An enzymatic fluorimetric assay for myo-inositol

Amperometric determination of Myo-inositol using a glassy carbon electrode modified with nanostructured copper sulfide

A method for the amperometric determination of Myo-inositol is presented. Nanostructured copper sulfide material was synthesized by solvothermal method and utilized as sensor matrix. The physico-chemical analysis using XRD, Raman, FE-SEM, TEM, and XPS confirmed the formation of CuS material. The voltammetric response of CuS-modified glassy carbon electrode for a successive Myo-inositol (0.5 μM) addition confirmed that the reaction takes place at the surface of the electrode. The modified electrode resulted in signal enhancement for a linear response ranging from 0.5-8.5 μM at an applied overpotential of 0.65 V with a correlation coefficient value (R²) of 0.99. The sensitivity and limit of detection of the modified electrode were 7.87 μA μM^-1 cm^-2 and 0.24 μM, respectively. The interfering effect of various compounds present in real samples was examined. Graphical abstract Schematic representation of synthetic protocol of nanostructured CuS and Myo-inositol oxidation on CuS-modified glassy carbon electrode in basic medium.

Quantification of plasma myo-inositol using gas chromatography-mass spectrometry

BACKGROUND

Myo-inositol (MI) deficiency is associated with an increased risk for neural tube defects (NTDs), mental disorders and metabolic diseases. We developed a gas chromatography-mass spectrometry (GC-MS) method to detect MI in human plasma, which was accurate, relatively efficient and convenient for clinical application.

METHODS

An external standard method was used for determination of plasma MI. Samples were analyzed by GC-MS after derivatization. The stable-isotope labeled internal standard approach was used to validate the method's accuracy. Alpha fetal protein (AFP) was detected by chemiluminescence immunoassay.

RESULTS

The method was validated by determining the linearity, sensitivity and recovery rate. There was a good agreement between the internal standard approach and the present method. The NTD-affected pregnancies showed lower plasma MI (P=0.024) and higher AFP levels (P=0.001) than control. Maternal MI level showed a better discrimination in spina bifida subgroup, while AFP level showed a better discrimination in anencephaly subgroup after stratification analysis.

CONCLUSIONS

We developed a sensitive and reliable method for the detection of clinical plasma MI, which might be a marker for NTDs screening, and established fundamental knowledge for clinical diagnosis and prevention for the diseases related to disturbed MI metabolism.

The diabetic rat kidney mediates inosituria and selective urinary partitioning of D-chiro-inositol

Diabetic nephropathy is a serious complication of diabetes mellitus with a pressing need for effective metabolic markers to detect renal impairment. Of potential significance are the inositol compounds, myo-inositol (MI), and the less abundant stereoisomer, D-chiro-inositol (DCI), which are excreted at increased levels in the urine in diabetes mellitus, a phenomenon known as inosituria. There is also a selective urinary excretion of DCI compared to MI. As the biological origins of altered inositol metabolism in diabetes mellitus are unknown, the aim of this study was to determine whether the diabetic kidney was directly responsible. Kidneys isolated from four-week streptozotocin-induced diabetic rats were characterized by a 3-fold reduction in glomerular filtration rate (GFR) compared to matched non-diabetic kidneys. When perfused with fixed quantities of MI (50 µM) and DCI (5 µM) under normoglycemic conditions (5 mM glucose), GFR-normalized urinary excretion of MI was increased by 1.7-fold in diabetic vs. non-diabetic kidneys. By comparison, GFR-normalized urinary excretion of DCI was increased by 4-fold. Perfusion conditions replicating hyperglycemia (20 mM glucose) potentiated DCI but not MI urinary excretion in both non-diabetic and diabetic kidneys. Overall, there was a 2.4-fold increase in DCI urinary excretion compared to MI in diabetic kidneys that was independent of glucose ambience. This increased urinary excretion of DCI and MI in diabetic kidneys occurred despite increased renal expression of the inositol transporters, sodium myo-inositol transporter subtype 1 and 2 (SMIT1 and SMIT2). These findings show that the diabetic kidney primarily mediates inosituria and altered urinary partitioning of MI and DCI. Urinary inositol levels might therefore serve as an indicator of impaired renal function in diabetes mellitus with wider implications for monitoring chronic kidney disease.

Quantitative determination of free glycerol and myo-inositol from plasma and tissue by high-performance liquid chromatography

A high-performance liquid chromatographic method that accurately measures glycerol and myo-inositol from plasma and tissue is described. The method incorporates a pre-column derivatization reaction using aqueous extracts with benzoyl chloride as a modifying agent. The benzoylated derivatives are isolated by HPLC using reversed-phase gradient chromatography and quantified via absorbance detection at 231 nm. The benzoylated derivatives of glycerol and myo-inositol are well resolved from other known carbohydrates, internal standard and other contaminants encountered within samples and during incubation. The benzoylation of these analytes reach a maximum between 3.5 and 6 h of incubation and are stable for at least 24 days at 4 degrees C. The limit of quantization (LOQ) of glycerol was equal to 2.5 nmol/ml plasma and 6.4 nmol/g tissue and the LOQ of myo-inositol was 1.8 nmol/ml plasma and 3.6 nmol/g tissue. Incubation of known standards and samples with benzoyl chloride at 40 degrees C for 4 h showed fully benzoylated products as determined by mass spectral analysis. Calibration curves were linear between 2.7 and 174 nmol for glycerol and 1.4-89 nmol for myo-inositol. Comparison of tissue and plasma concentrations of glycerol and myo-inositol found using this method are in good agreement with other reported values using other techniques.

Expression profiles of polyhydroxyalkanoate synthesis-related genes in Paracoccus denitrificans

A facultative methylotrophic bacterium, Paracoccus denitrificans can synthesize polyhydroxyalkanoate acids (PHA) from various alcohols. Recently, six genes, phaA, B, C, P, R, and Z, related to PHA synthesis have been cloned and characterized. PHA synthesis and the expression of phaA, B, C, P, R, and Z in P. denitrificans were examined at the transcriptional and translational levels under both nitrogen-sufficient and nitrogen-deficient conditions. The results showed that PHA synthesis is not regulated at the mRNA or protein level in phaA, B, and C. We also observed the condensation of acetyl-coenzyme A (acetyl-CoA) in the cells by high-performance liquid chromatography (HPLC). The results suggest that the amount of acetyl-CoA would regulate PHA synthesis. Finally, we discuss a possible regulation mechanism for PHA synthesis in P. denitrificans.

