does not acidify the cell interior in response to an outward-directed Li+ gradient and has no detectable amiloride-sensitive Na+ influx measured either in Li+- or H+-loaded cells. Growth of these fibroblast clones lacking Na+/H+ antiport was found to be pH conditional in HCO3(-)-free medium. Whereas wild-type cells can grow over a wide range of external pHs (6.6-8.2), PS mutants cannot grow at neutral and acidic pHs (pH less than 7.2); their optimal growth occurs at alkaline pH values (pH 8-8.3). These findings strongly suggest that the Na+/H+ antiport activity through regulation of intracellular pH plays a crucial role in growth control.

A proposed membrane model for generation of sodium currents in squid giant axons

An experimental review to show that axonal undercoat and cytoskeletal structures underneath the axolemma of squid giant axons play an important role in generating sodium currents is presented. Correspondingly, two alternative membrane models are proposed; one is that the undercoat and cytoskeletal structures support the functioning of sodium channels and the other is that they are directly incorporated with the molecular mechanism of generating sodium currents. This latter model is probable in squid giant axons. The model of direct participation of the underlying cytoskeleton in the sodium activation mechanism modifies the sodium activation gating kinetics in the Hodgkin-Huxley scheme; that is, the transition velocities between the open and closed states of the activation gate depend not only on membrane potentials but also on the time after the onset of application of a potential step.

Beating activity of heterokaryons between myocardial and non-myocardial cells in culture

Cultured mouse myocardial cells grown as monolayers fused upon treatment with HVJ (Sendai virus). The myocardial cells also fused with quail myocardial cells, neuroblastoma cells and non-excitable cells, such as KB cells. The beating activity of these heterokaryons was studied in the present work. Heterokaryons composed of myocardial cells from different species maintained spontaneous beating activity for 2 days or more. Those of one myocardial and one neuroblastoma cell maintained the activity for 22-26 h, while those of one myocardial and one non-excitable cell, such as KB cell, lost the activity within 2-4 h after addition of HVJ. Heterokaryons that had stopped spontaneous beating did not contract on application of electrical-field stimulation. The ration of non-myocardial cells in the heterokaryons increased in inverse proportion to the decrease in beating activity of the heterokaryons. Study of the rapid disappearance of beating activity in heterokaryons composed of one myocardial and one KB cell showed that both excitability of the cell membrane and myofibril organization were rapidly lost.

Cell growth and net Na+ flux are inhibited by a protein produced by kidney epithelial cells in culture

Proliferation of confluent kidney epithelial cell cultures (BSC-1 line) is inhibited by a protein (Mr approximately equal to 24,000) that is secreted by the cells. The mechanism of action of this growth inhibitor was sought by studying its effect on net Na+ flux because increased availability of Na+ in the culture medium had been shown to stimulate cell growth. The increase in cell Na+ content observed during stimulation of the growth after a medium change was attenuated in the presence of the purified inhibitor. Inhibition of both cell Na+ accumulation and growth in the presence of the protein was reversed completely by addition of NaCl to the medium. These results suggest that control of net Na+ flux and growth in kidney epithelial cells could be mediated, at least in part, by a secreted cellular protein.

Conditioned medium from a clonal Rous sarcoma virus transformed cerebellar cell line induces process extension in glial lines

A Rous Sarcoma virus transformed cerebellar cell line, BC6, secretes a factor which causes clonal glial cell lines to rapidly (1 h) extend processes. The factor shows a degree of specificity since only 3 out of 10 lines which exhibit either glial or combined glial and neuronal properties respond. The active factor appears to be a soluble protein since it remains in the supernatant after centrifugation at 100,000 g for 2 h and is trypsin-sensitive. When conditioned medium is fractionated on a Sephadex G-100 column, activity elutes in and just behind the void volume. By several criteria the factor is distinct from glia maturation factor.

