Circulation Research
MAY
1984
VOL. 54
NO. 5
An Offidai Journal of the American Heart Association
BRIEF REVIEWS
The Effect of Hyponatremia on the Regulation of
Intracellular Volume and Solute Composition
Jared Grantham and Michael Linshaw
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From the Kidney and Urology Research Center, Departments of Medicine and Pediatrics, University of Kansas School of Medicine,
Kansas City, Kansas
HYPONATREMIA is a common fluid and electrolyte disturbance. The decrease in plasma sodium
concentration is usually associated with a reduction
in body fluid osmolality, a state that causes shifts of
water between the interstitium and the cells. This
review is addressed to the cellular response of tissues
to a reduction in plasma osmolality. We briefly
describe the current understanding of how hyponatremia is generated, how certain organs function
in response to hyponatremia, how the individual
cells within organs alter intracellular volume in response to hypoosmolality, and how the volume
adjustment is affected at the cellular level.
Genesis of Hyponatremia
Hyponatremia, associated with a decrease in
serum osmolality, generally occurs when water intake exceeds the capacity of the kidneys to generate
an appropriately dilute urine.
Hyponatremia may occur with decreased or increased total body fluid volume, as well as decreased
or increased total body sodium content. In states of
dehydration (decreased total body water), a deficit
of sodium out of proportion to water leads to hyponatremia. Conversely, in edema-producing disorders, if water intake is excessive compared to sodium
and if a sufficiently dilute urine is not excreted,
hyponatremia will develop even in the presence of
excessive total body sodium.
Under normal circumstances, the urinary diluting
mechanism is so efficient that it is unusual for
hyponatremia to be induced by drinking large quantities of water. For example, in the absence of vasopressin (diabetes insipidus), about 10-15% of the
glomerular filtrate, or 17-25 liters/day, can be excreted as maximally dilute urine. Since most patients
do not exceed this level of water intake in hypona-
tremic states, some derangement of the diluting
mechanism usually is present to prevent the excretion of urine sufficiently dilute to maintain isotonicity of body fluids. However, a defect in the diluting
ability will not in and of itself cause hyponatremia,
as there must also be a concomitant relative excess
of water intake.
Since urine is diluted along the ascending loop of
Henle, a decrease in the volume of filtrate delivered
to that segment secondary to a decrease in the
glomerular filtration rate or an increase in the
amount of fluid reabsorbed along the proximal tubule will ultimately decrease the potential load of
filtrate that can be excreted as water. Furthermore,
an alteration in the capability of the ascending portion of Henle's loop to reabsorb solute in excess of
water will minimize the elimination of a dilute urine.
Finally, since the urine is ultimately concentrated
along the collecting tubule in the presence of ADH,
any physiological osmotic or nonosmotic stimulus
for the continued secretion of ADH or the inappropriate secretion of ADH in the absence of such
physiological stimuli will cause the continued reabsorprion of water and may lead to hyponatremia if
water intake is excessive.
A mild degree of hyponatremia is well tolerated
by most patients. Adverse signs and symptoms develop with progressive hyponatremia, particularly
when the serum sodium drops below 125 mEq/liter.
When chronic hyponatremia develops over several
days to weeks, the symptomatology may be minimal
or absent, even with significant degrees of hyponatremia. When the decrease in serum sodium occurs
rapidly (within 12-24 hours), the signs are apt to be
much more dramatic (Arieff and Guisado, 1976).
The adverse effects are related primarily to the neuromuscular system—specifically, to the brain—
which is enclosed in a nondistensible skull and
484
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cannot tolerate sudden increases in cell size. Symptoms of hypoosmolality include muscle cramps,
varying degrees of behavioral and neurological dysfunction, and gastrointestinal symptoms, such as
vomiting and nausea. Symptoms of hyponatremia
are generally nonspecific, quite variable, and correlate better with the rate of decrease in serum sodium
than with the absolute degree of hyponatremia.
Symptoms vary from minimal confusion and headaches to seizures and coma. An acute drop in osmolality associated with a mean value of serum
sodium of 112 mEq/liter in less than 12 hours is
associated with a significant (50%) mortality (Arieff
et al., 1976). However, a similar degree of hyponatremia developed more chronically is associated with
only a 12% mortality, and in patients with chronic
hyponatremia with a mean serum sodium of 122
mEq/liter and no symptoms, the mortality rate is
virtually nil.
