Active transport of myo-inositol in rat pancreatic islets

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.

Sodium/myo-inositol cotransporter 1 and myo-inositol are essential for osteogenesis and bone formation

myo-Inositol (MI) plays an essential role in several important processes of cell physiology, is involved in the neural system, and provides an effective treatment for some psychiatric disorders. Its role in osteogenesis and bone formation nonetheless is unclear. Sodium/MI cotransporter 1 (SMIT1, the major cotransporter of MI) knockout (SMIT1(-/-)) mice with markedly reduced tissue MI levels were used to characterize the essential roles of MI and SMIT1 in osteogenesis. SMIT1(-/-) embryos had a dramatic delay in prenatal mineralization and died soon after birth owing to respiratory failure, but this could be rescued by maternal MI supplementation. The rescued SMIT1(-/-) mice had shorter limbs, decreased bone density, and abnormal bone architecture in adulthood. Deletion of SMIT1 resulted in retarded postnatal osteoblastic differentiation and bone formation in vivo and in vitro. Continuous MI supplementation partially restored the abnormal bone phenotypes in adult SMIT1(-/-) mice and strengthened bone structure in SMIT1(+/+) mice. Although MI content was much lower in SMIT1(-/-) mesenchymal cells (MSCs), the I(1,4,5)P(3) signaling pathway was excluded as the means by which SMIT1 and MI affected osteogenesis. PCR expression array revealed Fgf4, leptin, Sele, Selp, and Nos2 as novel target genes of SMIT1 and MI. SMIT1 was constitutively expressed in multipotential C3H10T1/2 and preosteoblastic MC3T3-E1 cells and could be upregulated during bone morphogenetic protein 2 (BMP-2)-induced osteogenesis. Collectively, this study demonstrated that deficiency in SMIT1 and MI has a detrimental impact on prenatal skeletal development and postnatal bone remodeling and confirmed their essential roles in osteogenesis, bone formation, and bone mineral density (BMD) determination.

Sodium-dependent Transport of [3H](1d)chiro-Inositol by Tetrahymena

The transport characteristics of (1D)chiro-inositol by the ciliate Tetrahymena were examined in competition studies employing [3H](1D)chiro-inositol. (1D)chiro-Inositol transport was competed by unlabeled (1D)chiro-inositol, myo-inositol, scyllo-inositol, and D-glucose in a concentration-dependent manner. Conversely, (1D)chiro-inositol competed for [3H]myo- and [3H]scyllo-inositol transport. Lineweaver-Burke analysis of the competition data indicated a Km of 10.3 mM and a Bmax of 4.7 nmol/min/mg for (1D)chiro-inositol. Transport of (1D)chiro-inositol was inhibited by cytochalasin B, an inhibitor of facilitated glucose transporters, and phlorizin, an inhibitor of sodium-dependent transporters. Removal of sodium from the radiolabeling buffer also inhibited uptake. The presence of 0.64 mM calcium or magnesium ions exerted negligible effects on transport, although potassium was inhibitory. [3H](1D)chiro-Inositol was shown to be incorporated into Tetrahymena phosphoinositides.

Differential expression, activity and regulation of the sodium/myo-inositol cotransporter in astrocyte cultures from different regions of the rat brain

The high-affinity sodium/myo-inositol cotransporter (SMIT) is involved in osmoregulation in several cells and tissues. In the CNS the activity of SMIT also determines the individual susceptibility of neural cells to the inositol depleting effect of lithium, which is considered to be important in lithium's therapeutic effects in manic-depressive illness. Among neural cells SMIT is particularly active in astrocytes. In the present work we have cloned the cDNA of SMIT of the rat and assessed its activity, expression and regulation in primary astroglia cultures derived from five different rat brain regions: cerebellum, cortex, diencephalon, hippocampus and tegmentum. After an incubation period of 24 h in medium containing 3[H]labeled myo-inositol different steady-state concentrations were detected which were dependent on the brain region from which the astrocytes were cultured. In addition, myo-inositol uptake in astrocytes from different areas was characterized by two different Km values (27 microM for cerebellum and diencephalon, 50 microM for cortex, hippocampus and tegmentum) and by three different v(max) values (approx. 200 pmol/mg protein/min for astrocytes from cerebellum and tegmentum, 298 for hippocampus and 465 for cortex), indicating that the active myo-inositol uptake into astroglial cells is distinct in the various brain regions. The efficacy of uptake as determined by v(max) values of 3[H]myo-inositol uptake correlated with the level of mRNA of SMIT in the astrocyte cultures from the various brain regions as determined by semiquantitative reverse transcription-polymerase chain reaction (RT-PCR). Both 3[H]myo-inositol uptake and SMIT mRNA content was upregulated by incubation of astrocytes in medium of increased osmolarity. In astrocytes from cerebellum, cortex, hippocampus and tegmentum 3[H]myo-inositol uptake was downregulated by chronic incubation with 400 microM inositol. This effect was not observed in astrocytes from diencephalon. Furthermore, in astrocytes from cortex and hippocampus but not from cerebellum, diencephalon and tegmentum incubation with corticosterone for three days upregulated 3[H]myo-inositol uptake. It is concluded that SMIT is differentially expressed and regulated in astrocytes from distinct brain regions. These regional differences suggest particular consideration of localized effects in investigations of the role of myo-inositol in the mechanism of action of antibipolar drugs.

