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

A transmural gradient in the cardiac Na/K pump generates a transmural gradient in Na/Ca exchange

We previously demonstrated a transmural gradient in Na/K pump current (I (P)) and [Na(+)]( i ), with the highest maximum I (P) and lowest [Na(+)]( i ) in epicardium. The present study examines the relationship between the transmural gradient in I (P) and Na/Ca exchange (NCX). Myocytes were isolated from canine left ventricle. Whole-cell patch clamp was used to measure current generated by NCX (I (NCX)) and inward background calcium current (I (ibCa)), defined as inward current through Ca(2+) channels less outward current through Ca(2+)-ATPase. When resting myocytes from endocardium (Endo), midmyocardium (Mid) or epicardium (Epi) were studied in the same conditions, I (NCX) was the same and I (ibCa) was zero. Moreover, Western blots were consistent with NCX protein being uniform across the wall. However, the gradient in [Na(+)]( i ), with I (ibCa) = 0, should create a gradient in [Ca(2+)]( i ). To test this hypothesis, we measured resting [Ca(2+)]( i ) using two methods, based on either transport or the Ca(2+)-sensitive dye Fura2. Both methods demonstrated a significant transmural gradient in resting [Ca(2+)]( i ), with Endo > Mid > Epi. This gradient was eliminated by exposing Epi to sufficient ouabain to partially inhibit Na/K pumps, thus increasing [Na(+)]( i ) to values similar to those in Endo. These data support the existence of a transmural gradient for Ca(2+) removal by NCX. This gradient is not due to differences in expression of NCX; rather, it is generated by a transmural gradient in [Na(+)]( i ), which is due to a transmural gradient in plasma membrane expression of the Na/K pump.

Transmural gradients in Na/K pump activity and [Na+]I in canine ventricle

There are well-documented differences in ion channel activity and action potential shape between epicardial (EPI), midmyocardial (MID), and endocardial (ENDO) ventricular myocytes. The purpose of this study was to determine if differences exist in Na/K pump activity. The whole cell patch-clamp was used to measure Na/K pump current (I(P)) and inward background Na(+)-current (I(inb)) in cells isolated from canine left ventricle. All currents were normalized to membrane capacitance. I(P) was measured as the current blocked by a saturating concentration of dihydro-ouabain. [Na(+)](i) was measured using SBFI-AM. I(P)(ENDO) (0.34 +/- 0.04 pA/pF, n = 17) was smaller than I(P)(EPI) (0.68 +/- 0.09 pA/pF, n = 38); the ratio was 0.50 with I(P)(MID) being intermediate (0.53 +/- 0.13 pA/pF, n = 19). The dependence of I(P) on [Na(+)](i) or voltage was essentially identical in EPI and ENDO (half-maximal activation at 9-10 mM [Na(+)](i) or approximately -90 mV). Increasing [K(+)](o) from 5.4 to 15 mM caused both I(P)(ENDO) and I(P)(EPI) to increase, but the ratio remained approximately 0.5. I(inb) in EPI and ENDO were nearly identical ( approximately 0.6 pA/pF). Physiological [Na(+)](i) was lower in EPI (7 +/- 2 mM, n = 31) than ENDO (12 +/- 3 mM, n = 29), with MID being intermediate (9 +/- 3 mM, n = 22). When cells were paced at 2 Hz, [Na(+)](i) increased but the differences persisted (ENDO 14 +/- 3 mM, n = 10; EPI 9 +/- 2 mM, n = 10; and MID intermediate, 11 +/- 2 mM, n = 9). Based on these results, the larger I(P) in EPI appears to reflect a higher maximum turnover rate, which implies either a larger number of active pumps or a higher turnover rate per pump protein. The transmural gradient in [Na(+)](i) means physiological I(P) is approximately uniform across the ventricular wall, whereas transporters that utilize the transmembrane electrochemical gradient for Na(+), such as Na/Ca exchange, have a larger driving force in EPI than ENDO.

Isoform-specific stimulation of cardiac Na/K pumps by nanomolar concentrations of glycosides

It is well-known that micromolar to millimolar concentrations of cardiac glycosides inhibit Na/K pump activity, however, some early reports suggested nanomolar concentrations of these glycosides stimulate activity. These early reports were based on indirect measurements in multicellular preparations, hence, there was some uncertainty whether ion accumulation/depletion rather than pump stimulation caused the observations. Here, we utilize the whole-cell patch-clamp technique on isolated cardiac myocytes to directly measure Na/K pump current (I(P)) in conditions that minimize the possibility of ion accumulation/depletion causing the observed effects. In guinea pig ventricular myocytes, nanomolar concentrations of dihydro-ouabain (DHO) caused an outward current that appeared to be due to stimulation of I(P) because of the following: (1) it was absent in 0 mM [K(+)](o), as was I(P); (2) it was absent in 0 mM [Na(+)](i), as was I(P); (3) at reduced [Na(+)](i), the outward current was reduced in proportion to the reduction in I(P); (4) it was eliminated by intracellular vanadate, as was I(P). Our previous work suggested guinea pig ventricular myocytes coexpress the alpha(1)- and alpha(2)-isoforms of the Na/K pumps. The stimulation of I(P) appears to be through stimulation of the high glycoside affinity alpha(2)-isoform and not the alpha(1)-isoform because of the following: (1) regulatory signals that specifically increased activity of the alpha(2)-isoform increased the amplitude of the stimulation; (2) regulatory signals that specifically altered the activity of the alpha(1)-isoform did not affect the stimulation; (3) changes in [K(+)](o) that affected activity of the alpha(1)-isoform, but not the alpha(2)-isoform, did not affect the stimulation; (4) myocytes from one group of guinea pigs expressed the alpha(1)-isoform but not the alpha(2)-isoform, and these myocytes did not show the stimulation. At 10 nM DHO, total I(P) increased by 35 +/- 10% (mean +/- SD, n = 18). If one accepts the hypothesis that this increase is due to stimulation of just the alpha(2)-isoform, then activity of the alpha(2)-isoform increased by 107 +/- 30%. In the guinea pig myocytes, nanomolar ouabain as well as DHO stimulated the alpha(2)-isoform, but both the stimulatory and inhibitory concentrations of ouabain were approximately 10-fold lower than those for DHO. Stimulation of I(P) by nanomolar DHO was observed in canine atrial and ventricular myocytes, which express the alpha(1)- and alpha(3)-isoforms of the Na/K pumps, suggesting the other high glycoside affinity isoform (the alpha(3)-isoform) also was stimulated by nanomolar concentrations of DHO. Human atrial and ventricular myocytes express all three isoforms, but isoform affinity for glycosides is too similar to separate their activity. Nevertheless, nanomolar DHO caused a stimulation of I(P) that was very similar to that seen in other species. Thus, in all species studied, nanomolar DHO caused stimulation of I(P), and where the contributions of the high glycoside affinity alpha(2)- and alpha(3)-isoforms could be separated from that of the alpha(1)-isoform, it was only the high glycoside affinity isoform that was stimulated. These observations support early reports that nanomolar concentrations of glycosides stimulate Na/K pump activity, and suggest a novel mechanism of isoform-specific regulation of I(P) in heart by nanomolar concentrations of endogenous ouabain-like molecules.

