Intracellular Electrical Potential Research Articles

Vertical Nanowire Electrode Array for Enhanced Neurogenesis of Human Neural Stem Cells via Intracellular Electrical Stimulation.

Extracellular electrical stimulation (ES) can provide electrical potential from outside the cell membrane, but it is often ineffective due to interference from external factors such as culture medium resistance and membrane capacitance. To address this, we developed a vertical nanowire electrode array (VNEA) to directly provide intracellular electrical potential and current to cells through nanoelectrodes. Using this approach, the cell membrane resistivity and capacitance could be excluded, allowing effective ES. Human fetal neural stem cells (hfNSCs) were cultured on the VNEA for intracellular ES. Combining the structural properties of VNEA and VNEA-mediated ES, transient nanoscale perforation of the electrode was induced, promoting cell penetration and delivering current to the cell. Intracellular ES using VNEA improved the neuronal differentiation of hfNSCs more effectively than extracellular ES and facilitated electrophysiological functional maturation of hfNSCs because of the enhanced voltage-dependent ion-channel activity. The results demonstrate that VNEA with advanced nanoelectrodes serves as a highly effective culture and stimulation platform for stem-cell neurogenesis.

Clearance of Fluid From Airspaces of Newborns and Infants

After completing this article, readers should be able to: 1. Describe fetal lung fluid secretion, its application on lung development, and its reabsorption immediately after birth. 2. List the primary cellular mechanism responsible for lung fluid absorption. 3. Describe clinical conditions in newborns and infants associated with impaired sodium transport. 4. Describe how sodium transport can be influenced by fetal lung maturity. When the airspaces of the lungs are filled with fluid, normal gas exchange cannot take place, and there is marked respiratory distress and the need for intensive care. This occurs in only two situations: failure to clear fetal lung liquid at birth and leakage of fluid from the vasculature filling the alveoli (alveolar pulmonary edema). Fluid that is secreted into the fetal airspaces to enable normal lung development must be absorbed at the time of birth for a normal transition from fetal to postnatal life. Failure to do so can lead to two well-known clinical syndromes: transient tachypnea of the newborn (TTN) and when there is coexistent relative surfactant deficiency, neonatal respiratory distress syndrome (nRDS). Pulmonary edema can be caused postnatally by many disorders; those that occur frequently during the neonatal period are patent ductus arteriosus or structural congenital heart disease with marked left-to-right shunting. It is not only biologically reasonable to assume that clearance of such airspace fluid would be beneficial to the patient, but studies in adults suffering from pulmonary edema have shown that the lung’s ability to clear such fluid correlates with survival. In this article, we review how the epithelium of the lung uses the active transport of sodium (Na+), followed by chloride (Cl−) and water, from the apical to basolateral side of the alveolar lining cells to clear fluid and how the underlying mechanisms for active Na+ transport are influenced by the degree of …

Cortical Intracellular Electrical Potential in Roots of Unstressed and Stressed Sunflower Seedlings. I. Dependence on Water Status

The intracellular electrical potential (Ec) was measured with a microelectrode in cortical cells of intact roots of sunflower (Helianthus annuus) seedlings subjected to differing levels of water stress and illumination. Water stress was rapidly imposed and relieved by lowering and raising the level of nutrient solution in a tank which contained all the seedling roots except the lateral root in which Ec was measured. The base value of Ec for unstressed, illuminated seedlings was –120 mV. When placed in the dark, a small reversible depolarisation of approximately 10 mV could be measured in roots of seedlings grown under low levels of radiation but was not evident in seedlings grown under high levels of radiation. The imposition of and relief from extreme water stress (leaves and apex severely wilted) resulted in a reversible depolarisation of approximately 50 mV. The effect of water stress was greatly reduced by the presence of sucrose and glucose in the nutrient solution perfusing the 30 mm portion of the lateral root in which Ec was measured. It appears likely that the variation in potential was mediated by a reduction in the supply of photosynthate to the roots.