Alternate splicing in human Na+-MI cotransporter gene yields differentially regulated transport isoforms

myo-Inositol is a ubiquitous intracellular organic osmolyte and phosphoinositide precursor maintained at millimolar intracellular concentrations through the action of membrane-associated Na+-myo-inositol cotransporters (SMIT). Functional cloning and expression of a canine SMIT cDNA, which conferred SMIT activity in Xenopus oocytes, predicted a 718-amino acid peptide homologous to the Na+-glucose cotransporter with a potential protein kinase A phosphorylation site and multiple protein kinase C phosphorylation sites. A consistent approximately 1.0- to 13.5-kb array of transcripts hybridizing with this cDNA are osmotically induced in a variety of mammalian cells and species, yet SMIT activity appears to vary among different tissues and species. An open reading frame on human chromosome 21 (SLC5A3) homologous to that of the canine cDNA (96.5%) is thought to comprise an intronless human SMIT gene. Recently, this laboratory ascribed multiply sized, osmotically induced SMIT transcripts in human retinal pigment epithelial cells to the alternate utilization of several 3'-untranslated SMIT exons. This article describes an alternate splice donor site within the coding region that extends the open reading frame into the otherwise untranslated 3' exons, potentially generating novel SMIT isoforms. In these isoforms, the last putative transmembrane domain is replaced with intracellular carboxy termini containing a novel potential protein kinase A phosphorylation site and multiple protein kinase C phosphorylation sites, and this could explain the heterogeneity in the regulation and structure of the SMIT.

Effect of osmolality and myo-inositol deprivation on the transport properties of myo-inositol in primary astrocyte cultures

myo-Inositol uptake measured in primary astrocyte cultures was saturable in the presence of Na+ with a Km of 13-18 microM and a Vmax of 9.4 nmoles/mg protein/hour in myo-inositol-fed cells, indicating a high affinity transport system. In myo-inositol-deprived cells, Km was about 53 microM with a Vmax of 13.2 nmoles/mg protein/hour. Decreasing osmolality decreased the Vmax to about 1.9 nmoles/mg protein/hour whereas increasing osmolality increased Vmax about 5-fold, while Kms were essentially unchanged in myo-inositol fed cells. In cells deprived of myo-inositol, Vmax decreased in hypotonic medium and increased in hypertonic medium almost 10-fold, but with more than a doubling of the Km regardless of the osmolality. Glucose (25 mM) inhibited myo-inositol uptake 51% whereas the other hexoses used inhibited uptake much less. Our findings indicate that myo-inositol uptake in astrocytes occurs through an efficient carrier-mediated Na(+)-dependent co-transport system that is different from that of glucose and its kinetic properties are affected by myo-inositol availability and osmotic stress.

Regulation of myo-inositol transport during the growth and differentiation of thyrocytes: a link with thyroid-stimulating hormone-induced phospholipase A2 activity

The Vmax of myo-inositol transport increased 3-fold during epidermal growth factor (EGF)-induced growth and thyroid-stimulating hormone. (TSH)-induced differentiation in primary cultures of sheep and human thyrocytes. The Km remained unaltered. This up-regulation required the presence of insulin. The TSH-induced rise in myo-inositol transport commenced 8 to 16 h after the initial stimulus and achieved a plateau at 24 h. In human thyrocytes the change in Vmax was accompanied by an increase in the steady-state levels of mRNA for the myo-inositol transporter following treatment with either ligand. Examination of the metabolites of myo-inositol showed few significant changes after treatment of sheep thyrocytes with EGF for 24 h. This is consistent with maintenance of the intracellular concentration of myo-inositol as the cells enlarge in preparation for cell division. In TSH-treated cells, however, up-regulation of myo-inositol transport was linked with increased myo-inositol cycling across the cell membrane, increased phospholipase A2-mediated turnover of phosphatidylinositol and a concomitant increase in arachidonic acid turnover. Increased levels of myo-inositol phosphates were also noted 24 h after TSH treatment. These results indicate the initiation of secondary signalling events many hours after the primary stimulus.