Tetrodotoxin-sensitive Na+ channels in isolated single cultured rat myocardial cells

We studied the existence of tetrodotoxin (TTX)-sensitive fast Na+ channels in isolated single (verified by dye injection) myocardial cells compared with small multiple-cell groups from 1- to 3-day-old rat ventricles in cell culture. For single cells, average values were -63 mV maximum membrane potential, 32 mV overshoot, and 65 V/s maximum rate of rise of the action potentials (+Vmax). These values were comparable to values from groups of multiple (2-10) cells. TTX strongly depressed +Vmax dose dependently, with no difference between single and multiple cells. +Vmax was also decreased by lowering extracellular Na+ concentration but not by D 600 or lowering extracellular Ca2+ concentration. These results suggest that 1) isolated single cells possess TTX-sensitive fast Na+ channels, 2) culturing per se does not alter TTX sensitivity, and 3) TTX sensitivity is not modified by cell density.

High-affinity binding of alpha-scorpion toxin: a neuronal property

alpha-Scorpion toxin binding to its receptor--one component of the voltage-sensitive sodium channel--was studied in an attempt to define its phenotypic specificity. To this end we investigated the ability of neuronal, glial myogenic and fibroblastic cell lines to bind alpha-toxin II, purified from venom of the scorpion Androctonus australis Hector. A single class of saturable high-affinity (Kd congruent to 1 nM) binding sites, was present only in cell lines exhibiting some of the characteristics of normal neuronal cells, such as the N18, NIE-115, NS20, BN10-10, NG108-15 and T28 cell lines. NIA-103, which is an electrically non-excitable neuronal cell, gave negative results. In glial (G26-20, TR6B, C6) myogenic (T984) or fibroblastic (L) cell lines, we were unable to detect high-affinity binding sites for alpha-scorpion toxin. Primary cultures of rat skeletal muscle cells were also negative. Thus specific binding in the nanomolar range seems to be selectively associated with the neuronal phenotype. alpha-Scorpion toxin binding was tested before and after induction of neurites: in N18, NIE-115, NS20 cell lines, the differentiation brought on an increase in the number of binding sites but had little effect on the dissociation constant; in the hybrids NG108-15 and T28 high affinity saturable binding sites were detectable after but not prior to morphological differentiation.

Sodium-channels in non-excitable glioma cells, shown by the influence of veratridine, scorpion toxin, and tetrodotoxin on membrane potential and on ion transport

Veratridine induces membrane potential oscillations in non-excitable glioma cells, which are not affected by ouabain (2 mM) or by D600 (0.1 mM). In the presence of veratridine, scorpion toxin causes depolarization of the glioma cells to a positive value of the membrane potential. These effects of veratridine and of scorpion toxin are observed in Na+ but not in choline medium and are inhibited by tetrodotoxin. The response of the glioma cells to bradykinin has also been studied during these experiments. Previously bradykinin has been shown in these cells to induce a hyperpolarizing response caused by an increase in K+ conductance. This response to bradykinin can still be seen during the veratridine-induced oscillations of the membrane potential. In the glioma cells the uptake of guanidinium, a substitute for Na+, is enhanced by veratridine plus scorpion toxin. This stimulation is tetrodotoxin-sensitive. However, in the excitable neuroblastoma X glioma hybrid cells studied for comparison, veratridine causes membrane potential oscillations accompanied at the rising phase by one action potential or a train of action potentials. The results demonstrate that in non-excitable glioma cells tetrodotoxin-sensitive Na+ channels can be activated by veratridine and by scorpion toxin.

