Anesthetic and calcium action in the voltage-clamped squid giant axon

Tetrodotoxin: a brief history

Tetrodotoxin (TTX), contained in puffer, has become an extremely popular chemical tool in the physiological and pharmacological laboratories since our discovery of its channel blocking action in the early 1960s. This brief review describes the history of discovery of TTX action on sodium channels, and represents a story primarily of my own work. TTX inhibits voltage-gated sodium channels in a highly potent and selective manner without effects on any other receptor and ion channel systems. TTX blocks the sodium channel only from outside of the nerve membrane, and is due to binding to the selectivity filter resulting in prevention of sodium ion flow. It does not impairs the channel gating mechanism. More recently, the TTX-resistant sodium channels have been discovered in the nervous system and received much attention because of their role in pain sensation. TTX is now known to be produced not by puffer but by bacteria, and reaches various species of animals via food chain.(Communicated by Masanori OTSUKA, M.J.A.).

Action of external divalent ion reduction on sodium movement in the squid giant axon

Voltage clamp measurements of the sodium potential have been made on the resting squid giant axon to study the effect of variations in external divalent ion concentration upon net sodium flux. From these measurements the intracellular sodium concentration and the net sodium inflow were calculated using the Nernst relation and constant activity coefficients. While an axon bathed in artificial sea water shows a slow increase in internal sodium concentration, the rate of sodium accumulation is increased about two times by reducing external calcium and magnesium concentrations to 0.1 times their normal values. The mean inward net sodium flux increases from a mean control value of 97 pmole/cm² sec. to 186 pmole/cm² sec. in low divalent solution. Associated with these effects of external divalent ion reduction are a marked decrease in action potential amplitude, little or no change in resting potential, and a shift along the voltage axis of the curve relating peak sodium conductance to membrane potential similar to that obtained by Frankenhaeuser and Hodgkin (1957). These results implicate divalent ions in long term (minutes to hours) sodium permeability.

Interactions of calcium with sodium and potassium in membrane potentials of the lobster giant axon

Experiments were performed on the lobster giant axon to determine the relation between intracellular spike amplitude and external calcium ion concentration. Action potential decline in low external calcium is greatly accelerated by simultaneous removal of external sodium ion. Correlation of the time course of spike decline in low calcium-low sodium solution with the time courses of spike decline in low calcium alone and in low sodium alone indicates that the effect of simultaneous removal of both ions is significantly greater than the sum of the individual effects. For a given time of treatment, spike amplitude was a function of external calcium concentration. While spike height is proportional to the log of the external calcium concentration over the range 2.5 to 50 millimolar, the proportionality constant is dependent upon the sodium concentration. Under the conditions of low external sodium (50 per cent reduction) the slope of the linear relationship between the spike height and the log of the external calcium concentration is about 5 times greater than in normal external sodium. Decreasing external calcium concentration and simultaneously increasing external potassium concentration produce a greater spike reduction than the arithmetic sum of spike reductions in low calcium alone and in high potassium alone. It is suggested that calcium interacts strongly with sodium and potassium in the spike-generating mechanism. A theoretical basis for these results is discussed.

Action of the calcium antagonists cocaine and ethanol on contraction and potassium efflux of smooth muscle

Isolated longitudinal smooth muscle from guinea pig ileum exposed to a high potassium depolarizing medium exhibited a sustained increase in muscle tone and an increase in potassium efflux. When the concentration of calcium ion in the medium was elevated the increase in muscle tone was enhanced, but the change in potassium efflux was reduced slightly. Lowering the calcium concentration diminished the increase in muscle tone. Both cocaine and ethanol completely inhibited the sustained contraction of potassium-depolarized fibers. Addition of excess calcium ion reversed these inhibitions. Cocaine acted primarily like a competitive antagonist; and ethanol, like an indirect antagonist of calcium, ion. Under certain conditions acetylcholine potentiated the reversal by calcium ion of the drug-induced inhibitions. The two inhibitory drugs had dissimilar effects on potassium efflux from smooth muscle fibers immersed in Tyrode solution. Cocaine depressed and ethanol enhanced this membrane process. However, the increase in potassium efflux induced by acetylcholine was inhibited by ethanol. This inhibition also was reversed by increasing the concentration of calcium ion in the medium. The data suggested that calcium activates and cocaine and ethanol inhibit a cellular reaction which occurs beyond the point of membrane depolarization and is essential for smooth muscle contraction. Furthermore, calcium serves to depress membrane excitability, but appears to have a specific stimulatory role in the acetylcholine-induced increase in potassium efflux from longitudinal fibers.

