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

Mechanism of monensin-induced hyperpolarization of neuroblastoma-glioma hybrid NG108-15

Addition of the ionophore monensin to mouse neuroblastoma-rat glioma hybrid NG108-15 cells leads to a 20 to 30-mV increase in the electrical potential across the plasma membrane as shown by direct intracellular recording techniques and by distribution studies with the lipophilic cation [3H]-tetraphenylphosphonium+ (TPP+) [Lichtshtein, D., Kaback, H.R. & Blume, A.J. (1979) Proc. Natl. Acad. Sci. USA 76, 650-654]. The effect is not observed with cells suspended in high K+ medium, is dependent upon the presence of Na+ externally, and the concentration of monensin that induces half-maximal stimulation of TPP+ accumulation is approximately 1 microM. The ionophore also causes rapid influx of Na+, a transient increase in intracellular pH, and a decrease in extracellular pH, all of which are consistent with the known ability of monensin to catalyze the transmembrane exchange of H+ for Na+. Although ouabain has no immediate effect on the membrane potential, the cardiac glycoside completely blocks the increase in TPP+ accumulation observed in the presence of monensin. Thus, the hyperpolarizing effect of monensin is mediated apparently by an increase in intracellular Na+ that acts to stimulate the electrogenic activity of the Na+,K+-ATPase. Because monensin stimulates TPP+ accumulation in a number of other cultured cell lines in addition to NG108-15, the techniques described may be of general use for studying the Na+,K+ pump and its regulation in situ.

Effect of external potassium on the coupled sodium: potassium transport ratio of axons

1. Resting membrane potential and the current-voltage relation were measured in crayfish giant axons bathed in various potassium solutions with and without ouabain. 2. Ouabain caused a depolarization of the membrane at each [K]o used but did not affect membrane resistance. 3. The ouabain-sensitive transport current was least (3 microamperemeter/cm2) in 0 mM [K]o and greatest (7 microamperemeter/cm2) in 16.2 and 21.6 mM [K]o. 4. The assumption was made an some indirect evidence presented that axons equilibrated in various potassium solutions maintain constant internal sodium and potassium concentrations for up to 3 h. 5. On the basis of this assumption, the apparent ratio of coupled Na : K transport was calculated. It was found to be least (-1.3/1) in 0 mM [K]o and to approach infinity in 16.2 and 21.6 mM [K]o. 6. The data indicate that the apparent variability of the Na : K exchange ratio likely represents an intrinsic property of the exchange mechanism and is less likely to be explained by a fixed-ratio coupled Na : K transport operating in parallel with electro-neutral Na : Na or K : K exchange.

Effect of sodium ion on levels of cyclic adenosine 3',5'-monophosphate in guinea pig cerebral slices

Accumulation of cyclic AMP was studied in guinea pig cerebral slices when Na+ levels in the bathing medium were varied or agents which affect tissue Na+ content were added. When NaCl was gradually replaced with Tris-HCl or choline chloride, cyclic AMP formation was progressively enhanced. ;when Na+ was below 30 mM, cyclic AMP formation reached the maximum (approximately 30 fold), but this increment was not blocked by tetrodotoxin. The stimulatory effect of high K+ was nearly linear over 120 mM and became much more prominent when Na+ also was not blocked by tetrodotoxin. Ouabain (10(-4) M), electrical pulses and glutamate (5 x 10(-3) M), each stimulated cyclic AMP formation about 17-, 7- and 5-fold, respectively. Tetrodotoxin (2 x 10(-6) M) completely blocked the effects of electrical pulses and partially blocked the effects of glutamate and oubain. It is suggested that the increase of cyclic AMP in cerebral cortical slices may be related to the decrease in Na+ gradient across the cell membrane.

