The independence of electrogenic sodium transport and membrane potential in a molluscan neurone

Millimeter waves thermally alter the firing rate of the Lymnaea pacemaker neuron

The effects of millimeter waves (mm-waves, 75 GHz) and temperature elevation on the firing rate of the BP-4 pacemaker neuron of the pond snail Lymnaea stagnalis were studied by using microelectrode techniques. The open end to a rectangular waveguide covered with a thin Teflon film served as a radiator. Specific absorption rates (SARs), measured in physiological solution at the radiator outlet, ranged from 600 to 4,200 W/kg, causing temperatures rises from 0.3 to 2.2 degrees C, respectively. Irradiation at an SAR of 4200 W/kg caused a biphasic change in the firing rate, i.e., a transient decrease in the firing rate (69 +/- 22% below control) followed by a gradual increase to a new level that was 68 +/- 21% above control. The biphasic changes in the firing rate were reproduced by heating under the condition that the magnitude (2 degrees C) and the rate of temperature rise (0.96 degrees C/s) were equal to those produced by the irradiation (for an SAR of 4,030 W/kg). The addition of 0.05 mM of ouabain caused the disappearance of transient responses of the neuron to the irradiation. It was shown that the rate of temperature rise played an important role in the development of a transient neuronal response. The threshold stimulus for a transient response of the BP-4 neuron found in warming experiments was a temperature rise of 0.0025 degrees C/s.

Role of the sodium pump in pacemaker generation in dog colonic smooth muscle

1. The role of the Na+ pump in the generation of slow wave activity in circular muscle of the dog colon was investigated using a partitioned 'Abe-Tomita' type chamber for voltage control. 2. Blockade of the Na+ pump by omission of extracellular K+, by ouabain, or the combination of 0 mM-Na+ and ouabain, depolarized the membrane up to approximately -40 mV and abolished the slow wave activity. Repolarization back to the control membrane potential by hyperpolarizing current restored the slow wave activity. 3. Slow waves continued to be present in 0 Na+, Li+ HEPES solution. 4. The depolarization induced by the procedures to block Na+ pump activity was associated with an increase in input membrane resistance. 5. Voltage-current relationships show the presence of an inward rectification. 6. Reduction of temperature depolarized the membrane, and decreased the slow wave frequency and amplitude. The slow wave amplitude was restored by repolarization of the membrane. 7. Brief depolarizing pulses evoked premature slow waves. Brief hyperpolarizing pulses terminated the slow waves. 8. We conclude that abolition of slow wave activity by Na+ pump blockade is a direct effect of membrane depolarization and that the Na+ pump is not responsible for the generation of the slow wave. 9. Our results are consistent with the hypothesis that pacemaker activity in smooth muscle is a consequence of membrane conductance changes which are metabolically dependent.

The plasma membrane ATPase of Neurospora: a proton-pumping electroenzyme

Probably the best marker enzyme for plasma membranes of eukaryotic cells is a magnesium-dependent, vanadate-inhibited ATPase whose primary function is the transmembrane transport of cations. In animal cells, different species of the enzyme transport different cations: sodium ions released in unequal exchange for potassium ions, calcium ions extruded alone (perhaps), or protons secreted in equal exchange for potassium ions. But in plants and fungi only proton secretion has been clearly demonstrated. A useful model cell for studying the proton-secreting ATPase has been the ascomycete fungus Neurospora, in which the enzyme drives an outward current of protons that can exceed 50 microA/cm2 and can support membrane potentials greater than 300 mV. Both thermodynamic and kinetic studies have shown that the proton-pumping ATPase of Neurospora normally transports only a single proton for each ATP molecule split; and kinetic modelling studies have suggested (contrary to conventional assumptions) that the fast steps in the overall reaction are transmembrane transit of the proton and its dissociation following transport, while the slow steps are the binding of protons and/or ATP. The primary structure of the Neurospora enzyme, recently deduced by gene sequencing, is very close to that of the yeast (Saccharomyces) enzyme, and the hydropathic patterns for both closely resemble those for the animal-cell plasma-membrane ATPases. All of these enzymes appear to have 6-10 membrane-spanning alpha-helices, plus a large cytoplasmic headgroup which bears the catalytic nucleotide-binding site. Structural data, taken together with the electrical-kinetic behavior, suggest that the catalytic headgroup functions as an energized gate for protons. From a geometric point of view, action of such a gate would transfer the membrane field across the "transported" ion, rather than vice versa.

