Action Potential Height Research Articles

Mechanical and electrical correlates of isometric muscle fatigue in skeletal muscle in the cat.

The amplitude and duration of motor unit action potentials, the rise time, peak tension, and half relaxation time of an isometric twitch, the force-velocity relationship, and tetanic tension were measured at the beginning (fresh muscle) and at the end of a fatiguing isometric contraction at a tension of either 40 or 70% of the initial strength in the soleus (a slow twitch muscle) and the medial gastrocnemius (a fast twitch muscle) of the cat. These same parameters were also measured at set intervals following these contractions to assess their rate of recovery to pre-exercise values. At The end of a fatiguing contraction at either tension examined, Vmx, twitch tension and tetanic tension, were all reduced while there was a prolonged twitch duration and duration of the motor unit action potential for both types of muscle. The height of the motor unit action potential was only marginally effected by muscle fatigue. Following the fatiguing contraction, the endurance required several hours to recover in the medial gastrocnemius muscle but recovered fully within 15 min after either tension in the soleus muscle. Tetanic tension and twitch tension both required less than 10 min for full recovery in the medial gastrocnemius muscle but recovered fully to the pre-exercise values within 3 min following fatiguing isometric contractions in the soleus muscles. Vmx, and the height and duration of the motor unit action potential both recovered within 1 min following the end of the exercise.

Failure of peptides (angiotensin, bradykinin, substance P, physalaemin, Met-enkephalin) to influence the cardiac action potential.

Angiotensin, bradykinin, substance P, physalaemin, and Met-enkephalin were tested for their ability to affect the action potential (AP) of the isolated guinea pig papillary muscle driven at 1/sec and 3/sec. Variables studied were height and duration of AP, dV/dtmax, half-life of recovery of dV/dtmax, and refractory period. At relevant concentrations the peptides were not effective.

Single cell isolation: A way to examine network interactions

A procedure for isolating identified, small neurons from snail ganglia is described. The technique allows a particular neuron, previously identified by morphological and electrophysiological characteristics, to be marked and then isolated from the ganglia. This procedure was developed to permit the detailed comparison of the electrical characteristics of a neuron before and after isolation from an intact system. An earlier description has appeared. The cell somata is marked intracellularly by the iontophoretic injection of Procion navy blue H3RS which visually differentiates the cell from other cells in the ganglion. The ganglion is then treated with a trypsin-haluronidase solution to soften the ganglion sheath, which is then removed. The cells are gently shaken to isolate them from the ganglion and then examined electrophysiologically. A comparison of membrane properties, such as action potential height, duration and rate of rise and decay was made before and after all treatments were applied to assess deleterious effect. An analysis of network properties, such as burst duration, number of spikes per burst and presynaptic activity was also performed after each phase of the procedure. No significant differences were noted after dye injection, enzyme treatment, and where appropriate, after isolation. An increase in input resistance and corresponding decrease in the slope of the steady state current--voltage plot (I--V plot) were observed after isolation of a cell. These were expected results of removing the 'load' (i.e. axon or electrical coupling) from the cell soma. This method may be applied to many other systems to study the effects of network interactions on the properties of a single cell and should therefore facilitate the analysis of neuronal networks as well as single cell properties.

Effect of stretch on conduction velocity and cable properties of cardiac Purkinje fibers.

Cardiac Purkinje fibers were studied before and after stretch to 30% and 50% in excess of their slack length, and membrane properties and conduction velocity were measured in relation to the stretch. Conduction velocity increased by 26% with 50% stretch. The resting potentials averaged -77 mV and did not change with stretch. In addition, the action potential height, maximal upstroke velocity, duration, and time constant of the foot did not change. These results suggested that the increase in conduction velocity was not due to a change in membrane excitability. Two geometric models of stretch, called unfolding and uncoiling, were considered. Cable analyses were performed to distinguish between these models. The effects were mixed, with about 2/3 of the length change resulting from unfolding and 1/3 from uncoiling. These results support the concept of redundancy of the surface membrane by folding. The changes are likely to be of importance in activation of the heart by the His-Purkinje system, especially in cardiac dilatation due to disease.