Homologous and heterologous regulation of receptor-stimulated phosphoinositide hydrolysis

Signal transduction at a diverse range of pharmacologically distinct receptors is effected by the enhanced turnover of inositol phospholipids, with the attendant formation of inositol 1,4,5-trisphosphate and diacylglycerol. Although considerable progress has been made in recent years towards the identification and characterization of the individual components of this pathway, much less is known of mechanisms that may underlie its regulation. In this review, evidence is presented for the potential regulation of inositol lipid turnover at the level of receptor, phosphoinositide-specific phospholipase C and substrate availability in response to either homologous or heterologous stimuli. Available data indicate that the extent of receptor-stimulated inositol lipid hydrolysis is regulated by multiple mechanisms that operate at different levels of the signal transduction pathway.

Mitogen-activated protein kinase and its activator are regulated by hypertonic stress in Madin-Darby canine kidney cells

Madin-Darby canine kidney cells behave like the renal medulla and accumulate small organic solutes (osmolytes) in a hypertonic environment. The accumulation of osmolytes is primarily dependent on changes in gene expression of enzymes that synthesize osmolytes (sorbitol) or transporters that uptake them (myo-inositol, betaine, and taurine). The mechanism by which hypertonicity increases the transcription of these genes, however, remains unclear. Recently, it has been reported that yeast mitogen-activated protein (MAP) kinase and its activator, MAP kinase-kinase, are involved in osmosensing signal transduction and that mutants in these kinases fail to accumulate glycerol, a yeast osmolyte. No information is available in mammals regarding the role of MAP kinase in the cellular response to hypertonicity. We have examined whether MAP kinase and MAP kinase-kinase are regulated by extracellular osmolarity in Madin-Darby canine kidney cells. Both kinases were activated by hypertonic stress in a time- and osmolarity-dependent manner and reached their maximal activity within 10 min. Additionally, it was suggested that MAP kinase was activated in a protein kinase C-dependent manner. These results indicate that MAP kinase and MAP kinase-kinase(s) are regulated by extracellular osmolarity.

Osmotic regulation of myo-inositol uptake in primary astrocyte cultures

Uptake of myo-inositol by astrocytes in hypertonic medium (440 mosm/kg H2O) was increased near 3-fold after incubation for 24 hours, which continued for 72 hours, as compared with the uptake by cells cultured in isotonic medium (38 nmoles/mg protein). myo-Inositol uptake by astrocytes cultured in hypotonic medium (180 mosm/kg H2O) for periods up to 72 hours was reduced by 74% to 8 to 10 nmoles/mg protein. Astrocytes incubated in either hypotonic or hypertonic medium for 24 hours and then placed in isotonic medium reversed the initial down- or up-regulation of uptake. Activation of chronic RVD and RVI correlates with regulation of myo-inositol uptake. A 30 to 40 mosm/kg H2O deviation from physiological osmolality can influence myo-inositol homeostasis. The intracellular content of myo-inositol in astrocytes in isotonic medium was 25.6 +/- 1.3 micrograms/mg protein (28 mM). This level of myo-inositol is sufficient for this compound to function as an osmoregulator in primary astrocytes and it is likely to contribute to the maintenance of brain volume.

Age-dependent neurotoxicity in rats chronically exposed to low level lead ingestion: phospholipid metabolism in synaptosomes and microvessels

The uptake of [3H]Ch and [3H]MI by synaptosomes or microvessels, the concentration of membrane phospholipids, and the incorporation of [3H]Ch or [3H]MI into the respective phospholipids in synaptosomes or microvessels, were studied in samples obtained from the brain of control rats and rats exposed to a low-level lead ingestion starting prenatally, neonatally or at an adult age. The Vmax values for the uptake of [3H]Ch by control-neonatal and control-adult samples were significantly different. However, there was no significant difference in the Vmax values for the uptake of [3H]MI by control-neonatal and control-adult samples. The same was true for the Km values for the uptake of [3H]Ch or [3H]MI. Chronic exposure of embryonic and neonatal rats to a low-level lead ingestion inhibited the rate of uptake of [3H]Ch and [3H]Mi by the brain synaptosomes or microvessels, reduced the concentrations of Ch and MI phospholipids in membranes of these tissues, and did not effect the incorporation of [3H]Ch and [3H]MI into the respective membrane phospholipids. In adult rats, these changes were not observed following chronic exposure. These observations suggest that Ch and MI transport mechanisms in the brain of embryonic and neonatal rats are sensitive to chronic low-level lead ingestion but Ch and MI transport mechanisms in the brain of adult rats are not. A lead-induced decrease in the availability of Ch and MI in the brain may be responsible for the observed decrease in the concentrations of phospholipids.

