An identification of the K activated Na pump current in sheep Purkinje fibres

Sodium-pump potentials and currents in guinea-pig ventricular muscles and myocytes

When guinea-pig papillary muscles were depolarized to ca. -30 mV by superfusion with K+-free Tyrode's solution supplemented with Ba2+, Ni2+, and D600, addition of Cs+ transiently hyperpolarized the membrane in a reproducible manner. The size of the hyperpolarization (pump potential) depended on the duration of the preceding K+-free exposure; peak amplitudes (Epmax) elicited by 10 mM Cs+ after 5-, 10-, and 15-min K+-free exposures were 12.9, 17.7, and 23.2 mV, respectively. Pump potentials were unaffected by external Cl- but suppressed by cardiac glycosides, hyperosmotic conditions, and low-Na+ solution. Using Epmax as an indicator of Na+ pump activation, the half-maximal concentration for activation by Cs+ was 12-16.3 mM. At 6 mM, Cs+ was three times less potent than Rb+ or K+ and five times more potent than Li+. From these findings, and correlative voltage-clamp data from myocytes, we calculate that (i) a pump current of 7.8 nA/cm2 generates an Epmax of 1 mV and (ii) resting pump current in normally polarized muscle (~0.16 µA/cm2) is five times smaller than previously estimated.Key words: sodium pump, cesium, rubidium, sodium pump current.

Voltage-dependent inhibition of the Na+,K+ pump by tetraethylammonium

Tetraethylammonium (TEA+) is an effective inhibitor of a variety of K+ channels, and has been widely used to reduce K+-sensitive background conductances in electrophysiological investigations of the Na+,K+-ATPase. Here we demonstrate by combination of two-electrode voltage clamp (TEVC) and giant patch clamp of Xenopus oocytes, and measurements of the activity of purified ATPase of pig kidney that TEA+ directly inhibits the Na+,K+-ATPase from the outside. The KI value in TEVC experiments at 0 mV is about 10 mM increasing with more negative potentials. A similar voltage-dependent inhibition by TEA+ was observed in the excised membrane patches except that the apparent KI value at 0 mV is about 100 mM, a value nearly identical to that found for inhibition of purified kidney ATPase. The voltage-dependent inhibition can be described by an effective valency of 0.39 and is attributed to an interference with the voltage-dependent binding of K+ at an external access channel. The apparent dielectric length of the access channel for K+ is not affected by TEA+.

Charge transport of ion pumps on lipid bilayer membranes

Ion pumps create ion gradients across cell membranes while consuming light energy or chemical energy. The ion gradients are used by the corresponding cell types for passive-ion transport via ion channels or carriers or for accumulation of nutrients like sugar or amino acids via cotransport systems or antiporters.

Voltage-dependent stimulation of Na+/K(+)-pump current by external cations: selectivity of different K+ congeners

Currents generated by the endogenous Na+/K+ pump in the oocytes of Xenopus laevis were determined under voltage-clamp as currents activated by different K+ congeners. The voltage dependence of the pump current reflects voltage-dependent steps in the reaction cycle. The decrease of K(+)-activated pump current at positive potentials has been attributed to voltage-dependent stimulation by the external K+ (Rakowski, Vasilets, LaTona and Schwarz (1991) J. Membr. Biol. 121, 177-187). In Na(+)-free solution, activation of the pump by external cations seems to be the dominating voltage-dependent and rate-determining step in the reaction cycle. Under these conditions, the voltage dependence of apparent Km values for pump activation can be analyzed. The dependence suggests voltage-dependent binding of extracellular cations assuming that an effective charge of about 0.4 of an elementary charge is moved in the electrical field during a step associated with the cation binding. The apparent Km values at 0 mV differ for various cations that stimulate pump activity. The values are in mM: 0.10 for Tl+, 0.63 for K+, 0.71 for Rb+, 9.3 for NH4+, and 12.9 for Cs+. The corresponding apparent affinities follow the same sequence as the cation permeability of the K(+)-selective delayed rectifier channel of nerve cells. The results are compatible with the interpretation that the cations have to pass an ion-selective access channel to reach their binding sites in the pump molecule.

Properties of the pacemaker current (If) in latent pacemaker cells isolated from cat right atrium

