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

Antiarrhythmic properties of acetaminophen in the dog

Mongrel dogs bred for research and weighing 25 +/- 3 kg were used to test the hypothesis that acetaminophen has antiar-rhythmic properties. Only ventricular arrhythmias defined by the Lambeth Conventions were investigated. Dogs were exposed either to 60 mins of regional myocardial ischemia followed by 180 mins of reperfusion (n = 14) or were administered a high dose of ouabain (n = 14). Both groups of 14 dogs were further divided into vehicle and acetaminophen treatment groups (n = 7 in each). During selected 10-min intervals, we recorded the numbers of ventricular premature beats, ventricular salvos, ventricular bigeminy, ventricular tachycardia (nonsustained and sustained), and we recorded the heart rate, systemic arterial blood pressure, and left ventricular function. Neither heart rate nor the number of ventricular arrhythmias differed significantly under baseline conditions. Conversely, the combined average number of ventricular ectopic beats during ischemia and reperfusion was significantly less in the presence of acetaminophen (28 +/- 4 vs. 6 +/- 1; P < 0.05). Similarly, percent ectopy during reperfusion in vehicle- and acetaminophen-treated dogs was 1.4 +/- 0.4 and 0.4 +/- 0.2, respectively (P < 0.05). The number of all ventricular ectopic beats except ventricular salvos was also significantly reduced in the presence of acetaminophen. Similar results were obtained with ouabain. Our results reveal that systemic administration of a therapeutic dose of acetaminophen has previously unreported antiarrhythmic effects in the dog.

Diurnal modulation of the Na+/K+-ATPase and spontaneous firing in the rat retinorecipient clock neurons

The ventral "core" suprachiasmatic nucleus (vSCN) neurons are the retinorecipient neurons in the mammalian circadian clock and maintain a diurnal firing rhythm in reduced preparations. We tested the possibility that daily changes in Na+/K+-ATPase accompany diurnal variation in spontaneous electrical activity. In control, bath application of 9 microM strophanthidin increased the spontaneous firing both at day and night but to different extents. In the presence of 1 mM Ni2+ to block spontaneous firing, addition of 9 microM strophanthidin, but not higher concentrations (6.5-20 mM) of external K+, induced the silenced cells to fire action potentials in a diurnal rhythm, suggesting a diurnal change in Na+/K+-ATPase activity. Consistently, voltage-clamp recordings demonstrated that the pump current blocked by 9 microM strophanthidin was approximately three times larger in daytime than nighttime and was little affected by the presence of 1 mM Ni2+. Experiments with various concentrations of strophanthidin further suggests day-night differences in maximum Na+/K+-ATPase activity, amounting to 6 pA of pump current at day and down to 3.5 pA at night, and in its half-block concentrations, changing from a daytime value of 4 microM to a nighttime value of 8 microM. Our results indicate that the vSCN neurons exhibit a diurnal rhythm in the Na+/K+-ATPase the activity of which is higher during the day when the firing rate is also higher. Mechanistically, the modulation could be accounted for in terms of changes in the maximum activity of Na+/K+-ATPase and its ability to block by strophanthidin.

Properties of the Na+/K+ pump current in small neurons from adult rat dorsal root ganglia

1 The present investigation was undertaken to characterize the Na(+)/K(+) pump current in small ( Collapse

Key Words

• na⁺/k⁺ pump current
• dorsal root ganglion
• neuron
• ouabain
• α1 isoform
• α3 isoform

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MESH Headings

• Animals
• Cell Size/drug effects
• Cell Size/physiology
• Cells, Cultured
• Dose-Response Relationship, Drug
• Ganglia, Spinal/cytology
• Ganglia, Spinal/drug effects
• Ganglia, Spinal/physiology
• Male
• Membrane Potentials/drug effects
• Membrane Potentials/physiology
• Neurons/cytology
• Neurons/drug effects
• Neurons/physiology
• Ouabain/pharmacology
• Rats
• Rats, Sprague-Dawley
• Sodium-Potassium-Exchanging ATPase/physiology

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Affiliation(s)

Kanako Hamada Division of Neurology, Department of Medicine, Shiga University of Medical Science, Otsu, Shiga 520-2192, Japan
Hiroshi Matsuura Department of Physiology, Shiga University of Medical Science, Otsu, Shiga 520-2192, Japan
Mitsuru Sanada Division of Neurology, Department of Medicine, Shiga University of Medical Science, Otsu, Shiga 520-2192, Japan
Futoshi Toyoda Department of Physiology, Shiga University of Medical Science, Otsu, Shiga 520-2192, Japan
Mariko Omatsu-Kanbe Department of Physiology, Shiga University of Medical Science, Otsu, Shiga 520-2192, Japan
Atsunori Kashiwagi Division of Endocrinology and Metabolism, Department of Medicine, Otsu, Shiga University of Medical Science, Shiga 520-2192, Japan
Hitoshi Yasuda Division of Neurology, Department of Medicine, Shiga University of Medical Science, Otsu, Shiga 520-2192, Japan

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Patch-clamping of primary cardiac cells with micro-openings in polyimide films

Patch-clamping is a powerful method for investigating the function and regulation of ionic channels. Currently, great efforts are being made to automate this method. As a step towards this goal, the feasibility of patch-clamping primary cells with a microscopic opening in a planar substrate was tested. Using standard microfabrication and ion beam technology, small-diameter openings (2 and 4 microm) were formed in polyimide films (thickness 6.5 microm). Single cells (sheep Purkinje heart cells, Chinese hamster ovary cells) in a suspension were positioned on top of the opening and sucked towards the opening to improve adhesion of the cell to the planar substrate, hence increasing the seal resistance. Voltage/current measurements yielded a median seal resistance of 1.3 Mohms with 4 microm openings (n=24) and 26.0 Mohms with 2 microm openings (n = 75), respectively. With 2 microm openings, successful loose-patch recordings of TTX-sensitive inward currents and action potentials in sheep Purkinje heart cells (n = 18) were made. In rare cases, gigaseals (n = 4) were also measured, and a whole-cell configuration (n = 1) could be established. It was concluded that the simple planar patch approach is suitable for automated loose-patch recordings from cells in suspension but will hardly be suitable for high-throughput whole-cell patch-clamping with high-resistance seals.

