The effect of veratridine on excitable membranes of nerve and muscle

Effect of tetracaine on veratrine-mediated influx of sodium into rat brain synaptosomes

Depolarization of synaptosomes with veratrine (0.1 mg/ml) or 50 mmol/l of KCl results in the release of radioactivity from 14C-choline loaded synaptosomes with partial dependence on external Ca2+. Like tetrodotoxin, tetracaine prevented veratrine but not KCl action, with a half-maximal effect at approximately 10(-5) mol/l of anesthetic. A similar half-maximal value was obtained for tetracaine blockade of veratrine-stimulated tetrodotoxin-sensitive 22Na influx into synaptosomes, complete blockage being achieved at 10(-4) mol/l. At this concentration tetracaine failed to modify Ca2+ channels measured by KCl-induced 45Ca uptake. Microviscosity of a lipid bilayer in synaptic membranes evaluated with 5- and 16-doxylstearate spin labels decreased at tetracaine concentrations exceeding 10(-3) mol/l. It is suggested that Na+ channels of synaptosomes are blocked by direct action of anesthetic or through changes in the channel annular lipids.

Blockade by neurotransmitter antagonists of veratridine-activated ion channels in neuronal cell lines

The voltage-dependent Na+ ionophore of various neuronal cells is permeable not only to Na+ ions but also to guanidinium ions. Therefore, the veratridine- (or aconitine-)stimulated influx of [14C]guanidinium in neuroblastoma x glioma hybrid cells was measured to characterize the Na+ ionophore of these cells. Half-maximal stimulation of guanidinium uptake was seen at 30 microM veratridine. At 1 mM guanidinium, the veratridine-stimulated uptake of guanidinium was lowered to 50% by approximately 60 mM Li+, Na+, or K+ and by a few millimolar Mn2+, Co2+, or Ni2+. The basal, as well as the veratridine-stimulated, uptake of guanidinium was inhibited by the cholinergic antagonists (+)-tubocurarine (Ki = 50 to 500 nM) and atropine (Ki = 5 to 30 microM) and the adrenergic antagonists phentolamine (Ki = 5 microM) and propranolol (Ki = 60 microM). The specificity of the inhibitory effects of these agents is stressed by the ineffectiveness of various other neurotransmitter antagonists. However, the corresponding ionophore in neuroblastoma cells (clone N1E-115) seems to be regulated differently. While phentolamine and propranolol inhibit the veratridine-activated uptake as in the hybrid cells, (+)-tubocurarine and atropine exert only a slight effect.

Acetylation of synaptosomal protein: inhibition by veratridine

Incubation of synaptosomes with [3H]acetate results in rapid labeling of protein. Labeling is decreased in the presence of veratridine, and the effect of veratridine is blocked by tetrodotoxin. Most of the radioactivity can be removed by base or acid hydrolysis, and is probably incorporated as acetate; it is this fraction that is affected by the veratridine. The data suggest that veratridine stimulates deacetylation is involved in membrane function.

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.

Carnosine release from olfactory bulb synaptosomes is calcium-dependent and depolarization-stimulated

The dipeptide carnosine (beta-alanyl-L-histidine) has been proposed as a neurotransmitter in the mammalian olfactory pathway. Therefore, the efflux of in vivo-synthesized [14C]carnosine from mouse olfactory bulb synaptosomes was investigated. Carnosine was found to be released from the olfactory bulb synaptosomes by two mechanisms. The first is a slow spontaneous process that is independent of depolarization. The rate of this release was doubled in the presence of 1 mM external carnosine. Release by the second mechanism was markedly stimulated in the presence of calcium by depolarization with either 60 mM K+ or 300 microM veratridine. Omission of calcium abolished the stimulatory effect of both of these agents. Further, blockage of the veratridine-induced depolarization by tetrodotoxin also inhibited carnosine release. These results are consistent with the hypothesis that carnosine acts as a neurotransmitter in the mouse olfactory pathway.

Enhancement of calcium current during digitalis inotropy in mammalian heart: positive feed-back regulation by intracellular calcium?

