Membrane current and intracellular sodium changes in a snail neurone during extrusion of injected sodium

The distribution of sodium and potassium in amphibian embryos during early development

1. Intracellular and intercellular concentrations of sodium and potassium have been measured in pregastrular embryos of Xenopus laevis and Amblystoma mexicanum. Calcium and magnesium contents have also been determined.2. Between egg and gastrula stages of development the potassium concentration is near to 60 m-mole/l. embryo. Up to the blastula stage the sodium concentration is near to 50 m-mole/l. embryo; the amount of sodium in the embryo begins to fall just before gastrulation.3. Embryo calcium and magnesium concentrations show no significant variations prior to gastrulation. Average calcium concentrations for the different stages range from 5 to 8 m-mole/l. embryo; magnesium concentrations lie between 12.4 and 17 m-mole/l. embryo.4. The intercellular fluid contains 100 mM sodium and 1 mM potassium; the majority of the sodium is ionically active. As no net uptake of sodium or potassium occurs before gastrulation these cations must have been transferred from the cells to the cavity.5. At all developmental stages studied, the intracellular potassium concentration is close to 100 m-mole/l. cell water. Intracellular sodium falls steadily from 80 m-mole/l. cell water in eggs to 30 m-mole/l. cell water at the beginning of gastrulation.6. The intracellular sodium activity, measured with sodium sensitive intracellular micro-electrodes, is relatively constant between egg and blastula stages at about 14 mM. During each cell division cycle the intracellular sodium activity rises transiently by 2-3 mM.

Conductance changes, an electrogenic pump and the hyperpolarization of leech neurones following impulses

Following trains of impulses, sensory neurones in the C.N.S. of the leech show a prolonged hyperpolarization, which lasts for seconds or minutes. In the present investigation the mechanisms that underly this hyperpolarization have been studied by recording intracellularly. Two factors have been found to be responsible. One is the activity of an electrogenic pump (see Baylor & Nicholls, 1969b); the other is a long-lasting change in K conductance.1. Additional evidence that an electrogenic pump contributes to a slow after-hyperpolarization of leech sensory neurones is provided by the effects of injecting Na intracellularly. This leads to an increase in membrane potential that is blocked by the cardiac glycoside strophanthidin. Furthermore, after a train of impulses, reducing the K concentration in the external fluid characteristically reduces the hyperpolarizing action of the pump.2. The hyperpolarization following impulses is associated with a reduction of the cell membrane resistance that can persist for several minutes.3. Several lines of evidence suggest that the reduction in input resistance during the hyperpolarization is mainly due to an increased permeability to K. Thus, when the K concentration in Ringer fluid is reduced, the peak amplitude of the hyperpolarization following a train becomes larger. Furthermore, the conductance dependent part of the after-hyperpolarization has a reversal potential close to the equilibrium potential for K (E(K)). Substitution of Cl by SO(4) has little effect either on the after-hyperpolarization or on the conductance change following a train.4. Increased external Ca concentrations lead to a marked increase in the hyperpolarization that follows impulse activity. The enhanced hyperpolarization in high Ca is associated with a corresponding reduction in input resistance. The amplitude and duration of the hyperpolarization following a brief train of impulses can be increased by a factor of 5 or more in Ringer fluid containing 10 mM-Ca instead of the usual 1.8 mM. The hyperpolarization and resistance changes still occur in solutions containing 20 mM-Mg.5. To augment the hyperpolarization the increased concentration of Ca must be present during the train of impulses.6. The relative contributions of the K conductance increase and of the electrogenic pump for generating the hyperpolarization after impulse activity are different in the three types of sensory cell responding to touch, pressure and noxious stimulation.

Effects of electrogenic sodium pumping on the membrane potential of longitudinal smooth muscle from terminal ileum of guinea-pig