There is a paucity of information concerning any
deleterious effects that hyponatremia may have on
organs other than the brain and peripheral nerves.
Other tissues exposed to hypoosmolar plasma are
subject to the same kind of cell water and cell
volume alterations as the central nervous system.
However, in both adults and children, cardiac, hepatic, pulmonary, renal, immune, and other organ
systems are not prominently altered by hyponatremia.
Response of Whole Organs to Decreased
Osmolality
As mentioned above, an alteration in function is
most notable in the central nervous system. The
brain responds to hyponatremia by altering the cellular content of water and solutes. Responses differ,
depending on the rapidity of the drop in serum
sodium. Studies to illustrate these differences were
performed in rabbits subjected to either acute or
chronic hyponatremia by fluid infusion and the
administration of ADH (Arieff et al., 1976). Brain
water and intracellular solute were quite different
between animals made hyponatremic, either acutely
or chronically. For example, when the animals were
made acutely hyponatremic by dropping their serum
sodium to 119 mEq/liter within 12 hours, brain
water increased 19% above control and the solute
content (sodium + potassium + chloride) was unchanged. In animals made chronically hyponatremic
over a 3V2- or 10-day period by dropping the sodium
to 122 or 99 mEq/liter, respectively, the brain water
content increased only 7% above control. In animals
made hyponatremic over a 3V2-day period, the brain
solute content remained similar to control, but those
made severely hyponatremic over 16 days had 1737% less brain solute than control animals. The brain
tissue had adapted to the hypoosmolality by decreasing its cellular solute content, thereby minimizing the retention of cellular water. When brain,
muscle, and liver tissue were compared in rats made
Circulation Research/Vo/. 54, No. 5, May 1984
hyponatremic by injection of pitressin and distilled
water, all tissues increased the water content proportionately with graded increases in water intake.
However, the percentage increase in brain water
was less than other tissues, because brain tissue
extruded solute and cellular water more efficiently
(Wasterlain and Posner, 1968).
Hyponatremia does affect the resistance of several
vascular beds, including kidney, skeletal muscle,
lung, and heart. However, renal, cardiac, and pulmonary function are generally not adversely affected by the hyponatremic state. There are cardiac
compensatory responses to hyponatremia. Acute salt
depletion lowers plasma osmolality and leads to
osmotic movement of water from the extracellular
to the intracellular compartment (Brace et al., 1975).
Plasma volume, cardiac output, and blood pressure
generally decrease in this condition. The low serum
osmolality, rather than low salt concentration, is
probably responsible for these initial hemodynamic
alterations, since blood pressure does not fall during
acute salt depletion without water loss when the
plasma osmolality is held constant with sucrose in
experimental animals (Haddy and Scott, 1971).
Tachycardia and an increase in total peripheral resistance subsequently develop for reasons that are
not entirely clear. The effect of hypoosmolality on
the resistance of muscle and coronary vessels was
studied in intact dogs, by means of a non-dilution
dialysis technique in which systemic osmolality was
kept constant but the osmolality of blood entering
the gracilis and coronary vascular beds was decreased (Brace et al., 1975). Hypoosmolality led to
increased gracilis artery perfusion pressure and increased coronary artery vascular resistance. There
was an increased contractile force associated with
the increased vascular resistance during perfusion
of the hypoosmolal solution. Vascular resistance
from skeletal muscle and coronary artery, as well as
myocardial contractile force, all increased concomitantly with the acute decrease in plasma osmolality
induced by partial removal of sodium chloride. Similar changes have been reported with renal vessels
(Gasitua et al., 1969). Hypoosmotic perfusion of
renal arteries in dogs leads to an increase in the
vascular resistance of renal arteries, probably due to
the osmotic shift of water into the vascular smooth
muscle and endothelial cells. The direct effect of
decreased osmolality on vascular and cardiac muscles may contribute to mechanisms that regulate
blood pressure in the hypoosmolar state. However,
there is no evidence that such changes in the vascular beds of muscle, heart, or kidneys lead to alteration in the functions of these organs. With respect
to renal function, hyponatremia may enhance tubular sodium reabsorption, but this point is not
settled (Earlye and Schrier, 1973). We are not aware
of any reports of decreased renal blood flow or
glomerular filtration rate occurring as a direct consequence of the hyponatremic state.
