In vivo osmoregulation of Na/myo-inositol cotransporter mRNA in rat kidney medulla

AMP-Activated Protein Kinase (AMPK)-Dependent Regulation of Renal Transport

AMP-activated kinase (AMPK) is a serine/threonine kinase that is expressed in most cells and activated by a high cellular AMP/ATP ratio (indicating energy deficiency) or by Ca²⁺. In general, AMPK turns on energy-generating pathways (e.g., glucose uptake, glycolysis, fatty acid oxidation) and stops energy-consuming processes (e.g., lipogenesis, glycogenesis), thereby helping cells survive low energy states. The functional element of the kidney, the nephron, consists of the glomerulus, where the primary urine is filtered, and the proximal tubule, Henle's loop, the distal tubule, and the collecting duct. In the tubular system of the kidney, the composition of primary urine is modified by the reabsorption and secretion of ions and molecules to yield final excreted urine. The underlying membrane transport processes are mainly energy-consuming (active transport) and in some cases passive. Since active transport accounts for a large part of the cell's ATP demands, it is an important target for AMPK. Here, we review the AMPK-dependent regulation of membrane transport along nephron segments and discuss physiological and pathophysiological implications.

Colon Ascendens Stent Peritonitis (CASP) Induces Excessive Inflammation and Systemic Metabolic Dysfunction in a Septic Rat Model

The colon ascendens stent peritonitis (CASP) surgery induces a leakage of gut contents, causing polymicrobial sepsis related to post-operative multiple organ failure and death in surgical patient. To evaluate the effects of CASP on multiple organs, we analyzed the systemic metabolic consequences in liver, kidney, lung, and heart of rats after CASP by employing a combination of metabolomics, clinical chemistry, and biological assays. We found that CASP surgery after 18 h resulted in striking elevations of lipid, amino acids, acetate, choline, PC, and GPC in rat liver together with significant depletion of glucose and glycogen. Marked elevations of organic acids including lactate, acetate, and creatine and amino acids accompanied by decline of glucose, betaine, TMAO, choline metabolites (PC and GPC) nucleotides, and a range of organic osmolytes such as myo-inositol are observed in the kidney of 18 h post-operative rat. Furthermore, 18 h post-operative rats exhibited accumulations of lipid, amino acids, and depletions of taurine, myo-inositol, choline, PC, and GPC and some nucleotides including uridine, inosine, and adenosine in the lung. In addition, significant elevations of some amino acids, uracil, betaine, and choline metabolites, together with depletion of inosine-5'-monophosphate, were only observed in the heart of 18 h post-operative rats. These results provide new insights into pathological consequences of CASP surgery, which are important for timely prognosis of sepsis.

Identification and Functional Implications of Sodium/Myo-Inositol Cotransporter 1 in Pancreatic β-Cells and Type 2 Diabetes

Myo-inositol (MI), the precursor of the second messenger phosphoinositide (PI), mediates multiple cellular events. Rat islets exhibit active transport of MI, although the mechanism involved remains elusive. Here, we report, for the first time, the expression of sodium/myo-inositol cotransporter 1 (SMIT1) in rat islets and, specifically, β-cells. Genetic or pharmacological inhibition of SMIT1 impaired glucose-stimulated insulin secretion by INS-1E cells, probably via downregulation of PI signaling. In addition, SMIT1 expression in INS-1E cells and isolated islets was augmented by acute high-glucose exposure and reduced in chronic hyperglycemia conditions. In corroboration, chronic MI treatment improved the disease phenotypes of diabetic rats and islets. On the basis of our results, we postulate that the MI transporter SMIT1 is required to maintain a stable PI pool in β-cells in order that PI remains available despite its rapid turnover.

Osmoadaptation of Mammalian cells - an orchestrated network of protective genes

In mammals, the cells of the renal medulla are physiologically exposed to interstitial osmolalities several-fold higher that found in any other tissue. Nevertheless, these cells not only have the ability to survive in this harsh environment, but also to function normally, which is critical for maintenance of systemic electrolyte and fluid homeostasis. Over the last two decades, a substantial body of evidence has accumulated, indicating that sequential and well orchestrated genomic responses are required to provide tolerance to osmotic stress. This includes the enhanced expression and action of immediate-early genes, growth arrest and DNA damage inducible genes (GADDs), genes involved in cell cycle control and apoptosis, heat shock proteins, and ultimately that of genes involved in the intracellular accumulation of nonperturbing organic osmolytes. The present review summarizes the sequence of genomic responses conferring resistance against osmotic stress. In addition, the regulatory mechanisms mediating the coordinated genomic response to osmotic stress will be highlighted.

The Caenorhabditis elegans mucin-like protein OSM-8 negatively regulates osmosensitive physiology via the transmembrane protein PTR-23

The molecular mechanisms of animal cell osmoregulation are poorly understood. Genetic studies of osmoregulation in yeast have identified mucin-like proteins as critical regulators of osmosensitive signaling and gene expression. Whether mucins play similar roles in higher organisms is not known. Here, we show that mutations in the Caenorhabditis elegans mucin-like gene osm-8 specifically disrupt osmoregulatory physiological processes. In osm-8 mutants, normal physiological responses to hypertonic stress, such as the accumulation of organic osmolytes and activation of osmoresponsive gene expression, are constitutively activated. As a result, osm-8 mutants exhibit resistance to normally lethal levels of hypertonic stress and have an osmotic stress resistance (Osr) phenotype. To identify genes required for Osm-8 phenotypes, we performed a genome-wide RNAi osm-8 suppressor screen. After screening ∼18,000 gene knockdowns, we identified 27 suppressors that specifically affect the constitutive osmosensitive gene expression and Osr phenotypes of osm-8 mutants. We found that one suppressor, the transmembrane protein PTR-23, is co-expressed with osm-8 in the hypodermis and strongly suppresses several Osm-8 phenotypes, including the transcriptional activation of many osmosensitive mRNAs, constitutive glycerol accumulation, and osmotic stress resistance. Our studies are the first to show that an extracellular mucin-like protein plays an important role in animal osmoregulation in a manner that requires the activity of a novel transmembrane protein. Given that mucins and transmembrane proteins play similar roles in yeast osmoregulation, our findings suggest a possible evolutionarily conserved role for the mucin-plasma membrane interface in eukaryotic osmoregulation.

