Simulations of macromolecules in protective and denaturing osmolytes: properties of mixed solvent systems and their effects on water and protein structure and dynamics

Rarely is any solution simply solute and water. In vivo, solutes, such as proteins and nucleic acids, swim in a sea of water, salts, ions, small molecules, and lipids, not to mention other macromolecules. In vitro, virtually all solutions contain a mixture of aqueous solvents, or "cosolvents" [i.e., solvent(s) in addition to water], that can alter the dynamics, behavior, solubility, and stability of proteins and nucleic acids. We have developed models for a number of cosolvents, including the denaturant urea and the small chemical chaperone trimethylamine N-oxide (TMAO). This chapter examines the models for these two cosolvents in the context of experimental data. The direct and indirect effects of these molecules on water and protein are studied with molecular dynamics simulations. These observations and conclusions are drawn from simulations of these molecules in pure water and as a cosolvent for the protein chymotrypsin inhibitor 2. Urea-induced denaturation occurs initially through attack of the protein by water and hydration of hydrophobic protein moieties as a result of disruption of the hydrogen bonding network of water by urea. This indirect denaturing effect of urea is followed by more direct action as urea replaces some waters involved in the initial hydration of the hydrophobic core and subsequently binds to polar residues and the protein main chain to compete with the intraprotein hydrogen bonds. In the case of TMAO, we find that it encourages water-water interactions, thereby stabilizing the protein as a result of the increased penalty for the hydration of hydrophobic residues.

Methods of Changing Biopolymer Volume Fraction and Cytoplasmic Solute Concentrations for In Vivo Biophysical Studies

In vitro changes in polymer volume fraction (macromolecular crowding) and changes in solute or salt concentration typically have large effects on protein and nucleic acid processes (e.g., folding, binding, assembly, precipitation, crystallization). However, the large changes in these concentration variables, which occur in vivo as part of cellular responses to osmotic stress, appear to have much less dramatic effects on cellular biopolymer processes. Methods of changing intracellular concentrations by varying the extracellular osmolality or the concentration of a permeable solute or by titrating cells with an impermeable solute (plasmolysis) under conditions where an active response is suppressed are reviewed. The first in vivo biophysical studies of protein folding and protein diffusion performed as a function of these variables are also discussed.

Intracellular localization of integrin-like protein and its roles in osmotic stress-induced abscisic acid biosynthesis in Zea mays

Plants have evolved many mechanisms to cope with adverse environmental stresses. Abscisic acid (ABA) accumulates significantly in plant cells in response to drought conditions, and this is believed to be a major mechanism through which plants enhance drought tolerance. In this study, we explore the possible mechanisms of osmotic stress perception by plant cells and the consequent induction of ABA biosynthesis. Immunoblotting and immunofluorescence localization experiments, using a polyclonal antibody against human integrin beta1, revealed the presence of a protein in Zea mays roots that is similar to the integrin proteins of animals and mainly localized in the plasma membrane. Treatment with GRGDS, a synthetic pentapeptide containing an RGD domain, which interacted specifically with the integrin protein and thus blocked the cell wall-plasma membrane interaction, significantly inhibited osmotic stress-induced ABA biosynthesis in cells, and the GRGDS analog which does not contain the RGD domain had no effect. Our results show that a strong interaction exists between the cell wall and plasma membrane and that this interaction is largely mediated by integrin-like proteins. They also imply that the cell wall and/or cell wall-plasma membrane interaction plays important roles in the perception of osmotic stress. Accordingly, we conclude that the cell wall and/or cell wall-plasma membrane interaction mediated by the integrin-like protein plays important roles in osmotic stress-induced ABA biosynthesis in Zea mays.

Molecular Basis of Osmolyte Effects on Protein and Metabolites

Osmolytes can have strong effects on biochemical reactions, such as protein folding or protein-ligand interaction. These effects are mediated through solvation-the nonspecific interaction between the solution components. Therefore, understanding the impact of osmolytes on cellular biochemistry requires an understanding of the underlying solvation processes. This chapter discusses the thermodynamic effects of osmolytes on proteins and small organic molecules in terms of the solvation of these molecules, as derived from Kirkwood-Buff theory. This approach allows experimental determination of solvation properties from thermodynamic data. Knowledge of solvation at this level provides insight into the observed behavior of proteins and small molecules in osmolyte solution on a microscopic level. As examples, we provide solvation effects on protein folding, ligand binding, and osmolyte thermodynamics.

