The effect of dihydro-ouabain and lithium-ions on the outward current in cardiac Purkinje fibers. Evidence for electrogenicity of active transport

From Bernstein's rheotome to Neher-Sakmann's patch electrode. The action potential

The aim of this review was to provide an overview of the most important stages in the development of cellular electrophysiology. The period covered starts with Bernstein's formulation of the membrane hypothesis and the measurement of the nerve and muscle action potential. Technical innovations make discoveries possible. This was the case with the use of the squid giant axon, allowing the insertion of "large" intracellular electrodes and derivation of transmembrane potentials. Application of the newly developed voltage clamp method for measuring ionic currents, resulted in the formulation of the ionic theory. At the same time transmembrane measurements were made possible in smaller cells by the introduction of the microelectrode. An improvement of this electrode was the next major (r)evolution. The patch electrode made it possible to descend to the molecular level and record single ionic channel activity. The patch technique has been proven to be exceptionally versatile. In its whole-cell configuration it was the solution to measure voltage clamp currents in small cells. See also: https://doi.org/10.14814/phy2.13860 & https://doi.org/10.14814/phy2.13862.

The Plastic Glial-Synaptic Dynamics within the Neuropil: A Self-Organizing System Composed of Polyelectrolytes in Phase Transition

Several explanations have been proposed to account for the mechanisms of neuroglial interactions involved in neural plasticity. We review experimental results addressing plastic nonlinear interactions between glial membranes and synaptic terminals. These results indicate the necessity of elaborating on a model based on the dynamics of hydroionic waves within the neuropil. These waves have been detected in a small scale experimental model of the central nervous system, the in vitro retina. We suggest that the brain, as the heart and kidney, is a system for which the state of water is functional. The use of nonlinear thermodynamics supports experiments at convenient biological spatiotemporal scales, while an understanding of the properties of ions and their interactions with water requires explanations based on quantum theories. In our approach, neural plasticity is seen as part of a larger process that encompasses higher brain functions; in this regard, hydroionic waves within the neuropil are considered to carry both physiological and cognitive functions.

Relevance of excitable media theory and retinal spreading depression experiments in preclinical pharmacological research

In preclinical neuropharmacological research, molecular, cell-based, and systems using animals are well established. On the tissue level the situation is less comfortable, although during the last decades some effort went into establishing such systems, i.e. using slices of the vertebrate brain together with optical and electrophysiological techniques. However, these methods are neither fast, nor can they be automated or upscaled. By contrast, the chicken retina can be used as a suitable model. It is easy accessible and can be kept alive in vitro for hours up to days. Due to its structure, in addition the retina displays remarkable intrinsic optical signals, which can be easily used in experiments. Also to electrophysiological methods the retina is well accessible. In excitable tissue, to which the brain and the retina belong, propagating excitation waves can be expected, and the spreading depression is such a phenomenon. It has been first observed in the forties of the last century. Later, Martins-Ferreira established it in the chicken retina (retinal spreading depression or RSD). The electrophysiological characteristics of it are identical with those of the cortical SD. The metabolic differences are known and can be taken into account. The experimental advantage of the RSD compared to the cortical SD is the pronounced intrinsic optical signal (IOS) associated with the travelling wave. This is due to the maximum transparency of retinal tissue in the functional state; thus any physiological event will change it markedly and therefore can be easily seen even by naked eye. The theory can explain wave spread in one (action potentials), two (RSDs) and three dimensions (one heart beat). In this review we present the experimental and the excitable media context for the data interpretation using as example the cholinergic pharmacology in relation to functional syndromes. We also discuss the intrinsic optical signal and how to use it in pre-clinical research.

Quaternary organic amines inhibit Na,K pump current in a voltage-dependent manner: direct evidence of an extracellular access channel in the Na,K-ATPase

The effects of organic quaternary amines, tetraethylammonium (TEA) chloride and benzyltriethylammonium (BTEA) chloride, on Na,K pump current were examined in rat cardiac myocytes superfused in extracellular Na(+)-free solutions and whole-cell voltage-clamped with patch electrodes containing a high Na(+)-salt solution. Extracellular application of these quaternary amines competitively inhibited extracellular K(+) (K(+)(o)) activation of Na,K pump current; however, the concentration for half maximal inhibition of Na,K pump current at 0 mV (K(0)(Q)) by BTEA, 4.0 +/- 0.3 mM, was much lower than the K(0)(Q) for TEA, 26.6 +/- 0.7 mM. Even so, the fraction of the membrane electric field dissipated during K(+)(o) activation of Na,K pump current (lambda(K)), 39 +/- 1%, was similar to lambda(K) determined in the presence of TEA (37 +/- 2%) and BTEA (35 +/- 2%), an indication that the membrane potential (V(M)) dependence for K(+)(o) activation of the Na,K pump current was unaffected by TEA and BTEA. TEA was found to inhibit the Na,K pump current in a V(M)-independent manner, i.e., inhibition of current dissipated 4 +/- 2% of the membrane electric field. In contrast, BTEA dissipated 40 +/- 5% of the membrane electric field during inhibition of Na,K pump current. Thus, BTEA inhibition of the Na,K-ATPase is V(M)-dependent. The competitive nature of inhibition as well as the similar fractions of the membrane electric field dissipated during K(+)(o)-dependent activation and BTEA-dependent inhibition of Na,K pump current suggest that BTEA inhibits the Na,K-ATPase at or very near the enzyme's K(+)(o) binding site(s) located in the membrane electric field. Given previous findings that organic quaternary amines are not occluded by the Na,K-ATPase, these data clearly demonstrate that an ion channel-like structure provides access to K(+)(o) binding sites in the enzyme.

Electrophysiology of the sodium-potassium-ATPase in cardiac cells

Like several other ion transporters, the Na(+)-K(+) pump of animal cells is electrogenic. The pump generates the pump current I(p). Under physiological conditions, I(p) is an outward current. It can be measured by electrophysiological methods. These methods permit the study of characteristics of the Na(+)-K(+) pump in its physiological environment, i.e., in the cell membrane. The cell membrane, across which a potential gradient exists, separates the cytosol and extracellular medium, which have distinctly different ionic compositions. The introduction of the patch-clamp techniques and the enzymatic isolation of cells have facilitated the investigation of I(p) in single cardiac myocytes. This review summarizes and discusses the results obtained from I(p) measurements in isolated cardiac cells. These results offer new exciting insights into the voltage and ionic dependence of the Na(+)-K(+) pump activity, its effect on membrane potential, and its modulation by hormones, transmitters, and drugs. They are fundamental for our current understanding of Na(+)-K(+) pumping in electrically excitable cells.

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.

