595
Brief Review
Importance of Organic Osmolytes for
Osmoregulation by Renal Medullary Cells
Arlyn Garcia-Perez and Maurice B. Burg
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The cells in the renal medulla protect themselves from the extracellular hypertonicity in that
region of the kidney by accumulating large amounts of sorbitol, inositol, grycerophosphorylcholine, and betaine. The system is uniquely active in this part of the body, but it represents a
throwback to primitive mechanisms by which cells in virtually all organisms, including
bacteria, yeasts, plants, and lower animals counteract water stress. In this brief review, we
summarize how these "compatible organic osmolytes" help the renal medullary cells to survive,
the mechanisms by which the organic osmolytes are accumulated, and how the accumulation is
controlled to adjust for changing extracellular NaCl and urea concentrations. The compatible
organic osmolytes are all intermediates in important biochemical pathways, and although the
medical consequences are not yet fully worked out, it is already apparent that inappropriate
accumulation of these solutes has major pathophysiological consequences. (Hypertension
1990;16:595-602)
C
ells in the medulla of the kidney contain high
concentrations of glycerophosphoiylcholine
(GVC),1-2 inositol,2^ sorbitol,4 and betaine.5
All four compounds are intermediates in important
biochemical pathways, but that is not the reason
there is so much of them in these particular cells.
Rather, these "organic osmolytes" protect the cells
from the hyperosmotic environment of the renal
medulla.
The balance between extracellular and intracellular solutes in the renal medulla of rats during
antidiuresis is illustrated in Figure 1. The extracellular fluid contains high concentrations of NaCl and
urea coincident with operation of the urinary concentrating mechanism. The osmolality and urea
concentration are just as high in the cells as outside.
However, the electrolyte concentration is much
lower in the cells,6-7 similar to the level in cells
elsewhere in the body. The osmotic difference is
made up by organic osmolytes, which evidently help
regulate cell volume by opposing external hypertonicity. The question remains, however, why the cells
accumulate these particular solutes, rather than
inorganic salts, which are more generally used to
regulate the volume of mammalian cells. The answer5-8 comes from theories derived by comparative
physiologists and biochemists.
From the Laboratory of Kidney and Electrolyte Metabolism,
National Heart, Lung, and Blood Institute, National Institutes of
Health, Bethesda, Md.
Address for correspondence: Maurice B. Burg, MD, National
Institutes of Health, Building 10, Room 6N307, Bethesda, MD
20892.
Received March 20,1990; accepted in revised form May 2,1990.
Polyols, Certain Amino Acids, and Methylamines in
Combination With Urea Are Compatible Solutes
Whose Accumulation Does Not Perturb
Intracellular Macromolecules as Does Excessive
Accumulation of Inorganic Salts
Three classes of intracellular organic osmolytes are
accumulated during water stress by a wide variety of
organisms (bacteria, plants, animals)9: 1) polyhydric
alcohols (polyols), 2) neutral free amino acids and
related solutes, and 3) a combination of urea and
methylamines. The organic solutes in renal medullas
fit these categories. Sorbitol and inositol are polyols.
Betaine is both an amino acid derivative and a
methylamine. GPC is a methylamine. Based on in
vitro studies of enzyme activity and other macromolecular functions, compatible solutes do not significantly perturb protein function over a wide range of
concentrations, in contrast to inorganic salts, which
do. The compatible osmolytes hypothesis states that
polyols, certain amino acids, and methylamines in
combination with urea can be safely accumulated to
high concentrations by cells without significant effects on cell activities, and therefore should be commonly found in osmotically stressed organisms. In
support of this theory, bacteria and yeast do not grow
as well when they are prevented from accumulating
compatible solutes during water stress.
The level of compatible solutes in cells ideally
should vary with extracellular tonicity (mainly, NaCl
concentration) in such a way that the intracellular
inorganic salts remain at a relatively constant and
normal concentration. Renal medullary extracellular
NaCl varies both with location in the medulla and
596
Hypertension
Vol 16, No 6, December 1990
400
3OOO
25OO •
Sorbitol
Betaine
Inositol
2O00 •
Na K, Cl
GPC
Na,
15OO
K
Cl
•
SodUin
Urea
Sorbitol
InoeJtol
Betaine
GPC
FIGURE 3. Bar graph showing osmofyte content of tissue at
the tip of rabbit renal papillas. Data are from Reference 10.
