Clinical Science (1979) 51.289-293
~
EDITORIAL REVIEW
J . P A T R I C K A N D P. J . H I L T O N
Medical Unit, St Thomas5 Hospital, London, and The Tropical Metabolism Research Unit,
University of the West Indies. Jamaica, West Indies
change in the activity of the sodium pump as
opposed to alterations in other transport processes. We have not attempted to review the literature on sodium-transport disorders, but have
selected examples to point out the distinctions
which we believe could help our understanding of
these disorders. Before proceeding to the detailed
argument, it is as well to make some general points
concerning the terms to be used and also the nature
of the techniques involved in the determination of
the transport rates under discussion.
The cardinal feature of the sodium pump and the
flux from which it derives its name is the transport
of sodium from the cell interior to the extracellular
fluid. The flux is measured as follows: the cells
under study are incubated in a medium containing
radioactive sodium until they have accumulated
sufficient radioactivity for satisfactory counting.
The cells are then washed and suspended in a
medium free of radioactive sodium. Over an appropriate period of time either the loss of radioactivity
from the cells or the gain of radioactivity by the
extracellular fluid is observed. When the residual
radioactivity in the cells is plotted against time, a
smooth curve results, which converts to a straight
line if the natural logarithm of radioactivity is
substituted for radioactivity on the y axis. The
slope of this line is the pseudo-first-order rate
constant for sodium efflux and has the dimensions
time-'. This rate constant cannot be identified with
the activity of the sodium pump alone as a
proportion of the radioactive tracer leaving the cells
does so by pathways independent of the sodium
pump. The value for this latter rate constant may
be determined by performing an identical experi-
The high potassium content and low sodium
content of most animal cells differ strikingly from
the contents in the extracellular fluid surrounding
them. This arrangement is so widespread that it
probably represents the optimal condition for intracellular processes. Such a system of cells and extracellular fluid is intrinsically unstable and will inevitably run downhill, with net movements of sodium
into the cell. The weight of evidence suggests that
the mechanism responsible for the maintenance of
these ion gradients is an active transport system
located in the cell membrane, which extrudes
sodium from the cell while simultaneously moving
potassium in the reverse direction. This process is
thermodynamically 'uphill' and consequently
energy-requiring; the immediate source of energy is
ATP, which is continuously replenished by the processes of intermediary metabolism. The transport
system, which has been most clearly defined,
appears to consist principally of an ATPase inhibitable by cardiac glycosides and usually referred
to as the sodium pump (Glynn & Karlish, 1975;
Jorgensen, 1975).
The purpose of this review is to consider the
different ways in which the transport of sodium and
potassium may be altered by disease and therapy.
We are particularly concerned to emphasize the
time-dependent features of such alterations and
also the very different implications of a primary
Key words: active transport, adenosine triphosphate, cell-membrane ,permeability, sodium pump,
potassium.
Correspondence: Dr J. Patrick, Renal Laboratory,
Medical Unit, St Thomas's Hospital, London, S.E.1.
21
289
290
J. Patrick and P. J . Hilton
ment in the presence of a sufficient concentration of
ouabain to produce maximal inhibition of the
pump. This also gives rise to a first-order rate constant which, when subtracted from the rate constant for total sodium efflux, gives the ouabainsensitive sodium-efflux rate constant. It should be
stressed that what has been measured or calculated
is a rate constant, or the fractional loss of radioactive sodium with time. To derive the actual
quantity of sodium actively transported by this process, the intracellular sodium content is multiplied
by the corresponding rate constant. This figure is
usually referred to as a unidirectional flux.
Potassium efflux is measured in a similar way,
with radioactive potassium. Sodium and potassium influx may be determined by exposing the
cells to an extracellular fluid containing the appropriate radioisotope for a known period of time. The
gain in radioactivity (after correction for re-efflux)
can be converted directly into an ionic flux since
the specific radioactivity of the extracellular fluid is
both constant and readily measured. Like sodium
efflux, potassium influx is usually determined in the
presence and absence of ouabain in maximally
inhibitory concentrations.
The results obtained from the study of sodium
transport in diseased cells are usually presented as
the intracellular electrolyte values and the appropriate rate constant derived from the movement of the
radioactive tracer. This contrasts with the physiological literature, where more commonly one finds
the value of the unidirectional flux reported. The
implications of this different mode of presentation
of data are discussed below.
The reduction in cellular potassium and the increase in sodium which follows total inhibition of
the sodium pump are well known, but in clinical
investigation the problem is usually to describe the
behaviour of the partially inhibited or impaired
pump. Even a cursory examination of the literature shows that, in these clinical studies, although
intracellular sodium is usually increased and the
rate constant for sodium efflux is often decreased,
the unidirectional flux of both sodium and intracellular potassium may be normal. This rather
unexpected picture demonstrates that the wellknown immediate effects of acute inhibition of
sodium transport are only a stage on the way to a
new steady state. A brief theoretical discussion of
the effects of a reduction in the ouabain-sensitive
rate constant for sodium efflux may help to show
why the expected reduction in potassium is not
always found.
