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Physiological Reviews, Vol. 81, No. 4, October 2001, pp. 1791-1826
Copyright ©2001 by the American Physiological Society
Arbeitsgruppe Muskelphysiologie, Fakultät für Biologie, Ruhr-Universität Bochum, Bochum, Germany
I. INTRODUCTION
A. Definition of Electrogenic Na+-K+ Pumping and Historical Background
B. Physiological Significance of Electrophysiological Studies on the Cardiac Na+-K+ Pump
II. ACTIVATION OF THE CARDIAC PUMP CURRENT BY MONOVALENT CATIONS
A. Activation of Ip by Intracellular Na+
B. Activation of Ip by Extracellular K+ and Its Congeners
C. Li+ Binds to Extra- and Intracellular Binding Sites of the Cardiac Na+-K+ Pump and Activates Ip
D. Pump Current Densities, Pump Site Densities, and Maximum Turnover Rate
III. THE REVERSAL POTENTIAL OF THE CARDIAC PUMP CURRENT
A. Theoretical Considerations
B. Evidence for Backward Running Na+-K+ Pump and Determination of the Na+-K+ Pump Erev
IV. VOLTAGE DEPENDENCE OF CARDIAC PUMP CURRENTS
A. Cardiac Steady-State Ip-V Relationships
B. Transient Pump Currents
V. DEPENDENCE OF PUMP CURRENT ON INTRACELLULAR ATP
VI. TEMPERATURE DEPENDENCE OF STEADY-STATE AND TRANSIENT PUMP CURRENTS
VII. SIGNIFICANCE OF ELECTROGENIC SODIUM-POTASSIUM PUMPING FOR THE MEMBRANE POTENTIAL OF CARDIAC CELLS
A. Contribution of Electrogenic Na+-K+ Pumping to the Cardiac Resting Potential
B. Importance of Electrogenic Na+-K+ Pumping for the Cardiac Action Potential
VIII. MODULATION OF CARDIAC PUMP CURRENT BY AUTONOMIC TRANSMITTERS AND RELATED COMPOUNDS
A. Effect of Adrenergic Agonists on Ip
B. Modulation of Cardiac Ip by Acetylcholine
IX. CARDIAC GLYCOSIDES AND CARDIAC PUMP CURRENT
A. Binding of Cardiac Glycosides to Various Isoforms of the Cardiac Na+-K+ Pump Is Species Dependent
B. Kinetics of Cardiac Steroid Binding to the Cardiac Na+-K+ Pump
C. Cardiac Glycosides Alter the Cardiac Ip-V Relationship
X. MODULATION OF THE CARDIAC SODIUM-POTASSIUM PUMP BY HORMONES
A. Aldosterone
B. Angiotensin-Converting Enzyme Inhibition
C. Thyroid Status and Cardiac Na+-K+ Pump
D. Insulin Changes the Cardiac Ip-V Curve
XI. MISCELLANEOUS
A. Anisosmolar External Solution Affects the Activity of the Cardiac Na+-K+ Pump
B. Dietary Cholesterol Alters Cardiac Na+-K+ Pumping
C. Amiodarone Inhibits the Cardiac Na+-K+ Pump Following Acute and Chronic Treatment by Different Mechanisms
XII. EFFECTS OF THE CARDIAC SODIUM-POTASSIUM PUMP ON ION TRANSPORTERS AND CHANNELS MEASURED BY ELECTROPHYSIOLOGICAL TECHNIQUES
A. Modulation of the Cardiac Na+/Ca2+ Exchange
B. Interaction Between the Cardiac Na+-K+ Pump and KATP Channels
C. Effects of the Cardiac Na+-K+ Pump on IK(Na)
D. Blockade of the Na+-K+ Pump Activates IK(ACh) in Atrial Myocytes
XIII. CONCLUSION
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ABSTRACT |
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Glitsch, Helfried Günther
Electrophysiology of the Sodium-Potassium-ATPase in
Cardiac Cells. Physiol. Rev. 81: 1791-1826, 2001.
Like several other ion transporters, the
Na+-K+ pump of animal cells is electrogenic.
The pump generates the pump current Ip. Under
physiological conditions, Ip is an outward
current. It can be measured by electrophysiological methods. These
methods permit the study of characteristics of the
Na+-K+ pump in its physiological environment,
i.e., in the cell membrane. The cell membrane, across which a potential
gradient exists, separates the cytosol and extracellular medium, which
have distinctly different ionic compositions. The introduction of the
patch-clamp techniques and the enzymatic isolation of cells have
facilitated the investigation of Ip in single
cardiac myocytes. This review summarizes and discusses the results
obtained from Ip measurements in isolated
cardiac cells. These results offer new exciting insights into the
voltage and ionic dependence of the Na+-K+ pump
activity, its effect on membrane potential, and its modulation by
hormones, transmitters, and drugs. They are fundamental for our current
understanding of Na+-K+ pumping in electrically
excitable cells.
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I. INTRODUCTION |
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A. Definition of Electrogenic Na+-K+ Pumping and Historical Background
The Mg2+-dependent, Na+- and
K+-activated ATPase (EC 3.6.1.37; Ref. 165) is the
molecular basis of the Na+-K+ pump in animal
cell membranes. The elucidation of the amino acid sequence of the
Na+-K+-ATPase
-subunit (105,
163) and of the
-subunit (106,
162) of various species (180) has prompted
numerous studies on ATPase molecules modified by mutagenesis and
heterologously expressed in various cells to investigate the
relationship between structure and function of the
Na+-K+-ATPase (98,
117, 180). Furthermore, the existence of
Na+/K+ isozymes has been studied in a large
variety of species and tissues (19, 174). The
Na+-K+-ATPase consists of at least two subunit
proteins in stoichiometric amounts, the
- and
-subunit. The
-subunit exhibits a molecular mass of ~110 kDa and probably spans
10 times the cell membrane. Both NH2 and COOH termini face
the cytoplasma. The
-subunit contains the binding sites for ATP,
Na+, K+, cardiac glycosides, specific
inhibitors of the enzyme, and the phosphorylation site. Thus the
-subunit is largely responsible for the catalytic, transport, and
pharmacological characteristics of the ATPase. The smaller
-unit,
with a molecular mass of ~50 kDa (depending on the degree of
glycosylation), has only one transmembrane domain. The COOH terminus is
located at the large ectodomain of the subunit, whereas the
NH2 terminus is exposed to the cytoplasma. The activity of
the Na+-K+-ATPase requires the
-subunit. The
subunit modulates the transport characteristics of the ATPase and plays
an important role in the maturation and proper membrane insertion of
the Na+-K+-ATPase (19). It is
still unclear whether the enzyme in vivo works as an(
)-monomer or
an (
)2-diprotomer (154). A small, hydrophobic protein of ~12 kDa, termed the
-subunit, copurifies with the
- and
-subunits of the
Na+-K+-ATPase. It has been found in various
tissues of different species (124). The physiological
function of the
-subunit is not yet known. Like other cellular
proteins,
- and
-subunits are expressed in various isoforms. At
present four
-subunits (
1-
4) and three
-subunits (
1-
3) have been identified.
The
1,
2, and
3 are expressed in a variety of tissues, whereas the
4-protein
has been detected so far only in the rat testis (186).
Both the
- and
-isoforms of the
Na+-K+-ATPase are expressed in a
tissue-specific pattern (reviewed in Ref. 19). As to the
-subunits, the
1-isoform is expressed ubiquitously,
whereas the
2-expression is predominant in cardiac and
skeletal muscle, brain, and adipocytes. The
3-isoform is abundant in neural tissues and in the ovary (19,
186). The tissue-specific expression of ATPase
isoforms can be altered during development and by hormones. Any
combination between one of the
-subunits
1-
3 and one of the
-subunits
1-
3 may result in an active
Na+-K+-ATPase isoform (27). The
isoforms differ in their kinetic characteristics with regard to
activation by Na+, K+, ATP, and inhibition by
cardiac glycosides. For example, the rat
1
1-isoform expressed in Sf9 insect cells
shows a higher Na+ and K+ affinity but a lower
affinity for ATP and a lower sensitivity toward cardiac glycosides than
the
3
1-isoform of the
Na+-K+-ATPase (19). Because of
their different kinetic characteristics and tissue-specific
expression, the various Na+-K+-ATPase isoforms
probably meet different physiological demands. On the one hand, the
1
1-isoform may be a general, housekeeping enzyme, since it is ubiquitously expressed and exhibits suitable kinetic properties with relatively high Na+ and
K+ affinities. On the other hand,
3-isoforms
are especially suited to restore the Na+ and K+
gradients across the cell membrane of electrically excitable cells due
to their lower Na+ and K+ affinities and higher
ATP affinity. With regard to the heart, the expression of the
Na+-K+-ATPase isoforms is species specific.
There is a marked variation of
2- and
3-expression among the species, whereas the
1-isoform of the ATPase is present in cardiac tissue of
all species studied. For instance, ventricular cells from adult human
or macaque hearts express three
-isoforms, ventricular myocytes from
the adult rat heart contain mainly
1- and
2-isoforms of the Na+-K+-ATPase,
whereas the sheep heart expresses only the
1-isoform (175). Whether guinea pig ventricular cells exclusively
contains the
1-isoform (175) or additional
2 (62) is still controversial. Because the
-subunits are specific cardiac steroid receptors exhibiting
different sensitivity for these drugs, this controversy echos in
contradictory reports on the number of different glycoside receptors
present in guinea pig cardiac ventricular myocytes (see below). For
further information about the isoforms of the
Na+-K+-ATPase, the reader is referred to
pertinent reviews (19, 174).
The Na+-K+ pump maintains the
Na+ and K+ gradients between the cytosol and
extracellular medium. The maintenance of the gradients is a
prerequisite for the ionic homeostasis of the cells, for cell volume
regulation, and for secondary active transports of amino acids, sugars,
bile acids, neurotransmitters, ions, and other solutes across the cell
boundary. Furthermore, in electrically excitable cells, creation and
maintenance of Na+ and K+ gradients across the
membrane are required for the generation of the resting potential and
the generation and propagation of action potentials. It is clear that
Na+-K+ pumping is of prime functional
significance in cells displaying relatively frequent electrical
discharges over a long period of time, like cardiac cells. Our present
understanding of Na+-K+ pumping is outlined in
the Post-Albers cycle (3, 137-139). For detailled information about the experimental basis and the scientific elaboration of the concept, the reader is referred to excellent, introductory reviews (72, 111). The
simplified scheme of the pump cycle, shown in Figure
1, may facilitate the appreciation of the
findings and ideas discussed below. According to the Post-Albers cycle, the Na+-K+ pump essentially exists in
two conformations, E1 and E2, which may be
phosphorylated (E1-P; P-E2 in the Fig. 1) or
dephosphorylated. In the E1 ATP conformation, the
cation-binding sites of the pump face the cytoplasm and
preferentially bind Na+, whereas in the P-E2
confirmation the binding sites face the extracellular space and
preferably bind extracellular K+ (K
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Electrogenicity denotes the characteristic of a biological transport mechanism to produce electrical current. The concept of electrogenic Na+-K+ pumping slowly emerged during the 1950s (176). However, a most influential paper on active Na+-K+ transport in axons of Sepia and Loligo (94) lent little support to this idea. Connelly (26) was the first to conclude on the basis of experimental evidence that the Na+-K+ pump in nerve fibres is electrogenic. "... since 1960 more and more evidence has accumulated showing that the pump is probably always at least partly electrogenic, with more sodium being extruded than potassium taken up" (176). Today, it is generally accepted that the Na+-K+ pump of animal cells is electrogenic and generates the pump current Ip. Since, under physiological conditions, three Na+ are removed from the cell but only two K+ are taken up per pump cycle, Ip is an outward current. The existence of an electrogenic Na+-K+ pump in cardiac cells was first suggested to explain the high temperature sensitivity of the cardiac resting potential (29). The early experimental data demonstrating electrogenic Na+ pumping in nerve and muscle have been reviewed in detail several years ago (119, 176). The experimental evidence for the electrogenicity of the cardiac Na+-K+ pump from multicellular preparations has been presented in various reviews (45, 47, 63, 65, 181).
The introduction of new methods and techniques into the
electrophysiology of the Na+-K+-ATPase during
the last two decades has markedly improved our knowledge of
electrogenic Na+ pumping. The isolation of single cardiac
cells (140) rendered possible Ip
measurements (48) by means of patch-clamp techniques (77). Reliable Ip measurements
require patch pipettes with a large tip diameter of ~4-5 µm and a
low resistance (<2 M
). In addition, membrane currents other than
Ip have to be suppressed by adequate
experimental conditions. The measurements demonstrated the activation
of Ip by various intra- and extracellular
cations and established the voltage dependence of
Ip under a variety of conditions. Furthermore,
they revealed the existence of transient pump currents in the
Na+ and K+ limb of the
Na+-K+ pump cycle (130,
135). Clearly, electrophysiological investigations of the
Na+-K+ pump in noncardiac preparations have
likewise produced exciting new insight into active cation transport.
For example, studies of Ip in Xenopus
oocytes (110, 147) and combined
electrophysiological and tracer flux measurements in squid axons
(53, 144) provided important data for our
current understanding of structural and functional properties of the
Na+-K+ pump in animal cells (111,
145, 150, 180).
B. Physiological Significance of Electrophysiological Studies on the Cardiac Na+-K+ Pump
The electrogenicity of ion pumps cannot be considered an epiphenomenon which is inevitably linked to ion pumping. Although the electrical potential difference set up by the pump across the cell membrane is thermodynamically equivalent to the simultaneously generated osmotic gradient, the former and the latter display quite different kinetic characteristics. As a consequence, the pump-generated membrane potential is, under many conditions, more efficient as a driving force for secondary active transports than the osmotic (ionic) gradient produced by the pump (see Ref. 111, p. 13-14). In addition, the pump current Ip directly affects the automaticity, the resting and action potential, and thereby the conduction of electrical impulses in excitable membranes. Ip is a direct indicator of Na+-K+ pumping, since the coupling ratio 3Na+:2K+:1ATP per pump cycle remains constant under a variety of conditions including changes of the intracellular Na+ or extracellular K+ concentration and of the membrane potential. This applies for cardiac (46, 65) and noncardiac tissues (reviewed in Ref. 32). Measurements of Ip by means of electrophysiological methods offer the possibility to study the Na+-K+ pump in its physiological environment, i.e., in the cell membrane separating two compartments of different ionic compositions (intra- and extracellular space) and to measure pump-mediated Na+ and K+ fluxes. In cardiac myocytes as in most animal cells, a membrane potential exists across the cell membrane. Because a translocation of electrical charge across the membrane constitutes the pump current, the Ip amplitude must depend on the membrane potential. The interaction between the electrogenic Na+-K+ pump and the membrane potential is best studied by electrophysiological techniques. The measured voltage dependence of external and internal cation binding to the cardiac pump has inspired our imagination of the molecular shape of the Na+-K+ pump as a channel-like structure. Furthermore, electrophysiological studies have rendered possible the identification of additional partial reactions in the pump cycle displaying voltage sensitivity. They also revealed effects of Na+-K+ pumping on currents produced by cardiac ionic channels or transporters in the vicinity of pump molecules. A major advantage of electrophysiological methods for studies on Na+-K+ pump-generated Na+ and K+ fluxes across the sarcolemma over, for instance, tracer measurements is the much better time resolution (up to the microsecond range). Corresponding measurements of Ip provided new insights into the kinetics of the interaction between the Na+-K+ pump and drugs with hitherto unrivalled precision.
This paper reviews some characteristics of the Na+-K+ pump as a current-generating molecule in single cardiac cells. The pump current has been investigated in cells isolated from various regions of the heart of different mammalian species. These regions differ in morphology and function. They include the primary pacemaker (sinoatrial node), the cardiac conducting system (Purkinje fibres), and the working myocardium (atrial and ventricles). Especially the ventricular myocytes are easy to isolate and exhibit a high density of pump molecules in the cell membrane. Their cellular geometry is adequate for studies on the Na+-K+ pump by means of patch-clamp techniques. For these reasons, several investigations that are pivotal for our understanding of the electrogenicity of the Na+-K+ pump have been carried out on these cells.
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II. ACTIVATION OF THE CARDIAC PUMP CURRENT BY MONOVALENT CATIONS |
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A. Activation of Ip by Intracellular
Na+
1. Mean affinity constant values for Ip activation
by intracellular Na+ solution vary according to the
experimental conditions
Earlier studies in multicellular cardiac preparations showed that
intracellular Na+ (Na

