Perspectives
Sorbitol, Osmoregulation, and the Complications of Diabetes
Maurice B. Burg and Peter F. Kador
National Heart, Lung and Blood Institute, and National Eye Institute, National Institutes of Health, Bethesda, Maryland 20892
Introduction
Sorbitol, glycerophosphoryicholine, inositol, and betaine are
"compatible" or "nonperturbing" osmotically active organic
solutes (organic "osmolytes"). These organic osmolytes accumulate to high levels in renal medullary cells in response to the
elevated concentration of salt in the renal medullary interstitium (1).
Similar organic osmolytes accumulate in cells of a wide
variety of organisms exposed to high salt environments (2).
The organic osmolytes elevate intracellular osmolality to
match the high external osmolality, while keeping the intracellular levels of sodium and potassium salts and cell volume
near normal. High levels of sodium and potassium salts perturb intracellular macromolecules much more than do comparable levels of organic osmolytes. Thus, organic osmolytes help
maintain the volume of cells and their internal milieu when
they face a high and variable extracellular salt concentration.
These compounds are also intermediates in a variety of biochemical pathways that are not obviously related to osmoregulation, and they occur in cells outside of the renal medulla.
In view of their role in the renal medulla, it is plausible that
they may also act as organic osmolytes in other tissues, but this
has not been generally established.
In ocular and neural tissues increased levels of sorbitol
along with concomitantly decreased levels of inositol are associated with the onset of diabetic complications (3, 4). Sorbitol
is produced in cells from glucose by a reaction catalyzed by
aldose reductase. In uncontrolled diabetes both the concentration of glucose in blood and the levels of aldose reductase
increase in some tissues. Then, more sorbitol is produced. The
increased sorbitol upsets osmoregulation in these cells and
damages them. Inositol generally is actively transported into
cells. Its lower level in diabetes appears to be linked to the
higher levels of sorbitol and glucose. Low intracellular inositol
has been associated with diabetic complications in nerve (4).
Because of the experimental evidence that links increased
levels of sorbitol with the onset of diabetic pathology, a series
of clinical trials are underway with various aldose reductase
inhibitors. The aim is to block the intracellular conversion of
glucose to sorbitol and thus prevent some complications of
diabetes.
The seemingly disparate studies on the renal medullary
cells and on the complications of diabetes converge on the role
of sorbitol in osmoregulation, both normal in the renal medulAddress reprint requests to Dr. Burg, Building 10, Room 6N307, National Institutes of Health, Bethesda, MD 20892.
Receivedfor publication 1 December 1987.
The Journal of Clinical Investigation, Inc.
Volume 81, March 1988, 635-640
lary cells, and abnormal in the neural and ocular tissues affected in diabetes.
Organic osmolytes and osmoregulation
Since cell membranes are incapable of sustaining significant
osmotic pressure differences, the osmolality inside a cell must
remain close to that of its surroundings. Therefore, when the
extracellular salt concentration rises, intracellular solute concentration increases as well. This occurs in all animal cells,
including those in the renal inner medulla. The initial response
to increased salt in the environment is loss of water by osmosis,
accompanied by an increase in the concentration ofthe solutes
that remain behind in the cells. Many types of osmotically
shrunken cells regulate back towards their normal volume by
accumulating sodium and potassium salts, followed by uptake
of water. However, this phase of osmoregulation is only temporary since in the long term, cells in general (2), and renal
medullary cells in particular (1, 5), do not balance high extracellular salt concentrations with equally high intracellular
concentrations of sodium and potassium salts. Instead, they
accumulate organic osmolytes either by uptake from their surroundings or by synthesis. Thus, cellular osmoregulation, after
an increase in extracellular salt concentration, occurs in two
phases: (i) volume regulatory increase by influx of sodium and
potassium salts, followed by (ii) accumulation of organic osmolytes, replacing the sodium and potassium salts and thus
restoring the intracellular milieu.
A theoretical basis for understanding organic osmolytes
was first established in explaining the high level ofamino acids
in the cells of some euryhaline animals (2). This accumulation
is restricted to neutral and acidic amino acids and is directly
related to environmental salinity. The particular neutral and
acidic amino acids that are accumulated resemble the cations
and anions in the Hofmeister or lyotropic series that favor
"6native" macromolecular structure and function. In contrast,
high concentrations of sodium and potassium chloride tend to
denature macromolecules. Polyhydric alcohols, like sorbitol
and inositol, are analogous in this respect to neutral and acidic
amino acids (2). Osmotically active organic solutes with these
properties are called "compatible solutes" or "nonperturbing
solutes" (2).
The difference between perturbing and nonperturbing solutes is exemplified by their effects on enzyme function (2, 6).