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.

Dynamics and consequences of potassium shifts in skeletal muscle and heart during exercise

Since it became clear that K(+) shifts with exercise are extensive and can cause more than a doubling of the extracellular [K(+)] ([K(+)](s)) as reviewed here, it has been suggested that these shifts may cause fatigue through the effect on muscle excitability and action potentials (AP). The cause of the K(+) shifts is a transient or long-lasting mismatch between outward repolarizing K(+) currents and K(+) influx carried by the Na(+)-K(+) pump. Several factors modify the effect of raised [K(+)](s) during exercise on membrane potential (E(m)) and force production. 1) Membrane conductance to K(+) is variable and controlled by various K(+) channels. Low relative K(+) conductance will reduce the contribution of [K(+)](s) to the E(m). In addition, high Cl(-) conductance may stabilize the E(m) during brief periods of large K(+) shifts. 2) The Na(+)-K(+) pump contributes with a hyperpolarizing current. 3) Cell swelling accompanies muscle contractions especially in fast-twitch muscle, although little in the heart. This will contribute considerably to the lowering of intracellular [K(+)] ([K(+)](c)) and will attenuate the exercise-induced rise of intracellular [Na(+)] ([Na(+)](c)). 4) The rise of [Na(+)](c) is sufficient to activate the Na(+)-K(+) pump to completely compensate increased K(+) release in the heart, yet not in skeletal muscle. In skeletal muscle there is strong evidence for control of pump activity not only through hormones, but through a hitherto unidentified mechanism. 5) Ionic shifts within the skeletal muscle t tubules and in the heart in extracellular clefts may markedly affect excitation-contraction coupling. 6) Age and state of training together with nutritional state modify muscle K(+) content and the abundance of Na(+)-K(+) pumps. We conclude that despite modifying factors coming into play during muscle activity, the K(+) shifts with high-intensity exercise may contribute substantially to fatigue in skeletal muscle, whereas in the heart, except during ischemia, the K(+) balance is controlled much more effectively.

Ionic basis for action potential prolongation by phenylephrine in canine epicardial myocytes

INTRODUCTION

In canine ventricle, alpha-adrenergic agonists prolong action potential duration (APD) without any effect on the action potential notch, suggesting that, in this species, the effect on repolarization might be independent of inhibition of I(to). The present study investigated the action of the alpha-adrenergic agonist phenylephrine on the action potential and the repolarizing currents I(to) and I(K) in isolated canine epicardial myocytes.

METHODS AND RESULTS

Isolated cells from canine epicardial tissue, and Purkinje fibers, were studied with the whole cell, voltage clamp method. Phenylephrine 0.1 microM increased APD by 13% +/- 4% at 90% repolarization without affecting the notch or amplitude. Under voltage clamp, concentrations of phenylephrine as high as 10 microM had no effect on I(to) in canine epicardial myocytes. However, I(to) of isolated canine Purkinje myocytes was reduced to 69% +/- 7% of control by 1 microM phenylephrine. Further studies in canine epicardial myocytes revealed an action of phenylephrine to inhibit I(K), and in particular I(Ks). Using a voltage protocol that included a two-step repolarization to separate I(Ks) and I(Kr) tail components, the largely I(Kr) component was not significantly affected by 1 microM phenylephrine, whereas the largely I(Ks) component was reduced to 81% +/- 5% of control value.

CONCLUSION

Alpha-adrenergic prolongation of repolarization in canine epicardium does not result from inhibition of I(to). Rather, it appears that reduction of I(Ks) contributes to the action of phenylephrine. The unresponsiveness of epicardial I(to) is not a general characteristic of the canine heart, because Purkinje myocyte I(to) was inhibited, suggesting regional differences in the molecular basis of I(to) and/or alpha-adrenergic signaling in the canine heart.

Sodium-potassium pump current in smooth muscle cells from mesenteric resistance arteries of the guinea-pig

1. The Na+-K+ pump current was studied in smooth muscle cells from mesenteric resistance arteries of guinea-pigs by the use of the perforated patch-clamp technique in the presence of blockers for various ion channels and exchangers. 2. When the Na+ concentration in the pipette solution ([Na+]i) was 50 mM, an increase in the extracellular K+ concentration ([K+]o) from 0 to 10 mM caused an outward current. Both the removal of K+ from the bath solution and the application of 10 microM ouabain abolished this current. Thus, this K+-induced and ouabain-sensitive current was considered to be the Na+-K+ pump current. 3. The amplitude of the Na+-K+ pump current increased as the membrane potential was made more positive until around 0 mV, while the amplitude saturated at more positive potentials than 0 mV. 4. An increase in [K+]o or [Na+]i amplified the Na+-K+ pump current. For [K+]o, the binding constant (Kd) was 1.6+/-0.3 mM and the Hill coefficient (nH) was 1.1+/-0.2 (n = 6). For [Na+]i, Kd was 22+/-5 mM and nH was 1.7+/-0.5 (n = 4-19). 5. The presence of various monovalent cations other than Na+ in the bath solution also evoked the Na+-K+ pump current. The order of potency was K+ >= Rb+ > Cs+ >> Li+. 6. Ouabain inhibited the Na+-K+ pump current in a dose-dependent manner with a Kd of 0.35+/-0.03 microM and an nH of 1.2+/-0.1 (n = 6-8). 7. The Na+-K+ pump current increased as temperature increased. The temperature coefficient (Q10; 26-36 C) was 1.87 (n = 9). 8. In summary the present study characterized for the first time the Na+-K+ pump current in vascular smooth muscle cells by the use of the voltage-clamp method. The use of this method should provide essential information for Na+,K+-ATPase-mediated changes in the cell functions of vascular smooth muscle 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.