INHIBITION OF CAFFEINE RIGOR AND RADIOCALCIUM MOVEMENTS BY LOCAL ANESTHETICS IN FROG SARTORIUS MUSCLE

Local anesthetics have been found to act as competitive inhibitors of caffeine in frog sartorius muscle. They block caffeine-induced rigor and the attendant increase in Ca⁴⁵ influx and efflux. Increased net uptake of sodium, loss of potassium, and concurrent increase in oxygen consumption are all effectively blocked by procaine. Evidence is presented that the inhibitory effect of the local anesthetics cannot be explained by the formation of molecular complexes with caffeine. Increased efflux of Ca⁴⁵ produced by changing from zero calcium Ringer's to 0.1 mM or 1 mM calcium Ringer's is inhibited by procaine and tetracaine. EDTA-stimulated calcium efflux is not affected by either local anesthetic. Caffeine rigor develops in frog muscle depolarized with KCl or rendered electrically inexcitable by sodium lack. Both the rigor and the increased calcium fluxes are inhibited by local anesthetics in depolarized muscle.

TETRODOTOXIN BLOCKAGE OF SODIUM CONDUCTANCE INCREASE IN LOBSTER GIANT AXONS

Previous studies suggested that tetrodotoxin, a poison from the puffer fish, blocks conduction of nerve and muscle through its rather selective inhibition of the sodium-carrying mechanism. In order to verify this hypothesis, observations have been made of sodium and potassium currents in the lobster giant axons treated with tetrodotoxin by means of the sucrose-gap voltage-clamp technique. Tetrodotoxin at concentrations of 1 x 10^-7 to 5 x 10^-9 gm/ml blocked the action potential but had no effect on the resting potential. Partial or complete recovery might have occurred on washing with normal medium. The increase in sodium conductance normally occurring upon depolarization was very effectively suppressed when the action potential was blocked after tetrodotoxin, while the delayed increase in potassium conductance underwent no change. It is concluded that tetrodotoxin, at very low concentrations, blocks the action potential production through its selective inhibition of the sodium-carrying mechanism while keeping the potassium-carrying mechanism intact.

REACTION OF LOCAL ANESTHETICS WITH PHOSPHOLIPIDS. A POSSIBLE CHEMICAL BASIS FOR ANESTHESIA

Local anesthetics (LA) have been found to interact with phospholipids and lipids extracted from nerve and muscle. This reaction is demonstrated by: (a) Inhibition by LA of phospholipid (and tissue lipid) facilitated transport of calcium from a methanol: water phase into chloroform. This action is dependent upon the cationic form of the LA. (b) LA increase the electrical resistance of "membranes" prepared by impregnating Millipore filters with cephalin:cholesterol or tissue lipid extracts and bathed with NaCl or KCl solutions. (c) LA coagulate aqueous dispersions of cephalin, phosphatidyl serine, phosphatidyl ethanolamine, and inositide, an action shared by calcium. The order of potency in coagulating cephalin sols is tetracaine > calcium > butacaine > procaine. Na⁺ and K⁺ do not coagulate phospholipid dispersions at 0.1 M concentration and antagonize the effect of Ca²⁺. (d) LA produce a marked fall in the pH of cephalin sols equivalent to that produced by calcium, (e) Ca²⁺ and LA form 1:2 molar complexes with phospholipids probably by ion-ion and ion-induced polar type of binding at the phosphate groups of the lipid. It is suggested that such reactions with cell membrane phospholipids may underlie inhibitory effects of LA on cellular ion fluxes and provide a chemical basis for anesthetic action.

THE ACTION OF MORPHINE-LIKE DRUGS ON IMPULSE TRANSMISSION IN MAMMALIAN NERVE FIBRES

Experiments on nerves in situ and on isolated nerves provide no evidence that morphine interferes with impulse transmission in myelinated or nonmyelinated nerve fibres. The concentrations used in experiments on isolated nerves were 10- to 100-times as high as those required to depress transmission at autonomic nerve-effector cell junctions. Examination of the resting membrane potential, the action potential and the positive after-potential, the conduction velocity, the time courses of the recovery of the size of the action potential and of the excitability after a conditioning stimulus, the ability of the axons to sustain repetitive activity and the posttetanic hyperpolarization gave no indication that morphine affects either the mechanisms involved in the initiation of the propagated impulse or those leading to restoration of the resting state after activity. Analgesic drugs, such as pethidine and methadone which have a local anaesthetic action, may cause a reversible decrease in the size of the compound action potential and in the conduction velocity of A-B and C fibres.