Determination of transport stoichiometry for two cation-coupled myo-inositol cotransporters: SMIT2 and HMIT

Three different mammalian myo-inositol cotransporters are currently known; two are Na+-coupled (SMIT1 and SMIT2) and one is proton-coupled (HMIT). Although their transport stoichiometries have not been directly determined, significant cooperativities in the Na+ activation of SMIT1 and SMIT2 suggest that more than one Na+ ion drives the transport of each myo-inositol. The two techniques used here to determine transport stoichiometry take advantage of the electrogenicity of both SMIT2 and HMIT expressed in Xenopus oocytes. The first method compares the measurement of charge transferred into voltage-clamped oocytes with the simultaneous uptake of radiolabelled substrate. The second approach uses high accuracy volume measurements to determine the transport-dependent osmolyte uptake and compares it to the amount of charge transported. This method was calibrated using a potassium channel (ROMK2) and was validated with the Na+/glucose cotransporter SGLT1, which has a known stoichiometry of 2 : 1. Volume measurements indicated a stoichiometric ratio of 1.78 +/- 0.27 ion per alpha-methyl-glucose (alphaMG) for SGLT1 whereas the radiotracer uptake method indicated 2.14 +/- 0.05. The two methods yielded a SMIT2 stoichiometry measurement of 1.75 +/- 0.30 and 1.82 +/- 0.10, both in agreement with a 2 Na+:1 myo-inositol stoichiometry. For HMIT, the flux ratio was 1.02 +/- 0.04 charge per myo-inositol, but the volumetric method suggested 0.67 +/- 0.05 charge per myo-inositol molecule. This last value is presumed to be an underestimate of the true stoichiometry of one proton for one myo-inositol molecule due to some proton exchange for osmotically active species. This hypothesis was confirmed by using SGLT1 as a proton-driven glucose cotransporter. In conclusion, despite the inherent difficulty in estimating the osmotic effect of a proton influx, the volumetric method was found valuable as it has the unique capacity of detecting unidentified transported substrates.

Determination of myo-inositol in biological samples by liquid chromatography–mass spectrometry

Due to the absence of HPLC methods to determine myo-inositol using mass detection and considering its sensitivity and selectivity, a high performance liquid chromatography-mass spectrometry method for the analysis of myo-inositol is described and applied to its direct determination in urine and saliva samples. Successful resolution of myo-inositol and its related substances was achieved with a stationary phase Aminex HPX-87C Column with milli-Q water as mobile phase and 5 mM ammonium acetate added post-column. The detector counted positive ions by monitoring m/z = 198, which corresponds to the myo-inositol adduct with ammonium cation. Urine and saliva samples were previously purified by passing through an anion-exchange resin. Concentrations as low as 138 and 461 microg/l in saliva and urine could be respectively quantified. Intra-day R.S.D. ranged from 0.83 to 1.02%, whereas inter-day R.S.D. was between 1.54 and 3.58%.

Specific determination of myo-inositol in multivitamin pharmaceutical preparations by a flow injection system using a myo-inositol dehydrogenase reactor coupled with a glucose eliminating enzyme reactor

A flow injection system for myo-inositol determination in multivitamin pharmaceutical preparations using two enzyme reactors was developed. Myo-inositol was detected using a fluorophotometer, to measure the fluorescence of NADH produced from NAD+ by a myo-inositol dehydrogenase reactor (IDR) containing myo-inositol dehydrogenase immobilized on porous glass. Enhanced interference due to excess glucose included in a multivitamin pharmaceutical preparation as a sweetener was eliminated by a glucose eliminating reactor (GER) co-immobilized with three enzymes (glucose oxidase, mutarotase and catalase). The calibration coefficient for the standard curve was 0.9993 for myo-inositol detection in the range of 1-5 microg/ml. Myo-inositol was determined even in the presence of glucose concentrations of 140-420 microg/ml. The recovery of myo-inositol added to the multivitamin pharmaceutical preparation was 99.6% (n=9).

Determination of urinary myo-inositol concentration by an improved enzymatic cycling method using myo-inositol dehydrogenase from Flavobacterium sp

BACKGROUND

To determine myo-inositol more accurately, we improved the enzymatic cycling method.

METHODS

We screened myo-inositol dehydrogenase (MIDH; EC.1.1.1.18) from Flavobacterium sp., which was highly specific to myo-inositol. We measured urinary myo-inositol/creatinine ratio 2 h after 75-g oral glucose tolerance test (2 h MI) of 71 volunteers, and investigated the relationship between diabetes and urinary myo-inositol concentration.

RESULTS

The calibration curve was linear (r = 1.00) up to 2000 micromol/l, and the detection limit was 10 micromol/l. Within-run and between-run CVs were 0.5-1.1% and 0.4-1.3%, respectively. The 2 h MI of impaired fasting glycemia (IFG; 65.1 +/- 46.6 mg/g Cr, P < 0.005), impaired glucose tolerance (IGT; 85.0 +/- 73.7 mg/g Cr, P < 0.001) and diabetes (163.4 +/- 73.7 mg/g Cr, P < 0.0001) increased significantly compared with that of normal glucose tolerance (NGT; 24.0 +/- 14.4 mg/g Cr). From receiver operating characteristic analyses on 2 h MI, with 50 mg/g Cr as a tentative cutoff value to detect diabetes, the sensitivity and specificity were 100% and 77%, respectively. With 40 mg/g Cr as a tentative cutoff value to detect NGT, the sensitivity and specificity were 74% and 85%, respectively.

CONCLUSIONS

The myo-inositol measurement method demonstrated high specificity and yielded accurate results. The results of clinical trials suggested that 2 h MI could not only determine diabetes but also distinguish IFG and IGT from NGT.

Identification of a novel Na+/myo-inositol cotransporter

rkST1, an orphan cDNA of the SLC5 family (43% identical in sequence to the sodium myo-inositol cotransporter SMIT), was expressed in Xenopus laevis oocytes that were subsequently voltage-clamped and exposed to likely substrates. Whereas superfusion with glucose and other sugars produced a small inward current, the largest current was observed with myo-inositol. The expressed protein, which we have named SMIT2, cotransports myo-inositol with a K(m) of 120 microm and displays a current-voltage relationship similar to that seen with SMIT (now called SMIT1). The transport is Na(+)-dependent, with a K(m) of 13 mm. SMIT2 exhibits phlorizin-inhibitable presteady-state currents and substrate-independent "Na(+) leak" currents similar to those of related cotransporters. The steady-state cotransport current is also phlorizin-inhibitable with a K(i) of 76 microm. SMIT2 exhibits stereospecific cotransport of both d-glucose and d-xylose but does not transport fucose. In addition, SMIT2 (but not SMIT1) transports d-chiro-inositol. Based on previous publications, the tissue distribution of SMIT2 is different from that of SMIT1, and the existence of this second cotransporter may explain much of the heterogeneity that has been reported for inositol transport.

