The effect of tetraethylammonium chloride on the muscle membrane examined with an intracellular microelectrode

Modulation of voltage-gated ion channels by sialylation

Control and modulation of electrical signaling is vital to normal physiology, particularly in neurons, cardiac myocytes, and skeletal muscle. The orchestrated activities of variable sets of ion channels and transporters, including voltage-gated ion channels (VGICs), are responsible for initiation, conduction, and termination of the action potential (AP) in excitable cells. Slight changes in VGIC activity can lead to severe pathologies including arrhythmias, epilepsies, and paralyses, while normal excitability depends on the precise tuning of the AP waveform. VGICs are heavily posttranslationally modified, with upward of 30% of the mature channel mass consisting of N- and O-glycans. These glycans are terminated typically by negatively charged sialic acid residues that modulate voltage-dependent channel gating directly. The data indicate that sialic acids alter VGIC activity in isoform-specific manners, dependent in part, on the number/location of channel sialic acids attached to the pore-forming alpha and/or auxiliary subunits that often act through saturating electrostatic mechanisms. Additionally, cell-specific regulation of sialylation can affect VGIC gating distinctly. Thus, channel sialylation is likely regulated through two mechanisms that together contribute to a dynamic spectrum of possible gating motifs: a subunit-specific mechanism and regulated (aberrant) changes in the ability of the cell to glycosylate. Recent studies showed that neuronal and cardiac excitability is modulated through regulated changes in voltage-gated Na(+) channel sialylation, suggesting that both mechanisms of differential VGIC sialylation contribute to electrical signaling in the brain and heart. Together, the data provide insight into an important and novel paradigm involved in the control and modulation of electrical signaling.

Graded and all-or-none electrogenesis in arthropod muscle. II. The effects of alkali-earth and onium ions on lobster muscle fibers

Conversion of graded responsiveness of lobster muscle fibers to all-or-none activity by alkali-earth and tetraethylammonium (TEA) ions appears to be due to a combination of effects. The membrane is hyperpolarized, its resistance is increased, and its sensitivity to external K⁺ is diminished, all effects which indicate diminished K⁺ conductance. While the spikes are prolonged, the conductance is higher throughout the response than it is in the resting membrane. Repetitive activity becomes prominent. These effects indicate maintained high conductance for an ion which causes depolarization. This is normally Na⁺, since its presence in low concentrations potentiates the effects of Ba⁺⁺, but the alkali-earth ions and TEA can also carry inward charge. Ba⁺⁺, Sr⁺⁺, and TEA appear to be more effective than is Ca⁺⁺ in its normal role, which is probably to depress K⁺ conductance and Na inactivation. Thus, conversion of graded to all-or-none responsiveness appears to occur because of the relative increase of depolarizing inward ion flux and decrease of repolarizing outward flux.

Permeability of frog skeletal muscle cells to choline

Using choline-methyl-C¹⁴ as a tracer, it has been shown that choline⁺ penetrates into the cells of resting frog skeletal muscle at a rate similar to that of Na⁺, and that it escapes from these cells much more slowly than does Na⁺. Some implications of these findings are discussed.

Neuromuscular blocking activity of methanolic extract of Piper sarmentosum leaves in the rat phrenic nerve-hemidiaphragm preparation

Methanolic extract of Piper sarmentosum Roxb. (Piperaceae) leaves was studied for the neuromuscular blocking activity in rat phrenic nerve-hemidiaphragm preparations. The plant extract, at concentrations of 3.2, 4.0, 4.8 and 6.4 mg/ml, exhibited an initially transient increase in twitch tension which was followed by a marked dose-related neurally-evoked twitch depression. The neuromuscular blocking effect produced by the plant extract was compared with d-tubocurarine (dTC) and succinylcholine (SCh). The EC50 for neurally-evoked twitch depression of the extract, dTC and SCh was 4.07 mg/ml, 1.1 microM and 15 microM, respectively. The neurally-evoked twitch depression produced by the extract was partially antagonized by tetraethylammonium (TEA) but not by neostigmine (NS). These findings suggested that the plant extract possessed a marked neuromuscular blocking activity at the neuromuscular junction and a possible mechanism which was likely to inhibit neurotransmitter (acetylcholine) release at the presynaptic terminal.