Dynamic Ca2+-induced inward rectification of K+ current during the ventricular action potential

Inward rectification, an important determinant of cell excitability, can result from channel blockade by intracellular cations, including Ca2+. However, mostly on the basis of indirect arguments, Ca2+-mediated rectification of inward rectifier K+ current (IK1) is claimed to play no role in the mammalian heart. The present study investigates Ca2+-mediated IK1 rectification during the mammalian ventricular action potential. Guinea pig ventricular myocytes were patch-clamped in the whole-cell configuration. The action potential waveform was recorded and then applied to reproduce normal excitation under voltage-clamp conditions. Subtraction currents obtained during blockade of K+ currents by either 1 mmol/L Ba2+ (IBa) or K+-free solution (I0K) were used to estimate IK1. Similar time courses were observed for IBa and I0K; both currents were strongly reduced during depolarization (inward rectification). Blockade of L-type Ca2+ current by dihydropyridines (DHPs) increased systolic IBa and I0K by 50.7% and 254.5%, respectively. beta-Adrenergic stimulation, when tested on I0K, had an opposite effect; ie, it reduced this current by 66.5%. Ryanodine, an inhibitor of sarcoplasmic Ca2+ release, increased systolic IBa by 47.7%, with effects similar to those of DHPs. Intracellular Ca2+ buffering (BAPTA-AM) increased systolic IBa by 87.7% and blunted the effect of DHPs. Thus, IK1 may be significantly reduced by physiological Ca2+ transients determined by both Ca2+ influx and release. Although Ca2+-induced effects may represent only a small fraction of total IK1 rectification, they are large enough to affect excitability and repolarization. They may also contribute to facilitation of early afterdepolarizations by conditions increasing Ca2+ influx.

Regulation of Na(+)-K+ pump activity in contracting rat muscle

1. In rat soleus muscle, high frequency electrical stimulation produced a rapid increase in intracellular Na+ (Na+i) content. This was considerably larger in muscles contracting without developing tension than in muscles contracting isometrically. During subsequent rest a net extrusion of Na+ took place at rates which, depending on the frequency and duration of stimulation, approached the maximum transport capacity of the Na(+)-K+ pumps present in the muscle. 2. In isometrically contracting muscles, the net extrusion of Na+ continued for up to 10 min after stimulation, reducing Na+i to values 30% below the resting level (P < 0.001). This undershoot in Na+i, seen in both soleus and extensor digitorum longus muscles, could be maintained for up to 30 min and was blocked by ouabain or cooling to 0 degree C. 3. The undershoot in Na+i could be elicited by direct stimulation as well as by tubocurarine-suppressible stimulation via the motor endplate. It could not be attributed to a decrease in Na+ influx, to effects of noradrenaline or calcitonin gene-related peptide released from nerve endings, to an increase in extracellular K+ or the formation of nitric oxide. 4. The results indicate that excitation rapidly activates the Na(+)-K+ pump, partly via a change in its transport characteristics and partly via an increase in intracellular Na+ concentration. This activation allows an approximately 20-fold increase in the rate of Na+ efflux to take place within 10 s. 5. The excitation-induced activation of the Na(+)-K+ pump may represent a feed-forward mechanism that protects the Na(+)-K+ gradients and the membrane potential in working muscle. Contrary to previous assumptions, the Na(+)-K+ pump seems to play a dynamic role in maintenance of excitability during contractile activity.

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.

Amino acid substitutions in the rat Na+, K(+)-ATPase alpha 2-subunit alter the cation regulation of pump current expressed in HeLa cells

1. To study the functional role of negatively charged amino acids (E327 and D925) located in the transmembrane region of the rat alpha 2-isoform of the Na+, K(+)-ATPase (rat alpha 2*) in ion transport, the effects of mutations on external K+ dependence and internal Na+ dependence of pump currents were assessed by the patch-clamp technique in combination with a system for rapid solution changes. 2. Amino acid residues were replaced by glutamine (E327Q) or leucine (D925L) and were introduced into rat alpha 2* cDNA which encodes a ouabain-resistant isoform. These mutant enzymes were stably expressed in HeLa cells. The endogenous ouabain-sensitive HeLa cell Na+, K(+)-ATPase activity was selectively inhibited by 1 microM ouabain present in both the growing media and the assay solution. 3. External K(+)- and internal Na(+)-dependent pump activation was observed in all cells expressing rat alpha 2*, E327Q or D925L; however, the apparent affinities were significantly reduced by the mutations. 4. In E327Q, the activation of pump current was slightly slower than for rat alpha 2*, whereas the deactivation rate was faster. In contrast, D925L produced pump current having dramatically slower activation and deactivation kinetics. 5. These results indicate that these negatively charged amino acids (E327 and D925) are important in cation-induced conformational changes of the protein, which are intermediate steps in the pump mechanism.

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.

The Na/K pump, resting potential and selective permeability in canine Purkinje fibres at physiologic and room temperatures

All mammalian cells maintain a resting potential generated by ions moving down concentration gradients. In excitable cells, the inside potential is negative relative to outside. In order to maintain this electrochemical gradient, the sodium potassium (Na/K) pump actively transports out three sodium ions for every two potassium ions it brings in. This process generates a net outward current and thus hyperpolarizes the resting potential. I employed dihydroouabain (DHO) to inhibit the Na/K pump and thus measure its contribution to the resting potential. It contributed 9.0 mV at 34 degrees C and 3.8 mV at 25 degrees C. The PK/PNa ratios were calculated at both temperatures before and after subtracting the Na/K pump contribution. These ratios also suggested a decreased contribution of the Na/K pump under hypothermia. Taken together, these results suggest that the pump contribution to the resting potential is more significant at physiologic temperatures (34 degrees C) than at room temperature (25 degrees C), and that estimates of selective permeability can only be accurately obtained after assessing and eliminating the Na/K pump contribution to the resting potential.