Grantham and Linshaw/Hyponatremia
485
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Response of Specific Cells to Acute
Hypoosmolality
Because of the complex anatomic interrelations
between the extracellular and intracellular fluid
compartments in whole animals, the description of
how specific cells within organs respond to changes
in extracellular fluid osmolality derives principally
from studies in vitro (Grantham, 1977; Hoffman,
1977). The cellular response to hypoosmolality is
usually examined by making a sudden reduction in
the osmolality of the external bathing medium. As
sodium is the principal nonpermeant cation in the
extracellular fluid, the concentration of NaCl usually
is decreased to lower the extracellular fluid osmolality. Cell volume, as measured directly from the
external dimensions of the cells or gravimetrically,
is observed to change rapidly (Fig. 1). The change
in cellular volume can usually be resolved into two
distinct phases. In the first, or osmometric phase, the
cells rapidly swell as water diffuses into the cytoplasm. A maximal increase in size is reached when
the water activities of the intracellular and extracellular fluids achieve equilibrium. The magnitude of
the initial rapid increase in cell size depends on four
major factors: (1) the intracellular content of osmotically active solutes, (2) the constancy of osmotic
solutes within the cells during the osmometric
phase, (3) the relative stiffness of the cell plasma
membrane, and (4) the osmolality of the bathing
medium.
A few seconds after the cells enlarge to their
maximal extent in a given hypoosmotic medium,
many cells begin to adjust the intracellular volume
Ui
I80T0NIC BATH
HVPOTONIC BATH
l
I80T0NIC BATH
> 2.0
E
\
1.5
ui
OSM0UETRIC
RESPONSE
u
1-0
z
UI
0.5
UI
E
TIME
FIGURE 1. General pattern of cell volume response to a sudden change
in osmolality of the extracellular fluid. In this hypothetical example,
the relative intracellular volume is the cell volume in test hypotonic
medium divided by the cell volume in isotonic medium. Hypotonic
medium causes the cells to swell rapidly, the osmometric phase. This
is followed by a steady decrease in cell volume, the volume regulatory
decrease (VRD) phase. Replacement of hypotonic with isotonic medium causes a second osmometric response, this time a shrinkage of
the cells, the magnitude of which reflects the amount of intracellular
solute lost during VRD. The cells gradually enlarge back to their
original size during the volume regulatory increase (VRI) phase.
Although the osmometric phases last only a few seconds, in most
mammalian cells, the duration of the VRD and VRI phases is variable,
depending on the cell type of species (See Table 1).
back toward the original cell size. This begins the
volume regulatory decrease (VRD) phase illustrated in
Figure 1. Although different types of cells swell
osmometrically in a few seconds, these same cells
regulate the volume back to the baseline at widely
different rates (Table 1). Figure 1 also illustrates the
reponse of cells in hypotonic medium to re-exposure
to isotonic medium. This causes a second osmometric phase, only, in this case, the cells shrink below
the original baseline volume. This is followed by a
volume regulatory increase (VRI) phase during which
the cell volume returns to the original baseline. In
this review, we will focus the discussion on the VRD
portion of the response of a variety of cells to
hypoosmotic extracellular media (Table 1).
Lymphocytic Cells
Osmometric and VRD phases in response to hypotonic medium have been repeatedly demonstrated
in lymphocytic cells (Table 1). Leukemic white cells
from mice rapidly swell and complete the VRD
period in about 20-30 minutes. VRD is accompanied
by the loss of intracellular K+ and Cl~ to a major
extent, and Na+, to a lesser degree. Human lymphocytic cells also exhibit osmometric and VRD behavior. Peripheral mononuclear cells, principally T lymphocytes, complete the VRD cycle in about 20-30
minutes, and lose K+ and Cl~ primarily in the process. Interestingly, tonsillar cells, which are principally B lymphocytes, show osmometric responses,
but the VRD process is virtually absent.
Ouabain, an inhibitor of Na,K-ATPase, has confusing effects on the VRD response of lymphocytes
(Iichtman et al., 1972). If the cells are exposed to
ouabain for only a few minutes before the subsequent hypotonic challenge, the VRD process is unaffected. In contrast, exposure to ouabain for several
hours markedly attenuates the VRD process (Rosenberg et al., 1972; Roti Ron' and Rothstein, 1973;
Shank et al., 1973; Bui and Wiley, 1981; Cheung et
al., 1982a). One can explain ouabain's inconsistent
effect on the VRD process by supposing that the
action of the sodium pump (Na,K-ATPase) is not
directly tied to the VRD response. If the transmembrane gradients for cations and anions are instrumental in driving the VRD process, short exposure
to ouabain may not result in a change in the cellular
content of electrolytes, whereas relatively long exposure will cause the loss of intracellular K+ and the
net addition of NaCl.