SOME EFFECTS OF CALCIUM IONS ON THE ACTION POTENTIAL OF SINGLE NODES OF RANVIER

Action potentials of single frog nerve fibers were recorded with the air-gap method in "low Ca" (0.26 mM) and "high Ca" (4.2 mM) solutions and compared to spikes in normal Ringer's (1.05 mM Ca). On increasing (Ca)(o) the action potentials became shorter, the "knee" during the falling phase as well as the threshold for abolition moved to internal potentials more positive, and the spike recovery during the relative refractory period was faster. Outward current pulses applied during an action potential affected its configuration more in low Ca than in high Ca. The onset of the delayed rectification (in the absence of Na) was found faster in high Ga. After-potentials during anelectrotonus declined more rapidly in high Ca than in low Ca. The results are compared primarily with the voltage-clamp analysis of Ca effects on squid axons and satisfactory qualitative agreement is reached.

EFFECT OF DETERGENT ON ELECTRICAL PROPERTIES OF SQUID AXON MEMBRANE

The effects of detergents on squid giant axon action and resting potentials as well as membrane conductances in the voltage clamp have been studied. Anionic detergents (sodium lauryl sulfate, 0.1 to 1.0 mM; dimethyl benzene sulfonate, 1 to 20 mM, pH 7.6) cause a temporary increase and a later decrease of action potential height and the value of the resting potential. Cationic detergent (cetyl trimethyl ammonium chloride, 6 x 10^-5M or more, pH 7.6) generally brings about immediate and irreversible decreases in the action and resting potentials. Non-ionic detergent (tween 80, 0.1 M, pH 7.6) causes a slight reversible reduction of action potential height without affecting the value of the resting potential. Both anionic and cationic detergents generally decrease the sodium and potassium conductances irreversibly. The effect of non-ionic detergent is to decrease the sodium conductance reversibly, leaving the potassium conductance almost unchanged.

GABA receptor-channel complex as a target site of mercury, copper, zinc, and lanthanides

1. The GABAA receptor-chloride channel complex has been shown to be modulated by a variety of chemicals. Scores of chemicals with diverse and unrelated structures augment the GABA-induced chloride current, while some other chemicals suppress the current. Certain heavy metals and a variety of polyvalent cations increase or decrease the current in a potent and efficacious manner. 2. We have studied the mechanisms whereby mercury, copper, zinc, and lanthanides modulated the GABA system by whole-cell and single-channel patch clamp techniques as applied to the rat dorsal root ganglion neurons in primary culture. 3. Mercuric chloride augmented the GABA-induced current to 115% of control at 0.1 microM and to 270% of control at 100 microM. It also generated a slowly developing inward current carried by a variety of ions. In contrast, methylmercury suppressed the GABA-induced current. The potent stimulation of the GABA system by mercuric chloride is deemed important in mercury intoxication. 4. Copper and zinc suppressed the GABA-induced current with an EC50 of 16 and 19 microM, respectively. They bound to a common site on the external surface of the GABA receptor-channel complex. 5. Lanthanum augmented the GABA-induced current with an EC50 of 230 microM by increasing the affinity of the receptor for GABA. It bound to a site on or near the external surface of the GABA receptor-channel complex which is different from the sites for GABA, barbiturates, benzodiazepines, picrotoxin, and copper/zinc. 6. Six other lanthanides with larger atomic numbers also exerted the same stimulatory effect with their efficacies increasing with the atomic number. 7. Single-channel analyses have revealed that the augmentation of whole-cell current by terbium, a lanthanide, is due to three actions: an increase in the overall mean open time, a decrease in the overall mean closed time, and an increase in the overall mean burst time.

Medial vestibular neurons are endogenous pacemakers whose discharge is modulated by neurotransmitters