The ability to sense and respond to changes in cell volume is a process termed osmoregulation and is an essential prerequisite for cellular life. While the molecular details of this physiological process are well described in unicellular organisms such as yeast and bacteria, the mechanisms that govern osmoregulation in animals are poorly understood. Using a genetic approach in the nematode C. elegans, we identified the mucin-like protein OSM-8 as a critical regulator of osmoregulation. Disruption of the osm-8 gene results in the activation of physiological responses that are normally only activated in response to hyperosmotic stress, suggesting that osm-8 is a negative regulator of C. elegans osmoregulatory physiology. Through a genome-wide RNAi suppressor screen, we also identified a transmembrane protein, PTR-23, that is required for osm-8 mutants to activate osmoregulatory physiological responses. Together with previous findings from yeast, our data define an important and possibly evolutionarily conserved role for the plasma membrane-mucin matrix interface in eukaryotic osmoregulation. Our findings also illustrate the value of studying cell physiological processes such as osmoregulation in a live animal model, in which complex and dynamic extracellular matrix structures are preserved.

A role for inositol monophosphatase 1 (IMPA1) in salinity adaptation in the euryhaline eel (Anguilla anguilla)

This study investigated the expression and tissue distribution of inositol monophosphatase (IMPA1) and characterized its role in salinity adaptation in the eel. The coding sequence of eel IMPA1 was determined and confirmed to be orthologous to the mammalian gene/enzyme by phylogenetic analysis and structural modeling. Quantitative real-time PCR and Western blot techniques indicated up to 17-fold increases in mRNA expression and 2-fold increases in protein abundance in major osmoregulatory tissues following transfer of fish to seawater (SW). This was accompanied by up to 5-fold increases in enzyme activity, and 1.8- and 3-fold increases in inositol contents within the gill and kidney, respectively. Immunohistological studies revealed that IMPA1 protein expression predominated in SW-acclimated fish within basal epithelial/epidermal layers of the gill, esophagus, intestine, skin, and fins. SW transfer also induced a 10-fold increase in inositol content in the fin. IMPA1 immunoreactivity was also identified in chondrocytes within the cartilagenous matrix of the gills and fins, as well as in clusters of interstitial cells surrounding the kidney tubules. The observed increases in expression of IMPA1 highlight a protective role for inositol within various eel tissues following SW acclimation. This constitutes an adaptive mechanism in teleost fish naturally exposed to hypertonic environments.

Tonicity-dependent induction of Sgk1 expression has a potential role in dehydration-induced natriuresis in rodents

In various mammalian species, including humans, water restriction leads to an acute increase in urinary sodium excretion. This process, known as dehydration natriuresis, helps prevent further accentuation of hypernatremia and the accompanying rise in extracellular tonicity. Serum- and glucocorticoid-inducible kinase (Sgk1), which is expressed in the renal medulla, is regulated by extracellular tonicity. However, the mechanism of its regulation and the physiological role of hypertonicity-induced SGK1 gene expression remain unclear. Here, we identified a tonicity-responsive enhancer (TonE) upstream of the rat Sgk1 transcriptional start site. The transcription factor NFAT5 associated with TonE in a tonicity-dependent fashion in cultured rat renal medullary cells, and selective blockade of NFAT5 activity resulted in suppression of the osmotic induction of the Sgk1 promoter. In vivo, water restriction of rats or mice led to increased urine osmolality, increased Sgk1 expression, increased expression of the type A natriuretic peptide receptor (NPR-A), and dehydration natriuresis. In cultured rat renal medullary cells, siRNA-mediated Sgk1 knockdown blocked the osmotic induction of natriuretic peptide receptor 1 (Npr1) gene expression. Furthermore, Npr1-/- mice were resistant to dehydration natriuresis, which suggests that Sgk1-dependent activation of the NPR-A pathway may contribute to this response. Collectively, these findings define a specific mechanistic pathway for the osmotic regulation of Sgk1 gene expression and suggest that Sgk1 may play an important role in promoting the physiological response of the kidney to elevations in extracellular tonicity.

Genome-wide RNAi screening identifies protein damage as a regulator of osmoprotective gene expression

The detection, stabilization, and repair of stress-induced damage are essential requirements for cellular life. All cells respond to osmotic stress-induced water loss with increased expression of genes that mediate accumulation of organic osmolytes, solutes that function as chemical chaperones and restore osmotic homeostasis. The signals and signaling mechanisms that regulate osmoprotective gene expression in animal cells are poorly understood. Here, we show that gpdh-1 and gpdh-2, genes that mediate the accumulation of the organic osmolyte glycerol, are essential for survival of the nematode Caenorhabditis elegans during osmotic stress. Expression of GFP driven by the gpdh-1 promoter (P(gpdh-1)::GFP) is detected only during hypertonic stress but is not induced by other stressors. Using P(gpdh-1)::GFP expression as a phenotype, we screened approximately 16,000 genes by RNAi feeding and identified 122 that cause constitutive activation of gpdh-1 expression and glycerol accumulation. Many of these genes function to regulate protein translation and cotranslational protein folding and to target and degrade denatured proteins, suggesting that the accumulation of misfolded proteins functions as a signal to activate osmoprotective gene expression and organic osmolyte accumulation in animal cells. Consistent with this hypothesis, 73% of these protein-homeostasis genes have been shown to slow age-dependent protein aggregation in C. elegans. Because diverse environmental stressors and numerous disease states result in protein misfolding, mechanisms must exist that discriminate between osmotically induced and other forms of stress-induced protein damage. Our findings provide a foundation for understanding how these damage-selectivity mechanisms function.

Survival in Hostile Environments: Strategies of Renal Medullary Cells

Cells in the renal medulla exist in a hostile milieu characterized by wide variations in extracellular solute concentrations, low oxygen tensions, and abundant reactive oxygen species. This article reviews the strategies adopted by these cells to allow them to survive and fulfill their functions under these extreme conditions.

Novel expression of sodium/myo-inositol co-transporter in podocytes in puromycin aminonucleoside nephrosis

BACKGROUND

How podocytes respond to injury is poorly understood, although podocyte injury in the glomerulus has been proposed as the crucial mechanism in the pathogenesis of proteinuria and focal segmental glomerulosclerosis. An increase in sodium/myo-inositol co-transporter (SMIT) transcripts, an osmoprotective gene, has been demonstrated in a variety of brain injury models. In the present study, we investigated SMIT expression in podocytes in experimental nephrosis.

METHODS

Two types of nephrosis were induced in rats: puromycin aminonucleoside (PAN) nephrosis and monoclonal antibody (mAb) 5-1-6 nephropathy. Podocyte injury was morphologically distinct in the former type of nephrosis and limited to a minimum in the latter. SMIT expression in isolated glomeruli was estimated by ribonuclease protection assay. Localization of SMIT-expressing cells in glomeruli was examined by in situ hybridization.