) of
the patch pipette is a prerequisite for correct measurements of
Ip as a function of
[Na+]i (121). There is evidence
suggesting the existence of a "fuzzy space" (112), in
which the ionic concentration deviates from that in the bulk cytosolic
solution (23, 159). As to the cardiac Ip-[Na+]pip
relationship, it was demonstrated that the subsarcolemmal [Na+] is not always controlled by the [Na+]
of the pipette solution. This is true not only during strong Ip activation but also in the steady state, at
least in certain cells (16). Furthermore, a relatively
high patch-pipette resistance might cause an additional
Na+ gradient across the cytosol if active
Na+/K+ exchange is strongly activated. Thus a
thoughtful procedure is required to obtain reliable results. In
addition, intracellular K+ are known to be competitive
inhibitors of Na+ at intracellular Na+-binding
sites of the Na+-K+ pump (73).
Consequently, one would expect a lower
[Na+]pip for half-maximal
Ip activation (K0.5
value) from measurements where the main cation of the pipette solution
is a weaker competitor than K+ [Cs+ or even
tetraethylammonium ion (TEA+)]. It might be helpful to
remember these points when reading the data presented in Table
1. They were mainly obtained at
30-37°C. Since, under physiological conditions, K+ is
the main cation in cardiac cells, the "physiological"
K0.5 value for Ip
activation by Na

Table 1.
Activation of Ip by
Na 
Figure 2 compares the Ip activation as a function of [Na+]pip using pipette solutions containing different main cations in guinea pig ventricular myocytes at 0 mV holding potential. Obviously, half-maximal Ip activation occurs at higher [Na+]pip if Cs+ instead of TEA+ or N-methyl-D-glucamine ion (NMDG+) is used as main pipette cation.
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In summary, cardiac Ip is activated by intracellular Na+. The [Na+]pip reported for half-maximal Ip activation varies widely according to the experimental procedure and the ionic conditions chosen.
2. Is Ip activation by intracellular Na+ voltage dependent?
Whether or not binding of intracellular Na+ to the
cardiac Na+-K+ pump is voltage sensitive is
still a point of controversy. The sensitivity of the pump in guinea pig
ventricular cells to [Na+]pip in the range
between 3 and 50 mM increased with depolarization (131).
An e-fold drop of the K0.5 value
(i.e., [Na+]pip for half-maximal
Ip activation) was estimated for a
depolarization by 250 mV. However, the effect was present only in
Na+-rich superfusion media but absent in
Na+-free solution where the apparent affinity of the pump
to Na

5 mM
Na
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B. Activation of Ip by Extracellular
K+ and Its Congeners
1. Measurements in Na+-containing solution
Under physiological conditions, the Na+ pump of animal
cells is activated by extracellular K+. The activation of
the pump by external K+ follows sigmoid saturation kinetics
that can be described by a Hill equation. Earlier studies on the pump
activation by extracellular K+ in multicellular cardiac
preparations have been reviewed in useful articles (45,
65). The majority of the data suggest half-maximal Ip activation by [K+]o
in the low millimolar range. Similar K0.5 values
were obtained from experiments on single cardiac myocytes. The data
obtained at 30-37°C are presented in Table
2. The Ip
activation by two K+ congeners, Tl+ and
NH


Rb+ > Cs+
Li+
(151). The order of potency is the same as that observed
for the activation of the isolated Mg2+-dependent,
Na+- and K+-activated ATPase, which is the
molecular basis of the Na+-K+ pump
(155). In addition, the order of potency confirmed the sequence deduced from voltage-clamp measurement on guinea pig papillary muscles and sheep Purkinje fibers (39).
Table 2.
Activation of Ip by
K 
2. K0.5 values for Ip activation by external cations in Na+-free media
Since the early observation on erythrocytes it is known that
external Na+ and K+ compete for common binding
sites (138). As a consequence, the K0.5 value for pump activation by extracellular
K+ is appreciably lower in Na+-free than in
Na+-containing media. Thus one would expect for cardiac
cells a lower K0.5 value for the
Ip activation by K+ and its
congeners in Na+-free than in Na+-containing
solution. In fact, a K0.5 value of 0.22 mM was
derived for the activation of Ip by
K












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3. Activation of Ip by K
The negative slope of the Ip-V
relationship in Xenopus oocytes is probably due to
voltage-sensitive binding of extracellular K+ to the
Na+-K+ pump (147). It is
considered that K










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(1) |
is a steepness factor, and F, Vc, R, and T have their
usual meanings. K0.5(Vc=0
mV) values for Ip activation by various
external cations and the respective
-values are collected in Table 3
(from Ref. 18). Because the steps subsequent to K

-values and the
Hill coefficients derived, it can be estimated that the external
activator cations sense ~0.2 of the membrane potential at their
binding sites (see Ref. 150). In summary, the binding of
K
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C. Li+ Binds to Extra- and Intracellular Binding Sites of the Cardiac Na+-K+ Pump and Activates Ip
It is known from earlier studies on multicellular cardiac
preparations that Li+ is a weak external pump activator
cation (see Ref. 64 for references). As to isolated cardiac cells,
Ip is activated by Li







20 mV) (87). Half-maximal
Ip activation was observed at 23 or 73 mM
Li



D. Pump Current Densities, Pump Site Densities, and Maximum Turnover Rate
Usually, in experiments designed to measure
Ip, the Na+-K+ pump of
cells is strongly activated by nearly saturating concentrations of
Na
45 mV and near body temperature. They agree
reasonably well with earlier data for the active Na+ efflux
via the cardiac Na+ pump obtained by various
nonelectrophysiological methods (63). (An
Ip density of 1 µA/cm2 translates
to a pump-mediated flux of ~30
pmol·cm
2·s
1). They are also in line
with previous estimates of Ip density in
multicellular cardiac preparations (28). For comparison, simultaneous measurements of Ip density and
active 22Na efflux in squid axons yielded 0.89 µA/cm2 and ~ 25 pmol·cm
2·s
1 (144), again
in accordance with earlier flux data (94).
From the maximum quantity of movable charge derived from measurements
of transient pump currents in single cardiac cells (see sect.
IVB), the pump site density of the myocytes has
been estimated assuming a single charge (1.6 × 10
19
C) to be transfered per pump molecule and the specific membrane capacitance to be 1 µF/cm2. The first number obtained by
this procedure for the pump site density of single cardiac cells was
published by Nakao and Gadsby (130). According to the
authors, the pump site density of guinea pig ventricular myocytes is
~1,200/µm2. Later estimates include slightly higher
numbers of 2,200 to 2,800 pumps/µm2 for guinea pig
ventricular cells (108) and ~ 2,600 sites/µm2 for rat ventricular myocytes (35).
These values are higher by a factor 2 to 4 than an earlier number
derived from Ip measurements in a guinea pig
multicellular ventricular preparation (28).
The maximum turnover rate of the charge transfer by the cardiac Na+-K+ pump can be calculated if the maximum Ip density and the pump site density are known. Accordingly, a maximum turnover rate of ~80/s has been obtained from the data mentioned above for guinea pig ventricular myocytes (51). A higher maximum turnover rate of ~200/s has been derived from measurements of transient pump currents in excised patches of rat ventricle cells (42). Both numbers apply to turnover rates at 0 mV and 36°C.
The pump site densities reported above are within the range of earlier estimates in a variety of cell species by means of different methods, mainly cardiac glycoside binding (see Table 3 in Ref. 32). These estimates vary considerably among the cell types, whereas the calculated turnover rates are much more similar and amount to ~100/s at body temperature (32).
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III. THE REVERSAL POTENTIAL OF THE CARDIAC PUMP CURRENT |
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A. Theoretical Considerations
The free energy of intracellular ATP hydrolysis
(
GATP) fuels the active
Na+/K+ transport by the
Na+-K+ pump. It amounts to about
60 kJ/mol in
many animal cells (31). The energy is used to transport 3 mol Na+ and 2 mol K+ against their respective
concentration gradient (osmotic work) and the electric charge of 1 mol
Na+ against the electrical field across the cell membrane
(electrical work). Thus the physiological active
Na+/K+ transport proceeds as long as
|
(2) |
GATP equals the
energy required for the active cation transport. This potential is
called the reversal potential (Erev) of the
Na+-K+ pump. No active
Na+/K+ fluxes occur at
Erev, and Ip vanishes. If
the membrane potential becomes more negative than
Erev, the Na+-K+ pump
runs backward producing an active Na+ influx into the cell
and generating an inwardly directed Ip. Erev can be derived from Equation 2
by
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(3) |
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(3a) |
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GATP and
steepening the ionic gradients.
GATP is given
by
|
(4) |
G
GATP can be obtained by increasing the
[ADP] · [Pi]/[ATP] ratio in the cell studied.
B. Evidence for Backward Running Na+-K+ Pump and Determination of the Na+-K+ Pump Erev
The procedure outlined above was first applied to squid
giant axons, and an inwardly directed Ip was
demonstrated (34). Under similar experimental conditions,
an inwardly directed Ip was measured in guinea
pig ventricular myocytes over a range of membrane potentials between
+40 and
120 mV (6). Figure
6 presents some of the results . Figure
6A (bottom trace) demonstrates that the
strophanthidin-inhibited pump current Ip of
a guinea pig ventricular myocyte is inwardly directed under the
conditions chosen. As shown in Figure 6B,
Ip remains an inward current even at positive
potentials. Figure 6C displays I-V
curves before, during, and after application of the cardiac steroid, a
specific inhibitor of the Na+-K+ pump. The
I-V relationships were derived from the
experiment illustrated in Figure 6A. Figure 6D
exhibits the Ip-V curve of the
backward running Na+-K+ pump obtained from the
strophanthidin-sensitive current Ip at various potentials, as illustrated in Figure 6B.
Ip was small at +40 mV, increased with
hyperpolarization, and reached an apparent plateau level of about
0.32 µA/cm2 near
100 mV. By means of a similar
approach, an inward Ip between +30 mV and
110
mV that did not reach a plateau at the most negative membrane potential
tested was observed in cardiac Purkinje cells. The
Ip density amounted to
0.13
µA/cm2 at
95 mV (71). Furthermore,
Erev shifted to more positive potentials at less
negative
GATP values. Shifting
GATP to less negative values also diminished
Ip over the entire voltage range studied.
However, the concentration of extracellular Cs+ required
for half-maximal Ip activation remained
unchanged. Equation 3 predicts changes of
Erev by variation of the transmembrane gradients of the pumped cations at constant
GATP. This
prediction was verified (69). Flattening the ionic
gradients increased Ip over the entire voltage
range studied and shifted Erev toward more
negative potentials. Conversely, steepening the gradients diminished
Ip and shifted Erev to
more positive potentials.
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The Ip-V curve of the backward
running Na+-K+ pump was also studied in
internally dialyzed squid axons under voltage clamp (143). There was a steady decline of Ip density from an
apparent plateau at
80 to
100 mV (
0.24 µA/cm2) to
practically zero at +30 mV. In contrast to earlier observations (34), a negative slope of the
Ip-V relationship was not found. In
K+-free solution, an inwardly directed
Ip was measured in Xenopus oocytes
with a reduced intracellular [Na] and an augmented
[ADP] · [Pi]/[ATP] ratio. The
Ip density amounted to about
0.1
µA/cm2 at
100 mV without an apparent plateau and
declined with depolarization. Under these conditions the
Ip reversal potential was obviously at positive
membrane potentials (38).
In summary, the studies demonstrate under suitable conditions in various cell species a backward running Na+-K+ pump generating an inward Ip, in line with thermodynamic considerations.
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IV. VOLTAGE DEPENDENCE OF CARDIAC PUMP CURRENTS |
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A. Cardiac Steady-State
Ip-V Relationships
1. Basic characteristics of the Ip-V curve
Whole cell recording is a mode of the patch-clamp technique
(77) which permits a much better control of the membrane
potential and the ionic composition of the intracellular compartment
than earlier voltage-clamp techniques applied to multicellular
preparations. Gadsby et al. (48) were the first to study
the voltage dependence of Ip by whole cell
recording from isolated, single guinea pig ventricular cells in
Na+-containing solution after minimizing passive
Na+, Ca2+, and K+ currents.
According to the authors, the Ip-V
relationship of the myocytes is sigmoid in shape with a steep positive
slope between
100 and 0 mV, a less steep slope at more negative
potentials, and nearly no voltage dependence of
Ip at positive membrane potentials (see also
Fig. 7A, circles). A further
careful study by the authors confirmed these characteristics of the
cardiac Ip-V curve (51). However, a region of negative slope in the
Ip-V curve was not observed. A
decrease of Ip with hyperpolarization in guinea
pig ventricular cells was also reported in 1987 by others
(123) in abstract form. The pump current of adult isolated
rat cardiac myocytes displays a similar voltage dependence as in guinea
pig myocytes (167). This is also true for the voltage
dependence of Ip in rabbit ventricular cells
dialyzed with a pipette solution containing 80 mM Na+
(79). For unknown reasons, some authors were unable to
detect any voltage dependence of Ip in guinea
pig ventricular myocytes (127). In the cardiac conducting
system, the Ip-V curve shows little
voltage dependence at membrane potenials positive to
20 mV. The pump
current declines with hyperpolarization (66).

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Fig. 7.
The effect of Na 



), then 1.5 mM (
), and then 150 mM again (
). Steady
current levels were measured near the end of 160-ms voltage pulses from
the holding potential
40 mV. Pump currents were obtained by
subtracting currents recorded in the presence of 2 mM strophanthidin
from the average of currents recorded just before, and just after, the
brief exposure to strophanthindin. [Adapted from Gadsby and Nakao
(50).] B and C:
Ip-V relationships at various
[Na+]pip in Na+-free solution.
Na+-K+ pump currents normalized to cell
capacitance are plotted versus membrane potential. B:
Ip-V relationships at 0 mM
(
), 0.5 mM (
), 2 mM (
),
5 mM (
), and 50 mM (
)
Na
5 mM)
Na

2. Effects of internal Na+ on the Ip-V relationship of cardiac cells
Already in 1987 it was reported that lowering internal
[Na+] diminishes the pump current of guinea pig
ventricular cells in Na+-rich solution and shifts the
Ip-V curve to the right (to more positive potentials), whereas the voltage dependence of
Ip persists (50). However, in the
absence of external Na+, lowering
[Na+]pip from 50 to
17 mM scaled down the
Ip-V relationship of these cells
without a marked shift toward more positive potentials
(131). A similar shift of the normalized
Ip-V curve to the right and a
reduction of Ip if [Na]pip was
lowered from 50 to 5 mM occurred in sheep cardiac Purkinje cells
superfused with Na+-rich medium (66). Reducing
[Na+]pip from 85 to 20 mM scales down the
Ip-V relationship of rat myocytes at
145 mM Na
5 mM (main cation in
pipette: TEA+ or NMDG+) (9). Some
of the findings are displayed in Figure 7, B and C. Lowering [Na+]pip from 50 to 5 mM or
below reveals the effect of [Na+]pip on the
cardiac Ip-V curve in myocytes
containing NMDG+ (Fig. 7B) or TEA+
(Fig. 7C) as main cation. The relative
Ip activation at low
[Na+]pip increases with depolarization, i.e.,
the K0.5 value for Ip activation by [Na+]pip declines (Fig. 3). In
Na+-free pipette solutions, Ip is
absent in the entire voltage range tested.
In summary, reducing [Na+]pip to values well below the K0.5 value reveals not only a scaling down of the Ip-V relationship in cardiac cells but also a rightward shift of the Ip-V curve to more positive membrane potentials. The effect is observed in Na+-free and Na+-containing solution. The reason why Nakao and Gadsby (131) did not detect the shift of the Ip-V curve to the right at low [Na+]pip and [Na+]o remains unclear. The shift may be produced by the voltage sensitivity of internal Na+ binding to the Na+-K+ pump. At [Na+]pip, lower than saturating concentrations, negative potentials will inhibit the binding of internal Na+ to the pump and thereby the activation of Ip. As a consequence, the normalized Ip-V relationship is shifted toward more positive membrane potentials. However, without additional information it cannot be excluded that the voltage dependence of internal Na+ binding is only apparent and is due to the voltage sensitivity of any slower step subsequent to Na+ binding in the pump cycle.
3. The voltage dependence of Ip varies with the extracellular Na+ concentration
The sigmoid shape of the cardiac
Ip-V relationship in
Na+-containing solution is characterized by a steep
positive slope of the curve at negative membrane potentials up to
100
mV and a less positive slope at even more negative potentials. Lowering [Na+]o reduces the voltage dependence of
Ip in cardiac cells in a concentration-dependent manner, as first reported from measurements on isolated guinea pig ventricular myocytes (50,
131). Figure 7A shows, for example, that
Ip of a guinea pig ventricle cell in
Na+-poor solution (1.5 mM Na





