Influence of batrachotoxin, veratridine, grayanotoxin 1 and tetrodotoxin on uptake of Na-22 by rat brain membrane preparations

Thiamin and derivatives as modulators of rat brain chloride channels

Several membrane fractions were prepared from rat brain by differential and sucrose density gradient centrifugation. Most fractions took up 36Cl- rapidly at a rate linear with time during the first 30-60 s, then the rate progressively slowed down. The lowest rate of uptake was found in the mitochondrial fraction. Oxythiamin partially inhibited 36Cl- uptake in all fractions. In P2 (crude synaptosomal fraction), oxythiamin decreased the initial rate of uptake by 32%, the apparent Ki being 1.5 mM. Thiamin and amprolium were less effective as inhibitors. 4,4'-Diisothiocyanostilbene-2,2'-disulfonic acid (0.1-1 mM) inhibited 36Cl- uptake by 40-50%. In the presence of this compound at a concentration > or = 5 x 10(-4) M, oxythiamin became ineffective. 36Cl- uptake was increased by GABA (0.1 mM) and this effect was antagonized by picrotoxin as expected, but not by oxythiamin. The rate of 36Cl- uptake did not appreciably depend on the external chloride concentration and was unaffected by bumetanide or by replacement of external Na+ by choline. Taken together, these data suggest that the oxythiamin-sensitive 36Cl- influx is essentially diffusional and is not related to the GABA receptor or the Na:K:2Cl co-transport. Partial replacement of external Na+ by K+ or treatment with 0.1 mM veratridine (which should both result in membrane depolarization) increased 36Cl- uptake by 50 and 30% respectively; the inhibitory effect of oxythiamin was enhanced to the same proportion.(ABSTRACT TRUNCATED AT 250 WORDS)

The sodium channel of excitable and non-excitable cells

Excitation and conduction in the majority of excitable cells, as originally described in the squid axon, are initiated by a transient and highly selective increase of the membrane Na conductance, which allows this ion to move passively down its electrochemical gradient (Hodgkin & Katz, 1949; Hodgkin & Huxley, 1952). The term ‘Na channel’ was introduced to describe the mechanism involved in this conductance change (Hodgkin & Keynes, 1955).

Tetrodotoxin-insensitive central depression by grayanotoxin-III in mice

The effect of systemic administration of grayanotoxin (GTX)-III, a constituent in leaves of Pieris japonica D. Don with an ability to activate the voltage-sensitive sodium channels in excitable tissues, on general behaviors of animals was studied using Std-ddy mice. Intraperitoneal administration of the toxin (0.1-0.25 mg/kg b. wt.) resulted in a dose-dependent manner in a significant and reversible muscle relaxation, and a profound and long lasting (greater than or equal to 60 min) depression of locomotor activity. Pretreatment with GTX-III caused a profound potentiation of the duration of loss of righting reflex by pentobarbital with a concomitant delay of the onset of convulsive seizures by various convulsants such as strychnine, picrotoxin and pentetrazol. Neither tetrodotoxin (1-5 micrograms/kg, i.p.) nor Ro15-1788 (1-5 mg/kg, i.p.) prevented the GTX-III-induced suppression of locomotor activity. These results suggest that GTX-III may elicit a central depressant action in mice through a molecular mechanism other than activation of the voltage-sensitive sodium channels in the brain.

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.

Mouse brain synaptosomal sodium channels: activation by aconitine, batrachotoxin, and veratridine, and inhibition by tetrodotoxin

Batrachotoxin, veratridine and aconitine, activators of the voltage-dependent sodium channel in excitable cell membranes, increase the rate of 22Na+ uptake by mouse brain synaptosomes. Batrachotoxin was both the most potent (K0.5, 0.49 microM) and most effective activator of specific 22Na+ uptake. Veratridine (K0.5, 34.5 microM) and aconitine (K0.5, 19.6 microM) produced maximal stimulations of 22Na+ uptake that were 73% and 46%, respectively, of that produced by batrachotoxin. Activation of 22Na+ uptake by veratridine was completely inhibited by tetrodotoxin (I50, 6 nM ), a specific blocker of nerve membrane sodium channels. These results identify appropriate conditions for measuring sodium channel-dependent 22Na+ flux in mouse brain synaptosomes. The pharmacological properties of mouse brain synaptosomal sodium channels described here are distinct from those previously described for sodium channels in rat brain synaptosomes and mouse neuroblastoma cells.