Extraneuronal saxitoxin binding sites in rabbit myelinated nerve

The changes in binding of 3H-labeled saxitoxin (STX) to rabbit sciatic nerve during axonal regeneration (after nerve crush) and during axonal degeneration (after nerve section) were measured and compared with the corresponding changes in the sciatic nerves of other mammals (rat, guinea pig, and cat). In the rabbit and rat, regeneration after nerve crush is associated with a 2- to 4-fold increase in STX binding capacity, consistent with the known corresponding increase in the number of nodes of Ranvier in regenerating nerve. Furthermore, consistent with the disappearance of nodes that occurs with Wallerian degeneration, nerve section leads to a disappearance of all, or most, of the STX binding in rat and guinea pig nerve, similar to that previously found for cat nerve. However, in the rabbit, nerve section leads to a large maintained increase in STX binding. Intraneural injection of diphtheria toxin, which is known to damage Schwann cells and which causes an increase in STX binding in intact nerves, abolishes the binding in cut nerves. It is suggested that the increased binding in cut nerves is to nonneuronal sites situated on the surface membrane of the Schwann cells, which have greatly proliferated in number as axonal degeneration has progressed. The reason for the difference between rabbits and other species and the possibility that the binding sites of rabbit Schwann cells represent functional sodium channels remain to be investigated.

Thrombin-induced protein phosphorylation in resting platelets and fibroblasts: evidence for common post-receptor molecular events

We have investigated the thrombin-stimulated protein phosphorylation associated with the activation of two cellular processes: 1) reinitiation of DNA synthesis in G0-arrested hamster fibroblasts and 2) stimulation of serotonin release in platelets. We found a rapid 4- to 6-fold increased phosphorylation of a peptide with apparent Mr = 27,000 in SDS-PAGE. In both cell systems, the 27,000 dalton phosphopeptide is cytosolic, is resolved by isoelectric focusing as multiple variants (pHi 5 to 6), and thrombin stimulation generates the most acidic phosphorylated forms. This result and the fact that the 27,000 dalton peptide is also stimulated by a variety of growth factors in fibroblasts, strongly suggests that thrombin action in platelets and growth factor-induced mitogenesis, share common post-receptor molecular events.

Tetrodotoxin-sensitive ion channels characterized in glial and neuronal cells from rat brain

Ion channels were studied in primary neuronal and in primary glial cultures from rat brain by measuring the uptake of guanidinium, an ion that can permeate the Na+ channel. Neuronal cells exhibit a veratridine-stimulated (EC50 30 microM) guanidinium uptake, which is blocked by tetrodotoxin (IC50 30nM). This demonstrates the presence of a voltage-dependent Na+ channel. In glial cells veratridine + scorpion toxin, but not veratridine or scorpion toxin alone can stimulate a tetrodotoxin-sensitive ion uptake, thus indicating a 'silent' Na+ channel in the glial cells. Phentolamine, propranolol and various local anesthetic drugs (e.g. tetracaine, dibucaine) blocked the two different kinds of Na+ channels in the two cell populations investigated.

Differential action of tetrodotoxin on identified leech neurons

1. In leech segmental ganglia, the maximum rate of depolarization of action potentials was found to depend largely on Na in the Retzius (R) cell, the mechanosensory P, N and T cells and an identifiable neuron of unknown function, the X cell. 2. Tetrodotoxin (TTX) 15 100 mumol/l had little or no effect on R and X cells. In contrast, membrane excitation in N, P and T cells was depressed in dose- and use-dependent fashion. 3. The data imply the existence of two kinds of Na channels in normal, fully differentiated leech neurons. Correlation of such differences should lead to a better understanding of how particular neurons perform different functions.

Growth factor activation of an amiloride-sensitive Na+/H+ exchange system in quiescent fibroblasts: coupling to ribosomal protein S6 phosphorylation

Chinese hamster lung fibroblast cells (CCl39) enter the G0/G1 nonproliferative state after serum deprivation. In this report, we show that reinitiation of DNA synthesis by serum or the combination of purified human thrombin and insulin (1-10 microgram/ml) is preceded by very early stimulation of ionic fluxes (Na+/Rb+) and protein phosphorylation (27,000 daltons, 62,000 daltons, and the ribosomal S6 proteins). The potentiating action of insulin on thrombin-stimulated DNA synthesis is also observed on thrombin-stimulated Na+ influx, Rb+ influx, and protein S6 phosphorylation. Moreover, we demonstrate that CCl39 cells possess a Na+/H+ exchange system sensitive to amiloride. Half-maximal inhibition of growth factor-activated Na+ influx and Na+-dependent H+ efflux is obtained with 3-10 microM amiloride. Two lines of evidence indicate that the extrusion of H+ via the activation of the Na+/H+ exchanger is coupled to protein S6 phosphorylation: serum-stimulated phosphorylation is blocked by (i) amiloride at a concentration that abolishes serum-stimulated Na+ influx and (ii) protonophores that acidify the cell interior. The present data support the idea that the regulation of intracellular pH is a key event in the mechanism of growth factor action.