RESULTS

SMIT transcripts in glomeruli increased conspicuously in the nephrotic stage of PAN nephrosis, whereas the transcripts in cortices and medullae did not show significant changes. In situ hybridization revealed that podocytes were predominant cells expressing SMIT in the glomerulus. Significant increase of SMIT mRNA in the glomeruli was detected before the onset of massive proteinuria. In contrast, up-regulation of SMIT expression was not observed in mAb 5-1-6 nephropathy, whose urinary protein levels were comparable with those in the nephrotic stage of PAN nephrosis.

CONCLUSIONS

These findings suggest that SMIT expression in podocytes is not provoked by an effect of massive proteinuria but by extensive cellular injury.

pH dependence of Na+/myo-inositol cotransporters in rat thick limb cells

BACKGROUND

To balance medullary interstitium hypertonicity generated by transepithelial NaCl absorption, medullary thick ascending limb (MTAL) cells accumulate myo-inositol (MI). Expression of Na+-MI cotransporter (SMIT) mRNA in TAL is correlated with the NaCl absorption rate. Our present study aimed to determine the plasma membrane location and functional properties of the Na+-MI cotransporter in MTAL cells.

METHODS

Preparation of basolateral (BLMV) and luminal (LMV) membrane vesicles were simultaneously isolated from purified rat MTAL suspension, and uptake of [3H]myo-inositol ([3H]MI) was used to assess Na+-MI cotransport activity.

RESULTS

In the presence of an inside-negative membrane potential, imposing an inwardly-directed Na+-gradient versus tetramethylammonium (TMA) stimulated the initial [3H]MI uptake in BLMV and LMV. Phlorizin inhibited Na+ gradient-dependent initial [3H]MI uptake in both preparations, with IC50 values of 565 and 29 micromol/L in BLMV and LMV, respectively. 2-0,C-methylene myo-inositol (MMI), a competitive inhibitor of MI transport, only inhibited the BLMV Na+-MI cotransporter. Phlorizin-sensitive Na+ gradient-dependent initial [3H]MI uptake showed Michaelis-Menten kinetics in both preparations, with similar Vmax but different Km values of 51 and 107 micromol/L in BLMV and LMV, respectively. Finally, BLMV but not LMV Na+-MI cotransporter exhibited a marked pH dependence with sigmoidal patterns of activation, as intravesicular pH (pHi) was decreased from 8.0 to 6.0 at extravesicular pH (pHe) 8.0, and as pHe was increased from 6.0 to 8.0 at pHi 6.0. Maximal activation was observed at pHi 6.5 and pHe 7.5.

CONCLUSIONS

In rat MTAL cells, Na+-MI cotransporter activity is present in both BLM and LM, and has markedly different functional properties, indicating the presence of distinct transporters. Basolateral Na+-MI cotransporter activity is maximal at physiological pH values of MTAL cells and interstitium, and a powerful modulation of the transporter activity may be exerted by pHe and pHi.

Adaptation of kidney medulla to hypertonicity: role of the transcription factor TonEBP

The osmolality of the mammalian kidney medulla is very high. The high osmolality provides the driving force for water reabsorption and urinary concentration, key functions of the kidney for maintaining proper body fluid volume and blood pressure. Salt and urea are the major solutes in the renal medullary interstitium. Unfortunately, high salt (hypertonicity) causes DNA damage and cell death. In response, the renal medullary cells adapt to the hypertonicity by accumulating compatible osmolytes. A regulatory protein, tonicity-responsive enhancer binding protein (TonEBP), plays a central role in the accumulation of these compatible osmolytes by stimulating genes whose products either actively transport or synthesize the appropriate osmolytes. TonEBP is active under isotonic conditions. It responds to both an increase and a decrease in ambient tonicity, in opposite directions, which involves changes in its abundance and nucleocytoplasmic distribution. In the kidney medulla, however, nucleocytoplasmic distribution is the major site of control, under normal conditions of diuresis and antidiuresis.

Osmoregulation of endothelial nitric-oxide synthase gene expression in inner medullary collecting duct cells. Role in activation of the type A natriuretic peptide receptor

Previously, we showed that increased extracellular tonicity promotes increased type A natriuretic peptide receptor (NPR-A) expression through a p38 MAPKbeta pathway in inner medullary collecting duct cells. The endothelial and inducible nitric-oxide synthase (eNOS and iNOS respectively) genes are also expressed in this nephron segment and are thought to play a role in regulating urinary sodium concentration. We sought to determine whether changes in tonicity might regulate NOS gene expression, and if so, whether these latter changes might be linked mechanistically to the increase in NPR-A gene expression. Increased extracellular tonicity effected a time-dependent reduction in eNOS and iNOS protein levels, eNOS mRNA levels, and eNOS gene promoter activity over the first 8 h of the incubation. Although levels of the eNOS mRNA and promoter activity had returned to normal after 24 h, eNOS protein levels remained low at 24-36 h, and recovery was not complete even at 48 h. The decrease in eNOS expression was signaled in large part through a p38 MAPK-dependent mechanism. Reduction in eNOS expression together with the concomitant decline in intracellular cyclic GMP levels appears to account for a significant portion of the p38 MAPK-dependent osmotic stimulation of NPR-A gene expression noted previously. Collectively, these findings support the existence of a complex regulatory circuitry in the cells of the inner medullary collecting duct linking two independent cyclic GMP-generating signal transduction systems involved in regulation of urinary sodium concentration.

Osmoregulation of natriuretic peptide receptor signaling in inner medullary collecting duct. A requirement for p38 MAPK

In the inner medullary collecting duct of the terminal nephron, the type A natriuretic peptide receptor (NPR-A) plays a major role in determining urinary sodium content. This nephron segment, by virtue of its medullary location, is subject to very high levels of extracellular tonicity. We have examined the ability of medium tonicity to regulate the activity and expression of this receptor in cultured rat inner medullary collecting duct cells. We found that NaCl (75 mm) and sucrose (150 mm), but not urea (150 mm), increased natriuretic peptide receptor activity, gene expression, and promoter activity. The osmotic stimulus also activated extracellular signal-regulated kinase (ERK), c-Jun NH(2)-terminal kinase (JNK), and p38 mitogen-activated protein kinase (p38 MAPK). In the latter instance the beta isoform was selectively activated. Inhibition of p38 MAPK with SB203580 blocked the osmotic induction of receptor activity and expression, as well as receptor gene promoter activity, whereas inhibition of ERK with PD98059 had no effect. Cotransfection of p38 beta MAPK together with the receptor gene promoter resulted in amplification of the osmotic stimulation of the latter, whereas cotransfection of dominant negative MKK6, but not dominant-negative MEK, completely blocked the osmotic induction of receptor promoter activity. Collectively, the data indicate that extracellular osmolality stimulates receptor activity and receptor gene expression through a specific p38 beta-dependent mechanism, raising the possibility that changes in medullary tonicity could play an important role in the regulation of renal sodium handling in the terminal nephron.