Ionic effects on the membrane potential of hyperpolarizing photoreceptors in scallop retina

1. The effects of different external ionic conditions and of metabolic inhibitors on the membrane potential of hyperpolarizing photoreceptors in the retina of the scallop Pecten irradians were examined in the presence and absence of light.2. Changes in extracellular K(+) have a greater effect on membrane potential in the light than in darkness. The receptor potential is increased in amplitude when [K](o) is reduced and decreased when [K](o) is elevated. It is hyperpolarizing when [K](o) is less than the estimated value for [K](i) and depolarizing when this condition is reversed.3. The complete replacement of [Na](o) causes a significant hyperpolarization of membrane potential in darkness, whereas it has a much smaller hyperpolarizing effect on the peak of the receptor potential.4. The ratio of Na(+) to K(+) permeabilities (P(Na)/P(K)) decreases during bright illumination. Our results suggest that P(K) is seven times that for P(Na) in the dark but is 57 times greater than P(Na) in light.5. The metabolic inhibitors DNP and NaCN cause membrane potential in the dark to hyperpolarize. This hyperpolarization is associated with a decrease in the P(Na)/P(K) ratio similar to that found during illumination.6. High [Ca(+)](o) also causes membrane potential in the dark to hyperpolarize. This hyperpolarization is associated with an increase in membrane conductance.7. The results indicate that the hyperpolarizing receptor potential of the distal photoreceptor is produced by a light-evoked increase in K(+) permeability.

Role of lipids in the Neurospora crassa membrane. II. Membrane potential and resistance studies; the effect of altered fatty acid composition on the electrical properties of the cell membrane

The effect of doubling the saturated fatty acid content on the electrophysiology of Neurospora crassa membranes was studied. Intracellular membrane input resistance (Rm) and potential (Em) were measured for wild-type (w/t) and cel- (Tween 40) organisms as a function of temperature. Over the 0 to 40 degrees C temperature range studied, mean Em values of both w/t and cel- (Tw 40) organisms increased from -160 to -210 mV. This difference is greater than that expected from Nernst potential considerations, indicating an active component of Em. This active component is insensitive to a doubling of the saturated fatty acid content. Rm exhibits a temperature dependence and hysteresis. Averaged data indicate an increase in Rm with decreased temperature. The slope of the temperature dependence varies among individual hyphae. Above 17.5 degrees C cel- (Tw 40) hyphae averaged greater than 70% higher values of Rm than w/t. Below 17.5 degrees C w/t Rm data divided into low and high temperature dependence groups, while cel- data exhibited a low temperature dependence. The results are discussed in relation to gel-liquid crystal phase transitions, membrane fluidity, and the contribution of fatty acid structure to membrane electrical properties.

Metabolic component in the epithelial intracellular potential of rabbit cornea

Effects of the change of external ionic composition and the addition of metabolic inhibitors on rabbit cornea were studied by recording the epithelial intracellular potential. High K and Li Ringer's solutions, applied to the corneal endothelial side, caused a marked depolarization of the epithelial cells, but no potential change was seen when applied to the epithelial side. Ouabain, MIAA and NaCN applied to the endothelial side reduced the epithelial potential, while those applied to the epithelial side did not change the potential. DNP and FDNB also had no effect when applied to the epithelial side only. The thermal dependence of the epithelial intracellular potentials of whole eye (0.85 mV/degrees C) and excised cornea (2.01 mV/degrees C) preparations were greater than about 0.2 mV/degrees C predicted by the Nernst equation. It is concluded that the epithelial cell layer of rabbit cornea act as a tight barrier against diffusion of K ion and metabolic inhibitors from the tear side to the epithelial basal cell. A high thermal dependence of the epithelial intracellular potential may depend greatly on the pump ingibition.

The influence of cardioactive steroids, metabolic inhibitors, temperature and sodium on membrane conductance and potential of crayfish giant axons

1. The resting membrane potential and the current-voltage relation were measured in crayfish giant axons before and after treatment with cardioactive steroids, metabolic inhibitors, extracellular sodium depletion and low temperature. 2. The membrane resistance of axons treated with cardioactive steroids, metabolic inhibitors, and low extracellular sodium was reduced by 30-53% depending on the treatment. Low temperature also caused a decrease in the membrane resistance of the axon but the decrease was limited to potentials around the resting membrane potential. The temperature response of sodium depleted or ouabain treated axons was an increase in resistance at all points along the current-voltage relation. 3. All inhibitors and low temperature caused a depolarization of the membrane potential. Ouabain and strophanthidin were the most effective, reducing the membrane potential by an average of 9.6 mV in 10-20 min. Low sodium did not cause a depolarization but consistently reduced the membrane resistance by an average of 30%. 4. The data suggest that there is an interaction between the activity of the ouabain-sensitive transport system and resting membrane resistance in the crayfish axon.