1CO0
Urea
Urea
500 •
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Extracellular
Collecting
duct cells
FIGURE 1. Osmofytes at the tip of the renal papilla in
antidiuretic rats. Data for urea and electrolytes are from
Reference 7.
with the state of diuresis. We expect the organic
osmolytes to vary in the same manner if they are to
serve as compatible solutes. Figure 2 shows the
distribution of osmolytes in kidneys of antidiuretic
rabbits. The results in other studies of rabbits and
additional species are similar.5-1118 NaCl increases
toward the tip of the renal papilla. The distribution
of sorbitol, betaine, and GPC closely follows this
gradient. Inositol, on the other hand, is higher in the
outer than in the inner medulla, suggesting an additional role for inositol, besides acting as a compatible
Methylamines Protect Against Denaturing Effects of
High Urea Concentrations
The concentration of urea in renal medullary cells
varies considerably and generally is high. Urea is a
potent destabilizer of protein structure and an inhibitor of function. Methylamines are effective stabilizers
of protein structure and generally activators of functional properties (K^, V ^ ) . When combined with
urea at a concentration ratio of about 1 = 2 methylamine^urea, their effects can often offset those of
100-
6O-
eo-
4030-
osmolyte. Figure 3 shows how osmolytes differ between diuresis and antidiuresis in rabbits. Sodium is
higher during antidiuresis, as are the compatible
organic osmolytes. This result is also similar in the
other species studied. Considering both location in
the kidney and the state of diuresis, the sum of the
renal organic osmolytes correlates strongly with
extracellular sodium, consistent with their role as
compatible solutes.1017 Interestingly, free amino acids are abundant in renal medullary cells, but their
level does not change with antidiuresis.11
NO6ITOL
eo-
20-
4O-
10-
20-
O-
0«
CORTEX
OUTER
fcBXJLLA
K/BXJLLA
SORBITOL
CORTEX
CUTER
KGXJLLA
FIGURE 2. Line graphs showing osmofytes in the kidneys of antidiuretic rabbits. Data are from Reference 10.
Garcia-Perez and Burg
300
MOCK
200
•§
1OO
era Isotonic
[77! Hypertonic to
7OO mosmo)
1
Osmoregulation by Renal Medullary Cells
Glucose
597
Sorbitol
1
Inositol
•
Betaine
•
• •
•
(O)
Choline
2OO
PAP-HT2
GPC
•
> <—^—>
15O
100
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Betaine bnosrto) GPC
Sorbito)
FIGURE 4. Bar graphs showing organic osmolytes in renal
cells grown in tissue culture medium that is isotonic (*=>300
mosmol/kg) or hypertonic to <=700 mosmol/kg with added
NaCl and urea. Data for MDCK are from Reference 22;
PAP-HT25 from Reference 23.
9 19
urea. - Thus, an additional role as "counteracting"
osmolytes has been proposed for methylamines. Renal
medullary urea varies both with location in the medulla and with the state of diuresis (Figures 2 and 3).
We expect betaine and GPC to vary in the same
manner if they are to serve as counteracting osmolytes. In fact, in chronic states renal medullary
GPC concentration correlates very well with urea, and
much better than betaine does.10 Based on this and
other studies,17 GPC apparently is the main counteracting organic osmolyte in kidneys.
Renal Cells in Tissue Culture Accumulate Organic
Osmolytes When Exposed to High NaCl and Urea
It is much easier to determine the mechanisms
involved in accumulation of organic osmolytes and
how they are controlled in tissue culture than in vivo.
Two renal epithelial cell lines have mainly been used,
PAP-HT25 (from rabbit inner medulla)20 and
MDCK (from dog kidney).21 When the medium NaCl
and urea levels are high, PAP-HT25 cells accumulate
all four renal organic osmolytes and MDCK accumulate all but sorbitol (Figure 4).