High levels of perturbing solutes, such as NaCl and KC1, increase the Km of some enzymes or decrease their Vma.,, whereas
even higher levels of nonperturbing solutes do not. In addition
to enzymes, perturbing solutes also affect other types of proteins and nucleic acids (7) by altering the association-dissociation equilibria between macromolecules, the stability of macroscopic fibrous structures made up of proteins or nucleic
acids, the conformation of nucleic acids and globular proteins,
Sorbitol, Osmoregulation, and the Complications of Diabetes
635
and the rates of macromolecular transconformational reactions.
Organic osmolytes, which are nonperturbing or compatible
solutes, have been identified during water stress in a wide variety of bacteria, plants, and animals (2). They fall into three
classes: (a) polyhydric alcohols (polyols), (b) free amino acids
and their derivatives, and (c) combinations of urea and methylamines. The predominant osmolytes in renal medullas fit
these categories. Sorbitol and inositol are polyols, betaine is
both an amino acid derivative and a methylamine, and glycerophosphoryicholine is both a polyol and methylamine (1, 5).
The concentration of organic osmolytes in renal medullary
cells generally varies directly with the concentration of the
urine (1, 8). The link presumably is the renal medullary extracellular salt concentration, which must be high to concentrate
the urine and often varies with urine concentration. Thus, the
accumulation of osmolytes in renal medullary cells must be
regulated. Of the different organic osmolytes in the renal medulla, the control of sorbitol accumulation has been most completely studied.
Regulation ofsorbitol in renal medullary cells after
changes in extracellular NaCi concentration
In mammalian tissues sorbitol is produced from glucose in a
reaction catalyzed by the enzyme aldose reductase (low Km
aldehyde reductase; alditol/NAD(P)+ 1-oxidoreductase, EC
1.1.1.21) (3): glucose + NADPH -- sorbitol + NADP'.
Sorbitol may in turn be oxidized by sorbitol dehydrogenase
(l-iditol dehydrogenase, EC 1.1.1.14) to fructose (3). Sorbitol
+ NAD+-. fructose + NADH. This two-step conversion of
glucose to fructose is known as the sorbitol or polyol pathway.
High levels of aldose reductase have been found in renal
medullas (3). As determined by immunofluorescence microscopy with specific antibodies, aldose reductase is abundant in
the terminal part of the collecting duct, thin limbs of Henle's
loop, papillary surface epithelium, and interstitial cells. No
staining was observed, however, in collecting ducts from the
outer part of the inner medulla. Also, immunoreactive staining in the outer medulla was spotty and difficult to distinguish.
In rats deprived of water there is a gradient of sorbitol from
5 mmol/kg wet weight in the outer medulla to - 60 mmol/kg
wet weight at the tip of the papilla (8). The concentration of
sorbitol in cell water is presumably even higher, because the
sorbitol is mostly intracellular and the cell water is only a
fraction of the wet weight. When rats (8) and rabbits (1) were
given access to water the levels ofsorbitol in the inner medulla
decreased by 20-40%. It is difficult to determine whether
the change is consistent with the proposed role of sorbitol as an
osmolyte since the resulting diuresis was modest, and the extent and duration of changes in medullary extracellular NaCl
concentrations were not measured. It is difficult to accurately
control and measure extracellular salt concentrations in the
inner medulla in vivo. Therefore, the control of sorbitol levels
and aldose reductase activity in renal cells has been most thoroughly investigated using cell culture.
GRB-PAP1 is a line of rabbit renal cells derived from the
papillary surface epithelium (9), which is one of the renal medullary tissues that contains a high level of immunoreactive
aldose reductase protein (3). These cells contain large amounts
ofaldose reductase and sorbitol when they are grown in hyperosmotic medium (500-600 mosmol/kg), but not when grown
in medium with a normal osmolality (300 mosmol/kg) (10).
-
636
M. B. Burg and P. F. Kador
The intracellular sorbitol concentration was 240 mM in the
600 mosmol/kg medium, which is sufficiently high to balance
most of the 300 mosmol/kg excess of NaCl added to make the
medium hyperosmotic. The cell water content and sodium
and potassium concentrations in these cells did not differ significantly between the 300 mosmol/kg medium and the 600
mosmol/kg medium. Thus, the accumulation of sorbitol apparently increased the intracellular solute concentration
enough to balance the high external osmolality, while the cells
maintained normal volume and concentrations of sodium and
potassium salts. No significant amounts of osmolytes other
than sorbitol were identified. The increase in aldose reductase
activity in hyperosmotic medium is due to an increase in the
amount of enzyme protein (11). The enzyme, which has a
weight of 39 kD in this tissue, increased from low levels under
isosmotic conditions to > 10% of the soluble cell protein in
600 mosmol/kg medium.