At this stage, we should state two fundamental
assumptions, which are generally made in sodiumand potassium-transport studies. First, that
substantially all the sodium and the potassium in
the cell can be regarded as freely available to the
transport mechanisms; secondly, that for every
three sodium ions removed from the cell by the
sodium pump two potassium ions enter by the
same mechanism (Post & Jolly, 1957; Garrahan &
Glynn, 1967). There have been some reports of a
small non-exchangeable pool of sodium (Keynes &
Swan, 1959; Mullins & Frumento, 1963), but in
human erythrocytes and leucocytes this has not
been reported. In the presence of normal extracellular concentrations of sodium and potassium
the 3 :2 stoicheiometric relationship also seems to
apply to leucocytes (Hilton, Edmondson, Thomas
& Patrick, 1975).
If we postulate an acute reduction in the
ouabain-sensitive rate constant for sodium efflux
without alteration of sodium influx, then the
cellular sodium concentration must increase so that
the product of intracellular sodium and the rate
constant for sodium efflux again balance sodium
influx. Thus the decrease in sodium efflux which
accompanied the initial decrease in the rate
constant for sodium efflux is not a permanent
effect. Similarly, the decrease in cellular potassium
associated with the initial fall in sodium transport
will be temporary and cellular potassium will return
towards a value only a little less than normal as the
sodium transport rate is restored. This conclusion
is substantiated by the results of Welt, Sachs &
McManus ( 1 964), who investigated the transport
rates for sodium and potassium of the erythrocytes from subjects with renal failure. Those with
high intracellular sodium concentrations and
depression of the sodium-efflux rate constant exhibited a normal intracellular potassium and a
normal rate of ouabain-sensitive potassium influx
at this high intracellular sodium. If the sodium was
artifically reduced to a normal value then
potassium influx became subnormal. They commented that: ‘It is our assumption that a defect in
transport permits the accumulation of sodium up to
a point where the impaired pump is able to maintain a new steady-state at the expense of a higher
intracellular sodium concentration’. Funder &
Wieth (1974) studied the effect on erythrocyte
composition of clinical doses of cardiac glycosides
sufficient to induce partial inhibition of the sodium
pump. Although a consistent rise in intracellular
sodium was found, intracellular potassium
remained within the normal range. This study provides the clearest evidence for the contention that
Intracellular electrolytes and ion transport
intracellular potassium need not be greatly affected
by moderate reduction in the ouabain-sensitive rate
constant for sodium efflux. It should be noted that
in the erythrocyte approximately 2-3 weeks were
required for the new steady state to be achieved.
Other cells with more active sodium-transport
systems will achieve a new potassium equilibrium
more rapidly. In this respect it is noteworthy that
other workers, who have shown reductions in
erythrocyte potassium after treatment with digoxin
(Clifford & Beautyman, 1958; Astrup, 1974), had
made their observations in the first few days of
treatment, possibly before a new steady state had
been achieved.
The changes in cell sodium and potassium we
have discussed relate to a constant reference value
(cell dry weight). It has to be accepted that the increase in cell cation consequent on an inhibition of
the sodium pump will lead to an increase in cell
water. If cell sodium and potassium and their transport rates are related to the variable of cell water or
cell volume rather than dry weight, these changes
will be at least partially obscured.
Reductions in the rate constant for ouabainsensitive sodium efflux associated with an increase
in intracellular sodium and a somewhat lessconstant change in intracellular potassium have
been reported in a number of conditions, including
uraemia (Welt et al., 1964; Welt, Smith, Dunn,
Czerwinski, Proctor, Cole, Balfe & Gitelman,
1967; Edmondson, Hilton, Jones, Patrick &
Thomas, 1975), hypertension (Edmondson,
Thomas, Hilton, Patrick & Jones, 1975), hyperthyroidism (Smith & Samuel, 1970), malnutrition
(Patrick & Golden, 1977) and liver disease (Alam,
Wilkinson, Poston, Moodie 8c Williams, 1977;
Alam, Poston, Wilkinson, Golindano & Williams,
1978; Owen & McIntyre, 1978). In most of these
examples, a depressed rate constant for sodium
efflux was associated with an elevated intracellular
sodium and no change in the unidirectional flux for
sodium. In erythrocytes from hyperthyroid
patients, Smith & Samuel (1970) noted an increase
in intracellular sodium coupled with a decrease in
the rate constant for sodium efflux. Calculated
sodium efflux was increased and, although influx
was not measured in this study, it would clearly
have been increased if we assume the cells were in a
steady state. These results imply that hyperthyroidism alters not only the sodium pump but also
the permeability of the erythrocyte membrane to
sodium.