1. Single latent pacemaker cells were isolated from the Eustachian ridge of cat right atrium using Langendorff perfusion and enzyme dispersion techniques. Whole-cell patch-clamp techniques were used to study the hyperpolarization-activated inward current (I(f)). 2. All cells studied beat rhythmically. Pacemaker activity was recorded in the voltage range -68 +/- 1 to -54 +/- 2 mV and its cycle length was 901 +/- 67 ms (72 +/- 5 beats min-1) at 34-36 degrees C. Cells were elongated with tapered ends, and appeared bent or crinkled without obvious striations. Mean cell diameter and length were 7.4 +/- 0.5 microns and 93.1 +/- 5.9 microns, respectively (n = 15). Input resistance and total membrane capacitance were 2.2 +/- 0.2 G omega and 27.8 +/- 3.1 pF, respectively. 3. Hyperpolarizing clamp steps more negative than -50 mV elicited a time-dependent increasing inward current that was maximally activated at -120 mV. Activation of I(f) was well within the pacemaker voltage range. Half-maximal activation voltage and slope factor were calculated, using a Boltzmann function, to be -80.5 mV and 8.4, respectively. 4. The fully activated current-voltage (I-V) relationship was approximately linear at voltages more negative than -30 mV and showed outward rectification at more positive voltages. The reversal potential of I(f) was -26 mV and the fully activated conductance was 1.75 +/- 0.14 nS (n = 21). Caesium (2 mM) blocked I(f) at voltages more negative than the reversal potential. Reducing extracellular Na+ or K+ shifted the reversal potential more negative, and increasing extracellular K+ exerted the opposite effect. Reducing extracellular Na+ decreased I(f) amplitude and the slope of the fully activated I-V relationship, and elevated extracellular K+ increased I(f) amplitude and the slope of the fully activated I-V relationship. 5. Some pacemaker cells exhibited a short delay in the onset of I(f) activation whereas other pacemaker cells exhibited little, if any, delay in activation. I(f) currents exhibiting no delay in activation were best fitted by a single exponential function with a mean time constant of 3.20 +/- 1.03 s at -70 mV (n = 4). 6. A nystatin-permeabilized patch recording method was used to record spontaneous pacemaker action potentials and I(f) from the same pacemaker cell. Caesium (2 mM) inhibited I(f) by more than 90% (at -70 mV), and decreased the slope of diastolic depolarization, resulting in a 48 +/- 5% decrease in spontaneous rate.(ABSTRACT TRUNCATED AT 400 WORDS)

Influence of glycogen content, temperature, and Euro Collins solution on membrane potential and sodium activity of superfused porcine liver slices

The influence of glycogen content, temperature, and Euro Collins (EC) solution on membrane potential (Vm) and intracellular sodium activity (aNai) were measured in cells of superfused porcine liver slices by means of double-barrelled ion-sensitive microelectrodes. Vm was -26.1mV in fasted pigs and -20.6mV after glucose feeding, when measured in HEPES-buffered solution (P less than 0.0001). aNai was not measurably affected by glucose feeding. During superfusion with Tyrode solution, lowering the temperature from 35.5 degrees C to 15.5 degrees C led to a fast Vm decrease of roughly 2mV followed by an increase of 1-3mV. At the same time, aNai increased from 12.8 to 18.2mM within 10 min. Superfusion with EC solution for 10 min caused comparable changes in fed and fasted pigs. Vm depolarized at either temperature by about 16mV. At 35.5 degrees C the initial aNai of 17.5mM was roughly halved, whereas at 15.5 degrees C it decreased from 21.0 to 14.3mM. The results suggest that the nutritional state markedly affects the electric properties of liver. However, the effect on membrane potential of high-potassium organ-protective solutions seems to be distinctly more pronounced. Moreover, cellular Na+ activity decreases in consequence of an extracellular Na+ reduction with protective solutions, which might be balanced to some extent by a simultaneous temperature decrease.

Changes of the subsarcolemmal Na+ concentration in internally perfused cardiac cells

Transients of Na+/K+ pump and of Na+/Ca2+ exchange current occur during whole-cell recording from cardiac cells upon quick changes of active Na+ efflux. The transients reflect a temporary loss of control of the subsarcolemmal Na+ concentration. Even in the steady state the control is not complete is certain cells. Quantitative studies on ion transport by whole-cell recording are meaningful only if an adequate control of the submembranal ionic composition is demonstrated.

Ionic currents activated during hyperpolarization of single right atrial myocytes from cat heart

Whole-cell recording techniques were used on single right atrial myocytes to study the ionic currents that may be responsible for the diverse diastolic voltage characteristics of atrial tissue. Ionic currents were activated by hyperpolarizing voltage pulses negative to -30 mV. In general, four different types of cells were identified based primarily on the ionic currents elicited during hyperpolarization. The first cell type exhibited an inward current that decayed with time at more negative voltages, reversed near the potassium equilibrium potential, inwardly rectified at more positive voltages, increased in elevated extracellular potassium, and was blocked by 3 mM barium or 10 mM cesium. This current was identified as the potassium current iK1. A second cell type exhibited a time-dependent inward current that increased at more negative voltages, had an activation range between -50 and -110 mV, had a reversal potential of -26 mV, and was blocked by 3 mM cesium. This current was identified as an if current. A third cell type exhibited an inward current that initially decayed and then became more inward with time. Barium (3 mM) abolished the initial inward current and revealed a time-dependent increasing inward current that was blocked by 3 mM cesium. This current was composed of both the iK1 and if currents. A fourth cell type exhibited only small time-independent leak currents in response to hyperpolarization. These results indicate that individual cells within the right atrium are electrophysiologically heterogeneous with respect to the types of ionic channels present in their sarcolemmal membranes. This specialization in ionic currents partially explains the diverse diastolic voltage characteristics and functional properties of atrial tissue.

