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

Sodium sensitivity of KNa channels in mouse CA1 neurons

Potassium channels play an important role regulating transmembrane electrical activity in essentially all cell types. We were especially interested in those that determine the intrinsic electrical properties of mammalian central neurons. Over 30 different potassium channels have been molecularly identified in brain neurons, but there often is not a clear distinction between molecular structure and the function of a particular channel in the cell. Using patch-clamp methods to search for single potassium channels in excised inside-out (ISO) somatic patches with symmetrical potassium, we found that nearly all patches contained non-voltage-inactivating channels with a single-channel conductance of 100-200 pS. This conductance range is consistent with the family of sodium-activated potassium channels (Slo2.1, Slo2.2, or collectively, K_Na). The activity of these channels was positively correlated with a low cytoplasmic Na⁺ concentration (2-20 mM). Cell-attached recordings from intact neurons, however, showed little or no activity of this K⁺ channel. Attempts to increase channel activity by increasing intracellular sodium concentration ([Na⁺]_i) with bursts of action potentials or direct perfusion of Na⁺ through a whole cell pipette had little effect on K_Na channel activity. Furthermore, excised outside-out (OSO) patches across a range of intracellular [Na⁺] showed less channel activity than we had seen with excised ISO patches. Blocking the Na⁺/K⁺ pump with ouabain increased the activity of the K_Na channels in excised OSO patches to levels comparable with ISO-excised patches. Our results suggest that despite their apparent high levels of expression, the activity of somatic K_Na channels is tightly regulated by the activity of the Na⁺/K⁺ pump.NEW & NOTEWORTHY We studied K_Na channels in mouse hippocampal CA1 neurons. Excised inside-out patches showed the channels to be prevalent and active in most patches in the presence of Na⁺. Cell-attached recordings from intact neurons, however, showed little channel activity. Increasing cytoplasmic sodium in intact cells showed a small effect on channel activity compared with that seen in inside-out excised patches. Blockade of the Na⁺/K⁺ pump with ouabain, however, restored the activity of the channels to that seen in inside-out patches.

Chansporter complexes in cell signaling

Ion channels facilitate diffusion of ions across cell membranes for such diverse purposes as neuronal signaling, muscular contraction, and fluid homeostasis. Solute transporters often utilize ionic gradients to move aqueous solutes up their concentration gradient, also fulfilling a wide variety of tasks. Recently, an increasing number of ion channel-transporter ('chansporter') complexes have been discovered. Chansporter complex formation may overcome what could otherwise be considerable spatial barriers to rapid signal integration and feedback between channels and transporters, the ions and other substrates they transport, and environmental factors to which they must respond. Here, current knowledge in this field is summarized, covering both heterologous expression structure/function findings and potential mechanisms by which chansporter complexes fulfill contrasting roles in cell signaling in vivo.

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.

Effect of mercuric chloride on the kinetics of cationic and substrate activation of the rat brain microsomal ATPase system

Mercuric chloride (HgCl2), a neurotoxic compound, inhibited the adenosine triphosphatase (ATPase) system in a concentration-dependent manner. Hydrolysis of ATP was linear with time with or without HgCl2 in the reaction mixtures. Higher inhibition of (Na(+)-K+)ATPase activity by HgCl2 was observed in alkaline (8.0 to 9.0) pH and at lower temperatures (17 to 32 degrees). Activation energy values were increased slightly in the presence of HgCl2. Activation of (Na(+)-K+)ATPase by ATP in the presence of HgCl2 showed a decrease in Vmax from 15.29 to 5.0 mumol of inorganic phosphate (Pi)/mg protein/hr with no change in Km. Similarly, activation of K(+)-stimulated p-nitrophenyl phosphatase (K(+)-PNPPase) in the presence of HgCl2 showed a decrease in Vmax from 3.26 to 1.35 mumols of p-nitrophenol (PNP)/mg protein/hr with no change in Km. K(+)-activation kinetic studies indicated that HgCl2 decreased Vmax from 14.01 to 4.30 mumols Pi/mg protein/hr in the case of (Na(+)-K+)ATPase and from 3.45 to 2.40 mumols PNP/mg protein/hr in the case of K(+)-PNPPase with no changes in Km. Na(+)-activation of (Na(+)-K+)ATPase in the presence of HgCl2 showed a decrease in Vmax from 11.06 to 3.23 mumols Pi/mg protein/hr and an increase in Km from 1.06 to 2.08 mM. Preincubation of microsomes with sulfhydryl (SH) agents dithiothreitol, cysteine and glutathione protected HgCl2-inhibition of (Na(+)-K+)ATPase. The data suggest that HgCl2 inhibited (Na(+)-K+)ATPase by interfering with the dephosphorylation of the enzyme-phosphoryl complex.

