Characterization and purification of a mammalian osmoregulatory protein, aldose reductase, induced in renal medullary cells by high extracellular NaCl

Role of ion transport in control of apoptotic cell death

Cell shrinkage is a hallmark and contributes to signaling of apoptosis. Apoptotic cell shrinkage requires ion transport across the cell membrane involving K(+) channels, Cl(-) or anion channels, Na(+)/H(+) exchange, Na(+),K(+),Cl(-) cotransport, and Na(+)/K(+)ATPase. Activation of K(+) channels fosters K(+) exit with decrease of cytosolic K(+) concentration, activation of anion channels triggers exit of Cl(-), organic osmolytes, and HCO3(-). Cellular loss of K(+) and organic osmolytes as well as cytosolic acidification favor apoptosis. Ca(2+) entry through Ca(2+)-permeable cation channels may result in apoptosis by affecting mitochondrial integrity, stimulating proteinases, inducing cell shrinkage due to activation of Ca(2+)-sensitive K(+) channels, and triggering cell-membrane scrambling. Signaling involved in the modification of cell-volume regulatory ion transport during apoptosis include mitogen-activated kinases p38, JNK, ERK1/2, MEKK1, MKK4, the small G proteins Cdc42, and/or Rac and the transcription factor p53. Osmosensing involves integrin receptors, focal adhesion kinases, and tyrosine kinase receptors. Hyperosmotic shock leads to vesicular acidification followed by activation of acid sphingomyelinase, ceramide formation, release of reactive oxygen species, activation of the tyrosine kinase Yes with subsequent stimulation of CD95 trafficking to the cell membrane. Apoptosis is counteracted by mechanisms involved in regulatory volume increase (RVI), by organic osmolytes, by focal adhesion kinase, and by heat-shock proteins. Clearly, our knowledge on the interplay between cell-volume regulatory mechanisms and suicidal cell death is still far from complete and substantial additional experimental effort is needed to elucidate the role of cell-volume regulatory mechanisms in suicidal cell death.

Cellular Response to Hyperosmotic Stresses

Cells in the renal inner medulla are normally exposed to extraordinarily high levels of NaCl and urea. The osmotic stress causes numerous perturbations because of the hypertonic effect of high NaCl and the direct denaturation of cellular macromolecules by high urea. High NaCl and urea elevate reactive oxygen species, cause cytoskeletal rearrangement, inhibit DNA replication and transcription, inhibit translation, depolarize mitochondria, and damage DNA and proteins. Nevertheless, cells can accommodate by changes that include accumulation of organic osmolytes and increased expression of heat shock proteins. Failure to accommodate results in cell death by apoptosis. Although the adapted cells survive and function, many of the original perturbations persist, and even contribute to signaling the adaptive responses. This review addresses both the perturbing effects of high NaCl and urea and the adaptive responses. We speculate on the sensors of osmolality and document the multiple pathways that signal activation of the transcription factor TonEBP/OREBP, which directs many aspects of adaptation. The facts that numerous cellular functions are altered by hyperosmolality and remain so, even after adaptation, indicate that both the effects of hyperosmolality and adaptation to it involve profound alterations of the state of the cells.

Insights from modeling the 3D structure of NAD(P)H-dependent D-xylose reductase of Pichia stipitis and its binding interactions with NAD and NADP

NAD(P)H-dependent d-xylose reductase is a homodimeric oxidoreductase that belongs to the aldo-keto reductase superfamily. The enzyme has the special function to catalyze the first step in the assimilation of xylose into yeast metabolic pathways. Performing this function via reducing the open chain xylose to xylitol, the xylose reductase of Pichia stipitis is one of the most important enzymes that can be used to construct recombinant Saccharomyces cerevisiae strain for utilizing xylose and producing alcohol. To investigate into the interaction mechanism of the enzyme with its ligand NAD and NADP, the 3D structure was developed for the NAD(P)H-dependent d-xylose reductase from P. stipitis. With the 3D structure, the molecular docking operations were conducted to find the most stable bindings of the enzyme with NAD and NADP, respectively. Based on these results, the binding pockets of the enzyme for NAD and NADP have been explicitly defined. It has been found that the residues in forming the binding pockets for both NAD and NADP are almost the same and mainly hydrophilic. These findings may be used to guide mutagenesis studies, providing useful clues to modify the enzyme to improve the utilization of xylose for producing alcohol. Also, because human aldose reductases have the function to reduce the open chain form of glucose to sorbitol, a process physiologically significant for diabetic patients at the time that their blood glucose levels are elevated, the information gained through this study may also stimulate the development of new strategies for therapeutic treatment of diabetes.

Impaired ability to increase water excretion in mice lacking the taurine transporter gene TAUT

Cellular taurine uptake or release counteracts alterations of cell volume. Na+-coupled taurine transporter TAUT mediates concentrative cellular uptake of taurine. Inhibition of vasopressin secretion by hypotonicity may involve taurine release from glial cells of supraoptic nucleus. We compared renal function of mice lacking TAUT (taut-/-) and wild-type littermates (taut+/+). We observed renal taurine loss and subsequent hypotaurinemia in taut-/- mice. With free access to water, plasma and urine osmolality, urinary flow rate as well as urinary excretion and plasma concentrations of Na+ and K+ were similar in taut-/- and taut+/+ mice, whereas plasma concentrations of urea were enhanced in taut-/- mice. An oral water load (1 ml/16 g body weight) induced a similar diuresis in both genotypes. Repeating the oral water load immediately after normalization of urine flow rate, however, resulted in delayed diuresis and higher urinary vasopressin/creatinine ratios in taut-/- mice. In comparison, the repeated diuretic response to vasopressin V2 receptor blockade was not different between genotypes. Water deprivation for 36 h led to similar antidiuresis and increases of urinary osmolality in both genotypes. Upon free access to water after deprivation, taut-/- mice continued to concentrate urine up to 6 days, while taut+/+ mice rapidly returned to normal urinary osmolality. Urinary vasopressin/creatinine ratios and plasma aldosterone concentrations were not different under basal conditions but were significantly higher in taut-/- mice than in taut+/+ mice at 6 days after water deprivation. In conclusion, taut-/- mice suffer from renal taurine loss and impaired ability to lower urine osmolality and to increase urinary water excretion. The latter defect could reside extrarenally and result from a role of taurine in the suppression of vasopressin release which may be attenuated in taut-/- mice.