Cortical Intracellular Electrical Potential in Roots of Unstressed and Stressed Sunflower Seedlings. II. Radial Profiles and Oscillations

Various models have been proposed to explain how plants can extract water from dry soil against a gradient in water potential. According to one model, it is hypothesised that water uptake from dry soil is associated with a radial gradient in the intracellular electrical potential (Ec) of root cortical cells such that the potential in the outer cells is relatively hyperpolarised. As a partial test of this model, a microelectrode was used to measure radial profiles of Ec in intact roots of sunflower (Helianthus annuus) seedlings. The seedlings were subjected to either a control (unstressed) or one of two stress treatments. Water stress was rapidly imposed and relieved by lowering and raising the level of nutrient solution in a tank which contained all the seedling roots except the lateral root in which Ec was measured. There was a gradient in Ec in roots of unstressed plants with the potential of the outer cortical cells being relatively depolarised. The imposition of stress shifted the gradient in accordance with the hypothesis. Increasing the magnitude of the osmotic potential of the solution perfusing the measured portion of a lateral root of a stressed seedling resulted in a discontinuity in the profile between the second and third cortical layers. The gradients in the profile of Ec indicate there is a limitation in intercellular electrical coupling and the appearance of a discontinuity may indicate a decrease in coupling. Three types of occasional, spontaneous oscillations in Ec are characterised. One type of oscillation may be additional evidence that the electrical coupling between the cortical cell layers is variable and depends on the transport and status of water in the roots.

Effect of artificial electron receptors on the non-linearity of the current-induced voltage response in the membrane of the green alga Hydrodictyon reticulatum

The non-linear character of the voltage response of the intracellular electrical potential of the alga Hydrodictyon reticulatum to current injection was studied using a microelectrode technique. The phenomenon was strongly affected by the potassium ion concentration in the medium and by artificial electron acceptors (ferricyanide and the tetrathialfulvalene cation), by inhibitors of photosystem II and membrane H +-ATPase, by ethanol and by barium ions. Consideration of all the results suggests that the membrane potential dependence of potassium channels as well as changes in the ion concentration gradients in unstirred layers are responsible for the non-linear response phenomenon.

Ion transport across the epithelium of the rabbit caecum

The isolated rabbit caecum was studied in vitro. Under our experimental conditions, the rabbit caecum secreted potassium and chloride and absorbed sodium. To characterize the transport properties of the apical and the basolateral barriers, transepithelial electrical and flux ( 22Na, 36Cl and 86Rb) measurements and their sensitivity to transport inhibitors (furosemide, DIDS, ouabain and barium) are presented together with intracellular measurements with double-barrelled microelectrodes of intracellular electrical potentials and ionic activities. The fluxes of sodium and chloride were insensitive to DIDS and furosemide. The secretion of potassium and the absorption of sodium were both inhibited by ouabain, indicating that they are coupled through the sodium pump. Ouabain induced a slow fall in the chloride net fluxes, suggesting that these fluxes are also driven by the sodium pump, albeit indirectly. The basolateral to apical fluxes of potassium are insensitive to barium added to the apical side, but are accelerated by the replacement of chloride by gluconate on the apical side, suggesting the presence of a K + Cl − symport in the apical barrier.

In vitro culture ofDrosophila ovarian follicles: The influence of different media on development, RNA synthesis, protein synthesis and potassium uptake

Contradictory electrophysiological results were recently obtained inDrosophila ovarian follicles kept in different salines or complete media during measurements. Therefore, I checked follicles maintained in various solutions using morphological, physiological and biochemical criteria. Defined complete media were the best for supporting development from stage 10 to stage 14 (end of oogenesis). Supplementation of the solutions with insect pupal haemolymph had negative effects. For the maintenance of RNA synthesis, complete media were again superior to simple salines. Total protein synthesis was not very sensitive to the culture conditions during short incubation periods, but electrophoretic protein patterns were slightly less complex in the salines than in complete media. Furthermore, some major proteins (e.g. chorion proteins) synthesized during longterm culture failed to appear in the salines. In view of extrafollicular electrical currents and intracellular electrical potentials, potassium uptake experiments were conducted with several inhibitors, using rubidium-86 as a probe. Both potassium concentration and osmolarity were found to exert strong influences on total potassium uptake of the follicles. In the tested media and salines differing amounts of potassium were taken up via (Na+, K+)-ATPase, via other K+-pump(s) or passively. The possible influences of several parameters on the outcome of earlier in vitro experiments withDrosophila follicles are discussed. For further in vitro studies the chemically defined complete R-14 medium seems to be the most suitable.