The mechanisms of osmoregulatory accumulation
of the various renal organic osmolytes, as defined in
tissue culture studies, are summarized in Figure 5.
Sorbitol is synthesized from glucose in a reaction
catalyzed by aldose reductase.24 GPC is synthesized
from choline.25 Inositol26 and betaine22 are accumulated by transport. The tissue culture studies have
provided considerable information about how accumulation is regulated in response to changes in
NaCl and urea. Some of the processes discovered in
A Transporters ^
A Tonicity
A Leak
A Enzymes
(Slow)
' (Fast)
FIGURE 5. Schematic drawing showing mechanisms of organic osmolyte accumulation and loss in renal medullary cells.
When extracellular osmolality is altered, transporters and
enzymes change (A) slowly (at left), but permeability changes
rapidly (at right). PC, phosphatidylcholine; GPC, gfycerophosphorylcholine; AR, aldose reductase.
these tissue culture experiments have also been
found in vivo, but confirmation of the others awaits
further evidence.
Sorbitol Is Synthesized From Glucose in a Reaction
Catalyzed by Aldose Reductase, the Amount of
Which Is Osmotically Regulated
When PAP-HT25 cells, grown at a normal osmolality, are switched to high NaCl medium, aldose
reductase activity rises. The induction of aldose
reductase activity and subsequent sorbitol accumulation are relatively slow.27 There is a lag of 6 hours
before a rise in enzyme activity is detected and
sorbitol continues to accumulate for 3-4 days. The
signal for induction of aldose reductase activity appears to be an increase in intracellular ionic strength
(mainly potassium salts).27 When extracellular NaCl
increases, the cells shrink and their internal ionic
strength rises, remaining elevated for at least 24
hours. The increase in ionic strength apparently
triggers a series of events that result in accumulation
of sorbitol. As the sorbitol accumulates, the ionic
strength is reduced toward normal. Increasing the
medium osmolality with raffinose, which also raises
intracellular ionic strength, has the same effect as
adding NaCl, but increasing urea or glycerol, which
do not raise the intracellular ionic strength, does not
elevate aldose reductase or sorbitol.
The earliest event that has been characterized is a
rise in aldose reductase messenger RNA (mRNA)
abundance.28 The level of mRNA begins to rise after
3 hours, peaks at 18-24 hours, then falls to a steady
level some six times higher than in isotonic medium,
as sorbitol is accumulated by the cells. The increase
in aldose reductase mRNA involves more rapid
transcription of the aldose reductase gene (A. Garcia-Perez, F.L. Smardo, J.D. Ferraris, B.M. Martin,
and M.B. Burg, unpublished observations). This im-
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plies that ionic strength may specifically regulate
transcription of a particular gene. Osmotic regulation
of transcription in bacterial cells is an active area of
investigation,19 and the existence and mechanism of
such gene regulation in mammalian cells is an intriguing subject for future research.
The rate of synthesis of aldose reductase protein
rises, then falls, proportional to the changes in aldose
reductase mRNA.30 As more protein is synthesized,
the amount in the cells increases,31 accounting for the
rise in aldose reductase activity. Degradation of
aldose reductase protein, on the other hand, is slow
(half-life of approximately 1 week) and is unaffected
by osmolality.30-32 Therefore, the increase in enzyme
protein is entirely due to more rapid production.
As already mentioned, the rate of synthesis of
aldose reductase protein peaks 18-24 hours after
medium NaCl is increased, then falls. The fall coincides with accumulation of sorbitol. If the rise in
sorbitol is prevented by inhibiting aldose reductase,
the rate of synthesis continues to increase for more
than 24 hours.30 The most likely link between sorbitol
and aldose reductase protein synthesis rate is the
ionic strength, which apparently regulates transcription of the aldose reductase gene.27
Expression of aldose reductase mRNA and the enzyme activity vary in vivo in response to changes in renal
medullary extracellular N a d concentration,33 similar
to the responses observed in tissue culture. Because
Brattleboro rats lack vasopressin, they excrete large
quantities of dilute urine and have low renal medullary
Na+. After they have been antidiuretic for 5 days
because of treatment with vasopressin, renal medullary
Na+, aldose reductase mRNA, and aldose reductase
activity are all greatly increased. The response of the
enzyme system is relatively slow, however. When normal rats become diuretic because they are given furosemide, renal medullary Na+ and sorbitol decrease
markedly within 2 hours, but aldose reductase mRNA
and activity are not affected at that time, similar to the
results in tissue culture.