Hydration status affects nuclear distribution of transcription factor tonicity responsive enhancer binding protein in rat kidney

Tonicity responsive enhancer binding protein (TonEBP) is the transcription factor that regulates tonicity responsive expression of proteins that catalyze cellular accumulation of compatible osmolytes. In cultured MDCK cells, hypertonicity stimulates the activity of TonEBP via a combination of increased protein abundance and increased nuclear localization. For investigating regulation of TonEBP in the kidney, rats were subjected to water loading or dehydration. Water loading lowered urine osmolality and mRNA expression of sodium/myo-inositol cotransporter (SMIT), a target gene of TonEBP, in the renal medulla; dehydration doubled the urine osmolality and increased SMIT mRNA expression. In contrast, overall abundance of TonEBP and its mRNA measured by immunoblot and ribonuclease protection assay, respectively, was not affected. Immunohistochemical analysis, however, revealed that nuclear distribution of TonEBP is generally increased throughout the medulla in dehydrated animals compared with water loaded animals. Increased nuclear localization was particularly dramatic in thin limbs. Notable exceptions were the middle to terminal portions of the inner medullary collecting ducts and blood vessels, where a change in TonEBP distribution was not evident. Immunohistochemical detection of SMIT mRNA revealed that the changes in nuclear distribution of TonEBP correlate with expression of SMIT. It is concluded that under physiologic conditions, nucleocytoplasmic distribution is the dominant mode of regulation of TonEBP in the renal medulla.

Diffuse brain injury induces local expression of Na+/myo-inositol cotransporter in the rat brain

We studied expression of an osmoprotective gene, sodium/myo-inositol cotransporter (SMIT) in Marmarou's animal model for human diffuse brain injury by in situ hybridization and immunohistochemistry. In rats with diffuse brain injury, transient upregulation of SMIT mRNA was exclusively observed in the lateral area of pyramidal tract in lower brainstem. The expression was induced at 1 h after injury, peaked at 24 h, and returned to almost control levels at 48 h. Upregulated expression was found mainly in small glia-like cells. By immunohistochemistry using antibodies to phosphorylated mitogen-activated protein (MAP) kinases, inductions of phosphorylated p44/42 MAP kinase were also observed after diffuse brain injury. Interestingly, the distribution patterns of induced phosphorylated p44/42 MAP kinase were completely coincident with those of upregulated SMIT mRNA after diffuse brain injury. These results suggest that diffuse brain injury induces local expression of SMIT by activation of p44/42 MAP kinase cascade. The confined SMIT induction may reflect regional differences of damage and/or cellular differences in sensitivity to neuropathological stresses caused by this injury.

Kainic acid-induced seizure upregulates Na(+)/myo-inositol cotransporter mRNA in rat brain

A major organic osmolyte, myo-inositol protects cells from perturbing effects of high intracellular concentrations of electrolytes. Myo-inositol is accumulated into cells through Na(+)/myo-inositol cotransporter (SMIT). In order to investigate the regulation of SMIT in generalized seizure, we employed Northern blot analysis and in situ hybridization to study the changes in SMIT mRNA expression in kainic acid-injected rats. Northern blot analysis demonstrated that SMIT mRNA began to increase in the brain 2 h after onset of seizure, and peaked at 12 h. In situ hybridization revealed rapid increase of SMIT mRNA (2 h of seizure) in the CA3 hippocampal pyramidal cells and in the dentate granular cells. Then, at 4-6 h SMIT mRNA expression was observed in the other limbic structure such as amygdala and piriform cortex. Finally, in neocortex and in CA1 pyramidal cells, SMIT mRNA was slowly increased and peaked at 12 h. Microautoradiogram demonstrated that cells expressed SMIT mRNA were mainly neurons. These results suggest that SMIT mRNA is upregulated by kainic acid-induced seizure primarily in structures involved in seizure activity.

High water intake ameliorates tubulointerstitial injury in rats with subtotal nephrectomy: possible role of TGF-beta

BACKGROUND

It has been shown that tubulointerstitial injury correlates well with a decline of renal function. In this study, we investigated the effect of high water intake (HWI) on functional and structural parameters in rats with subtotal nephrectomy.

METHODS

Two weeks after the ablative procedure, rats were divided into two groups. One group received the treatment with HWI (3% sucrose added to drinking water) for eight weeks. Functional parameters were compared with sham-operated control (CONT) or nephrectomized rats without treatment (NX). Remnant kidneys were then assessed histologically for evidence of interstitial fibrosis and glomerulosclerosis.

RESULTS

Creatinine clearance was significantly improved in HWI rats compared with NX rats. Simultaneously, urinary protein was also significantly reduced in HWI rats. HWI predominantly ameliorated interstitial lesions and, to a lesser extent, glomerular lesions. Northern blot analysis demonstrated that transforming growth factor-beta (TGF-beta) mRNA expression was significantly suppressed in HWI rats. In situ hybridization revealed that HWI suppressed TGF-beta mRNA expression mainly in the outer medulla. Fibronectin mRNA was also reduced by the HWI treatment. The changes in TGF-beta and fibronectin mRNA were in parallel with Na+/myo-inositol cotransporter (SMIT) mRNA, which is regulated by extracellular osmolarity. Immunohistochemistry demonstrated that protein expression of TGF-beta and fibronectin coincided with the mRNA expression.

CONCLUSION

These results suggest that HWI reduces TGF-beta mRNA expression in medullary interstitium and ameliorates tubulointerstitial injury in rats with reduced renal mass.

Neuroprotective role of Na+/myo-inositol cotransporter against veratridine cytotoxicity

Na+/myo-inositol cotransporter has been shown to protect cells from the perturbing effects of hypertonic stress by the accumulation of myo-inositol. Here we report a regulatory mechanism for the cotransporter. Induction of myo-inositol cotransporter mRNA was observed after exposure to veratridine, a voltage-gated sodium channel opener. The veratridine-elicited induction was inhibited when Na+ was eliminated from the bath, although calcium chelation failed to modify the gene expression. Veratridine evoked an accumulation of Na+ in the cells, which paralleled the abundance of the mRNA. These results strongly suggested that an increase in Na+ influx due to sodium channel opening affected transcription of the cotransporter gene. Activity of the myo-inositol cotransporter was also up-regulated after veratridine exposure. To clarify the possible roles of myoinositol accumulation under veratridine exposure, we next examined the neurotoxic effects of veratridine when myo-inositol uptake was blocked. Neither 30 microM veratridine nor 500 microM 2-O,C-methylene myo-inositol, a competitive inhibitor of myo-inositol, elicited apparent cytotoxicity. However, a combination of these agents markedly increased cytotoxicity in culture, suggesting that an adequate amount of myo-inositol was necessary when the cells were stimulated with veratridine.