Interpretation of steady-state current-voltage curves: consequences and implications of current subtraction in transport studies

A problem often confronted in analyses of charge-carrying transport processes in vivo lies in identifying porter-specific component currents and their dependence on membrane potential. Frequently, current-voltage (I-V)--or more precisely, difference-current-voltage (dI-V)--relations, both for primary and for secondary transport processes, have been extracted from the overall membrane current-voltage profiles by subtracting currents measured before and after experimental manipulations expected to alter the porter characteristics only. This paper examines the consequences of current subtraction within the context of a generalized kinetic carrier model for Class I transport mechanisms (U.-P. Hansen, D. Gradmann, D. Sanders and C.L. Slayman, 1981, J. Membrane Biol. 63:165-190). Attention is focused primarily on dI-V profiles associated with ion-driven secondary transport for which external solute concentrations usually serve as the experimental variable, but precisely analogous results and the same conclusions are indicated in relation to studies of primary electrogenesis. The model comprises a single transport loop linking n (3 or more) discrete states of a carrier 'molecule.' State transitions include one membrane charge-transport step and one solute-binding step. Fundamental properties of dI-V relations are derived analytically for all n-state formulations by analogy to common experimental designs. Additional features are revealed through analysis of a "reduced" 2-state empirical form, and numerical examples, computed using this and a "minimum" 4-state formulation, illustrate dI-V curves under principle limiting conditions. Class I models generate a wide range of dI-V profiles which can accommodate essentially all of the data now extant for primary and secondary transport systems, including difference current relations showing regions of negative slope conductance. The particular features exhibited by the curves depend on the relative magnitudes and orderings of reaction rate constants within the transport loop. Two distinct classes of dI-V curves result which reflect the relative rates of membrane charge transit and carrier recycling steps. Also evident in difference current relations are contributions from 'hidden' carrier states not directly associated with charge translocation in circumstances which can give rise to observations of counterflow or exchange diffusion. Conductance-voltage relations provide a semi-quantitative means to obtaining pairs of empirical rate parameters.(ABSTRACT TRUNCATED AT 400 WORDS)

Incorporation of membrane potential into theoretical analysis of electrogenic ion pumps

The transport rate of an electrogenic ion pump, and therefore also the current generated by the pump, depends on the potential difference (delta psi) between the two sides of the membrane. This dependence arises from at least three sources: (i) charges carried across the membrane by the transported ions; (ii) protein charges in the ion binding sites that alternate between exposure to (and therefore electrical contact with) the two sides of the membrane; (iii) protein charges or dipoles that move within the domain of the membrane as a result of conformational changes linked to the transport cycle. Quantitative prediction of these separate effects requires presently unavailable molecular information, so that there is great freedom in assigning voltage dependence to individual steps of a transport cycle when one attempts to make theoretical calculations of physiological behavior for an ion pump for which biochemical data (mechanism, rate constants, etc.) are already established. The need to make kinetic behavior consistent with thermodynamic laws, however, limits this freedom, and in most cases two points on a curve of rate versus delta psi will be fixed points independent of how voltage dependence is assigned. Theoretical discussion of these principles is illustrated by reference to ATP-driven Na,K pumps. Physiological data for this system suggest that all three of the possible mechanisms for generating voltage dependence do in fact make significant contributions.