Inositol and higher inositol phosphates in neural tissues: homeostasis, metabolism and functional significance

Inositol phospholipids and inositol phosphates mediate well-established functions in signal transduction and in Ca2+ homeostasis in the CNS and non-neural tissues. More recently, there has been renewed interest in other roles that both myo-inositol and its highly phosphorylated forms may play in neural function. We review evidence that myo-inositol serves as a clinically relevant osmolyte in the CNS, and that its hexakisphosphate and pyrophosphorylated derivatives may play roles in such diverse cellular functions as DNA repair, nuclear RNA export and synaptic membrane trafficking.

An enzymatic cycling method for the measurement of myo-inositol in biological samples

INTRODUCTION

A sensitive and simple enzymatic cycling method is described for the quantitation of myo-inositol in biological samples.

METHODS

The method involves the use of a sensitive and simple enzymatic cycling method is described for the quantitation of myo-inositol in biological samples. The method involves use of thio-NAD(+), NADH and thermostable myo-inositol dehydrogenase (IDH; EC. 1.1.1.18) and measurement of the increase in absorbance at 405 nm of thio-NADH at 37 degrees C.

RESULTS

The calibration curve for myo-inositol was linear (r=1.00) between 10 and 400 micromol/l. Analytical recoveries of exogenous myo-inositol added to serum and urine were 100-105% and 98-103%, respectively. Within-run and between-run coefficient of variation (CV) were 0.6-2.1% and 1.1-3.0%, respectively. This method was free from interference by hemoglobin, bilirubin, ascorbate, chyle, various sugars, sugar alcohol and myo-inositol phosphates. With the use of myo-inositol as a standard solution, the serum myo-inositol concentration (mean+/-SD) was significantly greater in patients with diabetes mellitus (DM) without nephropathy (73.0+/-13.8 micromol/l, n=7) than in healthy individuals without DM (61.0+/-12.4 micromol/l, n=20). The urinary myo-inositol concentration was also significantly greater in patients with DM without nephropathy (793.3+/-870.3 micromol/l, n=7) than in healthy individuals without DM (76.0+/-63.0 micromol/l, n=13).

CONCLUSIONS

This new method is simple, sensitive and enables quantitative analysis of myo-inositol.

An enzymatic assay for myo-inositol in tissue samples

An enzymatic assay for myo-inositol (MI) was modified. The method is based on the oxidation of MI by NAD(+)-dependent MI dehydrogenase, coupled to reoxidation of NADH by iodonitrotetrazolium chloride and diaphorase. The resultant formazan is measured spectrophotometrically. In order to remove interference by glucose, preliminary phosphorylation of glucose by hexokinase was employed before the above reaction. The assay is quantitative for MI in amounts ranging from 1 to 20 nmol. This method gives a negligible blank, even in the measurement of rat serum. The reduced MI content in the sciatic nerve and lens of streptozotocin-induced diabetic rats recovered in a dose-dependent manner by treatment with a novel potent aldose reductase inhibitor, GP-1447 ¿3-[(4,5, 7-trifluorobenzothiazol-2-yl)methyl]-5-methylphenylacetic acid¿.

myo-Inositol is an osmolyte in rat liver macrophages (Kupffer cells) but not in RAW 264.7 mouse macrophages

The role of myo-inositol as an osmolyte was studied in cultured rat liver macrophages (Kupffer cells). Hyperosmotic exposure of Kupffer cells stimulated myo-inositol uptake and led to an increase in the mRNA levels for the sodium/myo-inositol cotransporter (SMIT). Conversely, hypo-osmotic (205 m-osM) exposure diminished myo-inositol uptake when compared with normo-osmotic (305 m-osM) control incubations. The hyperosmolarity-induced SMIT mRNA increase was counteracted by added myo-inositol or betaine. In contrast with Kupffer cells, there was only a slight hyperosmotic stimulation of myo-inositol uptake in RAW 264.7 mouse macrophages, and the myo-inositol transporter (SMIT) mRNA was not detectable. Further, a slight stimulation of taurine uptake and an increase in taurine transporter (TAUT) mRNA level by hyperosmolarity was observed in RAW 264.7 cells, whereas hypo-osmolarity led to a decrease in taurine uptake and TAUT mRNA level. When Kupffer cells were preloaded with myo-inositol, hypo-osmotic exposure led to a rapid efflux of myo-inositol from the cells. Myo-inositol efflux was also stimulated by phagocytosis of latex particles; however, latex was without effect on the hyperosmolarity-induced increase of SMIT mRNA levels. The results suggest a role of myo-inositol as an osmolyte in rat Kupffer cells but not in RAW 264.7 mouse macrophages. The functional relevance of this osmolyte strategy might lie in the maintenance of cell volume homeostasis during phagocytosis in Kupffer cells; however, the interplay with the other osmolytes betaine and taurine remains to be established.

Effect of propionyl-L-carnitine on oscillatory potentials in electroretinogram in streptozotocin-diabetic rats

The effect of propionyl-L-carnitine, an analogue of L-carnitine, and insulin on the oscillatory potentials of the electroretinogram was determined in rats with streptozotocin-induced diabetes. Propionyl-L-carnitine was administered at a daily dose of 0.5 g/kg by gavage for 4 weeks, while other rats were treated with subcutaneous injections of insulin (8-10 U/day). Both treatments shortened the peak latencies of the oscillatory potentials in the electroretinogram, which were significantly prolonged in untreated diabetic rats (O1, O2 and O3, and sigma (O1 + O2 + O3)) (P < 0.0001 vs. untreated normal rats). A significant decrease in the erythrocyte free carnitine level in diabetic rats was prevented by both treatments. Insulin produced a significant reduction of retinal glucose, sorbitol and fructose levels in diabetic rats, while propionyl-L-carnitine failed to do so. However, both treatments markedly reduced serum lipids levels in the diabetic rats. These findings provide information on the pathogenesis of diabetic retinopathy as well as suggesting the potential therapeutic value of propionyl-L-carnitine for retinopathy.

Neurotropin prevents neurophysiological abnormalities and ADP-induced hyperaggregability in rats with streptozotocin-induced diabetes

Neurotropin, a non-proteinaceous extract from the inflamed dermis of rabbits inoculated with vaccinia virus, was administered for 8 weeks to rats with streptozotocin-induced diabetes. The physiological and biochemical changes of the nerves were studied as well as ADP-induced platelet aggregation. Neurotropin improved the caudal motor nerve conduction velocity, R-R variability, sciatic nerve blood flow, and platelet hyperaggregability in diabetic rats, despite having no effect on sorbitol and fructose accumulation or myoinositol depletion in the sciatic nerve. The correlation between nerve conduction velocity, R-R variability, nerve blood flow, and platelet aggregation were significant between each two parameters (p < 0.0001). Thus, the mechanism of action of neurotropin differed from that of aldose reductase inhibitors. These findings suggest that vascular factors may play an important role in the development of diabetic neuropathy, and that neurotropin may be useful for the treatment of this condition.