Consequences of CO2 acidosis for transmembrane Na+ transport and membrane current in rabbit cardiac Purkinje fibres

1. The influence of sarcolemmal Na(+)-H+ exchange on intracellular Na+ activity (aiNa), intracellular pH (pHi) and membrane holding current (Ih) was investigated in rabbit cardiac Purkinje fibres. pHi and aiNa were measured with liquid sensor ion-selective microelectrodes. A two-microelectrode voltage clamp was used while recording pHi or aiNa.pHi was varied by alternating a nominally CO2-free HEPES buffer and a CO2-HCO3-buffer. 2. The intrinsic buffer capacity was calculated from the decrease in pHi after addition of CO2. The most accurate estimate was obtained when transmembrane pHi regulation was blocked and equalled 18.4 +/- 1.0 mequiv H+/pH unit (mean +/- S.E.M.). 3. aiNa started to rise when pHi fell below 7.0. A hyperpolarization paralleled the increase in aiNa. The magnitude of the rise in aiNa and the hyperpolarization were steeply dependent on pHi. 4. Inhibition of the Na(+)-K+ pump by K(+)-free superfusion increased aiNa. The rate of rise in aiNa was highly dependent on pHi. The rates of rise in 5% (pHi = 7.00 +/- 0.03), 7% (pHi = 6.89 +/- 0.04) and 15% CO2 (pHi = 6.74 +/- 0.02) at constant external pH (pHo) relative to the rate in HEPES solution (pHi = 7.24 +/- 0.02) were: 1.5 +/- 0.2, 2.4 +/- 0.1 and 3.1 +/- 0.2. The acid-induced rise in aiNa was abolished by 2 mM-amiloride. 5. Extracellular acidosis slowed down the recovery of pHi and depressed the rate of rise in aiNa upon intracellular acidification. When both pHi and pHo were decreased to 6.7 the acid-dependent rate increase fell to about 10% of the value found at pHo 7.4. 6. The tetrodotoxin (TTX)-sensitive Na+ current was not influenced by the change in pHi in the range 6.7-7.2. 7. Intracellular acidosis was associated with an early aiNa-independent depolarization and an inward shift in Ih. Current-voltage plots revealed that the initial inward current shift reversed at -81.5 mV on average, showed inward rectification and was largely depressed in the presence of 1 mM-Ba2+. These observations indicate a decrease in K+ conductance when pHi falls. 8. The increase in aiNa elicited a Na(+)-K+ pump-dependent outward current which could override the initial aiNa-independent current shift. At a pHo of 6.7 the initial fall in Ih remained, while the secondary outward current was largely depressed. 9. The rate of active Na+ extrusion and Na(+)-K+ pump current were suppressed by about 30% at pHi 6.7 compared to pHi 7.2.(ABSTRACT TRUNCATED AT 400 WORDS)

A model study of the contribution of active Na-K transport to membrane repolarization in cardiac cells

A biochemical model of active Na-K transport in cardiac cells was studied in conjunction with a representation of the passive membrane currents and ion concentration changes. The active transport model is based on the thermodynamic and kinetic properties of a six-step reaction scheme for the Na,K-ATPase. It has a fixed Na:K stoechiometry of 3:2, and its activation is governed by three parameters: membrane potential intracellular Na+ concentration, and interstitial K+ concentration. The Na-K pump current is directly proportional to the density of Na,K-ATPase molecules. The passive membrane currents and ion concentration changes involve only Na+ and K+ ions, and no attempt was made to provide a precise representation of Ca2+ currents or Ca2+ concentration changes. The surface-to-volume ratio of the interstitial compartment is 55 times larger than that of the intracellular compartment. The flux balance conditions are such that the original equilibrium concentration values are re-established at each stimulation cycle. The underlying assumptions of the model were checked against experimental measurements on Na-K pump activity in a variety of preparations. In addition, the qualitative validation of the model was carried out by comparing its behavior following sudden frequency shifts to corresponding experimental observations. The overall behavior of the model is quite satisfactory and it is used to provide the following indications: (1) when the intracellular and interstitial volumes are relatively large, the ion concentration transients are small and the pumping rate depends essentially on average concentration levels. (2) An increase in internal Na+ concentration potentiates the response of the Na-K pump to rapid membrane depolarizations. (3) When the internal Na+ concentration is large enough, the Na-K pump current transient plays an important role in shaping the plateau and repolarization phase of the action potential. (4) A rapid increase in external K+ concentration during voltage clamp in multicellular preparations could saturate the Na-K pump response and lead to a fairly linear dependence of the pump activity on the internal Na+ concentration.

The changes in CiNa of sheep cardiac Purkinje fibres in hyperosmotic solutions

1. The changes in intracellular sodium ion concentration (CiNa) of sheep cardiac Purkinje fibres in hyperosmotic solutions were studied using Na-sensitive liquid ion-exchanger microelectrodes. 2. CiNa was increased in hyperosmotic solutions containing different concentrations of sucrose from 0 to 300 mM. 3. The changes in resting membrane potential (RMP) in hyperosmotic solutions had no regularity. In most of the experiments there was hyperpolarization of the membrane but in a few cases a depolarization or no change of RMP were also observed. 4. The N-shape of I-V relations of the fibres became more pronounced in hyperosomotic solutions.