Relation between action potential duration and mechanical activity on rat diaphragm fibers. Effects of 3,4-diaminopyridine and tetraethylammonium

The aim of this work was to study the electrical and mechanical properties of small bundles of rat diaphragm muscle treated with two blockers of the delayed potassium rectification channels: 3,4-diaminopyridine (3,4-DAP, 2.5 mM) and tetraethylammonium (TEA, 20 mM). Twitch tension was significantly potentiated by TEA and 3,4-DAP (39% and 59% respectively). Maximal tetanic tension was not affected by both drugs. The voltage dependence of the tension vs the resting membrane potential was shifted to lower values in TEA and 3,4-DAP. 3,4-DAP increased the caffeine contracture tension (2.5-10 mM) and lowered the caffeine contracture threshold. The duration of the action potential (measured at the level of -40 mV) was increased by TEA and 3,4-DAP solutions. This change was a consequence of the decrease in the rate of repolarization of the action potential. In addition, TEA reduced the amplitude and the rate of rise of the action potential. We suggested that the increment in the duration of the action potential and the shift of the mechanical threshold to more negative values of membrane potential might be the factors involved in the twitch potentiation induced by the TEA and 3,4-DAP solutions.

Early effects of tetraethylammonium chloride on the contractile properties of isolated rabbit basilar arteries

The acute vascular effects of tetraethylammonium chloride (TEA) were examined on annular segments of rabbit basilar arteries. Contractions induced by the potassium channel blocker were compared with those obtained for potassium chloride, 5-hydroxytryptamine (5-HT) and norepinephrine (NE). The greater magnitude of the contractions was of the following order: [K+] greater than 5-HT greater than TEA greater than NE. High concentrations of TEA alone (10(-2) M) generated spontaneous oscillatory contractions in cerebral vessels that were normally quiescent. Low concentrations of TEA (10(-8)-10(-6) M), which had no vasomotor properties per se, enhanced the contractile response of submaximal concentrations of 5-HT (10(-7) M) and NE (3 X 10(-6) M) and attenuated the contraction produced by 60 mM [K+]. An increased vascular response to the amines was still evident up to 3 h after the addition of TEA despite frequent rinsing with fresh buffer solutions. On arteries precontracted with TEA (10(-2) M), but not high [K+], the subsequent addition of 5-HT (10(-7) M) still induced a powerful constriction. Repeated concentration-response curves for [K+] were reproducible and, in the presence of TEA (10(-8) or 10(-6) M), the curve was displaced to the right in a competitive manner. A higher concentration of TEA (10(-4) M) was devoid of any blocking properties on the [K+]-induced response whereas, at 10(-3) M TEA, the response was potentiated, as evidenced by a shift of the curve to the left. Interactions between TEA and the cumulative response to 5-HT were difficult to interpret. Repeated exposures of the artery to 5-HT resulted in an increased maximal response with each determination (EAm = 127 +/- 9% and 149 +/- 14% of control values following the second and third applications, respectively). With TEA (10(-6) M), the increase in the maximal contractile effect noted previously was not observed. Contractions induced by single concentrations of TEA (10(-2) M) or [K+] (60 mM) were calcium dependent, were abolished completely in a calcium-free medium, and were depressed by the calcium antagonist nimodipine. 5-Hydroxytryptamine-induced contractions (10(-5) M) were less sensitive to withdrawal of calcium from the extracellular medium (31 +/- 6% relative to the maximal response at 4 mM calcium). Hence, an acute reduction in potassium conductance in cerebrovascular smooth muscle produced by TEA has complex, concentration-dependent effects and reproduces only part of the spectrum of effects of cisternal injection of blood on cerebrovascular reactivity.