Intracellular Na(+) concentration is elevated in heart failure but Na/K pump function is unchanged

BACKGROUND

Intracellular sodium concentration ([Na(+)](i)) modulates cardiac contractile and electrical activity through Na/Ca exchange (NCX). Upregulation of NCX in heart failure (HF) may magnify the functional impact of altered [Na(+)](i).

METHODS AND RESULTS

We measured [Na(+)](i) by using sodium binding benzofuran isophthalate in control and HF rabbit ventricular myocytes (HF induced by aortic insufficiency and constriction). Resting [Na(+)](i) was 9.7+/-0.7 versus 6.6+/-0.5 mmol/L in HF versus control. In both cases, [Na(+)](i) increased by approximately 2 mmol/L when myocytes were stimulated (0.5 to 3 Hz). To identify the mechanisms responsible for [Na(+)](i) elevation in HF, we measured the [Na(+)](i) dependence of Na/K pump-mediated Na(+) extrusion. There was no difference in V(max) (8.3+/-0.7 versus 8.0+/-0.8 mmol/L/min) or K(m) (9.2+/-1.0 versus 9.9+/-0.8 mmol/L in HF and control, respectively). Therefore, at measured [Na(+)](i) levels, the Na/K pump rate is actually higher in HF. However, resting Na(+) influx was twice as high in HF versus control (2.3+/-0.3 versus 1.1+/-0.2 mmol/L/min), primarily the result of a tetrodotoxin-sensitive pathway.

CONCLUSIONS

Myocyte [Na(+)](i) is elevated in HF as a result of higher diastolic Na(+) influx (with unaltered Na/K-ATPase characteristics). In HF, the combined increased [Na(+)](i), decreased Ca(2+) transient, and prolonged action potential all profoundly affect cellular Ca(2+) regulation, promoting greater Ca(2+) influx through NCX during action potentials. Notably, the elevated [Na(+)](i) may be critical in limiting the contractile dysfunction observed in HF.

Intracellular [Na+] and Na+ pump rate in rat and rabbit ventricular myocytes

Intracellular [Na+] ([Na+]i) is centrally involved in regulation of cardiac Ca2+ and contractility via Na+-Ca2+ exchange (NCX) and Na+-H+ exchange (NHX). Previous work has indicated that [Na+]i is higher in rat than rabbit ventricular myocytes. This has major functional consequences, but the reason for the higher [Na+]i in rat is unknown. Here, resting [Na+]i was measured using the fluorescent indicator SBFI, with both traditional calibration and a novel null-point method (which circumvents many limitations of prior methods). In rabbit, resting [Na+]i was 4.5 +/- 0.4 mM (traditional calibration) and 4.4 mM (null-point). Resting [Na+]i in rat was significantly higher using both the traditional calibration (11.1 +/- 0.7 mM) and the null-point approach (11.2 mM). The rate of Na+ transport by the Na+ pump was measured as a function of [Na+]i in intact cells. Rat cells exhibited a higher V(max) than rabbit (7.7 +/- 1.1 vs. 4.0 +/- 0.5 mM x min(-1)) and a higher K(m) (10.2 +/- 1.2 vs. 7.5 +/- 1.1 mM). This results in little difference in pump activity for a given [Na+]i below 10 mM, but at measured resting [Na+]i levels the pump-mediated Na+ efflux is much higher in rat. Thus, Na+ pump rate cannot explain the higher [Na+]i in rat. Resting Na+ influx rate was two to four times higher in rat, and this accounts for the higher resting [Na+]i. Using tetrodotoxin, HOE-642 and Ni2+ to block Na+ channels, NHX and NCX, respectively, we found that all three pathways may contribute to the higher resting Na+ influx in rat (albeit differentially). We conclude that resting [Na+]i is higher in rat than in rabbit, that this is caused by higher resting Na+ influx in rat and that a higher Na+,K+-ATPase pumping rate in rat is a consequence of the higher [Na+]i.

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.

Voltage dependence of the apparent affinity for external Na(+) of the backward-running sodium pump

