CAT HEART MUSCLE IN VITRO. 8. ACTIVE TRANSPORT OF SODIUM IN PAPILLARY MUSCLES

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.

Hyperpolarization induced by sodium removal in rabbit sinoatrial node cells. Possible role of electrogenic sodium-calcium exchange

Spontaneously active rabbit sinoatrial node (SAN) cells were bathed in K-free solution or in K-free ouabain (20 microM)-containing solution to depress the electrogenic Na(+)-K+ pump activity. In SAN cells exposed to K-free solution, the automatic action potentials ceased with gradual depolarization, followed by an eventual steady-state membrane potential of -32 +/- 1 mV. Under conditions where the Na(+)-K+ pump was blocked, removal of external Na+ produced a large and rapid hyperpolarization in the membrane potential and the membrane was hyperpolarized by 23 +/- 0.5 mV. When the external Na+ was lowered, Na+ was replaced by Li+. The Na-free hyperpolarization was not affected by applications of verapamil (4 microM), lidocaine (1 mM), and quinidine (50 microM), but was inhibited by either quinacrine (50 microM) or Cd2+ (10 mM), which are blockers of Na(+)-Ca2+ exchange. In the absence of external K+, replacement of external NaCl by sucrose produced a hyperpolarization similar to that seen in the replacement of external Na+ by Li+. In the K-free ouabain (20 microM)-containing solution, removal of external Na+ also produced a hyperpolarization, and the membrane potential dropped from -29 +/- 1 to -48 +/- 1 mV. The intracellular acidification due to NH4Cl removal after exposure to NH4Cl (20 mM) produced a decrease in Na-free hyperpolarization, which in the presence of ouabain was inhibited by the application of Cd2+ (10 mM). Removal of external Ca2+ nearly completely blocked Na-free hyperpolarization. It can be concluded that Na-free hyperpolarizations are related to the functioning of an electrogenic Na(+)-Ca2+ exchange mechanism.

Trimetazidine inhibits Na+,K(+)-ATPase activity, and overdrive hyperpolarization in guinea-pig ventricular muscles

The effect of trimetazidine on Na+,K(+)-ATPase activity or the Na+,K+ pump was studied in guinea pig ventricular muscles with the use of biochemical and electrophysiological methods. The effect of trimetazidine on enzyme activity was compared with that in the liver, jejunum and kidney obtained from the same species. Na+,K(+)-ATPase activity in the heart and liver was significantly and concentration dependently decreased by trimetazidine (above 1.5 x 10(-5) M). Even the highest concentration (1.5 x 10(-4) M) of trimetazidine failed to decrease the Na+,K(+)-ATPase activity in the jejunum and kidney. The membrane potential was recorded in the ventricular muscle with a microelectrode. The hyperpolarization which followed 1-min overdrive stimulation (3.3 Hz) was decreased by trimetazidine (1.5 x 10(-4) M), but the depolarization during the stimulation was not affected by this drug. Ouabain, a potent Na+,K+ pump inhibitor, markedly decreased the overdrive hyperpolarization and increased the depolarization during the stimulation (10(-7), 5 x 10(-7), 10(-6) M). Therefore, the effect of trimetazidine and ouabain on the Na+,K+ pump-mediated alteration in the resting potential is different, suggesting that trimetazidine has additional direct membrane effects, e.g. a decrease in K+ conductance. In conclusion, trimetazidine inhibits Na+,K(+)-ATPase activity and thus the Na+,K+ pump in the ventricular muscles but with an inhibitory effect about 300 times less than that of ouabain. Trimetazidine inhibited the Na+,K(+)-ATPase in the liver as well, but not that in jejunum and kidney.

Suggestive evidence for inhibitory action of amiodarone on Na+, K+-pump activity in guinea pig heart

1. Effects of amiodarone, a powerful antiarrhythmic agent, on Na+, K+-pump activity were examined on ventricular papillary muscle of guinea pig, by means of conventional microelectrode technique. 2. The activity of Na+, K+-pump was measured by two methods. One of them was to measure the amplitudes of depolarization observed during overdrive stimulation (3.3 Hz) and of hyperpolarization observed after the overdrive stimulation (post-overdrive hyperpolarization), and the other, to measure the hyperpolarization observed following introduction of 10 mM K+, after exposure to K+ free solution for a certain duration. 3. Amiodarone significantly decreased the amplitude of depolarization during overdrive stimulation and the amplitude of post-overdrive hyperpolarization. 4. In the latter method, the deactivation process of hyperpolarization recorded by the introduction of 10 mM K+ following K+ depletion slowed down by amiodarone. 5. These findings suggest that amiodarone may inhibit, at least in part, the Na+, K+-pump activity in the ventricular muscle of guinea pig.