1. Neurons in the medial vestibular nucleus (MVN), recorded in a rat brain slice preparation, exhibit a highly regular, high-frequency (5- to 35-Hz) spontaneous discharge. The rhythmic firing rate was constant (< 5% variation) and sustained for a long time (maximum observation, 4 hr). 2. The rhythmic firing was evident even in neurons (n = 15) completely isolated from exogenous input fibers, suggesting that it is due to an endogenous pacemaker property. When recorded intracellularly, the discharge was found to be associated with a smooth, concave pacemaker prepotential, and the rate of firing was reduced in proportion to applied hyperpolarizing current, indicating that these are pacemaker discharges. 3. This conclusion is supported by the observation that perfusion with a low-calcium/high-magnesium Krebs-Ringer solution, which completely and reversibly blocks all synaptic transmission, did not abolish the spontaneous discharge. The low-calcium/high-magnesium solution increased spontaneous firing in some neurons and decreased in others, suggesting that the firing is synaptically modulated and the synaptic influence is tonically active. 4. Application of kynurenate (10 mM), an antagonist of the excitatory amino acid receptors, gradually reduced neuronal discharges in most neurons (22 of 25), while the addition of 10 mM sucrose as an osmotic control had no effect. Depression of neuronal discharges reached its minimum (an average of 60% of the control level) and was maintained at that level until gradually washed out. The rhythmic firing pattern persisted in all neurons even after the excitatory receptors were blocked. 5. When the GABAA receptor antagonist, bicuculline (20 microM), was applied, elevation of neuronal discharges was evident in most neurons (30 of 32) tested. The firing increased gradually, with a final control level of 130% (121-160%). In contrast, the GABAB receptor antagonist, phaclofen (20 microM and 100 microM), had no effect in most neurons (19 of 23) tested. Further, the excitatory and inhibitory action could be detected on the same neuron when bicuculline and kynurenate were both evaluated (n = 10). 6. These results indicate that the spontaneous discharge of MVN neurons is due to an endogenous pacemaker under the tonic influence of both inhibitory and excitatory transmitter actions. The bicuculline-sensitive GABAA receptors and the kynurenate-sensitive glutamate receptors both mediate the tonic modulation.

Ca-dependent action of cocaine on K current in freshly dissociated dorsal root ganglia from rats

The actions of cocaine on K currents were analyzed in enzymatically dissociated neurons from adult rat dorsal root ganglia. The whole cell voltage clamp configuration was used to determine that in the presence of Ca cocaine inhibited a K conductance that was activated from a holding potential of -55 mV. The cocaine concentration that decreased K currents half-maximally was 18 microM. At a concentration of 40 microM, cocaine had no significant effect on K current remaining in the absence of external Ca, whereas at 80 microM a significant 17% depression of this current was observed. Large transient K currents occurring with depolarizations from -90 mV were also resistant to 40 microM cocaine, suggesting that cocaine specifically inhibits a Ca-dependent K current. Ca currents elicited from a holding potential of -55 mV were largely unaltered by 40 microM cocaine. The data suggest that cocaine may be a useful tool for experimental blockage of Ca-dependent K currents and also provide additional insight into the overall actions and toxicity of the drug.

The effect of cocaine on membrane potential, on membrane depolarization by veratridine or elevated [K]o and on sodium/potassium permeability ratios in synaptosomes from the limbic cortex of the rat

Effects of cocaine on the synaptosomal membrane potential (Em), on membrane depolarization induced by veratridine or elevated [K]o and on sodium/potassium permeability ratios (pNa/pK), have been measured in buffer containing either low or high [Na]. Fluorescence of the dye rhodamine 6G was used to measure the membrane potential. Cocaine began to reduce the Em (depolarized) at concentrations between 10(-6) and 10(-5) M in low [Na] buffer and between 10(-5) and 10(-4) M in high [Na] buffer. Maximum depolarization (with 10(-3) M cocaine) was 21 mV in low [Na] buffer and 11 mV in high [Na] buffer. Cocaine also reduced the depolarization caused by veratridine or elevated [K]o; the effective concentration of cocaine in reducing the response to veratridine was one-tenth that necessary to reduce the response to elevated [K]o. The antagonism by cocaine of the response to veratridine was similar to that found by other investigators; however, this action would tend to oppose depolarization and thus cannot explain the depolarizing effect of cocaine alone. The antagonism by cocaine of the depolarization caused by elevated [K] was consistent with a reduction in pK; such a change in pK could explain the observed reduction in Em caused by cocaine alone. The effect of cocaine (10(-3) M) on the Em was also measured as a function of [K]o at low and high [Na]o. Cocaine caused membrane depolarization at all [K]o's (3.9-19.2 mV), an effect that was somewhat greater in the low [Na] medium. These measurements of Em were fitted to the Goldman equation and the ratio of pNa/pK estimated. The presence of cocaine increased the estimate of pNa/pK by 45.7%, presumably by reducing pK.

Some properties of inhibitory action of lidocaine on the Ca2+ current of single isolated frog sensory neurons

The action of lidocaine on the Ca2+ current (ICa) was studied on single isolated neurons of frog dorsal root ganglia using a 'concentration-clamp' technique which combines intracellular perfusion with a rapid external solution change. Lidocaine decreased the peak amplitude of ICa at a threshold concentration of 10 microM. Higher concentrations gave a dose-dependent increase in inhibition of ICa. Lidocaine also depressed the Na+ current (INa) at a threshold concentration 10 times lower than that for decreasing the amplitude of ICa of neurons isolated from same ganglia. The rate of inhibitory action on ICa was slowed at more negative holding potentials (VH). Lidocaine appears to block both closed and open Ca2+ channels, but acts more profoundly on open channels.