120 and
40 mV. The authors were able to fit their
Ip-V data obtained at various [Na+]o by a kinetic equation derived within
the framework of the pseudo two-state model of
Na+-K+ pumping (81; see sect.
IVA6). The mechanism by which
Na




4. Effect of K
In contrast to the sigmoid
Ip-V relationship of
cardiac cells (48), endogenous
Na+-K+ pumps of
Xenopus oocytes (110, 158) and
Torpedo pumps expressed in Xenopus oocytes
(157) display Ip-V
curves exhibiting a maximum and a region of negative slope positive to
+20 mV. The authors concluded that their measurements suggest at least
two voltage-dependent partial reactions oppositely affected by the
membrane potential in the Na+-K+ pump cycle
(110). It was considered that the region of negative slope
in the Ip-V curve might possibly be
an artifact caused by passive currents overlapping
Ip (146). A later study finally established beyond any reasonable doubt that the
Ip-V relationship of
Xenopus oocytes shows a region of negative slope
(147). According to the authors, the negative slope of the
Ip-V curve is most probably due to
voltage-dependent binding of K
Since then, Ip-V curves displaying a
region of negative slope were also observed under special conditions in
cardiac cells. A region of negative slope in the
Ip-V curve of rabbit cardiac Purkinje
cells was measured in Na+-containing media at low
[K+]o (less than K0.5
value) and positive membrane potentials. The region extends to negative
voltages in Na+-free solution containing low concentration
of K+ or its congeners Tl+,
NH
|
5. The 3Na+:2K+ stoichiometry of the Na+-K+ pump is voltage independent
For an adequate interpretation of the shape of
Ip-V relationships it is essential to
know whether the 3Na+:2K+ stoichiometry of the
Na+-K+ pump is voltage dependent. Two studies
of Ip-V curves in isolated noncardiac
cells have provided relevant informations. Simultaneous measurements of
22Na efflux and pump current revealed that the
Na+:K+ stoichiometry of Torpedo
californica pumps expressed in Xenopus oocytes is
voltage independent between +50 and
100 mV (157, 179). Comparable experiments on squid giant axons also
demonstrated that the Na+:K+ stoichiometry is
voltage independent (144). Both the pumped Na+
efflux and the Na+-K+ pump current declined by
roughly the same amount upon hyperpolarization. This voltage dependence
implies an effect of membrane potential on Na+
release/rebinding at the extracellular face of the cell membrane rather
than on a hypothetical reverse Na+ transport via
Na+-K+ pump.
6. Interpretation of Ip-V relationships
The sigmoid shape of the Ip-V curve in guinea pig ventricular myocytes (48) was interpreted in the following way (51). Since the pump rate (and therefore Ip) is nearly voltage independent at positive membrane potentials, a voltage-independent partial reaction rate limits the pump cycle in this potential range. At more negative potentials a step with voltage-sensitive transition rates limits the cycle rate, either directly, or via the control of the level of an intermediate that enters the rate-limiting partial reaction (see also Ref. 33) . In view of the experimental data only a single voltage-dependent partial reaction was assumed. Several studies on cells and cell-free systems (see Refs. 33, 142) suggested that the Na+-extruding limb of the pump cycle contains this voltage-sensitive step. More precisely, it seems likely that the deocclusion and release/rebinding of Na+ to/from the extracellular space are voltage dependent (see Ref. 4). This step in turn controls the concentration of an intermediate participating in the rate-limiting partial reaction, which, under the experimental conditions chosen, is probably the K+ translocation to the cell interior (see Ref. 33). Thus the shape of the Ip-V relationship in guinea pig ventricle cells was explained by assuming that depolarization of the sarcolemma enhances the voltage-dependent step of the Na+ translocation such that the voltage-insensitive K+ import becomes rate limiting for the Na+-K+ pump cycle at positive membrane potentials (6).
The quantitative kinetic analysis of the data (51) was based on a model mentioned above (81). According to this model for the interpretation of Ip-V curves, any multistate (unbranched) pump cycle containing a single voltage-sensitive step can be treated as a pseudo two-state cycle (scheme 1)
|
where E1 and E2
represent the two states of the pump cycle,
and
denote
empirical voltage-dependent rate constants, and c and
d signify lumped empirical rate constants that are voltage insensitive. If an asymmetrical Eyring barrier exists for charge translocation (see Ref. 111, p. 68-69) and a single charge is moved in
the pump cycle (130), the voltage-sensitive rate
constants are given by
=
0 · exp[
· V · F/R · T]
and
=
0 · exp[
(1
)V · F/R · T], where
0 and
0 represent the forward and
backward rate constant at zero potential, respectively;
indicates
the location of the barrier in the cell membrane; and V,
F, R, and T have their usual meaning.
By assuming numbers for the rate constants suggested by the
experimental data, and
= 0.1, it was possible to fit the
turnover rate-voltage relationship derived from the Ip-V curve by means of an equation
deduced on the basis of the pseudo two-state model for the pump
cycle (51).
As mentioned above, this interpretation assumes a single voltage-sensitive partial reaction in the Na+-K+ pump cycle. However, to understand Ip-V curves displaying a region of negative slope, an additional potential-dependent step in the pump cycle has to be postulated (147). It was hypothesized that binding of extracellular K+ to the Na+-K+ pump might occur within the cell membrane. To reach their binding sites, the K+ have to cross a part of the electrical field over the membrane. As a consequence, the membrane potential affects the local K+ concentration at the binding sites. Depolarization decreases the K+ concentration at the sites and thereby diminishes the pump cycle rate and Ip. This mechanism produces the negative slope of the Ip-V relationship. The reader is referred to a brilliant, introductory review (Ref. 111, p. 75-83) for a detailed discussion of this "high field, narrow access channel hypothesis." Some early observations (6, 74) suggested that the steps of K+ translocation by the pump beyond K+ binding may be voltage independent, and recent experimental evidence (135) supports this view.
B. Transient Pump Currents
1. Identification of transient pump currents during
electroneutral Na+/Na+ exchange in cardiac
cells
To identify voltage-dependent partial reactions in the pump
cycle, it is extremely helpful to constrain the active cation transport
to only a few steps of the cycle. The physiological Na



-adrenergic stimulation on the transient pump current was studied in
adult rat cardiac myocytes (35). According to the authors,
10-50 µM norepinephrine did not alter the total charge transferred
during the transient pump current. The density of pump molecules was
calculated from the maximal charge moved during the transient current.
It remained unchanged in the presence of the drug. Most recently, an
altered voltage dependence of the charge transfer by the the
Na+/K+ pump was observed upon application of
forskolin to guinea pig ventricular cells (108; see sect.
VIIIA1).

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Fig. 9.
Voltage dependence of charge movement (A) and of decay
rate constant (B) during transient,
strophanthidin-sensitive currents (inset in
A). Holding potential,
40 mV; temperature, 35°C.
A, inset: superimposed sample records of
strophanthidin-sensitive currents elicited by pulses to +60, +20,
20, and
60 mV; graph, time integral [Q(V)]
of current transients initiated by each pulse, plotted against pulse
potential (V).
, Charge measured directly
from the records;
, charge estimated by extrapolation
of the exponential current decay. Only "on" charge movement is
plotted. "Off" and "on" charge movements appear to be the same.
The smooth curve, derived from the Boltzmann relation, shows
Q(V)/
Qmax = 1/[1 + exp(V'
V)/k], where
Q(V) = Q(V)
Qmin,
Qmax = Qmax
Qmin;
V' is the potential at the midpoint, and k
provides a measure of the equivalent charge. Vertical bar, 250 pA;
horizontal bar, 25 ms. B: rate constants of exponential fits
to transient currents plotted against membrane potential:
, rate constants of "on" transients;
, rate constants of "off" transients, at
40 mV,
following repolarization from
60 mV < V < +80
mV. C: simplified kinetic scheme for ATP-dependent
Na+/Na+ exchange emphasizing the
voltage-sensitive step A
B, governed by
voltage-dependent forward and backward rate constants,
(V) and
(V). [Adapted from Nakao and Gadsby
(130).]
2. Hypothetical mechanism of the transient pump currents
The characteristics of the transient pump currents were explained
by the hypothesis that the Na+ binding sites of the pump
molecule provide two negative charges and thus exhibit one positive
charge if three Na+ are bound (130). As a
consequence, the rate of Na+ translocation by the pump is
expected to be voltage sensitive. Since a positive charge is moved, in
general the forward rate constant of the translocation should be
increased upon depolarization, whereas the backward rate constant
should be diminished. Vice versa, a hyperpolarization of the cell
membrane should reduce the forward rate constant and augment the
backward rate constant. Thus the steady-state concentrations of the
enzyme intermediates participating in the translocation step depend on
membrane potential. After a voltage jump, the rate constants
immediately reach their new values, whereas the concentrations of the
intermediates approach their new steady-state more slowly. During
this approach the forward and backward fluxes of the intermediates
differ and cause a transient pump current that declines
monoexponentially with a rate constant that is the sum of the new
forward and backward rate constants at the new membrane potential. The
rate constant of decline of the transient pump current increases
steeply with hyperpolarization but is nearly constant at positive
potentials (Fig. 9B). Since at positive voltages the
backward rate constant of the translocation step should be small, the
rate constant of current decline will be dominated by the forward rate
constant, which seems to be nearly voltage independent. Thus the
observed potential dependence of the rate constant of current decline
is largely due to the voltage dependence of the backward rate constant.
By comparing the forward and backward rate constants near zero
potential with data in the literature, the authors suggested that the
studied voltage-dependent step of the pump cycle might be the
translocation/deocclusion E1P (Na3)
E2P Na3. Furthermore, they discussed the
possibility that the voltage dependence of this partial reaction might
fully account for the electrogenicity and voltage sensitivity of the physiological Na+/K+ exchange carried out by
the Na+-K+ pump (130). By
application of a giant-patch technique (90) to guinea
pig ventricular myocytes, two components of transient pump current were
distinguished, suggesting Na+ release/rebinding from/to
E2P in two partial reactions with different rate constants,
amounts of charge Q moved and slope factors of the
corresponding Boltzmann-Fermi equations (91). Both
Qfast (within the first 100 µs during a
potential step) and Qslow (in the millisecond
range) depend on [Na+]o. These observations
were interpreted to mean that the Na+ binding sites of the
pump open to the exterior in two voltage-sensitive partial
reactions. Deocclusion of the first Na+ rate limits the
strongly voltage-dependent release of this ion constituting
Qslow, whereas a further electroneutral
conformational change enables the last two Na+ to
dissociate from their binding sites in a weakly voltage-sensitive partial reaction (Qfast). The further analysis
of transient pump currents during 50-µs potential steps revealed
additional current components probably related to Na+
release and rebinding from or to the E2P
Na3/E2P Na2 intermediates of the
pump cycle (92). Similarly, measurements on squid axons succeeded in differentiating at least three components of transient pump current which might be related to Na+ release to the
exterior (ultrafast component) and two distinct transitions involved in
the Na+ deocclusion/release (142).
High-speed voltage jumps revealed that the pre-steady-state charge
movements relax in three phases that represent the deocclusion and
release of the three Na+ in a strictly sequential order
(95).
3. Identification and properties of transient pump currents during electroneutral K+/K+ exchange
Transient pump currents might likewise occur under conditions that
constrain the Na+-K+ pump to
K



E2K2
K+ translocation is probably electroneutral, but, on the
other hand, they did not exclude voltage-dependent
K







| |
V. DEPENDENCE OF PUMP CURRENT ON INTRACELLULAR ATP |
|---|
|
|
|---|
The molecular basis of the Na+-K+ pump is
the Mg2+-dependent,
Na+-K+-activated ATPase. Thus one would expect
that the activity of the pump, including the generation of
Ip, depends on intracellular ATP. In 1983 it was
verified that in dog cardiac preparations both
Na+-K+ pump and sarcolemmal
Na+-K+-ATPase display identical dependencies on
Na+ and ATP (136). The ATP concentration
required for half-maximum pump activation in the sarcolemmal
vesicles studied was estimated to be ~210 µM. From studies on the
steady-state Ip in isolated cardiac Purkinje
cells it seems likely that the Na+-K+ pump is
preferentially fuelled by glycolytic ATP synthesis (70), although other ATP sources contribute to the ATP supply for the cardiac
Na+-K+ pump (see Ref. 141). The pump current of
giant patches from guinea pig ventricular cells is half-maximally
activated at ~80 µM cytosolic MgATP (93). Similarly, a
saturable ATP dependence of Ip with a Michaelis
constant (Km) value of ~150 µM was observed in giant patches from rat and guinea pig ventricular myocytes (42). Figure 10
illustrates these findings. Figure 10A displays outward
(pump) currents (bottom trace) evoked by various MgATP concentrations (top trace) at 0 mV in a giant patch from a
rat ventricular myocyte. The pump current increases with increasing [MgATP]. Figure 10B shows normalized mean
Ip values as a function of [MgATP]. The
experiments were carried out on giant patches from rat and guinea pig
ventricular cells under various ionic conditions at zero potential. The
dashed curve represents the fit of a Michaelis-Menten equation to
the data. The K0.5 values were estimated to be
146 to 165 µM MgATP and suggest ATP binding to the low-affinity
ATP binding site of the Na+-K+ pump. A strong
reduction of the transient pump currents evoked by voltage jumps was
noted in guinea pig cells upon internal perfusion with ATP-poor or
-free solutions (130). ATP concentration jumps induced by
photolytic ATP release from caged ATP activate cardiac Ip. The kinetics of the transient pump currents
induced by ATP jumps or voltage jumps were studied in giant patches
from guinea pig cardiomyocytes superfused with K+-free
media (43). The rate constants of the fast component from the transient pump currents evoked by an ATP concentration jump were
very similar to the rate constants of the transient pump currents
induced by voltage jumps to zero or positive potentials (~200
s
1 at 24°C) and displayed the same activation energy.
Furthermore, the two techniques revealed a comparable amount of charge
moved during the transient pump currents in the same giant patch. This result implies that the same charge-carrying mechanism was studied by both techniques.
|
| |
VI. TEMPERATURE DEPENDENCE OF STEADY-STATE AND TRANSIENT PUMP CURRENTS |
|---|
|
|
|---|
Because sarcolemmal Na+-K+ transport by the pump is performed by a vectorial enzyme reaction, a marked temperature dependence of the Na+-K+ pump activity is anticipated. As to the steady-state Ip of single rabbit sinoatrial node cells, a Q10 value of 2.1 in a temperature range between 25 and 37°C has been reported (151). This means a change of the Ip amplitude by a factor of 2.1 following an alteration of the temperature by 10°C. Similar Q10 values were found for the steady-state Ip of isolated sheep cardiac Purkinje cells (Q10 = 2.9) and single guinea pig ventricular myocytes (Q10 = 2.2) (187).
Concerning transient pump currents, the rate constants of current
decline during Na