The effects of beta-stimulation on the Na(+)-K+ pump current-voltage relationship in guinea-pig ventricular myocytes

1. The whole cell patch clamp technique was used to study effects of the beta agonist isoprenaline (Iso) on the current-voltage (I-V) relationship of the Na(+)-K+ pump current (Ip) in acutely isolated guinea-pig ventricular myocytes. 2. The effect of Iso on Ip at high [Ca2+]i (1.4 microM) was voltage dependent. The I-V relationship of Ip in Iso shifted by approximately 30 mV in the negative direction on the voltage axis, increasing Ip at negative voltages but leaving Ip unchanged at positive voltages. 3. Intracellular application of the calmodulin antagonist, calmodulin-dependent protein kinase II fragment 290-309, did not eliminate or reduce the Iso-induced voltage shift, suggesting calmodulin-dependent protein kinase II was not involved. 4. The Iso inhibition of Ip at low [Ca2+]i (15 nM) was not voltage dependent. Ip was reduced by 20 to 30% in the presence of Iso at each holding potential. 5. When the voltage dependence of Ip was largely reduced by substitution of N-methyl-D-glucamine+ for external Na+, the magnitude of the low [Ca2+]i, Iso-induced inhibition of Ip was progressively eliminated by increasing the [Ca2+]i. At a [Ca2+]i of 1.4 microM, this inhibition disappeared. 6. At intermediate values of [Ca2+]i, the I-V curves in Na(+)-containing solution in the presence and the absence of Iso crossed over. The higher the [Ca2+]i, the more positive the voltage at which the two I-V curves intersected. 7. During beta-adrenergic activation our results suggest intracellular Ca2+ has two effects: (a) It prevents protein kinase A (PKA) phosphorylation-induced inhibition of Ip. (b) It causes a PKA phosphorylation-induced shift of the pump I-V relationship in the negative direction on the voltage axis. These effects may have important physiological significance in the regulation of heart rate and cardiac contractility.

Depressive effects of arenobufagin on the delayed rectifier K+ current of guinea-pig cardiac myocytes

The effects of a bufadienolide isolated from toad venom, arenobufagin, a potent Na+/K+ pump inhibitor, were studied in single guinea-pig ventricular cells in the whole-cell patch-clamp configuration. Arenobufagin (50 microM) applied extracellularly decreased the amplitude of the delayed rectifier K+ current (IdK) by 30% without affecting the gating kinetics. The L-type Ca2+ current was also depressed, but to a lesser extent. The inward rectifier K+ current was hardly affected. Ouabain and the internal dialysis of cells with the solution containing 20 mM Na+ depressed IdK in a similar way as arenobufagin. On the other hand, arenobufagin also depressed IdK when the Na+/K+ pump was already inhibited in the K(+)-free Tyrode solution. Therefore, both a direct effect on the channel and an indirect effect through the inhibition of the Na+/K+ pump may be involved in the depression of IdK by arenobufagin.

Ouabain-induced excitation of colonic smooth muscle due to block of K+ conductance by intracellular Na+ ions

The mechanism by which ouabain causes excitation of canine colonic circular smooth muscle was investigated. Ouabain-induced depolarization and increase in contractility were related to the concentration of extracellular sodium and prevented by complete substitution of sodium ions with N-methyl-D-glucamine or lithium ions. Absence of external sodium ions did not prevent the depolarization and increase in contractility induced by tetraethylammonium. Exposure of the muscle strips to sodium-free solutions produced a transient hyperpolarization and decrease in the input membrane resistance consistent with the hypothesis that intracellular sodium blocks potassium conductance. The relationship between the membrane potential and the extracellular potassium concentration indicated that the resting membrane potential is mainly determined by the membrane potassium conductance. Our data suggest the following mechanism of action for ouabain: (a) ouabain blocks Na+/K+ pump thereby increasing the intracellular sodium concentration; (b) increase in intracellular sodium inhibits membrane potassium conductance, which depolarizes the membrane and prolongs the slow wave plateau, resulting in an increase of the force of contraction. The direct contribution of the sodium pump to the resting membrane potential, if any, can only be minor (< 6 mV).

Isoprenaline, Ca2+ and the Na(+)-K+ pump in guinea-pig ventricular myocytes

1. The whole-cell patch clamp technique was employed to study the effects of the beta-agonist isoprenaline (ISO) on the Na(+)=K+ pump current, Ip, in acutely isolated ventricular myocytes from guinea-pig hearts. Propranolol, a beta-adrenergic antagonist, was used to demonstrate that all of the effects of ISO, stimulatory or inhibitory, are mediated by beta-receptors. 2. Below about 150 nM [Ca2+]i, we find that ISO reduces Ip, while above this [Ca2+]i ISO increases Ip. The stimulatory and inhibitory effects of ISO on Ip are independent of either intracellular sodium ([Na+]i) or extracellular potassium ([K+]o). These results suggest that the end-effect of ISO is directly on the maximum pump turnover rate (Vmax) rather than indirectly through changes in [Na+]i or [K+]o or modulatory effects on Na+ or K+ affinity. 3. The maximum effect of ISO increases Ip by 25% when [Ca2+] is buffered at 1.4 microM. A half-maximal effect is reached at roughly 10 nM-ISO and a near-maximal effect by 0.5 microM. 4. The permeabilized patch technique, using amphotericin B (Horn & Marty, 1988; Rae, Cooper, Gates & Watsky, 1991), was employed to minimize changes in the normal second messenger systems and calcium buffers. In these experiments, we used a high intracellular sodium solution (pipette sodium was 50 mM), thus sodium-calcium exchange was depressed and we expected [Ca2+]i to be above 150 nM. ISO increases Ip in these conditions as in the dialysed cells. 5. Our results suggest that beta-stimulation can increase Ip, but only if [Ca2+]i is above about 150 nM. In the beating heart [Ca2+]i rises well above this value during systole and the average [Ca2+]i, which depends on heart rate, is expected to normally be above this level. During beta-stimulation, the increase in Ip along with a concomitant increase in IK (Giles, Nakajima, Ono & Shibata, 1989; Duchatelle-Gourdon, Hartzell & Lagrutta, 1989) helps prevent action potential lengthening and allows an increase in heart rate even in the presence of increased calcium current. Further, beta-stimulation will compensate for the effects on Ip of either hypokalaemia or digitalis toxicity, and so reduce the expected rise in both [Na+]i and [Ca2+]i.

Modeling the dynamic features of the electrogenic Na,K pump of cardiac cells

The purpose of this paper is to examine the dynamic features of the electrogenic Na,K pump of cardiac cells, based on a comparative analysis of a mechanistic model and an ad hoc mathematical description of the Na,K pump. Both representations are incorporated into a modified version of the Beeler-Reuter model for the ventricular membrane, and the resulting action potential models are studied under conditions of repetitive stimulation at steady rates between 0 and 3 Hz. The two Na,K pump representations have nearly identical steady-state characteristics of sensitivity to internal Na+ concentration, external K+ concentration, and membrane potential. Rapid voltage-dependent transient pump currents are present in the mechanistic model, while they are absent in the ad hoc mathematical description we used. The stimulation results show that a sizable peak of pump current caused by the action potential upstroke in the mechanistic model affects phase 1 repolarization, and that this effect is relatively independent of the stimulation rate. The pump current generated by our ad hoc mathematical description is constant during the action potential and does not affect directly the repolarization time course. While the two Na,K pump models show similar pumping efficiency at low stimulation rates, the mechanistic pump is more efficient at high rates of activity. In essence, the distinctive features of the mechanistic model are due to an energy barrier expressing the voltage dependence of the translocation step of the mechanism, and to the redistribution of the intermediates of the biochemical reactions during activity. In comparison, the ad hoc mathematical description exhibits a fixed dependence of the pump current on voltage and ionic concentrations.