myo-inositol transport and metabolism in fetal-bovine aortic endothelial cells

The myo-inositol transport system in confluent fetal-bovine aortic endothelial cells was characterized after 7-10 days in subculture, at which time the myo-inositol levels and rates of myo-[2-3H]-inositol uptake and incorporation into phospholipid had reached steady state. Kinetic analysis indicated that the uptake occurred by both a high-affinity transport system with an apparent Kt of 31 microM and Vmax. of 45 pmol/min per mg of protein, and a non-saturable low-affinity system. Uptake was competitively inhibited by phlorhizin, with a Ki of 50 microM; phloretin was a non-competitive inhibitor, with half-maximal inhibition between 0.2 and 0.5 mM. Glucose was a weak competitive inhibitor, with a Ki of 37 mM; galactose failed to inhibit uptake. A weak dependence on Na+ for the initial rate of uptake was observed at 11 microM myo-inositol. When fetal-bovine-serum (FBS)-supplemented medium, which contained 225 microM myo-inositol, was used, the cells contained about 200 nmol of myo-inositol/mg of DNA. With adult-bovine-serum (ABS)-supplemented medium, which contained 13 microM myo-inositol, the cells contained about 110 nmol/mg of DNA. Transport of 11 microM myo-[2-3]inositol was 18 and 125 pmol/min per mg of DNA for cells grown in FBS and ABS respectively. Kinetic analysis showed that for the cells grown in FBS the Vmax. of the high-affinity system was decreased by 64%, whereas the Kt remained essentially unchanged. Increased cell myo-inositol levels were not associated with an increased rate of phosphatidylinositol synthesis. After prolonged exposure of fetal endothelial cells to a myo-inositol concentration which approximated to a high fetal as opposed to a low adult blood level, cell myo-inositol levels doubled and high-affinity transport underwent down-regulation.

The characteristics, capacity and receptor regulation of inositol uptake in 1321N1 astrocytoma cells

The uptake of inositol into 1321N1 astrocytoma cells was studied by measurement of the accumulation of free [3H]inositol within the intracellular pool. Uptake occurs via a saturable transporter with apparent Km for inositol approximately 40 microM and Vmax approximately 180 pmol/min per mg of protein, which permits intracellular inositol concentrations to exceed those of the medium by a factor of approximately 500. At extracellular concentrations up to 500 microM, inositol uptake is highly dependent (> or = 85%) on the presence of Na+ in the medium, and at physiological extracellular inositol concentrations, allows inositol to achieve an intracellular concentration of approximately 20 mM, indicating an active process driven by the Na+ gradient. Despite this, uptake was only minimally impaired or was unaffected by ouabain (1 mM) or dinitrophenol (1 mM). Consistent with a carrier-mediated mechanism, uptake was competitively blocked by phlorhizin (K1 approximately 125 microM). Uptake was also inhibited by carbachol and histamine, which act respectively via muscarinic and H1 receptors in these cells to stimulate phospholipase C. Inhibition by carbachol was dose-dependent (EC50 approximately 3-30 microM) and blocked by atropine. Inhibition by carbachol (1 mM) was non-competitive, resulting from approximately 50% decrease in the Vmax for uptake without affecting the Km and was persistent over 30-90 min. Inhibition by carbachol and histamine was independent of extracellular Ca2+ and was reproduced by phorbol ester, but not by Ca2+ ionophore or stimulation of adenylate cyclase. These results imply that receptors which couple to phospholipase C may mediate inhibition of inositol uptake via protein kinase C. The data are discussed in relation to inositol homoeostasis in resting and stimulated cells.