1. Effects of digitalis compounds on slow inward Ca current I(si)) and contractile force were examined in ferret ventricular muscle (single sucrose-gap voltage clamp) and calf Purkinje fibres (two micro-electrode voltage clamp).2. In ventricular muscle, ouabain increased I(si) and inward current tails associated with I(si) conductance. The enhancement of I(si) followed a time course similar to the development of the positive inotropic effect, and it could be observed in the absence of aftercontractions or other signs of toxicit.3. The response of myocardial I(si) and twitch force to ouabain depended strongly on a previous history of driven action potentials.4. Veratridine, a toxin that promotes Na entry through tetrodotoxin-sensitive channels, also increased I(si) and twitch force in driven ventricular muscle preparations.5. The effects of ouabain, action potential stimulation and veratridine are consistent with reported effects of K-poor solutions in indicating that elevation of intracellular Na can lead to enhancement of I(si). Additional experiments suggest that the link between Na(i) and I(si) involves intracellular Ca.6. When Cs-loaded Purkinje fibres were bathed in solutions containing Sr instead of Ca, enhancement of I(si) by strophanthidin was abolished even though a positive inotropic response persisted.7. After intracellular injection of Purkinje fibres with EGTA, I(si) no longer increased with strophanthidin, although it remained responsive to adrenaline.8. Clear-cut increases in I(si) were seen in Cs-loaded Purkinje fibres even at very low concentrations of strophanthidin (20-50 nM), where the occurence of Na pump inhibition has been questioned.9. Positive regulation of Ca entry by intracellular Ca may act as a facilitory mechanism that amplifies myocardial responsiveness to digitalis and other inotropic interventions. Through changes in I(si), small rises in diastolic free Ca might lead to large increases in the activator Ca transient during contraction.

Neuronal and glial release of [3H]GABA from the rat olfactory bulb

GABA uptake and release mechanisms have been shown for neuronal as well as glial cells. To explore further neuronal versus glial components of the [3H]gamma-aminobutyric acid ([3H]GABA) release studies were performed with two different microdissected layers of the olfactory bulb of the rat: the olfactory nerve layer (ONL), consisting mainly of glial cells, and the external plexiform layer (EPL) with a high density of GABAergic dendritic terminals. In some experiments substantia nigra was used as a GABAergic axonal system and the trigeminal ganglia as a peripheral glial model. Spontaneous release of [3H]GABA was always lower in neuronal elements as compared with glial cells. A veratridine-evoked release was observed from the ONL but not from the trigeminal ganglia. Tetrodotoxin (TTX) abolished the veratridine-evoked release from the ONL, which also showed a partial inhibition when high magnesium concentrations were used in a Ca2+-free solution. beta-Alanine was strongly exchanged with [3H]GABA from the ONL of animals with the olfactory nerve lesioned and from animals with no lesion; but only a small heteroexchange was found from the external plexiform layer. The beta-alanine heteroexchange was able to deplete the releasable GABA store from the ONL of lesioned animals. In nonlesioned animals and the external plexiform layer, the veratridine-stimulated release of [3H]GABA was not significantly reduced after the beta-alanine heteroexchange. Stimulation of the [3H]GABA release by high concentrations of potassium elicited a higher release rate from axonal terminals than from dendrites or glia. Neurones and glia showed a similar inhibition of [3H]GABA release when a high magnesium concentration was added to a calcium-free solution. When D-600 was used as a calcium-flux blocker no inhibition of the release was observed in glial cells, whereas an almost complete blockage was found in both neuronal preparations (substantia nigra and EPL). These results provide further evidence for differential release mechanisms of GABA from CNS neurones and glial cells.

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.

On the mechanism by which veratridine causes a calcium-independent release of gamma-aminobutyric acid from brain slices

1 The mechanisms by which veratridine increases the release of gamma-aminobutyric acid (GABA) from brain slices have been studied.2 Exposure of superfused cerebro-cortical, nigral or cerebellar slices to veratridine (5 muM) or KCl (50 mM) caused large increases in the efflux of [(3)H]-GABA.3 Reduction of the external Ca concentration [Ca](o) to zero had strikingly different effects on the veratridine and K-evoked release of [(3)H]-GABA. The K-evoked release from all three areas was greatly reduced in Ca-free medium, but the veratridine-evoked release from cerebeller slices was not affected, and the release of [(3)H]-GABA from cortical and nigral slices was increased three fold. The potentiation of the veratridine evoked release of GABA which occurred in Ca-free medium was not due to the reduction in divalent ions, because it still occurred in medium in which the Ca was replaced by an equivalent amount of Mg.4 The veratridine-evoked release of [(14)C]-glycine from slices of spinal cord was also significantly increased in Ca-free medium. In contrast, the release of cortical [(3)H]-noradrenaline and [(14)C]-acetylcholine caused by the alkaloid was greatly diminished in Ca-free medium.5 The veratridine but not the K-evoked release of [(3)H]-GABA was abolished when the external Na concentration [Na](o) was reduced to zero and by tetrodotoxin (TTX) (0.2 muM). Cl-free medium did not affect the veratridine-evoked release of [(3)H]-GABA or its potentiation by Ca-free medium.6 Exposure of the tissue to depolarizing concentrations of external K ([K](o) = 120 mM) did not abolish the veratridine evoked release of [(3)H]-GABA or its potentiation by Ca-free medium.7 Pre-incubation of cortical slices with L-2,4, diaminobutyric acid (DABA), or substitution of Na in the superfusion medium with Li, did not affect the veratridine-evoked release of [(3)H]-GABA, indicating that the alkaloid does not stimulate GABA efflux by a carrier-mediated transport process.8 Exposure of the tissue to ruthenium red (10 muM) increased the veratridine evoked release of [(3)H]-GABA in both normal and in Ca-free medium but almost abolished the K-evoked release.9 It is suggested that veratridine causes GABA release by increasing the permeability of the nerve terminals to Na. In normal medium, the resulting influx of Ca(2+) ions through voltage-dependent Ca(2+) channels may be involved in triggering the release of GABA. However, a major part of the GABA efflux appears to be triggered by the release of Ca(2+) ions from intraterminal mitochondria, which results from the increase in[Na](i). Since Ca(2+) ions antagonize the action of veratridine, the potentiation of the drug-evoked release of GABA that occurs in Ca-free medium, might be due to the absence of the antagonistic Ca(2+) ions. The resulting greater increase in Na entry and [Ca](i) caused by Ca release from intracellular stores, must presumably more than balance the contribution normally made by any influx of extracellular Ca(2+).