The effect of strophanthidin on action potential, calcium current and contraction in isolated guinea-pig ventricular myocytes

1. A method is described for producing high yields of calcium-tolerant ventricular myocytes from guinea-pig hearts (73.4% rod-shaped cells, n = 19). Their action potential (AP) and membrane currents were recorded using conventional microelectrodes and cell shortening was measured optically using a linear photodiode array. 2. The sensitivity of the guinea-pig Na(+)-K+ pump to strophanthidin (a rapidly acting digitalis analogue) was determined by measuring the inhibition of outward pump current by different doses. The pump was found to have a dissociation constant (KD) for strophanthidin of 1.11 x 10(-5) M, and 5 x 10(-4) M-strophanthidin inhibited the pump maximally. 3. Exposure to strophanthidin resulted in an initial lengthening followed by a shortening of the AP, and an increased contraction. Initial AP lengthening was associated with a more positive AP plateau which became more negative as the AP shortened. 4. There was a reversible reduction of Ca2+ current (ICa) during exposure to strophanthidin. ICa changed reciprocally with contraction and with a similar time course. 5. Strophanthidin exposure caused a reduction of ICa at all activating voltages, suggesting that it resulted in a reduction of Ca2+ conductance with little change of its voltage dependence. 6. The role of an increase of intracellular calcium (Cai2+) was investigated by impaling myocytes with microelectrodes containing BAPTA 1,2-bis (2-amino-phenoxy)ethane-N,N,N',N'-tetraacetic acid, a calcium chelator) to increase Cai2+ buffering. Strophanthidin still shortened the AP when BAPTA was present, suggesting that a rise of Cai2+ is not a major cause of AP shortening. 7. Although AP shortening was little affected, the decline of ICa with strophanthidin was markedly reduced when BAPTA was present, suggesting that a rise of Cai2+ was the cause of the ICa decline with strophanthidin. 8. When barium ions carried the current through Ca2+ channels, strophanthidin did not reduce Ca2+ channel current, suggesting that this compound does not have a direct inhibitory effect on the channel. 9. The results suggest that strophanthidin causes a reduction of ICa by increasing Cai2+, via the mechanism of Cai(2+)-dependent inactivation of ICa. The reduction of ICa at least partially explains the AP shortening and more negative plateau with strophanthidin. 10. The shortening of the AP, more negative plateau and reduced ICa have negative inotropic effects which oppose the direct positive inotropic effect of strophanthidin.

Nifedipine antagonizes ouabain-induced ST-segment changes and derangement of epicardial activation pattern in isolated rabbit hearts

Isolated rabbit hearts, perfused according to the Langendorff technique were treated with 0.2 mumol/l ouabain or with a combination of 0.2 mumol/l ouabain and 3 nmol/l nifedipine. In a second series of experiments, a combination of 0.2 mumol/l ouabain with 0.2 mumol/l nitroglycerin was examined. Under this treatment, coronary flow and left ventricular pressure were continuously measured and, furthermore, the epicardial potential distribution was mapped with high temporal (4 kHz/channel) and spatial resolution (256 electrodes, arranged in 4 plates with 64 electrodes each, 8 x 8 matrix with 1 mm mesh size). Ouabain led to the expected positive inotropy of 10%, but also to a decrease in coronary flow of 15%. Besides these changes a nearly generalized ST-segment elevation could be observed. Moreover, the epicardial activation pattern was disturbed by changes in the location of epicardial breakthrough-points. Concomitantly, the epicardial activation vector field was deranged. Under the additional influence of nifedipine, coronary flow was reduced by only 5%, whereas the positive inotropic effect remained unchanged. The epicardial ST-elevation was diminished significantly and there was no derangement in the process of epicardial activation. Nitroglycerin led to an increase in relative coronary flow comparable to that observed under nifedipine but did not antagonize the disturbances of the process of activation by ouabain and only partially inhibited ST-elevation. Hence, it is concluded that ouabain already provokes coronary vasoconstriction in therapeutic concentrations leading to ST-elevation and prearrhythmic changes in the process of excitation. These changes could be diminished by additional treatment with nifedipine, but not with nitroglycerin.

Effect of norepinephrine on Na(+)-K+ pump and Na+ influx in sheep cardiac Purkinje fibers

Effects of norepinephrine and Ca+ on Na(+)-K+ pump and pacemaker current were investigated by simultaneous measurement of intracellular Na+ activity (aiNa) and membrane potential in driven (1 Hz) and quiescent sheep cardiac Purkinje fibers. Concurrently, twitch force was measured in driven fibers, in which norepinephrine (NE) produced a decrease in aiNa, a prolongation in action potential duration, and a hyperpolarization in diastolic membrane potential, Vdm. In contrast, in quiescent fibers, NE produced an increase in aiNa and a depolarization in resting membrane potential, Vm. The decrease in aiNa, prolongation in action potential duration, and hyperpolarization in Vdm produced by NE were blocked by 5 x 10(-6) M strophanthidin, presumably through inhibition of the Na(+)-K+ pump. The increase in aiNa and membrane depolarization caused by NE were abolished by high [K+]o or Cs+, presumably through inhibition of the pacemaker current, if. These results indicate that in driven fibers NE stimulates predominantly the Na(+)-K+ pump, producing a decrease in aiNa and that in quiescent fibers it increases predominantly if, producing an increase in aiNa. The effect of NE on driven and quiescent fibers differs because of the voltage dependence of if and perhaps the Na(+)-K+ pump. Consequently, the relative magnitude of the two opposing effects of NE on aiNa appears to be dependent on membrane potential. In quiescent fibers, Cs+ monotonically decreased aiNa to a steady-state value, while Cs+ hyperpolarized membrane potential and then slowly depolarized to a steady-state level, producing a transient hyperpolarization. In driven fibers, Cs+ decreased aiNa, shortened action potential duration, and depolarized Vdm. Cs+ decreased aiNa more in quiescent fibers than in driven fibers. The decrease in aiNa and hyperpolarization in membrane potential produced by Cs+ in quiescent fibers were abolished by depolarization induced by high K+ extracellular concentration (25.4 mM) but were not abolished or reduced by 5 x 10(-6) M strophanthidin. These results suggest that the decrease in aiNa and hyperpolarization in membrane potential by Cs+ are caused by blockage of if but not by stimulation of the Na(+)-K+ pump and that if is an important source of Na+ loading into cells.