Local anesthetics and pressure: a comparison of dibucaine binding to lipid monolayers and bilayers

The binding of the local anesthetic dibucaine to monolayers composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine was studied with a Langmuir trough at pH 5.5 (22 degrees C, 0.1 M NaCl). At this pH value only the charged form of the local anesthetic exists in solution. Charged dibucaine was found to be surface active and to penetrate into the lipid monolayer, with the hydrophobic part of the molecule being accommodated between the fatty acyl chains of the lipid. The dibucaine intercalation could be quantitated by measuring the expansion of the film area, delta A, at constant surface pressure, pi. At a given surface pressure, delta A increased with increasing dibucaine in the buffer phase. On the other hand, keeping the dibucaine concentration constant, the area increase, delta A, was strongly dependent on the surface pressure. The area increase, delta A, was large at low surface pressure and decreased with increasing surface pressure. A plot of the relative change in surface area, delta A/A, versus the surface pressure yielded straight lines in the pressure range of 25-36 mN/m for five different concentrations. The delta A/A vs. pi isotherms intersected at pi = 39.5 +/- 1 mN/m with delta A = O, indicating that charged dibucaine apparently can no longer penetrate into the monolayer film. By making judicial assumptions about the area requirement of dibucaine the monolayer expansion curves could be transformed into true binding isotherms. Dibucaine binding isotherms were constructed for different monolayer pressures and were compared to a bilayer binding isotherm measured under similar conditions with ultraviolet spectroscopy. The best agreement between monolayer and bilayer binding data was obtained for a monolayer held at a pressure of 30.7 to 32.5 mN/m, which can thus be considered as the bilayer-monolayer equivalence pressure. It is further suggested from this analogy that the binding of dibucaine does not change the internal pressure in the bilayer phase, at least not in the concentration range of physiological interest (0-2 mM dibucaine) but induces a lateral expansion. At higher molar ratios of cationic dibucaine to lipid, chi b, in the monolayer (chi b greater than 0.20) the area increase is larger than would be expected from the molecular dimensions of dibucaine. This is probably due to charge repulsion effects, which at still higher molar ratios (chi b greater than 0.6) lead to a micellisation. The pressure dependence of the intercalation of cationic dibucaine into lipid membranes may also be of relevance for the phenomenon of pressure reversal in anesthesia.

Histochemical study of calcium on T-tubule membranes and in the sarcoplasmic reticulum, in frog twitch muscle fibres at rest and during activity

The trigger calcium hypothesis of signal transmission between T-tubules and terminal cisternae (TC) of the sarcoplasmic reticulum (SR) in twitch muscle fibres implies the presence of calcium along T-tubule membranes at rest and its release upon excitation. To test this hypothesis, calcium was immobilised using a fixing and precipitating solution of glutaraldehyde in phosphate buffer at pH 8.0 and the calcium was substituted for by lead. Simultaneous tension recordings revealed the occurrence of contractions or a burst of twitches upon perfusion with the fixative. Procaine or tetrodotoxin (TTX) was used to inhibit this activity. In fibres without fixative-induced activity, precipitates were observed along T-tubules and in adjoining parts of TC. In activated fibres, tubular and TC precipitates were absent. These results are consistent with the trigger calcium hypothesis. In fibres activated by depolarisation, calcium returned to TC after passing successively through different parts of the SR.

A 2H-NMR study on the interactions of the local anesthetic tetracaine with membranes containing phosphatidylserine

The interaction of the local anesthetic tetracaine with phosphatidylserine-containing model membranes has been studied by 2H-NMR. Charged tetracaine exhibited an unusually large partition coefficient into multilamellar dispersions of phosphatidylserine. The 2H-NMR spectra consisted of a Pake doublet and a narrow line, with the former corresponding to tetracaine in the bilayer and the latter to tetracaine free in solution. A strong pH dependence of the quadrupole splittings indicated different membrane locations for charged and uncharged tetracaine. In equimolar mixtures of phosphatidylserine and phosphatidylcholine the partition coefficients and 2H-NMR spectra were much more like those observed in neat phosphatidylcholine than in neat phosphatidylserine. Dilution studies at pH 5.5 indicated that in phosphatidylserine/phosphatidylcholine mixtures tetracaine experiences a three-site exchange similar to that found earlier for tetracaine in phosphatidylcholine. Tetracaine is in fast exchange between sites weakly bound to membrane and free in solution, and in slow exchange with a strongly bound site in the membrane.