Expression of the Na+/myo-inositol cotransporter in the juxtaglomerular region

Myo-inositol is a major compatible osmolyte in the renal medulla and is accumulated in cells under hypertonic conditions by uptake via a Na+/myo-inositol cotransporter (SMIT). SMIT is regulated by extracellular osmolarity at the transcription level. We investigated localization of SMIT in rat kidney by immunohistochemical staining using an anti-SMIT-antibody raised against a synthetic peptide corresponding to part of SMIT and by in situ hybridization. SMIT protein localized predominantly to the basolateral membranes of cells of the thick ascending limb of Henle (TAL) and inner medullary collecting duct (IMCD). Macula densa (MD) cells, identified as the Tamm-Horsfall-protein (THP)-unreactive cells surrounded by THP-reactive TAL cells, also stained for anti-SMIT. In situ hybridization yielded the intense SMIT signals in the TAL and IMCD and also in the juxtaglomerular (JG) region. Prior loading of the animal with a high concentration of NaCl rapidly induced SMIT mRNA; furosemide down-regulated it. The high level of SMIT expression suggests that MD cells are exposed to hypertonicity at the basolateral surface. Because SMIT expression seemed to be proportional to the magnitude of NaCl reabsorption, it may be a good marker for examination of the tubuloglomerular feedback mechanism in vivo.

Transcription of the sodium/myo-inositol cotransporter gene is regulated by multiple tonicity-responsive enhancers spread over 50 kilobase pairs in the 5'-flanking region

The sodium/myo-inositol cotransporter is a plasma membrane protein responsible for concentrative cellular accumulation of myo-inositol in a variety of tissues. When cells in kidney and brain are exposed to a hyperosmolar salt condition (hypertonicity) due to the operation of urinary concentration mechanism and pathological conditions, respectively, they survive the stress of hypertonicity by raising the cellular concentration of myo-inositol. Transcription of the sodium/myo-inositol cotransporter gene is markedly stimulated in response to hypertonicity, leading to an increase in the activity of the cotransporter, which in turn drives the osmoprotective accumulation of myo-inositol. To understand the molecular mechanisms by which hypertonicity stimulates transcription, we analyzed the 5'-flanking region of the cotransporter gene for cis-acting regulatory sequences. We identified five tonicity-responsive enhancers that are scattered over 50 kilobase pairs. All the enhancers are variations of the same type of enhancer interacting with the transcription factor named tonicity-responsive enhancer binding protein. In vivo methylation experiments demonstrated that exposure of cells to hypertonicity increases the binding of tonicity-responsive enhancer binding protein to the enhancer sites, indicating that all of these enhancers are involved in the transcriptional stimulation. We conclude that the sodium/myo-inositol cotransporter gene is regulated by a large region (approximately 50 kilobase pairs) upstream of the gene.

Regulation of the polymeric immunoglobulin receptor by water intake and vasopressin in the rat kidney

The polymeric immunoglobulin receptor (pIgR) transports polymeric immunoglobulins (IgA) from the basolateral to the apical surface of epithelial cells. At the apical surface, its amino-terminal domain, termed secretory component (SC), is proteolytically cleaved and released either unbound (free SC) or bound to IgA. We examined the effects of changes in water balance and vasopressin on the production and secretion of the pIgR in the rat kidney in vivo. Water deprivation induced a 2.7-fold increase in the pIgR mRNA and a 2.2-fold increase in intracellular pIgR protein compared with water-loaded animals. Physiological doses of desmopressin reproduced the effects of water deprivation on mRNA and intracellular protein levels, suggesting that pIgR expression may be regulated by a vasopressin-coupled mechanism. Secretion of free SC and secretory IgA in the urine, however, correlated directly with water intake and urine flow. These results suggest that hydration status and vasopressin may affect the mucosal immunity of the kidney by regulating at different steps the epithelial cell production and secretion of the polymeric immunoglobulin transporter/ secretory component.

Human Na(+)-myo-inositol cotransporter gene: alternate splicing generates diverse transcripts

Na(+)-myo-inositol cotransport activity generally maintains millimolar intracellular concentrations of myo-inositol and specifically promotes transepithelial myo-inositol transport in kidney, intestine, retina, and choroid plexus. Glucose-induced, tissue-specific myo-inositol depletion and impaired Na(+)-myo-inositol cotransport activity are implicated in the pathogenesis of diabetic complications, a process modeled in vitro in cultured human retinal pigment epithelium (RPE) cells. To explore this process at the molecular level, a human RPE cDNA library was screened with a canine Na(+)-dependent myo-inositol cotransporter (SMIT) cDNA. Overlapping cDNAs spanning 3569 nt were cloned. The resulting cDNA sequence contained a 2154-nt open reading frame, 97% identical to the canine SMIT amino acid sequence. Genomic clones containing SMIT exons suggested that the cDNA is derived from at least five exons. Hypertonic stress induced a time-dependent increase, initially in a 16-kb transcript and subsequently in 11.5-, 9.8-, 8.5-, 3.8-, and approximately 1.2-kb SMIT transcripts, that was ascribed to alternate exon splicing using exon-specific probes and direct cDNA sequencing. The human SMIT gene is a complex multiexon transcriptional unit that by alternate exon splicing generates multiple SMIT transcripts that accumulate differentially in response to hypertonic stress.

Renal osmotic stress-induced cotransporter: expression in the newborn, adult and post-ischemic rat kidney

The renal osmotic stress-induced cotransporter (ROSIT), a new putative member of a family of organic solute transporters, is highly expressed in the kidney. Our in situ hybridization data now reveal that large amounts of ROSIT mRNA can be found in the S3 segment of the proximal tubule. In the developing kidney, ROSIT mRNA is expressed after the S-shaped body stage. Because the S3 segment is the major site of damage in the post-ischemic kidney, we evaluated alterations in ROSIT mRNA expression after ischemic acute tubular necrosis. Renal osmotic stress-induced cotransporter mRNA levels were already decreased eight hours post-ischemia. At seven days post-ischemia, ROSIT mRNA reappeared in a mosaic pattern in the regenerating S3 segment, being fully expressed three weeks after the insult except for focal areas. The exact localization of this putative osmolyte transporter in the kidney, together with that of other known osmolyte transporter will contribute to a better understanding of the mechanism of medullary osmolyte accumulation and its vectorial transport.

