Specific Sodium Channel Blocker Research Articles

Mode of action of an insecticidal peptide toxin from the venom of a weaving spider ( Diguetia canities)

A recently discovered spider toxin (DTX9.2) induced a rapid paralysis when injected into insects. In neurophysiological experiments, DTX9.2 elevated spike discharges in sensory nerves and at the neuromuscular junction, and also caused a depolarization of the membrane potential in cockroach giant axons. All these effects were reversed by treatment with the specific sodium channel blocker, tetrodotoxin. These findings suggest that DTX9.2 acts upon the voltage-dependent sodium channels of insect nerve membrane.

Endogenous bursting due to altered sodium channel function in rat hippocampal CA1 neurons

Intracellular recordings were obtained from pyramidal neurons in the rat hippocampal CA1 area in order to investigate membrane mechanisms involved in veratridine-induced epileptiform activity. Veratridine (0.03−0.2 μM) caused no changes in the passive membrane parameters including the resting potential, input resistance, and time constant. In the presence of small doses (0.03−0.1 μM) of veratridine, a single stimulus caused a relatively slow, large, synaptic-independent potential called the slow depolarizing after-potential (SDAP). When the hippocampal slice was treated with higher doses of veratridine (over 0.1 μM), bursting, or seizure-like activity (SLA) occurred in response to a brief super threshold intracellular stimulation. The duration of SLA bursting could be as long as ten seconds depending on the amplitude of SDAP, and was independent of the stimulus strength or duration. The frequency and configuration of SLA were sensitive to changes in membrane potential caused by applied DC current. At 0.3 μM or higher, veratridine induced spontaneous rhythmic bursting that was also sensitive to membrane potential changes. The evoked or spontaneous bursting is characterized by being: (1) independent of synaptic transmission in that it persisted after complete blockade of evoked synaptic potential with kynurenic acid (0.5 mM), (2) sensitive to selective inhibition by low doses of the specific sodium channel blockers tetrodotoxin (TTX) or cocaine with no apparent influence on the evoked action potential. These results indicate that endogenous SLA bursting can be induced in hippocampal CA1 pyramidal neurons when certain properties of sodium channels are altered by veratridine.

Muscarinic modulation of insulin secretion by single pancreatic β-cells

We have used a reverse hemolytic plaque assay and frequency distribution of immunoplaque areas to analyze the effect of carbachol (CCh, 100 nM), on insulin secretion by single pancreatic β-cells. The CCh effect was strongly dependent on the extracellular glucose concentration. Compared with the respective controls in each condition, when glucose was omitted from the incubation medium, CCh induced a 85% increase in the insulin secretion index. In 5.6 mM glucose, CCh induced a 100% increase in the insulin secretion index and this effect was characterized by (1) amplification of the response to glucose, and (2) recruitment of previously silent cells to secretory activity. However, at high glucose concentrations (20.6 mM), the insulin secretion index decreased 49%. CCh effects were blocked by atropine (1 μM). CCh effects were not uniform among β-cells. The functional subpopulation of β-cells with the highest secretion rate was preferentially affected by the muscarinic agonist. The specific sodium channel blocker tetrodotoxin prevented CCh-stimulated insulin secretion in basal media, suggesting that voltage-dependent sodium channels are involved in CCh stimulation-secretion coupling in single β-cells.

Role of activity in the selection of new electrical synapses between adult Helisoma neurons