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

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

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

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

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

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

Intra- and extracellular potassium activities, acetylcholine and resting potential in guinea pig atria

Intracellular potassium activity in guinea pig left atria was measured using potassium ion-selective microelectrodes and conventional microelectrodes. The effects of extracellular potassium concentration and acetylcholine on both intracellular potassium activity and the relationship between the resting membrane potential and the potassium equilibrium potential were investigated. Intracellular potassium activity was 102.1 mM in bathing media with a potassium concentration of 5 mM. Neither increasing extracellular potassium concentration to 10 mM nor exposure to acetylcholine (2 x 10(-6) to 10(-3) M) significantly altered intracellular potassium activity. In contrast, intracellular potassium activity decreased to 92.9 mM in 2.5 mM potassium concentration solutions. Resting membrane potential was 18.6, 9.6, and 7.3 mV positive to the potassium equilibrium potential in 2.5, 5, and 10 mM potassium, respectively. Acetylcholine caused a significant hyperpolarization at each extracellular potassium activity, confirming that resting membrane potential was positive to the potassium equilibrium potential. Even after exposure to 10(-3) M acetylcholine, the resting membrane potential apparently remained positive to the potassium equilibrium potential. If potassium accumulates in extracellular clefts during acetylcholine exposure, the calculated potassium equilibrium potentials are too negative, and the resting membrane potential might closely approximate the potassium equilibrium potential under these conditions. Fading of the acetylcholine-induced hyperpolarization and overshoot of the resting membrane potential on washout of acetylcholine were observed and are consistent with an accumulation of potassium during exposure to acetylcholine. In 5.0 mM potassium bathing solution, preparation-to-preparation variability of resting membrane potential can largely be explained by variability of intracellular potassium activity.(ABSTRACT TRUNCATED AT 250 WORDS)

The electrogenic sodium pump in guinea-pig ventricular muscle: inhibition of pump current by cardiac glycosides

1. The inhibition of the electrogenic sodium pump in guinea-pig ventricular muscle by cardiac glycosides was studied with a voltage-clamp technique.2. Superfusion of the preparation with dihydro-ouabain (DHO) produced a reversible depolarization of up to 7 mV. When the membrane potential was clamped to a constant value near the resting potential application of DHO produced a corresponding current change in the inward direction which reached a steady state in less than 1 min.3. The drug-induced current change (I(D)) was found to be the result of a parallel shift of the current-voltage relation. The contributions of a change in extracellular K or intracellular Na to the measured I(D) were shown to be very small. From these findings and the results summarized below it was concluded that I(D) represents the blockage of the electrogenic pump current by DHO and that it is proportional to the number of drug molecules bound to the Na-K-ATPase in the intact cell.4. The dependence of I(D) on the concentration of DHO applied (5 x 10(-6)-8 x 10(-4) M) was found to be consistent with the predictions of the law of mass action for reversible one-to-one binding of the drug to the Na-K pump under equilibrium conditions. From a Scatchard-type plot the equilibrium dissociation constant (K(D)) of DHO was determined to be 4.6 (+/-2.3) x 10(-5) M.5. The steady-state pump current in the resting preparation was calculated to be 0.81+/-0.26 muA/cm(2). It contributed 6.4+/-0.9 mV to the resting potential in Tyrode solution containing 3 mM-K.6. In the smallest preparations used the measured time course of the onset and decay of I(D) agreed with the chemical kinetics of binding and unbinding calculated for various DHO concentrations. The rate constant of unbinding (k(2)) was found to be 3.4 (+/-0.7) x 10(-2) S(-1) and the average rate constant of binding (k(1)) was 7.4 x 10(2) M(-1) S(-1).7. By comparing the effects of ouabain and DHO in the same preparation the following estimates of the chemical constants of ouabain binding to the Na-K pump were obtained: K(D) approximately 1.5 x 10(-6) M; k(1) approximately 4 x 10(3) M(-1) S(-1); k(2) approximately 6 x 10(-3) S(-1).8. An analysis of the transmembrane movements of Na and K in the steady state showed that the measured pump current density is consistent with a counter-transport of 3 Na and 2 K ions.

Intracellular potassium activity in guinea pig papillary muscle during prolonged hypoxia

During prolonged hypoxia, intracellular potassium concentration, [K](i) has been reported to fall by 70% with a concomitant decrease in the calculated potassium equilibrium potential, E(K). Nevertheless, resting membrane potential, V(m), declined only slightly. Because V(m) depolarized very little in relation to the calculated E(K), it was hypothesized that electrogenic Na-K pumping contributed up to 40 mV to V(m) during prolonged hypoxia. To further test this hypothesis we studied what changes prolonged hypoxia makes in the thermodynamically active fraction of cellular potassium, intracellular potassium activity, alpha(K) (i), and how change in alpha(K) (i) affects the relationship between V(m), E(K) and, by inference, the Na-K pump. Using double-barrel K-selective electrodes, V(m) and alpha(K) (i) were measured in quiescent guinea pig right ventricular papillary muscles superfused for 8 h with hypoxic Tyrode's solution. Over the 8-h period both V(m) and alpha(K) (i) decreased. However, the decline in V(m) was paralleled by a decrease in the E(K) calculated from alpha(K) (i). At no time was there hyperpolarization of V(m) beyond E(K). After 8 h the Na-K pump was inhibited by exposing the muscles to 0.1 mM ouabain. The onset of an increase in extracellular potassium activity, measured with a double-barrel electrode, was used to mark the amount of depolarization of V(m) due solely to pump inhibition. After hypoxia, V(m) depolarized 8.4+/-4.4 mV before extracellular potassium activity (alpha(K) (e)) increased. Thus, the decrease in alpha(K) (i) during hypoxia is much less than that reported for [K](i). The parallel decline in V(m) and E(K) and the small depolarization of V(m) with ouabain suggest that after prolonged hypoxia the Na-K pump continues to contribute to V(m), but the amount of this contribution is substantially less than previously reported.