Pacemaker current i(f) in adult canine cardiac ventricular myocytes

1. Single cells enzymatically isolated from canine ventricle and canine Purkinje fibres were studied with the whole-cell patch clamp technique, and the properties of the pacemaker current i(f) compared. 2. Steady-state i(f) activation occurred in canine ventricular myocytes at more negative potentials (-120 to -170 mV) than in canine Purkinje cells (-80 to -130 mV). 3. Reversal potentials were obtained in various extracellular Na+ (140, 79 or 37 mM) and K+ concentrations (25, 9 or 5.4 mM) to determine the ionic selectivity of i(f) in the ventricle. The results suggest that this current was carried by both sodium and potassium ions. 4. The plots of the time constants of i(f) activation against voltage were 'bell shaped' in both canine ventricular and Purkinje myocytes. The curve for the ventricular myocytes was shifted about 30 mV in the negative direction. In both ventricular and Purkinje myocytes, the fully activated I-V relationship exhibited outward rectification in 5.4 mM extracellular K+. 5. Calyculin A (0.5 microM) increased i(f) by shifting its activation to more positive potentials in ventricular myocytes. Protein kinase inhibition by H-7 (200 microM) or H-8 (100 microM) reversed the positive voltage shift of i(f) activation. This effect of calyculin A also occurred when the permeabilized patch was used for whole-cell recording. 6. These results indicate i(f) is present in ventricular myocytes. If shifted to more positive potentials i(f) could play a role in ischaemia-induced ventricular arrhythmias. The negative shift of i(f) in the ventricle might play a role in differentiating non-pacing regions of the heart from those regions that pace.

A dynamic model of the cardiac ventricular action potential. I. Simulations of ionic currents and concentration changes

A mathematical model of the cardiac ventricular action potential is presented. In our previous work, the membrane Na+ current and K+ currents were formulated. The present article focuses on processes that regulate intracellular Ca2+ and depend on its concentration. The model presented here for the mammalian ventricular action potential is based mostly on the guinea pig ventricular cell. However, it provides the framework for modeling other types of ventricular cells with appropriate modifications made to account for species differences. The following processes are formulated: Ca2+ current through the L-type channel (ICa), the Na(+)-Ca2+ exchanger, Ca2+ release and uptake by the sarcoplasmic reticulum (SR), buffering of Ca2+ in the SR and in the myoplasm, a Ca2+ pump in the sarcolemma, the Na(+)-K+ pump, and a nonspecific Ca(2+)-activated membrane current. Activation of ICa is an order of magnitude faster than in previous models. Inactivation of ICa depends on both the membrane voltage and [Ca2+]i. SR is divided into two subcompartments, a network SR (NSR) and a junctional SR (JSR). Functionally, Ca2+ enters the NSR and translocates to the JSR following a monoexponential function. Release of Ca2+ occurs at JSR and can be triggered by two different mechanisms, Ca(2+)-induced Ca2+ release and spontaneous release. The model provides the basis for the study of arrhythmogenic activity of the single myocyte including afterdepolarizations and triggered activity. It can simulate cellular responses under different degrees of Ca2+ overload. Such simulations are presented in our accompanying article in this issue of Circulation Research.

Electrogenic Na-K pump current in rat skeletal myoballs

Catecholamines and insulin have been reported to hyperpolarize skeletal muscle fibers via stimulation of the electrogenic Na-K pump (Flatman and Clausen, 1979, Nature, 281:580-581). Therefore, the electrogenic Na-K pump current was investigated in cultured colcemid-treated rat skeletal myoballs using whole-cell voltage clamp. Skeletal muscles were taken from newborn rat hindlegs, trypsin digested, and cultured. By day 7, all myoblast cells fused into myotubes. After treatment with the microtubule disrupter colcemid (10(-7) M) for 2 days, some of the myotubes became transformed into spherical myoballs, having an average diameter of 41.2 +/- 1.5 microns (n = 21). The resting membrane potential averaged -56.8 +/- 1.7 mV (n = 40). Ouabain (1 mM) quickly depolarized the myoballs to -51.1 +/- 1.1 mV (n = 27), showing the existence of an electrogenic Na-K pump in the skeletal myoball preparation. The values for the specific membrane resistance and capacitance were 5.5 +/- 1.0 K omega-cm2 (n = 21) and 3.7 +/- 0.3 microF/cm2 (n = 21), respectively. The pump current averaged 0.28 +/- 0.03 pA/pF (n = 10), with the membrane potential at -60 mV and 10 mM intrapipette Na+. The Na-K pump contribution to resting membrane potential was calculated to be 5.7 mV, matching the ouabain-induced rapid depolarization. When the Na-K pump was stimulated with 50 mM intrapipette Na+, the pump current was about doubled (0.52 +/- 0.08 pA/pF; n = 10). Isoproterenol (1 microM) and 8-Br-cAMP (1 mM) also significantly increased pump current by 50% (0.42 +/- 0.04 pA/pF; n = 9) and 64% (0.46 +/- 0.09 pA/pF; n = 7), respectively. In contrast, although insulin and phorbol ester also increased pump current, this increase was not statistically significant. The ineffectiveness of insulin and phorbol ester may be due to colcemid interfering with Na-K pump translocation from internal vesicles to the sarcolemma.

Effect of Napip on K0 activation of the Na-K pump in adult rat cardiac myocytes

To determine if environmental factors influence the external K (K0) dependence of Na-K pump current (Ip), we systematically varied internal (pipette) Na (Napip) and Na-K pump activity while measuring the K0 dependence in adult rat cardiac myocytes. For each Napip, reactivation of Ip by K0 was dose dependent. The maximal Ip (Ipmax) and apparent affinity for K0 binding to the Na-K pump (K0.5) increased as Napip increased. The results of making an equimolar substitution of tetramethylammonium for K and Cs, and partial Ip inhibition with ouabain, also showed that Ipmax and K0.5 increased as Napip increased. We simulated pump activity as a function of intracellular Na (Nai) and K0 using a cyclic model of the Na-K pump and found that the model predicts K0.5 for K0 binding increases as Na increases, even when the conditions are adjusted by removing pipette K and partial pump inhibition with ouabain.