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 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.

Fascicular parasystole associated with tachycardia-dependent right bundle branch block

A 43-year-old female with a chief complaint of palpitation was subjected to clinical electrophysiological studies. Initial standard 12-lead ECG revealed that her palpitation was caused by fascicular parasystole firing at the basic cycle length of 1.25-1.40 seconds, and that both sinus and parasystolic beats were associated with left anterior fascicular block and tachycardia-dependent RBBB. His-bundle electrocardiogram suggested that the parasystolic focus was located in the proximal portion of the anterior fascicle of the left bundle branch and that the site of tachycardia-dependent conduction block was located in the main right bundle branch. These findings suggest that diffuse pathological changes in the intraventricular conducting system were responsible for both the conduction block and automatic impulse formation in the present case.

Inhibition of Na-K pump current in guinea pig ventricular myocytes by dihydroouabain occurs at high- and low-affinity sites

Binding of cardiac glycosides to the Na+,K+-dependent ATPase has been shown to occur at both high- and low-affinity sites. However, recent reports suggest that glycoside-induced inhibition of electrogenic Na-K pump current occurs with simple first-order binding kinetics at relatively low-affinity sites. This implies that high-affinity binding sites have little to do with Na-K pump inhibition during exposure to cardiac glycosides. To better understand the role of the high-affinity site, we investigated the concentration dependence of Ipump inhibition by dihydroouabain (DHO) in guinea pig ventricular myocytes through use of wide-pore patch pipettes to "fix" internal Na+ activity at approximately 30 mM and to voltage clamp at -40 mV (T = 34 degrees C). DHO was found to have no effect on membrane conductance at a holding potential of -40 mV. Holding current was monitored and the difference between steady-state holding current before and during external exposure to nine concentrations (range, 0.01-1,000 microM) of DHO was measured and normalized to cellular membrane capacitance. The concentration dependence of the inhibition of Na-K pump current was biphasic and well fitted to a two-binding site model with inhibitory KD values of 0.05 microM and 64.5 microM. This is consistent with previously reported 3H-ouabain binding studies in guinea pig myocardium. These findings indicate that the electrogenic properties of the Na-K pump can be inhibited by glycoside binding to both high- and low-affinity sites.

Simultaneous measurement of changes in current and tracer flux in voltage-clamped squid giant axon

A method is described for the simultaneous measurement of changes in membrane current and unidirectional radiotracer flux in internally dialyzed voltage-clamped squid giant axons. The small currents that are produced by electrogenic transport processes or steady-state ionic currents can be resolved using this method. Because the use of grounded guard electrodes in the end pools is not, by itself, an adequate means of eliminating end-effects, two ancillary end pool clamp circuits are described to eliminate extraneous current flow from the ends of the axon. The end pool voltage-clamp circuits serve to minimize net current flow between the end pools and center pool, and employ stable, low-impedance calomel electrodes to monitor the potentials of the end and center pools. The adequacy of the method is demonstrated by experiments in which unidirectional 22Na efflux and current, flowing through tetrodotoxin (TTX)-sensitive Na channels into Na-free seawater, under K-free conditions, are shown to be equal. The equality of unidirectional TTX-sensitive flux and current is maintained over the entire range of membrane potentials examined (-60 to +20 mV). The method has been applied to a series of experiments in which the voltage dependence and stoichiometry of the Na/K pump have been measured (Rakowski et al., 1989), and can be applied in general to the simultaneous measurement of changes in current and flux of other electrogenic transport processes, and of currents through ionic channels that open under steady-state conditions.

Analysis of the hyperpolarizing effect of catecholamines on canine cardiac Purkinje fibres

1. The hyperpolarization induced by catecholamines on barium-depolarized (0.2-0.8 mM BaCl) canine cardiac Purkinje fibres, in vitro, was studied by use of conventional microelectrode recordings of transmembrane electrical potentials. 2. Noradrenaline, adrenaline and isoprenaline hyperpolarized Purkinje fibres in a concentration-dependent manner from a threshold concentration around 5 nM. The three catecholamines were shown to be approximately equipotent. Tachyphylaxis was observed when the interval between catecholamine applications was less than 15 min. 3. Atenolol (10 microM) blocked the hyperpolarization reversibly and theophylline (0.5 mM) potentiated it. 4. Tetrodotoxin (5 microM) did not affect the hyperpolarization induced by isoprenaline. Acetylcholine and histamine, up to 10 microM, were not effective in hyperpolarizing Purkinje fibres. 5. Low extracellular potassium concentrations (zero and 1 mM) did not affect the hyperpolarization, but high extracellular potassium concentrations (10-20 mM), markedly reduced the effect of isoprenaline (100 nM). 6. Reduction of the extracellular sodium concentration produced a roughly proportional reduction in the isoprenaline-induced hyperpolarization. The hyperpolarization was reversibly blocked in 34 mM sodium Tris-Tyrode solution. 7. The hyperpolarization was not reduced in Tyrode solution containing 0.6 mM calcium, but was drastically reduced in zero-calcium Tyrode solution. This effect was reversible. 8. Addition of verapamil (5-10 microM) diminished the hyperpolarization, in a concentration-dependent manner. This effect was partially reversed after washing. 9. Ouabain (0.7-1 microM) significantly reduced the isoprenaline-induced hyperpolarization, but 2,4-dinitrophenol (0.2 mM) did not affect it. 10. Caesium chloride (20 mM) abolished the hyperpolarization. The blockade was only partially reversed upon washing. 11. It is suggested that the hyperpolarization induced by a short exposure to catecholamines is mainly due to an increase in potassium permeability (PK). A mechanism involving calciumdependent potassium channels might underlie the increase in PK.

Role of aiNa in positive force-frequency staircase in guinea pig papillary muscle

In the ventricular papillary muscle of guinea pig heart, membrane potential, intracellular sodium activity (aiNa), and twitch force were measured simultaneously and continuously for many hours at stimulation rates of 0, 0.5, 1, 2, 3, 4, 5, and 6 Hz to investigate the relation of aiNa to twitch force and membrane potential both in the steady state and during the changes in these variables. After an increase in stimulation rate, both aiNa and twitch force increased progressively, reaching steady-state levels. The relation between twitch force and aiNa in the steady state was generally sigmoidal over the range of 0.5-5 Hz and steep in the 1- to 4-Hz range. After either increase or decrease in stimulation rate, the time course of change in aiNa was exponential and similar to that of change in twitch force. Moreover, the force-aiNa relation observed after increase in stimulation rate from 0.5 to 3 Hz resembled that observed after decrease in the rate from 3 to 0.5 Hz, indicating an absence of hysteresis in the relation. The results suggest that an increase in aiNa is an important factor involved in the force staircase. As stimulation rate was increased from 0.5 to higher rates (5 or 6 Hz) and then decreased back to 0.5 Hz, a hysteresis phenomenon was observed in the relation between twitch force and aiNa. This suggests that some secondary factor may alter the relation between twitch force and aiNa. As stimulation rate increased and aiNa rose, the steady-state diastolic membrane potential hyperpolarized. This result is consistent with the view that an increase in aiNa enhances the electrogenic Na+-K+ pump and hyperpolarizes the cell membrane.