Permeability properties and intracellular ion concentrations of epithelial cells in rat duodenum

Effects of the K+ concentration in the bathing fluid ([K+]l) on the intracellular K+, Na+ and Cl- concentrations ([K+]i [Na+]i and [Cl-]i) as well as on the electrical potential were studied in rat duodenum. Changes in the mucosal K+ concentration ([K+]m), bringing the sum of Na+ and K+ concentrations to 147.2 mM constant, had little effect on the transmural potential difference (PDt), but did induce marked changes in the mucosal membrane potential (Vm). As [K+]m increased, Vm was depolarized gradually and obeyed the Nernst equation for a potassium electrode in the range of [K+]m greater than approx. 60 mM. Experiments of ion analyses were carried out on strips of duodenum to determine the effect of changing the external K+ concentrations on [K+] i, [Na+]i and [Cl-]i. An increase in [K+]o resulted in increases in [K+]i and [Cl-]i and a decrease in [Na+]i, [K+]i approaching its maximum at [K+]o greater than 70 mM. Such changes in [K+]i and [Na+]i seem to correlate quantitatively with the changes in [K+]o and [Na+]o. The values of the ratio of permeability coefficients, Pna+/PK+ were estimated using the Vm values and intracellular ion concentrations measured in these experiments. The results suggested that there appeared a rather abrupt increase in the PNa+/PK+ ratio from 0 to approx. 0.1, as [K+]m decreased.

Potassium induced potential changes in rat diaphragm muscle

The effect of different potassium concentrations on the membrane potential and membrane resistance of rat diaphragm muscle fibres was measured by means of a double sucrose gap method and a microelectrode technique. Concentration measurements showed that the muscle fibres gained sodium and lost potassium in the equilibration period. In the absence of external chloride changing the external potassium concentration from 2.8 mM to potassium-free caused a depolarization of the membrane of about 30 mV and a small increase in membrane resistance. This K-dependent potential change (K-response) was induced by ouabain, K-strophanthin, 2,4-dinitrophenol and cyanide, indicating that an energy requiring process is involved. The temperature dependence of the K-response found is consistent with this assumption. Variation in potassium permeability in the absence and presence of external potassium could account for only 13% of the K-response. The K-response amplitude appeared to depend on the external potassium and the internal sodium concentration. Hyperpolarization of the membrane could not only be produced after readmission of potassium but also after addition of thallium, the latter being more potent. Raising the external chloride concentration resulted in a decrease of the K-response and membrane resistance. The current, generating the K-response was shown to be hardly influenced by conditional polarization of the membrane. It is concluded from these results that the K-response is mainly due to the operation of an electrogenic sodium pump.

Ionic permeabilities of an Aplysia giant neuron

In a giant neuron of Aplysia californica, permeabilities and conductances obtained by measuring net fluxes of Na+, K+ and Cl-minus with ion-specific microelectrodes were compared with those obtained by measuring transmembrane current and potential changes when the three ions were varied in the external solution. Net fluxes were measured with ion-specific microelectrodes, after blocking metabolic processes, thus allowing movement of ions down their electrochemical gradients. Premeabilities and conductances obtained from the "chemical" measurements (i.e., ion-specific electrodes) were generally comparable to the values obtained from "electrical" measurements (i.e., ion-specific electrodes) were generally comparable to the values obtained from "electrical" measurements. Where discrepancies occurred, they could be explained by showing that some of the assumptions necessary to use the "electrical" method were not quantitatively true in this system. The absolute magnitudes of the permeabilities are significantly less than those found in many axonal preparations. There is also a relatively high PNa/PK ratio. The selectivity of the membrane against ions such as Tris" and MeSO3-minus is not good, Tris+ being nearly as permeable as Na+ and MeSO3-minus about one-half as permeables as Cl-minus. These properties may be characteristic of somal membranes.