General anaesthetics: effects on transmitter release

In this report, the physiological effects, observations and events leading to transmitter release in both central nervous system and peripheral synapses are discussed. The presynaptic modulation of transmission at the central nervous system and at the neuromuscular junction was investigated using electrophysiological and neurochemical techniques. It was concluded that various general anaesthetics may affect the presynaptic mechanism of transmission, but these effects were controversial. For example, terminal excitability, which is taken as an index for presynaptic activity, could either be reduced or increased by general anaesthetics. Similar conflicting effects have been reported for the action of general anaesthetics on spontaneous and evoked release of ACh, uptake and release of intracellular Ca2+, and choline transport into the presynaptic nerve terminals.

Drug-ionic channel interactions: single-channel measurements

Ionic channels of excitable membranes are the basic site where ionic fluxes take place during the generation of action potentials. A variety of natural toxins, chemicals, and therapeutic drugs have been found to modify the gating kinetics of the Na+ channels, thereby altering the excitation pattern. Studies of such chemical modulations of Na+ channel gating provide the basis for understanding the mechanisms underlying the epilepsies and the actions of anticonvulsant drugs. Certain chemicals and toxins have been found to drastically slow the kinetics of the opening and closing of the Na+ channel. For example, batrachotoxin, the grayanotoxins, and the pyrethroids modify a population of the Na+ channels to give rise to an extremely slow opening and closing. Patch clamp techniques developed during the past few years permit measurements of the opening and closing of individual ionic channels. When an isolated membrane patch is depolarized, squared inward currents of about 1 picoampere in amplitude and 2 ms in duration are observed at 10 degrees C. After exposure of the membrane to batrachotoxin, open time is prolonged, single-current amplitude is greatly reduced, and channel opening is observed at large negative potentials, where no opening is expected to occur in normal preparations. In the batrachotoxin-poisoned membrane there are two separate groups of Na+ channels: one exhibiting normal characteristics and the other exhibiting a prolonged opening and reduced amplitude.

Further analysis of inhibitory effects of propranolol and local anaesthetics on the calcium current in Helix neurones

1 The effects of propranolol and local anaesthetics on Ca2+ current (Ica), individually separated from other ionic currents, in Helix neurones were studied under voltage clamp, using a suction pipette technique. 2 Increases in external Ca2+ concentrations overcame the inhibitory action of propranolol on Ica. Double reciprocal plots for peak Ica versus external Ca2+ concentrations in the presence or absence of propranolol did not intersect at the ordinate. 3 Internal application of propranolol (10-4M) inhibited Ica to about 40-60% of the control in a time-dependent manner. 4 Lignocaine and procaine at concentrations of 10-3-10-2 M inhibited Ica without shifting the threshold in the I-V relationships. Internal application of lignocaine (10-3-10-2M) also inhibited Ica: the ratio of depression of the Ica was almost equivalent to that of the agent applied externally. 5 The results provide evidence that propranolol inhibits Ica in the noncompetitive manner with Ca2+ at the cell membrane, and suggest that the agents may occupy the receptor site in the Ca2+-channel somewhere between the outer surface and inner phase of the membrane.

The effects of tetracaine on the membrane currents and contraction of frog atrial muscle

1. The effects of tetracaine on the membrane currents and contraction of isolated frog atrial trabeculae under voltage clamp has been investigated. 2. Tetracaine inhibits the slow inward current in a competitive way with a Ki of 0.13 mM, and the phasic component of the contractile response in a non-competitive manner with a Ki of 0.25 mM. 3. The tonic phase of contraction is little affects by tetracaine while the outward current is reduced. 4. The effects of tetracaine on contraction closely resemble those obtained previously with K contractures and further emphasize that two processes, both of which depend on the membrane potential, are involved in the initiation of contraction in amphibian heart. 5. The effect of tetracaine on the phasic tension would seem to be due to an inhibition of both the slow inward current and the release of Ca2+ from the sarcoplasmic reticulum.