Current-voltage relations of the basolateral membrane in tight amphibian epithelia: use of nystatin to depolarize the apical membrane

Exposure of the mucosal side of toad (Bufo bufo) urinary bladder and frog (Rana ridibunda) skin to the polyene ionophore nystatin, resulted in stable preparations in which the apical resistance was negligible compared to the basolateral resistance. The preparations support passive K currents in both directions and an amiloride-insensitive Na current in the apical-serosal direction which is blocked by ouabain. The nystatin-treated toad bladder was used to study the electrical properties of the basolateral membrane by means of current-voltage curves recorded transepithelially. The K current showed strong rectification at cellular potentials negative with respect to the interstitial space. The ouabain-sensitive current increased with membrane voltage at negative voltages but saturated above +20 mV.

From sea lemons to c-waves

This review of retinal pigment epithelial (RPE) physiology pays tribute to Anthony L. F. Gorman, who introduced the author to the giant neuron of Anisodoris nobilis (the sea lemon) and cellular neurobiology. The RPE is an epithelial monolayer with tight junctions, which controls the environment of the photoreceptor outer segments. The apical and basal membranes have different electrical properties and generate a standing potential across the eye. The RPE helps maintain adhesion between the retina and the wall of the eye. Adhesion is weakened by cyanide, low pH or low calcium, but enhanced by ouabain or acetazolamide. The RPE transports water from the subretinal space toward the choroid. This water movement is inhibited by hypoxia or cyanide but enhanced by ouabain or acetazolamide. The c-wave of the electroretinogram is a composite of a cornea-positive wave produced by hyperpolarization of the apical RPE membrane and a cornea-negative wave produced by the Muller cells, both in response to the fall in extracellular potassium that follows illumination of the photoreceptors. The "light response" of the standing potential is produced by depolarization of the basal membrane of the RPE. These examples illustrate how principles of cellular neurophysiology can be applied to questions of clinical relevance.

Effects of ATP on Na+ transport and membrane potential in inside-out renal basolateral vesicles

We have studied Na+ transport in inside-out basolateral membrane vesicles isolated from rabbit kidney cortex. The addition of ATP in the presence of Mg2+ to the outside of K+-loaded vesicles induced a rapid influx of Na+ against its chemical gradient. Whereas intravesicular K+ was required, extravesicular K+ was inhibitory. ATP-dependent Na+ uptake was inhibited by intravesicular, but not extravesicular ouabain, while extravesicular vanadate was inhibitory. Evaluation of changes in membrane potential using the lipophilic cation triphenylmethylphosphonium (TPMP+) demonstrated hyperpolarization of the membrane voltage after MgATP addition. Changing membrane potential from zero to -40 mV had no effect on ATP-dependent Na+ transport. The potential produced MgATP was inhibited by valinomycin and by protoinophores, but not by vanadate or ouabain. By contrast, the hyperpolarization that occurred in mitochondria after MgATP addition was inhibited by 75% by vanadate. We conclude that the properties of the renal basolateral sodium pump are qualitatively similar to those found in red cell and nerve and that these membranes probably also contain an electrogenic proton pump.

Interpretation of current-voltage relationships for "active" ion transport systems: I. Steady-state reaction-kinetic analysis of class-I mechanisms

This paper develops a simple reaction-kinetic model to describe electrogenic pumping and co- (or counter-) transport of ions. It uses the standard steady-state approach for cyclic enzyme- or carrier-mediated transport, but does not assume rate-limitation by any particular reaction step. Voltage-dependence is introduced, after the suggestion of Läuger and Stark (Biochim. Biophys. Acta 211:458-466, 1970), via a symmetric Eyring barrier, in which the charge-transit reaction constants are written as k12 = ko12 exp(zF delta psi/2RT) and k21 = ko21 exp(-zF delta psi/2RT). For interpretation of current-voltage relationships, all voltage-independent reaction steps are lumped together, so the model in its simplest form can be described as a pseudo-2-state model. It is characterized by the two voltage-dependent reaction constants, two lumped voltage-independent reaction constants (k12, k21), and two reserve factors (ri, ro) which formally take account of carrier states that are indistinguishable in the current-voltage (I-V) analysis. The model generates a wide range of I-V relationships, depending on the relative magnitudes of the four reaction constants, sufficient to describe essentially all I-V datas now available on "active" ion-transport systems. Algebraic and numerical analysis of the reserve factors, by means of expanded pseudo-3-, 4-, and 5-state models, shows them to be bounded and not large for most combinations of reaction constants in the lumped pathway. The most important exception to this rule occurs when carrier decharging immediately follows charge transit of the membrane and is very fast relative to other constituent voltage-independent reactions. Such a circumstance generates kinetic equivalence of chemical and electrical gradients, thus providing a consistent definition of ion-motive forces (e.g., proton-motive force, PMF). With appropriate restrictions, it also yields both linear and log-linear relationships between net transport velocity and either membrane potential or PMF. The model thus accommodates many known properties of proton-transport systems, particularly as observed in "chemiosmotic" or energy-coupling membranes.