Combined effect of betaine and trehalose on osmotic tolerance of Escherichia coli in mineral salts medium

In mineral salts medium, supplementing with betaine in combination with increased production of endogenous osmoprotectant from a second copy of the trehalose biosynthetic genes (otsBA) improved growth of E. coli and increased the MIC for xylose, glucose, sodium lactate and NaCl. With these compounds, this combination was more effective than either betaine or trehalose alone. With succinate, this combination was no more effective than betaine alone. Neither approach improved tolerance to ethanol. A combination of betaine and increased trehalose may improve strain productivity for many bioproducts by promoting growth in the presence of high sugar concentrations.

Glucosamine-induced increase in Akt phosphorylation corresponds to increased endoplasmic reticulum stress in astroglial cells

Increased glucose flux through the hexosamine biosynthetic pathway (HBP) is known to affect the activity of a number of signal transduction pathways and lead to insulin resistance. Although widely studied in insulin responsive tissues, the effect of increased HBP activity on largely insulin unresponsive tissues, such as the brain, remains relatively unknown. Herein, we investigate the effects of increased HBP flux on Akt activation in a human astroglial cells line using glucosamine, a compound commonly used to mimic hyperglycemic conditions by increasing HBP flux. Cellular treatment with 8 mM glucosamine resulted in a 96.8% +/- 24.6 increase in Akt phosphorylation after 5 h of treatment that remained elevated throughout the 9-h time course. Glucosamine treatment also resulted in modest increases in global levels of the O-GlcNAc protein modification. Increasing O-GlcNAc levels using the O-GlcNAcase inhibitor streptozotocin (STZ) also increased Akt phosphorylation by 96.8% +/- 11.0 after only 3 h although for a shorter duration than glucosamine; however, the more potent O-GlcNAcase inhibitors O-(2-acetamido-2-deoxy-D-glucopyranosylidene)amino-N-phenylcarbamate (PUGNAc) and 1,2-dideoxy-2'-propyl-alpha-D-glucopyranoso-[2,1-d]-Delta2'-thiazoline (NAGBT) failed to mimic the increases in phospho-Akt indicating that the Akt phosphorylation is not a result of increased O-GlcNAc protein modification. Further analysis indicated that this increased phosphorylation was also not due to increased osmotic stress and was not attenuated by N-acetylcysteine eliminating the potential role of oxidative stress in the observed phospho-Akt increases. Glucosamine treatment, but not STZ treatment, did correlate with a large increase in the expression of the endoplasmic reticulum (ER) stress marker GRP 78. Altogether, these results indicate that increased HBP flux in human astroglial cells results in a rapid, short-term phosphorylation of Akt that is likely a result of increased ER stress. The mechanism by which STZ increases Akt phosphorylation, however, remains unknown.

Dysfunction of regulatory volume increase is a key component of apoptosis

Sustained cell shrinkage is a major hallmark of apoptotic cell death. In apoptotic cells, whole cell volume reduction, called apoptotic volume decrease (AVD), proceeds until fragmentation of cells. Under non-apoptotic conditions, human epithelial HeLa cells exhibited a slow regulatory volume increase (RVI) after osmotic shrinkage induced by exposure to hypertonic solution. When AVD was induced by treatment with a Fas ligand, TNF-alpha or staurosporine, however, it was found that HeLa cells failed to undergo RVI. When RVI was inhibited by combined application of Na+/H+ exchanger (NHE) and anion exchanger blockers, hypertonic stress induced prolonged shrinkage followed by caspase-3 activation in HeLa cells. Hypertonicity also induced apoptosis in NHE1-deficient PS120 fibroblasts, which lack the RVI response. When RVI was restored by transfection of these cells with NHE1, hypertonicity-induced apoptosis was completely prevented. Thus, it is concluded that RVI dysfunction is indispensable for the persistence of AVD and induction of apoptosis.