Voltage dependence of Na/K pump current in Xenopus oocytes

Stage V and VI (Dumont, J.N., 1972, J. Morphol. 136:153-180) oocytes of Xenopus laevis were treated with collagenase to remove follicular cells and were placed in K-free solution for 2 to 4 days to elevate internal [Na]. Na/K pump activity was studied by restoring the eggs to normal 3 mM K Barth's solution and measuring membrane current-voltage (I-V) relationships before and after the addition of 10 microM dihydroouabain (DHO) using a two-microelectrode voltage clamp. Two pulse protocols were used to measure membrane I-V relationships, both allowing membrane currents to be determined twice at each of a series of membrane potentials: (i) a down-up-down sequence of 5 mV, 1-sec stair steps and (ii) a similar sequence of 1-sec voltage pulses but with consecutive pulses separated by 4-sec recovery periods at the holding potential (-40 mV). The resulting membrane I-V relationships determined both before and during exposure to DHO showed significant hysteresis between the first and second current measurements at each voltage. DHO difference curves also usually showed hysteresis indicating that DHO caused a change in a component of current that varied with time. Since, by definition, the steady-state Na/K pump I-V relationship must be free of hysteresis, the presence of hysteresis in DHO difference I-V curves can be used as a criterion for excluding such data from consideration as a valie measure of the Na/K pump I-V relationship. DHO difference I-V relationships that did not show hysteresis were sigmoid functions of membrane potential when measured in normal (90 mM) external Na solution. The Na/K pump current magnitude saturated near 0 mV at a value of 1.0-1.5 microA cm-2, without evidence of negative slope conductance for potentials up to +55 mV. The Na/K pump current magnitude in Na-free external solution was approximately voltage independent. Since these forward-going Na/K pump I-V relationships do not show a region of negative slope over the voltage range -110 to +55 mV, it is not necessary to postulate the existence of more than one voltage-dependent step in the reaction cycle of the forward-going Na/K pump.

Stimulation of cardiac alpha receptors increases Na/K pump current and decreases gK via a pertussis toxin-sensitive pathway

Alpha-adrenergic amines exert concentration-dependent actions on the automaticity of cardiac Purkinje fibers (Posner, P., E. L. Farrar, and C. R. Lambert. 1976. Am. J. Physiol. 231:1415-1420; Rosen, M. R., A. J. Hordof, J. P. Ilvento, and P. Danilo, Jr. 1977. Circ. Res. 40:390-400; Rosen, M. R., R. M. Weiss, and P. Danilo, Jr. 1984. J. Pharmacol. Exp. Ther. 231:1415-1420). At high concentrations they induce a largely beta adrenergic increase in the spontaneous firing rate of adult canine Purkinje fibers, whereas at concentrations less than 10(-6) M, their effect is mediated through alpha-adrenergic receptors and is seen predominantly as a decrease in the fibers' spontaneous firing rate. The mechanism for this decrease in spontaneous firing rate remains unexplained. We report here that phenylephrine (10(-7) M) increases the activity of the Na/K pump and decreases background gK in Purkinje myocytes. Both effects appear to be alpha-1 adrenergic and, in addition, are abolished on pretreatment with pertussis toxin. These results suggest that like the atrial muscarinic receptor (Pffafinger, P. J., J. M. Martin, D. D. Hunter, N. M. Nathanson, and B. Hille. 1985. Nature [Lond.]. 317:536-538; Breitwieser, G. E., and G. Szabo. 1985. Nature [Lond.]. 317:538-540) the Purkinje fiber alpha-1 receptor is coupled to background gK via a GTP-regulatory protein. Further, they suggest that the phenylephrine-induced decrease in spontaneous firing rate is due to stimulation of the Na/K pump via a novel coupling of the Na/K pump to a pertussis toxin-sensitive GTP regulatory protein.

Arrhythmogenic interaction between low potassium and ouabain in isolated guinea-pig ventricular myocytes

1. The two-microelectrode method of voltage clamp was used in single myocytes isolated from guinea-pig ventricles to investigate the mechanism underlying the arrhythmogenic interaction between low external K+ and digitalis. 2. We investigated the effects of ouabain (10(-6) M) with 4 mM-K+ or 2 mM-K+ on the peak magnitude of the inward component of oscillatory current (Iti) recorded upon repolarization to the resting potential after depolarizing clamps to -5 mV, and on the characteristics of the steady-state current-voltage relationship. 3. Whereas ouabain with 4 mM-K+ did not alter the current-voltage relationship from its control shape, ouabain with 2 mM-K+ caused marked changes in the curve: the zero-current intercept was shifted in a negative direction, the region of low slope conductance was extended to more negative potentials, and the curve was shifted downward relative to the control current-voltage relationship. The changes in the current-voltage relationship induced by ouabain with 2 mM-K+ were very similar to those induced by 2 mM-K+ alone. 4. The functional consequence of the changes in the current-voltage relationship induced by ouabain with 2 mM-K+ was a highly significant reduction in the amount of outward current (Ith) needed to reach the threshold for excitation; Ith was reduced from 1.6 +/- 0.4 nA (n = 6) in ouabain with 4 mM-K+ to 0.8 +/- 0.3 nA (n = 5) in ouabain with 2 mM-K+. 5. The mean value for Iti was larger in ouabain with 2 mM-K+ (0.58 +/- 0.41 nA, mean +/- S.D., n = 4) than in ouabain with 4 mM-K+ (0.42 +/- 0.38 nA, n = 5). Although the increase in Iti was not statistically significant because of the large variability of the measurement, it is possible that the increase might be physiologically significant. 6. Our results suggest that the arrhythmogenic interaction between digitalis and low K+ is due to the combined effects of low K+ on the current-voltage relationship and on the size of the peak inward current induced by ouabain. Whereas the effect of low K+ and ouabain on the inward current was highly variable, the effects on the current-voltage curve were far more consistent, pointing to an important role for alterations in the current-voltage relationship in the arrhythmogenic interaction between low K+ and ouabain.

Sodium-pump activity and its inhibition by extracellular calcium in cardiac myocytes of guinea pigs

Myocardial sodium-pump activity was examined from ouabain-sensitive 86Rb+ uptake using myocytes isolated from guinea-pig heart. Either sodium loading or the sodium ionophore, monensin, increased 86Rb+ uptake by over 400%, indicating that the amount of Na+ available to the pump is the primary determinant of its activity, and that the sodium pump has a substantial reserve capacity in quiescent myocytes. Moreover, the degree of the above stimulation is markedly higher than corresponding values reported with multicellular preparations, suggesting that diffusion barriers make it impossible to observe the capacity of the sodium pump in the latter preparations. Removal of extracellular Ca2+ increased ouabain-sensitive 86Rb+ uptake, probably by enhancing turnover of the sodium pump rather than increasing availability of Na+ to the pump.