Facilitation of noradrenaline release by gallamine in the rat salivary gland

The effect of gallamine on spontaneous and stimulation-evoked overflow of tritium was studied in the submandibular gland of the rat. The gland was perfused retrogradely and labeled with 3H-noradrenaline. The stimulation-evoked (1 Hz for 60 s) overflow of tritium was facilitated by increasing concentrations of gallamine (0.3-20 mM). None of the concentrations of gallamine increased the spontaneous overflow of the tritium. The facilitatory effect of gallamine was observed in 0.3 to 5 mM calcium medium; the maximum facilitation was observed at the normal concentration of calcium (2.5 mM). The facilitatory effect of gallamine was inversely related to the frequency of stimulation (10-fold facilitation at 1 Hz and 3-fold at 10 Hz). Stimulation of the salivary gland by a single pulse (1 ms duration) in the normal medium did not evoke an overflow of tritium; however, the same stimulus produced a marked increase in the overflow in the presence of gallamine. The facilitatory action of gallamine on the release of sympathetic transmitter is ascribed to the enhanced availability of calcium ions to the secretory process resulting from blockade of potassium conductance during nerve activity.

Inhibition of potassium conductance with external tetraethylammonium ion in Myxicola giant axons

In voltage clamp experiments, externally applied tetraethylammonium ion (TEA) was found to have minimal effects on transient sodium currents and to suppress steady-state potassium currents of Myxicola giant axons by causing a specific decrease in the maximum potassium conductance gK. The dose-response curve suggests a one-to-one stoichiometry for TEA-receptor binding with an apparent dissociation constant on 24 mM. The suppression of IK is essentially reversible. Experiments performed on high external potassium ion concentrations indicate that both outward and inward IK were blocked by external TEA. The results thus suggest the presence of TEA receptors on the outer surface of Myxicola axonal membrane similar to those reported in the frog node.

Simulated propagation of cardiac action potentials

We have used numerical methods for solving cable equations, combined with previously published mathematical models for the membrane properties of ventricular and Purkinje cells, to simulate the propagation of cardiac action potentials along a unidimensional strand. Two types of inhomogeneities have been simulated and the results compared with experimentally observed disturbances in cardiac action potential propagation. Changes in the membrane model for regions of the strand were introduced to simulate regions of decreased excitability. Regional changes in the intercellular coupling were also studied. The results illustrate and help to explain the disturbances in propagation which have been reported to occur at regions of decreased excitability, regions with changing action potential duration, or regions with changing intercellular coupling. The propagational disturbances seen at all of these regions are discussed in terms of the changing electrical load imposed upon the propagating impulse.

Propagation of action potentials in squid giant axons. Repetitive firing at regions of membrane inhomogeneities

Effects of reduction in potassium conductance on impulse conduction were studied in squid giant axons. Internal perfusion of axons with tetraethylammonium (TEA) ions reduces G K and causes the duration of action potential to be increased up to 300 ms. This prolongation of action potentials does not change their conduction velocity. The shape of these propagating action potentials is similar to membrane action potentials in TEA. Axons with regions of differing membrane potassium conductances are obtained by perfusing the axon trunk and one of its two main branches with TEA after the second branch has been filled with normal perfusing solution. Although the latter is initially free of TEA, this ion diffuses in slowly. Up until a large amount of TEA has diffused into the second branch, action potentials in the two branches have very different durations. During this period, membrane regions with prolonged action potentials are a source of depolarizing current for the other, and repetitive activity may be initiated at transitional regions. After a single stimulus in either axon region, interactions between action potentials of different durations usually led to rebound, or a short burst, of action potentials. Complex interactions between two axon regions whose action potentials have different durations resembles electric activity recorded during some cardiac arrhythmias.

Sodium currents in mammalian muscle

1. A method is described which allows the approximate computation of membrane current from measurements with three electrodes in the mid-region of a muscle fibre.2. Measurements of inward sodium current in frog muscle are compared with the results of previous clamping studies to test the validity of the new method.3. Sodium current in rat muscle (extensor digitorum longus) is in general similar to sodium current in frog muscle. Two differences in detail have been found between sodium current in rat and frog muscle: (a) at the same temperature (in the range 0-20 degrees C) inactivation is slower in the rat than in the frog; (b) in rat the steady-state activation is shifted negatively on the voltage axis by some 10-15 mV.4. Delayed outward current and charge movement (Schneider & Chandler, 1973) are present in rat muscle.5. Rat muscle fibres are more resistant than frog muscle fibres to the action of tetrodotoxin. Inward current is still detectable in rat muscle at 100 nM tetrodotoxin. We found no evidence to suggest the existence in rat muscle of two kinds of sodium channel, one sensitive and one less sensitive to tetrodotoxin.