The steady-state voltage and [Na(+)](o) dependence of the electrogenic sodium pump was investigated in voltage-clamped internally dialyzed giant axons of the squid, Loligo pealei, under conditions that promote the backward-running mode (K(+)-free seawater; ATP- and Na(+)-free internal solution containing ADP and orthophosphate). The ratio of pump-mediated (42)K(+) efflux to reverse pump current, I(pump) (both defined by sensitivity to dihydrodigitoxigenin, H(2)DTG), scaled by Faraday's constant, was -1.5 +/- 0.4 (n = 5; expected ratio for 2 K(+)/3 Na(+) stoichiometry is -2.0). Steady-state reverse pump current-voltage (I(pump)-V) relationships were obtained either from the shifts in holding current after repeated exposures of an axon clamped at various V(m) to H(2)DTG or from the difference between membrane I-V relationships obtained by imposing V(m) staircases in the presence or absence of H(2)DTG. With the second method, we also investigated the influence of [Na(+)](o) (up to 800 mM, for which hypertonic solutions were used) on the steady-state reverse I(pump)-V relationship. The reverse I(pump)-V relationship is sigmoid, I(pump) saturating at large negative V(m), and each doubling of [Na(+)](o) causes a fixed (29 mV) rightward parallel shift along the voltage axis of this Boltzmann partition function (apparent valence z = 0.80). These characteristics mirror those of steady-state (22)Na(+) efflux during electroneutral Na(+)/Na(+) exchange, and follow without additional postulates from the same simple high field access channel model (Gadsby, D.C., R.F. Rakowski, and P. De Weer, 1993. Science. 260:100-103). This model predicts valence z = nlambda, where n (1.33 +/- 0.05) is the Hill coefficient of Na binding, and lambda (0.61 +/- 0.03) is the fraction of the membrane electric field traversed by Na ions reaching their binding site. More elaborate alternative models can accommodate all the steady-state features of the reverse pumping and electroneutral Na(+)/Na(+) exchange modes only with additional assumptions that render them less likely.

Two types of action potential configuration in single cardiac Purkinje cells of sheep

Membrane potentials and currents of isolated sheep Purkinje and ventricular cells were compared using patch-clamp and microelectrode techniques. In approximately 50% of Purkinje cells, we observed action potentials that showed a prominent phase 1 repolarization and relatively negative plateau (LP cells). Action potential configuration of the remaining Purkinje cells was characterized by little phase 1 repolarization and relatively positive plateau (HP cells). Microelectrode impalement of Purkinje strands also revealed these two types of action potential configuration. In LP cells, the density of L-type Ca(2+) current (I(Ca,L)) was lower, whereas the density of transient outward K(+) current was higher, than in HP cells. Action potentials of HP cells strongly resembled those of ventricular cells. Densities of inward rectifier current and I(Ca,L) were significantly higher in ventricular cells compared with densities in both LP and HP Purkinje cells. Differences in current densities explain the striking differences in action potential configuration and the stimulus frequency dependency thereof that we observed in LP, HP, and ventricular cells. We conclude that LP Purkinje cells, HP Purkinje cells, and ventricular cells of sheep each have a unique action potential configuration.

Depolarization increases the apparent affinity of the Na+-K+ pump to cytoplasmic Na+ in isolated guinea-pig ventricular myocytes

1. In order to investigate the possible effect of membrane potential on cytoplasmic Na+ binding to the Na+-K+ pump, we studied Na+-K+ pump current-voltage relationships in single guinea-pig ventricular myocytes whole-cell voltage clamped with pipette solutions containing various concentrations of Na+ ([Na+]pip) and either tetraethylammonium (TEA+) or N-methyl-D-glucamine (NMDG+) as the main cation. The experiments were conducted at 30 C under conditions designed to abolish the known voltage dependence of other steps in the pump cycle, i.e. in Na+-free external media containing 20 mM Cs+. 2. Na+-K+ pump current (Ip) was absent in cells dialysed with Na+-free pipette solutions and was almost voltage independent at 50 mM Na+pip (potential range: -100 to +40 mV). By contrast, the activation of Ip by 0.5-5 mM Na+pip was clearly voltage sensitive and increased with depolarization, independently of the main intracellular cation species. 3. The apparent affinity of the Na+-K+ pump for cytoplasmic Na+ increased monotonically with depolarization. The [Na+]pip required for half-maximal Ip activation (K0.5 value) amounted to 5.6 mM at -100 mV and to 2.2 mM at +40 mV. 4. The results suggest that cytoplasmic Na+ binding and/or a subsequent partial reaction in the pump cycle prior to Na+ release is voltage dependent. From the voltage dependence of the K0.5 values the dielectric coefficient for intracellular Na+ binding/translocation was calculated to be approximately 0.08. The voltage-dependent mechanism might add to the activation of the cardiac Na+-K+ pump during cardiac excitation.

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.

Sodium dynamics underlying burst firing and putative mechanisms for the regulation of the firing pattern in midbrain dopamine neurons: a computational approach

A physiologically based multicompartmental computational model of a midbrain dopamine (DA) neuron, calibrated using data from the literature, was developed and used to test the hypothesis that sodium dynamics drive the generation of a slow oscillation postulated to underlie NMDA-evoked bursting activity in a slice preparation. The full compartmental model was reduced to three compartments and ultimately to two variables, while retaining the biophysical interpretation of all parameters. A phase-plane analysis then suggested two mechanisms for the regulation of the firing pattern: (1) bursting activity is favored by manipulations that enhance the region of negative slope in the whole-cell IV curve and inhibited by those manipulations, such as increasing linear currents, that tend to dampen this region and (2) assuming a region of negative slope is present in the IV curve, the bias of the system can be altered, either enabling or disabling bursting. The model provides a coherent framework for interpreting the effects of glutamate, aspartate, NMDA, and GABA agonists and antagonists under current-clamp conditions, as well as the effects of NMDA and barium under voltage-clamp conditions.