A mathematical model of rabbit cortical thick ascending limb of the Henle's loop

A mathematical version of the cell model of the cortical thick ascending limb of the rabbit proposed by Greger and Schlatter ((1983) Pflügers Arch. 396, 325–334) is described. Available data are sufficient to compute the most important parameters. Simulations of experiments with perfused tubules in which the transepithelial voltage and conductance, and the intracellular electrical potential were measured in the course of ionic substitutions in the perfusing baths or treatment with ouabain or furosemide are in good agreement with the experimental results with the exception of those relating to dilution potential experiments. The model can be used in the analysis and planning of experiments and is capable of predicting the instantaneous values of ionic fluxes and intracellular concentrations and of cell volume.

Oscillations of the Membrane Potential of Pulvinar Motor CellsIn Situin Relation to Leaflet Movements ofDesmodium motorium

The ultradian rhythmic movement of the lateral leaflets of Desmodium motorium is accompanied by rhythmic changes of the extra- and intracellular electrical potentials in the pulvinus, which are measured in situ in the pulvinus against the bathing solution of the petiole. Extra- and intracellular potentials oscillate with 180'b0 phase difference to each other, as shown by simultaneous measurements of both types of potentials in the abaxial part of the pulvinus. Light-induced changes of these potentials move in opposite directions. The in situ membrane potential of the motor cells of the pulvinus was calculated from the difference between the extra- and intracellular potentials. It was found to oscillate between −136 and −36 mV, in phase with the intracellular and inverse to the extracellular potential. The phase relationship between the leaflet movement rhythm and the in situ membrane potential rhythm was as follows: downward movement is preceded and accompanied by a strong depolarization, upward movement by hyperpolarization. Our results suggest that membrane depolarization in pulvinar motor cells of Desmodium motorium drives and controls potassium efflux and hyperpolarization potassium influx via potassium channels.

Functional Effects of Proximal Tubular Dopamine Production

Significant proximal tubular responses to exogenous dopamine require 0.1 to 10 mumol/L concentrations but endogenous peritubular dopamine and DOPA concentrations are in the picomolar to nanomolar range. Dopamine concentration approaches micromolar levels within proximal tubular cells and their brush borders, as a result of DOPA decarboxylation and secretion, and in collecting duct fluid, as a result of tubular fluid absorption. Thus dopamine probably acts either within the proximal tubule cell or brush border or from the collecting tubular lumen. DOPA and Na+ uptake are coupled; dopamine uptake is linked to intracellular electrical potential and its secretion to H+ counter-transport; therefore alterations in proximal tubular Na+ and H+ transport influence dopamine excretion. Haloperidol and SCH 23390 block dopamine excretion, therefore dopamine antagonists may inhibit tubular dopamine responses by lowering intracellular dopamine concentration as well as by receptor blockade. Evidence for an intracellular site of dopamine action can be deduced from the inhibitory effect of DOPA on oxygen consumption and 86Rb uptake in proximal tubule cells. We have confirmed these findings in isolated proximal tubule cells but not in proximal tubule fragments. The discrepant responses may be due to the fact that isolated cells loose their polarity while tubule fragments remain polarized. Dopamine inhibition of proximal tubular Na+, K(+)-ATPase is not reproduced by single dopamine agonists or inhibited by dopamine antagonists. Dopamine effects which are not linked to known dopamine receptors may be the result of redox cycling. Micromolar dopamine oxidizes sulfhydryl groups which may modify enzyme structure and activate protein kinase C.

Oscillations of electrical potential along a root of a higher plant

Higher plants exhibit an oscillation of electrical potential near the surface along the root. The oscillation was studied with the aid of both the usual intracellular microelectrode technique and the extracellular multielectrode technique, the latter making it possible to measure simultaneously electrical potentials along the root. It was found that the oscillation of extracellular surface potential showed the largest amplitude in the elongation region, and the phase of the oscillation in this region differed by 180 degrees from that in the mature region, where the oscillation appeared coherent. The measurement of the intracellular electrical potential suggested the existence of oscillatory components localized to the parenchyma/xylem interface in the elongation region. A theoretical analysis based on an electrical circuit network described the above-mentioned behavior. It was shown that the oscillation was propagated along the root over several centimeters without substantial decay in the mature region.