Electrogenic sodium extrusion can stop triggered activity in the canine coronary sinus

Soon after a burst of triggered activity in the canine coronary sinus begins, an initial fall in maximum diastolic potential and increase in rate gives way to an increase in maximum diastolic potential, reduction in rate, and eventual quiescence. This hyperpolarization, slowing, and subsequent quiescence might result from enhanced electrogenic sodium/potassium extrusion caused by the rise in intracellular sodium concentration ([Na+]i) associated with the high rate of firing. Triggered bursts can be terminated prematurely by a sudden increase in the rate of sodium extrusion, Brief exposure to K+- free fluid is known to cause [Na+]i to rise; reactivating the pump by switching back to K+-containing fluid causes immediate hyperpolarization, and within a few seconds, quiescence. Brief periods of overdrive, also thought to increase [Na+]i, are followed by hyperpolarization, slowing and, often, by premature termination of the burst. Inhibiting the sodium/potassium pump by exposure to 2 micrometer acetylstrophanthidin or to K+-free fluid (1) prevents or delays the hyperpolarization, (2) increases the rate of triggered activity and (3) prolongs bursts of activity when bursts last less than 2.5 minutes under control conditions. In the presence of 2 micrometer acetylstrophanthidine, neither brief exposures to K+- free fluid not overdrive causes sudden, premature termination of triggered bursts. Bursts do eventually stop in the presence of pump inhibitors; however, that termination is associated with an increase in rate and a decline in maximum diastolic potential and in action potential amplitude. We conclude that electrogenic Na+ extrusion plays an important role in the spontaneous termination of triggered activity.

The dependence of sodium pumping and tension on intracellular sodium activity in voltage-clamped sheep Purkinje fibres

1. Intracellular Na activity (aiNa) was measured in sheep cardiac Purkinje fibres using a recessed-tip Na+-sensitive micro-electrode. The membrane potentials was controlled with a two-micro-electrode voltage clamp. Tension was measured simultaneously. 2. Removing external K produced a rise of aiNa and both twitch and tonic tension. On adding 4-10 mM-[Rb]0 to reactivate the Na-K pump aiNa and tension declined. An electrogenic Na pump current transient accompanied the fall of aiNa. 3. The half-time of decay of the electrogenic Na pump current transient was similar to that of aiNa, (mean tNa0.5/tI0.5 = 0.97 +/- 0.03 (S.E.M.; n = 28)). Following the re-activation of the Na-K pump, the electrogenic Na pump current transient was linearly related to aiNa. 4. The duration of exposure to K-free, Rb-free solutions was varied to change the level of aiNa. On subsequently re-activating the Na-K pump with 10 mM-[Rb]0, the ratio of the charge extruded to the total change of aiNa was constant. It is concluded that the fraction of Na extruded electrogenically is unaffected by changes of aiNa. About 26% of the total Na extrusion appeared as charge transfer. 5. The relationship between tonic tension and aiNa was usually different during Na-K pump inhibition in a K-free, Rb-free solution compared with the relationship during Na-K pump re-activation. In general, a given aiNa was associated with a greater level of tonic tension during Na-K pump inhibition compared with that during pump re-activation. A similar hysteresis was often seen between twitch tension and aiNa.