Sodium pump evokes high density pump currents in rat midbrain dopamine neurons

1. Patch pipettes contained various concentrations of Na+ ([Na+]pip) in order to record strophanthidin-sensitive currents under voltage clamp in dopamine neurons in slices of rat substantia nigra and ventral tegmental area. 2. When [Na+]pip was 40 mM and the external K+ concentration ([K+]o) was 2.5 mM, strophanthidin (10 microM) evoked 461 +/- 121 pA of inward current. This effect was concentration dependent, with an EC50 of 7.1 +/- 2.6 microM. At potentials of -60 to -120 mV, strophanthidin-induced currents were not associated with significant changes in chord conductance. 3. Strophanthidin (10 microM) evoked 234 +/- 43 pA of inward current when [Na+]pip was 0.6 mM, and 513 +/- 77 pA when [Na+]pip was 80 mM. Despite higher pump currents with greater [Na+]pip, the strophanthidin EC50 was not significantly different for any of six different [Na+]pip. 4. Sodium pump currents were half-maximal when the [Na+]pip was about 1.3 mM. Maximum pump current was estimated at 830 pA (29 microA cm-2) at concentrations of intracellular Na+ that were assumed to be saturating (50-100 mM). 5. Strophanthidin currents were smaller in a reduced [K+]o (EC50 = 0.2 mM). 6. These data show that intracellular Na+ loading evokes relatively large pump currents. Our results are consistent with the physiological role of the sodium pump in burst firing in midbrain dopamine neurons

K+-dependence of electrogenic transport by the NaK-ATPase

Charge translocation by the NaK-ATPase from shark rectal gland was measured by adsorption of proteoliposomes to a planar lipid membrane. The proteoliposomes were prepared by reconstitution of purified NaK-ATPase into liposomes consisting of E. coli lipids. The protein was activated by applying an ATP concentration jump produced by photolysis of a protected derivative of ATP, caged ATP. K+ titrations were used to study the effect of K+ on the charge translocation kinetics of the protein. The time-dependent currents obtained after activation of the enzyme with caged ATP were analyzed with a simplified Albers-Post model (E1 (k1)-->E1ATP (k2)-->E2P (k3)-->E1) taking into account the capacitive coupling of the protein to the measuring system. The results of the K+ titrations show a strong dependence of the rate constant k3 on the K+ concentration at the extracellular side of the protein, indicating the K+ activated dephosphorylation reaction. In contrast, k1 and k2 remained constant. The K+ dependence of the rate k3 could be well described with a K+ binding model with two equivalent binding sites (E2P + 2K+ <==> E2P(K) + K+ <==> E2 P(2K)) followed by a rate limiting reaction (E2P(2K) --> E1(2K)). The half saturating K+ concentration K3,0.5 and the microscopic dissociation constant K3 for the K+ dependence of k3 were 4.5mM and 1.9mM respectively. At saturating K+ concentration the rate constant k3 was approximately 100 s(-1). The relative amount of net charge transported during the Na+ and the K+ dependent reactions could be determined from the experiments. Our results suggest electroneutral K+ translocation and do not support electrogenic K+ binding in an extracellular access channel. This is compatible with a model where 2 negative charges are cotransported with 3Na+ and 2K+ ions. Error analysis gives an upper limit of 20% charge transported during K+ translocation or during electrogenic K+ binding in a presumptive access channel compared to Na+ translocation.

Activation of the Na+/K+ pump current by intra- and extracellular Li ions in single guinea-pig cardiac cells

Li+ is the only ion that can replace the physiological intra- and extracellular activator cations of the Na+/K+ pump. In order to study this singular property of Li+ in some detail, the activation of the Na+/K+ pump current (Ip) by intra- and extracellular Li+ (Li+; Li[o]+) was measured in isolated guinea-pig ventricular myocytes by means of whole cell recording at 34 degrees C and a holding potential of -20 mV. Ip was identified as current blocked by dihydro-ouabain. Half-maximal Ip activation occurred at 23 mM Li(o)+ (K0.5 value) in cells containing Na+ (50 or 100 mM) and at 73 mM Li(o)+ in myocytes containing Li+ (100 mM). The K0.5 value of Ip activation by Li(o)+ increased with depolarisation, suggesting the transfer of 0.2 of an elementary charge across the electric field of the sacrolemma during Li(o)+-binding. An intracellular Li+ concentration of 36 mM caused half-maximal Ip activation in cells superfused with Na+- and Li+-free media containing 1 mM K+. In Na+-free solutions. the Ip-V curve displayed a positive slope at negative membrane potentials. A negative slope at positive potentials was observed in Li+-containing media. It is concluded that Li+ is less efficacious and potent than the physiological pump activator cations. The shape of the Ip-V curves in Na+-free solutions supports the view that the cardiac Na+/K+ pump contains a channel-like structure and suggests that there are voltage-sensitive steps in the pump cycle, apart from the binding of external cations.

Na/K pump current in guinea pig cardiac myocytes and the effect of Na leak

INTRODUCTION

Steady-state Na/K pump current (Ip) in adult guinea pig ventricular myocytes was studied to determine the effect on the Na/K pump of transmembrane Na leak, membrane potential, and pipette Na concentration.

METHODS AND RESULTS

Using conventional whole cell, patch clamp techniques, Ip was identified as either Ko-sensitive or ouabain-sensitive current when most other membrane currents were inhibited. Control experiments showed that there were no Ko-sensitive currents other than Ip under the conditions of our experiments. Ip was found to be similar to that reported by others being voltage dependent between -130 and 0 mV and having a half maximal activation by Nai of 28 mM. Ouabain sensitivity was also measured, and it was found that there were two binding sites with the high affinity site comprising 5% to 10% of the total and having an apparent affinity 1000-fold higher than the low affinity site. Apparent affinity of both sites was shifted about 10-fold (higher affinity) by increasing Nai from 10 to 85 mM. When internally perfused with 0 Na solution, Na leak through the membrane was found to be linearly related to Na/K pump activity. In contrast to prior suggestions, Ip was not correlated with series resistance when there was a large transmembrane Na gradient.

