Recovery from hypothermia in cardiac Purkinje fibers: considerations for an electrogenic mechanism

The modelling of a primitive 'sustainable' conservative cell

The simple sustainable or 'eternal' cell model, assuming preservation of all proteins, is designed as a building block, a primitive element upon which one can build more complete functional cell models of various types, representing various species. In the modelling we emphasize the electrophysiological aspects, in part because these are a well-developed component of cell models and because membrane potentials and their fluctuations have been generally omitted from metabolically oriented cell models in the past. Fluctuations in membrane potential deserve heightened consideration because probably all cells have negative intracellular potentials and most cells demonstrate electrical activity with vesicular extrusion, receptor occupancy, as well as with stimulated excitation resulting in regenerative depolarization. The emphasis is on the balances of mass, charge, and of chemical species while accounting for substrate uptake, metabolism and metabolite loss from the cell. By starting with a primitive representation we emphasize the conservation ideas. As more advanced models are generated they must adhere to the same basic principles as are required for the most primitive incomplete model.

Electrophysiological evidence of an increase in cold tolerance of cardiac muscles in mice after energy restriction

Life-prolonging energy restriction (ER) has been known to extend longevity. The heart was selected as the target organ of ER and the electrophysiological properties of ER on the heart were investigated. Action potential parameters were measured on ventricular papillary muscles of C57BL/6 mice (2-6 months of age). Resting membrane potential (Rm) did not change even when the temperature was lowered to 20 degrees C in ER mice (-67.5 +/- 0.8 mV), however, the membrane was depolarized in the control (-61.1 +/- 1.1 mV). Action potential duration measured at 30 and 50% repolarization was significantly prolonged in ER mice at 20-35 degrees C. Ouabain (10 microM) decreased Rm in ER mice at 20 degrees C (-68.6 +/- 1.0 to -63.6 +/- 0.8 mV), but failed to decrease Rm in the control (-60.6 +/- 1.8 to -62.1 +/- 1.2 mV). There were no significant differences in extracted Na, K-ATPase activity or affinity and binding capacity of ouabain between ER and control hearts. These results indicate that in ER mice the lack of effect of temperature on Rm was not due to a change in the physicochemical properties of Na, K-ATPase. The present study collectively suggests that ER increases cold tolerance in the heart of mice.

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.

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.

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).

Correlation between changes in membrane potential and intracellular sodium activity during K activated response in sheep Purkinje fibres

Following superfusion with a K free medium sheep Purkinje fibres show a strong correlation between changes in membrane potential and the intracellular Na activity in K containing solution. This correlation persists after changes in the internal Na or extracellular K activity and following substitution of Rb for external K.

Development of pump electrogenesis in hypokalaemic rat muscle

In hypokalaemic rats maintained on a potassium deficient diets for 10-50 days, the isolated "Na-loaded" and "K-depleted" ("Na-rich") muscle fibers showed the membrane potential less than -115 mV in "fresh" muscles of normal rats in K+-free Krebs solution. Upon adding 5 mM K+ to the K+-free medium bathing the soleus muscles, the measured potentials of "Na-rich" muscles always exceeded the membrane potentials of "fresh" muscles in 5 mM K+. The hyperpolarization was dependent on the amount of intracellular Na+ concentration (["na]i) accumulated during the potassium deficiency. The electrogenic Na-pump was activated by an increase of [Na]i of less than 5 mM. Further increases in [Na]i resulted in increases in membrane potential which appeared to approach a limit at [Na]i levels higher than 65 mM.

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.

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.

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.

Temperature sensitivity of outward current in cardiac Purkinje fibers. Evidence of electrogenicity of active transport

1. In cardiac Purkinje fibers the temperature sensitivity of the membrane current flowing after 2 sec in response to depolarizing clamp steps was recorded. When the temperature was quickly lowered (30 sec) from 37 degrees C to 20 degrees C for a period of 2 min the outward current was markedly reduced. The effect was immediately reversed upon rewarming. The reduction in outward current on cooling was most pronounced between 30 degrees C and 20 degrees C. 2. In the range of anomalous rectification cooling to 20 degrees C shifted the i.v. relation in a negative direction by a constant amount of 20 nA. Outside this potential range (negative to -80 mV and positive to -45 mV) the slope conductance was reduced with a Q10 of about 1.3. 3. In the presence of dihydroouabain cooling did not further reduce the outward current in the potential range of anomalous rectification. However, negative to -80 mV and positive to -40 mV the slope conductance was reduced. The results support the view that part of the outward current is generated by an electrogenic sodium pump which is inhibited by cooling.