1. The membrane potential of the separated longitudinal muscle of the guinea-pig terminal ileum was recorded intracellularly with glass micro-electrodes.2. In tissues kept at room temperature and then brought to 35 degrees C for 15-30 min or about 1 hr, the fall in membrane potential upon changing to potassium-free solution was 21.4 +/- 3.5 mV and 13.4 +/- 1.8 mV respectively. Ouabain (1.7 x 10(-6)M) produced a fall in membrane potential of 8.1 +/- 1.1 mV. Returning potassium to potassium-free solution, or changing from ouabain-containing to ouabain-free solution, resulted in an increase in membrane potential which was greater than the initial fall.3. Readmitting potassium to potassium-free solution produced an increase in membrane potential which began within 10 sec and reached a maximum within 15-30 sec. This response was reduced, abolished, or converted to a depolarization by ouabain. In chloride-deficient (13 mM) solution in which membrane resistance was increased, the response to readmitting potassium was increased 2(1/2)-fold so that the membrane potential sometimes exceeded -100 mV, which was probably more negative than E(K). On the basis of these results it was assumed that the response to readmitting potassium was due to the electrogenic activity of the sodium pump.4. The response to briefly readmitting a fixed concentration of potassium increased during the first 30 min in potassium-free solution. This increase was not due to an increase in membrane resistance as this fell with time in potassium-free solution. It was suggested that the increase in the response resulted from the progressive rise in internal sodium concentration which is known to occur in smooth muscle in potassium-free solution.5. Increasing the concentration of potassium over the range approximately 0.1-20 mM, increased the size of the electrogenic potential observed upon readmitting potassium to potassium-free solution. There was a fall in membrane resistance upon readmitting potassium (0.6, 5.9, or 20 mM) which was greater the larger the concentration of potassium. When allowance was made for the fall in membrane resistance, the dependency of the electrogenic response upon the concentration of potassium over the range 0.6-20 mM was much increased.6. The results indicate that the rate of electrogenic sodium pumping in this tissue is increased by increasing the external potassium concentration, and probably by increasing the internal sodium concentration. It was suggested that a rise in the latter could sensitize the pump to an increase in the former.

Active transport of potassium by the giant neuron of the aplysia abdominal ganglion

We measured the internal potassium activity, a(i) (K), and membrane potential, E(m), simultaneously in 111 R2 giant neurons of Aplysia californica. a(i) (K) was 165.3 +/- 3.4 mM, E(m) was -47.8 +/- 0.9 mv, and E(K) calculated using the Nernst equation was -76.9 +/- 0.05 mv. Such values were maintained for as long as 6 hr of continuous recording in untreated cells, a(i) (K) fell exponentially after the following treatments: cooling to 0.5 degrees -4 degrees C, ouabain, zero external potassium, 2,4-dinitrophenol, and cyanide. The effects of cooling and zero potassium were reversible. Potassium permeability was calculated from net potassium flux using the constant field equation and ranged from 2.6 to 18.5 x 10(-8) cm/sec. We conclude that potassium is actively transported into this neuron against a 30-40 mv electrochemical gradient.

Potential-dependent membrane current during the active transport of ions in snail neurones

1. The membrane current caused by the iontophoretic injection of sodium into giant neurones of the snail Helix pomatia was investigated under a long lasting voltage clamp. The inhibition of this current by ouabain (10(-4) M) and by cooling to + 7 degrees C confirmed its link with the active transport of ions. Therefore this current is called the pump current.2. Over the range of membrane potential -40 to -100 mV the changes in the steady current-voltage curves caused by the pump current development were investigated. The pump current was found to be potential-dependent. It decreased with increasing hyperpolarization of the neurone.3. With large hyperpolarizations the current-voltage curves obtained before the sodium injection and after eliciting the pump current coincided with each other. An increase in the membrane conductance was observed over the range of membrane potential corresponding to the pump current display.4. The applied sodium injections did not cause any marked changes in the passive permeability of the membrane. This fact made it possible to measure the charge transferred across the membrane during operation of the pump current. Unlike previous data, the ratio of this value to the charge used to inject sodium into the neurone appeared to be a variable.5. When the preparation was cooled to + 11 degrees C, and also during the first few minutes after the application of a potassium-free solution, both the pump current and the membrane potential at which it disappeared could increase.6. The pump current measurements during a number of transitions from one fixed level of the membrane potential to another showed that the current did not depend upon the potential at which it developed before each transition.7. The data presented allow the suggestion that the potential dependence of the pump current is determined by the changes in the rate of active transport of potassium, while the rate of active transport of sodium remains constant.

