Effects of lindane (gamma-hexachlorocyclohexane) on rat heart muscle contraction.

The effects of micromolar concentrations of lindane on the mechanical activity of cardiac left ventricular papillary muscles were studied in adult female rats. Lindane decreased the amplitude and duration of the contraction, and slowed down the time course of its ascending phase (i.e. decreased the maximum rate of rise of the initial phase (dC/dt(max))). Both amplitude and duration of the contraction, but not dC/dt(max), were restored by subsequent application of the rapid delayed outward K(+) current (I(Kr)) blocker E-4031 (10 nmol/l). Increasing the stimulation frequency from 1 to 3.3 Hz in the control solution produced a decrease in the amplitude of the first beat peak contraction while a slow recovery phase (srp) developed, as the result of the Na(+)-Ca(2+) exchanger activity. When the frequency was restored to 1 Hz, a post rest potentiation (prp) with a negative staircase (ns) developed due to the sarcoplasmic reticulum (SR) Ca(2+) refilling. Lindane increased the amplitude of both srp and prp, but did not affect ns, which indicates that SR Ca(2+) refilling was not altered by the pesticide. In conclusion, the results strongly suggest that some of the lindane-induced negative inotropic and chronotropic-like effects on the contraction are due to an increased I(Kr) while the decrease in dC/dt(max) (i.e. the rate of cross-bridge formation) results from lindane oxidative properties.

Micrometer scale ex vivo multiphoton imaging of unstained arterial wall structure.

BACKGROUND: We characterize the application of multiphoton microscopy to the observation of the extracellular matrix of fresh unstained vessels. METHOD: Combined two-photon-excited fluorescence (2PEF) and second harmonic generation (SHG) imaging of large arteries reveals the architecture of elastin and collagen fibers in the vessel wall with remarkable specificity. RESULTS: We present elastin/collagen imaging in unstained rat vessels at both micrometer and whole vessel scales, and we characterize the optical properties of rat carotid artery and aorta walls. We apply this method to evidence deleterious effects of residual doses of a pesticide on the vessel wall. CONCLUSION: This study illustrates the potential of 2PEF/SHG microscopy for pharmacological studies in unlabeled arteries.

Mechanical and electrophysiological effects of thiopental on rat cardiac left ventricular papillary muscle.

Thiopental induces a negative inotropic effect on mammalian heart muscle, where it decreases Ca2+ current and Ca2+ release from the sarcoplasmic reticulum and reduces K+ currents. We analysed the effects of thiopental on the mechanical and electrical activities of rat myocardium, which differ markedly from those of other mammals. The effects of thiopental on mechanical parameters and on the transmembrane resting (RP) and action (AP) potentials of rat left ventricular papillary muscle were investigated. These effects were also studied in the presence of atenolol, a beta-blocking agent, and 4-aminopyridine (4-AP), a blocker of the transient outward K+ current. Thiopental (3.8 x 10(-6), 3.8 x 10(-5) and 1.1 x 10(-4) M) induced a dose-dependent positive inotropic effect. This positive inotropic effect persisted in the presence of atenolol (1 x 10(-6) M) but did not develop in the presence of 1 mM 4-AP; 4-AP had a positive inotropic effect but not in the presence of thiopental. Moreover, thiopental (3.8 x 10(-5) M) lengthened the plateau and the slow repolarizing phase of the AP, while 1 mM 4-AP only prolonged the plateau duration. In rat myocardium, the positive inotropic effect of thiopental in part mimics that of 4-AP, and in part may be explained by the lengthening of the slow repolarizing phase of the AP.

Alterations of transmembrane currents in frog atrial heart muscle induced by photoexcited gymnochrome A purified from the crinoid, Gymnochrinus richeri.

The effects of gymnochrome A were tested on the electrical activity of the frog atrial heart muscle. Gymnochrome A (1-5 microM) did not alter the resting potential. Gymnochrome A (5 microM) slowed the initial depolarizing phase of the spontaneously beating action potential. Under voltage-clamp conditions gymnochrome A (5 microM) did not affect the electrical constant of the membrane and the kinetic parameters of the peak Na+ current (INa) recorded in the Ringer solution containing tetraethylammonium (2 mM) and Cd2+ (1 mM) but shifted the membrane potential at which the current both activated and reached its maximal value toward more negative membrane potentials. It did not alter the reversal potential for INa, indicating that the selectivity of the Na+ channels had not changed. These observations suggest that gymnochrome A binds to the membrane and shifts the activation of INa on the voltage axis by modifying the free negative fixed charges present at the membrane surface rather than by occupying a specific site on the Na+ channel. Photoexcited gymnochrome A transiently triggered an early outward current which lengthened the time-to-peak of INa and decreased its amplitude. In addition, photoexcited gymnochrome A blocked the background K+ current. This is, to our knowledge, the first time that such effects are reported on the cardiac muscle. These observations suggest that the photoexcitation of gymnochrome produces physico-chemical effects which lead to intracellular changes. Further experiments are required to determine their nature.