Characterization of the electrogenic sodium pump in cardiac Purkinje fibres

1. The Na pump is examined in sheep cardiac Purkinje fibres using a two micro-electrode voltage clamp technique.2. After reducing the external K concentration, [K](o), to zero for 2 min or more, subsequent addition of an ;activator cation' (known to activate the Na pump in other preparations) produces a transient increase of outward current. This outward current transient is abolished by 10(-5)M-strophanthidin (cf. Gadsby & Cranefield, 1979a).3. It is concluded that this transient increase of outward current is a result of a transient stimulation of the sodium pump by the raised [Na](i) following exposure to 0-K(o). Although this current transient may reflect the activity of an electrogenic Na pump, it is difficult to use K as the activator cation to establish this point. This is due to the extracellular K depletion that occurs during Na pump reactivation and the subsequent change that this K depletion produces in the current-voltage relationship of the Purkinje fibre.4. Rb(o) or Cs(o) have been used instead of K(o) to reactivate the Na pump when examining the transient increase of outward current. On adding either of these cations after exposing a preparation to a solution without such ;activator cations', the outward current transient is relatively voltage independent over a wide range of potentials (-90 to +10 mV). It is concluded that, following the addition of Rb(o) or Cs(o), the transient increase of outward current is a direct measure of the transient increase of the electrogenic Na pump current.5. Increasing [Rb](o) or [Cs](o) over the range of 0-40 mM increases the rate of decay of the electrogenic Na pump current transient. Using a simple model (cf. Rang & Ritchie, 1968), it is shown that the decay rate constant of the electrogenic Na pump current transient is a good measure of the degree of activation of the external site of the Na pump. At a given concentration of activator cation, Rb(o) produces a greater activation of the Na pump than does Cs(o). The K(0.5) for Rb(o) is 6.3 mM and for Cs(o) is 14.2 mM. Li(o) activates the Na pump more weakly than Rb(o) and Cs(o).6. The coupling ratio of the Na pump is shown to be independent of Rb(o) or Cs(o) over the range 2-40 mM. Furthermore, consistent with the results of Gadsby & Cranefield (1979a), the coupling ratio is independent of Na(i) over the range considered.7. The Q(10) for the electrogenic Na pump current transient varies between 1.6 and 2.3 over the range of temperature 26-46 degrees C.8. A maximum Na pump current of about 0.78 muA cm(-2) is obtained. Assuming a coupling ratio of 3Na/2K, the rate of Na ion transport into the cell is estimated to be about 23 p-mole cm(-2) sec(-1). Assuming a Na pump turnover of 150 sec(-1), we estimate that there are about 1000 Na pump sites per mum(2) of cell surface.9. We conclude that the electrogenic Na pump current transient provides a good measure of the activity of the Na pump when Rb or Cs are used as ;activator cations'. This measure can be used in the intact preparation to investigate the relationship between Na pump rate and other cellular events such as the regulation of tension (Eisner & Lederer, 1980).

Mechanism of monensin-induced hyperpolarization of neuroblastoma-glioma hybrid NG108-15

Addition of the ionophore monensin to mouse neuroblastoma-rat glioma hybrid NG108-15 cells leads to a 20 to 30-mV increase in the electrical potential across the plasma membrane as shown by direct intracellular recording techniques and by distribution studies with the lipophilic cation [3H]-tetraphenylphosphonium+ (TPP+) [Lichtshtein, D., Kaback, H.R. & Blume, A.J. (1979) Proc. Natl. Acad. Sci. USA 76, 650-654]. The effect is not observed with cells suspended in high K+ medium, is dependent upon the presence of Na+ externally, and the concentration of monensin that induces half-maximal stimulation of TPP+ accumulation is approximately 1 microM. The ionophore also causes rapid influx of Na+, a transient increase in intracellular pH, and a decrease in extracellular pH, all of which are consistent with the known ability of monensin to catalyze the transmembrane exchange of H+ for Na+. Although ouabain has no immediate effect on the membrane potential, the cardiac glycoside completely blocks the increase in TPP+ accumulation observed in the presence of monensin. Thus, the hyperpolarizing effect of monensin is mediated apparently by an increase in intracellular Na+ that acts to stimulate the electrogenic activity of the Na+,K+-ATPase. Because monensin stimulates TPP+ accumulation in a number of other cultured cell lines in addition to NG108-15, the techniques described may be of general use for studying the Na+,K+ pump and its regulation in situ.

Electrogenic sodium extrusion in cardiac Purkinje fibers

Thin canine cardiac Purkinje fibers in a fast flow chamber were exposed to K-free fluid for 15 s to 6 min to initiate "sodium loading," then returned to K-containing fluid to stimulate the sodium pump. The electrophysiological effects of enhanced pump activity may result from extracellular K depletion caused by enhanced cellular uptake of K or from an increase in the current generated as a result of unequal pumped movements of Na and K, or from both. The effects of pump stimulation were therefore studied under three conditions in which lowering the external K concentration ([K]0) causes changes opposite to those expected from an increase in pump current. First, the resting potential of Purkinje fibers may have either a "high" value of a "low" (less negative) value: at the low level of potential, experimental reduction of [K]0 causes depolarization, whereas an increase in pump current should cause hyperpolarization. Second, in regularly stimulated Purkinje fibers, lowering [K]0 prolongs the action potential, whereas an increase in outward pump current should shorten it. Finally, lowering [K]0 enhances spontaneous "pacemaker" activity in Purkinje fibers, whereas an increase in outward pump current should reduce or abolish spontaneous activity. Under all three conditions, we find that the effects of temporary stimulation of the sodium pump are those expected from a transient increase in outward pump current, not those expected from K depletion.