The action of tetraethyl-ammonium chloride on the response of the rat anococcygeus muscle to motor and inhibitory nerve stimulation and to some drugs

1 Tetraethylammonium chloride (TEA) 0.125 mM to 20 mM potentiates the response of the anococcygeus muscle to field stimulation of the motor adrenergic nerves without affecting the response to noradrenaline suggesting a pre-synaptic origin of potentiation. The potentiation is greatest at low, submaximal, frequencies (2 Hz) of stimulation and only slight at the higher frequency of 20 Hz. This difference is due to the restraint imposed on the demonstration of potentiation by maximal or near maximal motor responses since reduction of the mechanical response at 20 Hz by either phentolamine (post-synaptic block) or guanethidine (pre-synaptic block) resulted in a great increase in potentiation of the response at this frequency. 2 TEA in concentrations up to 1 mM similarly potentites the response to inhibitory nerve stimulation and again the greatest effect is at low frequencies. Higher concentrations (5-20 mM) progressively depress the inhibitory response. It is suggested that TEA may specifically antagonize the post-synaptic action of the inhibitory transmitter and that at higher concentrations of TEA this effect dominates the pre-synaptic action in increasing transmitter release. 3 TEA has no effect on the motor response to tyramine. 4 TEA (5-20 mM) causes a maintained rise in muscle tone. Part of this is abolished by phentolamine but part is resistant. A similar muscle stimulant action of TEA is observed in muscles from rats previously treated with 6-hydroxydopamine in which indirect sympathomimetic drugs and field stimulation could no longer produce a motor response. These results suggest that part of the motor effect of TEA is due to an increased spontaneous release of noradrenaline and part to a direct action on the muscle. 5 TEA 0.125 mM to 20 mM antagonize the stimulant action of carbachol. Dose-response curves show a parallel shift to the right with no change in the maximum response suggesting a competitive atropine-like action. Such an effect has previously been reported in amphibian tissue but not so far as we can determine in mammalian preparations. 6 The possible mode of action of TEA is discussed.

Mutants with reduced Ca activation in Paramecium aurelia

Two heat-sensitive "pawn" mutants of Paramecium aurelia are capable of avoiding reactions when grown at 23 degrees C but not at 35 degrees C. Electrophysiological analyses show that Ca activation is reduces in the mutants even when they are grown at 23 degrees C. The maximal rate of rise and the peak of the evoked action potential (Ca-spike) in the mutants are smaller than those of wild type in a K-solution. After suppression of K conductance by either TEA+ or Ba++, the action potentials of the mutants peak at the same level as that of wild type. However, the maximal rate of rise of the mutants remains only about half that of wild type. Thus, the mutations affect Ca activation but not K activation. Incubation at a high temperature (35 degrees C) further reduces Ca activation to almost zero in the mutants but has little or no effect on wild type. This almost complete loss of Ca activation explains the lack of avoiding reactions when the mutants are grown at high temperatures. A double mutant containing two heat-sensitive mutations shows extremely reduced Ca activation even when grown at 23 degrees C.

A mutant of Paramecium with increased relative resting potassium permeability

Fast-2, a membrane mutant of Paramecium aurelia, is due to a single-gene mutation and has behavioral abnormalities. Intracellular recordings through changes of external solutions were made. The mutant membrane hyperpolarized when it encountered solutions with low K+ concentration. This hyperpolarization and other associated activities were best observed in Ca- or Na-solutions devoid of K+. Membrane potential was plotted against the concentration of K+ (0.5 to 16 mM) in solutions of fixed Na+ or Ca++ concentration. The slopes of the curves for the mutant membrane were steeper than those for the wild type at the lower concentrations of K+. Inclusion of 2 mM tetraethylammonium chloride (TEA-Cl) counteracted the mutational effects. Spontaneous action potentials in Ba-solution and the electrically evoked action potentials in various solutions are normal in this mutant. We conclude that the resting permeability to K+ relative to the permeabilities to Na+ and Ca++ has been increased by the mutation.