Cardiovascular effects of lepadiformine, an alkaloid isolated from the ascidians Clavelina lepadiformis (Müller) and C. moluccensis (Sluiter).

The effects of lepadiformine, a natural marine alkaloid isolated from the ascidians Clavelina lepadiformis (Müller) and C. moluccensis (Sluiter), were studied in vivo by arterial blood pressure (aBP) recordings and electrocardiograms (ECG) in anaesthetised rats and in situ by peripheral vascular pressure recordings on perfused rabbit ear. Transmembrane resting (RP) and action (AP) potentials were also recorded by intracellular microelectrodes on electrically stimulated left ventricular papillary muscle and spontaneously beating atrium isolated from rat and frog hearts, respectively. Intravenous injection of lepadiformine (6mg/kg) produced marked bradycardia and a lengthening of ECG intervals as well as a transient decrease of aBP, which rapidly returned to normal. The decrease of aBP may have been related to a vasoconstrictor effect observed in the perfused ear experiment. Lepadiformine did not alter RP, but significantly lengthened the repolarising phase of AP in rat papillary muscle and frog atrium. Lepadiformine also mimicked the effect of Ba(2+) (0.2mM) on the rat AP repolarising phase. Moreover, the lengthening of the AP in frog atrium induced by lepadiformine still developed after the delayed outward K(+) current (I(K)) was blocked by tetraethylammonium (10mM). These observations suggest that lepadiformine-induced lengthening of AP duration was not due to a decrease of I(K), but may reasonably be attributed to a reduction of the inward rectifying K(+) current (I(K1)). This blockade of I(K1) could account for the cardiovascular effects of lepadiformine in vivo and in vitro and suggests that lepadiformine has antiarrhythmic properties.

Effects of halothane on the membrane potential in skeletal muscle of the frog.

Halothane has many effects on the resting membrane potential (V(m)) of excitable cells and exerts numerous effects on skeletal muscle one of which is the enhancement of Ca(2+) release by the sarcoplasmic reticulum (SR) resulting in a sustained contracture. The aim of this study was to analyse the effects of clinical doses of halothane on V(m), recorded using intracellular microelectrodes on cleaned and non stimulated sartorius muscle which was freshly isolated from the leg of the frog Rana esculenta. We assessed the mechanism of effects of superfused halothane on V(m) by the administration of selective antagonists of membrane bound Na(+), K(+) and Cl(-) channels and by inhibition of SR Ca(2+) release. Halothane (3%) induced an early and transient depolarization (4.5 mV within 7 min) and a delayed and sustained hyperpolarization (about 11 mV within 15 min) of V(m). The halothane-induced transient depolarization was sensitive to ryanodine (10 microM) and to 4-acetamido-4'-isothiocyanatostilbene 2,2' disulphonic acid (SITS, 1 mM). The hyperpolarization of V(m) induced by halothane (0.1 - 3%) was dose-dependent and reversible. It was insensitive to SITS (1 mM), tetrodotoxin (0.6 microM), and tetraethylammonium (10 mM) but was blocked and/or prevented by ryanodine (10 microM), charybdotoxin (CTX, 1 microM), and glibenclamide (10 nM). Our observations revealed that the effects of halothane on V(m) may be related to the increase in intracellular Ca(2+) concentration produced by the ryanodine-sensitive Ca(2+) release from the SR induced by the anaesthetic. The depolarization may be attributed to the activation of Ca(2+)-dependent Cl(-) (blocked by SITS) channels and the hyperpolarization to the activation of large conductance Ca(2+)-dependent K(+) channels, blocked by CTX, and to the opening of ATP-sensitive K(+) channels, inhibited by glibenclamide.

Effects of trachynilysin, a protein isolated from stonefish (Synanceia trachynis) venom, on frog atrial heart muscle.