The intracellular sodium activity of cardiac Purkinje fibres during inhibition and re-activation of the Na-K pump

1. The intracellular Na activity, aiNa, of sheep heart Purkinje fibres was continuously monitored using Na+-sensitive glass micro-electrodes. The effects of removal and restoration of external K, and of application and removal of various cardioactive steroids, were investigated. 2. The aiNa increased in K-free solutions and rapidly recovered on addition of external K. The rate of this recovery depended on both the external K concentration, [K]o, and the aiNa. The rate of aiNa recovery was found to be half maximally activated at a [K]o of about 10 mM. If corrections are applied to allow for changes in the net passive Na influx at various [K]o, then this value is increased to approximately 12.5 mM. 3. At a given [K]o, there appeared to be a linear relationship between the rate of aiNa recovery and the level to which aiNa had increased in K-free solution (over the range of aiNa from 7.5 to 31 mM). 4. Addition of the cardioactive steroids strophanthidin, acetylstrophanthidin, actodigin (AY 22,241) or dihydro-ouabain produced rapid changes of aiNa. At low concentrations, these compounds sometimes produced a small decrease in aiNa, while at concentrations above 10(-7) M they produced a dose-dependent increase. 5. The effects on aiNa of both low and high concentrations of all these cardioactive steroids were readily reversible within 120 min. The time course of the aiNa recovery mainly depended on the concentration of the cardioactive steroid applied, and on the level to which aiNa had increased. 6. Upon addition of a cardioactive steroid (above 10(-7) M, aiNa at first increased almost linearly with time. The rates of such an increase were measured during this period at various cardioactive steroid concentrations and used to produce dose-response curves. The concentrations that produced a half-maximum rate of aiNa increase were near to 10(-6) M for strophanthidin and acetylstrophanthidin, but near to 10(-5) M for actodigin and dihydro-ouabain. 7. The mean maximum rate of aiNa increase produced by the addition of a high cardioactive steroid concentration was 0.49 +/- 0.17 mM/min (+/-S.D., n = 21). This would indicate a net passive Na influx into the cells of approximately 2.8 p-mole/cm2sec. 8. This maximum rate of aiNa increase could be achieved by the addition of 10(-5) M-strophanthidin or acetylstrophanthidin, but 10(-4) to 10(-3) M-actodigin or dihydro-ouabain was required to produce a similar rate of increase. 9. The addition of these high cardioactive steroid concentrations produced an initially rapid increase of aiNa. After 15-30 min this aiNa increase slowed considerably. The aiNa appeared to reach a 'plateau' within 2-4 hr at levels much below those predicted for a Na electrochemical equilibrium across the cell membrane.

Activation of the electrogenic sodium pump in guinea-pig atria by external potassium ions

1. When cardiac preparations are rewarmed following prolonged hypothermia a transient hyperpolarization occurs in K-containing media. This hyperpolarization is correlated with the active Na efflux. It might be due to electrogenic Na pumping or to extracellular K depletion brought about by the activity of an electroneutral Na-K exchange pump. In order to distinguish between these mechanisms the effect of various extracellular K concentrations ([K](o)) on the membrane potential of guineapig atria was studied before and after hypothermia.2. The membrane potential increased with decreasing [K](o) before cooling. It reached values of -64 and -92 mV at 10.8 and 0 mM-K, respectively.3. The membrane hyperpolarized transiently after hypothermia beyong the potential observed before cooling. Maximal values of about -94 mV were obtained during rewarming in solutions containing 0.4-2.7 mM-K. The membrane potential was significantly lower (-88 mV) in K-free media. It was also diminished at [K](o) higher than 2.7 mM and was measured to be -74 mV at 10.8 mM-K.4. The hyperpolarization of the cell membrane during the first 20 min of rewarming was maximal at 2.7 mM-K and yielded 15.5 mV. The hyperpolarization amounted to 7.2 and 10 mV at 0.4 and 10.8 mM-K, respectively. No hyperpolarization occurred in K-free solutions.5. The rate of decline of the transient hyperpolarization increased with [K](o).6. Variations of membrane input resistance after changes in [K](o) were measured in rewarmed atrial trabecula. The measurements revealed an increase in membrane resistance in lower [K](o).7. It is concluded that the transient hyperpolarization of the cardiac cell membrane during rewarming is due to the activation of an electrogenic Na pump.8. The (relative) strength of the pump current at various [K](o) was derived from the observed dependence of the hyperpolarization and of the membrane input resistance on [K](o). The current is estimated to be half-maximal at about 1.5 mM-K.

The effect of heart rate on the membrane responsiveness of rabbit atrial muscle

The maximum rate of rise of action potentials in myocardial fibers of the rabbit atrium decreases with an increase in heart rate. This decrease of the dV/dt max is accompanied by a decrease of the diastolic transmembrane potential prior to the moment of activation (take-off potential). Comparison of the membrane responsiveness curve (relation between dV/dt max and take-off potential) as measured by varying the extracellular potassium concentration at a fixed rate of stimulation, with the effect of changes in the frequency of stimulation on dV/dt max and take-off potential made clear that the fall in dV/dt max after a sudden increase in heart rate was stronger than could be explained by the concomitant decrease of the take-off potential alone. This implicates that the membrane responsiveness itself is heart rate dependent. A possible explanation for this observation is that when heart rate is increased the active Na/K pump is not able to maintain the intracellular concentration of Na and K at the original level. Acceleration of the heart will lead to an intracellular loss of potassium and a gain of sodium. The first causes a diminishment of the diastolic membrane potential which according to the membrane responsiveness curve is attended with a decrease of the dV/dt max. The second results in a decrease of the sodium concentration gradient and therefore in a further reduction of the dV/dt max. This hypothesis was confirmed by experiments with ouabain added to the perfusion fluid. Ouabain, which is known to inhibit the Na/K pump, caused a decrease of both the take-off potential and dV/dt max that was completely comparable with the effects of an increase of the frequency of stimulation. In addition, observation of the time course of the changes in dV/dt max and membrane "resting" potential after a sudden change in the rate of stimulation, gave support to the electrogenic concept of the active Na/K pump in cardiac muscle.