The Clinical Character of Local Anesthetics: A Function of Frequency‐Dependent Conduction Block

It is clinically recognized fact that some local anesthetics have a proclivity for inhibition of motor nerves, while others preferentially affect sensory and sympathetic fibers. On the basis that sensory fibers have a frequency threshold for transmission of nociceptive stimuli and that somatic motor fibers have no such frequency threshold, we hypothesized that this variation may be due to differences in the effect of local anesthetics on axonal refractory period. Frog sciatic nerves were partially blocked with lidocaine, bupivacaine, tetracaine and etidocaine, and then stimulated in trains of 17 pulses, at frequencies between 3 and 100 Hz. The height and area of the last action potential in a train were measured. At a comparable level of partial block (50% at 100 Hz), tetracaine and etidocaine showed only a 10% difference between 3 and 100 Hz, while with bupivacaine and lidocaine there was a 30% drop between these two frequencies. This excellent correlation between the laboratory and clinical phenomenon supports our hypothesis. Local anesthetics which have a minimal effect on the refractory period yield enhanced motor block; whereas local anesthetics with a large effect on the refractory period are relatively more potent blockers of sensory and sympathetic transmission.

ACTION OF GUANETHIDINE ON RABBIT ATRIAL MEMBRANES

1 Intracellular potentials were recorded in driven left atria from reserpine-treated rabbits. Guanethidine 2 X 10(-5) M slightly increased Vmax and shortened the total duration (TD) of the action potential (AP) without causing hyperpolarization. For the first 30 min after 4 X 10(-4) M, Vmax increased without hyperpolarization and AP height increased slightly. Thereafter, Vmax and height decreased with a slight and gradual depolarization. This depolarization was irreversible. TD was increased after 15 minutes. Guanethidine 2 X 10(-3) M initially decreased Vmax and height before causing depolarization. 2. Pretreatment with tetrodotoxin (TTX) 1.6 X 10(-7) M prevented or reversed the initial increases in Vmax, height and TD induced by guanethidine (4 X 10(-4) M). 3 TTX 3.1 to 6.2 X 10(-6) M, added 15 or 30 min after guanethidine 4 X 10(-4) M, delayed or prevented depolarization by guanethidine. 4 Ouabain 10(-5) M incubated for 20 and 90 min greatly inhibited Na+, K+-adenosine triphosphatase and K+-phosphatase activities; guanethidine was without effect. 5 Guanethidine probably increases resting sodium permeability after the promotion of increases in sodium permeability during the AP. High doses of the drug decrease sodium permeability during the AP.

Membrane potential in the isolated papillary muscle of rats with experimental myotonia

The membrane potential was measured in isolated papillary muscles of rats, in which myotonia of skeletal muscle was inducedin vivo by 20,25-diazacholesterol. It was found a periodical conversion of the ordinary ventricular action potential into pacemaker like potentials; at subsequent and steady stimulation normal ventricular action potentials were measured in the same cells, and automaticity did not occur. At beginning of stimulation the membrane potential showed a prolonged after-depolarization, eventually turning into a second spontaneous fast upstroke, and returned to normal at steady stimulation. Resting potential, height of action potential, maximum rate of rise of depolarization and effective and absolute refractory period were normal. These parameters remained unchanged, when the spontaneous activity was inhibited in the presence of 2.92 × 10−5 M diphenylhydantoin or 1.87 × 10−5 M lidocaine. It is suggested, that the instability of the membrane potential in the isolated papillary muscle is due to an impaired repolarization probably as a result of a reduced chloride conductance.

A convenient bioassay for detecting nanomolar concentrations of tetrodotoxin

We describe a convenient as well as highly sensitive bioassay procedure for detecting nanomolar concentrations of tetrodotoxin. The method utilizes desheathed frog ( Rana pipiens) sciatic nerve mounted in a modified sucrose gap apparatus and depends on a measure of the reduction of the compound action potential height when the nerve bundle is exposed to the test solution ( ca. 1 ml). The concentration of TTX needed to effect a 50 per cent reduction in height of the compound action potential was found to be 8 nM. We have shown that the Oregon newt Taricha granulosa contains on the order of 1 mg of TTX per g of newt.

The effect of guanethidine pretreatment on transmission in the superior cervical ganglion.