Erythrocytes
The volume responses of a variety of red blood
cells have been extensively studied (Table 1). Human erythrocytes swell osmometrically in hypoosmotic media, and show a slow VRD phase requiring
several hours to reach a new steady state. Intracellular Na+ and K+ appear to be lost in the VRD phase.
Human red cells, because they hemolyze during
TABLE 1
Hypotonic Volume Regulatory Decrease (VRD) in Tissues and Cells
Relative
tonicity
Temp
(°C)
Mouse
Mouse
0.5
0.5
37.5
37
Mouse
Human
Human
0.5
0.7
Approximate
time to reach
new steady
state (min)
Type of solute lost
K+
Nat
Cr
30
20
X
X
X
X
37.5
37
20
20
X
X
X
0.6
0.6
RT
RT
5
35
0.7
0.7
0.7
RT
30
RT
RT
Infinite
30
X
0.7
RT
20
X
X
Grinstein et al., 1982a
Duck
0.6
37
90
X
X
Nucleated
Nucleated
Nucleated
Nonnucleated
Nonnudeated
Nonnudeated
FloundeT
Flounder
Salamander
Rabbit
Guinea pig
Human
0.8
0.6
0.7
RT
10
23
300
60
240
0.6
0.6
0.8
40
40
300
180
Nonnudeated
DOB
0.8
37
X
Kregenow, 1971; Kregenow,
1974
Fugelli, 1967
Cala, 1977
Cala, 1980
Davson, 1937
Davson, 1937
Poznansky and Solomon,
1972
Parker et al., 1975
0.5
37
X
Dellasega and Grantham,
1973; Grantham et al.,
1977
Grantham, unpublished
Cell Type
Lymphocytic
Leukemic
Leukemic
Leukemic
Mononuclear
Lymphocyte
Normal
Species
CLL
Lymphocytes
Peripheral
Tonsillar
Thymus
Lymphocyte
Peripheral
Erythrocytes
Nucleated
Ben-Sasson et al., 1975
Cheung et al., 1982a
Cheung et al., 1982b
X
Human
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Amino acid
X
X
X
X
X
>360
>1200
Collecting
tubule
Proximal tub.
Cortex slice
Cortex slice
Rabbit
0.5
35
30
Rabbit
0.5
Rabbit
0.75
0.45
39
37
25
5
Rat
Cortex slice
Rat
0.45
Cortex slice
Guinea pig
Cortex suspension
X
X
X
X
25
X
X
0.45
25
X
X
Rabbit
0.65
28
X
X
Polar epithelia
Urine bladder
Skin
Gallbladder
Lens
Toad
Frog
Necturus
Rat
0.70
0.50
0.80
RT
RT
RT
X
X
X
Nervous Tissue
Brain
Brain
Brain
Brain
Brain
Cat
Rat
Rat
Rabbit
Puppy
0.83
Rat
Cat
90
30
5
X
X
1-2 days
X
BT
BT
BT
BT
BT
24 hours
24 hours
Indeterminant
Indeterminant
Infinite
X
X
X
X
BT
27.5
Indeterminant
Indeterminant
Crab
0.73
0.54
0.50
20
180
Rat
0.70
BT
>24 hours
0.7
0.7
0.7
0.91
Gagnon et al., 1982
Gyory et al,, 1981
Hughes and MacKnight,
1976
Hughes and MacKnight,
1976
Hughes and MacKnight,
1976
Paillard et al., 1979
X
X
Skeletal
References
Rosenberg et al. 1972
Roti Roti and Rothstein,
1973
Shank et al., 1973
Bui and Wiley, 1981
Human
Kidney tubules (non-perfused)
Proximal tub.