Coupling of dual acid extrusion in the guinea-pig isolated ventricular myocyte to alpha 1- and beta-adrenoceptors

1. Intracellular pH (pHi) was recorded in single, isolated guinea-pig ventricular myocytes using the pH-sensitive fluorophore, carboxy-SNARF-1 (AM-loaded). 2. The dual acid extrusion system in this cell (Na(+)-H+ antiport and Na(+)-HCO3- symport) was activated by inducing an intracellular acid load, produced by addition and subsequent removal of extracellular 10 mM NH4Cl. Under these conditions, it is known that both acid-equivalent extruders are activated about equally. 3. Application of phenylephrine (100 microM; alpha-adrenergic agonist) resulted in an inhibition of pHi recovery from an acid load, recorded in HCO3-buffered medium containing 1.5 mM amiloride (amiloride inhibits Na(+)-H+ antiport; under these conditions pHi recovery is mediated through only the Na(+)-HCO3- symport carrier). This inhibitory effect of phenylephrine was prevented by the alpha 1-antagonist, prazosin (0.1 microM) and was unaffected by propranolol (1 microM). 4. Application of phenylephrine in Hepes-buffered medium (only Na(+)-H+ antiport is active under these conditions) elicited a stimulation of pHi recovery, again prevented by prazosin (0.1 microM). 5. These results point to an alpha 1 inhibition of Na(+)-HCO3- symport and an alpha 1 stimulation of Na+-H+ antiport. 6. Both adrenaline (1-5 microM) and noradrenaline (5 microM) slowed pHi recovery recorded in HCO3(-)-buffered solution containing amiloride (1.5 mM). The similarity of this result with that obtained previously using phenylephrine (paragraph 3) suggests that all three agonists inhibit the Na(+)-HCO3- symport through alpha 1 activation. 7. Isoprenaline (1 microM; beta-adrenergic agonist) slowed pHi recovery in Hepes-buffered solution but stimulated recovery in a HCO3(-)-buffered solution containing amiloride (1.5 mM). These results suggest that beta activation slows Na(+)-H+ antiport but stimulates Na(+)-HCO3- symport. 8. When both acid-equivalent extrusion carriers were inhibited in Na(+)-free, HCO3(-)-buffered medium, phenylephrine or isoprenaline had no effect on pHi, ruling out any effect of the adrenergic agonists on background acid-loading mechanisms. 9. Under physiological conditions (CO2/HCO3(-)-buffered solution, no amiloride), when both acid extruders would be activated by an intracellular acid load, application of phenylephrine, adrenaline or noradrenaline were found to slow pHi recovery. In contrast, isoprenaline stimulated pHi recovery under the same conditions.(ABSTRACT TRUNCATED AT 400 WORDS)

Cholinergic stimulation of the Na+/K+ adenosine triphosphatase as revealed by microphysiometry

The activation of a wide range of cellular receptors has been detected previously using a novel instrument, the microphysiometer. In this study microphysiometry was used to monitor the basal and cholinergic-stimulated activity of the Na+/K+ adenosine triphosphatase (ATPase) (the Na+/K+ pump) in the human rhabdomyosarcoma cell line TE671. Manipulations of Na+/K+ ATPase activity with ouabain or removal of extracellular K+ revealed that this ion pump was responsible for 8.8 +/- 0.7% of the total cellular energy utilization by those cells as monitored by the production of acid metabolites. Activation of the pump after a period of inhibition transiently increased the acidification rate above baseline, corresponding to increases in intracellular [Na+] ([Na+]i) occurring while the pump was off. The amplitude of this transient was a function of the total [Na+]i excursion in the absence of pump activity, which in turn depended on the duration of pump inhibition and the Na+ influx rate. Manipulations of the mode of energy metabolism in these cells by changes of the carbon substrate and use of metabolic inhibitors revealed that, unlike some other cells studied, the Na+/K+ ATPase in TE671 cells does not depend on any one mode of metabolism for its adenosine triphosphate source. Stimulation of cholinergic receptors in these cells with carbachol activated the Na+/K+ ATPase via an increase in [Na+]i rather than a direct activation of the ATPase.

Phosphatase inhibition by calyculin A increases i(f) in canine Purkinje fibers and myocytes

The actions of the phosphatase inhibitor calyculin A on the pacemaker current i(f) were studied in canine Purkinje fibers and myocytes. Calyculin A increased i(f) in response to hyperpolarizations toward the middle of the i(f) activation curve. A three pulse protocol indicated this increase was due to a positive shift of i(f) activation on the voltage axis. Taken together with our previous results (that kinase inhibition with H7 shifts i(f) activation in the negative direction on the voltage axis (2)), these results suggest that phosphorylation is an important regulator of the voltage dependence of i(f) activation.

Pacemaker current exists in ventricular myocytes

I(f), the "cardiac pacemaker current," is a nonselective cation channel activated on hyperpolarization in primary and secondary pacemaker regions of the mammalian heart. The cardiac pacing rate can be modulated by shifting the activation of I(f) to more positive (faster pacing) or more negative (slower pacing) voltages. We report for the first time the presence of this pacemaker current in ventricular myocytes. The potential importance of this observation to the mechanism of differentiation of nonpacing regions in the heart is discussed.