Electrophysiological observations on the digitalis-potassium interaction in canine Purkinje fibers

We studied the effects of elevating potassium concentration on the membrane potential of Purkinje cells exposed to toxic concentrations of acetylstrophanthidin or ouabain. Conventional intracellular microelectrode techniques were employed. Rapid elevation of [K+]o from 2.7 to 5.4 mEq/liter resulted in an initial increase (more negative) in membrane potential of cells demonstrating ouabain-induced phase 4 depolarization. The increase in maximal diastolic potential occurred initially without suppression of phase 4 depolarization. In cells rendered inexcitable by ouabain or acetylstrophanthidin, elevation of [K+]o consistently increased membrane potential and restored excitability. In four experiments automaticity was initiated within 2 minutes after the increase in [K]o. Although automaticity reappeared, as maximal diastolic potential increased, the automatic rate slowed and then pacemaker activity was suppressed. Studies with 3H-ouabain showed that the increase in membrane potential paralleled K+-induced release of 3H-ouabain from Purkinje cells. These studies suggest that elevation of [K+]o reverses digitalis toxic manifestations in canine Purkinje fibers by causing release of cardiac glycosides bound to the membrane. The observed increase in membrane potential of ouabain-treated Purkinje fibers that occurred after [K+]o elevation was considered to be mediated in part by restoration of the Na pump and by electrogenic pumping.

Effects of Na and K ions on the active Na transport in guinea-pig auricles

1. The effect of Na and K ions on active Na transport was studied in guinea-pig auricles by means of flame photometry. 2. The Na influx into preparations rewarmed in Tyrode's solution after cooling was estimated to be about 1.05 mmole/l fibre water - min (l.f.w.-min) or c. 8 pmole/cm2 - s. Intracellular Na ions enhanced the active Na efflux over a wide range of concentrations. A decrease in the extracellular Na concentration ([Na]o) had no major effect on the active Na efflux. 3. Extracellular K ions initiated an active Na efflux from rewarmed auricles with an elevated [Na[i over a narrow range of K concentrations ([K]o). 4. Assuming Michaelis-Menten kinetics the maximal active Na efflux activated by internal Na ions was calculated to be about 4 mmole/l.f.w. - min (30 pmole/cm2 - s). Half maximal Na efflux occurred at about 22 mmole/l.f.w. [Na]i. The maximal K-activated active - min (28 pmole/cm2 - s) and was half maximal at a [K]o of about 0.2 mM. 5. It is tentatively concluded that the maximal active Na efflux from guinea-pig atria is 3--4 times larger than the physiological flux. Under normal conditions active Na efflux in heart is mainly regulated by variations of [Na]i.

The effect of temperature on potassium chloride contracture in cat myocardium

1. Contracture was induced in cat myocardium by exposure to 140 mM-KC1 In isotonic Tyrode solution. Force of contracture expressed as mg/mm2 (muscle cross-sectional area) falls with increasing cross-sectional area. 2. The effect of temperature on isometric force developed during contracture was evaluated both in normal (untreated) atrial and ventricular muscle and following treatment with sympatholytic drugs. 3. The force of contracture was not significantly affected by sympatholytic drugs at 36 degrees C. 4. In normal atrial and ventricular muscle, force of contracture decreased when the muscle was cooled from 36 to either 29 or 20 degrees C. 5. In atrial muscle, the effect of temperature was not changed by sympatholytic drugs. In contrast, exposure to sympatholytic drugs increased contracture force developed by ventricular muscle at 20 degrees C. Also, contracture force was significantly greater at 20 than at 36 degrees C in ventricular muscle from reserpine-pretreated cats. 6. It is suggested that ventricular muscle becomes more sensitive to the relaxing effects of endogenous catecholamines at temperature is lowered. 7. The differences shown between atrial and ventricular muscle with respect to the effect of temperature and sympatholytic drugs on contracture force may result from the differing amounts of sarcoplasmic reticulum found in these types of cardiac muscle and also from different mechanisms of "excitation-contracture" coupling in atrial and ventricular muscle.