Abstract:Isolated superior cervical ganglia from rats pretreated with guanethidine sulphate intraperitoneally in single daily doses of 20‐40 mg/kg for 5‐21 days showed transmitted action potentials of normal form but diminished amplitude following single or repetitive stimulation of the preganglionic nerve. Curves relating the relative reduction in ganglionic action potential height with increasing frequency of stimulation were similar in preparations from control and guanethidine pretreated rats. Ganglia from guanethidine pretreated rats had the same sensitivity to exogenous acetylcholine as those from the controls as judged by doseidepolarization response curves. The absolute values of depolarization were, however, diminished. No impairment of the conduction of impulses in the preganglionic nerve or of the effect of antidromic stimulation was observed. The addition of guanethidine to the bath fluid resulted in an acute reversible ganglion block. A 50 % reduction of the ganglionic action potentials following single stimuli was observed at a concentration of 2 × 10−5M. At guanethidine concentrations of 5 × 10−5M maximum depolarizations obtained with acetylcholine or carbachol were reduced by approximately 50 % but the doselresponse curves were not shifted. Recovery was complete after washing. Retractions of the cat nictitating membrane following repetitive stimulation of the preganglionic nerve after pretreatment with guanethidine 5‐40 mg/kg for 14 days were preferentially decreased at low stimulus frequencies. The nictitating membrane was hypersensitive to intravenous adrenaline or noradrenaline. Contralateral ganglia from both rats and cats exhibited profound structural changes consisting of chromatolysis of the nerve cells and infiltration of small cells. The effects of guanethidine on the neurophysiological parameters are probably the result of a dose dependent inactivation of the majority of the ganglion cells.

A myotoxin secreted by some piscivorous Conus species.

1. Toxins isolated from the venom apparatus of Conus magus and Conus achatinus have the same pharmacological properties, but differ from the toxins of several other piscivorous species of cone shells.2. C. magus and C. achatinus toxins are heat labile at pH 8.5. A single lethal component with an approximate molecular weight of 10,000 was isolated from C. achatinus toxin by exclusion chromatography.3. Animals died from a characteristic spastic paralysis after intravenous injection of the toxin.4. Nerve transmission was unaffected by the toxin; skeletal muscle appeared to be the primary site of action. The toxin caused a persistent contracture of rat diaphragm muscle, and a dose-dependent decline in twitch tension. The contracture was potentiated by caffeine.5. The decline in twitch tension was associated with depolarization of the cell membrane. Action potential height and the maximum rate of rise declined, and spike propagation failed when the resting potential had declined to approximately 60 mV. The muscle recovered very slowly on washout of the toxin. The depolarization could be reversed by exposure of the preparation to 5 mM Na(+) solution or tetrodotoxin or saxitoxin.6. Miniature end-plate potential frequency in the rat diaphragm decreased, as did the quantal content of the end-plate potential. The acetylcholine-induced contraction and depolarization of the chronically denervated rat diaphragm were increased by low doses of toxin and reduced by higher, depolarizing toxin doses. The K(+)-induced contraction and depolarization of innervated diaphragm were similarly affected by the toxin.7. Cardiac and smooth muscle were relatively resistant to the toxin. The isotonic contraction of the isolated perfused guinea-pig heart was increased by the toxins from both Conidae. The heart rate decreased. Guinea-pig atrial cells showed a small decrease in action potential height and maximum rate of rise. Rabbit sino-atrial cells showed increases in action potential height, maximum diastolic potential and maximum rate of rise of the spike at low toxin levels, and no change in any of these parameters at high levels. There was a decrease in the rate of the spontaneously beating atrium. Atrioventricular nodal potentials showed no change other than a slight increase in the maximum rate of rise of the action potential.8. It is postulated that the action of the toxin may be related to a change in the Ca(++) permeability of the excitable membrane, which makes it unstable, leading to a secondary, depolarizing entry of Na(+). The effects of the toxin are compared with those of batrachotoxin, which it somewhat resembles.

The effect of amiodarone, a new anti‐anginal drug, on cardiac muscle

1. Amiodarone (2-butyl, 3-(4-diethylaminoethoxy, 3,5-diiodo, benzoyl) benzofuran hydrochloride), an anti-anginal drug which causes coronary dilatation and depresses myocardial oxygen consumption, was found to protect anaesthetized guinea-pigs against ouabain-induced ventricular fibrillation.2. A 5% (73.4 mM) solution of amiodarone had no local anaesthetic action on guinea-pig skin.3. Amiodarone, 20 mg/kg (29.4 mumol/kg) given daily for 6 weeks intraperitoneally, had no effect on the resting potential or action potential height, and only a small effect on the maximum rate of depolarization, of isolated rabbit atrial or ventricular muscle fibres as shown by intracellular recording. It caused a considerable prolongation of the action potential in both tissues.4. Simultaneous administration of thyroxine (5 mug; 6.26 nmol), given daily for 3 weeks intraperitoneally, prevented the prolongation by amiodarone of the duration of the action potential.5. Treatment of rabbits with 20 mg/kg of amiodarone daily intraperitoneally for 6 weeks had no effect on the weight of the thyroid gland, but was associated with a reduction in body growth rate.6. Treatment of rabbits with 10 mg/kg (60.3 mumol/kg) of potassium iodide (equal in its iodine content to that of 20 mg/kg of amiodarone), given daily for 6 weeks intraperitoneally, had no effect on body growth rate or the duration of cardiac action potentials.7. It was concluded that amiodarone had effects on cardiac action potentials similar to those which occur after thyroidectomy.