Glycerophosphorylchollne Is Synthesized
From Choline
GPC is higher in MDCK cells grown in medium
made hyperosmotic with NaG or NaCl plus urea
than in medium of normal osmolality.21 When
MDCK cells, grown at normal osmolality, are
switched to medium made hyperosmotic with NaCl
plus urea or urea alone, GPC rises over a week or
more.23 This increase in GPC was studied in detail to
determine the mechanism involved.
The signal that triggers GPC accumulation must
differ, at least in part, from that for sorbitol. Like
sorbitol, GPC increases when NaCl is added to the
medium.3* Unlike sorbitol, GPC increases in
MDCK2* and PAP-HT25 cells30 also when urea is
added to the medium. Elevated urea concentration
does not increase the intracellular ionic strength.34
Thus, although high intracellular ionic strength may
induce sorbitol accumulation (see above), it is not
necessary to induce GPC accumulation.
GPC is a methylamine. Although there is no direct
evidence that it counteracts the destabilizing effects
of urea, this is believed to be the case, judging from
its chemical structure.35 Assuming that methylamines
in the renal medulla counteract the destabilizing
effect of urea, it is understandable that GPC accumulation should be induced by high urea, even in the
absence of high NaCl.
Tissue culture medium ordinarily contains choline,
and it was present in the experiments listed above.
When cells grown in medium of normal osmolality
are switched to hyperosmotic medium that does not
contain choline, GPC rises slightly by the second day,
but it then falls back to the baseline level.34 In
contrast, if choline is present in the hyperosmotic
medium the level of GPC continues to rise. Thus,
GPC apparently is synthesized from choline taken up
from the medium, and even when there is no exogenous choline, some metabolic intermediate (probably
phosphatidylcholine) already formed in the cells
serves as the source for the transient increase in
GPC. Choline is taken up into MDCK cells by
sodium-independent transport, but this choline
transport is not affected by medium osmolality.
Therefore, substrate transport is not a control point
for osmoregulation of GPC accumulation.
The step that is osmotically regulated in the metabolism of GPC is unknown. The generally accepted
pathway of synthesis is from phosphatidylcholine
catalyzed by phospholipase A and lysophospholipase.36 GPC can be degraded by a specific diesterase.
Most attention has been directed to the possibility
that activity of this diesterase is controlled. The
original evidence was that the amount of GPC diesterase concentration is lower in renal medulla than
cortex and its activity is directly inhibited by NaCl
plus urea.37 Thus, a rise in medullary NaCl and urea
would inhibit the enzyme and increase GPC, as is
observed. There is little evidence for the theory,
however. When MDCK cells are adapted to high
NaCl or high NaCl plus urea, GPC diesterase activity
decreases approximately 25%, but this decrease is
small compared with the associated change in GPC
content.25 Also, GPC diesterase activity does not
differ significantly between antidiuretic and diuretic
rats.38 The question of osmotic regulation of GPC
diesterase remains to be resolved, and there are
numerous other possible control points in the GPC
metabolic pathways that need to be investigated.
Inositol Is Taken Up From the Medium by
Sodium-Dependent Transport, Which Is
Osmotically Regulated
MDCK21 and PAP-HT2530 cells accumulate increased amounts of inositol in medium made hypertonic by addition of NaCl. Inositol can be synthesized
by mammalian cells, but many types of cells accumulate it by sodium-dependent transport. Inositol usually is included in tissue culture media in plasmalike
Garcia-Perez and Burg
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concentrations (=0.1 mM). When MDCK cells are
switched to hypertonic medium, their inositol levels
rise over a week or more to approximately three
times the initial value. Increasing the osmolality with
NaCl, raffinose, sorbitol, or dextrose has essentially
the same effect.22"26 However, increasing the osmolality with urea22'26 or gh/cerol22 does not induce
inositol accumulation, which is similar to the result
for sorbitol.