Rabbit
Muscle
Heart
Heart
Skeletal
Other
Upton, 1972
Ussing, 1982
Persson and Spring, 1982
Patterson and Foumier,
1976
Yannet, 1940
Rymer and Fishman, 1973
Holliday et al., 1968
Arieff et al., 1976
Nattie and Edwards, 1981
X
X
X
X
X
X
Taurine
Amino adds
X
X
Welty et a]., 1976
Page and Storm, 1966
Lang and Gainer, 1969a,
1969b
Rymer and Fisham, 1973
RT =• room temperature; BT = body temperature; relative tonidty = hypo/isotonic; CLL «= chronic lymphocyte leukemia
486
487
Grantham and L/ns/iaw/Hyponatremia
prolonged incubation in vitro, have not been studied
in detail. However, a preliminary report indicates
that, in clinical hypoosmotic states, human erythrocytes adjust intracellular volume efficiently in vivo
(Stuewe et al., 1981).
Nucleated avaian erythrocytes show VRD over a
shorter time course than their non-nucleated mammalian counterparts. Kregenow (1971, 1974, 1981),
in an elegant series of studies, demonstrated that,
during VRD, duck erythrocytes lose K+ and an anion
(possibly C r , HCO3", or OH~) by a mechanism that
is insensitive to inhibition by ouabain. Salamander
and winter flounder erythrocytes also show osmometric and VRD behavior, and in these cells VRD is
accompanied by intracellular losses of K+ and Cl~,
and a small uptake of Na+.
Kidney Tubule Cells
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The kidney is an anatomically complex organ
comprised of tubules that are axially heterogeneous
(comprised of three distinct proximal tubule segments, a loop of Henle, several distinctly different
segments of distal tubule, glomeruli, and blood vessels). Studies of isolated segments of renal tubule in
vitro have defined the volume response to acute
hypoosmolality in the rabbit nephron (Dellasega and
Grantham, 1973; Gagnon et al., 1982; Gonzalez et
al., 1982; Welling et al., 1983b). The S, and S2
segments of the proximal tubule and the cortical
collecting tubule show the expected osmometric response to acute extracellular hypoosmolality, and all
of these segments exhibit VRD, but to a variable
degree. The proximal segments complete the VRD
cycle in less than 5 minutes, whereas the collecting
tubules take about 30 minutes. The in vitro studies
have used nonperfused isolated tubules, an experimental design that reduces or eliminates the influence of the lumen plasma membrane in transcellular
transport and permits the study of the basolateral
(peritubular) plasma membrane primarily. In isolated proximal and collecting segments of rabbit
tubule, the VRD process is accompanied by the loss
of K+, and, to a lesser extent, Na+, from the cells.
An anion undoubtedly is lost as well, and though
direct measurements have not been made, chloride
is probably lost with the cations. Studies of renal
cortex slices and suspensions have produced further
evidence that kidney cells regulate their volume in
response to acute hypoosmolality, but in these preparations at 25°C, NaCl, rather than KC1, is lost from
the cells during the VRD phase (Table 1). However,
in one study of rat renal cortex at 37°C, K+ was lost
during VRD, rather than Na+. The reasons for the
differences in extruded cation species among studies
of kidney cells is not known, but heterogeneity of
the preparations, different temperatures of study,
different media, and different species may account
for some of the confusion. One point emerges
clearly, however: renal tubules acutely exhibit VRD
in hypotonic media.
The effect of ouabain on the VRD phase in renal
tubules is perplexing. Ouabain quickly inhibits the
Na,K-ATPase of renal tubules from nearly all species. In an early report, Dellasega and Grantham
(1973) interpreted their findings to indicate that
ouabain did not block the VRD response in hypotonic medium, thus raising the possibility of a ouabain-insensitive volume-regulating mechanism in
kidney cells. Subsequent work (Linshaw and Stapleton, 1978; Linshaw and Grantham, 1980; Grantham
et al., 1981; Paillard et al., 1979; Gagnon et al.,
1982) has resolved some of the confusion about the
effect of ouabain. It appears that isolated rabbit renal
tubules enlarge to such an extent after inhibition of
the Na,K-ATPase with ouabain, that the constraint
offered by the relatively rigid tubule basement membrane causes extremely high transmembrane hydrostatic pressures to be generated during the osmometric phase in hypoosmotic medium. It is the hydrostatic pressure within the cells, not a ouabaininsensitive pump, that accounts for the 'apparent*
VRD of ouabain-treated proximal segments in hypoosmotic medium. This high pressure forces the
extrusion of intracellular NaCl and water by ultrafiltration through the relatively permeable basolateral plasma membrane. Current evidence favors the
view that the electrolyte gradients across the tubule
plasma membranes drive the VRD process, and that
the sodium pump participates in the VRD process
indirectly by generating and maintaining these gradients.