Cell volume regulation: osmolytes, osmolyte transport, and signal transduction

In recent years, it has become evident that the volume of a given cell is an important factor not only in defining its intracellular osmolality and its shape, but also in defining other cellular functions, such as transepithelial transport, cell migration, cell growth, cell death, and the regulation of intracellular metabolism. In addition, besides inorganic osmolytes, the existence of organic osmolytes in cells has been discovered. Osmolyte transport systems-channels and carriers alike-have been identified and characterized at a molecular level and also, to a certain extent, the intracellular signals regulating osmolyte movements across the plasma membrane. The current review reflects these developments and focuses on the contributions of inorganic and organic osmolytes and their transport systems in regulatory volume increase (RVI) and regulatory volume decrease (RVD) in a variety of cells. Furthermore, the current knowledge on signal transduction in volume regulation is compiled, revealing an astonishing diversity in transport systems, as well as of regulatory signals. The information available indicates the existence of intricate spatial and temporal networks that control cell volume and that we are just beginning to be able to investigate and to understand.

Cloning and characterization of the xyl1 gene, encoding an NADH-preferring xylose reductase from Candida parapsilosis, and its functional expression in Candida tropicalis

Xylose reductase (XR) is a key enzyme in D-xylose metabolism, catalyzing the reduction of D-xylose to xylitol. An NADH-preferring XR was purified to homogeneity from Candida parapsilosis KFCC-10875, and the xyl1 gene encoding a 324-amino-acid polypeptide with a molecular mass of 36,629 Da was subsequently isolated using internal amino acid sequences and 5' and 3' rapid amplification of cDNA ends. The C. parapsilosis XR showed high catalytic efficiency (kcat/Km = 1.46 s(-1) mM(-1)) for D-xylose and showed unusual coenzyme specificity, with greater catalytic efficiency with NADH (kcat/Km = 1.39 x 10(4) s(-1) mM(-1)) than with NADPH (kcat/Km = 1.27 x 10(2) s(-1) mM(-1)), unlike all other aldose reductases characterized. Studies of initial velocity and product inhibition suggest that the reaction proceeds via a sequentially ordered Bi Bi mechanism, which is typical of XRs. Candida tropicalis KFCC-10960 has been reported to have the highest xylitol production yield and rate. It has been suggested, however, that NADPH-dependent XRs, including the XR of C. tropicalis, are limited by the coenzyme availability and thus limit the production of xylitol. The C. parapsilosis xyl1 gene was placed under the control of an alcohol dehydrogenase promoter and integrated into the genome of C. tropicalis. The resulting recombinant yeast, C. tropicalis BN-1, showed higher yield and productivity (by 5 and 25%, respectively) than the wild strain and lower production of by-products, thus facilitating the purification process. The XRs partially purified from C. tropicalis BN-1 exhibited dual coenzyme specificity for both NADH and NADPH, indicating the functional expression of the C. parapsilosis xyl1 gene in C. tropicalis BN-1. This is the first report of the cloning of an xyl1 gene encoding an NADH-preferring XR and its functional expression in C. tropicalis, a yeast currently used for industrial production of xylitol.

NADPH-dependent D-aldose reductases and xylose fermentation in Fusarium oxysporum

Two aldose (xylose) reductases (ARI and ARII) from Fusarium oxysporum were purified and characterized. The native ARI was a monomer with M(r) 41000, pI 5.2 and showed a 52-fold preference for NADPH over NADH, while ARII was homodimeric with a subunit of M(r) 37000, pI 3.6 and a 60-fold preference for NADPH over NADH. In this study, the influence of aeration and the response to the addition of electron acceptors on xylose fermentation by F. oxysporum were also studied. The batch cultivation of F. oxysporum on xylose was performed under aerobic, anaerobic and oxygen-limited conditions in stirred tank reactors. Oxygen limitation had considerable influence on xylose metabolism. Under anaerobic conditions (0 vvm), xylitol was the main product with a maximum yield of 0.34 mole of xylitol/mole of xylose while the maximum ethanol yield (1.02 moles of ethanol/mole of xylose) was obtained under aerobic conditions (0.3 vvm). When the artificial electron acceptor acetoin was added to an anaerobic batch fermentation of xylose by F. oxysporum, the ethanol yield increased while xylitol excretion was also decreased.

Plasma 3-deoxyglucosone elevation in chronic renal failure is associated with increased aldose reductase in erythrocytes

BACKGROUND

Serum concentrations of 3-deoxyglucosone (3DG), a highly reactive dicarbonyl compound, are elevated in uremic patients. Aldose reductase (AR) is an enzyme involved in both the detoxification of 3DG and producing precursors of 3DG.

METHODS

We examined the relationship between plasma 3DG and erythrocyte AR content in uremic patients. Patients were divided into three groups: (1) progressive renal disease without hemodialysis (HD; chronic renal failure [CRF] group), (2) patients without diabetes mellitus (DM) treated with maintenance HD (HD group), and (3) patients with DM treated with maintenance HD (DM-HD group). High-performance liquid chromatography was used to measure 3DG, and erythrocyte AR was measured by means of enzyme-linked immunosorbent assay.

RESULTS

Both 3DG and erythrocyte AR levels were significantly greater in the CRF, HD, and DM-HD groups than in healthy controls. These results did not change after HD sessions in the HD or DM-HD groups. Serum creatinine levels correlated with 3DG and erythrocyte AR levels in the control and CRF groups (3DG: r = 0.67; P < 0.001; erythrocyte AR: r = 0.71; P < 0.001). Both erythrocyte AR and 3DG levels then increased as renal function declined. A positive correlation was seen between 3DG and erythrocyte AR levels in all groups (r = 0.65; P < 0.001), and also between plasma osmolality and erythrocyte AR level (r = 0.46; P < 0.001).

CONCLUSION

Both erythrocyte AR and 3DG levels are increased in uremic patients, and these increases could possibly contribute to the development of uremic symptoms.