Changes induced by hyperosmotic mannitol in cerebral endothelial cells: an atomic force microscopic study

Understanding the reaction of living cells in response to different extracellular stimuli, such as hyperosmotic stress, is of primordial importance. Mannitol, a cell-impermeable non-toxic alcohol, has been used successfully for reversible opening of the blood-brain barrier in hyperosmotic concentrations. In this study we analyzed the effect of hyperosmotic mannitol on the shape and surface structure of living cerebral endothelial cells by atomic force microscope imaging technique. Addition of clinically relevant concentrations of mannitol to the culture medium of the confluent cells induced a decrease of about 40% in the observed height of the cells. This change was consistent both at the nuclear and peripheral region of the cells. After mannitol treatment even a close examination of the contact surface between the cells did not reveal gap between them. We could observe the appearance of surface protrusions of about 100 nm. By force measurements the elasticity of the cells were estimated. While the Young's modulus of the control cells appeared to be 8.04 +/- 0.12 kPa, for the mannitol-treated cells it decreased to an estimated value of 0.93 +/- 0.04 kPa which points to large structural changes inside the cell.

Polyamine metabolism and biosynthetic gene expression in Arabidopsis thaliana under salt stress

In the present study we analysed polyamine metabolism in Arabidopsis thaliana (ecotype Columbia) rosette leaves collected at vegetative and reproductive stages from plants germinated and grown under increasing salt stress (0-75 mM NaCl) conditions. The expression level of the different isoforms of polyamine biosynthetic enzymes was analysed by reverse transcriptase-polymerase chain reaction (RT-PCR) and the polyamine biosynthetic enzyme activities were determined both in supernatant and pellet fractions. Free and perchloric acid (PCA)-conjugated (soluble and insoluble) polyamines, were measured. At vegetative stage, plants were able to adapt up to 50 mM NaCl, showing a significant growth inhibition only at 75 mM NaCl. At this growth stage and NaCl concentration there was an up-regulation of spermine biosynthesis. At reproductive stage, plants were able to flower up to 50 mM NaCl, even if with a delay of 7 days. On the contrary, at 75 mM NaCl two different phenotypes were isolated: 75/01 (salt sensitive) and 75/02 (salt tolerant). The sensitive plants (75/01) showed a severely stressed phenotype, compared to the tolerant ones (75/02), and the polyamine metabolism was up-regulated, with the increase of free putrescine and spermine.

Signalling pathways involved in hypertonicity- and acidification-induced activation of Na+/H+ exchange in trout hepatocytes

In trout hepatocytes, hypertonicity and cytosolic acidification are known to stimulate Na+/H+ exchanger (NHE) activity, which contributes to recovery of cell volume and intracellular pH (pHi),respectively. The present study investigated the signalling mechanisms underlying NHE activation under these conditions. Exposing trout hepatocytes to cariporide, a specific inhibitor of NHE-1, decreased baseline pHi,completely blocked the hypertonicity-induced increase of pHi and reduced the hypertonicity-induced proton secretion by 80%. Changing extracellular pH (pHe)above and below normal values, and allowing cells to adjust pHi accordingly,significantly delayed alkalinization during hypertonic exposure, whereas following an acid load an enhanced pHi recovery with increasing pHe was seen. Chelating Ca2+, and thereby preventing the hypertonicity-induced increase in intracellular Ca2+ ([Ca2+]i), significantly diminished hypertonic elevation of pHi, indicating that Ca2+signalling might be involved in NHE activation. A reduction in alkalinization and proton secretion was also observed in the presence of the protein kinase A(PKA) inhibitor H-89 or the calmodulin (CaM) inhibitor calmidazolium. A complete inhibition of hypertonic- and acidification-induced changes of pHi concurrent with an increase in hypertonically induced proton efflux was seen with the protein kinase C (PKC) inhibitor chelerythrine. Recovery of pHi following sodium propionate addition was reduced by more than 60% in the presence of cariporide, was sensitive to PKA inhibition, and tended to be reduced by CaM inhibition. In conclusion, we showed that NHE-1 is the main acid secretion mechanism during hypertonicity and recovery following acid loading. In addition, Ca2+-, PKA- and CaM-dependent pathways are involved in NHE-1 activation for recovery of cell volume and pHi. On the other hand, PKC appeared to have an impact on NHE-independent pathways affecting intracellular acid-base homeostasis.

HB-EGF: Increase in the ischemic rat retina and inhibition of osmotic glial cell swelling

We determined whether the expression of heparin-binding epidermal growth factor-like growth factor (HB-EGF) in the sensory rat retina alters during ischemia-reperfusion, and whether HB-EGF affects the osmotic swelling which is a characteristic feature of Müller glial cells after ischemia. Transient retinal ischemia was induced by elevation of the intraocular pressure for 1 h. Western blots revealed an upregulation of HB-EGF in the retina at 1, 3, and 7 days after reperfusion. HB-EGF inhibited the swelling of glial cells in retinal slices, via stimulation of the synaptic release of glutamate and subsequent activation of glial metabotropic glutamate receptors which resulted in an autocrine release of purinergic receptor agonists. Finally, activation of A1 receptors resulted in opening of glial K(+) and Cl(-) channels. It is suggested that the increased expression of HB-EGF and the inhibition of glial cell swelling may be parts of a protective role of HB-EGF in the ischemic retina.