Local anaesthetic and anti-arrhythmic actions of alprenolol relative to its effect on intracellular potentials and other properties of isolated cardiac muscle.

1. Alprenolol, a beta-adrenoceptor blocking drug reported to have the same potency as propranolol in vitro and in vivo, was found to be four times more active than procaine as a local anaesthetic on frog sciatic nerve.2. At doses of 0.125 mg/kg and above alprenolol protected anaesthetized guinea-pigs against ouabain-induced ventricular fibrillation.3. In isolated rabbit atria, resting potentials were unchanged, and the duration of the action potential was not prolonged, by concentrations of alprenolol up to 10.5 x 10(-6)M. The maximum rate of depolarization (MRD), however, was reduced by 30% at a concentration of 0.525 x 10(-6)M. This was 214 times less than the concentration which reduced the frog action potential height by 25%.4. The concentration of alprenolol required to produce more than 15% decrease in contraction or maximum driven frequency was 3.5 x 10(-6)M.5. As a test of the direct action of alprenolol on isolated cardiac muscle, MRD was also a more sensitive test than the measurement of electrical threshold or conduction velocity.

The effect of intracellular current pulses on membrane potentials in the moth heart

1. 1. A single microelectrode was used for simultaneously passing current pulses and recording bioelectric activity in single cells of the moth heart. 2. 2. The cell membrane responds to depolarizing and hyperpolarizing pulses as an ohmic resitor, showign no rectification with current flows up to 5 × 10 −9 A. 3. 3. A polarization resistance of 3.8±1 MΩ was measured and a specific membrane resistance of 1360Ω cm 2 was calculated. 4. 4. Action potential height was attenuated by depolarization and augmented by hyperpolarization. 5. 5. Although the slope of the prepotential declined in response to depolarizing pulses and increased in response to hyperpolarizing pulses, the rhythm of the spontaneous beat was not altered. 6. 6. The average rate of fall of the action potential was increased by hyper- polarization and diminished so markedly by depolarization that a plateau phase appeared where none had previously been detectable. The initial rising phase and the falling phase of the action potential could thus be altered by manipulating the membrane voltage.

THE EFFECT OF CYCLOPROPANE, HALOTHANE AND ETHER ON SYMPATHETIC GANGLIONIC TRANSMISSION

SUMMARY The actions of cyclopropane, halothane, and ether on sympathetic ganglionic transmission were studied by recording the compound action potential evoked in postganglionic nerves by single shock stimulation of the preganglionic pathways. The ganglia investigated were the superior cervical and inferior mesenteric in the rabbit, and the superior cervical and stellate ganglia in the cat. All three anaesthetics reduced the height of the postganglionic compound action potential. This effect is interpretable as partial ganglion block, but requires further study in view of the increased discharge rate measured in both pre- and postganglionic sympathetic nerves during associated studies with inhalation anaesthetics in rabbits.

EFFECT OF ETHANOL ON THE SODIUM AND POTASSIUM CONDUCTANCES OF THE SQUID AXON MEMBRANE.

The effects of ethanol on squid giant axons were studied by means of the sucrose-gap technique. The membrane action potential height is moderately reduced and the duration sometimes shortened by ethanol in sea water. Voltage clamp experiments showed that ethanol in sea water reduced the maximum membrane conductances for sodium (g'(Na)) and potassium (g'(K)). In experiments with multiple application of ethyl alcohol to the same spot of membrane, a reduction of g'(Na) to 82 per cent and of g'(K) to 80 per cent of their value in sea water was brought about by 3 per cent ethanol (by volume) while 6 per cent caused a decrease of g'(Na) to 59 per cent and of g'(K) to 69 per cent. Ethanol has no significant effect on the steady-state inactivation of g(Na) (as a function of conditioning membrane potential) or on such kinetic parameters as tau(h) or the time course of turning on gi g(Na) and g(K). It is concluded that ethanol mainly reduces g(Na) and g(K) in the Hodgkin-Huxley terminology.

PARACHLOROMERCURIBENZOATE ON ACTION POTENTIALS OF PURKINJE AND VENTRICULAR FIBERS OF HOG HEART.