Large amounts of sorbitol produced by the cells appear in
the medium (12). The amount of sorbitol in the medium after
24 h approximately equals the amount present in the cells at
any given time. Because of the relatively large volume of medium, however, the concentration of sorbitol in the medium
remains < 1 mM, which is much lower than the concentration
in the cells. Initially, the large amounts of sorbitol in the medium seemed surprising because sorbitol is generally regarded
as a nonpenetrating solute. Moreover, high cellular permeability to sorbitol would make sorbitol ineffective as an osmolyte.
Considering the large concentration gradient and the long time
involved (24 h), it can be readily calculated, however, that the
permeability to sorbitol actually is quite low.
The time course for changes in aldose reductase levels in
GRB-PAPl cells is relatively slow. When the medium was
switched from 300 to 500 mosmol/kg by adding NaCl, a time
lag of 12-24 h occurred before aldose reductase activity and
sorbitol concentrations began to increase. The increase was
half maximal after 2 d, and the highest levels were reached
only after 3 to 4 d (13). When GRB-PAP I cells were grown in a
hyperosmotic medium (600 mosmol/kg), and then switched to
normal osmolality, the rate of decrease in aldose reductase was
even slower. Both the amounts of aldose reductase protein and
the enzyme activity decreased by only one-half in a week and
required 2 wk to reach low levels (12).
In contrast to the slow fall in aldose reductase, sorbitol
concentration fell rapidly in cells switched from 600 mosmol/
kg to 300 mosmol/kg medium. After medium osmolality was
reduced, cell sorbitol decreased within 1 d to 10% of its initial
value. This fall in cell sorbitol is accounted for by the accelerated efflux ofsorbitol from the cells to the medium. This initial
flux is very rapid and a large fraction of the cell sorbitol appears in the medium within 5 min after the osmolality is decreased (Bagnasco, S., and M. Burg, unpublished observation).
Thus, changes in cell sorbitol in response to changes in medium osmolality occur by at least two mechanisms: slow
change in aldose reductase activity and a rapid change in sorbitol flux.
The nature of the stimulus that induces aldose reductase in
GRB-PAPl cells was investigated by increasing the osmolality
of the medium to the same extent with different solutes and
comparing the magnitude of the changes. The effectiveness of
the solutes depended on their molecular size (13). Raffinose, a
trisaccharide with a large molecular size, had essentially the
same effect as NaCI. On the other hand urea and glycerol, both
molecules with small molecular weights, did not cause increases in aldose reductase or sorbitol. The result apparently
depends on the osmotic effectiveness of the solutes. Hyperosmotic addition of NaCl or raffinose causes prolonged shrinkage of the cells, associated with increased intracellular Na and
K concentration. Urea, on the other hand, does not change
either cell volume or the concentration ofNa or K (except very
transiently) because it exerts little osmotic effect due to its
rapid penetration. Ouabain, which generally increases cell sodium, decreases cell potassium, and causes cells to swell, was
used to perturb the relations between cell potassium, sodium,
and water after increase in osmolality. Under those conditions,
aldose reductase activity and sorbitol accumulation were not
significantly related to cell sodium or water content. They were
somewhat correlated with cell potassium, but most strongly
related to the sum of cell sodium plus potassium concentration, i.e., ionic strength (Uchida, S., and M. Burg, unpublished
observation). In this respect it is interesting to note that elevated cell potassium and ionic strength have been linked with
high medium osmolality and increased transcription of the
osmoregulatory proU gene, which codes for a betaine trans-
porter, in Escherichia coli (14).
Role ofsorbitol and other sugar alcohols (polyols) in the
complications of diabetes
Although the accumulation of sorbitol may be beneficial for
renal medullary cells, sorbitol and other polyols do not normally accumulate to high levels in other tissues, and when they
do, cellular pathology may result. The best evidence for this is
the fact that damage occurs in tissues that accumulate the
polyol galactitol when animals are fed galactose, and this damage is preventable by aldose reductase inhibitors. Further,
there is increasing experimental evidence that aldose reductase-dependent accumulation of sorbitol is a common biochemical link in the pathogenesis of many late-onset diabetic
complications that are related to the control of blood glucose
level (3, 15, 16). The specific mechanisms by which the accumulation of polyols lead to the onset of these diabetic complications are the subject of continued investigation.