Kidney cell survival in high tonicity

The kidney medulla of mammals undergoes large changes in tonicity in parallel with the tonicity of the final urine that emerges from the kidney at the tip of the medulla. When the medulla is hypertonic, its cells accumulate the compatible osmolytes myo-inositol, betaine, taurine, sorbitol and glycerophosphorylcholine. The mechanisms by which the compatible osmolytes are accumulated have been explored extensively in kidney-derived cells in culture. Myo-inositol, betaine and taurine are accumulated by increased activity of specific sodium-coupled transporters, sorbitol by increased synthesis of aldose reductase that catalyses the synthesis of sorbitol from glucose. Glycerophosphorylcholine accumulates primarily because its degradation is reduced in cells in hypertonic medium. cDNAs for the cotransporters and for aldose reductase have been cloned and used to establish that hypertonicity increases the transcription of the genes for the cotransporters for myo-inositol, betaine and for aldose reductase. The region 5' to the promoter of the gene for the betaine cotransporter and for aldose reductase confer osmotic responsiveness to a heterologous promoter. The 12-bp sequence responsible for the transcriptional response to hypertonicity has been identified in the 5' region of the gene for the betaine cotransporter.

Induction of Na+/myo-inositol co-transporter mRNA after rat cryogenic injury

Myo-inositol is one of the major organic osmolytes in the brain. It is stored in the cells by the Na+/myo-inositol co-transporter (SMIT) which is regulated by extracellular osmolality. First, in order to confirm that local change of the osmolality induces alteration of the SMIT mRNA in brain, we examined change of SMIT mRNA of the animals with hypertonic NaCl application to the cortex. Application of hypertonic NaCl up-regulated the SMIT mRNA expression widely surrounding the application site. We next investigated the role of SMIT in brain during vasogenic edema, we examined expression of SMIT mRNA in the rat brain after cryogenic injury. The expression of SMIT mRNA was markedly increased 12 h after surgery and the induction of the mRNA extended to the entire cortex of the affected side. Up-regulated expression was found predominantly in the neurons in remote areas. The induction of SMIT mRNA was found until the 3rd day after surgery. These findings suggest that osmotic stress may spread over a wide area in the cortex in case of vasogenic edema produced by cryogenic injury and that the cells respond to this stress by increasing SMIT expression.

The canine sodium/myo-inositol cotransporter gene: structural organization and characterization of the promoter

The sodium/myo-inositol cotransporter (SMIT) is a plasma membrane protein catalyzing transfer of myo-inositol into cells against a considerable concentration gradient using the electrochemical potential of sodium across the cell membrane. Transcription of the SMIT gene is markedly stimulated when cells are exposed to a hypertonic environment resulting in increased abundance of SMIT mRNA and increased SMIT activity. The increased accumulation of myo-inositol protects cells from the deleterious effects of hypertonicity. In an effort toward understanding transcriptional regulation, we cloned canine genomic DNA fragments containing the SMIT gene. The gene is 37 kb in size consisting of 2 exons and a large intron of 25 kb. The entire open reading frame is in the second exon. The promoter of the gene is highly active due to a GC-rich sequence. Ribonuclease protection assay using a riboprobe complementary to the 5' end of the gene confirmed that the promoter of the gene is stimulated by hypertonicity. The promoters and regulatory sequences of the SMIT gene and the betaine transporter gene, another gene regulated by hypertonicity, appear to be different.

The differential osmoregulation and localization of taurine transporter mRNA and Na+/myo-inositol cotransporter mRNA in rat eyes

We studied the cellular localization and osmotic regulation of taurine transporter (TauT) mRNA in the rat eyes using in situ hybridization. TauT mRNA signals were expressed in the ciliary body, and the outer part of the inner nuclear layer (INL), the outer nuclear layer (ONL) and the inner segment (IS) of the adult rat retina. Chronic hypernatrema, induced by gavaging with 1 ml/100 g body weight of 5% NaCl every other day for 7 days, markedly increased in TauT mRNA in the retina compared with control rats. However, there was little change in TauT mRNA in the eyes in acute hypernatremic state that is induced by single injection of high concentration of NaCl. On the contrary, acute hypernatremic rats displayed markedly elevated Na+/myo-inositol cotransporter (SMIT) mRNA in the retina and the iris-ciliary body and the lens epithelium. Under chronic hypernatremic conditions, there was no significant increase in SMIT mRNA in rat eyes. These findings suggest that TauT mRNA is osmotically regulated in vivo to protect retinal neuronal function, especially against chronic hypernatremic conditions, in contrast to rapid up-regulation of SMIT mRNA in acute hypernatremic rats.

Induction of Na+/myo-inositol cotransporter mRNA after focal cerebral ischemia: evidence for extensive osmotic stress in remote areas

Myo-inositol is one of the major organic osmolytes in the brain. It is accumulated into cells through an Na+/ myo-inositol cotransporter (SMIT) that is regulated by extracellular tonicity. To investigate the role of SMIT in the brain after cerebral ischemia, we examined expression of SMIT mRNA in the rat brain after middle cerebral artery occlusion, which would reflect alteration of extracellular tonicity. The expression of SMIT mRNA was markedly increased 12 h after surgery in the cortex of the affected side and lasted until the second day. Increased expression was also found in the contralateral cingulate cortex. Up-regulated expression was found predominantly in the neurons in remote areas, although nonneuronal cells adjacent to the ischemic core also expressed this mRNA. These results suggest that cerebral ischemia causes extensive osmotic stress in brain and that the neuronal cells respond to this stress by increasing SMIT expression.

Localization and regulation of renal Na+/myo-inositol cotransporter in diabetic rats

We have examined the effect of diabetes on sodium/myo-inositol cotransporter (SMIT) mRNA levels and myo-inositol content in the kidney to test the hypothesis that diabetes-induced changes in renal myo-inositol levels are due to the regulation of SMIT mRNA levels. In streptozotocin-induced diabetic rats, after 3, 7 and 28 days of diabetes, SMIT mRNA levels in the whole kidney were increased three to fivefold, and remained increased by about twofold after six months of diabetes. Insulin treatment of diabetic rats normalized blood glucose levels and prevented the increase in SMIT mRNA levels. Treating diabetic rats with sorbinil, an aldose reductase inhibitor, corrected the abnormal accumulation of sorbitol but had no effect on the diabetes-induced increase in renal SMIT mRNA levels. The regional distribution of SMIT mRNA from normal rats showed a relative abundance in cortex, outer medulla, and inner medulla of 1.0:3.4:7.0. After seven days of diabetes, the levels of SMIT mRNA and myo-inositol content were significantly increased only in the outer medulla. In situ hybridization studies revealed that SMIT mRNA in the outer medulla was predominately localized to the medullary thick ascending limbs of Henle's loop and was not localized to any specific cell in the inner medulla. This distribution pattern was unchanged in diabetic rats. These studies show that diabetes causes an increase in renal SMIT mRNA, which is primarily localized to the outer medulla. Accumulation of myo-inositol by the thick ascending limb of Henle's loop may account for most of the increase caused by diabetes.