Subthreshold and near-threshold membrane currents in lobster stretch receptor neurones

1. The ion currents in the slowly and rapidly adapting stretch receptor neurone of lobster were investigated with respect to their nature and stationary kinetics in sub- and near-threshold voltage regions using electrophysiological and pharmacological techniques. 2. In both neurones the following currents were identified: (a) a tetrodotoxin-sensitive Na current, (b) a tetraethylammonium and 4-aminopyridine-sensitive K current, (c) a Co (or Mn)-sensitive Ca-dependent K current, (d) an ouabain-sensitive pump current and (e) a remaining leak current carried mainly by Na, K and Cl. 3. In suprathreshold voltage regions the balance between the individual membrane currents leads to the formation of a stationary negative conductance (negative slope in the voltage dependence of the ionic current) in the slowly, but not in the rapidly adapting cell. 4. These observations are compatible with the fact that during prolonged suprathreshold stimulation a stationary low frequency impulse during is possible in the slowly adapting cell, whereas in the rapidly adapting cell it is not.

Characterization of the electrogenic sodium pump in cardiac Purkinje fibres

1. The Na pump is examined in sheep cardiac Purkinje fibres using a two micro-electrode voltage clamp technique.2. After reducing the external K concentration, [K](o), to zero for 2 min or more, subsequent addition of an ;activator cation' (known to activate the Na pump in other preparations) produces a transient increase of outward current. This outward current transient is abolished by 10(-5)M-strophanthidin (cf. Gadsby & Cranefield, 1979a).3. It is concluded that this transient increase of outward current is a result of a transient stimulation of the sodium pump by the raised [Na](i) following exposure to 0-K(o). Although this current transient may reflect the activity of an electrogenic Na pump, it is difficult to use K as the activator cation to establish this point. This is due to the extracellular K depletion that occurs during Na pump reactivation and the subsequent change that this K depletion produces in the current-voltage relationship of the Purkinje fibre.4. Rb(o) or Cs(o) have been used instead of K(o) to reactivate the Na pump when examining the transient increase of outward current. On adding either of these cations after exposing a preparation to a solution without such ;activator cations', the outward current transient is relatively voltage independent over a wide range of potentials (-90 to +10 mV). It is concluded that, following the addition of Rb(o) or Cs(o), the transient increase of outward current is a direct measure of the transient increase of the electrogenic Na pump current.5. Increasing [Rb](o) or [Cs](o) over the range of 0-40 mM increases the rate of decay of the electrogenic Na pump current transient. Using a simple model (cf. Rang & Ritchie, 1968), it is shown that the decay rate constant of the electrogenic Na pump current transient is a good measure of the degree of activation of the external site of the Na pump. At a given concentration of activator cation, Rb(o) produces a greater activation of the Na pump than does Cs(o). The K(0.5) for Rb(o) is 6.3 mM and for Cs(o) is 14.2 mM. Li(o) activates the Na pump more weakly than Rb(o) and Cs(o).6. The coupling ratio of the Na pump is shown to be independent of Rb(o) or Cs(o) over the range 2-40 mM. Furthermore, consistent with the results of Gadsby & Cranefield (1979a), the coupling ratio is independent of Na(i) over the range considered.7. The Q(10) for the electrogenic Na pump current transient varies between 1.6 and 2.3 over the range of temperature 26-46 degrees C.8. A maximum Na pump current of about 0.78 muA cm(-2) is obtained. Assuming a coupling ratio of 3Na/2K, the rate of Na ion transport into the cell is estimated to be about 23 p-mole cm(-2) sec(-1). Assuming a Na pump turnover of 150 sec(-1), we estimate that there are about 1000 Na pump sites per mum(2) of cell surface.9. We conclude that the electrogenic Na pump current transient provides a good measure of the activity of the Na pump when Rb or Cs are used as ;activator cations'. This measure can be used in the intact preparation to investigate the relationship between Na pump rate and other cellular events such as the regulation of tension (Eisner & Lederer, 1980).