Inhibition by somatostatin of bovine growth hormone secretion following sodium channel activation

1. Growth hormone secretion, exchangeable cellular sodium and calcium concentrations measured by 22Na and 45Ca incorporation, and efflux of 45Ca were studied in dispersed bovine anterior pituitary cells. 2. Addition of veratridine (100 microM), an activator of sodium channels, increased exchangeable sodium and calcium concentrations in the cells, the efflux of 45Ca from prelabelled cells, and caused a biphasic stimulation of the rate of growth hormone secretion. Secretion of growth hormone was not stimulated when the extracellular calcium was decreased below 0.1 mM. 3. The increases in growth hormone secretion, exchangeable calcium concentration and 45Ca efflux from prelabelled cells caused by veratridine were abolished by addition of the calcium antagonist verapamil (20 microM). Verapamil also reduced the rise in exchangeable sodium caused by veratridine and increased the resting exchangeable sodium concentrations. 4. The peptide somatostatin (1 micrograms/ml) prevented veratridine-stimulated growth hormone secretion but did not inhibit the increases in exchangeable sodium and calcium caused by veratridine. The peptide itself elicited a transient increase in 45Ca efflux and subsequently partially inhibited veratridine-stimulated 45Ca efflux. 5. The data suggest that anterior pituitary cells possess voltage-sensitive sodium channels. Activation of these channels by veratridine may lead to depolarization and increased entry of calcium via potential-dependent calcium channels, which contributes to a rise in cytoplasmic calcium concentration and the subsequent stimulation of growth hormone secretion. We conclude that the calcium antagonist verapamil may also interact with sodium channels, and that the peptide somatostatin may act on growth-hormone-secreting cells either to prevent the rise in cytoplasmic calcium by hyperpolarizing the cells or to decrease the affinity of a population of calcium binding sites in the cells.

Ion channels and membrane potential in stimulus-secretion coupling in adrenal medulla cells

The role of Na+ channels and membrane potential in stimulus secretion coupling in adrenal medulla cell cultures was investigated. Veratridine, aconitine, batrachotoxin (BTX), and scorpion venom, which increase the flux of ions through tetrodotoxin(TTX)-sensitive Na+ channels, all evoke secretion of catecholamines that is blocked by TTX. TTX partially inhibits secretion induced by low concentrations of nicotine in Locke's solution but has no effect on high concentrations of nicotine (20 microM). In Ca2+-sucrose media TTX has no effect on secretion at either high or low concentrations of nicotine. Replacement of Na+ with Li+ in Locke's solution reduces the response to nicotine and to veratridine. Complete replacement of Na+ with hydrazine, diethanolamine, TRIS, and choline completely inhibits the response to nicotine and almost completely inhibits the response to veratridine. Following exposure of cells to 50 mM-100 mM-K+, nicotine does not stimulate catecholamine secretion unless the cells are resuspended in media containing less than 50 mM-K+. Neither dibutyryl-cyclic AMP nor dibutyryl-cyclic GMP evokes secretion. alpha-Bungarotoxin (1 microM) did not inhibit nicotine-induced secretion. These studies indicate that Na+ channels and acetylcholine (ACh) receptor ion channels are independently coupled to the influx of Ca2+. The membrane potential appears to affect nicotine- and veratridine-evoked secretion.

Membrane potentials in cell-free preparations from guinea pig cerebral cortex: effect of depolarizing agents and cyclic nucleotides

The distribution of [3H]triphenylmethylphosphonium ion between the medium and vesicular entities was examined in a cell-free, particulate preparation from guinea pig cerebral cortex. This distribution followed the Nernst relationship with regard to the external potassium ion concentration and, in physiological media, indicated the maintenance of a mean trans-membrane potential ranging from -58 to -78 mV. The neurotoxins batrachotoxin, veratridine, and grayanotoxin I, partially depolarized the preparation. Tetrodotoxin blocked the depolarization by batrachotoxin, veratridine, and gray-anotoxin I. The depolarization by these neurotoxins was potentiated by the presence of anemone toxin II and presumably reflected the response of vesicular components of neuronal origin. An additional potassium-sensitive depolarization probably represented the response of vesicular components of glial origin with an apparent transmembrane potential of -8 to -35 mV. No correlation could be demonstrated between changes in transmembrane potential and stimulation of cyclic AMP generation by a variety of agents in this preparation.