CONCLUSION

These data suggest that, under conditions of high transmembrane Na gradient, Na leak through the membrane plays a significant role in determining Na/K pump activity.

Role of intracellular sodium overload in the genesis of cardiac arrhythmias

A number of clinical cardiac disorders may be associated with a rise of the intracellular Na concentration (Na(i)) in heart muscle. A clear example is digitalis toxicity, in which excessive inhibition of the Na/K pump causes the Na(i) concentration to become raised above the normal level. Especially in digitalis toxicity, but also in many other situations, the rise of Na(i) may be an important (or contributory) cause of increased cardiac arrhythmias. In this review, we consider the mechanisms by which a raised Na(i) may cause cardiac arrhythmias. First, we describe the factors that regulate Na(i), and we demonstrate that the equilibrium level of Na(i) is determined by a balance between Na entry into the cell, and Na extrusion from the cell. A number of mechanisms are responsible for Na entry into the cell, whereas the Na/K pump appears to be the main mechanism for Na extrusion. We then consider the processes by which an increased level of Nai might contribute to cardiac arrhythmias. A rise of Na(i) is well known to result in an increase of intracellular Ca, via the important and influential Na/Ca exchange mechanism in the cell membrane of cardiac muscle cells. A rise of intracellular Ca modulates the activity of a number of sarcolemmal ion channels and affects release of intracellular Ca from the sarcoplasmic reticulum, all of which might be involved in causing arrhythmia. It is possible that the increase in contractile force that results from the rise of intracellular Ca may initiate or exacerbate arrhythmia, since this will increase wall stress and energy demands in the ventricle, and an increase in wall stress may be arrhythmogenic. In addition, the rise of Na(i) is anticipated to modulate directly a number of ion channels and to affect the regulation of intracellular pH, which also may be involved in causing arrhythmia. We also present experiments in this review, carried out on the working rat heart preparation, which suggest that a rise of Na(i) causes an increase of wall stress-induced arrhythmia in this model. In addition, we have investigated the effect on wall stress-induced arrhythmia of maneuvers that might be anticipated to change intracellular Ca, and this has allowed identification of some of the factors involved in causing arrhythmia in the working rat heart.

Comparison of ouabain-sensitive and -insensitive Na/K pumps in HEK293 cells

The Na/K pump current I(p) of single HEK293 cells either untransfected (endogenous I(p)) or transfected with the alpha1 subunit of the rat Na/K pump (exogenous I(p)) was investigated in Na-containing solution by means of whole-cell recording at 30 degrees C. The endogenous I(p) was irreversibly blocked by 10(-4) M ouabain or 2 x 10(-4) M dihydro-ouabain (DHO). Its density amounted to 0.33 pA pF(-1) at 0 mV and 5.4 mM K(o). It was half maximally activated at 1.5 mM K(o) and increased linearly with depolarization over the entire voltage range studied (-80 to +60 mV). In contrast, HEK293 cells stably transfected with cDNA for the cardiac glycoside-resistant alpha1 subunit of the rat Na/K pump showed an I(p) in the presence of 10(-4) M ouabain and 2 x 10(-4) M DHO, respectively. This exogenous I(p) was reversibly blocked by 10(-2) M ouabain. Half maximal activation of the exogenous I(p) occurred at 1.7 mM K(o). Its amplitude increased linearly with depolarization at negative voltages but remained almost constant at positive membrane potentials. Comparison with the I(p) of isolated rat cardiac ventricular myocytes strongly suggests that the exogenous I(p) in HEK293 cells is generated by the alpha1 subunit of the rat Na/K pump since it displays identical properties. Therefore, HEK293 cells represent an expression system well suited for the electrophysiological analysis of recombinant, cardiac glycoside-resistant Na/K pumps by means of whole-cell recording.

Mechanisms for depolarization by l-palmitoylcarnitine in single guinea pig ventricular myocytes

INTRODUCTION

The changes of the resting potential (RP) and of the current-voltage (I-V) relationship induced by l-palmitoylcarnitine (l-PC) in the presence of the IKI blocker, cesium, or in the presence of the INa/K blocker, ouabain, were tested in guinea pig ventricular myocytes to ascertain the relative contributions of IKI and INa/K suppression to the membrane depolarization caused by this amphiphile.

METHODS AND RESULTS

Ramp voltages were applied to myocytes with the whole cell, patch clamp technique. l-PC (10 microM) produced additional membrane depolarization in the presence of either 10 mM Cs+ or 30 microM ouabain. In the presence of Cs+, l-PC, like 3 microM ouabain, shifted current inward at potentials negative to -20 mV as a result of INa/K blockade. In the presence of 30 microM ouabain, l-PC, like Cs+, shifted current inward at potentials between -27 and -88 mV and outward at potentials negative to -88 mV. This is attributed to IKI block because the current was inwardly rectifying, with a reversal potential near EK. When l-PC or ouabain inhibited INa/K, the presence of an Ni(2+)-sensitive component attributed to INa/Ca distorted the membrane I-V relationship, particularly in the presence of Cs+. The relative contributions of IKI and INa/K block by l-PC were voltage dependent. At the RP, l-PC produced a greater block of INa/K than of IKI.

CONCLUSION

l-PC depolarizes the resting membrane by inhibiting both IKI and INa/K. It is concluded that suppression of INa/K by l-PC predominates over block of IKI to depolarize the membrane at the RP.