Effects of potassium ions and sodium ions on membrane potential of epithelial cells in rat duodenum

1. Mucosal and serosal membrane potentials (Vm and Vs) of epithelial cells in rat duodenum were recorded together with the transmural potential differences (PDt). 2. The value of Vm in rat duodenum at 37 degrees C was about -53mV, being considerably greater than the values reported hitherto for the small intestine of various species. 3. When Cl- in the mucosal medium was partially replaced with SO42- at fixed mucosal Na+ and K+ concentrations ([Na+]m and [K+]m), the membrane potential was scarcely affected in the steady state several minutes after replacement, whereas marked changes in the potential were observed with varying [K+]m or [Na+]m. 4. As the mucosal K+ concentration increased at constant [Na+]m, Vm was gradually decreased (depolarization), together with the increase in PDt. Such a change in Vm caused by varying [K+]m obeys Nernst's equation in the range of [K+]m higher than about 60 mM. 5. At constant [K+]m, an increase in [Na+]m also caused the decrease of Vm for the lower [K+]m region, whereas Vm was not affected by such changes in [Na+]m in the range of [K+]m higher than approx. 60 mM. 6. The values of PNa/PK were obtained from the modified Goldman equation under an appropriate assumption. The ratio of the permeability coefficients markedly increases from zero to approx. 0.07 with a decrease in [K+]m.

Contribution of an electrogenic sodium pump to the membrane potential in rabbit sinoatrial node cells

A study has been made of the transient hyperpolarization (K+-induced hyperpolarization) which developed following readmission of potassium after having pre-treated the rabbit sinoatrial node tissue with K+-depleted Tyrode solution for 4--5 min at 35 degrees C. Evidence is presented indicating that the K+-induced hyperpolarization results from the activity of an electrogenic sodium pump: The K+-induced hyperpolarization was inhibited by substituting Li+ for Na+ and by cooling the tissue. The amplitude of the K+-induced hyperpolarization was increased either by increasing K+ concentration in the recovery solution or by decreasing K+ concentration in the pre-treatment K+-depleted solution. By removing Cl- from the perfusates, the amplitude of the K+-induced hyperpolarization increased. In a Cl--depleted solution, the sinoatrial node cell membrane hyperpolarized by approximately 15 mV without a transient depolarization.

A further study of the fine structure and membrane properties of neuroglia in the optic nerve of Necturus

The optic nerve of Necturus maculosus consists of a homogeneous population of astroglia and bundles of unmyelinated axons. The glial cell processes ramify within the nerve roughly delineating fascicles of axons and come together at the periphery to form a complete external limiting membrane interrupted only by narrow clefts between adjacent processes. They are frequently "attached" to one another, forming specialized junctions. Blood vessels are entirely outside the nerve which is surrounded by a basal lamina. The temperature dependence of the glial membrane potential is accurately predicted by the Nernst relation. The membrane potential is unaffected by changes in Cl, Na, Li, and guanidinium which are apparently impermeant. The permeability of the glial membrane to other cations is in the sequence Tl greater than K greater than Rb greater than Cs greater than NH4. This suggests that the chemical nature of the site of potassium permeability in glial cells is similar to that in the neuron.

Activation of electrogenic sodium pump in mammalian skeletal muscle by external cations

The effect of change of the external ionic composition on "Na-loaded" and "K-depleted" soleus muscle fibres of K-deficient rats was investigated by recording resting membrane potentials. The addition of K, Rb, Cs and NH4 ions to K-free Krebs solution bathing "Na-rich" muscles resulted in a rapid hyperpolarization. The hyperpolarization was abolished by removing the above cations, cooling to ca. 4 degrees C, and adding 0.1 mM ouabain. The effectiveness of cations for activating the electrogenic Na pump was Rb greater than or equal to K greater than NH4 greater than Cs, and NH4 ions seemed to be unique in their stimulating action. The resting cell membrane of "Na-rich" muscles is permeable to cations in the order of Rb = K greater than Cs greater than NH4. Reducing Na ions in Krebs solution had no effect on the rate of Na-pumping in "Na-rich" muscle fibres at a given K concentration. It is concluded that the external K ions could be replaced by Rb, Cs and NH4 ions in activating the electrogenic Na pump in "Na-rich" soleus muscle fibres, but that the electrogenic Na pump in this tissue does not require the external Na ions.