Increase of cell inositol in hypertonic medium
requires that inositol be present in the medium.26
The cells accumulate inositol by active sodium-dependent transport. When the medium is made hypertonic by adding NaQ, the sodium gradient into the
cells rises for a while, which increases the sodiumdependent transport. This is only a small part of the
effect, however. Most of the increased transport is
still found when medium sodium is decreased back to
the normal level during transport measurement (hyperosmolality maintained with mannitol). Also, when
the osmolality of the tissue culture medium is increased with raffinose, the transport rate increases to
even higher levels than when osmolality is increased
with NaCl, although the sodium gradient is not
elevated. The rise in transport rate occurs over
several days. V m rises without change in Km.
Hypertonicity probably increases the number of
functioning inositol transporters. The evidence for
this is indirect. Increased V ^ is consistent with the
existence of more transporters but does not prove it.
Additional support for the theory comes from recent
experiments using toad oocytes as an expression
system.39 When mRNA from MDCK cells grown in
hypertonic medium is injected into the oocytes, sodium-dependent [3H] inositol uptake by the oocytes
increases, but this does not occur with mRNA from
oocytes grown under isotonic conditions. Most likely,
hypertonicity increases inositol transporter mRNA,
resulting in more inositol transporters being synthesized and activated.
There is some preliminary evidence for sodiumdependent inositol transport in rat renal outer40 and
inner41 medullas; however, there are no reports
indicating whether this process is osmotically regulated in vivo and fully accounts for medullary inositol
accumulation that is observed.
Betaine Is Taken Up From the Medium by
Sodium-Dependent Transport, Which Is
Osmotically Regulated
Osmoregulatory uptake of betaine by MDCK cells
is similar in most respects to that of inositol. In brief,
although there is betaine synthesis in some tissues,
this does not account for its osmoregulatory accumulation by renal cells in tissue culture.26 In these cells
the accumulation depends on the presence of betaine
in the medium and occurs by sodium-dependent
transport. The plasma betaine concentration probably is similar to that of inositol (=0.1 mM), but there
have been few measurements. Hypertonicity inCTeases betaine transport V^,, (but not Km). Only a
Osmoregulation by Renal Medullary Cells
599
small part of the increment in transport is due to
increased sodium gradient. High NaCl or raffinose
induces betaine uptake, but urea and glycerol do not.
The rise in betaine transport occurs over 12-24
hours.
Although urea has little effect on inositol in MDCK
cells, urea inhibits the accumulation of betaine. This
finding is striking in view of the theory of counteracting osmolytes, which was discussed earlier. Remember
that methylamines (including betaine) can reverse
harmful effects of urea. Yet urea does not induce
accumulation of betaine, as it does that of GPC.
Rather, urea suppresses betaine uptake. Evidently,
renal cells prefer to use GPC as a counteracting
osmolyte for reasons that we do not understand.
At present it is not clear that accumulation of
betaine by renal medullary cells in vivo is regulated
by transport from the extracellular fluid, as it is in
tissue culture. There is evidence that renal medullary
cells can not only synthesize betaine from choline42-43
but also that they can actively transport betaine.43
Neither process has been shown to be osmotically
regulated in vivo.
Accumulation of Organic Osmolytes Contributes
to the Survival and Growth of Renal Cells
Exposed to Hyperosmolality, but Excessive
Accumulation Can Be Harmful
The compatible and counteracting osmolytes hypotheses imply that cells exposed to high NaCl or
urea will be harmed if they do not accumulate
organic osmolytes. Study of cloning efficiency, which
measures survival and growth of cells in culture,
provides direct evidence for this and for a harmful
effect of excessive accumulation of organic osmolytes.