Ionic mechanisms of a two-component cholinergic inhibition in Aplysia neurones

1. A two-component inhibition, consisting of a rapid and slow i.p.s.p., has been observed in the medial cells of the pleural ganglion of Aplysia. Each i.p.s.p. has been shown to be mediated by a distinct cholinergic receptor. The ionic mechanisms of the two components of the inhibitory response (whether elicited synaptically or by ACh injection) are analysed in this paper.2. The inversion potential (typically -60 mV) of the rapid i.p.s.p. and of the rapid response to ACh injection is selectively altered by an intracellular injection of chloride or by partial substitution of the external chloride by impermeant anions. The shift caused by this last procedure is similar to that predicted for the chloride equilibrium potential (E(Cl)) by the Nernst equation.3. The slow i.p.s.p. and the slow response to ACh injection (both of which invert around -80 mV) are insensitive to changes in either internal or external chloride concentrations; on the contrary, with alterations of the concentration of potassium in the external medium, the inversion potential of the slow responses is altered in a way similar to that expected for the potassium equilibrium potential (E(K)).4. It is concluded that the rapid i.p.s.p. and the corresponding ACh potential are due to a change in chloride permeability of the post-synaptic membrane, whereas the slow responses are due to a selective change in potassium permeability.5. Additional data suggest that the fast, ;chloride' channel is impermeable to sulphate and methylsulphate, but slightly permeable to propionate and isethionate. The slow, ;potassium' channel is impermeable to caesium ions, whereas its permeability to rubidium ions is half that to potassium.6. The potassium permeability of both the non-synaptic and synaptic membrane is markedly reduced by an intracellular injection of either tetraethylammonium (TEA) or caesium. These ions not only block the cholinergic potassium currents (whether inward or outward) but likewise block the potassium currents activated in the same cells by an iontophoretic injection of dopamine.7. The potassium dependent synaptic potentials are also selectively affected by manipulations known to block the electrogenic sodium pump. In the presence of ouabain or in sea water in which sodium has been replaced by lithium, there is an apparent reduction of these potentials which was shown to be simply a reflexion of the movement of E(K) towards a less polarized level. This shift in inversion potential was not seen for the potassium dependent response to ACh iontophoretic injection. These results are interpreted in terms of accumulation of potassium ions assumed to occur in the extracellular spaces of the neuropile, but not in the thoroughly dissected somatic region.8. Cooling was shown to eliminate, selectively, the synaptic and ACh potential changes caused by an increase in potassium permeability.

Inhibitory and excitatory effects of dopamine on Aplysia neurones

1. Electrophoretic application of dopamine (DA) on Aplysia neurones elicits both excitatory and inhibitory effects, which in many cases are observed in the same neurone, and often result in a biphasic response.2. The DA receptors are localized predominantly on the axons. Desensitization, which occurs after repeated injections or with bath application of DA, is more marked for excitatory responses.3. Tubocurarine and strychnine block the DA excitatory responses without affecting the inhibitory ones, which can be selectively blocked by ergot derivatives. It is concluded that the excitatory and inhibitory effects are mediated by two distinct receptors.4. The two DA receptors can be pharmacologically separated from the three ACh receptors described in the same nervous system.5. In some neurones the dopamine inhibitory responses can be inverted by artificial hyperpolarization of the membrane at the potassium equilibrium potential, E(K), indicating that dopamine causes a selective increase in potassium permeability.6. In other neurones the reversal potential of dopamine inhibitory responses is at a more depolarized level than E(K), but can be brought to E(K) by pharmacological agents known to block the receptors mediating the excitatory effects of DA.7. In still other neurones, the hyperpolarization induced by DA cannot be inverted in normal conditions, but a reversal can be induced by ouabain or by the substitution of external sodium by lithium. These results are discussed in terms of an hypothesis in which dopamine increases the potassium permeability of a limited region of the axonal membrane.8. It is concluded that a selective increase in potassium permeability probably accounts for all dopamine inhibitory effects in the neurones studied.

Ionic permeability changes as the basis of the thermal dependence of the resting potential in barnacle muscle fibres

1. The thermal dependence of the resting potential of isolated barnacle muscle fibres was larger (1-2 mV/ degrees C) than predicted by Nernst's equation (about 0.2 mV/ degrees C). A comparative study was made of the influence on thermal dependence of parameters related to (a) passive permeability and to (b) Na extrusion.2. High [K](o) decreased the thermal dependence reversibly. [K(i)], [Na](i) and [Cl](i) were determined by chemical analysis, and Goldman's equation was fitted to data relating V to [K](o) at different temperatures, in the presence and absence of ouabain 5 x 10(-5)M. In both cases the behaviour of V when T was lowered from 20 to 4 degrees C was accounted for by increases in the calculated P(Na/PK) and P(Cl/PK) (from 0.006 to 0.043 and from 0.17 to 0.34 on the average, respectively.)3. Other parameters related to passive permeability (and which caused reversible depolarization): decreased [Cl](o) (methanesulphonate or gluconate substituted), and decreased pH(o) (below 5.0), also decreased the thermal dependence reversibly.4. Inhibitors (ouabain 5 x 10(-5)M, cyanide 2-10 x 10(-3)M, 2,4-dinitrophenol 2 x 10(-4)M) externally applied did not affect either resting potential or its thermal dependence for several hours.5. Increasing [Na](i) three- to fourfold by intracellular injection decreased both resting potential and its thermal dependence.6. Although a small effect by a Na electrogenic pump cannot be excluded, the largest part of the thermal effect on the resting potential is concluded to depend on temperature-induced variations in relative ionic permeabilities to cations and anions. A model is proposed which can account for the data assuming that (a) each permeant ion associates to a separate site in the membrane, and (b) the ion-site equilibrium is temperature-dependent.