Polarized Epithelia
The kidney tubules described above were studied
in the absence of lumen perfusion. Osmometric
swelling and VRD have been observed in epithelial
tissues exposed to symmetric and asymmetric solutions in the respective bathing media (Table 1). Frog
skin, for example, shows osmometric swelling and
VRD after the serosal medium tonicity is changed.
The VRD response in this tissue is relatively independent of the composition of the mucosal medium.
In contrast, Necturus gallbladder exhibits osmometric swelling and VRD when either the mucosal or
serosal osmolalities are reduced acutely. In this tissue, bicarbonate seems to have a central role in the
VRD mechanism (Fisher et al., 1981). The lens of
the eye also exhibits osmometric swelling and VRD.
Nervous Tissue
Brain and peripheral nervous tissues exhibit osmometric behavior in hypoosmotic extracellular media (Table 1). The function of the CNS is drastically
disturbed as extracellular osmolality is acutely decreased. In contrast, CNS function is reasonably well
maintained when hypoosmolality occurs gradually
over a period of several days, as may be seen in
congestive heart failure or in inappropriate antidiuresis syndromes. The excitation of axons is not
altered appreciably if the internal and external ba-
488
thing fluid osmolalities are changed symmetrically
and concurrently, suggesting that altered neural
function in hypotonic states may be related more to
the changes in cell volume than to changes in fluid
tonicity (Kukita and Yamagishi, 1979).
Nervous tissue exhibits VRD in hypoosmotic media, although the time course has not been clearly
defined. Most studies indicate that VRD requires
several hours and is accompanied by the cellular
loss of K+, Na+, and Cl~ in mammals, and, possibly,
amino acids in lower species.
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Muscle
Several studies show that skeletal and cardiac
muscle fibers swell osmometrically in hypoosmotic
extracellular media (Table 1). VRD has been observed in cat heart muscle and in skeletal muscle of
the blue crab, Callinectes. The time course of VRD
in mammalian muscle is not known, but in Callinectes the process takes several hours. Mammalian
heart muscle fibers lose Na + , Cl~, and taurine during
VRD, whereas Callinectes skeletal muscle loses K
and a variety of amino acids. Hypoosmolality stimulates the sodium pump in frog sartorius muscle
fibers in vitro, thereby favoring the extrusion of Na+
from the cytoplasm (Venosa, 1978). The force of
heart muscle contraction appears to increase as the
medium tonicity is decreased (Gulati and Babi,
1982).
Liver
liver cells, either in the intact organ (Lambotte
and Wojcik, 1976) or grown in vitro (Polefka et al.,
1981), show osmometric swelling in hypotonic media. The VRD response is not well defined, but some
preliminary evidence suggests that the intracellular
content of K+, Na+, and Cl~ are decreased and that
the membrane permeability of Na+ relative to K+
may be increased in hypotonic extracellular media.
Mechanisms of Osmometric Response and
Volume Regulatory Decrease (VRD)
In the foregoing section, evidence is presented to
support the view that most types of cells in animal
species respond acutely to hypoosmotic extracellular
media by swelling, and then returning the cell volume toward a more normal value. Although the
cells swell rapidly, the VRD phase extends from a
few minutes to several hours. The osmometric swelling phase (Fig. 1) is due to the rapid influx of water
into the cells through plasma membranes that are
highly permeable to water. The driving force for
water uptake is the difference in the effective osmotic pressure between the intracellular and extracellular fluids, due principally to the initial difference in the concentrations of crystalloid solutes K+,
Cl~, and HCO3~ inside the cells and Na+, Cl~, and
HCO3- in the extracellular fluid. Since the plasma
membranes are much less permeable to crystalloid
solutes than to water, the net osmotic driving force
Circulation Research/Vol. 54, No. 5, May 1984
is oriented to cause water to enter the cells. Whether
the water enters the cells by diffusing through the
lipid matrix of plasma membranes, or enters by bulk
flow through specialized protein channels or pores
in the membranes, has not been resolved. Most cells
behave as imperfect osmometers, i.e., the intracellular volume increases in proportion to the initial
osmotic gradient across the plasma membrane. As
the intracellular water content increases, cell volume
increases, until the effective osmolalities of the inside and outside solutions are virtually equal. In
situations in which no intracellular solute is lost or
gained during the osmometric phase, cell volume
reaches a maximal level and remains there for prolonged periods. This type of behavior is seen in socalled 'tight* cells, e.g., human erythrocytes, frog
skin epithelium, and cortical collecting tubules. In
these cells, the basal permeability of the plasma
membranes to solutes is so low and the increase in
permeability to ions due to osmometric swelling is
so small, that intracellular ions diffuse down electrochemical gradients very slowly during VRD. By contrast, in 'leaky* cells, e.g., renal proximal tubule,
circulating lymphocytes, and mononuclear cells and
Necturus gallbladder cells, intracellular solutes are
lost more rapidly during VRD.