Effects of thiamine antagonists on nerve conduction. I. Actions of antimetabolites and fern extract on propagated action potentials

To assess the hypothesis that thiamine is directly involved in the permeability changes at the sodium channel during nerve conduction, the effects of thiamine antagonists on lobster giant axon resting and action potentials were determined. Thiamine antimetabolites, in millimolar concentrations, reversibly decreased the maximum rate of rise and amplitude of the action potential while increasing its duration. In particular, thiamine tert-butyl disulfide (TTBD) elicited the formation of pronounced shoulders during repolarization, lengthening the action potential by 2-50 times, depending on dose. Antimetabolites also depolarized the resting membrane, but this change was poorly reversible and may indicate a dual mechanism for antimetabolite action. An extract of the fern, Pteris aquilina, reversibly decreased the maximum rate of rise of the action potential and depolarized the resting potential. It also elevated and prolonged the action potential after-depolarization, sometimes causing repetitive activity. The strength of these actions was correlated with the antithiamine potency of the extract, and was diminished by addition of thiamine to the extract.

The electrical basis of excitation and inhibition in the rat anoccygeus muscle

1. The transmembrane potential of smooth muscle cells of the rat anococcygeus muscle was studied with micro-electrodes. The muscle had a mean resting membrane potential of --61-5 mV and normally lacked spontaneous mechanical and electrical activity. 2. Stimulation of intramural nerves with single pulses produced a small depolarization and contraction. At frequencies greater than 0.5 Hz a second component occurred which had a maximum value of 35--51 mV with short trains at 30 Hz. Spike potentials were rarely seen. Depolarization was accompanied by a decrease in membrane resistance. 3. Noradrenaline (3 times 10-minus 5 M) and guanethidine (3 times 10-minus 5 M) both depolarized the membrane and produced contraction. Initially, oscillations in the membrane potential were oftern seen. 4. Intramural nerve stimulation in the presence of guanethidine damped the oscillations and produced relaxation. This was accompanied by neither hyperpolarization nor a change in membrane resistance. 5. TEA (1 mM) depolarized the membrane (--47-6 mV) and initiated spontaneous activity. Field stimulation evoked spikes with overshoot.

Effects of tetraethylammonium chloride on the membrane activity of guinea-pig stomach smooth muscle

1. The effects of tetraethylammonium (TEA) on the membrane activity of the antral circular muscle of the guinea-pig stomach were investigated with micro-electrode and double sucrose gap methods.2. In a concentration of 1-1.5 x 10(-3) g/ml. (3-5 mM), the membrane potential was not influenced; the membrane resistance measured by inward current pulses remained the same but the rectifying property of the membrane was suppressed.3. TEA (1-1.5 x 10(-3) g/ml.) enhanced the spike amplitude markedly even from fibres which generated graded responses.4. TEA (1-1.5 x 10(-3) g/ml.) did not increase the maximum rate of rise of the spike but decreased the maximum rate of fall of the spike markedly.5. In Na-free (Tris or sucrose) solution, in K-deficient and excess-K solutions, TEA (1-1.5 x 10(-3) g/ml.) suppressed the rectifying property of the membrane and enhanced the spike amplitude.6. Atropine (10(-6) g/ml.) had no effect on the enhancement of the spike amplitude produced by TEA.7. The minimum concentration of Ca ions required for the effect of TEA on the spike amplitude was one fifth of the normal concentration. TEA also enhanced the spike amplitude in Sr-Krebs.8. The possible role of TEA on the membrane activity is considered to be due to suppression of the K conductance when the membrane is depolarized. Alternative possible roles of TEA on the spike amplitude are also discussed.