An electrogenic sodium pump in Limulus ventral photoreceptor cells

A hyperpolarization can be recorded intracellularly following either a single bright light stimulus or the intracellular injection of Na(+). This after-hyperpolarization is abolished by bathing in 5 x 10(-6)M strophanthidin or removal of extracellular K(+). Both treatments also lead to a small, rapid depolarization of the dark-adapted cell. When either treatment is prolonged, light responses can still be elicited, although with repetitive stimuli the responses are slowly and progressively diminished in size. The rate of diminution is greater for higher values of [Ca(++)](out); with [Ca(++)](out) = 0.1 mM, there is almost no progressive diminution of repetitive responses produced by either K(+)-free seawater or strophanthidin. We propose that an electrogenic Na(+) pump contributes directly to dark-adapted membrane voltage and also generates the after-hyperpolarizations, but does not directly generate the receptor potential. Inhibition of this pump leads to intracellular accumulation of sodium ions, which in turn leads to an increase in intracellular Ca(++) (provided there is sufficient extracellular Ca(++)). This increase in intracellular calcium probably accounts for the progressive decrease in the size of the receptor potential seen when the pump is inhibited.

Origin of the after-hyperpolarization that follows removal of depolarizing agents from the isolated superior cervical ganglion of the rat

1. Potential changes in isolated rat superior cervical ganglia following addition and removal of depolarizing agents were recorded using a moving-fluid extracellular electrode system.2. Ganglionic negativity produced by carbachol was followed by a pronounced ganglionic positivity on washing. This after-positivity was attributed to hyperpolarization of the ganglion cells since it was unaffected by crushing the postganglionic trunk.3. The after-hyperpolarization was selectively depressed by (a) cooling (Q(10) 2.3), (b) metabolic inhibitors (cyanide, azide, 2,4-dinitrophenol), (c) reducing [K(+)](o) or substituting Cs(+) for K(+), (d) ouabain, and (e) substituting Li(+) for Na(+). This suggested a close dependence on active Na(+) transport.4. When K(+) was restored to K(+)-free solution, or the preparation was warmed rapidly, or when metabolic inhibitors were washed away, the hyperpolarization was rapidly regenerated. The effect of restoring K(+) indicated that the hyperpolarization was generated directly by the Na(+) pump.5. The hyperpolarization was not altered by replacing Cl(-) with isethionate, indicating that the voltage change produced by the Na(+) current was not modified by passive Cl(-) movements.6. Hexamethonium added to the washout fluid augmented the after-hyperpolarization, suggesting that there was a high (cationic) leak current due to continued receptor-activation on washing with normal Krebs solution.7. The hyperpolarization was reduced by omission of Ca(2+) and restored by addition of Mg(2+). This was considered to result from changes in passive membrane permeability.8. The time-course of post-carbachol hyperpolarization accorded with a Na(+) extrusion process whose rate was directly proportional to [Na(+)](i) with a rate constant of 0.38+/-0.02 min(-1) at 23-27 degrees C.9. With increasing concentrations of carbachol, the amplitude of the hyperpolarization increased in proportion to the preceding depolarization, but the rate constant of the hyperpolarization was unchanged.

Activation of the electrogenic sodium pump in guinea-pig auricles by internal sodium ions

1. The effect of various intracellular Na concentrations ([Na](i)) on the membrane potential after hypothermia was studied in guinea-pig auricles.2. For varying [Na](i), the atria were cooled for 4 hr at 4-6 degrees C in a K-poor solution with different concentrations of NaCl. The auricles were rewarmed in normal Tyrode solution at 35 degrees C.3. Extracellular space (ECS), intracellular Na and K concentrations ([Na](i) and [K](i)) and membrane potential of the atria were measured before and after hypothermia.4. The ECS, measured as inulin space, amounted to 350 ml./kg wet wt. at 35 degrees C and to 300 ml./kg wet wt. at 4-6 degrees C.5. [K](i) decreased during cooling and increased during rewarming the auricles. [Na](i) increased during hypothermia in bathing fluids containing NaCl, but decreased in NaCl- and Na-free solutions. At the beginning of rewarming a net Na transport occurred from cells with high [Na](i), while a net Na uptake took place in atria with low [Na](i).6. At the same time, the membrane potential of auricles with increased [Na](i) hyperpolarized beyond the steady-state value recorded at the end of rewarming, or even beyond the calculated K(+) equilibrium potential (E(K)). Afterwards, the hyperpolarization levelled off, while the E(K) values increased further. The membrane potential of atria with decreased [Na](i) showed no transitory hyperpolarization during rewarming.7. The hyperpolarization beyond the steady-state value of membrane potential in rewarmed auricles was significantly correlated to the active Na efflux.8. From these results it is concluded that the membrane potential of guinea-pig atria after hypothermia is affected by an active, electrogenic Na pump activated by intracellular Na ions.