[3H]noradrenaline release from rabbit pulmonary artery: sodium-pump-dependent sodium-calcium exchange

1. The release of [3H]noradrenaline ([3H]NA) from the isolated main pulmonary artery of the rabbit has been measured in the presence of neuronal (cocaine, 3 X 10(-5) M) and extraneuronal (corticosterone, 5 X 10(-5) M) uptake blockers. 2. K+ removal from the external medium increased the release of [3H]NA, an action transiently inhibited by Ca2+-free (+1 mM-EGTA) solution, i.e. after Ca2+ removal transmitter release was first abolished and then started to increase again after a delay lasting about 90-120 min. 3. Ca2+ readmission to arteries which had been kept in Ca2+- and 'K+-free' solution, markedly increased the [3H]NA release. The rate of transmitter release was dependent on the preceding perfusion period with 'K+-free' solution, being greater for longer exposure times. 4. When Ca2+ and K+ were readmitted together to K+-depleted and Na+-enriched preparations, the release of [3H]NA transiently increased. 5. If K+ was readmitted first, the subsequently applied Ca2+ was ineffective in producing transmitter release. 6. Different alkali metal ions (Rb+, Cs+ or Li+) were also readmitted as K+ substitutes together with Ca2+. In all cases the release of neurotransmitter transiently increased; however, the rate of release was dependent on the monovalent cation used. Thus, Rb+ ions were as effective as, Cs+ about one-third as effective as, and Li+ about one-fifth as effective as K+ in activating the Na+ pump. 7. It is concluded that in the absence of external Ca2+, and in response to Na+-pump inhibition, the release of Ca2+ from internal stores is responsible for the NA release observed. On readmission of Ca2+ the rate of transmitter release is dependent on the Na+ previously gained inside. Furthermore, the activity of the Na+ pump determines the rate of transmitter release through the Na-Ca exchange mechanism.

Active sodium transport and fluid secretion in the gall-bladder epithelium of Necturus

Intracellular Na, K and Cl activities (acNa, acK and acCl) and membrane potentials were measured in Necturus gall-bladder epithelium using double-barrelled ion-sensitive micro-electrodes. Mucosal membrane potential was about -55 mV and the mean control activities were acNa = 14.7 mM, acK = 91.6 mM and acCl = 20.3 mM. Replacing mucosal Na by K caused a fall in acNa that followed an exponential time course. The rate of change in acNa was linearly related to acNa above a certain value (congruent to 3 mM). acK and acCl both increased in K Ringer solution. From the change in all three ions the cell was estimated to swell at an initial rate of 0.13% s-1. From the initial rate of change in acNa, a net cell efflux of Na of 405 pmol cm-2 s-1 was calculated. Replacement of Na by Tris or choline led to a similar result. The transepithelial Na transport rate was for this group of animals 346 pmol cm-2 s-1. Ouabain (10(-3) M) produced an increase in acNa and acCl, whereas acK decreased. The cells were estimated to swell at an initial rate of 0.06% s-1. The initial Na influx after Na-pump inhibition was calculated to be 162 pmol cm-2 s-1. The parallel measure of the transepithelial rate of transport of Na gave a value of 189 pmol cm-2 s-1. Ouabain inhibited the decrease in acNa after replacement of Na by K by about 80%. A fast depolarization, ranging from 2 to 7 mV, occurred after the perfusion with ouabain. Em then slowly decreased from about 53 to 32 mV in 1 h. It is concluded that (a) the major fraction of the transepithelial transport of Na is transcellular and mediated by the Na pump, (b) the pumping rate is linearly dependent on internal Na within a certain range and (c) the Na pump is electrogenic under normal circumstances.

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.