Our aim was to determine whether neural activity in the form of sodium-dependent action potentials plays a role in the formation, maintenance and specificity of electrical synapses between regenerating neurons. We axotomized buccal neurons of the mollusc, Helisoma trivolvis, and placed ganglia into organ culture in the absence or presence of tetrodotoxin (TTX), a specific sodium channel blocker. Electrical coupling was measured using intracellular microelectrodes positioned within the soma of identified neurons. Neurite outgrowth was assessed by epifluorescence microscopy after filling neurons by iontophoresis with Lucifer yellow. Previous studies found that two days after axotomy transient electrical synapses form between heterologous neurons (e.g. buccal neurons 4 and 5). Five days after axotomy these transient connections disappeared and a new electrical synapse was stabilized between the paired buccal neurons 5. To determine whether blocking neural activity with TTX affected the specificity and formation of new electrical synapses, we examined electrical coupling between the heterologous neurons 4 and 5 two days after axotomy, and the paired buccal neurons 5 five days after axotomy. Our electrophysiological recordings indicated that different neurons in the buccal ganglion varied in their sensitivity to TTX (i.e. sensitivity of buccal neurons 19 > 5 > 4), but spontaneous activity was abolished in all 3 neurons by 2 × 10 −5 M TTX. Furthermore, the inhibitory effects of TTX occurred within seconds of superfusion and persisted for at least 6 days. Inhibition of activity by TTX could be reversed after superfusion with normal saline. Neurite outgrowth from axotomized neurons was not appreciably altered in the presence of TTX. Furthermore, no differences in the incidence of electrical coupling or the coupling resistance were detected between neurons 4 and 5 two days after axotomy and organ culture in the presence of TTX. However, electrical coupling between the symmetrically paired neurons 5 was elevated in the presence of TTX after 5 days. We conclude from these results that neural activity in the form of sodium-dependent action potentials does not play an important role in the formation or breaking of transient electrical synapses during neuronal regeneration in the mollusc Helisoma trivolvis.

Role of ion channels and intraterminal calcium homeostasis in the action of deltamethrin at presynaptic nerve terminals

Using a continuous perfusion system, synaptosomes prepared from rat brain released [ 3H]norepinephrine in a Ca 2+-dependent manner when pulse depolarized by briefly elevating external potassium concentrations. Tetrodotoxin (10 −7M), a sodium channel blocker, inhibited 48% of this pulsed release, and D595 (10 −5M), a phenethylamine-type calcium channel blocker, inhibited 21%. In combination, these two specific ion channel antagonists appear to function independently of each other in an additive fashion. Addition of deltamethrin to this preparation resulted in an enhanced release of [ 3H]norepinephrine which occurred in a biphasic fashion. At 10 −7M, deltamethrin produced a 42% enhancement in the first or initial peak of [ 3H]norepinephrine release and a 100% enhancement in the second or tailing peak. Addition of deltamethrin to tetrodotoxin-pretreated synaptosomes resulted in a net 37% enhancement of the initial peak release and a net increase of 277% in the tailing peak. Addition of deltamethrin to D595-pretreated synaptosomes produced no significant effect on enhanced [ 3H]norepinephrine release from either peak. Since tetrodotoxin is a specific sodium channel blocker, deltamethrin may be enhancing [ 3H]norepinephrine release by increasing the uptake of Ca 2 via other voltage-gated channels (e.g. calcium) or exchange mechanisms in addition to its action at voltage-gated sodium channels. To determine whether deltamethrin may also have an effect on intraterminal Ca 2+ homeostasis, external Ca 2+ was replaced with Ba 2+ and synaptosomes were depolarized with pentylenetetrazole (PTZ). At 10 −5 M, deltamethrin produced a 66% increase in neurotransmitter release over that produced by PTZ alone. An estimated ec 50 value of deltamethrin for PTZ-induced release was calculated to be 2.4 × 10 −10M.

Mexiletine-quinidine in isolated hearts: an interaction involving the sodium channel