The peripheral auditory apparatus

This review of the peripheral auditory apparatus represents an attempt to analyse critically recent developments in the field. The coverage is not exhaustive, the emphasis is on functional aspects and no attempt is made to review the anatomy of the ear. Particular emphasis is placed on three broad sections: the physiology of the middle ear, basilar membrane mechanics and the electrophysiology of the cochlea. It is in these areas that recent technical advances have led to experiments which throw doubt on traditionally held concepts. Such an advance is the application of the Mössbauer technique to the problem of middle- and inner-ear mechanics. Because of its novelty, this technique is discussed in detail. Other new methods such as laser holography are just starting to be used, as are the ion selective microelectrodes for measuring dynamic changes in endolymph concentration. After many years of slow progress there is a sudden spark of enthusiasm for peripheral auditory research, and the coming decade promises to be most exciting indeed.

Intracellular sodium activity and the sodium pump in snail neurones

1. Recessed-tip Na(+)-sensitive micro-electrodes were used to measure [Na(+)](i) continuously in snail neurones for experiments lasting up to several hours. The average resting [Na(+)](i) in twenty-two cells was 3.6 mM.2. Inhibition of the Na pump by ouabain caused [Na(+)](i) to increase at an average rate of 0.54 m-mole/min. This corresponds to a passive influx of Na quantitatively similar to that observed in squid axons.3. Changing external K over the range 1-8 mM had little effect on [Na(+)](i), but K-free or 0.25 mM-K Ringer caused a rise in [Na(+)](i).4. Increasing membrane potential by up to 90 mV caused an increased influx of Na, but did not inhibit the pump.5. Reducing external Na caused a decrease in [Na(+)](i) but did not affect the pump rate at a given [Na(+)](i). The pump rate at low [Na(+)](i) was proportional to [Na(+)](i) minus a threshold value of about 1 mM.6. The Na pump appeared still to be electrogenic at subnormal rates of activity.7. It is concluded that, given sufficient external K, the rate of the Na pump depends principally on [Na(+)](i). Changes in external Na or membrane potential appear to affect the pump only indirectly, by changing the Na influx and thus [Na(+)](i).

Membrane potential of smooth muscle cells in K-free solution

1. The changes of the ion content, the membrane potential and of the membrane permeability of taenia coli cells have been studied during exposure to K-free solutions. The relative value of the total membrane conductance was determined by measuring the electrotonic potential during constant current pulses with an intracellular electrode. The P(K) values were calculated from (42)K-efflux in K-free solutions.2. In solutions containing penetrating anions the cells initially depolarize. Thereafter they hyperpolarize to about - 85 mV and again depolarize after 90 min to - 5 mV. These potential changes are much smaller if large anions are used as chloride substitutes. Moreover, the final depolarization is only reached after 4-5 hr. This hyperpolarization is not inhibited by 10(-5)M ouabain.3. These potential changes are accompanied by a progressive exchange of intracellular K by Na. In solutions containing chloride or nitrate the relative value of the total membrane conductance increases to a maximal value, corresponding to the peak value of the calculated P(K). Such changes of the membrane conductance and of P(K) do not occur in K-free solutions containing large anions.4. It is proposed that the initial depolarization is probably caused by an inhibition of an electrogenic Na pump. In chloride or nitrate solution the hyperpolarization is due to an increase of the [K](i)/[K](o) ratio and to an increase of the K permeability. In the presence of large anions the hyperpolarization remains small because this increase of P(K) does not occur.