Polyols and sugar cataracts
The adverse effects of sorbitol were first observed by Kinoshita
in the lens (17). In this tissue, when extracellular glucose concentration rises, more sorbitol is produced, and is catalyzed by
an increased level of aldose reductase. The accumulation of
sorbitol leads to cell swelling, which in turn leads to altered
lens membrane permeability and biochemical changes associated with cataract formation. These observations led to the
development of the osmotic hypothesis, which states that lens
cell swelling in response to the aldose reductase-initiated accumulation of polyols leads to loss of cellular integrity.
The train of events is as follows (18). Accumulation of
sorbitol raises intracellular osmolality, which results in an influx of water into the lens. Cell swelling is accompanied by
increased membrane permeability, increased intracellular sodium, and decreased cell potassium. In addition, the levels of
reduced glutathione, myo-inositol, ATP, and free amino acids
decrease, and vacuoles form. As vacuole formation progresses,
cortical opacification results. Protein synthesis decreases and
the cells lose dry weight. Eventually, complete loss of osmotic
integrity occurs. Electrolytes, amino acids, and proteins freely
permeate the cell plasma membranes, and lens opacification
proceeds to the final nuclear cataract stage.
The osmotic hypothesis is supported by a variety of experimental observations including in vitro lens culture studies,
studies with various animal models, and the use of aldose
reductase inhibitors that prevent or significantly delay cataract
formation in proportion to their potency in preventing polyol
formation in the lens (18). A variety of structurally diverse
aldose reductase inhibitors have been used (15, 16, 18). Of
these, perhaps the best-known inhibitors undergoing clinical
trials are the hydantoin sorbinil and the carboxylic acids Aldrestat (tolrestat), Ponalrestat (Statil), and Epalrestat (ONO
2235). Administered either orally, by injection, or topically as
eye drops, these inhibitors delay or prevent the appearance of
diabetic or galactosemic (sugar) cataracts in animals. Animal
studies demonstrate that the rate of sugar cataract formation is
directly dependent upon both the levels of aldose reductase
present in the lens and on how well the sugar serves as a
substrate for this enzyme (18). Cataracts occur in galactose-fed
animals because aldose reductase reduces a variety of aldonic
sugars, including galactose. High levels of galactose cause
faster and greater damage than do high levels of glucose because galactose is a better substrate for aldose reductase, and
thus is reduced to galactitol (dulcitol) more rapidly than glucose is to sorbitol. Further, galactitol is more persistent than
sorbitol because galactitol is not further metabolized by sorbitol dehydrogenase. In addition to cataract formation, galactose-fed animals also display other cellular pathology similar
to diabetes, except that the pathology occurs earlier under galactosemic conditions and is more severe than in the diabetic
state. This has been useful in elucidating the relationship between aldose reductase and diabetic complications because the
galactosemic animals have normal insulin levels.
Aldose reductase activity and immunohistochemical staining of aldose reductase protein increase in diabetic rat lenses,
suggesting that the aldose reductase protein level is increased
(19), just as it is in renal medullary cells exposed to high NaCl.
Aldose reductase activity is also increased in diabetic human
lenses cultured in vitro, and these lenses accumulate higher
levels of polyols than those from nondiabetics (18). Moreover,
increased immunohistochemical staining for aldose reductase
has also been observed in both the anterior and posterior superficial cortical layers of cataractous lenses extracted from
diabetics, but not in those extracted from nondiabetics ( 19).
Polyol effects in the cornea
Aldose reductase is present in both the corneal epithelium and
endothelium, and experimental studies link deranged osmoregulation in both regions to damage that occurs in diabetic
and galactose-fed animals. Defects in the rate of reepithelialization occur in severely diabetic or galactosemic rats whose
corneas have been denuded by limbus to limbus scraping (3,
16, 20), and the resurfaced corneas of these rats appear hazy
and edematous. In addition, the epithelial cells appear distended and lack filopodia. Both the delay in reepithelialization
and the hazy, edematous appearance of the cornea are prevented by oral or topical administration of aldose reductase
inhibitors.
In the corneal endothelium, which consists of a monolayer
of hexagonally shaped cells, there are changes in endothelial
cell size (polymegathism) and shape (pleomorphism) in diabetic humans, rats, and dogs (8, 21, 22). The diabetic endotheSorbitol, Osmoregulation, and the Complications ofDiabetes
637
Hal monolayer may loose cells, and the remaining cells may
have to stretch and slide to cover the cell-free areas. Topical
administration of an aldose reductase inhibitor to diabetic rats
has been reported to significantly reduce the extent of these
morphological changes.