Effects of Goniopora toxin on guinea-pig blood vessels

Effects of a marine polypeptide, Goniopora toxin (GPT) (molecular weight 12,000), were examined in isolated blood vessels guinea pigs. GPT, ranging from 10-100 nM, augmented the contractile response to electrical transmural stimulation in the thoracic aorta, portal vein, and mesenteric and femoral arteries. The effects were abolished by tetrodotoxin and bretylium, and were markedly attenuated by phentolamine. As GTP did not affect the resting tension spontaneous rhythmicity or noradrenaline-induced contraction, the toxin appears to act on the neural elements in the vascular wall rather than on the smooth muscle. In the portal vein preloaded with 3H-noradrenaline, GPT enhanced the 3H-efflux in response to electrical transmural stimulation, yet had not effect on the spontaneous efflux. The increase in stimulation-evoked 3H-efflux caused by GPT was more than 15 times larger than the increase seen with cocaine or phentolamine. Tetrodotoxin completely blocked the 3H-efflux induced by electrical transmural stimulation. These data suggest that GPT acts on nerve components in guinea pig blood vessels and increases the release of noradrenaline evoked by electrical stimulation of the nerve fibers. These effects are probably associated with prolongation of the action potential duration and repetitive discharges in the adrenergic nerve fibers.

Sodium and calcium fluxes in a clonal nerve cell line

1. 22Na+ and 45Ca2+ fluxes were studied in the clonal nerve cell line PC12. Three distinct types of ion channels were found: (a) voltage-dependent Na+ channels, (b) voltage-dependent Ca2+ channels, and (c) acetylcholine-activated channels permeable to both ions. 2. 22Na+ uptake through voltage-dependent Na+ channels is induced by veratridine and scorpion venom, and is inhibited 50% by 5 X 10(-7) M-tetrodotoxin and greater than 98% by 5 X 10(-6) M-tetrodotoxin. 3. 45Ca2+ uptake through voltage-dependent Ca2+ channels is induced by depolarizing the cells in 50 mM-KCl. This flux is not dependent on the presence of Na+ in the medium and is insensitive to 5 X 10(-6) M-tetrodotoxin. However, 1 mM-Mn2+ causes a 95% inhibition of K+-induced 45Ca2+ uptake. 4. Veratridine and scorpion venom also induce voltage-dependent 45Ca2+ uptake which can be blocked by 1mM-Mn2+. In contrast to KCl-induced 45Ca2+ uptake, this flux is completely blocked by 5 X 10(-6) M-tetrodotoxin and is abolished by removal of Na+ from the medium. Thus the depolarizing stimulus for Ca2+ uptake in this case is Na+ from the medium. Thus the depolarizing stimulus for Ca2+ uptake in this case is Na+ influx through voltage-dependent Na+ channels. 5. Carbamylcholine induces both 22Na+ and 45Ca2+ fluxes which are blocked by nicotinic cholinergic antagonists with the exception of alpha-bungarotoxin. The 22Na+ flux occurs exclusively via acetylcholine receptor channels, as evidenced by the lack of effect of 5 X 10(-6) M-tetrodotoxin. In the presence of Na+, almost all of the 45Ca2+ uptake can be blocked by 1 mM-Mn2+ and thus occurs via voltage-dependent Ca2+ channels which are activated by the depolarizing Na+ influx. 6--8% of the total 45Ca2+ flux, however, is insensitive to 1 mM-Mn2+, suggesting that this portion of the uptake occurs via the acetylcholine receptor channels. In Na+-free medium, the Mn2+-resistant 45Ca2+ component increases to 40% of the total uptake, apparently due to lack of competition from Na+ for the acetylcholine receptor channels. This receptor-linked flux still causes sufficient depolarization to induce the additional 60% of the Ca2+ flux through voltage-dependent, Mn2+ sensitive Ca2+ channels. 6. Mn2+ inhibits Ca2+ flux through voltage-dependent Ca2+ channels by competing for entry through these channels. 50 mM-KCl induces 54Mn2+ fluxes in PC12 cells that are comparable in magnitude to 45Ca2+ fluxes. 7. In normal saline 45Ca2+ efflux from PC12 cells is several times more rapid than in Na+-free medium, indicating the presence of a Ca2+-Na+ exchange mechanism.