Effects of changes of ionic environment on the negative after-potential of the spike in rat uterine muscle

1. The spontaneous activity of the smooth muscle of rat uterus consists of bursts of spikes, each spike being followed by a negative after-potential. The effect of changes in ionic environment on the negative after-potential was investigated at various stages of pregnancy and in the non-pregnant condition.2. The amplitude of the negative after-potential was the same in spontaneously generated and electrically evoked spikes. During repetitive discharge, whether spontaneous or in response to depolarizing current application, the amplitude of the after-potential was smallest in the first spike of a burst and it increased gradually with repetition of discharge.3. The decay of the negative after-potential was slower than the passive return of the membrane potential to its resting level.4. The amplitude of the negative after-potential was larger in non-pregnant uterus than during late pregnancy.5. In pregnant uterus, the replacement of the Cl in the medium with benzene sulphonate transiently augmented the negative after-potential and then gradually reduced it. Eventually, the negative after-potential disappeared and, instead, a positive after-potential was observed. This conversion took place without a noticeable change in the resting potential or in the initial falling phase of the action potential itself. Replacement of Cl with NO(3) had no appreciable effect on the negative after-potential.6. In non-pregnant uterus, the conversion of the negative to a positive after-potential was never observed. However, in Cl-deficient solution the size and duration of the negative after-potential were reduced.7. In Cl-deficient solution (benzene sulphonate substitution), the decay of the electrotonic potential following the break of cathodal current became faster than that in normal solution. On the other hand the development of the anodic electrotonic potential became slower.8. Replacement of the NaCl in the medium with sucrose converted the negative after-potential to a positive after-potential. On the other hand, reduction of Na only by replacement of NaCl with Tris-Cl had no noticeable effect on the negative after-potential.9. It is concluded that the negative after-potential of the spike in rat uterine muscle is largely due to an increase of Cl conductance of the membrane.

The effect of the tetraethylammonium ion on the delayed currents of frog skeletal muscle

1. A method which permitted control of the membrane potential near the end of a muscle fibre and measurement of an approximation of the membrane current was used to investigate the effects of the tetraethylammonium (TEA) ion on the delayed outward potassium current obtained on depolarizing.2. Assuming R(i) to be 250 Omega cm and a fibre diameter of 80 mu, the mean value for the maximum potassium conductance (g(K)) was 23.2 +/- 3.2 mmho.cm(-2).3. 58 mM-TEA, in two series of experiments, reduced g(K) by about 90%. A concentration-effect relation for TEA in its action on the delayed rectifier could be fitted by a curve for a drug-receptor complex assuming one molecule of TEA to combine reversibly with one receptor, and a dissociation constant of 8 x 10(-3)M.4. TEA tended to shift the threshold for delayed rectification to slightly more negative membrane potentials. TEA caused a similar shift in the relation between n(infinity) and membrane potential, but did not much alter the form of the relation.5. The relation between n(infinity) and membrane potential and between tau(n) (-1) and membrane potential were well fitted by the model of Adrian, Chandler & Hodgkin (1970a) assuming that the Q(10) was 2.5.6. TEA slowed the rate of onset of the delayed potassium currents, decreasing tau(n) (-1) (the reciprocal of the time constant of the fourth power function which described the current's development) by about 80%.7. The inactivation of the delayed current with time was shown to follow a complex time course. A fast phase decays with a time constant of 270 msec and a slow phase with a time constant of 2.3 sec at a membrane potential of + 10 mV.8. The fast phase of the delayed current is much more susceptible to the action of TEA than the slow phase, and these are interpreted in terms of different potassium channels. TEA has little effect on the time constant with which either the fast current or the slow current inactivates.

The differential effects of tetraethylammonium and zinc ions on the resting conductance of frog skeletal muscle