Evidence for the Compatible Osmolytes Hypothesis
When PAP-HT25 cells are switched to medium
made hypertonic by adding NaCl under conditions in
which mainly sorbitol is accumulated (i.e., no betaine
or urea in the medium), prevention of the sorbitol
accumulation by inhibition of aldose reductase
greatly reduces cloning efficiency.44 This result was
surprising at first because in numerous tests of aldose
reductase inhibitors used for preventing complications of diabetes, no evidence of renal toxicity
emerged.45 The explanation is that when betaine is
available, more of it is accumulated by the cells
during aldose reductase inhibition, compensating for
the lower level of sorbitol.46 This accumulation of
betaine reverses the decrease in cloning efficiency
caused by aldose reductase inhibitors (T. Moriyama,
A. Garcia-Perez, and M.B. Burg, unpublished observation). The effect probably also occurs in vivo.
Antidiuretic rats given aldose reductase inhibitors
accumulate more betaine in their renal medullas,
compensating for the decrease in sorbitol (P. Yancey,
personal communication).
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Hypertension
Vol 16, No 6, December 1990
[NaCl]
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High Extracellular Glucose Damages Renal Cells
by Causing Excessive Accumulation of Sorbitol
Glucose is the substrate for synthesis of sorbitol.
When blood glucose is high, as in diabetes, cells in
the eyes and peripheral nerves that contain aldose
reductase are harmed by excess accumulation of
sorbitol (see Reference 45 for review). This is one
of the major complications of diabetes. Despite the
beneficial effects of sorbitol in renal cells, discussed
above, excessive accumulation of sorbitol also
harms them. When glucose concentration is elevated in the range found in uncontrolled diabetes,
PAP-HT25 cell sorbitol increases, and cloning efficiency falls.44 The fall in cloning efficiency is even
greater when medium NaCl is elevated than when it
is normal. Under these conditions, an aldose reductase inhibitor at concentrations that reduce cell
sorbitol to a level appropriate for the external NaCl
concentration increases cloning efficiency. However, with high medium NaCl, if the inhibition of
aldose reductase is too great and sorbitol falls too
much, cloning efficiency is greatly reduced again.
Evidently, osmoregulation requires appropriate accumulation of compatible organic osmolytes, and
either too little or too much can be harmful.
Evidence for the Counteracting Osmolytes Hypothesis
When MDCK cells are switched to medium containing a high concentration of urea, it accumulates
in the cells, and their cloning efficiency is greatly
reduced.47 Betaine, which when added to the medium increases the osmolality even further, accumulates in the cells and protects them from the harmful
effect of urea, as shown by an increase in cloning
efficiency from the low level with urea alone. Urea
stimulates the accumulation of GPC, but reduces
betaine (see above). Thus, methylamines protect
renal cells from harmful effects of urea, but GPC is
the methylamine that is used by kidney cells as the
principal counteracting osmolyte in vivo.
FIGURE 6. Schematic drawing showing osmoregulation of sorbitol in renal medullary cells. In the state
of chronic antidiuresis, the cells are adapted, meaning that they contain high levels of aldose reductase
(AR). During a transient diuresis, sorbitol concentration falls rapidly by efflux (steps 1 to 3). Then,
when antidiuresis is reestablished, sorbitol is accumulated again by synthesis and catalyzed by the
aldose reductase, which has persisted during the
diuresis (step 4). If the diuresis is prolonged, however, as in diabetes insipidus, aldose reductase falls,
and the cells become unadopted (step 5). Then, for
sorbitol to reaccumulate, antidiuresis must induce
synthesis of new aldose reductase (steps 6-8), a
process requiring more than 1 day. See text for
additional details. mRNA, messenger RNA.