Interaction between batrachotoxin and yohimbine

The neurotoxins, batrachotoxin and veratridine, are specific activators of sodium channels and cause an increase in the rate of 22Na uptake in neuroblastoma cells. Yohimbine, an indolakylamine alkaloid, inhibits this batrachotoxin-induced 22Na uptake. The dose-response curve of yohimbine suggest that the inhibitor acts reversibly on a single class of binding sites with dissociation constant of 3--4 x 10(-5) M. The dissociation constant is not affected by depolarization from--41 to 0 mV. Kinetic and equilibrium experiments indicate that yohimbine is a competitive inhibitor of the action of batrachotoxin. These results support the conclusion that yohimbine inhibitis the sodium flux by acting on the channel gating mechanism rather than by occluding the channels.

On the voltage-dependent action of tetrodotoxin

The use of the maximum rate-of-rise of the action potential (Vmax) as a measure of the sodium conductance in excitable membranes is invalid. In the case of membrane action potentials, Vmax depends on the total ionic current across the membrane; drugs or conditions that alter the potassium or leak conductances will also affect Vmax. Likewise, long-term depolarization of the membrane lessens the fraction of total ionic current that passes through the sodium channels by increasing potassium conductance and inactivating the sodium conductance, and thereby reduces the effect of Vmax of drugs that specifically block sodium channels. The resultant artifact, an apparent voltage-dependent potency of such drugs, is theoretically simulated for the effects of tetrodotoxin on the Hodgkin-Huxley squid axon.

Sea anemone toxin:a tool to study molecular mechanisms of nerve conduction and excitation-secretion coupling

The effects of polypeptide neurotoxin from Anemonia sulcata on nerve conduction in crayfish giant axons and on frog myelinated fibers have been analyzed. The main features of toxin action are the following: (i) the toxin acts at very low doses and its action is apparently irreversible. (ii) The toxin selectively affects the closing (inactivation) of the Na+ channel by slowing it down considerably; it does not alter the opening mechanism of the Na+ channel or the steady-state potassium conductance. (iii) The tetrodotoxin-receptor association is unaffected by previous treatment of the axonal membrane with the sea anemone toxin. (iv) Conversely, the sea anemone toxin can only associate with the membrane when the Na+ channel is open for Na+; it does not bind when the channel is previously blocked by tetrodotoxin. (v) Besides its effect on the action potential, the sea anemone toxin displays a veratridine-type depolarizing action at low Ca2+ concentration which can be suppressed by tetrodotoxin. The sea anemone toxin greatly stimulates the release of gamma-[3H]aminobutyric acid from neurotransmitter-loaded rat brain synaptosomes. The apparent dissociation constant of the neurotoxin-receptor complex in this system is 20 nM. The sea anemone toxin effect is antagonized by tetrodotoxin.

Molecular mechanism of cardiotoxin action on axonal membranes

Cardiotoxin isolated from Naja mossambica mossambica selectively deactivates the sodium-potassium activated adenosine triphosphatase of axonal membranes. Tetrodotoxin binding and acetylcholinesterase activities are unaffected by cardiotoxin treatment. The details of association of cardiotoxin with the axonal membrane were studied by following the deactivation of the sodium-potassium activated adenosine triphosphatase and by direct binding measurements with a tritiated derivative of the native cardiotoxin. The maximal binding capacity of the membrane is 42-50 nmol of cardiotoxin/mg of membrane protein. The high amount of binding suggests association of the toxin with the lipid phase of the membrane. It has been shown that cardiotoxin first associates rapidly and reversibly to membrane lipids, then, in a second step, it induces a rearrangement of the membrane structure which produces and irreversible deactivation of the sodium-potassium activated adenosine triphosphatase. Solubilization of the membrane-bound ATPase with Lubrol WX gives an active enzyme species that is resistant to cardiotoxin-induced deactivation. Cardiotoxin binding to the membrane is prevented by high concentrations of Ca 2+ and dibucaine. Although cardiotoxins and neurotoxins of cobra venom have large sequence homologies, their mode of action on membranes is very different. The cardiotoxin seems to bind to the lipid phase of the axonal membrane and inhibits the sodium-potassium activated adenosine triphosphatase, whereas the neurotoxin associates with a protein receptor in the post-synaptic membrane and blocks acetylcholine transmission.

