Electrogenic sodium pump in rabbit sinoatrial node cell

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

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

Ouabain actions on the spontaneous activity and ionic currents in rabbit sino-atrial node cells

1. The effects of ouabain on the action potentials and the membrane currents in spontaneously beating rabbit sino-atrial (SA) node cells were examined using the two-microelectrode technique. 2. Cumulative administrations of ouabain (10(-8) to 10(-6) M) caused a negative chronotropic effect in a concentration-dependent manner. The effect was not modified by atropine (10(-7) M). At 10(-6) M, ouabain prolonged the duration of action potentials, but other parameters were unaffected to any significant extent. Ouabain elicited an arrhythmia, and increasing concentrations increased the incidence of arrhythmia (75% at 3 x 10(-7) M). 3. Pretreatment with clonidine (10(-6) M), a selective agonist of presynaptic alpha 2-adrenoceptors, completely blocked the development of arrhythmia induced by ouabain (3 x 10(-7) M). Prazosin (10(-6) M), an alpha 1 antagonist, had similar effects, and yohimbine (10(-6) to 10(-5) M), an alpha 2 antagonist, did not affect the arrhythmias. 4. Ouabain (10(-8) to 10(-6) M) inhibited the slow inward and the time-dependent outward currents, but enhanced the hyperpolarization-activated inward current, in a concentration-dependent manner. The time course of inactivation phase for Isi was composed of two (fast and slow) components. Ouabain decreased the fast component and increased the slow component. The voltage of half-maximum activation for the outward current was not affected. Ouabain elicited a transient inward current on the repolarizing step, and also on the depolarizing step.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Differences in the determinants of overdrive suppression between sinus rhythm and slow atrioventricular junctional rhythm

Sinus node recovery time was compared to the recovery time of a slow atrioventricular junctional rhythm in each of the same seven pentobarbital anesthetized dogs. Recovery time and the first five cardiac cycles were examined after pacing atria and ventricles for 20, 40, and 60 seconds at four or more pacing cycle lengths. Data relating recovery times and return to control conditions to prepacing cycle length, pacing cycle length, duration of pacing, site of pacing, and origin of rhythms were analyzed by covariance analysis. From the analyses, the relative contribution of the determinants are: the prepacing cycle length 73%, the site of pacing 3.5%, the pacing cycle length 2%, and the interaction of the site of pacing and pacing cycle length 1% for sinus node recovery time; and for slow atrioventricular junctional rhythm recovery time, the duration of pacing 40%, the interactions between the duration of pacing and the pacing cycle length 27%, and the prepacing cycle length 9%. A modified exponential decay model predicted 8 beats for return to prepacing conditions during sinus rhythm and 66-100 beats during atrioventricular junctional rhythm. We conclude that the single most important determinant of sinus node recovery time is the prepacing cycle length. Pacing cycle length and site of pacing have a significant but small influence on sinus node recovery time and duration of pacing, beyond 20 seconds, has no significant influence. In contrast, duration of pacing is the most important determinant of slow atrioventricular junctional recovery time. Another major determinant of slow atrioventricular junctional recovery time is the interactions between pacing cycle length and duration of pacing. Prepacing cycle length has a minor influence, and site of pacing has no influence, on slow atrioventricular junctional recovery time.

Inhibition of the Na pump--a mechanism in the genesis of cardiac arrhythmias

An ATP-driven Na pump maintains the unsymmetrical Na and K distribution across the cell membrane of cardiac cells. An increase of the intracellular Na or extracellular K concentration enhances this active Na transport. About 35 per cent of the actively transported Na is ejected from the cells as a hyperpolarizing outward current. The Na pump influences the cardiac Ca metabolism via the Na-Ca exchange. Inhibition of the pump affects the generation and conduction of the cardiac action potential by various mechanisms. It seems to be involved in the genesis of cardiac arrhythmias.