Quantitative chromatographic analysis of inositol phospholipids and related compounds

The metabolism of phospholipids and the mobilization of second messengers such as inositol-1,4,5-trisphosphate, 1,2-diacylglycerol (DAG) and arachidonic acid (AA) from phospholipids is commonly studied by radiolabelling phospholipids with [3H]myo-inositol or [32P]ATP and measuring the incorporation of radioactivity in different phospholipids or their hydrolysis products. However, for the radiolabelling method to accurately reflect changes in the compound's mass, it is essential that the tissue is labelled to isotopic equilibrium which is difficult to achieve. To circumvent the disadvantages of the radiolabelling method, several analytical procedures have been developed for the mass analysis of phospholipids and inositolphosphates (IPs). Quantitation of the mass or the radiolabelling of phospholipids is a complex multi-step procedure that involves quantitative isolation of phospholipids, fractionation of individual phospholipids and either determination of radioactivity in each component or the measurement of their mass. Phospholipids, DAG and AA are extracted from tissue sample with organic solvents such as chloroform-methanol (2:1) containing HCl or formic acid. The extract is separated by TLC, cartridge-column chromatography or HPLC on a reversed-phase column. Phospholipids are quantitated by measuring inorganic phosphate, absorption at 200 nm or mass spectrometry. Inositol phosphates are extracted with perchloric acid or trichloroacetic acid and separated by ion-exchange cartridge-column or HPLC with an ion-exchange column. IPs are quantitated by measuring inorganic phosphate or by using enzymatic reaction, metal-dye coupling, NMR or mass spectrometry.

Sodium-independent carrier-mediated inositol transport in cultured renal epithelial (LLC-PK1) cells

In addition to the concentrative, Na(+)-dependent inositol transport system demonstrated in many cell types, carrier-mediated, Na(+)-independent inositol transport is also shown to exist in LLC-PK1 renal epithelia. Inhibition of inositol uptake in Na(+)-free saline by 0.1 mM phloretin, and self-inhibition by net concentrations of inositol exceeding 10 mM, demonstrate the carrier-mediation of the Na(+)-independent uptake and distinguish it from flux through anion channels. The Na(+)-dependent uptake exhibits higher affinity for inositol, as seen by the stronger self-inhibition at lower inositol concentrations in Na+ saline. Kinetic analyses indicate a Km of 178 microM and a Vmax of 2447 pmol/min per microgram DNA for the Na(+)-dependent system, whereas the lower affinity, lower capacity Na(+)-independent system manifests a Km of 5.2 mM and a Vmax of 249 pmol/min per microgram DNA. the Na(+)-independent uptake further differs from the Na(+)-dependent transport by the lack of inhibitory effect of 10 microM glucose, and the greater relative inhibition of phloretin compared to that of phlorizin. Both types of uptake appear to localize predominantly to the basal-lateral cell surface. The Na(+)-independent transport is bidirectional, functioning in efflux as well as influx of inositol.

Effect of a potent new aldose reductase inhibitor, (5-(3-thienyltetrazol-1-yl)acetic acid (TAT), on diabetic neuropathy in rats

(5-(3-Thienyl)tetrazol-1-yl)acetic acid (TAT), a novel potent aldose reductase inhibitor, was administered for 4 weeks to rats with streptozotocin-induced diabetes. Physiological and biochemical studies were subsequently conducted on rat nerve tissue and erythrocyte sorbitol content was estimated. Sciatic nerve blood flow (SNBF) was markedly lower (about 43.4%) in untreated diabetic (DC) rats than in non-diabetic controls (NC). A significant delay in caudal motor nerve conduction velocity (MNCV) and significantly higher glucose, sorbitol and fructose values were observed in the sciatic nerve, accompanied by a markedly higher sorbitol concentration in erythrocytes. In contrast, TAT-treated diabetic groups (DT-10, DT-40 and DT-200) had significantly higher SNBF, MNCV and sciatic nerve myo-inositol values and lower sciatic nerve sorbitol and fructose levels and erythrocyte sorbitol concentration than the DC group. There were good correlations between SNBF and MNCV (r = 0.672, P < 0.001) and between SNBF and erythrocyte sorbitol (r = 0.455, P < 0.003). These findings suggest that both vascular and metabolic factors play an important role in diabetic neuropathy and the effect of aldose reductase inhibitors on diabetic neuropathy may be mediated by at least these two factors.

Catalytic-rate improvement of a thermostable malate dehydrogenase by a subtle alteration in cofactor binding

The nucleotide-binding fold of many NAD(+)-dependent dehydrogenases contains a conserved acidic amino acid residue which hydrogen-bonds with the 2'- and 3'-hydroxy groups of the adenine-ribose of the cofactor. This residue is highly conserved as aspartate in malate dehydrogenases, except in the thermophilic enzyme from Thermus aquaticus B (TaqMDH), which has glutamic acid-41 in the equivalent position. The catalytic mechanism was dissected to investigate the functional significance of this difference in TaqMDH with respect to a mutant enzyme where glutamic acid-41 was replaced by aspartic acid. The mutant enzyme was found to retain a high degree of protein structural stability to both thermal and chemical denaturation. When compared with the wild-type enzyme the mutant had a higher Km and Kd for both reduced and oxidized cofactors (NADH and NAD+) and a 2-3-fold increase in steady-state kcat in both assay directions. The rate-determining step for the reduction of oxaloacetate by wild-type TaqMDH was shown to be the rate of NAD+ release, which was about 2.5-fold higher for the mutant enzyme. This correlates well with the 1.8-fold higher steady-state kcat of the mutant enzyme and represents an improvement in the steady-state kcat of a thermophilic enzyme at moderate temperature by a conservative amino acid substitution which increases the rate of product release.