Pharmacologic characterization of the Na+ ionophores in L6 myotubes

We present a pharmacologic characterization of the Na+ ionophores present in L6 myotubes in vitro. Action potentials are abolished by replacement of the external Na+ by Tris. The amplitude of the action potential is generally resistant to high concentrations of tetrodotoxin (10(-5) M) and saxitoxin (10(-6 M), but the effect of these agents is highly variable. Veratridine (10(-4 M) consistently induces, as a short-term effect, a marked prolongation of the falling phase of the action potential. As a long-term effect, veratridine consistently induces a Na+-dependent reduction in the resting potential of the cell. The effects of veratridine on the action potential are not antagonized by tetrodotoxin or saxitoxin. However, the effects of veratridine on the resting potential are strongly antagonized by tetrodotoxin (10(-5) M) and fully inhibited by saxitoxin (10(-6) M). Significantly, under conditions where saxitoxin has fully inhibited the effects of veratridine on the resting potential, the myotubes are capable of generating overshooting action potentials. In contrast to their sensitivity to veratridine, L6 myotubes are insensitive to 10(-5) M alpha-dihydro-grayanotoxin-II. These results are discussed in the contexts of developmental significance and current views about Na+ ionophores.

Sodium flux through the sodium channels of axon membrane fragments isolated from lobster nerves

The efflux of 22Na from vesicles formed by axolemma fragments isolated from lobster nerves was studied in the presence and in the absence of drugs having well-known action on the sodium channels. The vesicles were equilibrated 12-14 h at 4 degrees C with 22Na in lobster solution containing 1 mM ouabain. Afterwards the suspension was divided: one portion was used as control and the others were treated with veratrine (0.025-0.50 mg/ml), tetrodotoxin (1-2,000 nM) in the presence of veratrine, or tetrodotoxin alone. After 3 h at 20-22 degrees C, the suspensions were diluted into nonradioactive solutions and the 22Na efflux followed by a rapid filtration technique. The results revealed that veratrine increases the efflux rate and the additional application of tetrodotoxin abolishes it, e.g., 0.50 mg of veratrine/ml increases the rate, expressed in 10(-2) min(-1), from 0.59 +/- 0.04 (mean +/- SEM; n = 13) to 0.86 +/- 0.05 (n = 13), and the addition of 100 nM tetrodotoxin diminishes it to 0.48 +/- 0.07 (n = 4). This increase and diminution are statistically significant (P less than 0.005), but this is not the case between the control and the veratrine plus tetrodotoxin values (P greater than 0.05). 50% of the diminution is produced by 11.9 +/- 2.4 nM tetrodotoxin. Tetrodotoxin alone produces a slight diminution of the 22Na efflux. Batrachotoxin (0.50 muM) has an action similar to veratrine's. These findings are considered evidence of the presence of functioning sodium channels in the isolated axolemma fragments.

A transient excited state model for sodium permeability changes in excitable membranes

In this paper we explore the properties of a mathematical model for the passive sodium permeability system of excitable membranes. This model is distinguished by the explicit inclusion of a rate constant which depends not on instantaneous voltage, but on rate of voltage change. Actually, the model is a rather modest modification of the Hodgkin-Huxley model, but displays some behaviors which the H-H model does not. Among these behaviors are a pronounced inactivation shift (for certain parameter values), a difference between inactivation time constant as measured by turning off a sodium current under sustained depolarization and as measured by double pulse experiments, skip runs under sustained current stimulation, and accommodation to slowing rising currents.

Cooperative activation of action potential Na+ ionophore by neurotoxins

Four neurotoxins that activate the action potential Na+ ionophore of electrically excitable neuroblastoma cells interact with two distinct classes of sites, one specific for the alkaloids veratridine, batrachotoxin, and aconitine, and the second specific for scorpion toxin. Positive heterotropic cooperativity is observed between toxins bound at these two classes of sites. Tetrodotoxin is a noncompetitive inhibitor of activation by each of these toxins (KI = 4-8 nM). These results suggest the existence of three functionally separable components of the action potential Na+ ionophore: two regulatroy components, which bind activating neurotoxins and interact allosterically in controlling the activity of a third ion-transport component, which binds tetrodotoxin.

The relation between the effects of veratridine on action potential and contraction in mammalian ventricular myocardium

In the isolated papillary muscle of the guinea pig veratridine produces an increase of the force of contraction by increasing the rate of force development. Time to peak force is slightly reduced, whereas relaxation time is markedly prolonged. Threshold, half-maximally and maximally effective concentrations for the positive inotropic effect are 0.1, 0.4 and 1.6 muM, respectively. 2. The positive inotropic effect of the maximally effective concentration of veratridine amounts to 68% of the maximum positive inotropic effect of dihydro-ouabain tested on the same muscle (N = 12). 3. Veratridine prolongs the action potential (AP) by delaying repolarization. The effect is concentration-dependent (range: 0.4--3.2 muM); it requires 1--2 hrs of maintained exposure to reach a steady state and is only slowly reversible upon removal of the drug. A concentration causing a nearly maximal positive inotropic effect (0.8 muM) does not affect resting potential or rate of rise of the AP; the overshoot is slightly depressed. 4. Tetrodotoxin (5--16 muM) reversibly inhibits both AP prolongation and positive inotropic effect of veratridine by shifting the concentration-effect curves for these effects to higher concentrations of veratridine. It also prevents veratridine-induced spontaneous activity. 5. Dihydro-ouabain or reduction of [K]o below 5.9 mM augument the positive inotropic effect of veratridine, while the interaction between veratridine and noradrenaline is additive. 6. The positive inotropic effect of 1.6 muM veratridine declines progressively when the contraction frequency is reduced below 0.5 Hz; rested-state contractions (at 0.004 Hz) are not increased by 1.6 muM veratridine. 7. It is concluded that (a) veratridine delays repolarization by prolonging the Na permeability component which is mediated by the fast Na channels; (b) this specific sarcolemmal effect of veratridine is the sole cause for its positive inotropic action by effecting an increase of [Na]i which probably leads to a subsequent increase of Ca uptake.