Cardiac Na+ pump current-voltage relationships at various transmembrane gradients of the pumped cations

Thermodynamic considerations predict changes of the Na+ pump current (Ip)-voltage (V) relationship of animal cells upon variations of the electrochemical gradients against which cations must be pumped. Experimental data in support of the predictions are sparse. Therefore, the effect on the Ip-V relationship of various electrochemical gradients for pumped Na+ and Cs+ was studied at constant deltaGATP (approximately -39kJ/mol in cardioballs from sheep Purkinje fibres. Control of the subsarcolemmal ionic concentrations during whole-cell recording was ensured by activation of Ip below its half maximal activity or by measuring the initial Ip following reactivation of the Na+/K+ pump. With gradients close to physiological conditions Ip was outward over the entire voltage range and the Ip-V relationship showed a maximum near zero potential. Steepening the ionic gradients diminished the Ip amplitude and outward pump current was no longer detectable between -65 mV and -110 mV. Flattened ionic gradients increased the Ip amplitude and shifted apparently the reversal potential Erev to more negative values. These changes are in line with theoretical considerations. The measured Ip-V relationships were fitted by curves computed on the basis of a simplified Post-Albers scheme of Na+/Cs+ pumping. The increased Ip amplitude at flat ionic gradients was due to a decrease of [Cs+]o for half maximal Ip activation. The maximal Ip amplitude remained unaffected

Sodium--potassium pump current in rabbit sino-atrial node cells

1. The Na(+)-K+ pump current (Ip) was studied in sino-atrial (SA) node cells of rabbits using the whole-cell patch-clamp technique. 2. With 50 mM Na+ in the pipette solution ([Na+]pip), changing the external K+ concentration (-K+-o) from 0 to 5.4 mM caused the holding current to shift in an outward direction and reach a new steady state. The current-voltage relationships obtained by subtraction of current traces recorded at 0 mM Ko+ from those recorded at 5.4 mM Ko+ revealed time-independent and voltage-dependent characteristics. The external K(+)-induced current was completely blocked by external application of 10 microM ouabain, indicating the existence of Ip in SA node cells of rabbit heart. 3. Ip increased as [K+]o increased. With 30 mM Na+pip, Ip at 0 mV was activated by [K+]o with non-linear least-squares fit parameters for the Hill equation of K0.5 of 1.4 mM and a Hill coefficient (nH) of 1.2 (n = 7). 4. The cation dependence of the K+ site of the Na(+)-K+ pump was examined using various monovalent cations. The sequence was K+ > or = Rb+ > Cs+ > > > Li+. 5. Ip at 0 mV also increased as [Na+]pip was increased from 10 to 150 mM at 5.4 mM Ko+, with a K0.5 value of 14 mM and a nH of 1.3 (n = 54). 6. Ip at 0 mV was reduced by lowering the temperature from 37 to 25 degrees C with 30 mM Na+pip and 5.4 mM Ko+. The temperature coefficient (Q10) for Ip was 2.1 (n = 27). 7. With 10 mM Na+pip and 5.4 mM Ko+, the half-activation voltage of Ip was -52 +/- 16 mV and the current at this voltage was 22.5 +/- 3.5 pA (n = 10), indicating that Ip contributes significantly to the background outward current during the normal pacemaker potential of SA node cells.

Change of Na+ pump current reversal potential in sheep cardiac Purkinje cells with varying free energy of ATP hydrolysis

1. The Na(+)-K+ pump current, Ip, of cardioballs from isolated sheep cardiac Purkinje cells was measured at 30-34 degrees C by means of whole-cell recording. 2. Under physiological conditions Ip is an outward current. Experimental conditions which cause a less negative free energy of intracellular ATP hydrolysis (delta GATP) and steeper sarcolemmal gradients for the pumped Na+ and Cs+ ions evoked an Ip in the inward direction over a wide range of membrane potentials. The reversal of the Ip direction was reversible. 3. The inwardly directed Ip increased with increasingly negative membrane potentials and amounted to -0.13 +/- 0.03 microA cm-2 (mean +/- S.E.M.; n = 6) at -95 mV. 4. The reversal potential (Erev) of Ip was studied as a function of delta GATP at constant sarcolemmal gradients of the pumped cations. 5. In order to vary delta GATP the cell interior was dialysed with patch pipette solutions containing 10 mM ATP and different concentrations of ADP and inorganic phosphate. The media were composed to produce delta GATP levels of about -58, -49 and -39 kJ mol-1. 6. A less negative delta GATP shifted Erev to more positive membrane potentials. From measurements of Ip as a function of membrane potential Erev was estimated to be -195, -115 and -60 mV at delta GATP levels of approximately -58, -49 and -39 kJ mol-1, respectively. The calculated Erev amounted to -224 mV at delta GATP approximately -58 kJ mol-1, -126 mV at delta GATP approximately 49 kJ mol-1 and -24 mV at delta GATP approximately -39 kJ mol-1. 7. Possible reasons for the discrepancy between estimated and calculated Erev values are discussed. 8. Shifting delta GATP to less negative values not only altered Erev but also diminished Ip at each membrane potential tested. The maximal Ip (Ip,max), which can be activated by external Cs+ (Cs+o), decreased under these conditions, whereas [Cs+]o causing half-maximal Ip activation remained unchanged. Similarly, the voltage dependence of Ip activation by Cs+o was unaffected. 9. It is concluded that Erev of Ip varies with delta GATP at constant sarcolemmal gradients of the pumped cations. This agrees with thermodynamic considerations.