The net loss of osmotically active solutes from the
cells in VRD phase promotes the secondary loss of
water by osmosis, thereby leading to a decrease in
cell volume. Most cells that show this VRD response
do so in a way that causes cell volume to return to
or near the original baseline volume in isotonic
medium. This ability to return to normal cell size
invites speculation that the cells regulate their intracellular water content; hence, we refer to the phenomenon as volume regulatory decrease.
It is important to emphasize that there is no unifying mechanism that will explain the phenomenon of
VRD in the different cell types. In most mammalian
cells, the principal solutes lost from the cytoplasm
are K+ and Cl~, but, in some instances, Na and
Cl~—or Na+, K+, and Cl~—may be lost during VRD
(Table 1).
Figure 2 summarizes some of the mechanisms
postulated for VRD. In all cases, the Na,K,-ATPase
is considered to play a permissive role in VRD. In
other words, the sodium pump is essential to establish the gradients for ions that ultimately will be
dissipated during VRD by transport mechanisms
separate from the Na,K-ATPase.
It has been postulated that three principal dissipative mechanisms participate in the accelerated loss
of K+ and Cl~ from the cells during VRD: (1) uncoupled conductive fluxes, (2) coupled symport fluxes,
and (3) independent cation—anion antiport fluxes.
Uncoupled Conductive Fluxes. This mechanism has
been hypothesized on the basis of studies in Ehrlich
ascites tumor cells (Hoffman, 1978) and lymphocytes (Grinstein et al., 1982a). Shortly after osmometric swelling, there is an increase in the permea-
Grantham and Lmsfraw/Hyponatremia
A. UNCOUPLED CONDUCTIVE FLUXES
(LYMPHOCYTES. EHRJCH ACTTES TUMOR ran I %
RENAL PROXIMAL TUBUJEB. UAMMAUAN)
CYTOPLASM I
CHARACTERISTICS OF ION
MOVEMENTS DURING VRO
1) VOLTAGE SENSITIVE
2) BARUM SENSITIVE
3) DOS SENSmVE
4) C« 2+ SENSITIVE
B
COUPLED SYMPORT
(DOCK AND SHEEP EHYTWXXrYTHS)
1) VOLTAGE »4SENSmVE
2) FUROSEMD6 SENSITIVE
C. INDEPENDENT CATK3N-AMON ANTPOflT
(AtCHUUA ERYDAOCYTES)
1) VOLTAGE INSENSITIVE
2) MEMJMpH SENSITIVE
3) CYTOPLASM ACORCATION
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FIGURE 2. Postulated mechanisms of KCl extrusion during VRD in
different types of cells.
bility of the plasma membrane to both K+ and Cl".
Recent studies suggest that the cation and anion
move independently, but both are sensitive to transmembrane voltage. DIDS (2,2,-stilbene disulfonate),
an inhibitor of Cl~ movement, slows the VRD response in ascites cells and lymphocytes. Barium, a
K+ channel blocker, retards VRD in proximal tubules
(Welling et al., 1983a). Moreover, recent studies in
lymphocytes and ascites cells indicate that, in these
cells with low basal Cl~ permeability, the increase
in Cl" permeability may be rate limiting for VRD
(Grinstein et al., 1982b). The VRD response in lymphocytes and proximal tubules is extremely sensitive
to changes in intracellular calcium levels, suggesting
that elevated internal calcium levels may be required
to initiate the change in ion permeabilities leading
to VRD (Grinstein et al., 1982b).
Coupled Symport Fluxes. Duck and sheep erythrocytes extrude K+ and Cl~ during VRD by a process
that appears to couple the ions on passage through
the membrane (Kregenow and Caryk, 1979; Dunham and Ellory, 1981; Kregenow, 1981). This process, possibly a NaCl—KCl symport process, is voltage insensitive, and is inhibited by furosemide. Internal calcium does not appear to have a role in this
mechanism.