Potassium depletion modulates aldose reductase mRNA in rat renal inner medulla

The organic osmolytes present in renal inner medullary cells balance the extracellular hyperosmolality and protect the cell against the effects of high salts and urea. We previously demonstrated that a renal concentrating defect due to potassium depletion was associated with a decrease in organic osmolytes including sorbitol. However, we could not determine whether a reduction in medullary organic osmolyte would be cause or effect of urine concentration defect associated with potassium depletion. We focused on the synthesis of sorbitol catalyzed by the enzyme, aldose reductase. To clarify whether the treatment of potassium depletion would affect aldose reductase when extracellular tonicity, and medullary sodium or potassium was maintained at the level of control rats, we administered a hypertonic solution of NaCl or KCl to potassium-depleted rats and evaluated aldose reductase enzymatic activity and mRNA abundance as well as the medullary contents of organic osmolytes. Either infusion significantly reduced tissue sodium content in potassium-depleted rats. With KCl infusion protocol but not that of NaCl, sorbitol as well as aldose reductase mRNA abundance increased to the control level. Medullary contents of other organic osmolytes exhibited a pattern similar to sorbitol. Data suggested that aldose reductase mRNA abundance was reduced in potassium depletion irrespective of medullary sodium content. A decrease in sorbitol level may precede a urinary concentrating defect. Our finding constitutes the first demonstration of the relationship between a potassium deficiency and the abundance of aldose reductase mRNA, an osmoregulatory protein in the kidney.

Expression of betaine transporter mRNA: its unique localization and rapid regulation in rat kidney

Betaine is a major compatible osmolyte in the renal medulla. It is taken up into cells via the betaine gamma-amino-n-butyric acid transporter (BGT-1). We investigated the localization of BGT-1 mRNA and its acute regulation by NaCl and furosemide administration. In situ hybridization revealed that BGT-1 mRNA is predominantly present in the outer medulla and papilla. Less intense signals were seen in the inner medulla and no signals were found in the cortex. Microscopic examination suggested that intense signals were present in the medullary thick ascending limbs of Henle's loop (MTAL) and the inner medullary collecting ducts (IMCD). A reverse transcription and polymerase chain reaction assay of individual microdissected segments along the nephron confirmed its localization. Intraperitoneal administration of NaCl rapidly increased the signal in the MTAL, and furosemide prevented the increase in BGT-1 mRNA by NaCl loading. In contrast, BGT-1 mRNA in the IMCD is less sensitive to these kinds of acute regulation. These results suggest that BGT-1 expression in the MTAL is rapidly regulated in response to the magnitude of NaCl absorption, as suggested for the expression of Na+/myo-inositol cotransporter.

Rapid and transient up-regulation of Na+/myo-inositol cotransporter transcription in the brain of acute hypernatremic rats

The osmoregulatory system is well developed in the brain. Osmolytes contribute to maintenance of cell volume and cellular functions without changing intracellular ionic composition. Myo-inositol is regarded as one of the major osmolytes in the brain. In the present study, we investigated the changes in expressions of sodium myo-inositol cotransporter (SMIT) mRNA in the brain of acute hypernatremic rats by in-situ hybridization and Northern blot methods. Under moderate acute hypernatremic conditions, SMIT mRNA level increased markedly at 1 h and returned to almost control levels at 3 h, in accordance with plasma Na+ concentrations. Especially, distinct increases in SMIT mRNA expression were observed in the granule cells and glial cells in the cerebellum. These findings indicate that SMIT plays an important role in osmoregulation, especially in the early stages of acute hypernatremia in the brain.

Osmotic regulation of Na-myo-inositol cotransporter mRNA level and activity in endothelial and neural cells

Myo-inositol (MI) is an important factor in the synthesis of phosphoinositides, and as an osmolyte, MI contributes to the regulation of cell volume. In cells of renal origin, hypertonicity causes an increase in sodium-dependent MI transporter (SMIT) mRNA levels and MI transport. However, it is unknown whether changes in osmolarity regulate transport of MI in neural or endothelial cells. IN these studies, neural and endothelial cells were exposed to hyperosmotic medium for up to 48 h, and the effect on MI transport was determined. Transport of MI was maximally increased by exposing the cells to hyperosmotic medium for 24 h. Kinetic analysis of high-affinity MI transport demonstrated an increase in the apparent maximal velocity with no significant change in the apparent Km. The hyperosmotic induction of MI transport was blocked by the addition of cycloheximide, indicating a requirement for protein synthesis, and was associated with increased levels of SMIT mRNA. In contrast to the effect of hypertonicity, exposure of neural and endothelial cells to hypotonic conditions caused a decrease in SMIT mRNA levels and MI transport in endothelial cells. These studies demonstrate that, in extrarenal cell types, changes in osmolarity also regulate SMIT activity and mRNA levels.

Differential uptake of myo-inositol in vivo into rat brain areas

Oral inositol has been reported to have antidepressant and antipanic properties in humans. Inositol enters the brain poorly and high doses are required. Natural uptake processes and specific transporters are involved. We here report that intraperitoneally administered inositol is taken up differently by various brain areas and that brain areas have different baseline inositol levels. These effects could be important in understanding the differential effects of lithium-induced lowering of inositol and of behavioral effects of exogenous inositol.

Expression of Na+/myo-inositol cotransporter mRNA in normal and hypertonic stress rat eyes

We studied the localization of Na+/myo-inositol cotransporter (SMIT) mRNA in normal and hypertonic stress rat eyes by in situ hybridization histochemistry using cRNA probes. SMIT mRNA signals were observed in the iris-ciliary body, the lens epithelial cells, and the ganglion cell layer and the inner nuclear layer of the retina. There was a rapid increase on SMIT mRNA in the retina of hypertonic stress rats compared with control rats. These findings suggest that Na+/myo-inositol cotransporter gene expression is osmotically regulated in vivo to protect retinal neuronal function against hypertonic stress.