The membrane properties of the smooth muscle of the guinea-pig portal vein in isotonic and hypertonic solutions

The membrane properties of the longitudinal smooth muscle of the guinea-pig portal vein were investigated under various experimental conditions.1. In isotonic Krebs solution, the membrane potential (-48.7 mV), the maximum rates of rise and fall of the spike (4.6 and 2.3 V/sec respectively), the space constant (0.61 mm), the conduction velocity of excitation (0.97 cm/sec) and the time constant of the foot of the propagated spike (18.4 msec) were measured.2. The various parameters of the muscle membrane in the isotonic solution were compared with those in the hypertonic solution prepared by the addition of solid sucrose (twice the normal tonicity).3. When the muscles were perfused with hypertonic solution, marked depolarization of the membrane and increased membrane resistance occurred. These were probably due to reduction of the K permeability, increased internal resistance of the muscle and shrinkage of the muscle fibre.4. The membrane potential in isotonic and hypertonic solutions was analysed into two components, i.e. the metabolic (electrogenic Na-pump) and the ionic (electrical diffusion potential) component in the various environmental conditions.(a) In isotonic and hypertonic solutions, the membrane was depolarized by lowering the temperature or by removal of K ion from the solutions. When the tissues were rewarmed or on readdition of K ion, the membrane was markedly hyperpolarized. These hyperpolarizations of the membrane were suppressed by treatment with ouabain (10(-5) g/ml.), by warming to only 20 degrees C and by K-free solution.(b) The relationships between the membrane potential and the [K](o) in isotonic Krebs, in the hypertonic (sucrose) Krebs, in the Na-free (Tris) Krebs and in the Cl-deficient (C(6)H(5)SO(3)) Krebs were observed. The maximum slopes of the membrane depolarization against tenfold changes of [K](o) were much lower than that expected if it behaved like a K electrode.(c) In Na-free (Tris) solution, the membrane was not depolarized in isotonic condition but it was depolarized in hypertonic condition.5. The low membrane potential in hypertonic solution (-37 mV) compared with isotonic solution (-49 mV) was thought to be mainly due to suppression of K permeability of the membrane and not due to suppression of the metabolic component. The electrogenic Na-pump and the membrane potential of the portal vein was discussed in relation to other excitable cell membranes.

Hyperpolarization of a barnacle photoreceptor membrane following illumination

Membrane potential changes following illumination of a photoreceptor cell in the lateral ocellus of a barnacle (Balanus eburneus) were studied by means of intracellular recording and polarization techniques. Illumination produces a depolarizing response. When the illumination is terminated, the membrane potential temporarily becomes more negative than the resting potential prior to illumination. Although the amplitude of this postillumination hyperpolarization depends upon the intensity as well as the duration of the light pulse, the time course is fairly constant. The hyperpolarization is not associated with any significant membrane conductance increase and is abolished by 10(-5)M ouabain. It diminishes when the external Na or K ions are removed. An intracellular injection of Na ions produces a hyperpolarization similar to that following illumination. It is suggested that the postillumination hyperpolarization is produced by an electrogenic Na pump which is activated by the Na influx during illumination.

Prediction of repetitive firing behaviour from voltage clamp data on an isolated neurone soma

1. Membrane parameters of an isolated neural cell body have been determined by voltage clamp analysis. Data are expressed as membrane ion-specific conductances, leak conductance, and capacitance.2. Three ionic currents are present: Inward, I(I); and two operationally distinct outward currents, I(K) and I(A). Both outward currents are apparently carried by potassium ions.3. Hodgkin-Huxley-like equations were solved for the discharge of two sequential action potentials in response to a constant stimulus current. The digital computer solutions are compared with action potential data recorded from the investigated cell.4. The computed and experimentally measured relationships between firing frequency and stimulus current intensity are compared and are linear over the same portion of the total frequency range.5. Cell behaviour in the latter part of the interspike interval is dominated by the conductance g(A) while g(Na) and g(K) largely determine the character of the action potential and the initial portion of the interspike interval.6. Prehyperpolarization of the membrane activates g(A) and the membrane response to depolarizing current differs markedly from the response elicited when no prehyperpolarization is imposed.

Inhibition of impulse activity in a sensory neuron by an electrogenic pump

THE CRAYFISH TONIC STRETCH RECEPTOR NEURON MANIFESTS THREE PHENOMENA: (a) Impulse frequency in response to a depolarizing current decays exponentially to half the initial rate with a time constant of about 4 sec. (b) One or more extra impulses superimposed on steady activity result in a lengthening of the interspike interval immediately following the last extra impulse which is proportional to the number of extra impulses. However, above a "threshold' number of impulses the proportionality constant becomes abruptly larger. (c) Following trains of impulses, the resting potential of the cell is hyperpolarized by an amount proportional to impulse number. Such posttetanic hyperpolarization (PTH) decays approximately exponentially with a time constant of 11 sec, but this varies with membrane potential. These effects are attributed to the incremental increase of an inhibitory (hyperpolarizing) current with a long (relative to interspike interval) decay constant. We suggest that this inhibitory current is the result of increased electrogenic Na pumping stimulated by Na entering with each impulse. Evidence is presented that the three effects are reversibly inhibited by conditions which depress active Na transport: (a) Li substituted for Na in the bath; (b) application of strophanthidin; (c) K removal; (d) treatment with cyanide; (e) cooling. We conclude that a single process is responsible for the three responses described above and identify that process as electrogenic Na pumping. Our observations also indicate that electrogenic pumping contributes to this neuron's resting potential.