Inhibitory effects of propranolol on the calcium current of Helix neurones

1 Calcium current (ICa) and potassium current (IK) in isolated nerve cell bodies of Helix aspersa were independently separated after suppression of Na+ and K+ currents, and Na+ and Ca2+ currents, respectively. The suction pipette method was used. Under voltage clamp conditions, the effects of propranolol on both the ICa and IK were examined. 2 Propranolol inhibits the ICa at relatively low concentrations (10(-6)-10(-5) M). The inhibitory action was dose-dependent. 3 The IK was moderately suppressed by the drug at high concentrations (10(-5)-5 X 10(-1) M). 4 Results provide evidence for a new aspect of the pharmacological action of propranolol on the excitable cell membrane.

The effect of local anaesthetics on strophanthidin toxicity in canine cardiac Purkinje fibres

1. Canine Purkinje fibres were superfused in vitro and the electrical and mechanical effects of the local anaesthetics benzocaine and procaine were studied in the absence and in the presence of other agents.2. Both benzocaine (1 x 10(-4)-5 x 10(-4)m) and procaine (6 x 10(-5)-2.5 x 10(-4)m) decreased slightly the amplitude of the upstroke and markedly the duration of the action potential. The plateau was shifted to more negative values and the force of contraction was decreased. These effects were dose-dependent.3. The local anaesthetics abolished the spontaneous activity induced by strophanthidin (5 x 10(-7)m) by flattening the oscillatory potential in diastole and increased the force of contraction under these circumstances.4. The local anaesthetics significantly delayed the time of the onset of the spontaneous activity induced by strophanthidin (10(-6)m). Also, the intensity of the stimuli had to be increased and the rate of discharge of intoxicated fibres was slower in the presence of local anaesthetics. In contrast, the positive inotropic effect was little affected.5. The local anaesthetics reduced but did not block the inotropic action of norepinephrine and high Ca; and did not abolish (while Mn did) small action potential in 27 mm-K-depolarized fibres.6. In fibres treated with local anaesthetics, lowering [Ca](o) did not result in a force rebound and administration of caffeine or exposure to low Na resulted in a larger increase in force.7. In fibres loaded with Ca, local anaesthetics caused an increase in force.8. Local anaesthetics decreased the force more when the external Na concentration was lower.9. This work shows that the local anaesthetics alter the mechanical performance of Purkinje fibres and lead to a depression of the strophanthidin induced oscillatory potential. As for the mechanism for these changes, the present experiments support the hypothesis that local anaesthetic agents have an antiarrhythmic action by decreasing intracellular Na and therefore intracellular Ca as suggested by Perry, McKinney & DeWeer (1978) on the basis of experiments on nerve.

The intracellular sodium activity of sheep heart Purkinje fibres: effects of local anaesthetics and tetrodotoxin

1. The intracellular Na activity (a(Na) (i)) of quiescent sheep heart Purkinje fibres has been measured using Na(+)-sensitive glass micro-electrodes. The effects of local anaesthetics (procaine and lidocaine) and tetrodotoxin (TTX) have been investigated.2. Local anaesthetics reduced the steady-state level of the intracellular Na activity in a dose-dependent manner. The highest concentrations used (10(-2)M) reduced the intracellular Na activity by about 25%.3. TTX decreased the steady-state level of the intracellular Na activity. At a concentration of 10(-6) g/ml. (3.13 x 10(-6)M), TTX produced a decrease in intracellular Na activity of approximately 10%.4. The initial rate of rise of the intracellular Na activity upon addition of the cardioactive steroid strophanthidin (10(-5)M) was used to estimate the net passive Na influx.5. Procaine (5 x 10(-4)M) caused a 50% reduction of this rate of rise of the intracellular Na activity. The highest concentration of procaine used (10(-2)M) decreased the rate of rise by approximately 80%.6. Procaine (5 x 10(-3)M) also reduced the rate of rise of intracellular Na produced by the removal of external K (K(o)), and prevented the large depolarization associated with the absence of K(o).7. TTX also produced a decrease in the rate of rise of the intracellular Na activity that occurs upon addition of strophanthidin. A maximum effect was produced in our experiments at a TTX concentration of 10(-6) g/ml. At this concentration the rate of rise of intracellular Na activity was reduced by approximately 40% at a membrane potential of -70 mV.8. We conclude from our experiments that the effects of local anaesthetics and TTX on the intracellular Na activity are brought about by a reduction of the Na permeability of the cell membrane, and that at the normal resting potential, Na entry through TTX-sensitive channels contributes greatly to the total net Na influx.