Radio-label and mass determinations of inositol 1,3,4,5-tetrakisphosphate formation in rat cerebral cortical slices: differential effects of myo-inositol

To investigate the effects of increasing concentrations of myo-inositol (inositol) on receptor stimulated [3H]inositol polyphosphate formation in the absence of lithium, slices of rat cerebral cortex were incubated with various concentrations of [3H]inositol (1 to 30 microM). Carbachol stimulated formation of [3H]inositol trisphosphate (InsP3) and [3H]inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P4) increased several fold when the inositol concentration was increased reaching a plateau at approximately 12 microM inositol. Time course studies revealed that in the presence of low concentrations of inositol (1 microM), [3H]InsP3 and [3H]Ins(1,3,4,5)P4 formation in response to carbachol stimulation increased slowly over a 10 to 20 min time period, whereas in the presence of 4 and 12 microM inositol, carbachol stimulated [3H]InsP3 and [3H]Ins(1,3,4,5)P4 formation was rapid and essentially complete within 3 to 5 min after carbachol addition. Although the carbachol dose response in 12 microM inositol had a much greater maximal efficacy, there was no change in potency. Similar to the effects of carbachol on [3H]Ins(1,3,4,5)P4 formation from prelabeled phosphoinositides, muscarinic receptor stimulation increased Ins(1,3,4,5)P4 mass formation by seven fold. Furthermore, Li+ (8 mM) completely inhibited carbachol stimulated increases in Ins(1,3,4,5)P4 mass formation. In contrast to the effects of increasing inositol on carbachol stimulated formation of radiolabeled inositol phosphates, increasing inositol had no effect upon mass formation of Ins(1,3,4,5)P4. These results show that when measuring inositol polyphosphate formation by the radiolabeling technique in the absence of Li+, increasing the inositol concentration greatly increases the stimulated component of [3H]InsP3 and [3H]Ins(1,3,4,5)P4 formation. However, this inositol induced increase in agonist stimulated Ins(1,3,4,5)P4 formation is not reflected as an increase in mass formation.

Quantitative analysis of inositol lipids and inositol phosphates in synaptosomes and microvessels by column chromatography: comparison of the mass analysis and the radiolabelling methods

Chromatographic methods that measure both the mass and the radiolabelling of various inositol lipids and inositol phosphates in tissues have been developed. The mass of phosphatidylinositol (PtdIns), phosphatidylinositol-4-monophosphate [PtdIns(4)P] and phosphatidylinositol-4,5-bisphosphate [PtdIns(4,5)P2] was quantitated by measuring the inorganic phosphate, whereas inositol monophosphate (IP), inositol bisphosphate (IP2), inositol trisphosphate (IP3) and inositol tetrakisphosphate (IP4) were quantitated by using an enzymic method. The radiolabelling of various inositol lipids and inositol phosphates was determined by incubating the tissue samples with [3H]myo-inositol, separating individual inositol lipids and inositol phosphates, and measuring the radioactivity in each compound. Although the mass analysis method was sensitive enough to measure low levels of inositol lipids or inositol phosphates, the method was laborious and time-consuming. Compared with the enzymic method, the radiolabelling method was simple and fast, but it gave variable results. This study demonstrated differences in inositol lipid and inositol phosphate levels by radiolabelling and mass measurements, and agonist-stimulated phosphatidylinositol turnover of synaptosomes versus the blood-brain barrier as represented by microvessels. Although the mass of PtdIns, PtdIns(4)P and PtdIns(4,5)P2 was comparable in synaptosomes and microvessels, the incorporation of [3H]myo-inositol into phosphorylated PtdIns in microvessels was less than that in synaptosomes.

A sensitive radioisotopic assay of myo-inositol: its application to rat pancreatic islets

A radioisotopic procedure for the assay of myo-inositol is presented. It is based on the generation of NADH from NAD+ in the reaction catalyzed by myo-inositol dehydrogenase and the subsequent NADH-dependent conversion of 2-[U-14C]ketoglutarate to 14C-labeled L-glutamate in the reaction catalyzed by glutamate dehydrogenase. This method was applied to the measurement of myo-inositol in rat pancreatic islets. The myo-inositol islet content was decreased when the animals were fed a diet deprived of myo-inositol. When incubated in the absence of exogenous D-glucose, pancreatic islets, like parotid cells, released myo-inositol in the incubation medium. Over 90 min of incubation, a rise in extracellular D-glucose concentration increased the myo-inositol islet content, which was decreased, however, after incubation in the presence of carbamylcholine. These findings indicate that the myo-inositol content of islets is affected by nutritional and other environmental factors.

Inositol and inositol 1,4,5-trisphosphate content of Down syndrome fibroblasts exhibiting enhanced inositol uptake

Fibroblasts from individuals with Down syndrome (DS; trisomy 21) exhibit increased inositol uptake. Here we examine the relationship between this increase in uptake and mass levels of free inositol and inositol 1,4,5-trisphosphate (IP3) in DS fibroblasts. We report that human fibroblasts contain high levels of free inositol which are not significantly affected by the increase in inositol uptake associated with DS. In addition, increased uptake is accompanied by increased efflux of radiolabelled inositol from DS cells. Neither basal nor bradykinin-stimulated IP3 levels in DS cells differ significantly from normal values. This work highlights the usefulness of the DS cells in uncovering the role of transport across the plasma membrane in cellular inositol homeostasis.

Evidence for attenuation of myo-inositol uptake, phosphoinositide turnover and inositol phosphate production in aortic vasculature of rats during pregnancy

We postulated that vascular phosphoinositide metabolism is attenuated during pregnancy, and thereby could contribute to maternal vasodilation and reduced vascular reactivity. The basal rate of incorporation of [3H]myo-inositol and [3H]glycerol into phosphoinositides of aortae from pregnant rats in vitro was significantly reduced, when compared with vessels from virgin animals. After injection of [3H]myo-inositol intravenously into chronically instrumented conscious pregnant and virgin rats, the incorporation of the label by phosphatidylinositol was 66 +/- 4% less in aortae of gravid versus virgin animals (P less than 0.001), despite comparable plasma concentrations of radioactivity. Fold stimulation of total [3H]inositol phosphates by arginine vasopressin, norepinephrine, and angiotensin II over a 15-min period was not different between aortic segments from virgin and gravid rats, although both absolute basal and stimulated levels were significantly less in vessels from pregnant animals. After 45 s of incubation with 10(-7) M arginine vasopressin, however, the fold-stimulation of [3H]inositol trisplus tetrakisphosphate was reduced in aortae from gravid rats, when compared with vessels from virgin animals (P less than 0.005). By HPLC, greater than 90% of the radioactivity in the [3H]inositol trisplus tetrakisphosphate column fraction after 30 and 60 s of agonist stimulation was [3H]inositol-1,4,5-trisphosphate. We further observed that the rate of uptake of [3H]myo-inositol by aortic vasculature obtained from gravid rats was significantly (24%) less than uptake by vessels from virgin animals. Plasma myo-inositol concentrations were not significantly different, but presumably as a consequence of reduced uptake, aortic segments freshly isolated from pregnant rats contained 22 +/- 6% less myo-inositol than vessels from virgin controls as measured by gas chromatography-mass spectrometry (P less than 0.03). We conclude that myo-inositol uptake and content, phosphoinositide turnover, and inositol phosphate production are reduced in aortic vasculature of gravid rats.