The clinical manifestations of these defects are decreased
corneal sensitivity and decreased tolerance of the corneal epithelium to stress during therapies that include photocoagulation, vitrectomy, and even use of contact lenses. Traumatized
areas of the corneal epithelium tend to heal more slowly in
diabetics and require increased medical attention. Corneal
thickening and persistent stromal edema, suggestive of abnormal endothelial cell function, have also been observed clinically. Limited administration of aldose reductase inhibitors in
the United States and Japan has resulted in apparent improvement in the state of the corneal epithelium in diabetics
(23, 24).
Polyols and diabetic retinopathy
Retinopathy leads to loss of vision in a significant fraction of
diabetics. It is characterized by changes in the retinal capillary
bed with initial formation of microaneurysms, exudates, and
small intraretinal hemorrhages. Later, neovascularization, fibrovascular proliferation, and vitreous hemorrhages occur. Although the etiology of this complication remains uncertain,
considerable evidence implicates the abnormal accumulation
of polyols, which causes degeneration of retinal capillary pericytes. Trials are underway with aldose reductase inhibitors in
an attempt to prevent retinopathy.
A distinguishing feature of early retinal changes is selective
degeneration of pericytes (mural cells), whose long processes
encircle the retinal endothelial capillary cells. Loss of pericytes
is associated with decreased capillary tonicity, followed by formation ofmicroaneurysms, dilation of vessels, increased number of endothelial cells, and formation of some acellular vessels. There is evidence that sorbitol accumulates in pericytes
and causes them to degenerate into cell ghosts. Aldose reductase protein (detected immunohistochemically) is present in
the mural cells but not in the endothelial cells of human and
dog retinal capillaries (16, 25). Cultured human and dog retinal mural cells display aldose reductase activity, and the production of polyol in these cells is inhibited by aldose reductase
inhibitors (26). Finally, retinal vascular changes similar to
those observed in diabetic dogs and humans occur in galactose-fed dogs (27). Galactitol production in isolated dog retinal
vessels cultured in medium containing 30 mM galactose is
significantly reduced by the aldose reductase inhibitor sorbinil (28).
In addition to pericyte degeneration, retinal capillary basement membrane thickening is commonly observed in diabetic
animals and those fed galactose. This thickening can be prevented by aldose reductase inhibitors (29). The relationship
between aldose reductase and basement membrane thickening
remains unclear. Possibly, the membrane thickening is secondary to degeneration of the retinal pericytes.
Because of the considerable evidence linking retinopathy
to polyol accumulation, a number of clinical studies are being
conducted to see if aldose reductase inhibitors can delay or
arrest the progression of retinal changes in early diabetic retinopathy. No conclusive results are reported, as yet. However, a
possible indication of clinical improvement is the report that
638
M. B. Burg and P. F Kador
sorbinil reduces the blood retinal barrier permeability in diabetics, as monitored by vitreous fluorophotography (30).
Diabetic neuropathy
A variety of neurological disorders are associated with diabetes
mellitus, and a majority of all diabetics are to some degree
afflicted with neurological symptoms. Recent reviews have
discussed the relationship of aldose reductase to neurological
symptoms (3, 16).
Both sorbitol and myo-inositol appear to be involved in the
onset of neuropathy, and their relative roles are questioned.
The possible primacy of myo-inositol has been emphasized
elsewhere (4); the relationship to sorbitol accumulation will
mainly be considered in the present context. In peripheral
nerves such as the sciatic, there is aldose reductase in Schwann
cells, which are the cells responsible for the formation and
maintenance of the myelin sheath (3, 16). Sorbitol and galactitol accumulate in the sciatic nerves ofdiabetic and galactosemic rats, respectively, and cause these nerves to swell. This
occurs more rapidly in the galactose-fed than in the diabetic
rats, and can be reversed by aldose reductase inhibitors (31,
32). These changes resemble those that have been shown to
damage lenses (16). Polyol accumulation and nerve swelling
are associated with decreased nerve myo-inositol and amino
acid levels, decreased nerve conduction velocity, and decreased choline acetyltransferase. Polyol accumulation and the
other effects are prevented by aldose reductase inhibitors (3,
16, 33, 34).
Impairment of motor nerve conduction, a clinical hallmark of peripheral neuropathy, is among the earliest and most
easily quantifiable signs of diabetic neuropathy. Clinical improvement in nerve conduction has been reported with several
aldose reductase inhibitors. A small but significant increase in
motor nerve conduction was first reported in a multicenter
randomized, doubly masked, crossover trial with sorbinil (35).
This has been followed by other studies, many with inconclusive results (16). Nevertheless, the positive effects of sorbinil
treatment, as reported in the initial multicentered trial, were
also supported by morphological and morphometric examinations of sural nerve biopsies (36). Several studies indicate more
positive responses in patients with mild neuropathy compared
with those with severe neuropathy. Perhaps, aldose reductase
inhibitors improve nerve conduction velocities in patients
with mild, but not severe, neuropathy.