The structure of apo and holo forms of xylose reductase, a dimeric aldo-keto reductase from Candida tenuis

Xylose reductase is a homodimeric oxidoreductase dependent on NADPH or NADH and belongs to the largely monomeric aldo-keto reductase superfamily of proteins. It catalyzes the first step in the assimilation of xylose, an aldose found to be a major constituent monosaccharide of renewable plant hemicellulosic material, into yeast metabolic pathways. It does this by reducing open chain xylose to xylitol, which is reoxidized to xylulose by xylitol dehydrogenase and metabolically integrated via the pentose phosphate pathway. No structure has yet been determined for a xylose reductase, a dimeric aldo-keto reductase or a family 2 aldo-keto reductase. The structures of the Candida tenuis xylose reductase apo- and holoenzyme, which crystallize in spacegroup C2 with different unit cells, have been determined to 2.2 A resolution and an R-factor of 17.9 and 20.8%, respectively. Residues responsible for mediating the novel dimeric interface include Asp-178, Arg-181, Lys-202, Phe-206, Trp-313, and Pro-319. Alignments with other superfamily members indicate that these interactions are conserved in other dimeric xylose reductases but not throughout the remainder of the oligomeric aldo-keto reductases, predicting alternate modes of oligomerization for other families. An arrangement of side chains in a catalytic triad shows that Tyr-52 has a conserved function as a general acid. The loop that folds over the NAD(P)H cosubstrate is disordered in the apo form but becomes ordered upon cosubstrate binding. A slow conformational isomerization of this loop probably accounts for the observed rate-limiting step involving release of cosubstrate. Xylose binding (K(m) = 87 mM) is mediated by interactions with a binding pocket that is more polar than a typical aldo-keto reductase. Modeling of xylose into the active site of the holoenzyme using ordered waters as a guide for sugar hydroxyls suggests a convincing mode of substrate binding.

Noncovalent enzyme-substrate interactions in the catalytic mechanism of yeast aldose reductase

The role of noncovalent interactions in the catalytic mechanism of aldose reductase from the yeast Candida tenuis was determined by steady-state kinetic analysis of the NADH-dependent reduction of various aldehydes, differing in hydrophobicity and the hydrogen bonding capability with the binary enzyme-NADH complex. In a series of aliphatic aldehydes, substrate hydrophobicity contributes up to 13.7 kJ/mol of binding energy. The aldehyde binding site of aldose reductase appears to be 1.4 times more hydrophobic than n-octanol and can accommodate a linear alkyl chain with at least seven methylene groups (approximately 14 A in length). Binding energy resulting from interactions at positions 3-6 of the aldehyde is distributed between increasing the catalytic constant 2.6-fold and decreasing the apparent dissociation constant 59-fold. Hydrogen bonding interactions of the enzyme nucleotide complex with the C-2(R) hydroxyl group of the aldehyde are crucial to transition state binding and contribute up to 17 kJ/mol of binding energy. A comparison of the kinetic data of yeast aldose reductase, a key enzyme in the metabolism of D-xylose, and human aldose reductase, a presumably perfect detoxification catalyst [Grimshaw, C. E. (1992) Biochemistry 31, 10139], clearly reflects these differences in physiological function.

Functional significance of cell volume regulatory mechanisms

To survive, cells have to avoid excessive alterations of cell volume that jeopardize structural integrity and constancy of intracellular milieu. The function of cellular proteins seems specifically sensitive to dilution and concentration, determining the extent of macromolecular crowding. Even at constant extracellular osmolarity, volume constancy of any mammalian cell is permanently challenged by transport of osmotically active substances across the cell membrane and formation or disappearance of cellular osmolarity by metabolism. Thus cell volume constancy requires the continued operation of cell volume regulatory mechanisms, including ion transport across the cell membrane as well as accumulation or disposal of organic osmolytes and metabolites. The various cell volume regulatory mechanisms are triggered by a multitude of intracellular signaling events including alterations of cell membrane potential and of intracellular ion composition, various second messenger cascades, phosphorylation of diverse target proteins, and altered gene expression. Hormones and mediators have been shown to exploit the volume regulatory machinery to exert their effects. Thus cell volume may be considered a second message in the transmission of hormonal signals. Accordingly, alterations of cell volume and volume regulatory mechanisms participate in a wide variety of cellular functions including epithelial transport, metabolism, excitation, hormone release, migration, cell proliferation, and cell death.

Urea and methylamines have similar effects on aldose reductase activity

The concentration of urea in renal medullary cells is sufficiently high to inhibit activity of many enzymes, yet the cells survive and function. The generally accepted explanation is the counteracting osmolytes hypothesis, which holds that methylamines, such as glycerophosphorylcholine (GPC) and glycine betaine (betaine), found in the renal medulla stabilize biological macromolecules and oppose the effects of urea. The present study tests this hypothesis by determining the effects of urea and methylamines, singly and in combination, on the activity of aldose reductase, an enzyme that is important in renal medullas for catalyzing production of sorbitol from glucose. In apparent contradiction to the counteracting osmolytes hypothesis, urea (1.0 M) and three different methylamines (trimethylamine N-oxide, betaine, and GPC; 0.5 M) all have similar and partially additive inhibitory effects. They all decrease substantially both the Michaelis constant (K(m)) and the maximum velocity (Vmax). Also a high concentration (0.5 M) of other organic osmolytes that are abundant in the renal medulla, namely inositol, sorbitol, or taurine, has a similar but lesser effect. KCl (0.3 M) causes a small increase in activity. We discuss the significance of these findings with regard to function of aldose reductase in the renal medulla and the counteracting osmolytes hypothesis.

Protein patterns, osmolytes, and aldose reductase of L-929 cells exposed to hyperosmotic media

L-929 cells acclimated to media made hyperosmotic (600 mosmol/kgH2O) by addition of NaCl, sorbitol, or mannitol show, on SDS-polyacrylamide gels, a markedly enhanced protein band at 40 kDa, most likely corresponding to the enzyme aldose reductase. The effect was not observed in cells acclimated to a medium rendered hyperosmotic by addition of proline. The major organic osmolyte accumulated is sorbitol in cells acclimated to high-sorbitol or high-NaCl medium, proline in cells acclimated to high-proline medium. Cells acclimated to any of these hyperosmotic media display unaltered Na+ levels and similarly increased K+ levels and decreased Cl-levels. These results are interpreted in terms of the mechanisms involved in aldose reductase induction and in regulation of the enzyme activity in long-term acclimation to hyperosmotic media.

Coordinate regulation of organic osmolytes in renal cells

Adaptation of cells to prolonged hypertonicity generally involves accumulation of compatible organic osmolytes. Renal medullary cells in vivo and in tissue culture accumulate several different organic osmolytes, including sorbitol, inositol, betaine, and glycerophosphocholine (GPC) in response to hypertonicity. For the total concentration of these organic osmolytes to be appropriate for the ambient tonicity, an increase in one should cause the others to fall, minimizing changes in their total concentration. The experiments presented here demonstrate this in tissue culture and investigate the mechanisms involved. Sorbitol is synthesized from glucose, catalyzed by aldose reductase. Betaine is transported into the cells. Hypertonicity increases transcription of the aldose reductase and betaine transporter genes, ultimately elevating cell sorbitol and betaine. If aldose reductase is inhibited, which prevents accumulation of sorbitol, betaine transporter gene expression increases, resulting in a higher cell betaine that compensates for the lower sorbitol. Conversely, when cell betaine is altered by changing its concentration in the medium, aldose reductase transcription changes reciprocally, resulting in compensating changes in cell sorbitol. Hypertonicity increases GPC by inhibiting GPC:choline phosphodiesterase (GPC:PDE), an enzyme that degrades GPC. When cell betaine or inositol is increased by raising its concentration in the medium, GPC:PDE activity rises, reducing cell GPC. Thus, the total of the osmolytes, rather than the level of any individual one, is maintained.