Contribution of an electrogenic sodium pump to membrane potential in mammalian skeletal muscle fibres

1. Relationship between the resting membrane potential and the changes in the intraceullar Na and K concentrations ([Na]i and [K]i) was studied in 'Na-loaded' and K-depleted' soleus (SOL) muscles of rats which had fed a K-free diet for 40 and more days. 2. The extracellular space of the muscles was not significantly different between normal and K-deficient rats. The inulin space in both the 'fresh' and Na-rich' muscles can be determined by the same function relating the space to the muscle weight. 3. Presence of 2-5-15 mM-K in the recovery solution hyperpolarized the 'Na-rich' muscul fibres at the beginning of recovery. The hyperpolarized membrane potential exceeded, beyond the measured potential of 'fresh' muscle fibres, the theoretical potential derived from the ionic theory, or even beyond Ek. Then, the measured membrane potential declined progressively during the immersion in a recovery solution and returned to the steady-state value When a considerable Na extrusion and K uptake took place, the measured membrane potential became equal to Ek. 4.he maximal hyperpolarization occurring immediately after immersion in the recovery solution became smaller and had a shorter duration when increasing the external K concentration ([K]o) from 2-5 to 15mM. 5. The K-sensitive hyperpolarization was completely abolished on exposure to 0mM [K]o, on cooling to ca. 4 degrees C, and in the presence of oubain (10(-4) M). The inhibitory effects were reversed on returning to the control conditions. The membrane potential obtained after inhibition of the electrogenic Na-pump with cooling or ouabain agrees well with that predicted by the 'constant-field' equation. 7. The external Cl ions had a short-circuiting effect on the electrogenic Na-pumping activated on adding K ions. 8. The replacement of Na ions in a recovery solution with Li ions resulted in a faster rate of depolarization from the maximal hyperpolarizationp. It is concluded that the resting membrane potential of 'Na-loaded' and 'K-depleted' SOL muscle fibres is the sum of an ionic diffusion potential predicted by either the Nernst equation or the constant-field equation and of the potential produced by an electrogenic Na-pump.

Studies on bursting pacemaker potential activity in molluscan neurons. II. Regulations by divalanet cations

Identified cells in Aplysia (R15, R2, LPG) and cell 11 in Otala have been used to investigate the effects of divalent cations, temperature, pH and ouabain on neuronal activity. Divalent cations act primarily to regulate the appearance of bursting pacemaker potential (BPP) activity in these cells. These ions are necessary for B generation and will inhibit its appearance at high concentrations (Ca greater than Mg greater that greater than Sr). In addition, Ca is involved in the seasonal modulation of BPP activity in a neurosecretory cell in Otala. Monovalent cations play secondary roles as regulators of BPP activity by competing with divalent cations for the sites involved in the regulation of the (probable) monovalent conductances underlying BPPs. The effects of pH, temperature and ouabain on membrane properties and BPP activity are partly related to their interaction with divalent cations. The results described indicate important roles for divalent cations in the regulation of the expression of BPP activity and its underlying membrane properties both in different nerve cells and in the same cell during dormancy and activity of the snails.