Intracellular Na+ and Ca2+ in leech Retzius neurones during inhibition of the Na+-K+ pump

The intracellular Na activity, aNai, and the intracellular Ca activity, aCai, were measured with double-barrelled neutral carrier Na+- and Ca2+-sensitive microelectrodes in Retzius neurones in the central nervous system of the leech Hirudo medicinalis. The aNai was measured to be 8.0 mM (corrected for Ca interference), which corresponds to a cytoplasmic Na+ concentration of 10.7 mM, assuming a Na activity coefficient of 0.75. The calculated Na+ equilibrium potential was 59 mV, giving a total Na+ electrochemical gradient of approximately 102 mV. The aCai was found to range between 1 and 5 X 10(-7) M, from which a Ca2+ equilibrium potential near + 120 mV was estimated. When the Na+-K+ pump was inhibited by lowering the external K+ concentration or by adding the glycoside ouabain (5 X 10(-4) M), the aNai reversibly increased severalfold. When aNai increased to high levels following complete pump inhibition, the aCai increased above 10(-6) M, and the membrane input resistance decreased. After removal of ouabain, aNai, aCai and the membrane resistance recovered within 30 min after a delay of 20-40 min. Our results suggest that a large increase of aNai produces a rise in aCai, possibly by means of a Na+-Ca2+ exchange across the cell membrane. The elevation of the aCai may be responsible for the decrease in membrane resistance, and may also be related to the uncoupling of the paired Retzius neurones observed in the presence of Na+-K+ pump inhibitors.

Increased ouabain binding after repeated noradrenergic stimulation

Acute noradrenergic stimulation has previously been shown to stimulate brain (Na+, K+)-adenosine triphosphatase activity. Effects of repeated stimulation with piperoxane were examined in the present study. Daily piperoxane increased ouabain binding, measured 24 h after the last dose, after 4 days or 3 weeks treatment; K+-p-nitrophenylphosphatase was increased after 3 weeks. Prazosin, which, like piperoxane, activates presynaptic noradrenergic neurons but, unlike piperoxane, blocks postsynaptic receptors, did not increase K+-p-nitrophenylphosphatase, and decreased ouabain binding after 3 weeks.

Ionic shifts in myelinated nerve fibers during early stages of Wallerian degeneration

The distribution and relative mass fraction of the diffusible ions Na, K, Cl and Ca were determined by X-ray microanalysis in the axons of rat sciatic nerve 18 h and 36 h after crush and in control nerves. The investigations were performed in freeze-dried ultrathin cryosections after shock freezing of the nerves in liquified propane. 18 h after crush, no definite alteration of the ions were found compared to the control nerves. In electron micrographs of routinely processed nerves, no ultrastructural changes were seen. 36 h after crush, two types of ionic imbalance were found, the first characterized by decreased K and slightly increased Na in the axon, the second by concurrently increasing axonal Na and Cl, accompanied, in some fibers, by accumulating Ca. These types of ionic imbalance presumably represent two stages of axolemmal permeability alteration corresponding to early structural changes of axoplasm in electron micrographs of routinely processed nerves at that time.

Temperature acclimation: effects on membrane physiology of an identified snail neuron

The neuronal basis for thermal acclimation was examined by comparing the short- and long-term effects of temperature change on the physiological properties of an identified neuron in the isolated ganglion of Hexis aspersa. Using intracellular electrophysiological techniques, we found that the frequency of spontaneous action potentials and excitability of neurons from warm-acclimated animals was depressed by abruptly cooling from 20 to 5 degrees C. After a 2-wk period of acclimation to 5 degrees C, the levels of spontaneous activity and excitability were comparable to those of warm-acclimated neurons at 20 degrees C. Conversely, abrupt warming of neurons from cold-acclimated animals greatly increased the frequency of spontaneous activity, but after acclimation to 20 degrees C the frequency decreased. Although the duration of the action potential and the cell's electrogenic Na-K pump were temperature sensitive, thermal acclimation had no obvious effects on these parameters. Membrane permeability to Na and PNa/PK decreased with cooling, whereas PRb/PK and PCs/PK increased. Warming had the opposite effect on the relative alkali cation permeability (PX/PK). With acclimation PX/PK underwent compensatory changes.

Oscillations of membrane potential in L cells. IV. Role of intracellular Ca2+ in hyperpolarizing excitability

Effects of divalent cations on oscillations of membrane potentials (i.e., spontaneous repetitive hyperpolarizing responses) and on hyperpolarizing responses induced by electrical stimuli as well as on resting potentials were studied in large nondividing L cells. Deprivation of Ca2+ from the external medium inhibited these hyperpolarizing responses accompanying slight depolarization of the resting potential Sr2+ or Mn2+ applied to the external medium in place of Ca2+ was able to substitute for Ca2+ in the generation of hyperpolarizing responses, while Mg2+, Ba2+ or La3+ suppressed hyperpolarizing responses. The addition of A23187 to the bathing medium or intracellular injection of Ca2+, Sr2+, Mn2+ or La3+ induced membrane hyperpolarization. When the external Ca2+, Sr2+ or Mn2+ concentration was increased, the resting potential also hyperpolarized, in a saturating manner. The amplitude of maximum hyperpolarization produced by high external Ca2+ was of the same order of magnitude as those of hyperpolarizing responses and was dependent on the external K+ concentration. In the light of these experimental observations, it was deduced that the K+ conductance increase associated with the hyperpolarizing excitation is the result of an increase in the intracellular concentration of free Ca2+ mainly derived from the external solution.