Effects of fluorinated inositols on the proliferation of Swiss 3T3 fibroblasts

The six monodeoxyfluoro-myo-inositols (nFIns) have previously been synthesized as potential inhibitors of signalling pathways mediated by phosphoinositides and their derivatives. Each of the six nFIns isomers was introduced into Swiss 3T3 fibroblasts by the techniques of microinjection or scrape loading at intracellular concentrations of approx. 2-4 mM. Of the six nFIns analogues, only 3FIns and 5FIns inhibited the serum-stimulated proliferation of 3T3 fibroblasts assayed by cell counting. Proliferation was inhibited to a similar extent by 3FIns or 5FIns, irrespective of which technique was used to introduce the nFIns analogues into the cells. Proliferation of cells 35 h after serum stimulation (i.e. when the first cell cycle was completed in control cells) was inhibited by approx. 50% by both 3FIns and 5FIns, and entry into S phase in the first cell cycle was inhibited to the same extent. This indicated that the nFIns analogues were inhibiting proliferation in the G1 phase of the cell cycle. Proliferation during the second cell cycle (35-60 h after stimulation) was inhibited by 75-85%. The inhibitory nFIns analogues were not toxic to the cells, nor did they affect the cellular ATP/ADP ratio. The effectiveness of the nFIns analogues in inhibiting proliferation was directly correlated with their ability to be incorporated into phosphatidylinositol analogues, suggesting that they may act by modulating phosphoinositide signalling pathways or other functions essential for DNA synthesis.

Osmoregulation of Na(+)-inositol cotransporter activity and mRNA levels in brain glial cells

During plasma hypertonicity brain volume is regulated acutely by electrolyte uptake and chronically by accumulation of organic solutes such as inositol. Cultured rat C6 glioma cells, an astrocyte-like cell line, show a similar pattern of volume control. Volume regulatory accumulation of inositol requires external inositol, indicating that membrane transport plays a central role in this process. The inositol uptake pathway is Na+ dependent and exhibits Michaelis-Menten kinetics. Chronic hypertonic acclimation results in a twofold increase in the maximum velocity of the transporter without changing the Km. Hypertonic stress also results in a 17-fold increase in transporter mRNA. Elevation of mRNA levels precedes activation of the transporter by 4-6 h, suggesting that increased inositol uptake is mediated by synthesis and membrane insertion of new transport proteins. Reacclimation of hypertonic cells to isotonicity causes a rapid reduction of transporter mRNA levels to control levels within 4 h. In contrast, downregulation of transport activity does not begin until between 10 and 24 h after reexposure to isotonicity.

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.

In vivo use of neutral radiolabelled molecular probes to evaluate blood-ocular barrier integrity in normal and streptozotocin-diabetic rats

Evidence suggests that the consequences of diabetes mellitus are numerous and that net changes in ocular barrier permeability are necessarily complex functions of changes at specific anatomical loci. In this study we explore changes in blood-aqueous and vitreous permeability in streptozotocin-diabetic rats using five stable radiolabelled probes. Three probes, (3H)-L-glucose, (14C)-sucrose and (14C)-carboxylinulin are relatively large molecules and are expected to move into ocular humours via paracellular routes. Two probes, (14C)-urea and (14C)-glycerol, are small and likely have a trans-cellular component to permeability. Pulse-chase kinetic studies follow the appearance of test molecules into ocular humours with rate constants estimated via linear modelling. Larger neutral probes L-glucose, sucrose and carboxylinulin entered the aqueous humour of control rats slowly via routes that presumably circumvent tight-junctioned barriers. These slow-entry rates were found to increase in diabetic animals suggesting an increase in passive paracellular permeability with significant variation among animals. In contrast, aqueous entry rates of smaller probes urea and glycerol were decreased in diabetic animals suggesting that these probes cross membranes and cells less efficiently in diabetic animals. The magnitude of these changes increased with the length of exposure to diabetes. Paralleling the aqueous humour studies, we found a significant but variable increase in vitreous entry rate with L-glucose, sucrose and carboxylinulin, but a decrease in entry rates with small probes urea and glycerol. These results suggests that diabetes-related blood-ocular permeability changes are complex and depend on the size and properties of the probe as well as the degree of diabetes exposure.

myo-inositol transport in mouse astroglia-rich primary cultures

Uptake of radiolabeled myo-inositol was studied in astroglia-rich primary cultures derived from neonatal mouse brains. The uptake was saturable in the presence of Na+ with a Km of 25 microM and a Vmax of 60 pmol.min-1.(mg protein)-1, suggesting a high-affinity transport system for myo-inositol in astroglial cells. In addition, a Na(+)-independent, nonsaturable component was found. Carrier-mediated uptake was not inhibited by cytochalasin B (50 microM), but was reduced by depolarizing concentrations of K+ and, to different extents, in the presence of phloretin, ouabain, or amiloride (1 mM each). scyllo-Inositol, glucose, and galactose also reduced myo-inositol uptake; inhibition by the two hexoses was not reversed in the presence of 0.4 mM sorbinil. On the other hand, uptake of 2-deoxyglucose was not inhibited by high concentrations of myo-inositol. Preincubation of the cells with glucose-free or inositol-free medium stimulated uptake of myo-inositol and preincubation with 25 mM glucose in the presence of 0.4 mM sorbinil had no effect on the rate of uptake. The results suggest that myo-inositol is taken up into the astroglial cells by a transport mechanism that is distinct from that of glucose and probably is an active one. Sorbitol pathway activity does not interfere with myo-inositol uptake.