Measurement of picomole amounts of any inositol phosphate isomer separable by h.p.l.c. by means of a bioluminescence assay

An assay is described which allows the determination of the mass of any individual inositol phosphate (InsP) isomer by combining a popular h.p.l.c. separation method with simple desalting, dephosphorylation and final measurement of Ins liberated using an inositol dehydrogenase-NADH-linked bioluminescence reaction. The limit of sensitivity of this assay is about 1 pmol of Ins. routinely 5 pmol. About 40 mg wet wt. of guinea pig small intestine longitudinal smooth muscle contains 5 pmol of Ins(1,4,5)P3. For Inst(1,3,4)P3 or Ins(1,3,4,5)P4 slightly more smooth muscle is needed, and for major isomers of InsP1 or InsP2 10 mg wet wt. or less of tissue can be used. A 35 mm tissue culture plate with a confluent layer of rat fibroblasts contains about 30 pmol of Ins(1,4,5)P3. The method was applied to the measurement of the masses of Ins1P1. Ins4P1, Ins(1,4)P2, Ins(1,3,4)P3, Ins(1,4,5)P3, Ins(1,3,4,5)P4 and InsP5. The h.p.l.c. elution profiles of radiolabelled InsPS generated from [32P]Pi-labelled human Erythrocytes, [3H]Ins-labelled cultured rat fibroblasts and [3H]Ins-labelled smooth muscle fragments from guinea pig small intestine were compared with the h.p.l.c. elution profiles of their masses.

Effect of myo-inositol on renal Na-K-ATPase in experimental diabetes

The activity of Na-K-ATPase in the kidney is increased by experimental diabetes. Because the kidney is rich in myo-inositol and abnormal inositol metabolism has been implicated in early neural complications of diabetes, we studied the effect of myo-inositol supplementation on Na-K-ATPase activity in renal medullary and cortical homogenates of Sprague-Dawley rats made diabetic with streptozotocin. Myo-inositol (650 mg/kg) was administered by gavage daily for 1 and 2 weeks after induction of diabetes. Medullary Na-K-ATPase (mumol/mg protein/h) was increased at 1 week by approximately 60% in diabetic rats versus control (25.9 +/- 0.07 vs 16.3 +/- 0.7; P less than .01). This increase was completely prevented by myo-inositol supplementation, despite persistent hyperglycemia. At 2 weeks, similar results were seen; medullary Na-K-ATPase activity was increased by 50% in diabetic rats compared with control, and once again myo-inositol prevented this increase. Sorbinil, the aldose reductase inhibitor, was also administered by gavage (20 mg/kg) for 2 weeks and partially prevented the increase in medullary Na-K-ATPase activity (20.0 +/- 0.9; P less than .05). At both 7 and 14 days, Na-K-ATPase activity in the cortex of untreated diabetic rats was also significantly increased compared with nondiabetic control rats and the increase was prevented by myo-inositol or Sorbinil. Myo-inositol or Sorbinil did not reduce Na-K-ATPase activity of nondiabetic control rats, nor did they prevent the increase in medullary Na-K-ATPase in compensatory hypertrophy following uninephrectomy. Myo-inositol content of outer medulla was about five to six times that of cortex, but was unaltered by the diabetic state.(ABSTRACT TRUNCATED AT 250 WORDS)

The phosphatidylinositol synthase of proximal tubule cells

Phosphatidylinositol (PI) is a precursor for an important class of phospholipids, the phosphatidylinositol polyphosphates. Because renal myo-inositol levels may vary under both physiological (e.g., antidiuretic) and pathophysiological (e.g., diabetic) conditions, the formation of PI from CDP-diacylglycerol (CDP-DG) and myo-inositol via phosphatidylinositol synthase and the regulation of this enzyme have important implications for the cellular biology of renal epithelia. We sought to understand the role of PI synthase by determining its subcellular localization, kinetic properties and regulation in rabbit proximal tubule cells. Proximal tubule cells were isolated from New Zealand White rabbits. The subcellular synthesis of PI was assessed by [32P]orthophosphate labelling with subsequent subcellular fractionation. Labelling of PI was time-dependent and consistent with the rapid incorporation of 32PO4 into basolateral, brush-border, microsomal and nuclear fractions. Pulse-chase labelling of proximal tubule cells was consistent with the formation of PI in microsomal fraction of the proximal tubule cells in addition to both brush-border and basolateral membranes. Conversely, phosphatidylcholine, phosphatidylethanolamine and phosphatidylglycerol displayed radiolabelling patterns consistent with microsomal synthesis alone. The in situ formation of phosphatidylinositol was substantiated by the direct measurement of phosphatidylinositol synthase activity in basolateral, brush-border and microsomal fractions. The apparent Km values for myo-inositol were 0.32 +/- 0.19, 0.39 +/- 0.21 and 0.23 +/- 0.05 mM, and for CDP-DG were 0.12 +/- 0.02, 0.14 +/- 0.05 and 0.12 +/- 0.02 mM in basolateral, brush-border and microsomal fractions, respectively. Vmax values for phosphatidylinositol formation were slightly, but not significantly greater, in microsomal than for plasma membrane fractions. Moreover, based on enzymatic enrichment data, plasma membrane PI synthase activity could not be explained by microsomal cross-contamination alone. PI synthase activity was inhibited by co-incubation with PI without differences among the cellular fractions. Intracellular myo-inositol concentration in the proximal tubule cells as measured by gas-liquid chromatography was 20.5 mM, significantly greater than the apparent Km values for myo-inositol. In conclusion, the in situ synthesis of phosphatidylinositol occurs in several membrane fractions; the kinetic properties of phosphatidylinositol synthase appear to be similar in each fraction; and phosphatidylinositol synthase in proximal tubule cells is inhibited by its own formation product. These data suggest that myo-inositol concentration alone is unlikely to be an important regulator of the chemical mass of phosphatidylinositol at the levels of this polyol observed in rabbit kidney.

Competitive inhibition by glucose of myo-inositol incorporation into cultured porcine aortic endothelial cells

To explore the significance of hyperglycaemia as a causal factor for the appearance of diabetic angiopathies we investigated aspects of myo-inositol metabolism in porcine aortic endothelial cells. myo-Inositol was shown to be a long-living metabolite. Its uptake into the cells was mediated by a high-affinity, Na(+)-dependent uptake system inhibitable by ouabain with an apparent KM of 18.6 mumols/l, which was responsible for more than 80% of total uptake at physiological myo-inositol concentrations. Inhibition of inositol uptake by D-glucose was exclusively competitive with an apparent Ki of 24 mmol/l as shown by Lineweaver-Burk- and Dixon-plot analysis. The specificity of competitive inhibition was studied. L-Glucose which is stereochemically related to myo-inositol in the same way as the D-isomer proved to be an equally potent inhibitor. The hexoses D-galactose, D-mannose and D-fructose inhibited myo-inositol uptake to a minor extent. D-allose and 3-O-methyl-D-glucose had no inhibitory effect indicating that the OH-group of the carbon atom in 3 position is essential for the interaction with the carrier. The acyclic hexitol sorbitol also did not compete. As expected, the aldose reductase blocker sorbinil did not influence the carrier since there is no polyol pathway operating in porcine aortic endothelial cells. In accordance with the results of the uptake experiments, the incorporation of exogenous myo-inositol into membrane phosphatidylinositol was reduced at elevated extracellular glucose levels. The results raise the possibility that hyperglycaemia impairs endothelial inositol supply.