Effects of removing the external potassium on the smooth muscle of guinea-pig taenia coli

1. Removal of the external K ions depolarized the membrane of the guinea-pig taenia coli by about 5 mV, and increased the frequency of the spontaneous spike activity. After about 20 min in K-free solution, the frequency gradually decreased and spontaneous activity stopped after about 60 min.2. Readmission of K produced a transient hyperpolarization by 10-15 mV for 10-20 min, and lowered the excitability. The membrane resistance decreased during the hyperpolarization. When the external K concentration was returned to normal before the spontaneous activity had disappeared in K-free solution, there was a complete suppression of the spontaneous electrical and mechanical activities, accompanied by hyperpolarization of the membrane. The duration of this suppression of electrical and mechanical activities depended on the exposure time to K-free solution, the recovery being delayed by longer exposure.3. The effects of removing external K were essentially the same in the presence of tetrodotoxin (10(-7) g/ml.), of atropine (10(-6) g/ml.), and also in low Cl (2-7 mM).4. The effects of ouabain (10(-6) g/ml.) were similar to those of zero K but appeared more slowly. After treatment with ouabain, the membrane was hyperpolarized by removing the external K and there was no hyperpolarization nor suppression of the mechanical activity on readmission of external K.5. Rb substitution for K produced little effect and addition of 5.9 mM-Rb after K-removal produced a typical suppression of activity as the readmission of K. When K was replaced with an equimolar concentration of Cs, the effects were similar to those produced by K-free solution.6. The pattern of the response to a change in the external K concentration was different below 23 degrees C. The spontaneous activity was suppressed by removal of K, and increased by returning the external K.7. It is concluded that, in the taenia, the membrane potential can be modified by an electrogenic pump, the activity of which is inhibited by removing K, by ouabain, and by lowering the temperature to about 23 degrees C. During the recovery from the exposure to K-free solution or to ouabain, the pump activity is potentiated, causing hyperpolarization of the membrane by 10-20 mV. Rb can, Cs cannot, replace K in activating the pump at the same concentration as K.

Optical studies on the kinetics of the sodium pump in mammalian non-myelinated nerve fibres

1. A study has been made of the changes in the fluorescence of desheathed rabbit cervical vagus nerves that occur during and after electrical stimulation of its non-myelinated fibres.2. Stimulation for 5 sec at 30 shocks/sec produces a maximal decrease, of about 1% of the resting fluorescence. Stimulation for less than 0.5 sec fails to produce responses visible above the inherent noise in the recording system.3. A pharmacological dissection (with ouabain, metabolic inhibitors, and calcium) has revealed four phases of fluorescence change:(a) under conditions where the sodium pump is functioning, there is a prolonged decrease in the fluorescence following electrical activity;(b) even in the absence of pumping the mere entry of sodium into the nerve causes an initial decrease in fluorescence;(c) the entry of calcium ions with electrical activity also causes an initial rapid decrease in fluorescence;(d) following these phases of decreased fluorescence there is a phase of increased fluorescence.4. These changes in fluorescence are related to changes in the NADH concentration in the nerve resulting from:(a) the splitting of ATP during sodium extrusion;(b) the initial binding of sodium to the sodium- and potassium-dependent ATPase, which is the sodium pump;(c) the stimulation of mitochondrial respiration by calcium that has entered during the spike; and(d) an increased glycogenolysis as a result of the calcium entry during activity.

Temperature dependence of the sodium-potassium permeability ratio of a molluscan neurone

1. The temperature dependence of the membrane potential of a molluscan giant neurone was examined under conditions which block the electrogenic activity of the Na-K exchange pump.2. When the Na pump was blocked by ouabain or the removal of external K, the membrane potential depolarized as temperature was increased.3. This depolarization was prevented by the replacement of external Na with impermeant cations, but was greater when Na was replaced with Li.4. All observed effects of ouabain were attributable to inhibition of the Na pump. The depolarization in response to ouabain at warmer temperatures was completely reversible, and the rate of both onset and reversibility of the ouabain effect was dependent upon temperature.5. Using a modified form of the constant field equation, the internal K concentration and the Na-K permeability ratio, P(Na)/P(K), were calculated from the experimental data.6. P(Na)/P(K) was found to increase from 0.028 at 4 degrees C to 0.068 at 18 degrees C. It is suggested that this increase is due primarily to a change in P(Na).