It is instructive to consider the theoretical effects
of a primary increase in membrane permeability
29 1
to sodium. If the rate constant for sodium efflux
is held at a particular level, intracellular sodium
will increase until the product of the rate constant for sodium efflux and intracellular sodium
again equal the influx rate. If the stoicheiometry
of the sodium pump remains constant and
membrane permeability to potassium is also unchanged then cellular potassium would actually increase. We are unaware of any pathological state in
which these conditions obtain. However, there
are examples of primary increases in membrane
permeability, of which the haemolytic anaemia
reported by Zarkowsky, Oski, Sha’afi, Shohet &
Nathan (1968) is one of the most completely
documented examples. In this disease intracellular
sodium was approximately 100 mmol/l of cells and
potassium was approximately 40 mmol/l of cells.
Nevertheless, transport via the pump was increased lZfold, sodium influx was increased 30fold and potassium efilux was increased 15-fold.
There appeared to be no abnormality of the pump
despite the reversed sodium/potassium ratio and
the disease is primarily one of increased membrane
permeability. In view of the current interest in the
role of sodium transport in energy expenditure one
of the most interesting consequences of this condition was the threefold increase in glucose consumption and lactate production.
As we mentioned above, there appears to be a
stoicheiometric relationship between the transport
of sodium via the pump and the utilization of ATP,
with three sodium ions being extruded for the
hydrolysis of one molecule of ATP (Sen & Post,
1964). There has been considerable debate as to the
consequences for the whole organism of the
activity of the sodium pump. Estimates of the proportion of basal energy devoted to this process
vary from 5% (Chinet, Clausen & Girardier, 1977;
Folke & Sestoft, 1977) to 45% (Edelman, 1976).
Similarly experiments to assess the contribution of
altered sodium transport to thyroid thermogenesis
have also produced discrepant values varying from
90% (Ismail-Beigi & Edelman, 1970; Asano,
Liberman & Edelman, 1976) to 5% (Folke &
Sestoft, 1977). Results like these have even provoked the suggestion that the currently postulated
transport systems, of which sodium transport is
one of the most important, constitute a state of
caloric catastrophe in that the sum of their
energetic demands are greater than the total energy
supply (Ling, Miller & Ochsenfield, 1973; Minkoff
& Damadian, 1973). In debate Glynn has strongly
attacked the theoretical basis of this latter argument (Cope & Glynn, 1977).
292
J. Patrick and P. J . Hilton
It has to be acknowledged that there are major
methodological problems to be overcome in performing such experiments. The two approaches
most commonly used have been to study energy
consumption before and after inhibition of the
sodium pump, either by the application of ouabain
or by the removal of external sodium. Both these
manoeuvres (which incidentally alter intracellular
sodium in opposite directions) also cause cellular
potassium depletion. This will itself lead to effects
on intermediary metabolism, and in addition
ouabain has the potential for affecting intracellular
enzymes (Blatt, McVerry & Kimm, 1972; Horn,
Walaas & Walaas, 1973). In experiments of this
type, it may well be unsafe to assume that one is
studying simply the deletion of one transport process, and that secondary processes, also energydemanding, have not been activated. In general, the
highest estimates of the energetic costs of sodium
transport have come from studies on tissue slices,
lower estimates being derived from perfused tissues. Whatever the ultimate explanation of these
discrepancies, it remains a thermodynamic
necessity that energy be expended in the maintenance of transmembrane gradients for sodium and
potassium. Moreover, as Whittam (1961) pointed
out, the hydrolysis of ATP is related to the flux of
sodium via the pump (mmol unit time-' unit
mass-') and not to the rate constant itself. If we
accept these arguments, sodium influx appears to
be of prime importance as a pacemaker of sodium
transport and at any rate one portion of energy
consumption.
In many ways, the tissue most suited to the
exploration of this concept is brown fat, one role
for which is undoubtedly thermogenesis. Unfortunately no clear-cut explanation for the mechanism
of this process has yet emerged. Horwitz (1976)
has suggested an important if not dominant role for
catecholamine-induced increases in membrane
permeability leading to an increase in energy consumption and heat production. Himms-Hagen
(1976), however, has emphasized the multifactorial nature of thermogenesis and tends to the
view that changes in sodium transport play only a
minor role in this process.
In summary, there is evidence that abnormalities
of sodium and potassium transport are to be found
in a number of widely differing clinical conditions.
Such alterations are of importance in three major
respects. They may result in changes in intracellular electrolyte composition with far-reaching
secondary effects on cell metabolism. They may
directly affect the membrane potential and the
function of those specialized cells whose normal
working is dependent on this. Changes in sodium
transport involving an important alteration in the
number of ions moved in unit time will alter the
requirement for ATP at the plasma membrane.
By using theoretical arguments and examples
drawn from the more complex situation found in
clinical investigation, we have attempted to show
that there are problems in direct extrapolation from
the physiology laboratory to the patient but that,
with care, the study of intracellular composition
and electrolyte fluxes in disease can be rewarding in
terms of an increased understanding of the complex
of metabolic disorders that is a clinical entity in
man.
Acknowledgments
J.P. was supported by a grant from the Wellcome
Trust.
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