In the Antidiuretic State Renal Medullary
Cells Are Adapted to High NaCl and Urea and
Change Their Levels of Transporters and Enzymes
for Organic Osmolytes Slowly; However, They
Respond to Acute Decrease in NaCl by Rapid
Efflux of the Organic Osmolytes
Unadapted cells accumulate organic osmolytes
slowly, after exposure to hyperosmotic medium, limited
by slow induction of transporters and enzymes. When
cells in culture that have been grown at (and adapted
to) an osmolality of approximately 300 mosmol/kg are
switched to hyperosmotic medium, the accumulation of
organic osmolytes requires days, and at least part of the
delay is due to the slow rate at which enzymes and
transporters are induced (see above). The accumulation is also slow in vivo. Brattleboro rats have low levels
of organic osmolytes in their inner medullas because of
congenital diabetes insipidus.12-13-33 After treatment
with antidiuretic hormone, their urine rapidly becomes
more concentrated. The renal medullary organic osmolytes also increase, but much more slowly. The
osmolytes rise gradually during the first 3 days,13 and
most of them are even higher after 12 days.12
Decrease of transporters and enzymes also lags
after a decrease in hypertonicity. When PAP-HT25
cells that have been adapted to hypertonicity are
returned to isotonic medium, aldose reductase activity falls very slowly (half-time of 6 days).32 Although
the level of aldose reductase mRNA falls rapidly,28
the enzyme protein is long lived.48 In contrast, the
level of sorbitol decreases within minutes by efflux
from the cells.32 Similarly, in MDCK cells betaine
transport decreases over 1-2 days and inositol transport over 6-12 hours when the osmolality is reduced,
but both inositol and betaine efflux from the cells
within minutes,49 as does GPC.25 Similarly, organic
osmolytes efflux rapidly from cells in freshly prepared
suspensions of renal medullary tubules in vitro when
NaCl concentration is decreased.14-50 The organic
osmolytes also fall rapidly in vivo during furosemide
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diuresis,14-33'51 which acutely lowers renal medullary
extracellular NaCl concentration.10'1517^3
The mechanism of sorbitol efflux is a large, transient increase in permeability.32-52 While sorbitol is
exiting the cells at a fast rate, there is also a large
increase in its influx, measured with [l4C]sorbitol.
The rapid efflux of organic osmolytes is volume
regulatory. It limits cell swelling when osmolality
decreases abruptly. The response is analogous to the
rapid efflux of KC1 that occurs from many different
cell types during the volume regulatory decrease that
follows a fall in osmolality.53 Within 15 minutes after
the osmolality is decreased, cell sorbitol is greatly
reduced, and the sorbitol permeability falls, but not
back to its original low level in the hyperosmotic
medium. A slower efflux of sorbitol continues, which
balances its continued production (remember that
aldose reductase takes many days to fall), so that
excess sorbitol does not reaccumulate in the cells.
Mammals that do not consume large amounts of
fluid and are chronically antidiuretic adapt their
renal medullary cells in the sense that the cells
maintain high levels of the enzymes and transporters
responsible for organic osmolyte accumulation
(shown for aldose reductase in the model proposed
in Figure 6). Then, when renal medullary extracellular NaCl concentration falls during a diuresis, the
cells swell immediately (1 in Figure 6), but within
seconds there is an increase in the permeability to
organic osmolytes, including sorbitol (2 in Figure 6).
The efflux of solute, followed by water, restores cell
volume within minutes (3 in Figure 6). Under physiological conditions, diuresis is likely to be transient.
With the subsequent return to antidiuresis, sorbitol
can be accumulated again rapidly (4 in Figure 6)
because aldose reductase remains high. The same is
true of other organic osmolytes whose transporters
and enzymes have remained adapted.
If diuresis persists chronically, as in diabetes insipidus, the renal medullary cells become unadapted (5
in Figure 6). They lose the enzymes and transporters
required to accumulate organic osmolytes. Then,
should the animals become antidiuretic again, there
is a lag during which the enzymes and transporters
adapt again (6 through 8 in Figure 6). This transition
from the unadapted to adapted state has been informative to study in tissue culture and animal models,
but it is not the normal physiological regulation,
which depends heavily on changes in permeability
and is in fact much faster than the time required to
go from the unadapted to adapted state and back
again.
In conclusion, the large amounts of sorbitol, betaine, GPC, and inositol in renal medullary cells act
as compatible and counteracting osmolytes, protecting the cells from the high NaCl and urea in the
medulla. Their osmoregulation is important for
proper function of these cells.
2.
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KEY WORDS • sorbitol • inositol • ghycerophosphorylcholine
• betaine • osmotic regulation
Importance of organic osmolytes for osmoregulation by renal medullary cells.
A Garcia-Perez and M B Burg
Hypertension. 1990;16:595-602
doi: 10.1161/01.HYP.16.6.595
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