Myo-inositol transport into endothelial cells derived from nervous system microvessels

Myo-inositol, the precursor in the biosynthesis of inositol phospholipids and inositol phosphates, is found in many tissues at concentrations well above its concentration in the plasma, but the highest concentrations are in the central nervous system and the neuroretina. We describe an active, sodium gradient-dependent transport of myo-inositol into cultured endothelial cells derived from bovine retinal microvessels. Transport is inhibited by cytochalasin B, and phloridzin greater than phloretin. Mannitol, sorbitol, and fructose do not inhibit uptake, but D-galactose. inhibits uptake greater than L-glucose greater than D-glucose. The apparent Km of this transport system is 311 +/- 47 (S.D.) microM and the apparent Vmax is 40.8 +/- 2.8 (S.D.) pmol.mg protein-1.min-1. This transport system may be a key in the maintenance of this tissue concentrations as it could concentrate myo-inositol from the plasma into the extracellular spaces of the eye and central nervous system.

Product-precursor relationships amongst inositol polyphosphates. Incorporation of [32P]Pi into myo-inositol 1,3,4,6-tetrakisphosphate, myo-inositol 1,3,4,5-tetrakisphosphate, myo-inositol 3,4,5,6-tetrakisphosphate and myo-inositol 1,3,4,5,6-pentakisphosphate in intact avian erythrocytes

Avian erythrocytes were incubated with myo-[3H]inositol for 6-7 h and with [32P]Pi for the final 50-90 min of this period. An acid extract was prepared from the prelabelled erythrocytes, and the specific radioactivities of the gamma-phosphate of ATP and of both the myo-inositol moieties (3H, d.p.m./nmol) and the individual phosphate groups (32P, d.p.m./nmol) of [3H]Ins[32P](1,3,4,6)P4,[3H]Ins[32P](1,3,4,5)P4, [3H]Ins[32P](3,4,5,6)P4 and [3H]Ins[32P](1,3,4,5,6)P5 were determined. The results provide direct confirmation that one of the cellular InsP4 isomers is Ins(1,3,4,5)P4 which is synthesized by sequential phosphorylation of the 1,4,5 and 3 substitution sites of the myo-Ins moiety, precisely as previously deduced [Batty, Nahorski & Irvine (1985) Biochem. J. 232, 211-215; Irvine, Letcher, Heslop & Berridge (1986) Nature (London) 320, 631-634]. This is compatible with the proposed synthetic route from PtdIns via PtdIns4P, PtdIns(4,5)P2 and Ins(1,4,5)P3. The data also suggest that, in avian erythrocytes, the principle precursor of Ins(1,3,4,5,6)P5 is Ins(3,4,5,6)P4. Furthermore, if the gamma- (and/or beta-) phosphate of ATP is the precursor of the phosphate moieties of Ins(3,4,5,6)P4, then this isomer must be derived from the phosphorylation of Ins(3,4,6)P3. If the gamma- (and/or beta-) phosphate of ATP similarly acts as the ultimate precursor to all of the phosphates of Ins(1,3,4,6)P4, then, in intact avian erythrocytes, the main precursor of Ins(1,3,4,6)P4 is Ins(1,4,6)P3. This contrasts with the expectation, based on results with cell-free systems, that Ins(1,3,4,6)P4 is synthesized by the direct phosphorylation of Ins(1,3,4)P3.

Mass measurement of inositol phosphates

This review summarises the methods available for the mass measurement of inositol phosphates, i.e., use of radioactive inositol lipid precursors, optical techniques, gas chromatography, mass spectrometry, nuclear magnetic resonance, fast atom bombardment and assays specific for Ins(1,4,5)P3. Examples of the use of each method, its sensitivity, advantages and drawbacks are given.

Methods for the analysis of inositol phosphates

Interest in the inositol phospholipids was stimulated by the simultaneous discoveries that the products of hydrolysis of these lipids could serve as messengers to activate to synergistic signaling pathways in hormonally responsive cells, namely, inositol 1,4,5-trisphosphate which causes the release of Ca2+ from intracellular stores and diacylglycerol which promotes the activation of protein kinase C. At the same time, Berridge and co-workers introduced relatively simple approaches to study the inositol phospholipid cycle. These included the use of [3H]inositol to label the inositol metabolites, all of which are confined to this cycle, and of Li+ to decrease the rate of degradation of the inositol phosphates. Water-soluble inositol phosphates and chloroform-soluble inositol phospholipids could then be separated by solvent partition and the inositol phosphates further separated by use of an anion-exchange resin. However, the subsequent application of high-performance liquid chromatography as a separation technique indicated the existence of many isomers of the inositol phosphates formed by different pathways of dephosphorylation and phosphorylation. Mapping of these metabolic pathways may be substantially complete, but novel pathways may still be discovered. We review both old and new methods of analysis of the inositol phosphates for the measurement of mass and radioactivity. Although the complexity of the cycle sometimes demands the use of sophisticated methods of separation and rigorous identification, older and inexpensive methods may still be useful for some purposes.

Osmoregulatory changes in myo-inositol transport by renal cells

Renal medullary cells contain high concentrations of myo-inositol, sorbitol, betaine, and glycerophosphocholine, whose levels vary with urinary osmolality. Accumulation of these "compatible" organic osmolytes is believed to help the cells osmoregulate in response to the high extracellular osmolality that occurs as part of the urinary concentrating mechanism. MDCK cells (a line from dog kidney) were previously shown to accumulate myo-inositol in response to increased medium osmolality. We demonstrate here that this accumulation requires the presence of myo-inositol in the medium, implying that the myo-inositol is not synthesized by the cells but rather is transported into them from the extracellular solution. The MDCK cells contain sodium-dependent myo-inositol transporters. Relative to isotonic controls, sodium-dependent myo-inositol uptake is higher in cells exposed to increased osmolality either acutely (1-7 days) or chronically (greater than 1 year). Transport is further enhanced when the cells are cultured in myo-inositol-free medium. The transport has both high- and low-affinity components. The observed changes in transport involve changes in maximal velocity of the high-affinity component but not in its Km. We conclude that renal cells can osmoregulate by changing the number (or, less likely, the transport turnover rate) of functioning sodium-dependent myo-inositol transporters.