The membrane of giant molluscan neurons: electrophysiologic properties and the origin of the resting potential

The molluscan neuron, because of its large size and accessibility, has been an important model for studying the electrophysiology of nerve cells. This review catalogs data about specific molluscan neurons, but the greater importance of this material is in the broad picture of how a neuronal membrane maintains internal potential and is responsive to changes in the environment. Electrical properties of the membrane. The mechanisms which contribute to the resting potential in molluscan neurons can be separated into ionic and metabolic components. When the electrogenic sodium pump is eliminated experimentally, the ionic component of the potential follows the constant field equation quite closely. Many of the "constants" and "parameters" which characterize the membrane of molluscan neurons are actually variables which depend upon temperature, ionic environment, and membrane potential. The evaluation of the electrical parameters is complicated by extensive infoldings of the somatic membrane, and by large axons which drain current from the soma. Most molluscan neurons have a very high specific membrane resistance and a correspondingly low potassium permeability. Membrane capacitance is close to the 1 microF/cm2 value which characterizes biological membranes. The current-voltage relation of molluscan neurons may be complicated by inward-going rectification, but if that is inhibited the I-V curve follows the prediction of either the constant field equation or a simple electrical model. Factors which modify membrane behavior. The resting potential of molluscan neurons is very sensitive to changes in temperature and Ko, through a combination of effects upon the electrogenic sodium pump, inward-going rectification, and the membrane "parameters". Inward-going rectification depends upon a rectifying K conductance, and can be eliminated by cold or the removal of Ko. Strong or prolonged currents have time-dependent effects upon the membrane, and excessive polarization leads to a "high conductance state". The underlying (non-rectifying) K permeability of the membrane is relatively insensitive to temperature and ionic changes, whereas the Na permeability increases with warming. Membrane resistance varies with both temperature and ions (because the I-V curve is sensitive to these conditions) but membrane capacitance is relatively insensitive to external factors. Electrogenic sodium transport. Sodium transport is electrogenic in molluscan neurons. It can be stimulated by warm temperatures and an excess of substrate (e.g. high Nai); it can be inhibited by cold, by an absence of substrate (e.g. low Ko), or by pharmacologic agents such as cyanide or ouabain.(ABSTRACT TRUNCATED AT 400 WORDS)

Analysis of hyperpolarizations induced by glutamate and acetylcholine on Onchidium neurones

1. Four giant neurones, designated G-H cells, in the right pleural ganglion of the marine pulmonate mollusc, Onchidium verruculatum, showed characteristic membrane hyperpolarization during applications of either acetylcholine (ACh) or L-glutamate. In the presence of ACh the membrane was hyperpolarized only transiently, while in the presence of glutamate the response was maintained. Significant increases in membrane conductance accompanied the changes in membrane potential.2. In excess potassium sea water, a slight hyperpolarization occurred when the normal concentration was increased between one- and twofold. However, depolarization usually occurred when the concentration was increased tenfold except on a few occasions when a slight but definite hyperpolarization occurred. These changes were all accompanied by a substantial increase in the membrane conductance. This hyperpolarization was in all probability the result of an increase in chloride ion permeability caused by the release of an ACh-like transmitter from depolarized presynaptic nerve terminals.3. The reversal levels for glutamate- and ACh-induced hyperpolarization respectively were approximately - 20 and - 17 mV with respect to the resting membrane potential.4. By changing the external ion composition, glutamate- and ACh-induced hyperpolarization were shown to be the result of an increased permeability of the subsynaptic membrane to potassium and chloride ions respectively. It appears therefore that inhibition in the same G-H cells can be activated by two different transmitter substances and that each of them activates a change in the membrane permeability to a different ion.5. The relationship between the concentration of glutamate and the membrane conductance change was suggestive of two glutamate molecules reacting with a single receptor site.

Long-term effect of ouabain and sodium pump inhibition on a neuronal membrane

1. The long-term effects of ouabain on the membrane potential of the Anisodoris giant neurone (G cell) were examined in cells maintained for periods of up to 15 hr at 11-13 degrees C.2. In the presence of ouabain (5 x 10(-4)M), the membrane potential depolarized to a constant level for 1-4 hr, then hyperpolarized for 5-7 hr after which it gradually depolarized again.3. During the hyperpolarizing phase, after 6-8 hr in ouabain, [K](1) fell approximately 50%, [Na](1) increased 50-100% and the P(Na)/P(K) ratio decreased to 25% of its initial value.4. After 8 hr in ouabain the membrane conductance increased two- to fourfold. This increase was independent of temperature and membrane rectification.5. The K permeability (P(K)) was calculated from the constant field equation, and showed a fourfold increase after long-term treatment with ouabain. This rise in P(K) probably underlies the membrane hyperpolarization and the decrease in the P(Na)/P(K) ratio.6. It is suggested that inhibition of the Na(+) pump with ouabain causes a gradual rise in [Na](1) which secondarily leads to Ca(2+) uptake, an increase in [Ca](1), and thereby an increase in P(K).