Kinetics of myo-inositol transport in corneal endothelial cells: diverse effects of sugars and implications in corneal deutergensence [corrected]

Kinetics of myo-inositol (MI) uptake into primary cultures of bovine corneal endothelial cells (CEC) were studied. Confluent corneal endothelial cells accumulated 3H-MI in a time dependent and saturable process. At a narrow range of external concentrations of 3H-MI (4-50 microM), the Na(+)-dependent MI uptake followed saturation kinetics. The apparent Km value was 20 microM with a maximum velocity (Vmax) of 16 pmol/20 min/micrograms DNA. At low external 3H-MI concentrations the uptake was dependent on Na ions, but at higher levels the Na(+)-independent fraction of MI uptake significantly increased. The uptake was sensitive to removal of Ca ions and to the presence of inhibitors such as n-ethyl maleimide, phlorizin, ouabain, and amiloride (an inhibitor of Na+/H+ exchanger). The sensitivity of MI uptake toward inhibitors and ionic changes in the bathing media was reduced as external concentrations of 3H-MI increased. Citrate at 0.5 mM increased the uptake, suggesting involvement of mitochondrial oxidative metabolism in the MI uptake. Percent release of radioactivity by 2 min, after an initial 40-min incubation with 20 microM 3H-MI, was 6.6% +/- 0.8 or 35% +/- 4 when release media contained BSS alone or BSS containing 5 mM nonradioactive MI, respectively. Efflux of radioactivity from the cells also was enhanced when release media contained 40 mM glucose. Glucose and galactose as well as nonmetabolizable glucose analogues, such as 3O-methyl glucose or alpha-methyl glucose, at high concentrations (40 mM), acutely (in the incubation media) or chronically (in the growth media) inhibited MI uptake into CEC, and the extent of inhibition was inversely proportional to the external levels of 3H-MI. However, glucose at lower levels (less than or equal to 10 mM) slightly increased MI uptake. These studies indicated that the uptake of MI into corneal endothelial cells was an Na(+)-dependent active process at a narrow range of external radioactive MI concentrations. Higher levels of MI were taken up by the cells via a passive diffusion mechanism, independent of carrier protein(s). Glucose influenced the uptake of MI in a complex manner. The increased MI efflux by glucose or by MI was perhaps due to the limited capacity of CEC for accumulation or compartmentalization of this or other solutes/osmolytes, a phenomenon that may be related to the role of CEC in maintenance of corneal deutergence. High glucose-induced inhibition of Na(+)-dependent MI uptake may be in part due to glucose regulation of Na+ fluxes and cell volume.(ABSTRACT TRUNCATED AT 400 WORDS)

Inositol uptake in rat aorta

The purpose of this study was to investigate the mechanism of inositol uptake into rat thoracic aorta. 3H-inositol uptake into deendothelialized aorta was linear for at least 2 h and was composed of both a saturable, Na(+)-dependent, and a nonsaturable, Na(+)-independent component. The Na(+)-dependent component of inositol uptake had a Km of 50 microM and a Vmax of 289 pmol/mg prot/h. Exposure to LiCl, ouabain, or Ca2(+)-free Krebs-Ringer bicarbonate solution inhibited uptake. Metabolic poisoning with dinitrophenol, as well as incubation with phloretin, an inhibitor of carrier-mediated hexose transport, also inhibited uptake. Exposure to norepinephrine decreased inositol uptake, while phorbol myristate acetate was without effect. Isobutylmethylxanthine significantly increased inositol uptake, while the increased uptake due to dibutyryl cyclic AMP and forskolin were not statistically significant. Sodium nitroprusside, an activator of guanylate cyclase, and 8-bromo cyclic GMP, were without effect on uptake, as was methylene blue, an inhibitor of guanylate cyclase. Inositol uptake into the aorta was increased when the endothelium was allowed to remain intact, although this effect was likely due to uptake into both the endothelial and smooth muscle cells. These results suggest that the uptake of inositol into vascular smooth muscle is: (1) dependent upon an inward Na(+)-gradient; (2) carrier mediated, and (3) inhibited by alpha 1 adrenoceptor agonists.