Indirect effects of acetylcholine on the electrogenic sodium pump in bull-frog atrial muscle fibres

1. Effects of acetylcholine (ACh) on the activity of electrogenic Na+ pump in bullfrog atrial muscle fibres were examined using the single sucrose-gap voltage clamp technique. 2. In the K+-free solution, 10 microM-ACh induced a large outward current (ACh-induced current) with an increase in the membrane conductance. 3. The amplitude of the ACh-induced current decreased to 15% of the control 10 min after application of 1 microM-ouabain, suggesting the contribution of electrogenic Na+ pump to the ACh-induced current. The remaining ACh-induced current was not affected even if the concentration of ouabain was increased ten times. 4. The K+-activated current induced by an activation of the electrogenic Na+ pump was suppressed or reversed its direction during the course of the ACh-induced current. 5. The ACh-induced current was completely inhibited by applications of either atropine or barium ions while the K+-activated current was not affected. 6. Both ouabain-sensitive and -insensitive ACh-induced currents were decreased when the membrane was hyperpolarized and eliminated around -95 mV. 7. The ouabain-sensitive component was decreased by increasing the external K+ concentration [K+]o; the proportions of this current to ACh-induced current in 0.5, 0.75, 1 and 2 mM [K+]o were 54, 42, 34 and 14%, respectively. 8. The current-voltage (i-v) relation obtained in 2 or 4 mM [K+]o, where the currents carried by Na+ and Ca2+ were blocked by application of 1 microM-TTX and 1 mM-Cd2+, exhibits marked inward-going rectification but does not show a clear N-shaped feature. Ba2+ (1 mM) induced an inward current at the holding potential (-80 mV) and eliminated the inward-going rectification of the membrane. 9. These results suggest that the increase in the K+ permeability by ACh increases the concentration of K+ immediately outside of the membrane, which in turn stimulates the electrogenic Na+ pump mechanism. The physiological significance of the action of ACh on the electrogenic Na+ pump in bull-frog atrium is discussed in relation to the background K+ current (IK,1).

Contribution of the Na+/K+-pump to the membrane potential

The inward movement of sodium ions and the outward movement of potassium ions are passive and the reverse movements against the electrochemical gradients require the activity of a metabolism-driven Na+/K+-pump. The activity of the Na+/K+-pump influences the membrane potential directly and indirectly. Thus, the maintenance of a normal electrical function requires that the Na+/K+-pump maintain normal ionic concentrations within the cell. The activity of the Na+/K+-pump also influences the membrane potential directly by generating an outward sodium current that is larger when the Na+/K+-pump activity is greater. The activity of the Na+/K+-pump is regulated by several factors including the intracellular sodium concentration and the neuromediators norepinephrine and acetylcholine. The inhibition of the Na+/K+-pump can lead indirectly to the development of inward currents that may cause repetitive activity. Therefore, the Na+/K+-pump modifies the membrane potential in different ways both under normal and abnormal conditions and influences in an essential way many cardiac functions, including automaticity, conduction and contraction. Key words. Active transport of ions; cardiac tissues; electroneutral and electrogenic Na+/K/-pump; control of Na+/K+-pump; normal and abnormal electrical events.

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.

Electrophysiologic properties differ in the ventricular endocardium and epicardium of the Japanese monkey

We measured action potential duration (APD) from the endocardium (Endo) and epicardium (Epi) of the left ventricular free wall in Japanese monkey hearts and found that the APD of Endo is significantly longer than that of Epi at a stimulus cycle length of 1500 msec in normal Tyrode solution (control condition). We then hypothesized that shorter APD of Epi results from greater outward pump current and that the difference in the current may be due to a difference in membrane Na,K-ATPase activity between Endo and Epi. If this were the case, interventions which alter the Na,K-pump activity should alter electrophysiologic characteristics in the endocardium and the epicardium to different degrees. An application of ouabain (10(-6) M), an inhibitor of Na,K-ATPase, produced greater shortening of APD and greater depolarization of the resting potential in Endo as compared with Epi. Shortening of stimulus cycle length shortened the APD more markedly in Endo than Epi, resulting in a significantly longer APD in Epi than Endo at a stimulus cycle length of 200 msec. The hyperpolarization and the APD shortening produced when the tissues were returned to 5.4 mM K+ Tyrode solution from K+-free medium were also more marked in Endo than in Epi. Such findings suggest that endocardial cells are more liable to accumulate Na ions intracellularly and K ions extracellularly when the Na, K-pump is suppressed by ouabain or K+-free perfusion, presumably due to lower activity of Na, K-ATPase in Endo compared to Epi.(ABSTRACT TRUNCATED AT 250 WORDS)

Arrhythmogenic actions of antiarrhythmic drugs

Cardiac arrhythmias may result from abnormalities of impulse propagation or abnormalities of impulse initiation. When arrhythmias are initiated by antiarrhythmic drugs, the most common mechanisms appear to be conduction block or reentry, and abnormal impulse initiation, which may be triggered by afterdepolarizations. The effects of drugs on conduction may result from their actions on the fast sodium channel, the slow calcium channel or their ability to prolong repolarization. The extent to which a drug that depresses fast sodium or slow calcium entry will exert its toxic effects depends in large part on its binding characteristics to its channel receptor site. Such toxicity represents a continuum for the therapeutic effects of these drugs. The factors that control drug access to binding sites, including lipid solubility, molecular size and extent of ionization, are reviewed, as are the contributions to conduction abnormalities of drug-induced changes in repolarization. The mechanisms whereby drugs induce abnormalities of impulse initiation are still a matter of conjecture. Apparently, drugs that increase inward plateau currents or decrease repolarizing potassium ion currents carry increased risk. Moreover, there is evidence for the role of early after depolarizations occurring secondary to prolonged repolarization as a possible cause of arrhythmias, including torsades de pointes. The mechanisms whereby antiarrhythmic drugs may contribute to this type of tachyarrhythmia are reviewed.

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.