Na/K pump current in aggregates of cultured chick cardiac myocytes

Spontaneously beating aggregates of cultured embryonic chick cardiac myocytes, maintained at 37 degrees C, were voltage clamped using a single microelectrode switching clamp to measure the current generated by the Na/K pump (Ip). In resting, steady-state preparations an ouabain-sensitive current of 0.46 +/- 0.03 microA/cm2 (n = 22) was identified. This current was not affected by 1 mM Ba, which was used to reduce inward rectifier current (IK1) and linearize the current-voltage relationship. When K-free solution was used to block Ip, subsequent addition of Ko reactivated the Na/K pump, generating an outward reactivation current that was also ouabain sensitive. The reactivation current magnitude was a saturating function of Ko with a Hill coefficient of 1.7 and K0.5 of 1.9 mM in the presence of 144 mM Nao. The reactivation current was increased in magnitude when Nai was increased by lengthening the period of time that the preparation was exposed to K-free solution prior to reactivation. When Nai was raised by 3 microM monensin, steady-state Ip was increased more than threefold above the resting value to 1.74 +/- 0.09 microA/cm2 (n = 11). From these measurements and other published data we calculate that in a resting myocyte: (a) the steady-state Ip should hyperpolarize the membrane by 6.5 mV, (b) the turnover rate of the Na/K pump is 29 s-1, and (c) the Na influx is 14.3 pmol/cm2.s. We conclude that in cultured embryonic chick cardiac myocytes, the Na/K pump generates a measurable current which, under certain conditions, can be isolated from other membrane currents and has properties similar to those reported for adult cardiac cells.

The cardioplegic solution HTK: effects on membrane potential, intracellular K+ and Na+ activities in sheep cardiac Purkinje fibres

The effects of the cardioplegic solution HTK on membrane potential (EM) and intracellular K and Na activities (aiK, aiNa) were studied in sheep cardiac Purkinje fibres by means of conventional and ion-selective microelectrodes. HTK contains (mM): Na 15, K 10, Ca 0, Mg 4, histidine 180. (1) In control conditions EM was -74.3 +/- 3.3 mV (n = 25), aiK was 116.4 +/- 4.1 mM (n = 7) and aiNa was 8.2 +/- 1.4 mM (n = 15). (2) Exposure to HTK led to a depolarization to -59.7 +/- 3.6 mV (n = 25) which exceeded by about 5-7 mV that induced in a Tyrode solution of 10 mM K and in a modified HTK solution supplemented by 2 mM Ca (n = 6). (3) Addition of 0.5 mM barium eliminated the difference in the steady-state depolarization. (4) HTK superfusion increased aiK to 120.1 +/- 4.4 mM (n = 7) and decreased aiNa to 3.9 +/- 0.9 mM (n = 15). (5) The decrease in aiNa was insensitive to amiloride (1 mM) and to external alkalization but was slightly increased by addition of 2 mM calcium. (6) When the calcium in Tyrode solution was lowered from 2.0 mM to 0.05 mM, aiNa hardly decreased during subsequent exposure to unmodified HTK and it increased in the presence of 0.1 mM dihydroouabain. We propose the hypothesis (1) that the difference in membrane depolarization between HTK and a 10 mM K-Tyrode is caused by a decrease in K conductance by the HTK solution and (2) that the aiNa decline mainly results from a coupled Ca influx via Na-Ca exchange due to a delayed washout of external calcium.

Effect of isoprenaline on active Na transport in sheep cardiac Purkinje fibres

The effect of isoprenaline (ISO; 5.10(-8) to 10(-6) M) on active Na transport was studied in depolarized sheep cardiac Purkinje fibres. Membrane current (I) and intracellular Na activity were measured simultaneously during enhanced Na pumping in voltage clamped preparations. ISO stimulated enhanced active Na transport but did not affect membrane current or intracellular Na concentration (ciNa) in the steady state under the chosen experimental conditions. The stimulatory effect of ISO was mediated by beta 1-adrenoceptors via a cAMP dependent pathway. The effect depended on the extracellular K concentration (coK) and was inhibited by external Ba ions. Complementary experiments on isolated sheep Purkinje cells revealed no ISO induced alteration of the Na pump current. The mechanism of the ISO induced stimulation of enhanced Na pumping in sheep Purkinje fibres probably involves an augmented K efflux. A direct effect on the pump molecule seems unlikely.

Pump current and Na+/K+ coupling ratio of Na+/K+-ATPase in reconstituted lipid vesicles

A method is described for studying the coupling ratio of the Na+/K+ pump, i.e., the ratio of pump-mediated fluxes of Na+ and K+, in a reconstituted system. The method is based on the comparison of the pump-generated current with the rate of K+ transport. Na+/K+-ATPase from kidney is incorporated into the membrane of artificial lipid vesicles; ATPase molecules with outward-oriented ATP-binding site are activated by addition of ATP to the medium. Using oxonol VI as a potential-sensitive dye for measuring transmembrane voltage, the pump current is determined from the change of voltage with time t. In a second set of experiments, the membrane is made selectively K+-permeable by addition of valinomycin, so that the membrane voltage U is equal to the Nernst potential of K+. Under this condition, dU/dt reflects the change of intravesicular K+ concentration and thus the flux of K+. Values of the Na+/K+ coupling ratio determined in this way are close to 1.5 in the experimental range (10-75 mM) of extravesicular (cytoplasmic) Na+ concentrations.