Steady-state contribution of the sodium pump to the resting potential of a molluscan neurone

1. The electrogenic contribution of the Na(+)-K(+) exchange pump to the membrane potential of the Anisodoris giant neurone (G cell) was examined under steady-state and Na(+) loaded conditions.2. The membrane potential was variable for the first 1-4 hr after impalement, but, in the absence of experimental manipulation, remained constant thereafter. The average membrane potential for ten cells maintained at 11-13 degrees C and measured 5-36 hr after impalement was 55.8 +/- 1.0 mV (S.E. of mean).3. Low concentrations of external ACh caused a reversible increase in membrane Na(+) conductance. Brief exposure to ACh proved a fast and reversible technique to load the cell with Na(+) ions, and transiently stimulate the electrogenic Na(+) pump.4. In ten cells maintained from 5 to 36 hr at 11-13 degrees C the reduction in membrane potential produced by inhibition of the Na(+) pump with ouabain was remarkably constant between cells and averaged + 9.7 mV.5. Cells maintained under steady-state conditions (at 11-13 degrees C) for extended periods of time were shown to be relatively insensitive to changes in temperature and to small changes in external K(+).6. It is estimated that the Na(+)-K(+) exchange pump contributes approximately - 10 mV to the steady-state resting potential of the G cell, and that two Na(+) ions are extruded for every K(+) ion transported into the cell per pump cycle.

Ionic basis of hyperpolarizing receptor potential in scallop eye: increase in permeability to potassium ions

Photoreceptors of the distal retina of the scallop respond to light with a large hyperpolarizing receptor potential which is caused by a selective increase in permeability to potassium ions.

Cyclic variation of potassium conductance in a burst-generating neurone in Aplysia

1. The hyperpolarization between bursts in the R 15 cell of Aplysia is accompanied by an increase in membrane slope conductance.2. The post-burst hyperpolarization can be observed with ouabain, lithium, or potassium-free solution if artificial inward current is applied. The hyperpolarization can be observed with dinitrophenol or cooling to 10 degrees C, with no injected current. Thus, the hyperpolarization apparently is not due to the cyclic activity of an electrogenic pump.3. A reversal potential for the post-burst hyperpolarization can be demonstrated by passage of inward current during the inter-burst period. The reversal of direction of the potential depends on recent occurrence of a burst.4. The reversal potential varies with external potassium concentration, but not with chloride or sodium.5. The post-burst hyperpolarization is not blocked by external tetraethylammonium at a concentration which greatly prolongs the action potentials.6. During the onset of spike blockage by, and recovery from, calcium-free+tetrodotoxin saline, the bursts of action potentials appear to be driven by endogenous waves of membrane potential.7. The hyperpolarizing phase of the waves in calcium-free+tetrodotoxin medium is accompanied by an increased slope conductance.8. A reversal potential can be demonstrated for the hyperpolarization following a wave in calcium-free+tetrodotoxin medium by applying inward current during the interwave period.9. The waves in calcium-free+tetrodotoxin medium are blocked by ouabain but can be reinstated by artificial hyperpolarization.10. The post-burst hyperpolarization and the post-wave hyperpolarization appear to result from a periodic increase in membrane conductance, primarily to potassium ions.

Electrogenic sodium pump and high specific resistance in nerve cell bodies of the squid

An electrogenic sodium pump contributes to the membrane potential in squid nerve cell bodies, imparting a temperature dependence to the resting potential that is abolished by strophanthidin. The existence of a potential produced by the pump in the soma but not the axon is correlated with a higher membrane resistance in the soma. Thus, membranes from different parts of a neuron may have functionally significant differences in resistance.

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.