Aldose reductase inhibitors may be useful for the subjective relief of pain. First observed in early clinical trials with
alrestatin (37), this has become more evident in recent studies
with sorbinil, which is more potent. In the sorbinil trials patients who were unresponsive to conventional therapy experienced decreased pain, improved sensory perception, increased
muscle strength, and normalization of nerve conduction velocities (38, 39).
The initial clinical trials with alrestatin (37) also suggested
that aldose reductase inhibitors may be beneficial for symptoms of autonomic neuropathy, which include gastroparesis,
diarrhea, erectile dysfunction, abnormal sudomotor, pupillary
function, and weakness. Since then, these symptoms have all
been reported to qualitatively improve in selected patients receiving sorbinil (16, 39). Sorbinil also produced quantitative
improvements in the ratio of the R-R interval during expiration to that during inspiration (E/I ratio) and decreases in the
minimal heart rate (16, 39). Beneficial effects of sorbinil on
cardiac performance have also been reported (40).
Do aldose reductase and sorbitol have a normal
osmoregulatory role in nonrenal tissues affected
by the complications of diabetes?
In view of the evidence that aldose reductase and sorbitol help
regulate the volume and intracellular milieu of renal medullary cells after changes in external osmolality, it is worth considering whether they have a similar role in the lens and other
tissues. Recently, based on indirect evidence, it was proposed
that aldose reductase activity and intracellular sorbitol accumulation protect lenses against daily, diet- and disease-related
changes in osmotic pressure in the aqueous humor (41). This
theory was tested by incubating rabbit lenses for 4 h in medium made hyperosmotic with added glucose (55.5 mM) (42).
Sorbitol and fructose accumulated continuously during the
hyperosmotic period, but never reached a level high enough to
balance the hyperosmolality of the glucose added to the medium. Then, the glucose and osmolality were reduced to normal levels for four additional hours. Lens sorbitol and fructose
remained elevated at approximately a constant level during
this time. The authors concluded that the observed accumulation of sorbitol and fructose was beneficial for reducing dehydration of the lens during acute cyclical changes in blood glucose, but that continuous, rather than cyclical activity of the
pathway might be detrimental and cause cataracts. The putative acute osmoregulatory benefit of the sorbitol and fructose
may be questioned, however, considering that they accumulated in the lenses slowly (42), and that their level did not fall
when the glucose was reduced to isosmotic levels. In this respect renal medullary cells were strikingly different. Recall that
a large efflux of sorbitol reduced its level in PAP-HT25 cells
within minutes after the medium osmolality was decreased (9).
Based on these observations, the role of aldose reductase and
sorbitol in osmoregulation may well differ between renal and
lens cells, but an exact comparison is difficult because the
experimental conditions were so different in the various studies. In addition, it is not clear whether hyperosmolality associated with hyperglycemia causes the increase in aldose reductase activity observed in lenses of diabetic rats analogous to the
effect of hyperosmolality in renal medullary cells.
Summary and conclusions
Sorbitol, whose synthesis is catalyzed by aldose reductase, is
one of several organic solutes that renal medullary cells use to
osmoregulate in the face of high and variable osmolality in the
renal medullary extracellular fluid. Evidently, sorbitol accumulation is beneficial to renal medullary cells, and they are
able to control its level without obvious harm. There is no such
obvious normal role for the aldose reductase and sorbitol
which occur in lenses, corneas, retinal vessels, and peripheral
nerves, and there is considerable evidence that accumulation
of sorbitol in these tissues during uncontrolled diabetes causes
abnormal osmoregulation and subsequent cellular pathology.
Aldose reductase inhibitors may prevent harmful sorbitol accumulation in diabetes. The possible helpfulness ofthese drugs
in preventing complications of diabetes is being tested in clinical trials. Some benefit has been demonstrated in a limited
number of patients with diabetic neuropathy and corneal erosions. However, clinical trials studying these and other com-
plications, such as retinopathy and cataracts, are unfinished,
and will require years to complete because of the chronic and
erratic progression of these complications. Nonetheless, there
is considerable promise based on these early trials and on
well-documented studies in animals, and there is reason to
hope that aldose reductase inhibitors will prove clinically useful for preventing at least some ofthe distressing complications
of diabetes.
References
1. Bagnasco, S., R. Balaban, H. M. Fales, Y.-M. Yang, and M.
Burg. 1986. Predominant osmotically active organic solutes in rat and
rabbit renal medullas. J. Biol. Chem. 261:5872-5877.