Combination therapy with mexiletine and quinidine has been shown to be more effective than either monotherapy in treating patients with ventricular tachycardia. This enhanced efficacy was associated with prolongation of ventricular refractoriness and conduction time in the infarct zone. As sodium channel activity is a determinant of both conduction time and refractoriness we formed the hypothesis that the mexiletine-quinidine interaction was due at least in part to interactions involving the sodium channel. To assess the role of sodium channel blockade in the enhanced anti-arrhythmic activity of mexiletine-quinidine combination we determined whether the electrophysiological and anti-arrhythmic effects of tetrodotoxin combined with mexiletine or quinidine mimicked the effect seen with mexiletine combined with quinidine. Eighty isolated perfused rabbit hearts were treated with mexiletine, quinidine and tetrodotoxin either alone or in combination before and after circumflex occlusion-reperfusion. Ventricular fibrillation occurred in response to single extrastimuli in all 24 hearts treated with a saline control infusion. Combinations of mexiletine and quinidine at concentrations which alone had little electrophysiological activity produced anti-arrhythmic activity greater than that seen with high concentrations of mexiletine or quinidine alone. The combination of similarly low concentrations of tetrodotoxin and quinidine also produced enhanced anti-arrhythmic efficacy and enhanced prolongation of ventricular refractoriness and conduction which mimicked the effect of mexiletine and quinidine in combination. In contrast, the combination of mexiletine and tetrodotoxin did not produce enhanced anti-arrhythmic and electrophysiological activity. Since tetrodotoxin is a highly specific sodium channel blocker, these data suggest that the enhanced antiarrhythmic activity of mexiletine-quinidine combination therapy involves, at least in part, blockade of the cardiac sodium channel.

Frequency- and Voltage-Dependent Effects of Recainam on the Upstroke Velocity of Action Potential in Rabbit Ventricular Muscle

The effects of recainam (Wy 42,362) on transmembrane action potentials were examined in isolated rabbit right ventricular papillary muscles. Recainam (3 x 10(-5) to 3 x 10(-4) M) caused a concentration-dependent decrease in the Vmax of the action potential. At 3 x 10(-4) M, there was a slight decrease in the amplitude of the action potential. The resting potential and the action potential duration were not affected. Use-dependent block of Vmax was tested over a wide range of pacing frequencies (from 0.1 to 3.0 Hz). At 1.0 Hz, recainam 10(-4) M produced exponential decreases in Vmax with a rate constant of 0.17 per action potential and 39.8% reduction at steady state. This use-dependent block was augmented at the higher stimulation frequencies. The time constant for the recovery of Vmax from use-dependent block (offset) was 17.2 s. In papillary muscles depolarized with 10 mM [K+]0, the use-dependent block was augmented but tonic block and the rates of onset and offset of the use-dependent block were similar to those in normally polarized preparations in 4 mM [K+]0. The curves relating membrane potential and Vmax in preparations stimulated at a low frequency (0.01 Hz) were not shifted by 10(-4) M recainam. These findings suggest that recainam is a specific sodium-channel blocker and has kinetically slow but potent affinity for the channel during action potentials. This selective binding during action potential was further augmented by depolarization and is likely to play a significant role in the control of ventricular arrhythmias by the drug.

Frequency- and Voltage-Dependent Effects of Recainam on the Upstroke Velocity of Action Potential in Rabbit Ventricular Muscle

Summary: The effects of recainam (Wy 42,362) on transmembrane action potentials were examined in isolated rabbit right ventricular papillary muscles. Recainam (3 × 10−5 to 3 × 10−4M) caused a concentration-dependent decrease in the JOURNAL/jcph/04.03/00005344-198904000-00017/OV0312/v/2021-02-06T190858Z/r/image-pngmax of the action potential. At 3 × 10−4M, there was a slight decrease in the amplitude of the action potential. The resting potential and the action potential duration were not affected. Use-dependent block of JOURNAL/jcph/04.03/00005344-198904000-00017/OV0312/v/2021-02-06T190858Z/r/image-pngmax was tested over a wide range of pacing frequencies (from 0.1 to 3.0 Hz). At 1.0 Hz, recainam 10−4M produced exponential decreases in JOURNAL/jcph/04.03/00005344-198904000-00017/OV0312/v/2021-02-06T190858Z/r/image-pngmax with a rate constant of 0.17 per action potential and 39.8% reduction at steady state. This use-dependent block was augmented at the higher stimulation frequencies. The time constant for the recovery of JOURNAL/jcph/04.03/00005344-198904000-00017/OV0312/v/2021-02-06T190858Z/r/image-pngmax from use-dependent block (offset) was 17.2 s. In papillary muscles depolarized with 10 mM [K+]0, the use-dependent block was augmented but tonic block and the rates of onset and offset of the use-dependent block were similar to those in normally polarized preparations in 4 mM [K+]0. The curves relating membrane potential and JOURNAL/jcph/04.03/00005344-198904000-00017/OV0312/v/2021-02-06T190858Z/r/image-pngmax in preparations stimulated at a low frequency (0.01 Hz) were not shifted by 10−4M recainam. These findings suggest that recainam is a specific sodium-channel blocker and has kinetically slow but potent affinity for the channel during action potentials. This selective binding during action potential was further augmented by depolarization and is likely to play a significant role in the control of ventricular arrhythmias by the drug.