The effects of trachynilysin (TLY), a protein toxin isolated from stonefish (Synanceia trachynis) venom, were studied on the electrical and mechanical activities of frog atrial fibres. TLY (1 microg/ml) hyperpolarized the membrane, shortened the action potential (AP) duration (APD), exerted a negative inotropic effect and elicited contracture. These effects did not develop in the presence of atropine. TLY shortened the APD of fibres isolated from a frog completely paralyzed with botulinum type A toxin, in the presence of Ca2+ but not when Ca2+ was replaced by Sr2+. TLY increased the basal and the peak of the fluorescence ratio of stimulated fibres loaded with fura-2. Confocal laser scanning microscopy revealed the existence of a diffuse innervation in atrial tissue. Our results suggest that TLY enhances the release of acetylcholine from atrial cholinergic nerve terminals and activates indirectly muscarinic receptors leading to a shortening of APD. They also show that the mechanical effects induced by TLY are due to an increase of the Ca2+ influx and to a rise in intracellular Ca2+ levels which leads to (i) a slowing of the Na+/Ca2+ exchange activity, which accounts for the contracture and (ii) the activation of a Ca2+-dependent K+ current involved in the APD shortening.

Toxicity of French strains of the dinoflagellate Prorocentrum minimum experimental and natural contaminations of mussels.

Mediterranean strains of Prorocentrum minimum do not appear to have the same toxic component as Japanese strains since they showed no cytotoxicity for hepatocytes in culture. However, their toxic components, which appear to block calcium channels, were detectable by the immobilisation test on Diptera larvae. A bio-accumulation experiment in the laboratory showed that the toxins could accumulate in nearly equivalent amounts in the hepatopancreas and meat of cultured mussels. The same toxicity was found in natural samples collected in a period of bloom of P. minimum. These results suggest that P. minimum could be responsible for shellfish toxicity in the natural environment and thus present a risk for human health.

[Muscarinic modulation of cardiac activity]. / Modulation muscarinique de l'activite cardiaque.

The goal of the present review is to report information concerning cardiac innervation or more precisely to approach the modulation of cardiac electrical and mechanical activity by parasympathetic innervation. Acetylcholine (ACh) release by nerve endings from the vagus nerve hyperpolarizes the membrane, shortens action potential (AP) duration and has a negative inotropic effect on cardiac muscle. Toxins are usefull tools in the study of membrane signals. The Caribbean ciguatoxin (C-CTX-1) has a muscarinic effect on frog atrial fibres. The toxin evokes the release of ACh from motoneuron nerve terminals innervating this tissue which allows us to propose a model, similar to the one of the neuromuscular junction (nmj), to describe the events occurring during the triggering and release of ACh. Trachynilysin (TLY) is a proteic toxin which causes an influx of Ca2+ into the cells and releases ACh from nmj synaptic vesicles. TLY has a muscarinic effect on atrial fibres which is explicated in the release of neurotransmitter from the nerve endings generated by the TLY-induced Ca2+ influx. It is known that ACh release from nmj is known to be due to exocytosis of synaptic vesicles via the activation of a proteic complex blocked by botulinum toxins. One of these proteins SNAP-25 is the target of type A botulinum toxin (BoNT/A). The study of hearts isolated from BoNT/A poisoned frogs show that atrial AP is lengthened and reveals the presence of SNAP-25 in nerve endings of this tissue. Moreover, the electrical activity of ventricular muscle is markedly altered; in BoNT/A treated frog, an important outward current activated by internal Ca2+ develops. ACh released from nerve terminals binds to a G protein coupled membrane receptor and activates a K+ channel and other effectors. Five subtypes of muscarinic receptors have been cloned from different tissue (M1, M2, M3, M4) subtypes have been identified in cardiac tissues throughout many species. These receptors coupled with different G-proteins activate different effectors. M1 receptors modulate the cardiac plateau and therefore the magnitude of the peak contraction. M2 receptors are mainly involved in the repolarization phase of the AP and modulate the duration of the peak contraction. The roles of M3 and M4 are not yet clearly defined; however, they may activate K+ currents. In conclusion, ACh releases from parasympathetic nerve endings which innervate cardiac cells follows to similar events (Ca2+ influx; presence of a SNAP-25 protein) to those which produce ACh release from nmj, stimulates different G proteins coupled muscarinic receptors, and activates different effectors involved in the modulation of cardiac electrical and mechanical activity.