The inward rectifier K+ current underlies oscillatory membrane potential behaviour in bovine pigmented ciliary epithelial cells

1. Fresh bovine, pigmented ciliary epithelial cells possess an inward rectifier current activated by hyperpolarization. This current was investigated using whole-cell patch-clamp techniques. At the holding potential of -70 mV, and with EK (potassium equilibrium potential) set at -84 mV, a small outward current flowed through the inward rectifier that was sensitive to external K+, becoming more outward in 0.5 mM K+ and progressively more inward in 20 and 50 mM K+. 2. The inward rectifier showed V-EK dependence; increasing [K+]o increased the inward conductance from 1.28 nS in 5 mM K+ to 7.42 nS in 50 mM K+. The conductance at a given V-EK was proportional to the square root of [K+]o. 3. It was blocked by external Cs+ but replacing K+ in the pipette with Cs+ blocked only outward ion movement through the inward rectifier. Inward rectification was also blocked by Ba2+ (85% with Ki (concentration giving half-maximal inhibition) = 3.1 x 10(-5) M) and TEA+ (74% with Ki = 2.9 x 10(-4) M). 4. The activation time constant was voltage dependent, decreasing from 5 ms to 0.7 ms over the voltage range -90 to -170 mV. With increasing hyperpolarization the current exhibited time-dependent decay. The time constant for this process was voltage sensitive but the steady-state inactivation was independent of external [K+]. 5. The current disappeared in culture within 8 days. 6. Solution flow over the cell inactivated the inward rectifier, a property that may be related to [K+]o. 7. In current clamp the cells exhibited an unstable region at a potential of around -70 mV. Once in this region oscillations and regenerative hyperpolarizing potentials were observed. This behaviour was eliminated by treatments that blocked (Cs+, Ba2+) or removed (0.5 mM K+) active inward rectification. 8. It is suggested that these oscillations may represent a process of cation loading, the first step in the secretion of aqueous humour.

The kinetics of the inhibition by dihydroouabain of the sodium pump current in single rabbit cardiac Purkinje cells

The kinetics of the inhibition by dihydroouabain (DHO) of the Na pump current were studied in isolated rabbit cardiac Purkinje cells by means of whole-cell recording. A fast exchange of the extracellular solution at the membrane of the cell studied was performed via two multi-barrelled pipettes nearby. Judging from the steady-state inhibition of the Na pump current at various DHO concentrations half maximal inhibition occurred at 1 x 10(-5) mol/l DHO in a medium containing 2 mmol/l K+ and at 3.5 x 10(-5) mol/l DHO in a solution containing 10.8 mmol/l K+. Assuming a reversible one-to-one binding reaction, the time course of DHO binding and unbinding was analysed under a variety of conditions. It was shown that the antagonistic effect of K+ on the inhibition of the pump current was entirely due to a decrease of the association rate constant of DHO binding to the sodium pump at higher K+ concentrations. The association rate constant amounted to 1 x 10(4) (mol/l)-1.s-1 at 2 mmol/l K+ and to 2.8 x 10(3) (mol/l)-1.s-1 at 10.8 mmol/l K+. The dissociation rate constant of DHO unbinding remained unchanged (approximately 0.06 s-1). The equilibrium dissociation constant KD for the inhibition of the pump current by DHO decreased by a factor 3 to 5 if the Na+ concentration of the patch pipette solution was augmented from 5 to 50 mmol/l. Increasing DHO concentrations inhibited the pump current increasingly faster and to a larger extent. The KD values derived showed little variation with the concentration of DHO applied.(ABSTRACT TRUNCATED AT 250 WORDS)

The cardiac conduction system in the rat expresses the alpha 2 and alpha 3 isoforms of the Na+,K(+)-ATPase

The sodium pump is crucial for the function of the heart and of the cardiac conduction system, which initiates the heartbeat. The alpha (catalytic) subunit of this pump has three isoforms; the alpha 1 isoform is ubiquitous, but the alpha 2 and alpha 3 isoforms are localized to excitable tissue. Because rodent alpha 2 and alpha 3 isoforms are relatively sensitive to ouabain, which also slows cardiac conduction, we studied heart-cell-specific expression of pump isoform genes. Multiple conduction-system structures, including sinoatrial node, bundle branches, and Purkinje strands, had prominent, specific hybridization signal for alpha 2 and alpha 3 isoforms compared with adjacent working myocytes. This gene-expression approach may be useful for labeling conduction tissue and also for localizing specific membrane channels and receptors in this system.

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)

A novel action of isoproterenol to inactivate a cardiac K+ current is not blocked by beta and alpha adrenergic blockers

The K+ current iKl sets the resting potential in cardiac cells. Here we report that isoproterenol (ISO), a prototypical beta agonist, increases inactivation of iKl. This action of ISO on iKl is mimicked by permeant analogues of cAMP but is not blocked by the beta blockers propranolol and pindolol or the alpha blockers prazosin or yohimbine. We suggest that this novel action of ISO may contribute to pacemaker activity in the Purkinje strand and be mediated through a class of receptors different from classical beta's or alpha's.

The mechanism of rectification of iK1 in canine Purkinje myocytes

We have characterized the inward rectifying background potassium current, iK1, of canine cardiac Purkinje myocytes in terms of its reversal potential, voltage activation curve, and "steady-state" current-voltage relation. The latter parameter was defined from the difference current between holding currents in the presence and absence of 20 mM cesium. Our data suggest that iK1 rectification does not arise exclusively from voltage-dependent gating or exclusively from voltage-dependent blockade by internal magnesium ions. The voltage activation curve constructed from tail currents fit to a Boltzmann two-state model predicts less outward current than is actually observed. The magnesium-dependent rectification due to channel blockade is too fast to account for the time-dependent gating of iK1 that gives rise to the tail currents. We propose a new model of rectification that assumes that magnesium blockade of the channel occurs simultaneously with voltage-dependent gating. The new model incorporates the kinetic schema elaborated by Matsuda, H. (1988. J. Physiol. 397:237-258) to explain the appearance of subconducting states of the iK1 channel in the presence of blocking ions. That schema suggested that iK1 channels were composed of three parallel pores, each of which could be blocked independently. In our model we considered the consequences of partial blockade of the channel. If the channels are partially blocked at potentials where normally they are mostly gated closed, and if the partially blocked channels cannot close, then blockade will have the paradoxical result of enhancing the current carried by iK1.