A model of sino-atrial node electrical activity based on a modification of the DiFrancesco-Noble (1984) equations

DiFrancesco & Noble's (1984) equations (Phil. Trans. R. Soc. Lond. B (in the press.] have been modified to apply to the mammalian sino-atrial node. The modifications are based on recent experimental work. The modified equations successfully reproduce action potential and pacemaker activity in the node. Slightly different versions have been developed for peripheral regions that show a maximum diastolic potential near --75 mV and for central regions that do not hyperpolarize beyond --60 to --65 mV. Variations in extracellular potassium influence the frequency of pacemaker activity in the s.a. node model very much less than they do in the Purkinje fibre model. This corresponds well to the experimental observation that the node is less sensitive to external [K] than are Purkinje fibres. Activation of the Na-K exchange pump in the model by increasing intracellular sodium can suppress pacemaker activity. This phenomenon may contribute to the mechanism of overdrive suppression.

A mathematical model of the effects of acetylcholine pulses on sinoatrial pacemaker activity

A mathematical model of dynamic vagus-sinus interactions was devised based on Hodgkin and Huxley-type equations of time- and voltage-dependent membrane currents. Brief vagal pulses were modeled with a concentration-dependent, acetylcholine-activated, potassium current. Single acetylcholine ("vagal") pulses scanning the sinus cycle induced changes in pacemaker rhythm that depended on pulse magnitude, duration, and time of occurrence during the cycle. Phase-response curves summarizing these effects are strikingly similar to experimental results. Notably, appropriately timed acetylcholine pulses could produce an acceleratory response. With repetitive acetylcholine input, the model produced various patterns of synchronization of the sinus pacemaker. There was stable entrainment at harmonic (i.e., 1:1, 2:1, etc.) relations, as well as more complex arrhythmic patterns that depended on the relationship between the acetylcholine cycle length and the sinus pacemaker period. In some cases, shortening of the acetylcholine input cycle length led to "paradoxical" acceleration of the sinus pacemaker. Simulations suggest that many clinically observed sinus rhythm disturbances can be explained by dynamic vagus-sinus interactions.

The basis for the membrane potential of quiescent cells of the canine coronary sinus

During prolonged periods of quiescence, the membrane potential of cells in the isolated canine coronary sinus, exposed to normal Tyrode solution containing 4 mM-K, declines to about -60 mV. The nature of the resting potential was investigated, in small strips of coronary sinus tissue mounted in a fast-flow system, by recording the membrane potential responses to sudden changes in the extracellular ionic environment. At extracellular K concentrations ([K]o) from 0 to 64 mM the resting potential was little affected by replacing all but 1 mM of external Cl ions with isethionate and methylsulphate ions. At [K]o levels from 4 to 150 mM the resting potential was reasonably well described by the Goldman-Hodgkin-Katz equation on the assumption that the intracellular K concentration ([K]i) was 155 mM and that the ratio of membrane permeability coefficients for Na and K, PNa/PK, was 0.07. In the presence of a high concentration of acetylcholine or carbachol (greater than or equal to 1 microM), the resting potentials at [K]o levels from 1 to 150 mM approximated K equilibrium potentials (EK) calculated on the assumption that [K]i was 155 mM. At [K]o levels less than or equal to 8 mM replacing most of the external Na with sucrose or Tris caused a substantial hyperpolarization, whereas application of 1-2 microM-tetrodotoxin caused only slight hyperpolarization. A transient hyperpolarization, due to enhanced electrogenic Na extrusion, was recorded on switching back to 4 mM-K following brief exposures to K-free solution; no transient hyperpolarization was recorded in the presence of 5 microM-acetylstrophanthidin. The acetylstrophanthidin itself caused a rapid depolarization of several millivolts. Preliminary conductance measurements made with two micro-electrodes in some smaller preparations indicate that the steady-state current-voltage relationship is N-shaped. We conclude that the low membrane potential of quiescent coronary sinus cells reflects not a low [K]i but rather a relatively high ratio PNa/PK, of about 0.07: the Na ions flow into the cells via predominantly TTX-insensitive pathways and are extruded by the electrogenic Na/K exchange pump, which thereby makes a substantial contribution to the resting potential.