Current-voltage relationships for the plasma membrane and its principal electrogenic pump in Neurospora crassa: I. Steady-state conditions

The nonlinear membrane current-voltage relationship (I-V curve) for intact hyphae of Neurospora crassa has been determined by means of a 3-electrode voltage-clamp technique, plus "quasi-linear" cable theory. Under normal conditions of growth and respiration, the membrane I-V curve is best described as a parabolic segment convex in the direction of depolarizing current. At the average resting potential of - 174 mV, the membrane conductance is approximately 190 micronhos/cm2; conductance increase to approximately 240 micronhos/cm2 at -300 mV, and decreases to approximately 130 micronhos/cm2 at 0 mV. Irreversible membrane breakdown occurs at potentials beyond this range. Inhibition of the primary electrogenic pump in Neurospora by ATP withdrawal (with 1 mM KCN) depolarizes the membrane to the range of -40 to -70 mV and reduces the slope of the I-V curve by a fixed scaling factor of approximately 0.8. For wild-type Neurospora, compared under control conditions and during steady-state inhibition by cyanide, the I-V difference curve--presumed to define the current-voltage curve for the electrogenic pump--is a saturation function with maximal current of approximately 20 muA/cm2, a half saturation potential near -300 mV, and a projected reversal potential of ca. -400 mV. This value is close to the maximal free energy available to the pump from ATP hydrolysis, so that pump stoichiometry must be close to 1 H+ extruded:1 ATP split. The time-courses of change in membrane potential and resistance with cyanide are compatible with the steady-state I-V curves, under the assumption the cyanide has no major effects other than ATP withdrawal. Other inhibitors, uncouplers, and lowered temperature all have more complicated effects. The detailed temporal analysis of voltage-clamp data showed three time-constants in the clamping currents: one of 10 msec, for charging the membrane capacitance (0.9 muF/cm/2); a second of 50-75 msec; and a third of 20-30 sec, perhaps representing changes of intracellular composition.

An analysis of the influence of membrane potential and metabolic poisoning with azide on the sodium pump in skeletal muscle

1. Activation of the Na pump in muscle by the external K concentration, [K]O, is independent of the membrane potential (Em) as shown by experiments in which Em was either stabilized during variation of [K]O or varied by application of azide at constant or zero [K]O. 2. Application of azide to Na-enriched muscles causes a transient increase in 22Na efflux which occurs either in the presence or in the absence of external K. 3. The increased 22Na efflux induced by azide is abolished by addition of ouabain and is greatly reduced by removal of almost all of the external Na concentration, [Na]o. 4. Azide-treated muscles show a rather normal K sensitivity of 22Na efflux and [K]O induces a net Na extrusion from Na-enriched muscles in the presence of azide. 5. Azide reduces ouabain-sensitive K influx to low values thus interfering with K pump but not with the ability of K to activate the Na pump. 6. The experiments provide evidence that azide promotes a ouabainsensitive Na-Na exchange in Na-enriched muscles and that it partially uncouples the Na-K exchange normally observed.