Expression of Na+/myo-inositol cotransporter mRNA in the inner ear of the rat

We have demonstrated the cellular localization of Na+/myo-inositol cotransporter (SMIT) mRNA in the rat inner ear by in situ hybridization. In the cochlea, the most intense SMIT mRNA signals were observed in fibrocytes of the spiral ligament, moderate signals were found in the spiral limbus, inner hair cells and spiral ganglion cells, while the hybridization signals were almost undetectable in the marginal cells of the stria vascularis and outer hair cells. In the vestibular system, moderate hybridization signals were found in the sensory epithelium, fibrocytes and vestibular ganglion cells. These findings suggest that SMIT plays an important role in maintenance of intracellular ionic balance and cell volume in the inner ear, especially in the fibrocytes associated with generation of the ion gradients between the endolymph and perilymph.

Na+/myo-inositol transport is regulated by basolateral tonicity in Madin-Darby canine kidney cells

We investigated the effects of change in basolateral osmolality on Na(+)-dependent myo-inositol uptake in Madin-Darby canine kidney cells to test our hypothesis that the Na+/myo-inositol transporter (SMIT), an osmolyte transporter, is mainly regulated by osmolality on the basolateral surface. A significant osmotic gradient between both sides of the epithelium persisted at least 10 h after basolateral osmolality was increased. [3H]myo-inositol uptake increased in a basolateral osmolality-dependent manner. The magnitude of the increase is comparable to that for making both sides hypertonic. Apical hypertonicity also increased the uptake on the basal side, but the magnitude of the increase was significantly smaller than the basolateral or both sides hypertonicity. Betaine-gamma-amino-n-butyric acid transporter activity, measured by [3H]gamma-amino-n-butyric uptake, showed a pattern similar to SMIT activity in response to basolateral hypertonicity. The most plausible explanation for the polarized effect of hypertonicity is that the basal membrane is much more water permeable than the apical membrane. These results seem to be consistent with the localization and regulation of the SMIT in vivo.

Localization and rapid regulation of Na+/myo-inositol cotransporter in rat kidney

myo-inositol, a major compatible osmolyte in renal medulla, is accumulated in several kinds of cells under hypertonic conditions via Na+/myo-inositol cotransporter (SMIT). To investigate the physiological role of the SMIT, we sought to determine its localization by in situ hybridization and its acute regulation by NaCl and furosemide administration. Northern analysis demonstrated that SMIT is strongly expressed in the medulla and at low levels in the cortex of kidney. Intraperitoneal injection of NaCl rapidly induced SMIT mRNA in both the cortex and medulla, and furosemide completely abolished this induction. In situ hybridization revealed that SMIT it predominantly present in the medullary and cortical thick ascending limbs of Henle's loop (TALH) and macula densa cells. Less intense signals were seen in the inner medullary collecting ducts (IMCD). NaCl loading increased the signals throughout the TALH, and furosemide reduced the signals. SMIT in the IMCD is less sensitive to these kinds of acute regulation. Thus, the distribution pattern of SMIT does not correspond to the corticomedullary osmotic gradient, and SMIT in the TALH and macula densa cells is regulated very rapidly. These results suggest that SMIT expression in TALH may be regulated by intracellular and/or peritubular tonicity close to the basolateral membrane, which is supposed to be proportional to the magnitude of NaCl reabsorption.

Cell volume regulated transporters of compatible osmolytes

Virtually all cells respond to hypertonicity by accumulating certain small organic solutes (compatible osmolytes) that, in contrast to intracellular ions, do not perturb macromolecular function. Several important compatible osmolytes are accumulated by coupled transport. Transcription of genes encoding these cotransporters is increased by hypertonicity and a tonicity-responsive enhancer element has been identified. When cells return to an iso-osmotic environment, osmolytes are rapidly lost through a pathway that current evidence indicates may be a volume-sensitive chloride channel.

Effect of inositol and glycine with increasing sodium chloride and constant osmolality on development of rabbit embryos

PURPOSE

Many commercially available culture media have a high sodium content as sodium chloride and sodium bicarbonate. We examined the effects of osmolytes, inositol and glycine, on embryos cultured in synthetic RD medium with a lower concentration of NaCl (93 mM) and a higher concentration of NaCl (116 mM) with media held constant at 270 mosmols.

RESULTS

There were no significant effects of either glycine or inositol on embryo growth when the embryos were cultured in RD medium with 93 mM NaCl (P > 0.05). Culture of one-cell embryos for 72 h in RD medium with 93 mM NaCl, 116 mM NaCl and 116 mM NaCl containing 0.56 mM inositol resulted in 77%, 14% and 55% expanded blastocysts, respectively (P < 0.05). Corresponding values for two-cell embryos cultured for 67 h were 84%, 34% and 66% expanded blastocysts (P < 0.05). When one-cell and two-cell embryos were cultured in RD medium with 93 mM NaCl, 116 mM NaCl, 116 mM NaCl plus 1 mM glycine and 116 mM NaCl plus 1 mM glycine and 0.56 mM inositol, expanded blastocysts from one-cell embryos were 79%, 19%, 49%, and 48%, respectively (P < 0.05), and expanded blastocysts from two-cell embryos were 91%, 32%, 52%, and 53%, respectively (P < 0.05).

CONCLUSION

Inositol and glycine presumably behave as osmolytes in providing substantial protection for rabbit one-cell and two-cell embryos cultured in a medium with high NaCl concentration.

Na+/myo-inositol cotransport is regulated by tonicity in cultured rat mesangial cells

Mesangial cells are considered to be faced with osmotic stress under physiological (such as extraglomerular mesangial cells) and pathophysiological (for example, diabetes mellitus) conditions. To see if mesangial cells have an osmoregulatory mechanism, like renal medullary cells, we measured the intracellular contents of organic osmolytes in isotonic and hypertonic conditions. Cultured rat mesangial cells are well tolerant of acute increase in osmolality up to 500 mOsm/kg. The myo-inositol content increased in hypertonic cells more than six-fold the value in isotonic cells. The contents of glycerophosphorylcholine and sorbitol also increased but were less than that of myo-inositol. The Na(+)-dependent myo-inositol uptake in hypertonic cells was a 12-fold uptake in isotonic cells, reaching a maximum 24 hours after the switch to a hypertonic medium. The uptake rate increased as medium osmolality increased from 300 to 500 mOsm/kg. Raffinose is the most effective solute to increase the myo-inositol uptake. NaCl, glucose and mannitol also increased the uptake rate (NaCl > glucose > mannitol). The increased uptake by hypertonicity was the result of an increase in Vmax without change in Km and was dependent on RNA and protein synthesis. These results indicate that mesangial cells respond to extracellular hypertonicity by increasing myo-inositol transport activity and accumulating myo-inositol into the cells, suggesting that myo-inositol functions as an organic osmolyte in mesangial cells.