Mechanism of mesaconitine-induced contractile response in guinea pig vas deferens

Mesaconitine (MA) caused contraction of the isolate guinea pig vas deferens in a dose-dependent manner (3 x 10(-6) - 10-(-4) g/ml) and nonspecifically potentiated the contractile response to norepinephrine (NE), acetylcholine, histamine and tyramine. Tachyphylaxis developed during the repeated application of MA. The contraction produced by MA was prevented by treatment with phentolamine, phenoxybenzamine, procaine, bretylium or tetrodotoxin, pretreatment with reserpine or 6-hyroxydopamine, or denervation. Atropine or hexamethonium did not abolish the effect of MA, excluding the possibility that the muscarinic and nicotinic receptors are the site of action of MA. MA elicited a dose-dependent release of NE from the isolated vas deferens which had been blocked by treatment with tetrodotoxin, pretreatment with reserpine, or the exclusion of calcium ion from the bath medium. Consequently, it is concluded that MA-induced contraction was brought about by the release of NE from pre-synaptic neuronal sites and by excitation of the post-synaptic membrane.

[Changes in the synaptic contacts in the amygdaloid complex following unilateral lobectomy in rabbits]

Synaptic junctions in the rabbit amygdala were studied by electron microscopy. A definite distribution of morphologically different synapses on the neuronal surface was noticed. This distribution changed 3 days after ipsilateral lobectomy. A suggestion is made on transformation of a certain morphological form of synaptic contact into another one during the changed function.

Effect of local anesthetics on chloride transport in erythrocytes

The self-exchange of chloride isotopes across the human erythrocyte membrane was inhibited by tetracaine, benzocaine, and lidocaine. The inhibition by tetracaine was increased at higher pH values, but was not exclusively due to the uncharged form of tetracaine. The inhibition was effective within 5 seconds, was reversible, was non competitive with chloride ions, and was not reversed by calcium ions. These findings indicate that local anesthetics react with the erythrocyte chloride carrier at a site separate from the chloride site and cause inhibition of anion transport by a specific mechanism not involving changes in the surface charge density on the erythrocyte membrane.

Ionic mechanism of action of a spin-labeled local anesthetic on squid axon membranes

The ionic mechanism of action of a spin-labeled local anesthetic (SLA), 2-[N-methyl-N-(2,2,6,6-tetramethylpiperidonooxyl)]-ethyl 4-ethoxylbenzoate, was studied by means of voltage clamp technique with squid giant axons in comparison with the parent compound without spin label moiety, 2-(N,N-dimethyl)ethyl 4-ethoxylbenzoate (GS-01). Like other local anesthetics, they suppressed both sodium and potassium conductance increases. However, three remarkable differences have been noted between SLA and GS-01: (1) SLA is more effective than GS-01 in suppressing the sodium and potassium conductance increases; (2) SLA induces a potassium inactivation, whereas GS-01 is lacking this ability; (3) SLA has no effect on the time to peak sodium current, whereas GS-01 prolongs it. GS-01 resembles procaine with respect to (2) and (3) above. SLA will become a useful probe for the study of the molecular mechanism of local anesthetic aciton and of ionic channel function.

Local anesthetic activity of 1-(morpholino or piperidino)-2-propanol derivatives

The local anesthetic activities of 1-(morpholino or piperidino)-2-propanol derivatives were investigated. These activities were observed with nerve trunk anesthesia, rabbit's corneal anesthesia and intradermal injection anesthesia. Among these propanol derivatives, No. 24 and No. 25 which possess thioether-type sulfur were the most active and the effective potency was enhanced by increasing the dissociation constant (pKa) value in a series of the compounds. Effects on cat spinal reflex were also observed. These propanol derivatives revealed no selective depression of presynaptic depression or depression of the potential of monosynaptic reflex (MSR) and polysynaptic reflex (PSR). Procaine and lidocaine had similar actions. Effects on axonal membrane potential and interaction of the calcium on these derivatives were also investigated. These derivatives decreased the action potential without altering the resting membrane potential and when antagonized with calcium, sciatic nerve action potential was decreased. Procaine and idocaine also showed the same results.

Tricaine (MS-222): effects on ionic conductances of squid axon membranes

Experiments utilizing internally perfused squid giant axons have been carried out to determine the mode of action of tricaine (MS-222) in producing its anesthetic effects. Tricaine, in concentrations of 1 and 3 mM, was applied to the internal surface of the membrane via the perfusion system. Peak sodium current and steady-state potassium current were measured by the axial-wire voltage clamp technique. Tricaine suppressed both of these membrane ionic currents with the major effect being on the peak sodium current. Its action on nerve membrane appeared quite similar to other local anesthetics. One major difference was noted in that tricaine shifts sodium conductance curves in the direction of depolarization along the potential axis, an effect quite similar to that observed with increasing external Ca2+ concentration and with application of pentobarbital.