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

Effect of temperature on membrane potential and ionic fluxes in intact and dialysed barnacle muscle fibres

1. The temperature-dependent component of the resting potential in intact, cannulated and dialysed fibres from the muscle of the barnacle Balanus nubilus was studied under a variety of different experimental conditions. A decrease in temperature from 22 to 12 degrees C produced a mean depolarization of 10 mV.2. Neither addition of strophanthidin, nor replacement of external sodium by lithium affect the voltage shift induced by temperature. However, the magnitude of the voltage shift depends on the external chloride and potassium concentration.3. The dialysis technique was applied to measure the potassium, chloride and sodium fluxes as a function of temperature. The Q(10) for the passive fluxes of these ions was 1.9, 1.7, and 1.4 respectively.4. The temperature-dependent changes in the passive ionic fluxes combined with the inability of inhibitors of the sodium pump to alter the temperature dependence of the resting potential suggest that the change induced by temperature on the resting potential is primarily caused by a change in the passive permeability ratios, and is not related to active ion transport.

Effect of hypokalemia and hypomagnesemia produced by hemodialysis on vascular resistance in canine skeletal muscle: role of potassium in active hyperemia

Hemodialysis was used to study the effect of an acute local decrease in plasma [K + ] or [Mg 2+ ] or in both on vascular resistance in skeletal muscle. A dialyzer was placed in the arterial supply of the collateral-free gracilis muscle of the dog and blood flow was held constant while measuring perfusion pressure. Pressure increased linearly with decreased [K + ] down to 0.2 mEq/liter. A 50% decrease in [K + ] produced a 12% increase in resistance. Prolonged hypokalemia produced a sustained increase in perfusion pressure and a decreased responsiveness to close arterial injection of norepinephrine. Removal of up to 84% of the plasma Mg 2+ produced no effect, either alone or in conjunction with hypokalemia. When the potassium level of the perfusing blood was changed from normal to hypokalemic during the dilation brought on by simulated exercise, the resistance did not change. In addition, the magnitude of the resistance changes seen during exercise were much greater than could be induced by local changes in plasma [K + ] alone. It is concluded that hypokalemia produces active constriction of vascular smooth muscle. However, this study fails to lend support to the idea that potassium alone is responsible for exercise dilation.

Potassium ion accumulation near a pace-making cell of Aplysia

1. A delayed current decrease associated with prolonged depolarization was studied in R(15) (the parabolic burster) of Aplysia by using intracellular recording and voltage clamp techniques.2. For long duration command pulses (3 sec), the outward current shows a delayed decrease. The current goes from a maximum near 100 msec and falls until a steady-state outward current is reached between 1.5 and 2.5 sec after the beginning of the command step. This final steady-state current is usually only about 20-30% of the peak outward current.3. Double step voltage clamps show that this current decrease is associated with a large shift of e.m.f. Measurements of conductance, on the other hand, fail to show any significant difference in conductance associated with peak and steady-state currents.4. From the results of application of high K(+) ringer, the conclusion is reached that this shift in e.m.f. is due to an accumulation of K(+) near the exterior cell membrane. Several other experiments exclude the possibility of either metabolic events or compensating conductance changes producing the phenomenon.5. The location of the accumulation is considered on the basis of anatomical studies. It is concluded that the accumulation takes place in the extensive infoldings found in cells like R(15). An explanation of the difference in delayed current decrease between pace-makers and non-pace-makers is suggested, since the pace-makers apparently have more extensive invaginations than the non-pace-makers. This suggestion is lent support by measurements of capitance and current density.

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.

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).

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

1. The current-voltage relations of the Anisodoris giant neurone (G cell) were studied in the presence and absence of Na pump activity.2. Inhibition of the electrogenic Na pump with ouabain had no effect on either the presence at warm temperatures (10-15 degrees C), or absence at cold temperatures (0-5 degrees C), of inward-going rectification.3. Abolition of inward-going rectification in the warm, by replacement of external K with Rb, did not affect the electrogenic Na pump.4. The current generated by the electrogenic pump was essentially constant between the membrane potentials of - 30 and - 100 mV.5. The potential produced by the electrogenic pump can be predicted by a modification of the constant field equation.6. It is estimated that the energy required to extrude Na was between 3160 and 3700 cal/g-atom, and that uncoupled Na efflux during pump activity was typically between 0.2 and 4.0 p-mole/cm(2).sec.

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.

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.

Electrogenic sodium pump in smooth muscle cells of the guinea-pig's taenia coli

1. The changes of the membrane potential, of the K equilibrium potential, and of the membrane conductance during K accumulation by K-depleted tissues have been studied. Three subsequent characteristic periods can be described.2. Readmission of 5.9 mM-K after complete depletion results in a rapid extrusion of Na and uptake of K, and in a rapid hyperpolarization of the cells. Initially the time course of the K equilibrium potential and the membrane potential are similar except in propionate solution. This initial period is characterized by a high membrane conductance. No change of membrane potential occurs if 10(-5)M ouabain is present.3. After 5-7 min the membrane potential becomes more negative than the K equilibrium potential. The difference between both values is larger in solutions containing propionate or in hypertonic solutions. This second phase of the recovery period is characterized by a progressive decrease of the membrane conductance.4. In a third phase both the membrane potential and the membrane resistance return to their steady-state value.5. If the external K concentration in the recovery solution is increased, the maximal hyperpolarization is less and has a shorter duration. A decrease of the temperature of the recovery solution results in a slower initial rate of repolarization and in a decrease of the maximal value of the hyperpolarization.6. These observations demonstrate the existence of an electrogenic sodium pump in smooth muscle cells during stimulation of the Na pump. An analysis of the experimental data obtained under steady-state conditions in normal Krebs solution suggests that also under these conditions an electrogenic Na pump might take part in the maintenance of the resting potential.

The hyperpolarization of frog skeletal muscle fibres induced by removing potassium from the bathing medium

1. The time course of changes in resting potential after removing K(0) was studied in twenty-four single fibres and in 136 fibres from small bundle (two to four fibres) preparations of frog semitendinosus muscles.2. The initial resting potentials in the control saline ranged between -88 and -98 mV. The potentials returned to nearly the initial values when control conditions were reinstated after 3-8 hr of experimentation. All the fibres twitched at the end of the experiment.3. Only about one third of the fibres hyperpolarized for any length of time on exposure to a K-free saline at room temperature (20-28 degrees C). The hyperpolarization was reversed to depolarization after a variable delay. The resting potential could fall to -50 or -40 mV.4. The remainder of the fibres depolarized with little or no prior hyperpolarization.5. Both patterns of response could be replicated in the different fibres.6. Hyperpolarization induced by K-free solution was reduced or abolished on cooling to ca 10 degrees C; on substitution of Tris or Li for Na; and upon inhibition of the Na pump with DNP (0.025-0.2 mM) or ouabain (0.05 mM). The latter agent was not as effective as the other conditions.7. Only small, slowly developing depolarization occurred when Na was replaced with Tris or Li.8. The various effects in K-free solutions were reversed on returning to the control conditions.9. It is suggested that removal of K(0) itself has little or no direct effect on the resting potential and that the initial hyperpolarization is due to the pumped efflux of Na without a compensatory influx of K. Block of the pump electrogenesis is manifested by depolarization of the fibres as K(1) is depleted and Na(1) increased.10. The Na pump appears to be dependent upon the nutritional status of the frogs and variations of the latter probably cause the different responses of fibres to removal of K(0).

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.

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).