Ionizing radiation decreases veratridine-stimulated uptake of sodium in rat brain synaptosomes

Veratridine-stimulated uptake of sodium-22 in brain synaptosomes was significantly reduced by ionizing radiation over a dose range of 10 to 1000 rads. The response was dose-dependent and involved a decrease in the maximum effect of veratridine on uptake. The central nervous system may be more sensitive to ionizing radiation than generally thought, perhaps through a loss of the ability of the sodium channel to respond properly to stimulation.

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.

Effects of propranolol and a number of its analogues on sodium channels

To assess the relative contributions that the sodium channel blocking activity of propranolol may play in a variety of its therapeutic applications, its effects were examined in vitro with a sodium channel specific 22Na+ uptake system, using rat brain membranes. Propranolol inhibited 22Na+ uptake in the rat brain membrane preparation by acting as a competitive inhibitor of the binding of the sodium channel opening agent veratridine, with an IC50 for this action of 6.5 microM. This is approximately one order of magnitude higher in concentration than that necessary for expression of the beta-adrenergic antagonism of propranolol. The binding of propranolol and its action to block sodium channels were demonstrably different from those of the neurotoxins tetrodotoxin and saxitoxin. Propranolol had effects on sodium channels that are similar, although not identical to those of the local anesthetics procaine and lidocaine. The concentrations of propranolol and a number of its analogues which produced 50% inhibition of 22Na+ uptake (IC50 values ranging from 4 to greater than 100 microM) were similar to the concentrations of these same analogues which were required to produce negative inotropic and antiarrythmic effects (ED40) on isolated rabbit atria [D. O. Rauls and J. K. Baker, J. med. Chem, 22, 81 (1979)]. These effects showed correlations of 0.945 and 0.936, respectively, with the 22Na+ uptake inhibition. It is concluded from this information that a substantial proportion of the negative inotropic and antiarrythmic effects of propranolol is due to its action on sodium channels.

Batrachotoxinin-A 20-alpha-benzoate: a new radioactive ligand for voltage sensitive sodium channels

Batrachotoxinin-A 20-alpha-benzoate (BTX-B), an analog of the potent depolarizing agent batrachotoxin (BTX), was prepared by selective esterification of naturally occurring batrachotoxin-A with benzoic acid. BTX-B depolarized rat phrenic nerve-diaphragm preparations with a time course and concentration dependence virtually indistinguishable from that of BTX. A specific, saturable component of equilibrium binding of [3H]BTX-B to mouse cerebral cortex homogenates was measured, described by an equilibrium dissociation constant of 0.7 microM and a maximum number of binding sites of 90 pmol per gram of tissue (wet weight). Specific binding is inhibited by BTX and other BTX analogs, veratridine, and grayanotoxin but is unaffected by tetrodotoxin and cevine. Under conditions of this assay, neither crude Leiurus quinquestriatus scorpion venom nor purified sea anemone toxin have any effect of specific binding. The data support the conclusion that BTX-B interacts with a recognition site associated with voltage sensitive sodium channels which is identical to the recognition site for BTX.

Characterization of the electrogenic sodium channel from rat brain membranes using neurotoxin-dependent 22Na uptake

The sodium channel was studied in osmotically-sensitive membrane preparations from rat brain and in innervated and chronically denervated rat soleus and extensor digitorum longus muscles. These experiments were undertaken in order to define a set of parameters for sodium channel function at the subcellular level to be used as a measure of retention of channel integrity upon subsequent isolation of the channel. Various neurotoxins and drugs were employed to control the permeability of the brain membranes to 22Na and the sodium-conductance properties of the muscles. Batrachotoxin (ED50 = 0.2 micrometer), veratridine (ED50 = 1 micrometer), or grayanotoxin I (ED50 = 30 micrometers) stimulated 22Na uptake in brain membranes is inhibited in an apparently uncompetitive manner by the sodium channel blocking agents tetrodotoxin and saxitoxin in a simple competitive manner by Ca2+ and in a partial or allosteric competitive manner by lidocaine and procaine. This 22Na uptake assay, which can be equated to a measure of equilibrium toxin binding, shows dependence on the concentration of the membranes and is sensitive to pH, temperature, ionic strength, and the ionic composition of the media. Parallel biophysical studies on sodium channels in rat muscle show that the properties of the sodium channel are similarly affected by these agents.

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.