Time-dependent aspects of osmolyte changes in rat kidney, urine, blood and lens with sorbinil and galactose feeding

Sorbitol plus myo-inositol, betaine and glycerophosphorylcholine (GPC) are cellular osmolytes in the mammalian renal medulla. Galactosemia and hyperglycemia can cause excessive levels of galactitol or sorbitol in several organs via aldose reductase (AR) catalysis. AR inhibitors can reduce these polyols. To examine osmolyte responses to polyol perturbations, male Wistar rats were fed normal diet, the AR inhibitor sorbinil (at 40 mg/kg/d), 25% galactose, or a combination, for 10, 21 and 42 days. All animals at 21 days had higher apparent renal AR activity than at 10 or 42 days, possibly providing resistance to sorbinil. Sorbinil feeding alone tended to increase urinary, plasma and renal urea levels. It reduced AR activity and sorbitol contents in renal inner medulla, though less so at 21 days; other renal osmolytes, especially betaine, were elevated. Galactose feeding caused little change in renal AR activity, and resulted in high galactose and galactitol contents in renal medulla, urine, blood and lens (and higher renal Na+ contents at 10 days). Renal sorbitol, inositol and GPC decreased, while betaine contents trended higher at all times. Sorbinilgalactose feeding reduced renal AR activities and galactitol contents (again less so at 21 days), urine, blood and lens galactitol, and further reduced renal sorbitol contents. At 10 and 21 days it tended to raise renal betaine more, and restore inositol (but not GPC) contents to control levels. At 42 days it reduced renal and urinary Na+ and galactose, and decreased renal betaine to control levels. Under most conditions, total renal (non-urea) organic osmolyte contents (presumed to be mostly intracellular) and Na+ plus galactose contents (presumed mostly extracellular) changed together such that cell volumes may have been maintained. The exception was 10 days on galactose, where total osmolytes appeared too low. In galactose-fed animals, urine/plasma ratios suggest some renal galactitol efflux, and cellular galactitol probably helps maintain osmotic balance rather than cause swelling.

Long-term induction of an aldose reductase protein by basic fibroblast growth factor in rat astrocytes in vitro

Basic fibroblast growth factor (bFGF) is known to elicit various developmental-like effects on astrocytes in vitro, but these effects were studied mainly over short-term periods. In this work we asked the question whether bFGF could induce long-term effects on rat astrocytes in culture. This factor was found to induce only a transient mitogenic effect lasting less than 48 h, even when the treatment was carried on for 4 days. By contrast, it induced long-term effects on the rate of synthesis of several proteins as seen by two-dimensional polyacrylamide gel electrophoresis after labeling the cells with [35S]methionine. The most upregulated protein was extracted from preparative gels of soluble extracts of cultured bFGF-treated astrocytes and of normal brain. It was characterized by internal amino acid microsequencing. Two tryptic digest peptides had N-terminal sequences similar to rat lens aldose reductase. This protein was also expressed in oligodendroglial and neuronal cells in culture, but it was not upregulated by bFGF. Aldose reductase is thought to be involved in a minor pathway of glucose metabolism and in diabetic complications. Its long-term regulation by bFGF will possibly help in the understanding of its actual physiological role.

Purification and partial characterization of an aldo-keto reductase from Saccharomyces cerevisiae

A cytosolic aldo-keto reductase was purified from Saccharomyces cerevisiae ATCC 26602 to homogeneity by affinity chromatography, chromatofocusing, and hydroxylapatite chromatography. The relative molecular weights of the aldo-keto reductase as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and size exclusion chromatography were 36,800 and 35,000, respectively, indicating that the enzyme is monomeric. Amino acid composition and N-terminal sequence analysis revealed that the enzyme is closely related to the aldose reductases of xylose-fermenting yeasts and mammalian tissues. The enzyme was apparently immunologically unrelated to the aldose reductases of other xylose-fermenting yeasts. The aldo-keto reductase is NADPH specific and catalyzes the reduction of a variety of aldehydes. The best substrate for the enzyme is the aromatic aldehyde p-nitrobenzaldehyde (Km = 46 microM; kcat/Km = 52,100 s-1 M-1), whereas among the aldoses, DL-glyceraldehyde was the preferred substrate (Km = 1.44 mM; kcat/Km = 1,790 s-1 M-1). The enzyme failed to catalyze the reduction of menadione and p-benzoquinone, substrates for carbonyl reductase. The enzyme was inhibited only slightly by 2 mM sodium valproate and was activated by pyridoxal 5'-phosphate. The optimum pH of the enzyme is 5. These data indicate that the S. cerevisiae aldo-keto reductase is a monomeric NADPH-specific reductase with strong similarities to the aldose reductases.

Regulation of cell function by the cellular hydration state

Cellular hydration can change within minutes under the influence of hormones, nutrients, and oxidative stress. Such short-term modulation of cell volume within a narrow range acts per se as a potent signal which modifies cellular metabolism and gene expression. It appears that cell swelling and cell shrinkage lead to certain opposite patterns of cellular metabolic function. Apparently, hormones and amino acids can trigger those patterns simply by altering cell volume. Thus alterations of cellular hydration may represent another important mechanism for metabolic control and act as another second or third messenger linking cell function to hormonal and environmental alterations.

Is sorbitol dehydrogenase gene expression affected by streptozotocin-diabetes in the rat?