Thiazides stimulate calcium absorption in urinary bladder of winter flounder

Thiazides inhibit voltage-independent NaCl absorption in the urinary bladder of the winter flounder presumably by blocking an electroneutral mucosal Na/Cl co-transporter. As thiazides stimulate calcium absorption in mammalian distal convoluted tubule while inhibiting NaCl absorption, we studied the effects of hydrochlorothiazide (HCTZ) on unidirectional 45Ca fluxes and intracellular electrical potential in short-circuited bladders to examine possible mechanisms of HCTZ effects on calcium transport. Basal secretory calcium flux was, on average, slightly larger than absorptive flux, reflecting small net calcium secretion. Mucosal addition of HCTZ (10 −4 M) stimulated absorptive calcium flux by 46% while the secretory flux was unaltered. Thus, HCTZ tended to induce net calcium absorption. Pre-treatment with serosal ouabain (10 −4 M) attenuated the HCTZ-induced increase in absorptive calcium flux. Moreover, HCTZ hyperpolarized the mucosal membrane potential by 18% as measured by conventional open-tip microelectrodes. These effects of HCTZ are consistent with the hypothesis that HCTZ indirectly stimulates Na/Ca exchange located at the serosal membrane. In conclusion, HCTZ in flounder urinary bladder, as in mammalian distal convoluted tubule, simultaneously inhibits NaCl absorption and stimulates calcium absorption. This study expands on the functional similarities between the flounder urinary bladder and the mammalian distal convoluted tubule.

Intracellular electrical potential measurements in Drosophila follicles.

We measured the intracellular electrical potential in oocyte and nurse cells of Drosophila follicles at different developmental stages (6-14) and determined the intrafollicular potential difference. During stages 8-10B, when intrafollicular transport is known to occur, no significant potential difference was found. During late vitellogenic stages the nurse cells assume a more positive potential than the oocyte. This result contrasts with the published data on Hyalophora follicles, in which intercellular electrophoresis of negatively charged proteins occurs from nurse cells to oocyte as a result of an intrafollicular potential difference (nurse cells more negative than the oocyte). Such a potential difference was not observed in Drosophila follicles at any stage, not even after application of juvenile hormone. The extrafollicular electrical field is described with a dipole model. The hypothetical dipole is located in the long axis of the follicle and changes its calculated length stage-specifically.

Intracellular electrical potentials in the rabbit collecting ducts perfused in vitro.

Cortical collecting ducts (CCD) and inner stripe of outer medullary collecting ducts (OMCD) of the rabbit were perfused in vitro and the electrical potential was measured with 0.5 M KCl-filled conventional microelectrodes. In CCD, the transepithelial potential (Vte) was -1.9 +/- 0.5 (S.E.) mV and the intracellular potential (Vb) was -74.5 +/- 1.3 mV, while in OMCD, Vte was +2.0 +/- 0.4 mV and Vb was -24.6 +/- 1.0 mV. Acute reduction of peritubular HCO3- concentration from 25 to 5 mM (from pH 7.4 to 6.9) decreased Vb slowly but markedly from -26.3 +/- 1.8 to -18.9 +/- 1.6 mV (p less than 0.001) in OMCD, while it depolarized Vb slightly in CCD (from -75.6 +/- 1.9 to -74.3 +/- 2.1 mV, P less than 0.05). Acute peritubular pH reduction with HCO3- -free HEPES-buffered solution also depolarized Vb of OMCD from -24.7 +/- 2.1 to -15.9 +/- 1.6 mV (p less than 0.001). Although the mechanism(s) of Vb depolarization in OMCD is unclear at the moment, this depolarization suggests that OMCD is responsive to peritubular pH alterations. The results of this study provide the evidence that the collecting duct is heterogeneous in Vb values and its response to the peritubular pH change.

Electrical properties of cultured epithelioid cells (MDCK).

This is a study of the intracellular electrical potential, membrane resistance, and capacity of MDCK cells (epithelioid of renal origin) cultured in monolayers on a collagen couch. These monolayers have a transepithelial resistance of 256 +/- 12 (22) ohm cm2 (mean +/- standard error, and number of observations), and the cells have 61.6 +/- 6.3 (92) M omega across their plasma membrane. The electrical capacity of the cells is 45.1 +/- 2.9 (63) pF and is much higher than expected for a cell of its size (diameter 14 micrometers, height 5 micrometers) and cannot be attributed to intercellular coupling, as no evidence of this type of connection was found in 20 pairs of neighboring cells. On the contrary, the high capacity is in keeping with previous studies using electron microscopy showing microvilli and a high degree of lateral infolding. The relationship between resistance and capacity was 1981 +/- 177 (61) omega . microF. The cells have an intracellular potential of -40.5 +/- 15 (120) mV. Yet the shape of the distribution curve suggests that the actual value may be somewhat higher (some -50 mV). The current/voltage curve of the distribution curve suggests that the actual value may be somewhat higher (some -50 mV). The current/voltage curve shows a marked asymmetry, and in some cells the voltage becomes time-dependent for large, depolarizing current pulses.