Mechanism of nerve membrane depolarization caused by grayanotoxin I

1. The mechanism of depolarization of squid axon membranes caused by grayanotoxin I has been studied by means of internal perfusion and voltage clamp techniques.2. The depolarization induced by either internal or external application of grayanotoxin I was reversed by decreasing the external sodium concentration from 449 to 1 mm.3. No depolarization was observed when both external and internal media were devoid of sodium ions, indicating that the depolarization by grayanotoxin I in normal media is due to a specific increase in resting sodium permeability.4. The resting sodium permeability as measured by voltage clamp was increased to 1.31 x 10(-6) cm/sec by internal application of 1 x 10(-5)m grayanotoxin I, an increase by a factor of about 90.5. The apparent dissociation constant of internally applied grayanotoxin I in increasing the resting sodium permeability was estimated to be 4.12 x 10(-5)m, and the toxin interacts with the membrane receptor on a one-to-one stoichiometric basis.6. Tetrodotoxin antagonized the action of grayanotoxin I in increasing the resting sodium permeability in a non-competitive manner.

Ion fluxes through the sodium channels of garfish olfactory nerve membranes

1. The efflux of (22)Na from garfish olfactory nerves, after treatment with ouabain and equilibration with (22)Na, occurs in two components of almost equal size. One component represents efflux from the extracellular space and the other, much slower, component represents efflux from the intracellular space through the axonal membrane.2. The rate of efflux of (22)Na through the membrane is increased from 0.038 to 0.055 min(-1) by veratrine, and restored back to 0.036 min(-1) by the additional application of tetrodotoxin or saxitoxin (10(-7)M).3. 50% inhibition of this increased sodium ion flux occurs at tetrodotoxin and saxitoxin concentrations of 12 and 6 nM respectively.4. The tetrodotoxin-sensitive efflux of (22)Na is almost unchanged in nerves equilibrated with hypertonic 0.85 M-NaCl, whereas it is largely eliminated in nerves equilibrated with 0.85 M-LiCl. We interpret this to indicate that there exists within the sodium channel a specific metal cation site which is responsible for the co-ordination of the ions during their passage through the membrane. Lithium ions bind to this site relatively strongly, with a dissociation constant of 0.2-0.3 M, whereas sodium ions bind less strongly, with a dissociation constant greater than 0.75 M. Other evidence indicates that this site is the site at which tetrodotoxin and saxitoxin act.5. Batrachotoxin has a similar action to veratrine but is effective in concentrations at least 100 times lower.

Sodium uptake associated with activation of action potential ionophores of cultured neuroblastoma and muscle cells

Veratridine, an activator of action potential Na(+) ionophores, stimulated passive Na(+) uptake by electrically excitable neuroblastoma and muscle cells but had no effect on clonal cell lines defective in Na(+)-ionophore activity. Veratridine-dependent Na(+) uptake was completely inhibited by tetrodotoxin, a specific inhibitor of the action potential Na(+) ionophore. Half-maximal inhibition was obtained with 11 nM tetrodotoxin. Thus, veratridinedependent Na(+) uptake provides a specific and convenient means of assaying populations of cultured cells for action potential Na(+)-ionophore activity.

Effects of membrane depolarization on light scattering by cerebral cortical slices