The polyol pathway, which comprises the enzymes aldose reductase and sorbitol dehydrogenase, is recognised to play a major role in the pathogenesis of diabetic complications. Although there has been extensive research on aldose reductase, the role of sorbitol dehydrogenase has been overlooked. This study examined the response of sorbitol dehydrogenase gene expression to streptozotocin-diabetes (STZ-diabetes) in the rat and whether these changes were reversed by insulin. STZ-diabetes increased testicular sorbitol dehydrogenase gene expression in a manner that was not reversible by insulin but had no effect on gene expression in kidney and brain. A secondary question was the relationship between sorbitol dehydrogenase and aldose reductase gene expression in STZ-diabetes. STZ-diabetes increased renal aldose reductase gene expression in a manner that was not reversible by insulin but had no effect on gene expression in the brain, testes and muscle. Thus, STZ-diabetes causes changes in sorbitol dehydrogenase gene expression which do not parallel those in aldose reductase, implying that expression of the two genes is not regulated via a common mechanism. Furthermore, changes in sorbitol dehydrogenase and aldose reductase gene expression cannot be fully explained on the basis of the osmoregulatory hypothesis, suggesting that regulation is mediated via mechanisms that are multifactorial and tissue-specific.

A qualitative and quantitative protein database approach identifies individual and groups of functionally related proteins that are differentially regulated in simian virus 40 (SV40) transformed human keratinocytes: an overview of the functional changes associated with the transformed phenotype

A qualitative and quantitative two-dimensional (2-D) gel database approach has been used to identify individual and groups of proteins that are differentially regulated in simian virus 40 (SV40) transformed human keratinocytes (K14). Five hundred and sixty [35S]methionine-labeled proteins (462 isoelectric focusing, IEF; 98 nonequilibrium pH gradient electrophoresis, NEPHGE), out of the 3038 recorded in the master keratinocyte database, were excised from dry, silver-stained gels of normal proliferating primary keratinocytes and K14 cells and the radioactivity was determined by liquid scintillation counting. Two hundred and thirty five proteins were found to be either up- (177) or down-regulated (58) in the transformed cells by 50% or more, and of these, 115 corresponded to known proteins in the keratinocyte database (J.E. Celis et al., Electrophoresis 1993, 14, 1091-1198). The lowest abundance acidic protein quantitated was present in about 60,000 molecules per cell, assuming a value of 10(8) molecules per cell for total actin. The results identified individual, and groups of functionally related proteins that are differentially regulated in K14 keratinocytes and that play a role in a variety of cellular activities that include general metabolism, the cytoskeleton, DNA replication and cell proliferation, transcription and translation, protein folding, assembly, repair and turnover, membrane traffic, signal transduction, and differentiation. In addition, the results revealed several transformation sensitive proteins of unknown identity in the database as well as known proteins of yet undefined functions. Within the latter group, members of the S100 protein family--whose genes are clustered on human chromosome 1q21--were among the highest down-regulated proteins in K14 keratinocytes. Visual inspection of films exposed for different periods of time revealed only one new protein in the transformed K14 keratinocytes and this corresponded to keratin 18, a cytokeratin expressed mainly by simple epithelia. Besides providing with the first global overview of the functional changes associated with the transformed phenotype of human keratinocytes, the data strengthened previous evidence indicating that transformation results in the abnormal expression of normal genes rather than in the expression of new ones.

Molecular basis for osmoregulation of organic osmolytes in renal medullary cells

Renal medullary cells are naturally exposed to extremely high and variable interstitial concentrations of NaCl and urea, consequent to operation of the urinary concentrating mechanism. They respond by accumulating large and variable amounts of sorbitol, glycerophosphocholine (GPC), glycine betaine (betaine), myo-inositol (inositol), and taurine both in vivo and in cell cultures. Sorbitol is synthesized from glucose, catalyzed by aldose reductase. Hypertonicity increases aldose reductase activity by raising this enzyme's transcription, mRNA level, and translation, and thereby increases production of sorbitol. GPC is synthesized from choline via phosphatidylcholine. A combination of high NaCl plus urea does not increase GPC synthesis, but does reduce its degradation by inhibiting GPC:choline phosphodiesterase. Betaine, inositol and taurine are taken up into the cells, each by a different sodium-dependent transporter. Hypertonicity increases mRNAs of all three transporters. This is due to increased transcription (at least of the inositol and betaine transporters). The eventual result is greater betaine, inositol and taurine uptake and accumulation. Osmoregulation of net sorbitol and GPC synthesis and of betaine, inositol and taurine transport is slow, requiring hours to days. However, following an acute fall in tonicity, these organic osmolytes exit from the cells within minutes, via specialized efflux mechanisms. As demonstrated by cloning efficiency studies, renal cell survival and growth following hypertonicity depend on the sum of all organic osmolytes that are accumulated; altering one experimentally changes the others to maintain a nearly constant total. Methylamine accumulation protects these cells against high urea; the methylamine that is preferentially accumulated in response to high urea is GPC.

Control of sorbitol metabolism in renal inner medulla of diabetic rats: regulation by substrate, cosubstrate and products of the aldose reductase reaction

Streptozotocin diabetes induces a 4-fold increase in the maximal velocity of inner medullary aldose reductase as determined in vitro but increases sorbitol synthesis in intact inner medullary collecting duct (IMCD) cells only 1.3-fold. In order to resolve this discrepancy we investigated the importance of intracellular factors in controlling the role of cellular sorbitol synthesis. These factors include glucose concentration, sorbitol concentration, the activity of the NADPH-regenerating pentose phosphate pathway, intracellular NADP and NADPH content, and intracellular reduced (GSH) and oxidized glutathione (GSSG). It was found that the apparent Km of cellular sorbitol production for glucose was identical in control and diabetic rats (56 +/- 18 vs. 59 +/- 14 mmol/l D-glucose), whereas Vmax increased by 31% in diabetes. In inner medullary collecting duct cells of diabetic rats containing 146 +/- 5 mumol sorbitol/g protein, sorbitol synthesis was slightly lower (-15%), compared to cells which had been sorbitol-depleted prior to the experiment (87 +/- 4 mumol sorbitol/g protein). However, no inhibitory effect of sorbitol (up to 200 mmol/l) was observed on aldose reductase activity in vitro. In diabetic rats the content of NADPH was about 32% lower than in the control rats (3.8 +/- 0.3 vs. 5.6 +/- 0.4 mumol/g protein) and the ratio of NADPH/NADP was decreased from 25.6 +/- 5.1 to 8.6 +/- 1.7. In homogenates of the inner medulla the activity of 6-phospho-gluconate dehydrogenase (EC 1.1.1.43) was identical in both experimental groups, so the pentose phosphate shunt seems to be unaltered. GSH content in diabetic rats was also diminished (4.02 +/- 0.67 mumol/g protein vs. 7.41 +/- 0.5 mumol/g protein) and the GSH/GSSG ratio fell from 92.6 to 57.4. In enzyme tests in vitro an apparent Km of 7.3 +/- 1.9 mumol/l of the aldose reductase for NADPH was found; NADP acted as competitive inhibitor with an apparent K(i) of 183 +/- 31 mumol/l. Aldose reductase activity was also found to be strongly inhibited by the SH-group reagent p-chloromercurybenzoesulfonate (apparent K(i) = 0.85 x 10(-6) mol/l). Combining the results obtained on the properties of the aldose reductase in vitro and the observation made in the intact cells, the investigators suggest that the decrease in NADPH/NADP ratio, as well as changes in the redox state in the cells of diabetic animals, can play a significant role in the control of sorbitol synthesis.