Dynamic vagal control of pacemaker activity in the mammalian sinoatrial node

Dynamic heart rate control by parasympathetic nervous input involves feedback mechanisms and reflex bursting of efferent cardiac vagal fibers. Periodic vagal bursting induces phasic changes in sinoatrial cycle length and can entrain the pacemaker to beat at periods that may be identical to those of the vagal burst. We investigated the electrophysiological basis of these phenomena in isolated sinus node preparations (rabbit, cat, and sheep). In the presence of propranolol (3.9 X 10(-6)M), relatively brief (50-150 msec) trains of stimuli, applied onto the endocardial surface of the preparation, activated postganglionic vagal terminals and induced a brief hyperpolarization of sinoatrial pacemaker cells. This vagally mediated hyperpolarization could alter the pacemaker rhythm by an amount that depended on its duration and its position in the cycle, as well as on the duration of the free-running pacemaker period. When the free-running period was sufficiently long and the hyperpolarization was induced sufficiently early in the spontaneous cycle, a "paradoxical" acceleration of the pacemaker rhythm ensued. Phasic changes were plotted on phase-response curves, constructed by scanning systematically the sinoatrial pacemaker period with single or repetitive vagal trains. These phase-response curves enabled us to predict the entrainment characteristics and the levels of synchronization of the pacemaker to the vagal periodicity. The overall data explain the cellular mechanisms involved in the phasic effects of brief vagal discharges on sinoatrial periodicity, and provide conclusive evidence for the prediction that repetitive vagal input is capable of forcing the cardiac pacemaker to beat at rates that can be faster or slower than the intrinsic pacemaker rate. These data should improve our knowledge of the dynamic control of heart rate by neural reflexes and aid in our understanding of rhythm disturbances generated by the interaction of the cardiac pacemaker with vagal activity.

Electrogenic sodium pump in rabbit atrio-ventricular node cell

Electrogenicity of the Na pump was demonstrated in rabbit A-V node cells by analyzing the K-induced hyperpolarization occurring after a short period of K-free perfusion. The transient hyperpolarization was abolished completely by strophanthidin (10(-5)M). The membrane slope conductance remained unchanged during the transient hyperpolarization. On perfusion of 50 mM K and K-free incubation the transient hyperpolarization reached --69 mV which was more negative than the expected EK (about --28 mV). The order of potencies of monovalent cations to activate the K site of the Na pump was Tl greater than Rb equal to K greater than NH4 equal to Cs greater than Li, which was similar to the sequence reported in the literature. Michaelis-Menten type activation of the K site of the Na pump was suggested from the relationship between the decay rate constant of the K-induced outward current transient and [K]o. These findings obviously indicate that the Na pump in the A-V node cells shares common characteristics with those of other excitable tissues. Direct contribution of the pump activity to the membrane potential under physiological conditions was suggested by a significant depolarization occurring immediately after application of strophanthidin.

Activation of active Na transport in sheep Purkinje fibres by external K or Rb ions

The intracellular Na activity (aiNa) of sheep Purkinje fibres bathed in solutions with and without K (Rb) is studied by means of Na sensitive microelectrodes. Perfusion with K (Pb) free media increases aiNa. Upon reapplication of K (Pb) containing solutions aiNa decreases and cell membrane hyperpolarizes transiently. After an initial delay attributed to K (Rb) equilibration in the extracellular space, the decline of aiNa and membrane potential towards their respective resting values is approximately monoexponential and displays the same time constant (tau). The tau values and aiNa in the steady state vary with the extracellular K (Rb) concentration ([K]0, [Rb]0). According to a simple model the activation of the Na pump by external K (Rb) can be estimated from the time constants or the aiNa steady state values at various [K]0 ([Rb]0). Half maximal activation occurs at 1.6-3.7 mM K (Rb). External K and Rb ions are equipotent activators of the Na pump in sheep Purkinje fibres.

Effects of rapid stimulation on the transmembrane action potentials of rabbit sinus node pacemaker cells

We studied the mechanism of post-overdrive suppression in superfused rabbit sinus node pacemaker cells. Small specimens of sinus node tissue isolated from rabbit hearts were driven at a fast rate (overdrive) for 10-120 seconds using single sucrose gap methods. During the control perfusion (35 degrees C Tyrode's solution), overdrive caused a progressive decrease in maximum diastolic potential (MDP), overshoot (OS), and maximum rate of depolarization at phase 0 [dV/dt)max]. After cessation of the overdrive, the rate of diastolic depolarization decreased, and the spontaneous activity was suppressed temporarily (post-overdrive suppression). MDP, OS, (dV/dt)max, and the spontaneous activity returned within a few seconds to the level observed before overdrive. Atropine (2 x 10(-6) g/ml) did not influence the effects of overdrive. After ouabain administration (3 x 10(-7) g/ml) or in low temperature perfusate (25 degrees C), the effects of overdrive were accentuated, and a marked suppression of spontaneous activity with a long pause of over several seconds was seen following the overdrive. These results suggest that the post-overdrive suppression of sinus node is attributable, at least in part, to ionic shifts following overdrive, and may be potentiated by metabolic dysfunction of pacemaker cells.