Na+ pump current-voltage relationships of rabbit cardiac Purkinje cells in Na(+)-free solution

1. The Na+ pump current (Ip) of isolated, single rabbit cardiac Purkinje cells in Na(+)-free solution was measured at 32-34 degrees C by means of whole-cell recording. 2. The Ip amplitude was studied as a function of clamp potential (Vc) and external concentration of various monovalent cations known to activate the Na(+)-K+ pump. 3. Under conditions which strongly activated Ip the Ip-Vc curve of the cells displayed a positive slope at membrane potentials negative to -20 mV and little variation at more positive potentials. 4. The Ip-Vc relationship showed an extended region of negative slope at positive and negative potentials in solutions containing low concentrations of activator cations which caused little Ip activation. A positive slope of the Ip-Vc curve was occasionally observed at clamp potentials negative to -60 mV under these conditions. 5. The shape of the Ip-Vc relation was independent of the cation species used as external Ip activator. 6. At zero membrane potential half-maximum Ip activation (K0.5(Vc = 0 mV) occurred at 0.05 mM Tl+, 0.08 mM K+, 0.4 mM NH4+ and 1.5 mM Cs+. The Hill coefficient derived amounted to 0.9 for Tl+, 1.2 for K+, 1.04 for NH4+ and 1.5 for Cs+. 7. The concentrations of external activator cations required for half-maximum Ip activation increased with depolarization. The voltage dependence of the K0.5 values could be described by a single exponential function for clamp potentials positive to -40 mV. 8. The steepness of the function is determined by a factor alpha, indicating the apparent fraction of an elementary charge which moves in the electrical field across the sarcolemma when external monovalent cations bind to the Na(+)-K+ pump. 9. The alpha values were calculated to be 0.32 for Tl+, 0.24 for K+, 0.29 for NH4+ and 0.18 for Cs+. Possible interpretations of the alpha values are considered. 10. It is suggested that binding of external monovalent activator cations to the Na(+)-K+ pump (or a process related to the binding) is voltage dependent. This potential-dependent process determines mainly the shape of the Ip-Vc curve in cardiac Purkinje cells superfused with Na(+)-free media containing low concentrations (< K0.5(Vc = 0 mV)) of K+ or its congeners.

The Na+/K+ pump of cardiac Purkinje cells is preferentially fuelled by glycolytic ATP production

The role of glycolysis and oxidative phosphorylation in providing the ATP for the cardiac Na+/K+ pump was studied in cardioballs from sheep Purkinje fibres. As an indicator of the pump activity, the pump current Ip was measured at -20 mV and 30-33 degrees C by means of whole-cell recording. During intracellular perfusion with a pipette solution containing 5 mM ATP and 15 mM glucose Ip reached a maximum within 8 min and declined to 50% of this value within 27 min after gaining access to the cell interior. Perfusion with an ATP- and glucose-free medium barely enhanced the Ip decline. Inhibition of the oxidative phosphorylation by carbonylcyanide m-chlorophenylhydrazone (CCCP, 2 microM or 20 microM) moderately accelerated the effect of the ATP- and glucose-free pipette solution. Addition of 2 mM iodoacetic acid (an inhibitor of glycolysis) to the latter medium further enhanced the Ip decrease with time. Inhibition of the glycolytic ATP synthesis by 2-deoxy-D-glucose (5 mM) caused a dramatic decline of Ip to half of its maximum within 7.3 min. Pyruvate (5 mM) and inorganic phosphate (2 mM) did not affect the fast Ip decline evoked by the ATP- and glucose-free, 2-deoxyglucose-containing medium, whereas 2 microM CCCP still hastened the fast Ip decrease slightly. This effect of complete metabolic inhibition was reversed by switching to an inhibitor-free pipette solution containing 15 mM ATP. It is concluded that the Na+/K+ pump of cardiac Purkinje cells is preferentially fuelled by glycolytic ATP synthesis.

Dependence of Na+ pump current on external monovalent cations and membrane potential in rabbit cardiac Purkinje cells

1. The effect of membrane potential and various extracellular monovalent cations on the Na+ pump current (Ip) was studied on isolated, single Purkinje cells of the rabbit heart by means of whole-cell recording. 2. Ip was identified as current activated by external K+ or its congeners NH4+ and Tl+. The current was blocked by dihydroouabain (1-5 x 10(-4) M) over the whole range of membrane potentials tested. 3. In Na(+)-containing solution half-maximum Ip activation (K0.5) occurred at 0.4 mM-Tl+, 1.9 mM-K+ and 5.7 mM-NH4+ (holding potential, -20 mV). 4. The pump current (Ip)-voltage (V) relationship of the cells in Na(+)-containing media with K+ or its congeners at the tested concentrations greater than K0.5 displayed a steep positive slope at negative membrane potentials between -120 and -20 mV. Little voltage dependence of Ip was observed at more positive potentials up to +40 mV. At even more positive potentials Ip measured at 2 and 5.4 mM-K+ decreased. 5. Lowering the concentration of K+ or its congeners below the K0.5 value in Na(+)-containing solution induced a region of negative slope of the Ip-V curve at membrane potentials positive to -20 mV. 6. The shape of the Ip-V relationship remained unchanged when the K+ concentration (5.4 mM) of the Na(+)-containing medium was replaced by NH4+ or Tl+ concentrations of similar potency to activate Ip (20 mM-NH4+ or 2 mM-Tl+). 7. In Na(+)-free, choline-containing solution half-maximum Ip activation occurred at 0.13 mM-K+ (holding potential, -20 mV). 8. At negative membrane potentials the positive slope of the Ip-V curve was flatter in Na(+)-free than in Na(+)-containing media. A reduced voltage dependence of Ip persisted, regardless of whether choline ions or Li+ were used as a Na+ substitute. 9. Lowering the K+ concentration of the Na(+)-free, choline-containing solution to 0.05 mM evoked an extended region of negative slope in the Ip-V relationship at membrane potentials between -40 and +60 mV. 10. It is concluded that the apparent affinity of the Na(+)-K+ pump towards K+ in cardiac Purkinje cells depends on both the membrane potential and the extracellular Na+ concentration. 11. The region of negative slope of the Ip-V curve observed in cells which were superfused with media containing low concentrations of K+ or its congeners strongly suggests the existence of at least two voltage-sensitive steps in the cardiac Na(+)-K+ pump cycle.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Charge translocation by the Na,K-pump: I. Kinetics of local field changes studied by time-resolved fluorescence measurements