Metabolism of the 'organic osmolyte' glycerophosphorylcholine in isolated rat inner medullary collecting duct cells. II. Regulation by extracellular osmolality

In isolated inner medullary collecting duct (IMCD) cells requirements for the organic osmolyte glycerophosphorylcholine (GPC) vary with extracellular osmolality. To investigate mechanisms of osmotic adaptation GPC metabolism was studied under different osmotic conditions. In contrast to the GPC precursors choline and phosphatidylcholine (PC) cellular GPC was proportional to the osmolality. Hypotonic decrease in cellular GPC was mediated by fast initial release significantly exceeding the low hypertonic release. In long-term studies the total amount of GPC decreased significantly under hypotonic conditions but remained constant under hypertonic conditions resulting in a significant difference after 15 h. To investigate osmotic influences on GPC synthesis and GPC degradation studies with [methyl-3H]choline were performed. Pulse-chase experiments displayed no significant osmotic differences in PC synthesis or in PC degradation to GPC indicated by a similar specific activity of PC. This suggested that phospholipase A2 (PC degradation) was osmotically insensitive. A small and distinct metabolic PC pool may be responsible for high radioactive labeling of newly synthesized GPC which displayed a significantly higher specific activity under hypotonic conditions accompanied by a decrease in GPC amount. Therefore, a higher activity of glycerophosphorylcholine:choline phosphodiesterase (GPC:choline phosphodiesterase) (GPC degradation) under hypotonic conditions is proposed. Similar conclusions can be drawn from using phosphatidyl[methyl-3H]choline. As further evidence for osmotic regulation of GPC:choline phosphodiesterase the specific activity of choline displayed a significant hypotonic increase with chase time which may be equivalent to increased GPC degradation. Therefore, the in vitro experiments suggest that cellular GPC is regulated by an osmosensitive GPC:choline phosphodiesterase. Such a regulation also seems to be present during long-term in vivo experiments. No evidence was found for a genetic adaptation of GPC:choline phosphodiesterase in vivo.

Osmotically-induced nerve taurine depletion and the compatible osmolyte hypothesis in experimental diabetic neuropathy in the rat

Diabetic neuropathy results from progressive nerve fibre damage with blunted nerve regeneration and repair and may be complicated by nerve hyperexcitability resulting in pain. The naturally occurring amino acid taurine functions as an osmolyte, inhibitory neurotransmitter, and modulator of pain perception. It is also known to have neurotrophic actions. The compatible osmolyte hypothesis proposes that levels of intracellular organic osmolytes including taurine and myo-inositol, respond co-ordinately in response to changes in intracellular sorbitol or external osmolality to maintain the intracellular milieu. We hypothesize that glucose-induced sorbitol accumulation in diabetes mellitus will result in taurine depletion in peripheral nerve which may potentially impair nerve regeneration and precipitate neuronal hyperexcitability and pain. This study explored the relationships of taurine, myo-inositol and sorbitol in the rat nerve and their effects on nerve conduction velocity. Osmolyte levels and nerve conduction velocity were determined in sciatic nerve from non-diabetic and streptozotocin-induced diabetic rats, with or without dietary taurine or myo-inositol supplementation. Taurine levels decreased by 31% (p < 0.01) and myo-inositol decreased by 37% (p < 0.05) in diabetic nerve as sorbitol accumulated. Taurine supplementation of diabetic animals did not affect nerve conduction velocity but further reduced nerve myo-inositol levels. Prevention of sorbitol accumulation with the aldose reductase inhibitor sorbinil increased nerve taurine levels by 22% (p < 0.05) when compared with untreated diabetic animals. Thus, we have demonstrated an interdependence of organic osmolytes within the nerve. Abnormal accumulation of one osmolyte results in reciprocal depletion of others. Diabetic neuropathy may be an example of maladaptive osmoregulation, nerve damage and instability being aggravated by taurine depletion.

Decreased endoneurial fluid electrolytes in normal rat sciatic nerve after aldose reductase inhibition

The role of the enzyme aldose reductase in nerve homeostasis was examined by treating rats with an aldose reductase inhibitor. Female Sprague-Dawley rats were treated with Ponalrestat (25 mg/kg/day) or with excipient alone for 4 to 12 weeks before examining electrophysiologic function, endoneurial fluid electrolyte concentrations, nerve polyol levels, water content and (Na+,K+)-ATPase activity. Sorbitol, the product of glucose metabolism by aldose reductase, was detected in all nerves from control animals, whereas it was below detection limits in 7 of 11 nerves from Ponalrestat-treated rats. Ponalrestat treatment reduced endoneurial fluid sodium and chloride concentrations by 25% and 37%, respectively (both P < 0.001). No differences in nerve water content, conduction velocity, or ATPase activities were detected. These data, and previous studies demonstrating that increased flux through aldose reductase causes the accumulation of endoneurial electrolytes, suggest a role for this enzyme in modulation of the endoneurial microenvironment. However, short-term inhibition of aldose reductase does not appear to affect nerve function. Thus, our findings do not elicit concerns regarding the use of aldose reductase inhibitors in the treatment of clinical diabetic neuropathy.

Effects on rat renal osmolytes of extended treatment with an aldose reductase inhibitor

1. The mammalian renal medulla uses sorbitol, myo-inositol, betaine and glycerophosphorylcholine as intracellular osmolytes. 2. Sorbitol synthesis was inhibited by feeding male Wistar rats the aldose reductase inhibitor sorbinil at 40 mg/kg/day for 71 d, and renal inner medullas were extracted for analysis. 3. Aldose reductase activities and sorbitol contents were greatly reduced in sorbinil-treated animals, while betaine contents increased significantly (with no other osmolytes changing). 4. The betaine increase compensated for the sorbitol decrease such that the total organic osmolytes maintained the same ratio to sodium contents as controls. 5. These results are identical to the pattern previously reported for sorbinil treatment of rats for 10 d, but not for 21 d.