Further studies of the potential-dependence of the sodium-induced membrane current in snail neurones

1. The potential-dependence of the membrane current induced by intracellular injections of sodium ions was studied on giant neurones of the snail Helix pomatia. This current decreases with membrane hyperpolarization at room temperature and can be reversed at sufficiently negative holding potentials. The same injections at 7 degrees C, as well as injections of lithium or potassium ions do not induce membrane currents and do not increase membrane conductance. 2. An increase in the amount of injected sodium changes the potential-dependence of the induced membrane currents. Small injections (about 1 muC) induce a current that does not depend upon the membrane potential. Further increase in the injection size not only increases the induced current but also enhances its potential-dependence and often reveals the existence of a reversal potential. The latter reaches -60 to -65 mV with large sodium injections. 3. An increase in extracellular potassium concentration from 4 to 8 mM shifts the reversal potential 17 mV in the depolarizing direction, and a decrease from 4 to 2 mM shifts it 14 mV in the hyperpolarizing direction. Replacement of potassium by rubidium or elimination of sodium ions from the outside solution, does not affect the induced current or its potential-dependence. 4. The coefficient of electrogenicity (the ratio between the amount of charge transferred by the sodium-induced membrane current and the amount brought into the cell during the injection) increases with an increase in the injection size if the membrane potential is clamped near the resting potential level. This relation is reversed when the holding potential is -80 mV. The reversal takes place at holding potentials near -60 mV. 5. 10 mM TEA does not affect the induced current and its potential-dependence. 6. It is suggested that the potential-dependence of the sodium-induced membrane current is a result of a specific increase in the membrane potassium conductance that is coupled with high activity of the sodium pump.

The independence of membrane potential and potassium activation of the sodium pump in muscle

The membrane potential (Em) of sartorius muscle fibers was made insensitive to [K+] by equilibration in a 95 mM K+, 120 mM Na+ Ringer solution. Under these conditions a potassium-activated, ouabain-sensitive sodium efflux was observed which had characteristics similar to those seen in muscles with Em sensitive to [K+]. In addition, in the presence of 10 mM K+, these muscles were able to produce a net sodium extrusion against an electrochemical gradient which was also inhibited by 10- minus 4 M oubain. This suggests that the membrane potential does not play a major role in the potassium activation of the sodium pump in muscles.

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)

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.

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.

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.

The effects of temperature and ions on the current-voltage relation and electrical characteristics of a molluscan neurone

1. Current-voltage relations were generated in the Anisodoris giant neurone (G cell) by either current pulses or slow biphasic current ramps.2. Inward-going rectification occurred during hyperpolarization at warm temperatures (10-15 degrees C), but not at cold temperatures (0-5 degrees C) or in the absence of external K.3. Replacing external K with Rb eliminated inward-going rectification in the warm, but produced it in the cold. The removal of external Na, Cl or Ca had no effect upon inward-going rectification.4. At cold temperatures the I-V relation was linear when generated by current pulses, but was non-linear in accordance with the constant field hypothesis when generated by current ramps.5. A high conductance state developed when the membrane was hyperpolarized beyond a critical potential (approximately - 130 mV in the cold, and - 110 mV in the warm) which was dependent upon external Ca, but not upon K, Na or Cl.6. Hysteresis was observed in the ramp-generated I-V relation whenever the cell was polarized into the high conductance state.7. Rectification and the high conductance state appear to involve different mechanisms within the membrane. However, both are dependent upon absolute membrane potential and not the resting potential.8. The axonal-somatic conductance ratio for the G cell was calculated to be between 2 and 10.9. The membrane time constant (200-100 msec) and specific resistance (0.1-1.5 x 10(6) Omega cm(2)) varied with temperature, membrane potential, and external ions in a manner that correlated with changes in the shape of the I-V relation. In addition, the resistance was dependent upon external Ca.10. The K permeability (P(K)), measured during inhibition of inwardgoing rectification, was independent of temperature and membrane potential. However, P(Na) increased with warming.11. The specific capacitance was calculated to be 0.5-1.0 muF/cm(2). The capacitance increased slightly with warming, but was independent of membrane potential and unaffected by reductions in external K or Na.