Monitoring expression profiles of antioxidant genes to salinity, iron, oxidative, light and hyperosmotic stresses in the highly salt tolerant grey mangrove, Avicennia marina (Forsk.) Vierh. by mRNA analysis

Plant photosynthesis results in the production of molecular oxygen. An inevitable consequence of this normal process is the production of reactive oxygen species (ROS) by the transfer of electrons to molecular oxygen. Plants are adequately protected by the presence of multiple antioxidative enzymes in different organelles of the plant such as chloroplasts, cytosol, mitochondria and peroxisomes. Under high light and CO(2) limiting conditions caused by environmental stress like salinity, these antioxidative enzymes play an important role in scavenging toxic radicals. To investigate the functions of antioxidative enzymes in a mangrove plant, we isolated three cDNAs encoding cytosolic Cu-Zn SOD (Sod1), catalase (Cat1) and ferritin (Fer1) from Avicennia marina cDNA library. Sod1, Cat1 and Fer1 cDNA encoded full-length proteins with 152, 492 and 261 amino acids respectively. We studied the expression of these antioxidant genes in response to salt, iron, hydrogen peroxide, mannitol and light stress by mRNA expression analysis. Cat1, Fer1 showed short-term induction while Sod1 transcript was found to be unaltered in response to NaCl stress. A decrease in mRNA levels was observed for Sod1, Cat1 while Fer1 mRNA levels remained unaltered with osmotic stress treatment. Sod1, Cat1 and Fer1 mRNA levels were induced by iron, light stress and by direct H(2)O(2) stress treatment, thus confirming their role in oxidative stress response.

Neuronal control of the cardiac responses to osmotic stress in the gastropod limpet Patella caerulea

Mediterranean limpets Patella caerulea were exposed to different salinity conditions and treated with drugs interfering with neuronal control of heartbeat. Heart rate was monitored using a non-invasive method. Limpets were superfused with control (33 g l(-1)), hyposaline (0 and 10 g l(-1)) or hypersaline (56 and 66 g l(-1)) artificial seawater. Under osmotic stress the limpets showed an initial increase of heart rate, followed by acardia, particularly under hyposalinity. The tachycardia observed after exposure to 56 g l(-1) was abolished in the animals injected with a selective sodium channel blocker, tetrodotoxin (TTX), or with a serotoninergic antagonist, methysergide. Injection of TTX also partly prevented the acardia occurring at 0 g l(-1). The acardia was completely prevented after injection with atropine and benzoquinonium, two selective cholinergic antagonists. These findings indicate that cardiac responses of P. caerulea to variations in external salinity are regulated by an extrinsic neuronal control involving the serotoninergic and the cholinergic systems in the tachycardic and acardic responses, respectively.

Proteome analysis of salt stress response in the cyanobacteriumSynechocystis sp. strain PCC 6803

In the present study, changes in protein synthesis patterns after salt shock visualized by 35S-methionine labeling and the changed protein composition in salt-acclimated cells of the cyanobacterium Synechocystis sp. strain PCC 6803 were analyzed by a combination of 2-DE for protein separation and PMF for protein identification. As a basis for the differential analysis, a proteome map with 500 identified protein spots comprising 337 different protein species was established. Fifty-five proteins were found, which are induced by salt shock or accumulated after long-term salt acclimation. Some of the proteins are salt stress-specific, such as enzymes involved in the synthesis of the compatible solute glucosylglycerol, while most of them are involved in general stress acclimation. Particularly, heat-shock proteins and proteins acting against lesions by reactive oxygen species were found. Moreover, changes in enzymes involved in basic carbohydrate metabolism were detected. The dynamic of the proteome of salt-stressed Synechocystis cells was compared to previous data concerning transcriptome analysis revealing that 89% of the proteins induced shortly after salt shock were also found to be induced at the RNA level. However, 42% of the stably up-regulated proteins in salt-acclimated cells were not detected previously using DNA microarrays. The comparison of transcriptomic and proteomic analyses shows the significance of post-transcriptional regulatory mechanisms in acclimation of Synechocystis to high salt concentrations.