Studies on avian erythrocyte metabolism. XVII. Kinetics and transport properties of myo-inositol in chicken reticulocytes

The uptake of myo-inositol was determined in a reticulocyte-enriched fraction prepared from chicken blood and compared with uptake in mature erythrocytes. While reticulocytes accumulated inositol at levels more than threefold that of the plasma concentration, erythrocyte levels were only slightly higher than that of the plasma concentration. The rate of uptake in reticulocytes was approximately 66 mumol/ml rbc/h compared to 5 mumol/ml rbc/h in mature erythrocytes when measured at an inositol medium concentration of 250 microM. The kinetic analysis of inositol influx by reticulocytes reveals a two component system: saturable and nonsaturable. The saturable component, which has a Km for inositol of approximately 222 microM, is Na-dependent. This Na-dependent saturable component, which presumably reflects active transport of inositol, accounts for 30-35% of the transport process. The saturable component is completely inhibited by amiloride but to a lesser extent by ouabain and bumetanide. Moreover, in the course of reticulocyte maturation, the saturable component is lost concomitantly with the completion of the synthesis of myo-inositol pentakisphosphate and the drastic decrease in the membrane permeability to inositol. In addition, phloretin and cytochalasin B, which bind to hexose carriers and inhibit hexose sugar transport, also inhibited inositol transport. The uptake of inositol was not affected by excesses of 3-O-methylglucose (100 mM) or by physiological concentrations of D-glucose. Thus, the transport mechanism of myo-inositol appears distinct from that of D-glucose.

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.

Changes in extracellular calcium within the physiological range influence receptor-mediated inositol phosphate responses in brain and tracheal smooth muscle slices

The effect of changes in extracellular calcium concentration ([Ca2+]e) on the incorporation of myo-[2-3H]-inositol into phosphoinositides and agonist-stimulated 3H-inositol phosphates (3H-InsPs) was examined in rat cerebral cortex and bovine tracheal smooth muscle slices. In brain slices, reduction in [Ca2+]e from 2.4 to 1.2 mmol/l resulted in an approximate doubling of the carbachol and noradrenaline-stimulated 3H-InsP response with no effect on the EC50 values. An identical effect of varying [Ca2+]e was observed for carbachol-stimulated 3H-InsP formation in tracheal smooth muscle with a further increase in 3H-InsPs evident at [Ca2+]e 0.6 mmol/l. In this tissue the effect of changes in [Ca2+]e on the incorporation of myo-[2-3H]-inositol into the total phosphoinositide pool directly paralleled the changes in 3H-InsPs except in conditions of no added calcium when 3H-InsP responses were markedly impaired. Additional studies in brain slices using buffer where the added calcium varied between 0 and 2.4 mmol/l, showed that both the carbachol stimulated formation of separate inositol phosphates during short incubation periods and incorporation of myo-[2-3H]-inositol into PtdInsP and PtdInsP2 under basal conditions was maximal at [Ca2+]e 0.3 mmol/l. Omitting Ca2+ from the buffer resulted in maximal labelling of PtdIns but a decrease in PtdInsP and PtdInsP2 labelling (compared with the level at [Ca2+]e 0.3 mmol/l) and a markedly impaired inositol polyphosphate response. Alterations in [Ca2+]e following 3H-inositol labelling but immediately prior to carbachol stimulation did not influence 3H-inositol polyphosphate responses. It is therefore clear that even relatively small changes in [Ca2+]e markedly influence agonist-stimulated 3H-InsP responses in brain and tracheal smooth muscle slices and that these reflect changes in the labelling of substrate inositol lipids.(ABSTRACT TRUNCATED AT 250 WORDS)

Inositol uptake and metabolism in molluscan neuronal tissue

The uptake of myo-[3H]inositol into neurones from Lymnaea stagnalis has been demonstrated to be a sodium-dependent process, saturable with a Km of approximately 50 microM and shown to be linear with time for at least 120 min. The rate of transport of myo-inositol into the cell appears to influence directly its incorporation into neuronal lipids. Using anion-exchange high-performance liquid chromatography, we have demonstrated a high rate of breakdown of phosphatidylinositol 4,5-bisphosphate in Lymnaea nerve under basal conditions. Stimulation with carbamylcholine enhanced production of inositol 1-phosphate, inositol bisphosphate, inositol 1,4,5-trisphosphate, and inositol 1,3,4-trisphosphate. Formation of inositol tetrakisphosphate was not detected. Electrical stimulation also caused an increased formation of inositol phosphates. These results provide evidence for an active myo-inositol transport system in molluscan neurones and suggest that the hydrolysis of inositol lipids may play a role as an intracellular signalling system in this tissue.