Stoichiometry and voltage dependence of the sodium pump in voltage-clamped, internally dialyzed squid giant axon

The stoichiometry and voltage dependence of the Na/K pump were studied in internally dialyzed, voltage-clamped squid giant axons by simultaneously measuring, at various membrane potentials, the changes in Na efflux (delta phi Na) and holding current (delta I) induced by dihydrodigitoxigenin (H2DTG). H2DTG stops the Na/K pump without directly affecting other current pathways: (a) it causes no delta I when the pump lacks Na, K, Mg, or ATP, and (b) ouabain causes no delta I or delta phi Na in the presence of saturating H2DTG. External K (Ko) activates Na efflux with Michaelis-Menten kinetics (Km = 0.45 +/- 0.06 mM [SEM]) in Na-free seawater (SW), but with sigmoid kinetics in approximately 400 mM Na SW (Hill coefficient = 1.53 +/- 0.08, K1/2 = 3.92 +/- 0.29 mM). H2DTG inhibits less strongly (Ki = 6.1 +/- 0.3 microM) in 1 or 10 mM K Na-free SW than in 10 mM K, 390 mM Na SW (1.8 +/- 0.2 microM). Dialysis with 5 mM each ATP, phosphoenolpyruvate, and phosphoarginine reduced Na/Na exchange to at most 2% of the H2DTG-sensitive Na efflux. H2DTG sensitive but nonpump current caused by periaxonal K accumulation upon stopping the pump, was minimized by the K channel blockers 3,4-diaminopyridine (1 mM), tetraethylammonium (approximately 200 mM), and phenylpropyltriethylammonium (20-25 mM) whose adequacy was tested by varying [K]o (0-10 mM) with H2DTG present. Two ancillary clamp circuits suppressed stray current from the axon ends. Current and flux measured from the center pool derive from the same membrane area since, over the voltage range -60 to +20 mV, tetrodotoxin-sensitive current and Na efflux into Na-free SW, under K-free conditions, were equal. The stoichiometry and voltage dependence of pump Na/K exchange were examined at near-saturating [ATP], [K]o and [Na]i in both Na-free and 390 mM Na SW. The H2DTG-sensitive F delta phi Na/delta I ratio (F is Faraday's constant) of paired measurements corrected for membrane area match, was 2.86 +/- 0.09 (n = 8) at 0 mV and 3.05 +/- 0.13 (n = 6) at -60 to -90 mV in Na-free SW, and 2.72 +/- 0.09 (n = 7) at 0 mV and 2.91 +/- 0.21 (n = 4) at -60 mV in 390 mM Na SW. Its overall mean value was 2.87 +/- 0.07 (n = 25), which was not significantly different from the 3.0 expected of a 3 Na/2 K pump.(ABSTRACT TRUNCATED AT 400 WORDS)

The dependence of sodium pump current on internal Na concentration and membrane potential in cardioballs from sheep Purkinje fibres

The effect of various intracellular Na concentrations (CiNa) and membrane potentials on the Na pump current (Ip) was studied in isolated, cultured sheep cardiac Purkinje cells ('cardioballs'). Ip was identified as cardiac steroid sensitive current. The dependence of Ip on CiNa was investigated at a membrane potential of -40 mV by means of whole-cell recording from cardioballs internally perfused with media containing various Na concentrations. Internal perfusion with a Na free solution abolished Ip. The amplitude of Ip as a function of CiNa displayed saturation kinetics. Half maximal activation of Ip occurred at a CiNa of about 9 mM. The maximal Ip density was estimated to be 1.1 microA/cm2. The potential dependence of Ip was studied by conventional whole-cell recording under various ionic conditions. Generally Ip displayed little voltage dependence at membrane potentials positive to -20 mV. Ip declined at more negative potentials. The pump cycle probably includes only one voltage sensitive step. The potential dependence of Ip was more pronounced at lower CiNa or lower concentrations of the external pump activator Cs+. The findings are in line with the idea that increasingly steeper ionic gradients against which the cations are pumped strengthen the voltage dependence of Ip in the potential range studied. Other factors probably affecting the pump current-voltage (Ip-V) relation are discussed. The results suggest that Ip varies during electrical activity.

Role of the sodium pump and the background K+ channel in passive K+(Rb+) uptake by isolated cardiac sarcolemmal vesicles

A simple procedure was developed for the isolation of a sarcolemma-enriched membrane preparation from homogenates of bullfrog (Rana catesbeiana) heart. Crude microsomes obtained by differential centrifugation were fractionated in Hypaque density gradients. The fraction enriched in surface membrane markers consisted of 87% tightly sealed vesicles. The uptake of 86Rb+ by the preparation was measured in the presence of an opposing K+ gradient using a rapid ion exchange technique. At low extravesicular Rb+ concentrations, at least 50% of the uptake was blocked by addition of 1 mM ouabain to the assay medium. Orthovanadate (50 microM), ADP (2.5 mM) or Mg (1 mM) were also partial inhibitors of Rb+ uptake under these conditions, and produced a complete block of Rb+ influx in the presence of 1 mM ouabain. When 86Rb+ was used as a tracer of extravesicular K+ (Rb+0 less than or equal to 40 microM, K+0 = 0.1-5 mM) a distinct uptake pathway emerged, as detected by its inhibition by 1 mM Ba2+ (K0.5 = 20 microM). At a constant internal K+ concentration (K+in = 50 mM), the magnitude of the Ba2+-sensitive K+ uptake was found to depend on K+0 in a manner that closely resembles the K+ concentration dependence of the background K+ conductance (IK1) observed electrophysiologically in intact cardiac cells. We conclude that K+ permeates passively this preparation through two distinct pathways, the sodium pump and a system identifiable as the background potassium channel.