Barium inhibition of basolateral membrane potassium conductance in tracheal epithelium.

Addition of barium ion, Ba2+, to the submucosal bathing solution of canine tracheal epithelium reversibly decreased the short-circuit current and increased transepithelial resistance. The decrease in short-circuit current represented a decrease in the net rate of Cl secretion with no change in the rate of Na absorption. Intracellular microelectrode techniques and an equivalent electrical circuit analysis were used to localize the effect of Ba2+ to an inhibition of the permeability of the basolateral membrane to K. Ba2+ (2 mM) doubled basolateral membrane resistance, decreased the equivalent electromotive force at the basolateral membrane, and decreased the magnitude of the depolarization of basolateral membrane voltage produced by increasing the submucosal K concentration. The inhibition of the basolateral K permeability depolarized the negative intracellular voltage, resulting in both a decrease in the driving force for Cl exit and an estimated increase in intracellular Cl concentration. These studies indicate that there is a Ba2+-inhibitable K conductance at the basolateral membrane of tracheal epithelial cells and that the K permeability plays an important role in the generation of the negative intracellular electrical potential that provides the driving force for Cl exit from the cell.

Effects of Na-coupled alanine transport on intracellular K activities and the K conductance of the basolateral membranes of Necturus small intestine.

Intracellular electrical potentials and K activity, (K)c, were determined simultaneously in Necturus small intestine before and after the addition of alanine to the mucosal solution. As noted previously (Gunter-Smith, Grasset & Schultz, 1982), the addition of alanine to the mucosal solution resulted in a prompt depolarization of the electrical potential difference across the apical membrane (psi mc) and a decrease in the slope resistance of that barrier (rm). This initial response was followed by a slower repolarization of psi mc associated with a decrease in the slope resistance of the basolateral membrane (rs) so that when the steady state was achieved (rm/rs) did not differ significantly from control values in the absence of alanine. In the absence of alanine, psi mc averaged -32 mV and (K)c averaged 67 mM. When a steady state was achieved in the presence of alanine these values averaged -24 mV and 50 mM, respectively. The steady-state electrochemical potential differences for K across the basolateral membrane in the absence and presence of alanine did not differ significantly. Inasmuch as the rate of transcellular active Na transport or "pump activity" was increased two- to threefold in the presence of alanine, it follows that, if active Na extrusion across the basolateral membrane is coupled to active K uptake across that barrier with a fixed stoichiometry then, the decrease in rs must be due to an increase in the conductance of the basolateral membrane to K that parallels the increase in "pump activity". This "homocellular" regulatory mechanism serves to (i) prevent an increase in (K)c due to an increase in pump activity; and, (ii) repolarize psi mc and thus restore the electrical driving force for the rheogenic Na-coupled entry processes.

Epithelial potassium transport: tracer and electrophysiological studies in choroid plexus.