Activation of the electrogenic Na-pump of cardiac muscle fibres by ACh in K-free solutions

The ionic mechanism of the membrane outward current (ACh-current) of bullfrog atrium muscle, induced by acetylcholine in K-free solution, was analyzed by a voltage-clamp experiment. The results suggested that the ACh-current was induced not only by an increase in K-conductance but also by an activation of the electrogenic Na-pump.

Potassium permeability of embryonic avian heart cells in tissue culture

The relationship between the external potassium concentration ([K]o) and membrane permeability has been reexamined using a tissue-cultured preparation of embryonic chick heart cells in which diffusional limitations are minimal. The unidirectional K efflux and electrochemical gradients were determined as a function of [K]o, and the results showed that potassium permeability was constant within the range of 1-20 mM [K]o. Membrane potentials were obtained in K-free solutions and correlated with 42K efflux and intracellular ion content measurements under the same conditions. In contrast to preparations of the intact embryonic chick heart, 42K efflux does not decrease in K-free media. Simulations of tracer measurements at reduced [K]o from naturally occurring cardiac muscle indicate that the experimentally observed decrease in 42K efflux could result from diffusional limitations. This observation, when coupled with the experimental results, suggests that the effect of low [K]o on membrane permeability in homeothermic preparations of cardac muscle should be reevaluated.

The effect of brief vagal stimulation on the isolated rabbit sinus node

We developed an isolated rabbit atrial preparation which responds consistently and reproducibly to brief, submaximal stimulation of the autonomic nerves contained in it. In 6 of 11 preparations in the presence of propranolol (1 mg/liter), the time course of changes in the atrial rate following 120 msec vagal stimulation was bimodal. The maximal slowing occurred at 0.64 +/- 0.16 second, and the peak secondary slowing occurred at 2.3 +/- 1.0 seconds. An acceleratory component occurred between the first and second peaks between 0.8 and 1.6 seconds. The total time course of vagal effect lasted for 5.0 +/- 2.0 seconds. These changes in rate could not be explained by shifts in the location of the primary pacemaker. The acceleratory component was due to a 4.7 +/- 2.0 (SD) mV depolarization of the maximum diastolic membrane potential of the primary pacemaker of the sinus node which lasted for 1.8 +/- 0.3 seconds. Following vagal stimulation, there was an increase of 0.2 mM in the activity of potassium in the extracellular space recorded with a potassium-sensitive electrode; this peaked between 1.4 and 2.5 seconds and cleared with an exponential time course. The halftimes for recovery ranged between 2.8 and 4.6 seconds. The initial peak slowing of the bimodal time course and the acceleratory component therefore appear to be direct effects of acetylcholine. The secondary slowing occurs after acetylcholine presumably has been inactivated and occurs coincidently with the accumulation of potassium in the extracellular space.

Editorial: Electrical activity and excitation-contraction coupling in mammalian ventricular muscle

The modern view of the mammalian ventricular action potential is that Na-ions are responsible for depolarization, the plateau is maintained by an inward Ca current and repolarization is due to the inactivation of this inward Ca current against a background outward K current. The electrical activity spreads over the surface and T-tubules of the cell and the inflow of Ca-ions, and possibly also the electrical signal, cause, a further release of Ca from internal stores, the lateral cisternae of the sarcoplasmic reticulum (SR). This released Ca, by triggering the splitting of ATP, induces contraction. The released Ca is pumped back into the longitudinal SR and, as the Ca concentration is reduced, relaxation occurs. The Ca in the longitudinal SR is transported back to the storage site in the cisternae. This completes the cycle, release, uptake, transport back to the releasing site. On the basis of this scheme, the action of various drugs is briefly discussed.

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.