1. The effects of tetraethylammonium (TEA) and zinc ions on the resting conductance of frog muscle were examined using a method which permitted control of the membrane potential near the end of a muscle fibre and measurement of an approximation of the membrane current.2. TEA reduced the amplitude of the inactivating inward current obtained on hyperpolarizing, both when this was measured as initial current [I(m)(0)] less the estimated chloride current and as [I(m)(0) - I(m)(infinity)]. 115 mM-TEA reduced the amplitude of [I(m)(0) - I(m)(infinity)] by about 85%.3. TEA had little effect on the time constant with which the inward potassium currents inactivated. This finding appeared to be in conflict with the view that the inactivation is due to depletion of potassium from the T-system, though the results in standard Ringer were in good agreement with such an hypothesis.4. In the standard chloride Ringer, the resting membrane resistance was 3530 Omega cm(2). 115 mM-TEA increased this 1.6 times to 5580 Omega cm(2).5. The effect of TEA on voltage-current relations obtained in high K(2)SO(4) solutions was also examined. With large pulses, the voltage-current relations in TEA and control solutions tended to become linear. This linear part could be extrapolated and subtracted from the voltage-current relation.6. TEA, at a concentration of 150 mM, reversibly reduced by 88% the amplitude of the currents obtained during both depolarizing and hyperpolarizing pulses; this figure was obtained after subtraction of the linear part of the relation, which was itself unaffected by TEA and which represented a resistance of 5600 Omega cm(2).7. The reductions in the inactivating potassium current and in the current flowing through the inwardly rectifying potassium channel in high K(2)SO(4) solutions fit, fairly closely, a concentration-effect relation for TEA with a dissociation constant of 20 x 10(-3)M.8. Also investigated were the slow time-dependent changes in membrane potential occurring when [K](o) is altered from 2.5 to 10 mM at constant chloride (120 mM). The findings were consistent with the view that TEA reduced the potassium conductance without much effect on the chloride conductance. In particular, when muscles equilibrated for 1 hr in 10 mM-K Ringer were returned to 2.5 mM-K, the initial rate of repolarization was reduced, even though the driving force on K was slightly larger than in the control experiment.9. Zinc ions appeared to be without effect either on the amplitude of the inactivating potassium current obtained when square hyperpolarizing pulses were applied to the fibre membrane, or on its time constant.10. In standard Tris-buffered Ringer, the membrane resistance was 3350 Omega cm(2). Zinc Ringer (2.5 mM) increased this rather more than twice to 6830 Omega cm(2).11. It is concluded that the two agents, TEA(+) and Zn(2+), which both reduce delayed potassium currents in muscle, act in different ways on the resting conductance of frog muscle.

Actions of some cations on the electrical properties and mechanical threshold of frog sartorius muscle fibers

With the use of a point voltage-clamp technique, the effects of Zn(2+), UO(2) (2+), tetraethylammonium, and several other homologous quaternary ammonium ions on the electrical properties of the frog sartorius muscle and its mechanical threshold were studied. None of the agents separated the voltage thresholds for mechanical activation and delayed rectification. However, Zn(2+), UO(2) (2+), and TEA, which are known to potentiate the twitch, caused some inhibition of the normal increase in potassium conductance during delayed rectification. Zn(2+) and UO(2) (2+) also slowed the rate of development of the outward current. A strength-duration relation was studied for depolarization pulses capable of initiating contraction. With a depolarizing pulse of 2.5 msec the mechanical threshold is about -13 mv at about 20 degrees C. UO(2) (2+), 0.5 microM, which markedly reduced the outward current produced by such a short pulse, did not raise the mechanical threshold. All findings indicate that there is no direct causal relation between delayed rectification and mechanical activation.

The effect o f calcium on contraction and conductance thresholds in frog skeletal muscle

1. The effect of extracellular calcium and magnesium on the contraction threshold and on the thresholds for an increase in sodium and potassium conductance with depolarization was studied in voltage-clamped frog muscle fibres.2. A larger depolarization was required to reach each of the three thresholds when the concentration of divalent cation was increased.3. The contraction and potassium conductance thresholds appeared to shift in parallel with alterations in calcium over the concentration range 0.2-10.0 mM and in magnesium over the concentration range 5.4-90.0 mM. The shift amounted to about 4 mV for a threefold change in concentration of divalent cation.4. The sodium conductance threshold was much more sensitive to alterations in divalent cation concentration than was either the contraction or the potassium conductance threshold.

Actions of gallamine and tetraethylammonium at the frog neuromuscular junction

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Tetraethylammonium and tetrodotoxin: effects on cochlear potentials

Tetraethylammonium chloride, which is believed to decrease potassium conductance, and tetrodotoxin, which apparently decreases sodium conductance in nerve fibers, were introduced iontophoretically into the organ of Corti or the scala media of guinea pig cochlea. The former depressed the direct-current endocochlear potential and also the alternating-current cochlear microphonics (the receptor potential of the ear), but tetrodotoxin was ineffective except on the nerve impulses.