Fast charge translocations associated with partial reactions of the Na,K-pump: I. Current and voltage transients after photochemical release of ATP

Nonstationary electric currents are described which are generated by the Na,K-pump. Flat membrane sheets 0.2-1 micron in diameter containing a high density of oriented Na,K-ATPase molecules are bound to a planar lipid bilayer acting as a capacitive electrode. In the aqueous phase adjacent to the bound membrane sheets, ATP is released within milliseconds from an inactive, photolabile precursor ("caged" ATP) by an intense flash of light. After the ATP-concentration jump, transient current and voltage signals can be recorded in the external circuit corresponding to a translocation of positive charge across the pump protein from the cytoplasmic to the extracellular side. These electrical signals which can be suppressed by inhibitors of the Na,K-ATPase require the presence of Na+ but not of K+ in the aqueous medium. The intrinsic pump current Ip(t) can be evaluated from the recorded current signal, using estimated values of the circuit parameters of the compound membrane system. Ip(t) exhibits a biphasic behavior with a fast rising period, followed by a slower decline towards a small quasi-stationary current. The time constant of the rising phase of Ip(t) is found to depend on the rate of photochemical ATP release. Further information on the microscopic origin of the current transient can be obtained by double-flash experiments and by chymotrypsin modification of the protein. These and other experiments indicate that the observed charge-translocation is associated with early events in the normal transport cycle. After activation by ATP, the pump goes through the first steps of the cycle and then enters a long-lived state from which return to the initial state is slow.

The effects of membrane potential on active and passive sodium transport in Xenopus oocytes

1. The effects of membrane potential on the Na+-K+ pump were studied by measuring membrane current and 22Na+ efflux in voltage-clamped Xenopus oocytes. The effects of inhibiting the Na+-K+ pump with strophanthidin were examined. 2. Strophanthidin produced an inward shift of membrane current which reversed on removal of the drug. In control oocytes the magnitude of this current was not significantly affected by changing membrane potential over the range -20 to -160 mV. 3. In another series of experiments the intracellular Na+ concentration ([Na+]i) was elevated either by overnight Na+-K+ pump inhibition (strophanthidin or exposure to K+-free solutions) or by loading with nystatin. This Na+-loading increased the magnitude of the strophanthidin-sensitive current. The ratio of strophanthidin-sensitive 22Na+ efflux:strophanthidin-sensitive current was consistent with that expected from a 3Na+-2K+ exchange. 4. When [Na+]i was elevated the strophanthidin-sensitive current was sensitive to changes of membrane potential. Hyperpolarization from -20 to -80 mV decreased the current to 60% of control. It is suggested that the current is not sensitive to membrane potential at normal [Na+]i because the over-all reaction is rate limited by the availability of intracellular Na+. 5. The application of strophanthidin decreased the rate of 22Na+ efflux. Both the strophanthidin-insensitive and the strophanthidin-sensitive components of efflux were sensitive to changes of membrane potential. The strophanthidin-insensitive component was not greatly affected by hyperpolarization from -40 to -160 mV but was increased by depolarization to +40 mV. 6. In Na+-loaded oocytes, the strophanthidin-sensitive component of 22Na+ efflux was inhibited by hyperpolarization negative from -40 mV. Hyperpolarization from -40 to -160 mV decreased the efflux by 54 +/- 5%. Over the limited range of potentials for which a comparison could be made, the effects on 22Na+ efflux were somewhat less than on the electrogenic Na+-K+ pump current. On average there was no significant effect of depolarizing from 0 to +40 mV. However, in some experiments a clear inhibition of the efflux was observed. If the oocytes were not Na+ loaded there was no significant effect of membrane potential on the strophanthidin-sensitive Na+ efflux. 7. These results show that the effects of membrane potential on the net reaction of the Na+-K+ pump (as measured by the electrogenic current) result partly from an inhibition of the forward mode of operation. However, there is also evidence to suggest a contribution from stimulation of the reverse reaction.

On the effect of unstirred layers on K+-activated electrogenic Na+ pumping in cardiac Purkinje strands

Many studies of electrogenic Na+ pumping in Purkinje strands have involved intracellular Na+ loading by exposure to 0 mM K+, followed by reexposure to K+. For sheep Purkinje strands the K+ concentration for half-maximal stimulation (K0.5) in such studies is higher than K0.5 of canine Purkinje strands. A model was developed to determine if gradients in the K+ concentration of extracellular fluid layers during enhanced pump activity can account for the discrepancy. Pump activity was assumed linearly dependent on [Na+]i and dependent on [K+]o, according to Michaelis-Menten kinetics. The model simulated diffusion of K+ across unstirred layers and both depletion and accumulation of K+ in extracellular clefts of Purkinje strands during changes in the K+ concentration of the tissue bath. Errors in estimates of K0.5 occurred when delay in achieving a steady state extracellular K+ concentration was simulated. The simulations suggested that a linear relationship between pump current and intracellular Na+, a monoexponential decay of pump current, independence of the rate constants for the current decay on the initial Na+ load and holding potential, and apparent Michaelis-Menten K+ kinetics is not sufficient evidence against pump-induced interstitial K+ depletion having introduced errors in determination of K0.5. It is concluded that interstitial K+ depletion may account for the difference between determinations of K0.5 in sheep and canine Purkinje strands.