Interaction of intracellular ion buffering with transmembrane-coupled ion transport

The role of the Na/Ca exchanger in the control of cellular excitability and tension development is a subject of current interest in cardiac physiology. It has been suggested that this coupled transporter is responsible for rapid changes in intracellular calcium activity during single beats, generation of plateau currents, which control action potential duration, and control of intracellular sodium during Na/K pump suppression, which may occur during terminal states of ischemia. The actual behavior of this exchanger is likely to be complex for several reasons. First, the exchanger transports two ionic species and thus its instantaneous flux rate depends on both intracellular sodium and calcium activity. Secondly, the alteration in intracellular calcium activity, which is caused by a given transmembrane calcium flux, and which controls the subsequent exchanger rate, is a complex function of available intracellular calcium buffering. The buffers convert the ongoing transmembrane calcium fluxes into changes in activity that are a small and variable fraction of the change in total calcium concentration. Using a number of simple assumptions, we model changes in intracellular calcium and sodium concentration under the influence of Na/Ca exchange, Na/K ATPase and Ca-ATPase pumps, and passive sodium and calcium currents during periods of suppression and reactivation of the Na/K ATPase pump. The goal is to see whether and to what extent general notions of the role of the Na/Ca exchanger used in planning and interpreting experimental studies are consistent with its function as derived from current mechanistic assumptions about the exchanger. We find, for example, that based on even very high estimates of intracellular calcium buffering, it is unlikely that Na/Ca exchange alone can control intracellular sodium during prolonged Na/K pump blockade. It is also shown that Na/Ca exchange can contaminate measurements of Na/K pump currents under a variety of experimental conditions. The way in which these and other functions are affected by the dissociation constants and total capacity of the intracellular calcium buffers are also explored in detail.

Intracellular sodium affects ouabain interaction with the Na/K pump in cultured chick cardiac myocytes

Whether a given dose of ouabain will produce inotropic or toxic effects depends on factors that affect the apparent affinity (K0.5) of the Na/K pump for ouabain. To accurately resolve these factors, especially the effect of intracellular Na concentration (Nai), we have applied three complementary techniques for measuring the K0.5 for ouabain in cultured embryonic chick cardiac myocytes. Under control conditions with 5.4 mM Ko, the value of the K0.5 for ouabain was 20.6 +/- 1.2, 12.3 +/- 1.7, and 6.6 +/- 0.4 microM, measured by voltage-clamp, Na-selective microelectrode, and equilibrium [3H]ouabain-binding techniques, respectively. A significant difference in the three techniques was the time of exposure to ouabain (30 s-30 min). Since increased duration of exposure to ouabain would increase Nai, monensin was used to raise Nai to investigate what effect Nai might have on the apparent affinity of block by ouabain. Monensin enhanced the rise in Na content induced by 1 microM ouabain. In the presence of 1 microM [3H]ouabain, total binding was found to be a saturating function of Na content. Using the voltage-clamp method, we found that the value of the K0.5 for ouabain was lowered by nearly an order of magnitude in the presence of 3 microM monensin to 2.4 +/- 0.2 microM and the magnitude of the Na/K pump current was increased about threefold. Modeling the Na/K pump as a cyclic sequence of states with a single state having high affinity for ouabain shows that changes in Nai alone are sufficient to cause a 10-fold change in K0.5. These results suggest that Nai reduces the value of the apparent affinity of the Na/K pump for ouabain in 5.4 mM Ko by increasing its turnover rate, thus increasing the availability of the conformation of the Na/K pump that binds ouabain with high affinity.

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

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

[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)

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.

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.

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

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

Internal and external K+ help gate the inward rectifier

Recent investigations have demonstrated substantial reductions in internal [K+] in cardiac Purkinje fibers during myocardial ischemia (Dresdner, K.P., R.P. Kline, and A.L. Wit. 1987, Circ. Res. 60: 122-132). We investigated the possible role these changes in internal K+ might play in abnormal electrical activity by studying the effects of both internal and external [K+] on the gating of the inward rectifier iK1 in isolated Purkinje myocytes with the whole-cell patch-clamp technique. Increasing external [K+] had similar effects on the inward rectifier in the Purkinje myocyte as it does in other preparations: increasing peak conductance and shifting the activation curve in parallel with the potassium reversal potential. A reduction in pipette [K+] from 145 to 25 mM, however, had several dramatic previously unreported effects. It decreased the rate of activation of iK1 at a given voltage by several-fold, reversed the voltage dependence of recovery from deactivation, so that the deactivation rate decreased with depolarization, and caused a positive shift in the midpoint of the activation curve of iK1 that was severalfold smaller than the associated shift of reversal potential. These changes suggest an important role of internal K+ in gating iK1 and may contribute to changes in the electrical properties of the myocardium that occur during ischemia.

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.

Sympathetic neural and alpha-adrenergic modulation of arrhythmias

alpha 1-Adrenergic stimulation of the neonatal heart may induce either an increase or a decrease in ventricular automaticity, with the latter response predominating as age increases. We used isolated tissues from the hearts of neonatal and adult dogs and rats, as well as rat myocytes in tissue culture alone or in coculture with sympathetic nerves, to study the role of sympathetic innervation in modulating the alpha-adrenergic response. In the absence of sympathetic innervation, alpha-adrenergic stimulation uniformly increases automaticity. As the myocyte is innervated, an increased quantity of a GTP regulatory protein is detectable. That this protein is an essential transducer of alpha-adrenergic inhibition of automaticity is evidenced by the conversion of the alpha response from excitatory to inhibitory as the protein develops. ADP-ribosylation of the protein with pertussis toxin causes the alpha response to revert to excitation in both adult canine hearts and innervated myocytes in tissue culture. Hence, we have evidence for sympathetic modulation of cardiac rhythm via a regulatory protein whose function depends on normal neuronal development. Abnormal development of innervation may predispose to arrhythmogenesis via persistence of a primitive response to alpha stimulation.