Prolactin-releasing peptide is essential to maintain the prolactin level and osmotic balance in freshwater teleost fish

We administered prolactin-releasing peptide (PrRP) or anti-PrRP antiserum to goldfish in fresh water and analyzed their effects on prolactin and osmoregulatory mechanisms. The pituitary mRNA level of prolactin increased by PrRP but decreased by anti-PrRP. The rate of water inflow in the gills decreased by PrRP and increased by anti-PrRP, showing that PrRP restricts branchial water permeability, as also restricted by prolactin. PrRP also expanded the mucous cell layers on the scales, which may restrict efficiently water inflow by the mucous system. Eventually, the plasma osmotic pressure decreased by anti-PrRP. We conclude that PrRP is essential to maintain prolactin levels and osmotic balance in fresh water.

Influence of high salinity on biofilm formation and benzoate assimilation by Pseudomonas aeruginosa

Pseudomonas species were used in bioremediation technologies. In situ conditions, such as marine salinity, could limit the degradation of hydrocarbons and aromatic compounds by the bacteria. Biofilm ability to tolerate environmental stress could be used to increase biorestoration. In this report, we used scanning confocal laser microscopy and microtiter dish assay to analyse the impact of hyperosmotic stress on biofilm formation by Pseudomonas aeruginosa. We used benzoate as the sole carbon source and the effect of the stress on its degradation was also studied. Hyperosmotic shock inhibited the biofilm development and decreased the degradation of benzoate. The osmoprotectant glycine betaine partially restored both the biofilm formation and benzoate degradation, suggesting that it could be used as a complement in bioremediation processes.

Coupled K+-water flux through the HERG potassium channel measured by an osmotic pulse method

The streaming potential (V(stream)) is a signature feature of ion channels in which permeating ions and water molecules move in a single file. V(stream) provides a quantitative measure of the ion and water flux (the water-ion coupling ratio), the knowledge of which is a prerequisite for elucidating the mechanisms of ion permeation. We have developed a method to measure V(stream) with the whole-cell patch-clamp configuration. A HEK293 cell stably expressing the HERG potassium channel was voltage clamped and exposed to hyperosmotic solutions for short periods of time (<1 s) by an ultrafast solution switching system (the osmotic pulse [quick jump-and-away] method). The reversal potentials were monitored by a series of voltage ramps before, during, and after the osmotic pulse. The shifts of the reversal potentials immediately after the osmotic jump gave V(stream). In symmetrical K+ solutions (10 mM), the V(stream)s measured at different osmolalities showed a linear relationship with a slope of -0.7 mV/DeltaOsm, from which the water-ion coupling ratio (n, the ratio of the flux of water to the flux of cations; Levitt, D.G., S.R. Elias, and J.M. Hautman. 1978. Biochim. Biophys. Acta. 512:436-451) was calculated to be 1.4. In symmetrical 100 mM K+ solutions, the coupling ratio was decreased significantly (n = 0.9), indicating that the permeation process through states with increased ion occupancy became significant. We presented a diagrammatic representation linking the water-ion coupling ratio to the mode of ion permeation and suggested that the coupling ratio of one may represent the least hydrated ion flux in the single-file pore.

Na+/K+ selectivity of leaf sheath in wheat cultivars differing in salt tolerance

A salt-tolerant wheat cultivar (DK961) and a salt-sensitive one (LM15) were exposed to 200 mmol/L NaCl for 3 d to study selectivity in transport of K(+) and Na(+) of the leaf sheath. Na(+) content of leaf sheath and K(+) content of leaf blade of DK961 were significantly higher than those of LM15, which led to a lower Na(+)/K(+) ratio of leaf blade of DK961 than that of LM15. These results suggested that leaf sheath of DK961 had much stronger abilities of limiting Na(+) transport and allowing K(+) transport from leaf sheath to leaf blade than that of LM15 to maintain lower Na(+)/K(+) ratio of leaf blade under NaCl stress, indicating that leaf sheath may play an important role in wheat salt tolerance.