1. A system is described for simultaneously measuring the respiration and the reflectance of a tissue slice and is applied to a study of guinea-pig cerebral cortical slices.2. Reducing bathing medium osmolarity led to a reversible decrease in reflectance of these slices (as well as slices from liver and kidney cortex). In half isotonic solutions reflectance was reduced by 31%.3. Anoxia led to a decreased reflectance which was eliminated if all the Cl was substituted by the larger glucuronate anion.4. It is concluded that slice reflectance is lowered when cellular volumes are increased by water or isotonic solution influx.5. Membrane depolarization effected by ouabain, high (60 mM) K bathing medium, veratridine or repeated electrical pulses led to rapid decreases in reflectance of 25, 27, 31 and 7.5% respectively. Turning off the electrical pulses caused reflectance to return to control values. Reversibility of the chemical effectors was not tested.6. Substitution of Cl by glucuronate abolished the reflectance changes, although it did not inhibit the increased respiration induced by the depolarizing stimuli.7. Tetrodotoxin abolished both the respiratory and reflectance effects of veratridine and electrical pulses but had no effect upon those of high K or ouabain.8. The decrease in reflectance began about 1 sec after initiation of the pulses and was half maximal by 8 sec.9. Titrating reflectance against [K] showed that an increase of 5 mM-K led to a 4% decrease in reflectance and that reflectance became minimal between 60 and 80 mM-K(+).10. It is concluded that membrane depolarization in excitable cells of the cerebral cortex (and also, possibly, in the glia) causes rapid increases in cell volume due to influx of isotonic solution.11. The results indicate, more specifically, that changes in intercellular K concentrations of size and duration thought to occur following nervous activity in the C.N.S. cause cell volume changes large enough to drastically reduce the intercellular volumes and so, transiently, increase extracellular molecular and ionic concentrations. Increases of extracellular [K] and [Ca] have significant effects upon synaptic transmission and upon spontaneous nervous activity. It is suggested that nervous activity in one cell (or portion of it) might, in this way, strongly influence function in neighbouring elements.

Locations of amino acids in brain slices from the rat. Tetrodotoxin-sensitive release of amino acids

1. Amino acids, particularly glutamate, gamma-aminobutyrate, aspartate and glycine, were released from rat brain slices on incubation with protoveratrine (especially in a Ca(2+)-deficient medium) or with ouabain or in the absence of glucose. Release was partially or wholly suppressed by tetrodotoxin. 2. Tetrodotoxin did not affect the release of glutamine under various incubation conditions, nor did protoveratrine accelerate it. 3. Protoveratrine caused an increased rate of formation of glutamine in incubated brain slices. 4. Increased K(+) in the incubation medium caused release of gamma-aminobutyrate, the process being partly suppressed by tetrodotoxin. 5. Incubation of brain slices in a glucose-free medium led to increased production of aspartate and to diminished tissue contents of glutamates, glutamine and glycine. 6. Use of tetrodotoxin to suppress the release of amino acids from neurons in slices caused by the joint action of protoveratrine and ouabain (the latter being added to diminish reuptake of amino acids), it was shown that the major pools of glutamate, aspartate, glycine, serine and probably gamma-aminobutyrate are in the neurons. 7. The major pool of glutamine lies not in the neurons but in the glia. 8. The tricarboxylic cycle inhibitors, fluoroacetate and malonate, exerted different effects on amino acid contents in, and on amino acid release from, brain slices incubated in the presence of protoveratrine. Fluoroacetate (3mm) diminished the content of glutamine, increased that of glutamate and gamma-aminobutyrate and did not affect respiration. Malonate (2mm) diminished aspartate and gamma-aminobutyrate content, suppressed respiration and did not affect glutamine content. It is suggested that malonate acts mainly on the neurons, and that fluoroacetate acts mainly on the glia, at the concentrations quoted. 9. Glutamine was more effective than glutamate as a precursor of gamma-aminobutyrate. 10. It is suggested that glutamate released from neurons is partly taken up by glia and converted there into glutamine. This is returned to the neurons where it is hydrolysed and converted into glutamate and gamma-aminobutyrate.

Chloride conductance in normal and myotonic muscle fibres and the action of monocarboxylic aromatic acids

1. Cable parameters, component conductances, excitability and membrane potentials in isolated external intercostal fibre bundles at 38 degrees C from normal and myotonic goats were measured in normal and low-chloride Ringer, and in the presence of monocarboxylic aromatic acids that produce myotonic responses in mammalian muscle.2. The mean resting chloride conductance in mumho/cm(2) of myotonic fibres (range 0-147) was significantly less than that of normal fibres (range 376-951). The mean resting potassium conductance was higher in myotonic fibres (range 123-285) than in normal fibres (range 44-132). Potassium conductance increased about 10 mumho/cm(2) per mV increase in absolute resting potential.3. In normal fibres in normal Ringer 3-chloro-2,5,6-trimethylbenzoic acid; 5,6-dihydro-5,5-dimethyl-7-carboxybenz[c]acridine; phenanthrene-9-carboxylic acid; and anthracene-9-carboxylic acid at 10(-5)-10(-4)M decreased membrane conductance without consistently changing diameter or capacitance. In low-chloride Ringer 3-chloro-2,5,6-trimethylbenzoic acid (5 x 10(-5)M) increased potassium conductance in myotonic and normal fibres. It is concluded that these compounds block chloride conductance.4. The carboxylic acids produced myotonia in normal fibres similar to that in untreated myotonic fibres.5. Anthracene-9-carboxylic acid intravenously (8 mg/kg) in normal goats produced acutely a condition resembling myotonia congenita. The carboxylic acids produced no myotonic effects in frog muscle.