No correlation between glycemic control and an increase in erythrocyte aldose reductase activity in type I and type II diabetic patients

Aldose reductase, the first enzyme of the polyol pathway, has been related to the pathogenesis of diabetic complications. The regulation of the enzyme in diabetes patients, however, has not yet been clarified. We recently reported that the activity of aldose reductase was increased in erythrocytes of insulin-dependent diabetes mellitus patients but short-term hyperglycemia did not affect the enzyme activity. It is still unclear, however, whether or not the increase in the enzyme activity is caused by long-term hyperglycemia and thus would be seen equally in both type I (insulin-dependent diabetes mellitus) and type 2 (non-insulin-dependent diabetes mellitus) individuals. To further clarify these issues we measured erythrocyte aldose reductase activity in 46 type I patients and 30 type II patients who had variable glucose control and in 16 nondiabetic subjects. We compared the enzyme activity with plasma glucose levels and hemoglobin A1c levels. The results show that erythrocyte aldose reductase activity is increased in both type I and type II patients as compared with nondiabetic subjects (7.1 +/- 0.3 U/L and 6.8 +/- 0.4 U/L erythrocytes versus 5.6 +/- 0.2 U/L erythrocytes, p less than 0.001 and p less than 0.01, respectively), but there were no significant differences between the two groups of diabetic patients. The enzyme activity varied by approximately four times among the diabetic individuals but there was no correlation between the enzyme activity and plasma glucose or hemoglobin A1c levels. We conclude that the increased activity of erythrocyte aldose reductase seen in diabetes is not related to hyperglycemia.

Hypertonic stress induces alpha B-crystallin expression

Alpha B-crystallin, a major lens protein, was induced in primary cultures of dog lens epithelial cells and glomerular endothelial cells when they were grown under conditions of hypertonic stress. With Western blot analysis using a specific alpha B-crystallin antibody, we observed a significant increase in the concentration of alpha B-crystallin protein in cells grown for 4-6 days in media supplemented with 150 mM NaCl or 250 mM cellobiose. These supplements increased the osmolarity of the medium from 300 to 550-600 mosmol kg-1. Alpha B-crystallin mRNA was also increased reaching a maximum four-fold increase in lens and 16-fold increase in kidney cells within 1-2 days. These studies demonstrate a type of regulation of alpha B-crystallin expression in cells from lenticular and non-lenticular tissues.

Amino acids as volume-regulatory osmolytes in mammalian cells

1. This review summarizes current knowledge relating to the volume-regulatory and osmoprotective functions of amino acids in mammalian cells exposed to anisosmotic fluids. 2. Experiments in vivo and in vitro have established that they play a significant role in regulating brain cell volume under these conditions, and that taurine may be of particular importance in this respect. 3. Their possible role in renal medulla is discussed, and it is suggested that they may protect cells against acute (but not long-term) osmotic variation. 4. Evidence is briefly presented regarding adaptive changes in amino acid content of other cell types.

Role of organic osmolytes in adaptation of renal cells to high osmolality

Kidney cells accumulate organic osmolytes in order to protect themselves from the high concentrations of NaCl and urea in the blood and interstitial fluid of the renal medulla. The renal medullary organic osmolytes are sorbitol, inositol, betaine and GPC. The concentrations of these solutes in renal medullary NaCl and urea concentration, as summarized in Fig. 8 (the putative controlled steps are highlighted). Sorbitol accumulates by synthesis from glucose, catalyzed by aldose reductase. Hypertonicity increases the transcription of the gene that encodes this enzyme. GPC is synthesized from choline, and the amount retained apparently may be controlled by the activity of GPC diesterase, an enzyme that catabolizes GPC. Inositol and betaine are taken up from the medium by sodium-dependent transport, and this transport is increased by hypertonicity. Control of these processes is slow (hours to days), but a decrease in tonicity causes a transient, rapid efflux of the solutes, which prevents the cells from becoming overly distended. Similar strategies are used by all types of cells, including bacteria and those in plants and animals, that can adapt to hyperosmotic stress.

Importance of organic osmolytes for osmoregulation by renal medullary cells

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, glycerophosphorylcholine, 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.

Increase in aldose reductase mRNA in dog lens epithelial cells under hypertonic conditions

Aldose reductase (AR) mRNA levels increase when dog lens epithelial cells are exposed to hypertonic conditions. Hybridization of mRNA to an AR cDNA, using Northern and slot blots, showed that AR mRNA is elevated at least fourfold when primary dog lens epithelial cells are grown in media (300 mosmol kg-1) supplemented with 150 mM NaCl (600 mosmol kg-1 final). A time course showed an increase in AR mRNA of approximately twofold by 24 hr with a maximum increase of between four- and eightfold by 48 hr. AR mRNA remained elevated for the duration of the experiment, 8 days. The addition of Tolrestat, an inhibitor of aldose reductase, had no effect on the increased level of AR mRNA in these hypertonically stressed cells. Cells grown in media supplemented with 250 mM sorbitol also showed a substantial increase in AR mRNA. These data indicate, as in other cell types, the lens, a target tissue of diabetes, responds to hypersomotic stress with an induction of AR expression and suggests that AR may play a role in intracellular osmotic regulation.

Induction of aldose reductase expression in rat kidney mesangial cells and Chinese hamster ovary cells under hypertonic conditions

Rat kidney cortex mesangial cells (MES) and Chinese hamster ovary cells (CHO) responded to hypertonicity (600 mosmol/kg) in culture by accumulating sorbitol. The accumulation of sorbitol was due to increased aldose reductase (AR) activity, apparently brought about by increased levels of AR mRNA and protein. The levels of AR mRNA increased approximately 60-fold in MES cells and 30-fold in CHO cells by 24 h in culture media (300 mosmol/kg supplemented with 150 mM NaCl, 600 mosmol/kg total). AR activity also markedly increased (14- to 16-fold above control), but MES took 4 days and CHO 6 days to reach this maximum. Other osmolytes, raffinose and sorbitol (at concentrations of 250 to 300 mM) elicited the same response as that of 150 mM NaCl. These data show that AR expression is induced in MES and CHO cells under hypertonic conditions. Of special interest is the induction of large amounts of AR in rat kidney cortex mesangial cells, a target tissue of diabetes and a site where excessive accumulation of sorbitol is suspected to be a critical factor in diabetic nephropathy.