Contributions of the sodium pump and ionic gradients to the membrane potential of a molluscan neurone

1. The membrane potential of the gastro-oesophageal giant neurone of the marine mollusc, Anisodoris nobilis, was examined during changes of temperature and of the ionic medium.2. The response of the membrane potential to rapid changes in the external K concentration was prompt, stable, and reversible up to 200 mM-K, and was independent of the external Cl concentration.3. Warming the cell produced a prompt hyperpolarization that was approximately 10 times greater than predicted by the Nernst or constant field equations. Electrogenic activity of the Na-K exchange pump was shown to be responsible for this effect.4. At temperatures below 5 degrees C, the relationship between the membrane potential and the external K concentration could be predicted by a constant field equation.5. At temperatures above 5 degrees C, the membrane potential could not be predicted by the constant field equation except after inhibition of the electrogenic Na pump with ouabain or the reduction of internal Na.6. Inhibition of the electrogenic Na pump by low external K concentrations was dependent upon the external Na concentration.7. It is concluded that the membrane potential is the sum of ionic and metabolic components, and that the behaviour of the ionic component can be predicted by a constant field type equation.

Post-tetanic hyperpolarization produced by an electrogenic pump in dorsal spinocerebellar tract neurones of the cat

1. Hyperpolarization following single and repetitive excitation of dorsal spinocerebellar tract (DSCT) neurones of the cat was studied by intracellular recording.2. Hyperpolarization following an antidromic action potential consisted of an initial, brief phase (undershoot) and a late, prolonged phase. The latter hyperpolarization was independent of the membrane potential, whereas the former was reversed in polarity by hyperpolarizing pulses applied across the DSCT cell membrane.3. DSCT neurones showed a prolonged hyperpolarization after a train of antidromic action potentials. The amplitude and duration of the hyperpolarization were dependent on the number and the frequency of action potentials. A similar hyperpolarization was observed following a train of impulses evoked by depolarizing pulses applied through the intracellular electrode.4. There was no detectable conductance change during the post-tetanic hyperpolarization. The latter showed no reversal potential when the membrane potential was altered.5. The half-decay time of the post-tetanic hyperpolarization was lengthened when the cord temperature was lowered. The temperature coefficient (Q(10)) was 2.4 within the range of 31-40 degrees C.6. The amplitude of the undershoot following each action potential was assumed to provide a criterion for the accumulation of the extracellular K(+). Alterations in the amplitude of undershoots during repetitive excitation suggested that the duration of post-tetanic hyperpolarization depends on the accumulation of extracellular K(+) as well as of intracellular Na(+) associated with a train of impulses.7. It is suggested that post-tetanic hyperpolarization is produced by an electrogenic sodium pump. A possible significance of such a hyperpolarization in impulse coding is discussed.

Characteristics of electrogenic sodium pumping in rat myometrium

Sodium-rich myometrium, obtained from the uteri of pregnant rats, rapidly hyperpolarized when 4.6-120 mM potassium was added to the bathing medium at 37 degrees C. Hyperpolarization was due to sodium pumping since the process was markedly temperature dependent, was abolished by ouabain, and required both intracellular sodium and extracellular potassium. The observed membrane potential exceeded the calculated potassium equilibrium potential during hyperpolarization providing evidence that sodium pumping was electrogenic. Hyperpolarization was reduced in the presence of chloride. The rate of sodium pumping may influence potassium permeability since potassium apparently did not short-circuit the pump during hyperpolarization.

The effect of altered thyroid state on atrial intracellular potentials

1. A group of rabbits was made hypothyroid by thyroidectomy, and another group was injected daily with L-thyroxine. After an appropriate interval respective alterations in thyroid state were confirmed by measurement of heart weight and of plasma iodine, and the animals' atria were isolated for recording.2. Measurements were made of atrial contractions, conduction velocity, spontaneous heart rate and maximum driven frequency, and action potentials were recorded with intracellular micro-electrodes.3. The resting potential and action potential heights were not affected by differences of thyroid state.4. Atrial arrhythmias are common in hyperthyroidism, rare in myxoedema. The possibility that hypothyroidism might reduce the rate of rise of the action potential, as do anti-arrhythmic drugs, and hyperthyroidism increase it, was investigated. Although the rate of rise was slower in hypothyroid atria at some driving frequencies, this could not alone account for an anti-arrhythmic effect, because at frequencies near the spontaneous heart rate the rate of rise of the action potential was not reduced.5. The duration of the repolarization phase of the action potential was greatly prolonged in atria from thyroidectomized rabbits, and was shortened in hyperthyroid atria. These changes could account for a reduced probability of arrhythmias in hypothyroidism, and the converse in hyperthyroidism.