Transport of potassium by bullfrog choroid plexus was studied using tracers and ion-selective microelectrodes. 1) Tracers: We measured unidirectional uptakes of42K across each surface of the plexus, efflux of42K from loaded tissues, intracellular potassium pools, and [3H]-ouabain binding.42K uptake across the brush border membrane was composed of diffusional and saturable components. The saturable component exhibited kinetics of a two-site, model withKm's of 0.3mm and aVmax of 8 μmol cm−2 hr−1. Ouabain inhibited uptake across the brush border membrane with aKi of 1×10−7m. Ethacrynic acid, phloretin, amiloride, and low sodium concentrations inhibited uptake, whereas bicarbonate ions increased transport up to 100%. The rate of ouabain-sensitive uptake across the serosal surface was only 6% of that across the ventricular surface. The efflux of potassium across the brush border membrane could account for most of the efflux from the epithelium. Potassium was accumulated within the plexus to a concentration in excess of 100mm.42K in the extracellular compartments exchanged with 40–55% of the intracellular potassium. Ouabain bound to the brush border membrane with aKm of 8×10−7m, ak1 of 3.8×104 mol−1 min−1 and ak−1 of 3×10−2 min−1. Ouabain binding was blocked by cymarin and gitoxigin. 2) Electrodes: Under control conditions the intracellular electrical potential,Evc, was −45 mV and the apparent intracellular K+-concentration, Kc+, was 90mm. K+ ions appeared to be actively accumulated within the epithelium.Evc and Kc+ were followed under three experimental conditions: (i) treating the tissue with ouabain; (ii) varying the ventricular K+ concentration; and (iii) passing transmural currents. It is concluded that the permeability of the ventricular membrane to potassium (1–5 × 10−5 cm sec−1)_was much greater than the permeability of the serosal membrane and that the rate of K pumping into the epithelium was 0.35–0.55×10−9 mol cm−2 sec−1. 96% of the transmural current was paracellular, and the resistance of the serosal membrane was nine times greater than that of the ventricular membrane. Increasing Kc+ produced Nernstian changes inEvc andEKvc, but the cells only depolarized by 0.13 mV for each mV increase in the chemical potential of the ventricular solutions.

Potassium activity in photoreceptors, glial cells and extracellular space in the drone retina: changes during photostimulation.

1. A double-barrelled potassium-sensitive micro-electrode was developed that was fine enough to record intracellular electrical potentials and potassium activities (aK) in the drone retina. 2. aK was measured in the photoreceptor cells, in the pigment (glial) cells, and in the extracellular space, in the superfused, cut, retina. The effect of photostimulation was studied: 20 msec light flashes, intense enough to evoke receptor potentials of maximum amplitude were presented, 1/sec, in a train lasting about 2 min. 3. In photoreceptors with membrane potentials greater than or equal to 50 mV aK in the dark was 79 mM, S.D. = 27 mM, n = 11. During photostimulation aK fell by 21.5 +/- 9.5 mM with a half-time of 30 +/- 22 sec. (A tentative conversion from activities to free concentrations can be made by taking the activity coefficient as 0.70 its value in the Ringer solution). 4. In pigment cells with membrane potentials greater than or equal to 50 mV, aK in the dark was 52 mM, S.D. = 13 mM, n = 11. During photostimulation aK increased by 14 +/- 5 mM. 5. In the extracellular space aK increased during photostimulation with a mean half-time of less than 1.3 sec to a maximum (mean value 14 mM, S.D. = 8.4 mM, n = 22), and then fell to a plateau. 6. It is estimated from the anatomy that the photoreceptors occupy approximately 38% of the total volume of the retina, the pigment cells 57%, and extracellular space 5%. Hence, it seems possible that during photostimulation nearly all the net loss of potassium from the photoreceptors is temporarily stored in the pigment cells. 7. Recordings were made in the extracellular space of the intact animal by passing the electrode through a hole in the cornea. The mean aK in the dark was 7.7 mM, S.E. = 0.4 mM, n = 22. In the superfused retina, aK in the dark was 6.3 mM, S.E. = 0.7 mM, n = 22, even though aK in the Ringer solution was 2.2 mM. Increasing the aK of the Ringer solution to 7.0 mM had no apparent effect on aK in the extracellular space at depths greater than 20 micron. 8. In the intact animal the amplitude and time course of the change in extracellular aK evoked by the standard pattern of photostimulation were within the range observed in the superfused preparation.

An electrogenic Na +/K + pump in the choroid plexus

Intracellular electrical potential and potassium activity was measured by means of microelectrodes in the epithelial cells of choroid plexus from bullfrogs ( Rana catesbeiana). Ouabain applied from the ventricular side caused an abrupt depolarisation of 10 mV but only a gradual loss of potassium from the cells. Readministration of potassium to the ventricular solution of plexuses which were previously depleted of potassium, caused a hyperpolarisation of about 4 mV. These two experiments are consistent with the notion of an electrogenic Na +/K + pump situated at the ventricular membrane and which pumps potassium into the cell and sodium into the ventricle. The numerical values obtained suggest that 3 sodium ions are pumped for 2 potassium ions. The permeability coefficient for potassium exit from the cell is calculated to be 1.24 · 10 −5 cm −1 · s −1 expressed per cm 2 of flat epithelium.