Developmental aspects of potassium flux and permeability of the embryonic chick heart

1. The rate coefficient of 42K efflux, the transmembrane potential, the intracellular concentrations of Na and K and the volume/surface area have been measured in embryonic chick hearts of different ages. 2. With respect to age, the rate coefficient for 42K efflux was minimal for preparations from 6-8 day old embryos, and distinctly higher values were obtained for the hearts of 3-5 and 18-20 days. With respect to the effect of external K concentration (Ko), all age groups showed a five- to sevenfold increase in rate coefficient between 2-5 and 140 mM-Ko. The effect of Ko was found to be indepedent of extracellular Na, except in the 18-20 day hearts bathed in K-free solution. 3. Intracellular concentrations of K and Na were found to decrease, membrane potential to increase with age. The volume/surface area measured by stereologic and morphometric techniques did not change with age. 4. The permeability coefficient for K (PK), calculated from the absolute K flux and the measured membrane potentials, was fairly constant for a given age between 2-5 and 20 mM-Ko. In K-free solution, PK was markedly reduced (factor 4). At a given Ko, PK increased twofold between 6-8 and 18-20 days while PNa remained relatively constant.

Contribution of an electrogenic sodium pump to the membrane potential in rabbit sinoatrial node cells

A study has been made of the transient hyperpolarization (K+-induced hyperpolarization) which developed following readmission of potassium after having pre-treated the rabbit sinoatrial node tissue with K+-depleted Tyrode solution for 4--5 min at 35 degrees C. Evidence is presented indicating that the K+-induced hyperpolarization results from the activity of an electrogenic sodium pump: The K+-induced hyperpolarization was inhibited by substituting Li+ for Na+ and by cooling the tissue. The amplitude of the K+-induced hyperpolarization was increased either by increasing K+ concentration in the recovery solution or by decreasing K+ concentration in the pre-treatment K+-depleted solution. By removing Cl- from the perfusates, the amplitude of the K+-induced hyperpolarization increased. In a Cl--depleted solution, the sinoatrial node cell membrane hyperpolarized by approximately 15 mV without a transient depolarization.

Effect on membrane potential and electrical activity of adding sodium to sodium-depleted cardiac purkinje fibers

Canine cardiac Purkinje fibers exposed to Na-free solutions containing 128 mM TEA and 16 mM Ca show resting potentials in the range -50 to -90 mV; if the concentration of Na in the perfusate is raised from 0 to 4 to 24 mM, hyperpolarization follows. If the initial resting potential is low, the hyperpolarization tends to be greater; the average increase in the presence of 8 mM Na is 14 mV. Such hyperpolarization is not induced by adding Na to K-free solutions, is not seen in cooled fibers, or in fibers exposed to 10(-3) M ouabain, nor is it induced by adding Li and thus may result from electrogenic sodium extrusion. Fibers exposed to Na-free solutions are often spontaneously active; if they are quiescent they often show repetitive activity during depolarizing pulses. Such spontaneous or repetitive activity is suppressed by the addition of Na. This suppression may or may not be related to the hyperpolarization.

Metabolism and the electrical activity of anoxic ventricular muscle

1. The action potential duration of anoxic guinea-pig ventricular muscle was related to ATP generated by glycolysis. In 50 mM glucose medium the action potential duration was maintained; in 5 mM glucose medium the action potential duration shortened, the glycolytic rate declined and the ATP content was reduced.2. The action potential amplitude was related to the metabolic state of the muscle but not to the intracellular sodium concentration.3. It is suggested that changes in the action potential duration and overshoot in anoxic muscle may be due to an influence of metabolism on the slow inward current.4. Anoxic muscle incubated for 8 hr in 5 mM glucose medium had an E(m) of -77.1 mV compared to -81.1 mV in fresh muscle. The calculated E(k) of anoxic muscle was -47.4 mV.5. The resting potential of anoxic muscle was separated into two components, one dependent on potassium distribution and the other on the activity of an electrogenic sodium pump.6. The electrogenic pump component was stimulated upon raising the glucose concentration of the medium or upon raising the external potassium concentration.7. The electrogenic pump component was inhibited by ouabain or by reduction of the temperature from 35 to 8 degrees C.

Ion levels and membrane potential in chick heart tissue and cultured cells

Intracellular concentrations of sodium and potassium as well as resting potentials and overshoots have been determined in heart tissue from chick embryos aged 2-18 days. Intracellular potassium declined from 167 mM at day 2 to 117-119 mM at days 14-18. Intracellular sodium remained nearly constant at 30-35 mM during the same period. The mean resting potential increased from -61.8 mV at day 3 to about -80 mV at days 14-18. The mean overshoot during the same period increased from 12 to 30 mV. P(Na)/P(K) calculated from the ion data and resting potentials declined from 0.08 at day 3 to 0.01 at days 14-18. Thus, the development of embryonic chick heart during days 2-14 is characterized by a declining intracellular potassium concentration and an increasing resting potential and overshoot. Heart cells from 7- to 8-day embryos, cultured either in monolayer or reassociated into aggregates, were compared with intact tissue of the same age. The intracellular concentrations of sodium and potassium were similar in the three preparations and cultured cells responded to incubation in low potassium medium or treatment with ouabain in a manner similar to that of intact tissue. Resting potentials and overshoots were also similar in the three preparations.