Uptake and metabolism of myo-inositol by L1210 leukaemia cells

The initial rate of uptake of [3H]myo-inositol by L1210 murine leukaemia cells is directly proportional to the extracellular concentration and unaffected by several analogues of myo-inositol even at millimolar concentrations. Scyllitol, a geometric isomer of myo-inositol, partially inhibited the uptake of myo-inositol (40% at 0.1 mM). A portion of the uptake of myo-inositol was not inhibited even at 5 mM-scyllitol. At steady-state the intracellular concentration of [3H]myo-inositol is directly proportional to the extracellular concentration. Addition of myo-inositol to medium does not enhance the growth of L1210 cells; these cells can maintain an extracellular concentration of 20 microM-myo-inositol even when grown in myo-inositol-free medium. Synthesis of myo-inositol from glucose by L1210 cells was demonstrated by use of [13C]glucose and m.s. L1210 cells maintain myo-inositol pools by a combination of synthesis de novo and uptake of exogenous myo-inositol by either passive diffusion or a low affinity carrier.

Characterization of glucose transport in an insulin-secreting cell line

The rat insulinoma-derived RINm5F cell line retains many differentiated functions of islet beta-cells. However, it fails to recognize glucose as an insulin secretagogue in the physiological concentration range. With this cell line, glucose-transport kinetics were investigated, by using a double-label technique with the non-metabolizable glucose analogue 3-O-methylglucose (OMG). RINm5F cells possess a passive glucose-transport system with high capacity and low affinity. Equilibration across the plasma membrane of extracellular OMG concentrations up to at least 20 mM is achieved within 2 min at 37 degrees C. The half-saturation of OMG uptake occurs at 32 mM. At lower temperatures OMG uptake is markedly retarded, with a temperature coefficient (Q10) of 2.9. As indicated by efflux measurements, transport is symmetrical. Cytochalasin B at micromolar concentrations and phlorrhizin in millimolar concentrations are potent inhibitors of OMG uptake. Neutralization of the secreted insulin with antibodies does not alter OMG uptake kinetics. The glucose metabolism of RINm5F cells is much exaggerated compared with that of islet beta-cells. Nonetheless, when measured in parallel to uptake, transport exceeds by far the rate of metabolism at glucose concentrations above 3 mM. Measurements of intracellular D-glucose reveal a lower intracellular glucose concentration relative to the extracellular in RINm5F cells. This seems to be due to abnormalities in the subsequent steps of glucose metabolism, rather than to abnormalities in hexose uptake. The loss of glucose-induced insulin release in RINm5F cells cannot be explained by alterations in hexose transport.

Signal transduction in insulin secretion: comparison between fuel stimuli and receptor agonists

The initial events in signal transduction in insulin-secreting cells are summarized in FIGURE 8. Both nutrient stimuli, such as glucose and amino acids and the muscarinic agonist carbachol (carbamylcholine) raise [Ca2+]i. Although the rise in [Ca2+]i precedes the stimulation of insulin release, it is not a moment-to-moment regulator of release. The metabolizable fuel stimuli cause Ca2+ influx through voltage-dependent Ca2+ channels following depolarization of the membrane potential. In contrast, carbachol, which does not depolarize, elicits Ptd Ins 4,5-P2 hydrolysis, a reaction catalyzed by phospholipase C. The generation of Ins 1,4,5-P3 in this instance is Ca2+ independent, but appears to involve a GTP-binding protein. However, this protein is not a substrate for pertussis toxin. The levels of Ins 1,4,5-P3, which releases Ca2+ from an ATP-dependent Ca2+ pool of the endoplasmic reticulum, are increased prior to the rise in [Ca2+]i. The mitochondria may take up Ca2+ after large increases in [Ca2+]i. A previously proposed second messenger, arachidonic acid, is much less selective than Ins 1,4,5-P3 in that it releases Ca2+ from mitochondria as well as from the endoplasmic reticulum in a slow and irreversible manner. As Ins 1,4,5-P3 is also generated during glucose stimulation of islets, albeit in a Ca2+-dependent manner, this metabolite could mediate not only the action of carbachol but also contribute to amplifying the [Ca2+]i rise in response to glucose.