[Effect of ciguatoxins on the cardiocirculatory system]. / Effet des ciguatoxines sur le système cardio-circulatoire.

The aim of the present review was to collect the main observations reported until now concerning the cardio-circulatory effects of polyether toxins, called ciguatoxins, which are involved in an endemic intoxication named ciguatera found in tropical and subtropical countries. Ciguatera is caused by the ingestion of fishes contaminated with the dinoflagellate Gamberdiscus toxicus. Due to both tropical fish exportation destined for food and tourism, the disease has now spread out to temperate areas. Several toxins have been isolated and purified from different fish species living in different geographical areas. They are classified into three main groups by the nature of certain cycles of their carbon skeleton. Clinical reports show evidence that ciguatera intoxication affect both electrocardiograms and blood pressure. In most cases, ciguateric intoxication mainly evoked bradycardia, hypotension, and the alteration of S-T segment in the electrocardiogram. Isolated and purified ciguatoxins strongly altered the morphology of cardiac tissue inducing swelling of the cells and alterations of cellular organelles. These toxins impair the conduction of cardiac nerves and increase the opening probability of Na+ channels in intracardiac ganglions. Depending on the concentration applied, the substances exerted either a fast positive inotropic effect or a negative inotropic effect on the contraction of mammalian atrial and ventricular cardiac muscle. These effects were attributed to a release of noradrenaline and acetylcholine from neural terminals of the autonomic nervous system present in cardiac tissue. They also exert a slow delayed inotropic effect on the contraction which has been attributed to a direct effect of the toxins on tetrodotoxin-sensitive voltage-dependent Na+ channels of cardiac membranes. Ciguatoxins depolarized the membrane of mammalian atrial and ventricular preparations and shifted the threshold of sodium current activation to more negative membrane potentials. In conclusion, the inotropic effects of ciguatoxins on cardiac tissues mainly depend on the toxin concentration sensitivity of autonomic nerve terminals, which released noradrenaline and/or acetylcholine, while the ciguatoxin-induced increase of the sodium influx could be involved in the cardiac cell swelling which coincides with reports in which ciguatoxins induced a mannitol-inhibited swelling of the Node of Ranvier.

The effects of gadolinium, a stretch-sensitive channel blocker, on diaphragm muscle.

Stretch-activated channels (SAC) have been identified in many cell types including striated muscles. In diaphragm muscle, the influence of SAC on the length-active tension relationship remains unknown. Patch clamp experiments were performed on single fibres (n = 10). In isolated diaphragm muscle from adult hamsters, the effects of gadolinium (Gd3+), the most potent inhibitor of SAC blocker, on tension response to stretch at baseline were studied (n = 10), after pretreatment of the muscle with 1 nmol isoproterenol (n = 10), 0.5 micromol forskolin (n = 6), or 0.1 mmol dibutyryl cyclic adenosine monophosphate (cAMP) (n = 10). Results were compared to those obtained in low [Na+]e (n = 10), Ca2+-free medium (n = 6) or after 5 micromol nifedipine (n = 8). Gd3+ reduced active tension measured over a range of initial muscle lengths in a concentration-dependent manner (10 and 50 micromol). In isolated fibres, mechanical stretch generated a membrane current that was sensitive to Gd3+. In muscles, lowering [Na+]e mimicked the effects of Gd3+, while no change in the length-tension relationship was observed in Ca2+-free medium or after nifedipine. Drugs which increase cAMP prevented the effects of Gd3+ on active tension. In the diaphragm, gadolinium-sensitive channels are activated during physiological changes in length and influence tension development. Moreover, cyclic adenosine monophosphate content modulates the effects of gadolinium on stretch-activated channels.

Cardiotoxicity of verrucotoxin, a protein isolated from the venom of Synanceia verrucosa.

Freshly purified but unstable verrucotoxin (VTX) and a more stable proteic complex of the toxin (p-VTX) were isolated from the venom of the stonefish Synanceia verrucosa and applied to frog atrial fibres. VTX and p-VTX decreased the amplitude and the duration of the stimulated peak tension and accelerated the relaxation phase of the contraction. The negative inotropic effect of p-VTX decreased with increasing the external Ca concentration ([Ca]o) in the Ringer solution. The negative chronotropic effect induced by p-VTX was insensitive to change in [Ca]o. It is reversed by glibenclamide. p-VTX shortened the duration of the plateau and the repolarizing phase of the action potential. Glibenclamide but not tetraethylammonium reversed the p-VTX-induced shortening of the AP repolarizing phase. The data suggest that the toxin isolated from the venom of S. verrucosa inhibits Ca channels and might activate ATP-sensitive potassium channels in frog atrial heart muscle.