Expression analysis of barley (Hordeum vulgare L.) during salinity stress

Barley (Hordeum vulgare L.) is a salt-tolerant crop species with considerable economic importance in salinity-affected arid and semiarid regions of the world. In this work, barley cultivar Morex was used for transcriptional profiling during salinity stress using a microarray containing approximately 22,750 probe sets. The experiment was designed to target the early responses of genes to a salinity stress at seedling stage. We found a comparable number of probe sets up-regulated and down-regulated in response to salinity. The differentially expressed genes were broadly characterized using gene ontology and through expression-based hierarchical clustering to identify interesting features in the data. A prominent feature of the response to salinity was the induction of genes involved in jasmonic acid biosynthesis and genes known to respond to jasmonic acid treatment. A large number of abiotic stress (heat, drought, and low temperature) related genes were also found to be responsive to salinity stress. Our results also indicate osmoprotection to be an early response of barley under salinity stress. Additionally, we compared the results of our studies with two other reports characterizing gene expression of barley under salinity stress and found very few genes in common.

Activator-specific recruitment of Mediator in vivo

The Mediator complex associates with eukaryotic RNA polymerase (Pol) II and is recruited to transcriptional enhancers by activator proteins. It is believed that Mediator is a general component of the Pol II machinery that is crucial to connect enhancer-bound activators to basic transcription factors. However, we show that Mediator does not detectably associate with many highly active Pol II promoters in yeast cells. Furthermore, in response to stress conditions, Mediator association is not directly related to Pol II association and in some cases is not detectable at highly activated promoters. Thus, Mediator is recruited to enhancers in an activator-specific manner, and it does not seem to be a stoichiometric component of the basic Pol II machinery in vivo. Mediator is recruited by many activators involved in stress responses, but not by the major activators that function under optimal conditions.

Regulatory volume increase (RVI) by in situ and isolated bovine articular chondrocytes

Metabolism of the matrix by chondrocytes is sensitive to alterations in cell volume that occur, for example, during static loading and osteoarthritis. The ability of chondrocytes to respond to changes in volume could be important, and this study was aimed at testing the hypothesis that chondrocytes can regulate their volume following cell shrinking by regulatory volume increase (RVI). We used single cell fluorescence imaging of in situ bovine articular chondrocytes, cells freshly isolated into 280 or 380 mOsm, or 2-D cultured chondrocytes loaded with calcein or fura-2, to investigate RVI and changes to [Ca2+]i during shrinkage. Following a 42% hyperosmotic challenge, chondrocytes rapidly shrunk, however, only approximately 6% of the in situ or freshly isolated chondrocytes demonstrated RVI. This contrasted with 2D-cultured chondrocytes where approximately 54% of the cells exhibited RVI. The rate of RVI was the same for all preparations. During the 'post-RVD/RVI protocol', approximately 60% of the in situ and freshly isolated chondrocytes demonstrated RVD, but only approximately 5% showed RVI. There was no relationship between [Ca2+]i and RVI either during hyperosmotic challenge, or during RVD suggesting that changes to [Ca2+]i were not required for RVI. Depolymerisation of the actin cytoskeleton by latrunculin, increased RVI by freshly isolated chondrocytes, in a bumetanide-sensitive manner. The results showed that in situ and freshly isolated articular chondrocytes have only limited RVI capacity. However, RVI was stimulated by treating freshly isolated chondrocytes with latrunculin B and following 2D culture of chondrocytes, suggesting that cytoskeletal integrity plays a role in regulating RVI activity which appears to be mediated principally by the Na+ - K+ -2Cl- cotransporter.

Transient Osmotic Absorption of Fluid in Microvessels Exposed to Low Concentrations of Dimethyl Sulfoxide

Dimethyl Sulfoxide (DMSO) is a common solvent for pharmacological agents. It is a small, lipophilic molecule thought to be relatively highly permeable through the cell membrane. While measuring the effect of low concentrations of DMSO (0.05-0.5% v/v) on capillary hydraulic conductivity as a vehicle control for pharmacological agents, the authors noticed what appeared to be an unusual transient absorption of fluid across the vessel wall. This absorption occurred during occlusion of the vessel, but dissipated quickly (1.7-8.6 s). The transient reabsorption reappeared upon each successive occlusion. To determine the nature of this transient absorption, the authors have measured the effect of increasing the pressure of the perfusing solution, of the concentration and time of perfusion of DMSO, and of superfusing the DMSO. They found that the absorption rate, but not the filtration rate, was concentration dependent, and was significantly correlated with the osmotic pressure of the DMSO. Moreover, the time taken for completion of the transient, i.e., time to reversal of flow, was inversely proportional to the hydraulic conductivity of the vessel. Furthermore, the transient absorption could be reduced and eventually abolished by increasing the hydrostatic pressure. These results strongly suggested that perfusion with low concentrations of DMSO could set up a significant osmotic pressure gradient across the vessel wall. This proposed mechanism for the absorption was confirmed by the measurement of a significant osmotic reflection coefficient of the vessel wall to DMSO (0.11 +/- 0.01). Relatively low concentrations (0.05-0.5%) of DMSO were therefore able to stimulate a significant osmotic transient across the blood vessel walls.