| |
VII. SIGNIFICANCE OF ELECTROGENIC SODIUM-POTASSIUM PUMPING FOR THE MEMBRANE POTENTIAL OF CARDIAC CELLS |
|---|
|
|
|---|
A. Contribution of Electrogenic Na+-K+
Pumping to the Cardiac Resting Potential
1. Theoretical note
The Na+-K+ pump directly generates a
potential difference across the cell membrane. Under physiological
conditions this potential difference Ep
hyperpolarizes the cell membrane and, therefore, increases the absolute
value of the resting potential. According to Ohm's law, the magnitude
of Ep depends on both the
Ip amplitude and the membrane resistance. The
Ip amplitude is determined by the pump molecule
density in the cell membrane and by the turnover rate of the pump. The
membrane resistance depends on the density and characteristics of
mainly the ionic channels in the membrane. Accordingly, the
contribution of electrogenic Na+-K+ pumping to
the resting potential of various cells varies widely. There are animal
cells in which the resting potential is predominantly generated by the
Na+-K+ pump. These cells include T lymphocytes
of mice (100), rat mast cells (21), and
vomeronasal chemoreceptor neurons of the frog (177). In
cells where the passive Na+ and K+ fluxes
crucially contribute to the setting of the resting potential, the
effect of electrogenic Na+-K+ pumping on the
resting potential (Em) may be described by the Mullins-Noda equation (129)
|
(5) |
|
(6) |
2. Experimental data
Experimental estimations of the pump contribution to the cardiac resting potential in multicellular preparations yielded 5-10% of the resting potential (reviewed in Refs. 40, 65). As to single cells, an Ep of 4.2 mV was derived contributing to the resting potential of canine cardiac Purkinje cells (25). In chick cardiac myocytes, Ip hyperpolarizes the sarcolemma by 6.5 mV (170). An Ep value of only 0.4 mV was reported from measurements on guinea pig ventricular myocytes (114). A much higher Ep of ~20 mV can be calculated from data obtained in experiments on isolated rabbit sinoatrial node cells (151). Although, in general, the contribution of electrogenic Na+-K+ pumping to the cardiac resting potential amounts to only a few millivolts, it is of physiological significance. This is because the steady-state inactivation of Na+ (and Ca2+) channels and thus their availability for the production of action or pacemaker potentials depends steeply on membrane voltage near the resting (or maximal diastolic) potential.
B. Importance of Electrogenic Na+-K+ Pumping for the Cardiac Action Potential
As mentioned above, the potential difference Ep contributed by the electrogenic Na+-K+ pump to the cardiac membrane potential depends on the Ip amplitude and the membrane resistance. Compared with the values at resting potential, both factors are increased at the plateau of the cardiac action potential. The higher membrane restistance is mainly due to a reduced potassium conductance at the plateau level (see Ref. 132, p. 28). The cardiac Ip-V relationship predicts an increase of Ip by a factor ~2 at the action potential plateau (15, 131) (see also Fig. 7A). Furthermore, the enhanced Na+ influx during an action potential additionally augments Ip via an increased subsarcolemmal Na+ concentration. For these reasons one would expect that electrogenic Na+-K+ pumping markedly affects the shape of the cardiac action potential during the plateau phase. Indeed, in the first voltage-clamp study of the cardiac Ip in a multicellular preparation, a prolongation of the action potential was observed in Purkinje fibers as an early effect of the Na+-K+ pump inhibition by the cardiac glycoside DHO (99). Vice versa, the activation of the pump in Purkinje fibers following a short period of increased stimulation frequency or in K+-free solution considerably shortens the action potential duration (47).
In isolated cardiac cells too, an early effect of cardiac glycosides is a lengthening of the action potential. Figure 11 shows an example. The action potential of an isolated guinea pig ventricular myocyte is prolonged by ouabain in a concentration-dependent manner. The higher the concentration applied, the stronger is the Na+-K+ pump inhibition and the longer is the action potential duration (at 90% repolarization; APD90) since the repolarizing Ip is increasingly diminished. Comparable effects of other cardiac steroids have been reported (e.g., Ref. 113). The effects are in accordance with calculations based on the membrane resistance and Ip amplitude at the plateau level of the action potential (113, 115, 116). These calculations predict a plateau depolarization of up to 9-16 mV following a blockade of Ip. The significance of Ip and its modulation by adrenergic transmitters for the shape of the cardiac action potential under physiological conditions have recently been reemphasized (183).
|
| |
VIII. MODULATION OF CARDIAC PUMP CURRENT BY AUTONOMIC TRANSMITTERS AND RELATED COMPOUNDS |
|---|
|
|
|---|
It seems likely that autonomic transmitters adjust the
Na+-K+ pump activity to the functional demands
of the cells and the organism. In fact, various effects of the
transmitters (and related compounds) on Na+-K+
pumping of single cardiac cells via adrenergic and muscariner-gic acetylcholine receptors have been described. The mechanisms of the
intracellular signal transduction from receptor stimulation to the
cardiac Na+-K+ pump are outlined in scheme 2, which is partially hypothetical.
-Adrenergic agonists exert their
effects via
-receptors in the sarcolemma. The receptors are coupled
to Gs proteins which activate adenyl cyclase (AC) to
facilitate the synthesis of cAMP
|
cAMP stimulates protein kinase A (PKA) which enhances active
Na+/K+ exchange by phosphorylation of
Na+-K+ pump molecules. Stimulation of
adrenergic
1-receptors causes, via a G protein, the
activation of phospholipase C (PLC) which cleaves phosphatidylinositol
4,5-diphosphate (PIP2) to diacylglycerol (DAG) and inositol
trisphosphate (IP3). DAG and Ca2+ (released
from the sarcoplasmic reticulum by IP3) activate protein kinase C, which in turn stimulates cardiac
Na+-K+ pumping by pump phosphorylation. Binding
of acetylcholine to a muscarinergic acetylcholine receptor
(M2 receptor) causes inhibition of adenylyl cyclase,
decrease of [cAMP], and thereby inhibition of PKA and
Na+-K+ pump activity.
A. Effect of Adrenergic Agonists on Ip
1. A stimulatory action of these substances on the
Na+-K+ pump in multicellular cardiac
preparations has been reported by various authors during the last 45 years (for references, see Ref. 67). However, the mechanism of action
remained unclear. More recent studies on isolated cardiac cells failed
so far to elucidate the mechanism. Even worse, according to the
information available at present, it seems questionable whether a
universal mechanism of catecholamine action exists for cardiac cells of
different animal species. Apart from species differences, technical
problems [impaired receptor function following cell isolation, use of
patch pipettes with relatively high resistance (>2 M Isoprenaline (10 In contrast to the just described observations on guinea pig ventricle
myocytes, a study on rat ventricular cells did not detect any
-Adrenergic stimulation of cardiac Ip
)] may also
contribute to the inconsistent observations.
7 to 10
6 M) induced a
decrease of the intracellular Na+ activity
(a

-adrenergic stimulation of the Na+-K+ pump in guinea pig ventricular myocytes
(120). However, other observations on the same cell
species revealed a direct
-adrenergic effect (56). The
-adrenergic modulation of Ip was not due to drug-induced changes of the intracellular Na+ or
extracellular K+ concentration but directly to the
variation of the maximum pump turnover rate. Interestingly,
Ip was reduced by isoprenaline at low (<150 nM)
intracellular [Ca2+], but increased at higher
[Ca2+]i. In a further patch-clamp study
(54) it was shown that the
-adrenergic increase of
Ip at high [Ca2+]i was
mediated by a phosphorylation step via the cAMP-PKA cascade. A
study of the
-adrenergic effects of isoprenaline on the
Ip-V relationship in guinea pig
ventricular cells (59) showed that the increase of
Ip at high [Ca2+]i
(1.4 µM) was voltage dependent. At positive voltages isoprenaline had
little effect on the Ip amplitude, whereas the
drug increased Ip at negative membrane
potentials. However, the inhibition of Ip caused
by isoprenaline at low [Ca2+]i (15 nM) was
voltage independent. The authors suggested two effects of intracellular
Ca2+ on the
-adrenergic modulation of cardiac
Ip. First, it counteracts the inhibiton of
Ip induced by protein phosphorylation via PKA activation (55), and second, it shifts the
Ip-V curve in the negative direction
during the
-adrenergic, PKA-mediated phosphorylation of the
cardiac Na+-K+ pump. The physiological
significance of the increase of Ip during
-adrenergic stimulation at high [Ca2+]i
may consist of a partial compensation for
-adrenergic effects on
membrane conductances, which tend to prolong the cardiac action potential. Most recently, an increase of Ip by
the adenylyl cyclase activator forskolin in guinea pig ventricular
myocytes has been reported. The increase was observed at nanomolar and
subnanomalar [Ca2+]i and was mediated, at
least in part, by facilitation of a partial reaction in the
Na+ limb of the pump cycle, different from
Na
-adrenergic regulation of the Na+-K+-ATPase
(101). Surprisingly, other experiments using the same cell
species, similar solutions, and electrophysiological techniques but a
different protocol showed an Ip stimulation by
norepinephrine and isoprenaline (35). Figure
12 illustrates some of these results. Under the conditions chosen, the membrane current of a rat myocyte clamped at
40 mV is nearly identical to the ouabain-sensitive Ip (Fig. 12, A and B).
Norepinephrine (10 µM) and isoprenaline (10 µM) increase
Ip. This is shown in Figure 12, A and
B, respectively. Mean values of the
Ip activation by the two catecholamines are presented in Figure 12C. Both substances augment
Ip by ~40%. The stimulation of
Ip occurred at low
[Ca2+]i (20 nM), was voltage independent, and
was not due to Na



-adrenergic Ip stimulation in rat myocytes might be caused by a drug-induced facilitation of
K+ deocclusion and K+ release into the cell
interior. Furthermore, the adrenergic modulation of
Ip was examined in adult rat cardiac myocytes in
short-term culture (up to 4 days). The Ip
stimulation by norepinephrine and isoprenaline (at 20 nM free
Ca
-receptor mediated
(166). In contrast to experiments on guinea pig
ventricular cells (56, 59), a reduction
rather than an increase of the adrenergic Ip
stimulation with increasing [Ca2+]i was
observed, and no effect of adrenergic stimulation on the shape or
position of the Ip-V relationship
could be detected. Because there was no change in the voltage
dependence of Ip during
-adrenergic
stimulation of the Na+-K+ pump, it was
concluded that the Na+ release from the pump to the
extracellular medium is not modulated by
-agonists and that the
mechanism of
-adrenergic Ip modulation in rat
ventricular cells is qualitatively different from that proposed for
guinea pig cardiac myocytes (166).

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Fig. 12.
Stimulation of Ip by norepinephrine (NA) and
isoprenaline (Iso) in a rat cardiac myocyte. A: example
recording of the holding current (Ih) at
40 mV
as affected by 10 µM NA (marked by arrows) and 1 mM ouabain (Ou). The
peak effect of NA was measured as shown by arrow labeled
INa. Ouabain was applied to the cell for a short
time before and after NA application to measure
Ip as shown by arrow Ip.
B: example record of Ih from a
different myocyte illustrating the whole cell current response to 10 µM Iso. Protocol, conditions, and labels are the same as in
A. C: mean amplitudes ± SE of
Ip before and during the application of NA
(1-10 µM, columns on left) or Iso (1-10 µM, columns on
right); n indicates the number of cells studied.
[Adapted from Dobretsov et al. (35).]
2.
-Adrenergic stimulation of Ip in cardiac
cells
In canine Purkinje myocytes, the
-adrenergic drug phenylephrine
increases the Na+-K+ pump activity and thereby
Ip and decreases the background K+
conductance. Both effects are mediated by
1-adrenoceptors and are blocked by a pretreatment of the
Purkinje fibers with pertussis toxin. The
-antagonist propranolol is
ineffective. The effect of phenylephrine is abolished by
10
4 M DHO, a specific blocker of the
Na+-K+ pump, in Ba2+-containing
external solution. The known negative chronotropic effect of
phenylephrine might be due to activation of electrogenic Na+-K+ pumping. The pump seems to be coupled to
a pertussis toxin-sensitive G protein (160).
Norepinephrine and the
-adrenergic agonists phenylephrine,
methoxamine, and metaraminol increase Ip of
guinea pig ventricular myocytes in propranolol-containing media.
The increase is blocked by the
1-antagonist prazosin and
is unaffected by the
2-antagonist yohimbine. The
stimulation of Ip is not due to accumulation of
intracellular Na+ or extracellular K+ and is
independent of voltage. The norepinephrine concentration required for
half-maximal Ip stimulation
(K0.5 value) depends on
[Ca2+]i and decreases from 219 nM at 15 nM
Ca

1-adrenoceptors via PKC. The sensitivity of the coupling
depends on [Ca2+]i. The maximal
increase in Ip is independent of membrane
potential and [Ca2+]i (183). An
electropharmacological study demonstrated that the
1-adrenoceptor stimulation of Ip
in rat ventricular myocytes is mediated by the
1b-subtype of the
1-adrenergic receptors (185). It is helpful to note that adrenergic modulation of
the Na+-K+ pump
(Na+-K+-ATPase) might result in different
effects, depending on cell species and/or experimental conditions. For
further discussion and additional references, the reader is referred to
a relevant review (180). In a most recent paper, evidence
is presented that two isoforms of the
-subunit of the
Na+-K+-ATPase (
1, 82%;
2, 18%) are present in guinea pig ventricles. The
-adrenergic, PKA-mediated effects on the cardiac
Na+-K+ pump are targeted to the
1-isoform, whereas the
-adrenergic, PKC-dependent
effects are targeted to the
2-isoform (62).
B. Modulation of Cardiac Ip by Acetylcholine
To our knowledge, so far only one report has been published on the
action of acetylcholine (ACh) on the cardiac Ip
(58). According to this study ACh does not modulate the
basal Ip at any voltage in guinea pig
ventricular myocytes containing a high or low
[Ca2+]i. However, it reverses the effect of
isoprenaline on Ip, regardless of
[Ca2+]i, with a K0.5
value of ~16 nM. Figure 13 presents
an original record of membrane current from a guinea pig ventricular
cell clamped at
60 mV (58). Ip is
estimated as current blocked by 1 mM DHO. At low internal
[Ca2+] (15 nM), isoprenaline (1 µM) decreases
Ip. This effect is abolished by acetylcholine
(ACh; 1 µM). The ACh effect is mediated by muscarinic receptors since
it is blocked by atropine. The ACh-induced stimulation of the
receptor leads most probably via a Gi protein to an
inhibition of adenylyl cyclase and in consequence to a decrease in
[cAMP] (see scheme 2). Thus a high vagal tone per se does not
modulate the cardiac Na+-K+ pump, but
activation of muscarinic receptors reverses the modulation of the pump
induced by a high sympathetic tone.
|
| |
IX. CARDIAC GLYCOSIDES AND CARDIAC PUMP CURRENT |
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|
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A. Binding of Cardiac Glycosides to Various Isoforms of the Cardiac Na+-K+ Pump Is Species Dependent
As mentioned in section I, different isoforms of the
-subunit of the Na+-K+-ATPase are expressed
in the cardiac cells of various animal species. In general, two of
three
-subunit isoforms are detected in the heart. There is
agreement in the literature that rat myocytes (e.g., Refs. 12, 164,
175) and myocytes from many other species, including human cells (e.g.,
Refs. 175, 182), express more than one Na+-K+
pump isoform (see Ref. 175). Because the
-isoforms represent the
receptors for cardiac glycosides and normally differ in their sensitivity for the drugs (see Ref. 19, but also Ref. 27), more than
one receptor class for cardiac steroids should be found in heart cells.
However, only a single
-isoform is expressed in sheep cardiac cells
(175) and, perhaps, in guinea pig ventricular myocytes
(175; but see Refs. 11, 62). Thus only one receptor class may be
present in the heart cells of these two species. The sensitivity of the
1-isoforms for cardiac steroids varies widely among the
animal species. For instance, the rat
1-subunit is known
to be extremely resistant to the drugs.
The chapter below describes the interaction between cardiac glycosides and the Na+-K+ pump as studied by electrophysiological techniques in isolated cardiac cells. It is not in the focus of the present article to review the immense literature on cellular and subcellular actions of cardiac steroids in other preparations and/or investigated by other physiological or by biochemical methods. Clearly, this literature is at least as important for our understanding of the glycoside action as the few observations reported here. For a more extensive overview the reader is referred to relevant reviews (2, 40, 78, 116).
Ever since the pump current was first described in a cardiac
preparation, cardiac glycosides, which are specific inhibitors of the
Na+-K+ pump (153), have been used
to identify cardiac Ip (99). For example, in the first paper on the voltage dependence of
Ip in single cardiac cells, the pump current of
guinea pig ventricular myocytes was measured as the
ouabain-blockable current (48). As just described, it
is still controversial whether in these cells more than one
-subunit
isoform of the Na+-K+-ATPase (more than one
class of cardiac steroid-receptors) is expressed and functioning. On
the one hand, two components of the total Ip
with different sensitivities to cardiac glycosides have been described
(57, 127). According to the authors, the highly sensitive component contributes up to 30-40% of the total Ip under quasi-physiological conditions.
This is in line with a biochemical-immunological study
(11) which described a component with high affinity toward
cardiac glycosides (dissociation constant ~10 nM ouabain, DHO, or
digitoxigenin) that contributes as much as 55% to the total
Na+-K+-ATPase activity of guinea pig
ventricular cells. Furthermore, a recent paper (62) showed
by a molecular biological approach that 18% of the pump mRNA consists
of mRNA for the
2-subunit of the pump which exhibits a
high affinity for cardiac glycosides. On the other hand,
Ip inhibition by strophanthidin occurs as
reversible 1:1 binding to a single class of pumps in guinea pig
ventricular cells (114). Similarly, the
Ip inhibition by DHO in rat and guinea pig
cardiac myocytes in a range between ~20 and 95% inhibition can be
analyzed by assuming simple saturation kinetics and a single population
of pump molecules (86). Of course, this does not exclude
the presence of a minor class of pumps (additional
-isoform), displaying a different affinity to cardiac glycosides. However, application of immunologic methods did not detect any
2-isoenzyme of the Na+-K+ pump
in guinea pig ventricular myocytes (122,
175). Finally, the Ip
inhibition-concentration curve (range 10
8 to 5 × 10
5 M) for guinea pig ventricular
myocytes could not be fitted by assuming two isozymes showing different
sensitivities toward DHO (84). So far, the reason for the
conflicting results with guinea pig cardiac cells is completely
unclear. A qualitatively or quantitatively different expression of
-subunit isoforms of the Na+-K+ pump in
various strains of guinea pig may be involved.
By way of contrast, two components were clearly identified in the Ip inhibition-concentration curve in rat myocytes. The high-affinity component amounted to ~32% of the total Ip in cells from young animals but only to ~15% in myocytes from adult rats (84).
B. Kinetics of Cardiac Steroid Binding to the Cardiac
Na+-K+ Pump
1. Reversible one-to-one binding to a single class of
receptors on the pump
To study the inhibition of the cardiac Ip
by cardiac glycosides, DHO is often used since its action on the
Na+-K+ pump is readily reversible. The
inhibition of Ip by DHO was investigated in
isolated canine Purkinje myocytes at 8 mM K
|
(7) |
|
(8) |
on, the time constant of the process of DHO binding,
by
|
(9) |
|
(10) |
off is the time constant of DHO unbinding. By
applying the above equations to their data, an apparent
KD value (i.e., the [DHO] which causes
half-maximal inhibition of Ip) of 3.7 × 10
6 M DHO (K0.5 value), an
apparent association rate constant k'1 of
2.5 × 103 (M × s)
1 and a
dissociation rate constant k'2 of 0.009 s
1 were calculated (25). The agreement
between measured steady-state Ip inhibition
as a function of [DHO] and calculated
(k'2/k'1) K0.5 value suggested that the kinetic model
outlined above is reasonable (25). A similar but more
detailed study was carried out on single rabbit cardiac Purkinje cells
(17). Half-maximal inhibition of Ip
by 1 × 10
5 M DHO was found in a medium
containing 2 mM K
5 M DHO at 10.8 mM
K