Pharmacological and electrophysiological characterization of lithium ion flux through the action potential sodium channel in neuroblastoma X glioma hybrid cells

Interaction of Li+ with the voltage-dependent Na+ channel has been analyzed in neuroblastoma X glioma hybrid cells. The cells were able to generate action potentials in media containing Li+ instead of Na+. The uptake of Li+ into the hybrid cells was investigated for the pharmacological analysis of Li+ permeation through voltage-dependent Na+ channels. Veratridine and aconitine increased the uptake of Li+ to the same degree (EC50 30 microM). This increase was blocked by tetrodotoxin (IC50 20 nM). Veratridine and aconitine did not act synergistically; however, the veratridine-stimulated influx was further enhanced by the toxin of the scorpion Leiurus quinquestriatus (EC50 0.06 micrograms/ml). This stimulation was also blocked by tetrodotoxin. Thus, the voltage-dependent Na+ channel of the hybrid cells accepts both Li+ and Na+ in a similar manner.

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.

Neurotoxins as probes in the study of neuronal development

We have investigated the expression of surface membrane binding sites for tetanus toxin and alpha-scorpion toxin (AaHII) on cells of the in vivo developing mouse nervous system. There is a close temporal correlation in the pattern of emergence and accumulation of tetanus toxin binding cells (TBC) and that of post-mitotic neurons. In different nervous system areas, the fluctuations in relative TBC abundance reflect regional changes in the dynamics of neuronal subpopulations. The results indicate that the acquisition of membrane tetanus toxin binding sites may represent one of the earliest detectable characteristics of nascent neurons. The Na+ channel-associated scorpion toxin become detectable in fetal mouse brain two days after the appearance of TBC. Their density increases with fetal age without change in receptor properties. At all stages, scorpion toxin binds to a single class of noninteracting sites with a KD = 0.1 - 0.5 nM. The affinity of binding is voltage-dependent. Studies on brain cells and various cell lines grown in vitro suggest a selective association of the high affinity scorpion toxin receptors with neuronal phenotype. In culture, as in vivo, there is a time dependent increase in receptor density. These results indicate that both tetanus toxin and scorpion toxin can be used as qualitative markers of neuronal differentiation; moreover, estimates of the density of scorpion toxin binding sites provide a quantitative index of neuronal maturation.

Differentiation of the fast Na+ channel in embryonic heart cells: interaction of the channel with neurotoxins

The sensitivity of embryonic cardiac cells to tetrodotoxin (TTX) increases with age. At the early embryonic stage, the maximum upstroke velocity is not affected by the presence of TTX. In the course of both in ovo and in vitro development, this velocity reaches an adult-like value of 90-120 V/sec, which is decreased in the presence of TTX to 5-10 V/sec. The differentiation of the Na+ channel has been followed by using three types of specific toxins: (i) TTX or a tritiated derivative of it, (ii) a polypeptide toxin extracted from sea anemone, and (iii) the alkaloidic toxins veratridine and batrachotoxin. Electrophysiological, including voltage-clamp experiments, and biochemical studies have shown (i) that the TTX receptor and the fast Na+ channel machinery exist even when action potentials are insensitive to TTX--the channel is then in a nonfunctional or silent form that is revealed (or chemically activated) by both the alkaloids and the polypeptide toxin--and (ii) that the total number of Na+ channels increases during development by a factor of 4 or 5. In monolayers of cardiac cells insensitive to TTX in which all Na+ channels are in a nonfunctional form, the rate of degradation of the TTX receptor follows first-order kinetics with a half-time of 9 hr. In aggregates fully sensitive to TTX, the number of TTX receptors remains perfectly stable 24 hr after blockade of protein synthesis.