Passive electrical properties and electrogenic sodium transport of cultured guinea-pig coronary endothelial cells

1. Coronary endothelial cells were isolated from adult guinea-pig hearts (Nees, Gerbes & Gerlach, 1981) and the electrical properties of primary cultures were studied using the tight-seal whole-cell recording mode of the patch clamp technique. 2. On the third or fourth day in culture whole-cell clamp records from single coronary endothelial cells were obtained at 37 degrees C. The resting potential was -33 +/- 6 mV (n = 10). The membrane time constant determined with rectangular current pulses was 68 +/- 22 ms (n = 10). 3. In voltage clamp experiments, no time-dependent membrane conductance changes were found in the range -80 to +40 mV. The current-voltage relation was linear in normal physiological salt solution containing 5.4 mM-K+. The input resistance was 1.7 +/- 0.4 G omega. When the external K+ concentration was increased to 116 mM the cells depolarized to about -3 mV and the clamp currents showed marked inward rectification. 4. Between days four and seven in culture the endothelial cells formed confluent monolayers which showed the characteristic 'cobblestone' morphology. The input resistance of cells in a monolayer was 8 +/- 3 M omega, i.e. a factor of 200 lower than that found in single cells. It was concluded that the cells in the confluent monolayer are coupled electrically by gap junctions. 5. Exposure of coronary endothelial cells to K+-free solution for 5 min produced a depolarization of about 8 mV. Upon readmission of normal external K+ the cells transiently hyperpolarized by up to 20 mV. This transient hyperpolarization decayed with a time constant of 1.9 +/- 0.3 min. 6. The transient hyperpolarization could be abolished by application of 2 x 10(-4) M-dihydro-ouabain (DHO). Application of DHO in the steady state produced a depolarization of 8 +/- 1 mV. From these findings it was concluded that coronary endothelial cells possess an electrogenic sodium pump which contributes about -8 mV to the resting potential. 7. From the passive electrical properties of single cells and the morphological data available it was calculated that endothelium in situ may have a large electrical space constant, probably between 250 and 550 microns. 8. The functional implications of the large space constant of the endothelial monolayer are discussed. It is suggested that intra-endothelial conduction of electrical signals from capillaries to the resistance vessels may be involved in the local regulation of blood flow in the intact heart.

Na,K pump stimulation by intracellular Na in isolated, intact sheep cardiac Purkinje fibers

Regulation of the Na,K pump in intact cells is strongly associated with the level of intracellular Na+. Experiments were carried out on intact, isolated sheep Purkinje strands at 37 degrees C. Membrane potential (Vm) was measured by an open-tipped glass electrode and intracellular Na+ activity (aNai) was calculated from the voltage difference between an Na+-selective microelectrode (ETH 227) and Vm. In some experiments, intracellular potassium (aiK) or chloride (aCli) was measured by a third separate microelectrode. Strands were loaded by Na,K pump inhibition produced by K+ removal and by increasing Na+ leak by removing Mg++ and lowering free Ca++ to 10(-8) M. Equilibrium with outside levels of Na+ was reached within 30-60 min. During sequential addition of 6 mM Mg++ and reduction of Na+ to 2.4 mM, the cells maintained a stable aNai ranging between 25 and 90 mM and Vm was -30.8 +/- 2.2 mV. The Na,K pump was reactivated with 30 mM Rb+ or K+. Vm increased over 50-60 s to -77.4 +/- 5.9 mV with Rb+ activation and to -66.0 +/- 7.7 mV with K+ activation. aiNa decreased in both cases to 0.5 +/- 0.2 mM in 5-15 min. The maximum rate of aiNa decline (maximum delta aNai/delta t) was the same with K+ and Rb+ at concentrations greater than 20 mM. The response was abolished by 10(-5) M acetylstrophantidin. Maximum delta aNai/delta t was independent of outside Na+, while aKi was negatively correlated with aNai (aKi = 88.4 - 0.86.aNai). aCli decreased by at most 3 mM during reactivation, which indicates that volume changes did not seriously affect aNai. This model provided a functional isolation of the Na,K pump, so that the relation between the pump rate (delta aNai/delta t) and aiNa could be examined. A Hill plot allowed calculation of Vmax ranging from 5.5 to 27 mM/min, which on average is equal to 25 pmol.cm-2.s-1.K 0.5 was 10.5 +/- 0.6 mM (the aNai that gives delta aNai/delta t = Vmax/2) and n equaled 1.94 +/- 0.13 (the Hill coefficient). These values were not different with K+ or Rb+ as an external activator. The number of ouabain-binding sites equaled 400 pmol.g-1, giving a maximum Na+ turnover of 300 s-1. The Na,K pump in intact Purkinje strands exhibited typical sigmoidal saturation kinetics with regard to aNai as described by the equation upsilon/Vmax = aNai(1.94)/(95.2 + aNai(1.94)). The maximum sensitivity of the Na,K pump to aiNa occurred at approximately 6 mM.

Background K+ current in isolated canine cardiac Purkinje myocytes

The current-voltage (I-V) relation of the background current, IK1, was studied in isolated canine cardiac Purkinje myocytes using the whole-cell, patch-clamp technique. Since Ba2+ and Cs+ block IK1, these cations were used to separate the I-V relation of IK1 from that of the whole cell. The I-V relation of IK1 was measured as the difference between the I-V relations of the cell in normal Tyrode (control solution) and in the presence of either Ba2+ (1 mM) or Cs+ (10 mM). Our results indicate that IK1 is an inwardly rectifying K+ current whose conductance depends on extracellular potassium concentration. In different [K+]0's the I-V relations of IK1 exhibit crossover. In addition the I-V relation of IK1 contains a region of negative slope (even when that of the whole cell does not). We also examined the relationship between the resting potential of the myocyte, Vm, and [K+]0 and found that it exhibits the characteristic anomalous behavior first reported in Purkinje strands (Weidmann, S., 1956, Elektrophysiologie der Herzmuskelfaser, Med. Verlag H. Huber), where lowering [K+]0 below 4 mM results in a depolarization.

Models of the Na-K pump in cardiac muscle predict the wrong intracellular Na+ activity

The Na-K pump in cardiac Purkinje strands has been carefully studied with voltage clamp and Na+-selective microelectrodes. In three of these studies both the rate of change of intracellular Na+ activity, a(Nai), after pump blockade, and the time constant of reduction of a(Nai) after an Na+ load were measured. These two parameters can be employed with a formalism relating pump activity to a(Nai) in order to predict the a(Nai) in the steady state. Several formalisms were tested: (a) a first-order dependence on a(Nai); (b) a model based on the assumption of a single, saturable, Na+-binding site that must be occupied for transport to occur; (c) a model based on n equivalent, saturable, Na+ binding sites per pump molecule all of which must be occupied for transport to occur. The first two models predicted an a(Nai) that is far below the value of about 6 mM that is experimentally obtained. The third model would work for n greater than or equal to 4. These results suggest that either the cardiac Na-K pump is not well described by available Na-K pump models for n less than 4 or that the measured Na+ influx rate, extrusion rate or a(Nai) are in error.