[Effect of neurotoxins on the electrical activity and contraction of the heart muscle]. / Effet des neurotoxines sur I'activité électrique et la contraction du muscle cardiaque.

The goal of the present review is to report the effect of the main neurotoxins known on the electrical and mechanical activity of heart muscle. Toxins which block the sodium channel (tetrodotoxin, saxitoxin) shorten the action potential (AP) duration and decrease the initial depolarizing phase of the AP. Toxins which occupy different sites in the channel and alter the gating mechanisms of the Na channel (aconitine, batrachotoxin, veratridine, sea anemone and scorpion toxins, brevetoxin and ciguatoxin) depolarize, lengthen the AP duration, increase the contraction and cause arrhythmias. Ca channel agonists (atrotoxin, maitotoxin) increase the amplitude of the cardiac plateau. Ca channel antagonists (TCX, omega-conotoxin) decrease the magnitude of the plateau and exert a negative inotropic effect. Okadaic acid increases the Ca current leading to an increase in the plateau amplitude and a lengthening in the AP duration and the development of a positive inotropic effect on the contraction. Toxin affecting voltage-dependent K channels on heart muscle and the actual knowledge concerning the effect and the mode of action of palytoxin have also been reviewed. It is concluded that toxins, used as tools to analyze and characterize the structure and the function of ionic channels involved in the development of the electrical activity of excitable cells exhibit numerous effects on cardiac muscle. Some of these effects might not only be due to a direct action of these substances on membrane channels but they might also be the result of the release of neuromediators from nervous endings surrounding cardiac cells.

Effect of an unknown toxin isolated from Dinophysis sp.-contaminated French mussels, on the electrical and mechanical activity of frog heart.

The effects of an unknown toxin, isolated along with okadaic acid from the hepatopancreas of French mussels contaminated by Dinophysis sp., producing ataxia, neurologic symptoms, bradycardia, arrhythmias, electrocardiographic changes, and cardiac arrest, have been studied in terms of the electrical and mechanical activity of frog atrial heart muscle. The toxin, in a dose-dependent manner, decreased the amplitude of the stimulated peak tension of isolated fibers. The toxin (1-36 micrograms/ml) did not modify the membrane resting potential but decreased the amplitude of the plateau and shortened the duration of the action potential. The toxin inhibited the Cd-sensitive L-type Ca current and increased a 4-aminopyridine-sensitive outward current in voltage-clamped cardiac myocytes. The data show that the cardiac effect of the toxin is markedly different from that of okadaic acid.

Does crude venom of the stonefish (Synanceia verrucosa) activate beta-adrenoceptors in the frog heart muscle?

Venom isolated from the stonefish Synanceia verrucosa was assayed in concentrations of 0.07 and 5.7 micrograms/ml on frog atrial fibres and myocytes. Venom, less than 2.9 micrograms/ml, dose-dependently increased the amplitude and the duration of the stimulated peak tension, lengthened the time constant of the relaxation phase and shortened the duration of the action potential (AP). The concentration of venom 5.7 micrograms/ml decreased the amplitude of the peak tension, induced a contracture, reduced the amplitude of the plateau and shortened its duration as well as the repolarizing phase of the AP. The positive inotropic effect induced by the venom (2.9 micrograms/ml) on the contraction was inhibited dose-dependently by propranolol but was unchanged by the alpha-adrenergic antagonists urapidil and yohimbine, the adenyl cyclase activity remaining sensitive to forskolin. Venom, adrenalin and propranolol competed for a common site. Venom (2.9 micrograms/ml) increased both the Ca and the delayed outward K currents of enzymatically isolated atrial myocytes. The data suggest that the venom activates adrenoceptors, essentially beta-adrenoceptors.

Pachymatisma johnstonii extract opens ATP-sensitive potassium channels in cardiac myocytes of the frog heart.