4 M DHO. The steroid inhibits Ip
and therefore shifts the membrane current in the inward directions. A
new stable current level is reached in ~10 s. Short intermittent test
pulses of K+-free medium block Ip
and indicate the current level in the absence of
Na+-K+ pumping. It is clear from the current
trace that 10
4 M DHO inhibits ~90% of
Ip at 2 mM K

4 M DHO proceeds more slowly to a final level of only
~65% of Ip in drug-free solution. Figure
14B emphasizes that the time constant of
Ip decline in DHO-containing solution is
smaller at 2 mM K

1 at 2 mM
K
1 at 10.8 mM K

1 at both [K+]o. Increasing
[Na+] of the patch-pipette solution (for
intracellular perfusion) from 5 to 50 mM increased the
k'1 value but left the
k'2 value unchanged. As a consequence,
the K0.5 value decreased by a factor of 3-5.
The main changes occurred after an increase of
[Na+]pip from 5 to 15 mM. The observed
dependence of K0.5 on
[Na+]pip is reminiscent of an earlier paper
(169) which demonstrated a decrease of the
K0.5 value of ouabain for the pump inhibition in
chick cardiac myocytes upon an increase in the intracellular Na+ concentration. In accordance with a model published
previously (88), it was concluded that an increase of
Na
3 and 1.4 × 10
5 M DHO, respectively (K
20 mV; Napip, 50 or 100 mM). The
higher K0.5 value for binding of DHO to rat
cells was due to a smaller apparent association rate constant and a
larger dissociation rate constant. There was little evidence for the
presence of an isoform with higher DHO affinity in guinea pig myocytes,
and only a few percent of the pump molecules in rat cells displayed a
higher affinity to DHO (86). Half-maximal
Ip activation by K





|
2. Binding of cardiac glycosides to two classes of pump molecules
In line with reports on two isoforms of the
-subunit present in
guinea pig ventricular myocytes (11, 62),
inhibition of Ip in these cells has been
occasionally described to display a biphasic concentration dependence
(127). The data were fitted best by the equation
|
(11) |
8 M DHO, and KDl was derived to
be 6.5 × 10
5 M DHO. Under the conditions chosen
(temperature, 34°C; Na

40 mV), the pumps
with a high DHO affinity contributed as much as 40% to the total
Ip. A detailed analysis of electrogenic
Na+-K+ pumping in guinea pig ventricular
myocytes also indicated the presence of more than one
-subunit
isoform (57). Obviously, two classes of pump molecules
displaying different affinities to DHO contribute to the total pump
current at
60 mV holding potential. The high-affinity pumps are
half-maximally inhibited by 7.5 × 10
7 M DHO and
the low-affinity pump molecules by 7.2 × 10
5 M
DHO. Although the latter K0.5 value agrees
nicely with the corresponding KDl value above
(127), the former is larger by a factor of 15. The
high-affinity pumps produced ~30% of the total pump current.
They were half-maximally activated by 0.4 mM K

As reported above, a well-established kinetic competition
exists between K


3. At therapeutic concentrations cardiac glycosides preferentially bind to high-affinity pump molecules in human cardiac tissue
Studies on electrogenic Na+-K+ pumping of
human atrial tissue in media containing various acetylstrophanthidin
concentrations (149) or of atrial tissue from
digoxin-treated patients (148) suggest that at least
two classes of pump molecules displaying different glycoside
sensitivity are present in the human heart. If administered
therapeutically, cardiac glycosides seem to bind mainly to
high-affinity pumps that may represent up to ~33% of the pumps
(149). These findings are in qualitative agreement with
observations on Ip inhibition by cardiac
glycosides in isolated ventricular myocytes from the human failing
heart (182). According to the latter authors, the
Ip inhibition-concentration curve for DHO is
biphasic. The K0.5 value for inhibition of the
high-affinity pumps amounted to 6 × 10
9 M DHO.
This class of pump molecules generated ~14% of the total Ip under the conditions chosen. The
low-affinity pumps displayed a K0.5 value of
2 × 10
6 M DHO, 10 times lower than for pumps
in guinea pig ventricular cells. The difference is obviously due to a
larger dissociation rate constant for DHO in guinea pig myocytes. The
K0.5 values of digoxin for high-affinity
Na+-K+ pumps were estimated to be
~10
9 M and for low-affinity pumps 3 × 10
7 M, respectively (at
[Na+]pip = 100 mM). Thus this report
suggests that therapeutic doses probably block preferentially
high-affinity pump molecules and inhibit 7-10% of the total
Ip or Na+-K+ pump
activity (182).
4. Evaluation of new cardiac steroids as Na+-K+ pump modulators by measurements of the cardiac Ip
A convenient method to study the modulating effect of new cardiac steroids on the cardiac Na+-K+ pump is to measure the change of Ip caused by the drugs in cardiac cells. This method was used in an attempt to identify C-22-substituted derivatives of digitoxigenin, which might be suitable for affinity labeling of the binding site(s) of the lactone ring on the Na+-K+-ATPase. Whole cell recording from sheep cardiac Purkinje cells revealed that esters derived from 22-hydroxy-digitoxigenin are useful for this purpose (41).
C. Cardiac Glycosides Alter the Cardiac Ip-V Relationship
Alterations of the cardiac Ip-V
curve by cardiac steroids were shown when the interaction between DHO
or ouabain, and the Na+-K+ pump of rat and
guinea pig ventricular myocytes was studied at various
[K+]o and membrane potentials by means of
whole cell recording (85). The glycoside-induced
changes of the cardiac Ip-V
relationship are probably mediated by potential-evoked alterations
of the local [K+]o, near the K+
binding sites of the Na+-K+ pump at the bottom
of an extracellular access channel. The alterations were estimated by
means of the Boltzmann-Fermi equation and compared with the effect
of various [K+]o on the binding of DHO to the
cardiac Na+-K+ pump at 0 mV. Upon a change of
[K+] in the access channel, a new equilibrium of
glycoside binding to the pump is reached. The time required to
establish the new binding equilibrium depends on the glycoside, the
concentration of the cardioactive steroid, and on the sensitivity of
the cells toward the drug. The findings lend further support to the
access-channel hypothesis and thus suggest that most probably
K
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X. MODULATION OF THE CARDIAC SODIUM-POTASSIUM PUMP BY HORMONES |
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A. Aldosterone
A short-term exposure of cardiac cells to the
mineralocorticoid aldosterone affects the
Na+-K+ pump. This was shown in isolated rabbit
ventricular myocytes by whole cell recording of
Ip and measurements of
a


gradients. There was an
aldosterone-induced increase of Ip if these
gradients were steeply inward. Flattening the Na+ and
Cl
gradients resulted in an aldosterone-evoked
decrease of Ip. Bumetanide (10 µM), an
inhibitor of the Na+-K+-2Cl
transporter, abolished the increase of Ip and
Na+ influx evoked by aldosterone. Similarly, the
mineralocorticoid receptor blocker potassium canrenoate completely
blocked the aldosterone-induced Ip increase.
The data strongly suggest that stimulation of Ip and Na+ influx in cardiac myocytes exposed to aldosterone
in vitro is most probably due to activation of the
Na+-K+-2Cl
transporter by the
hormone (125).
Interestingly, when the hormone (50 µg/kg body wt) was administered
to rabbits for 7 days, Ip of isolated
ventricular cells decreased at 10 mM but not at 80 mM
Na



B. Angiotensin-Converting Enzyme Inhibition
By means of the same methods and cardiac preparations, a decrease
of a
1,
2,
1) of
the Na+-K+ pump in myocytes of
captopril-treated rabbits compared with control. This and other
findings suggest an action of captropril on already existing pump
molecules rather than an interference with the pump synthesis. The
reduction of Ip by angiotensin in ventricular
myocytes from rabbits was blocked by pertussis toxin and the PKC
inhibitors staurosporine and bis-indolylmaleimide I. It was
mimicked by the PKC activator phorbol 12-myristate 13-acetate (PMA).
Thus modulation of Ip (and cardiac
Na+-K+ pump activity) by angiotensin II is
mediated via the angiotensin II type 1 receptor, a G protein, and PKC.
Figure 15 is from a recent paper on the
subject (22) and shows that the increase of
Ip caused by captopril or losartan can be
blocked by PMA-induced PKC stimulation, by inclusion of the PKC
fragment PKCF (see legend to Fig. 15) in the patch pipette solution, or
by exposure of myocytes from captropril-treated rabbits to
angiotensin II. The modulation of cardiac Ip by
angiotensin II is probably based on a decrease of the pump's
selectivity for Na

|
C. Thyroid Status and Cardiac Na+-K+ Pump
The dependence of the cardiac Na+-K+ pump
activity on the thyroid status was studied in isolated ventricular
myocytes and papillary muscles from the rabbit (37).
Ip of single ventricular cells was measured by
means of whole cell recording and a
40 mV and 10 mM Na




D. Insulin Changes the Cardiac Ip-V Curve
Insulin (100 mU/ml) flattens the
Ip-V curve of rabbit ventricular
myocytes at negative and positive membrane potentials
(80). The flattening, observed at 10 mM
Na


40 mV. The hormone most probably facilitates the voltage-dependent binding of
Na
The effects of catecholamines on cardiac electrogenic Na+-K+ pumping have already been reviewed in section VIIIA.
| |
XI. MISCELLANEOUS |
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A. Anisosmolar External Solution Affects the Activity of the Cardiac Na+-K+ Pump
The role of the Na+-K+-ATPase in the
short-term volume regulation of cardiac and other cells is
controversial. The effect of hypo- and hyperosmolar bathing solutions
on Ip and a








40 mV holding
potential. Cell swelling also induced a flattening of the
Ip-V curve at negative potentials and
a region of negative slope at positive voltages. Thus
Ip stimulation by hypotonic solution is voltage
dependent. The effect on Ip at the holding
potential was independent of [Ca2+]o and
[Na+]o. In contrast to the decrease of
Ip upon reexposure to isosmolar solution,
Ip stimulation in hyposmotic media was not
mediated by PKC. It was blocked by the tyrosine kinase inhibitor
tyrphostin A-25, which also completely inhibited the variation of the
Ip-V relationship observed in
hypotonic solution. Furthermore, LY-294002, a specific inhibitor of
phosphatidylinositol 3-kinase, and okadaic acid, an inhibitor of
protein phosphatase 1 (PP1) and 2A (PP2A), also blocked the
Ip stimulation at
40 mV. Tyrosine kinase has been shown to activate protein phosphatase 1, whereas
phosphatidylinositol 3-kinase is activated in tyrosine
kinase-dependent intracellular signaling. The results suggest that
Ip stimulation by cell swelling involves the
activation of tyrosine kinase, phosphatidylinositol 3-kinase, and
protein phosphatase 1 and may be mediated by a dephosphorylation of the
Na+-K+-ATPase. In contrast to the findings
reported above (184), an Ip
stimulation at high [Na+]pip (>35 mM) was
observed in about one-half of the isolated guinea pig ventricular
myocytes exposed to hyposmotic medium and an Ip decrease at comparable [Na+]pip in 50% of
the myocytes superfused with hypertonic solution. However, only 1 of 10 experiments with cells dialyzed with 10 mM Na
|
B. Dietary Cholesterol Alters Cardiac Na+-K+ Pumping
A modest diet-induced increase in serum cholesterol (from
~0.9 to ~4-6 mM) augmented Ip at 10 mM
Na
40 mV from ~0.3 to ~0.5 pA/pF in
isolated rabbit ventricular myocytes (75). The
Ip-[Na+]pip
relationship revealed an increased Ip in a range
of Na



C. Amiodarone Inhibits the Cardiac Na+-K+ Pump Following Acute and Chronic Treatment by Different Mechanisms
The antiarrhythmic agent amiodarone produces (among other effects)
a state of cellular hypothyroidism. Because the cardiac Na+-K+ pump is sensitive to changes of the
thyroid status (see above), the effect of the antiarrhythmic drug on
the pump was studied by whole cell recording of
Ip from isolated rabbit ventricular cells and by
a



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XII. EFFECTS OF THE CARDIAC SODIUM-POTASSIUM PUMP ON ION TRANSPORTERS AND CHANNELS MEASURED BY ELECTROPHYSIOLOGICAL TECHNIQUES |
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A. Modulation of the Cardiac Na+/Ca2+ Exchange
During the past decade, the interaction of
Na+-K+ pumping with currents generated by other
cardiac ion transporters and channels has been studied by
patch-clamp methods. In internally dialyzed isolated cardiac cells,
alterations of the Na+-K+ pump activity cause
changes not only of the pump current Ip but also
of the Na+/Ca2+ exchange current
INa/Ca. Figure
17 presents an example. Figure 17A shows the membrane current of a rabbit cardiac Purkinje
cell at
20 mV holding potential. After 8 min in K+-free
solution, electrogenic Na+-K+ pumping is
initiated by application of a medium containing 10.8 mM K+
(uppermost line). The K+-rich solution immediately evokes
an outward current (largely Ip) that declines
from an initial peak toward a steady-state level. The
Na+/Ca2+ exchanger current
INa/Ca is simultaneously measured as current blocked by 5 mM Ni
3- and
2-subunits (astrocytes) of the
Na+-K+ pump and the
Na+/Ca2+ exchanger are expressed in a reticular
distribution within the cell membrane overlying functional sarcoplasmic
(endoplasmic) reticulum. Cardiac steroids (and endogenous
digitalis-like factors) may exert their pharmacological (hormonal)
effect by binding to these specialized plasmalemmal domains, modulating
[Na+]i and [Ca2+]i
only in the cleft between cell membrane and sarcoplasmic reticulum ("plasmerosome") and thereby the Ca2+ content of the
adjacent sarcoplasmic reticulum. By way of contrast, Na+-K+ pump molecules containing the cardiac
glycoside-insensitive "housekeeping"
1-isoform
are uniformly expressed in the rat cell membranes studied (20, 102).
|
B. Interaction Between the Cardiac Na+-K+ Pump and KATP Channels
The effect of altered Na+-K+ pump activity
on IK(ATP) was investigated by means of
patch-clamp measurements on guinea pig ventricular myocytes
(141). Inhibition of the forward-running pump (i.e., the physiological mode) by the cardioactive steroid strophanthidin, K+-free bathing solution, or strong hyperpolarization
decreased IK(ATP). Correspondingly, blocking the
backward-running (ATP-synthesizing) pump opened the
KATP channels. From the effect of bath-applied strophanthidin on single KATP channels in cell-attached
patches, it was deduced that the interaction between the
KATP channels and the Na+-K+ pump
is not restricted to a fuzzy space of submicrometric dimensions and
that the cytosolic ATP concentration controls the subsarcolemmal ATP
concentration. A similar study on the modulation of
IK(ATP) by the Na+-K+
pump was carried out in giant patches and single guinea pig ventricular cells (103). Inhibition of the
Na+-K+ pump by replacing Li+ or
NMDG+ for cytosolic Na+ decreased
IK(ATP) in giant patches at 1 mM ATP by ~30%
and lowered the K0.5 value of ATP for the
opening of K(ATP) channels by ~40%. Inhibiton of the
pump by Na+ substitution had little effect on
IK(ATP) at low (<0.1 mM) or high (10 mM) ATP
concentration, presumably because the activity of either the pump or of
the K(ATP) channels, respectively, is low under these
conditions. Similarly, whole cell recording revealed that inhibition of
the Na+-K+ pump by 5 × 10
5
M ouabain strongly decreased an inward rectifying current at 1 mM but
not at 10 mM ATP in the pipette solution. Under the latter condition,
IK(ATP) is absent. The pump activity probably
modulates IK(ATP) via variation of [ATP] in a
subsarcolemmal space, where ATP diffusion is restricted. Modulation of
IK(ATP) by the Na+-K+
pump through changes of the subsarcolemmal [ATP] was also proposed on
the basis of patch-clamp measurements on rabbit ventricular myocytes as the mechanism of the attenuation by digoxin of the infarct
size-limiting effect of ischemic preconditioning (82). The same mechanism seems to govern the effect of the inhibiton of the
Na+-K+ pump by ouabain on
IK(ATP) of guinea pig ventricular myocytes. Furthermore, the mechanism might be the basis for the
time-dependent alleviation by DHO of the action potential
shortening in coronary perfused guinea pig ventricular muscles during
metabolic inhibiton (1).
C. Effects of the Cardiac Na+-K+ Pump on IK(Na)
A K+ channel that is gated by intracellular
[Na+] >20 mM exists in guinea pig ventricular myocytes.
The probability of opening for this channel depends on
Na