Two-step high-performance liquid chromatography method for the determination of myo-inositol and sorbitol

A simple two-step HPLC method for the separation and quantitation of myo-inositol and sorbitol in extracts of glomeruli from rat kidneys is described. The limit of detection is 2 ng. The procedure involves fractionation of the sugar alcohols on a Waters Sugar Pak column, preparation of the p-nitrobenzoate derivatives, and further purification with quantitation by absorbance at 254 nm using a Waters mu Porasil column. The applicability of the procedure to determination of sorbitol and myo-inositol in biological samples was demonstrated by the finding of marked alterations in sorbitol and myo-inositol content of glomeruli isolated from diabetic compared to that from normal rat kidneys.

Enzyme assay of L-myo-inositol 1-phosphate synthetase based on high-performance liquid chromatography of benzoylated inositol

A procedure for the determination of inositol by reversed-phase HPLC is described which is based on a precolumn benzoylation and detection at 230 nm. This procedure was used to assay the activity of L-myo-inositol 1-phosphate synthetase (EC 5.5.1.4) after treatment of the enzymatic product by a phosphatase.

Differential regulation of protein kinase C and (Na,K)-adenosine triphosphatase activities by elevated glucose levels in retinal capillary endothelial cells

Elevated cellular sorbitol levels resulting from conversion of increased glucose by aldose reductase might deplete cellular myoinositol content, which could then lower inositol phosphates (InsPs) and diacylglycerol levels, key regulators of protein kinase C (PKC). Secondary to altered PKC activity, other cellular enzymes such as (Na,K)-ATPase could be affected. To test this hypothesis we examined the association between PKC activity, (Na,K)-ATPase activity, and sorbitol, myoinositol, and InsP levels in cultured bovine retinal capillary endothelial cells, a cell type prominently involved in diabetic retinopathy. Elevating glucose concentration in culture media from 100 to 400 mg/dl led to a 100% increase in sorbitol levels, which could be inhibited completely by sorbinil, an aldose reductase inhibitor. In contrast, no changes were observed in myoinositol or InsP levels. Subfractionated PKC activities showed a 100% increase in the membranous pool with a parallel decrease in the cytosolic fraction. Adding sorbinil did not affect PKC activity, whereas the PKC agonist, phorbol myristate acetate (PMA), stimulated translocation of PKC. Ouabain-inhibitable (Na,K)-ATPase activity was decreased 70% by elevated glucose levels. This decrease could be prevented by adding either PMA or sorbinil. Thus, in retinal capillary endothelial cells elevated glucose concentration can affect PKC and (Na,K)-ATPase activities, probably via different mechanisms.

Chronic exposure to high glucose decreases myo-inositol in cultured cerebral microvascular pericytes but not in endothelium

It has been proposed that the development of diabetic complications may involve a depletion of cellular myo-inositol due to an increase in polyol (sorbitol) formation. We therefore initially examined the effect of diabetes on levels of these metabolites in isolated cerebral microvessels. Compared with microvessels from control rats, microvessels from diabetic animals showed no detectable alteration in myo-inositol levels and a small increase in sorbitol content. To assess whether myo-inositol depletion might occur in only certain microvascular cells, cultured bovine cerebral microvascular pericytes and endothelium were grown for 3 or 18-20 days at 1.1, 5.5, or 22.2 mmol/l glucose. Increased medium glucose concentration resulted in increased sorbitol content in both cell types after both periods of incubation (p less than 0.05). In contrast, a significant decrease in myo-inositol content (22%, p less than 0.01) was observed only in pericytes grown for 18-20 days in the high glucose medium. Neither the adenosine 5'-triphosphate content nor the adenosine 5'-triphosphate/adenosine 5'-diphosphate (ATP/ADP) ratio of the pericytes was affected by the medium glucose concentration, indicating that the decrease in myo-inositol was not caused by a deficiency in the cellular energy state affecting the active transport of myo-inositol. These data suggest that myo-inositol depletion occurs selectively in the pericyte, a cell type known to be the site of early morphological changes in diabetes. Furthermore, the depletion apparently requires prolonged exposure to high glucose and is not due to a change in energy state.

Survey of osmolytes in renal cell lines

In renal medullas during antidiuresis, the extracellular fluid is hyperosmotic because of high concentrations of NaCl and urea. Under those conditions, the cells contain high concentrations of organic osmolytes, namely sorbitol, myo-inositol, glycerophosphorylcholine (GPC), and betaine to balance the extracellular hyperosmolality. These organic osmolytes increase cell osmolality without perturbing the intracellular milieu in ways that would degrade the function of cellular macromolecules. The present study surveyed a number of cell lines for the ability to survive in media with high concentrations of NaCl and/or urea and for the accumulation of organic osmolytes. Of the renal cell lines tested, MDCK, GRB-MAL1, and A6 cells proliferated in hyperosmotic media, but medullary interstitial cells LLC-PK1 and LLC-PK3 did not proliferate, nor did nonrenal HTC-BH cells, MDCK, LLC-PK1, and LLC-PK3 cells contained higher concentrations of myo-inositol, GPC, and betaine when cultured in media containing high NaCl (with or without high urea) and much lower or undetectable levels of these osmolytes when grown in isosmotic media. Sorbitol, and to a lesser extent myo-inositol, were elevated in GRB-MAL1 cells in media hyperosmotic with NaCl but not in isosmotic media. There was less accumulation of organic osmolytes when only urea was added to increase osmolality. Thus the same osmolytes were accumulated by one or another cell line in vitro as were previously found in renal medullas. These cell lines provide models for studying osmolyte accumulation.

Bradykinin-stimulated changes in inositol phosphate mass in renal papillary collecting tubule cells

The effect of bradykinin on changes in the chemical levels of myo-inositol polyphosphates in renal papillary collecting tubules was investigated. Myo-inositol phosphate mass was determined by means of an enzymatic, fluorometric assay. Bradykinin induced increases in myo-inositol mono-, bis-, and trisphosphate which were both time and concentration dependent. Furthermore, the magnitude of the chemical levels of myo-inositol monophosphate formed were unlikely to be accounted for solely by the formation and degradation of myo-inositol trisphosphate. These observations are consistent with the concomitant hydrolysis of phosphatidylinositol and phosphatidylinositol bisphosphate. This study also confirms, in freshly isolated renal tubules, observations regarding bradykinin-induced phosphatidylinositol bisphosphate hydrolysis made previously in radiolabeled cultures.