Induction of the enzyme aldose reductase in a lens epithelial cell line from a transgenic mouse

A lens epithelial cell line established from a transgenic mouse synthesizes high levels of the enzyme aldose reductase which converts sugars to polyols. This enzyme has been implicated in the formation of sugar cataracts in animals and with diabetic complications in man. The mouse aldose reductase has been characterized and by sodium dodecyl sulfate-polyacrylamide gel electrophoresis has an apparent molecular mass of 38,000, similar to the enzyme in rat and man. The cellular enzyme is inhibited by two aldose reductase inhibitors: Sorbinil (IC50 = 1.8 X 10(-7) M) and Alcon 1576 (IC50 = 7.8 X 10(-8) M). The amount and the specific activity of the aldose reductase can be further increased in the cells by raising the osmolarity of the medium to 500 mOSM. Although the amount of aldose reductase is increased approximately sevenfold under these conditions, alpha-crystallin, one of the main lens specific proteins, remained at about the same concentration. No detectable increase in sorbitol was found within the cells, in contrast to published reports on renal cells in which this polyol increases under similar hyperosmotic conditions; however, in the lens cells there was a five-fold increase in the inositol content, suggesting that this polyol rather than sorbitol may be used to compensate for some of the changes in the osmolarity. The induction of the enzyme aldose reductase without the apparent accumulation of its product suggests a complex mechanism for osmoregulation in the lens cells.

Localization of aldose and aldehyde reductase in the kidney

The distribution of NADPH-dependent reductase activity in the rat cortex, outer medulla and inner medulla was investigated through biochemical and histochemical methods. Biochemical studies revealed reductase activity to be present in all three regions of the kidney with the highest specific activity observed in the inner medulla, followed by the cortex and the outer medulla. Activity in all three regions was inhibited by the aldose reductase inhibitors sorbinil, tolrestat and 7-hydroxychromone-2-carboxylic acid. Based on substrate utilization and response to sulfate on the inhibitors, the inner medulla contains primarily aldose reductase (EC 1.1.1.21) while the cortex contains primarily aldehyde reductase (EC 1.1.1.2). The outer medulla contains a mixture of both enzymes. This distribution was confirmed by a radioimmunoassay for aldose reductase. Immunohistochemical investigations of the rat kidney with antibodies against rat lens aldose reductase and rat kidney aldehyde reductase revealed a similar distribution of these enzymes. Aldehyde reductase was immunohistochemically detected only in the cortex where it was localized in the proximal convoluted tubules. Immunoreactive aldose reductase was detected in Henle's loop at both the inner stripe of the outer medulla and in the inner medulla, and in the collecting tubules and the epithelial cell lining the pelvis of the inner medulla near the papilla. No specific immunohistochemical staining for aldose reductase was observed in the cortex. A similar immunohistochemical distribution of aldose reductase was also observed in the human kidney with antibodies against human placental aldose reductase.

Osmoregulation of glycerophosphorylcholine content of mammalian renal cells

Renal medullary cells contain high concentrations of "compatible" organic osmolytes such as glycerophosphorylcholine (GPC), betaine, myo-inositol, and sorbitol. The organic osmolytes occur as an osmoregulatory response to the high and variable interstitial NaCl concentration that is part of the urinary concentrating mechanism. Madin-Darby canine kidney (MDCK) cells in culture were previously shown to accumulate GPC in response to increased osmolality. We demonstrate here that this accumulation occurs in response to elevated extracellular urea concentration as well as to elevated NaCl. GPC does not accumulate unless either choline or GPC is present in the medium. Thus the accumulation results from osmoregulated synthesis of GPC from choline and, possibly, also osmoregulated uptake of extracellular GPC. When the osmolality is decreased from high to normal levels, cell GPC concentration decreases greatly over 24 h, accompanied by efflux of GPC and choline into the medium.

Anisosmotic cell volume regulation: a comparative view

A variety of organisms and cell types spanning the five taxonomic kingdoms are exposed, either naturally or through experimental means, to osmotic stresses. A common physiological response to these challenges is maintenance of cell volume through changes in the concentration of intracellular inorganic and organic solutes, collectively termed osmolytes. Research on the mechanisms by which the concentration of these solutes is regulated has proceeded along several experimental lines. Extensive studies on osmotically activated ion transport pathways have been carried out in vertebrate cells and tissues. Much of our knowledge on organic osmolytes has come from investigations on invertebrates, bacteria, and protists. The relative simplicity of bacterial genetics has provided a powerful and elegant tool to explore the modifications of gene expression during volume regulation. An implication of this diverse experimental approach is that phylogenetically divergent organisms employ uniquely adapted mechanisms of cell volume regulation. Given the probability that changes in extracellular osmolality were physiological stresses faced by the earliest organisms, it is more likely that cell volume regulation proceeds by highly conserved physiological processes. We review volume regulation from a comparative perspective, drawing examples from all five taxonomic kingdoms. Specifically, we discuss the role of inorganic and organic solutes in volume maintenance and the mechanisms by which the concentrations of these osmolytes are regulated. In addition, the processes that may transduce volume perturbations into regulatory responses, such as stretch activation of ion channels, intracellular signaling, and genomic regulation, are discussed. Throughout this review we emphasize areas we feel are important for future research.

Intracellular sorbitol content in isolated rat inner medullary collecting duct cells. Regulation by extracellular osmolarity

In order to study the mechanisms involved in the regulation of renal inner medullary sorbitol content, collecting duct cells were isolated from rat inner medulla and the effect of extracellular osmolarity on sorbitol synthesis and sorbitol content was investigated. Cells isolated at 300 mosmol/l and incubated up to 24 h as primary cultures in 300 mosmol/l media or in media made 600 mosmol/l by the addition of 150 mM NaCl showed no difference in total synthesis. Intracellular sorbitol content was, however, 2.3-fold higher in the cells kept in the higher osmotic medium. Cells isolated at 600 mosmol/l released sorbitol about 8 times faster when transferred into hypoosmotic medium (300 mosmol/l) than when transferred into isoosmotic (600 mosmol/l) media. Cells exposed to hyperosmotic media (900 mosmol/l with NaCl) maintained a higher intracellular sorbitol content than cells incubated in isoosmotic media. Changes of intracellular sorbitol content could not be attributed entirely to cell lysis--as demonstrated by determination of cellular content of lactate and lactate dehydrogenase. The alteration in sorbitol membrane permeability was reversible and was only observed when poorly permeable solutes (such as NaCl and sucrose) were used for the experiments, changes in urea elicited no effect. It is proposed that rapid changes in membrane permeability to sorbitol play an important role in the adjustment of intracellular sorbitol concentration in inner medullary collecting duct cells to changes in extracellular osmolarity.