Properties of an electrogenic sodium-potassium pump in isolated canine Purkinje myocytes

1. Purkinje myocytes were isolated from canine Purkinje strands by collagenase exposure and gentle trituration. The myocytes were studied by a switched single-micro-electrode voltage-clamp technique at 37 degrees C in Tyrode solution containing 8 mM-K+ and 2 mM-Ca2+. 2. The dose-response relation for the cardiotonic steroid dihydroouabain (DHO) was obtained by measuring the change in membrane current caused by application of concentrations of 1-100 microM. The KD obtained in fourteen experiments was 3.7 +/- 1.1 microM (mean +/- S.E. of mean). 3. We employed 100 microM-DHO (a concentration more than 25-fold greater than the KD) to estimate the resting pump current (Ip) in the isolated myocytes. A value of 0.27 +/- 0.02 microA microF-1 (mean +/- S.E. of mean, n = 32) was obtained. 4. Myocytes were also exposed to K+-free solution for a period of 200 s. On return to K+-containing Tyrode solution there was a slowly decaying outward current. The time constant of decay of this pump current transient was 87 +/- 8 s (mean +/- S.E. of mean, n = 8). The integral beneath this transient was used to obtain a second estimate of the resting pump current. In four preparations where exposures in DHO and in K+-free solutions were employed the ratio Ip, DHO/Ip, K-free was 1.76 +/- 0.15 (mean +/- S.E. of mean). 5. From the magnitude of resting pump current, in the presence of total pump blockade the Na+ activity should rise at a rate of 1.3 mM min-1. 6. Reducing [K+]o from 8 to 1 mM reduced Ip by more than 40% initially. Ip then slowly increased over the next 30 min. These results suggest that the steady-state inward background current is not greatly altered by changes in [K+]o, and that [Na+]i rises to a new level. The changes in Ip obtained at early times following reduction of [K+]o to 1 or 0.5 mM (t less than 1.75 min) were used to estimate the Km for external K+; a value of 0.8 mM was obtained. 7. The results suggest that the properties of the Na+-K+ pump in isolated canine Purkinje myocytes are similar to those in canine Purkinje strands. This argues against major distortions of measured pump properties in the canine Purkinje strand and for the physiological state of the Na+-K+ pump in the isolated Purkinje myocyte.

Opposite effects of adrenaline and ouabain on the resting potential of rat atrial cells

In the left rat atrium changes in diastolic potential (E max) evoked by sudden stimulation train were modified by adrenaline and ouabain. The early stimulation depolarization phase (SDP) of E max occurring on stimulation was shortened by adrenaline, but lengthened and strongly enhanced by ouabain. The stimulation repolarization phase (SRP) following SDP was markedly inhibited by ouabain, while accelerated and increased by adrenaline. In continuously stimulated atria E max was decreased by ouabain and augmented by adrenaline. The adrenaline-induced hyperpolarization was reduced or suppressed in the presence of 10-4 M or 10-3 M ouabain, respectively. The present data suggest that adrenaline could stimulate the electrogenic sodium pump in the rat atrium.

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.

Incorporation of membrane potential into theoretical analysis of electrogenic ion pumps

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

Membrane current through adenosine-triphosphate-regulated potassium channels in guinea-pig ventricular cells

The question whether activation of the ATP-regulated K channel is responsible for macroscopic anoxia-induced outward currents was examined in ventricular cells isolated enzymatically from guinea-pig heart. Gigaseal patch-clamp electrodes were used for a whole-cell voltage clamp. Membrane currents were compared in the same cell while the cell interior was dialysed by perfusing the electrode with different solutions. When the cell was dialysed with various ATP-deficient (less than or equal to 2 mM) internal solutions, the Ca current decreased in a dose-dependent manner to less than 10% of control at 0.5 mM-ATP. A slight (ca. 25%) decrease of the slope conductance for hyperpolarizing current was observed. When a delayed rectification on depolarization followed by a marked outward current tail on repolarization was present under control conditions, this time-dependent outward current was also depressed. An increase in a time-independent outward current was observed accompanied by marked current fluctuations. The outward current showed a reversal potential near the K equilibrium potential, inward rectification, and no relaxation on voltage jumps. The power density spectrum of the current fluctuations showed a pattern similar to the spectrum calculated from the single-channel currents of ATP-regulated K channels. The amplitude of the single-channel current, estimated from the fluctuations, was almost equal to that of the single-channel current. The total number of channels within one cell was estimated as 2000-3000. It is concluded that the ATP-regulated K channels are responsible for the increase in the outward current and the shortening of the action potential duration under various anoxic conditions.

Voltage dependence of Na/K pump current in isolated heart cells

The Na/K pump usually pumps more Na+ out of the cell than K+ in, and so generates an outward component of membrane current which, in the heart, can be an important modulator of the frequency and shape of the cardiac impulse. Because it is electrogenic, Na/K pump activity ought to be sensitive to membrane potential, and it should decline with hyperpolarization. However, such voltage dependence of outward pump current has yet to be demonstrated, one reason being the technical difficulty of accurately measuring pump current over a sufficiently wide voltage range. The whole-cell patch-clamp technique allows effective control of both intracellular and extracellular solutions as well as membrane voltage. Applying this technique to myocardial cells isolated from guinea pig ventricle, we have measured Na/K pump current between -140 mV and +60 mV, after minimizing passive currents flowing through Ca2+, K+ and Na+ channels. We report here that strongly activated pump current shows marked voltage dependence; it declines steadily from a maximal level near 0 mV, becoming very small at -140 mV. Pump current-voltage relationships will provide essential information for testing models of the Na/K pump mechanism and for predicting pump-mediated changes in the electrical activity of excitable cells.

Effect of imipramine on calcium and potassium currents in isolated bovine ventricular myocytes

Isolated bovine ventricular myocytes were investigated with a two-microelectrode voltage clamp technique. The clamp currents were analyzed in terms of ICa and IK. Possible effects on INa were avoided by superfusing the cells with a Na-free medium. Imipramine (IMI) was applied at a concentration of 3.6 microM. Within the initial 3 min (early phase), IMI reduced peak ICa by 38 +/- 9% but IMI did not change the time constants of inactivation, the voltage dependence of peak ICa or its reversal potential. Therefore, we conclude that IMI reduced calcium conductance. After 10 min of exposure (late phase), IMI can also reduce the reversal potential of ICa. The inward rectifying potassium current (IK1) was transiently enhanced by 15 +/- 8% but later (8-10 min) reduced by 19 +/- 4%. Washout of IMI completely reversed all the effects within 10 min. Reduction of ICa diminished the rate of rise and the overshoot of the slow action potential and can explain the shortening of the AP seen in both Na-free and Na-containing media. Possible clinical implications are discussed.