Effects of chronic bretylium treatment on the sympathetic neuron and the smooth musculature of the rat

The effects of chronic i.p. injection of high doses of bretylium on sympathetic nerves on the smooth musculature of the vas deferens of adult and newborn rats were examined using fluorescence histochemistry, light and electron microscopy and organ bath physiological techniques. Bretylium treatment caused mitochondrial swelling, loss of cristae and the formation of electron-dense inclusions in the mitochondria of sympathetic neurons. However, neuron degeneration was not observed and fluorescent histochemical appearance of adrenergic neurons was normal. A small transient supersensitivity of the isolated vas deferens of bretylium-treated rats to noradrenaline, but not to acetylcholine, occurred. There was, however, considerable increase in the maximal contractile response to both noradrenaline and acetylcholine. In high calcium concentrations acetylcholine-induced contractions of vasa deferentia from bretylium-treated rats were significantly greater than control; there was no difference in magnitude of noradrenaline-induced contractions.

Role of membrane-bound Ca in ghost permeability to Na and K

The permeability of red cell ghosts to K is determined by the amount of membrane-bound Mg which, in turn, depends on internal Mg. Contrasting with such effect, an increase in cellular Ca raises K permeability. To test whether this action is due to a competitive displacement of membrane Mg, the free Ca content of human red cell ghosts was altered by means of Ca-EGTA buffers. Net Na and K movements as well as Ca and Mg bindings were assessed after incubation in a Na-medium at 37 degrees C. Raising Ca from 3 X 10(-7) to 1 X 10(-2) M caused a large K efflux with very little Na gain. Under similar conditions, Ca binding was increased without affecting membrane-bound Mg. Both Ca binding and K loss were markedly diminished by either adding ATP to the hemolytic medium or increasing internal Mg at a fixed Ca concentration. A Scatchard analysis showed three Ca binding sites, two of them having high affinity. It is concluded that Ca action does not arise from a displacement of membrane-bound Mg but from binding to different sites in the membrane. Presumably, high affinity sites are involved in the control of K permeability.

An electrogenic sodium pump and baroreceptor function in normotensive and spontaneously hypertensive rats

Postexcitatory depression (PED) and adaptation were examined in slowly adapting aortic baroreceptors of normotensive rats (NTR) and spontaneously hypertensive rats (SHR); an aortic arch-aortic nerve preparation in vitro was used. PED was elicited either mechanically by employing single or double pressure steps, or electrically by antidromic stimulation of the aortic nerve. During PED the axon of the receptor was capable of conducting action potentials and the receptor itself could respond to increased pressures. The relationship between duration of PED and number of impulses preceding it was hyperbolic. In NTR's and SHR's PED was abolished by ouabain or solutions containing no potassium, neither of which affected the steady state pressure-volume relationship of the aorta. These interventions, which are known to block electrogenic pumps, also lowered the pressure threshold and increased the curvature of the steady state pressure-frequency curve. Furthermore, lithium, another agent that blocks electrogenic pumps, also abolished PED. Thus, PED is attributed mainly to an electrogenic sodium pump which operates normally in baroreceptors. We found that adaptation from peak transient to steady state frequency did not appear to be altered significantly when the pump was blocked. Blockage of the pump by ouabain is responsible for the baroreceptor reflex effects elicited by this drug. We conclude that resetting and reduced sensitivity in SHR baroreceptors are not attributed to significant differences in electrogenic pump activity.

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 effects of lithium and sodium on the potassium conductance of snail neurones

1. The iontophoretic injection of lithium into snail neurones reversibly increased the resting relative potassium permeability (PK). 2. Long exposures to snail Ringer containing 25 mM-Li and correspondingly reduced Na also caused an increase in PK. This did not occur with Ringer in which the same reduction of Na was made by replacing it with Tris. 3. Replacement of part of the Ringer Na by either Li or Tris led to proportional decreases in internal Na. 4. Injecting large quantities of Na into ouabain-treated cells caused effects similar to those of Li injection. Without ouabain, Na injection stimulated the electrogenic Na pump. 5. A number of tests failed to produce any clear evidence that intracellular Ca was involved in the response to Li.

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)

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.