The contribution of Na and K ions to the pacemaker current in sheep cardiac Purkinje fibres

The ionic components of the pacemaker current are quantitatively analysed in sheep cardiac Purkinje fibres by simultaneous measurements of the intracellular Na activity (alpha iNa) and the membrane current under voltage clamp. The pacemaker current is operationally defined as the Cs inhibited membrane current (ICs) in Ba containing media at clamp potentials negative to -60 mV. At these potentials solutions containing CsCl (0.2-5 mM) shift the holding membrane current into the outward direction and simultaneously decrease alpha iNa. The Cs effects on membrane current and alpha iNa display a similar voltage dependence. A Cs inhibited Na influx contributes to ICs. The ratio ICs/(Cs inhibited Na influx in electrical units) is less than 1 at membrane potentials positive to the potassium equilibrium potential EK and greater than 1 at potentials negative to EK. The ratio is close to 1 at EK suggesting Na ions to be the only carriers of the current at EK whereas K ions contribute to ICs at potentials different from EK. The effects of Cs on the Cs inhibited Na influx and ICs show a very similar dose dependence. The effect is half maximum at approximately 0.2 mM CsCl (in 21.6 mM K; clamp potential: -85 mV). An increase of the external K concentration augments ICs and the Cs inhibited Ca influx. Na and K ions carrying ICs probably cross the membrane via an identical channel. The permeability of the channel for K+ is about 10-20 times larger than for Na+. The ICs reversal potential of a fibre bathed in a medium containing 5.4 mM K is estimated to be -50 to -60 mV.

An effect of noradrenaline on resting potential and Na activity in sheep cardiac Purkinje fibres

Noradrenaline (NA; 10(-9) to 10(-6) M) depolarizes the cell membrane of quiescent sheep Purkinje fibres at resting potential level. A corresponding inward shift of the holding current occurs in voltage clamped preparations in a potential range between ca. -40 and -100 mV. The depolarizing effect is present in phentolamine (1.5 X 10(-6) M) containing solution, mimicked by isoprenaline, blocked by propranolol (5 X 10(-6) M) and is therefore supposed to be beta-adrenoceptor mediated. The inward shift of the holding current is half maximum at 60 nM noradrenaline. The shift is accompanied with an increase of the intracellular Na activity (aiNa) of the fibres as measured by Na sensitive microelectrodes. Both, the inward shift of the holding current and the accompanying aiNa increased are strongly inhibited by 2 mM CsCl. It is concluded that the depolarizing action of noradrenaline is mainly caused by the known catecholamine induced shift of the steady state activation curve of the pacemaker current (if). The shift increases if in a potential range between -40 and -100 mV and augments thereby aiNa.

Influence of Na/K pump current on action potentials in Purkinje fibers

Moderate changes in the size of the outward (hyperpolarizing) current that is generated directly by the electrogenic Na/K exchange pump in the surface membrane of cardiac Purkinje fibers can cause substantial alterations in the shape of the action potential, in the level of the diastolic potential, or of the resting potential of quiescent cells, or in the rate of firing of spontaneously active preparations. Transient increments in Na/K pump current, of suitable magnitude, can be elicited experimentally in small canine Purkinje fibers by causing a transient increase in their intracellular Na concentration, [Na]i, and, thereby, a transient increase in the rate of electrogenic Na extrusion. Two techniques were used to increase [Na]i: in the first, the rate of Na extrusion from the cells was temporarily reduced by omitting K ions from the bathing fluid for short periods of time, in the second, the rate of Na entry into the cells was temporarily increased by electrically stimulating the preparations rapidly (e.g., greater than or equal to 2 Hz) for brief periods. After the extracellular K concentration was restored, or after electrical stimulation was stopped, respectively, use of a two-microelectrode voltage-clamp technique allowed the resulting increments in pump current to be measured directly, as changes in holding current. Increments in pump current elicited by these two methods in the same preparation decline with the same exponential time-course. In preparations stimulated electrically at a regular, low rate (e.g., less than or equal to 1 Hz) both methods of temporarily stimulating the Na/K pump cause a marked, transient reduction in the duration of the action potential. A closely similar reduction in action-potential duration to that observed during enhanced pump activity can be elicited by injecting, from an external source, a steady hyperpolarizing current of magnitude similar to that of the increment in pump current recorded in the same preparation under voltage clamp.