D. Blockade of the Na+-K+ Pump Activates IK(ACh) in Atrial Myocytes
A novel gating mechanism of the cardiac muscarinic K+
channel, independent of G protein activation, was identified in chick atrial cells (173). The mechanism involves two steps.
First, the functional state of the channel is modified by ATP
hydrolysis. Second, [Na+]i >3 mM gates the
modified channel with a K0.5 value of ~40 mM. Inhibition of the Na+-K+ pump either in
K+-free medium or by ouabain (5 × 10
4
M) activates IK(ACh) in the same manner as an
increase of [Na+]i and implies that the
blockade of the pump activates IK(ACh) via an
augmented subsarcolemmal [Na+]. The findings demonstrate
for the first time a Na
| |
XIII. CONCLUSION |
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The Na+-K+ pump of animal cells generates the pump current Ip. Under physiological conditions Ip is an outward current. It can be measured by electrophysiological methods. The introduction of the patch-clamp techniques has rendered possible a hitherto unequaled experimental control of membrane potential and composition of the intracellular medium in single cells. The techniques of whole cell recording and recording from giant patches permit the study of Na+-K+ pump characteristics by measuring Ip. For this purpose, cardiac cells are especially suitable, since they exhibit a high density of sarcolemmal pump molecules and an adequate cellular geometry and are easily obtainable. In fact, measurements of the cardiac Ip have been pivotal for the understanding of Na+-K+ pumping in electrically excitable cells. The cardiac Na+-K+ pump can be studied in its physiological environment, i.e., in the sarcolemma which separates cytosol and extracellular medium with their different ionic compositions and across which a membrane potential exists. The effect of intra- and extracellular cations on the cardiac pump, the interaction between membrane potential and Na+-K+ pump, and the modulation of the pump activity by transmitters, hormones, and drugs have been studied with unrivaled precision and time resolution by means of patch-clamp techniques. Through the recording of Ip and transient pump currents, several voltage-dependent partial reactions of the pump cycle have been identified, including the binding and unbinding of monovalent cations to or from the pump in an access channel. The fundamental mechanism of ion translocation by the pump, however, remains to be clarified. Furthermore, the quantitative contribution of electrogenic Na+-K+ pumping to the cardiac action potential under physiological conditions has still to be assessed. The knowledge of pump modulation by transmitters, hormones, and drugs in cardiac cells is preliminary and requires further detailed studies. Finally, the interaction between Na+-K+ pump molecules and ion channels or transporters colocalized in the sarcolemma is a promising field of future research.
| |
ACKNOWLEDGMENTS |
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I am grateful to Profs. H. C. Lüttgau (Bochum), W. Schwarz (Frankfurt), and F. Verdonck (Kortrijk/Leuven) for friendly support and advice over many years and for critical reading of an earlier draft of the manuscript. I also thank Drs. S. Erlenkamp and S. Zillikens for their pertinent comments on the manuscript. I owe a pleasant, excellent cooperation for more than 30 years to A. Balzer-Ferrai, W. Grabowski, P. Greger, and Dr. H. Pusch (Dept. of Cell Physiology, Ruhr-University Bochum). Last but not least, I thank P. Lerch for invaluable secretarial help.
The financial support by the Deutsche Forschungsgemeinschaft for nearly three decades is gratefully acknowledged.
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FOOTNOTES |
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Address for reprint requests and other correspondence: H. G. Glitsch, Buchenberger Str. 6, 78126 Königsfeld, Germany.
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REFERENCES |
|---|
|
|
|---|
| 1. | Abe T, Sato T, Kiyosue T, Saikawa T, Sakata T, and Arita M. Inhibiton of Na+-K+ pump alleviates the shortening of action potential duration caused by metabolic inhibition via blockade of KATP channels in coronary perfused ventricular muscles of guinea- pigs. J Mol Cell Cardiol 31: 533-542, 1999[Web of Science][Medline]. |
| 2. | Akera T, and Brody TM. The role of Na+,K+-ATPase in the inotropic action of digitalis. Pharmacol Rev 29: 187-220, 1978[Web of Science][Medline]. |
| 3. |
Albers RW,
Fahn S, and Koval GJ.
The role of sodium ions in the activation of Electrophorus electric organ adenosine triphosphatase.
Proc Natl Acad Sci USA
50: 474-481, 1963 |
| 4. | Apell H-J. Electrogenic properties of the Na,K pump. J Membr Biol 110: 103-114, 1989[Web of Science][Medline]. |
| 5. | Apell HJ, Borlinghaus R, and Läuger P. Fast charge translocations associated with partial reactions of the Na,K-pump. II. Microscopic analysis of transient currents. J Membr Biol 97: 179-191, 1987[Web of Science][Medline]. |
| 6. |
Bahinski A,
Nakao M, and Gadsby DC.
Potassium translocation by the Na+/K+ pump is voltage insensitive.
Proc Natl Acad Sci USA
85: 3412-3416, 1988 |
| 7. |
Baker PF,
Blaustein MP,
Hodgkin AL, and Steinhardt RA.
The influence of calcium on sodium efflux in squid axons.
J Physiol (Lond)
200: 431-468, 1969 |
| 8. | Baker PF, and Willis JS. Potassium ions and the binding of cardiac glycosides to mammalian cells. Nature 226: 521-523, 1970[Medline]. |
| 9. |
Barmashenko G,
Kockskämper J, and Glitsch HG.
Depolarization increases the apparent affinity of the Na+-K+ pump to cytoplasmic Na+ in isolated guinea-pig ventricular myocytes.
J Physiol (Lond)
517: 691-698, 1999 |
| 10. | Baumgarten CM, and Fozzard HA. Cardiac resting and pacemaker potentials. In: The Heart and Cardiovascular System (2nd ed.), edited by Fozzard HA, Haber E, Jennings RB, and Katz AM. New York: Raven, 1992, vol. 1, p. 963-1001. |
| 11. | Berrebi-Bertrand I, Maixent J-M, Guede FG, Gerbi A, Charlemagne D, and Lelievre LG. Two functional Na+/K+-ATPase isoforms in the left ventricle of guinea pig heart. Eur J Biochem 196: 129-133, 1991[Web of Science][Medline]. |
| 12. | Berlin JR, Fleming AJ, and Ishizuka N. Identification of low and high affinity ouabain-sensitive Na pump current in voltage-clamped rat cardiac myocytes. Ion-motive ATPases: structure, function, and regulation. Ann NY Acad Sci 671: 440-442, 1992[Medline]. |
| 13. | Berlin JR, and Peluffo RD. Mechanism of electrogenic reaction steps during K+ transport by the Na,K-ATPase. Na/K-ATPase and related transport ATPases: structure, mechanism, and regulation. Ann NY Acad Sci 834: 251-259, 1997[Web of Science][Medline]. |
| 14. |
Bewick NL,
Fernandes C,
Pitt AD,
Rasmussen HH, and Whalley DW.
Mechanisms of Na+-K+ pump regulation in cardiac myocytes during hyposmolar swelling.
Am J Physiol Cell Physiol
276: C1091-C1099, 1999 |
| 15. |
Bielen FV,
Glitsch HG, and Verdonck F.
Dependence of Na+ pump current on external monovalent cations and membrane potential in rabbit cardiac Purkinje cells.
J Physiol (Lond)
442: 169-189, 1991 |
| 16. | Bielen FV, Glitsch HG, and Verdonck F. Changes of the subsarcolemmal Na+ concentration in internally perfused cardiac cells. Biochim Biophys Acta 1065: 269-271, 1991[Medline]. |
| 17. | Bielen FV, Glitsch HG, and Verdonck F. The kinetics of the inhibition by dihydroouabain of the sodium pump current in single rabbit cardiac Purkinje cells. Naunyn-Schmiedeberg's Arch Pharmacol 345: 100-107, 1992[Web of Science][Medline]. |
| 18. |
Bielen FV,
Glitsch HG, and Verdonck F.
Na+ pump current-voltage relationships of rabbit cardiac Purkinje cells in Na+-free solution.
J Physiol (Lond)
465: 699-714, 1993 |
| 19. |
Blanco G, and Mercer RW.
Isozymes of the Na-K-ATPase: heterogeneity in structure, diversity in function.
Am J Physiol Renal Physiol
275: F633-F650, 1998 |
| 20. | Blaustein MP, Juhaszova M, and Golovina VA. The cellular mechanism of action of cardiotonic steroids: a new hypothesis. Clin Exp Hypertens 20: 691-703, 1998. |
| 21. | Bronner C, Mousli M, Eleno N, and Landry Y. Resting plasma membrane potential of rat peritoneal mast cells is set predominantly by the sodium pump. FEBS Lett 255: 401-404, 1989[Web of Science][Medline]. |
| 22. |
Buhagiar KA,
Hansen PS,
Gray DF,
Mihailidou AS, and Rasmussen HH.
Angiotensin regulates the selectivity of the Na+-K+ pump for intracellular Na+.
Am J Physiol Cell Physiol
277: C461-C468, 1999 |
| 23. | Carmeliet E. A fuzzy subsarcolemmal space for intracellular Na+ in cardiac cells? Cardiovasc Res 26: 433-442, 1992[Web of Science][Medline]. |
| 24. | Carmeliet E, and Luk H-N. The Na+-activated K+ channel in cardiac cells. In: Intracellular Regulation of Ion Channels, edited by Morad M, and Agus Z. Berlin: Springer, 1992, vol. H60, p. 53-59. (NATO ASI Ser.) |
| 25. |
Cohen IS,
Datyner NB,
Gintant GA,
Mulrine NK, and Pennefather P.
Properties of electrogenic sodium-potassium pump in isolated canine Purkinje myocytes.
J Physiol (Lond)
383: 251-267, 1987 |
| 26. | Connelly CM. Recovery processes and metabolism of nerve. Rev Mod Phys 31: 475-484, 1959[Web of Science]. |
| 27. |
Crambert G,
Hasler U,
Beggah AT,
Yu C,
Modyanov NN,
Horisberger J-D,
Lelievre L, and Geering K.
Transport and pharmacological properties of nine different human Na,K-ATPase isozymes.
J Biol Chem
275: 1976-1986, 2000 |
| 28. |
Daut J, and Rüdel R.
The electrogenic sodium pump in guinea-pig ventricular muscle: inhibiton of pump current by cardiac glycosides.
J Physiol (Lond)
330: 243-264, 1982 |
| 29. |
Délèze J.
Possible reasons for drop of resting potential of mammalian heart preparations during hypothermia.
Circ Res
8: 553-557, 1960 |
| 30. | Désilets M, and Baumgarten CM. Isoproterenol directly stimulates the Na+-K+ pump in isolated cardiac myocytes. Am J Physiol Heart Circ Physiol 251: H218-H225, 1986. |
| 31. | De Weer P. The electrogenic sodium pump: thermodynamics and kinetics. In: Membrane Control of Cellular Activity, edited by Lüttgau HC. Stuttgart: Fischer, 1986, vol. 33, p. 387-399. |
| 32. | De Weer P. Cellular sodium-potassium transport. In: The Kidney: Physiology and Pathophysiology (2nd ed.), edited by Seldin DW, and Giebisch G. New York: Raven, 1992, p. 93-112. |
| 33. | De Weer P, Gadsby DC, and Rakowski RF. Voltage dependence of the Na-K pump. Annu Rev Physiol 50: 225-241, 1988[Web of Science][Medline]. |
| 34. | De Weer P, and Rakowski RF. Current generated by backward-running electrogenic Na pump in squid giant axons. Nature 309: 450-452, 1984[Medline]. |
| 35. |
Dobretsov M,
Hastings SL, and Stimers JR.
Na+-K+ pump cycle during -adrenergic stimulation of adult rat cardiac myocytes.
J Physiol (Lond)
507: 527-539, 1998 |
| 36. |
Doohan MM,
Gray DF,
Hool LC,
Robinson BG, and Rasmussen HH.
Thyroid status and regulation of intracellular sodium in rabbit heart.
Am J Physiol Heart Circ Physiol
272: H1589-H1597, 1997 |
| 37. |
Doohan MM,
Hool LC, and Rasmussen HH.
Thyroid status and Na+-K+ pump current, intracellular sodium, and action potential duration in rabbit heart.
Am J Physiol Heart Circ Physiol
268: H1838-H1846, 1995 |
| 38. | Efthymiadis A, and Schwarz W. Conditions for a backward-running Na+/K+ pump in Xenopus oocytes. Biochim Biophys Acta 1068: 73-76, 1991[Medline]. |
| 39. |
Eisner DA, and Lederer WJ.
The role of the sodium pump in the effects of potassium-depleted solutions on mammalian cardiac muscle.
J Physiol (Lond)
294: 279-301, 1979 |
| 40. | Eisner DA, and Smith TW. The Na-K pump and its effectors in cardiac muscle. In: The Heart and Cardiovascular System (2nd ed.), edited by Fozzard HA, Haber E, Jennings RB, and Katz AH. New York: Raven, 1992, vol. 1, p. 863-902. |
| 41. | Erlenkamp S, Gretzer B, Zillikens S, Glitsch HG, Pusch H, Staroske T, and Welzel P. Na+/K+ pump inhibition and positive inotropic effect of digitoxigenin and some C-22-substituted derivatives in sheep cardiac preparations. Naunyn-Schmiedeberg's Arch Pharmacol 357: 54-62, 1998[Web of Science][Medline]. |
| 42. | Friedrich T, Bamberg E, and Nagel G. Na+,K+-ATPase pump currents in giant excised patches activated by an ATP concentration jump. Biophys J 71: 2486-2500, 1996[Web of Science][Medline]. |
| 43. | Friedrich T, and Nagel G. Comparison of Na+/K+-ATPase pump currents activated by ATP concentration or voltage jumps. Biophys J 73: 186-194, 1997[Web of Science][Medline]. |
| 44. |
Fujioka Y,
Matsuoka S,
Ban T, and Noma A.
Interaction of the Na+-K+ pump and Na+-Ca2+ exchange via [Na+]i in a restricted space of guinea-pig ventricular cells.
J Physiol (Lond)
509: 457-470, 1998 |
| 45. | Gadsby DC. The Na/K pump of cardiac cells. Annu Rev Biophys Bioeng 13: 373-398, 1984[Web of Science][Medline]. |
| 46. | Gadsby DC. The Na/K pump of cardiac myocytes. In: Cardiac Electrophysiology and Arrhythmias, From Cell to Bedside, edited by Zipes DP, and Jalife J. New York: Saunders, 1991, p. 35-51. |
| 47. | Gadsby DC, and Cranefield PF. Effects of electrogenic sodium extrusion on the membrane potential of cardiac Purkinje fibers. In: Normal and Abnormal Conduction in the Heart, edited by Paes de Carvalho A, Hoffman BF, and Lieberman M. Mount Kisko, NY: Futura, 1982, p. 225-247. |
| 48. | Gadsby DC, Kimura J, and Noma A. Voltage dependence of Na/K pump current in isolated heart cells. Nature 315: 63-65, 1985[Medline]. |