The effects of a proteic extract (PachyTX), isolated from the sponge Pachymatisma johnstonii, were studied respectively with intracellular microelectrode and the "whole cell" patch clamp techniques on fibers and myocytes isolated from the sinoatrial region of frog heart. PachyTX significantly and reversibly shortened the action potential duration and decreased the stimulated peak contraction. Under voltage clamp, PachyTX increased, in a dose-dependent manner, a tetraethylammonium-insensitive but glibenclamide-inhibited outward current. This current failed to develop when adenosine 5'-triphosphate (ATP) was intracellularly applied to the cell by diffusion through the patch electrode. PachyTX mimics the effect of the ATP-sensitive K channel opener SR 44866 on frog atrial action potential and peak contraction. From this evidence, it can be concluded that PachyTX is a new natural potent opener of ATP-sensitive K channels in frog atrial heart muscle and that the opening of these channels is associated with the inhibition of the contraction of cardiac muscle.

Influence of Bistramide A on the twitch tension in rat atrial heart muscle.

Bistramide A, a new toxin isolated from the Urochordate Lissoclinum bistratum Sluiter, was applied to rat auricular heart muscle bundles. At a stimulation frequency of 0.2 Hz, the toxin induces a dose-dependent reduction of the stimulated twitch tension force; it decreases Vmax and shortens the duration of the plateau and the slow repolarizing phase of the action potential. In the control solution, switching from a stimulation frequency of 0.2 Hz to 1 Hz decreases the force with which a positive potentiation develops either at a maintained high frequency or after switching from 1 Hz to 0.2 Hz. Bistramide A reduces both the force evoked at 1 Hz and the potentiation. The data suggest that Bistramide A blocks Na+ conductance; inhibits Ca++ channels in a time- and frequency-dependent manner; reduces Na(+)-Ca++ exchange activity; but does not modify the ability of the sarcoplasmic reticulum to be refilled although the rate of Ca++ accumulation is decreased.

Alteration of the voltage-dependence of the twitch tension in frog skeletal muscle fibres by a polyether, Bistramide A.

Bistramide A, a toxin isolated from Lissoclinum bistratum Sluiter, was applied to frog skeletal muscle fibres. Micromolar concentrations of toxin decreased the amplitude of the twitch tension. Bistramide A shifted the activation and the steady-state inactivation characteristics of the tension on the voltage axis towards more positive and negative membrane potentials respectively. The data suggest that Bistramide A binds to the activated and inactivated state of the dihydropyridine-receptor which senses the T-tubule membrane potential.

The polyether Bistramide A affects the calcium sensitivity of the contractile proteins in frog atrial heart muscle.

The effects of Bistramide A, a new toxin isolated from the Urochordate Lissoclinum bistratum Sluiter have been studied on the mechanical activity of frog heart atrial muscle preparations. The peak tension of isolated trabeculae was sensitive to nanomolar concentrations of Bistramide A. Lineweaver-Burk relationships suggest that Bistramide A competes with Ca for a common site. In voltage-clamped trabeculae, the toxin inhibited both the cadmium-sensitive Ca current and the phasic component of the tension with a dissociation constant of 3.3 microM and a stoichiometry of 2. Bistramide A decreased the isometric tension of skinned fibres in a dose-dependent manner with a dissociation constant of 400 nM and a stoichiometry of 2. The toxin reduced the maximum Ca activated force and decreased the sensitivity of the contractile proteins to Ca. The data suggest that Bistramide A decreases the Ca-sensitivity of contractile proteins prior to blocking the Ca current.

Blockade of sodium channels by Bistramide A in voltage-clamped frog skeletal muscle fibres.

The effect of Bistramide A, a toxin isolated from Bistratum lissoclinum Sluiter (Urochordata), on the peak sodium current (INa) of frog skeletal muscle fibres was studied with the double sucrose gap voltage clamp technique. External or internal application of Bistramide A inhibited INa without alteration of the kinetic parameters of the current nor of the apparent reversal potential for Na. The steady-state activation curve of INa was unchanged while the steady-state inactivation curve of INa was shifted towards more negative membrane potentials. Dose-response curves indicated an apparent dissociation constant for Bistramide A of 3.3 microM and a Hill coefficient of 1.2 which suggested a one to one relation between the toxin and Na channel. The inhibition of INa occurred at rest, and was more important at more positive holding potentials. Bistramide A exhibited only a weak frequency-dependent effect. The toxin did not interact with the use-dependent effect of lidocaine. It mainly blocked Na channels at more depolarized holding potentials. The toxin blocked Na channels when it was internally applyed and when the inactivation gating system has been previously destroyed by internal diffusion of iodate. The data suggest that Bistramide A inhibited the Na channel both at rest and in the inactivated state and occupied a site which was not located on the inactivation gate.