2. Yancey, P. H., M. E. Clark, S. C. Hand, R. D. Bowlus, and G. N.
Somero. 1982. Living with water stress: evolution of osmolyte systems.
Science(Wash. DC). 217:1214-1222.
3. Dvornik, D. 1987. Aldose Reductase Inhibition: An Approach to
the Prevention of Diabetic Complications. McGraw-Hill, Inc., New
York.
4. Green, D. A., S. A. Lattimer, and A. A. F. Sima. 1987. Sorbitol,
phosphoinositidesk and sodium-potassium-ATPase in the pathogenesis of diabetic complications. N. Engl. J. Med. 316:599-605.
5. Balaban, R. S., and M. B. Burg. 1987. Osmotically active organic
solutes in the renal inner medulla. Kidney Int. 31:562-564.
6. Somero G. N. 1986. Protons, osmolytes, and fitness of internal
milieu for protein function. Am. J. Physiol. 251 :R197-R213.
7. Von Hippel, P. H., and T. Schleich. 1969. The effects of neutral
salts on the structure and conformational stability of macromolecules
in solution. In Structure and Stability of Biological Macromolecules.
S. N. Timasheff and G. D. Fasman, editors. Marcel Dekker, Inc., New
York. 417-574.
8. Oates, P. J., and K. J. Goddu. 1987. A sorbitol gradient in rat
inner medulla. Kidney Int. 31:448.
9. Uchida, S., N. Green, H. Coon, T. Triche, S. Mims, and M.
Burg. 1987. High NaCl induces stable changes in phenotype and
karyotype of renal cells in culture. Am. J. Physiol. 253(Cell Physiol.
22):C230-C242.
10. Bagnasco, S. M., S. Uchida, R. S. Balaban, P. F. Kador, and
M. B. Burg. 1987. Induction of aldose reductase and sorbitol in renal
inner medullary cells by elevated extracellular NaCl. Proc. Natl. Acad.
Sci. USA. 84:1718-1720.
11. Bedford, J. J., S. M. Bagnasco, P. Kador, H. W. Harris, and
M. B. Burg. 1987. Characterization and purification of a mammalian
osmoregulatory protein, aldose reductase, induced in renal medullary
cells by high extracellular NaCl. J. Bio. Chem. 262:14255-14259.
12. Bagnasco, S., J. Bedford, and M. Burg. 1987. Slow changes in
aldose reductase and rapid changes in sorbitol flux mediate osmoregulation by renal medullary cells. Fed. Proc. 46:1229. (Abstr.)
13. Uchida, S., A. Garcia-Perez, and M. Burg. 1987. Mechanism by
which high NaCl induces aldose reductase activity in cells cultures
from kidney medulla. Fed. Proc. 46:1229. (Abstr.)
14. Higgins, F. H., J. Cairney, D. A. Stirling, L. Sutherland, and
I. R. Booth. 1987. Osmotic regulation of gene expression: ionic
strength as an intracellular signal? Trends Biolog. Science. 12:339-344.
15. Kador, P. F., N. E. Sharpless, and J. H. Kinoshita. 1985. Perspective series: aldose reductase inhibitors: a potential new class of
agents for the pharmacological control of certain diabetic complications. J. Med. Chem. 28:841-849.
16. Kador, P. F. 1987. The role of aldose reductase in the development of diabetic complications. Med. IRes. Rev. In press.
17. Kinoshita, J. H. 1974. Mechanism initiating cataract formation. Invest. Ophthalmol. 13:713-724.
18. Kador, P. F., and J. H. Kinoshita. 1984. Diabetic galactosemic
cataracts. Ciba Foundation Symposium No. 106. Human Cataract
Formation. 106:1 10.
Sorbitol, Osmoregulation, and the Complications ofDiabetes
639
19. Akagi, Y., P. F. Kador, and J. H. Kinoshita. 1986. Immunohistochemical localization for aldose reductase in diabetic lenses. Invest.
Ophthalmol. Visual Sci. 28:163-167.
20. Datiles, M. B., P. F. Kador, H. N. Fukui, T. S. Hu, and J. H.
Kinoshita. 1983. Corneal re-epithelialization in galactosemic rats. Invest. Ophthalmol. Visual Sci. 24:563-569.
21. Schultz, R. O., M. Matsuda, R. W. Yee, H. F. Edelhauser, and
K. J. Schultz. 1984. Corneal endothelial changes in type I and type II
diabetes. Am. J. Ophthalmol. 98:401.
22. Matsuda, M., T. Awata, Y. Ohashi, M. Inaba, M. Fukuda, and
M. Reizo. 1987. The effects of aldose reductase inhibition on endothelial cell morphology in diabetic rats. Current Eye Res. 6:391-392.