Inhibition of the Na pump--a mechanism in the genesis of cardiac arrhythmias

An ATP-driven Na pump maintains the unsymmetrical Na and K distribution across the cell membrane of cardiac cells. An increase of the intracellular Na or extracellular K concentration enhances this active Na transport. About 35 per cent of the actively transported Na is ejected from the cells as a hyperpolarizing outward current. The Na pump influences the cardiac Ca metabolism via the Na-Ca exchange. Inhibition of the pump affects the generation and conduction of the cardiac action potential by various mechanisms. It seems to be involved in the genesis of cardiac arrhythmias.

On the temperature dependence of the Na pump in sheep Purkinje fibres

The temperature dependence of cardiac active Na transport is studied in voltage clamped sheep Purkinje fibres by means of simultaneous measurements of the membrane current (I) and the intracellular Na activity (alpha iNa). During activation of the Na pump a transient outward current (delta I) and alpha iNa decline exponentially with an identical time constant (tau). The transient outward current and the decline in alpha iNa are blocked by 10(-4) M dihydroouabain (DHO). Lowering the temperature from 42 degrees C to 17 degrees C prolongs tau. The electrogenic fraction (e.f.) of the active Na efflux remains unaffected. The Q10 value of the active Na transport derived from the changes of tau varies within the temperature range studied. The Q10 amounts to approximately 1.2 between 42 degrees C and 35 degrees C, to approximately 2.4 between 35 degrees C and 22 degrees C and to approximately 2.1 between 35 degrees C and 17 degrees C. Correspondingly the activation energy of the active Na transport is not constant between 42 degrees C and 17 degrees C. It is calculated to be 3.4 kcal/mol between 42 degrees C and 35 degrees C, 15.9 kcal/mol between 35 degrees C and 22 degrees C and 12.4 kcal/mol between 35 degrees C and 17 degrees C. Variations in temperature change the maximal rate constant of the active Na transport, whereas the sensitivity of the Na pump towards the extracellular K concentration (Ko) is little affected.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in the electrical activity of dog cardiac Purkinje fibres at high heart rates

Rate-dependent changes in the electrical activity of dog Purkinje fibres have been studied. At high rates of stimulation the rate of repolarization is greater, the action potential is shorter, the maximum diastolic potential is increased, the pace-maker potential is reduced in amplitude, and on cessation of rapid stimulation there can be a suppression of spontaneous activity. After an increase of the stimulus frequency there is an abrupt shortening of the action potential, which can be attributed to incomplete recovery of the plateau currents; this is followed by a progressive decline in action potential duration over the next several hundred seconds. The factor responsible for the slow changes in duration could also be responsible for the accompanying increase in maximum diastolic potential because this develops along a similar time course. These slow changes in electrical activity have been investigated with the phase-plane technique. They are the result of an increase in the net outward current over a wide range of potentials (approximately -10 to approximately -90 mV) during the repolarization phase of the action potential. In voltage-clamp experiments background current has been observed to be strongly rate dependent: the background current during a test voltage-clamp pulse after a train of action potentials is more outward at higher stimulus frequencies. When the frequency is increased, background current slowly becomes more outward over several hundred seconds, and this change therefore occurs along the appropriate time course to explain the slow alteration in electrical activity under these conditions. The extra outward background current at high rates is relatively independent of membrane potential in the range from -110 to -40 mV (more circumstantial evidence indicates that this range may extend to at least +10 mV); this potential dependence is similar to that of the Na-K-pump current (Eisner & Lederer, 1980). Strophanthidin and ouabain, agents known to block the Na-K pump, alter both the changes in background current and the slow rate-dependent changes in electrical activity. Although after an increase in rate there is a gradual change in background current that can be explained by an increase in electrogenic Na-K-pump activity, the initial effect of switching rate is to produce a change in current that is consistent with an increase of the extracellular K concentration. A transient increase in the K concentration of restricted extracellular clefts has been recorded under these conditions in dog Purkinje strands by Kline & Kupersmith (1982) using K-sensitive microelectrodes. The effect on electrical activity of these changes is discussed.(ABSTRACT TRUNCATED AT 400 WORDS)

Properties of an inward rectifying K channel in the membrane of guinea-pig atrial cardioballs

Single channel outward current fluctuations are recorded in excised (outside-out) membrane patches of isolated atrial cells in culture (cardioballs) from hearts of adult guinea-pigs. The ionic channel displays a high selectivity to K ions. Accordingly the reversal potential of the single channel current is close to the K equilibrium potential. The open channel conductance is unaffected by the membrane potential but depends on the K concentration of the outside solution (19.7pS at 2 mM Ko to 30.7pS at 20 mM Ko). The open state probability (Po) of the channel shows a marked voltage dependence. Po amounts to c.0.9 at -40 mV and decreases to c.0.1 at +40 mV. Under the assumption of no channel interaction a macroscopic steady state current voltage relationship is reconstructed from the single channel data. The relationship displays inward-going rectification. The rectification is due to the voltage dependence of Po. The I-V curve displays a negative slope at membrane potentials positive to -15 mV. In bathing solutions containing Ba ions (0.2 mM) Po is reduced by rapid closures which interrupt the open state events. The unit channel conductance is unaffected by Ba ions. The channel block exerted by Ba ions is augmented with increasing membrane hyperpolarization. The results suggest that the channel studied may represent a background K conductance.