Effect of sodium and sodium-substitutes on the active ion transport and on the membrane potential of smooth muscle cells

1. The changes of the ion content and of the membrane potential of taenia coli cells have been studied during prolonged exposure to Na-deficient solutions containing either Li or choline.2. A K-free solution containing either 71 mM-Na-71 mM-Li or 71 mM-Na-71 mM choline causes a slower loss of cellular K than a 142 mM-Na solution. In both these Na-deficient solutions the membrane hyperpolarizes to about -100 mV for periods up to 6 hr. This hyperpolarization is partially abolished by 2 x 10(-5)M ouabain.3. Replacing all extracellular Na by Li and maintaining 5.9 mM-K causes a fast loss of all Na and a progressive replacement of K by Li. These changes of the intracellular ion content are accompanied by a depolarization of the cells, suggesting that intracellular Li cannot substitute for Na in activating the ion pump.4. Exposing K-depleted cells to a K-free 71 mM-Na-71 mM-Li solution results in a ouabain sensitive transport of Na and Li against their electro-chemical gradient.5. The K-uptake by K-depleted cells from a solution containing 0.59 mM-K is increased by reducing [Na](o) to half of its normal value. This finding indicates that external Na inhibits the active Na-K exchange.6. In Na-enriched tissues half of the Na efflux is due to a ouabain insensitive Na-exchange diffusion. If Li is used as a Na substitute, the Na-Li exchange compensates for the diminution of the Na-exchange diffusion unless ouabain is added.

The passive electrical properties of the membrane of a molluscan neurone

1. The passive electrical properties of the membrane of the gastrooesophageal giant neurone (G cell) of the marine mollusc, Anisodoris nobilis were studied with small current steps.2. The membrane transient response can be fitted with a theoretical curve assuming as a model for the cell a sphere (soma) connected to a cable (axon). The axo-somatic conductance ratio (rho), determined by applying this model, is large (approximately 5) and the membrane time constant (tau) is long (approximately 1 sec).3. When the actual surface area of the cell, corrected for surface infoldings, and the spread of current along its axon is taken into account, the electrical measurements imply a specific resistance of the membrane of approximately 1.0 MOmega.cm(2).4. Estimates of specific membrane capacity, either from measurements of the initial portion of the membrane transient or from the ratio of the time constant to the specific membrane resistance are close to the value of 1 muF/cm(2) expected for biological membranes.5. Thus, our measurements of specific capacitance, time constant, length constant and axo-somatic conductance ratio all indicate that the value found for the specific membrane resistance of the G cell, while unexpectedly large, is valid.6. The magnitude of this value suggests that the conductance (permeability) of its membrane to ions is much smaller than that previously assumed for nerve membranes; this small conductance may be related to the larger surface-to-volume ratio of the G cell.

Active transport of chloride by the giant neuron of the Aplysia abdominal ganglion

Internal chloride activity, a(i) (Cl), and membrane potential, E(m), were measured simultaneously in 120 R2 giant neurons of Aplysia californica. a(i) (Cl) was 37.0 +/- 0.8 mM, E(m) was -49.3 +/- 0.4 mv, and E(Cl) calculated using the Nernst equation was -56.2 +/- 0.5 mv. Such values were maintained for as long as 6 hr of continuous recording in untreated neurons. Cooling to 1 degrees -4 degrees C caused a(i) (Cl) to increase at such a rate that 30-80 min after cooling began, E(Cl) equalled E(m). The two then remained equal for as long as 6 hr. Rewarming to 20 degrees C caused a(i) (Cl) to decline, and E(Cl) became more negative than E(m) once again. Exposure to 100 mM K(+)-artificial seawater caused a rapid increase of a(i) (Cl). Upon return to control seawater, a(i) (Cl) declined despite an unfavorable electrochemical gradient and returned to its control values. Therefore, we conclude that chloride is actively transported out of this neuron. The effects of ouabain and 2,4-dinitrophenol were consistent with a partial inhibitory effect. Chloride permeability calculated from net chloride flux using the constant field equation ranged from 4.0 to 36 x 10(-8) cm/sec.

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.