| 49. | Gadsby DC, and Nakao M. Dependence of Na/K pump current on intracellular [Na] in isolated cells from guinea-pig ventricle (Abstract). J Physiol (Lond) 371: 201P, 1986. |
| 50. | Gadsby DC, and Nakao M. [Na] dependence of the Na/K pump current-voltage relationship in isolated cells from guinea-pig ventricle (Abstract). J Physiol (Lond) 382: 106P, 1987. |
| 51. |
Gadsby DC, and Nakao M.
Steady-state current-voltage relationship of the Na/K pump in guinea pig ventricular myocytes.
J Gen Physiol
94: 511-537, 1989 |
| 52. | Gadsby DC, Nakao M, Bahinski A, Nagel G, and Svenson M. Charge movements via the cardiac Na,K-ATPase. Acta Physiol Scand 146: 111-123, 1992[Web of Science]. |
| 53. |
Gadsby DC,
Rakowski RF, and De Weer P.
Extracellular access to the Na,K pump: pathway similar to ion channel.
Science
260: 100-103, 1993 |
| 54. |
Gao J,
Cohen IS,
Mathias RT, and Baldo GJ.
Regulation of the -stimulation of the Na+-K+ pump current in guinea-pig ventricular myocytes by a cAMP-dependent PKA pathway.
J Physiol (Lond)
477: 373-380, 1994 |
| 55. |
Gao J,
Cohen IS,
Mathias RT, and Baldo GJ.
The inhibitory effect of -stimulation on the Na/K pump current in guinea pig ventricular myocytes is mediated by a cAMP-dependent PKA pathway.
Pflügers Arch
435: 479-484, 1998[Web of Science][Medline].
|
| 56. |
Gao J,
Mathias RT,
Cohen IS, and Baldo GJ.
Isoprenaline, Ca2+ and the Na+-K+ pump in guinea-pig ventricular myocytes.
J Physiol (Lond)
449: 689-704, 1992 |
| 57. |
Gao J,
Mathias RT,
Cohen IS, and Baldo GJ.
Two functionally different Na/K pumps in cardiac ventricular myocytes.
J Gen Physiol
106: 995-1030, 1995 |
| 58. |
Gao J,
Mathias RT,
Cohen IS, and Baldo GJ.
Effects of acetylcholine on the Na+-K+ pump current in guinea-pig ventricular myocytes.
J Physiol (Lond)
501: 527-535, 1997 |
| 59. |
Gao J,
Mathias RT,
Cohen IS,
Shi J, and Baldo GJ.
The effects of -stimulation on the Na+-K+ pump current-voltage relationship in guinea-pig ventricular myocytes.
J Physiol (Lond)
494: 697-708, 1996 |
| 60. | Gao J, Mathias RT, Cohen IS, Sun X, and Baldo GJ. Modulators of PKC affect Na/K pummp current in guinea pig ventricular myocytes (Abstract). Biophys J 72: A51, 1997. |
| 61. | Gao J, Mathias RT, Cohen IS, Wang Y, Sun X, and Baldo GJ. Activation of PKC increases Na+-K+ pump current in ventricular myocytes from guinea pig heart. Pflügers Arch 437: 643-651, 1999[Web of Science][Medline]. |
| 62. |
Gao J,
Wymore R,
Wymore RT,
Wang Y,
McKinnon D,
Dixon JE,
Mathias RT,
Cohen IS, and Baldo GJ.
Isoform-specific regulation of the sodium pump by - and -adrenergic agonists in the guinea-pig ventricle J.
Physiol (Lond)
516: 377-383, 1999 |
| 63. | Glitsch HG. Characteristics of active Na transport in intact cardiac cells. Am J Physiol Heart Circ Physiol 236: H189-H199, 1979. |
| 64. | Glitsch HG. Electrogenic Na pumping in the heart. Annu Rev Physiol 44: 389-400, 1982[Web of Science][Medline]. |
| 65. | Glitsch HG, and Krahn T. The cardiac electrogenic Na pump. In: Membrane Control of Cellular Activity, edited by Lüttgau HC. Stuttgart: Fischer, 1986, vol. 33, p. 401-417. |
| 66. | Glitsch HG, Krahn T, and Pusch H. The dependence of sodium pump current on internal Na concentration and membrane potential in cardioballs from sheep Purkinje fibres. Pflügers Arch 414: 52-58, 1989[Web of Science][Medline]. |
| 67. | Glitsch HG, Krahn T, Pusch H, and Suleymanian M. Effect of isoprenaline on active Na transport in sheep cardiac Purkinje fibres. Pflügers Arch 415: 88-94, 1989[Web of Science][Medline]. |
| 68. | Glitsch HG, Reuter H, and Scholz H. The effect of the internal sodium concentration on calcium fluxes in isolated guinea-pig auricles. J Physiol (Lond) 209: 25-43, 1970. |
| 69. | Glitsch HG, Schwarz W, and Tappe A. Cardiac Na+ pump current-voltage relationships at various transmembrane gradients of the pumped cations. Biochim Biophys Acta 1278: 137-146, 1996[Medline]. |
| 70. | Glitsch HG, and Tappe A. The Na+/K+ pump of cardiac Purkinje cells is preferentially fuelled by glycolytic ATP production. Pflügers Arch 422: 380-385, 1993[Web of Science][Medline]. |
| 71. |
Glitsch HG, and Tappe A.
Change of Na+ pump current reversal potential in sheep cardiac Purkinje cells with varying free energy of ATP hydrolysis.
J Physiol (Lond)
484: 605-616, 1995 |
| 72. |
Glynn IM.
"All hands to the sodium pump."
J Physiol (Lond)
462: 1-30, 1993 |
| 73. | Glynn IM, and Karlish SJD. The sodium pump. Annu Rev Physiol 37: 13-55, 1975[Web of Science][Medline]. |
| 74. |
Goldshlegger R,
Karlish SJD,
Rephaeli A, and Stein WD.
The effect of membrane potential on the mammalian sodium-potassium pump reconstituted into phospholipid vesicles.
J Physiol (Lond)
387: 331-355, 1987 |
| 75. |
Gray DF,
Hansen PS,
Doohan MM,
Hool LC, and Rasmussen HH.
Dietary cholesterol affects Na+-K+ pump function in rabbit cardiac myocytes.
Am J Physiol Heart Circ Physiol
272: H1680-H1689, 1997 |
| 76. |
Gray DF,
Mihailidou AS,
Hansen PS,
Buhagiar KA,
Bewick NL,
Rasmussen HH, and Whalley DW.
Amiodarone inhibits the Na+-K+ pump in rabbit cardiac myocytes after acute and chronic treatment.
J Pharmacol Exp Ther
284: 75-82, 1998 |
| 77. | Hamill OP, Marty A, Neher E, Sakmann B, and Sigworth FJ. Improved patch-clamp techniques for high-resolution current recording from cell and cell-free membrane patches. Pflügers Arch 391: 85-100, 1981[Web of Science][Medline]. |
| 78. | Hansen O. Interaction of cardiac glycosides with (Na++K+)-activated ATPase. A biochemical link to digitalis-induced inotropy. Pharmacol Rev 36: 143-163, 1984[Web of Science][Medline]. |
| 79. |
Hansen PS,
Buhagiar KA,
Gray DF, and Rasmussen HH.
Voltage- dependent stimulation of the Na+-K+ pump by insulin in rabbit cardiac myocytes.
Am J Physiol Cell Physiol
278: C546-C553, 2000 |
| 80. | Hansen PS, Gray DF, and Rasmussen HH. Insulin regulates the voltage dependence of the sarcolemmal Na+-K+ pump (Abstract). Circulation 92, Suppl I: I-638, 1995. |
| 81. | Hansen U-P, Gradmann D, Sanders D, and Slayman CL. Interpretation of current-voltage relationships for "active" ion transport systems: I. Steady-state reaction-kinetic analysis of class-I mechanisms. J Membr Biol 63: 165-190, 1981[Web of Science][Medline]. |
| 82. |
Haruna T,
Horie M,
Kouchi I,
Nawada R,
Tsuchiya K,
Akao M,
Otani H,
Murakami T, and Sasayama S.
Coordinate interaction between ATP-sensitive K+ channel and Na+,K+-ATPase modulates ischemic preconditioning.
Circulation
98: 2905-2910, 1998 |
| 83. |
Hemsworth PD,
Whalley DW, and Rasmussen HH.
Electrogenic Li+/Li+ exchange mediated by the Na+-K+ pump in rabbit cardiac myocytes.
Am J Physiol Cell Physiol
272: C1186-C1192, 1997 |
| 84. | Hermans AN. Ion Translocation by the Na+/K+ Pump: Influence of Membrane Potential and Cardiac Glycosides (MD thesis). Leuven, Belgium: Acta Biomed Lovan, 1997, vol. 143, p. 1-140. |
| 85. |
Hermans AN,
Glitsch HG, and Verdonck F.
The effect of cardiac glycosides on the Na+ pump current-voltage relationship of isolated rat and guinea-pig heart cells.
J Physiol (Lond)
481: 279-291, 1994 |
| 86. |
Hermans AN,
Glitsch HG, and Verdonck F.
The antagonistic effect of K![]() |
| 87. | Hermans AN, Glitsch HG, and Verdonck F. Activation of the Na+/K+ pump current by intra- and extracellular Li ions in single guinea-pig cardiac cells. Biochim Biophys Acta 1330: 83-93, 1997[Medline]. |
| 88. | Herzig S, Lüllmann H, and Mohr K. On the cooperativity of ouabain-binding to intact myocardium. J Mol Cell Cardiol 17: 1095-1104, 1985[Web of Science][Medline]. |
| 89. |
Heyse S,
Wuddel I,
Apell H-J, and Stürmer W.
Partial reactions of the Na,K-ATPase: determination of rate constants.
J Gen Physiol
104: 197-240, 1994 |
| 90. | Hilgemann DW. Giant excised cardiac sarcolemmal membrane patches: sodium and sodium-calcium exchange currents. Pflügers Arch 415: 247-249, 1989[Web of Science][Medline]. |
| 91. |
Hilgemann DW.
Channel-like function of the Na,K pump probed at microsecond resolution in giant membrane patches.
Science
263: 1429-1432, 1994 |
| 92. | Hilgemann DW. Recent electrical snapshots of the cardiac Na,K pump. Na/K-ATPase and related transport ATPases: structure, mechanism, and regulation. Ann NY Acad Sci 834: 260-269, 1997[Web of Science][Medline]. |
| 93. | Hilgemann DW, Nagel GA, and Gadsby DC. Na/K pump current in giant membrane patches excised from ventricular myocytes. In: The Sodium Pump: Recent Developments, edited by Kaplan JH, and De Weer P. New York: Rockefeller Univ. Press, 1991, p. 543-547. |
| 94. | Hodgkin AL, and Keynes RD. Active transport of cations in giant axons from Sepia and Loligo. J Physiol (Lond) 128: 28-60, 1955. |
| 95. | Holmgren M, Wagg J, Bezanilla F, Rakowski RF, De Weer P, and Gadsby DC. Three distinct and sequential steps in the release of sodium ions by the Na+/K+-ATPase. Nature 403: 898-901, 2000[Medline]. |
| 96. |
Hool LC,
Gray DF,
Robinson BG, and Rasmussen HH.
Angiotensin-converting enzyme inhibitors regulate the Na+-K+ pump via effects on angiotensin metabolism.
Am J Physiol Cell Physiol
271: C172-C180, 1996 |
| 97. |
Hool LC,
Whalley DW,
Doohan MM, and Rasmussen HH.
Angiotensin-converting enzyme inhibiton, intracellular Na+, and Na+-K+ pumping in cardiac myocytes.
Am J Physiol Cell Physiol
268: C366-C375, 1995 |
| 98. | Horisberger J-D, Lemas V, Kraehenbühl J-P, and Rossier BC. Structure-function relationship of Na,K-ATPase. Annu Rev Physiol 53: 565-584, 1991[Web of Science][Medline]. |
| 99. | Isenberg G, and Trautwein W. The effect of dihydro-ouabain and lithium-ions on the outward current in cardiac Purkinje fibres. Pflügers Arch 350: 41-54, 1974[Web of Science][Medline]. |
| 100. | Ishida Y, and Chused TM. Lack of voltage sensitive potassium channels and generation of membrane potential by sodium potassium ATPase in murine T lymphocytes. J Immunol 151: 610-620, 1993[Abstract]. |
| 101. |
Ishizuka N, and Berlin JR.
-Adrenergic stimulation does not regulate Na pump function in voltage-clamped ventricular myocytes of the rat heart.
Pflügers Arch
424: 361-363, 1993[Web of Science][Medline].
|
| 102. |
Juhaszova M, and Blaustein MP.
Na+ pump low and high ouabain affinity subunit isoforms are differently distributed in cells.
Proc Natl Acad Sci USA
94: 1800-1805, 1997 |
| 103. | Kabakov AY. Activation of KATP channels by Na/K pump in isolated cardiac myocytes and giant membrane patches. Biophys J 75: 2858-2867, 1998[Web of Science][Medline]. |
| 104. | Kameyama M, Kakei M, Sato R, Shibasaki T, Matsuda H, and Irisawa H. Intracellular Na+ activates a K+ channel in mammalian cardiac cells. Nature 309: 354-356, 1984[Medline]. |
| 105. |
Kawakami K,
Noguchi S,
Noda M,
Takahashi H,
Ohta T,
Kawamura M,
Nojima H,
Nagano K,
Hirose T,
Inayama S,
Hayashida H,
Miyata T, and Numa S.
Primary structure of the -subunit of Torpedo californica (Na+ + K+) ATPase deduced from cDNA sequence.
Nature
316: 733-736, 1985[Medline].
|
| 106. |
Kawakami K,
Nojima H,
Ohta T, and Nagano K.
Molecular cloning and sequence analysis of human Na, K-ATPase -subunit.
Nucleic Acids Res
14: 2833-2844, 1986 |
| 107. |
Kinard TA,
Liu X-Y,
Liu S, and Stimers JR.
Effect of Napip on Ko activation of the Na-K pump in adult rat cardiac myocytes.
Am J Physiol Cell Physiol
266: C37-C41, 1994 |
| 108. |
Kockskämper J,
Erlenkamp S, and Glitsch HG.
Activation of the cAMP-protein kinase A pathway facilitates Na+ translocation by the Na+-K+ pump in guinea-pig ventricular myocytes.
J Physiol (Lond)
523: 561-574, 2000 |
| 109. | Kockskämper J, and Glitsch HG. Sodium pump of cultured guinea pig atrial myocytes. Na/K-ATPase and related transport ATPases: structure, mechanism, and regulation. Ann NY Acad Sci 834: 354-356, 1997[Web of Science][Medline]. |
| 110. | Lafaire AV, and Schwarz W. Voltage dependence of the rheogenic Na+/K+ ATPase in the membrane of oocytes of Xenopus laevis. J Membr Biol 91: 43-51, 1986[Web of Science][Medline]. |
| 111. | Läuger P. Electrogenic Ion Pumps. Sunderland, MA: Sinauer, 1991, p. 1-313. |
| 112. |
Lederer WJ,
Niggli E, and Hadley RW.
Sodium-calcium exchange in excitable cells: fuzzy space.
Science
248: 283, 1990 |
| 113. |
Levi AJ.
The effect of strophanthidin on action potential, calcium current and contraction in isolated guinea-pig ventricular myocytes.
J Physiol (Lond)
443: 1-23, 1991 |
| 114. | Levi AJ. The electrogenic sodium/potassium pump and passive sodium influx of isolated guinea pig ventricular myocytes. J Cardiovasc Electrophysiol 3: 225-238, 1992. |
| 115. |
Levi AJ.
A role for sodium/calcium exchange in the action potential shortening caused by strophanthidin in guinea pig ventricular myocytes.
Cardiovasc Res
27: 471-481, 1993 |
| 116. | Levi AJ, Boyett MR, and Lee CO. The cellular actions of digitalis glycosides on the heart. Prog Biophys Mol Biol 62: 1-54, 1994[Web of Science][Medline]. |
| 117. | Lingrel JB, Argüello JM, Van Huysse J, and Kuntzweiler TA. Cation and cardiac glycoside binding sites of the Na, K-ATPase. Na/K-ATPase and related transport ATPases: structure, mechanism, and regulation. Ann NY Acad Sci 834: 194-206, 1997[Web of Science][Medline]. |
| 118. | Luk H-N, and Carmeliet E. Na+-activated K+ current in cardiac cells: rectification, open probabilitiy, block and role in digitalis toxicity. Pflügers Arch 416: 766-768, 1990[Web of Science][Medline]. |
| 119. | Lüttgau HC, and Glitsch H. Membrane physiology of nerve and muscle fibres. Prog Zool 24: 1-132, 1976. |