[Computer modeling the hydrostatic pressure characteristics of the membrane potential for polymeric membrane, separated non-homogeneous electrolyte solutions]

On the basis of model equation depending the membrane potential deltapsis, on mechanical pressure difference (deltaP), concentration polarization coefficient (zetas), concentration Rayleigh number (RC) and ratio concentration of solutions separated by membrane (Ch/Cl), the characteristics deltapsis = f(deltaP)zetas,RC,Ch/Cl for steady values of zetas, RC and Ch/Cl in single-membrane system were calculated. In this system neutral and isotropic polymeric membrane oriented in horizontal plane, the non-homogeneous binary electrolytic solutions of various concentrations were separated. Nonhomogeneity of solutions is results from creations of the concentration boundary layers on both sides of the membrane. Calculations were made for the case where on a one side of the membrane aqueous solution of NaCl at steady concentration 10(-3) mol x l(-1) (Cl) was placed and on the other aqueous solutions of NaCl at concentrations from 10(-3) mol x l(-1) to 2 x 10(-2) mol x l(-1) (Ch). Their densities were greater than NaCl solution's at 10(-3) mol x l(-1). It was shown that membrane potential depends on hydrodynamic state of a complex concentration boundary layer-membrane-concentration boundary layer, what is controlled by deltaP, Ch/Cl, RC and zetas.

The developmental expression of K+ channels in retinal glial cells is associated with a decrease of osmotic cell swelling

A major function of glial cells is the control of osmotic and ionic homeostasis, mediated by K+ and water movements predominantly through inwardly rectifying K+ (Kir) and aquaporin water channels. It has been suggested that K+ currents through Kir channels are implicated in the regulation of glial cell volume. Here, we investigated whether the developmental increase in Kir channel expression in Müller glial cells of the rat retina is associated with an alteration of cell volume regulation under anisoosmotic conditions. Around the time of eye opening at postnatal day (P) 15, developing retinal glial cells fully alter the profile of their membrane conductances, from a current pattern with prominent fast transient K+ and Na+ currents to a pattern of noninactivating currents through Kir and delayed rectifier K+ channels. Concomitantly, aquaporins-1 and -4 are expressed in the developing retina. This is accompanied by a conspicuous alteration of the swelling characteristics of cells; somata of immature glial cells in early postnatal retinas (P5-P15) swell under hypotonic stress but no swelling is inducible in mature cells at P18 and thereafter. However, glial cells at all developmental stages swell when their Kir channels are blocked by Ba2+. The postnatal maturation of Kir channel currents and volume regulation in retinal glial cells is delayed by visual deprivation. The data suggest that Kir channels are crucially involved in osmotic volume homeostasis of mature glial cells, and that the absence of Kir channels in immature cells is a major cause of their insufficient volume regulation.

Effect of duration of osmotic downshock and coexisting glutamate on survival and uptake of ectoine in halotolerant Brevibacterium sp. JCM 6894

Halotolerant Brevibacterium sp. JCM 6894 that was subjected to an osmotic downshock (0.7 M NaCl to 0 M) was examined for its survival and uptake of ectoine in the presence of ectoine and/or carbon sources. In the presence of ectoine alone, the rates of ectoine uptake by the 1 h-downshocked cells were low and high in the absence and presence of 0.7 M NaCl, respectively, which were in parallel with the rates of cell growth. The presence of glutamate or amino acids together with ectoine exerted a stimulative effect on the survival of downshocked cells. The incubation time of the cells subjected to osmotic downshock strongly affected ectoine uptake as well as the cell growth of this strain, suggesting that the transporter of ectoine in the strain JCM 6894 was stimulated during the osmotic downshock for about 1 h. Different downshock strengths had marked effects on the rate of ectoine uptake when the downshocked cells were incubated in the presence of NaCl.