Membrane fragments containing a high density of Na,K-ATPase can be noncovalently labeled with amphiphilic styryl dyes (e.g., RH 421). Phosphorylation of the Na,K-ATPase by ATP in the presence of Na+ and in the absence of K+ leads to a large increase of the fluorescence of RH 421 (up to 100%). In this paper evidence is presented that the styryl dye mainly responds to changes of the electric field strength in the membrane, resulting from charge movements during the pumping cycle: (i) The spectral characteristic of the ATP-induced dye response essentially agrees with the predictions for an electrochromic shift of the absorption peak. (ii) Adsorption of lipophilic anions to Na,K-ATPase membranes leads to an increase, adsorption of lipophilic cations to the decrease of dye fluorescence. These ions are known to bind to the hydrophobic interior of the membrane and to change the electric field strength in the boundary layer close to the interface. (iii) The fluorescence change that is normally observed upon phosphorylation by ATP is abolished at high concentrations of lipophilic ions. Lipophilic ions are thought to redistribute between the adsorption sites and water and to neutralize in this way the change of field strength caused by ion translocation in the pump protein. (iv) Changes of the fluorescence of RH 421 correlate with known electrogenic transitions in the pumping cycle, whereas transitions that are known to be electrically silent do not lead to fluorescence changes. The information obtained from experiments with amphiphilic styryl dyes is complementary to the results of electrophysiological investigations in which pump currents are measured as a function of transmembrane voltage. In particular, electrochromic dyes can be used for studying electrogenic processes in microsomal membrane preparations which are not amenable to electrophysiological techniques.

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)

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.

[Na] and [K] dependence of the Na/K pump current-voltage relationship in guinea pig ventricular myocytes

Na/K pump current was determined between -140 and +60 mV as steady-state, strophanthidin-sensitive, whole-cell current in guinea pig ventricular myocytes, voltage-clamped and internally dialyzed via wide-tipped pipettes. Solutions were designed to minimize all other components of membrane current. A device for exchanging the solution inside the pipette permitted investigation of Na/K pump current-voltage (I-V) relationships at several levels of pipette [Na] [( Na]pip) in a single cell; the effects of changes in external [Na] [( Na]o) or external [K] [( K]o) were also studied. At 50 mM [Na]pip, 5.4 mM [K]o, and approximately 150 mM [Na]o, Na/K pump current was steeply voltage dependent at negative potentials but was approximately constant at positive potentials. Under those conditions, reduction of [Na]o enhanced pump current at negative potentials but had little effect at positive potentials: at zero [Na]o, pump current was only weakly voltage dependent. At 5.4 mM [K]o and approximately 150 mM [Na]o, reduction of [Na]pip from 50 mM scaled down the sigmoid pump I-V relationship and shifted it slightly to the right (toward more positive potentials). Pump current at 0 mV was activated by [Na]pip according to the Hill equation with best-fit K0.5 approximately equal to 11 mM and Hill coefficient nH approximately equal to 1.4. At zero [Na]o, reduction of [Na]pip seemed to simply scale down the relatively flat pump I-V relationship: Hill fit parameters for pump activation by [Na]pip at 0 mV were K0.5 approximately equal to 10 mM, nH approximately equal to 1.4. At 50 mM [Na]pip and high [Na]o, reduction of [K]o from 5.4 mM scaled down the sigmoid I-V relationship and shifted it slightly to the right: at 0 mV, K0.5 approximately equal to 1.5 mM and nH approximately equal to 1.0. At zero [Na]o, lowering [K]o simply scaled down the flat pump I-V relationships yielding, at 0 mV, K0.5 approximately equal to 0.2 mM, nH approximately equal to 1.1. The voltage-independent activation of Na/K pump current by both intracellular Na ions and extracellular K ions, at zero [Na]o, suggests that neither ion binds within the membrane field. Extracellular Na ions, however, seem to have both a voltage-dependent and a voltage-independent influence on the Na/K pump: they inhibit outward Na/K pump current in a strongly voltage-dependent fashion, with higher apparent affinity at more negative potentials (K0.5 approximately equal to 90 mM at -120 mV, and approximately 170 mM at -80 mV), and they compete with extracellular K ions in a seemingly voltage-independent manner.(ABSTRACT TRUNCATED AT 400 WORDS)

Activation of the Na pump current by external K and Cs ions in cardioballs from sheep Purkinje fibres

The Na pump current (Ip) was measured at 30-33 degrees C as a function of the extracellular K+ or Cs+ concentrations ([K]o; [Cs]o) by means of whole cell recording from cardioballs isolated from sheep Purkinje fibres. The results show that the magnitude of Ip is an s-shaped, saturating function of [K]o or [Cs]o, respectively. Half maximal Ip activation occurs at 1.2 mM Ko or 3.2 mM Cso (clamp potential: -20 mV). These values are appreciably lower than earlier measurements on sheep Purkinje fibres indicated. The Hill coefficients for Ip activation by external K+ or Cs+ were calculated to be 1.94 and 1.73, respectively. The numbers suggest that Ip activation requires at least two external activator cations.