| 120. |
Main MJ,
Grantham CJ, and Cannell MB.
Changes in subsarcolemmal sodium concentration measured by Na-Ca exchanger activity during Na-pump inhibition and -adrenergic stimulation in guinea-pig ventricular myocytes.
Pflügers Arch
435: 112-118, 1997[Web of Science][Medline].
|
| 121. | Mathias RT, Cohen IS, and Oliva C. Limitations of the whole cell patch clamp technique in the control of intracellular concentrations. Biophys J 58: 759-770, 1990[Web of Science][Medline]. |
| 122. |
McDonough AA,
Zhang Y,
Shin V, and Frank JS.
Subcellular distribution of sodium pump isoform subunits in mammalian cardiac myocytes.
Am J Physiol Cell Physiol
270: C1221-C1227, 1996 |
| 123. | Mehrke G, Daut J, Harmann S, and Dischner A. The effects of dihydro-ouabain on the current-voltage relation of guinea-pig cardiomyocytes (Abstract). Pflügers Arch 408 Suppl 1: R10, 1987. |
| 124. |
Mercer RW,
Biemesderfer D,
Bliss DP,
Collins JH, and Forbush B.
Molecular cloning and immunological characterization of the -polypeptide, a small protein associated with the Na,K-ATPase.
J Cell Biol
121: 579-586, 1993 |
| 125. |
Mihailidou AS,
Buhagiar KA, and Rasmussen HH.
Na+ influx and Na+-K+ pump activation during short-term exposure of cardiac myocytes to aldosterone.
Am J Physiol Cell Physiol
274: C175-C181, 1998 |
| 126. |
Mihailidou AS,
Bundgaard H,
Mardini M,
Hansen PS,
Kjeldsen K, and Rasmussen HH.
Hyperaldosteronemia in rabbits inhibits the cardiac sarcolemmal Na+-K+ pump.
Circ Res
86: 37-42, 2000 |
| 127. |
Mogul DJ,
Rasmussen HH,
Singer DH, and Ten Eick RE.
Inhibition of Na-K pump current in guinea pig ventricular myocytes by dihydroouabain occurs at high- and low-affinity sites.
Circ Res
64: 1063-1069, 1989 |
| 128. |
Mogul DJ,
Singer DH, and Ten Eick RE.
Dependence of Na-K pump current on internal Na+ in mammalian cardiac myocytes.
Am J Physiol Heart Circ Physiol
259: H488-H496, 1990 |
| 129. |
Mullins LJ, and Noda K.
The influence of sodium-free solutions on the membrane potential of frog muscle fibers.
J Gen Physiol
47: 117-132, 1963 |
| 130. | Nakao M, and Gadsby DC. Voltage dependence of Na translocation by the Na/K pump. Nature 323: 628-630, 1986[Medline]. |
| 131. |
Nakao M, and Gadsby DC.
[Na] and [K] dependence of the Na/K pump current-voltage relationship in guinea pig ventricular myocytes.
J Gen Physiol
94: 539-565, 1989 |
| 132. | Noble D. The Initiation of the Heartbeat. Oxford UK: Clarendon, 1975, p. 1-156. |
| 133. | Omay HS, and Schwarz W. Voltage-dependent stimulation of Na+/K+-pump current by external cations: selectivity of different K+ congeners. Biochim Biophys Acta 1104: 167-173, 1992[Medline]. |
| 134. |
Or E,
Goldshleger R, and Karlish SJD.
An effect of voltage on binding of Na+ at the cytoplasmic surface of the Na+-K+ pump.
J Biol Chem
271: 2470-2477, 1996 |
| 135. |
Peluffo RD, and Berlin JR.
Electrogenic K+ transport by the Na+-K+ pump in rat cardiac ventricular myocytes.
J Physiol (Lond)
501: 33-40, 1997 |
| 136. | Philipson KD, and Nishimoto AY. ATP-dependent Na+ transport in cardiac sarcolemmal vesicles. Biochim Biophys Acta 733: 133-141, 1983[Medline]. |
| 137. |
Post RL,
Hegyvary C, and Kume S.
Activation by adenosine triphosphate in the phosphorylation kinetics of sodium and potassium ion transport adenosine triphosphatase.
J Biol Chem
247: 6530-6540, 1972 |
| 138. |
Post RL,
Merrit CR,
Kinsolving CR, and Albright CD.
Membrane adenosine triphosphatase as a participant in the active transport of sodium and potassium in the human erythrocyte.
J Biol Chem
235: 1796-1802, 1960 |
| 139. |
Post RL,
Sen AK, and Rosenthal AS.
A phosphorylated intermediate in adenosine triphosphatase-dependent sodium and potassium transport across kidney membranes.
J Biol Chem
240: 1437-1445, 1965 |
| 140. | Powell T, and Twist VW. A rapid technique for the isolation and purification of adult cardiac muscle cells having respiratory control and a tolerance to calcium. Biochem Biophys Res Commun 72: 327-333, 1976[Web of Science][Medline]. |
| 141. |
Priebe L,
Friedrich M, and Benndorf K.
Functional interaction between KATP channels and the Na+-K+ pump in metabolically inhibited heart cells of the guinea-pig.
J Physiol (Lond)
492: 405-417, 1996 |
| 142. | Rakowski RF, Bezanilla F, De Weer P, Gadsby DC, Holmgren M, and Wagg J. Charge translocation by the Na/K pump. Na/K-ATPase and related transport ATPases: structure, mechanism, and regulation. Ann NY Acad Sci 834: 231-243, 1997[Web of Science][Medline]. |
| 143. | Rakowski RF, De Weer P, and Gadsby DC. Current-voltage relationship of the backward-running Na/K pump in voltage-clamped internally-dialyzed squid giant axons (Abstract). J Biophys 53: 223a, 1988. |
| 144. |
Rakowski RF,
Gadsby DC, and De Weer P.
Stoichiometry and voltage dependence of the sodium pump in voltage-clamped, internally dialyzed squid giant axon.
J Gen Physiol
93: 903-941, 1989 |
| 145. | Rakowski RF, Gadsby DC, and De Weer P. Voltage dependence of the Na/K pump. J Membr Biol 155: 105-112, 1997[Web of Science][Medline]. |
| 146. | Rakowski RF, and Paxson CL. Voltage dependence of Na/K pump current in Xenopus oocytes. J Membr Biol 106: 173-182, 1988[Web of Science][Medline]. |
| 147. | Rakowski RF, Vasilets LA, Latona J, and Schwarz W. A negative slope in the current-voltage relationship of the Na+/K+ pump in Xenopus oocytes produced by reduction of external [K+]. J Membr Biol 121: 177-187, 1991[Web of Science][Medline]. |
| 148. |
Rasmussen HH,
Okita GT,
Hartz RS, and Ten Eick RE.
Inhibiton of electrogenic Na+-pumping in isolated atrial tissue from patients treated with digoxin.
J Pharmacol Exp Ther
252: 60-64, 1990 |
| 149. |
Rasmussen HH,
Ten Eick RE,
Okita GT,
Hartz RS, and Singer DH.
Inhibiton of electrogenic Na-pumping attributable to binding of cardiac steroids to high-affinity pump sites in human atrium.
J Pharmacol Exp Ther
235: 629-635, 1985 |
| 150. |
Sagar A, and Rakowski RF.
Access channel model for the voltage dependence of the forward-running Na+/K+ pump.
J Gen Physiol
103: 869-894, 1994 |
| 151. |
Sakai R,
Hagiwara N,
Matsuda N,
Kasanuki H, and Hosoda S.
Sodium-potassium pump current in rabbit sino-atrial node cells.
J Physiol (Lond)
490: 51-62, 1996 |
| 152. |
Sasaki N,
Mitsuiye T,
Wang Z, and Noma A.
Increase of the delayed rectifier K+ and Na+-K+ pump currents by hypotonic solutions in guinea pig cardiac myocytes.
Circ Res
75: 887-895, 1994 |
| 153. | Schatzmann H-J. Herzglykoside als Hemmstoffe für den aktiven Kalium- und Natriumtransport durch die Erythrocytenmembran. Helv Physiol Pharmacol Acta 11: 346-354, 1953[Web of Science][Medline]. |
| 154. | Schoner W, Thönges D, Hamer E, Antolovic R, Buxbaum E, Willeke M, Serpersu EH, and Scheiner-Bobis G. Is the sodium pump a functional dimer? In: The Sodium Pump-Structure Mechanism, Hormonal Control and Its Role in Disease, edited by Bamberg E, and Schoner W. New York: Springer, 1994, p. 332-341. |
| 155. |
Schuurmans Steckhoven F, and Bonting SL.
Transport adenosine triphosphatases: properties and functions.
Physiol Rev
61: 1-76, 1981 |
| 156. | Schwartz A, Lindenmayer GE, and Allen JC. The sodium-potassium adenosine triphosphatase: pharmacological, physiological and biochemical aspects. Pharmacol Rev 27: 3-134, 1975. |
| 157. | Schwarz W, and Gu Q. Characteristics of the Na+/K+-ATPase from Torpedo californica expressed in Xenopus oocytes: a combination of tracer flux measurements with electrophsiological measurements. Biochim Biophys Acta 945: 167-174, 1988[Medline]. |
| 158. | Schweigert B, Lafaire AV, and Schwarz W. Voltage dependence of the Na-K ATPase: measurements of ouabain-dependent membrane current and ouabain binding in oocytes of Xenopus laevis. Pflügers Arch 412: 579-588, 1988[Web of Science][Medline]. |
| 159. | Semb SO, and Sejersted OM. Fuzzy space and control of Na+,K+-pump rate in heart and skeletal muscle. Acta Physiol Scand 156: 213-225, 1996[Web of Science][Medline]. |
| 160. | Shah A, Cohen IS, and Rosen MR. Stimulation of cardiac alpha receptors increases Na/K pump current and decreases gK via a pertussis toxin-sensitive pathway. Biophys J 54: 219-225, 1988[Web of Science][Medline]. |
| 161. |
Shattock MJ, and Matsuura H.
Measurement of Na+-K+ pump current in isolated rabbit ventricular myocytes using the whole-cell voltage-clamp technique.
Circ Res
72: 91-101, 1993 |
| 162. |
Shull GE,
Lane LK, and Lingrel JB.
Amino-acid sequence of the -subunit of the (Na+ + K+) ATPase deduced from a cDNA.
Nature
321: 429-431, 1986[Medline].
|
| 163. | Shull GE, Schwartz A, and Lingrel JB. Amino-acid sequence of the catalytic subunit of the (Na+ + K+) ATPase deduced from a complementary DNA. Nature 316: 691-695, 1985[Medline]. |
| 164. |
Shyjan AW, and Levenson R.
Antisera specific for the 1, 2, 3, and subunits of the Na,K-ATPase: differential expression of and subunits in rat tissue membranes.
Biochemistry
28: 4531-4535, 1989[Medline].
|
| 165. | Skou JC. The influence of some cations on an adenosine triphosphatase from peripheral nerves. Biochim Biophys Acta 23: 394-401, 1957[Medline]. |
| 166. | Stimers JR, and Dobretsov M. Adrenergic stimulation of Na/K pump current in adult rat cardiac myocytes in short-term culture. J Membr Biol 163: 205-216, 1998[Web of Science][Medline]. |
| 167. | Stimers JR, Liu S, and Kinard TA. Effect of Nai on activity and voltage dependence of the Na/K pump in adult rat cardiac myocytes. J Membr Biol 135: 39-47, 1993[Web of Science][Medline]. |
| 168. |
Stimers JR,
Liu S, and Lieberman M.
Apparent affinity of the Na/K pump for ouabain in cultured chick cardiac myocytes.
J Gen Physiol
98: 815-833, 1991 |
| 169. |
Stimers JR,
Lobaugh LA,
Liu S,
Shigeto N, and Lieberman M.
Intracellular sodium affects ouabain interaction with the Na/K pump in cultured chick cardiac myocytes.
J Gen Physiol
95: 77-95, 1990 |
| 170. |
Stimers JR,
Shigeto N, and Lieberman M.
Na/K pump current in aggregates of cultured chick cardiac myocytes.
J Gen Physiol
95: 61-76, 1990 |
| 171. | Stürmer W, Bühler R, Apell H-J, and Läuger P. Charge translocation by the Na,K pump. II. Ion binding and release at the extracellular face. J Membr Biol 121: 163-176, 1991[Web of Science][Medline]. |
| 172. |
Su Z,
Zou A,
Nonaka A,
Zubair I,
Sanguinetti MC, and Barry WH.
Influence of prior Na+ pump activity on pump and Na+/Ca2+ exchange currents in mouse ventricular myocytes.
Am J Physiol Heart Circ Physiol
275: H1808-H1817, 1998 |
| 173. |
Sui JL,
Chan KW, and Logothetis DE.
Na+ activation of the muscarinic K+ channel by a G-protein-independent mechanism.
J Gen Physiol
108: 381-391, 1996 |
| 174. | Sweadner KJ. Isozymes of the Na+/K+-ATPase. Biochim Biophys Acta 988: 185-200, 1989[Medline]. |
| 175. |
Sweadner KJ,
Herrera VLM,
Amato S,
Moellmann A,
Gibbons DK, and Repke KRH.
Immunologic identification of Na+,K+-ATPase isoforms in myocardium-isoform change in deoxycorticosterone acetate-salt hypertension.
Circ Res
74: 669-678, 1994 |
| 176. |
Thomas RC.
Electrogenic sodium pump in nerve and muscle cells.
Physiol Rev
52: 563-594, 1972 |
| 177. |
Trotier D, and Døving KB.
Direct influence of the sodium pump on the membrane potential of vomeronasal chemoreceptor neurones in frog.
J Physiol (Lond)
490: 611-621, 1996 |
| 178. | Tsuchiya K, Horie M, Haruna T, Ai T, Nishimoto T, Fujiwara H, and Sasayama S. Functional communication between cardiac ATP-sensitive K+ channel and Na/K ATPase. J Cardiovasc Electrophysiol 9: 415-422, 1998[Web of Science][Medline]. |
| 179. | Vasilets LA, and Schwarz W. Regulation of endogenous and expressed Na+/K+ pumps in Xenopus oocytes by membrane potential and stimulation of protein kinases. J Membr Biol 125: 119-132, 1992[Web of Science][Medline]. |
| 180. | Vasilets LA, and Schwarz W. Structure-function relationships of cation binding in the Na+/K+-ATPase. Biochim Biophys Acta 1154: 201-222, 1993[Medline]. |
| 181. | Vassalle M. The role of the electrogenic sodium pump in controlling excitability in nerve and cardiac fibers. In: Current Topics in Membranes and Transport, edited by Slayman CL. New York: Academic, 1982, vol. 16, p. 467-483. |
| 182. | Verdonck F, Vanhaecke J, Mubagwa K, Flameng W, Stankovicova T, and Sipido K. Sensitivity of the Na+/K+ pump current to cardiac glycosides in ventricular myocytes from human failing heart (Abstract). Biophys J 74: A354, 1998. |
| 183. |
Wang Y,
Gao J,
Mathias RT,
Cohen IS,
Sun X, and Baldo GJ.
-Adrenergic effects on Na+-K+ pump current in guinea-pig ventricular myocytes.
J Physiol (Lond)
509: 117-128, 1998 |
| 184. |
Whalley DW,
Hool LC,
Ten Eick RE, and Rasmussen HH.
Effect of osmotic swelling and shrinkage on Na+-K+ pump activity in mammalian cardiac myocytes.
Am J Physiol Cell Physiol
265: C1201-C1210, 1993 |
| 185. |
Williamson AP,
Kennedy RH,
Seifen E,
Lindemann JP, and Stimers JR.
1b-Adrenoceptor-mediated stimulation of Na-K pump current in adult rat ventricular myocytes.
Am J Physiol Heart Circ Physiol
264: H1315-H1318, 1993 |
| 186. |
Woo AL,
James PF, and Lingrel JB.
Characterization of the fourth isoform of the Na,K-ATPase.
J Membr Biol
169: 39-44, 1999[Web of Science][Medline].
|
| 187. | Zillikens S. Die Wirkung von Digitoxigenin und Digitoxigenin-Derivaten auf die Na+/K+-ATPase des Warmblüterherzens (PhD thesis). Bochum, Germany: Ruhr-Universität, 1999, p. 1-141. |
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