A model of cardiac electrical activity incorporating ionic pumps and concentration changes

Equations have been developed to describe cardiac action potentials and pacemaker activity. The model takes account of extensive developments in experimental work since the formulation of the M.N.T. (R. E. McAllister, D. Noble and R. W. Tsien, J. Physiol., Lond. 251, 1-59 (1975)) and B.R. (G. W. Beeler and H. Reuter, J. Physiol., Lond . 268, 177-210 (1977)) equations. The current mechanism i K2 has been replaced by the hyperpolarizing-activated current, i f . Depletion and accumulation of potassium ions in the extracellular space are represented either by partial differential equations for diffusion in cylindrical or spherical preparations or, when such accuracy is not essential, by a three-compartment model in which the extracellular concentration in the intercellular space is uniform. The description of the delayed K current, i K , remains based on the work of D. Noble and R. W. Tsien ( J. Physiol., Lond . 200, 205-231 (1969 a )). The instantaneous inward-rectifier, i K1 , is based on S. Hagiwara and K. Takahashi’s equation ( J. Membrane Biol . 18, 61-80 (1974)) and on the patch clamp studies ofB. Sakmann and G. Trube ( J. Physiol., Lond . 347, 641-658 (1984)) and of Y. Momose, G. Szabo and W. R. Giles ( Biophys. J . 41, 311a (1983)). The equations successfully account for all the properties formerly attributed to i K2 , as well as giving more complete descriptions of i K1 and i K . The sodium current equations are based on experimental data of T. J. Colatsky ( J.Physiol., Lond. 305, 215-234 (1980)) and A. M. Brown, K. S. Lee and T. Powell ( J.Physiol., Lond. , Lond. 318, 479-500 (1981)). The equations correctly reproduce the range and magnitude of the sodium ‘window’ current. The second inward current is based in part on the data of H. Reuter and H. Scholz ( J. Physiol., Lond . 264, 17-47 (1977)) and K. S. Lee and R. W. Tsien ( Nature, Lond . 297,498-501 (1982)) so far as the ion selectivity is concerned. However, the activation and inactivation gating kinetics have been greatly speeded up to reproduce the very much faster currents recorded in recent work. A major consequence of this change is that Ca current inactivation mostly occurs very early in the action potential plateau. The sodium-potassium exchange pump equations are based on data reported by D. C. Gadsby ( Proc. natn. Acad. Sci. U. S. A. 77, 4035-4039 (1980)) and by D. A. Eisner and W. J. Lederer ( J. Physiol., Lond . 303, 441-474 (1980)). The sodium-calcium' exchange current is based on L. J. Mullins’ equations ( J. gen.. Physiol. 70, 681-695 (1977)). Intracellular calcium sequestration is represented by simple equations for uptake into a reticulum store which then reprimes a release store. The repriming equations use the data of W. R. Gibbons & H. A. Fozzard ( J. gen. Physiol . 65, 367-384 (1975 b )). Following Fabiato & Fabiato’s work ( J. Physiol., Lond. 249, 469-495 (I975)), Ca release is assumed to be triggered by intracellular free calcium. The equations reproduce the essential features of intracellular free calcium transients as measured with aequorin. The explanatory range of the model entirely includes and greatly extends that of the M.N.T. equations. Despite the major changes made, the overall time-course of the conductance changes to potassium ions strongly resembles that of the M.N.T. model. There are however important differences in the time courses of Na and Ca conductance changes. The Na conductance now includes a component due to the hyperpolarizing-activated current, i r , which slowly increases during the pacemaker depolarization. The Ca conductance changes are very much faster than in the M.N.T. model so that in action potentials longer than about 50 ms the primary contribution of the fast gated calcium channel to the plateau is due to a steady-state ‘window’ current or non-inactivated component. Slower calcium or Ca-activated currents, such as the Na-Ca exchange current, or Ca-gated currents, or a much slower Ca channel must then play the dynamic role previously attributed to the kinetics of a single type of calcium channel. This feature of the model in turn means that the repolarization process should be related to the inotropic state, as indicated by experimental work. The model successfully reproduces intracellular sodium concentration changes produced by variations in [Na]0, or Na-K pump block. The sodium dependence of the overshoot potential is well reproduced despite the fact that steady state intracellular Na is proportional to extracellular Na, as in the experimental results of D. Ellis J. Physiol., Lond . 274, 211-240 (1977)). The model reproduces the responses to current pulses applied during the plateau and pacemaker phases. In particular, a substantial net decrease in conductance is predicted during the pacemaker depolarization despite the fact that the controlling process is an increase in conductance for the hyperpolarizing-activated current. The immediate effects of changing extracellular [K] are reproduced, including: (i) the shortening of action potential duration and suppression of pacemaker activity at high [K ]; (ii) the increased automaticity at moderately low [K ]; and (iii) the depolarization to the plateau range with premature depolarizations and low voltage oscillations at very low [K]. The ionic currents attributed to changes in Na-K pump activity are well reproduced. It is shown that the apparent K m for K activation of the pump depends strongly on the size of the restricted extracellular space. With a 30% space (as in canine Purkinje fibres) the apparent K m is close to the assumed real value of 1 mM . When the extracellular space is reduced to below 5% , the apparent K m increases by up to an order of magnitude. A substantial part of the pump is then not available for inhibition by low [K] b . These results can explain the apparent discrepancies in the literature concerning the K m for pump activation.

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.

The effects of the Anemonia sulcata toxin (ATX II) on membrane currents of isolated mammalian myocytes

The effects of Anemonia sulcata toxin (ATX II) on action potentials and membrane currents were studied in single myocytes isolated from guinea-pig or bovine ventricles. Addition of ATX II (2-20 nM) prolonged the action potential duration without a significant change in resting membrane potential. Concentrations of 40 nM-ATX II or more induced after-depolarizations and triggered automaticity. The effects were reversible after washing or upon addition of 60 microM-tetrodotoxin (TTX). 5 mM-Ni did not modify the effects. The single patch-electrode voltage-clamp technique of Hamill, Marty, Neher, Sakmann & Sigworth (1981) was applied to record membrane currents in response to 8.4 S long depolarizations starting from a holding potential of -90 mV. Currents flowing later than 5 ms after the depolarizing step were analysed. The fast events could not be considered because of insufficient voltage homogeneity. After 2 min of exposure to ATX II (20 nM) the changes in net membrane currents were measured. The difference between the currents in the presence of ATX II and during control was defined as the 'ATX-II-induced current' (iATX). After 4 min of wash iATX disappeared. Within 10 S of exposure to 60 microM-TTX, iATX was blocked completely. At potentials positive to -60 mV, iATX was inwardly directed and decayed slowly but incompletely during the 8.4 S long depolarizing pulse. The rate of decay was faster during clamp pulses to more positive potentials. A high amplitude noise was superimposed on the current trace; its amplitude decreased with more positive potentials. We analysed the voltage dependence of iATX with 'isochronous' current-voltage relations. The 0.1 S isochrone of iATX was characterized by a 'threshold' for negative currents at -60 mV, a branch with a negative slope (k = -7 mV, potential of half-maximal activation (V0.5) = -38 mV, bovine cells) leading to a maximum inward current at -20 mV, and an ascending branch which led to an apparent reversal potential (Erev) around +40 mV. The values measured in guinea-pig myocytes were similar though not identical (k = -5.5 mV, V0.5 = -30 mV, maximum of inward current at -5 mV, Erev = +50 mV). Erev shifted to less positive potentials in later isochrones. Holding the membrane at -45 mV prevented the induction of extra current by ATX II. When the holding potential was then changed to -85 mV, iATX developed within some 2 min. Returning the holding potential to -45 mV blocked iATX with a similar slow time course.(ABSTRACT TRUNCATED AT 400 WORDS)

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.