23. Cobo, L. M. 1984. Aldose reductase and diabetic keratopathy.
In Aldose Reductase and Complications of Diabetes. Ann. Intern.
Med. 101:87.
24. Ohashi, Y., T. Mano, H., Umemoto, R. Hosotani, Y. Tamada,
Y. Tano, M. Matsuda, M. Fukuda, and R. Manabe. Effects of aldose
reductase inhibitor (CT-1 12) on diabetic keratoepitheliopathy. Exerpta Med. Congr Ser. In press.
25. Akagi, Y., H. Terubayashi, J. Millen, P. F. Kador, and J. H.
Kinoshita. 1986. Aldose reductase localization in dog retinal mural
cells. Current Eye Res. 5:833-886.
26. Hohman, T. C., C. Nishimura, W. G. Robison, Jr., and J. H.
Kinoshita. 1986. Aldose reductase in cultured human and canine retinal capillary pericytes. Invest. Ophthalmol. Visual Sci. 26:328.
27. Engerman, R. L., and T. S. Kern. 1984. Experimental galactosemia produces diabetic-like retinopathy. Diabetes. 33:97-100.
28. Kern, T. S., and R. L. Engerman. 1985. Hexitol production by
canine retinal microvessels. Invest. Ophthalmol. Visual Sci. 26:382384.
29. Robison, W. G., Jr., P. F. Kador, Y. Akagi, J. H. Kinoshita, R.
Gonzales, and D. Dvornik. 1986. Prevention of basement membrane
thickening in retinal capillaries by a novel aldose reductase inhibitor,
tolrestat. Diabetes. 35:295-299.
30. Cunha-Vaz, J. G., C. C. Mota, E. C. Leite, J. R. Abreu, and
M. A. Ruas. 1986. Effect of Sorbinil on blood-retinal barrier in early
diabetic retinopathy. Diabetes. 35:574-578.
640
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M. B. Burg and P. F. Kador
31. Gabbay, K. H. 1973. Role of sorbitol pathway in neuropathy.
Adv. Metab. Disord. 417-424.
32. Griffey, R., R. P. Eaton, C. Gaspavoric, and W. Sibbit. 1987.
Galactose neuropathy. Diabetes. 36:776-778.
33. Tomlinson, D. R., P. R. Holmes, and J. H. Mayer. 1982.
Reversal, by treatment with an aldose reductase inhibitor, of impaired
axonal transport and motor nerve conduction velocity in experimental
diabetes mellitus. Neurosci. Lett. 31:189-193.
34. Yue, D. K., M. A. Hanwell, P. M. Satchell, and J. R. Turtle.
1982. The effect of aldose reductase inhibition on motor nerve conduction velocity in diabetic rats. Diabetes. 31:789-794.
35. Judzewitsch, R., J. B. Jaspan, K. S. Polonsky, C. R. Weinberg,
J. B. Halter, E. Halar, M. A. Pfeifer, C. Vukadinovic, L. Bernstein, and
M. Schneider, et al. 1983. Aldose reductase inhibition improves motor
nerve conduction velocity in diabetic patients. N. Engl. J. Med.
308:119-125.
36. Sima, A. A. F., V. Bril, V. Nathanial, and D. A. Greene. The
effect of sorbinil treatment on diabetic neuropathy. Exerpta Med.
Congr. Ser. In press.
37. Fagius, J., and S. Jameson. 1981. Effects of aldose reductase
inhibitor treatment in diabetic polyneuropathy: a clinical and neurophysiological study. J. Neurol. Neurosurg. Psychiatry. 44:991-1001.
38. Young, R. J., D. J. Ewing, and B. F. Clarke. 1983. A controlled
trial of sorbinil, an aldose reductase inhibitor, in chronic painful diabetic neuropathy. Diabetes. 32:938-942.
39. Jaspan, J. B., V. L. Towle, R. Maselli, and K. Herold. 1986.
Clinical studies with an aldose reductase inhibitor in the autonomic
and somatic neuropathies of diabetes. Metab. Clin. Exp. 35:83-92.
40. Pfeifer, M., H. Snider, H. Peterson, J. Cyrus, and V. Broadstone. Improvement in sympathetic nervous system and cardiovascular performance by sorbinil in diabetics. Exerpta Med. Congr. Ser. In
press.
41. Seland, J. H., and L. T. Chylack. 1986. Acute glucose-derived
osmotic stress in rabbit lens. Acta Ophthalmol. 64:533-539.
42. Chylack, L. T., W. Tung, and R. Harding. 1986. Sorbitol production in the lens: a means of counteracting glucose-derived osmotic
stress. Ophthalmic Res. 18:313-320.