Effects of ozone on airway epithelial permeability and ion transport

Hartley guinea pigs had a 3-h sham exposure or a 3-h exposure to 1.0 ppm ozone. Within 2 h tracheas were excised and mounted in conventional Ussing chambers. Potential difference (PD), short circuit current (I sc), conductance (G) and unidirectional fluxes of 22Na, 36Cl and [ 14C]mannitol were measured in blinded experiments. Compared to the tracheal epithelia of sham-exposed animals, those from exposed animals had a larger PD (45%), an increased I sc (41%) and no change in G. In both groups, the I sc was approximately equivalent to net sodium absorption. The higher current in the exposed animal's tissues was disproportionately more sensitive to inhibition by the specific sodium channel blocker amiloride (44% vs. 13%). These observations suggest that early consequences of exposure of airway epithelium to ozone include increases in the density or open time of sodium channels in the luminal cell membrane or a change in sodium pump activity that increases the gradient for sodium entry across the luminal cell membrane. By 24 h after exposure these changes had been obscured by an increase in epithelial permeability, perhaps as the result of an inflammatory response to the ozone exposure.

Effect of ion channel blockers on germination ofBacillus megateriumspores

We surveyed 23 drugs that can interact with membrane components, such as ion channels, for their effect on spore germination. The results showed that triggering of spore germination was inhibited by specific calcium (Ca2+) potassium (K+) and sodium (Na+) channel blockers.

Effect of ion channel blockers on germination of Bacillus megaterium spores

We surveyed 23 drugs that can interact with membrane components, such as ion channels, for their effect on spore germination. The results showed that triggering of spore germination was inhibited by specific calcium (Ca2+) potassium (K+) and sodium (Na+) channel blockers.

Comparison between bretylium and diphenylhydantoin interaction with mucosal sodium-channels

The antifibrillatory drug bretylium and the antiepileptic drug diphenylhydantoin cause an increase in the potential different and in the short-circuit current (SCC) across frog skin when added to the outer surface. The effect of both drugs depends upon the presence of sodium ions in the outer medium and is blocked by the specific sodium channel blocker, amiloride. Quantitative analysis shows that amiloride binds to open as well as closed mucosal sodium channel with the same affinity. The effects of diphenylhydantoin and bretylium differ with respect to their dependence on external pH. The diphenylhydantoin or the bretylium stimulatory effects are additive to the effects of oxytocin. In most cases the diphenylhydantoin and bretylium effects are also additive. It is concluded that the external side of the mucosal Na + channels contains sites which interact specifically with either bretylium or diphenylhydantoin and thus remove the sodium induced closure of the channels.

Bretylium opens mucosal amiloride-sensitive sodium channels

Addition of the quanternary ammonium compound, bretylium, to the outer surface of a frog skin leads to an increase in the potential difference and in the short circuit current across the skin. Bretylium does not have any effect when applied to the inside face of the frog skin. The effect of bretylium is dependent upon the presence of sodium ions in the outer medium; it is depressed when sodium is replaced by choline or potassium but not when lithium substitutes for sodium. The bretylium effect is blocked by the specific sodium channel blocker, amiloride. It is proposed that bretylium opens mucosal, amiloride-sensitive sodium channels.