Independent Cation—Anion Antiport. Studies of
Amphimuma erythrocytes indicate that separate cation and anion antiporters are central mechanisms
in VRD (Cala, 1980). In these cells, the ion movements are electrically silent, but seem remarkably
sensitive to medium HCO3~ levels. During VRD, the
489
medium is alkalinized, indicating that the interior of
the cells is acidified.
The triggering mechanism that initiates the VRD
process is unknown. Leading candidates for this
central role include membrane stretching, altered
transmembrane electrical potential difference (PD),
and altered intracellular ion concentrations. Extreme
membrane stretching can certainly cause solute loss,
as evinced by the hemolysis of erythrocytes in very
dilute medium. But membrane stretching and nonspecific loss of solute seems unlikely in modest states
of hypoosmolality, since some cells can be arrested
in a fully swollen position for prolonged periods in
noncytolytic hypotonic medium. Altered transmembrane PD seems unlikely, since measurements of
PD disclose no significant patterns of alteration,
although it must be noted that these measurements
do not include any demonstrations of transient
changes in PD. Altered intracellular ionic concentrations would seem logical triggers for VRD, but which
ions? Changes in intracellular pH have been implicated in Amphiuma cells, and recent studies in lymphocytes (Grinstein et al., 1982b) and isolated proximal tubules (Neufeld et al., 1983) implicate intracellular Ca++ in the triggering process. Removal of
extracellular calcium, or blockade of calcium entry
into the cells by verapamil, arrests the VRD response
to hypotonic medium in these two cell types. The
possible relation between intracellular Ca + and the
increase in K+ permeability is attractive because of
the well-described propensity of Ca ++ to cause increased K+ conductance in a number of cells.
Whether or not increased intracellular Ca++ leads to
a change in membrane Cl~ permeability remains to
be determined.
Implications of VRD in Pathophysiological
States
In this brief review, we have collated the present
state of knowledge about the cellular response to
acute hypoosmolality. Overwhelming evidence is in
hand to show that most cells in the animal kingdom
possess mechanisms to guard against sustained cellular swelling in the face of acute hyponatremia
(hypoosmolality). One may ask whether or not these
mechanisms are ever called into play in intact animals, and, if so, under what conditions. In humans,
acute hyponatremia can result from exaggerated
ingestion of water or beer, as a consequence of
drowning in fresh water, or inadvertent infusion of
water during surgical or medical procedures. It is
doubtful, however, that man's cells have been endowed with mechanisms for cell volume regulation
for acquired hypoosmolality states that are so 'unnatural." Rather, the cells may have these latent
mechanisms as a kind of evolutionary hangover
from our phylogenetic ancestors who lived in estuaries or who crawled between brackish and sweet
pools of water on land. Indeed, a vast amount of
information decribes the volume regulatory re-
Circulation Research/Vol. 54, No. 5, May 1984
490
sponses of lower animals (Hoffman, 1977) and even
algae (Gutknect et al., 1978) to acute changes in
environmental salinity. The 'lower' animals have
efficient mechanisms for regulating intracellular volume in the face of a changing external osmolality,
through the loss of electrolytes or amino acids, or,
in the case of algae, by the generation of high
internal hydrostatic pressures. And, as seems to be
the case in mammalian brain tissue, the ancient
mechanisms for VRD and the phylogenetically more
advanced mechanisms for ion extrusion may have a
crucial role in the normalization of cell volume in
chronic hyponatremic states as typified by the syndromes of inappropriate antidiuresis and congestive
heart failure.
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We thank Janet Hartley for secretarial assistance, and Jim Bramich
of Merck, Sharpe, and Dohme for assistance with the literature starch.
Supported in part by grants from the National Institutes of Health
[AM 13476 (Crantham) and AM 25748 (Linshaw).]
Address for reprints: Dr. fared Crantham, University of Kansas,
School of Medicine, 4015 Sudler, 39th and Rainbow Boulevard, Kansas
City, Kansas 66103.
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INDEX TERMS: Cell volume • Volume regulation • Cell, Na, K,
Cl • Hypoosmolality • Water intoxication
The effect of hyponatremia on the regulation of intracellular volume and solute composition.
J Grantham and M Linshaw
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Circ Res. 1984;54:483-491
doi: 10.1161/01.RES.54.5.483
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