Parachloromercuribenzoate (0.5 to 2.6 x 10–4 m) was administered to a hog Purkinje fiber ventricular muscle preparation and its effects on transmembrane action potentials of the two types of fibers were studied. Parachloromercuribenzoate produced changes in action potential of Purkinje but little change in that of ventricular fiber. The earliest alteration in Purkinje fiber was a decrease in overshoot. This was followed by a decrease in height of action potential and slowing in rate of depolarization. There was a decline in diastolic resting potential but this occurred late. The difference in response of the two types of fibers is not readily explainable. One possible difference is that the sulfhydryl groups in Purkinje fiber may be more available for binding with p-chloromercuribenzoate. Another hypothesis is predicated on differences in metabolism in two types of fibers. There is evidence to indicate that Purkinje fiber utilizes anaerobic glycolysis predominantly. A greater sensitivity of the glycolytic-dependent sulfhydryl enzymes to action of this agent would interfere with metabolism in Purkinje fiber and cause a decrease in its resting potential.

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(-5)M 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.

Electrophysiologic antagonism and synergism of potassium and antiarrhythmic agents

Interactions of potassium and antiarrhythmic agents on ventricular myocardial electrophysiology were studied in 25 isolated, perfused rabbit hearts by utilizing an intracellular microelectrode technic. Perfusion of quinidine (0.01 mg./ml.) in a normal concentration of potassium (5.6 mEq./L.) caused a significant decrease in the maximal rate of depolarization (−69.1%), heart rate (−40.4%) and the action potential duration per unit of time (−13.8%). Reduction in the height of action and resting potentials was insignificant. Lowering potassium to 0.8 mEq./L., in the presence of the same concentration of quinidine, significantly increased the maximal rate of depolarization (+82.5%), the height of action potential (+8.5%) and resting potential (+9.9%). The action potential duration per unit of time also showed a significant increase (+15.2%) to control value, although the heart rate was further decreased. Initial perfusion of a low potassium solution caused a significant increase in the height of action potential (+3.3%) and resting potential (+7.0%) as well as a significant decrease in the action potential duration per unit of time (−19.2%). The maximal rate of depolarization and the heart rate showed no significant changes. Addition of quinidine to this low potassium perfusate did not produce any appreciable changes in the maximal rate of depolarization, or in the height of action and resting potentials. The action potential duration per unit of time again returned to its control level. Hence, the electrophysiologic action of quinidine was nullified by a low potassium concentration in both groups of experiments. Quite a similar antagonism was demonstrated between low potassium concentration and other antiarrhythmic agents (antazoline and SU-11636), while high potassium concentration (12 mEq./L.) enhanced the depressive effects of these three drugs. The electrophysiologic mechanism of antiarrhythmic agents was discussed, and the clinical importance of controlling serum potassium concentration during the administration of these agents was suggested.

Alteration of peripheral nerve function in the rat after prolonged outdoor cold exposure

Electrical characteristics of in vitro preparations of the ventral tail nerve were compared in rats exposed for 6 months to outdoor winter cold and control rats kept indoors at 24 C. Determinations of conduction velocity, excitability, and action potential height demonstrated changes in nerve function following outdoor cold exposure. Differences between nerves of outdoor and indoor rats were greatest at higher nerve temperatures (35 C), becoming less at lower temperatures, until no differences were present at 5 C. Conduction velocity showed the greatest changes with cold exposure. Excitability changes could only be demonstrated with stimulus durations less than .1 msec. No differences in absolutely refractory period were seen between cold-exposed and control rats. In general, changes that occurred with cold exposure resulted only in a decrease in the functions examined. The results show that changes in peripheral nerve function can occur in nonhibernating mammals following prolonged exposure to climatic cold, but that these changes are not necessarily of an adaptive nature.

Posttetanic changes in the human neuromuscular system.

Percutaneous stimulation of the human ulnar nerve produced muscle action potentials from the adductor pollicis muscle and twitch tension responses from thumb adduction. With increase in the stimulus strength, approximately proportional increases were found in the height of the muscle action potential and twitch tension. Tetanization can produce a long lasting potentiation of twitch tension which is accompanied by either increased, decreased or unchanged amplitude of muscle action potentials. With increase in the duration, frequency or intensity of the tetanus, an additional effect producing decreased muscle action potentials and twitch tension responses appeared, particularly at 20–30 seconds after the cessation of the tetanus. Possible mechanisms and sites of action of posttetanic potentiation in the neuromuscular system are mentioned. Submitted on December 28, 1956