Ionic permeability changes as the basis of the thermal dependence of the resting potential in barnacle muscle fibres

1. The thermal dependence of the resting potential of isolated barnacle muscle fibres was larger (1-2 mV/ degrees C) than predicted by Nernst's equation (about 0.2 mV/ degrees C). A comparative study was made of the influence on thermal dependence of parameters related to (a) passive permeability and to (b) Na extrusion.2. High [K](o) decreased the thermal dependence reversibly. [K(i)], [Na](i) and [Cl](i) were determined by chemical analysis, and Goldman's equation was fitted to data relating V to [K](o) at different temperatures, in the presence and absence of ouabain 5 x 10(-5)M. In both cases the behaviour of V when T was lowered from 20 to 4 degrees C was accounted for by increases in the calculated P(Na/PK) and P(Cl/PK) (from 0.006 to 0.043 and from 0.17 to 0.34 on the average, respectively.)3. Other parameters related to passive permeability (and which caused reversible depolarization): decreased [Cl](o) (methanesulphonate or gluconate substituted), and decreased pH(o) (below 5.0), also decreased the thermal dependence reversibly.4. Inhibitors (ouabain 5 x 10(-5)M, cyanide 2-10 x 10(-3)M, 2,4-dinitrophenol 2 x 10(-4)M) externally applied did not affect either resting potential or its thermal dependence for several hours.5. Increasing [Na](i) three- to fourfold by intracellular injection decreased both resting potential and its thermal dependence.6. Although a small effect by a Na electrogenic pump cannot be excluded, the largest part of the thermal effect on the resting potential is concluded to depend on temperature-induced variations in relative ionic permeabilities to cations and anions. A model is proposed which can account for the data assuming that (a) each permeant ion associates to a separate site in the membrane, and (b) the ion-site equilibrium is temperature-dependent.

The effect of 2,4-dinitrophenol on electrical and mechanical activity, metabolism and ion movements in guinea-pig ventricular muscle

1. DNP (2,4-dinitrophenol) reduced the duration of the action potential of guinea-pig ventricular muscle at a greater rate than did anoxia. The effect was dose-dependent and was modified by the concentration of glucose in the medium. DNP (0.1 mM) reduced the amplitude of the action potential of muscles incubated with 5 mM glucose; on raising the glucose concentration to 50 mM the effect was reversed.2. A large dose-dependent loss of K(+) occurred within 15 min of incubation with DNP and was attributed to increased efflux. K(+) loss was not related to Na(+) gain during the first 60 min of incubation; during the first 30 min DNP-treated muscle did not gain any Na(+). Although the shortening of the action potential by DNP during aerobic incubation was similar to that of muscles incubated under anaerobic conditions in glucose-free medium, the anaerobic incubation was not associated with increased (42)K efflux.3. It was concluded that the reduction in duration of the action potential was not necessarily the result of an increased K(+) efflux. The effect of DNP on (42)K efflux is considered to result from a direct effect on the cell membrane; the effect on electrical activity may be a combination of the increase in K(+) efflux and a reduction in the inward current due to Na(+) and Ca(++) previously assumed to be dependent on the glycolytic production of ATP.4. Electrogenic Na(+) pumping may contribute to the maintenance of resting potential in K(+)-depleted, DNP-treated cardiac muscle.

Activation of the electrogenic sodium pump in guinea-pig auricles by internal sodium ions

1. The effect of various intracellular Na concentrations ([Na](i)) on the membrane potential after hypothermia was studied in guinea-pig auricles.2. For varying [Na](i), the atria were cooled for 4 hr at 4-6 degrees C in a K-poor solution with different concentrations of NaCl. The auricles were rewarmed in normal Tyrode solution at 35 degrees C.3. Extracellular space (ECS), intracellular Na and K concentrations ([Na](i) and [K](i)) and membrane potential of the atria were measured before and after hypothermia.4. The ECS, measured as inulin space, amounted to 350 ml./kg wet wt. at 35 degrees C and to 300 ml./kg wet wt. at 4-6 degrees C.5. [K](i) decreased during cooling and increased during rewarming the auricles. [Na](i) increased during hypothermia in bathing fluids containing NaCl, but decreased in NaCl- and Na-free solutions. At the beginning of rewarming a net Na transport occurred from cells with high [Na](i), while a net Na uptake took place in atria with low [Na](i).6. At the same time, the membrane potential of auricles with increased [Na](i) hyperpolarized beyond the steady-state value recorded at the end of rewarming, or even beyond the calculated K(+) equilibrium potential (E(K)). Afterwards, the hyperpolarization levelled off, while the E(K) values increased further. The membrane potential of atria with decreased [Na](i) showed no transitory hyperpolarization during rewarming.7. The hyperpolarization beyond the steady-state value of membrane potential in rewarmed auricles was significantly correlated to the active Na efflux.8. From these results it is concluded that the membrane potential of guinea-pig atria after hypothermia is affected by an active, electrogenic Na pump activated by intracellular Na ions.